Internet DRAFT - draft-ietf-idr-bgp4
draft-ietf-idr-bgp4
Network Working Group Y. Rekhter
INTERNET DRAFT T.Li
Obsoletes: RFC1771 S. Hares
Editors
A Border Gateway Protocol 4 (BGP-4)
<draft-ietf-idr-bgp4-26.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol.
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the list of
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Autonomous Systems (ASs) that reachability information traverses.
This information is sufficient to construct a graph of AS
connectivity for this reachability from which routing loops may be
pruned and some policy decisions at the AS level may be enforced.
BGP-4 provides a set of mechanisms for supporting Classless Inter-
Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include
support for advertising a set of destinations as an IP prefix, and
eliminating the concept of network "class" within BGP. BGP-4 also
introduces mechanisms which allow aggregation of routes, including
aggregation of AS paths.
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy
decisions that can (and can not) be enforced using BGP. BGP can
support only the policies conforming to the destination-based
forwarding paradigm.
This specification covers only the exchange of IP version 4 network
reachability information.
This document obsoletes RFC1771.
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Table of Contents
1. Definition of commonly used terms . . . . . . . . . . . . . . 5
2. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
Specification of Requirements . . . . . . . . . . . . . . . . . . 8
3. Summary of Operation . . . . . . . . . . . . . . . . . . . . . 8
3.1 Routes: Advertisement and Storage . . . . . . . . . . . . . . 9
3.2 Routing Information Bases . . . . . . . . . . . . . . . . . . 10
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 12
4.1 Message Header Format . . . . . . . . . . . . . . . . . . . . 12
4.2 OPEN Message Format . . . . . . . . . . . . . . . . . . . . . 13
4.3 UPDATE Message Format . . . . . . . . . . . . . . . . . . . . 15
4.4 KEEPALIVE Message Format . . . . . . . . . . . . . . . . . . 22
4.5 NOTIFICATION Message Format . . . . . . . . . . . . . . . . . 22
5. Path Attributes . . . . . . . . . . . . . . . . . . . . . . . 24
5.1 Path Attribute Usage . . . . . . . . . . . . . . . . . . . . 26
5.1.1 ORIGIN . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1.2 AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1.3 NEXT_HOP . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.1.4 MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . . . 29
5.1.5 LOCAL_PREF . . . . . . . . . . . . . . . . . . . . . . . . 30
5.1.6 ATOMIC_AGGREGATE . . . . . . . . . . . . . . . . . . . . . 30
5.1.7 AGGREGATOR . . . . . . . . . . . . . . . . . . . . . . . . 31
6. BGP Error Handling . . . . . . . . . . . . . . . . . . . . . . 31
6.1 Message Header error handling . . . . . . . . . . . . . . . . 31
6.2 OPEN message error handling . . . . . . . . . . . . . . . . . 32
6.3 UPDATE message error handling . . . . . . . . . . . . . . . . 33
6.4 NOTIFICATION message error handling . . . . . . . . . . . . . 35
6.5 Hold Timer Expired error handling . . . . . . . . . . . . . . 35
6.6 Finite State Machine error handling . . . . . . . . . . . . . 35
6.7 Cease . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6.8 BGP connection collision detection . . . . . . . . . . . . . 36
7. BGP Version Negotiation . . . . . . . . . . . . . . . . . . . 37
8. BGP Finite State machine . . . . . . . . . . . . . . . . . . . 38
8.1 Events for the BGP FSM . . . . . . . . . . . . . . . . . . . 39
8.1.1 Optional Events linked to Optional Session attributes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
8.1.2 Administrative Events . . . . . . . . . . . . . . . . . . 44
8.1.3 Timer Events . . . . . . . . . . . . . . . . . . . . . . . 47
8.1.4 TCP connection based Events . . . . . . . . . . . . . . . . 49
8.1.5 BGP Messages based Events . . . . . . . . . . . . . . . . . 51
8.2 Description of FSM . . . . . . . . . . . . . . . . . . . . . 53
8.2.1 FSM Definition . . . . . . . . . . . . . . . . . . . . . . 53
8.2.1.1 Terms "active" and "passive" . . . . . . . . . . . . . . 54
8.2.1.2 FSM and collision detection . . . . . . . . . . . . . . . 54
8.2.1.3 FSM and Optional Attributes . . . . . . . . . . . . . . 55
8.2.1.4 FSM Event numbers . . . . . . . . . . . . . . . . . . . . 55
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8.2.1.5 FSM actions that are implementation dependent . . . . . . 56
8.2.2 Finite State Machine . . . . . . . . . . . . . . . . . . . 56
9. UPDATE Message Handling . . . . . . . . . . . . . . . . . . . 72
9.1 Decision Process . . . . . . . . . . . . . . . . . . . . . . 73
9.1.1 Phase 1: Calculation of Degree of Preference . . . . . . . 74
9.1.2 Phase 2: Route Selection . . . . . . . . . . . . . . . . . 74
9.1.2.1 Route Resolvability Condition . . . . . . . . . . . . . . 76
9.1.2.2 Breaking Ties (Phase 2) . . . . . . . . . . . . . . . . . 77
9.1.3 Phase 3: Route Dissemination . . . . . . . . . . . . . . . 79
9.1.4 Overlapping Routes . . . . . . . . . . . . . . . . . . . . 80
9.2 Update-Send Process . . . . . . . . . . . . . . . . . . . . . 81
9.2.1 Controlling Routing Traffic Overhead . . . . . . . . . . . 82
9.2.1.1 Frequency of Route Advertisement . . . . . . . . . . . . 82
9.2.1.2 Frequency of Route Origination . . . . . . . . . . . . . 83
9.2.2 Efficient Organization of Routing Information . . . . . . . 83
9.2.2.1 Information Reduction . . . . . . . . . . . . . . . . . . 83
9.2.2.2 Aggregating Routing Information . . . . . . . . . . . . . 84
9.3 Route Selection Criteria . . . . . . . . . . . . . . . . . . 86
9.4 Originating BGP routes . . . . . . . . . . . . . . . . . . . 87
10. BGP Timers . . . . . . . . . . . . . . . . . . . . . . . . . 87
Appendix A. Comparison with RFC1771 . . . . . . . . . . . . . . . 88
Appendix B. Comparison with RFC1267 . . . . . . . . . . . . . . . 89
Appendix C. Comparison with RFC 1163 . . . . . . . . . . . . . . 90
Appendix D. Comparison with RFC 1105 . . . . . . . . . . . . . . 90
Appendix E. TCP options that may be used with BGP . . . . . . . . 91
Appendix F. Implementation Recommendations . . . . . . . . . . . 91
Appendix F.1 Multiple Networks Per Message . . . . . . . . . . . 91
Appendix F.2 Reducing route flapping . . . . . . . . . . . . . . 92
Appendix F.3 Path attribute ordering . . . . . . . . . . . . . . 92
Appendix F.4 AS_SET sorting . . . . . . . . . . . . . . . . . . . 92
Appendix F.5 Control over version negotiation . . . . . . . . . . 93
Appendix F.6 Complex AS_PATH aggregation . . . . . . . . . . . . 93
Security Considerations . . . . . . . . . . . . . . . . . . . . . 94
IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 95
IPR Disclosure Acknowledgement . . . . . . . . . . . . . . . . . 97
Copyright Notice . . . . . . . . . . . . . . . . . . . . . . . . 98
Normative References . . . . . . . . . . . . . . . . . . . . . . 98
Non-normative References . . . . . . . . . . . . . . . . . . . . 99
Authors Information . . . . . . . . . . . . . . . . . . . . . . . 100
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Abstract
The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
ing protocol.
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reacha-
bility information includes information on the list of Autonomous
Systems (ASs) that reachability information traverses. This informa-
tion is sufficient to construct a graph of AS connectivity for this
reachability from which routing loops may be pruned and some policy
decisions at the AS level may be enforced.
BGP-4 provides a set of mechanisms for supporting Classless Inter-
Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include
support for advertising a set of destinations as an IP prefix and
eliminating the concept of network "class" within BGP. BGP-4 also
introduces mechanisms which allow aggregation of routes, including
aggregation of AS paths.
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy deci-
sions that can (and can not) be enforced using BGP. BGP can support
only the policies conforming to the destination-based forwarding par-
adigm.
1. Definition of commonly used terms
This section provides definition for terms that have a specific mean-
ing to the BGP protocol and that are used throughout the text.
Adj-RIB-In
The Adj-RIBs-In contain unprocessed routing information that has
been advertised to the local BGP speaker by its peers.
Adj-RIB-Out
The Adj-RIBs-Out contains the routes for advertisement to specific
peers by means of the local speaker's UPDATE messages.
Autonomous System (AS)
The classic definition of an Autonomous System is a set of routers
under a single technical administration, using an interior gateway
protocol (IGP) and common metrics to determine how to route pack-
ets within the AS, and using an inter-AS routing protocol to
determine how to route packets to other ASs. Since this classic
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definition was developed, it has become common for a single AS to
use several IGPs and sometimes several sets of metrics within an
AS. The use of the term Autonomous System here stresses the fact
that, even when multiple IGPs and metrics are used, the adminis-
tration of an AS appears to other ASs to have a single coherent
interior routing plan and presents a consistent picture of what
destinations are reachable through it.
BGP Identifier
A 4-octet unsigned integer indicating the BGP Identifier of the
sender of BGP messages. A given BGP speaker sets the value of its
BGP Identifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer.
BGP speaker
A router that implements BGP.
EBGP
External BGP (BGP connection between external peers).
External peer
Peer that is in a different Autonomous System than the local sys-
tem.
Feasible route
An advertised route that is available for use by the recipient.
IBGP
Internal BGP (BGP connection between internal peers).
Internal peer
Peer that is in the same Autonomous System as the local system.
IGP
Interior Gateway Protocol - a routing protocol used to exchange
routing information among routers within a single Autonomous Sys-
tem.
Loc-RIB
The Loc-RIB contains the routes that have been selected by the
local BGP speaker's Decision Process.
NLRI
Network Layer Reachability Information.
Route
A unit of information that pairs a set of destinations with the
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attributes of a path to those destinations. The set of destina-
tions are systems whose IP addresses are contained in one IP
address prefix carried in the Network Layer Reachability Informa-
tion (NLRI) field of an UPDATE message. The path is the informa-
tion reported in the path attributes field of the same UPDATE mes-
sage.
RIB
Routing Information Base.
Unfeasible route
A previously advertised feasible route that is no longer available
for use.
2. Acknowledgments
This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed and Yakov Rekhter.
We would like to express our thanks to Guy Almes, Len Bosack, and
Jeffrey C. Honig for their contributions to the earlier version
(BGP-1) of this document.
We would like to specially acknowledge numerous contributions by Den-
nis Ferguson to the earlier version of this document.
We like to explicitly thank Bob Braden for the review of the earlier
version (BGP-2) of this document as well as his constructive and
valuable comments.
We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he
assembled to review the earlier version (BGP-2) of this document.
This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
with a strong combination of toughness, professionalism, and cour-
tesy.
Certain sections of the document borrowed heavily from IDRP
[IS10747], which is the OSI counterpart of BGP. For this credit
should be given to the ANSI X3S3.3 group chaired by Lyman Chapin and
to Charles Kunzinger who was the IDRP editor within that group.
We would also like to thank Benjamin Abarbanel, Enke Chen, Edward
Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey Haas, Dimitry
Haskin, Stephen Kent, John Krawczyk, David LeRoy, Dan Massey,
Jonathan Natale, Dan Pei, Mathew Richardson, John Scudder, John
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Stewart III, Dave Thaler, Paul Traina, Russ White, Curtis Villamizar,
and Alex Zinin for their comments.
We would like to specially acknowledge Andrew Lange for his help in
preparing the final version of this document.
Finally, we would like to thank all the members of the IDR Working
Group for their ideas and support they have given to this document.
Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
3. Summary of Operation
The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
ing protocol. It is built on experience gained with EGP as defined in
[RFC904] and EGP usage in the NSFNET Backbone as described in
[RFC1092] and [RFC1093].
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reacha-
bility information includes information on the list of Autonomous
Systems (ASs) that reachability information traverses. This informa-
tion is sufficient to construct a graph of AS connectivity from which
routing loops may be pruned and some policy decisions at the AS level
may be enforced.
In the context of this document we assume that a BGP speaker adver-
tises to its peers only those routes that it itself uses (in this
context a BGP speaker is said to "use" a BGP route if it is the most
preferred BGP route and is used in forwarding). All other cases are
outside the scope of this document.
In the context of this document the term "IP address" refers to an IP
Version 4 address [RFC791].
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy deci-
sions that can (and can not) be enforced using BGP. Note that some
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policies can not be supported by the destination-based forwarding
paradigm, and thus require techniques such as source routing (aka
explicit routing) to be enforced. Such policies can not be enforced
using BGP either. For example, BGP does not enable one AS to send
traffic to a neighboring AS for forwarding to some destination
(reachable through but) beyond that neighboring AS intending that the
traffic take a different route to that taken by the traffic originat-
ing in the neighboring AS (for that same destination). On the other
hand, BGP can support any policy conforming to the destination-based
forwarding paradigm.
BGP-4 provides a new set of mechanisms for supporting Classless
Inter-Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms
include support for advertising a set of destinations as an IP prefix
and eliminating the concept of network "class" within BGP. BGP-4
also introduces mechanisms which allow aggregation of routes, includ-
ing aggregation of AS paths.
This document uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
(IGP) and common metrics to determine how to route packets within the
AS, and using an inter-AS routing protocol to determine how to route
packets to other ASs. Since this classic definition was developed, it
has become common for a single AS to use several IGPs and sometimes
several sets of metrics within an AS. The use of the term Autonomous
System here stresses the fact that, even when multiple IGPs and met-
rics are used, the administration of an AS appears to other ASs to
have a single coherent interior routing plan and presents a consis-
tent picture of what destinations are reachable through it.
BGP uses TCP [RFC793] as its transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgment, and sequencing. BGP listens on TCP port 179. The
error notification mechanism used in BGP assumes that TCP supports a
"graceful" close, i.e., that all outstanding data will be delivered
before the connection is closed.
Two systems form a TCP connection between one another. They exchange
messages to open and confirm the connection parameters.
The initial data flow is the portion of the BGP routing table that is
allowed by the export policy, called the Adj-Ribs-Out (see 3.2).
Incremental updates are sent as the routing tables change. BGP does
not require periodic refresh of the routing table. To allow local
policy changes to have the correct effect without resetting any BGP
connections, a BGP speaker SHOULD either (a) retain the current ver-
sion of the routes advertised to it by all of its peers for the
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duration of the connection, or (b) make use of the Route Refresh
extension [RFC2918].
KEEPALIVE messages may be sent periodically to ensure the liveness of
the connection. NOTIFICATION messages are sent in response to errors
or special conditions. If a connection encounters an error condition,
a NOTIFICATION message is sent and the connection is closed.
A peer in a different AS is referred to as an external peer, while a
peer in the same AS is referred to as an internal peer. Internal BGP
and external BGP are commonly abbreviated IBGP and EBGP.
If a particular AS has multiple BGP speakers and is providing transit
service for other ASs, then care must be taken to ensure a consistent
view of routing within the AS. A consistent view of the interior
routes of the AS is provided by the IGP used within the AS. For the
purpose of this document, it is assumed that a consistent view of the
routes exterior to the AS is provided by having all BGP speakers
within the AS maintain IBGP with each other.
This document specifies the base behavior of the BGP protocol. This
behavior can and is modified by extension specifications. When the
protocol is extended the new behavior is fully documented in the
extension specifications.
3.1 Routes: Advertisement and Storage
For the purpose of this protocol, a route is defined as a unit of
information that pairs a set of destinations with the attributes of a
path to those destinations. The set of destinations are systems whose
IP addresses are contained in one IP address prefix carried in the
Network Layer Reachability Information (NLRI) field of an UPDATE mes-
sage, and the path is the information reported in the path attributes
field of the same UPDATE message.
Routes are advertised between BGP speakers in UPDATE messages. Mul-
tiple routes that have the same path attributes can be advertised in
a single UPDATE message by including multiple prefixes in the NLRI
field of the UPDATE message.
Routes are stored in the Routing Information Bases (RIBs): namely,
the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out, as described in
Section 3.2.
If a BGP speaker chooses to advertise a previously received route, it
MAY add to or modify the path attributes of the route before adver-
tising it to a peer.
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BGP provides mechanisms by which a BGP speaker can inform its peer
that a previously advertised route is no longer available for use.
There are three methods by which a given BGP speaker can indicate
that a route has been withdrawn from service:
a) the IP prefix that expresses the destination for a previously
advertised route can be advertised in the WITHDRAWN ROUTES field
in the UPDATE message, thus marking the associated route as being
no longer available for use
b) a replacement route with the same NLRI can be advertised, or
c) the BGP speaker - BGP speaker connection can be closed, which
implicitly removes from service all routes which the pair of
speakers had advertised to each other.
Changing attribute(s) of a route is accomplished by advertising a
replacement route. The replacement route carries new (changed)
attributes and has the same address prefix as the original route.
3.2 Routing Information Base
The Routing Information Base (RIB) within a BGP speaker consists of
three distinct parts:
a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
been learned from inbound UPDATE messages received from other BGP
speakers. Their contents represent routes that are available as an
input to the Decision Process.
b) Loc-RIB: The Loc-RIB contains the local routing information
that the BGP speaker has selected by applying its local policies
to the routing information contained in its Adj-RIBs-In. These are
the routes that will be used by the local BGP speaker. The next
hop for each of these routes MUST be resolvable via the local BGP
speaker's Routing Table.
c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
local BGP speaker has selected for advertisement to its peers. The
routing information stored in the Adj-RIBs-Out will be carried in
the local BGP speaker's UPDATE messages and advertised to its
peers.
In summary, the Adj-RIBs-In contain unprocessed routing information
that has been advertised to the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local BGP
speaker's Decision Process; and the Adj-RIBs-Out organize the routes
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for advertisement to specific peers by means of the local speaker's
UPDATE messages.
Although the conceptual model distinguishes between Adj-RIBs-In, Loc-
RIB, and Adj-RIBs-Out, this neither implies nor requires that an
implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of
the information vs 1 copy with pointers) is not constrained by the
protocol.
Routing information that the BGP speaker uses to forward packets (or
to construct the forwarding table that is used for packet forwarding)
is maintained in the Routing Table. The Routing Table accumulates
routes to directly connected networks, static routes, routes learned
from the IGP protocols, and routes learned from BGP. Whether or not
a specific BGP route should be installed in the Routing Table, and
whether a BGP route should override a route to the same destination
installed by another source is a local policy decision, not specified
in this document. Besides actual packet forwarding, the Routing Table
is used for resolution of the next-hop addresses specified in BGP
updates (see Section 5.1.3).
4. Message Formats
This section describes message formats used by BGP.
BGP messages are sent over a TCP connection. A message is processed
only after it is entirely received. The maximum message size is 4096
octets. All implementations are required to support this maximum mes-
sage size. The smallest message that may be sent consists of a BGP
header without a data portion, or 19 octets.
All multi-octet fields are in network byte order.
4.1 Message Header Format
Each message has a fixed-size header. There may or may not be a data
portion following the header, depending on the message type. The lay-
out of these fields is shown below:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ +
| Marker |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker:
This 16-octet field is included for compatibility; it MUST be
set to all ones.
Length:
This 2-octet unsigned integer indicates the total length of the
message, including the header, in octets. Thus, e.g., it allows
one to locate in the TCP stream the (Marker field of the) next
message. The value of the Length field MUST always be at least
19 and no greater than 4096, and MAY be further constrained,
depending on the message type. No "padding" of extra data after
the message is allowed, so the Length field MUST have the
smallest value required given the rest of the message.
Type:
This 1-octet unsigned integer indicates the type code of the
message. This document defines the following type codes:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
[RFC2918] defines one more type code.
4.2 OPEN Message Format
After a TCP connection is established, the first message sent by each
side is an OPEN message. If the OPEN message is acceptable, a
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KEEPALIVE message confirming the OPEN is sent back.
In addition to the fixed-size BGP header, the OPEN message contains
the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version:
This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 4.
My Autonomous System:
This 2-octet unsigned integer indicates the Autonomous System
number of the sender.
Hold Time:
This 2-octet unsigned integer indicates the number of seconds
that the sender proposes for the value of the Hold Timer. Upon
receipt of an OPEN message, a BGP speaker MUST calculate the
value of the Hold Timer by using the smaller of its configured
Hold Time and the Hold Time received in the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An
implementation MAY reject connections on the basis of the Hold
Time. The calculated value indicates the maximum number of
seconds that may elapse between the receipt of successive
KEEPALIVE, and/or UPDATE messages by the sender.
BGP Identifier:
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This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP Iden-
tifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer.
Optional Parameters Length:
This 1-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this field
is zero, no Optional Parameters are present.
Optional Parameters:
This field contains a list of optional parameters, where each
parameter is encoded as a <Parameter Type, Parameter Length,
Parameter Value> triplet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
Parameter Type is a one octet field that unambiguously identi-
fies individual parameters. Parameter Length is a one octet
field that contains the length of the Parameter Value field in
octets. Parameter Value is a variable length field that is
interpreted according to the value of the Parameter Type field.
[RFC3392] defines the Capabilities Optional Parameter.
The minimum length of the OPEN message is 29 octets (including mes-
sage header).
4.3 UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE message can be used to construct
a graph describing the relationships of the various Autonomous Sys-
tems. By applying rules to be discussed, routing information loops
and some other anomalies may be detected and removed from inter-AS
routing.
An UPDATE message is used to advertise feasible routes sharing common
path attributes to a peer, or to withdraw multiple unfeasible routes
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from service (see 3.1). An UPDATE message MAY simultaneously adver-
tise a feasible route and withdraw multiple unfeasible routes from
service. The UPDATE message always includes the fixed-size BGP
header, and also includes the other fields as shown below (note, some
of the shown fields may not be present in every UPDATE message):
+-----------------------------------------------------+
| Withdrawn Routes Length (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+
Withdrawn Routes Length:
This 2-octets unsigned integer indicates the total length of
the Withdrawn Routes field in octets. Its value allows the
length of the Network Layer Reachability Information field to
be determined as specified below.
A value of 0 indicates that no routes are being withdrawn from
service, and that the WITHDRAWN ROUTES field is not present in
this UPDATE message.
Withdrawn Routes:
This is a variable length field that contains a list of IP
address prefixes for the routes that are being withdrawn from
service. Each IP address prefix is encoded as a 2-tuple of the
form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
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The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains an IP address prefix followed by
the minimum number of trailing bits needed to make the end
of the field fall on an octet boundary. Note that the value
of trailing bits is irrelevant.
Total Path Attribute Length:
This 2-octet unsigned integer indicates the total length of the
Path Attributes field in octets. Its value allows the length of
the Network Layer Reachability field to be determined as speci-
fied below.
A value of 0 indicates that neither the Network Layer Reacha-
bility Information field, nor the Path Attribute field is
present in this UPDATE message.
Path Attributes:
A variable length sequence of path attributes is present in
every UPDATE message, except for an UPDATE message that carries
only the withdrawn routes. Each path attribute is a triple
<attribute type, attribute length, attribute value> of variable
length.
Attribute Type is a two-octet field that consists of the
Attribute Flags octet followed by the Attribute Type Code
octet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The high-order bit (bit 0) of the Attribute Flags octet is the
Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0).
The second high-order bit (bit 1) of the Attribute Flags octet
is the Transitive bit. It defines whether an optional attribute
is transitive (if set to 1) or non-transitive (if set to 0).
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For well-known attributes, the Transitive bit MUST be set to 1.
(See Section 5 for a discussion of transitive attributes.)
The third high-order bit (bit 2) of the Attribute Flags octet
is the Partial bit. It defines whether the information con-
tained in the optional transitive attribute is partial (if set
to 1) or complete (if set to 0). For well-known attributes and
for optional non-transitive attributes the Partial bit MUST be
set to 0.
The fourth high-order bit (bit 3) of the Attribute Flags octet
is the Extended Length bit. It defines whether the Attribute
Length is one octet (if set to 0) or two octets (if set to 1).
The lower-order four bits of the Attribute Flags octet are
unused. They MUST be zero when sent and MUST be ignored when
received.
The Attribute Type Code octet contains the Attribute Type Code.
Currently defined Attribute Type Codes are discussed in Section
5.
If the Extended Length bit of the Attribute Flags octet is set
to 0, the third octet of the Path Attribute contains the length
of the attribute data in octets.
If the Extended Length bit of the Attribute Flags octet is set
to 1, then the third and the fourth octets of the path
attribute contain the length of the attribute data in octets.
The remaining octets of the Path Attribute represent the
attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute Type
Codes, their attribute values and uses are the following:
a) ORIGIN (Type Code 1):
ORIGIN is a well-known mandatory attribute that defines the
origin of the path information. The data octet can assume
the following values:
Value Meaning
0 IGP - Network Layer Reachability Information
is interior to the originating AS
1 EGP - Network Layer Reachability Information
learned via the EGP protocol [RFC904]
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2 INCOMPLETE - Network Layer Reachability
Information learned by some other means
Usage of this attribute is defined in 5.1.1.
b) AS_PATH (Type Code 2):
AS_PATH is a well-known mandatory attribute that is composed
of a sequence of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment
length, path segment value>.
The path segment type is a 1-octet long field with the fol-
lowing values defined:
Value Segment Type
1 AS_SET: unordered set of ASs a route in the
UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASs a route in
the UPDATE message has traversed
The path segment length is a 1-octet long field containing
the number of ASs (not the number of octets) in the path
segment value field.
The path segment value field contains one or more AS num-
bers, each encoded as a 2-octets long field.
Usage of this attribute is defined in 5.1.2.
c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the
(unicast) IP address of the router that SHOULD be used as
the next hop to the destinations listed in the Network Layer
Reachability Information field of the UPDATE message.
Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a four
octet unsigned integer. The value of this attribute MAY be
used by a BGP speaker's Decision Process to discriminate
among multiple entry points to a neighboring autonomous
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system.
Usage of this attribute is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known attribute that is a four octet
unsigned integer. A BGP speaker uses it to inform its other
internal peers of the advertising speaker's degree of pref-
erence for an advertised route.
Usage of this attribute is defined in 5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0.
Usage of this attribute is defined in 5.1.6.
g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the
aggregate route (encoded as 2 octets), followed by the IP
address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). This SHOULD be the same address as
the one used for the BGP Identifier of the speaker.
Usage of this attribute is defined in 5.1.7.
Network Layer Reachability Information:
This variable length field contains a list of IP address pre-
fixes. The length in octets of the Network Layer Reachability
Information is not encoded explicitly, but can be calculated
as:
UPDATE message Length - 23 - Total Path Attributes Length -
Withdrawn Routes Length
where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Withdrawn
Routes Length are the values encoded in the variable part of
the UPDATE message, and 23 is a combined length of the fixed-
size BGP header, the Total Path Attribute Length field and the
Withdrawn Routes Length field.
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Reachability information is encoded as one or more 2-tuples of
the form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains an IP address prefix followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is
irrelevant.
The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Withdrawn Routes Length + 2
octets for the Total Path Attribute Length (the value of Withdrawn
Routes Length is 0 and the value of Total Path Attribute Length is
0).
An UPDATE message can advertise at most one set of path attributes,
but multiple destinations, provided that the destinations share these
attributes. All path attributes contained in a given UPDATE message
apply to all destinations carried in the NLRI field of the UPDATE
message.
An UPDATE message can list multiple routes to be withdrawn from ser-
vice. Each such route is identified by its destination (expressed as
an IP prefix), which unambiguously identifies the route in the con-
text of the BGP speaker - BGP speaker connection to which it has been
previously advertised.
An UPDATE message might advertise only routes to be withdrawn from
service, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a
feasible route, in which case the WITHDRAWN ROUTES field need not be
present.
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An UPDATE message SHOULD NOT include the same address prefix in the
WITHDRAWN ROUTES and Network Layer Reachability Information fields,
however a BGP speaker MUST be able to process UPDATE messages in this
form. A BGP speaker SHOULD treat an UPDATE message of this form as if
the WITHDRAWN ROUTES doesn't contain the address prefix.
4.4 KEEPALIVE Message Format
BGP does not use any TCP-based keep-alive mechanism to determine if
peers are reachable. Instead, KEEPALIVE messages are exchanged
between peers often enough as not to cause the Hold Timer to expire.
A reasonable maximum time between KEEPALIVE messages would be one
third of the Hold Time interval. KEEPALIVE messages MUST NOT be sent
more frequently than one per second. An implementation MAY adjust the
rate at which it sends KEEPALIVE messages as a function of the Hold
Time interval.
If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.
A KEEPALIVE message consists of only message header and has a length
of 19 octets.
4.5 NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after sending it.
In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code:
This 1-octet unsigned integer indicates the type of NOTIFICA-
TION. The following Error Codes have been defined:
Error Code Symbolic Name Reference
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1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2
3 UPDATE Message Error Section 6.3
4 Hold Timer Expired Section 6.5
5 Finite State Machine Error Section 6.6
6 Cease Section 6.7
Error subcode:
This 1-octet unsigned integer provides more specific informa-
tion about the nature of the reported error. Each Error Code
may have one or more Error Subcodes associated with it. If no
appropriate Error Subcode is defined, then a zero (Unspecific)
value is used for the Error Subcode field.
Message Header Error subcodes:
1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.
OPEN Message Error subcodes:
1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter.
5 - [Deprecated - see Appendix A].
6 - Unacceptable Hold Time.
UPDATE Message Error subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute.
7 - [Deprecated - see Appendix A].
8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.
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11 - Malformed AS_PATH.
Data:
This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 below for more
details.
Note that the length of the Data field can be determined from
the message Length field by the formula:
Message Length = 21 + Data Length
The minimum length of the NOTIFICATION message is 21 octets (includ-
ing message header).
5. Path Attributes
This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories:
1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.
BGP implementations MUST recognize all well-known attributes. Some
of these attributes are mandatory and MUST be included in every
UPDATE message that contains NLRI. Others are discretionary and MAY
or MAY NOT be sent in a particular UPDATE message.
Once a BGP peer has updated any well-known attributes, it MUST pass
these attributes in any updates it transmits to its peers.
In addition to well-known attributes, each path MAY contain one or
more optional attributes. It is not required or expected that all BGP
implementations support all optional attributes. The handling of an
unrecognized optional attribute is determined by the setting of the
Transitive bit in the attribute flags octet. Paths with unrecognized
transitive optional attributes SHOULD be accepted. If a path with
unrecognized transitive optional attribute is accepted and passed
along to other BGP peers, then the unrecognized transitive optional
attribute of that path MUST be passed along with the path to other
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BGP peers with the Partial bit in the Attribute Flags octet set to 1.
If a path with recognized transitive optional attribute is accepted
and passed along to other BGP peers and the Partial bit in the
Attribute Flags octet is set to 1 by some previous AS, it MUST NOT be
set back to 0 by the current AS. Unrecognized non-transitive optional
attributes MUST be quietly ignored and not passed along to other BGP
peers.
New transitive optional attributes MAY be attached to the path by the
originator or by any other BGP speaker in the path. If they are not
attached by the originator, the Partial bit in the Attribute Flags
octet is set to 1. The rules for attaching new non-transitive
optional attributes will depend on the nature of the specific
attribute. The documentation of each new non-transitive optional
attribute will be expected to include such rules. (The description of
the MULTI_EXIT_DISC attribute gives an example.) All optional
attributes (both transitive and non-transitive) MAY be updated (if
appropriate) by BGP speakers in the path.
The sender of an UPDATE message SHOULD order path attributes within
the UPDATE message in ascending order of attribute type. The receiver
of an UPDATE message MUST be prepared to handle path attributes
within the UPDATE message that are out of order.
The same attribute (attribute with the same type) can not appear more
than once within the Path Attributes field of a particular UPDATE
message.
The mandatory category refers to an attribute which MUST be present
in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
message. Attributes classified as optional for the purpose of the
protocol extension mechanism may be purely discretionary, or discre-
tionary, required, or disallowed in certain contexts.
attribute EBGP IBGP
ORIGIN mandatory mandatory
AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory
MULTI_EXIT_DISC discretionary discretionary
LOCAL_PREF see Section 5.1.5 required
ATOMIC_AGGREGATE see Section 5.1.6 and 9.1.4
AGGREGATOR discretionary discretionary
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5.1 Path Attribute Usage
The usage of each BGP path attribute is described in the following
clauses.
5.1.1 ORIGIN
ORIGIN is a well-known mandatory attribute. The ORIGIN attribute is
generated by the speaker that originates the associated routing
information. Its value SHOULD NOT be changed by any other speaker.
5.1.2 AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute identi-
fies the autonomous systems through which routing information carried
in this UPDATE message has passed. The components of this list can be
AS_SETs or AS_SEQUENCEs.
When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it modifies the route's AS_PATH
attribute based on the location of the BGP speaker to which the route
will be sent:
a) When a given BGP speaker advertises the route to an internal
peer, the advertising speaker SHALL NOT modify the AS_PATH
attribute associated with the route.
b) When a given BGP speaker advertises the route to an external
peer, then the advertising speaker updates the AS_PATH attribute
as follows:
1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system prepends its own AS number as the
last element of the sequence (put it in the leftmost position
with respect to the position of octets in the protocol mes-
sage). If the act of prepending will cause an overflow in the
AS_PATH segment, i.e. more than 255 ASs, it SHOULD prepend a
new segment of type AS_SEQUENCE and prepend its own AS number
to this new segment.
2) if the first path segment of the AS_PATH is of type AS_SET,
the local system prepends a new path segment of type
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AS_SEQUENCE to the AS_PATH, including its own AS number in that
segment.
3) if the AS_PATH is empty, the local system creates a path
segment of type AS_SEQUENCE, places its own AS into that seg-
ment, and places that segment into the AS_PATH.
When a BGP speaker originates a route then:
a) the originating speaker includes its own AS number in a path
segment of type AS_SEQUENCE in the AS_PATH attribute of all UPDATE
messages sent to an external peer. (In this case, the AS number of
the originating speaker's autonomous system will be the only entry
the path segment, and this path segment will be the only segment
in the AS_PATH attribute).
b) the originating speaker includes an empty AS_PATH attribute in
all UPDATE messages sent to internal peers. (An empty AS_PATH
attribute is one whose length field contains the value zero).
Whenever the modification of the AS_PATH attribute calls for includ-
ing or prepending the AS number of the local system, the local system
MAY include/prepend more than one instance of its own AS number in
the AS_PATH attribute. This is controlled via local configuration.
5.1.3 NEXT_HOP
The NEXT_HOP is a well-known mandatory attribute that defines the IP
address of the router that SHOULD be used as the next hop to the des-
tinations listed in the UPDATE message. The NEXT_HOP attribute is
calculated as follows.
1) When sending a message to an internal peer, if the route is not
locally originated the BGP speaker SHOULD NOT modify the NEXT_HOP
attribute, unless it has been explicitly configured to announce
its own IP address as the NEXT_HOP. When announcing a locally
originated route to an internal peer, the BGP speaker SHOULD use
as the NEXT_HOP the interface address of the router through which
the announced network is reachable for the speaker; if the route
is directly connected to the speaker, or the interface address of
the router through which the announced network is reachable for
the speaker is the internal peer's address, then the BGP speaker
SHOULD use for the NEXT_HOP attribute its own IP address (the
address of the interface that is used to reach the peer).
2) When sending a message to an external peer X, and the peer is
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one IP hop away from the speaker:
- If the route being announced was learned from an internal
peer or is locally originated, the BGP speaker can use for the
NEXT_HOP attribute an interface address of the internal peer
router (or the internal router) through which the announced
network is reachable for the speaker, provided that peer X
shares a common subnet with this address. This is a form of
"third party" NEXT_HOP attribute.
- Otherwise, if the route being announced was learned from an
external peer, the speaker can use in the NEXT_HOP attribute an
IP address of any adjacent router (known from the received
NEXT_HOP attribute) that the speaker itself uses for local
route calculation, provided that peer X shares a common subnet
with this address. This is a second form of "third party"
NEXT_HOP attribute.
- Otherwise, if the external peer to which the route is being
advertised shares a common subnet with one of the interfaces of
the announcing BGP speaker, the speaker MAY use the IP address
associated with such an interface in the NEXT_HOP attribute.
This is known as a "first party" NEXT_HOP attribute.
- By default (if none of the above conditions apply), the BGP
speaker SHOULD use in the NEXT_HOP attribute the IP address of
the interface that the speaker uses to establish the BGP con-
nection to peer X.
3) When sending a message to an external peer X, and the peer is
multiple IP hops away from the speaker (aka "multihop EBGP"):
- The speaker MAY be configured to propagate the NEXT_HOP
attribute. In this case when advertising a route that the
speaker learned from one of its peers, the NEXT_HOP attribute
of the advertised route is exactly the same as the NEXT_HOP
attribute of the learned route (the speaker just doesn't modify
the NEXT_HOP attribute).
- By default, the BGP speaker SHOULD use in the NEXT_HOP
attribute the IP address of the interface that the speaker uses
to establish the BGP connection to peer X.
Normally the NEXT_HOP attribute is chosen such that the shortest
available path will be taken. A BGP speaker MUST be able to support
disabling advertisement of third party NEXT_HOP attributes to handle
imperfectly bridged media.
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A route originated by a BGP speaker SHALL NOT be advertised to a peer
using an address of that peer as NEXT_HOP. A BGP speaker SHALL NOT
install a route with itself as the next hop.
The NEXT_HOP attribute is used by the BGP speaker to determine the
actual outbound interface and immediate next-hop address that SHOULD
be used to forward transit packets to the associated destinations.
The immediate next-hop address is determined by performing a recur-
sive route lookup operation for the IP address in the NEXT_HOP
attribute using the contents of the Routing Table, selecting one
entry if multiple entries of equal cost exist. The Routing Table
entry which resolves the IP address in the NEXT_HOP attribute will
always specify the outbound interface. If the entry specifies an
attached subnet, but does not specify a next-hop address, then the
address in the NEXT_HOP attribute SHOULD be used as the immediate
next-hop address. If the entry also specifies the next-hop address,
this address SHOULD be used as the immediate next-hop address for
packet forwarding.
5.1.4 MULTI_EXIT_DISC
The MULTI_EXIT_DISC is an optional non-transitive attribute which is
intended to be used on external (inter-AS) links to discriminate
among multiple exit or entry points to the same neighboring AS. The
value of the MULTI_EXIT_DISC attribute is a four octet unsigned num-
ber which is called a metric. All other factors being equal, the exit
point with lower metric SHOULD be preferred. If received over EBGP,
the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other
BGP speakers within the same AS (see also 9.1.2.2). The
MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be
propagated to other neighboring ASs.
A BGP speaker MUST implement a mechanism based on local configuration
which allows the MULTI_EXIT_DISC attribute to be removed from a
route. If a BGP speaker is configured to remove the MULTI_EXIT_DISC
attribute from a route, then this removal MUST be done prior to
determining the degree of preference of the route and performing
route selection (Decision Process phases 1 and 2).
An implementation MAY also (based on local configuration) alter the
value of the MULTI_EXIT_DISC attribute received over EBGP. If a BGP
speaker is configured to alter the value of the MULTI_EXIT_DISC
attribute received over EBGP, then altering the value MUST be done
prior to determining the degree of preference of the route and per-
forming route selection (Decision Process phases 1 and 2). See
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Section 9.1.2.2 for necessary restrictions on this.
5.1.5 LOCAL_PREF
LOCAL_PREF is a well-known attribute that SHALL be included in all
UPDATE messages that a given BGP speaker sends to the other internal
peers. A BGP speaker SHALL calculate the degree of preference for
each external route based on the locally configured policy, and
include the degree of preference when advertising a route to its
internal peers. The higher degree of preference MUST be preferred. A
BGP speaker uses the degree of preference learned via LOCAL_PREF in
its Decision Process (see Section 9.1.1).
A BGP speaker MUST NOT include this attribute in UPDATE messages that
it sends to external peers, except for the case of BGP Confederations
[RFC3065]. If it is contained in an UPDATE message that is received
from an external peer, then this attribute MUST be ignored by the
receiving speaker, except for the case of BGP Confederations
[RF3065].
5.1.6 ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute.
When a BGP speaker aggregates several routes for the purpose of
advertisement to a particular peer, the AS_PATH of the aggregated
route normally includes an AS_SET formed from the set of ASs from
which the aggregate was formed. In many cases the network adminis-
trator can determine that the aggregate can safely be advertised
without the AS_SET and not form route loops.
If an aggregate excludes at least some of the AS numbers present in
the AS_PATH of the routes that are aggregated as a result of dropping
the AS_SET, the aggregated route, when advertised to the peer, SHOULD
include the ATOMIC_AGGREGATE attribute.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute SHOULD NOT remove the attribute from the route when propa-
gating it to other speakers.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute MUST NOT make any NLRI of that route more specific (as
defined in 9.1.4) when advertising this route to other BGP speakers.
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A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute needs to be aware of the fact that the actual path to des-
tinations, as specified in the NLRI of the route, while having the
loop-free property, may not be the path specified in the AS_PATH
attribute of the route.
5.1.7 AGGREGATOR
AGGREGATOR is an optional transitive attribute which MAY be included
in updates which are formed by aggregation (see Section 9.2.2.2). A
BGP speaker which performs route aggregation MAY add the AGGREGATOR
attribute which SHALL contain its own AS number and IP address. The
IP address SHOULD be the same as the BGP Identifier of the speaker.
6. BGP Error Handling.
This section describes actions to be taken when errors are detected
while processing BGP messages.
When any of the conditions described here are detected, a NOTIFICA-
TION message with the indicated Error Code, Error Subcode, and Data
fields is sent, and the BGP connection is closed, unless it is
explicitly stated that no NOTIFICATION message is to be sent and the
BGP connection is not to be closed. If no Error Subcode is specified,
then a zero MUST be used.
The phrase "the BGP connection is closed" means that the TCP connec-
tion has been closed, the associated Adj-RIB-In has been cleared, and
that all resources for that BGP connection have been deallocated.
Entries in the Loc-RIB associated with the remote peer are marked as
invalid. The local system recalculates its best routes for the des-
tinations of the routes marked as invalid, and before the invalid
routes are deleted from the system advertises to its peers either
withdraws for the routes marked as invalid, or the new best routes
before the invalid routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION mes-
sage that is sent to indicate an error is empty.
6.1 Message Header error handling.
All errors detected while processing the Message Header MUST be
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indicated by sending the NOTIFICATION message with Error Code Message
Header Error. The Error Subcode elaborates on the specific nature of
the error.
The expected value of the Marker field of the message header is all
ones. If the Marker field of the message header is not as expected,
then a synchronization error has occurred and the Error Subcode MUST
be set to Connection Not Synchronized.
If at least one of the following is true:
- if the Length field of the message header is less than 19 or
greater than 4096, or
- if the Length field of an OPEN message is less than the minimum
length of the OPEN message, or
- if the Length field of an UPDATE message is less than the mini-
mum length of the UPDATE message, or
- if the Length field of a KEEPALIVE message is not equal to 19,
or
- if the Length field of a NOTIFICATION message is less than the
minimum length of the NOTIFICATION message,
then the Error Subcode MUST be set to Bad Message Length. The Data
field MUST contain the erroneous Length field.
If the Type field of the message header is not recognized, then the
Error Subcode MUST be set to Bad Message Type. The Data field MUST
contain the erroneous Type field.
6.2 OPEN message error handling.
All errors detected while processing the OPEN message MUST be indi-
cated by sending the NOTIFICATION message with Error Code OPEN Mes-
sage Error. The Error Subcode elaborates on the specific nature of
the error.
If the version number contained in the Version field of the received
OPEN message is not supported, then the Error Subcode MUST be set to
Unsupported Version Number. The Data field is a 2-octets unsigned
integer, which indicates the largest locally supported version number
less than the version the remote BGP peer bid (as indicated in the
received OPEN message), or if the smallest locally supported version
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number is greater than the version the remote BGP peer bid, then the
smallest locally supported version number.
If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode MUST be set to Bad Peer AS. The determination
of acceptable Autonomous System numbers is outside the scope of this
protocol.
If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An implementa-
tion MUST reject Hold Time values of one or two seconds. An imple-
mentation MAY reject any proposed Hold Time. An implementation which
accepts a Hold Time MUST use the negotiated value for the Hold Time.
If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode MUST be set to Bad BGP Identifier.
Syntactic correctness means that the BGP Identifier field represents
a valid unicast IP host address.
If one of the Optional Parameters in the OPEN message is not recog-
nized, then the Error Subcode MUST be set to Unsupported Optional
Parameters.
If one of the Optional Parameters in the OPEN message is recognized,
but is malformed, then the Error Subcode MUST be set to 0 (Unspe-
cific).
6.3 UPDATE message error handling.
All errors detected while processing the UPDATE message MUST be indi-
cated by sending the NOTIFICATION message with Error Code UPDATE Mes-
sage Error. The error subcode elaborates on the specific nature of
the error.
Error checking of an UPDATE message begins by examining the path
attributes. If the Withdrawn Routes Length or Total Attribute Length
is too large (i.e., if Withdrawn Routes Length + Total Attribute
Length + 23 exceeds the message Length), then the Error Subcode MUST
be set to Malformed Attribute List.
If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode MUST be set to
Attribute Flags Error. The Data field MUST contain the erroneous
attribute (type, length and value).
If any recognized attribute has Attribute Length that conflicts with
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the expected length (based on the attribute type code), then the
Error Subcode MUST be set to Attribute Length Error. The Data field
MUST contain the erroneous attribute (type, length and value).
If any of the mandatory well-known attributes are not present, then
the Error Subcode MUST be set to Missing Well-known Attribute. The
Data field MUST contain the Attribute Type Code of the missing well-
known attribute.
If any of the mandatory well-known attributes are not recognized,
then the Error Subcode MUST be set to Unrecognized Well-known
Attribute. The Data field MUST contain the unrecognized attribute
(type, length and value).
If the ORIGIN attribute has an undefined value, then the Error Sub-
code MUST be set to Invalid Origin Attribute. The Data field MUST
contain the unrecognized attribute (type, length and value).
If the NEXT_HOP attribute field is syntactically incorrect, then the
Error Subcode MUST be set to Invalid NEXT_HOP Attribute. The Data
field MUST contain the incorrect attribute (type, length and value).
Syntactic correctness means that the NEXT_HOP attribute represents a
valid IP host address.
The IP address in the NEXT_HOP MUST meet the following criteria to be
considered semantically correct:
a) It MUST NOT be the IP address of the receiving speaker
b) In the case of an EBGP where the sender and receiver are one IP
hop away from each other, either the IP address in the NEXT_HOP
MUST be the sender's IP address (that is used to establish the BGP
connection), or the interface associated with the NEXT_HOP IP
address MUST share a common subnet with the receiving BGP speaker.
If the NEXT_HOP attribute is semantically incorrect, the error SHOULD
be logged, and the route SHOULD be ignored. In this case, a NOTIFICA-
TION message SHOULD NOT be sent, and connection SHOULD NOT be closed.
The AS_PATH attribute is checked for syntactic correctness. If the
path is syntactically incorrect, then the Error Subcode MUST be set
to Malformed AS_PATH.
If the UPDATE message is received from an external peer, the local
system MAY check whether the leftmost (with respect to the position
of octets in the protocol message) AS in the AS_PATH attribute is
equal to the autonomous system number of the peer that sent the mes-
sage. If the check determines that this is not the case, the Error
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Subcode MUST be set to Malformed AS_PATH.
If an optional attribute is recognized, then the value of this
attribute MUST be checked. If an error is detected, the attribute
MUST be discarded, and the Error Subcode MUST be set to Optional
Attribute Error. The Data field MUST contain the attribute (type,
length and value).
If any attribute appears more than once in the UPDATE message, then
the Error Subcode MUST be set to Malformed Attribute List.
The NLRI field in the UPDATE message is checked for syntactic valid-
ity. If the field is syntactically incorrect, then the Error Subcode
MUST be set to Invalid Network Field.
If a prefix in the NLRI field is semantically incorrect (e.g., an
unexpected multicast IP address), an error SHOULD be logged locally,
and the prefix SHOULD be ignored.
An UPDATE message that contains correct path attributes, but no NLRI,
SHALL be treated as a valid UPDATE message.
6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and the receiver of the mes-
sage detects an error in that message, the receiver can not use a
NOTIFICATION message to report this error back to the peer. Any such
error, such as an unrecognized Error Code or Error Subcode, SHOULD be
noticed, logged locally, and brought to the attention of the adminis-
tration of the peer. The means to do this, however, lies outside the
scope of this document.
6.5 Hold Timer Expired error handling.
If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code is sent and the BGP connection is
closed.
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6.6 Finite State Machine error handling.
Any error detected by the BGP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the NOTIFICATION message
with Error Code Finite State Machine Error.
6.7 Cease.
In absence of any fatal errors (that are indicated in this section),
a BGP peer MAY choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message MUST NOT be used when a fatal error
indicated by this section does exist.
A BGP speaker MAY support the ability to impose an (locally config-
ured) upper bound on the number of address prefixes the speaker is
willing to accept from a neighbor. When the upper bound is reached,
the speaker (under control of local configuration) either (a) dis-
cards new address prefixes from the neighbor (while maintaining BGP
connection with the neighbor), or (b) terminates the BGP connection
with the neighbor. If the BGP speaker decides to terminate its BGP
connection with a neighbor because the number of address prefixes
received from the neighbor exceeds the locally configured upper
bound, then the speaker MUST send to the neighbor a NOTIFICATION mes-
sage with the Error Code Cease. The speaker MAY also log this
locally.
6.8 BGP connection collision detection.
If a pair of BGP speakers try simultaneously to establish a BGP con-
nection to each other, then two parallel connections between this
pair of speakers might well be formed. If the source IP address used
by one of these connections is the same as the destination IP address
used by the other, and the destination IP address used by the first
connection is the same as the source IP address used by the other, we
refer to this situation as connection collision. Clearly in the
presence of connection collision, one of these connections MUST be
closed.
Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a colli-
sion does occur. The convention is to compare the BGP Identifiers of
the peers involved in the collision and to retain only the connection
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initiated by the BGP speaker with the higher-valued BGP Identifier.
Upon receipt of an OPEN message, the local system MUST examine all of
its connections that are in the OpenConfirm state. A BGP speaker MAY
also examine connections in an OpenSent state if it knows the BGP
Identifier of the peer by means outside of the protocol. If among
these connections there is a connection to a remote BGP speaker whose
BGP Identifier equals the one in the OPEN message, and this connec-
tion collides with the connection over which the OPEN message is
received then the local system performs the following collision reso-
lution procedure:
1. The BGP Identifier of the local system is compared to the BGP
Identifier of the remote system (as specified in the OPEN mes-
sage). Comparing BGP Identifiers is done by converting them to
host byte order and treating them as (4-octet long) unsigned inte-
gers.
2. If the value of the local BGP Identifier is less than the
remote one, the local system closes the BGP connection that
already exists (the one that is already in the OpenConfirm state),
and accepts the BGP connection initiated by the remote system.
3. Otherwise, the local system closes newly created BGP connection
(the one associated with the newly received OPEN message), and
continues to use the existing one (the one that is already in the
OpenConfirm state).
Unless allowed via configuration, a connection collision with an
existing BGP connection that is in Established state causes closing
of the newly created connection.
Note that a connection collision can not be detected with connections
that are in Idle, or Connect, or Active states.
Closing the BGP connection (that results from the collision resolu-
tion procedure) is accomplished by sending the NOTIFICATION message
with the Error Code Cease.
7. BGP Version Negotiation
BGP speakers MAY negotiate the version of the protocol by making mul-
tiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it
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tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common versions,
then this will allow them to rapidly determine the highest common
version. In order to support BGP version negotiation, future versions
of BGP MUST retain the format of the OPEN and NOTIFICATION messages.
8. BGP Finite State machine (FSM)
The data structures and FSM described in this document are
conceptual and do not have to be implemented precisely as described
here, as long as the implementations support the described
functionality and their externally visible behavior is the same.
This section specifies the BGP operation in terms of a Finite State
Machine (FSM). The section falls into 2 parts:
1) Description of Events for the State machine (Section 8.1)
2) Description of the FSM (Section 8.2)
Session attributes required (mandatory) for each connection are:
1) State
2) ConnectRetryCounter
3) ConnectRetryTimer
4) ConnectRetryTime
5) HoldTimer
6) HoldTime
7) KeepaliveTimer
8) KeepaliveTime
The state session attribute indicates what state the BGP FSM
is in. The ConnectRetryCounter indicates the number of times
a BGP peer has tried to establish a peer session.
The mandatory attributes related to timers are described in
section 10. Each timer has a "timer" and a "time" (the initial
value).
The optional Session attributes are listed below. These optional
attributes may be supported either per connection or per local sys-
tem:
1) AcceptConnectionsUnconfiguredPeers
2) AllowAutomaticStart
3) AllowAutomaticStop
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4) CollisionDetectEstablishedState
5) DampPeerOscillations
6) DelayOpen
7) DelayOpenTime
8) DelayOpenTimer
9) IdleHoldTime
10) IdleHoldTimer
11) PassiveTcpEstablishment
12) SendNOTIFICATIONwithoutOPEN
13) TrackTcpState
The optional session attributes support different features of the BGP
functionality that have implications for the BGP FSM state
transitions. Two groups of the attributes which relate to timers are:
group 1: DelayOpen, DelayOpenTime, DelayOpenTimer
group 2: DampPeerOscillations, IdleHoldTime, IdleHoldTimer
The first parameter (DelayOpen, DampPeerOscillations) is an
optional attribute that indicates that the Timer function is
active. The "Time" value specifies the initial value for "Timer"
(DelayOpenTime, IdleHoldTime). The "Timer" specifies the actual timer.
Please refer to section 8.1.1 for an explanation
of the interaction between these optional attributes and the events
signaled to the state machine. Section 8.2.1.3 also provides
a short overview of the different types of optional attributes
(flags or timers).
8.1 Events for the BGP FSM
8.1.1 Optional Events linked to Optional Session attributes
The Inputs to the BGP FSM are events. Events can either be
mandatory or optional. Some optional events are linked to
optional session attributes. Optional session attributes enable
several groups of FSM functionality.
The description below describes the linkage between FSM
functionality, events and the optional session attributes.
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Group 1: Automatic Administrative Events (Start/Stop)
Optional Session Attributes: AllowAutomaticStart, AllowAutomaticStop,
DampPeerOscillations, IdleHoldTime,
IdleHoldTimer
Option 1: AllowAutomaticStart
Description: A BGP peer connection can be started and stopped
by administrative control. This administrative
control can either be manual, based on
operator intervention, or under the control
of logic specific to a BGP implementation.
The term "automatic" refers to a start being
issued to the BGP peer connection FSM when
such logic determines that the BGP peer
connection should be restarted.
The AllowAutomaticStart attribute specifies
that this BGP connection supports automatic
starting of the BGP connection.
If the BGP implementation supports
AllowAutomaticStart, the peer may be
repeatedly restarted. Three other options
control the rate at which the automatic
restart occurs: DampPeerOscillations,
IdleHoldTime, and the IdleHoldTimer.
The DampPeerOscillations option specifies
that the implementation engages additional
logic to damp the oscillations of BGP peers
in the face of sequences of automatic start
and automatic stop. IdleHoldTime specifies
how long the BGP peer is held in the Idle
state prior to allowing the next automatic
restart. The IdleHoldTimer is the timer
that runs to hold the peer in Idle state.
An example of DampPeerOscillations logic
is an increase of the IdleHoldTime value
if a BGP peer oscillates connectivity
(connected/disconnected) repeatedly
within a time period. To engage this
logic, a peer could connect and disconnect
10 times within 5 minutes. The IdleHoldTime
value would be reset from 0 to 120 seconds.
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Values: TRUE or FALSE
Option 2: AllowAutomaticStop
Description: This BGP peer session optional attribute
indicates that the BGP connection allows
"automatic" stopping of the BGP connection.
An "automatic" stop is defined as a stop under
the control of implementation specific logic.
The implementation specific logic is outside
the scope of this specification.
Values: TRUE or FALSE
Option 3: DampPeerOscillations
Description: The DampPeerOscillations optional session
attribute indicates that this BGP connection
is using logic that damps BGP peer oscillations
in the Idle State.
Value: TRUE or FALSE
Option 4: IdleHoldTime
Description: The IdleHoldTime is the value
that is set in the IdleHoldTimer.
Values: Time in seconds
Option 5: IdleHoldTimer
Description: The IdleHoldTimer aids in controlling BGP peer
oscillation. The IdleHoldTimer is used to keep
the BGP peer in Idle for a particular duration.
The IdleHoldTimer_Expires event is described
in section 8.1.3.
Values: Time in seconds
Group 2: Unconfigured Peers
Optional Session Attributes: AcceptConnectionsUnconfiguredPeers
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Option 1: AcceptConnectionsUnconfiguredPeers
Description: The BGP FSM optionally allows the acceptance of BGP
peer connections from neighbors that are not
pre-configured. The
"AcceptConnectionsUnconfiguredPeers" optional
session attribute allows the FSM to support
the state transitions that allow the
implementation to accept or reject these
unconfigured peers.
The AcceptConnectionsUnconfiguredPeers has
security implications. Please refer to the
BGP Vulnerabilities document[BGP_VULN] for
details.
Value: True or False
Group 3: TCP processing
Optional Session Attributes: PassiveTcpEstablishment, TrackTcpState
Option 1: PassiveTcpEstablishment
Description: This option indicates that the BGP FSM will passively
wait for the remote BGP peer to establish the BGP
TCP connection.
value: TRUE or FALSE
Option 2: TrackTcpState
Description: The BGP FSM normally tracks the end result of a TCP
connection attempt rather than individual TCP messages.
Optionally, the BGP FSM can support additional
interaction with the TCP connection negotiation. The
interaction with the TCP events may increase the
amount of logging the BGP peer connection
requires and the number of BGP FSM changes.
Value: TRUE or FALSE
Group 4: BGP Message Processing
Optional Session Attributes: DelayOpen, DelayOpenTime,
DelayOpenTimer,
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SendNOTIFICATIONwithoutOPEN,
CollisionDetectEstablishedState
Option 1: DelayOpen
Description: The DelayOpen optional session attribute allows
implementations to be configured to delay
sending an OPEN message for a specific time
period (DelayOpenTime). The delay allows
the remote BGP Peer time to send the first
OPEN message.
Value: TRUE or FALSE
Option 2: DelayOpenTime
Description: The DelayOpenTime is the initial value that is
set in the DelayOpenTimer.
Value: Time in seconds
Option 3: DelayOpenTimer
Description: The DelayOpenTimer optional session attribute
is used to delay the sending of an OPEN message
on a connection. The DelayOpenTimer_Expires event
(Event 12) is described in section 8.1.3.
Value: Time in seconds
Option 4: SendNOTIFICATIONwithoutOPEN
Description: The SendNOTIFICATIONwithoutOPEN allows a peer to
send a NOTIFICATION without first sending an
OPEN message. Without this optional session
attribute, the BGP connection assumes that an
OPEN message must be sent by a peer prior
to the peer sending a NOTIFICATION message.
Value: True or False
Option 5: CollisionDetectEstablishedState
Description: Normally, a Detect Collision (6.8) will
be ignored in the Established state. This
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optional session attribute indicates that
this BGP connection processes
collisions in the Established state.
Value: True or False
Note: The optional session attributes clarify the BGP FSM description
for existing features of BGP implementations. The optional
session attributes may be pre-defined for an implementation
and not readable via management interfaces for existing
correct implementations. As newer BGP MIBs (version 2
and beyond) are supported, these fields will be accessible
via a management interface.
8.1.2 Administrative Events
An administrative event is an event in which the operator interface
and BGP Policy engine signal the BGP finite state machine to start or
stop the BGP state machine. The basic start and stop indication are
augmented by optional connection attributes to signal a certain type
of start or stop mechanism to the BGP FSM. An example of this combi-
nation is Event 5, AutomaticStart_with_PassiveTcpEstablishment. With
this event, the BGP implementation signals to the BGP FSM that the
implementation is using an Automatic Start with option to use a Pas-
sive TCP Establishment. The Passive TCP establishment signals that
this BGP FSM will wait for the remote side to start the TCP estab-
lishment.
Please note that only Event 1 (ManualStart) and Event 2 (ManualStop)
are mandatory administrative events. All other administrative events
are optional (Events 3-8). Each event below has a name, definition,
status (mandatory or optional), and what optional session attributes
SHOULD be set at each stage. When generating Event 1 through Event 8
for the BGP FSM, the conditions specified in the "Optional Attribute
Status" section are verified. If any of these conditions are not
satisfied, then the local system should log a FSM error.
The settings of optional session attributes may be implicit in some
implementations and therefore may not be set explicitly by an exter-
nal operator action. Section 8.2.1.5 describes these implicit set-
tings of the optional session attributes. The administrative states
described below may also be implicit in some implementations and not
directly configurable by an external operator.
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Event 1: ManualStart
Definition: Local system administrator manually starts peer
connection.
Status: Mandatory
Optional
Attribute
Status: The PassiveTcpEstablishment attribute SHOULD be
set to FALSE.
Event 2: ManualStop
Definition: Local system administrator manually
stops the peer connection.
Status: Mandatory
Optional
Attribute
Status: No interaction with any optional attributes.
Event 3: AutomaticStart
Definition: Local system automatically starts the
BGP connection.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE if this event occurs.
2) If the PassiveTcpEstablishment optional session
attribute is supported, it SHOULD be set to FALSE.
3) If the DampPeerOscillations is supported, it
SHOULD be set to FALSE when this event occurs.
Event 4: ManualStart_with_PassiveTcpEstablishment
Definition: Local system administrator manually starts peer
connection, but has PassiveTcpEstablishment
enabled. The PassiveTcpEstablishment optional
attribute indicates that the peer will listen prior
to establishing the connection.
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Status: Optional, depending on local system
Optional
Attribute
Status: 1) The PassiveTcpEstablishment attribute SHOULD
be set to TRUE if this event occurs.
2) The DampPeerOscillations attribute SHOULD be
set to FALSE when this event occurs.
Event 5: AutomaticStart_with_PassiveTcpEstablishment
Definition: Local system automatically starts the
BGP connection with the PassiveTcpEstablishment
enabled. The PassiveTcpEstablishment
optional attribute indicates
that the peer will listen prior to
establishing a connection.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD
be set to TRUE.
2) The PassiveTcpEstablishment attribute SHOULD
be set to TRUE
3) If the DampPeerOscillations attribute is
supported, the DampPeerOscillations SHOULD
be set to FALSE.
Event 6: AutomaticStart_with_DampPeerOscillations
Definition: Local system automatically starts the
BGP peer connection with peer oscillation
damping enabled. The exact method of damping
persistent peer oscillations is left up to the
implementation and is outside the scope of
this document.
Status: Optional, depending on local system.
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD
be set to TRUE.
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2) The DampPeerOscillations attribute SHOULD
be set to TRUE.
3) The PassiveTcpEstablishment attribute
SHOULD be set to FALSE.
Event 7: AutomaticStart_with_DampPeerOscillations_and_
PassiveTcpEstablishment
Definition: Local system automatically starts the
BGP peer connection with peer oscillation
damping enabled and PassiveTcpEstablishment
enabled. The exact method of damping
persistent peer oscillations is left up to the
implementation and is outside the scope of
this document.
Status: Optional, depending on local system
Optional
Attributes
Status: 1) The AllowAutomaticStart attribute
SHOULD be set to TRUE.
2) The DampPeerOscillations attribute SHOULD
be set to TRUE.
3) The PassiveTcpEstablishment attribute
SHOULD be set to TRUE.
Event 8: AutomaticStop
Definition: Local system automatically stops the
BGP connection.
An example of an automatic stop event is
exceeding the number of prefixes for a given
peer and the local system automatically
disconnecting the peer.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStop attribute
SHOULD be TRUE
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8.1.3 Timer Events
Event 9: ConnectRetryTimer_Expires
Definition: An event generated when the ConnectRetryTimer
expires.
Status: Mandatory
Event 10: HoldTimer_Expires
Definition: An event generated when the HoldTimer expires.
Status: Mandatory
Event 11: KeepaliveTimer_Expires
Definition: An event generated when the KeepaliveTimer expires.
Status: Mandatory
Event 12: DelayOpenTimer_Expires
Definition: An event generated when the DelayOpenTimer expires.
Status: Optional
Optional
Attribute
Status: If this event occurs,
1) DelayOpen attribute SHOULD be set to TRUE,
2) DelayOpenTime attribute SHOULD be supported,
3) DelayOpenTimer SHOULD be supported,
Event 13: IdleHoldTimer_Expires
Definition: An event generated when the IdleHoldTimer
expires indicating that the BGP connection has
completed waiting for the back-off period
to prevent BGP peer oscillation.
The IdleHoldTimer is only used when the
persistent peer oscillation damping
function is enabled by setting the
DampPeerOscillations optional attribute
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to TRUE.
Implementations not implementing the
persistent peer oscillation damping
function may not have the IdleHoldTimer.
Status: Optional
Optional
Attribute
Status: If this event occurs:
1) DampPeerOscillations attribute SHOULD be set
to TRUE.
2) IdleHoldTimer SHOULD have just expired.
8.1.4 TCP Connection based Events
Event 14: TcpConnection_Valid
Definition: Event indicating the local system reception of
a TCP connection request with a valid
source IP address and TCP port and a valid
destination IP address and TCP Port. The
definition of invalid source and invalid
destination IP address is left to the
implementation.
BGP's destination port SHOULD be port 179
as defined by IANA.
TCP connection request is denoted by the
local system receiving a TCP SYN.
Status: Optional
Optional
Attribute
Status: 1) The TrackTcpState attribute SHOULD be set to
TRUE if this event occurs.
Event 15: Tcp_CR_Invalid
Definition: Event indicating the local system reception
of a TCP connection request with either
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an invalid source address or port
number or an invalid destination
address or port number.
BGP destination port number SHOULD be 179
as defined by IANA.
A TCP connection request occurs when
the local system receives a TCP
SYN.
Status: Optional
Optional
Attribute
Status: 1) The TrackTcpState attribute should be set to
TRUE if this event occurs.
Event 16: Tcp_CR_Acked
Definition: Event indicating the local system's request
to establish a TCP connection to the remote
peer.
The local system's TCP connection sent a TCP
SYN, and received a TCP SYN/ACK message,
and sent a TCP ACK.
Status: Mandatory
Event 17: TcpConnectionConfirmed
Definition: Event indicating that the local system has
received a confirmation that the TCP
connection has been established by the
remote site.
The remote peer's TCP engine sent a TCP SYN.
The local peer's TCP engine sent a SYN, ACK
message and now has received a final ACK.
Status: Mandatory
Event 18: TcpConnectionFails
Definition: Event indicating that the local system has
received a TCP connection failure notice.
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The remote BGP peer's TCP machine could have
sent a FIN. The local peer would respond
with a FIN-ACK. Another alternative is that
the local peer indicated a timeout in the
TCP connection and downed the connection.
Status: Mandatory
8.1.5 BGP Message-based Events
Event 19: BGPOpen
Definition: An event is generated when a valid OPEN
message has been received.
Status: Mandatory
Optional
Attribute
Status: 1) The DelayOpen optional attribute SHOULD
be set to FALSE.
2) The DelayOpenTimer SHOULD not be running.
Event 20: BGPOpen with DelayOpenTimer running
Definition: An event is generated when a valid OPEN
message has been received for a peer
that has a successfully established
transport connection and is currently
delaying the sending of a BGP open
message.
Status: Optional
Optional
Attribute
Status: 1) The DelayOpen attribute SHOULD be
set to TRUE.
2) The DelayOpenTimer SHOULD be running.
Event 21: BGPHeaderErr
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Definition: An event is generated when a received
BGP message header is not valid.
Status: Mandatory
Event 22: BGPOpenMsgErr
Definition: An event is generated when an OPEN message
has been received with errors.
Status: Mandatory
Event 23: OpenCollisionDump
Definition: An event generated administratively
when a connection collision has been
detected while processing an incoming
OPEN message and this connection has been
selected to be disconnected. See section
6.8 for more information on collision
detection.
Event 23 is an administrative action
generated by implementation logic
that determines that this connection
needs to be dropped per the rules in
section 6.8. This event may occur if the FSM
is implemented as two linked state machines.
Status: Optional
Optional
Attribute
Status: If the state machine is to process this
event in Established state,
1) CollisionDetectEstablishedState
optional attribute SHOULD be set to TRUE
Please note: The OpenCollisionDump event can occur
in Idle, Connect, Active, OpenSent, OpenConfirm
without any optional attributes being set.
Event 24: NotifMsgVerErr
Definition: An event is generated when a
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NOTIFICATION message with "version
error" is received.
Status: Mandatory
Event 25: NotifMsg
Definition: An event is generated when a
NOTIFICATION message is received and
the error code is anything but
"version error".
Status: Mandatory
Event 26: KeepAliveMsg
Definition: An event is generated when a KEEPALIVE
message is received.
Status: Mandatory
Event 27: UpdateMsg
Definition: An event is generated when a valid
UPDATE message is received.
Status: Mandatory
Event 28: UpdateMsgErr
Definition: An event is generated when an invalid
UPDATE message is received.
Status: Mandatory
8.2 Description of FSM
8.2.1 FSM Definition
BGP MUST maintain a separate FSM for each configured peer. Each BGP
peer paired in a potential connection, unless configured to remain in
the idle state, or configured to remain passive, will attempt to con-
nect to the other. For the purpose of this discussion, the active or
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connecting side of the TCP connection (the side of a TCP connection
sending the first TCP SYN packet) is called outgoing. The passive or
listening side (the sender of the first SYN/ACK) is called an incom-
ing connection. (See Section 8.2.1.1 for information on the terms
active and passive used below.)
A BGP implementation MUST connect to and listen on TCP port 179 for
incoming connections in addition to trying to connect to peers. For
each incoming connection, a state machine MUST be instantiated.
There exists a period in which the identity of the peer on the other
end of an incoming connection is known, but the BGP identifier is not
known. During this time, both an incoming and an outgoing connection
for the same configured peering may exist. This is referred to as a
connection collision. (See Section 6.8.)
A BGP implementation will have at most one FSM for each configured
peering plus one FSM for each incoming TCP connection for which the
peer has not yet been identified. Each FSM corresponds to exactly one
TCP connection.
There may be more than one connection between a pair of peers if the
connections are configured to use a different pair of IP addresses.
This is referred to as multiple "configured peerings" to the same
peer.
8.2.1.1 Terms "active" and "passive"
The terms active and passive have been in the Internet operator's
vocabulary for almost a decade and have proven useful. The words
active and passive have slightly different meanings applied to a TCP
connection or applied to a peer. There is only one active side and
one passive side to any one TCP connection per the definition above
and the state machine below. When a BGP speaker is configured active,
it may end up on either the active or passive side of the connection
that eventually gets established. Once the TCP connection is com-
pleted, it doesn't matter which end was active and which end was pas-
sive. The only difference is which side of the TCP connection has
port number 179.
8.2.1.2 FSM and collision detection
There is one FSM per BGP connection. When the connection collision
occurs prior to determining what peer a connection is associated
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with, there may be two connections for one peer. After the connec-
tion collision is resolved (see Section 6.8) the FSM for the connec-
tion that is closed SHOULD be disposed of.
8.2.1.3 FSM and Optional Session Attributes
Optional Session Attributes specify either attributes that act
as flags (TRUE or FALSE) or optional timers. For optional
attributes that act as flags, if the optional session attribute
can be set to TRUE on the system, the corresponding the BGP FSM
actions must be supported. For example, if the following options
can be set in a BGP implementation: AutoStart and
PassiveTcpEstablishment, then the Events 3, 4 and 5 must be
supported. If an Optional Session attribute cannot be set to
TRUE, the events supporting that set of options do not have to
be supported.
Each of the optional timers (DelayOpenTimer and IdleHoldTimer),
has a group of attributes that are:
- flag indicating support,
- Time set in Timer
- Timer.
The two optional timers show this format:
DelayOpenTimer: DelayOpen, DelayOpenTime, DelayOpenTimer
IdleHoldTimer: DampPeerOscillations, IdleHoldTime,
IdleHoldTimer
If the flag indicating support for an optional timer
(DelayOpen or DampPeerOscillations), cannot be set to TRUE,
the timers and events supporting that
option do not have to be supported.
8.2.1.4 FSM Event numbers
The Event numbers (1-28) utilized in this state machine description
aid in specifying the behavior of the BGP state machine. Implementa-
tions MAY use these numbers to provide network management informa-
tion. The exact form of a FSM or the FSM events are specific to each
implementation.
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8.2.1.5 FSM actions that are implementation dependent.
The BGP FSM specifies at certain points that BGP initialization will
occur or that BGP resources will be deleted. The initialization of
the BGP FSM and the associated resources depend on the policy portion
of the BGP implementation. The details of these actions are outside
the scope of the FSM document.
8.2.2 Finite State Machine
Idle state:
Initially the BGP peer FSM is in the Idle state. (Hereafter
the BGP peer FSM will be shortened to BGP FSM.)
In this state BGP FSM refuses all incoming BGP
connections for this peer. No resources are allocated to the peer.
In response to a ManualStart event (Event 1) or an
AutomaticStart event (Event 3), the local system:
- initializes all BGP resources for the peer connection,
- sets ConnectRetryCounter to zero,
- starts the ConnectRetryTimer with initial value,
- initiates a TCP connection to the other BGP peer,
- listens for a connection that may be initiated by
the remote BGP peer, and
- changes its state to Connect.
The ManualStop event (Event 2) and AutomaticStop (Event 8) event
are ignored in the Idle state.
In response to a ManualStart_with_PassiveTcpEstablishment event
(Event 4) or AutomaticStart_with_PassiveTcpEstablishment event
(Event 5), the local system:
- initializes all BGP resources,
- sets the ConnectRetryCounter to zero,
- starts the ConnectRetryTimer with initial value,
- listens for a connection that may be initiated by
the remote peer, and
- changes its state to Active.
The exact value of the ConnectRetryTimer is a local
matter, but it SHOULD be sufficiently large to allow TCP
initialization.
If the DampPeerOscillations attribute is set to TRUE,
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the following three additional events may occur
within Idle state:
- AutomaticStart_with_DampPeerOscillations (Event 6),
- AutomaticStart_with_DampPeerOscillations_and_
PassiveTcpEstablishment (Event 7),
- IdleHoldTimer_Expires (Event 13).
Upon receiving these 3 events, the local system will
use these events to prevent peer oscillations.
The method of preventing persistent peer oscillation is
outside the scope of this document.
Any other event (Events 9-12, 15-28) received in the Idle state
does not cause change in the state of the local system.
Connect State:
In this state, BGP FSM is waiting for the TCP connection to
be completed.
The start events (Events 1, 3-7) are ignored in connect
state.
In response to a ManualStop event (Event 2), the local system:
- drops the TCP connection,
- releases all BGP resources,
- sets ConnectRetryCounter to zero,
- stops the ConnectRetryTimer and sets ConnectRetryTimer
to zero, and
- changes its state to Idle.
In response to the ConnectRetryTimer_Expires event (Event 9),
the local system:
- drops the TCP connection,
- restarts the ConnectRetryTimer,
- stops the DelayOpenTimer and resets the timer to zero,
- initiates a TCP connection to the other BGP peer,
- continues to listen for a connection that may be
initiated by the remote BGP peer, and
- stays in Connect state.
If the DelayOpenTimer_Expires event (Event 12) occurs in the
Connect state, the local system:
- sends an OPEN message to its peer,
- sets the HoldTimer to a large value, and
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- changes its state to OpenSent.
If the BGP FSM receives a TcpConnection_Valid event
(Event 14), the TCP connection is processed, and
the connection remains in the Connect state.
If the BGP FSM receives a Tcp_CR_Invalid event (Event 15),
the local system rejects the TCP connection, and the connection
remains in the Connect state.
If the TCP connection succeeds (Event 16 or Event 17),
the local system checks the DelayOpen attribute prior
to processing. If the DelayOpen attribute is set to TRUE,
the local system:
- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- sets the DelayOpenTimer to the initial value, and
- stays in the Connect state.
If the DelayOpen attribute is set to FALSE, the local system:
- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- completes BGP initialization
- sends an OPEN message to its peer,
- sets HoldTimer to a large value, and
- changes its state to OpenSent.
A HoldTimer value of 4 minutes is suggested.
If the TCP connection fails (Event 18), the local system
checks the DelayOpenTimer. If the DelayOpenTimer is running,
the local system:
- restarts the ConnectRetryTimer with initial value,
- stops the DelayOpenTimer and resets its value to zero,
- continues to listen for a connection that may be
initiated by the remote BGP peer, and
- changes its state to Active.
If the DelayOpenTimer is not running, the local system:
- stops the ConnectRetryTimer to zero,
- drops the TCP connection,
- releases all BGP resources, and
- changes its state to Idle.
If an OPEN message is received while the DelayOpenTimer is
running (Event 20), the local system:
- stops the ConnectRetryTimer (if running) and
sets the ConnectRetryTimer to zero,
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- completes the BGP initialization,
- stops and clears the DelayOpenTimer
(sets the value to zero),
- sends an OPEN message,
- sends a KEEPALIVE message,
- if the HoldTimer initial value is non-zero,
- starts the KeepaliveTimer with the initial value and
- resets the HoldTimer to the negotiated value,
else if HoldTimer initial value is zero,
- resets the KeepaliveTimer and
- resets the HoldTimer value to zero,
- and changes its state to OpenConfirm.
If the value of the autonomous system field is the same as the local
Autonomous System number, set the connection status to an internal
connection; otherwise it is "external".
If BGP message header checking detects an error (Event 21) or
OPEN message checking detects an error (Event 22) (see section
6.2), the local system:
- (optionally) If the SendNOTIFICATIONwithoutOPEN attribute
is set to TRUE, then the local system first sends
a NOTIFICATION message with the appropriate error
code, and then
- stops the ConnectRetryTimer (if running)
and sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping
if the DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If a NOTIFICATION message is received with a version
error (Event 24), the local system checks the DelayOpenTimer.
If the DelayOpenTimer is running, the local system:
- stops the ConnectRetryTimer (if running)
and sets the ConnectRetryTimer to zero,
- stops and resets the DelayOpenTimer (sets to zero),
- releases all BGP resources,
- drops the TCP connection, and
- changes its state to Idle.
If the DelayOpenTimer is not running, the local system:
- stops the ConnectRetryTimer and sets the
ConnectRetryTimer to zero,
- releases all BGP resources,
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- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- performs peer oscillation damping if the
DampPeerOscillations attribute is set to True, and
- changes its state to Idle.
In response to any other events (Events 8,10-11,13,19,23,
25-28) the local system:
- if the ConnectRetryTimer is running,
stops and resets the ConnectRetryTimer (sets to zero),
- if the DelayOpenTimer is running,
stops and resets the DelayOpenTimer (sets to zero),
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- performs peer oscillation damping if the
DampPeerOscillations attribute is set to True, and
- changes its state to Idle.
Active State:
In this state BGP FSM is trying to acquire a peer by listening
for and accepting a TCP connection.
The start events (Events 1, 3-7) are ignored in the Active
state.
In response to a ManualStop event (Event 2), the local system:
- If the DelayOpenTimer is running and the
SendNOTIFICATIONwithoutOPEN session attribute is set,
the local system sends a NOTIFICATION with a Cease,
- releases all BGP resources including
stopping the DelayOpenTimer
- drops the TCP connection,
- sets ConnectRetryCounter to zero,
- stops the ConnectRetryTimer and sets the
ConnectRetryTimer to zero, and
- changes its state to Idle.
In response to a ConnectRetryTimer_Expires event (Event 9),
the local system:
- restarts the ConnectRetryTimer (with initial value),
- initiates a TCP connection to the other BGP peer,
- continues to listen for TCP connection that may be
initiated by remote BGP peer, and
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- changes its state to Connect.
If the local system receives a DelayOpenTimer_Expires event
(Event 12), the local system:
- sets the ConnectRetryTimer to zero,
- stops and clears the DelayOpenTimer (set to zero),
- completes the BGP initialization,
- sends the OPEN message to its remote peer,
- sets its hold timer to a large value, and
- changes its state to OpenSent.
A HoldTimer value of 4 minutes is also suggested for this
state transition.
If the local system receives a TcpConnection_Valid event
(Event 14), the local system processes the TCP connection
flags and stays in Active state.
If the local system receives an Tcp_CR_Invalid event (Event 15):
the local system rejects the TCP connection and stays in
the Active State.
In response to a TCP connection succeeding (Event 16 or Event 17),
the local system checks the DelayOpen optional attribute prior to
processing.
If the DelayOpen attribute is set to TRUE, the local
system:
- stops the ConnectRetryTimer and sets the
ConnectRetryTimer to zero,
- sets the DelayOpenTimer to the initial value
(DelayOpenTime), and
- stays in the Active state.
If the DelayOpen attribute is set to FALSE, the local
system:
- sets the ConnectRetryTimer to zero,
- completes the BGP initialization,
- sends the OPEN message to its peer,
- sets its HoldTimer to a large value, and
- changes its state to OpenSent.
A HoldTimer value of 4 minutes is suggested as a "large value" for
the HoldTimer.
If the local system receives a TcpConnectionFails event (Event 18),
the local system:
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- restarts ConnectRetryTimer (with initial value),
- stops and clears the DelayOpenTimer (sets the value to zero),
- releases all BGP resource,
- increments ConnectRetryCounter by 1,
- optionally performs peer oscillation damping if
the DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If an OPEN message is received and the DelayOpenTimer is
running (Event 20), the local system:
- stops ConnectRetryTimer (if running) and sets
the ConnectRetryTimer to zero,
- stops and clears DelayOpenTimer (sets to zero),
- completes the BGP initialization,
- sends an OPEN message,
- sends a KEEPALIVE message,
- if the HoldTimer value is non-zero,
- starts the KeepaliveTimer to initial value,
- resets the HoldTimer to the negotiated value,
else if the HoldTimer is zero
- resets the KeepaliveTimer (set to zero),
- resets the HoldTimer to zero, and
- changes its state to OpenConfirm.
If the value of the autonomous system field is the same as
the local Autonomous System number, set the connection status
to an internal connection; otherwise it is external.
If BGP message header checking detects an error (Event 21)
or OPEN message checking detects an error (Event 22) (see
section 6.2), the local system:
- (optionally) sends a NOTIFICATION message with the
appropriate error code if the SendNOTIFICATIONwithoutOPEN
attribute is set to TRUE,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If a NOTIFICATION message is received with a version
error (Event 24), the local system checks the DelayOpenTimer.
If the DelayOpenTimer is running, the local system:
- stops the ConnectRetryTimer (if running) and
sets the ConnectRetryTimer to zero,
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- stops and resets the DelayOpenTimer (sets to zero),
- releases all BGP resources,
- drops the TCP connection, and
- changes its state to Idle.
If the DelayOpenTimer is not running, the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping
if the DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
In response to any other event (Events 8,10-11,13,19,23,25-28),
the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by one,
- (optionally) performs peer oscillation damping if
the DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
OpenSent:
In this state BGP FSM waits for an OPEN message from its peer.
The start events (Events 1, 3-7) are ignored in the OpenSent
state.
If a ManualStop event (Event 2) is issued in OpenSent
state, the local system:
- sends the NOTIFICATION with a cease,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- sets the ConnectRetryCounter to zero, and
- changes its state to Idle.
If an AutomaticStop event (Event 8) is issued in OpenSent
state, the local system:
- sends the NOTIFICATION with a cease,
- sets the ConnectRetryTimer to zero,
- releases all the BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
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DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the HoldTimer_Expires (Event 10), the local system:
- sends a NOTIFICATION message with error code Hold
Timer Expired,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If a TcpConnection_Valid (Event 14) or Tcp_CR_Acked (Event 16)
is received, or a TcpConnectionConfirmed event (Event 17) is
received, a second TCP connection may be in progress. This
second TCP connection is tracked per Connection Collision
processing (Section 6.8) until an OPEN message is received.
A TCP Connection Request for an Invalid port
(Tcp_CR_Invalid (Event 15)) is ignored.
If a TcpConnectionFails event (Event 18) is received,
the local system:
- closes the BGP connection,
- restarts the ConnectRetryTimer,
- continues to listen for a connection that may be
initiated by the remote BGP peer, and
- changes its state to Active.
When an OPEN message is received, all fields are checked
for correctness. If there are no errors in the OPEN message
(Event 19), the local system:
- resets the DelayOpenTimer to zero,
- sets the BGP ConnectRetryTimer to zero,
- sends a KEEPALIVE message, and
- sets a KeepaliveTimer (via the text below)
- sets the HoldTimer according to the negotiated value
(see Section 4.2),
- changes its state to OpenConfirm.
If the negotiated hold time value is zero, then the HoldTimer and
KeepaliveTimer are not started. If the value of the Autonomous
System field is the same as the local Autonomous System number,
then the connection is an "internal" connection; otherwise, it
is an "external" connection. (This will impact UPDATE processing
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as described below.)
If the BGP message header checking (Event 21) or OPEN message
check detects an error (Event 22)(see Section 6.2), the local system:
- sends a NOTIFICATION message with appropriate error
code,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is TRUE, and
- changes its state to Idle.
Collision detection mechanisms (Section 6.8) need to be
applied when a valid BGP OPEN message is received (Event 19 or
Event 20). Please refer to Section 6.8 for the details of
the comparison. A CollisionDetectDump event occurs when the
BGP implementation determines, by a means outside the scope of
this document, that a connection collision has occurred.
If a connection in OpenSent state is determined to be the
connection that must be closed, an OpenCollisionDump (Event 23)
is signaled to the state machine. If such an event is
received in OpenSent state, the local system:
- sends a NOTIFICATION with a Cease
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If a NOTIFICATION message is received with a version
error (Event 24), the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection, and
- changes its state to Idle.
In response to any other event (Events 9, 11-13,20,25-28),
the local system:
- sends the NOTIFICATION with the Error Code Finite
state machine error,
- sets the ConnectRetryTimer to zero,
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- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
OpenConfirm State:
In this state BGP waits for a KEEPALIVE or NOTIFICATION
message.
Any start event (Events 1, 3-7) is ignored in the OpenConfirm
state.
In response to a ManualStop event (Event 2) initiated by
the operator, the local system:
- sends the NOTIFICATION message with Cease,
- releases all BGP resources,
- drops the TCP connection,
- sets the ConnectRetryCounter to zero,
- sets the ConnectRetryTimer to zero, and
- changes its state to Idle.
In response to the AutomaticStop event initiated by the
system (Event 8), the local system:
- sends the NOTIFICATION message with Cease,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping
if the DampPeerOscillations attribute is set to TRUE,
and
- changes its state to Idle.
If the HoldTimer_Expires event (Event 10) occurs before a KEEPALIVE
message is received, the local system:
- sends the NOTIFICATION message with the error code,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if
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the DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the local system receives a KeepaliveTimer_Expires
event (Event 11), the system:
- sends a KEEPALIVE message,
- restarts the KeepaliveTimer, and
- remains in OpenConfirmed state.
In the event of TcpConnection_Valid event (Event 14), or TCP
connection succeeding (Event 16 or Event 17) while in OpenConfirm,
the local system needs to track the second connection.
If a TCP connection is attempted to an invalid port (Event 15),
the local system will ignore the second connection
attempt.
If the local system receives a TcpConnectionFails event
(Event 18) from the underlying TCP or a NOTIFICATION
message (Event 25), the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the local system receives a NOTIFICATION message with a
version error (NotifMsgVerErr (Event 24)), the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection, and
- changes its state to Idle.
If the local system receives a valid OPEN message
(BGPOpen (Event 19)), the collision detect function is
processed per Section 6.8. If this connection is to be
dropped due to connection collision, the local system:
- sends a NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection (send TCP FIN),
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
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DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If an OPEN message is received, all fields are checked for
correctness. If the BGP message header checking
(BGPHeaderErr (Event 21)) or OPEN message check detects
an error (see Section 6.2) (BGPOpenMsgErr (Event 22)), the
local system:
- sends a NOTIFICATION message with appropriate error
code,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If during the processing of another OPEN message, the BGP
implementation determines by a means outside the scope of
this document that a connection collision has occurred and
this connection is to be closed, the local system will
issue an OpenCollisionDump event (Event 23). When the local
system receives an OpenCollisionDump event (Event 23), the
local system:
- sends a NOTIFICATION with a Cease
- sets the ConnectRetryTimer to zero,
- releases all BGP resources
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the local system receives a KEEPALIVE message
(KeepAliveMsg (Event 26)), the local system:
- restarts the HoldTimer and
- changes its state to Established.
In response to any other event (Events 9, 12-13, 20, 27-28),
the local system:
- sends a NOTIFICATION with a code of Finite State
Machine Error,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
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- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
Established State:
In the Established state, the BGP FSM can exchange UPDATE,
NOTIFICATION, and KEEPALIVE messages with its peer.
Any Start event (Events 1, 3-7) is ignored in the
Established state.
In response to a ManualStop event (initiated by an
operator) (Event 2), the local system:
- sends the NOTIFICATION message with Cease,
- sets the ConnectRetryTimer to zero,
- deletes all routes associated with this connection,
- releases BGP resources,
- drops the TCP connection,
- sets ConnectRetryCounter to zero, and
- changes its state to Idle.
In response to an AutomaticStop event (Event 8), the local system:
- sends a NOTIFICATION with Cease,
- sets the ConnectRetryTimer to zero
- deletes all routes associated with this connection,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
One reason for an AutomaticStop event is: A BGP receives
UPDATE messages with number of prefixes for a given
peer so that the total prefixes received exceeds the
maximum number of prefixes configured. The local system
automatically disconnects the peer.
If the HoldTimer_Expires event occurs (Event 10), the
local system:
- sends a NOTIFICATION message with Error Code Hold
Timer Expired,
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- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the KeepaliveTimer_Expires event occurs (Event 11),
the local system:
- sends a KEEPALIVE message, and
- restarts its KeepaliveTimer unless the negotiated
HoldTime value is zero.
Each time the local system sends a KEEPALIVE or UPDATE
message, it restarts its KeepaliveTimer, unless the
negotiated HoldTime value is zero.
A TcpConnection_Valid (Event 14) received for a
valid port will cause the second connection to be
tracked.
An invalid TCP connection (Tcp_CR_Invalid event
(Event 15)), will be ignored.
In response to an indication that the TCP connection
is successfully established (Event 16 or Event 17),
the second connection SHALL be tracked until
it sends an OPEN message.
If a valid OPEN message (BGPOpen (Event 19)) is received,
and if the CollisionDetectEstablishedState optional
attribute is TRUE, the OPEN message will be checked
to see if it collides (Section 6.8) with any other connection.
If the BGP implementation determines that this connection
needs to be terminated, it will process an OpenCollisionDump
event (Event 23). If this connection needs to be
terminated, the local system:
- sends a NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero,
- deletes all routes associated with this connection,
- releases all BGP resources,
- drops the TCP connection,
- increments ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations is set to TRUE, and
- changes its state to Idle.
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If the local system receives a NOTIFICATION message
(Event 24 or Event 25) or a TcpConnectionFails (Event 18)
from the underlying TCP, it:
- sets the ConnectRetryTimer to zero,
- deletes all routes associated with this connection,
- releases all the BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- changes its state to Idle.
If the local system receives a KEEPALIVE message
(Event 26), the local system:
- restarts its HoldTimer, if the negotiated HoldTime
value is non-zero, and
- remains in the Established state.
If the local system receives an UPDATE message (Event 27),
the local system:
- processes the message,
- restarts its HoldTimer if the negotiated HoldTime
value is non-zero, and
- remains in the Established state.
If the local system receives an UPDATE message, and the
UPDATE message error handling procedure (see Section 6.3)
detects an error (Event 28), the local system:
- sends a NOTIFICATION message with Update error,
- sets the ConnectRetryTimer to zero,
- deletes all routes associated with this connection,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
In response to any other event (Events 9, 12-13, 20-22) the
local system:
- sends a NOTIFICATION message with Error Code Finite
State Machine Error,
- deletes all routes associated with this connection,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
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- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
9. UPDATE Message Handling
An UPDATE message may be received only in the Established state.
Receiving an UPDATE message in any other state is an error. When an
UPDATE message is received, each field is checked for validity as
specified in Section 6.3.
If an optional non-transitive attribute is unrecognized, it is qui-
etly ignored. If an optional transitive attribute is unrecognized,
the Partial bit (the third high-order bit) in the attribute flags
octet is set to 1, and the attribute is retained for propagation to
other BGP speakers.
If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible propaga-
tion to other BGP speakers.
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as IP
prefixes) contained in this field SHALL be removed from the Adj-RIB-
In. This BGP speaker SHALL run its Decision Process since the previ-
ously advertised route is no longer available for use.
If the UPDATE message contains a feasible route, the Adj-RIB-In will
be updated with this route as follows: if the NLRI of the new route
is identical to the one of the route currently stored in the Adj-RIB-
In, then the new route SHALL replace the older route in the Adj-RIB-
In, thus implicitly withdrawing the older route from service. Other-
wise, if the Adj-RIB-In has no route with NLRI identical to the new
route, the new route SHALL be placed in the Adj-RIB-In.
Once the BGP speaker updates the Adj-RIB-In, the speaker SHALL run
its Decision Process.
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9.1 Decision Process
The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIBs-In. The output of the Decision
Process is the set of routes that will be advertised to peers; the
selected routes will be stored in the local speaker's Adj-RIBs-Out
according to policy.
The BGP Decision Process described here is conceptual, and does not
have to be implemented precisely as described here, as long as the
implementations support the described functionality and their exter-
nally visible behavior is the same.
The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns either (a)
a non-negative integer denoting the degree of preference for the
route, or (b) a value denoting that this route is ineligible to be
installed in Loc-RIB and will be excluded from the next phase of
route selection.
The function that calculates the degree of preference for a given
route SHALL NOT use as its inputs any of the following: the existence
of other routes, the non-existence of other routes, or the path
attributes of other routes. Route selection then consists of individ-
ual application of the degree of preference function to each feasible
route, followed by the choice of the one with the highest degree of
preference.
The Decision Process operates on routes contained in the Adj-RIBs-In,
and is responsible for:
- selection of routes to be used locally by the speaker
- selection of routes to be advertised to other BGP peers
- route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each trig-
gered by a different event:
a) Phase 1 is responsible for calculating the degree of preference
for each route received from a peer.
b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into
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the Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each peer,
according to the policies contained in the PIB. Route aggregation
and information reduction can optionally be performed within this
phase.
9.1.1 Phase 1: Calculation of Degree of Preference
The Phase 1 decision function is invoked whenever the local BGP
speaker receives from a peer an UPDATE message that advertises a new
route, a replacement route, or withdrawn routes.
The Phase 1 decision function is a separate process which completes
when it has no further work to do.
The Phase 1 decision function locks an Adj-RIB-In prior to operating
on any route contained within it, and unlocks it after operating on
all new or unfeasible routes contained within it.
For each newly received or replacement feasible route, the local BGP
speaker determines a degree of preference as follows:
If the route is learned from an internal peer, either the value of
the LOCAL_PREF attribute is taken as the degree of preference, or
the local system computes the degree of preference of the route
based on preconfigured policy information. Note that the latter
(computing the degree of preference based on preconfigured policy
information) may result in formation of persistent routing loops.
If the route is learned from an external peer, then the local BGP
speaker computes the degree of preference based on preconfigured
policy information. If the return value indicates that the route
is ineligible, the route MAY NOT serve as an input to the next
phase of route selection; otherwise the return value MUST be used
as the LOCAL_PREF value in any IBGP readvertisement.
The exact nature of this policy information and the computation
involved is a local matter.
9.1.2 Phase 2: Route Selection
The Phase 2 decision function is invoked on completion of Phase 1.
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The Phase 2 function is a separate process which completes when it
has no further work to do. The Phase 2 process considers all routes
that are eligible in the Adj-RIBs-In.
The Phase 2 decision function is blocked from running while the Phase
3 decision function is in process. The Phase 2 function locks all
Adj-RIBs-In prior to commencing its function, and unlocks them on
completion.
If the NEXT_HOP attribute of a BGP route depicts an address that is
not resolvable, or it would become unresolvable if the route was
installed in the routing table the BGP route MUST be excluded from
the Phase 2 decision function.
If the AS_PATH attribute of a BGP route contains an AS loop, the BGP
route should be excluded from the Phase 2 decision function. AS loop
detection is done by scanning the full AS path (as specified in the
AS_PATH attribute), and checking that the autonomous system number of
the local system does not appear in the AS path. Operations of a BGP
speaker that is configured to accept routes with its own autonomous
system number in the AS path are outside the scope of this document.
It is critical that BGP speakers within an AS do not make conflicting
decisions regarding route selection that would cause forwarding loops
to occur.
For each set of destinations for which a feasible route exists in the
Adj-RIBs-In, the local BGP speaker identifies the route that has:
a) the highest degree of preference of any route to the same set
of destinations, or
b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules spec-
ified in 9.1.2.2.
The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being
held in the Loc-RIB. When the new BGP route is installed in the Rout-
ing Table, care must be taken to ensure that existing routes to the
same destination that are now considered invalid are removed from the
Routing Table. Whether or not the new BGP route replaces an existing
non-BGP route in the Routing Table depends on the policy configured
on the BGP speaker.
The local speaker MUST determine the immediate next-hop address from
the NEXT_HOP attribute of the selected route (see Section 5.1.3). If
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either the immediate next hop or the IGP cost to the NEXT_HOP (where
the NEXT_HOP is resolved through an IGP route) changes, Phase 2 Route
Selection MUST be performed again.
Notice that even though BGP routes do not have to be installed in the
Routing Table with the immediate next hop(s), implementations MUST
take care that before any packets are forwarded along a BGP route,
its associated NEXT_HOP address is resolved to the immediate
(directly connected) next-hop address and this address (or multiple
addresses) is finally used for actual packet forwarding.
Unresolvable routes SHALL be removed from the Loc-RIB and the routing
table. However, corresponding unresolvable routes SHOULD be kept in
the Adj-RIBs-In (in case they become resolvable).
9.1.2.1 Route Resolvability Condition
As indicated in Section 9.1.2, BGP speakers SHOULD exclude unresolv-
able routes from the Phase 2 decision. This ensures that only valid
routes are installed in Loc-RIB and the Routing Table.
The route resolvability condition is defined as follows.
1. A route Rte1, referencing only the intermediate network
address, is considered resolvable if the Routing Table contains at
least one resolvable route Rte2 that matches Rte1's intermediate
network address and is not recursively resolved (directly or indi-
rectly) through Rte1. If multiple matching routes are available,
only the longest matching route SHOULD be considered.
2. Routes referencing interfaces (with or without intermediate
addresses) are considered resolvable if the state of the refer-
enced interface is up and IP processing is enabled on this inter-
face.
BGP routes do not refer to interfaces, but can be resolved through
the routes in the Routing Table that can be of both types (those that
specify interfaces or those that do not). IGP routes and routes to
directly connected networks are expected to specify the outbound
interface. Static routes can specify the outbound interface, or the
intermediate address, or both.
Note that a BGP route is considered unresolvable not only in situa-
tions where the BGP speaker's Routing Table contains no route match-
ing the BGP route's NEXT_HOP. Mutually recursive routes (routes
resolving each other or themselves), also fail the resolvability
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check.
It is also important that implementations do not consider feasible
routes that would become unresolvable if they were installed in the
Routing Table even if their NEXT_HOPs are resolvable using the cur-
rent contents of the Routing Table (an example of such routes would
be mutually recursive routes). This check ensures that a BGP speaker
does not install in the Routing Table routes that will be removed and
not used by the speaker. Therefore, in addition to local Routing Ta-
ble stability, this check also improves behavior of the protocol in
the network.
Whenever a BGP speaker identifies a route that fails the resolvabil-
ity check because of mutual recursion, an error message SHOULD be
logged.
9.1.2.2 Breaking Ties (Phase 2)
In its Adj-RIBs-In a BGP speaker may have several routes to the same
destination that have the same degree of preference. The local
speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all routes with the
same degrees of preference, both those received from internal peers,
and those received from external peers.
The following tie-breaking procedure assumes that for each candidate
route all the BGP speakers within an autonomous system can ascertain
the cost of a path (interior distance) to the address depicted by the
NEXT_HOP attribute of the route, and follow the same route selection
algorithm.
The tie-breaking algorithm begins by considering all equally prefer-
able routes to the same destination, and then selects routes to be
removed from consideration. The algorithm terminates as soon as only
one route remains in consideration. The criteria MUST be applied in
the order specified.
Several of the criteria are described using pseudo-code. Note that
the pseudo-code shown was chosen for clarity, not efficiency. It is
not intended to specify any particular implementation. BGP implemen-
tations MAY use any algorithm which produces the same results as
those described here.
a) Remove from consideration all routes which are not tied for
having the smallest number of AS numbers present in their AS_PATH
attributes. Note, that when counting this number, an AS_SET counts
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as 1, no matter how many ASs are in the set.
b) Remove from consideration all routes which are not tied for
having the lowest Origin number in their Origin attribute.
c) Remove from consideration routes with less-preferred
MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
between routes learned from the same neighboring AS (the neighbor-
ing AS is determined from the AS_PATH attribute). Routes which do
not have the MULTI_EXIT_DISC attribute are considered to have the
lowest possible MULTI_EXIT_DISC value.
This is also described in the following procedure:
for m = all routes still under consideration
for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration
In the pseudo-code above, MED(n) is a function which returns the
value of route n's MULTI_EXIT_DISC attribute. If route n has no
MULTI_EXIT_DISC attribute, the function returns the lowest possi-
ble MULTI_EXIT_DISC value, i.e. 0.
Similarly, neighborAS(n) is a function which returns the neighbor
AS from which the route was received. If the route is learned via
IBGP, and the other IBGP speaker didn't originate the route, it is
the neighbor AS from which the other IBGP speaker learned the
route. If the route is learned via IBGP, and the other IBGP
speaker either (a) originated the route, or (b) created the route
by aggregation and the AS_PATH attribute of the aggregate route is
either empty or begins with an AS_SET, it is the local AS.
If a MULTI_EXIT_DISC attribute is removed before re-advertising a
route into IBGP, then comparison based on the received EBGP
MULTI_EXIT_DISC attribute MAY still be performed. If an implemen-
tation chooses to remove MULTI_EXIT_DISC, then the optional com-
parison on MULTI_EXIT_DISC if performed at all MUST be performed
only among EBGP learned routes. The best EBGP learned route may
then be compared with IBGP learned routes after the removal of the
MULTI_EXIT_DISC attribute. If MULTI_EXIT_DISC is removed from a
subset of EBGP learned routes and the selected "best" EBGP learned
route will not have MULTI_EXIT_DISC removed, then the
MULTI_EXIT_DISC must be used in the comparison with IBGP learned
routes. For IBGP learned routes the MULTI_EXIT_DISC MUST be used
in route comparisons which reach this step in the Decision
Process. Including the MULTI_EXIT_DISC of an EBGP learned route
in the comparison with an IBGP learned route, then removing the
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MULTI_EXIT_DISC attribute and advertising the route has been
proven to cause route loops.
d) If at least one of the candidate routes was received via EBGP,
remove from consideration all routes which were received via IBGP.
e) Remove from consideration any routes with less-preferred inte-
rior cost. The interior cost of a route is determined by calcu-
lating the metric to the NEXT_HOP for the route using the Routing
Table. If the NEXT_HOP hop for a route is reachable, but no cost
can be determined, then this step should be skipped (equivalently,
consider all routes to have equal costs).
This is also described in the following procedure.
for m = all routes still under consideration
for n = all routes in still under consideration
if (cost(n) is lower than cost(m))
remove m from consideration
In the pseudo-code above, cost(n) is a function which returns the
cost of the path (interior distance) to the address given in the
NEXT_HOP attribute of the route.
f) Remove from consideration all routes other than the route that
was advertised by the BGP speaker whose BGP Identifier has the
lowest value.
g) Prefer the route received from the lowest peer address.
9.1.3 Phase 3: Route Dissemination
The Phase 3 decision function is invoked on completion of Phase 2, or
when any of the following events occur:
a) when routes in the Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP
have changed
c) when a new BGP speaker - BGP speaker connection has been estab-
lished
The Phase 3 function is a separate process which completes when it
has no further work to do. The Phase 3 Routing Decision function is
blocked from running while the Phase 2 decision function is in
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process.
All routes in the Loc-RIB are processed into Adj-RIBs-Out according
to configured policy. This policy MAY exclude a route in the Loc-RIB
from being installed in a particular Adj-RIB-Out. A route SHALL NOT
be installed in the Adj-Rib-Out unless the destination and NEXT_HOP
described by this route may be forwarded appropriately by the Routing
Table. If a route in Loc-RIB is excluded from a particular Adj-RIB-
Out the previously advertised route in that Adj-RIB-Out MUST be with-
drawn from service by means of an UPDATE message (see 9.2).
Route aggregation and information reduction techniques (see 9.2.2.1)
may optionally be applied.
Any local policy which results in routes being added to an Adj-RIB-
Out without also being added to the local BGP speaker's forwarding
table, is outside the scope of this document.
When the updating of the Adj-RIBs-Out and the Routing Table is com-
plete, the local BGP speaker runs the Update-Send process of 9.2.
9.1.4 Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI overlap
occurs when a set of destinations are identified in non-matching mul-
tiple routes. Since BGP encodes NLRI using IP prefixes, overlap will
always exhibit subset relationships. A route describing a smaller
set of destinations (a longer prefix) is said to be more specific
than a route describing a larger set of destinations (a shorter pre-
fix); similarly, a route describing a larger set of destinations is
said to be less specific than a route describing a smaller set of
destinations.
The precedence relationship effectively decomposes less specific
routes into two parts:
- a set of destinations described only by the less specific route,
and
- a set of destinations described by the overlap of the less spe-
cific and the more specific routes
The set of destinations described by the overlap represents a portion
of the less specific route that is feasible, but is not currently in
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use. If a more specific route is later withdrawn, the set of desti-
nations described by the overlap will still be reachable using the
less specific route.
If a BGP speaker receives overlapping routes, the Decision Process
MUST consider both routes based on the configured acceptance policy.
If both a less and a more specific route are accepted, then the Deci-
sion Process MUST install in Loc-RIB either both the less and the
more specific routes or aggregate the two routes and install in Loc-
RIB the aggregated route, provided that both routes have the same
value of the NEXT_HOP attribute.
If a BGP speaker chooses to aggregate, then it SHOULD either include
all AS used to form the aggregate in an AS_SET or add the
ATOMIC_AGGREGATE attribute to the route. This attribute is now pri-
marily informational. With the elimination of IP routing protocols
that do not support classless routing and the elimination of router
and host implementations that do not support classless routing, there
is no longer a need to de-aggregate. Routes SHOULD NOT be de-aggre-
gated. A route that carries ATOMIC_AGGREGATE attribute in particular
MUST NOT be de-aggregated. That is, the NLRI of this route can not be
made more specific. Forwarding along such a route does not guarantee
that IP packets will actually traverse only ASs listed in the AS_PATH
attribute of the route.
9.2 Update-Send Process
The Update-Send process is responsible for advertising UPDATE mes-
sages to all peers. For example, it distributes the routes chosen by
the Decision Process to other BGP speakers which may be located in
either the same autonomous system or a neighboring autonomous system.
When a BGP speaker receives an UPDATE message from an internal peer,
the receiving BGP speaker SHALL NOT re-distribute the routing infor-
mation contained in that UPDATE message to other internal peers
(unless the speaker acts as a BGP Route Reflector [RFC2796]).
As part of Phase 3 of the route selection process, the BGP speaker
has updated its Adj-RIBs-Out. All newly installed routes and all
newly unfeasible routes for which there is no replacement route SHALL
be advertised to its peers by means of an UPDATE message.
A BGP speaker SHOULD NOT advertise a given feasible BGP route from
its Adj-RIB-Out if it would produce an UPDATE message containing the
same BGP route as was previously advertised.
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Any routes in the Loc-RIB marked as unfeasible SHALL be removed.
Changes to the reachable destinations within its own autonomous sys-
tem SHALL also be advertised in an UPDATE message.
If due to the limits on the maximum size of an UPDATE message (see
Section 4) a single route doesn't fit into the message, the BGP
speaker MUST not advertise the route to its peers and MAY choose to
log an error locally.
9.2.1 Controlling Routing Traffic Overhead
The BGP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed to
advertise UPDATE messages and the processing power needed by the
Decision Process to digest the information contained in the UPDATE
messages.
9.2.1.1 Frequency of Route Advertisement
The parameter MinRouteAdvertisementIntervalTimer determines the mini-
mum amount of time that must elapse between advertisement and/or
withdrawal of routes to a particular destination by a BGP speaker to
a peer. This rate limiting procedure applies on a per-destination
basis, although the value of MinRouteAdvertisementIntervalTimer is
set on a per BGP peer basis.
Two UPDATE messages sent by a BGP speaker to a peer that advertise
feasible routes and/or withdrawal of unfeasible routes to some common
set of destinations MUST be separated by at least MinRouteAdvertise-
mentIntervalTimer. Clearly, this can only be achieved precisely by
keeping a separate timer for each common set of destinations. This
would be unwarranted overhead. Any technique which ensures that the
interval between two UPDATE messages sent from a BGP speaker to a
peer that advertise feasible routes and/or withdrawal of unfeasible
routes to some common set of destinations will be at least Min-
RouteAdvertisementIntervalTimer, and will also ensure a constant
upper bound on the interval is acceptable.
Since fast convergence is needed within an autonomous system, either
(a) the MinRouteAdvertisementIntervalTimer used for internal peers
SHOULD be shorter than the MinRouteAdvertisementIntervalTimer used
for external peers, or (b) the procedure describe in this section
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SHOULD NOT apply for routes sent to internal peers.
This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval-
Timer, the last route selected SHALL be advertised at the end of Min-
RouteAdvertisementIntervalTimer.
9.2.1.2 Frequency of Route Origination
The parameter MinASOriginationIntervalTimer determines the minimum
amount of time that must elapse between successive advertisements of
UPDATE messages that report changes within the advertising BGP
speaker's own autonomous systems.
9.2.2 Efficient Organization of Routing Information
Having selected the routing information which it will advertise, a
BGP speaker may avail itself of several methods to organize this
information in an efficient manner.
9.2.2.1 Information Reduction
Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information
that it will place in the Adj-RIBs-Out by any of the following meth-
ods:
a) Network Layer Reachability Information (NLRI):
Destination IP addresses can be represented as IP address pre-
fixes. In cases where there is a correspondence between the
address structure and the systems under control of an autonomous
system administrator, it will be possible to reduce the size of
the NLRI carried in the UPDATE messages.
b) AS_PATHs:
AS path information can be represented as ordered AS_SEQUENCEs or
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unordered AS_SETs. AS_SETs are used in the route aggregation algo-
rithm described in 9.2.2.2. They reduce the size of the AS_PATH
information by listing each AS number only once, regardless of how
many times it may have appeared in multiple AS_PATHs that were
aggregated.
An AS_SET implies that the destinations listed in the NLRI can be
reached through paths that traverse at least some of the con-
stituent autonomous systems. AS_SETs provide sufficient informa-
tion to avoid routing information looping; however their use may
prune potentially feasible paths, since such paths are no longer
listed individually as in the form of AS_SEQUENCEs. In practice
this is not likely to be a problem, since once an IP packet
arrives at the edge of a group of autonomous systems, the BGP
speaker at that point is likely to have more detailed path infor-
mation and can distinguish individual paths to destinations.
9.2.2.2 Aggregating Routing Information
Aggregation is the process of combining the characteristics of sev-
eral different routes in such a way that a single route can be adver-
tised. Aggregation can occur as part of the Decision Process to
reduce the amount of routing information that will be placed in the
Adj-RIBs-Out.
Aggregation reduces the amount of information that a BGP speaker must
store and exchange with other BGP speakers. Routes can be aggregated
by applying the following procedure separately to path attributes of
the same type and to the Network Layer Reachability Information.
Routes that have different MULTI_EXIT_DISC attribute SHALL NOT be
aggregated.
If the aggregated route has an AS_SET as the first element in its
AS_PATH attribute, then the router that originates the route SHOULD
NOT advertise the MULTI_EXIT_DISC attribute with this route.
Path attributes that have different type codes can not be aggregated
together. Path attributes of the same type code may be aggregated,
according to the following rules:
NEXT_HOP:
When aggregating routes that have different NEXT_HOP attribute,
the NEXT_HOP attribute of the aggregated route SHALL identify
an interface on the BGP speaker that performs the aggregation.
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ORIGIN attribute:
If at least one route among routes that are aggregated has ORI-
GIN with the value INCOMPLETE, then the aggregated route MUST
have the ORIGIN attribute with the value INCOMPLETE. Other-
wise, if at least one route among routes that are aggregated
has ORIGIN with the value EGP, then the aggregated route MUST
have the ORIGIN attribute with the value EGP. In all other
cases the value of the ORIGIN attribute of the aggregated route
is IGP.
AS_PATH attribute:
If routes to be aggregated have identical AS_PATH attributes,
then the aggregated route has the same AS_PATH attribute as
each individual route.
For the purpose of aggregating AS_PATH attributes we model each
AS within the AS_PATH attribute as a tuple <type, value>, where
"type" identifies a type of the path segment the AS belongs to
(e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. If
the routes to be aggregated have different AS_PATH attributes,
then the aggregated AS_PATH attribute SHALL satisfy all of the
following conditions:
- all tuples of type AS_SEQUENCE in the aggregated AS_PATH
SHALL appear in all of the AS_PATH in the initial set of
routes to be aggregated.
- all tuples of type AS_SET in the aggregated AS_PATH SHALL
appear in at least one of the AS_PATH in the initial set
(they may appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of type AS_SEQUENCE in the aggregated
AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
precedes Y in each AS_PATH in the initial set which contains
Y, regardless of the type of Y.
- No tuple of type AS_SET with the same value SHALL appear
more than once in the aggregated AS_PATH.
- Multiple tuples of type AS_SEQUENCE with the same value
may appear in the aggregated AS_PATH only when adjacent to
another tuple of the same type and value.
An implementation may choose any algorithm which conforms to
these rules. At a minimum a conformant implementation SHALL be
able to perform the following algorithm that meets all of the
above conditions:
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- determine the longest leading sequence of tuples (as
defined above) common to all the AS_PATH attributes of the
routes to be aggregated. Make this sequence the leading
sequence of the aggregated AS_PATH attribute.
- set the type of the rest of the tuples from the AS_PATH
attributes of the routes to be aggregated to AS_SET, and
append them to the aggregated AS_PATH attribute.
- if the aggregated AS_PATH has more than one tuple with the
same value (regardless of tuple's type), eliminate all, but
one such tuple by deleting tuples of the type AS_SET from
the aggregated AS_PATH attribute.
- for each pair of adjacent tuples in the aggregated
AS_PATH, if both tuples have the same type, merge them
together, as long as doing so will not cause a segment with
length greater than 255 to be generated.
Appendix F, Section F.6 presents another algorithm that satis-
fies the conditions and allows for more complex policy configu-
rations.
ATOMIC_AGGREGATE:
If at least one of the routes to be aggregated has
ATOMIC_AGGREGATE path attribute, then the aggregated route
SHALL have this attribute as well.
AGGREGATOR:
Any AGGREGATOR attributes from the routes to be aggregated MUST
NOT be included in the aggregated route. The BGP speaker per-
forming the route aggregation MAY attach a new AGGREGATOR
attribute (see Section 5.1.7).
9.3 Route Selection Criteria
Generally speaking, additional rules for comparing routes among sev-
eral alternatives are outside the scope of this document. There are
two exceptions:
- If the local AS appears in the AS path of the new route being
considered, then that new route can not be viewed as better than
any other route (provided that the speaker is configured to accept
such routes). If such a route were ever used, a routing loop could
result.
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- In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an AS
SHOULD avoid using unstable routes, and it SHOULD NOT make rapid
spontaneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require expe-
rience, but the principle is clear. Routes that are unstable can
be "penalized" (e.g., by using the procedures described in
[RFC2439]).
9.4 Originating BGP routes
A BGP speaker may originate BGP routes by injecting routing informa-
tion acquired by some other means (e.g. via an IGP) into BGP. A BGP
speaker that originates BGP routes assigns the degree of preference
(e.g., according to local configuration) to these routes by passing
them through the Decision Process (see Section 9.1). These routes MAY
also be distributed to other BGP speakers within the local AS as part
of the update process (see Section 9.2). The decision whether to dis-
tribute non-BGP acquired routes within an AS via BGP or not depends
on the environment within the AS (e.g. type of IGP) and SHOULD be
controlled via configuration.
10 BGP Timers
BGP employs five timers: ConnectRetryTimer (see Section 8), HoldTimer
(see Section 4.2), KeepaliveTimer (see Section 8), MinASOrigination-
IntervalTimer (see Section 9.2.1.2), and MinRouteAdvertisementInter-
valTimer (see Section 9.2.1.1).
Two optional timers MAY be supported: DelayOpenTimer, IdleHoldTimer
by BGP (see section 8). Section 8 describes their use. The full oper-
ation of these optional timers is outside the scope of this document.
ConnectRetryTime is a mandatory FSM attribute that stores the initial
value for the ConnectRetryTimer. The suggested default value for the
ConnectRetryTime is 120 seconds.
HoldTime is a mandatory FSM attribute that stores the initial value
for the HoldTimer. The suggested default value for the HoldTime is 90
seconds.
During some portions of the state machine (see Section 8), the Hold-
Timer is set to a large value. The suggested default for this large
value is 4 minutes.
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The KeepaliveTime is a mandatory FSM attribute that stores the ini-
tial value for the KeepaliveTimer. The suggested default value for
the KeepaliveTime is 1/3 of the HoldTime.
The suggested default value for the MinASOriginationIntervalTimer is
15 seconds.
The suggested default value for the MinRouteAdvertisementInterval-
Timer on EBGP connections is 30 seconds.
The suggested default value for the MinRouteAdvertisementInterval-
Timer on IBGP connections is 5 seconds.
An implementation of BGP MUST allow the HoldTimer to be configurable
on a per peer basis, and MAY allow the other timers to be config-
urable.
To minimize the likelihood that the distribution of BGP messages by a
given BGP speaker will contain peaks, jitter SHOULD be applied to the
timers associated with MinASOriginationIntervalTimer, KeepaliveTimer,
MinRouteAdvertisementIntervalTimer, and ConnectRetryTimer. A given
BGP speaker MAY apply the same jitter to each of these quantities
regardless of the destinations to which the updates are being sent;
that is, jitter need not be configured on a "per peer" basis.
The suggested default amount of jitter SHALL be determined by multi-
plying the base value of the appropriate timer by a random factor
which is uniformly distributed in the range from 0.75 to 1.0. A new
random value SHOULD be picked each time the timer is set. The range
of the jitter random value MAY be configurable.
Appendix A. Comparison with RFC1771
There are numerous editorial changes (too many to list here).
The following list the technical changes:
Changes to reflect the usages of such features as TCP MD5
[RFC2385], BGP Route Reflectors [RFC2796], BGP Confederations
[RFC3065], and BGP Route Refresh [RFC2918].
Clarification on the use of the BGP Identifier in the AGGREGATOR
attribute.
Procedures for imposing an upper bound on the number of prefixes
that a BGP speaker would accept from a peer.
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The ability of a BGP speaker to include more than one instance of
its own AS in the AS_PATH attribute for the purpose of inter-AS
traffic engineering.
Clarifications on the various types of NEXT_HOPs.
Clarifications to the use of the ATOMIC_AGGREGATE attribute.
The relationship between the immediate next hop, and the next hop
as specified in the NEXT_HOP path attribute.
Clarifications on the tie-breaking procedures.
Clarifications on the frequency of route advertisements.
Optional Parameter Type 1 (Authentication Information) has been
deprecated.
UPDATE Message Error subcode 7 (AS Routing Loop) has been depre-
cated.
OPEN Message Error subcode 5 (Authentication Failure) has been
deprecated.
Use of the Marker field for authentication has been deprecated.
Implementations MUST support TCP MD5 [RFC2385] for authentication.
Clarification of BGP FSM.
Appendix B. Comparison with RFC1267
All the changes listed in Appendix A, plus the following.
BGP-4 is capable of operating in an environment where a set of reach-
able destinations may be expressed via a single IP prefix. The con-
cept of network classes, or subnetting is foreign to BGP-4. To
accommodate these capabilities BGP-4 changes semantics and encoding
associated with the AS_PATH attribute. New text has been added to
define semantics associated with IP prefixes. These abilities allow
BGP-4 to support the proposed supernetting scheme [9].
To simplify configuration this version introduces a new attribute,
LOCAL_PREF, that facilitates route selection procedures.
The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.
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A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
certain aggregates are not de-aggregated. Another new attribute,
AGGREGATOR, can be added to aggregate routes in order to advertise
which AS and which BGP speaker within that AS caused the aggregation.
To insure that Hold Timers are symmetric, the Hold Timer is now nego-
tiated on a per-connection basis. Hold Timers of zero are now sup-
ported.
Appendix C. Comparison with RFC 1163
All of the changes listed in Appendices A and B, plus the following.
To detect and recover from BGP connection collision, a new field (BGP
Identifier) has been added to the OPEN message. New text (Section
6.8) has been added to specify the procedure for detecting and recov-
ering from collision.
The new document no longer restricts the router that is passed in the
NEXT_HOP path attribute to be part of the same Autonomous System as
the BGP Speaker.
New document optimizes and simplifies the exchange of the information
about previously reachable routes.
Appendix D. Comparison with RFC 1105
All of the changes listed in Appendices A, B and C, plus the follow-
ing.
Minor changes to the RFC1105 Finite State Machine were necessary to
accommodate the TCP user interface provided by 4.3 BSD.
The notion of Up/Down/Horizontal relations present in RFC1105 has
been removed from the protocol.
The changes in the message format from RFC1105 are as follows:
1. The Hold Time field has been removed from the BGP header and
added to the OPEN message.
2. The version field has been removed from the BGP header and
added to the OPEN message.
3. The Link Type field has been removed from the OPEN message.
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4. The OPEN CONFIRM message has been eliminated and replaced with
implicit confirmation provided by the KEEPALIVE message.
5. The format of the UPDATE message has been changed signifi-
cantly. New fields were added to the UPDATE message to support
multiple path attributes.
6. The Marker field has been expanded and its role broadened to
support authentication.
Note that quite often BGP, as specified in RFC 1105, is referred
to as BGP-1, BGP, as specified in RFC 1163, is referred to as
BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and
BGP, as specified in this document is referred to as BGP-4.
Appendix E. TCP options that may be used with BGP
If a local system TCP user interface supports TCP PUSH function, then
each BGP message SHOULD be transmitted with PUSH flag set. Setting
PUSH flag forces BGP messages to be transmitted promptly to the
receiver.
If a local system TCP user interface supports setting of the DSCP
field [RFC2474] for TCP connections, then the TCP connection used by
BGP SHOULD be opened with bits 0-2 of the DSCP field set to 110
(binary).
An implementation MUST support TCP MD5 option [RFC2385].
Appendix F. Implementation Recommendations
This section presents some implementation recommendations.
Appendix F.1 Multiple Networks Per Message
The BGP protocol allows for multiple address prefixes with the same
path attributes to be specified in one message. Making use of this
capability is highly recommended. With one address prefix per message
there is a substantial increase in overhead in the receiver. Not only
does the system overhead increase due to the reception of multiple
messages, but the overhead of scanning the routing table for updates
to BGP peers and other routing protocols (and sending the associated
messages) is incurred multiple times as well.
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One method of building messages containing many address prefixes per
a path attribute set from a routing table that is not organized on a
per path attribute set basis is to build many messages as the routing
table is scanned. As each address prefix is processed, a message for
the associated set of path attributes is allocated, if it does not
exist, and the new address prefix is added to it. If such a message
exists, the new address prefix is just appended to it. If the message
lacks the space to hold the new address prefix, it is transmitted, a
new message is allocated, and the new address prefix is inserted into
the new message. When the entire routing table has been scanned, all
allocated messages are sent and their resources released. Maximum
compression is achieved when all the destinations covered by the
address prefixes share a common set of path attributes making it pos-
sible to send many address prefixes in one 4096-byte message.
When peering with a BGP implementation that does not compress multi-
ple address prefixes into one message, it may be necessary to take
steps to reduce the overhead from the flood of data received when a
peer is acquired or a significant network topology change occurs. One
method of doing this is to limit the rate of updates. This will
eliminate the redundant scanning of the routing table to provide
flash updates for BGP peers and other routing protocols. A disadvan-
tage of this approach is that it increases the propagation latency of
routing information. By choosing a minimum flash update interval
that is not much greater than the time it takes to process the multi-
ple messages this latency should be minimized. A better method would
be to read all received messages before sending updates.
Appendix F.2 Reducing route flapping
To avoid excessive route flapping a BGP speaker which needs to with-
draw a destination and send an update about a more specific or less
specific route should combine them into the same UPDATE message.
Appendix F.3 Path attribute ordering
Implementations which combine update messages as described above in
6.1 may prefer to see all path attributes presented in a known order.
This permits them to quickly identify sets of attributes from differ-
ent update messages which are semantically identical. To facilitate
this, it is a useful optimization to order the path attributes
according to type code. This optimization is entirely optional.
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Appendix F.4 AS_SET sorting
Another useful optimization that can be done to simplify this situa-
tion is to sort the AS numbers found in an AS_SET. This optimization
is entirely optional.
Appendix F.5 Control over version negotiation
Since BGP-4 is capable of carrying aggregated routes which can not be
properly represented in BGP-3, an implementation which supports BGP-4
and another BGP version should provide the capability to only speak
BGP-4 on a per-peer basis.
Appendix F.6 Complex AS_PATH aggregation
An implementation which chooses to provide a path aggregation algo-
rithm which retains significant amounts of path information may wish
to use the following procedure:
For the purpose of aggregating AS_PATH attributes of two routes,
we model each AS as a tuple <type, value>, where "type" identifies
a type of the path segment the AS belongs to (e.g. AS_SEQUENCE,
AS_SET), and "value" is the AS number. Two ASs are said to be the
same if their corresponding <type, value> tuples are the same.
The algorithm to aggregate two AS_PATH attributes works as fol-
lows:
a) Identify the same ASs (as defined above) within each AS_PATH
attribute that are in the same relative order within both
AS_PATH attributes. Two ASs, X and Y, are said to be in the
same order if either:
- X precedes Y in both AS_PATH attributes, or - Y precedes X
in both AS_PATH attributes.
b) The aggregated AS_PATH attribute consists of ASs identified
in (a) in exactly the same order as they appear in the AS_PATH
attributes to be aggregated. If two consecutive ASs identified
in (a) do not immediately follow each other in both of the
AS_PATH attributes to be aggregated, then the intervening ASs
(ASs that are between the two consecutive ASs that are the
same) in both attributes are combined into an AS_SET path seg-
ment that consists of the intervening ASs from both AS_PATH
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attributes; this segment is then placed in between the two con-
secutive ASs identified in (a) of the aggregated attribute. If
two consecutive ASs identified in (a) immediately follow each
other in one attribute, but do not follow in another, then the
intervening ASs of the latter are combined into an AS_SET path
segment; this segment is then placed in between the two consec-
utive ASs identified in (a) of the aggregated attribute.
c) For each pair of adjacent tuples in the aggregated AS_PATH,
if both tuples have the same type, merge them together, as long
as doing so will not cause a segment with length greater than
255 to be generated.
If as a result of the above procedure a given AS number appears
more than once within the aggregated AS_PATH attribute, all, but
the last instance (rightmost occurrence) of that AS number should
be removed from the aggregated AS_PATH attribute.
Security Considerations
A BGP implementation MUST support the authentication mechanism speci-
fied in RFC 2385 [RFC2385]. The authentication provided by this mech-
anism could be done on a per peer basis.
BGP makes use of TCP for reliable transport of its traffic between
peer routers. To provide connection-oriented integrity and data ori-
gin authentication, on a point-to-point basis, BGP specifies use of
the mechanism defined in RFC 2385. These services are intended to
detect and reject active wiretapping attacks against the inter-router
TCP connections. Absent use of mechanisms that effect these security
services, attackers can disrupt these TCP connections and/or masquer-
ade as a legitimate peer router. Because the mechanism defined in the
RFC does not provide peer-entity authentication, these connections
may be subject to some forms of replay attacks that will not be
detected at the TCP layer. Such attacks might result in delivery
(from TCP) of "broken" or "spoofed" BGP messages.
The mechanism defined in RFC 2385 augments the normal TCP checksum
with a 16-byte message authentication code (MAC) that is computed
over the same data as the TCP checksum. This MAC is based on a one-
way hash function (MD5) and use of a secret key. The key is shared
between peer routers and is used to generate MAC values that are not
readily computed by an attacker who does not have access to the key.
A compliant implementation must support this mechanism, and must
allow a network administrator to activate it on a per-peer basis.
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RFC 2385 does not specify a means of managing (e.g., generating, dis-
tributing, and replacing) the keys used to compute the MAC. RFC 3562
[RFC3562] (an informational document) provides some guidance in this
area, and provides rationale to support this guidance. It notes that
a distinct key should be used for communication with each protected
peer. If the same key is used for multiple peers, the offered secu-
rity services may be degraded, e.g., due to increased risk of compro-
mise at one router adversely affecting other routers.
The keys used for MAC computation should be changed periodically, to
minimize the impact of a key compromise or successful cryptanalytic
attack. RFC 3562 suggests a crypto period (the interval during which
a key is employed) of at most 90 days. More frequent key changes
reduce the likelihood that replay attacks (as described above) will
be feasible. However, absent a standard mechanism for effecting such
changes in a coordinated fashion between peers, one cannot assume
that BGP-4 implementations complying with this RFC will support fre-
quent key changes.
Obviously, each key also should be chosen so as to be hard for an
attacker to guess. The techniques specified in RFC 1750 for random
number generation provide a guide for generation of values that could
be used as keys. RFC 2385 calls for implementations to support keys
"composed of a string of printable ASCII of 80 bytes or less." RFC
3562 suggests keys used in this context be 12 to 24 bytes of random
(pseudo-random) bits. This is fairly consistent with suggestions for
analogous MAC algorithms, which typically employ keys in the range of
16-20 bytes. RFC 3562 also observes that, to provide enough random
bits at the low end of this range, a typical ACSII text string would
have to be close to the upper bound for key length specified in RFC
2385.
BGP vulnerabilities analysis is discussed in [BGP_VULN].
IANA Considerations
All the BGP messages contain an 8-bit message type, for which IANA is
to create and maintain a registry entitled "BGP Message Types". This
document defines the following message types:
Name Value Definition
---- ----- ----------
OPEN 1 See Section 4.2
UPDATE 2 See Section 4.3
KEEPALIVE 3 See Section 4.4
NOTIFICATION 4 See Section 4.5
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Future assignment are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [kompella-zinin]. Assignments consist of a name and the
value.
The BGP UPDATE messages may carry one or more Path Attributes, where
each Attribute contains an 8-bit Attribute Type Code. IANA is already
maintaining such a registry, entitled "BGP Path Attributes". [note to
IANA, the registry already exists at http://www.iana.org/assign-
ments/bgp-parameters, but should be renamed per this document. XXX to
be removed upon RFC publication.] This document defines the following
Path Attributes Type Codes:
Name Value Definition
---- ----- ----------
ORIGIN 1 See Section 5.1.1
AS_PATH 2 See Section 5.1.2
NEXT_HOP 3 See Section 5.1.3
MULTI_EXIT_DISC 4 See Section 5.1.4
LOCAL_PREF 5 See Section 5.1.5
ATOMIC_AGGREGATE 6 See Section 5.1.6
AGGREGATOR 7 See Section 5.1.7
Future assignment are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [kompella-zinin]. Assignments consist of a name and the
value.
The BGP NOTIFICATION message carries an 8-bit Error Code, for which
IANA is to create and maintain a registry entitled "BGP Error Codes".
This document defines the following Error Codes:
Name Value Definition
------------ ----- ----------
Message Header Error 1 Section 6.1
OPEN Message Error 2 Section 6.2
UPDATE Message Error 3 Section 6.3
Hold Timer Expired 4 Section 6.5
Finite State Machine Error 5 Section 6.6
Cease 6 Section 6.7
Future assignment are to be made using either the Standards Action process
defined in [RFC2434], or the Early IANA Allocation process defined
in [kompella-zinin]. Assignments consist of a name and the value.
The BGP NOTIFICATION message carries an 8-bit Error Subcode, where
each Subcode has to be defined within the context of a particular
Error Code, and thus has to be unique only within that context.
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IANA is to create and maintain a set of registries, "Error Subcodes",
with a separate registry for each BGP Error Code. Future assignment are
to be made using either the Standards Action process defined in [RFC2434],
or the Early IANA Allocation process defined in [kompella-zinin].
Assignments consist of a name and the value.
This document defines the following Message Header Error subcodes:
Name Value Definition
-------------------- ----- ----------
Connection Not Synchronized 1 See Section 6.1
Bad Message Length 2 See Section 6.1
Bad Message Type 3 See Section 6.1
This document defines the following OPEN Message Error subcodes:
Name Value Definition
-------------------- ----- ----------
Unsupported Version Number 1 See Section 6.2
Bad Peer AS 2 See Section 6.2
Bad BGP Identifier 3 See Section 6.2
Unsupported Optional Parameter 4 See Section 6.2
[Deprecated] 5 See Appendix A
Unacceptable Hold Time 6 See Section 6.2
This document defines the following UPDATE Message Error subcodes:
Name Value Definition
-------------------- --- ----------
Malformed Attribute List 1 See Section 6.3
Unrecognized Well-known Attribute 2 See Section 6.3
Missing Well-known Attribute 3 See Section 6.3
Attribute Flags Error 4 See Section 6.3
Attribute Length Error 5 See Section 6.3
Invalid ORIGIN Attribute 6 See Section 6.3
[Deprecated] 7 See Appendix A
Invalid NEXT_HOP Attribute 8 See Section 6.3
Optional Attribute Error 9 See Section 6.3
Invalid Network Field 10 See Section 6.3
Malformed AS_PATH 11 See Section 6.3
IPR Disclosure Acknowledgement
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
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RFC 3668.
Copyright Notice
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to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Additional copyright notices are not permitted in IETF Documents
except in the case where such document is the product of a joint
development effort between the IETF and another standards development
organization or the document is a republication of the work of
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an individual basis by the IAB.
Disclaimer
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFOR-
MATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Normative References
[RFC791] Postel, J., "Internet Protocol - DARPA Internet Program Pro-
tocol Specification", RFC791, September 1981.
[RFC793] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC2385, August 1998.
[RFC2434] Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC2434, October 1998
[RFC2474] Nichols, K., et al.,"Definition of the Differentiated
Expiration Date April 2005 [Page 98]
RFC DRAFT October 2004
Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC2474,
December 1998
Non-normative References
[RFC904] Mills, D., "Exterior Gateway Protocol Formal Specification",
RFC904, April 1984.
[RFC1092] Rekhter, Y., "EGP and Policy Based Routing in the New
NSFNET Backbone", RFC1092, February 1989.
[RFC1093] Braun, H-W., "The NSFNET Routing Architecture", RFC1093,
February 1989.
[RFC1772] Rekhter, Y., and P. Gross, "Application of the Border Gate-
way Protocol in the Internet", RFC1772, March 1995.
[RFC1518] Rekhter, Y., Li, T., "An Architecture for IP Address Allo-
cation with CIDR", RFC 1518, September 1993.
[RFC1519] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless
Inter-Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, September 1993.
[RFC1930] Hawkinson, J., Bates, T.,"Guidelines for creation, selec-
tion, and registration of an Autonomous System (AS)", RFC1930, March
1996.
[RFC1997] R. Chandra, P. Traina, T. Li, "BGP Communities Attribute",
RFC 1997, August 1996.
[RFC2439] C. Villamizar, R. Chandra, R. Govindan, "BGP Route Flap
Damping", RFC2439, November 1998.
[RFC2796] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection -
An Alternative to Full Mesh IBGP", RFC2796, April 2000.
[RFC3392] R. Chandra, J. Scudder, "Capabilities Advertisement with
BGP-4", RFC2842.
[RFC2858] T. Bates, R. Chandra, D. Katz, Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC2858.
[RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC2918,
September 2000.
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RFC DRAFT October 2004
[RFC3065] Traina, P, McPherson, D., Scudder, J., "Autonomous System
Confederations for BGP", RFC3065, February 2001.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC3562, July 2003.
3563 Cooperative Agreement Between the ISOC/IETF and ISO/IEC Joint
[IS10747] "Information Processing Systems - Telecommunications and
Information Exchange between Systems - Protocol for Exchange of
Inter-domain Routeing Information among Intermediate Systems to Sup-
port Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993
[BGP_VULN] Murphy, S., "BGP Security Vulnerabilities Analysis",
draft-ietf-idr-bgp-vuln-00.txt, work in progress
[kompella-zinin] Kompella, K., Zinin, A., "Early IANA Allocation of
Standards Track Codepoints", Work in progress
Editors' Addresses
Yakov Rekhter
Juniper Networks
email: yakov@juniper.net
Tony Li
email: tony.li@tony.li
Susan Hares
NextHop Technologies, Inc.
email: skh@nexthop.com
Expiration Date April 2005 [Page 100]
