Internet DRAFT - draft-ietf-idr-ix-bgp-route-server
draft-ietf-idr-ix-bgp-route-server
IDR Working Group E. Jasinska
Internet-Draft BigWave IT
Intended status: Standards Track N. Hilliard
Expires: October 31, 2016 INEX
R. Raszuk
Bloomberg LP
N. Bakker
Akamai Technologies B.V.
April 29, 2016
Internet Exchange BGP Route Server
draft-ietf-idr-ix-bgp-route-server-10
Abstract
This document outlines a specification for multilateral
interconnections at Internet exchange points (IXPs). Multilateral
interconnection is a method of exchanging routing information between
three or more external BGP speakers using a single intermediate
broker system, referred to as a route server. Route servers are
typically used on shared access media networks, such as Internet
exchange points (IXPs), to facilitate simplified interconnection
between multiple Internet routers.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on October 31, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction to Multilateral Interconnection . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Technical Considerations for Route Server Implementations . . 3
2.1. Client UPDATE Messages . . . . . . . . . . . . . . . . . 3
2.2. Attribute Transparency . . . . . . . . . . . . . . . . . 4
2.2.1. NEXT_HOP Attribute . . . . . . . . . . . . . . . . . 4
2.2.2. AS_PATH Attribute . . . . . . . . . . . . . . . . . . 4
2.2.3. MULTI_EXIT_DISC Attribute . . . . . . . . . . . . . . 5
2.2.4. Communities Attributes . . . . . . . . . . . . . . . 5
2.3. Per-Client Policy Control in Multilateral Interconnection 5
2.3.1. Path Hiding on a Route Server . . . . . . . . . . . . 5
2.3.2. Mitigation of Path Hiding . . . . . . . . . . . . . . 7
2.3.2.1. Multiple Route Server RIBs . . . . . . . . . . . 7
2.3.2.2. Advertising Multiple Paths . . . . . . . . . . . 7
2.3.3. Implementation Suggestions . . . . . . . . . . . . . 8
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction to Multilateral Interconnection
Internet exchange points (IXPs) provide IP data interconnection
facilities for their participants, typically using shared Layer-2
networking media such as Ethernet. The Border Gateway Protocol (BGP)
[RFC4271], an inter-Autonomous System routing protocol, is commonly
used to facilitate exchange of network reachability information over
such media.
While bilateral external BGP sessions between exchange participants
were previously the most common means of exchanging reachability
information, the overhead associated with dense interconnection can
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cause substantial operational scaling problems for partipants of
larger Internet exchange points.
Multilateral interconnection is a method of interconnecting BGP
speaking routers using a third party brokering system, commonly
referred to as a route server and typically managed by the IXP
operator. Each of the multilateral interconnection participants
(usually referred to as route server clients) announces network
reachability information to the route server using external BGP, and
the route server in turn forwards this information to each other
route server client connected to it, according to its configuration.
Although a route server uses BGP to exchange reachability information
with each of its clients, it does not forward traffic itself and is
therefore not a router.
A route server can be viewed as similar in function to an [RFC4456]
route reflector, except that it operates using EBGP instead of iBGP.
Certain adaptions to [RFC4271] are required to enable an EBGP router
to operate as a route server; these are outlined in Section 2 of this
document. Route server functionality is not mandatory in BGP
implementations.``
The term "route server" is often in a different context used to
describe a BGP node whose purpose is to accept BGP feeds from
multiple clients for the purpose of operational analysis and
troubleshooting. A system of this form may alternatively be known as
a "route collector" or a "route-views server". This document uses
the term "route server" exclusively to describe multilateral peering
brokerage systems.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
2. Technical Considerations for Route Server Implementations
2.1. Client UPDATE Messages
A route server MUST accept all UPDATE messages received from each of
its clients for inclusion in its Adj-RIB-In. These UPDATE messages
MAY be omitted from the route server's Loc-RIB or Loc-RIBs, due to
filters configured for the purposes of implementing routing policy.
The route server SHOULD perform one or more BGP Decision Processes to
select routes for subsequent advertisement to its clients, taking
into account possible configuration to provide multiple NLRI paths to
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a particular client as described in Section 2.3.2.2 or multiple Loc-
RIBs as described in Section 2.3.2.1. The route server SHOULD
forward UPDATE messages from its Loc-RIB or Loc-RIBs to its clients
as determined by local policy.
2.2. Attribute Transparency
As a route server primarily performs a brokering service,
modification of attributes could cause route server clients to alter
their BGP Decision Process for received prefix reachability
information, thereby changing the intended routing policies of
exchange participants. Therefore, contrary to what is specified in
section 5. of [RFC4271], route servers SHOULD NOT by default (unless
explicitly configured) update well-known BGP attributes received from
route server clients before redistributing them to their other route
server clients. Optional recognized and unrecognized BGP attributes,
whether transitive or non-transitive, SHOULD NOT be updated by the
route server (unless enforced by local IXP operator configuration)
and SHOULD be passed on to other route server clients.
2.2.1. NEXT_HOP Attribute
The NEXT_HOP is a well-known mandatory BGP attribute which defines
the IP address of the router used as the next hop to the destinations
listed in the Network Layer Reachability Information field of the
UPDATE message. As the route server does not participate in the
actual routing of traffic, the NEXT_HOP attribute MUST be passed
unmodified to the route server clients, similar to the "third party"
next hop feature described in section 5.1.3. of [RFC4271].
2.2.2. AS_PATH Attribute
AS_PATH is a well-known mandatory attribute which identifies the
autonomous systems through which routing information carried in the
UPDATE message has passed.
As a route server does not participate in the process of forwarding
data between client routers, and because modification of the AS_PATH
attribute could affect route server client BGP Decision Process, the
route server SHOULD NOT prepend its own AS number to the AS_PATH
segment nor modify the AS_PATH segment in any other way. This
differs from the behaviour specified in section 5.1.2 of [RFC4271],
which requires that the BGP speaker prepends its own AS number as the
last element of the AS_PATH segment.
In contrast to what is recommended in section 6.3 of [RFC4271], route
server clients need to be able to accept UPDATE messages where the
leftmost AS in the AS_PATH attribute is not equal to the AS number of
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the route server that sent the UPDATE message. If the route server
client BGP system has implemented a check for this, the BGP
implementation MUST allow this check to be disabled and SHOULD allow
the check to be disabled on a per-peer basis.
2.2.3. MULTI_EXIT_DISC Attribute
MULTI_EXIT_DISC is an optional non-transitive attribute intended to
be used on external (inter-AS) links to discriminate among multiple
exit or entry points to the same neighboring AS. Contrary to section
5.1.4 of [RFC4271], if applied to an NLRI UPDATE sent to a route
server, this attribute SHOULD be propagated to other route server
clients and the route server SHOULD NOT modify its value.
2.2.4. Communities Attributes
The BGP COMMUNITIES ([RFC1997]) and Extended Communities ([RFC4360])
attributes are attributes intended for labeling information carried
in BGP UPDATE messages. Transitive as well as non-transitive
Communities attributes applied to an NLRI UPDATE sent to a route
server SHOULD NOT be modified, processed or removed, except as
defined by local policy. If a Communities Attribute is intended for
processing by the route server itself, as determined by local policy,
it MAY be modified or removed.
2.3. Per-Client Policy Control in Multilateral Interconnection
While IXP participants often use route servers with the intention of
interconnecting with as many other route server participants as
possible, there are circumstances where control of path distribution
on a per-client basis is important to ensure that desired
interconnection policies are met.
The control of path distribution on a per-client basis can lead to a
path being hidden from the route server client. We refer to this as
"path hiding".
Neither Section 2.3 nor its subsections form part of the normative
specification of this document, but are included for information
purposes only.
2.3.1. Path Hiding on a Route Server
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___ ___
/ \ / \
..| AS1 |..| AS2 |..
: \___/ \___/ :
: \ / | :
: \ / | :
: IXP \/ | :
: /\ | :
: / \ | :
: ___/____\_|_ :
: / \ / \ :
..| AS3 |..| AS4 |..
\___/ \___/
Figure 1: Per-Client Policy Controlled Interconnection at an IXP
Using the example in Figure 1, AS1 does not directly exchange prefix
information with either AS2 or AS3 at the IXP, but only interconnects
with AS4.
In the traditional bilateral interconnection model, per-client policy
control to a third party exchange participant is accomplished either
by not engaging in a bilateral interconnection with that participant
or else by implementing outbound filtering on the BGP session towards
that participant. However, in a multilateral interconnection
environment, only the route server can perform outbound filtering in
the direction of the route server client; route server clients depend
on the route server to perform their outbound filtering for them.
Assuming the [RFC4271] BGP Decision Process is used, when the same
prefix is advertised to a route server from multiple route server
clients, the route server will select a single path for propagation
to all connected clients. If, however, the route server has been
configured to filter the calculated best path from reaching a
particular route server client, then that client will not receive a
path for that prefix, although alternate paths received by the route
server might have been policy compliant for that client. This
phenomenon is referred to as "path hiding".
For example, in Figure 1, if the same prefix were sent to the route
server via AS2 and AS4, and the route via AS2 was preferred according
to the BGP Decision Process on the route server, but AS2's policy
prevented the route server from sending the path to AS1, then AS1
would never receive a path to this prefix, even though the route
server had previously received a valid alternative path via AS4.
This happens because the BGP Decision Process is performed only once
on the route server for all clients.
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Path hiding will only occur on route servers which employ per-client
policy control; if an IXP operator deploys a route server without
implementing a per-client routing policy control system, then path
hiding does not occur as all paths are considered equally valid from
the point of view of the route server.
2.3.2. Mitigation of Path Hiding
There are several approaches which can be taken to mitigate against
path hiding.
2.3.2.1. Multiple Route Server RIBs
The most portable method to allow for per-client policy control
without the occurrence of path hiding, is by using a route server BGP
implementation which performs the per-client best path calculation
for each set of paths to a prefix, which results after the route
server's client policies have been taken into consideration. This
can be implemented by using per-client Loc-RIBs, with path filtering
implemented between the Adj-RIB-In and the per-client Loc-RIB.
Implementations can optimize this by maintaining paths not subject to
filtering policies in a global Loc-RIB, with per-client Loc-RIBs
stored as deltas.
This implementation is highly portable, as it makes no assumptions
about the feature capabilities of the route server clients.
2.3.2.2. Advertising Multiple Paths
The path distribution model described above assumes standard BGP
session encoding where the route server sends a single path to its
client for any given prefix. This path is selected using the BGP
path selection decision process described in [RFC4271]. If, however,
it were possible for the route server to send more than a single path
to a route server client, then route server clients would no longer
depend on receiving a single path to a particular prefix;
consequently, the path hiding problem described in Section 2.3.1
would disappear.
We present two methods which describe how such increased path
diversity could be implemented.
2.3.2.2.1. Diverse BGP Path Approach
The Diverse BGP Path proposal as defined in [RFC6774] is a simple way
to distribute multiple prefix paths from a route server to a route
server client by using a separate BGP session from the route server
to a client for each different path.
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The number of paths which may be distributed to a client is
constrained by the number of BGP sessions which the server and the
client are willing to establish with each other. The distributed
paths may be established from the global BGP Loc-RIB on the route
server in addition to any per-client Loc-RIB. As there may be more
potential paths to a given prefix than configured BGP sessions, this
method is not guaranteed to eliminate the path hiding problem in all
situations. Furthermore, this method may significantly increase the
number of BGP sessions handled by the route server, which may
negatively impact its performance.
2.3.2.2.2. BGP ADD-PATH Approach
The [I-D.ietf-idr-add-paths] Internet draft proposes a different
approach to multiple path propagation, by allowing a BGP speaker to
forward multiple paths for the same prefix on a single BGP session.
As [RFC4271] specifies that a BGP listener must implement an implicit
withdraw when it receives an UPDATE message for a prefix which
already exists in its Adj-RIB-In, this approach requires explicit
support for the feature both on the route server and on its clients.
If the ADD-PATH capability is negotiated bidirectionally between the
route server and a route server client, and the route server client
propagates multiple paths for the same prefix to the route server,
then this could potentially cause the propagation of inactive,
invalid or suboptimal paths to the route server, thereby causing loss
of reachability to other route server clients. For this reason, ADD-
PATH implementations on a route server should enforce send-only mode
with the route server clients, which would result in negotiating
receive-only mode from the client to the route server.
2.3.3. Implementation Suggestions
Route server implementation authors may wish to consider one of the
methods described in Section 2.3.2 to allow per-client route server
policy control without "path hiding".
3. Security Considerations
The path hiding problem outlined in section Section 2.3.1 can be used
in certain circumstances to proactively block third party path
announcements from other route server clients. Route server
operators should be aware that security issues may arise unless steps
are taken to mitigate against path hiding.
The AS_PATH check described in Section 2.2.2 is normally enabled in
order to check for malformed AS paths. If this check is disabled,
the route server client loses the ability to check incoming UPDATE
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messages for certain categories of problems. This could potentially
cause corrupted BGP UPDATE messages to be propagated where they might
not be propagated if the check were enabled. Regardless of any
problems relating to malformed UPDATE messages, this check is also
used to detect BGP loops, so removing the check could potentially
cause routing loops to be formed. Consequently, this check SHOULD
NOT be disabled by IXP participants unless it is needed to establish
BGP sessions with a route server, and if possible should only be
disabled for peers which are route servers.
Route server operators should carefully consider the security
practices discussed in [RFC7454], "BGP Operations and Security".
4. IANA Considerations
The new set of mechanisms for route servers does not require any new
allocations from IANA.
5. Acknowledgments
The authors would like to thank Ryan Bickhart, Steven Bakker, Martin
Pels, Chris Hall, Aleksi Suhonen, Bruno Decraene, Pierre Francois and
Eduardo Ascenco Reis for their valuable input.
In addition, the authors would like to acknowledge the developers of
BIRD, OpenBGPD, Quagga and IOS whose BGP implementations include
route server capabilities which are compliant with this document.
Route server functionality was described in 1995 in [RFC1863] and
modern route server implementations are based on concepts developed
in the 1990s by the Routing Arbiter Project and the Route Server Next
Generation Project, managed by ISI and Merit. Although the original
RSNG code is no longer in use at any IXPs, the IXP community owes a
debt of gratitude to the many people who were involved in route
server development in the 1990s. Note that [RFC1863] was made
historical by [RFC4223].
6. References
6.1. Normative References
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<http://www.rfc-editor.org/info/rfc1997>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <http://www.rfc-editor.org/info/rfc4360>.
6.2. Informative References
[I-D.ietf-idr-add-paths]
Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", draft-ietf-idr-
add-paths-13 (work in progress), December 2015.
[RFC1863] Haskin, D., "A BGP/IDRP Route Server alternative to a full
mesh routing", RFC 1863, DOI 10.17487/RFC1863, October
1995, <http://www.rfc-editor.org/info/rfc1863>.
[RFC4223] Savola, P., "Reclassification of RFC 1863 to Historic",
RFC 4223, DOI 10.17487/RFC4223, October 2005,
<http://www.rfc-editor.org/info/rfc4223>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<http://www.rfc-editor.org/info/rfc4456>.
[RFC6774] Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
and K. Kumaki, "Distribution of Diverse BGP Paths",
RFC 6774, DOI 10.17487/RFC6774, November 2012,
<http://www.rfc-editor.org/info/rfc6774>.
[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
February 2015, <http://www.rfc-editor.org/info/rfc7454>.
Authors' Addresses
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Elisa Jasinska
BigWave IT
ul. Skawinska 27/7
Krakow, MP 31-066
Poland
Email: elisa@bigwaveit.org
Nick Hilliard
INEX
4027 Kingswood Road
Dublin 24
IE
Email: nick@inex.ie
Robert Raszuk
Bloomberg LP
731 Lexington Ave
New York City, NY 10022
USA
Email: robert@raszuk.net
Niels Bakker
Akamai Technologies B.V.
Kingsfordweg 151
Amsterdam 1043 GR
NL
Email: nbakker@akamai.com
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