Internet DRAFT - draft-ietf-idr-tunnel-encaps
draft-ietf-idr-tunnel-encaps
IDR Working Group K. Patel
Internet-Draft Arrcus, Inc
Obsoletes: 5512, 5566 (if approved) G. Van de Velde
Updates: 5640 (if approved) Nokia
Intended status: Standards Track S. Sangli
Expires: July 11, 2021 J. Scudder
Juniper Networks
January 7, 2021
The BGP Tunnel Encapsulation Attribute
draft-ietf-idr-tunnel-encaps-22
Abstract
This document defines a BGP Path Attribute known as the "Tunnel
Encapsulation Attribute", which can be used with BGP UPDATEs of
various SAFIs to provide information needed to create tunnels and
their corresponding encapsulation headers. It provides encodings for
a number of Tunnel Types along with procedures for choosing between
alternate tunnels and routing packets into tunnels.
This document obsoletes RFC 5512, which provided an earlier
definition of the Tunnel Encapsulation Attribute. RFC 5512 was never
deployed in production. Since RFC 5566 relies on RFC 5512, it is
likewise obsoleted. This document updates RFC 5640 by indicating
that the Load-Balancing Block sub-TLV may be included in any Tunnel
Encapsulation Attribute where load balancing is desired.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 11, 2021.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Brief Summary of RFC 5512 . . . . . . . . . . . . . . . . 4
1.2. Deficiencies in RFC 5512 . . . . . . . . . . . . . . . . 4
1.3. Use Case for The Tunnel Encapsulation Attribute . . . . . 5
1.4. Brief Summary of Changes from RFC 5512 . . . . . . . . . 6
1.5. Update to RFC 5640 . . . . . . . . . . . . . . . . . . . 7
1.6. Effects of Obsoleting RFC 5566 . . . . . . . . . . . . . 7
2. The Tunnel Encapsulation Attribute . . . . . . . . . . . . . 8
3. Tunnel Encapsulation Attribute Sub-TLVs . . . . . . . . . . . 9
3.1. The Tunnel Egress Endpoint Sub-TLV (type code 6) . . . . 9
3.1.1. Validating the Address Subfield . . . . . . . . . . . 11
3.2. Encapsulation Sub-TLVs for Particular Tunnel Types (type
code 1) . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.1. VXLAN (tunnel type 8) . . . . . . . . . . . . . . . . 12
3.2.2. NVGRE (tunnel type 9) . . . . . . . . . . . . . . . . 14
3.2.3. L2TPv3 (tunnel type 1) . . . . . . . . . . . . . . . 16
3.2.4. GRE (tunnel type 2) . . . . . . . . . . . . . . . . . 16
3.2.5. MPLS-in-GRE (tunnel type 11) . . . . . . . . . . . . 17
3.3. Outer Encapsulation Sub-TLVs . . . . . . . . . . . . . . 17
3.3.1. DS Field (type code 7) . . . . . . . . . . . . . . . 18
3.3.2. UDP Destination Port (type code 8) . . . . . . . . . 18
3.4. Sub-TLVs for Aiding Tunnel Selection . . . . . . . . . . 19
3.4.1. Protocol Type Sub-TLV (type code 2) . . . . . . . . . 19
3.4.2. Color Sub-TLV (type code 4) . . . . . . . . . . . . . 20
3.5. Embedded Label Handling Sub-TLV (type code 9) . . . . . . 20
3.6. MPLS Label Stack Sub-TLV (type code 10) . . . . . . . . . 21
3.7. Prefix-SID Sub-TLV (type code 11) . . . . . . . . . . . . 23
4. Extended Communities Related to the Tunnel Encapsulation
Attribute . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.1. Encapsulation Extended Community . . . . . . . . . . . . 24
4.2. Router's MAC Extended Community . . . . . . . . . . . . . 25
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4.3. Color Extended Community . . . . . . . . . . . . . . . . 26
5. Special Considerations for IP-in-IP Tunnels . . . . . . . . . 26
6. Semantics and Usage of the Tunnel Encapsulation attribute . . 26
7. Routing Considerations . . . . . . . . . . . . . . . . . . . 29
7.1. Impact on the BGP Decision Process . . . . . . . . . . . 29
7.2. Looping, Mutual Recursion, Etc. . . . . . . . . . . . . . 29
8. Recursive Next Hop Resolution . . . . . . . . . . . . . . . . 30
9. Use of Virtual Network Identifiers and Embedded Labels when
Imposing a Tunnel Encapsulation . . . . . . . . . . . . . . . 31
9.1. Tunnel Types without a Virtual Network Identifier Field . 31
9.2. Tunnel Types with a Virtual Network Identifier Field . . 31
9.2.1. Unlabeled Address Families . . . . . . . . . . . . . 32
9.2.2. Labeled Address Families . . . . . . . . . . . . . . 32
10. Applicability Restrictions . . . . . . . . . . . . . . . . . 33
11. Scoping . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
12. Operational Considerations . . . . . . . . . . . . . . . . . 35
13. Validation and Error Handling . . . . . . . . . . . . . . . . 35
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
14.1. Obsoleting RFC 5512 . . . . . . . . . . . . . . . . . . 36
14.2. Obsoleting Code Points Assigned by RFCs 5566 . . . . . . 37
14.3. BGP Tunnel Encapsulation Parameters Grouping . . . . . . 37
14.4. BGP Tunnel Encapsulation Attribute Tunnel Types . . . . 37
14.5. Subsequent Address Family Identifiers . . . . . . . . . 37
14.6. BGP Tunnel Encapsulation Attribute Sub-TLVs . . . . . . 37
14.7. Flags Field of VXLAN Encapsulation sub-TLV . . . . . . . 38
14.8. Flags Field of NVGRE Encapsulation sub-TLV . . . . . . . 39
14.9. Embedded Label Handling sub-TLV . . . . . . . . . . . . 39
14.10. Color Extended Community Flags . . . . . . . . . . . . . 39
15. Security Considerations . . . . . . . . . . . . . . . . . . . 40
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 41
17. Contributor Addresses . . . . . . . . . . . . . . . . . . . . 41
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
18.1. Normative References . . . . . . . . . . . . . . . . . . 42
18.2. Informative References . . . . . . . . . . . . . . . . . 44
Appendix A. Impact on RFC 8365 . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46
1. Introduction
This document obsoletes RFC 5512. The deficiencies of RFC 5512, and
a summary of the changes made, are discussed in Sections 1.1-1.3.
The material from RFC 5512 that is retained has been incorporated
into this document. Since [RFC5566] relies on RFC 5512, it is
likewise obsoleted.
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 BCP
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14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.1. Brief Summary of RFC 5512
[RFC5512] defines a BGP Path Attribute known as the Tunnel
Encapsulation attribute. This attribute consists of one or more
TLVs. Each TLV identifies a particular type of tunnel. Each TLV
also contains one or more sub-TLVs. Some of the sub-TLVs, for
example, the "Encapsulation sub-TLV", contain information that may be
used to form the encapsulation header for the specified Tunnel Type.
Other sub- TLVs, for example, the "color sub-TLV" and the "protocol
sub-TLV", contain information that aids in determining whether
particular packets should be sent through the tunnel that the TLV
identifies.
[RFC5512] only allows the Tunnel Encapsulation attribute to be
attached to BGP UPDATE messages of the Encapsulation Address Family.
These UPDATE messages have an AFI (Address Family Identifier) of 1 or
2, and a SAFI of 7. In an UPDATE of the Encapsulation SAFI, the NLRI
(Network Layer Reachability Information) is an address of the BGP
speaker originating the UPDATE. Consider the following scenario:
o BGP speaker R1 has received and selected UPDATE U for local use;
o UPDATE U's SAFI is the Encapsulation SAFI;
o UPDATE U has the address R2 as its NLRI;
o UPDATE U has a Tunnel Encapsulation attribute.
o R1 has a packet, P, to transmit to destination D;
o R1's best route to D is a BGP route that has R2 as its next hop;
In this scenario, when R1 transmits packet P, it should transmit it
to R2 through one of the tunnels specified in U's Tunnel
Encapsulation attribute. The IP address of the tunnel egress
endpoint of each such tunnel is R2. Packet P is known as the
tunnel's "payload".
1.2. Deficiencies in RFC 5512
While the ability to specify tunnel information in a BGP UPDATE is
useful, the procedures of [RFC5512] have certain limitations:
o The requirement to use the "Encapsulation SAFI" presents an
unfortunate operational cost, as each BGP session that may need to
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carry tunnel encapsulation information needs to be reconfigured to
support the Encapsulation SAFI. The Encapsulation SAFI has never
been used, and this requirement has served only to discourage the
use of the Tunnel Encapsulation attribute.
o There is no way to use the Tunnel Encapsulation attribute to
specify the tunnel egress endpoint address of a given tunnel;
[RFC5512] assumes that the tunnel egress endpoint of each tunnel
is specified as the NLRI of an UPDATE of the Encapsulation SAFI.
o If the respective best routes to two different address prefixes
have the same next hop, [RFC5512] does not provide a
straightforward method to associate each prefix with a different
tunnel.
o If a particular Tunnel Type requires an outer IP or UDP
encapsulation, there is no way to signal the values of any of the
fields of the outer encapsulation.
o In [RFC5512]'s specification of the sub-TLVs, each sub-TLV has
one-octet length field. In some cases, where a sub-TLV may
require more than 255 octets for its encoding, a two-octet length
field may be needed.
1.3. Use Case for The Tunnel Encapsulation Attribute
Consider the case of a router R1 forwarding an IP packet P. Let D be
P's IP destination address. R1 must look up D in its forwarding
table. Suppose that the "best match" route for D is route Q, where Q
is a BGP-distributed route whose "BGP next hop" is router R2. And
suppose further that the routers along the path from R1 to R2 have
entries for R2 in their forwarding tables, but do NOT have entries
for D in their forwarding tables. For example, the path from R1 to
R2 may be part of a "BGP-free core", where there are no BGP-
distributed routes at all in the core. Or, as in [RFC5565], D may be
an IPv4 address while the intermediate routers along the path from R1
to R2 may support only IPv6.
In cases such as this, in order for R1 to properly forward packet P,
it must encapsulate P and send P "through a tunnel" to R2. For
example, R1 may encapsulate P using GRE, L2TPv3, IP in IP, etc.,
where the destination IP address of the encapsulation header is the
address of R2.
In order for R1 to encapsulate P for transport to R2, R1 must know
what encapsulation protocol to use for transporting different sorts
of packets to R2. R1 must also know how to fill in the various
fields of the encapsulation header. With certain encapsulation
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types, this knowledge may be acquired by default or through manual
configuration. Other encapsulation protocols have fields such as
session id, key, or cookie that must be filled in. It would not be
desirable to require every BGP speaker to be manually configured with
the encapsulation information for every one of its BGP next hops.
This document specifies a way in which BGP itself can be used by a
given BGP speaker to tell other BGP speakers, "if you need to
encapsulate packets to be sent to me, here's the information you need
to properly form the encapsulation header". A BGP speaker signals
this information to other BGP speakers by using a new BGP attribute
type value, the BGP Tunnel Encapsulation Attribute. This attribute
specifies the encapsulation protocols that may be used as well as
whatever additional information (if any) is needed in order to
properly use those protocols. Other attributes, for example,
communities or extended communities, may also be included.
1.4. Brief Summary of Changes from RFC 5512
This document addresses the deficiencies identified in Section 1.2
by:
o Deprecating the Encapsulation SAFI.
o Defining a new "Tunnel Egress Endpoint sub-TLV" (Section 3.1) that
can be included in any of the TLVs contained in the Tunnel
Encapsulation attribute. This sub-TLV can be used to specify the
remote endpoint address of a particular tunnel.
o Allowing the Tunnel Encapsulation attribute to be carried by BGP
UPDATEs of additional AFI/SAFIs. Appropriate semantics are
provided for this way of using the attribute.
o Defining a number of new sub-TLVs that provide additional
information that is useful when forming the encapsulation header
used to send a packet through a particular tunnel.
o Defining the sub-TLV type field so that a sub-TLV whose type is in
the range from 0 to 127 inclusive has a one-octet length field,
but a sub-TLV whose type is in the range from 128 to 255 inclusive
has a two-octet length field.
One of the sub-TLVs defined in [RFC5512] is the "Encapsulation sub-
TLV". For a given tunnel, the Encapsulation sub-TLV specifies some
of the information needed to construct the encapsulation header used
when sending packets through that tunnel. This document defines
Encapsulation sub-TLVs for a number of tunnel types not discussed in
[RFC5512]: VXLAN (Virtual Extensible Local Area Network, [RFC7348]),
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NVGRE (Network Virtualization Using Generic Routing Encapsulation
[RFC7637]), and MPLS-in-GRE (MPLS in Generic Routing Encapsulation
[RFC4023]). MPLS-in-UDP [RFC7510] is also supported, but an
Encapsulation sub-TLV for it is not needed since there are no
additional parameters to be signaled.
Some of the encapsulations mentioned in the previous paragraph need
to be further encapsulated inside UDP and/or IP. [RFC5512] provides
no way to specify that certain information is to appear in these
outer IP and/or UDP encapsulations. This document provides a
framework for including such information in the TLVs of the Tunnel
Encapsulation attribute.
When the Tunnel Encapsulation attribute is attached to a BGP UPDATE
whose AFI/SAFI identifies one of the labeled address families, it is
not always obvious whether the label embedded in the NLRI is to
appear somewhere in the tunnel encapsulation header (and if so,
where), or whether it is to appear in the payload, or whether it can
be omitted altogether. This is especially true if the tunnel
encapsulation header itself contains a "virtual network identifier".
This document provides a mechanism that allows one to signal (by
using sub-TLVs of the Tunnel Encapsulation attribute) how one wants
to use the embedded label when the tunnel encapsulation has its own
virtual network identifier field.
[RFC5512] defines a Tunnel Encapsulation Extended Community that can
be used instead of the Tunnel Encapsulation attribute under certain
circumstances. This document describes (Section 4.1) how the Tunnel
Encapsulation Extended Community can be used in a backwards-
compatible fashion. It is possible to combine Tunnel Encapsulation
Extended Communities and Tunnel Encapsulation attributes in the same
BGP UPDATE in this manner.
1.5. Update to RFC 5640
This document updates [RFC5640] by indicating that the Load-Balancing
Block sub-TLV MAY be included in any Tunnel Encapsulation Attribute
where loadbalancing is desired.
1.6. Effects of Obsoleting RFC 5566
This specification obsoletes RFC 5566. This has the effect of, in
turn, obsoleting a number of code points defined in that document.
From the "BGP Tunnel Encapsulation Attribute Tunnel Types" registry,
"Transmit tunnel endpoint" (type code 3), "IPsec in Tunnel-mode"
(type code 4), "IP in IP tunnel with IPsec Transport Mode" (type code
5), and "MPLS-in-IP tunnel with IPsec Transport Mode" (type code 6)
are obsoleted. From the "BGP Tunnel Encapsulation Attribute Sub-
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TLVs" registry, "IPsec Tunnel Authenticator" (type code 3) is
obsoleted. See Section 14.2.
2. The Tunnel Encapsulation Attribute
The Tunnel Encapsulation attribute is an optional transitive BGP Path
attribute. IANA has assigned the value 23 as the type code of the
attribute. The attribute is composed of a set of Type-Length-Value
(TLV) encodings. Each TLV contains information corresponding to a
particular Tunnel Type. A Tunnel Encapsulation TLV, also known as
Tunnel TLV, is structured as shown in Figure 1:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel Type (2 Octets) | Length (2 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Value (Variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Tunnel Encapsulation TLV Value Field
o Tunnel Type (2 octets): identifies a type of tunnel. The field
contains values from the IANA Registry "BGP Tunnel Encapsulation
Attribute Tunnel Types". See Section 3.4.1 for discussion of
special treatment of tunnel types with names of the form "X-in-Y".
o Length (2 octets): the total number of octets of the Value field.
o Value (variable): comprised of multiple sub-TLVs.
Each sub-TLV consists of three fields: a 1-octet type, a 1-octet or
2-octet length field (depending on the type), and zero or more octets
of value. A sub-TLV is structured as shown in Figure 2:
+--------------------------------+
| Sub-TLV Type (1 Octet) |
+--------------------------------+
| Sub-TLV Length (1 or 2 Octets) |
+--------------------------------+
| Sub-TLV Value (Variable) |
+--------------------------------+
Figure 2: Encapsulation Sub-TLV Value Field
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o Sub-TLV Type (1 octet): each sub-TLV type defines a certain
property about the Tunnel TLV that contains this sub-TLV. The
field contains values from the IANA Registry "BGP Tunnel
Encapsulation Attribute Sub-TLVs".
o Sub-TLV Length (1 or 2 octets): the total number of octets of the
sub-TLV Value field. The Sub-TLV Length field contains 1 octet if
the Sub-TLV Type field contains a value in the range from 0-127.
The Sub-TLV Length field contains two octets if the Sub-TLV Type
field contains a value in the range from 128-255.
o Sub-TLV Value (variable): encodings of the Value field depend on
the sub-TLV type as enumerated above. The following sub-sections
define the encoding in detail.
3. Tunnel Encapsulation Attribute Sub-TLVs
This section specifies a number of sub-TLVs. These sub-TLVs can be
included in a TLV of the Tunnel Encapsulation attribute.
3.1. The Tunnel Egress Endpoint Sub-TLV (type code 6)
The Tunnel Egress Endpoint sub-TLV specifies the address of the
egress endpoint of the tunnel, that is, the address of the router
that will decapsulate the payload. Its Value field contains three
subfields:
1. a reserved subfield
2. a two-octet Address Family subfield
3. an Address subfield, whose length depends upon the Address
Family.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family (2 octets) | Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ (Variable) +
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Tunnel Egress Endpoint Sub-TLV Value Field
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The Reserved subfield SHOULD be originated as zero. It MUST be
disregarded on receipt, and it MUST be propagated unchanged.
The Address Family subfield contains a value from IANA's "Address
Family Numbers" registry. This document assumes that the Address
Family is either IPv4 or IPv6; use of other address families is
outside the scope of this document.
If the Address Family subfield contains the value for IPv4, the
Address subfield MUST contain an IPv4 address (a /32 IPv4 prefix).
If the Address Family subfield contains the value for IPv6, the
Address subfield MUST contain an IPv6 address (a /128 IPv6 prefix).
In a given BGP UPDATE, the address family (IPv4 or IPv6) of a Tunnel
Egress Endpoint sub-TLV is independent of the address family of the
UPDATE itself. For example, an UPDATE whose NLRI is an IPv4 address
may have a Tunnel Encapsulation attribute containing Tunnel Egress
Endpoint sub-TLVs that contain IPv6 addresses. Also, different
tunnels represented in the Tunnel Encapsulation attribute may have
tunnel egress endpoints of different address families.
There is one special case: the Tunnel Egress Endpoint sub-TLV MAY
have a Value field whose Address Family subfield contains 0. This
means that the tunnel's egress endpoint is the address of the next
hop. If the Address Family subfield contains 0, the Address subfield
is omitted. In this case, the Length field of Tunnel Egress Endpoint
sub-TLV MUST contain the value 6 (0x06).
When the Tunnel Encapsulation attribute is carried in an UPDATE
message of one of the AFI/SAFIs specified in this document (see the
second paragraph of Section 6), each TLV MUST have one, and one only,
Tunnel Egress Endpoint sub-TLV. If a TLV does not have a Tunnel
Egress Endpoint sub-TLV, that TLV should be treated as if it had a
malformed Tunnel Egress Endpoint sub-TLV (see below).
In the context of this specification, if the Address Family subfield
has any value other than IPv4, IPv6, or the special value 0, the
Tunnel Egress Endpoint sub-TLV is considered "unrecognized" (see
Section 13). If any of the following conditions hold, the Tunnel
Egress Endpoint sub-TLV is considered to be "malformed":
o The length of the sub-TLV's Value field is other than 6 added to
the defined length for the address family given in its Address
Family subfield. Therefore, for address family behaviors defined
in this document, the permitted values are:
* 10, if the Address Family subfield contains the value for IPv4.
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* 22, if the Address Family subfield contains the value for IPv6.
* 6, if the Address Family subfield contains the value zero.
o The IP address in the sub-TLV's Address subfield lies within a
block listed in the relevant Special-Purpose IP Address Registry
[RFC6890] with either a "destination" attribute value or a
"forwardable" attribute value of "false". (Such routes are
sometimes colloquially known as "Martians".) This restriction MAY
be relaxed by explicit configuration.
o It can be determined that the IP address in the sub-TLV's Address
subfield does not belong to the Autonomous System (AS) that
originated the route that contains the attribute. Section 3.1.1
describes an optional procedure to make this determination.
Error Handling is specified in Section 13.
If the Tunnel Egress Endpoint sub-TLV contains an IPv4 or IPv6
address that is valid but not reachable, the sub-TLV is not
considered to be malformed.
3.1.1. Validating the Address Subfield
This section provides a procedure that MAY be applied to validate
that the IP address in the sub-TLV's Address subfield belongs to the
AS that originated the route that contains the attribute. (The
notion of "belonging to" an AS is expanded on below.) Doing this is
thought to increase confidence that when traffic is sent to the IP
address depicted in the Address subfield, it will go to the same AS
as it would go to if the Tunnel Encapsulation Attribute were not
present, although of course it cannot guarantee it. See Section 15
for discussion of the limitations of this procedure. The principal
applicability of this procedure is in deployments that are not
strictly scoped. In deployments with strict scope, and especially
those scoped to a single AS, these procedures may not add substantial
benefit beyond those discussed in Section 11.
The Route Origin ASN (Autonomous System Number) of a BGP route that
includes a Tunnel Encapsulation Attribute can be determined by
inspection of the AS_PATH attribute, according to the procedure
specified in [RFC6811] Section 2. Call this value Route_AS.
In order to determine the Route Origin ASN of the address depicted in
the Address subfield of the Tunnel Egress Endpoint sub-TLV, it is
necessary to consider the forwarding route, that is, the route that
will be used to forward traffic toward that address. This route is
determined by a recursive route lookup operation for that address, as
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discussed in [RFC4271] Section 5.1.3. The relevant AS Path to
consider is the last one encountered while performing the recursive
lookup; the procedures of RFC6811 Section 2 are applied to that AS
Path to determine the Route Origin ASN. If no AS Path is encountered
at all, for example if that route's source is a protocol other than
BGP, the Route Origin ASN is the BGP speaker's own AS number. Call
this value Egress_AS.
If Route_AS does not equal Egress_AS, then the Tunnel Egress Endpoint
sub-TLV is considered not to be valid. In some cases a network
operator who controls a set of Autonomous Systems might wish to allow
a Tunnel Egress Endpoint to reside in an AS other than Route_AS;
configuration MAY allow for such a case, in which case the check
becomes, if Egress_AS is not within the configured set of permitted
AS numbers, then the Tunnel Egress Endpoint sub-TLV is considered to
be "malformed".
Note that if the forwarding route changes, this procedure MUST be
reapplied. As a result, a sub-TLV that was formerly considered valid
might become not valid, or vice-versa.
3.2. Encapsulation Sub-TLVs for Particular Tunnel Types (type code 1)
This section defines Encapsulation sub-TLVs for the following tunnel
types: VXLAN ([RFC7348]), NVGRE ([RFC7637]), MPLS-in-GRE ([RFC4023]),
L2TPv3 ([RFC3931]), and GRE ([RFC2784]).
Rules for forming the encapsulation based on the information in a
given TLV are given in Section 6 and Section 9.
Recall that the tunnel type itself is identified by the Tunnel Type
field in the attribute header (Section 2); the Encapsulation sub-
TLV's structure is inferred from this. Regardless of the Tunnel
Type, the sub-TLV type of the Encapsulation sub-TLV is 1. There are
also tunnel types for which it is not necessary to define an
Encapsulation sub-TLV, because there are no fields in the
encapsulation header whose values need to be signaled from the tunnel
egress endpoint.
3.2.1. VXLAN (tunnel type 8)
This document defines an Encapsulation sub-TLV for VXLAN [RFC7348]
tunnels. When the Tunnel Type is VXLAN, the length of the sub-TLV is
12 octets. The following is the structure of the Value field in the
Encapsulation sub-TLV:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V|M|R|R|R|R|R|R| VN-ID (3 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address (4 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address (2 Octets) | Reserved (2 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: VXLAN Encapsulation Sub-TLV Value Field
V: This bit is set to 1 to indicate that a VN-ID (Virtual Network
Identifier) is present in the Encapsulation sub-TLV. If set to 0,
the VN-ID field is disregarded. Please see Section 9.
M: This bit is set to 1 to indicate that a MAC Address is present
in the Encapsulation sub-TLV. If set to 0, the MAC Address field
is disregarded.
R: The remaining bits in the 8-bit flags field are reserved for
further use. They MUST always be set to 0 by the originator of
the sub-TLV. Intermediate routers MUST propagate them without
modification. Any receiving routers MUST ignore these bits upon
receipt.
VN-ID: If the V bit is set, the VN-ID field contains a 3 octet VN-
ID value. If the V bit is not set, the VN-ID field MUST be set to
zero on transmission and disregarded on receipt.
MAC Address: If the M bit is set, this field contains a 6 octet
Ethernet MAC address. If the M bit is not set, this field MUST be
set to all zeroes on transmission and disregarded on receipt.
Reserved: MUST be set to zero on transmission and disregarded on
receipt.
When forming the VXLAN encapsulation header:
o The values of the V, M, and R bits are NOT copied into the flags
field of the VXLAN header. The flags field of the VXLAN header is
set as per [RFC7348].
o If the M bit is set, the MAC Address is copied into the Inner
Destination MAC Address field of the Inner Ethernet Header (see
section 5 of [RFC7348]).
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If the M bit is not set, and the payload being sent through the
VXLAN tunnel is an Ethernet frame, the Destination MAC Address
field of the Inner Ethernet Header is just the Destination MAC
Address field of the payload's Ethernet header.
If the M bit is not set, and the payload being sent through the
VXLAN tunnel is an IP or MPLS packet, the Inner Destination MAC
Address field is set to a configured value; if there is no
configured value, the VXLAN tunnel cannot be used.
o If the V bit is not set, and the BGP UPDATE message has AFI/SAFI
other than Ethernet VPNs (SAFI 70, "BGP EVPNs") then the VXLAN
tunnel cannot be used.
o Section 9 describes how the VNI field of the VXLAN encapsulation
header is set.
Note that in order to send an IP packet or an MPLS packet through a
VXLAN tunnel, the packet must first be encapsulated in an Ethernet
header, which becomes the "inner Ethernet header" described in
[RFC7348]. The VXLAN Encapsulation sub-TLV may contain information
(for example,the MAC address) that is used to form this Ethernet
header.
3.2.2. NVGRE (tunnel type 9)
This document defines an Encapsulation sub-TLV for NVGRE [RFC7637]
tunnels. When the Tunnel Type is NVGRE, the length of the sub-TLV is
12 octets. The following is the structure of the Value field in the
Encapsulation sub-TLV:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V|M|R|R|R|R|R|R| VN-ID (3 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address (4 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address (2 Octets) | Reserved (2 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: NVGRE Encapsulation Sub-TLV Value Field
V: This bit is set to 1 to indicate that a VN-ID is present in the
Encapsulation sub-TLV. If set to 0, the VN-ID field is
disregarded. Please see Section 9.
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M: This bit is set to 1 to indicate that a MAC Address is present
in the Encapsulation sub-TLV. If set to 0, the MAC Address field
is disregarded.
R: The remaining bits in the 8-bit flags field are reserved for
further use. They MUST always be set to 0 by the originator of
the sub-TLV. Intermediate routers MUST propagate them without
modification. Any receiving routers MUST ignore these bits upon
receipt.
VN-ID: If the V bit is set, the VN-ID field contains a 3 octet VN-
ID value, used to set the NVGRE VSID (see Section 9). If the V
bit is not set, the VN-ID field MUST be set to zero on
transmission and disregarded on receipt.
MAC Address: If the M bit is set, this field contains a 6 octet
Ethernet MAC address. If the M bit is not set, this field MUST be
set to all zeroes on transmission and disregarded on receipt.
Reserved: MUST be set to zero on transmission and disregarded on
receipt.
When forming the NVGRE encapsulation header:
o The values of the V, M, and R bits are NOT copied into the flags
field of the NVGRE header. The flags field of the NVGRE header is
set as per [RFC7637].
o If the M bit is set, the MAC Address is copied into the Inner
Destination MAC Address field of the Inner Ethernet Header (see
section 3.2 of [RFC7637]).
If the M bit is not set, and the payload being sent through the
NVGRE tunnel is an Ethernet frame, the Destination MAC Address
field of the Inner Ethernet Header is just the Destination MAC
Address field of the payload's Ethernet header.
If the M bit is not set, and the payload being sent through the
NVGRE tunnel is an IP or MPLS packet, the Inner Destination MAC
Address field is set to a configured value; if there is no
configured value, the NVGRE tunnel cannot be used.
o If the V bit is not set, and the BGP UPDATE message has AFI/SAFI
other than Ethernet VPNs (EVPN) then the NVGRE tunnel cannot be
used.
o Section 9 describes how the VSID (Virtual Subnet Identifier) field
of the NVGRE encapsulation header is set.
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3.2.3. L2TPv3 (tunnel type 1)
When the Tunnel Type of the TLV is L2TPv3 over IP [RFC3931], the
length of the sub-TLV is between 4 and 12 octets, depending on the
length of the cookie. The following is the structure of the Value
field of the Encapsulation sub-TLV:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Cookie (Variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: L2TPv3 Encapsulation Sub-TLV Value Field
Session ID: a non-zero 4-octet value locally assigned by the
advertising router that serves as a lookup key for the incoming
packet's context.
Cookie: an optional, variable length (encoded in octets -- 0 to 8
octets) value used by L2TPv3 to check the association of a
received data message with the session identified by the Session
ID. Generation and usage of the cookie value is as specified in
[RFC3931].
The length of the cookie is not encoded explicitly, but can be
calculated as (sub-TLV length - 4).
3.2.4. GRE (tunnel type 2)
When the Tunnel Type of the TLV is GRE [RFC2784], the length of the
sub-TLV is 4 octets. The following is the structure of the Value
field of the Encapsulation sub-TLV:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Key (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: GRE Encapsulation Sub-TLV
GRE Key: 4-octet field [RFC2890] that is generated by the
advertising router. Note that the key is optional. Unless a key
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value is being advertised, the GRE Encapsulation sub-TLV MUST NOT
be present.
3.2.5. MPLS-in-GRE (tunnel type 11)
When the Tunnel Type is MPLS-in-GRE [RFC4023], the length of the sub-
TLV is 4 octets. The following is the structure of the Value field
of the Encapsulation sub-TLV:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE-Key (4 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: MPLS-in-GRE Encapsulation Sub-TLV Value Field
GRE-Key: 4-octet field [RFC2890] that is generated by the
advertising router. Note that the key is optional. Unless a key
value is being advertised, the MPLS-in-GRE Encapsulation sub-TLV
MUST NOT be present.
Note that the GRE Tunnel Type defined in Section 3.2.4 can be used
instead of the MPLS-in-GRE Tunnel Type when it is necessary to
encapsulate MPLS in GRE. Including a TLV of the MPLS-in-GRE tunnel
type is equivalent to including a TLV of the GRE Tunnel Type that
also includes a Protocol Type sub-TLV (Section 3.4.1) specifying MPLS
as the protocol to be encapsulated.
Although the MPLS-in-GRE tunnel type is just a special case of the
GRE tunnel type and thus is not strictly necessary, it is included
for reasons of backwards compatibility with, for example,
implementations of [RFC8365].
3.3. Outer Encapsulation Sub-TLVs
The Encapsulation sub-TLV for a particular Tunnel Type allows one to
specify the values that are to be placed in certain fields of the
encapsulation header for that Tunnel Type. However, some tunnel
types require an outer IP encapsulation, and some also require an
outer UDP encapsulation. The Encapsulation sub-TLV for a given
Tunnel Type does not usually provide a way to specify values for
fields of the outer IP and/or UDP encapsulations. If it is necessary
to specify values for fields of the outer encapsulation, additional
sub-TLVs must be used. This document defines two such sub-TLVs.
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If an outer Encapsulation sub-TLV occurs in a TLV for a Tunnel Type
that does not use the corresponding outer encapsulation, the sub-TLV
MUST be treated as if it were an unrecognized type of sub-TLV.
3.3.1. DS Field (type code 7)
Most of the tunnel types that can be specified in the Tunnel
Encapsulation attribute require an outer IP encapsulation. The
Differentiated Services (DS) Field sub-TLV can be carried in the TLV
of any such Tunnel Type. It specifies the setting of the one-octet
Differentiated Services field in the outer IPv4 or IPv6 encapsulation
(see [RFC2474]). Any one-octet value can be transported; the
semantics of the DSCP field is beyond the scope of this document.
The Value field is always a single octet.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| DS value |
+-+-+-+-+-+-+-+-+
Figure 9: DS Field Sub-TLV Value Field
Because the interpretation of the DSCP field at the recipient may be
different from its interpretation at the originator, an
implementation MAY provide a facility to use policy to filter or
modify the DS Field.
3.3.2. UDP Destination Port (type code 8)
Some of the tunnel types that can be specified in the Tunnel
Encapsulation attribute require an outer UDP encapsulation.
Generally there is a standard UDP Destination Port value for a
particular Tunnel Type. However, sometimes it is useful to be able
to use a non-standard UDP destination port. If a particular tunnel
type requires an outer UDP encapsulation, and it is desired to use a
UDP destination port other than the standard one, the port to be used
can be specified by including a UDP Destination Port sub-TLV. The
Value field of this sub-TLV is always a two-octet field, containing
the port value. Any two-octet value other than zero can be
transported. If the reserved value zero is received, the sub-TLV
MUST be treated as malformed according to the rules of Section 13.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Port (2 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: UDP Destination Port Sub-TLV Value Field
3.4. Sub-TLVs for Aiding Tunnel Selection
3.4.1. Protocol Type Sub-TLV (type code 2)
The Protocol Type sub-TLV MAY be included in a given TLV to indicate
the type of the payload packets that are allowed to be encapsulated
with the tunnel parameters that are being signaled in the TLV.
Packets with other payload types MUST NOT be encapsulated in the
relevant tunnel. The Value field of the sub-TLV contains a 2-octet
value from IANA's "ETHER TYPES" registry [Ethertypes]. If the
reserved value 0xFFFF is received, the sub-TLV MUST be treated as
malformed according to the rules of Section 13.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype (2 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Protocol Type Sub-TLV Value Field
For example, if there are three L2TPv3 sessions, one carrying IPv4
packets, one carrying IPv6 packets, and one carrying MPLS packets,
the egress router will include three TLVs of L2TPv3 encapsulation
type, each specifying a different Session ID and a different payload
type. The Protocol Type sub-TLV for these will be IPv4 (protocol
type = 0x0800), IPv6 (protocol type = 0x86dd), and MPLS (protocol
type = 0x8847), respectively. This informs the ingress routers of
the appropriate encapsulation information to use with each of the
given protocol types. Insertion of the specified Session ID at the
ingress routers allows the egress to process the incoming packets
correctly, according to their protocol type.
Note that for tunnel types whose names are of the form "X-in-Y", for
example, "MPLS-in-GRE", only packets of the specified payload type
"X" are to be carried through the tunnel of type "Y". This is the
equivalent of specifying a Tunnel Type "Y" and including in its TLV a
Protocol Type sub-TLV (see Section 3.4.1) specifying protocol "X".
If the Tunnel Type is "X-in-Y", it is unnecessary, though harmless,
to explicitly include a Protocol Type sub-TLV specifying "X". Also,
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for "X-in-Y" type tunnels, a Protocol Type sub-TLV specifying
anything other than "X" MUST be ignored; this is discussed further in
Section 13.
3.4.2. Color Sub-TLV (type code 4)
The Color sub-TLV MAY be used as a way to "color" the corresponding
Tunnel TLV. The Value field of the sub-TLV is eight octets long, and
consists of a Color Extended Community, as defined in Section 4.3.
For the use of this sub-TLV and Extended Community, please see
Section 8.
The format of the Value field is depicted in Figure 15.
If the Length field of a Color sub-TLV has a value other than 8, or
the first two octets of its Value field are not 0x030b, the sub-TLV
MUST be treated as if it were an unrecognized sub-TLV (see
Section 13).
3.5. Embedded Label Handling Sub-TLV (type code 9)
Certain BGP address families (corresponding to particular AFI/SAFI
pairs, for example, 1/4, 2/4, 1/128, 2/128) have MPLS labels embedded
in their NLRIs. The term "embedded label" is used to refer to the
MPLS label that is embedded in an NLRI, and the term "labeled address
family" to refer to any AFI/SAFI that has embedded labels.
Some of the tunnel types (for example, VXLAN and NVGRE) that can be
specified in the Tunnel Encapsulation attribute have an encapsulation
header containing a "Virtual Network" identifier of some sort. The
Encapsulation sub-TLVs for these tunnel types may optionally specify
a value for the virtual network identifier.
Suppose a Tunnel Encapsulation attribute is attached to an UPDATE of
a labeled address family, and it is decided to use a particular
tunnel (specified in one of the attribute's TLVs) for transmitting a
packet that is being forwarded according to that UPDATE. When
forming the encapsulation header for that packet, different
deployment scenarios require different handling of the embedded label
and/or the virtual network identifier. The Embedded Label Handling
sub-TLV can be used to control the placement of the embedded label
and/or the virtual network identifier in the encapsulation.
The Embedded Label Handling sub-TLV may be included in any TLV of the
Tunnel Encapsulation attribute. If the Tunnel Encapsulation
attribute is attached to an UPDATE of a non-labeled address family,
then the sub-TLV MUST be disregarded. If the sub-TLV is contained in
a TLV whose Tunnel Type does not have a virtual network identifier in
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its encapsulation header, the sub-TLV MUST be disregarded. In those
cases where the sub-TLV is ignored, it MUST NOT be stripped from the
TLV before the route is propagated.
The sub-TLV's Length field always contains the value 1, and its Value
field consists of a single octet. The following values are defined:
1: The payload will be an MPLS packet with the embedded label at
the top of its label stack.
2: The embedded label is not carried in the payload, but is carried
either in the virtual network identifier field of the
encapsulation header, or else is ignored entirely.
If any value other than 1 or 2 is carried, the sub-TLV MUST be
considered malformed, according to the procedures of Section 13.
Please see Section 9 for the details of how this sub-TLV is used when
it is carried by an UPDATE of a labeled address family.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| 1 or 2 |
+-+-+-+-+-+-+-+-+
Figure 12: Embedded Label Handling Sub-TLV Value Field
3.6. MPLS Label Stack Sub-TLV (type code 10)
This sub-TLV allows an MPLS label stack ([RFC3032]) to be associated
with a particular tunnel.
The length of the sub-TLV is a multiple of 4 octets and the Value
field of this sub-TLV is a sequence of MPLS label stack entries. The
first entry in the sequence is the "topmost" label, the final entry
in the sequence is the "bottommost" label. When this label stack is
pushed onto a packet, this ordering MUST be preserved.
Each label stack entry has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: MPLS Label Stack Sub-TLV Value Field
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The fields are as defined in [RFC3032], [RFC5462].
If a packet is to be sent through the tunnel identified in a
particular TLV, and if that TLV contains an MPLS Label Stack sub-TLV,
then the label stack appearing in the sub-TLV MUST be pushed onto the
packet before any other labels are pushed onto the packet. (See
Section 6 for further discussion.)
In particular, if the Tunnel Encapsulation attribute is attached to a
BGP UPDATE of a labeled address family, the contents of the MPLS
Label Stack sub-TLV MUST be pushed onto the packet before the label
embedded in the NLRI is pushed onto the packet.
If the MPLS Label Stack sub-TLV is included in a TLV identifying a
Tunnel Type that uses virtual network identifiers (see Section 9),
the contents of the MPLS Label Stack sub-TLV MUST be pushed onto the
packet before the procedures of Section 9 are applied.
The number of label stack entries in the sub-TLV MUST be determined
from the sub-TLV length field. Thus it is not necessary to set the S
bit in any of the label stack entries of the sub-TLV, and the setting
of the S bit is ignored when parsing the sub-TLV. When the label
stack entries are pushed onto a packet that already has a label
stack, the S bits of all the entries being pushed MUST be cleared.
When the label stack entries are pushed onto a packet that does not
already have a label stack, the S bit of the bottommost label stack
entry MUST be set, and the S bit of all the other label stack entries
MUST be cleared.
The TC (Traffic Class) field ([RFC3270], [RFC5129]) of each label
stack entry SHOULD be set to 0, unless changed by policy at the
originator of the sub-TLV. When pushing the label stack onto a
packet, the TC of each label stack SHOULD be preserved, unless local
policy results in a modification.
The TTL (Time to Live) field of each label stack entry SHOULD be set
to 255, unless changed to some other non-zero value by policy at the
originator of the sub-TLV. When pushing the label stack onto a
packet, the TTL of each label stack entry SHOULD be preserved, unless
local policy results in a modification to some other non-zero value.
If any label stack entry in the sub-TLV has a TTL value of zero, the
router that is pushing the stack on a packet MUST change the value to
a non-zero value, either 255 or some other value as determined by
policy as discussed above.
Note that this sub-TLV can appear within a TLV identifying any type
of tunnel, not just within a TLV identifying an MPLS tunnel.
However, if this sub-TLV appears within a TLV identifying an MPLS
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tunnel (or an MPLS-in-X tunnel), this sub-TLV plays the same role
that would be played by an MPLS Encapsulation sub-TLV. Therefore, an
MPLS Encapsulation sub-TLV is not defined.
Although this specification does not supply detailed instructions for
validating the received label stack, implementations might impose
restrictions on the label stack they can support. If an invalid or
unsupported label stack is received, the tunnel MAY be treated as not
feasible according to the procedures of Section 6.
3.7. Prefix-SID Sub-TLV (type code 11)
[RFC8669] defines a BGP Path attribute known as the "Prefix-SID
Attribute". This attribute is defined to contain a sequence of one
or more TLVs, where each TLV is either a "Label-Index" TLV, or an
"Originator SRGB (Source Routing Global Block)" TLV.
This document defines a Prefix-SID sub-TLV. The Value field of the
Prefix-SID sub-TLV can be set to any permitted value of the Value
field of a BGP Prefix-SID attribute [RFC8669].
[RFC8669] only defines behavior when the Prefix-SID Attribute is
attached to routes of type IPv4/IPv6 Labeled Unicast ([RFC4760],
[RFC8277]), and it only defines values of the Prefix-SID Attribute
for those cases. Therefore, similar limitations exist for the
Prefix-SID sub-TLV: it SHOULD only be included in a BGP UPDATE
message for one of the address families defined in [RFC8669]. If
included in a BGP UPDATE for any other address family then it MUST be
ignored.
The Prefix-SID sub-TLV can occur in a TLV identifying any type of
tunnel. If an Originator SRGB is specified in the sub-TLV, that SRGB
MUST be interpreted to be the SRGB used by the tunnel's egress
endpoint. The Label-Index, if present, is the Segment Routing SID
that the tunnel's egress endpoint uses to represent the prefix
appearing in the NLRI field of the BGP UPDATE to which the Tunnel
Encapsulation attribute is attached.
If a Label-Index is present in the Prefix-SID sub-TLV, then when a
packet is sent through the tunnel identified by the TLV, if that
tunnel is from a labeled address family, the corresponding MPLS label
MUST be pushed on the packet's label stack. The corresponding MPLS
label is computed from the Label-Index value and the SRGB of the
route's originator, as specified in section 4.1 of [RFC8669].
The corresponding MPLS label is pushed on after the processing of the
MPLS Label Stack sub-TLV, if present, as specified in Section 3.6.
It is pushed on before any other labels (for example, a label
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embedded in UPDATE's NLRI, or a label determined by the procedures of
Section 9), are pushed on the stack.
The Prefix-SID sub-TLV has slightly different semantics than the
Prefix-SID attribute. When the Prefix-SID attribute is attached to a
given route, the BGP speaker that originally attached the attribute
is expected to be in the same Segment Routing domain as the BGP
speakers who receive the route with the attached attribute. The
Label-Index tells the receiving BGP speakers what the prefix-SID is
for the advertised prefix in that Segment Routing domain. When the
Prefix-SID sub-TLV is used, there is no implication that the prefix-
SID for the advertised prefix is the same in the Segment Routing
domains of the BGP speaker that originated the sub-TLV and the BGP
speaker that received it.
4. Extended Communities Related to the Tunnel Encapsulation Attribute
4.1. Encapsulation Extended Community
The Encapsulation Extended Community is a Transitive Opaque Extended
Community.
The Encapsulation Extended Community encoding is as shown below
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x03 (1 Octet)| 0x0c (1 Octet)| Reserved (2 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (2 Octets) | Tunnel Type (2 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Encapsulation Extended Community
The value of the high-order octet of the extended type field is 0x03,
which indicates it's transitive. The value of the low-order octet of
the extended type field is 0x0c.
The last two octets of the Value field encode a tunnel type.
This Extended Community may be attached to a route of any AFI/SAFI to
which the Tunnel Encapsulation attribute may be attached. Each such
Extended Community identifies a particular Tunnel Type, its semantics
are the same as semantics of a Tunnel Encapsulation attribute Tunnel
TLV for which the following three conditions all hold:
1. it identifies the same Tunnel Type,
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2. it has a Tunnel Egress Endpoint sub-TLV for which one of the
following two conditions holds:
A. its "Address Family" subfield contains zero, or
B. its "Address" subfield contains the address of the next hop
field of the route to which the Tunnel Encapsulation
attribute is attached
3. it has no other sub-TLVs.
Such a Tunnel TLV is called a "barebones" Tunnel TLV.
The Encapsulation Extended Community was first defined in [RFC5512].
While it provides only a small subset of the functionality of the
Tunnel Encapsulation attribute, it is used in a number of deployed
applications, and is still needed for backwards compatibility. In
situations where a tunnel could be encoded using a barebones TLV, it
MUST be encoded using the corresponding Encapsulation Extended
Community. Notwithstanding, an implementation MUST be prepared to
process a tunnel received encoded as a barebones TLV.
Note that for tunnel types of the form "X-in-Y", for example, MPLS-
in-GRE, the Encapsulation Extended Community implies that only
packets of the specified payload type "X" are to be carried through
the tunnel of type "Y". Packets with other payload types MUST NOT be
carried through such tunnels. See also Section 2.
In the remainder of this specification, when a route is referred to
as containing a Tunnel Encapsulation attribute with a TLV identifying
a particular Tunnel Type, it implicitly includes the case where the
route contains a Tunnel Encapsulation Extended Community identifying
that Tunnel Type.
4.2. Router's MAC Extended Community
[I-D.ietf-bess-evpn-inter-subnet-forwarding] defines a Router's MAC
Extended Community. This Extended Community, as its name implies,
carries the MAC address of the advertising router. Since the VXLAN
and NVGRE Encapsulation Sub-TLVs can also optionally carry a router's
MAC, a conflict can arise if both the Router's MAC Extended Community
and such an Encapsulation Sub-TLV are present at the same time but
have different values. In case of such a conflict, the information
in the Router's MAC Extended Community MUST be used.
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4.3. Color Extended Community
The Color Extended Community is a Transitive Opaque Extended
Community with the following encoding:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x03 (1 Octet)| 0x0b (1 Octet)| Flags (2 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Color Value (4 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Color Extended Community
The value of the high-order octet of the extended type field is 0x03,
which indicates it is transitive. The value of the low-order octet
of the extended type field for this community is 0x0b. The color
value is user defined and configured locally. No flags are defined
in this document; this field MUST be set to zero by the originator
and ignored by the receiver; the value MUST NOT be changed when
propagating this Extended Community. The Color Value field is
encoded as 4 octet value by the administrator and is outside the
scope of this document. For the use of this Extended Community
please see Section 8.
5. Special Considerations for IP-in-IP Tunnels
In certain situations with an IP fabric underlay, one could have a
tunnel overlay with the tunnel type IP-in-IP. The egress BGP speaker
can advertise the IP-in-IP tunnel endpoint address in the Tunnel
Egress Endpoint sub-TLV. When the Tunnel type of the TLV is IP-in-
IP, it will not have a Virtual Network Identifier. However, the
tunnel egress endpoint address can be used in identifying the
forwarding table to use for making the forwarding decisions to
forward the payload.
6. Semantics and Usage of the Tunnel Encapsulation attribute
The BGP Tunnel Encapsulation attribute MAY be carried in any BGP
UPDATE message whose AFI/SAFI is 1/1 (IPv4 Unicast), 2/1 (IPv6
Unicast), 1/4 (IPv4 Labeled Unicast), 2/4 (IPv6 Labeled Unicast),
1/128 (VPN-IPv4 Labeled Unicast), 2/128 (VPN-IPv6 Labeled Unicast),
or 25/70 (Ethernet VPN, usually known as EVPN)). Use of the Tunnel
Encapsulation attribute in BGP UPDATE messages of other AFI/SAFIs is
outside the scope of this document.
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There is no significance to the order in which the TLVs occur within
the Tunnel Encapsulation attribute. Multiple TLVs may occur for a
given Tunnel Type; each such TLV is regarded as describing a
different tunnel. (This also applies if the Tunnel Encapsulation
Extended Community encoding is used.)
The decision to attach a Tunnel Encapsulation attribute to a given
BGP UPDATE is determined by policy. The set of TLVs and sub-TLVs
contained in the attribute is also determined by policy.
Suppose that:
o a given packet P must be forwarded by router R;
o the path along which P is to be forwarded is determined by BGP
UPDATE U;
o UPDATE U has a Tunnel Encapsulation attribute, containing at least
one TLV that identifies a "feasible tunnel" for packet P. A
tunnel is considered feasible if it has the following four
properties:
* The Tunnel Type is supported (that is, router R knows how to
set up tunnels of that type, how to create the encapsulation
header for tunnels of that type, etc.)
* The tunnel is of a type that can be used to carry packet P (for
example, an MPLS-in-UDP tunnel would not be a feasible tunnel
for carrying an IP packet, unless the IP packet can first be
encapsulated in a MPLS packet).
* The tunnel is specified in a TLV whose Tunnel Egress Endpoint
sub-TLV identifies an IP address that is reachable. The
reachability condition is evaluated as per [RFC4271]. If the
IP address is reachable via more than one forwarding table,
local policy is used to determine which table to use.
* There is no local policy that prevents the use of the tunnel.
Then router R MUST send packet P through one of the feasible tunnels
identified in the Tunnel Encapsulation attribute of UPDATE U.
If the Tunnel Encapsulation attribute contains several TLVs (that is,
if it specifies several feasible tunnels), router R may choose any
one of those tunnels, based upon local policy. If any Tunnel TLV
contains one or more Color sub-TLVs (Section 3.4.2) and/or the
Protocol Type sub-TLV (Section 3.4.1), the choice of tunnel may be
influenced by these sub-TLVs. Many other factors, for example
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minimization of encapsulation header overhead, could also be used to
influence selection.
The reachability to the address of the egress endpoint of the tunnel
may change over time, directly impacting the feasibility of the
tunnel. A tunnel that is not feasible at some moment, may become
feasible at a later time when its egress endpoint address is
reachable. The router may start using the newly feasible tunnel
instead of an existing one. How this decision is made is outside the
scope of this document.
Once it is determined to send a packet through the tunnel specified
in a particular Tunnel TLV of a particular Tunnel Encapsulation
attribute, then the tunnel's egress endpoint address is the IP
address contained in the Tunnel Egress Endpoint sub-TLV. If the
Tunnel TLV contains a Tunnel Egress Endpoint sub-TLV whose Value
field is all zeroes, then the tunnel's egress endpoint is the address
of the Next Hop of the BGP Update containing the Tunnel Encapsulation
attribute. The address of the tunnel egress endpoint generally
appears in a "destination address" field of the encapsulation.
The full set of procedures for sending a packet through a particular
Tunnel Type to a particular tunnel egress endpoint depends upon the
tunnel type, and is outside the scope of this document. Note that
some tunnel types may require the execution of an explicit tunnel
setup protocol before they can be used for carrying data. Other
tunnel types may not require any tunnel setup protocol.
Sending a packet through a tunnel always requires that the packet be
encapsulated, with an encapsulation header that is appropriate for
the Tunnel Type. The contents of the tunnel encapsulation header may
be influenced by the Encapsulation sub-TLV. If there is no
Encapsulation sub-TLV present, the router transmitting the packet
through the tunnel must have a priori knowledge (for example, by
provisioning) of how to fill in the various fields in the
encapsulation header.
A Tunnel Encapsulation attribute may contain several TLVs that all
specify the same Tunnel Type. Each TLV should be considered as
specifying a different tunnel. Two tunnels of the same type may have
different Tunnel Egress Endpoint sub-TLVs, different Encapsulation
sub-TLVs, etc. Choosing between two such tunnels is a matter of
local policy.
Once router R has decided to send packet P through a particular
tunnel, it encapsulates packet P appropriately and then forwards it
according to the route that leads to the tunnel's egress endpoint.
This route may itself be a BGP route with a Tunnel Encapsulation
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attribute. If so, the encapsulated packet is treated as the payload
and is encapsulated according to the Tunnel Encapsulation attribute
of that route. That is, tunnels may be "stacked".
Notwithstanding anything said in this document, a BGP speaker MAY
have local policy that influences the choice of tunnel, and the way
the encapsulation is formed. A BGP speaker MAY also have a local
policy that tells it to ignore the Tunnel Encapsulation attribute
entirely or in part. Of course, interoperability issues must be
considered when such policies are put into place.
See also Section 13, which provides further specification regarding
validation and exception cases.
7. Routing Considerations
7.1. Impact on the BGP Decision Process
The presence of the Tunnel Encapsulation attribute affects the BGP
best route selection algorithm. If a route includes the Tunnel
Encapsulation attribute, and if that attribute includes no tunnel
which is feasible, then that route MUST NOT be considered resolvable
for the purposes of Route Resolvability Condition [RFC4271]
Section 9.1.2.1.
7.2. Looping, Mutual Recursion, Etc.
Consider a packet destined for address X. Suppose a BGP UPDATE for
address prefix X carries a Tunnel Encapsulation attribute that
specifies a tunnel egress endpoint of Y, and suppose that a BGP
UPDATE for address prefix Y carries a Tunnel Encapsulation attribute
that specifies a tunnel egress endpoint of X. It is easy to see that
this can have no good outcome. [RFC4271] describes an analogous case
as mutually recursive routes.
This could happen as a result of misconfiguration, either accidental
or intentional. It could also happen if the Tunnel Encapsulation
attribute were altered by a malicious agent. Implementations should
be aware that such an attack will result in unresolvable BGP routes
due to the mutually recursive relationship. This document does not
specify a maximum number of recursions; that is an implementation-
specific matter.
Improper setting (or malicious altering) of the Tunnel Encapsulation
attribute could also cause data packets to loop. Suppose a BGP
UPDATE for address prefix X carries a Tunnel Encapsulation attribute
that specifies a tunnel egress endpoint of Y. Suppose router R
receives and processes the advertisement. When router R receives a
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packet destined for X, it will apply the encapsulation and send the
encapsulated packet to Y. Y will decapsulate the packet and forward
it further. If Y is further away from X than is router R, it is
possible that the path from Y to X will traverse R. This would cause
a long-lasting routing loop. The control plane itself cannot detect
this situation, though a TTL field in the payload packets would
prevent any given packet from looping infinitely.
During the deployment of techniques as described in this document,
operators are encouraged to avoid mutually recursive route and/or
tunnel dependencies. There is greater potential for such scenarios
to arise when the tunnel egress endpoint for a given prefix differs
from the address of the next hop for that prefix.
8. Recursive Next Hop Resolution
Suppose that:
o a given packet P must be forwarded by router R1;
o the path along which P is to be forwarded is determined by BGP
UPDATE U1;
o UPDATE U1 does not have a Tunnel Encapsulation attribute;
o the address of the next hop of UPDATE U1 is router R2;
o the best route to router R2 is a BGP route that was advertised in
UPDATE U2;
o UPDATE U2 has a Tunnel Encapsulation attribute.
Then packet P MUST be sent through one of the tunnels identified in
the Tunnel Encapsulation attribute of UPDATE U2. See Section 6 for
further details.
However, suppose that one of the TLVs in U2's Tunnel Encapsulation
attribute contains one or more Color Sub-TLVs. In that case, packet
P MUST NOT be sent through the tunnel contained in that TLV, unless
U1 is carrying a Color Extended Community that is identified in one
of U2's Color Sub-TLVs.
The procedures in this section presuppose that U1's address of the
next hop resolves to a BGP route, and that U2's next hop resolves
(perhaps after further recursion) to a non-BGP route.
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9. Use of Virtual Network Identifiers and Embedded Labels when Imposing
a Tunnel Encapsulation
If the TLV specifying a tunnel contains an MPLS Label Stack sub-TLV,
then when sending a packet through that tunnel, the procedures of
Section 3.6 are applied before the procedures of this section.
If the TLV specifying a tunnel contains a Prefix-SID sub-TLV, the
procedures of Section 3.7 are applied before the procedures of this
section. If the TLV also contains an MPLS Label Stack sub-TLV, the
procedures of Section 3.6 are applied before the procedures of
Section 3.7.
9.1. Tunnel Types without a Virtual Network Identifier Field
If a Tunnel Encapsulation attribute is attached to an UPDATE of a
labeled address family, there will be one or more labels specified in
the UPDATE's NLRI. When a packet is sent through a tunnel specified
in one of the attribute's TLVs, and that tunnel type does not contain
a virtual network identifier field, the label or labels from the NLRI
are pushed on the packet's label stack. The resulting MPLS packet is
then further encapsulated, as specified by the TLV.
9.2. Tunnel Types with a Virtual Network Identifier Field
Two of the tunnel types that can be specified in a Tunnel
Encapsulation TLV have virtual network identifier fields in their
encapsulation headers. In the VXLAN encapsulation, this field is
called the VNI (VXLAN Network Identifier) field; in the NVGRE
encapsulation, this field is called the VSID (Virtual Subnet
Identifier) field.
When one of these tunnel encapsulations is imposed on a packet, the
setting of the virtual network identifier field in the encapsulation
header depends upon the contents of the Encapsulation sub-TLV (if one
is present). When the Tunnel Encapsulation attribute is being
carried in a BGP UPDATE of a labeled address family, the setting of
the virtual network identifier field also depends upon the contents
of the Embedded Label Handling sub-TLV (if present).
This section specifies the procedures for choosing the value to set
in the virtual network identifier field of the encapsulation header.
These procedures apply only when the Tunnel Type is VXLAN or NVGRE.
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9.2.1. Unlabeled Address Families
This sub-section applies when:
o the Tunnel Encapsulation attribute is carried in a BGP UPDATE of
an unlabeled address family, and
o at least one of the attribute's TLVs identifies a Tunnel Type that
uses a virtual network identifier, and
o it has been determined to send a packet through one of those
tunnels.
If the TLV identifying the tunnel contains an Encapsulation sub-TLV
whose V bit is set, the virtual network identifier field of the
encapsulation header is set to the value of the virtual network
identifier field of the Encapsulation sub-TLV.
Otherwise, the virtual network identifier field of the encapsulation
header is set to a configured value; if there is no configured value,
the tunnel cannot be used.
9.2.2. Labeled Address Families
This sub-section applies when:
o the Tunnel Encapsulation attribute is carried in a BGP UPDATE of a
labeled address family, and
o at least one of the attribute's TLVs identifies a Tunnel Type that
uses a virtual network identifier, and
o it has been determined to send a packet through one of those
tunnels.
9.2.2.1. When a Valid VNI has been Signaled
If the TLV identifying the tunnel contains an Encapsulation sub-TLV
whose V bit is set, the virtual network identifier field of the
encapsulation header is set to the value of the virtual network
identifier field of the Encapsulation sub-TLV. However, the Embedded
Label Handling sub-TLV will determine label processing as described
below.
o If the TLV contains an Embedded Label Handling sub-TLV whose value
is 1, the embedded label (from the NLRI of the route that is
carrying the Tunnel Encapsulation attribute) appears at the top of
the MPLS label stack in the encapsulation payload.
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o If the TLV does not contain an Embedded Label Handling sub-TLV, or
it contains an Embedded Label Handling sub-TLV whose value is 2,
the embedded label is ignored entirely.
9.2.2.2. When a Valid VNI has not been Signaled
If the TLV identifying the tunnel does not contain an Encapsulation
sub-TLV whose V bit is set, the virtual network identifier field of
the encapsulation header is set as follows:
o If the TLV contains an Embedded Label Handling sub-TLV whose value
is 1, then the virtual network identifier field of the
encapsulation header is set to a configured value.
If there is no configured value, the tunnel cannot be used.
The embedded label (from the NLRI of the route that is carrying
the Tunnel Encapsulation attribute) appears at the top of the MPLS
label stack in the encapsulation payload.
o If the TLV does not contain an Embedded Label Handling sub-TLV, or
if it contains an Embedded Label Handling sub-TLV whose value is
2, the embedded label is copied into the lower 3 octets of the
virtual network identifier field of the encapsulation header.
In this case, the payload may or may not contain an MPLS label
stack, depending upon other factors. If the payload does contain
an MPLS label stack, the embedded label does not appear in that
stack.
10. Applicability Restrictions
In a given UPDATE of a labeled address family, the label embedded in
the NLRI is generally a label that is meaningful only to the router
represented by the address of the next hop. Certain of the
procedures of Section 9.2.2.1 or Section 9.2.2.2 cause the embedded
label to be carried by a data packet to the router whose address
appears in the Tunnel Egress Endpoint sub-TLV. If the Tunnel Egress
Endpoint sub-TLV does not identify the same router represented by the
address of the next hop, sending the packet through the tunnel may
cause the label to be misinterpreted at the tunnel's egress endpoint.
This may cause misdelivery of the packet. Avoidance of this
unfortunate outcome is a matter of network planning and design, and
is outside the scope of this document.
Note that if the Tunnel Encapsulation attribute is attached to a VPN-
IP route [RFC4364], and if Inter-AS "option b" (see section 10 of
[RFC4364]) is being used, and if the Tunnel Egress Endpoint sub-TLV
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contains an IP address that is not in same AS as the router receiving
the route, it is very likely that the embedded label has been
changed. Therefore use of the Tunnel Encapsulation attribute in an
"Inter-AS option b" scenario is not recommended.
Other documents may define other ways to signal tunnel information in
BGP. For example, [RFC6514] defines the "P-Multicast Service
Interface Tunnel" (PMSI Tunnel) attribute. In this specification, we
do not consider the effects of advertising the Tunnel Encapsulation
Attribute in conjunction with other forms of signaling tunnels. Any
document specifying such joint use MUST provide details as to how
interactions should be handled.
11. Scoping
The Tunnel Encapsulation attribute is defined as a transitive
attribute, so that it may be passed along by BGP speakers that do not
recognize it. However the Tunnel Encapsulation attribute MUST be
used only within a well-defined scope, for example, within a set of
Autonomous Systems that belong to a single administrative entity. If
the attribute is distributed beyond its intended scope, packets may
be sent through tunnels in a manner that is not intended.
To prevent the Tunnel Encapsulation attribute from being distributed
beyond its intended scope, any BGP speaker that understands the
attribute MUST be able to filter the attribute from incoming BGP
UPDATE messages. When the attribute is filtered from an incoming
UPDATE, the attribute is neither processed nor distributed. This
filtering SHOULD be possible on a per-BGP-session basis; finer
granularities (for example, per route and/or per attribute TLV) MAY
be supported. For each external BGP (EBGP) session, filtering of the
attribute on incoming UPDATEs MUST be enabled by default.
In addition, any BGP speaker that understands the attribute MUST be
able to filter the attribute from outgoing BGP UPDATE messages. This
filtering SHOULD be possible on a per-BGP-session basis. For each
EBGP session, filtering of the attribute on outgoing UPDATEs MUST be
enabled by default.
Since the Tunnel Encapsulation Extended Community provides a subset
of the functionality of the Tunnel Encapsulation attribute, these
considerations apply equally in its case: any BGP speaker that
understands it MUST be able to filter it from incoming BGP UPDATE
messages, it MUST be possible to filter the Tunnel Encapsulation
Extended Community from outgoing messages, and in both cases this
filtering MUST be enabled by default for EBGP sessions.
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12. Operational Considerations
A potential operational difficulty arises when tunnels are used, if
the size of packets entering the tunnel exceeds the maximum
transmission unit (MTU) the tunnel is capable of supporting. This
difficulty can be exacerbated by stacking multiple tunnels, since
each stacked tunnel header further reduces the supportable MTU. This
issue is long-standing and well-known. The tunnel signaling provided
in this specification does nothing to address this issue, nor to
aggravate it (except insofar as it may further increase the
popularity of tunneling).
13. Validation and Error Handling
The Tunnel Encapsulation attribute is a sequence of TLVs, each of
which is a sequence of sub-TLVs. The final octet of a TLV is
determined by its length field. Similarly, the final octet of a sub-
TLV is determined by its length field. The final octet of a TLV MUST
also be the final octet of its final sub-TLV. If this is not the
case, the TLV MUST be considered to be malformed, and the "Treat-as-
withdraw" procedure of [RFC7606] is applied.
If a Tunnel Encapsulation attribute does not have any valid TLVs, or
it does not have the transitive bit set, the "Treat-as-withdraw"
procedure of [RFC7606] is applied.
If a Tunnel Encapsulation attribute can be parsed correctly, but
contains a TLV whose Tunnel Type is not recognized by a particular
BGP speaker, that BGP speaker MUST NOT consider the attribute to be
malformed. Rather, it MUST interpret the attribute as if that TLV
had not been present. If the route carrying the Tunnel Encapsulation
attribute is propagated with the attribute, the unrecognized TLV MUST
remain in the attribute.
The following sub-TLVs defined in this document MUST NOT occur more
than once in a given Tunnel TLV: Tunnel Egress Endpoint (discussed
below), Encapsulation, DS, UDP Destination Port, Embedded Label
Handling, MPLS Label Stack, Prefix-SID. If a Tunnel TLV has more
than one of any of these sub-TLVs, all but the first occurrence of
each such sub-TLV type MUST be disregarded. However, the Tunnel TLV
containing them MUST NOT be considered to be malformed, and all the
sub-TLVs MUST be propagated if the route carrying the Tunnel
Encapsulation attribute is propagated.
The following sub-TLVs defined in this document may appear zero or
more times in a given Tunnel TLV: Protocol Type, Color. Each
occurrence of such sub-TLVs is meaningful. For example, the Color
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sub-TLV may appear multiple times to assign multiple colors to a
tunnel.
If a TLV of a Tunnel Encapsulation attribute contains a sub-TLV that
is not recognized by a particular BGP speaker, the BGP speaker MUST
process that TLV as if the unrecognized sub-TLV had not been present.
If the route carrying the Tunnel Encapsulation attribute is
propagated with the attribute, the unrecognized sub-TLV MUST remain
in the attribute.
In general, if a TLV contains a sub-TLV that is malformed, the sub-
TLV MUST be treated as if it were an unrecognized sub-TLV. There is
one exception to this rule -- if a TLV contains a malformed Tunnel
Egress Endpoint sub-TLV (as defined in Section 3.1), the entire TLV
MUST be ignored, and MUST be removed from the Tunnel Encapsulation
attribute before the route carrying that attribute is distributed.
Within a Tunnel Encapsulation attribute that is carried by a BGP
UPDATE whose AFI/SAFI is one of those explicitly listed in the second
paragraph of Section 6, a TLV that does not contain exactly one
Tunnel Egress Endpoint sub-TLV MUST be treated as if it contained a
malformed Tunnel Egress Endpoint sub-TLV.
A TLV identifying a particular Tunnel Type may contain a sub-TLV that
is meaningless for that Tunnel Type. For example, perhaps the TLV
contains a UDP Destination Port sub-TLV, but the identified tunnel
type does not use UDP encapsulation at all, or a tunnel of the form
"X-in-Y" contains a Protocol Type sub-TLV that specifies something
other than "X". Sub-TLVs of this sort MUST be disregarded. That is,
they MUST NOT affect the creation of the encapsulation header.
However, the sub-TLV MUST NOT be considered to be malformed, and MUST
NOT be removed from the TLV before the route carrying the Tunnel
Encapsulation attribute is distributed. An implementation MAY log a
message when it encounters such a sub-TLV.
14. IANA Considerations
This document makes the following requests of IANA. (All
registration procedures listed below are per their definitions in
[RFC8126].)
14.1. Obsoleting RFC 5512
Because this document obsoletes RFC 5512, change all registration
information that references [RFC5512] to instead reference this
document.
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14.2. Obsoleting Code Points Assigned by RFCs 5566
Since this document obsoletes RFC 5566, the code points assigned by
that RFC are similarly obsoleted. Specifically, the following code
points should be marked as deprecated.
In the "BGP Tunnel Encapsulation Attribute Tunnel Types" registry:
+-------+---------------------------------------------+
| Value | Name |
+-------+---------------------------------------------+
| 3 | Transmit tunnel endpoint |
| 4 | IPsec in Tunnel-mode |
| 5 | IP in IP tunnel with IPsec Transport Mode |
| 6 | MPLS-in-IP tunnel with IPsec Transport Mode |
+-------+---------------------------------------------+
And in the "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry:
+-------+----------------------------+
| Value | Name |
+-------+----------------------------+
| 3 | IPsec Tunnel Authenticator |
+-------+----------------------------+
14.3. BGP Tunnel Encapsulation Parameters Grouping
Create a new registry grouping, to be named "BGP Tunnel Encapsulation
Parameters".
14.4. BGP Tunnel Encapsulation Attribute Tunnel Types
Relocate the "BGP Tunnel Encapsulation Attribute Tunnel Types"
registry to be under the "BGP Tunnel Encapsulation Parameters"
grouping.
14.5. Subsequent Address Family Identifiers
Modify the "Subsequent Address Family Identifiers" registry to
indicate that the Encapsulation SAFI (value 7) is obsoleted. This
document should be the reference.
14.6. BGP Tunnel Encapsulation Attribute Sub-TLVs
Relocate the "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry
to be under the "BGP Tunnel Encapsulation Parameters" grouping.
Add the following note to the registry:
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If the Sub-TLV Type is in the range from 0 to 127 inclusive, the
Sub-TLV Length field contains one octet. If the Sub-TLV Type is
in the range from 128-255 inclusive, the Sub-TLV Length field
contains two octets.
Change the registration policy of the registry to the following:
+----------+-------------------------+
| Value(s) | Registration Procedure |
+----------+-------------------------+
| 0 | Reserved |
| 1-63 | Standards Action |
| 64-125 | First Come First Served |
| 126-127 | Experimental Use |
| 128-191 | Standards Action |
| 192-252 | First Come First Served |
| 253-254 | Experimental Use |
| 255 | Reserved |
+----------+-------------------------+
Rename the following entries within the registry:
+-------+-----------------+------------------------+
| Value | Old Name | New Name |
+-------+-----------------+------------------------+
| 6 | Remote Endpoint | Tunnel Egress Endpoint |
| 7 | IPv4 DS Field | DS Field |
+-------+-----------------+------------------------+
14.7. Flags Field of VXLAN Encapsulation sub-TLV
Create a registry named "Flags Field of VXLAN Encapsulation sub-TLV"
under the "BGP Tunnel Encapsulation Parameters" grouping. The
registration policy for this registry is "Standards Action". The
minimum possible value is 0, the maximum is 7.
The initial values for this new registry are indicated below.
+--------------+-----------------+-----------------+
| Bit Position | Description | Reference |
+--------------+-----------------+-----------------+
| 0 | V (VN-ID) | (this document) |
| 1 | M (MAC Address) | (this document) |
+--------------+-----------------+-----------------+
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14.8. Flags Field of NVGRE Encapsulation sub-TLV
Create a registry named "Flags Field of NVGRE Encapsulation sub-TLV"
under the "BGP Tunnel Encapsulation Parameters" grouping. The
registration policy for this registry is "Standards Action". The
minimum possible value is 0, the maximum is 7.
The initial values for this new registry are indicated below.
+--------------+-----------------+-----------------+
| Bit Position | Description | Reference |
+--------------+-----------------+-----------------+
| 0 | V (VN-ID) | (this document) |
| 1 | M (MAC Address) | (this document) |
+--------------+-----------------+-----------------+
14.9. Embedded Label Handling sub-TLV
Create a registry named "Embedded Label Handling sub-TLV" under the
"BGP Tunnel Encapsulation Parameters" grouping. The registration
policy for this registry is "Standards Action". The minimum possible
value is 0, the maximum is 255.
The initial values for this new registry are indicated below.
+-------+-------------------------------------+-----------------+
| Value | Description | Reference |
+-------+-------------------------------------+-----------------+
| 0 | Reserved | (this document) |
| 1 | Payload of MPLS with embedded label | (this document) |
| 2 | no embedded label in payload | (this document) |
+-------+-------------------------------------+-----------------+
14.10. Color Extended Community Flags
Create a registry named "Color Extended Community Flags" under the
"BGP Tunnel Encapsulation Parameters" grouping. The registration
policy for this registry is "Standards Action". The minimum possible
value is 0, the maximum is 15.
No initial values are to be registered. The format of the registry
is shown below.
+--------------+-------------+-----------+
| Bit Position | Description | Reference |
+--------------+-------------+-----------+
+--------------+-------------+-----------+
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15. Security Considerations
As Section 11 discusses, it is intended that the Tunnel Encapsulation
attribute be used only within a well-defined scope, for example,
within a set of Autonomous Systems that belong to a single
administrative entity. As long as the filtering mechanisms discussed
in that section are applied diligently, an attacker outside the scope
would not be able to use the Tunnel Encapsulation attribute in an
attack. This leaves open the questions of attackers within the scope
(for example, a compromised router) and failures in filtering that
allow an external attack to succeed.
As [RFC4272] discusses, BGP is vulnerable to traffic diversion
attacks. The Tunnel Encapsulation attribute adds a new means by
which an attacker could cause traffic to be diverted from its normal
path, especially when the Tunnel Egress Endpoint sub-TLV is used.
Such an attack would differ from pre-existing vulnerabilities in that
traffic could be tunneled to a distant target across intervening
network infrastructure, allowing an attack to potentially succeed
more easily, since less infrastructure would have to be subverted.
Potential consequences include "hijacking" of traffic (insertion of
an undesired node in the path allowing for inspection or modification
of traffic, or avoidance of security controls) or denial of service
(directing traffic to a node that doesn't desire to receive it).
In order to further mitigate the risk of diversion of traffic from
its intended destination, Section 3.1.1 provides an optional
procedure to check that the destination given in a Tunnel Egress
Endpoint sub-TLV is within the AS that was the source of the route.
One then has some level of assurance that the tunneled traffic is
going to the same destination AS that it would have gone to had the
Tunnel Encapsulation attribute not been present. As RFC 4272
discusses, it's possible for an attacker to announce an inaccurate
AS_PATH, therefore an attacker with the ability to inject a Tunnel
Egress Endpoint sub-TLV could equally craft an AS_PATH that would
pass the validation procedures of Section 3.1.1. BGP Origin
Validation [RFC6811] and BGPsec [RFC8205] provide means to increase
assurance that the origins being validated have not been falsified.
Many tunnels carry traffic that embeds a destination address that
comes from a non-global namespace. One example is MPLS VPNs. If a
tunnel crosses from one namespace to another, without the necessary
translation being performed for the embedded address(es), there
exists a risk of misdelivery of traffic. If the traffic contains
confidential data that's not otherwise protected (for example, by
end-to-end encryption) then confidential information could be
revealed. The restriction of applicability of the Tunnel
Encapsulation attribute to a well-defined scope limits the likelihood
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of this occurring. See the discussion of "option b" in Section 10
for further discussion of one such scenario.
RFC 8402 specifies that "SR domain boundary routers MUST filter any
external traffic" ([RFC8402] Section 8.1). For these purposes,
traffic introduced into a SR domain using the Prefix-SID sub-TLV lies
within the SR domain, even though the prefix-SIDs used by the routers
at the two ends of the tunnel may be different, as discussed in
Section 3.7. This implies that the duty to filter external traffic
extends to all routers participating in such tunnels.
16. Acknowledgments
This document contains text from RFC 5512, authored by Pradosh
Mohapatra and Eric Rosen. The authors of the current document wish
to thank them for their contribution. RFC 5512 itself built upon
prior work by Gargi Nalawade, Ruchi Kapoor, Dan Tappan, David Ward,
Scott Wainner, Simon Barber, Lili Wang, and Chris Metz, whom the
authors also thank for their contributions. Eric Rosen was the
principal author of earlier versions of this document.
The authors wish to thank Lou Berger, Ron Bonica, Martin Djernaes,
John Drake, Susan Hares, Satoru Matsushima, Thomas Morin, Dhananjaya
Rao, Ravi Singh, Harish Sitaraman, Brian Trammell, Xiaohu Xu, and
Zhaohui Zhang for their review, comments, and/or helpful discussions.
Alvaro Retana provided an especially comprehensive review.
17. Contributor Addresses
Below is a list of other contributing authors in alphabetical order:
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Randy Bush
Internet Initiative Japan
5147 Crystal Springs
Bainbridge Island, Washington 98110
United States
Email: randy@psg.com
Robert Raszuk
Bloomberg LP
731 Lexington Ave
New York City, NY 10022
United States
Email: robert@raszuk.net
Eric C. Rosen
18. References
18.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<https://www.rfc-editor.org/info/rfc2784>.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, DOI 10.17487/RFC2890, September 2000,
<https://www.rfc-editor.org/info/rfc2890>.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.
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[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, DOI 10.17487/RFC3270, May 2002,
<https://www.rfc-editor.org/info/rfc3270>.
[RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
"Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
RFC 3931, DOI 10.17487/RFC3931, March 2005,
<https://www.rfc-editor.org/info/rfc3931>.
[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed.,
"Encapsulating MPLS in IP or Generic Routing Encapsulation
(GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
<https://www.rfc-editor.org/info/rfc4023>.
[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,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
2008, <https://www.rfc-editor.org/info/rfc5129>.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
2009, <https://www.rfc-editor.org/info/rfc5462>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153,
RFC 6890, DOI 10.17487/RFC6890, April 2013,
<https://www.rfc-editor.org/info/rfc6890>.
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[RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
eXtensible Local Area Network (VXLAN): A Framework for
Overlaying Virtualized Layer 2 Networks over Layer 3
Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
<https://www.rfc-editor.org/info/rfc7348>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[RFC7637] Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
Virtualization Using Generic Routing Encapsulation",
RFC 7637, DOI 10.17487/RFC7637, September 2015,
<https://www.rfc-editor.org/info/rfc7637>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8669] Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah,
A., and H. Gredler, "Segment Routing Prefix Segment
Identifier Extensions for BGP", RFC 8669,
DOI 10.17487/RFC8669, December 2019,
<https://www.rfc-editor.org/info/rfc8669>.
18.2. Informative References
[Ethertypes]
"IANA Ethertype Registry",
<http://www.iana.org/assignments/ieee-802-numbers/ieee-
802-numbers.xhtml>.
[I-D.ietf-bess-evpn-inter-subnet-forwarding]
Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
Rabadan, "Integrated Routing and Bridging in EVPN", draft-
ietf-bess-evpn-inter-subnet-forwarding-11 (work in
progress), October 2020.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
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[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[RFC5512] Mohapatra, P. and E. Rosen, "The BGP Encapsulation
Subsequent Address Family Identifier (SAFI) and the BGP
Tunnel Encapsulation Attribute", RFC 5512,
DOI 10.17487/RFC5512, April 2009,
<https://www.rfc-editor.org/info/rfc5512>.
[RFC5565] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, DOI 10.17487/RFC5565, June 2009,
<https://www.rfc-editor.org/info/rfc5565>.
[RFC5566] Berger, L., White, R., and E. Rosen, "BGP IPsec Tunnel
Encapsulation Attribute", RFC 5566, DOI 10.17487/RFC5566,
June 2009, <https://www.rfc-editor.org/info/rfc5566>.
[RFC5640] Filsfils, C., Mohapatra, P., and C. Pignataro, "Load-
Balancing for Mesh Softwires", RFC 5640,
DOI 10.17487/RFC5640, August 2009,
<https://www.rfc-editor.org/info/rfc5640>.
[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
<https://www.rfc-editor.org/info/rfc6514>.
[RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
"Encapsulating MPLS in UDP", RFC 7510,
DOI 10.17487/RFC7510, April 2015,
<https://www.rfc-editor.org/info/rfc7510>.
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.
[RFC8365] Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
Uttaro, J., and W. Henderickx, "A Network Virtualization
Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
DOI 10.17487/RFC8365, March 2018,
<https://www.rfc-editor.org/info/rfc8365>.
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[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
Appendix A. Impact on RFC 8365
[RFC8365] references RFC 5512 for its definition of the BGP
Encapsulation Extended Community. That extended community is now
defined in this document, in a way consistent with its previous
definition.
RFC 8365 talks in Section 6 about the use of the Encapsulation
Extended Community to allow Network Virtualization Edge devices
(NVEs) to signal their supported encapsulations. We note that with
the introduction of this specification, the Tunnel Encapsulation
Attribute can also be used for this purpose. For purposes where RFC
8365 talks about "advertising supported encapsulations" (for example,
in the second paragraph of Section 6), encapsulations advertised
using the Tunnel Encapsulation Attribute should be considered equally
with those advertised using the Encapsulation Extended Community.
In particular, a review of Section 8.3.1 of RFC 8365 is called for,
to consider whether the introduction of the Tunnel Encapsulation
Attribute creates a need for any revisions to the split horizon
procedures.
RFC 8365 also refers to a draft version of this specification in the
final paragraph of section 5.1.3. That paragraph references
Section 8.2.2.2 of the draft. In this version of the document the
correct reference would be Section 9.2.2.2. There are no substantive
differences between the section in the referenced draft, and that in
this document.
Authors' Addresses
Keyur Patel
Arrcus, Inc
2077 Gateway Pl
San Jose, CA 95110
United States
Email: keyur@arrcus.com
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Gunter Van de Velde
Nokia
Copernicuslaan 50
Antwerpen 2018
Belgium
Email: gunter.van_de_velde@nokia.com
Srihari R. Sangli
Juniper Networks
Email: ssangli@juniper.net
John Scudder
Juniper Networks
Email: jgs@juniper.net
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