BGP Prefix-SID: Segment Routing Prefix Segment Identifier Extensions for BGP
A BGP attribute that hands out a numbered label index for each prefix, so Segment Routing can run over BGP in MPLS networks.
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>.
@misc{rfc8669,
series = {Request for Comments},
number = 8669,
howpublished = {RFC 8669},
publisher = {RFC Editor},
doi = {10.17487/RFC8669},
url = {https://www.rfc-editor.org/info/rfc8669},
author = {S. Previdi and C. Filsfils and A. Lindem and A. Sreekantiah and H. Gredler},
title = {{Segment Routing Prefix Segment Identifier Extensions for BGP}},
pagetotal = 15,
year = 2019,
month = dec,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Previdi
Request for Comments: 8669 Huawei Technologies
Category: Standards Track C. Filsfils
ISSN: 2070-1721 A. Lindem, Ed.
Cisco Systems
A. Sreekantiah
H. Gredler
RtBrick Inc.
December 2019
Segment Routing Prefix Segment Identifier Extensions for BGP
Abstract
Segment Routing (SR) leverages the source-routing paradigm. A node
steers a packet through an ordered list of instructions called
"segments". A segment can represent any instruction, topological or
service based. The ingress node prepends an SR header to a packet
containing a set of segment identifiers (SIDs). Each SID represents
a topological or service-based instruction. Per-flow state is
maintained only on the ingress node of the SR domain. An "SR domain"
is defined as a single administrative domain for global SID
assignment.
This document defines an optional, transitive BGP attribute for
announcing information about BGP Prefix Segment Identifiers (BGP
Prefix-SIDs) and the specification for SR-MPLS SIDs.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8669.
Copyright Notice
Copyright (c) 2019 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. MPLS BGP Prefix-SID
3. BGP Prefix-SID Attribute
3.1. Label-Index TLV
3.2. Originator SRGB TLV
4. Receiving BGP Prefix-SID Attribute
4.1. MPLS Data Plane: Labeled Unicast
5. Advertising BGP Prefix-SID Attribute
5.1. MPLS Data Plane: Labeled Unicast
6. Error Handling of BGP Prefix-SID Attribute
7. IANA Considerations
8. Manageability Considerations
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
The Segment Routing (SR) architecture leverages the source-routing
paradigm. A segment represents either a topological instruction,
such as "go to prefix P following shortest path", or a service
instruction. Other types of segments may be defined in the future.
A segment is identified through a Segment Identifier (SID). An "SR
domain" is defined as a single administrative domain for global SID
assignment. It may be comprised of a single Autonomous System (AS)
or multiple ASes under consolidated global SID administration.
Typically, the ingress node of the SR domain prepends an SR header
containing SIDs to an incoming packet.
As described in [RFC8402], when SR is applied to the MPLS data plane
([RFC8660]), the SID consists of a label.
[RFC8402] also describes how Segment Routing can be applied to an
IPv6 data plane (SRv6) using an IPv6 routing header containing a
stack of SR SIDs encoded as IPv6 addresses [IPv6-SRH]. The
applicability and support for Segment Routing over IPv6 is beyond the
scope of this document.
A BGP Prefix Segment is a BGP prefix with a Prefix-SID attached. A
BGP Prefix-SID is always a global SID ([RFC8402]) within the SR
domain and identifies an instruction to forward the packet over the
Equal-Cost Multipath (ECMP) best path computed by BGP to the related
prefix. The BGP Prefix-SID is the identifier of the BGP Prefix
Segment. In this document, we always refer to the BGP Prefix Segment
by the BGP Prefix-SID.
This document describes the BGP extensions to signal the BGP Prefix-
SID. Specifically, this document defines a BGP attribute known as
the "BGP Prefix-SID attribute" and specifies the rules to originate,
receive, and handle error conditions for the attribute.
The BGP Prefix-SID attribute defined in this document can be attached
to prefixes from Multiprotocol BGP IPv4/IPv6 Labeled Unicast
([RFC4760] [RFC8277]). Usage of the BGP Prefix-SID attribute for
other Address Family Identifier (AFI) / Subsequent Address Family
Identifier (SAFI) combinations is not defined herein but may be
specified in future specifications.
[RFC8670] describes example use cases where the BGP Prefix-SID is
used for the above AFI/SAFI combinations.
It should be noted that:
* A BGP Prefix-SID will be global across ASes when the
interconnected ASes are part of the same SR domain.
Alternatively, when interconnecting ASes, the ASBRs of each domain
will have to handle the advertisement of unique SIDs. The
mechanisms for such interconnection are outside the scope of the
protocol extensions defined in this document.
* A BGP Prefix-SID MAY be attached to a BGP prefix. This implies
that each prefix is advertised individually, reducing the ability
to pack BGP advertisements (when sharing common attributes).
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
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. MPLS BGP Prefix-SID
The BGP Prefix-SID is realized on the MPLS data plane ([RFC8660]) in
the following way:
The operator assigns a globally unique label index, L_I, to a
locally originated prefix of a BGP speaker N, which is advertised
to all other BGP speakers in the SR domain.
According to [RFC8402], each BGP speaker is configured with a
label block called the Segment Routing Global Block (SRGB). While
[RFC8402] recommends using the same SRGB across all the nodes
within the SR domain, the SRGB of a node is a local property and
could be different on different speakers. The drawbacks of the
use case where BGP speakers have different SRGBs are documented in
[RFC8402] and [RFC8670].
If traffic engineering within the SR domain is required, each node
may also be required to advertise topological information and Peer
SIDs for each of its links and peers. This information is
required to perform the explicit path computation and to express
an explicit path as a list of SIDs. The advertisement of
topological information and peer segments (Peer SIDs) is done
through [BGPLS-SR-EPE].
If a prefix segment is to be included in an MPLS label stack,
e.g., for traffic-engineering purposes, knowledge of the prefix
originator's SRGB is required in order to compute the local label
used by the originator.
This document assumes that Border Gateway Protocol - Link State
(BGP-LS) is the preferred method for a collecting both peer
segments (Peer SIDs) and SRGB information through [RFC7752],
[BGPLS-SR-EPE], and [BGPLS-SR-EXT]. However, as an optional
alternative for the advertisement of the local SRGB without the
topology or the peer SIDs and, therefore, without applicability
for TE, the Originator SRGB TLV of the BGP Prefix-SID attribute is
specified in Section 3.2 of this document.
A BGP speaker will derive its local MPLS label L from the label
index L_I and its local SRGB as described in [RFC8660]. The BGP
speaker then programs the MPLS label L in its MPLS data plane as
its incoming/local label for the prefix. See Section 4.1 for more
details.
The outgoing label for the prefix is found in the Network Layer
Reachability Information (NLRI) of the Multiprotocol BGP IPv4/IPv6
Labeled Unicast prefix advertisement as defined in [RFC8277]. The
label index L_I is only used as a hint to derive the local/
incoming label.
Section 3.1 of this document specifies the Label-Index TLV of the
BGP Prefix-SID attribute; this TLV can be used to advertise the
label index for a given prefix.
3. BGP Prefix-SID Attribute
The BGP Prefix-SID attribute is an optional, transitive BGP path
attribute. The attribute type code 40 has been assigned by IANA (see
Section 7).
The BGP Prefix-SID attribute is defined here to be a set of elements
encoded as "Type/Length/Value" tuples (i.e., a set of TLVs). All BGP
Prefix-SID attribute TLVs will start with a 1-octet type and a
2-octet length. The following TLVs are defined in this document:
* Label-Index TLV
* Originator SRGB TLV
The Label-Index and Originator SRGB TLVs are used only when SR is
applied to the MPLS data plane.
For future extensibility, unknown TLVs MUST be ignored and propagated
unmodified.
3.1. Label-Index TLV
The Label-Index TLV MUST be present in the BGP Prefix-SID attribute
attached to IPv4/IPv6 Labeled Unicast prefixes ([RFC8277]). It MUST
be ignored when received for other BGP AFI/SAFI combinations. The
Label-Index TLV 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Label Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: 7, the total length in octets of the value portion of the
TLV.
RESERVED: 8-bit field. It MUST be clear on transmission and MUST
be ignored on reception.
Flags: 16 bits of flags. None are defined by this document. The
Flags field MUST be clear on transmission and MUST be ignored
on reception.
Label Index: 32-bit value representing the index value in the
SRGB space.
3.2. Originator SRGB TLV
The Originator SRGB TLV is an optional TLV and 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRGB 1 (6 octets) |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRGB n (6 octets) |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3
Length: The total length in octets of the value portion of the
TLV: 2 + (non-zero multiple of 6).
Flags: 16 bits of flags. None are defined in this document. The
Flags field MUST be clear on transmission and MUST be ignored
on reception.
SRGB: 3 octets specifying the first label in the range followed
by 3 octets specifying the number of labels in the range. Note
that the SRGB field MAY appear multiple times. If the SRGB
field appears multiple times, the SRGB consists of multiple
ranges that are concatenated.
The Originator SRGB TLV contains the SRGB of the node originating the
prefix to which the BGP Prefix-SID is attached. The Originator SRGB
TLV MUST NOT be changed during the propagation of the BGP update. It
is used to build SR policies when different SRGBs are used in the
fabric, for example, [RFC8670].
Examples of how the receiving routers concatenate the ranges and
build their neighbor's Segment Routing Global Block (SRGB) are
included in [RFC8660].
The Originator SRGB TLV may only appear in a BGP Prefix-SID attribute
attached to IPv4/IPv6 Labeled Unicast prefixes ([RFC8277]). It MUST
be ignored when received for other BGP AFI/SAFI combinations. Since
the Label-Index TLV is required for IPv4/IPv6 prefix applicability,
the Originator SRGB TLV will be ignored if it is not specified in a
manner consistent with Section 6.
If a BGP speaker receives a node's SRGB as an attribute of the BGP-LS
Node NLRI and the BGP speaker also receives the same node's SRGB in a
BGP Prefix-SID attribute, then the received values should be the
same. If the values are different, the values advertised in the BGP-
LS NLRI SHOULD be preferred, and an error should be logged.
4. Receiving BGP Prefix-SID Attribute
A BGP speaker receiving a BGP Prefix-SID attribute from an External
BGP (EBGP) neighbor residing outside the boundaries of the SR domain
MUST discard the attribute unless it is configured to accept the
attribute from the EBGP neighbor. A BGP speaker SHOULD log an error
for further analysis when discarding an attribute.
4.1. MPLS Data Plane: Labeled Unicast
A BGP session supporting the Multiprotocol BGP Labeled IPv4 or IPv6
Unicast ([RFC8277]) AFI/SAFI is required.
When the BGP Prefix-SID attribute is attached to a BGP Labeled IPv4
or IPv6 Unicast [RFC8277] AFI/SAFI, it MUST contain the Label-Index
TLV and MAY contain the Originator SRGB TLV. A BGP Prefix-SID
attribute received without a Label-Index TLV MUST be considered to be
"invalid" by the receiving speaker.
The label index provides guidance to the receiving BGP speaker as to
the incoming label that SHOULD be allocated to the prefix.
A BGP speaker may be locally configured with an SRGB=[SRGB_Start,
SRGB_End]. The preferred method for deriving the SRGB is a matter of
local node configuration.
The mechanisms through which a given label-index value is assigned to
a given prefix are outside the scope of this document.
Given a label index L_I, we refer to (L = L_I + SRGB_Start) as the
derived label. A BGP Prefix-SID attribute is designated
"conflicting" for a speaker M if the derived label value L lies
outside the SRGB configured on M. Otherwise, the Label-Index TLV is
designated "acceptable" to speaker M.
If multiple different prefixes are received with the same label
index, all of the different prefixes MUST have their BGP Prefix-SID
attribute considered to be "conflicting".
If multiple valid paths for the same prefix are received from
multiple BGP speakers or, in the case of [RFC7911], from the same BGP
speaker, and the BGP Prefix-SID attributes do not contain the same
label index, then the label index from the best path BGP Prefix-SID
attribute SHOULD be chosen with a notable exception being when
[RFC5004] is being used to dampen route changes.
When a BGP speaker receives a path from a neighbor with an
"acceptable" BGP Prefix-SID attribute and that path is selected as
the best path, it SHOULD program the derived label as the label for
the prefix in its local MPLS data plane.
When a BGP speaker receives a path from a neighbor with an "invalid"
or "conflicting" BGP Prefix-SID attribute, or when a BGP speaker
receives a path from a neighbor with a BGP Prefix-SID attribute but
is unable to process it (e.g., local policy disables the
functionality), it MUST ignore the BGP Prefix-SID attribute. For the
purposes of label allocation, a BGP speaker MUST assign a local (also
called dynamic) label (non-SRGB) for such a prefix as per classic
Multiprotocol BGP IPv4/IPv6 Labeled Unicast ([RFC8277]) operation.
In the case of an "invalid" BGP Prefix-SID attribute, a BGP speaker
MUST follow the error-handling rules specified in Section 6. A BGP
speaker SHOULD log an error for further analysis. In the case of a
"conflicting" BGP Prefix-SID attribute, a BGP speaker SHOULD NOT
treat it as an error and SHOULD propagate the attribute unchanged. A
BGP speaker SHOULD log a warning for further analysis, i.e., in the
case the conflict is not due to a label-index transition.
When a BGP Prefix-SID attribute changes and transitions from
"conflicting" to "acceptable", the BGP Prefix-SID attributes for
other prefixes may also transition to "acceptable" as well.
Implementations SHOULD ensure all impacted prefixes revert to using
the label indices corresponding to these newly "acceptable" BGP
Prefix-SID attributes.
The outgoing label is always programmed as per classic Multiprotocol
BGP IPv4/IPv6 Labeled Unicast ([RFC8277]) operation. Specifically, a
BGP speaker receiving a prefix with a BGP Prefix-SID attribute and a
label NLRI field of Implicit NULL [RFC3032] from a neighbor MUST
adhere to standard behavior and program its MPLS data plane to pop
the top label when forwarding traffic to the prefix. The label NLRI
defines the outbound label that MUST be used by the receiving node.
5. Advertising BGP Prefix-SID Attribute
The BGP Prefix-SID attribute MAY be attached to BGP IPv4/IPv6 Labeled
Unicast prefixes [RFC8277]. In order to prevent distribution of the
BGP Prefix-SID attribute beyond its intended scope of applicability,
attribute filtering SHOULD be deployed to remove the BGP Prefix-SID
attribute at the administrative boundary of the SR domain.
A BGP speaker that advertises a path received from one of its
neighbors SHOULD advertise the BGP Prefix-SID received with the path
without modification as long as the BGP Prefix-SID was acceptable.
If the path did not come with a BGP Prefix-SID attribute, the speaker
MAY attach a BGP Prefix-SID to the path if configured to do so. The
content of the TLVs present in the BGP Prefix-SID is determined by
the configuration.
5.1. MPLS Data Plane: Labeled Unicast
A BGP speaker that originates a prefix attaches the BGP Prefix-SID
attribute when it advertises the prefix to its neighbors via
Multiprotocol BGP IPv4/IPv6 Labeled Unicast ([RFC8277]). The value
of the label index in the Label-Index TLV is determined by
configuration.
A BGP speaker that originates a BGP Prefix-SID attribute MAY
optionally announce the Originator SRGB TLV along with the mandatory
Label-Index TLV. The content of the Originator SRGB TLV is
determined by configuration.
Since the label-index value must be unique within an SR domain, by
default an implementation SHOULD NOT advertise the BGP Prefix-SID
attribute outside an AS unless it is explicitly configured to do so.
In all cases, the Label field of the advertised NLRI ([RFC8277]
[RFC4364]) MUST be set to the local/incoming label programmed in the
MPLS data plane for the given advertised prefix. If the prefix is
associated with one of the BGP speaker's interfaces, this is the
usual MPLS label (such as the Implicit or Explicit NULL label
[RFC3032]).
6. Error Handling of BGP Prefix-SID Attribute
When a BGP speaker receives a BGP UPDATE message containing a
malformed or invalid BGP Prefix-SID attribute attached to an IPv4/
IPv6 Labeled Unicast prefix ([RFC8277]), it MUST ignore the received
BGP Prefix-SID attribute and not advertise it to other BGP peers. In
this context, a malformed BGP Prefix-SID attribute is one that cannot
be parsed due to not meeting the minimum attribute length
requirement, containing a TLV length that doesn't conform to the
length constraints for the TLV, or containing a TLV length that would
extend beyond the end of the attribute (as defined by the attribute
length). This is equivalent to the "Attribute discard" action
specified in [RFC7606]. When discarding an attribute, a BGP speaker
SHOULD log an error for further analysis.
As per [RFC7606], if the BGP Prefix-SID attribute appears more than
once in an UPDATE message, all the occurrences of the attribute other
than the first one SHALL be discarded and the UPDATE message will
continue to be processed. Similarly, if a recognized TLV appears
more than once in a BGP Prefix-SID attribute while the specification
only allows for a single occurrence, then all the occurrences of the
TLV other than the first one SHALL be discarded and the Prefix-SID
attribute will continue to be processed.
For future extensibility, unknown TLVs MUST be ignored and propagated
unmodified.
7. IANA Considerations
This document defines a BGP path attribute known as the BGP Prefix-
SID attribute. IANA has assigned attribute code type 40 to the BGP
Prefix-SID attribute from the "BGP Path Attributes" registry.
This document defines two TLVs for the BGP Prefix-SID attribute.
These TLVs have been registered with IANA. IANA has created a
registry for BGP Prefix-SID Attribute TLVs as follows:
Under the "Border Gateway Protocol (BGP) Parameters" registry, the
new registry titled "BGP Prefix-SID TLV Types" has been created and
points to this document as the reference.
Registration Procedure(s):
Values 1-254, Expert Review as defined in [RFC8126]
Values 0 and 255, Reserved
+-------+-----------------+---------------+
| Value | Type | Reference |
+=======+=================+===============+
| 0 | Reserved | This document |
+-------+-----------------+---------------+
| 1 | Label-Index | This document |
+-------+-----------------+---------------+
| 2 | Deprecated | This document |
+-------+-----------------+---------------+
| 3 | Originator SRGB | This document |
+-------+-----------------+---------------+
| 4-254 | Unassigned | |
+-------+-----------------+---------------+
| 255 | Reserved | This document |
+-------+-----------------+---------------+
Table 1: BGP Prefix-SID TLV Types
The value 2 previously corresponded to the IPv6 SID TLV, which was
specified in previous versions of this document. It was removed, and
use of the BGP Prefix-SID for Segment Routing over the IPv6 data
plane [RFC8402] has been deferred to future specifications.
IANA has also created the "BGP Prefix-SID Label-Index TLV Flags"
registry under the "Border Gateway Protocol (BGP) Parameters"
registry, with a reference to this document. Initially, this 16-bit
flags registry is empty. The registration policy for flag bits is
Expert Review [RFC8126], consistent with the "BGP Prefix-SID TLV
Types" registry.
Finally, IANA has created the "BGP Prefix-SID Originator SRGB TLV
Flags" registry under the "Border Gateway Protocol (BGP) Parameters"
registry, with a reference to this document. Initially, this 16-bit
flags registry is empty. The registration policy for flag bits is
Expert Review [RFC8126] consistent with the BGP Prefix-SID TLV Types
registry.
The designated experts must be good and faithful stewards of the
above registries, ensuring that each request is legitimate and
corresponds to a viable use case. Given the limited number of bits
in the flags registries and the applicability to a single TLV,
additional scrutiny should be afforded to requests for flag-bit
allocation. In general, no single use case should require more than
one flag bit and, should the use case require more, alternate
encodings using new TLVs should be considered.
8. Manageability Considerations
This document defines a BGP attribute to address use cases such as
the one described in [RFC8670]. It is assumed that advertisement of
the BGP Prefix-SID attribute is controlled by the operator in order
to:
* Prevent undesired origination/advertisement of the BGP Prefix-SID
attribute. By default, a BGP Prefix-SID attribute SHOULD NOT be
attached to a prefix and advertised. Hence, BGP Prefix-SID
Advertisement SHOULD require explicit enablement.
* Prevent any undesired propagation of the BGP Prefix-SID attribute.
By default, the BGP Prefix-SID is not advertised outside the
boundary of a single SR/administrative domain that may include one
or more ASes. The propagation to other ASes MUST be explicitly
configured.
The deployment model described in [RFC8670] assumes multiple ASes
under a common administrative domain. For this use case, the BGP
Prefix-SID Advertisement is applicable to the inter-AS context, i.e.,
EBGP, while it is confined to a single administrative domain.
9. Security Considerations
This document introduces a BGP attribute (BGP Prefix-SID), which
inherits the security considerations expressed in: [RFC4271],
[RFC8277], and [RFC8402].
When advertised using BGPsec as described in [RFC8205], the BGP
Prefix-SID attribute doesn't impose any unique security
considerations. It should be noted that the BGP Prefix-SID attribute
is not protected by the BGPsec signatures.
It should be noted that, as described in Section 8, this document
refers to a deployment model where all nodes are under the single
administrative domain. In this context, we assume that the operator
doesn't want to leak any information related to internal prefixes and
topology outside of the administrative domain. The internal
information includes the BGP Prefix-SID. In order to prevent such
leaking, the common BGP mechanisms (filters) are applied at the
boundary of the SR/administrative domain. Local BGP-attribute-
filtering policies and mechanisms are not standardized and,
consequently, are beyond the scope of this document.
To prevent a Denial-of-Service (DoS) or Distributed-Denial-of-Service
(DDoS) attack due to excessive BGP updates with an invalid or
conflicting BGP Prefix-SID attribute, error log message rate limiting
as well as suppression of duplicate error log messages SHOULD be
deployed.
Since BGP-LS is the preferred method for advertising SRGB
information, the BGP speaker SHOULD log an error if a BGP Prefix-SID
attribute is received with SRGB information different from that
received as an attribute of the same node's BGP-LS Node NLRI.
10. References
10.1. 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>.
[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>.
[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>.
[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>.
[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>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[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>.
[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>.
[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>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
10.2. Informative References
[BGPLS-SR-EPE]
Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
S., and J. Dong, "BGP-LS extensions for Segment Routing
BGP Egress Peer Engineering", Work in Progress, Internet-
Draft, draft-ietf-idr-bgpls-segment-routing-epe-19, 16 May
2019, <https://tools.ietf.org/html/draft-ietf-idr-bgpls-
segment-routing-epe-19>.
[BGPLS-SR-EXT]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", Work in Progress, Internet-Draft, draft-ietf-
idr-bgp-ls-segment-routing-ext-16, 27 June 2019,
<https://tools.ietf.org/html/draft-ietf-idr-bgp-ls-
segment-routing-ext-16>.
[IPv6-SRH] Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", Work in Progress, Internet-Draft, draft-ietf-6man-
segment-routing-header-26, 22 October 2019,
<https://tools.ietf.org/html/draft-ietf-6man-segment-
routing-header-26>.
[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>.
[RFC5004] Chen, E. and S. Sangli, "Avoid BGP Best Path Transitions
from One External to Another", RFC 5004,
DOI 10.17487/RFC5004, September 2007,
<https://www.rfc-editor.org/info/rfc5004>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[RFC8670] Filsfils, C., Ed., Previdi, S., Dawra, G., Aries, E., and
P. Lapukhov, "BGP Prefix Segment in Large-Scale Data
Centers", RFC 8670, DOI 10.17487/RFC8670, December 2019,
<https://www.rfc-editor.org/info/rfc8670>.
Acknowledgements
The authors would like to thank Satya Mohanty for his contribution to
this document.
The authors would like to thank Alvaro Retana for substantive
comments as part of the Routing AD review.
The authors would like to thank Bruno Decraene for substantive
comments and suggested text as part of the Routing Directorate
review.
The authors would like to thank Shyam Sethuram for comments and
discussion of TLV processing and validation.
The authors would like to thank Robert Raszuk for comments and
suggestions regarding the MPLS data-plane behavior.
The authors would like to thank Krishna Deevi, Juan Alcaide, Howard
Yang, and Jakob Heitz for discussions on conflicting BGP Prefix-SID
label indices and BGP add paths.
The authors would like to thank Peter Yee, Tony Przygienda, Mirja
Kuhlewind, Alexey Melnikov, Eric Rescorla, Suresh Krishnan, Warren
Kumari, Ben Campbell Sue Hares, and Martin Vigoureux for IDR Working
Group last call, IETF Last Call, directorate, and IESG reviews.
Contributors
Keyur Patel
Arrcus, Inc.
United States of America
Email: Keyur@arrcus.com
Saikat Ray
Unaffiliated
United States of America
Email: raysaikat@gmail.com
Authors' Addresses
Stefano Previdi
Huawei Technologies
Italy
Email: stefano@previdi.net
Clarence Filsfils
Cisco Systems
Brussels
Belgium
Email: cfilsfil@cisco.com
Acee Lindem (editor)
Cisco Systems
301 Midenhall Way
Cary, NC, 27513
United States of America
Email: acee@cisco.com
Arjun Sreekantiah
Email: arjunhrs@gmail.com
Hannes Gredler
RtBrick Inc.
Email: hannes@rtbrick.com
ISO Plain Language
On this page
§1What a BGP Prefix-SID Is
When Segment Routing is applied to the MPLS data plane, the segment identifier consists of a label. Segment Routing can also be applied to an IPv6 data plane, using an IPv6 routing header containing a stack of segment identifiers encoded as IPv6 addresses, but the applicability of Segment Routing over IPv6 is beyond the scope of this document.
A BGP Prefix Segment is a BGP prefix with a Prefix-SID attached. A BGP Prefix-SID is always a global segment identifier within the Segment Routing domain, and it identifies an instruction to forward the packet over the Equal-Cost Multipath best path computed by BGP to the related prefix. The BGP Prefix-SID is the identifier of the BGP Prefix Segment, and the document always refers to the segment by that identifier.
This document defines the BGP extensions that signal the BGP Prefix-SID: a BGP attribute known as the BGP Prefix-SID attribute, and the rules to originate, receive and handle error conditions for it. The attribute can be attached to prefixes from Multiprotocol BGP IPv4 and IPv6 Labeled Unicast. Its use with other address family combinations is not defined here.
Two points are worth noting. A BGP Prefix-SID will be global across autonomous systems when the interconnected systems are part of the same Segment Routing domain; otherwise the border routers of each domain have to handle the advertisement of unique identifiers, and the mechanisms for that interconnection are outside the scope of the document. And a BGP Prefix-SID may be attached to a BGP prefix, which implies each prefix is advertised individually, reducing the ability to pack BGP advertisements with shared attributes.
A segment identifier tells the network what instruction to apply to a packet. In an MPLS network that instruction is a label. This RFC attaches such an identifier to a prefix carried in BGP, so that networks whose prefixes live in BGP can also build paths out of instructions.
§2From a Label Index to a Label
The operator assigns a globally unique label index to a locally originated prefix of a BGP speaker, and advertises it to the other BGP speakers in the Segment Routing domain. Each BGP speaker is configured with a label block called the Segment Routing Global Block. While the Segment Routing architecture recommends using the same block across all nodes in the domain, the block of a node is a local property and could differ between speakers.
If traffic engineering within the domain is required, each node may also have to advertise topological information and Peer SIDs for its links and peers, which is done through the BGP Link State extensions.
Where a prefix segment is to be included in an MPLS label stack, for example for traffic engineering, the originator's block is needed in order to compute the local label used by the originator. The document assumes that BGP-LS is the preferred method for collecting peer segments and block information, but as an optional alternative for advertising the local block alone, without topology and without applicability to traffic engineering, it defines the Originator SRGB TLV.
A BGP speaker derives its local MPLS label L from the label index L_I and its local block, and then programs that label in its MPLS data plane as the incoming, or local, label for the prefix. The outgoing label is found in the NLRI of the Multiprotocol BGP IPv4 or IPv6 Labeled Unicast prefix advertisement; the label index is only used as a hint to derive the local label.
Each router keeps a private range of labels, its block. The number that travels in BGP is an offset into that range, not a label in itself, so the same number can be used by different routers for different things. A receiving router adds the offset to the start of its own block and programs the result as the label for that prefix.
§3The Attribute and Its Two TLVs
The BGP Prefix-SID attribute is an optional, transitive BGP path attribute with the type code 40 assigned by IANA. It is defined as a set of elements encoded as Type, Length, Value tuples. Every TLV starts with a 1-octet type and a 2-octet length. Two TLVs are defined: the Label-Index TLV and the Originator SRGB TLV, both used only when Segment Routing is applied to the MPLS data plane. For future extensibility, unknown TLVs must be ignored and propagated unmodified.
The Label-Index TLV must be present in the attribute attached to IPv4 or IPv6 Labeled Unicast prefixes, and must be ignored when received for other address family combinations. It has a type of 1 and a length of 7 octets. The value holds an 8-bit reserved field, which must be clear on transmission and ignored on reception; a 16-bit flags field, none of which are defined by this document and which must also be clear on transmission and ignored on reception; and a 32-bit label index, the index value in the block space.
The Originator SRGB TLV is optional, with a type of 3. Its length is 2 plus a non-zero multiple of 6 octets. It holds a 16-bit flags field, clear on transmission and ignored on reception, and then one or more ranges, each made of 3 octets giving the first label in the range and 3 octets giving the number of labels in it. The field may appear several times, in which case the block consists of the concatenated ranges.
The Originator SRGB TLV carries the block of the node that originated the prefix, and it must not be changed during the propagation of the update. It is used to build SR policies where different blocks are in use. It may only appear on IPv4 or IPv6 Labeled Unicast prefixes, and is ignored elsewhere.
IANA has created the BGP Prefix-SID TLV Types registry: 0 and 255 are reserved, 1 is Label-Index, 2 is deprecated, 3 is Originator SRGB, and 4 to 254 are unassigned, with Expert Review for new values. The value 2 previously corresponded to the IPv6 SID TLV in earlier versions of the document; it was removed and use of the attribute for Segment Routing over IPv6 is deferred to future specifications. Two empty 16-bit flags registries were also created, one for the Label-Index TLV and one for the Originator SRGB TLV.
The attribute is a small container holding two optional pieces of information: the offset for the prefix, which is mandatory, and the block the originator used, which is optional. Both are written as tagged blocks so that future versions can add more without breaking older readers.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | TLV type, the value 1 for Label-Index. | |
LENGTH | Length | 2 bytes | Length of the value portion, 7 octets. | |
RESERVED | RESERVED | 1 byte | Must be clear on transmission and ignored on reception. | |
FLAGS | Flags | 2 bytes | No flags are defined by this document; clear on transmission. | |
LABEL-INDEX | Label Index | 4 bytes | The index value in the Segment Routing Global Block space. |
- 0Reserved
- 1Label-Index
- 2Deprecated
- 3Originator SRGB
- 4-254Unassigned
- 255Reserved
§4Receiving the Attribute
A BGP speaker receiving the attribute from an EBGP neighbour outside the boundaries of the Segment Routing domain must discard it unless it is configured to accept it from that neighbour, and should log an error when discarding it.
A session supporting the Multiprotocol BGP Labeled IPv4 or IPv6 Unicast address family is required. When the attribute is attached to such a prefix it must contain the Label-Index TLV and may contain the Originator SRGB TLV; an attribute received without a Label-Index TLV must be considered invalid.
The label index gives the receiving speaker guidance about the incoming label to allocate for the prefix. Given a label index L_I, the derived label is L_I plus SRGB_Start. The attribute is conflicting for a speaker M when the derived label lies outside the block configured on M, and otherwise acceptable. If several different prefixes are received with the same label index, all of them must be considered conflicting.
If several valid paths for the same prefix are received from multiple speakers, or from the same speaker where multiple paths are advertised, and the attributes do not contain the same label index, then the label index from the best path should be chosen, with the exception of route dampening. When a speaker receives a path with an acceptable attribute and selects it as the best path, it should program the derived label as the label for the prefix in its local MPLS data plane.
When a speaker receives a path with an invalid or conflicting attribute, or one it cannot process, it must ignore the attribute and assign a local, dynamic label that is not in the block, as in ordinary labelled unicast operation. An invalid attribute must follow the error handling rules, and an error should be logged. A conflicting attribute should not be treated as an error and should be propagated unchanged, with a warning logged for analysis. When an attribute changes from conflicting to acceptable, other prefixes may also become acceptable, and implementations should ensure that the affected prefixes revert to the newly acceptable label indices.
On receipt, the receiver checks the offset against its own range. If the resulting label is inside the range, the receiver installs it. If it falls outside, or if two prefixes claim the same offset, the receiver treats the attribute as unusable and falls back to a local label of its own choosing. The attribute is not an error, it simply cannot be trusted for that prefix.
| State | What causes it | What the receiver does |
|---|---|---|
| Acceptable | The derived label lies inside the receiver's own block | Programs the derived label for the prefix |
| Conflicting | The derived label lies outside the block, or two prefixes claim the same index | Ignores the attribute and allocates a local label |
| Invalid | No Label-Index TLV, or the attribute is malformed | Ignores the attribute and follows the error handling rules |
§5Advertising the Attribute
The attribute may be attached to BGP IPv4 or IPv6 Labeled Unicast prefixes. To prevent distribution of the attribute beyond its intended scope, attribute filtering should be deployed to remove it at the administrative boundary of the Segment Routing domain.
A speaker that advertises a path received from one of its neighbours should advertise the BGP Prefix-SID it received with that path, without modification, as long as the attribute was acceptable. If the path did not come with an attribute, the speaker may attach one if it is configured to do so, and the content of the TLVs is then determined by configuration.
A speaker that originates a prefix attaches the attribute when it advertises the prefix to its neighbours via Multiprotocol BGP IPv4 or IPv6 Labeled Unicast, and the value of the label index is determined by configuration. It may optionally announce the Originator SRGB TLV along with the mandatory Label-Index TLV.
Since the label index must be unique within the Segment Routing domain, by default an implementation should not advertise the attribute outside an autonomous system unless it is explicitly configured to do so. In all cases the label field of the advertised NLRI must be set to the local or incoming label programmed in the MPLS data plane for the prefix. If the prefix is associated with one of the speaker's interfaces, this is the usual MPLS label, such as the Implicit or Explicit NULL label.
A router that originates a prefix decides which offset to use and puts it in the advertisement. A router that passes the advertisement on passes the attribute through unchanged, as long as it found it usable. By default the attribute stops at the edge of the operator's own network, so the numbering scheme inside is not leaked to other networks.
§6Error Handling
When a speaker receives an UPDATE containing a malformed or invalid BGP Prefix-SID attribute attached to an IPv4 or IPv6 Labeled Unicast prefix, it must ignore the attribute and not advertise it to other peers. A malformed attribute is one that cannot be parsed because it does not meet the minimum attribute length requirement, contains a TLV length that does not conform to the length constraints for the TLV, or contains a TLV length that would extend beyond the end of the attribute. This is equivalent to the attribute discard action of RFC 7606, and an error should be logged when an attribute is discarded.
As in RFC 7606, if the attribute appears more than once in an UPDATE message, all occurrences other than the first are discarded and the message continues to be processed. The same applies when a recognised TLV appears more than once inside the attribute but the specification allows only one occurrence: the later occurrences are discarded and the attribute continues to be processed. Unknown TLVs must be ignored and propagated unmodified.
If the attribute cannot be parsed, the receiver drops it and says so in its log, but it still processes the rest of the route update. The same is true of a repeated copy of the attribute or of a repeated copy of one of its parts: the first copy is used and the rest are dropped.
§8Manageability Considerations
The document assumes that advertisement of the attribute is controlled by the operator, for two reasons. First, to prevent undesired origination and advertisement: by default the attribute should not be attached to a prefix and advertised, so advertisement should require explicit enablement. Second, to prevent undesired propagation: by default the attribute is not advertised outside the boundary of a single Segment Routing or administrative domain, which may include one or more autonomous systems, and propagation to other autonomous systems must be explicitly configured.
The deployment model described in RFC 8670 assumes multiple autonomous systems under a common administrative domain. For that use case the advertisement is applicable in the inter-AS context, that is to EBGP, while it stays confined to a single administrative domain.
Both the decision to publish the attribute at all and the decision to let it cross the edge of your own network belong to the operator. The defaults are off, so nothing changes until someone turns it on deliberately.
- OriginationBy default no prefix carries the attribute; enabling it is an explicit choice.
- PropagationThe attribute stops at the boundary of the administrative domain unless it is configured otherwise.
- FilteringAttribute filtering at the boundary keeps the internal numbering scheme private.
§9Security Considerations
The attribute inherits the security considerations of RFC 4271, RFC 8277 and RFC 8402.
When advertised using BGPsec, as described in RFC 8205, the attribute imposes no unique security considerations, but it is not protected by the BGPsec signatures.
The deployment model assumes all nodes are under a single administrative domain, and it assumes the operator does not want to leak information about internal prefixes and topology outside that domain. The BGP Prefix-SID is part of that internal information, so the common BGP filtering mechanisms are applied at the boundary of the domain. Local attribute filtering policies are not standardized and are beyond the scope of the document.
To prevent a denial-of-service attack caused by excessive BGP updates carrying an invalid or conflicting attribute, rate limiting of error log messages and suppression of duplicate error messages should be deployed. Since BGP-LS is the preferred method for advertising block information, a speaker should log an error if it receives an attribute with block information different from the block it received for the same node in BGP-LS.
The attribute says something about the inside of your network, so it is filtered at the boundary by default. It is not covered by BGPsec signatures, which means a signed route can still carry an attribute that was tampered with, so the checks in the receiving rules matter.