Internet DRAFT - draft-ietf-idr-segment-routing-te-policy
draft-ietf-idr-segment-routing-te-policy
Network Working Group S. Previdi
Internet-Draft Huawei Technologies
Updates: 9012 (if approved) C. Filsfils
Intended status: Standards Track K. Talaulikar, Ed.
Expires: 25 April 2024 Cisco Systems
P. Mattes
Microsoft
D. Jain
Google
23 October 2023
Advertising Segment Routing Policies in BGP
draft-ietf-idr-segment-routing-te-policy-26
Abstract
This document introduces a BGP SAFI with two NLRIs to advertise a
candidate path of a Segment Routing (SR) Policy. An SR Policy is an
ordered list of segments (i.e., instructions) that represent a
source-routed policy. An SR Policy consists of one or more candidate
paths, each consisting of one or more segment lists. A headend may
be provisioned with candidate paths for an SR Policy via several
different mechanisms, e.g., CLI, NETCONF, PCEP, or BGP. This
document specifies how BGP may be used to distribute SR Policy
candidate paths. It defines sub-TLVs for the Tunnel Encapsulation
Attribute for signaling information about these candidate paths.
This documents updates RFC9012 with extensions to the Color Extended
Community to support additional steering modes over SR Policy.
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 25 April 2024.
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Copyright Notice
Copyright (c) 2023 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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. SR Policy Encoding . . . . . . . . . . . . . . . . . . . . . 5
2.1. SR Policy SAFI and NLRI . . . . . . . . . . . . . . . . . 6
2.2. SR Policy and Tunnel Encapsulation Attribute . . . . . . 8
2.3. Remote Endpoint and Color . . . . . . . . . . . . . . . . 9
2.4. SR Policy Sub-TLVs . . . . . . . . . . . . . . . . . . . 9
2.4.1. Preference Sub-TLV . . . . . . . . . . . . . . . . . 9
2.4.2. Binding SID Sub-TLV . . . . . . . . . . . . . . . . . 10
2.4.3. SRv6 Binding SID Sub-TLV . . . . . . . . . . . . . . 12
2.4.4. Segment List Sub-TLV . . . . . . . . . . . . . . . . 13
2.4.5. Explicit NULL Label Policy Sub-TLV . . . . . . . . . 19
2.4.6. Policy Priority Sub-TLV . . . . . . . . . . . . . . . 21
2.4.7. Policy Candidate Path Name Sub-TLV . . . . . . . . . 22
2.4.8. Policy Name Sub-TLV . . . . . . . . . . . . . . . . . 23
3. Color Extended Community . . . . . . . . . . . . . . . . . . 24
4. SR Policy Operations . . . . . . . . . . . . . . . . . . . . 25
4.1. Advertisement of SR Policies . . . . . . . . . . . . . . 25
4.2. Reception of an SR Policy NLRI . . . . . . . . . . . . . 26
4.2.1. Validation of an SR Policy NLRI . . . . . . . . . . . 26
4.2.2. Eligibility for Local Use of an SR Policy NLRI . . . 26
4.2.3. Propagation of an SR Policy . . . . . . . . . . . . . 27
5. Error Handling and Fault Management . . . . . . . . . . . . . 28
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
6.1. Existing Registry: Subsequent Address Family Identifiers
(SAFI) Parameters . . . . . . . . . . . . . . . . . . . . 30
6.2. Existing Registry: BGP Tunnel Encapsulation Attribute
Tunnel Types . . . . . . . . . . . . . . . . . . . . . . 30
6.3. Existing Registry: BGP Tunnel Encapsulation Attribute
sub-TLVs . . . . . . . . . . . . . . . . . . . . . . . . 30
6.4. Existing Registry: Color Extended Community Flags . . . . 31
6.5. New Registry: SR Policy Segment List Sub-TLVs . . . . . . 31
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6.6. New Registry: SR Policy Binding SID Flags . . . . . . . . 31
6.7. New Registry: SR Policy SRv6 Binding SID Flags . . . . . 32
6.8. New Registry: SR Policy Segment Flags . . . . . . . . . . 32
6.9. New Registry: Color Extended Community Color-Only
Types . . . . . . . . . . . . . . . . . . . . . . . . . . 33
7. Security Considerations . . . . . . . . . . . . . . . . . . . 33
8. Manageability Considerations . . . . . . . . . . . . . . . . 34
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 34
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
11.1. Normative References . . . . . . . . . . . . . . . . . . 35
11.2. Informational References . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
Segment Routing (SR) [RFC8402] allows a headend node to steer a
packet flow along a specific path. Intermediate per-path states are
eliminated thanks to source routing.
The headend node is said to steer a flow into an SR Policy [RFC8402].
The packets steered into an SR Policy carry an ordered list of
segments associated with that SR Policy.
[RFC9256] further details the concepts of SR Policy and steering into
an SR Policy. These apply equally to the SR-MPLS and Segment Routing
for IPv6 (SRv6) data-plane instantiations of Segment Routing using
SR-MPLS and SRv6 Segment Identifiers (SIDs) as described in
[RFC8402]. [RFC8660] describes the representation and processing of
this ordered list of segments as an MPLS label stack for SR-MPLS.
While [RFC8754] and [RFC8986] describe the same for SRv6 with the use
of the Segment Routing Header (SRH).
The SR Policy related functionality described in [RFC9256] can be
conceptually viewed as being incorporated in an SR Policy Module
(SRPM). Following is a reminder of the high-level functionality of
SRPM:
* Learning multiple candidate paths for an SR Policy via various
mechanisms (CLI, NETCONF, PCEP, or BGP).
* Selection of the best candidate path for an SR Policy.
* Associating a Binding SID (BSID) to the selected candidate path of
an SR Policy.
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* Installation of the selected candidate path and its BSID in the
forwarding plane.
This document specifies the use of BGP to distribute one or more of
the candidate paths of an SR Policy to the headend of that policy.
The document describes the functionality provided by BGP and, as
appropriate, provides references for the functionality which is
outside the scope of BGP (i.e. resides within SRPM on the headend
node).
This document specifies a way of representing SR Policy candidate
paths in BGP UPDATE messages. BGP can then be used to propagate the
SR Policy candidate paths to the headend nodes in a network. The
usual BGP rules for BGP propagation and best-path selection are used.
At the headend of a specific policy, this will result in one or more
candidate paths being installed into the "BGP table". These paths
are then passed to the SRPM. The SRPM may compare them to candidate
paths learned via other mechanisms and will choose one or more paths
to be installed in the data plane. BGP itself does not install SR
Policy candidate paths into the data plane.
This document introduces a BGP subsequent address family (SAFI) for
IPv4 and IPv6 address families. In UPDATE messages of those AFI/
SAFIs, the NLRI identifies an SR Policy Candidate Path while the
attributes encode the segment lists and other details of that SR
Policy Candidate Path.
While for simplicity we may write that BGP advertises an SR Policy,
it has to be understood that BGP advertises a candidate path of an SR
policy and that this SR Policy might have several other candidate
paths provided via BGP (via an NLRI with a different distinguisher as
defined in Section 2.1), PCEP, NETCONF, or local policy
configuration.
Typically, a controller defines the set of policies and advertises
them to policy headend routers (typically ingress routers). These
policy advertisements use the BGP extensions defined in this
document. The policy advertisement is, in most but not all cases,
tailored for a specific policy headend. In this case, the
advertisement may be sent on a BGP session to that headend and not
propagated any further.
Alternatively, a router (i.e., a BGP egress router) advertises SR
Policies representing paths to itself. In this case, it is possible
to send the policy to each headend over a BGP session to that
headend, without requiring any further propagation of the policy.
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An SR Policy intended only for the receiver will, in most cases, not
traverse any Route Reflector (RR, [RFC4456]).
In some situations, it is undesirable for a controller or BGP egress
router to have a BGP session to each policy headend. In these
situations, BGP Route Reflectors may be used to propagate the
advertisements. In certain other deployments, it may be necessary
for the advertisement to propagate through a sequence of one or more
ASes within an SR Domain (refer to Section 7 for the associated
security considerations). To make this possible, an attribute needs
to be attached to the advertisement that enables a BGP speaker to
determine whether it is intended to be a headend for the advertised
policy. This is done by attaching one or more Route Target Extended
Communities to the advertisement [RFC4360].
The BGP extensions for the advertisement of SR Policies include
following components:
* A Subsequent Address Family Identifier (SAFI) whose NLRIs
identifies an SR Policy candidate path.
* A Tunnel Type identifier for SR Policy, and a set of sub-TLVs to
be inserted into the Tunnel Encapsulation Attribute (as defined in
[RFC9012]) specifying segment lists of the SR Policy candidate
path, as well as other information about the SR Policy.
* One or more IPv4 address format route target extended community
([RFC4360]) attached to the SR Policy advertisement and that
indicates the intended headend of such an SR Policy advertisement.
The Color Extended Community (as defined in [RFC9012]) is used to
steer traffic into an SR Policy, as described in section 8.8 of
[RFC9256]. The Section 3 of this document updates [RFC9012] with
modifications to the format of the Flags field of the Color Extended
Community by using the two leftmost bits of that field.
1.1. Requirements Language
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. SR Policy Encoding
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2.1. SR Policy SAFI and NLRI
A SAFI is introduced in this document: the SR Policy SAFI with
codepoint 73. The AFI used MUST be IPv4(1) or IPv6(2).
The SR Policy SAFI uses the NLRI format defined as follows:
+------------------+
| NLRI Length | 1 octet
+------------------+
| Distinguisher | 4 octets
+------------------+
| Policy Color | 4 octets
+------------------+
| Endpoint | 4 or 16 octets
+------------------+
Figure 1: SR Policy SAFI Format
where:
* NLRI Length: 1 octet indicating the length expressed in bits as
defined in [RFC4760]. When AFI = 1 the value MUST be 96 and when
AFI = 2 the value MUST be 192.
* Distinguisher: 4-octet value uniquely identifying the policy in
the context of <color, endpoint> tuple. The distinguisher has no
semantic value and is solely used by the SR Policy originator to
make unique (from an NLRI perspective) both for multiple candidate
paths of the same SR Policy as well as candidate paths of
different SR Policies (i.e. with different segment lists) with the
same Color and Endpoint but meant for different headends.
* Policy Color: 4-octet value identifying (with the endpoint) the
policy. The color is used to match the color of the destination
prefixes to steer traffic into the SR Policy as specified in
section 8 of [RFC9256].
* Endpoint: value identifies the endpoint of a policy. The Endpoint
may represent a single node or a set of nodes (e.g., an anycast
address). The Endpoint is an IPv4 (4-octet) address or an IPv6
(16-octet) address according to the AFI of the NLRI. The address
can be either a unicast or an unspecified address (0.0.0.0 for
IPv4, :: for IPv6) as specified in section 2.1 of [RFC9256].
The color and endpoint are used to automate the steering of BGP
service routes on SR Policy as described in section 8 of [RFC9256].
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The NLRI containing an SR Policy candidate path is carried in a BGP
UPDATE message [RFC4271] using BGP multi-protocol extensions
[RFC4760] with an AFI of 1 or 2 (IPv4 or IPv6) and with a SAFI of 73.
The fault management and error handling in the encoding of the NLRI
is specified in Section 5.
An update message that carries the MP_REACH_NLRI or MP_UNREACH_NLRI
attribute with the SR Policy SAFI MUST also carry the BGP mandatory
attributes. In addition, the BGP update message MAY also contain any
of the BGP optional attributes.
The next-hop network address field in SR Policy SAFI (73) updates may
be either a 4-octet IPv4 address or a 16-octet IPv6 address,
independent of the SR Policy AFI. The length field of the next-hop
address specifies the next-hop address family. If the next-hop
length is 4, then the next-hop is an IPv4 address; if the next-hop
length is 16, then it is a global IPv6 address; if the next-hop
length is 32, then it has a global IPv6 address followed by a link-
local IPv6 address. The setting of the next-hop field and its
attendant processing is governed by standard BGP procedures as
described in section 3 of [RFC4760] and section 3 of [RFC2545].
It is important to note that any BGP speaker receiving a BGP message
with an SR Policy NLRI, will process it only if the NLRI is among the
best paths as per the BGP best-path selection algorithm. In other
words, this document leverages the existing BGP propagation and best-
path selection rules. Details of the procedures are described in
Section 4.
It has to be noted that if several candidate paths of the same SR
Policy (endpoint, color) are signaled via BGP to a headend, then it
is RECOMMENDED that each NLRI uses a different distinguisher. If BGP
has installed into the BGP table two advertisements whose respective
NLRIs have the same color and endpoint, but different distinguishers,
both advertisements are passed to the SRPM as different candidate
paths along with their respective originator information (i.e., ASN
and BGP Router-ID) as described in section 2.4 of [RFC9256]. The ASN
would be the ASN of the origin and the BGP Router-ID is determined in
the following order:
* From the Route Origin Community [RFC4360] if present and carrying
an IP Address, or
* As the BGP Originator ID [RFC4456] if present, or
* As the BGP Router-ID of the peer from which the update was
received as a last resort.
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2.2. SR Policy and Tunnel Encapsulation Attribute
The content of the SR Policy Candidate Path is encoded in the Tunnel
Encapsulation Attribute defined in [RFC9012] using a Tunnel-Type
called SR Policy Type with codepoint 15. The use of SR Policy
Tunnel-type is applicable only for the AFI/SAFI pairs of (1/73,
2/73).
The SR Policy Encoding structure is as follows:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encapsulation Attribute (23)
Tunnel Type: SR Policy (15)
Binding SID
SRv6 Binding SID
Preference
Priority
Policy Name
Policy Candidate Path Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Segment
Segment
...
...
Figure 2: SR Policy Encoding
where:
* SR Policy SAFI NLRI is defined in Section 2.1.
* Tunnel Encapsulation Attribute is defined in [RFC9012].
* Tunnel-Type is set to 15.
* Preference, Binding SID, SRv6 Binding SID, Priority, Policy Name,
Policy Candidate Path Name, ENLP, Segment-List, Weight, and
Segment sub-TLVs are defined in Section 2.4.
* Additional sub-TLVs may be defined in the future.
A Tunnel Encapsulation Attribute MUST NOT contain more than one TLV
of type "SR Policy".
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2.3. Remote Endpoint and Color
The Tunnel Egress Endpoint and Color sub-TLVs, as defined in
[RFC9012], may also be present in the SR Policy encodings.
The Tunnel Egress Endpoint and Color Sub-TLVs of the Tunnel
Encapsulation Attribute are not used for SR Policy encodings and
therefore their value is irrelevant in the context of the SR Policy
SAFI NLRI. If present, the Tunnel Egress Endpoint sub-TLV and the
Color sub-TLV MUST be ignored by the BGP speaker and MUST NOT be
removed from the Tunnel Encapsulation Attribute during propagation.
2.4. SR Policy Sub-TLVs
This section specifies the sub-TLVs defined for encoding the
information about the SR Policy Candidate Path.
Preference, Binding SID, SRv6 Binding SID, Segment-List, Priority,
Policy Name, Policy Candidate Path Name, and Explicit NULL Label
Policy are the sub-TLVs introduced for the BGP Tunnel Encapsulation
Attribute [RFC9012] being defined in this section.
Weight and Segment are sub-TLVs of the Segment-List sub-TLV mentioned
above.
The fault management and error handling in the encoding of the sub-
TLVs defined in this section are specified in Section 5.
None of the sub-TLVs defined in the following sub-sections have any
effect on the BGP best-path selection or propagation procedures.
These sub-TLVs are not used by the BGP path selection process and are
instead passed on to SRPM as SR Policy Candidate Path information for
further processing described in section 2 of [RFC9256].
The use of SR Policy Sub-TLVs is applicable only for the AFI/SAFI
pairs of (1/73, 2/73). Future documents may extend their
applicability to other AFI/SAFI.
2.4.1. Preference Sub-TLV
The Preference sub-TLV is used to carry the Preference of an SR
Policy candidate path. The contents of this sub-TLV are used by the
SRPM as described in section 2.7 of [RFC9256].
The Preference sub-TLV is optional and it MUST NOT appear more than
once in the SR Policy encoding.
The Preference sub-TLV has following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preference (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Preference sub-TLV
where:
* Type: 12
* Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
MUST be 6.
* Flags: 1 octet of flags. No flags are defined in this document.
The Flags field MUST be set to zero on transmission and MUST be
ignored on receipt.
* RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
* Preference: a 4-octet value indicating the Preference of the SR
Policy Candidate Path as described in section 2.7 of [RFC9256].
2.4.2. Binding SID Sub-TLV
The Binding SID sub-TLV is used to signal the binding SID related
information of the SR Policy candidate path. The contents of this
sub-TLV are used by the SRPM as described in section 6 in [RFC9256].
The Binding SID sub-TLV is optional and it MUST NOT appear more than
once in the SR Policy encoding.
When the Binding SID sub-TLV is used to signal an SRv6 SID, the
choice of its SRv6 Endpoint Behavior [RFC8986] to be instantiated is
left to the headend node. It is RECOMMENDED that the SRv6 Binding
SID sub-TLV defined in Section 2.4.3, that enables the specification
of the SRv6 Endpoint Behavior, be used for signaling of an SRv6
Binding SID for an SR Policy candidate path.
The Binding SID sub-TLV has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Binding SID (variable, optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Binding SID sub-TLV
where:
* Type: 13
* Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
MUST be one of: 18 when a SRv6 BSID is present, 6 when a SR-MPLS
BSID is present, or 2 when no BSID is present.
* Flags: 1 octet of flags. The following flags are defined in the
registry "SR Policy Binding SID Flags" as described in
Section 6.6:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S|I| |
+-+-+-+-+-+-+-+-+
Figure 5: Binding SID Flags
where:
- S-Flag: This flag encodes the "Specified-BSID-only" behavior.
It is used by SRPM as described in section 6.2.3 in [RFC9256].
- I-Flag: This flag encodes the "Drop Upon Invalid" behavior. It
is used by SRPM as described in section 8.2 in [RFC9256].
- The unassigned bits in the Flag octet MUST be set to zero upon
transmission and MUST be ignored upon receipt.
* RESERVED: 1 octet of reserved bits. MUST be set to zero on
transmission and MUST be ignored on receipt.
* Binding SID: If the length is 2, then no Binding SID is present.
If the length is 6 then the Binding SID is encoded in 4 octets
using the format below. Traffic Class (TC), S, and TTL (Total of
12 bits) are RESERVED and MUST be set to zero and MUST be ignored.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Binding SID Label Encoding
If the length is 18 then the Binding SID contains a 16-octet SRv6
SID.
2.4.3. SRv6 Binding SID Sub-TLV
The SRv6 Binding SID sub-TLV is used to signal the SRv6 Binding SID
related information of an SR Policy candidate path. It enables the
specification of the SRv6 Endpoint Behavior [RFC8986] to be
instantiated on the headend node. The contents of this sub-TLV are
used by the SRPM as described in section 6 in [RFC9256].
The SRv6 Binding SID sub-TLV is optional. More than one SRv6 Binding
SID sub-TLVs MAY be signaled in the same SR Policy encoding to
indicate one or more SRv6 SIDs, each with potentially different SRv6
Endpoint Behaviors to be instantiated.
The SRv6 Binding SID sub-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 | Flags | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRv6 Binding SID (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// SRv6 Endpoint Behavior and SID Structure (optional) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: SRv6 Binding SID sub-TLV
where:
* Type: 20
* Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
MUST be 26 when the SRv6 Endpoint Behavior and SID Structure is
present else it MUST be 18.
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* Flags: 1 octet of flags. The following flags are defined in the
registry "SR Policy Binding SID Flags" as described in
Section 6.7:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S|I|B| |
+-+-+-+-+-+-+-+-+
Figure 8: SRv6 Binding SID Flags
where:
- S-Flag: This flag encodes the "Specified-BSID-only" behavior.
It is used by SRPM as described in section 6.2.3 in [RFC9256].
- I-Flag: This flag encodes the "Drop Upon Invalid" behavior. It
is used by SRPM as described in section 8.2 in [RFC9256].
- B-Flag: This flag, when set, indicates the presence of the SRv6
Endpoint Behavior and SID Structure encoding specified in
Section 2.4.4.2.4.
- The unassigned bits in the Flag octet MUST be set to zero upon
transmission and MUST be ignored upon receipt.
* RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
* SRv6 Binding SID: Contains a 16-octet SRv6 SID.
* SRv6 Endpoint Behavior and SID Structure: Optional, as defined in
Section 2.4.4.2.4.
2.4.4. Segment List Sub-TLV
The Segment List sub-TLV encodes a single explicit path towards the
endpoint as described in section 5.1 of [RFC9256]. The Segment List
sub-TLV includes the elements of the paths (i.e., segments) as well
as an optional Weight sub-TLV.
The Segment List sub-TLV may exceed 255 bytes in length due to a
large number of segments. A 2-octet length is thus required.
According to section 2 of [RFC9012], the sub-TLV type defines the
size of the length field. Therefore, for the Segment List sub-TLV, a
code point of 128 or higher is used.
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The Segment List sub-TLV is optional and MAY appear multiple times in
the SR Policy encoding. The ordering of Segment List sub-TLVs does
not matter since each sub-TLV encodes a Segment List.
The Segment List sub-TLV contains zero or more Segment sub-TLVs and
MAY contain a Weight sub-TLV.
The Segment List sub-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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// sub-TLVs //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Segment List sub-TLV
where:
* Type: 128.
* Length: the total length (not including the Type and Length
fields) of the sub-TLVs encoded within the Segment List sub-TLV in
terms of octets.
* RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
* sub-TLVs currently defined:
- An optional single Weight sub-TLV.
- Zero or more Segment sub-TLVs.
Validation of an explicit path encoded by the Segment List sub-TLV is
beyond the scope of BGP and performed by the SRPM as described in
section 5 of [RFC9256].
2.4.4.1. Weight Sub-TLV
The Weight sub-TLV specifies the weight associated with a given
segment list. The contents of this sub-TLV are used only by the SRPM
as described in section 2.11 of [RFC9256].
The Weight sub-TLV is optional and it MUST NOT appear more than once
inside the Segment List sub-TLV.
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The Weight sub-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 | Flags | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Weight sub-TLV
where:
* Type: 9.
* Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
MUST be 6.
* Flags: 1 octet of flags. No flags are defined in this document.
The Flags field MUST be set to zero on transmission and MUST be
ignored on receipt.
* RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
* Weight: 4 octets value indicating the weight associated with a
segment list as described in section 2.11 of [RFC9256].
2.4.4.2. Segment Sub-TLVs
A Segment sub-TLV describes a single segment in a segment list (i.e.,
a single element of the explicit path). One or more Segment sub-TLVs
constitute an explicit path of the SR Policy candidate path. The
contents of these sub-TLVs are used only by the SRPM as described in
section 4 in [RFC9256].
The Segment sub-TLVs are optional and MAY appear multiple times in
the Segment List sub-TLV.
Section 4 of [RFC9256] defines several Segment Types:
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Type A: SR-MPLS Label
Type B: SRv6 SID
Type C: IPv4 Prefix with optional SR Algorithm
Type D: IPv6 Global Prefix with optional SR Algorithm for SR-MPLS
Type E: IPv4 Prefix with Local Interface ID
Type F: IPv4 Addresses for link endpoints as Local, Remote pair
Type G: IPv6 Prefix and Interface ID for link endpoints as Local,
Remote pair for SR-MPLS
Type H: IPv6 Addresses for link endpoints as Local, Remote pair
for SR-MPLS
Type I: IPv6 Global Prefix with optional SR Algorithm for SRv6
Type J: IPv6 Prefix and Interface ID for link endpoints as Local,
Remote pair for SRv6
Type K: IPv6 Addresses for link endpoints as Local, Remote pair
for SRv6
The following sub-sections specify the sub-TLVs used for Segment
Types A and B. The other segment types are specified in
[I-D.ietf-idr-bgp-sr-segtypes-ext].
2.4.4.2.1. Segment Type A
The Type A Segment Sub-TLV encodes a single SR-MPLS SID. The format
is as follows and is used to encode MPLS Label fields as specified in
[RFC3032] [RFC5462].:
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 | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Type A Segment sub-TLV
where:
* Type: 1.
* Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
MUST be 6.
* Flags: 1 octet of flags as defined in Section 2.4.4.2.3.
* RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
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* Label: 20 bits of label value.
* TC: 3 bits of traffic class.
* S: 1 bit of bottom-of-stack.
* TTL: 1 octet of TTL.
The following applies to the Type-1 Segment sub-TLV:
* The S bit MUST be zero upon transmission and MUST be ignored upon
reception.
* If the originator wants the receiver to choose the TC value, it
sets the TC field to zero.
* If the originator wants the receiver to choose the TTL value, it
sets the TTL field to 255.
* If the originator wants to recommend a value for these fields, it
puts those values in the TC and/or TTL fields.
* The receiver MAY override the originator's values for these
fields. This would be determined by local policy at the receiver.
One possible policy would be to override the fields only if the
fields have the default values specified above.
2.4.4.2.2. Segment Type B
The Type B Segment Sub-TLV encodes a single SRv6 SID. The format is
as follows:
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 | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// SRv6 SID (16 octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// SRv6 Endpoint Behavior and SID Structure (optional) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Type B Segment sub-TLV
where:
* Type: 13.
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* Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
MUST be 26 when the SRv6 Endpoint Behavior and SID Structure is
present else it MUST be 18.
* Flags: 1 octet of flags as defined in Section 2.4.4.2.3.
* RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
* SRv6 SID: 16 octets of IPv6 address.
* SRv6 Endpoint Behavior and SID Structure: Optional, as defined in
Section 2.4.4.2.4.
The TLV 2 defined for the advertisement of Segment Type B in the
earlier versions of this document has been deprecated to avoid
backward compatibility issues.
2.4.4.2.3. Segment Flags
The Segment Types sub-TLVs described above may contain the following
flags in the "Flags" field defined in Section 6.8:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|V| |B| |
+-+-+-+-+-+-+-+-+
Figure 22: Segment Flags
where:
V-Flag: This flag, when set, is used by SRPM for "SID
verification" as described in Section 5.1 of [RFC9256].
B-Flag: This flag, when set, indicates the presence of the SRv6
Endpoint Behavior and SID Structure encoding specified in
Section 2.4.4.2.4.
The unassigned bits in the Flag octet MUST be set to zero upon
transmission and MUST be ignored upon receipt.
The following applies to the Segment Flags:
* V-Flag applies to all Segment Types.
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* B-Flag applies to Segment Type B. If B-Flag appears with Segment
Type A it MUST be ignored.
2.4.4.2.4. SRv6 SID Endpoint Behavior and Structure
The Segment Types sub-TLVs described above MAY contain the SRv6
Endpoint Behavior and SID Structure [RFC8986] encoding as described
below:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Endpoint Behavior | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LB Length | LN Length | Fun. Length | Arg. Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23: SRv6 SID Endpoint Behavior and Structure
where:
Endpoint Behavior: 2 octets. It carries the SRv6 Endpoint
Behavior code point for this SRv6 SID as defined in section 9.2 of
[RFC8986]. When set with the value 0xFFFF (i.e., Opaque), the
choice of SRv6 Endpoint Behavior is left to the headend.
Reserved: 2 octets of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
Locator Block Length: 1 octet. SRv6 SID Locator Block length in
bits.
Locator Node Length: 1 octet. SRv6 SID Locator Node length in
bits.
Function Length: 1 octet. SRv6 SID Function length in bits.
Argument Length: 1 octet. SRv6 SID Arguments length in bits.
The total of the locator block, locator node, function, and argument
lengths MUST be less than or equal to 128.
2.4.5. Explicit NULL Label Policy Sub-TLV
To steer an unlabeled IP packet into an SR policy, it is necessary to
create a label stack for that packet, and push one or more labels
onto that stack.
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The Explicit NULL Label Policy (ENLP) sub-TLV is used to indicate
whether an Explicit NULL Label [RFC3032] must be pushed on an
unlabeled IP packet before any other labels.
If an ENLP Sub-TLV is not present, the decision of whether to push an
Explicit NULL label on a given packet is a matter of local
configuration.
The ENLP sub-TLV is optional and it MUST NOT appear more than once in
the SR Policy encoding.
The contents of this sub-TLV are used by the SRPM as described in
section 4.1 of [RFC9256].
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 | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ENLP |
+-+-+-+-+-+-+-+-+
Figure 24: ELNP sub-TLV
Where:
Type: 14.
Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
MUST be 3.
Flags: 1 octet of flags. No flags are defined in this document.
The Flags field MUST be set to zero on transmission and MUST be
ignored on receipt.
RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
ENLP (Explicit NULL Label Policy): Indicates whether Explicit NULL
labels are to be pushed on unlabeled IP packets that are being
steered into a given SR policy. This field has one of the
following values:
- 0: Reserved.
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- 1: Push an IPv4 Explicit NULL label on an unlabeled IPv4
packet, but do not push an IPv6 Explicit NULL label on an
unlabeled IPv6 packet.
- 2: Push an IPv6 Explicit NULL label on an unlabeled IPv6
packet, but do not push an IPv4 Explicit NULL label on an
unlabeled IPv4 packet.
- 3: Push an IPv4 Explicit NULL label on an unlabeled IPv4
packet, and push an IPv6 Explicit NULL label on an unlabeled
IPv6 packet.
- 4: Do not push an Explicit NULL label.
- 5 - 255: Reserved.
The ENLP reserved values may be used for future extensions and
implementations SHOULD ignore the ENLP Sub-TLV with these values.
The behavior signaled in this Sub-TLV MAY be overridden by local
configuration. The section 4.1 of [RFC9256] describes the
behavior on the headend for the handling of the explicit null
label.
2.4.6. Policy Priority Sub-TLV
An operator MAY set the Policy Priority sub-TLV to indicate the order
in which the SR policies are re-computed upon topological change.
The contents of this sub-TLV are used by the SRPM as described in
section 2.12 of [RFC9256].
The Priority sub-TLV is optional and it MUST NOT appear more than
once in the SR Policy encoding.
The Priority sub-TLV has 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 | Priority | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: Priority sub-TLV
Where:
Type: 15
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Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets.The value
MUST be 2.
Priority: a 1-octet value.
RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
2.4.7. Policy Candidate Path Name Sub-TLV
An operator MAY set the Policy Candidate Path Name sub-TLV to attach
a symbolic name to the SR Policy candidate path.
Usage of Policy Candidate Path Name sub-TLV is described in section
2.6 of [RFC9256].
The Policy Candidate Path Name sub-TLV may exceed 255 bytes in length
due to a long name. A 2-octet length is thus required. According to
section 2 of [RFC9012], the sub-TLV type defines the size of the
length field. Therefore, for the Policy Candidate Path Name sub-TLV
a code point of 128 or higher is used.
It is RECOMMENDED that the size of the symbolic name for the
candidate path is limited to 255 bytes. Implementations MAY choose
to truncate long names to 255 bytes when signaling via BGP.
The Policy Candidate Path Name sub-TLV is optional and it MUST NOT
appear more than once in the SR Policy encoding.
The Policy Candidate Path Name sub-TLV has 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Policy Candidate Path Name //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: Policy Candidate Path Name sub-TLV
Where:
Type: 129.
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Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
is variable.
RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
Policy Candidate Path Name: Symbolic name for the SR Policy
candidate path without a NULL terminator as specified in section
2.6 of [RFC9256].
2.4.8. Policy Name Sub-TLV
An operator MAY set the Policy Name sub-TLV to associate a symbolic
name with the SR Policy for which the candidate path is being
advertised via the SR Policy NLRI.
Usage of Policy Name sub-TLV is described in section 2.1 of
[RFC9256].
The Policy Name sub-TLV may exceed 255 bytes in length due to a long
policy name. A 2-octet length is thus required. According to
section 2 of [RFC9012], the sub-TLV type defines the size of the
length field. Therefore, for the Policy Name sub-TLV a code point of
128 or higher is used.
It is RECOMMENDED that the size of the symbolic name for the SR
Policy is limited to 255 bytes. Implementations MAY choose to
truncate long names to 255 bytes when signaling via BGP.
The Policy Name sub-TLV is optional and it MUST NOT appear more than
once in the SR Policy encoding.
The Policy Name sub-TLV has 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Policy Name //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27: Policy Name sub-TLV
Where:
Type: 130
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Length: Specifies the length of the value field (i.e., not
including Type and Length fields) in terms of octets. The value
is variable.
RESERVED: 1 octet of reserved bits. This field MUST be set to
zero on transmission and MUST be ignored on receipt.
Policy Name: Symbolic name for the policy. It SHOULD be a string
of printable ASCII characters, without a NULL terminator.
3. Color Extended Community
The Color Extended Community [RFC9012] is used to steer traffic
corresponding to BGP routes into an SR Policy with matching color
value. The Color Extended Community 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 Color Extended
Community in BGP UPDATE messages of other AFI/SAFIs is outside the
scope of this document.
Two bits from the Flags field of the Color Extended Community are
used as follows to support the requirements of Color-Only steering as
specified in Section 8.8 of [RFC9256]:
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C O| Unassigned |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: Color Extended Community Flags
The CO bits together form the Color-Only Type field which indicates
the various matching criteria between BGP NH and SR Policy endpoint
in addition to the matching of the color value. Following types are
defined:
* Type 0: Specific Endpoint Match: Request match for the endpoint
that is the BGP NH
* Type 1: Specific or Null Endpoint Match: Request match for either
the endpoint that is the BGP NH or a null endpoint (e.g., like a
default gateway)
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* Type 2: Specific, Null, or Any Endpoint Match: Request match for
either the endpoint that is the BGP NH or with a null or any
endpoint
* Type 3: reserved for future use and SHOULD NOT be used. Upon
reception, an implementation MUST treat it like Type 0.
The details of the SR Policy steering mechanisms based on these
Color-Only types are specified in section 8.8 of [RFC9256].
One or more Color Extended Communities MAY be associated with a BGP
route update. Sections 8.4.1, 8.5.1, and 8.8.2 of [RFC9256] specify
the steering behaviors over SR Policies when multiple Color Extended
Communities are associated with a BGP route.
4. SR Policy Operations
As mentioned in Section 1, BGP is not the actual consumer of an SR
Policy NLRI. BGP is in charge of the origination and propagation of
the SR Policy NLRI but its installation and use are outside the scope
of BGP. The details of SR Policy installation and use are specified
in [RFC9256].
4.1. Advertisement of SR Policies
Typically, but not limited to, an SR Policy is computed by a
controller or a path computation engine (PCE) and originated by a BGP
speaker on its behalf.
Multiple SR Policy NLRIs may be present with the same <color,
endpoint> tuple but with different content when these SR policies are
intended for different headends.
The distinguisher of each SR Policy NLRI prevents undesired BGP route
selection among these SR Policy NLRIs and allows their propagation
across route reflectors [RFC4456].
Moreover, one or more route targets SHOULD be attached to the
advertisement, where each route target identifies one or more
intended headends for the advertised SR Policy update.
If no route target is attached to the SR Policy NLRI, then it is
assumed that the originator sends the SR Policy update directly
(e.g., through a BGP session) to the intended receiver. In such a
case, the NO_ADVERTISE community [RFC1997] MUST be attached to the SR
Policy update (see further details in Section 4.2.3).
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4.2. Reception of an SR Policy NLRI
On reception of an SR Policy NLRI, a BGP speaker first determines if
it is valid as described in Section 4.2.1 and then performs the
decision process for selection of the best route (Section 9.1 of
[RFC4271]). The key difference from the base BGP decision process is
that BGP does not download the selected best routes of SR Policy SAFI
into the forwarding and instead considers them "usable" for passing
on to the SRPM for further processing as described in Section 4.2.2.
The selected best route is “propagated” (Section 9.1.3 of [RFC4271])
as described in Section 4.2.3 irrespective of its "usability" by the
local router.
4.2.1. Validation of an SR Policy NLRI
When a BGP speaker receives an SR Policy NLRI from a neighbor it MUST
first perform validation based on the following rules in addition to
the validation described in Section 5:
* The SR Policy NLRI MUST include a distinguisher, color, and
endpoint field which implies that the length of the NLRI MUST be
either 12 or 24 octets (depending on the address family of the
endpoint).
* The SR Policy update MUST have either the NO_ADVERTISE community
or at least one route target extended community in IPv4-address
format or both. If a router supporting this specification
receives an SR Policy update with no route target extended
communities and no NO_ADVERTISE community, the update MUST be
considered as malformed.
* The Tunnel Encapsulation Attribute MUST be attached to the BGP
Update and MUST have a Tunnel Type TLV set to SR Policy (codepoint
is 15).
A router that receives an SR Policy update that is not valid
according to these criteria MUST treat the update as malformed and
the SR Policy candidate path MUST NOT be passed to the SRPM.
4.2.2. Eligibility for Local Use of an SR Policy NLRI
An SR Policy NLRI update without any route target extended community
but having the NO_ADVERTISE community is considered usable.
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If one or more route targets are present, then at least one route
target MUST match the BGP Identifier of the receiver for the update
to be considered usable. The BGP Identifier is defined in [RFC4271]
as a 4-octet IPv4 address. Therefore, the route target extended
community MUST be of the same format.
If one or more route targets are present and none matches the local
BGP Identifier, then, while the SR Policy NLRI is valid, it is not
usable on the receiver node.
When the SR Policy tunnel type includes any sub-TLV that is
unrecognized or unsupported, the update SHOULD NOT be considered
usable. An implementation MAY provide an option for ignoring
unsupported sub-TLVs.
Once BGP on the receiving node has determined that the SR Policy NLRI
is usable, it passes the SR Policy candidate path to the SRPM. Note
that, along with the candidate path details, BGP also passes the
originator information for breaking ties in the candidate path
selection process as described in section 2.4 of [RFC9256].
When an update for an SR Policy NLRI results in its becoming
unusable, BGP MUST delete its corresponding SR Policy candidate path
from the SRPM.
The SRPM applies the rules defined in section 2 of [RFC9256] to
determine whether the SR Policy candidate path is valid and to select
the best candidate path among the valid ones for a given SR Policy.
4.2.3. Propagation of an SR Policy
SR Policy NLRIs that have the NO_ADVERTISE community attached to them
MUST NOT be propagated.
By default, a BGP node receiving an SR Policy NLRI MUST NOT propagate
it to any EBGP neighbor. An implementation MAY provide an explicit
configuration to override this and enable the propagation of valid SR
Policy NLRIs to specific EBGP neighbors where the SR domain comprises
multiple-ASes within a single service provider domain (see Section 7
for details).
A BGP node advertises a received SR Policy NLRI to its IBGP neighbors
according to normal IBGP propagation rules.
By default, a BGP node receiving an SR Policy NLRI SHOULD NOT remove
route target extended community before propagation. An
implementation MAY provide support for configuration to filter and/or
remove route target extended community before propagation.
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A BGP node MUST NOT alter the SR Policy information carried in the
Tunnel Encapsulation Attribute during propagation.
5. Error Handling and Fault Management
This section describes the error handling actions, as described in
[RFC7606], that are to be performed for the handling of the BGP
update messages for BGP SR Policy SAFI.
A BGP Speaker MUST perform the following syntactic validation of the
SR Policy NLRI to determine if it is malformed. This includes the
validation of the length of each NLRI and the total length of the
MP_REACH_NLRI and MP_UNREACH_NLRI attributes. It also includes the
validation of the consistency of the NLRI length with the AFI and the
endpoint address as specified in Section 2.1.
When the error determined allows for the router to skip the malformed
NLRI(s) and continue the processing of the rest of the update
message, then it MUST handle such malformed NLRIs as 'Treat-as-
withdraw'. In other cases, where the error in the NLRI encoding
results in the inability to process the BGP update message (e.g.
length related encoding errors), then the router SHOULD handle such
malformed NLRIs as 'AFI/SAFI disable' when other AFI/SAFI besides SR
Policy are being advertised over the same session. Alternately, the
router MUST perform 'session reset' when the session is only being
used for SR Policy or when it 'AFI/SAFI disable' action is not
possible.
The validation of the TLVs/sub-TLVs introduced in this document and
defined in their respective sub-sections of Section 2.4 MUST be
performed to determine if they are malformed or invalid. The
validation of the Tunnel Encapsulation Attribute itself and the other
TLVs/sub-TLVs specified in [RFC9012] MUST be done as described in
that document. In case of any error detected, either at the
attribute or its TLV/sub-TLV level, the "treat-as-withdraw" strategy
MUST be applied. This is because an SR Policy update without a valid
Tunnel Encapsulation Attribute (comprising of all valid TLVs/sub-
TLVs) is not usable.
An SR Policy update that is determined to be not valid, and therefore
malformed, based on rules described in Section 4.2.1 MUST be handled
by the "treat-as-withdraw" strategy.
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The validation of the individual fields of the TLVs/sub-TLVs defined
in Section 2.4 are beyond the scope of BGP as they are handled by the
SRPM as described in the individual TLV/sub-TLV sub-sections. A BGP
implementation MUST NOT perform semantic verification of such fields
nor consider the SR Policy update to be invalid or not usable based
on such validation.
An implementation SHOULD log any errors found during the above
validation for further analysis.
6. IANA Considerations
This document uses code point allocations from the following existing
registries:
* Subsequent Address Family Identifiers (SAFI) Parameters registry
* BGP Tunnel Encapsulation Attribute Tunnel Types registry under the
BGP Tunnel Encapsulation registry
* BGP Tunnel Encapsulation Attribute sub-TLVs registry under the BGP
Tunnel Encapsulation registry
* Color Extended Community Flags registry under the BGP Tunnel
Encapsulation registry
This document also requests the creation of the following new
registries:
* SR Policy Segment List Sub-TLVs under the BGP Tunnel Encapsulation
registry
* SR Policy Binding SID Flags under the BGP Tunnel Encapsulation
registry
* SR Policy SRv6 Binding SID Flags under the BGP Tunnel
Encapsulation registry
* SR Policy Segment Flags under the BGP Tunnel Encapsulation
registry
* Color Extended Community Color-Only Types registry under the BGP
Tunnel Encapsulation registry
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6.1. Existing Registry: Subsequent Address Family Identifiers (SAFI)
Parameters
This document introduces a SAFI in the registry "Subsequent Address
Family Identifiers (SAFI) Parameters" that has been assigned a code
point by IANA. The entry needs to be updated as follows:
Code Point Description Reference
-----------------------------------------------
73 SR Policy SAFI This document
Table 1: BGP SAFI Code Point
6.2. Existing Registry: BGP Tunnel Encapsulation Attribute Tunnel Types
This document introduces a Tunnel-Type in the registry "BGP Tunnel
Encapsulation Attribute Tunnel Types" that has been assigned a
codepoint by IANA. The entry needs to be updated as follows:
Code Point Description Reference
--------------------------------------------------
15 SR Policy This document
Table 2: Tunnel Type Code Point
6.3. Existing Registry: BGP Tunnel Encapsulation Attribute sub-TLVs
This document defines sub-TLVs in the registry "BGP Tunnel
Encapsulation Attribute sub-TLVs" that have been assigned code points
by IANA as follows via the early allocation process which needs to be
made permanent:
Code Point Description Reference
------------------------------------------------------------
12 Preference sub-TLV This document
13 Binding SID sub-TLV This document
14 ENLP sub-TLV This document
15 Priority sub-TLV This document
20 SRv6 Binding SID sub-TLV This document
128 Segment List sub-TLV This document
129 Policy Candidate Path Name sub-TLV This document
130 Policy Name sub-TLV This document
Table 3: BGP Tunnel Encapsulation Attribute Code Points
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6.4. Existing Registry: Color Extended Community Flags
This document defines the use of 2 bits in the registry called "Color
Extended Community Flags" under the "BGP Tunnel Encapsulation"
registry that have been assigned by IANA via the early allocation
process to form the Color-Only Types field which needs to be made
permanent:
Bit
Position Description Reference
------------------------------------------------------------------
0-1 Color-only Types Field This document
Table 4: Color Extended Community Flag Bits
6.5. New Registry: SR Policy Segment List Sub-TLVs
This document requests the creation of a new registry called "SR
Policy Segment List Sub-TLVs" under the "BGP Tunnel Encapsulation"
registry. The allocation policy of this registry is "Standards
Action" according to [RFC8126].
Following initial Sub-TLV codepoints are assigned by this document:
Value Description Reference
-----------------------------------------------------
0 Reserved This document
1 Segment Type A sub-TLV This document
2 Deprecated This document
3-8 Unassigned
9 Weight sub-TLV This document
10 Deprecated This document
11 Deprecated This document
12 Deprecated This document
13 Segment Type B sub-TLV This document
14-255 Unassigned
Table 5: SR Policy Segment List Code Points
6.6. New Registry: SR Policy Binding SID Flags
This document requests the creation of a new registry called "SR
Policy Binding SID Flags" under the "BGP Tunnel Encapsulation"
registry. The allocation policy of this registry is "Standards
Action" according to [RFC8126].
The following flags are defined:
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Bit Description Reference
-----------------------------------------------------------------
0 Specified-BSID-Only Flag (S-Flag) This document
1 Drop Upon Invalid Flag (I-Flag) This document
2-7 Unassigned
Table 6: SR Policy Binding SID Flags
6.7. New Registry: SR Policy SRv6 Binding SID Flags
This document requests the creation of a new registry called "SR
Policy SRv6 Binding SID Flags" under the "BGP Tunnel Encapsulation"
registry. The allocation policy of this registry is "Standards
Action" according to [RFC8126].
The following flags are defined:
Bit Description Reference
-----------------------------------------------------------------
0 Specified-BSID-Only Flag (S-Flag) This document
1 Drop Upon Invalid Flag (I-Flag) This document
2 SRv6 Endpoint Behavior &
SID Structure Flag (B-Flag) This document
3-7 Unassigned
Table 7: SR Policy SRv6 Binding SID Flags
6.8. New Registry: SR Policy Segment Flags
This document requests the creation of a new registry called "SR
Policy Segment Flags" under the "BGP Tunnel Encapsulation" registry.
The allocation policy of this registry is "Standards Action"
according to [RFC8126].
The following flags are defined:
Bit Description Reference
------------------------------------------------------------------
0 Segment Verification Flag (V-Flag) This document
1-2 Unassigned
3 SRv6 Endpoint Behavior &
SID Structure Flag (B-Flag) This document
4-7 Unassigned
Table 8: SR Policy Segment Flags
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6.9. New Registry: Color Extended Community Color-Only Types
This document requests the creation of a new registry called "Color
Extended Community Color-Only Types" under the "BGP Tunnel
Encapsulation" registry for assignment of codepoints (values 0
through 3) in the Color-Only Type field of the Color Extended
Community Flags field. The allocation policy of this registry is
"Standards Action" according to [RFC8126].
The following types are defined:
Type Description Reference
-----------------------------------------------------------
0 Specific Endpoint Match This document
1 Specific or Null Endpoint Match This document
2 Specific, Null, or Any Endpoint Match This document
3 Unassigned This document
Table 9: Color Extended Community Color-Only Types
7. Security Considerations
The security mechanisms of the base BGP security model apply to the
extensions described in this document as well. See the Security
Considerations section of [RFC4271] for a discussion of BGP security.
Also, refer to [RFC4272] and [RFC6952] for analysis of security
issues for BGP.
The BGP SR Policy extensions specified in this document enable
traffic engineering and service programming use-cases within an SR
domain as described in [RFC9256]. SR operates within a trusted SR
domain [RFC8402] and its security considerations also apply to BGP
sessions when carrying SR Policy information. The SR Policies
distributed by BGP are expected to be used entirely within this
trusted SR domain which comprises a single AS or multiple ASes/
domains within a single provider network. Therefore, precaution is
necessary to ensure that the SR Policy information advertised via BGP
sessions is limited to nodes in a secure manner within this trusted
SR domain. BGP peering sessions for address-families other than SR
Policy SAFI may be set up to routers outside the SR domain. The
isolation of BGP SR Policy SAFI peering sessions may be used to
ensure that the SR Policy information is not advertised by accident
or error to an EBGP peering session outside the SR domain.
Additionally, it may be considered that the export of SR Policy
information, as described in this document, constitutes a risk to
confidentiality of mission-critical or commercially sensitive
information about the network (more specifically endpoint/node
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addresses, SR SIDs, and the SR Policies deployed). BGP peerings are
not automatic and require configuration; thus, it is the
responsibility of the network operator to ensure that only trusted
nodes (that include both routers and controller applications) within
the SR domain are configured to receive such information.
8. Manageability Considerations
The specification of BGP models is an ongoing work based on
[I-D.ietf-idr-bgp-model] and its future extensions are expected to
cover the SR Policy SAFI. Existing BGP operational procedures also
apply to the SAFI specified in this document. The management,
operations, and monitoring of BGP speakers and the SR Policy SAFI
sessions between them are not very different from other BGP sessions
and can be managed using the same data models.
The YANG model for the operation and management of SR Policies
[I-D.ietf-spring-sr-policy-yang] reports the SR Policies provisioned
via BGP SR Policy SAFI along with their operational states.
9. Acknowledgments
The authors of this document would like to thank Shyam Sethuram, John
Scudder, Przemyslaw Krol, Alex Bogdanov, Nandan Saha, Bruno Decraene,
Gurusiddesh Nidasesi, Kausik Majumdar, Zafar Ali, Swadesh Agarwal,
Jakob Heitz, Viral Patel, Peng Shaofu, Cheng Li, Martin Vigoureux,
John Scudder, Vincent Roca, Brian Haberman, Mohamed Boucadair and
Shunwan Zhuang for their comments and review of this document. The
authors would like to thank Sue Hares for her detailed shepherd
review that helped in improving the document. The authors would also
like to thank Andrew Alston for his detailed AD review and comments.
10. Contributors
Eric Rosen
Juniper Networks
US
Email: erosen@juniper.net
Arjun Sreekantiah
Cisco Systems
US
Email: asreekan@cisco.com
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Acee Lindem
Cisco Systems
US
Email: acee@cisco.com
Siva Sivabalan
Cisco Systems
US
Email: msiva@cisco.com
Imtiyaz Mohammad
Arista Networks
India
Email: imtiyaz@arista.com
Gaurav Dawra
Cisco Systems
US
Email: gdawra.ietf@gmail.com
Peng Shaofu
ZTE Corporation
China
Email: peng.shaofu@zte.com.cn
Steven Lin
Calix
USA
Email: steven.lin@calix.com
11. References
11.1. Normative References
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<https://www.rfc-editor.org/info/rfc1997>.
[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>.
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[RFC2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing", RFC 2545,
DOI 10.17487/RFC2545, March 1999,
<https://www.rfc-editor.org/info/rfc2545>.
[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>.
[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>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <https://www.rfc-editor.org/info/rfc4360>.
[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>.
[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>.
[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>.
[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>.
[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>.
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[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>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
11.2. Informational References
[I-D.ietf-idr-bgp-model]
Jethanandani, M., Patel, K., Hares, S., and J. Haas, "YANG
Model for Border Gateway Protocol (BGP-4)", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-model-17, 5
July 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-idr-bgp-model-17>.
[I-D.ietf-idr-bgp-sr-segtypes-ext]
Talaulikar, K., Filsfils, C., Previdi, S., Mattes, P., and
D. Jain, "Segment Routing Segment Types Extensions for BGP
SR Policy", Work in Progress, Internet-Draft, draft-ietf-
idr-bgp-sr-segtypes-ext-01, 26 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
sr-segtypes-ext-01>.
[I-D.ietf-spring-sr-policy-yang]
Raza, S., Sawaya, R., Shunwan, Z., Voyer, D., Durrani, M.,
Matsushima, S., and V. P. Beeram, "YANG Data Model for
Segment Routing Policy", Work in Progress, Internet-Draft,
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draft-ietf-spring-sr-policy-yang-02, 23 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
sr-policy-yang-02>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
Authors' Addresses
Stefano Previdi
Huawei Technologies
Italy
Email: stefano@previdi.net
Clarence Filsfils
Cisco Systems
Brussels
Belgium
Email: cfilsfil@cisco.com
Ketan Talaulikar (editor)
Cisco Systems
India
Email: ketant.ietf@gmail.com
Paul Mattes
Microsoft
One Microsoft Way
Redmond, WA 98052
United States of America
Email: pamattes@microsoft.com
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Dhanendra Jain
Google
Email: dhanendra.ietf@gmail.com
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