SR Policy Extensions for Path Segment and Bidirectional Path
draft-ietf-idr-sr-policy-path-segment-13
| Document | Type | Active Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | Cheng Li , Zhenbin Li , Yuanyang Yin , Weiqiang Cheng , Ketan Talaulikar | ||
| Last updated | 2024-10-02 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Susan Hares | ||
| Shepherd write-up | Show Last changed 2024-06-07 | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | shares@ndzh.com |
draft-ietf-idr-sr-policy-path-segment-13
Interdomain Routing Working Group C. Li
Internet-Draft Z. Li
Intended status: Standards Track Huawei Technologies
Expires: 5 April 2025 Y. Yin
China Telecom
W. Cheng
China Mobile
K. Talaulikar
Cisco Systems
2 October 2024
SR Policy Extensions for Path Segment and Bidirectional Path
draft-ietf-idr-sr-policy-path-segment-13
Abstract
A Segment Routing(SR) policy identifies a set of candidate SR paths
Each SR path is passed in BGP as the SR Policy SAFI NLRI accompanied
with the Tunnel Encapsulation attribute (Tunnel-encaps). Each SR
Path (tunnel) uses a set of TLVs in the Tunnel-encaps attribute to
describe the characteristics of the SR Policy tunnel. One of the
TLVs that describes the tunnel is the Segment list TLV which provides
a list of segments contained in the tunnel.
This document specifies a new Path Segment Sub-TLV to associate a
Path Segment ID to the SR Segment List. The Path Segment ID can be
used for performance measurement, path correlation, and end-2-end
path protection. This Path Segment identifier can be also be used to
correlate two unidirectional SR paths into a bidirectional SR path.
Bidirection SR path may be required in some scenarios such as mobile
backhaul transport network.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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/.
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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 5 April 2025.
Copyright Notice
Copyright (c) 2024 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. Path Segment in SR Policy . . . . . . . . . . . . . . . . . . 4
3.1. SR Path Segment Sub-TLV . . . . . . . . . . . . . . . . . 5
4. SR Policy for Bidirectional Path . . . . . . . . . . . . . . 7
4.1. Reverse Path Segment List Sub-TLV . . . . . . . . . . . . 8
5. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Error Handling and Fault Management . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. Existing Registry: BGP Tunnel Encapsulation Attribute
sub-TLVs . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Segment routing (SR) [RFC8402] is a source routing paradigm that
explicitly indicates the forwarding path for packets at the ingress
node. The ingress node steers packets into a specific path according
to the Segment Routing Policy ( SR Policy) as defined in [RFC9256].
For distributing SR policies to the headend, [RFC9256] specifies a
mechanism by using BGP, and new Sub-TLVs are defined for SR Policies
in BGP UPDATE message.
In many use cases such as performance measurement, the path to which
the packets belong is required to be identified. In some scenarios,
(e.g., Mobile backhaul transport networks), there are Requirements to
support bidirectional path. However, there is no path identification
information for each Segment List in the SR Policies defined in
[RFC9256]. Also, the SR Policies defined in [RFC9256] only supports
unidirectional SR paths.
Therefore, this document defines the extension to SR policies that
carry Path Segment in the Segment List and support bidirectional
path. The Path Segment can be a Path Segment in SR-MPLS [RFC9545]
and SRv6 [I-D.ietf-spring-srv6-path-segment], or other IDs that can
identify a path. Also, this document defines extensions to BGP to
distribute SR policies carrying Path Segment and bidirectional path
information.
2. Terminology
This document makes use of the terms defined in [RFC8402] and
[RFC9256]. Some terms are listed below for reference.
* SR: Segment Routing.
* SR-MPLS: Segment Routing over MPLS data plane.
* SRv6: Segment Routing over IPv6 data plane.
* PSID: Path Segment Identifier.
* SRPM: SR Policy Module [I-D.ietf-idr-sr-policy-safi].
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2.1. 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.
3. Path Segment in SR Policy
As defined in [I-D.ietf-idr-sr-policy-safi] , the SR Policy encoding
structure is as follows:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Segment
Segment
...
...
An SR path can be specified by an Segment List Sub-TLV that contains
a set of segment Sub-TLVs and other Sub-TLVs as shown above. As
defined in [RFC9256], a candidate path includes multiple SR paths
specified by SID list. The Path Segment can be used for identifying
an SR path (specified by SID list) from the headend and the tailend.
Also, it can be used for identifying an SR candidate path in some use
cases if needed. This document defines a new Path Segment Sub-TLV
within Segment List Sub-TLV, the details will be described at section
3.1. The new SR Policy encoding structure with Path Segment Sub-TLV
is expressed as below:
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SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Path Segment
Segment
Segment
...
Segment List
Weight
Path Segment
Segment
Segment
...
...
The Path Segment is used to identified an SR path, and it can be used
in OAM or IOAM use cases. When all the SID Lists within a candidate
path share the same Path Segment ID, the Path Segment can be used to
collect the aggregated information of the candidate path. Multiple
Path Segment MAY be included in a Segment List for different use
cases. In SR-MPLS, one, or some or all of them MAY be inserted into
the SID List as the requirement of the use case. However, in SRv6,
only one Path Segment ID can be encoded in a SRH. Therefore, an
implementation MUST decide how to choose a Path Segment ID from the
multiple Path Segment IDs. In order to simplify the implementation,
this document suggests to encode only one Path Segment Sub-TLV for a
segment list, while the rest Path Segment SHOULD be ignored.
3.1. SR Path Segment Sub-TLV
This section defines an SR Path Segment Sub-TLV.
An SR Path Segment Sub-TLV is included in the segment list Sub-TLV to
identify an SID list. It 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Segment ID (Variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// SRv6 Endpoint Behavior and SID Structure (optional) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. Path Segment Sub-TLV
Where:
* Type (TBA1): SR Path Segment Sub-TLV (to be assigned by IANA).
* Length: the total length of the value field not including Type and
Length fields.
* Flags: 8 bits of flags. Following flags are defined:
0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+
| Reserved |B |L |
+--+--+--+--+--+--+--+--+
* - L-Flag: Local flag. Set when the Path Segment has local
significance on an SR node.
- 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. of [I-D.ietf-idr-sr-policy-safi]. It MUST
be ignored when the value of length field is smaller than 18.
- The rest bits of Flag are reserved and MUST be set to 0 on
transmission and MUST be ignored on receipt.
* Path Segment ID: if the length is 2, then no Path Segment ID is
present. If the length is 6 then the Path Segment ID is encoded
in 4 octets [RFC9545] using the format below. TC, S, TTL (Total
of 12 bits) are RESERVED and SHOULD 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Segment Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. SR-MPLS Path Segment Sub-TLV
If the length is 18 then the Path Segment ID contains a 16-octet SRv6
Path Segment ID [I-D.ietf-spring-srv6-path-segment].
If the length is larger than 18 and B-flag is set, then SRv6 Endpoint
Behavior and SID Structure TLVs is included as per Section 2.4.4.2.4.
of [I-D.ietf-idr-sr-policy-safi].
4. SR Policy for Bidirectional Path
In some scenariose, for example, mobile backhaul transport network,
there are requirements to support bidirectional path. In SR, a
bidirectional path can be represented as a binding of two
unidirectional SR paths. This document also defines a Reverse
Segment List Sub-TLV to describe the reverse path associated with the
forward path specified by the Segment List. An SR policy carrying SR
bidirectional path information is expressed as below:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes: Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Path Segment
Segment
Segment
...
Reverse Segment List
Path Segment
Segment
Segment
...
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4.1. Reverse Path Segment List Sub-TLV
A Reverse Path Segment List Sub-TLV is defined to specify an SR
reverse path associated with the path specified by the Segment List,
and it 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 (Variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. SR Reverse Path Segment List Sub-TLV
where:
Type (TBA2): Reverse Path Segment List Sub-TLV (to be assigned by
IANA).
Length: the total length of the Sub-TLVs encoded within the Reverse
Path Segment List Sub-TLV not including the Type and Length fields.
RESERVED: 1 octet of reserved bits. SHOULD be unset on transmission
and MUST be ignored on receipt.
Sub-TLVs, reuse the Sub-TLVs in Segment List defined in
[I-D.ietf-idr-sr-policy-safi] and [I-D.ietf-idr-bgp-sr-segtypes-ext].
* One or more mandatory SR Path Segment Sub-TLVs that contains the
Path Segments of the reverse SR path.
* One or more Segment Sub-TLVs to specify the reverse SR path.
The Segment sub-TLVs in the Reverse Path Segment List sub-TLV
provides the information of the reverse SR path. This Reverse Path
Segment list can be used for directing egress BFD peer to use
specific path for the reverse direction of the BFD session [RFC9612]
or other applications.
5. Operations
The document defines a new Sub-TLV under the extensions for SR policy
defined in [I-D.ietf-idr-sr-policy-safi], therefore, the description
of operations defined in [I-D.ietf-idr-sr-policy-safi], can apply to
this document directly, including advertisement of SR policies and
reception of SR policy NLRI.
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Typically but not limit to, the unidirectional or bidirectional SR
policies carrying path identification infomation are configured by a
controller.
After configuration, the unidirectional or bidirectional SR policies
carrying path identification infomation will be advertised by BGP
update messages. The operation of advertisement this SR policy is
the same as defined in [I-D.ietf-idr-sr-policy-safi], as well as the
reception.
The consumer of the unidirectional or bidirectional SR policies is
not the BGP process, it can be any applications, such as performance
measurement [I-D.ietf-spring-stamp-srpm]. The operation of sending
information to consumers is out of scope of this document.
6. Error Handling and Fault Management
The document defines a new Sub-TLV under the extensions for SR policy
defined in [I-D.ietf-idr-sr-policy-safi], therefore, the error
handling defined in [I-D.ietf-idr-sr-policy-safi] can apply to this
document. Also, the error handling as defined in [RFC7606] applies
to new Sub-TLVs as well as SAFI context, therefore, the error
handling in [RFC7606] also applies to this document.
Specifically, a BGP Speaker MUST perform Syntax validation of the
Tunnel Encapsulation Attribute following the error handling defined
in [RFC7606] and [I-D.ietf-idr-sr-policy-safi].
In addition, a BGP Speaker MUST perform Syntax validation of the new
Path Segment Sub-TLV to determine if it is malformed. This includes
the validation of the length of the Sub-TLV and the range of the
value fileds. If any validation check fails, the Update message MUST
be handle as 'Treat-as-withdraw'
In addition, the validation of the individual fields of the TLVs/Sub-
TLVs of the associated segment list are beyond the scope of BGP as
they are handled by the SR Policy Module
[I-D.ietf-idr-sr-policy-safi] as described in the individual TLV/Sub-
TLV sub-sections. Therefore this part is out of the scope of this
document. 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 [I-D.ietf-idr-sr-policy-safi].
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7. IANA Considerations
This document defines new Sub-TLVs in following registries:
7.1. Existing Registry: BGP Tunnel Encapsulation Attribute sub-TLVs
This document defines new Sub-TLVs in the registry "SR Policy List
Sub-TLVs" [I-D.ietf-idr-sr-policy-safi] to be assigned by IANA:
Codepoint Description Reference
-------------------------------------------------------------
TBA(17) Path Segment Sub-TLV This document
TBA(18) Reverse Segment List Sub-TLV This document
8. Security Considerations
Similar to [I-D.ietf-idr-sr-policy-safi], the security mechanisms of
the base BGP security model [RFC4271] apply to the extensions
described in this document. Also, the new security considerations
defined in [I-D.ietf-idr-sr-policy-safi] also apply to this document.
The Path Segment extension is included in the SR Policy extension
[I-D.ietf-idr-sr-policy-safi], so it does not introduce extra
security problems comparing the existing SR policy entension. The
Path Segment information is critical to the path, and a wrong Path
Segment ID may cause unexpected forwarding actions and results.
An implementation needs to make sure that the value of Path Segment
ID is correct to avoid unexpected forwarding actions and results,
especially in an SR-MPLS network. In addition, the Path Segment
information distribution from a controller to an ingress router has
to be protected. The security considereations in
[I-D.ietf-idr-sr-policy-safi] apply to this distribution procedure.
9. Contributors
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Mach(Guoyi) Chen
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: Mach.chen@huawei.com
Jie Dong
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: jie.dong@huawei.com
James N Guichard
Futurewei Technologies
2330 Central Express Way
Santa Clara
USA
Email: james.n.guichard@futurewei.com
Huanan Chen
China Telecom
109 West Zhongshan Ave
Guangzhou
China
Email: chenhuan6@chinatelecom.cn
10. Acknowledgements
Many thanks to Shraddha Hedge, Susan Hares for their detailed reviews
and professional comments.
11. References
11.1. Normative References
[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-
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idr-bgp-sr-segtypes-ext-04, 30 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
sr-segtypes-ext-04>.
[I-D.ietf-idr-sr-policy-safi]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
Work in Progress, Internet-Draft, draft-ietf-idr-sr-
policy-safi-06, 26 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-safi-06>.
[I-D.ietf-spring-srv6-path-segment]
Li, C., Cheng, W., Chen, M., Dhody, D., and Y. Zhu, "Path
Segment Identifier (PSID) in SRv6 (Segment Routing in
IPv6)", Work in Progress, Internet-Draft, draft-ietf-
spring-srv6-path-segment-11, 18 September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
srv6-path-segment-11>.
[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>.
[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>.
[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>.
[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>.
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[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>.
[RFC9545] Cheng, W., Ed., Li, H., Li, C., Ed., Gandhi, R., and R.
Zigler, "Path Segment Identifier in MPLS-Based Segment
Routing Networks", RFC 9545, DOI 10.17487/RFC9545,
February 2024, <https://www.rfc-editor.org/info/rfc9545>.
11.2. Informative References
[I-D.ietf-spring-stamp-srpm]
Gandhi, R., Filsfils, C., Voyer, D., Chen, M., and R. F.
Foote, "Performance Measurement Using Simple Two-Way
Active Measurement Protocol (STAMP) for Segment Routing
Networks", Work in Progress, Internet-Draft, draft-ietf-
spring-stamp-srpm-15, 24 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
stamp-srpm-15>.
[RFC9612] Mirsky, G., Tantsura, J., Varlashkin, I., and M. Chen,
"Bidirectional Forwarding Detection (BFD) Reverse Path for
MPLS Label Switched Paths (LSPs)", RFC 9612,
DOI 10.17487/RFC9612, July 2024,
<https://www.rfc-editor.org/info/rfc9612>.
Authors' Addresses
Cheng Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing
100095
China
Email: c.l@huawei.com
Zhenbin Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing
100095
China
Email: lizhenbin@huawei.com
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Yuanyang Yin
China Telecom
Guangzhou
China
Email: yinyuany@chinatelecom.cn
Weiqiang Cheng
China Mobile
Beijing
China
Email: chengweiqiang@chinamobile.com
Ketan Talaulikar
Cisco Systems
Email: ketant.ietf@gmail.com
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