Internet DRAFT - draft-zhang-idr-sr-policy-metric
draft-zhang-idr-sr-policy-metric
Network Working Group K. Zhang
Internet-Draft J. Dong
Intended status: Standards Track Huawei
Expires: 14 September 2023 K. Talaulikar
Cisco Systems
13 March 2023
BGP SR Policy Extensions for metric
draft-zhang-idr-sr-policy-metric-04
Abstract
SR Policy candidate paths can be represented in BGP UPDATE messages.
BGP can then be used to propagate the SR Policy candidate paths to
the headend nodes in the network. After SR Policy is installed on
the ingress node, the packets can be steered into SR Policy through
route selection. Therefore, route selection may be performed on the
ingress node of the SR Policy. If there are multiple routes to the
same destination, the route selection node can select routes based on
the local policy. The local policy may use the IGP metric of the
selected path, which is the IGP Metric of the SR Policy. Thus the
BGP UPDATE message need carry the metric of each segment list of the
SR Policy Candidate Path, which can be used in path selection of
routing.
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 BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown
here.
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."
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This Internet-Draft will expire on 14 September 2023.
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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. SR Policy and Tunnel Encapsulation Attribute Update . . . . . 3
4.1. Metric sub-TLV . . . . . . . . . . . . . . . . . . . . . 4
5. Metric process of SR Policy segment list . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
[I-D.ietf-idr-segment-routing-te-policy]defines SR Policy and Tunnel
Encapsulation Attributes. It defines the segment list of the SR
policies. Each segment list of an SR Policy is an segment routing
path, which may be calculated by path compuation element and
delivered to the head node of the device by BGP Update Message. On
the ingress node, when steer traffic to an SR Policy, the ingress
node may need to select between multiple SR Policy paths. And the
selection policy may need the path metric information. Therefore,
BGP needs to carry the metric of each path when delivering the
semgnet list of the SR Policy through Update messages to facilitate
route selection on the device.
2. Terminology
The following terminology is used in this document.
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SR Policy: An ordered list of segments.
Candidate Path: the unit for signaling of an SR Policy to a headend
via protocol PCEP or BGP, which is defined in
[I-D.ietf-pce-segment-routing-policy-cp] and
[I-D.ietf-idr-segment-routing-te-policy].
SRPM: SR Policy Module.
3. Motivation
In route selection scenarios, the metric of the SR Policy segment
list may be required.
The specific scenarios are as follows:
+--+ +--+ +---+
_ _ _ _ _|P1|_ _ _ _ _|P2|_ _ _ _ _|PE2|_ _ _ _
| +--+ +--+ +---+ |
| |
+---+ +---+ +---+
|CE1|_ _ _ _ |PE1| |CE1|
+---+ +---+ +---+
| +--+ +--+ +---+ |
|_ _ _ _ _|P3|_ _ _ _ _|P4|_ _ _ _ _|PE3|_ _ _ _|
+--+ +--+ +---+
On PE1, the route prefix to CE1 has two diffierent next hop, PE2 and
PE3. The next hop to PE1 uses an SR Policy1 on PE1, the endpoint of
SR Policy1 is PE2. The next hop to PE2 uses an SR Policy2 on PE1,
the endpoint of SR Policy2 is PE3. The prefix to CE1 want to choose
a next hop based on the IGP metric of the route PE1 to PE2 and PE1
and PE3, which uses SR Policy1 and SR Policy2. Thus need the IGP
metric of SR Policy semgent list on PE1.
4. SR Policy and Tunnel Encapsulation Attribute Update
As the metric is defined, the tunnel attribute encapsulation of the
BGP SR Policy needs to be updated.
The SR Policy Encoding structure is as follows:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
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Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Policy Candidate Path Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Metric
Segment
Segment
....
....
Where metric indicates the metric for the segment list.
4.1. Metric sub-TLV
A new sub-TLV called Metric sub-TLV is defined. Metric sub-TLV
specifies the metric of an SR policy Segment List. Each sub-TLV is
encoded as shown in Figure 1. More than one Metric Sub-TLVs may be
present in one Segment List to refer to the metric values of
different metric type.
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0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Metric Type | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Metric Sub-TLV
* Type: Metric, 1 octet, TBD.
* Length: 6 octets.
* Metric Type: 1-octet field which identifies the type of the metric
being used. The metric type code points are listed as follows. The
"Metric Type" code point is the same as the SR Segment List Metric
Sub-TLV "Metric Type" defined in [I-D.ietf-idr-bgp-ls-sr-policy], and
can re-use the IANA registry of it.
+------------+------------------------------------------+
| Code Point | Metric Type |
+------------+------------------------------------------+
| 0 | IGP Metric |
| 1 | Min Unidirectional Link Delay [RFC7471] |
| 2 | TE Metric [RFC3630] |
| 3 | Hop Count (refer [RFC5440]) |
| 4 | SID List Length |
| 5-250 | Unassigned |
| 251-255 | Private Use (not to be assigned by IANA) |
+------------+------------------------------------------+
Figure 2: Metric Type Code Point
* Flags: None are defined at this stage. Flags SHOULD be set to zero
on transmission and MUST be ignored on receipt.
* Metric Value: 4-octet value which indicates the metric of the
computed path.
5. Metric process of SR Policy segment list
When SR Policy headend get the SR Policy segment list with metric,
how to process the metric is local policy.
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The active candidate path of SR Policy may have several segment
lists, each segment list may have different metric type and value.
The candidate path can choose one metric type to use. If the metric
value of the Segment Lists in the candidate path is different, the
candidate path can use the maximum value as the metric of candidate
path. And the SR Policy can use the metric type and metric value of
the active candidate path.
6. Acknowledgements
TBD.
7. IANA Considerations
This document defines a new Sub-TLV in requests registries "SR Policy
List Sub- TLVs" [I-D.ietf-idr-segment-routing-te-policy]:
Value Description Reference
---------------------- ---------------------------- --------------
TBD Metric This document
Figure 3: Metric sub-TLV
8. Security Considerations
These extensions to BGP SR Policy do not add any new security issues
to the existing protocol.
9. References
[I-D.ietf-idr-bgp-ls-sr-policy]
Previdi, S., Talaulikar, K., Dong, J., Gredler, H., and J.
Tantsura, "Advertisement of Segment Routing Policies using
BGP Link-State", Work in Progress, Internet-Draft, draft-
ietf-idr-bgp-ls-sr-policy-00, 9 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
ls-sr-policy-00>.
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
Jain, D., and S. Lin, "Advertising Segment Routing
Policies in BGP", Work in Progress, Internet-Draft, draft-
ietf-idr-segment-routing-te-policy-20, 27 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
segment-routing-te-policy-20>.
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[I-D.ietf-pce-segment-routing-policy-cp]
Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H.
Bidgoli, "PCEP extension to support Segment Routing Policy
Candidate Paths", Work in Progress, Internet-Draft, draft-
ietf-pce-segment-routing-policy-cp-09, 7 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
segment-routing-policy-cp-09>.
[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>.
[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>.
Authors' Addresses
Ka Zhang
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
Email: zhangka@huawei.com
Jie Dong
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
Email: dongjie@huawei.com
Ketan Talaulikar
Cisco Systems
India
Email: ketant.ietf@gmail.com
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