Network Working Group K. Zhang
Internet-Draft J. Dong
Intended status: Informational Huawei
Expires: 10 May 2023 6 November 2022
BGP SR Policy Extensions for metric
draft-zhang-idr-sr-policy-metric-01
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."
This Internet-Draft will expire on 10 May 2023.
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Copyright Notice
Copyright (c) 2022 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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provided without warranty as described in the Revised BSD License.
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 . . . . . . . . . . 6
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.
SR Policy: An ordered list of segments.
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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>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
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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. The Metric
sub-TLV format is the same as the SR Segment List Metric Sub-TLV
defined in [I-D.ietf-idr-te-lsp-distribution].
Each sub-TLV is encoded as shown in Figure 1.
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 Margin |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric Bound |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Metric Sub-TLV
* Type: Metric, 1 octet, TBD.
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* Length: 12 octets.
* Metric Type: 1-octet field which identifies the type of the metric
being used. The metric type code points are listed as follows.
+============+=========================================+
| 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 |
+------------+-----------------------------------------+
Table 1: Metric Sub-TLV
* Flags: 1-octet field that indicates the validity of the metric
fields and their semantics. The following bit positions are
defined and the other bits MUST be cleared by the originator and
MUST be ignored by a receiver.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|M|A|B|V| |
+-+-+-+-+-+-+-+-+
where:
- M-Flag: Indicates that the metric margin allowed for this path
computation is specified when set
- A-Flag: Indicates that the metric margin is specified as an
absolute value when set and is expressed as a percentage of the
minimum metric when clear.
- B-Flag: Indicates that the metric bound allowed for the path is
specified when set.
- V-Flag: Indicates that the metric value computed is being reported
when set.
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* Metric Margin: 4-octet value which indicates the metric margin
value when the M-flag is set. The metric margin is specified
as either an absolute value or as a percentage of the minimum
computed path metric based on the A-flag. The metric margin
loosens the criteria for minimum metric path calculation up to
the specified metric to accomodate for other factors such as
bandwidth availability, minimal SID stack depth, and maximizing
of ECMP for the SR path computed.
* Bound: 4-octet value which indicates the maximum metric
value that is allowed when the B-flag is set. If the computed
path metric crosses the specified bound value then the path is
considered invalid.
* Metric Value: 4-octet value which indicates the metric of the
computed path when the V-flag is set. This value is available
and reported when the computation is successful and a valid
path is available.
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.
The active candidate path of SR Policy may have several segment
lists, each segment list have different metric. It is recommended
that the segment lists in one candidate path have the same metric
type. If the metric value of segment lists in one candidate path is
different, the candidate path metric can use the maximum value as the
metric of candidate path. And the SR Policy metric use the metric
value of active candidate path.
6. Acknowledgements
TBD.
7. IANA Considerations
This document requests that IANA allocates a new codepoints allocated
to the field defined in Section 4.1 from the "Sub-TLVs for SR Policy"
registry as specified.
8. Security Considerations
These extensions to BGP SR Policy do not add any new security issues
to the existing protocol.
9. References
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[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://www.ietf.org/archive/id/draft-ietf-idr-segment-
routing-te-policy-20.txt>.
[I-D.ietf-idr-te-lsp-distribution]
Previdi, S., Talaulikar, K., Dong, J., Chen, M., Gredler,
H., and J. Tantsura, "Distribution of Traffic Engineering
(TE) Policies and State using BGP-LS", Work in Progress,
Internet-Draft, draft-ietf-idr-te-lsp-distribution-18, 22
August 2022, <https://www.ietf.org/archive/id/draft-ietf-
idr-te-lsp-distribution-18.txt>.
[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-08, 24 October 2022,
<https://www.ietf.org/archive/id/draft-ietf-pce-segment-
routing-policy-cp-08.txt>.
[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>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
Authors' Addresses
Ka Zhang
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
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Jie Dong
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing
100095
China
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