Schedule for Segment Routing Policy
draft-zzd-spring-schedule-sr-policy-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Li Zhang , Jie Dong , Tianran Zhou | ||
| Last updated | 2024-09-29 | ||
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| Intended RFC status | (None) | ||
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| Stream | Stream state | (No stream defined) | |
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draft-zzd-spring-schedule-sr-policy-00
SPRING L. Zhang, Ed.
Internet-Draft J. Dong
Intended status: Standards Track T. Zhou
Expires: 2 April 2025 Huawei
29 September 2024
Schedule for Segment Routing Policy
draft-zzd-spring-schedule-sr-policy-00
Abstract
This document defines the Segment Routing (SR) Policy extension for
schedule (called SR-Schedule). It applies to both Segment Routing
over IPv6 (SRv6) and SR-MPLS. This document specifies the framework
of SR policy schedule including the SR-Schedule definition,
acquisition, computation, enforcement, and the handling behaviors on
the headend.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 2 April 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. SR-Schedule Definition for SR Policy . . . . . . . . . . . . 3
2.1. SR-Schedule of a Segment List . . . . . . . . . . . . . . 4
2.2. SR-Schedule of a Candidate Path . . . . . . . . . . . . . 4
3. The Framework of SR-Schedule for SR Policy . . . . . . . . . 4
3.1. Schedule Information Acquisition . . . . . . . . . . . . 5
3.2. SR-Schedule Computation . . . . . . . . . . . . . . . . . 5
3.2.1. SR-Schedule Computation with Flow Constraint . . . . 5
3.2.2. SR-Schedule Computation with Topology Constraint . . 5
3.2.3. SR-Schedule Computation with Mixed Constraint . . . . 5
3.3. SR-Schedule Enforcement . . . . . . . . . . . . . . . . . 6
3.4. Handling Behaviors on the Headend . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Segment Routing (SR) policy [RFC9256] is a set of candidate SR paths
consisting of one or more segment lists and necessary path
attributes. It enables instantiation of an ordered list of segments
with a specific intent for traffic steering.
[I-D.ietf-idr-segment-routing-te-policy] specifies how BGP may be
used to distribute SR Policy candidate paths. It introduces a BGP
SAFI with new NLRI to advertise a candidate path of a Segment Routing
(SR) Policy.
[I-D.ietf-tvr-use-cases] introduces a set of use cases where the
topology or attributes of the network changes predictably. The
topology changes may influence tha validity of some paths, and lead
to path reselection or even recalculation. However, the reselection
or recalculation takes a period of time, which will influence packet
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forwarding and cause problems such as packet disorder and packet
loss. However, on a network with predictable topology changes, the
headend knows future topology changes, it can schedule the forwarding
paths in advance, and steer flows to different set of paths based on
time to prevent packet forwarding from being influenced by topology
changes.
In the scenario of SR-policy-based TE paths, the resource allocation
efficiency is a big challenge. Some flows just last for a short
time, but the TE paths resources for the flows are usually reserved
for a long time and cannot be used by other services. Therefore, the
SR policy originator can generate a policy with time-limited paths
and resources, the headend schedules these paths over time to improve
the utilization of resources.
This document extends SR Policy to include the schedule time of
candidate paths/SR lists and applies to both SRv6 and SR-MPLS.
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-Schedule Definition for SR Policy
The Segment Routing policy architecture is specified in [RFC9256].
An SR Policy is associated with one or more candidate paths. A
candidate path is either dynamic, explicit, or composite. The
related concepts with the SR-Schedule definition in this document are
listed as follows.
An explicit/dynamic candidate path is expressed as a Segment-List or
a set of Segment-Lists directly or by computation. If a candidate
path is associated with a set of Segment-Lists, each Segment-List is
associated with weight for weighted load balancing. The default
weight is 1.
A composite candidate path is defined in [RFC9256].
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2.1. SR-Schedule of a Segment List
A segment list represents a specific source-routed path to send
traffic from the headend to the endpoint of the corresponding SR
policy [RFC9256]. The SR-Schedule of a segment list is defined as
the valid period of the path between a source node and a destination
node.
2.2. SR-Schedule of a Candidate Path
In the case of an explicit/dynamic candidate path, if it is expressed
as a single Segment-List, then the SR-Schedule of the candidate path
is the same as that of the SR-Schedule of the segment list as
described in Section 2.1.
In the case of an explicit/dynamic candidate path, if it is expressed
as a set of Segment-Lists (for load-balancing), then the SR-Schedule
of the candidate path is defined as the valid period of all the
Segment-Lists in the set.
3. The Framework of SR-Schedule for SR Policy
The framework of SR-Schedule for SR Policy includes schedule information acquisition, SR-Schedule computation, SR-Schedule enforcement, and handling behaviors on the headend.
+------------------+
| Network Manager |
+------------------+
|
Schedule information acquisition
|
+--------\|/-------+
+------------|Network Controller| SR-Schedule computation
| +--------/|\-------+
| |
SR-Schedule enforcement Schedule information acquisition
| |
+-\|/-+ +-----------|-----------+ +-----+
Handling |Head |-------| Segment Routing |---|End |
behaviors |end | | Network Domain | |Point|
+-----+ +-----------------------+ +-----+
Figure 1: Framework of SR-Schedule for SR Policy
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3.1. Schedule Information Acquisition
The SR-Schedule of a segment list is defined as the valid period of
the path, see Section 2.1. The valid period of a path can be limited
by a variety of factors, for example the flow’s predicted variation,
the predicted availability of nodes and links. The schedule
information could be acquired from the Network Manager or network
devices by YANG data model or routing protocol advertisement.
3.2. SR-Schedule Computation
The schedule information is sent to the network controller or the
headend where the SR-Schedule is computed. Depending upon the source
of time constraint, the computation methods are different, which are
described in the following subsections.
3.2.1. SR-Schedule Computation with Flow Constraint
When the flow with predictable time limit requests a SR path and the
topology is stable, the SR-Schedule calculation only needs to
consider the time limit of the flow. During calculation, the
controller needs to calculate the optimal path that meets flow
requirements, and then generate the SR-Schedule based on the flow
variation regularity.
3.2.2. SR-Schedule Computation with Topology Constraint
When the flow is constant but the network topology changes
predictably, the SR-Schedule calculation only needs to consider the
topology change. During calculation, multiple paths need to be
calculated based on flow requirements. The valid time of each path
is the intersection of the available time of all nodes and links in
the path. There must be no gap between two paths adjacent in time to
ensure the transmission will not be interrupted.
3.2.3. SR-Schedule Computation with Mixed Constraint
When the requested flow has a predicable time limit and the topology
also changes predictably, the SR-Schedule calculation needs to
consider both the flow and topology time constraints. During
calculation, the controller first obtains the period that needs to be
covered according to the flow time constraint. Then, the controller
calculates multiple paths that meet the flow's requirements. The
valid time of each path is the intersection of the available time of
all nodes and links in the path. There must be no gap between two
paths adjacent in time to ensure the transmission will not be
interrupted.
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3.3. SR-Schedule Enforcement
SR Policy as per [RFC9256] does not include SR-Schedule in the SR
Policy encoding structure. As specified in
[I-D.zzd-idr-sr-policy-scheduling], the SR-Schedule is encoded in the
SR policy structure as shown in Figure 2 and Figure 3. After the SR-
Schedule computation, the SR-Schedule is enforced along with the SR
Policy to the headend of the corresponding path.
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
-----> Scheduling Time Information (SR-Schedule)
Segment List
Weight
Segment
Segment
...
...
Figure 2: SR Policy Structure with SR-Schedule at Candidate Path
Level
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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
Segment List
--------> Scheduling Time Information (SR-Schedule)
Weight
Segment
Segment
...
...
Figure 3: SR Policy Structure with SR-Schedule at Segment List Level
3.4. Handling Behaviors on the Headend
After the SR Policy with SR-Schedule is computed, the headend
performs the handling behaviors.
After obtaining the SR Policy with SR-Schedule, the headend needs to
select the available paths at this moment according to the effective
time of the candidate path and the segment list.
When a packet arrives, the headend encapsulates the segment list of
the forwarding path in the packet header based on the available
paths.
If no path is available at a certain time point, the SR Policy is
considered malformed and should be logged with error.
4. Security Considerations
[RFC9256] specifies in detail the SR Policy construct (introduced in
[RFC8402]) and its security considerations. The additional SR-
Schedule attribute information can be sensitive in some deployments
and could influence SR path setup, selection and switching with
adverse effect. The protocol extensions that include SR-Schedule
need to take this into consideration. This document does not define
any new protocol extensions and thus does not introduce any further
security considerations.
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5. IANA Considerations
6. Acknowledgement
7. References
7.1. Normative References
[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>.
[I-D.ietf-idr-segment-routing-te-policy]
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-segment-
routing-te-policy-26, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
segment-routing-te-policy-26>.
[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>.
7.2. Informative References
[I-D.ietf-tvr-use-cases]
Birrane, E. J., Kuhn, N., Qu, Y., Taylor, R., and L.
Zhang, "TVR (Time-Variant Routing) Use Cases", Work in
Progress, Internet-Draft, draft-ietf-tvr-use-cases-09, 29
February 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-tvr-use-cases-09>.
[I-D.zzd-idr-sr-policy-scheduling]
Zhang, L., Zhou, T., Dong, J., Wang, M., and N. Nzima,
"BGP SR Policy Extensions for Path Scheduling", Work in
Progress, Internet-Draft, draft-zzd-idr-sr-policy-
scheduling-05, 29 August 2024,
<https://datatracker.ietf.org/doc/html/draft-zzd-idr-sr-
policy-scheduling-05>.
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[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>.
Authors' Addresses
Li Zhang (editor)
Huawei
Beiqing Road
Beijing
China
Email: zhangli344@huawei.com
Jie Dong
Huawei
Email: jie.dong@huawei.com
Tianran Zhou
Huawei
Email: zhoutianran@huawei.com
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