Interdomain Routing Working Group C. Li
Internet-Draft Z. Li
Intended status: Standards Track Huawei Technologies
Expires: May 6, 2021 H. Chen
China Telecom
W. Cheng
China Mobile
K. Talaulikar
Cisco Systems
November 2, 2020
SR Policy Extensions for Path Segment and Bidirectional Path
draft-ietf-idr-sr-policy-path-segment-02
Abstract
A Segment Routing (SR) policy is a set of candidate SR paths
consisting of one or more segment lists with necessary path
attributes. For each SR path, it may also have its own path
attributes, and Path Segment is one of them. A Path Segment is
defined to identify an SR path, which can be used for performance
measurement, path correlation, and end-2-end path protection. Path
Segment can be also used to correlate two unidirectional SR paths
into a bidirectional SR path which is required in some scenarios, for
example, mobile backhaul transport network.
This document defines extensions to BGP to distribute SR policies
carrying Path Segment and bidirectional path information.
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 6, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
3. Path Segment in SR Policy . . . . . . . . . . . . . . . . . . 3
3.1. SR Path Segment Sub-TLV . . . . . . . . . . . . . . . . . 5
4. SR Policy for Bidirectional Path . . . . . . . . . . . . . . 6
4.1. Reverse Path Segment List Sub-TLV . . . . . . . . . . . . 7
5. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. Existing Registry: BGP Tunnel Encapsulation Attribute
sub-TLVs . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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
[I-D.ietf-spring-segment-routing-policy]. For distributing SR
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policies to the headend, [I-D.ietf-idr-segment-routing-te-policy]
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. Futhermore, in some
scenarios, for example, mobile backhaul transport network, there are
requirements to support bidirectional path. However, there is no
path identification information for each Segment List in the SR
Policies defined in [I-D.ietf-spring-segment-routing-policy]. Also,
the SR Policies defined in [I-D.ietf-spring-segment-routing-policy]
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
[I-D.ietf-spring-mpls-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 memo makes use of the terms defined in [RFC8402] and
[I-D.ietf-idr-segment-routing-te-policy].
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-segment-routing-te-policy] , the SR
Policy encoding structure is as follows:
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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
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 [I-D.ietf-spring-segment-routing-policy], 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 Segmentg 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, all of them SHOULD be inserted into the SID List.
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:
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 | ST |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Segment ID (Variable depends on ST) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. Path Segment sub-TLV
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Where:
Type: 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 |G |
+--+--+--+--+--+--+--+--+
G-Flag: Global flag. Set when the Path Segment is global within an
SR domain. The rest bits of Flag are reserved and MUST be set to 0
on transmission and MUST be ignored on receipt.
ST: Segment type, specifies the type of the Path Segment, and it has
following types:
o 0: SR-MPLS Path Segment
o 1-255:Reserved
Path Segment ID: The Path Segment ID of an SR path. The Path Segment
type is indicated by the Segment Type(ST) field. It can be a Path
Segment in SR-MPLS [I-D.ietf-spring-mpls-path-segment], or other IDs
that identifies a path. When ST is 0, the Path Segment ID is a SR-
MPLS Path Segment, and format is shown below.
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 | ST=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Segment Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. SR-MPLS Path Segment sub-TLV
4. SR Policy for Bidirectional Path
In some scenarios, 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
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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
...
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: TBA.
Length: the total length of the sub-TLVs encoded within the Reverse
Path Segment List Sub-TLV not including the Type and Length fields.
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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-segment-routing-te-policy].
o One or more mandatory SR Path Segment sub-TLVs that contains the
Path Segments of the reverse SR path.
o 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, which can be used
for directing egress BFD peer to use specific path for the reverse
direction of the BFD session [I-D.ietf-mpls-bfd-directed] or other
applications.
5. Operations
The document does not bring new operation beyond the description of
operations defined in [I-D.ietf-idr-segment-routing-te-policy]. The
existing operations defined in
[I-D.ietf-idr-segment-routing-te-policy] can apply to this document
directly.
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 is the same as
defined in [I-D.ietf-idr-segment-routing-te-policy], 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.gandhi-spring-udp-pm]. The operation of sending
information to consumers is out of scope of this document.
6. IANA Considerations
This document defines new Sub-TLVs in following registries:
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6.1. 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-segment-routing-te-policy] to be assigned by
IANA:
Codepoint Description Reference
-------------------------------------------------------------
TBA Path Segment sub-TLV This document
TBA Reverse Segment List sub-TLV This document
7. Security Considerations
TBA
8. Contributors
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
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9. Acknowledgements
Many thanks to Shraddha Hedge for her detailed review and
professional comments.
10. References
10.1. Normative References
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
Rosen, E., Jain, D., and S. Lin, "Advertising Segment
Routing Policies in BGP", draft-ietf-idr-segment-routing-
te-policy-09 (work in progress), May 2020.
[I-D.ietf-spring-mpls-path-segment]
Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler,
"Path Segment in MPLS Based Segment Routing Network",
draft-ietf-spring-mpls-path-segment-03 (work in progress),
September 2020.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-09 (work in progress),
November 2020.
[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>.
[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>.
10.2. Informative References
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[I-D.gandhi-spring-udp-pm]
Gandhi, R., Filsfils, C., daniel.voyer@bell.ca, d.,
Salsano, S., Ventre, P., and M. Chen, "UDP Path for In-
band Performance Measurement for Segment Routing
Networks", draft-gandhi-spring-udp-pm-02 (work in
progress), September 2018.
[I-D.ietf-mpls-bfd-directed]
Mirsky, G., Tantsura, J., Varlashkin, I., and M. Chen,
"Bidirectional Forwarding Detection (BFD) Directed Return
Path for MPLS Label Switched Paths (LSPs)", draft-ietf-
mpls-bfd-directed-15 (work in progress), August 2020.
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
Huanan Chen
China Telecom
109 West Zhongshan Ave
Guangzhou
China
Email: chenhuan6@chinatelecom.cn
Weiqiang Cheng
China Mobile
Beijing
China
Email: chengweiqiang@chinamobile.com
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Ketan Talaulikar
Cisco Systems
Email: ketant@cisco.com
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