IGP Extensions for Optimized SRv6 SID Advertisement
draft-cheng-lsr-extension-opt-srv6-sid-adv-00
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draft-cheng-lsr-extension-opt-srv6-sid-adv-00
Network Working Group W. Cheng
Internet Draft L. Gong
Intended status: Standards Track China Mobile
Expires: December 29, 2024 C. Lin
New H3C Technologies
Louis Chan
Individual
June 29, 2024
IGP Extensions for Optimized SRv6 SID Advertisement
draft-cheng-lsr-extension-opt-srv6-sid-adv-00
Abstract
When the IGP runs SRv6 Flex-Algo or performs QoS resource
allocation, it needs to assign a large number of END.X SIDs, which
can significantly impact IGP LSDB advertisements and overall
performance.
This document proposes a simplified method for advertising a large
number of SRv6 SIDs. This method is particularly useful in scenarios
that require generating many END.X SIDs, such as when supporting
numerous Flex-Algo algorithms. It helps reduce the size of LSDB
advertisements and improves IGP advertisement efficiency and
operational performance.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 29, 2024.
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
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction...................................................3
1.1. Conventions and Terminology...............................4
2. Motivation.....................................................4
3. Solution.......................................................4
3.1. IS-IS extension...........................................6
3.2. OSPFv3 extension..........................................7
4. Security Considerations........................................8
5. Compatibility considerations...................................8
6. IANA Considerations............................................9
6.1. IS-IS SRv6 Locator LSA Sub-TLVs...........................9
6.2. OSPFv3 SRv6 Locator LSA Sub-TLVs..........................9
7. References.....................................................9
7.1. Normative References......................................9
7.2. Informative References...................................10
Authors' Addresses...............................................11
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1. Introduction
The Segment Routing (SR) allows for a flexible definition of end-to-
end paths by encoding paths as sequences of topological sub-paths,
called "segments". As defined in [RFC8402] and [RFC8986], an SRv6
Segment Identifier (SID) is an IPv6 address explicitly associated
with the segment and consists of Locator, Function and Argument
parts.
[RFC9352] defines the SRv6 End SID sub-TLV, the SRv6 End.X SID sub-
TLV, and the SRv6 LAN End.X SID sub-TLV in IS-IS.
The SRv6 End SID sub-TLV is used to advertise an SRv6 SID with
Endpoint behaviors which do not require a particular neighbor. The
SRv6 End SID sub-TLV is used to advertise an SRv6 SID associated
with a point to point adjacency. The SRv6 LAN End.X SID sub-TLV sub-
TLV is used to advertise an SRv6 SID associated with a LAN
adjacency. Each of these sub-TLVs contains a complete 128-bit SID
and the sub-TLV length is quite long.
Multiple SRv6 End.X SIDs can be associated with the same point to
point adjacency or the same physical LAN neighbor. Each SID is
advertised in a single SRv6 End.X SID sub-TLV or SRv6 LAN End.X SID
sub-TLV. These SIDs are possibly associated to the same Locator,
therefore the main differences among the sub-TLVs may be a few bits
in the Function part of SID and the Endpoint Behavior value
indicating different flavors.
The number of End.X SIDs has a positive correlation with the number
of neighbors. Assume that, each neighbor is assigned with End.X
SIDs, and each End.X behavior has several different flavors, such as
PSP, USP, USD, no PSP/USP/USD, etc. Then, the number of End.X SIDs
will be at least the number of neighbors multiplied by the number of
flavors.
If Flexible-Algorithm is applied on SRv6 forwarding plane as defined
in [RFC9350], a node generally advertises a Flex-Algorithm specific
locator for each Flex-Algorithm it participates in and also
advertises associated SRv6 END.X SIDs for every link that has not
been pruned from the Flex-Algorithm computation.
This document proposes a minimal advertisement method for
advertising the bulk generation of specific END.X SIDs. Other
routers can use the baseline END.X SID to generate specific END.X
SIDs in bulk for particular scenarios and use them in the
computation of paths for SR-TE or TI-LFA.
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The most common practice is to generate bulk END.X SIDs based on the
Flex-Algo algorithm. After generating the END.X SID in algorithm 0,
the source device uses it as the baseline END.X SID, directly
inherits the Func field from the baseline SID, and generates END.X
SIDs in other algorithms. When notifying the END.X SID, the source
router only needs to advertise the END.X SID in algorithm 0. Other
routers calculate END.X SIDs in other algorithms based on the same
algorithm.This method can also be used in other similar scenarios
for bulk generating END.X SIDs, such as in HQoS queues scenarios.
1.1. Conventions and Terminology
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. Motivation
In SRv6, the IGP protocol assigns an END.X SID to each neighbor.
SRv6 TE or TI-LFA functionality is achieved by specifying the actual
path of traffic using END.X SIDs as the path for a specified segment
routing.
The IGP protocol advertises neighbor information, which includes the
assigned END.X SID for each neighbor. Other routers extract the
END.X SID from the neighbor information to obtain the END.X SID
associated with that neighbor.
To support Flex-Algo, an END.X SID will be assigned for each Flex-
Algo algorithm associated with a neighbor. In scenarios with a large
number of neighbors and support for numerous Flex-Algo algorithms,
advertising the corresponding END.X SIDs for each neighbor can
result in a large LSDB, which in turn can lead to extensive LSDB
flooding.
To address this issue, this document describes a mechanism where,
for a given neighbor, it is sufficient to advertise the END.X SIDs
associated with the common Flex-Algo algorithm and the relationships
between the END.X SIDs in other Flex-Algo algorithms and the END.X
SIDs in the common Flex-Algo algorithm. This significantly reduces
the size of the IGP LSDB and improves operational performance.
3. Solution
Segment Identifier (SID) - A 128-bit IPv6 address that represents
the SRv6 instruction. It consists of locator ,function, args, and
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MBZ parts, where the locator is used to identify the destination
node, and the function identifier indicates the specific operation
to be performed on packets at that node. As shown in figure 1:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block | Node ID | Function | Agruments | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|<-------Locator------->|
Figure 1: SRv6 SID Structure
In general, each Flex-Algo algorithm specifies an SRv6 Locator, and
designates a prefix length via the Locator. When assigning adjacency
labels to neighbors for this Flex-Algo algorithm, the specified
Locator prefix is followed, and a Function is assigned to form an
adjacency label, with the args and MBZ fields typically left
unspecified. This approach allows for the allocation of an SRv6
adjacency label for each supported Flex-Algo.
For example, the common algorithm FA0 assigns END.X SID as
Locator0.Func0.Args, algorithm FA1 assigns Locator1.Func1.Args, and
algorithm FA2 assigns Locator2.Func2.Args. When announcing to the
outside, it is necessary to publish the adjacency labels from each
Flex-Algo algorithm. The range of Flex-Algo algorithm is from 128 to
255. In the given examples, we have omitted the MBZ section. Please
refer to Figure 2 for the specific allocation of adjacency SIDs.
+==========+====================+=========================+
|Flex-Algo | Locator | END.X SID |
+==========+====================+=========================+
| FA0 | Locator0 | Locator0.Func0.Args |
+----------+--------------------+-------------------------+
| FA1 | Locator1 | Locator1.Func1.Args |
+----------+--------------------+-------------------------+
| FAx | Locatorx | Locatorx.Func2.Args |
+----------+--------------------+-------------------------+
Figure 2: END.X SID Allocation in Flex-Algo
When there are numerous neighbors and support for a large number of
Flex-Algo algorithms, announcing a large number of SRv6 adjacency
SIDs can significantly impact IGP LSDB advertisements and overall
performance.
To address this issue, we propose utilizing the adjacency SID from
the common algorithm to automatically calculate the corresponding
adjacency SIDs in the respective of the Flex-Algo algorithms.
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When announcing the adjacency SIDs externally, we only need to
announce the adjacency SIDs corresponding to the common Flex-Algo
algorithm. The adjacency SIDs for other Flex-Algo algorithms will be
generated based on this information. The method of generation
involves inheriting the Func section of the Func section.
+==========+=========+========================+
|Flex-Algo |Locator | END.X SID |
+==========+=========+========================+
|FA0 |Locator0 | Locator0.Func.Args |
|(Common) | | |
+----------+---------+------------------------+
| FA1 |Locator1 | Locator1.Func.Args |
+----------+----------------------------------+
| FAx |Locatorx | Locatorx.Func.Args |
+----------+---------+------------------------+
Figure 3: Bulk END.X SID Allocation in Flex-Algo
Furthermore, when advertising the Locator information used by each
Flex-Algo, it is necessary to include positions for inheriting the
Func segment from the common adjacency SID.
Typically, the length of the Func of the benchmark END.X SID should
be the same as the length of the Func of the automatically generated
END.X SID.
The specific generation process is as follows: based on the locator
address information originally advertised by the algorithm, and
using the 16-byte END.X SID mask to perform a bitwise AND operation
with the base END.X SID, extract the Func field. Then, combine this
Func field with the locator address information to generate a new
END.X SID as defined in the algorithm.
Taking IS-IS as an example, assuming there are 100 neighbors, and
each interface supports 128 Flex-Algo algorithms. Without batch
generation, it would be necessary to advertise 12,800 adjacency
SIDs. Assuming each SID occupies 30 bytes and each LSP 1,500 bytes,
256 LSP fragments would need to be generated at this point.
With the introduction of the flex-algo extension, it is only
necessary to advertise 128 SID information, resulting in the
creation of only 3 LSP fragments.
3.1. IS-IS extension
[RFC9352] defines the format of the Locator TLV. In order to support
bulk adjacency SID generation, it is necessary to extend the Locator
TLV to include the 16 bytes SID Mask information. Use the 16-byte
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END.X SID mask to perform a bitwise AND operation with the base
END.X SID and extract the Func field. Then, combine the Func field
with the locator address information to generate a new END.X SID.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sid Mask (16 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: ISIS Adjacent-Sid-Offset Sub-Sub-TLV
where:
o Type: TBD. Single octet, as defined in Section 9 of [ISO10589].
o Length: Single octet, as defined in Section 9 of [ISO10589].
o Sid Mask: 16 octets. Use the 16-byte END.X SID mask to perform
a bitwise AND operation with the base END.X SID and extract the
Func field. Then, combine the Func field with the locator address
information to generate a new END.X SID.
3.2. OSPFv3 extension
[RFC9513] defines the format of the Locator TLV. In order to support
bulk adjacency SID generation, it is necessary to extend the Locator
TLV to include the 16 bytes SID Mask information. Use the 16-byte
END.X SID mask to perform a bitwise AND operation with the base
END.X SID and extract the Func field. Then, combine the Func field
with the locator address information to generate a new END.X SID.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sid Mask (16 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: OSPFv3 Adjacent-Sid-Offset Sub-Sub-TLV
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where:
o Type: TBD. 2-octet field.
o Length: 2-octet field.
o Sid Mask: 16 octets. Use the 16-byte END.X SID mask to perform
a bitwise AND operation with the base END.X SID and extract the
Func field. Then, combine the Func field with the locator address
information to generate a new END.X SID.
4. Security Considerations
TBD.
5. Compatibility considerations
This functionality consists of two parts:
Firstly, during the allocation of SRv6 END.X SIDs on routers, END.X
SIDs are allocated in accordance with the batch allocation rules
described in this document.
Secondly, when a router allocates its assigned END.X SIDs for
external advertisement, only the END.X SIDs allocated within the
base algorithm are explicitly advertised. END.X SIDs allocated
within other algorithms are not extensively advertised. Instead, the
receiving router generates the END.X SIDs within other algorithms
based on the same rules as the assigning router.
The first part of the functionality does not pose any compatibility
issues. However, the second part requires support for this
functionality on the receiving router. In real deployments, if all
routers within the IGP domain that support SRv6 functionality also
support this feature, then batch END.X SID advertisement can be
performed. Conversely, if some routers do not support this feature,
the assigning router needs to advertise all END.X SIDs externally.
Therefore, for the sake of compatibility, routers that support this
feature should provide a configuration command to disable the bulk
advertisement of END.X SIDs and instead advertise detailed
information.
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6. IANA Considerations
6.1. IS-IS SRv6 Locator LSA Sub-TLVs
This document defines a new sub-sub-tlv for the IS-IS SRv6 Locator
TLV.
+=======+====================+=======================+
| Type | Description | Reference |
+=======+====================+=======================+
| TBD | Adjacent-Sid-Offset| This Document |
+-------+--------------------+-----------------------+
6.2. OSPFv3 SRv6 Locator LSA Sub-TLVs
This document defines a new sub-sub-tlv for the OSPFv3 SRv6 Locator
TLV.
+=======+====================+=======================+
| Value | Description | Reference |
+=======+====================+=======================+
| TBD | Adjacent-Sid-Offset| This Document |
+-------+--------------------+-----------------------+
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[RFC9352] Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
and Z. Hu, "IS-IS Extensions to Support Segment Routing
over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
February 2023, <https://www.rfc-editor.org/info/rfc9352>.
[ISO10589] International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", Nov 2002.
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7.2. Informative References
[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>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986, DOI
10.17487/RFC8986, February 2021, <https://www.rfc-
editor.org/info/rfc8986>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI
10.17487/RFC9350, February 2023, <https://www.rfc-
editor.org/info/rfc9350>.
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Authors' Addresses
Weiqiang Cheng
China Mobile
China
Email: chengweiqiang@chinamobile.com
Liyan Gong
China Mobile
China
Email: gongliyan@chinamobile.com
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Louis Chan
Individual
Email: lwtchan@gmail.com
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