SPRING W. Cheng, Ed.
Internet-Draft China Mobile
Intended status: Standards Track C. Filsfils
Expires: 21 May 2021 Cisco Systems, Inc.
Z. Li
Huawei Technologies
D. Cai
Alibaba
D. Voyer
Bell Canada
F. Clad, Ed.
Cisco Systems, Inc.
S. Zadok
Broadcom
J. Guichard
Futurewei Technologies Ltd.
L. Aihua
ZTE Corporation
17 November 2020
Compressed SRv6 Segment List Encoding in SRH
draft-filsfilscheng-spring-srv6-srh-comp-sl-enc-02
Abstract
This document defines a compressed SRv6 Segment List Encoding in the
SRH. This solution does not require any SRH data plane change nor
any SRv6 control plane change. This solution leverages the SRv6
Network Programming model.
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
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Drafts is at https://datatracker.ietf.org/drafts/current/.
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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 21 May 2021.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . 3
4. SR Endpoint Flavors . . . . . . . . . . . . . . . . . . . . . 4
4.1. NEXT-C-SID Flavor . . . . . . . . . . . . . . . . . . . . 5
4.2. REPLACE-C-SID Flavor . . . . . . . . . . . . . . . . . . 6
4.3. Combined NEXT-and-REPLACE-C-SID Flavor . . . . . . . . . 6
5. GIB, LIB, global C-SID and local C-SID . . . . . . . . . . . 8
5.1. Global C-SID . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Local C-SID . . . . . . . . . . . . . . . . . . . . . . . 8
6. C-SID and Block Length . . . . . . . . . . . . . . . . . . . 9
6.1. C-SID Length . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Block Length . . . . . . . . . . . . . . . . . . . . . . 9
6.3. GIB/LIB Usage . . . . . . . . . . . . . . . . . . . . . . 9
7. Efficient SID-list Encoding . . . . . . . . . . . . . . . . . 10
8. Control Plane . . . . . . . . . . . . . . . . . . . . . . . . 10
9. Illustrations . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Interoperability Status . . . . . . . . . . . . . . . . . . . 10
11. Security Considerations . . . . . . . . . . . . . . . . . . . 10
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
13.1. Normative References . . . . . . . . . . . . . . . . . . 11
13.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Segment Routing architecture is defined in [RFC8402].
SRv6 Network Programming [I-D.ietf-spring-srv6-network-programming]
defines a framework to build a network program with topological and
service segments carried in a Segment Routing header (SRH) [RFC8754].
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This document adds new flavors to the SR endpoint behaviors defined
in [I-D.ietf-spring-srv6-network-programming]. These flavors enable
a compressed encoding of the SRv6 Segment-List in the SRH and
therefore address the requirements described in
[I-D.cheng-spring-shorter-srv6-sid-requirement].
The flavors defined in this document leverage the SRH data plane
without any change and do not require any SRv6 control plane change.
2. Terminology
This document leverages the terms defined in [RFC8402], [RFC8754] and
[I-D.ietf-spring-srv6-network-programming]. The reader is assumed to
be familiar with this terminology.
This document introduces the following new terms:
* Compressed-SID (C-SID): A C-SID is a short encoding of a SID in
SRv6 packet that does not include the SID block bits (locator
block).
* Compressed-SID container (C-SID container): An entry of the SRH
Segment-List field (128 bits) that contains a sequence of C-SIDs.
* Compressed-SID sequence (C-SID sequence): A group of one or more
C-SID containers in a segment list that share the same SRv6 SID
block.
* Uncompressed SID sequence: A group of one or more uncompressed
SIDs in a segment list.
* Compressed Segment List encoding: A segment list encoding that
reduces the packet header length thanks to one or more C-SID
sequences. A compressed Segment List encoding may also contain
any number of uncompressed SID sequences.
3. Basic Concepts
In an SRv6 domain, the SIDs are allocated from a particular IPv6
prefix: the SRv6 SID block. Therefore, all SRv6 SIDs instantiated
from the same SRv6 SID block share the same most significant bits.
These common bits are named Locator-Block in
[I-D.ietf-spring-srv6-network-programming]. Furthermore, when the
combined length of the SRv6 SID Locator, Function and Argument is
smaller than 128 bits, the trailing bits are set to zero.
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When a sequence of consecutive SIDs in a Segment List shares a common
Locator-Block, a compressed SRv6 Segment-List encoding can optimize
the packet header length by avoiding the repetition of the Locator-
Block and trailing bits with each individual SID.
The compressed Segment List encoding is fully compliant with the
specifications in [RFC8402], [RFC8754] and
[I-D.ietf-spring-srv6-network-programming]. Efficient encoding is
achieved by combining a compressed Segment List encoding logic on the
SR policy headend with new flavors of the base SRv6 endpoint
behaviors that decode this compressed encoding. No SRv6 SRH data
plane change nor control plane extension is required.
A Segment List can be encoded in the packet header using any
combination of compressed and uncompressed sequences. The C-SID
sequences leverage the flavors defined in this document, while the
uncompressed sequences use behaviors and flavors defined in other
documents, such as [I-D.ietf-spring-srv6-network-programming]. An SR
Policy headend constructs and compresses the SID-list depending on
the capabilities of each SR endpoint node that the packet should
traverse, as well as its own compression capabilities.
It is expected that compressed encoding flavors be available on
devices with limited packet manipulation capabilities, such as legacy
ASICs.
The compressed Segment List encoding supports any SRv6 SID Block
allocation. While other options are supported and may provide higher
efficiency, each routing domain can be allocated a /48 prefix from a
global IPv6 block (see Section 6.2).
4. SR Endpoint Flavors
This section defines several options to achieve compressed Segment
List encoding, in the form of two new flavors for the END, END.X and
END.T behaviors of [I-D.ietf-spring-srv6-network-programming]. These
flavors could also be combined with behaviors defined in other
documents.
The compressed encoding can be achieved by leveraging any of these SR
endpoint flavors. The NEXT-C-SID flavor and the REPLACE-C-SID flavor
expose the same high-level behavior in their use of the SID argument
to determine the next segment to be processed, but they have
different low-level characteristics that can make one more or less
efficient that the other for a particular SRv6 deployment. The NEXT-
and-REPLACE-C-SID flavor is the combination of the NEXT-C-SID flavor
and the REPLACE-C-SID flavor. It provides the best efficiency in
terms of encapsulation size at the cost of increased complexity.
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It is recommended for ease of operation that a single compressed
encoding flavor be used in a given SRv6 domain. However, in a multi-
domain deployment, different flavors can be used in different
domains.
All three flavors leverage the following variables:
* Variable B is the Locator Block length of the SID.
* Variable NF is the sum of the Locator Node and the Function
lengths of the SID. It is also referred to as C-SID length.
* Variable A is the Argument length of the SID.
4.1. NEXT-C-SID Flavor
A SID instantiated with the NEXT-C-SID flavor takes an argument that
carries the remaining C-SIDs in the current C-SID container.
The length A of the argument is equal to 128-B-NF and should be a
multiple of NF.
+----------------------------------------------------+
| Locator-Block | Locator-Node | Function | Argument |
+----------------------------------------------------+
<----- B -----> <--------- NF ----------> <-- A --->
Pseudo-code:
1. If (DA.Argument != 0) {
2. Copy DA.Argument into the bits [B..(B+A-1)] of the
Destination Address of the IPv6 header.
3. Set the bits [(B+A)..127] of the Destination Address
of the IPv6 header to zero.
4. } Else {
5. Decrement Segments Left by 1.
6. Copy Segment List[Segments Left] from the SRH to the
Destination Address of the IPv6 header.
7. }
Note: "DA.Argument" identifies the bits "[(B+NF)..127]" in the
Destination Address of the IPv6 header.
The NEXT-C-SID flavor has been previously documented in
[I-D.filsfils-spring-net-pgm-extension-srv6-usid] under the name
"SHIFT" flavor. In that context, a C-SID and a C-SID-sequence are
respectively named a Micro-Segment (uSID) and a Micro-Program.
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4.2. REPLACE-C-SID Flavor
A SID instantiated with the REPLACE-C-SID flavor takes an argument,
which is used to determine the index of the next C-SID in the
appropriate container.
All SIDs that are part of a C-SID sequence using the REPLACE-C-SID
flavor have the same C-SID length NF.
The length A of the argument should be at least ceil(log_2(128/NF)).
+----------------------------------------------------+
| Locator-Block | Locator-Node | Function | Argument |
+----------------------------------------------------+
<----- B -----> <--------- NF ----------> <-- A --->
Pseudo-code:
1. If (DA.Argument != 0) {
2. Decrement DA.Argument by 1.
3. } Else {
4. Decrement Segments Left by 1.
5. Set DA.Argument to (128/NF - 1).
6. }
7. Copy Segment List[Segments Left][DA.Argument] into the bits
[B..B+NF-1] of the Destination Address of the IPv6 header.
Notes:
* "DA.Argument" identifies the bits "[(B+NF)..(B+NF+A-1)]" in the
Destination Address of the IPv6 header.
* "Segment List[Segments Left][DA.Argument]" identifies the bits
"[DA.Argument*NF..(DA.Argument+1)*NF-1]" in the SRH Segment List
entry at index Segments Left.
The REPLACE-C-SID flavor has been previously documented in
[I-D.cl-spring-generalized-srv6-for-cmpr] under the name
"COC(Continue of Compression)" flavor. In that context, a C-SID and
a C-SID-sequence are respectively named a G-SID and G-SRv6
compression sub-path.
4.3. Combined NEXT-and-REPLACE-C-SID Flavor
A SID instantiated with the NEXT-and-REPLACE-C-SID flavor takes a
two-parts argument comprising, Arg.Next and Arg.Index, and encoded in
the SID in this order.
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The length A_I of Arg.Index is equal to ceil(log_2(128/NF)).
The length A_N of Arg.Next is equal to 128-B-NF-A_I and must be a
multiple of NF.
The total SID argument length A is the sum of A_I and A_N.
The NEXT-and-REPLACE-C-SID flavor also leverages an additional
variable, C_DA, that is equal to (1 + (A_N/NF)) and represents the
number of C-SID's that can be encoded in the IPv6 Destination
Address.
All SIDs that are part of a C-SID sequence using the NEXT-and-
REPLACE-C-SID flavor must have the same C-SID length NF.
Furthermore, this NF must be a divisor of 128.
+----------------------------------------------------------------+
| Locator-Block | Locator-Node | Function | Arg.Next | Arg.Index |
+----------------------------------------------------------------+
<----- B -----> <--------- NF ----------> <- A_N --> <-- A_I -->
Pseudo-code:
1. If (DA.Arg.Next != 0) {
2. Copy DA.Arg.Next into the bits [B..(B+A_N-1)] of the
Destination Address of the IPv6 header.
3. Set the bits [(B+A_N)..(B+NF+A_N-1)] of the Destination Address
of the IPv6 header to zero.
4. } Else If (DA.Arg.Index >= C_DA) {
5. Decrement DA.Arg.Index by C_DA.
6. Copy C_DA*NF bits from Segment List[Segments Left][DA.Arg.Index]
into the bits [B..B+C_DA*NF-1] of the Destination Address of
the IPv6 header.
7. } Else If (Segments Left != 0) {
8. Decrement Segments Left by 1.
9. Set DA.Arg.Index to ((DA.Arg.Index - C_DA) % (128/NF)).
10. Copy C_DA*NF bits from Segment List[Segments Left][DA.Arg.Index]
into the bits [B..B+C_DA*NF-1] of the Destination Address of
the IPv6 header.
11. } Else {
12. Copy DA.Arg.Index*NF bits from Segment List[0][0] into the bits
[B..B+DA.Arg.Index*NF-1] of the Destination Address of the
IPv6 header.
13. Set the bits [B+DA.Arg.Index*NF..B+NF+A_N-1] of the Destination
Address of the IPv6 header to zero.
14. Set DA.Arg.Index to 0.
15. }
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Notes:
* "DA.Arg.Next" identifies the bits "[(B+NF)..(B+NF+A_N-1)]" in the
Destination Address of the IPv6 header.
* "DA.Arg.Index" identifies the bits "[(B+NF+A_N)..(B+NF+A_N+A_I-
1)]" in the Destination Address of the IPv6 header.
* "Segment List[Segments Left][DA.Arg.Index]" identifies the bits
"[DA.Arg.Index*NF..(DA.Arg.Index+1)*NF-1]" in the SRH Segment List
entry at index Segments Left.
5. GIB, LIB, global C-SID and local C-SID
GIB: The set of IDs available for global C-SID allocation.
LIB: The set of IDs available for local C-SID allocation.
5.1. Global C-SID
A C-SID from the GIB.
A Global C-SID typically identifies a shortest-path to a node in the
SRv6 domain. An IP route is advertised by the parent node to each of
its global C-SID's, under the associated C-SID block. The parent
node executes a variant of the END behavior.
A node can have multiple global C-SID's under the same C-SID blocks
(e.g. one per IGP flexible algorithm). Multiple nodes may share the
same global C-SID (anycast).
5.2. Local C-SID
A C-SID from the LIB.
A local C-SID may identify a cross-connect to a direct neighbor over
a specific interface or a VPN context.
No IP route is advertised by a parent node for its local C-SID's.
If N1 and N2 are two different physical nodes of the SRv6 domain and
I is a local C-SID value, then N1 and N2 may bind two different
behaviors to I.
The concept of LIB is applicable to SRv6 and specifically to its
NEXT-C-SID and REPLACE-C-SID flavors. The shorter the SID/C-SID, the
more benefit the LIB brings.
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The allocation of C-SID's from the GIB and LIB depends on the C-SID
length (see Section 6.3).
6. C-SID and Block Length
6.1. C-SID Length
The NEXT-C-SID flavor supports both 16- and 32-bit C-SID lengths. A
C-SID length of 16-bit is recommended.
The REPLACE-C-SID flavor supports both 16- and 32-bit C-SID lengths.
A C-SID length of 32-bit is recommended.
6.2. Block Length
The compressed Segment List encoding supports any SRv6 SID Block
allocation either from GUA or LUA space.
The recommended SRv6 SID block sizes for the NEXT-C-SID flavor are
16, 32 or 48 bits. The smaller the block, the higher the compression
efficiency.
The recommended SRv6 SID block size for the REPLACE-C-SID flavor can
be 48, 56, 64, 72 or 80 bits, depending on the needs of the operator.
6.3. GIB/LIB Usage
The previous block and C-SID length recommendations, call for the
following GIB/LIB usage:
* NEXT-C-SID:
- GIB: END.NEXT-C-SID
- LIB: END.X.NEXT-C-SID, END.DX.NEXT-C-SID, END.DT.NEXT-C-SID
- LIB: END.DX.NEXT-C-SID for large-scale PW support
* REPLACE-C-SID:
- GIB: END.REPLACE-C-SID, END.X.REPLACE-C-SID, END.DX.REPLACE-
C-SID, END.DT.REPLACE-C-SID
- LIB: END.DX.REPLACE-C-SID for large-scale PW support
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7. Efficient SID-list Encoding
The compressed SID-list encoding logic is a local behavior of the SR
Policy headend node and hence out of the scope of this document.
8. Control Plane
This document does not require any control plane modification.
9. Illustrations
Illustrations will be provided in a separate document.
10. Interoperability Status
In November 2020, China Mobile successfully validated multiple
interoperable implementations of the NEXT-C-SID and REPLACE-C-SID
flavors defined in this document.
This testing covered two different implementations of the SRv6
endpoint flavors defined in this document:
* Hardware implementation in Cisco ASR 9000 running IOS XR
* Software implementation in Cisco IOS XRv9000 virtual appliance
* Hardware implementation in Huawei NE40E and NE5000E running VRP
The interoperability was validated for the following scenario:
* Packet forwarding through a traffic engineering segment list
combining, in the same SRH ([RFC8754]), SRv6 SIDs bound to an
endpoint behavior with the NEXT-C-SID flavor and SRv6 SIDs bound
to an endpoint behavior with the REPLACE-C-SID flavor.
Further interoperability testing is ongoing and will be reported in
this document as the work progresses.
11. Security Considerations
TBD
12. Acknowledgements
The authors would like to thank Bruno Decraene, Cheng Li, Kamran
Raza, Xing Jiang and YuanChao Su.
13. References
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13.1. Normative References
[I-D.ietf-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
Matsushima, S., and Z. Li, "SRv6 Network Programming",
Work in Progress, Internet-Draft, draft-ietf-spring-srv6-
network-programming-24, 7 October 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-spring-
srv6-network-programming-24.txt>.
[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>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
13.2. Informative References
[I-D.cheng-spring-shorter-srv6-sid-requirement]
Cheng, W., Xie, C., Pang, R., Li, Z., Chen, R., Zu, L.,
Duan, X., Mirsky, G., Dukes, D., and S. Zadok, "Shorter
SRv6 SID Requirements", Work in Progress, Internet-Draft,
draft-cheng-spring-shorter-srv6-sid-requirement-02, 13
July 2020, <http://www.ietf.org/internet-drafts/draft-
cheng-spring-shorter-srv6-sid-requirement-02.txt>.
[I-D.cl-spring-generalized-srv6-for-cmpr]
Cheng, W., Li, Z., Li, C., Clad, F., Aihua, L., Xie, C.,
Liu, Y., and S. Zadok, "Generalized SRv6 Network
Programming for SRv6 Compression", Work in Progress,
Internet-Draft, draft-cl-spring-generalized-srv6-for-cmpr-
01, 19 May 2020, <http://www.ietf.org/internet-drafts/
draft-cl-spring-generalized-srv6-for-cmpr-01.txt>.
[I-D.filsfils-spring-net-pgm-extension-srv6-usid]
Filsfils, C., Camarillo, P., Cai, D., Voyer, D., Meilik,
I., Patel, K., Henderickx, W., Jonnalagadda, P., Melman,
D., Liu, Y., and J. Guichard, "Network Programming
extension: SRv6 uSID instruction", Work in Progress,
Internet-Draft, draft-filsfils-spring-net-pgm-extension-
srv6-usid-08, 2 November 2020, <http://www.ietf.org/
internet-drafts/draft-filsfils-spring-net-pgm-extension-
srv6-usid-08.txt>.
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Authors' Addresses
Weiqiang Cheng (editor)
China Mobile
China
Email: chengweiqiang@chinamobile.com
Clarence Filsfils
Cisco Systems, Inc.
Belgium
Email: cf@cisco.com
Zhenbin Li
Huawei Technologies
China
Email: lizhenbin@huawei.com
Dennis Cai
Alibaba
United States of America
Email: d.cai@alibaba-inc.com
Daniel Voyer
Bell Canada
Canada
Email: daniel.voyer@bell.ca
Francois Clad (editor)
Cisco Systems, Inc.
France
Email: fclad@cisco.com
Shay Zadok
Broadcom
Israel
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Email: shay.zadok@broadcom.com
James N Guichard
Futurewei Technologies Ltd.
United States of America
Email: james.n.guichard@futurewei.com
Liu Aihua
ZTE Corporation
China
Email: liu.aihua@zte.com.cn
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