SPRING W. Cheng, Ed. Internet-Draft China Mobile Intended status: Standards Track C. Filsfils Expires: January 7, 2022 Cisco Systems, Inc. Z. Li Huawei Technologies B. Decraene Orange 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 R. Raszuk NTT Network Innovations C. Li Huawei Technologies July 06, 2021 Compressed SRv6 Segment List Encoding in SRH draft-filsfilscheng-spring-srv6-srh-compression-00 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 working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Cheng, et al. Expires January 7, 2022 [Page 1] Internet-Draft SRv6 Segment List Compression in SRH July 2021 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 January 7, 2022. Copyright Notice Copyright (c) 2021 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 and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . 4 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 . . . . . . . . . 7 5. GIB, LIB, global C-SID and local C-SID . . . . . . . . . . . 8 5.1. Global C-SID . . . . . . . . . . . . . . . . . . . . . . 8 5.2. Local C-SID . . . . . . . . . . . . . . . . . . . . . . . 9 6. C-SID and Block Length . . . . . . . . . . . . . . . . . . . 9 6.1. C-SID Length . . . . . . . . . . . . . . . . . . . . . . 9 6.2. Block Length . . . . . . . . . . . . . . . . . . . . . . 9 6.3. GIB/LIB Usage . . . . . . . . . . . . . . . . . . . . . . 10 7. Efficient SID-list Encoding . . . . . . . . . . . . . . . . . 10 8. Control Plane . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Illustrations . . . . . . . . . . . . . . . . . . . . . . . . 10 10. Interoperability Status . . . . . . . . . . . . . . . . . . . 10 11. Security Considerations . . . . . . . . . . . . . . . . . . . 11 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 13.1. Normative References . . . . . . . . . . . . . . . . . . 11 13.2. Informative References . . . . . . . . . . . . . . . . . 11 Appendix A. Inter Routing Domains with the End.XPS behavior . . 12 Cheng, et al. Expires January 7, 2022 [Page 2] Internet-Draft SRv6 Segment List Compression in SRH July 2021 Appendix B. Larger C-SID length by combining k C-SIDs to identify a behavior . . . . . . . . . . . . . . . . 14 B.1. Combining k C-SIDs in a NEXT-C-SID sequence . . . . . . . 14 B.2. Combining k C-SIDs in a REPLACE-C-SID sequence . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 1. Introduction The Segment Routing architecture is defined in [RFC8402]. SRv6 Network Programming [RFC8986] defines a framework to build a network program with topological and service segments carried in a Segment Routing header (SRH) [RFC8754]. This document adds new flavors to the SR endpoint behaviors defined in [RFC8986]. 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 [RFC8986]. The reader is assumed to be familiar with this terminology. This document introduces the following new terms: o 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). o Compressed-SID container (C-SID container): An entry of the SRH Segment-List field (128 bits) that contains a sequence of C-SIDs. o 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. o Uncompressed SID sequence: A group of one or more uncompressed SIDs in a segment list. o 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. Cheng, et al. Expires January 7, 2022 [Page 3] Internet-Draft SRv6 Segment List Compression in SRH July 2021 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 [RFC8986]. 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. 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 [RFC8986]. 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 [RFC8986]. 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 [RFC8986]. 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 Cheng, et al. Expires January 7, 2022 [Page 4] Internet-Draft SRv6 Segment List Compression in SRH July 2021 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. 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: o Variable B is the Locator Block length of the SID. o 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. o 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. } Cheng, et al. Expires January 7, 2022 [Page 5] Internet-Draft SRv6 Segment List Compression in SRH July 2021 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. 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: o "DA.Argument" identifies the bits "[(B+NF)..(B+NF+A-1)]" in the Destination Address of the IPv6 header. o "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 Cheng, et al. Expires January 7, 2022 [Page 6] Internet-Draft SRv6 Segment List Compression in SRH July 2021 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. 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: Cheng, et al. Expires January 7, 2022 [Page 7] Internet-Draft SRv6 Segment List Compression in SRH July 2021 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. } Notes: o "DA.Arg.Next" identifies the bits "[(B+NF)..(B+NF+A_N-1)]" in the Destination Address of the IPv6 header. o "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. o "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. Cheng, et al. Expires January 7, 2022 [Page 8] Internet-Draft SRv6 Segment List Compression in SRH July 2021 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. 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. Cheng, et al. Expires January 7, 2022 [Page 9] Internet-Draft SRv6 Segment List Compression in SRH July 2021 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: o 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 o 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 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: o Hardware implementation in Cisco ASR 9000 running IOS XR o Software implementation in Cisco IOS XRv9000 virtual appliance Cheng, et al. Expires January 7, 2022 [Page 10] Internet-Draft SRv6 Segment List Compression in SRH July 2021 o Hardware implementation in Huawei NE40E and NE5000E running VRP The interoperability was validated for the following scenario: o 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 Kamran Raza, Xing Jiang, YuanChao Su, Han Li and Yisong Liu. 13. References 13.1. Normative 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, . [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, . [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, . 13.2. Informative References [I-D.cheng-spring-shorter-srv6-sid-requirement] Cheng, W., Chongfeng, Pang, R., Li, Z., Chen, R., Lijun, Duan, X., Mirsk, G., Dukes, D., and S. Zadok, "Shorter SRv6 SID Requirements", draft-cheng-spring-shorter-srv6- sid-requirement-02 (work in progress), July 2020. Cheng, et al. Expires January 7, 2022 [Page 11] Internet-Draft SRv6 Segment List Compression in SRH July 2021 [I-D.cl-spring-generalized-srv6-for-cmpr] Cheng, W., Li, Z., Li, C., Clad, F., Liu, A., Xie, C., Liu, Y., and S. Zadok, "Generalized SRv6 Network Programming for SRv6 Compression", draft-cl-spring- generalized-srv6-for-cmpr-03 (work in progress), April 2021. [I-D.filsfils-spring-net-pgm-extension-srv6-usid] Filsfils, C., Garvia, P. C., 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", draft-filsfils-spring- net-pgm-extension-srv6-usid-10 (work in progress), March 2021. [RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream Label Assignment and Context-Specific Label Space", RFC 5331, DOI 10.17487/RFC5331, August 2008, . Appendix A. Inter Routing Domains with the End.XPS behavior Some SRv6 traffic may need to cross multiple routing domains, such as different Autonomous Systems (ASes) or different routing areas. Different routing domains may use different addressing schema and SRv6 SID blocks. This section defines an optional solution and SID behavior allowing for the use of different SRv6 SID blocks between routing domains. The solution requires a new SID behavior, called "Endpoint with cross-connect to an array of layer-3 adjacencies and SRv6 Prefix Swap" (End.XPS for short) allowing for this transition of SRv6 SID block between two routing domains. End.XPS is a variant of End.X, performing both "End.X Layer-3 Cross- Connect" and the translation of the SRv6 SID block between the two routing domains. The processing takes as an additional parameter the prefix B2/m corresponding the SRv6 SID block in the second domain. This parameter is a property of the (received) SID and is given as a result of the lookup on the IPv6 destination address which identifies the SRv6 SID and its properties. The End.XPS behavior is compatible with the NEXT-C-SID, REPLACE- C-SID, and NEXT-and-REPLACE-C-SID flavors described in this document. Cheng, et al. Expires January 7, 2022 [Page 12] Internet-Draft SRv6 Segment List Compression in SRH July 2021 When a router R receives a packet whose IPv6 DA matches a local End.XPS SID with the NEXT-C-SID flavor, that is associated with a set J of one or more Layer-3 adjacencies and the SRv6 SID block B2/m of the neighbor routing domain, R processes the packet as follows. 1. If (DA.Argument != 0) { 2. Write B2 into the most significant bits of the Destination Address of the IPv6 header. 3. Write DA.Argument into the bits [m..(m+A-1)] of the Destination Address of the IPv6 header. 4. Set the bits [(m+A)..127] of the Destination Address of the IPv6 header to zero. 5. } Else { 6. Decrement Segments Left by 1. 7. Copy Segment List[Segments Left] from the SRH to the Destination Address of the IPv6 header. 8. } 9. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. When a router R receives a packet whose IPv6 DA matches a local End.XPS SID with the REPLACE-C-SID flavor, that is associated with a set J of one or more Layer-3 adjacencies and the SRv6 SID block B2/m of the neighbor routing domain, R processes the packet as follows. 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. Write B2 into the most significant bits of the Destination Address of the IPv6 header. 8. Write Segment List[Segments Left][DA.Argument] into the bits [m..m+NF-1] of the Destination Address of the IPv6 header. 9. Write DA.Argument into the bits [m+NF..m+NF+A-1] of the Destination Address of the IPv6 header. 10. Set the bits [(m+NF+A)..127] of the Destination Address of the IPv6 header to zero. 11. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. Note: the way the SRv6 SID Block B2 of the next routing domain is known is out of scope of this document. As examples, it could be learnt via configuration, or using a signaling protocol either with the peer domain or with a central controller (e.g. PCE). Cheng, et al. Expires January 7, 2022 [Page 13] Internet-Draft SRv6 Segment List Compression in SRH July 2021 When End.XPS SID behavior is used, the restriction on the C-SID length for the REPLACE-C-SID and the NEXT-and-REPLACE-C-SID flavors is relaxed and becomes: all SID the are part of a C-SID sequence *within a domain* MUST have the same SID length NF. Appendix B. Larger C-SID length by combining k C-SIDs to identify a behavior When an SR endpoint node needs more bits than allowed by the C-SID length (NF) to identify a behavior, it MAY combine two (resp. k) C-SIDs to effectively benefit from a C-SID of length "2*NF" (resp. "k*NF"). This is a local behavior of this SR endpoint node that does not require any specific encoding in the SRH. The C-SID sequence only needs to have at least two (resp. k) remaining C-SIDs, instead of a single one. When two C-SIDs are combined, the first C-SID may be seen as having the role of a "Context SID" identifying a context-specific SID space or table, with the second SID being looked up in this context- specific table. This is similar to the Context-Specific label space defined in the section 3 of [RFC5331]. B.1. Combining k C-SIDs in a NEXT-C-SID sequence As part of a NEXT-C-SID sequence, k C-SIDs encoded in the same C-SID container can be seamlessly combined together by installing a more specific entry in FIB. This entry would match the concatenation of the SRv6 SID block and the k C-SIDs to be combined. For example, the 16-bit C-SIDs 1111, 2222 and 3333 in the SRv6 SID block 2001:db8::/48 can be combined as one 48-bit C-SID on router R by installing a FIB entry for the IPv6 prefix 2001:db8:0:1111:2222:3333::/96. This capability does not require any change to the existing SRv6 SID behaviors. B.2. Combining k C-SIDs in a REPLACE-C-SID sequence As part of a REPLACE-C-SID sequence, k C-SIDs can be combined together using the COMBINE-k-C-SID flavor. A SID instantiated with the COMBINE-k-C-SID flavor takes a SID argument that indicates the index of the next C-SID in the current C-SID container, and a local parameter k >= 2. The parameter k is a property of the C-SID and is given as a result of the FIB lookup that identifies the IPv6 Destination Address as an SRv6 SID. Cheng, et al. Expires January 7, 2022 [Page 14] Internet-Draft SRv6 Segment List Compression in SRH July 2021 This section describes the COMBINE-k-C-SID flavor applied to the End.DX2 SID behavior, but the flavor is equally applicable to others SID behaviors. When a router R receives a packet whose IPv6 DA matches a local End.DX2 with a COMBINE-k-C-SID flavor, a local parameter k and a SID argument DA.Argument, R processes the packet as per Section 4.9 of [RFC8986], with the following modifications. Line S02 of the SRH processing behavior S02. If (Segments Left != 0) { is replaced with S02. If (Segments Left + floor((DA.Argument - k + 1) * NF/128) != 0){ and lines S02-S03 of the upper-layer header processing behavior S02. Remove the outer IPv6 Header with all its extension headers S03. Forward the Ethernet frame to the OIF I are replaced with S02.a. Read the k-1 next C-SIDs located at positions Segment List[Segments Left + floor((DA.Argument - k + 1) * NF/128)] [(DA.Argument - k + 1) % (128/NF)] to Segment Left[Segments Left + floor((DA.Argument - 1) * NF/128)][(DA.Argument - 1) % (128/NF)] in the SRH. S02.b. Remove the outer IPv6 Header with all its extension headers S03. Forward the Ethernet frame to the OIF identified by the k-1 next C-SIDs, in the context of the matched IPv6 Destination Address. Authors' Addresses Weiqiang Cheng (editor) China Mobile China Email: chengweiqiang@chinamobile.com Clarence Filsfils Cisco Systems, Inc. Belgium Email: cf@cisco.com Cheng, et al. Expires January 7, 2022 [Page 15] Internet-Draft SRv6 Segment List Compression in SRH July 2021 Zhenbin Li Huawei Technologies China Email: lizhenbin@huawei.com Bruno Decraene Orange France Email: bruno.decraene@orange.com Dennis Cai Alibaba USA 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 Email: shay.zadok@broadcom.com James N Guichard Futurewei Technologies Ltd. USA Email: james.n.guichard@futurewei.com Cheng, et al. Expires January 7, 2022 [Page 16] Internet-Draft SRv6 Segment List Compression in SRH July 2021 Liu Aihua ZTE Corporation China Email: liu.aihua@zte.com.cn Robert Raszuk NTT Network Innovations USA Email: robert@raszuk.net Cheng Li Huawei Technologies China Email: chengli13@huawei.com Cheng, et al. Expires January 7, 2022 [Page 17]