]> China Mobile

China chengweiqiang@chinamobile.com

Cisco Systems, Inc.

Belgium cf@cisco.com

Huawei Technologies

China lizhenbin@huawei.com

Orange

France bruno.decraene@orange.com

Alibaba

USA d.cai@alibaba-inc.com

Bell Canada

Canada daniel.voyer@bell.ca

Cisco Systems, Inc.

France fclad@cisco.com

Broadcom

Israel shay.zadok@broadcom.com

Futurewei Technologies Ltd.

USA james.n.guichard@futurewei.com

ZTE Corporation

China liu.aihua@zte.com.cn

NTT Network Innovations

USA robert@raszuk.net

Huawei Technologies

China chengli13@huawei.com

General SPRING Internet-Draft This document specifies new flavors for the SR endpoint behaviors defined in RFC 8986, which enable a compressed SRv6 Segment-List encoding in the Segment Routing Header (SRH).

Introduction The Segment Routing (SR) architecture and SR for IPv6 (SRv6) are defined in . SRv6 Network Programming defines a framework to build a network program with topological and service segments (also referred to by their segment identifier (SID)) carried in a Segment Routing header (SRH) . This document specifies new flavors to the SR endpoint behaviors defined in Section 4 of . These flavors enable a compressed encoding of the SRv6 Segment-List in the SRH and therefore address the requirements described in . The flavors defined in this document leverage the SRv6 data plane defined in and , and are compatible with the SRv6 control plane extensions for IS-IS , OSPF , and BGP .

Terminology This document leverages the terms defined in , , and . The reader is assumed to be familiar with this terminology. This document introduces the following new terms: Locator-Block: The bits of an SRv6 SID Locator that identify the SRv6 SID Block, as defined in Section 3.1 of Section 3.1. Locator-Node: The bits of an SRv6 SID Locator that identify the parent node instantiating the SID, as defined in Section 3.1 of . Compressed-SID (C-SID): The Locator-Node and Function bits of a SID that supports compressed encoding of SIDs. C-SID container: A 128-bit container holding a list of C-SIDs. C-SID sequence: A group of one or more C-SID containers in a segment list. 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 contain any number of uncompressed SID sequences.

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 when, and only when, they appear in all capitals, as shown here.

Basic Concepts In an SRv6 domain, the SIDs are allocated from a particular IPv6 prefix: the Locator-Block. All SRv6 SIDs instantiated from the same Locator-Block share the same most significant bits. 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 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 , , and . 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. 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 . 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 Locator-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 ).

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 . 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 than 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: 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.

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.

<- NF -> <------------- A --------------> ]]>

End with NEXT-C-SID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End SID with the NEXT-C-SID flavor, the procedure described in Section 4.1 of is executed with the following modifications. The below pseudocode is inserted between lines S01 and S02 of the SRH processing in Section 4.1 of , and a second time before line S01 of the upper-layer header processing in Section 4.1.1 of , or prior to processing any extension header other than Hop-by-Hop or Destination Option.

Notes: DA.Argument identifies the bits [(B+NF)..127] in the Destination Address of the IPv6 header. The value in the Segments Left field of the SRH is not modified when DA.Argument in the received packet has a non-zero value.

End.X with NEXT-C-SID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.X SID with the NEXT-C-SID flavor, the procedure described in Section 4.2 of is executed with the same modifications as in of this document, except for line S08 that is replaced as follows.

Combination with PSP, USP and USD flavors PSP: The PSP flavor defined in Section 4.16.1 of is unchanged when combined with the NEXT-C-SID flavor. USP: The USP flavor defined in Section 4.16.2 of is unchanged when combined with the NEXT-C-SID flavor. USD: The USD flavor is unchanged when combined with the NEXT-C-SID flavor. The pseudocodes defined in and of this document are inserted at the beginning of the modified upper-layer header processing defined in Section 4.16.3 of for End and End.X, respectively.

REPLACE-C-SID Flavor A SID instantiated with the REPLACE-C-SID flavor takes an argument that indicates the index of the next C-SID in the appropriate C-SID container. The length A of the argument should be at least ceil(log_2(128/NF)). All SIDs that are part of a C-SID sequence using the REPLACE-C-SID flavor have the same C-SID length NF.

<----- NF -----> <- A --> ]]>

End with REPLACE-C-SID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End SID with the REPLACE-C-SID flavor, the SRH processing described in Section 4.1 of is replaced as follows.

max_LE) or (Segments Left > Last Entry)) { S11. Send an ICMP Parameter Problem to the Source Address, Code 0 (Erroneous header field encountered), Pointer set to the Segments Left field, interrupt packet processing and discard the packet. S12. } S13. Decrement DA.Argument by 1. S14. } Else { S15. If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){ S16. Send an ICMP Parameter Problem to the Source Address, Code 0 (Erroneous header field encountered), Pointer set to the Segments Left field, interrupt packet processing and discard the packet. S17. } S18. Decrement Segments Left by 1. S19. Set DA.Argument to (128/NF - 1). S20. } S21. Decrement IPv6 Hop Limit by 1 S22. Write Segment List[Segments Left][DA.Argument] into the bits [B..B+NF-1] of the Destination Address of the IPv6 header. S23. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. S24. } ]]>

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 upper-layer header processing described in Section 4.1.1 of is unchanged.

End.X with REPLACE-C-SID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.X SID with the REPLACE-C-SID flavor, the procedure described in Section 4.2 of is executed with the same modifications as in of this document, except for line S23 that is replaced as follows.

Combination with PSP, USP, and USD flavors PSP: When combined with the REPLACE-C-SID flavor, the additional PSP flavor instructions defined in Section 4.16.1.2 of are inserted after line S22 of the pseudocode in , and the first line of the inserted instructions is modified as follows.

Note: Segment List[Segments Left][DA.Argument-1] identifies the bits [(DA.Argument-1)*NF..DA.Argument*NF-1] in the SRH Segment List entry at index Segments Left. USP: When combined with the REPLACE-C-SID flavor, the lines S02-S04 of the pseudocode in are substituted by the USP flavor instructions defined in Section 4.16.2 of , with the following modification.

USD: The USD flavor defined in Section 4.16.3 of is unchanged when combined with the REPLACE-C-SID flavor.

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 should be at least 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-SIDs 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.

<- NF -> <-------- A_N ---------> <- A_I -> ]]>

Pseudo-code:

= 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: 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.

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.

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-SIDs, under the associated Locator-Block. The parent node executes a variant of the End behavior. A node can have multiple global C-SIDs under the same Locator-Block (e.g., one per IGP flexible algorithm). Multiple nodes may share the same global C-SID (anycast).

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-SIDs. 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-SIDs from the GIB and LIB depends on the C-SID length (see ).

C-SID and Locator-Block Length

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.

Locator-Block Length The RECOMMENDED Locator-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 Locator-Block size for the REPLACE-C-SID flavor can be 48, 56, 64, 72, or 80 bits, depending on the needs of the operator.

GIB/LIB Usage GIB and LIB usage is a local implementation and/or configuration decision, however, some guidelines for determining usage for specific SID behaviors and recommendations are provided. The GIB number space is shared among all segment endpoint nodes within a Locator-Block. The more SIDs assigned from this space, per node, the faster it is exhausted. Therefore its use is prioritized for SIDs that identify a node, like End behavior SIDs. The LIB number space is unique per node. Each node is able to fully utilize the entire LIB number space without consideration of assignments at other nodes. Therefore its use is prioritized for SIDs that identify services (of which there may be many) at nodes, like cross-connects, adjacencies, etc. While a longer C-SID length permits more flexibility in which SID behaviors may be assigned from the GIB, it also reduces compression. Given the previous Locator-Block and C-SID length recommendations, the following GIB/LIB usage is RECOMMENDED: 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

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.

Inter Routing Domains with the End.XPS behavior The End.XPS behavior described in this section is OPTIONAL. 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 Locator-Blocks. This section defines an optional solution and SID behavior allowing for the use of different Locator-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 Locator-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 Locator-Block between the two routing domains. The processing takes as an additional parameter the prefix B2/m corresponding the Locator-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. 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 Locator-Block B2/m of the neighbor routing domain, R processes the packet as follows.

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 Locator-Block B2/m of the neighbor routing domain, R processes the packet as follows.

Note: the way the Locator-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. Path Computation Element (PCE)). 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.

Control Plane This document does not require any control plane modification.

Illustrations Illustrations for the functionalities defined in this document are provided in .

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 (), 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.

Deployment Model Section 5 of defines the intra-SR-domain deployment model and associated security procedures. The same deployment model applies to the SIDs defined in this document.

Security Considerations The security requirements and mechanisms described in and also apply to this document. This document does not introduce any new security considerations.

Acknowledgements The authors would like to thank Kamran Raza, Xing Jiang, YuanChao Su, Han Li and Yisong Liu.

Contributors Darren Dukes
Cisco Systems, Inc.
Canada
ddukes@cisco.com

&RFC8402; &RFC8754; &RFC8986; &RFC2119; &RFC8174; &I-D.srcompdt-spring-compression-requirement; &I-D.ietf-lsr-isis-srv6-extensions; &I-D.ietf-lsr-ospfv3-srv6-extensions; &I-D.ietf-bess-srv6-services; &I-D.clad-spring-srv6-srh-compression-illus;

Open Issues This section was added as requested by the SPRING chair in . Issues raised during and after the adoption call for this draft are tracked in an issue tracker. The remainder of this section identifies the most significant open issues, from the adoption call, for the working group to keep track of. As a reminder to those reading this section, this document is a work in progress, and subject to change by the working group. As noted at the front of this document, "It is inappropriate to use Internet-Drafts as reference material" Given that the working group has said that it wants to standardize one data plane solution, and given that the document contains multiple SRv6 EndPoint behaviors that some WG members have stated are multiple data plane solutions, the working group will address whether this is valid and coherent with its one data plane solution objective. As reminded in the conclusion of the adoption call, this document is subject to the policy announced by the SPRING chairs in . In particular, this means that this document can not go to WG last call until 6man completes handling of an Internet Draft that deals with the relationship of C-SIDs to RFC 4291. It is hoped and expected that said resolution will be a WG last call and document approval in 6man of a document providing for the way that C-SIDs use the IPv6 destination address field.