Combined NEXT-CSID and REPLACE-CSID flavor in SRv6

Combined NEXT-CSID and REPLACE-CSID flavor in SRv6 China Mobile

China chengweiqiang@chinamobile.com

Huawei Technologies

China c.l@huawei.com

General SPRING Internet-Draft In order to reduce the size of SRv6 SID, NEXT-CSID and REPLACE-CSID flavors for SRv6 endpoint behaviors are proposed. Similar to PSP and USD, NEXT-CSID and REPLACE-CSID can be combined just like PSP & USD. This document defines the combined NEXT&REPLACE-CSID flavor, which can provide more efficient compression for SRv6 Segment-List encoding in the Segment Routing Header (SRH).

Introduction SRv6 Network Programming defines a framework to build a network program with topological and service segments carried in a Segment Routing header (SRH) . specifies NEXT-CSID and REPLACE-CSID 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 . NEXT-CSID flavor can enable an efficient segment list encoding when using a short locator block. However, when the locator block is long, the compression efficency is reduced, because fewer CSIDs can be encoded in a CSID container. This will affect all the containers because NEXT-CSID flavor requires to encode the locator block in each CSID container. REPLACE-CSID flavor can enable an efficient segment list encoding no matter the length of locator block. However, when the CSID sequence is short, the compression efficiency is not ideal, because the first container of the CSID sequence will only contain a single CSID without compression, and the compression begins from the second container and the block is not encoded in a container. Similar to PSP, USP, USD defined in , NEXT-CSID and REPLACE-CSID flavor can be combined in use. By combining NEXT-CSID and REPLACE-CSID falvor, the compression efficiency can be enhanced comparing to both NEXT-CSID and REPLACE-CSID flavor. Originally, the Combined NEXT&REPLACE-CSID flavor was defined in , and moved out of the document in order to accelerate the standardizatin progress of . This document defines the combined NEXT&REPLACE-CSID falvor, which leverages the SRv6 data plane defined in and , and are compatible with the SRv6 control plane extensions.

Terminology This document leverages the terminologies defined in , , and . Readers are assumed to be familiar with terminologies.

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.

NEXT&REPLACE-CSID Flavor {#sec-next&replace-flavor} This section defines a new flavor NEXT&REPLACE-CSID, which is the combination of NEXT-CSID and REPALCE-CSID . The NEXT&REPLACE-CSID can be applied for END, END.X, END.T, End.B6.Encaps, End.B6.Encaps.Red, End.BM, End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, End.DT2U, and End.DT2M behaviors defined in . Similar to NEXT-CSID flavor and the REPLACE-CSID flavor, the combined NEXT&REPLACE flavor leverages the SID Argument to determine the next SID to be processed. A SID instantiated with the NEXT&REPLACE-CSID flavor can take an argument to

carry the remaining CSIDs in the current CSID container, same as NEXT-CSID flavor .
indicate the index of the next CSID in the appropriate CSID container, same as REPLACE-CSID flavor .

The length of the argument equals to 128-LBL-LNFL.

Example of a NEXT&REPLACE-CSID flavored SID structure using a 48-bit Locator-Block, 16-bit combined locator and function, and 64-bit argument < LNFL > <------------- AL -------------> ]]> The encoding rules of the NEXT&REPLACE-CSID flavor is a combination of NEXT-CSID and REPLACE-CSID.

First container: follows the rules of encoding the fist container of NEXT-CSID flavor .
Other containers: follows the rules of encoding the other containers of REPLACE-CSID flavor .

The following figure shows an example of encoding a segment list using NEXT&REPLACE-CSID flavor SIDs.

NEXT&REPLACE-CSID SIDs Encoding The NEXT&REPLACE-CSID flavor supports both 16- and 32-bit CSID lengths. A CSID length of 16-bit is RECOMMENDED. The length of a CSID is determined by its behavior and LNFL by reusing the rules of REPLACE-CSID flavor defined in section 4.2 of .

End with NEXT&REPLACE-CSID {#sec-next&replace-end} When processing an IPv6 packet that matches a FIB entry locally instantiated as an End SID with the NEXT&REPLACE-CSID flavor, the procedure described in Section 4.1 of is executed with the following modifications. The S01 of NEXT-CSID flavor pseudo code is replaced by the below pseudo code, and the updated pseudo code is inserted between lines S01 and S02 of the REPLACE-CSID flavor pseudo code , 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.Arg.Next identifies the bits [(LBL+LNFL)..(LBL+2*LNFL-1)] in the DA of the IPv6 header, which is the next CSID if it appears.

According to the pseudo code, when the next CSID in the DA is not zero, then the node performs NEXT-CSID processing, otherwise, it performs the REPLACE-CSID processing. Therefore, the best compression can be achieved by combining the advantages of NEXT-CSID flavor and REPLACE-CSID flavor. An high-level pseudo code is provided for better understanding. A rendering of the complete pseudocode of END with NEXT&REPLACE-CSID is provided in . This modification applies to other behaviors such as END.X, END.T, END.B6.Encaps, and End.B6.Encaps.Red when they combine with NEXT&REPLACE-CSID flavor. All the complete pseudocode of END behaviors are defined in .

SR Source Node The NEXT&REPLACE-CSID does not bring new requirements comparing to NEXT-CSID and REPLACE-CSID falvor defined in section 6 of .

SID Validation for Compression The rules defined in section 6.1 of apply to NEXT&REPLACE-CSID.

Segment List Compression An SR source node MAY compress a segment list when it includes NEXT&REPLACE-CSID flavor SIDs. If an SR source node chooses to compress the segment list, a method is defined below.

First container: reuses the rules of NEXT-CSID flavor for the first container.
Other containers: reuses the rules of of REPLACE-CSID flavor for the other containers(except the first container).

Please see the figure {NEXT&REPLACE-CSID SIDs Encoding} for an example of encoding a segment list using NEXT&REPLACE-CSID flavor SIDs.

Upper-Layer Checksums The rules defined in section 6.5 of apply to NEXT&REPLACE-CSID.

Control Plane The rules defined in section 8 of apply to NEXT&REPLACE-CSID.

Operational Considerations The rules defined in section 9 of apply to NEXT&REPLACE-CSID. In order to enhace the compression efficiency, this document recommends to use 16-bit NEXT&REPLACE-CSID, which can provide the best compression.

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

IANA Considerations

SRv6 Endpoint Behaviors This I-D. requests IANA to make the following registrations from the "SRv6 Endpoint Behaviors" sub-registry under the top-level "Segment Routing" registry (https://www.iana.org/assignments/segment-routing/): Registration List

Value	Description	Reference
201	End with NEXT&REPLACE-CSID	This I-D.
202	End with NEXT&REPLACE-CSID & PSP	This I-D.
203	End with NEXT&REPLACE-CSID & USP	This I-D.
204	End with NEXT&REPLACE-CSID, PSP & USP	This I-D.
205	End.X with NEXT&REPLACE-CSID	This I-D.
206	End.X with NEXT&REPLACE-CSID & PSP	This I-D.
207	End.X with NEXT&REPLACE-CSID & USP	This I-D.
208	End.X with NEXT&REPLACE-CSID, PSP & USP	This I-D.
209	End.T with NEXT&REPLACE-CSID	This I-D.
210	End.T with NEXT&REPLACE-CSID & PSP	This I-D.
211	End.T with NEXT&REPLACE-CSID & USP	This I-D.
212	End.T with NEXT&REPLACE-CSID, PSP & USP	This I-D.
214	End.B6.Encaps with NEXT&REPLACE-CSID	This I-D.
215	End.BM with NEXT&REPLACE-CSID	This I-D.
216	End.DX6 with NEXT&REPLACE-CSID	This I-D.
217	End.DX4 with NEXT&REPLACE-CSID	This I-D.
218	End.DT6 with NEXT&REPLACE-CSID	This I-D.
219	End.DT4 with NEXT&REPLACE-CSID	This I-D.
220	End.DT46 with NEXT&REPLACE-CSID	This I-D.
221	End.DX2 with NEXT&REPLACE-CSID	This I-D.
222	End.DX2V with NEXT&REPLACE-CSID	This I-D.
223	End.DX2U with NEXT&REPLACE-CSID	This I-D.
224	End.DT2M with NEXT&REPLACE-CSID	This I-D.
227	End.B6.Encaps.Red with NEXT&REPLACE-CSID	This I-D.
228	End with NEXT&REPLACE-CSID & USD	This I-D.
229	End with NEXT&REPLACE-CSID, PSP & USD	This I-D.
230	End with NEXT&REPLACE-CSID, USP & USD	This I-D.
231	End with NEXT&REPLACE-CSID, PSP, USP & USD	This I-D.
232	End.X with NEXT&REPLACE-CSID & USD	This I-D.
233	End.X with NEXT&REPLACE-CSID, PSP & USD	This I-D.
234	End.X with NEXT&REPLACE-CSID, USP & USD	This I-D.
235	End.X with NEXT&REPLACE-CSID, PSP, USP & USD	This I-D.
236	End.T with NEXT&REPLACE-CSID & USD	This I-D.
237	End.T with NEXT&REPLACE-CSID, PSP & USD	This I-D.
238	End.T with NEXT&REPLACE-CSID, USP & USD	This I-D.
239	End.T with NEXT&REPLACE-CSID, PSP, USP & USD	This I-D.

Acknowledgements Many thanks to Yang Xia, Ka Zhang, Chenxi Li, Fang Qiu, Weidi Shi, Chao Wang, Yang Huang, Shuying Liu, Yisong Liu, Xinyue Zhang, Weier Li for their professional supports.

References Normative References Segment Routing Architecture Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through an ordered list of instructions, called "segments". A segment can represent any instruction, topological or service based. A segment can have a semantic local to an SR node or global within an SR domain. SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain. SR can be directly applied to the MPLS architecture with no change to the forwarding plane. A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. SR can be applied to the IPv6 architecture, with a new type of routing header. A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header. The active segment is indicated by the Destination Address (DA) of the packet. The next active segment is indicated by a pointer in the new routing header. IPv6 Segment Routing Header (SRH) Segment Routing can be applied to the IPv6 data plane using a new type of Routing Extension Header called the Segment Routing Header (SRH). This document describes the SRH and how it is used by nodes that are Segment Routing (SR) capable. Segment Routing over IPv6 (SRv6) Network Programming The Segment Routing over IPv6 (SRv6) Network Programming framework enables a network operator or an application to specify a packet processing program by encoding a sequence of instructions in the IPv6 packet header. Each instruction is implemented on one or several nodes in the network and identified by an SRv6 Segment Identifier in the packet. This document defines the SRv6 Network Programming concept and specifies the base set of SRv6 behaviors that enables the creation of interoperable overlays with underlay optimization. Compressed SRv6 Segment List Encoding Segment Routing over IPv6 (SRv6) is the instantiation of Segment Routing (SR) on the IPv6 data plane. This document specifies new flavors for the SRv6 endpoint behaviors defined in RFC 8986, which enable the compression of an SRv6 segment list. Such compression significantly reduces the size of the SRv6 encapsulation needed to steer packets over long segment lists. This document updates RFC 8754 by allowing a Segment List entry in the Segment Routing Header (SRH) to be either an IPv6 address, as specified in RFC 8754, or a REPLACE-CSID container in packed format, as specified in this document. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Compressed SRv6 SID List Requirements China Mobile China Telecom Juniper Cisco Systems Huawei ZTE Nokia This document specifies requirements for solutions to compress SRv6 SID lists.

Complete Pseudocodes The content of this section is informative rendering of the pseudocodes of with the modifications in this document. This rendering may not be used as a reference.

End with NEXT&REPLACE-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End SID with the NEXT&REPLACE-CSID flavor: 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.Arg.Index by 1. S14. If (Segment List[Segments Left][DA.Arg.Index] == 0) { S15. Decrement Segments Left by 1. S16. Decrement IPv6 Hop Limit by 1. S17. Update IPv6 DA with Segment List[Segments Left] S18. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. S19. } S20. } Else { S21. If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){ S22. 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. S23. } S24. Decrement Segments Left by 1. S25. Set DA.Arg.Index to (128/NF - 1). S26. } S27. Decrement IPv6 Hop Limit by 1. S28. Write Segment List[Segments Left][DA.Arg.Index] into the bits [B..B+NF-1] of the Destination Address of the IPv6 header. S29. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. S30. } ]]> Before processing the Upper-Layer header or any IPv6 extension header other than Hop-by-Hop or Destination Option of a packet matching a FIB entry locally instantiated as an End SID with the NEXT&REPLACE-CSID flavor: When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End SID with the NEXT&REPLACE-CSID flavor:

End.X with NEXT&REPLACE-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT&REPLACE-CSID flavor: Before processing the Upper-Layer header or any IPv6 extension header other than Hop-by-Hop or Destination Option of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT&REPLACE-CSID flavor: When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT&REPLACE-CSID flavor, it reuses the pseudo code of NEXT-CSID flavor .

Other End behaviors with NEXT&REPLACE-CSID Similar to END and END.X, the other behaviors including END.T, End.B6.Encaps, End.B6.Encaps.Red, End.BM, End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, End.DT2U, and End.DT2M are following the same logic of combining NEXT-CSID and REPLACE-CSID flavor pseudo code. When processing the SRH of a packet matching a FIB entry locally instantiated as one of the END.T, End.B6.Encaps, End.B6.Encaps.Red, End.BM, End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, End.DT2U, and End.DT2M SID with the NEXT&REPLACE-CSID flavor: Before processing the Upper-Layer header or any IPv6 extension header other than Hop-by-Hop or Destination Option of a packet matching a FIB entry locally instantiated as one of the END.T, End.B6.Encaps, End.B6.Encaps.Red, End.BM, End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, End.DT2U, and End.DT2M SID with the NEXT&REPLACE-CSID flavor: When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as one of the END.T, End.B6.Encaps, End.B6.Encaps.Red, End.BM, End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, End.DT2U, and End.DT2M SID with the NEXT&REPLACE-CSID flavor, it reuses the pseudo code of NEXT-CSID flavor .