Combined NEXT-CSID and REPLACE-CSID flavor in SRv6

2.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3.1. 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 [RFC8986] is executed with the following modifications.¶

The S01 of NEXT-CSID flavor pseudo code [RFC9800] 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 [RFC9800], and a second time before line S01 of the upper-layer header processing in Section 4.1.1 of [RFC8986], or prior to processing any extension header other than Hop-by-Hop or Destination Option.¶

S01. If (DA.Arg.Next != 0) {

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.¶

S01. If (DA.Arg.Next != 0) {
S02.      NEXT-CSID flavor processing
S03. } else{
S04.      REPLACE-CSID flavor processing
S05. }

A rendering of the complete pseudocode of END with NEXT&REPLACE-CSID is provided in Appendix A.1. 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 Appendix A.1.¶

4.1. SID Validation for Compression

The rules defined in section 6.1 of [RFC9800] apply to NEXT&REPLACE-CSID.¶

4.2. 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.¶

4.3. Upper-Layer Checksums

The rules defined in section 6.5 of [RFC9800] apply to NEXT&REPLACE-CSID.¶

8.1. 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/):¶

10.1. Normative References

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[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>.

[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>.

[RFC9800]

Cheng, W., Ed., Filsfils, C., Li, Z., Decraene, B., and F. Clad, Ed., "Compressed SRv6 Segment List Encoding", RFC 9800, DOI 10.17487/RFC9800, June 2025, <https://www.rfc-editor.org/info/rfc9800>.

10.2. Informative References

[I-D.ietf-spring-compression-requirement]

Cheng, W., Xie, C., Bonica, R., Dukes, D., Li, C., Peng, S., and W. Henderickx, "Compressed SRv6 SID List Requirements", Work in Progress, Internet-Draft, draft-ietf-spring-compression-requirement-03, 3 April 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-spring-compression-requirement-03>.

A.1. 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:¶

S01. When an SRH is processed {
S1.1   If (DA.Arg.Next != 0) {             //NEXT-CSID Processing
S1.2.    If (IPv6 Hop Limit <= 1) {
S1.3.       Send an ICMP Time Exceeded message to the Source Address
              with Code 0 (Hop limit exceeded in transit),
              interrupt packet processing, and discard the packet.
S1.4.    }
S1.5.    Copy the value of DA.Argument into the bits [LBL..(LBL+AL-1)]
            of the Destination Address.
S1.6.    Set the bits [(LBL+AL)..127] of the Destination Address to
            zero.
S1.7.    Decrement IPv6 Hop Limit by 1.
S1.8.    Submit the packet to the egress IPv6 FIB lookup for
            transmission to the next destination.
S1.9.  }                        //Below is REPLACE-CSID Processing
S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
           Segment List[0][DA.Arg.Index-1] == 0)) {
S03.     Stop processing the SRH, and proceed to process the next
           header in the packet, whose type is identified by
           the Next Header field in the routing header.
S04.   }
S05.   If (IPv6 Hop Limit <= 1) {
S06.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
S07.   }
S08.   max_LE = (Hdr Ext Len / 2) - 1
S09.   If (DA.Arg.Index != 0) {
S10.     If ((Last Entry > 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:¶

S01. If (DA.Arg.Next != 0) {
S02.   If (IPv6 Hop Limit <= 1) {
S03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
S04.   }
S05.   Copy the value of DA.Argument into the bits [LBL..(LBL+AL-1)]
         of the Destination Address.
S06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
S07.   Decrement Hop Limit by 1.
S08.   Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.
S09. }

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:¶

S01. If (Upper-Layer header type is allowed by local configuration) {
S02.   Proceed to process the Upper-Layer header
S03. } Else {
S04.   Send an ICMP Parameter Problem to the Source Address
          with Code 4 (SR Upper-layer Header Error)
          and Pointer set to the offset of the Upper-Layer header,
          interrupt packet processing, and discard the packet.
S05. }

A.2. 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:¶

S01. When an SRH is processed {
S02.  If (DA.Arg.Next != 0) {             //NEXT-CSID Processing
S03.      NEXT-CSID flavor END.X processing as per {{RFC9800}}
S04.  }                        //Below is REPLACE-CSID Processing
S05.      REPLACE-CSID flavor END.X processing as per {{RFC9800}}.
S06. }

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:¶

S01. If (DA.Arg.Next != 0) {
S02.     NEXT-CSID flavor END.X processing as per {{RFC9800}}
S03. }

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 [RFC9800].¶

S01. If (Upper-Layer header type is allowed by local configuration) {
S02.   Proceed to process the Upper-Layer header
S03. } Else {
S04.   Send an ICMP Parameter Problem to the Source Address
          with Code 4 (SR Upper-layer Header Error)
          and Pointer set to the offset of the Upper-Layer header,
          interrupt packet processing, and discard the packet.
S05. }

A.3. 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:¶

S01. When an SRH is processed {
S02.   If (DA.Arg.Next != 0) {             //NEXT-CSID Processing
S03.      NEXT-CSID flavor processing as per {{RFC9800}}
S04.   }                        //Below is REPLACE-CSID Processing
S05.      REPLACE-CSID flavor processing as per {{RFC9800}}.
S06. }

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:¶

S01. If (DA.Arg.Next != 0) {
S02.     NEXT-CSID flavor END.X processing as per {{RFC9800}}
S03. }

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 [RFC9800].¶

S01. If (Upper-Layer header type is allowed by local configuration) {
S02.   Proceed to process the Upper-Layer header
S03. } Else {
S04.   Send an ICMP Parameter Problem to the Source Address
          with Code 4 (SR Upper-layer Header Error)
          and Pointer set to the offset of the Upper-Layer header,
          interrupt packet processing, and discard the packet.
S05. }