Compressed SRv6 Segment List Encoding 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
Cisco Systems, Inc.
France fclad.ietf@gmail.com
RTG spring Segment Routing IPv6 Segment Routing Compressed SID CSID NEXT-CSID REPLACE-SID SRH Compression 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.
Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .
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Table of Contents
  • .  Introduction
  • .  Terminology
    • .  Requirements Language
  • .  Basic Concepts
  • .  SR Segment Endpoint Flavors
    • .  NEXT-CSID Flavor
      • .  End with NEXT-CSID
      • .  End.X with NEXT-CSID
      • .  End.T with NEXT-CSID
      • .  End.B6.Encaps with NEXT-CSID
      • .  End.B6.Encaps.Red with NEXT-CSID
      • .  End.BM with NEXT-CSID
      • .  Combination with PSP, USP, and USD Flavors
    • .  REPLACE-CSID Flavor
      • .  End with REPLACE-CSID
      • .  End.X with REPLACE-CSID
      • .  End.T with REPLACE-CSID
      • .  End.B6.Encaps with REPLACE-CSID
      • .  End.B6.Encaps.Red with REPLACE-CSID
      • .  End.BM with REPLACE-CSID
      • .  End.DX and End.DT with REPLACE-CSID
      • .  Combination with PSP, USP, and USD Flavors
  • .  CSID Allocation
    • .  Global CSID
    • .  Local CSID
    • .  Recommended Installation of CSIDs in FIB
  • .  SR Source Node
    • .  SID Validation for Compression
    • .  Segment List Compression
    • .  Rules for Segment Lists Containing NEXT-CSID Flavor SIDs
    • .  Rules for Segment Lists Containing REPLACE-CSID Flavor SIDs
    • .  Upper-Layer Checksums
  • .  Inter-Domain Compression
    • .  End.LBS: Locator-Block Swap
      • .  End.LBS with NEXT-CSID
      • .  End.LBS with REPLACE-CSID
    • .  End.XLBS: L3 Cross-Connect and Locator-Block Swap
      • .  End.XLBS with NEXT-CSID
      • .  End.XLBS with REPLACE-CSID
  • .  Control Plane
  • .  Operational Considerations
    • .  Flavor, Block, and CSID Length
    • .  GIB/LIB Usage
    • .  Pinging a SID
    • .  ICMP Error Processing
  • . Applicability to Other SRv6 Endpoint Behaviors
  • . Security Considerations
  • . IANA Considerations
    • .  SRv6 Endpoint Behaviors
  • . References
    • .  Normative References
    • .  Informative References
  • .  Complete Pseudocodes
    • .  End with NEXT-CSID
    • .  End.X with NEXT-CSID
    • .  End.T with NEXT-CSID
    • .  End.B6.Encaps with NEXT-CSID
    • .  End.BM with NEXT-CSID
    • .  End with REPLACE-CSID
    • .  End.X with REPLACE-CSID
    • .  End.T with REPLACE-CSID
    • .  End.B6.Encaps with REPLACE-CSID
    • . End.BM with REPLACE-CSID
  • Acknowledgements
  • Contributors
  • Authors' Addresses
Introduction The Segment Routing (SR) architecture describes two data plane instantiations of SR: SR over MPLS (SR-MPLS) and SR over IPv6 (SRv6). SRv6 Network Programming builds upon the IPv6 Segment Routing Header (SRH) to define a framework for constructing a network program with topological and service segments. Some SRv6 applications, such as strict path traffic engineering, may require long segment lists. Compressing the encoding of these long segment lists in the packet header can significantly reduce the header size. This document specifies new flavors to the SRv6 endpoint behaviors defined in that enable a compressed encoding of the SRv6 segment list. This document also specifies new SRv6 endpoint behaviors to preserve the compression efficiency in multi-domain environments. The SRv6 endpoint behaviors defined in this document leverage the SRv6 data plane defined in and ; the behaviors are compatible with the SRv6 control plane extensions for IS-IS , OSPF , and BGP . This document updates by allowing a Segment List entry in the SRH to be either an IPv6 address, as specified in , or a REPLACE-CSID container in packed format, as specified in .
Terminology This document leverages the terms defined in , , and , in particular segment, segment list, Segment Identifier (SID), SID list, SR policy, prefix segment, adjacency segment, SRH, SR domain, SR source node, SR segment endpoint node, transit node, SRv6 endpoint behavior, flavor, SID block, locator, function, and argument. The reader is assumed to be familiar with this terminology. This document introduces the following new terms:
Locator-Block:
The most significant bits of a SID locator that represent the SRv6 SID block. The Locator-Block is referred to as "B" in .
Locator-Node:
The least significant bits of a SID locator that identify the SR segment endpoint node instantiating the SID. The Locator-Node is referred to as "N" in .
Compressed-SID (CSID):
A compressed encoding of a SID. The CSID includes the Locator-Node and Function bits of the SID being compressed. If either constituent of the SID is empty (zero length), then the same applies to its CSID encoding.
CSID container:
A 128-bit IPv6 address that functions as a container holding a list of one or more CSIDs and the Argument (if any) of the last CSID.
CSID sequence:
A group of one or more consecutive SID list entries encoding the common Locator-Block and at least one CSID container.
Compressed SID list:
A segment list encoding that reduces the packet header length thanks to one or more CSID sequences. A compressed SID list also contains zero, one, or more uncompressed SIDs.
Global Identifiers Block (GIB):
The pool of CSID values available for global allocation.
Local Identifiers Block (LIB):
The pool of CSID values available for local allocation.
In this document, the length of each constituent part of a SID is referred to as follows:
  • LBL is the Locator-Block length of the SID.
  • LNL is the Locator-Node length of the SID.
  • FL is the Function length of the SID.
  • AL is the Argument length of the SID.
In addition, the Locator-Node and Function length (LNFL) is the sum of the LNL and the FL of the SID. It is also referred to as the "CSID length".
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 SR domain, all SRv6 SIDs instantiated from the same Locator-Block share the same most significant bits. In addition, when the combined length of the SRv6 SID Locator, Function, and Argument is smaller than 128 bits, the least significant bits of the SID are padded with zeros. The compressed segment list encoding seeks to decrease the packet header length by avoiding the repetition of the same Locator-Block and reducing the use of padding bits. Building upon, and fully compatible with the mechanisms specified in and , the compressed segment list encoding leverages a SID list compression logic at the SR source node (see ) in combination with new flavors of the SRv6 endpoint behaviors that process the compressed SID list (see ). An SR source node constructs and compresses the SID list depending on the SIDs instantiated on each SR segment endpoint node that the packet is intended to traverse, as well as its own compression capabilities. The resulting compressed SID list is a combination of CSID sequences, for the SIDs that the SR source node was able to compress, and uncompressed SIDs, which could not be compressed. In case the SR source node is able to compress all the SIDs in the SID list, the compressed SID list comprises only CSID sequences (one or more) and no uncompressed SIDs. Conversely, the compressed SID list comprises only uncompressed SIDs when the SR source is unable to compress any of the constituent SIDs.
SR Segment Endpoint Flavors This section defines two SR segment endpoint flavors: NEXT-CSID and REPLACE-CSID, for the End, End.X, End.T, End.B6.Encaps, End.B6.Encaps.Red, and End.BM behaviors of . This section also defines a REPLACE-CSID flavor for the End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, End.DT2U, and End.DT2M behaviors of . A counterpart NEXT-CSID flavor is not defined for these behaviors. Any SID can be the last element of a CSID sequence compressed using the NEXT-CSID flavor (see ) and the aforementioned SRv6 endpoint behaviors are always in the last position in a SID list; thus, there is no need for any modification of the behaviors defined in . Future documents may extend the applicability of the NEXT-CSID and REPLACE-CSID flavors to other SRv6 endpoint behaviors (see ). The use of these flavors, either individually or in combination, enables the compressed segment list encoding. The NEXT-CSID flavor and the REPLACE-CSID flavor both leverage the SID Argument to determine the next SID to be processed, but employ different SID list compression schemes. With the NEXT-CSID flavor, each CSID container is a fully formed SRv6 SID with the common Locator-Block for all the CSIDs in the CSID container, a Locator-Node and Function that are those of the first CSID, and an Argument carrying the subsequent CSIDs. With the REPLACE-CSID flavor, only the first element in a CSID sequence is a fully formed SRv6 SID. It has the common Locator-Block for all the CSIDs in the CSID sequence, and a Locator-Node and Function that are those of the first CSID. The remaining elements in the CSID sequence are CSID containers carrying the subsequent CSIDs without the Locator-Block. Regardless of which flavor is used, the IPv6 address carried in the Destination Address field of the IPv6 header is a valid SRv6 SID conforming to . In the remainder of this document, the term "a SID of this document" refers to any End, End.X, End.T, End.B6.Encaps, End.B6.Encaps.Red, or End.BM SID with the NEXT-CSID or the REPLACE-CSID flavor and with any combination of Penultimate Segment Pop (PSP), Ultimate Segment Pop (USP), and Ultimate Segment Decapsulation (USD) flavor, or any End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, End.DT2U, or End.DT2M with the REPLACE-CSID flavor. All the SRv6 endpoint behaviors introduced in this document are listed in . In the remainder of this document, the terms "NEXT-CSID flavor SID" and "REPLACE-CSID flavor SID" refer to any SID of this document with the NEXT-CSID flavor and with the REPLACE-CSID flavor, respectively.
NEXT-CSID Flavor A CSID sequence compressed using the mechanism of the NEXT-CSID flavor comprises one or more CSID containers. Each CSID container is a fully formed 128-bit SID structured as shown in . It carries a Locator-Block followed by a series of CSIDs. The Locator-Node and Function of the CSID container are those of the first CSID, and its Argument is the contiguous series of subsequent CSIDs. The second CSID is encoded in the most significant bits of the CSID container Argument. The third CSID is encoded in the bits of the Argument that immediately follow the second CSID, and so on. When all CSIDs have the same length, a CSID container can carry up to K CSIDs, where K is computed as floor((128-LBL)/LNFL) (floor(x) is the greatest integer less than or equal to x ). Each CSID container for NEXT-CSID is independent, such that contiguous CSID containers in a CSID sequence can be considered to be separate CSID sequences. When a CSID sequence compressed using the NEXT-CSID flavor comprises at least two CSIDs, the last CSID in the sequence is not required to have the NEXT-CSID flavor. It can be bound to any SRv6 endpoint behavior, including behaviors and REPLACE-CSID flavor, as long as the updated Destination Address resulting from the processing of the previous CSID in the sequence is a valid form for that last SID. Line S12 of the first pseudocode in provides sufficient conditions to ensure this property.
Structure of a NEXT-CSID Flavor SID (Scaled for a 48-Bit Locator‑Block, 16-Bit Combined Locator-Node and Function, and 64-Bit Argument) Locator-Block Loc-Node Argument Function LBL LNFL AL +------------------------------------------------------------------+ | Locator-Block |Loc-Node| Argument | | |Function| | +------------------------------------------------------------------+ <----------------------> <------> <------------------------------> LBL LNFL AL
illustrates a compressed SID list as could be produced by an SR source node steering a packet into an SR policy with a SID list of eight NEXT-CSID flavor SIDs. All SIDs in this example have a 48-bit Locator-Block, 16-bit combined Locator-Node and Function, and 64-bit Argument. The SR source node compresses the SR policy SID list as a compressed SID list of two CSID containers. The first CSID container carries a Locator-Block and the first five CSIDs. The second CSID container carries a Locator-Block and the sixth, seventh, and eighth CSIDs. Since the SR source node does not use the second CSID container at full capacity, it sets the 32 least significant bits to zero. The SR source node sets the IPv6 Destination Address (DA) with the value of the first CSID container and the first element of the SRH Segment List with the value of the second CSID container. Without reduced SRH (see ), the SR source node also writes the first CSID container as the second element of the SRH Segment List. Note that the CSIDs within a given CSID container appear in forward order to leverage the longest-prefix match IP forwarding, while the entries in the SRH Segment List appear in reversed order of their processing, as specified in .
Compressed SID List of Eight NEXT-CSID Flavor SIDs with a 48-Bit Locator‑Block, 16-Bit Combined Locator-Node and Function, and 64-Bit Argument 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Locator-Block 1st CSID 2nd CSID 3rd CSID 4th CSID 5th CSID First CSID container 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Locator-Block 6th CSID 7th CSID 8th CSID 0 Second CSID container 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Locator-Block +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | 1st CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2nd CSID | 3rd CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4th CSID | 5th CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ First CSID Container 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Locator-Block +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | 6th CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 7th CSID | 8th CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Second CSID Container
An implementation MUST support a 32-bit LBL and a 16-bit CSID length (LNFL) for NEXT-CSID flavor SIDs, and it MAY support any additional Locator-Block and CSID length. The AL for NEXT-CSID flavor SIDs is equal to 128-LBL-LNFL. When processing an IPv6 packet that matches a Forwarding Information Base (FIB) entry locally instantiated as a SID with the NEXT-CSID flavor, the SR segment endpoint node applies the procedure specified in the following subsection that corresponds to the SID behavior. If the SID also has the PSP, USP, or USD flavor, the procedure is modified as described in . An SR segment endpoint node instantiating a SID of this document with the NEXT-CSID flavor MUST accept any Argument value for that SID. At a high level, for any SID with the NEXT-CSID flavor, the SR segment endpoint node determines the next SID of the SID list as follows. If the Argument value of the active SID is non-zero, the SR segment endpoint node constructs the next SID from the active SID by copying the entire SID Argument value to the bits that immediately follow the Locator-Block, thus overwriting the active SID Locator-Node and Function with those of the next CSID, and filling the least significant LNFL bits of the Argument with zeros. Otherwise (if the Argument value is 0), the SR segment endpoint node copies the next 128-bit Segment List entry from the SRH to the Destination Address field of the IPv6 header.
End with NEXT-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End SID with the NEXT-CSID flavor, the procedure described in is executed with the following modifications. The below pseudocode is inserted between lines S01 and S02 of the SRH processing in . In addition, this pseudocode is executed before processing the first header in the IPv6 extension header chain that is not an SRH, a Hop-by-Hop header, or a Destination Options header. If the IPv6 extension header chain does not include any header matching this criterion, this pseudocode is executed before processing the upper-layer header. N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. Send an ICMP Time Exceeded message to the Source Address, Code 0 (Hop limit exceeded in transit), interrupt packet processing and discard the packet. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. N09. } A rendering of the complete pseudocode is provided in .
End.X with NEXT-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.X SID with the NEXT-CSID flavor, the procedure described in is executed with the following modifications. The pseudocode in of this document is modified by replacing line N08 as shown below. N08. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. The resulting pseudocode is inserted between lines S01 and S02 of the SRH processing in after applying the modification described in . In addition, this pseudocode is executed before processing the first header in the IPv6 extension header chain that is not an SRH, a Hop-by-Hop header, or a Destination Options header. If the IPv6 extension header chain does not include any header matching this criterion, this pseudocode is executed before processing the upper-layer header. A rendering of the complete pseudocode is provided in .
End.T with NEXT-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.T SID with the NEXT-CSID flavor, the procedure described in is executed with the following modifications. The pseudocode in of this document is modified by replacing line N08 as shown below. N08.1. Set the packet's associated FIB table to T. N08.2. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. The resulting pseudocode is inserted between lines S01 and S02 of the SRH processing in after applying the modification described in . In addition, this pseudocode is executed before processing the first header in the IPv6 extension header chain that is not an SRH, a Hop-by-Hop header, or a Destination Options header. If the IPv6 extension header chain does not include any header matching this criterion, this pseudocode is executed before processing the upper-layer header. A rendering of the complete pseudocode is provided in .
End.B6.Encaps with NEXT-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-CSID flavor, the procedure described in is executed with the following modifications. The pseudocode in of this document is modified by replacing line N08 as shown below. N08.1. Push a new IPv6 header with its own SRH containing B. N08.2. Set the outer IPv6 SA to A. N08.3. Set the outer IPv6 DA to the first SID of B. N08.4. Set the outer Payload Length, Traffic Class, Flow Label, Hop Limit, and Next Header fields. N08.5. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. The resulting pseudocode is inserted between lines S01 and S02 of the SRH processing in . In addition, this pseudocode is executed before processing the first header in the IPv6 extension header chain that is not an SRH, a Hop-by-Hop header, or a Destination Options header. If the IPv6 extension header chain does not include any header matching this criterion, this pseudocode is executed before processing the upper-layer header. A rendering of the complete pseudocode is provided in . Similar to the base End.B6.Encaps SID defined in , the NEXT-CSID flavor variant updates the Destination Address field of the inner IPv6 header to the next SID in the original segment list before encapsulating the packet with the segment list of SR Policy B. At the endpoint of SR Policy B, the encapsulation is removed and the inner packet is forwarded towards the exposed Destination Address, which already contains the next SID in the original segment list.
End.B6.Encaps.Red with NEXT-CSID This is an optimization of the End.B6.Encaps with NEXT-CSID behavior. When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.B6.Encaps.Red SID with the NEXT-CSID flavor, the procedure described in of this document is executed with the modifications in .
End.BM with NEXT-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.BM SID with the NEXT-CSID flavor, the procedure described in is executed with the following modifications. The pseudocode in of this document is modified by replacing line N08 as shown below. N08.1. Push the MPLS label stack for B. N08.2. Submit the packet to the MPLS engine for transmission. The resulting pseudocode is inserted between lines S01 and S02 of the SRH processing in . In addition, this pseudocode is executed before processing the first header in the IPv6 extension header chain that is not an SRH, a Hop-by-Hop header, or a Destination Options header. If the IPv6 extension header chain does not include any header matching this criterion, this pseudocode is executed before processing the upper-layer header. A rendering of the complete pseudocode is provided in .
Combination with PSP, USP, and USD Flavors
PSP:
The PSP flavor defined in is unchanged when combined with the NEXT-CSID flavor.
USP:
The USP flavor defined in is unchanged when combined with the NEXT-CSID flavor.
USD:
The USD flavor defined in is unchanged when combined with the NEXT-CSID flavor.
REPLACE-CSID Flavor A CSID sequence compressed using the mechanism of the REPLACE-CSID flavor starts with a CSID container in fully formed 128-bit SID format. The Locator-Block of this SID is the common Locator-Block for all the CSIDs in the CSID sequence, its Locator-Node and Function are those of the first CSID, and its Argument carries the index of the current CSID in the current CSID container. The Argument value is initially 0. When more segments are present in the segment list, the CSID sequence continues with one or more CSID containers in packed format carrying the series of subsequent CSIDs. Each container in packed format is a 128-bit Segment List entry split into K "positions" of LNFL bits, where K is computed as floor(128/LNFL). If LNFL does not divide into 128 perfectly, a zero pad is added in the least significant bits of the CSID container to fill the bits left over. The second CSID in the CSID sequence is encoded in the least significant bit position of the first CSID container in packed format (position K-1), the third CSID is encoded in position K-2, and so on. The last CSID in the CSID sequence is not required to have the REPLACE-CSID flavor. It can be bound to any SRv6 endpoint behavior, including the behaviors described in and NEXT-CSID flavor, as long as it meets the conditions defined in . The structure of a SID with the REPLACE-CSID flavor is shown in . The same structure is also that of the CSID container for REPLACE-CSID in fully formed 128-bit SID format.
Structure of a REPLACE-CSID Flavor SID (Scaled for a 48-Bit Locator‑Block, 32-Bit Combined Locator-Node and Function, and 48-Bit Argument) Locator-Block Locator-Node Argument + Function LBL LNFL AL +-------------------------------------------------------------------+ | Locator-Block | Locator-Node | Argument | | | + Function | | +-------------------------------------------------------------------+ <----------------------> <--------------> <-----------------------> LBL LNFL AL
The structure of a CSID container for REPLACE-CSID in packed format is shown in .
Structure of a CSID Container for REPLACE-CSID Using a 32-Bit CSID Length (K = 4) Fourth CSID Third CSID Second CSID First CSID (position 0) (position 1) (position 2) (position 3) LNFL LNFL LNFL LNFL +-------------------------------------------------------------------+ | Fourth CSID | Third CSID | Second CSID | First CSID | | (position 0) | (position 1) | (position 2) | (position 3) | +-------------------------------------------------------------------+ <--------------> <--------------> <--------------> <--------------> LNFL LNFL LNFL LNFL
illustrates a compressed SID list as could be produced by an SR source node steering a packet into an SR policy SID list of seven REPLACE-CSID flavor SIDs. All SIDs in this example have a 48-bit Locator-Block, 32-bit combined Locator-Node and Function, and 48-bit Argument. The SR source node compresses the SR policy SID list as a compressed SID list of three CSID containers. The first CSID container is in fully formed 128-bit SID format. It carries a Locator-Block, the first CSID, and the argument value zero. The second and third CSID containers are in packed format. The second CSID container carries the second, third, fourth, and fifth CSIDs. The third CSID container carries the sixth and seventh CSIDs. Since the SR source node does not use the third CSID container at full capacity, it sets the 64 least significant bits to zero. The SR source node sets the IPv6 DA with the value of the first CSID container, sets the first element in the SRH Segment List with the value of the third CSID container, and sets the second element of the SRH Segment List with the value of the second CSID container (the elements in the SRH Segment List appear in reversed order of their processing, as specified in ). Without reduced SRH, the SR source node also writes the first CSID container as the third element of the SRH Segment List.
Compressed SID List of Seven REPLACE-CSID Flavor SIDs with a 48-Bit Locator‑Block, 32-Bit Combined Locator-Node and Function, and 48-Bit Argument 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Locator-Block 1st CSID 1st CSID continued 0 First CSID container 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 5th CSID 4th CSID 3rd CSID 2nd CSID Second CSID container 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 7th CSID 6th CSID Third CSID container 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Locator-Block +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | 1st CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1st CSID continued | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 0 + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ First CSID Container 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 5th CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4th CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 3rd CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2nd CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Second CSID Container 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + 0 + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 7th CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6th CSID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Third CSID Container
This document updates by allowing each entry in the SRH Segment List to be either an IPv6 address or a REPLACE-CSID container in packed format. The SRv6 endpoint behaviors specified herein ensure that this entry is never copied as is to the IPv6 header and that the Destination Address field of the IPv6 header is always a valid SRv6 SID conforming to . The REPLACE-CSID flavor SIDs support any LBL, depending on the needs of the operator, as long as it does not exceed 128-LNFL-ceiling(log_2(128/LNFL)) (ceiling(x) is the least integer greater than or equal to x ), so that enough bits remain available for the CSID and Argument. An LBL of 48, 56, 64, 72, or 80 bits is recommended for easier reading in operation. This document defines the REPLACE-CSID flavor for 16-bit and 32-bit CSID lengths (LNFL). An implementation MUST support a 32-bit CSID length for REPLACE-CSID flavor SIDs. The AL for REPLACE-CSID flavor SIDs is equal to 128-LBL-LNFL. The index value is encoded in the least significant X bits of the Argument, where X is computed as ceiling(log_2(128/LNFL)). When processing an IPv6 packet that matches a FIB entry locally instantiated as a SID with the REPLACE-CSID flavor, the SR segment endpoint node applies the procedure specified in the following subsection that corresponds to the SID behavior. If the SID also has the PSP, USP, or USD flavor, the procedure is modified as described in . At a high level, at the start of a CSID sequence using the REPLACE-CSID flavor, the first CSID container in fully formed 128-bit SID format is copied to the Destination Address of the IPv6 header. Then, for any SID with the REPLACE-CSID flavor, the SR segment endpoint node determines the next SID of the SID list as follows. When an SRH is present, the SR segment endpoint node decrements the index value in the Argument of the active SID if the index value is not 0 or, if it is 0, decrements the Segments Left value in the SRH and sets the index value in the Argument of the active SID to K-1. The updated index value indicates the position of the next CSID within the CSID container in packed format at the "Segment List" index "Segments Left" in the SRH. The SR segment endpoint node then constructs the next SID by copying this next CSID to the bits that immediately follow the Locator-Block in the Destination Address field of the IPv6 header, thus overwriting the active SID Locator-Node and Function with those of the next CSID. If no SRH is present, the SR segment endpoint node ignores the index value in the SID Argument (except End.DT2M, see ) and processes the upper-layer header as per . The CSID sequence ends with a last CSID in the last CSID container that does not have the REPLACE-CSID flavor, or with the special CSID value 0, or when reaching the end of the segment list, whichever comes first.
End with REPLACE-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End SID with the REPLACE-CSID flavor, the SRH processing described in is executed with the following modifications. Line S02 of SRH processing in is replaced as follows. S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or Segment List[0][DA.Arg.Index-1] == 0)) { Lines S09 to S15 are replaced by the following pseudocode. R01. If (DA.Arg.Index != 0) { R02. If ((Last Entry > max_LE) or (Segments Left > Last Entry)) { R03. 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. R04. } R05. Decrement DA.Arg.Index by 1. R06. If (Segment List[Segments Left][DA.Arg.Index] == 0) { R07. Decrement Segments Left by 1. R08. Decrement IPv6 Hop Limit by 1. R09. Update IPv6 DA with Segment List[Segments Left] R10. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. R11. } R12. } Else { R13. If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){ R14. 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. R15. } R16. Decrement Segments Left by 1. R17. Set DA.Arg.Index to (floor(128/LNFL) - 1). R18. } R19. Decrement IPv6 Hop Limit by 1. R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits [LBL..LBL+LNFL-1] of the Destination Address of the IPv6 header. R21. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. The upper-layer header processing described in is unchanged. A rendering of the complete pseudocode is provided in .
End.X with REPLACE-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.X SID with the REPLACE-CSID flavor, the procedure described in is executed with the following modifications. The pseudocode in of this document is modified by replacing lines R10 and R21 as shown below. R10. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. R21. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. The SRH processing in is replaced with the resulting pseudocode. The upper-layer header processing is unchanged. A rendering of the complete pseudocode is provided in .
End.T with REPLACE-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.T SID with the REPLACE-CSID flavor, the procedure described in is executed with the following modifications. The pseudocode in of this document is modified by replacing lines R10 and R21 as shown below. R10.1. Set the packet's associated FIB table to T. R10.2. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. R21.1. Set the packet's associated FIB table to T. R21.2. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. The SRH processing in is replaced with the resulting pseudocode. The upper-layer header processing is unchanged. A rendering of the complete pseudocode is provided in .
End.B6.Encaps with REPLACE-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.B6.Encaps SID with the REPLACE-CSID flavor, the procedure described in is executed with the following modifications. The pseudocode in of this document is modified by replacing lines R10 and R21 as shown below. R10.1. Push a new IPv6 header with its own SRH containing B. R10.2. Set the outer IPv6 SA to A. R10.3. Set the outer IPv6 DA to the first SID of B. R10.4. Set the outer Payload Length, Traffic Class, Flow Label, Hop Limit, and Next Header fields. R10.5. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. R21.1. Push a new IPv6 header with its own SRH containing B. R21.2. Set the outer IPv6 SA to A. R21.3. Set the outer IPv6 DA to the first SID of B. R21.4. Set the outer Payload Length, Traffic Class, Flow Label, Hop Limit, and Next Header fields. R21.5. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. The SRH processing in is replaced with the resulting pseudocode. The upper-layer header processing is unchanged. A rendering of the complete pseudocode is provided in .
End.B6.Encaps.Red with REPLACE-CSID This is an optimization of the End.B6.Encaps with REPLACE-CSID behavior. When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.B6.Encaps.Red SID with the REPLACE-CSID flavor, the procedure described in of this document is executed with the modifications in .
End.BM with REPLACE-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.BM SID with the REPLACE-CSID flavor, the procedure described in is executed with the following modifications. The pseudocode in of this document is modified by replacing lines R10 and R21 as shown below. R10.1. Push the MPLS label stack for B. R10.2. Submit the packet to the MPLS engine for transmission. R21.1. Push the MPLS label stack for B. R21.2. Submit the packet to the MPLS engine for transmission. The SRH processing in is replaced with the resulting pseudocode. The upper-layer header processing is unchanged. A rendering of the complete pseudocode is provided in .
End.DX and End.DT with REPLACE-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.DX6, End.DX4, End.DT6, End.DT4, End.DT46, End.DX2, End.DX2V, or End.DT2U SID with the REPLACE-CSID flavor, the corresponding procedure described in Sections through of is executed. These SIDs differ from those defined in by the presence of an Argument as part of the SID structure. The Argument value is ignored by the SR segment endpoint node. When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.DT2M SID with the REPLACE-CSID flavor, the procedure described in is executed with the following modification. For any End.DT2M SID with the REPLACE-CSID flavor, the value of Arg.FE2 is 16 bits long. The SR segment endpoint node obtains the value Arg.FE2 from the 16 most significant bits of DA.Argument if DA.Arg.Index is zero or from the 16 least significant bits of the next position in the current CSID container (Segment List[Segments Left][DA.Arg.Index-1]) otherwise (DA.Arg.Index is non-zero).
Combination with PSP, USP, and USD Flavors PSP: When combined with the REPLACE-CSID flavor, the additional PSP flavor instructions defined in are inserted after lines R09 and R20 of the pseudocode in , and the first line of the inserted instructions after R20 is modified as follows. R20.1. If (Segments Left == 0 and (DA.Arg.Index == 0 or Segment List[0][DA.Arg.Index-1] == 0)) {
USP:
When combined with the REPLACE-CSID flavor, the line S03 of the pseudocode in are substituted by the USP flavor instructions S03.1 to S03.4 defined in . Note that S03 is shown in the complete pseudocode in .
USD:
The USD flavor defined in is unchanged when combined with the REPLACE-CSID flavor.
CSID Allocation The CSID value of 0 is reserved. It is used to indicate the end of a CSID container. In order to efficiently manage the CSID numbering space, a deployment may divide it into two non-overlapping sub-spaces: a GIB and a LIB. The CSID values that are allocated from the GIB have a global semantic within the Locator-Block, while those that are allocated from the LIB have a local semantic on an SR segment endpoint node and within the scope of the Locator-Block. The concept of LIB is applicable to SRv6 and specifically to its NEXT-CSID and REPLACE-CSID flavors. The shorter the CSID, the more benefit the LIB brings. The opportunity to use these sub-spaces, their size, and their CSID allocation policy depends on the CSID length relative to the size of the network (e.g., number of nodes, links, service routes). Some guidelines for a typical deployment scenario are provided in the below subsections.
Global CSID A global CSID is a CSID allocated from the GIB. A global CSID identifies a segment defined at the Locator-Block level. The tuple (Locator-Block, CSID) identifies the same segment across all nodes of the SR domain. A typical example is a prefix segment bound to the End behavior. A node can have multiple global CSIDs under the same Locator-Block (e.g., one per IGP flexible algorithm ()). Multiple nodes may share the same global CSID (e.g., anycast ).
Local CSID A local CSID is a CSID allocated from the LIB. A local CSID identifies a segment defined at the node level and within the scope of a particular Locator-Block. The tuple (Locator-Block, CSID) identifies a different segment on each node of the SR domain. A typical example is a non-routed Adjacency segment bound to the End.X behavior. Let N1 and N2 be two different physical nodes of the SR domain and I a local CSID value: N1 may allocate value I to SID S1 and N2 may allocate the same value I to SID S2.
Recommended Installation of CSIDs in FIB defines how an SR segment endpoint node identifies a locally instantiated SRv6 SID. To ensure that any valid argument value is accepted, an SR segment endpoint node instantiating a NEXT-CSID or REPLACE-CSID flavor SID should install a corresponding FIB entry that matches only the Locator and Function parts of the SID (i.e., with a prefix length of LBL + LNL + FL). In addition, an SR segment endpoint node instantiating NEXT-CSID flavor SIDs from both the GIB and LIB may install combined "Global + Local" FIB entries to match a sequence of global and local CSIDs in a single longest-prefix match (LPM) lookup. For example, let us consider an SR segment endpoint node 10 instantiating the following two NEXT-CSID flavor SIDs according to the CSID length, LBL, and GIB/LIB recommendations in this section.
  • The SID 2001:db8:b1:10:: bound to the End behavior with the NEXT-CSID flavor is instantiated from a GIB with:
    • LBL = 48 (Locator-Block value 0x20010db800b1),
    • LNL = 16 (Locator-Node value 0x0010),
    • FL = 0, and
    • AL = 64.
  • The SID 2001:db8:b1:f123:: bound to the End.X behavior for its local IGP adjacency 123 with the NEXT-CSID flavor is instantiated from a LIB with:
    • LBL = 48 (Locator-Block value 0x20010db800b1),
    • LNL = 0,
    • FL = 16 (Function value 0xf123), and
    • AL = 64.
For SID 2001:db8:b1:10::, Node 10 would install the FIB entry 2001:db8:b1:10::/64 bound to the End SID with the NEXT-CSID flavor. For SID 2001:db8:b1:f123::, Node 10 would install the FIB entry 2001:db8:b1:f123::/64 bound to the End.X SID for adjacency 123 with the NEXT-CSID flavor. In addition, Node 10 may also install the combined FIB entry 2001:db8:b1:10:f123::/80 bound to the End.X SID for adjacency 123 with the NEXT-CSID flavor. As another example, let us consider an SR segment endpoint node 20 instantiating the following two REPLACE-CSID flavor SIDs according to the CSID length, LBL, and GIB/LIB recommendations in this section.
  • 2001:db8:b2:20:1:: from a GIB with LBL = 48, LNL = 16, FL = 16, AL = 48, and bound to the End behavior with the REPLACE-CSID flavor.
  • 2001:db8:b2:20:123:: from a GIB with LBL = 48, LNL = 16, FL = 16, AL = 48, and bound to the End.X behavior for its local IGP adjacency 123 with the REPLACE-CSID flavor.
For SID 2001:db8:b2:20:1::, Node 20 would install the FIB entry 2001:db8:b2:20:1::/80 bound to the End SID with the REPLACE-CSID flavor. For SID 2001:db8:b2:20:123::, Node 20 would install the FIB entry 2001:db8:b2:20:123::/80 bound to the End.X SID for adjacency 123 with the REPLACE-CSID flavor.
SR Source Node An SR source node may learn from a control plane protocol (see ) or local configuration the SIDs that it can use in a segment list, along with their respective SRv6 endpoint behavior, structure, and any other relevant attribute (e.g., the set of L3 adjacencies associated with an End.X SID).
SID Validation for Compression As part of the compression process or as a preliminary step, the SR source node MUST validate the SID structure of each SID of this document in the segment list. The SR source node does so regardless of whether the segment list is explicitly configured, locally computed, or advertised by a controller (e.g., via BGP or PCEP ). A SID structure is valid for compression if it meets all the following conditions:
  • The LBL is not 0.
  • The LNFL is not 0.
  • The AL is equal to 128-LBL-LNL-FL.
When compressing a SID list, the SR source node MUST treat an invalid SID structure as unknown. A SID with an unknown SID structure is not compressible. discusses how the SIDs of this document and their structure can be advertised to the SR source node through various control plane protocols. The SID structure may also be learned through configuration or other management protocols. The details of such mechanisms are outside the scope of this document.
Segment List Compression An SR source node MAY compress a SID list when it includes NEXT-CSID and/or REPLACE-CSID flavor SIDs to reduce the packet header length. It is out of the scope of this document to describe the mechanism through which an uncompressed SID list is derived, since such a mechanism may include a wide range of considerations independent of compression (e.g., minimizing a specific metric, excluding certain links, or providing a loop-free fast-reroute path). As general guidance for implementation or future specification, such a mechanism should aim to select the combination of SIDs that would result in the shortest compressed SID list. For example, by selecting a CSID flavor SID over an equivalent non-CSID flavor SID or by consistently selecting SIDs of the same CSID flavor within each routing domain. The SID list that the SR source node pushes onto the packet MUST comply with the rules in Sections and and express the same list of segments as the original SID list. If these rules are not followed, the packet may get dropped or misrouted. If an SR source node chooses to compress the SID list, one method is described below for illustrative purposes. Any other method producing a compressed SID list of equal or shorter length than the uncompressed SID list MAY be used. This method walks the uncompressed SID list and compresses each series of consecutive NEXT-CSID flavor SIDs and each series of consecutive REPLACE-CSID flavor SIDs.
  • When the compression method encounters a series of one or more consecutive compressible NEXT-CSID flavor SIDs, it compresses the series as follows. A SID with the NEXT-CSID flavor is compressible if its structure is known to the SR source node and its Argument value is 0. S01. Initialize a NEXT-CSID container equal to the first SID in the series and initialize the remaining capacity of the CSID container to the AL of that SID S02. For each subsequent SID in the series { S03. If the current SID Locator-Block matches that of the CSID container and the current SID LNFL is lower than or equal to the remaining capacity of the NEXT-CSID container { S04. Copy the current SID Locator-Node and Function to the most significant remaining Argument bits of the NEXT-CSID container and decrement the remaining capacity by LNFL S05. } Else { S06. Push the NEXT-CSID container onto the compressed SID list S07. Initialize a new NEXT-CSID container equal to the current SID in the series and initialize the remaining capacity of the NEXT-CSID container to the AL of that SID S08. } // End If S09. } // End For S10. If at least one SID remains in the uncompressed SID list (following the series of compressible NEXT-CSID flavor SIDs) { S11. Set S to the next SID in the uncompressed SID list S12. If S is advertised with a SID structure, and the Locator-Block of S matches that of the NEXT-CSID container, and the sum of the Locator-Node, Function, and Argument length of S is lower than or equal to the remaining capacity of the CSID container { S13. Copy the Locator-Node, Function, and Argument of S to the most significant remaining Argument bits of the CSID container S14. } // End If S15. } // End If S16. Push the NEXT-CSID container onto the compressed SID list
  • When the compression method encounters a series of REPLACE-CSID flavor SIDs of the same CSID length in the uncompressed SID list, it compresses the series as per the following high-level pseudocode. A compression checking function ComCheck(F, S) is defined to check if two SIDs F and S share the same SID structure and Locator-Block value, and if S has either no Argument or an Argument with value 0. If the check passes, then ComCheck(F,S) returns true. S01. Initialize a REPLACE-CSID container in full SID format equal to the first SID in the series S02. Push the REPLACE-CSID container onto the compressed SID list S03. Initialize a new REPLACE-CSID container in packed format if there are more than one SIDs and initialize the remaining capacity of the REPLACE-CSID container to 128 bits S04. For each subsequent SID in the uncompressed SID list { S05. Set S to the current SID in the uncompressed SID list S06. If ComCheck(First SID, S) { S07. If the LNFL of S is lower than or equal to the remaining capacity of the REPLACE-CSID container { S08. Copy the Locator-Node and Function of S to the least significant remaining bits of the REPLACE-CSID container and decrement the remaining capacity by LNFL // Note S09. } Else { S10. Push the REPLACE-CSID container onto the compressed SID list S11. Initialize a new REPLACE-CSID container in packed format with all bits set to 0 S12. Copy the Locator-Node and Function of S to the least significant remaining bits of the REPLACE-CSID container and decrement the remaining capacity by LNFL // Note S13. } S14. If S is not a REPLACE-CSID flavor SID, then break S15. } Else { S16. Break S17. } // End If S18. } // End For S19. Push the REPLACE-CSID container (if it is not empty) onto the compressed SID list
In all remaining cases (i.e., when the compression method encounters a SID in the uncompressed SID list that is not handled by any of the previous subroutines), it pushes this SID as is onto the compressed SID list. Regardless of how a compressed SID list is produced, the SR source node writes it in the IPv6 packet as described in Sections and of . The text is reproduced below for reference.
A source node steers a packet into an SR Policy. If the SR Policy results in a Segment List containing a single segment, and there is no need to add information to the SRH flag or add TLV; the DA is set to the single Segment List entry, and the SRH MAY be omitted. When needed, the SRH is created as follows: The Next Header and Hdr Ext Len fields are set as specified in . The Routing Type field is set to 4. The DA of the packet is set with the value of the first segment. The first element of the SRH Segment List is the ultimate segment. The second element is the penultimate segment, and so on. The Segments Left field is set to n-1, where n is the number of elements in the SR Policy. The Last Entry field is set to n-1, where n is the number of elements in the SR Policy. TLVs (including HMAC) may be set according to their specification. The packet is forwarded toward the packet's Destination Address (the first segment). When a source does not require the entire SID list to be preserved in the SRH, a reduced SRH may be used. A reduced SRH does not contain the first segment of the related SR Policy (the first segment is the one already in the DA of the IPv6 header), and the Last Entry field is set to n-2, where n is the number of elements in the SR Policy.
Rules for Segment Lists Containing NEXT-CSID Flavor SIDs
  1. If a Destination Options header would follow an SRH with a segment list of more than one segment compressed as a single NEXT-CSID container, the SR source node MUST NOT omit the SRH.
  2. When the last Segment List entry (index 0) in the SRH is a NEXT-CSID container representing more than one segment and the segment S preceding the first segment of this NEXT-CSID container in the segment list has the PSP flavor, then the PSP operation is performed at the SR segment endpoint node of S. If the PSP behavior should instead be performed at the penultimate segment along the path, then the SR source node MUST NOT compress the ultimate SID of the SID list into a NEXT-CSID container.
  3. If a Destination Options header would follow an SRH with a last Segment List entry being a NEXT-CSID container representing more than one segment, the SR source node MUST ensure that the PSP operation is not performed before the penultimate SR segment endpoint node along the path.
  4. When the Argument of a NEXT-CSID container is not used to full capacity, the remaining least significant bits of that Argument MUST be set to 0.
Rules for Segment Lists Containing REPLACE-CSID Flavor SIDs
  1. All SIDs compressed in a REPLACE-CSID sequence MUST share the same Locator-Block and the same compression scheme.
  2. All SIDs except the last one in a CSID sequence for REPLACE-CSID MUST have the REPLACE-CSID flavor. If the last REPLACE-CSID container is fully filled (i.e., the last CSID is at position 0 in the REPLACE-CSID container) and the last SID in the CSID sequence is not the last segment in the segment list, the last SID in the CSID sequence MUST NOT have the REPLACE-CSID flavor.
  3. When a REPLACE-CSID flavor CSID is present as the last SID in a container that is not the last Segment List entry (index 0) in the SRH, the next element in the SID list MUST be a REPLACE-CSID container in packed format carrying at least one CSID.
The SR source node determines the compression scheme of REPLACE-CSID flavor SIDs as follows. When receiving a SID advertisement for a REPLACE-CSID flavor SID with LNL = 16, FL = 0, AL = 128-LBL-LNFL, and all zeros as the value of the Argument, the SR source node marks both the SID and its locator as using 16-bit compression. All other SIDs allocated from this locator with LNL = 16, FL = 16, AL = 128-LBL-LNFL, and all zeros as the value of the Argument are also marked as using 16-bit compression. When receiving a SID advertisement for a REPLACE-CSID flavor SID with LNFL = 32, AL = 128-LBL-LNFL, and all zeros as the value of the Argument, the SR source node marks both the SID and its locator as using 32-bit compression.
Upper-Layer Checksums The Destination Address used in the IPv6 pseudo-header () is that of the ultimate destination. At the SR source node, that address will be the Destination Address as it is expected to be received by the ultimate destination. When the last element in the compressed SID list is a CSID container, this address can be obtained from the last element in the uncompressed SID list or by repeatedly applying the segment behavior as described in . This applies regardless of whether an SRH is present in the IPv6 packet or is omitted. At the ultimate destination(s), that address will be in the Destination Address field of the IPv6 header.
Inter-Domain Compression Some SRv6 traffic may need to cross multiple routing domains, such as different Autonomous Systems (ASes) or different routing areas within an SR domain. Different routing domains may use different addressing schema and Locator-Blocks. A property of a CSID sequence is that all CSIDs in the sequence share the same Locator-Block. Therefore, a segment list that spans multiple routing domains using different Locator-Blocks may need a separate CSID sequence for each domain. This section defines a solution to improve the efficiency of CSID compression in multi-domain environments by enabling a CSID sequence to combine CSIDs having different Locator-Blocks. The solution leverages two new SRv6 endpoint behaviors, "Endpoint with SRv6 Locator-Block Swap" ("End.LBS" for short) and "Endpoint with L3 cross-connect and SRv6 Locator-Block Swap" ("End.XLBS" for short), that enable modifying the Locator-Block for the next CSID in the CSID sequence at the routing domain boundary.
End.LBS: Locator-Block Swap The End.LBS behavior is a variant of the End behavior that modifies the Locator-Block of the active CSID sequence. This document defines the End.LBS behavior with the NEXT-CSID flavor and the End.LBS behavior with the REPLACE-CSID flavor. An End.LBS SID is used to transition to a new Locator-Block when the routing domain boundary is on the SR segment endpoint node. Each instance of an End.LBS SID is associated with a target Locator-Block B2/m, where B2 is an IPv6 address prefix and m is the associated prefix length. The original and target Locator-Blocks can have different prefix lengths as long as the new Destination Address formed by combining the target Locator-Block with the Locator-Node, Function, and Argument as described in the pseudocode of Sections and is a valid IPv6 address. The target Locator-Block is a local property of the End.LBS SID on the SR segment endpoint node. The means by which an SR source node learns the target Locator-Block associated with an End.LBS SID are outside the scope of this document. As examples, it could be learned via configuration or signaled by a controller.
End.LBS with NEXT-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.LBS SID with the NEXT-CSID flavor and associated with the target Locator-Block B2/m, the SR segment endpoint node applies the procedure specified in with the lines N05 to N06 replaced as follows. N05.1. Initialize an IPv6 address A equal to B2. N05.2. Copy DA.Argument into the bits [m..(m+AL-1)] of A. N06. Copy A to the Destination Address of the IPv6 header.
End.LBS with REPLACE-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.LBS SID with the REPLACE-CSID flavor and associated with the target Locator-Block B2/m, the SR segment endpoint node applies the procedure specified in with the line R20 replaced as follows. R20.1. Initialize an IPv6 address A equal to B2. R20.2. Write Segment List[Segments Left][DA.Arg.Index] into the bits [m..m+LNFL-1] of A. R20.3. Write DA.Arg.Index into the bits [(128-ceiling(log_2(128/LNFL)))..127] of A. R20.4. Copy A to the Destination Address of the IPv6 header.
End.XLBS: L3 Cross-Connect and Locator-Block Swap The End.XLBS behavior is a variant of the End.X behavior that modifies the Locator-Block of the active CSID sequence. This document defines the End.XLBS behavior with the NEXT-CSID flavor and the End.XLBS behavior with the REPLACE-CSID flavor. An End.XLBS SID is used to transition to a new Locator-Block when the routing domain boundary is on a link adjacent to the SR segment endpoint node. Each instance of an End.XLBS SID is associated with a target Locator-Block B2/m and a set, J, of one or more L3 adjacencies. The original and target Locator-Blocks can have different prefix lengths as long as the new Destination Address formed by combining the target Locator-Block with the Locator-Node, Function, and Argument as described in the pseudocode of Sections and is a valid IPv6 address. The target Locator-Block and set of adjacencies are local properties of the End.XLBS SID on the SR segment endpoint node. The means by which an SR source node learns the target Locator-Block associated with an End.XLBS SID are outside the scope of this document. As examples, it could be learned via configuration or signaled by a controller.
End.XLBS with NEXT-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.XLBS SID with the NEXT-CSID flavor and associated with the target Locator-Block B2/m, the SR segment endpoint node applies the procedure specified in with the lines N05 to N06 (of the pseudocode in ) replaced as follows. N05.1. Initialize an IPv6 address A equal to B2. N05.2. Copy DA.Argument into the bits [m..(m+AL-1)] of A. N06. Copy A to the Destination Address of the IPv6 header.
End.XLBS with REPLACE-CSID When processing an IPv6 packet that matches a FIB entry locally instantiated as an End.XLBS SID with the REPLACE-CSID flavor and associated with the target Locator-Block B2/m, the SR segment endpoint node applies the procedure specified in with the line R20 (of the pseudocode in ) replaced as follows. R20.1. Initialize an IPv6 address A equal to B2. R20.2. Write Segment List[Segments Left][DA.Arg.Index] into the bits [m..m+LNFL-1] of A. R20.3. Write DA.Arg.Index into the bits [(128-ceiling(log_2(128/LNFL)))..127] of A. R20.4. Copy A to the Destination Address of the IPv6 header.
Control Plane provides an overview of the control plane protocols used for signaling of the SRv6 endpoint behaviors introduced by that document, including the base SRv6 endpoint behaviors that are extended in the present document. The CSID-flavored behaviors introduced by this document are advertised in the same manner as their base SRv6 endpoint behaviors using the SRv6 extensions for various routing protocols, such as:
  • IS-IS
  • OSPFv3
  • BGP , ,
  • BGP-LS
  • PCEP
The SR segment endpoint node MUST set the SID Argument bits to 0 when advertising a locally instantiated SID of this document in the routing protocol (e.g., IS-IS , OSPF , or BGP-LS ). Signaling the SRv6 SID Structure is REQUIRED for all the SIDs introduced in this document. It is used by an SR source node to compress a SID list as described in . The node initiating the SID advertisement MUST set the length values in the SRv6 SID Structure to match the format of the SID on the SR segment endpoint node. For example, for a SID of this document instantiated from a /48 SRv6 SID block and a /64 Locator, and having a 16-bit Function, the SRv6 SID Structure advertisement carries the following values.
  • LBL: 48
  • LNL: 16
  • FL: 16
  • AL: 48 (= 128-48-16-16)
A local CSID may be advertised in the control plane individually and/or in combination with a global CSID instantiated on the same SR segment endpoint node, with the End behavior, and the same Locator-Block and flavor as the local CSID. A combined global and local CSID is advertised as follows:
  • The SID Locator-Block is that shared by the global and local CSIDs
  • The SID Locator-Node is that of the global CSID
  • The SID Function is that of the local CSID
  • The SID AL is equal to 128-LBL-LNL-FL and the SID Argument value is 0
  • All other attributes of the SID (e.g., SRv6 endpoint behavior or algorithm) are those of the local CSID
The combined advertisement of local CSIDs with a global CSID is needed in particular for control plane protocols mandating that the SID is a subnet of a locator advertised in the same protocol (e.g., and for advertising Adjacency SIDs in IS-IS and OSPFv3, respectively). For a segment list computed by a controller and signaled to an SR source node (e.g., via BGP or PCEP ), the controller provides the ordered segment list comprising the uncompressed SIDs, with their respective behavior and structure, to the SR source node. The SR source node may then compress the SID list as described in . When a node receives an advertisement of a SID of this document that it does not support, it handles the advertisement as described in the corresponding control plane specification (e.g., Sections , , and of , Sections , , and of , and ).
Operational Considerations
Flavor, Block, and CSID Length SRv6 is intended for use in a variety of networks that require different prefix lengths and SID numbering spaces. Each of the two flavors introduced in this document comes with its own recommendations for Locator-Block and CSID length, as specified in Sections and . These flavors are best suited for different environments, depending on the requirements of the network. For instance, larger CSID lengths may be more suitable for networks requiring ample SID numbering space, while smaller CSID lengths are better for compression efficiency. The two compression flavors allow the compressed segment list encoding to adapt to a range of requirements, with support for multiple compression levels. Network operators can choose the flavor that best suits their use case, deployment design, and network scale. Both CSID flavors can coexist in the same SR domain, on the same SR segment endpoint node, and even in the same segment list. However, operators should generally avoid instantiating SIDs of different CSID flavors within the same routing domain or Locator-Block since these SIDs have different length and allocation recommendations (see Sections , , and ). In a multi-domain deployment, different flavors may be used in different routing domains of the SR domain. A deployment should use consistent LBLs and CSID lengths for all SIDs within a routing domain. Heterogeneous lengths, while possible, may impact the compression efficiency. The compressed segment list encoding works with various Locator-Block allocations. For example, each routing domain within the SR domain can be allocated a /48 Locator-Block from a global IPv6 block available to the operator or from a prefix allocated to SRv6 SIDs as discussed in .
GIB/LIB Usage GIB and LIB usage is a local implementation and/or configuration decision; however, some guidelines for determining usage for specific SRv6 endpoint behaviors and recommendations are provided. The GIB number space is shared among all SR segment endpoint nodes using SRv6 locators under a Locator-Block space. The more SIDs assigned from this space, per node, the faster it is exhausted. Therefore, its use is prioritized for global segments, such as SIDs that identify a node. The LIB number space is unique per node. Each node can fully utilize the entire LIB number space without consideration for assignments at other nodes. Therefore, its use is prioritized for local segments, such as SIDs that identify services (of which there may be many) at nodes, cross-connects, or adjacencies. While a longer CSID length permits more flexibility in which SRv6 endpoint behaviors may be assigned from the GIB, it also reduces the compression efficiency. Given the previous Locator-Block and CSID length recommendations, the following GIB/LIB usage is recommended:
  • NEXT-CSID:
    • GIB: End
    • LIB: End.X, End.T, End.DT4/6/46/2U/2M, End.DX4/6/2/2V (including large-scale pseudowire), End.B6.Encaps, End.B6.Encaps.Red, End.BM, End.LBS, and End.XLBS
  • REPLACE-CSID:
    • GIB: End, End.X, End.T, End.DT4/6/46/2U/2M, End.DX4/6/2/2V, End.B6.Encaps, End.B6.Encaps.Red, End.BM, End.LBS, and End.XLBS
    • LIB: End.DX2/2V for large-scale pseudowire
Any other allocation is possible but may lead to a suboptimal use of the CSID numbering space.
Pinging a SID An SR source node may ping an SRv6 SID by sending an ICMPv6 echo request packet destined to the SRv6 SID. The SR source node may ping the target SID with a SID list comprising only that target SID or with a longer one that comprises two or more SIDs. In that case, the target SID is the last element in the SID list. This operation is illustrated in . When pinging a SID of this document, the SR source node MUST construct the IPv6 packet as described in , including computing the ICMPv6 checksum as described in . In particular, when pinging a SID of this document with a SID list comprising only the target SID, the SR source node places the SID with Argument value 0 in the Destination Address of the ICMPv6 echo request and computes the ICMPv6 checksum using this SID as the Destination Address in the IPv6 pseudo-header. The Argument value 0 allows the SID SR segment endpoint node () to identify itself as the ultimate destination of the packet and process the ICMPv6 payload. Therefore, any existing IPv6 ping implementation can originate ICMP echo requests to a NEXT-CSID or REPLACE-CSID flavor SID with a SID list comprising only the target SID, provided that the user ensures that the SID Argument is 0.
ICMP Error Processing When an IPv6 node encounters an error while processing a packet, it may report that error by sending an IPv6 error message to the packet source with an enclosed copy of the invoking packet. For the source of an invoking packet to process the ICMP error message, the ultimate Destination Address of the IPv6 header may be required. defines the logic that an SR source node follows to determine the ultimate destination of an invoking packet containing an SRH. For an SR source node that supports the compressed segment list encoding defined in this document, the logic to determine the ultimate destination is generalized as follows.
  • If the Destination Address of the invoking IPv6 packet matches a known SRv6 SID, modify the invoking IPv6 packet by applying the SRv6 endpoint behavior associated with the matched SRv6 SID;
  • Repeat until the application of the SRv6 endpoint behavior would result in the processing of the upper-layer header.
The Destination Address of the resulting IPv6 packet may be used as the ultimate destination of the invoking IPv6 packet. Since the SR source node that needs to determine the ultimate destination is the same node that originally built the SID list in the invoking packet, it can perform this operation for all the SIDs in the packet.
Applicability to Other SRv6 Endpoint Behaviors Future documents may extend the applicability of the NEXT-CSID and REPLACE-CSID flavors to other SRv6 endpoint behaviors. For an SRv6 endpoint behavior that can be used before the last position of a segment list, a CSID flavor is defined by reproducing the same logic as described in Sections and to determine the next SID in the SID list.
Security Considerations discusses the security considerations for Segment Routing. describes the intra-SR-domain deployment model and how to secure it. describes the threats applicable to SRv6 and how to mitigate them. discusses the security considerations applicable to the SRv6 network programming framework, as well as the SR source node and SR segment endpoint node behaviors that it defines. This document introduces two new flavors, NEXT-CSID and REPLACE-CSID, for some of the SRv6 endpoint behaviors defined in and a method by which an SR source node may leverage the SIDs of these flavors to produce a compressed segment list encoding. This document also introduces two new SRv6 endpoint behaviors, End.LBS and End.XLBS, to preserve the efficiency of CSID compression in multi-domain environments. An SR source node constructs an IPv6 packet with a compressed segment list encoding as defined in Sections and of and . The paths that an SR source node may enforce using a compressed segment list encoding are the same, from a topology and service perspective, as those that an SR source node could enforce using the SIDs of . An SR segment endpoint node processes an IPv6 packet matching a locally instantiated SID as defined in , with the pseudocode modifications in of this document. These modifications change how the SR segment endpoint node determines the next SID in the packet but not the semantic of either the active or the next SID. For example, an adjacency segment instantiated with the End.X behavior remains an adjacency segment regardless of whether it uses the base End.X behavior defined in or a CSID flavor of that behavior. This document does not introduce any new SID semantic. Any other transit node processes the packet as described in . This document defines a new method of encoding the SIDs inside a SID list at the SR source node () and decoding them at the SR segment endpoint node (see Sections and ), but it does not change how the SID list itself is encoded in the IPv6 packet nor the semantic of any segment that it comprises. Therefore, this document is subject to the same security considerations that are discussed in , , and .
IANA Considerations
SRv6 Endpoint Behaviors IANA has updated the reference of the following registrations from the "SRv6 Endpoint Behaviors" registry under the "Segment Routing" registry group () to point to this document and transfer change control to the IETF. SRv6 Endpoint Behaviors Registration List
Value Description Reference
43 End with NEXT-CSID RFC 9800
44 End with NEXT-CSID & PSP RFC 9800
45 End with NEXT-CSID & USP RFC 9800
46 End with NEXT-CSID, PSP & USP RFC 9800
47 End with NEXT-CSID & USD RFC 9800
48 End with NEXT-CSID, PSP & USD RFC 9800
49 End with NEXT-CSID, USP & USD RFC 9800
50 End with NEXT-CSID, PSP, USP & USD RFC 9800
52 End.X with NEXT-CSID RFC 9800
53 End.X with NEXT-CSID & PSP RFC 9800
54 End.X with NEXT-CSID & USP RFC 9800
55 End.X with NEXT-CSID, PSP & USP RFC 9800
56 End.X with NEXT-CSID & USD RFC 9800
57 End.X with NEXT-CSID, PSP & USD RFC 9800
58 End.X with NEXT-CSID, USP & USD RFC 9800
59 End.X with NEXT-CSID, PSP, USP & USD RFC 9800
85 End.T with NEXT-CSID RFC 9800
86 End.T with NEXT-CSID & PSP RFC 9800
87 End.T with NEXT-CSID & USP RFC 9800
88 End.T with NEXT-CSID, PSP & USP RFC 9800
89 End.T with NEXT-CSID & USD RFC 9800
90 End.T with NEXT-CSID, PSP & USD RFC 9800
91 End.T with NEXT-CSID, USP & USD RFC 9800
92 End.T with NEXT-CSID, PSP, USP & USD RFC 9800
93 End.B6.Encaps with NEXT-CSID RFC 9800
94 End.B6.Encaps.Red with NEXT-CSID RFC 9800
95 End.BM with NEXT-CSID RFC 9800
96 End.LBS with NEXT-CSID RFC 9800
97 End.XLBS with NEXT-CSID RFC 9800
101 End with REPLACE-CSID RFC 9800
102 End with REPLACE-CSID & PSP RFC 9800
103 End with REPLACE-CSID & USP RFC 9800
104 End with REPLACE-CSID, PSP & USP RFC 9800
105 End.X with REPLACE-CSID RFC 9800
106 End.X with REPLACE-CSID & PSP RFC 9800
107 End.X with REPLACE-CSID & USP RFC 9800
108 End.X with REPLACE-CSID, PSP & USP RFC 9800
109 End.T with REPLACE-CSID RFC 9800
110 End.T with REPLACE-CSID & PSP RFC 9800
111 End.T with REPLACE-CSID & USP RFC 9800
112 End.T with REPLACE-CSID, PSP & USP RFC 9800
114 End.B6.Encaps with REPLACE-CSID RFC 9800
115 End.BM with REPLACE-CSID RFC 9800
116 End.DX6 with REPLACE-CSID RFC 9800
117 End.DX4 with REPLACE-CSID RFC 9800
118 End.DT6 with REPLACE-CSID RFC 9800
119 End.DT4 with REPLACE-CSID RFC 9800
120 End.DT46 with REPLACE-CSID RFC 9800
121 End.DX2 with REPLACE-CSID RFC 9800
122 End.DX2V with REPLACE-CSID RFC 9800
123 End.DT2U with REPLACE-CSID RFC 9800
124 End.DT2M with REPLACE-CSID RFC 9800
127 End.B6.Encaps.Red with REPLACE-CSID RFC 9800
128 End with REPLACE-CSID & USD RFC 9800
129 End with REPLACE-CSID, PSP & USD RFC 9800
130 End with REPLACE-CSID, USP & USD RFC 9800
131 End with REPLACE-CSID, PSP, USP & USD RFC 9800
132 End.X with REPLACE-CSID & USD RFC 9800
133 End.X with REPLACE-CSID, PSP & USD RFC 9800
134 End.X with REPLACE-CSID, USP & USD RFC 9800
135 End.X with REPLACE-CSID, PSP, USP & USD RFC 9800
136 End.T with REPLACE-CSID & USD RFC 9800
137 End.T with REPLACE-CSID, PSP & USD RFC 9800
138 End.T with REPLACE-CSID, USP & USD RFC 9800
139 End.T with REPLACE-CSID, PSP, USP & USD RFC 9800
140 End.LBS with REPLACE-CSID RFC 9800
141 End.XLBS with REPLACE-CSID RFC 9800
References Normative References 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. Internet Protocol, Version 6 (IPv6) Specification This document specifies version 6 of the Internet Protocol (IPv6). It obsoletes RFC 2460. 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. Informative References Advertisement of Segment Routing Policies using BGP Link-State Individual Cisco Systems Huawei Technologies RtBrick Inc. Nvidia Work in Progress Advertising Segment Routing Policies in BGP Huawei Technologies Cisco Systems Cisco Systems Microsoft Google Work in Progress Concrete Mathematics: A Foundation for Computer Science Operation of Anycast Services As the Internet has grown, and as systems and networked services within enterprises have become more pervasive, many services with high availability requirements have emerged. These requirements have increased the demands on the reliability of the infrastructure on which those services rely. Various techniques have been employed to increase the availability of services deployed on the Internet. This document presents commentary and recommendations for distribution of services using anycast. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. BGP Overlay Services Based on Segment Routing over IPv6 (SRv6) This document defines procedures and messages for SRv6-based BGP services, including Layer 3 Virtual Private Network (L3VPN), Ethernet VPN (EVPN), and Internet services. It builds on "BGP/MPLS IP Virtual Private Networks (VPNs)" (RFC 4364) and "BGP MPLS-Based Ethernet VPN" (RFC 7432). Operations, Administration, and Maintenance (OAM) in Segment Routing over IPv6 (SRv6) This document describes how the existing IPv6 mechanisms for ping and traceroute can be used in a Segment Routing over IPv6 (SRv6) network. The document also specifies the OAM flag (O-flag) in the Segment Routing Header (SRH) for performing controllable and predictable flow sampling from segment endpoints. In addition, the document describes how a centralized monitoring system performs a path continuity check between any nodes within an SRv6 domain. IGP Flexible Algorithm IGP protocols historically compute the best paths over the network based on the IGP metric assigned to the links. Many network deployments use RSVP-TE or Segment Routing - Traffic Engineering (SR-TE) to steer traffic over a path that is computed using different metrics or constraints than the shortest IGP path. This document specifies a solution that allows IGPs themselves to compute constraint-based paths over the network. This document also specifies a way of using Segment Routing (SR) Prefix-SIDs and SRv6 locators to steer packets along the constraint-based paths. IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane The Segment Routing (SR) architecture allows a flexible definition of the end-to-end path by encoding it as a sequence of topological elements called "segments". It can be implemented over the MPLS or the IPv6 data plane. This document describes the IS-IS extensions required to support SR over the IPv6 data plane. This document updates RFC 7370 by modifying an existing registry. OSPFv3 Extensions for Segment Routing over IPv6 (SRv6) The Segment Routing (SR) architecture allows a flexible definition of the end-to-end path by encoding it as a sequence of topological elements called segments. It can be implemented over an MPLS or IPv6 data plane. This document describes the OSPFv3 extensions required to support SR over the IPv6 data plane. Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment Routing over IPv6 (SRv6) Segment Routing over IPv6 (SRv6) allows for a flexible definition of end-to-end paths within various topologies by encoding paths as sequences of topological or functional sub-paths called "segments". These segments are advertised by various protocols such as BGP, IS-IS, and OSPFv3. This document defines extensions to BGP - Link State (BGP-LS) to advertise SRv6 segments along with their behaviors and other attributes via BGP. The BGP-LS address-family solution for SRv6 described in this document is similar to BGP-LS for SR for the MPLS data plane, which is defined in RFC 9085. Segment Routing over IPv6 (SRv6) Segment Identifiers in the IPv6 Addressing Architecture Segment Routing over IPv6 (SRv6) uses IPv6 as the underlying data plane. Thus, Segment Identifiers (SIDs) used by SRv6 can resemble IPv6 addresses and behave like them while exhibiting slightly different behaviors in some situations. This document explores the characteristics of SRv6 SIDs and focuses on the relationship of SRv6 SIDs to the IPv6 Addressing Architecture. This document allocates and makes a dedicated prefix available for SRv6 SIDs. Path Computation Element Communication Protocol (PCEP) Extensions for IPv6 Segment Routing Segment Routing (SR) can be used to steer packets through a network using the IPv6 or MPLS data plane, employing the source routing paradigm. An SR Path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), explicit configuration, or a Path Computation Element (PCE). Since SR can be applied to both MPLS and IPv6 data planes, a PCE should be able to compute an SR Path for both MPLS and IPv6 data planes. The Path Computation Element Communication Protocol (PCEP) extension and mechanisms to support SR-MPLS have been defined. This document outlines the necessary extensions to support SR for the IPv6 data plane within PCEP.
Complete Pseudocodes The content of this section is purely informative rendering of the pseudocodes of with the modifications in this document. This rendering may not be used as a reference.
End with NEXT-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. 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. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. N09. } S02. If (Segments Left == 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 with 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 ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address with Code 0 (Erroneous header field encountered) and Pointer set to the Segments Left field, interrupt packet processing, and discard the packet. S11. } S12. Decrement IPv6 Hop Limit by 1. S13. Decrement Segments Left by 1. S14. Update IPv6 DA with Segment List[Segments Left]. S15. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. Before processing the upper-layer header or any IPv6 extension header other than Hop-by-Hop or Destination Options of a packet matching a FIB entry locally instantiated as an End SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. Send an ICMP Time Exceeded message to the Source Address, Code 0 (Hop limit exceeded in transit), interrupt packet processing and discard the packet. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. N09. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End SID with the NEXT-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. }
End.X with NEXT-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. 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. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. N09. } S02. If (Segments Left == 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 with 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 ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address with Code 0 (Erroneous header field encountered) and Pointer set to the Segments Left field, interrupt packet processing, and discard the packet. S11. } S12. Decrement IPv6 Hop Limit by 1. S13. Decrement Segments Left by 1. S14. Update IPv6 DA with Segment List[Segments Left]. S15. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. Before processing the upper-layer header or any IPv6 extension header other than Hop-by-Hop or Destination Options of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. Send an ICMP Time Exceeded message to the Source Address, Code 0 (Hop limit exceeded in transit), interrupt packet processing and discard the packet. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. N09. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT-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. }
End.T with NEXT-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.T SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. 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. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08.1. Set the packet's associated FIB table to T. N08.2. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. N09. } S02. If (Segments Left == 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 with 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 ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address with Code 0 (Erroneous header field encountered) and Pointer set to the Segments Left field, interrupt packet processing, and discard the packet. S11. } S12. Decrement IPv6 Hop Limit by 1. S13. Decrement Segments Left by 1. S14. Update IPv6 DA with Segment List[Segments Left]. S15.1. Set the packet's associated FIB table to T. S15.2. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. Before processing the upper-layer header or any IPv6 extension header other than Hop-by-Hop or Destination Options of a packet matching a FIB entry locally instantiated as an End.T SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. Send an ICMP Time Exceeded message to the Source Address, Code 0 (Hop limit exceeded in transit), interrupt packet processing and discard the packet. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08.1. Set the packet's associated FIB table to T. N08.2. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. N09. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End.T SID with the NEXT-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. }
End.B6.Encaps with NEXT-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. Send an ICMP Time Exceeded message to the Source Address, Code 0 (Hop limit exceeded in transit), interrupt packet processing and discard the packet. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08.1. Push a new IPv6 header with its own SRH containing B. N08.2. Set the outer IPv6 SA to A. N08.3. Set the outer IPv6 DA to the first SID of B. N08.4. Set the outer Payload Length, Traffic Class, Flow Label, Hop Limit, and Next Header fields. N08.5. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. N09. } S02. If (Segments Left == 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 with 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 ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address with Code 0 (Erroneous header field encountered) and Pointer set to the Segments Left field, interrupt packet processing, and discard the packet. S11. } S12. Decrement IPv6 Hop Limit by 1. S13. Decrement Segments Left by 1. S14. Update IPv6 DA with Segment List[Segments Left]. S15. Push a new IPv6 header with its own SRH containing B. S16. Set the outer IPv6 SA to A. S17. Set the outer IPv6 DA to the first SID of B. S18. Set the outer Payload Length, Traffic Class, Flow Label, Hop Limit, and Next Header fields. S19. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. Before processing the upper-layer header or any IPv6 extension header other than Hop-by-Hop or Destination Options of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. Send an ICMP Time Exceeded message to the Source Address, Code 0 (Hop limit exceeded in transit), interrupt packet processing and discard the packet. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08.1. Push a new IPv6 header with its own SRH containing B. N08.2. Set the outer IPv6 SA to A. N08.3. Set the outer IPv6 DA to the first SID of B. N08.4. Set the outer Payload Length, Traffic Class, Flow Label, Hop Limit, and Next Header fields. N08.5. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. N09. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-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. }
End.BM with NEXT-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.BM SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. Send an ICMP Time Exceeded message to the Source Address, Code 0 (Hop limit exceeded in transit), interrupt packet processing and discard the packet. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08.1. Push the MPLS label stack for B. N08.2. Submit the packet to the MPLS engine for transmission. N09. } S02. If (Segments Left == 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 with 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 ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address with Code 0 (Erroneous header field encountered) and Pointer set to the Segments Left field, interrupt packet processing, and discard the packet. S11. } S12. Decrement IPv6 Hop Limit by 1. S13. Decrement Segments Left by 1. S14. Update IPv6 DA with Segment List[Segments Left]. S15. Push the MPLS label stack for B. S16. Submit the packet to the MPLS engine for transmission. Before processing the upper-layer header or any IPv6 extension header other than Hop-by-Hop or Destination Options of a packet matching a FIB entry locally instantiated as an End.BM SID with the NEXT-CSID flavor: N01. If (DA.Argument != 0) { N02. If (IPv6 Hop Limit <= 1) { N03. Send an ICMP Time Exceeded message to the Source Address, Code 0 (Hop limit exceeded in transit), interrupt packet processing and discard the packet. N04. } N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the Destination Address. N06. Set the bits [(LBL+AL)..127] of the Destination Address to zero. N07. Decrement IPv6 Hop Limit by 1. N08.1. Push the MPLS label stack for B. N08.2. Submit the packet to the MPLS engine for transmission. N09. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End.BM SID with the NEXT-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. }
End with REPLACE-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End SID with the REPLACE-CSID flavor: S01. When an SRH is processed { 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 R01. If (DA.Arg.Index != 0) { R02. If ((Last Entry > max_LE) or (Segments Left > Last Entry)) { R03. 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. R04. } R05. Decrement DA.Arg.Index by 1. R06. If (Segment List[Segments Left][DA.Arg.Index] == 0) { R07. Decrement Segments Left by 1. R08. Decrement IPv6 Hop Limit by 1. R09. Update IPv6 DA with Segment List[Segments Left] R10. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. R11. } R12. } Else { R13. If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){ R14. 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. R15. } R16. Decrement Segments Left by 1. R17. Set DA.Arg.Index to (128/LNFL - 1). R18. } R19. Decrement IPv6 Hop Limit by 1. R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits [LBL..LBL+LNFL-1] of the Destination Address of the IPv6 header. R21. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. S16. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End SID with the 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. }
End.X with REPLACE-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.X SID with the REPLACE-CSID flavor: S01. When an SRH is processed { 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 R01. If (DA.Arg.Index != 0) { R02. If ((Last Entry > max_LE) or (Segments Left > Last Entry)) { R03. 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. R04. } R05. Decrement DA.Arg.Index by 1. R06. If (Segment List[Segments Left][DA.Arg.Index] == 0) { R07. Decrement Segments Left by 1. R08. Decrement IPv6 Hop Limit by 1. R09. Update IPv6 DA with Segment List[Segments Left] R10. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. R11. } R12. } Else { R13. If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){ R14. 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. R15. } R16. Decrement Segments Left by 1. R17. Set DA.Arg.Index to (128/LNFL - 1). R18. } R19. Decrement IPv6 Hop Limit by 1. R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits [LBL..LBL+LNFL-1] of the Destination Address of the IPv6 header. R21. Submit the packet to the IPv6 module for transmission to the new destination via a member of J. S16. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End.X SID with the 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. }
End.T with REPLACE-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.T SID with the REPLACE-CSID flavor: S01. When an SRH is processed { 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 R01. If (DA.Arg.Index != 0) { R02. If ((Last Entry > max_LE) or (Segments Left > Last Entry)) { R03. 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. R04. } R05. Decrement DA.Arg.Index by 1. R06. If (Segment List[Segments Left][DA.Arg.Index] == 0) { R07. Decrement Segments Left by 1. R08. Decrement IPv6 Hop Limit by 1. R09. Update IPv6 DA with Segment List[Segments Left] R10.1. Set the packet's associated FIB table to T. R10.2. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. R11. } R12. } Else { R13. If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){ R14. 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. R15. } R16. Decrement Segments Left by 1. R17. Set DA.Arg.Index to (128/LNFL - 1). R18. } R19. Decrement IPv6 Hop Limit by 1. R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits [LBL..LBL+LNFL-1] of the Destination Address of the IPv6 header. R21.1. Set the packet's associated FIB table to T. R21.2. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination. S16. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End.T SID with the 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. }
End.B6.Encaps with REPLACE-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the REPLACE-CSID flavor: S01. When an SRH is processed { 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 R01. If (DA.Arg.Index != 0) { R02. If ((Last Entry > max_LE) or (Segments Left > Last Entry)) { R03. 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. R04. } R05. Decrement DA.Arg.Index by 1. R06. If (Segment List[Segments Left][DA.Arg.Index] == 0) { R07. Decrement Segments Left by 1. R08. Decrement IPv6 Hop Limit by 1. R09. Update IPv6 DA with Segment List[Segments Left] R10.1. Push a new IPv6 header with its own SRH containing B. R10.2. Set the outer IPv6 SA to A. R10.3. Set the outer IPv6 DA to the first SID of B. R10.4. Set the outer Payload Length, Traffic Class, Flow Label, Hop Limit, and Next Header fields. R10.5. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. R11. } R12. } Else { R13. If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){ R14. 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. R15. } R16. Decrement Segments Left by 1. R17. Set DA.Arg.Index to (128/LNFL - 1). R18. } R19. Decrement IPv6 Hop Limit by 1. R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits [LBL..LBL+LNFL-1] of the Destination Address of the IPv6 header. R21.1. Push a new IPv6 header with its own SRH containing B. R21.2. Set the outer IPv6 SA to A. R21.3. Set the outer IPv6 DA to the first SID of B. R21.4. Set the outer Payload Length, Traffic Class, Flow Label, Hop Limit, and Next Header fields. R21.5. Submit the packet to the egress IPv6 FIB lookup for transmission to the next destination. S16. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the 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. }
End.BM with REPLACE-CSID When processing the SRH of a packet matching a FIB entry locally instantiated as an End.BM SID with the REPLACE-CSID flavor: S01. When an SRH is processed { 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 R01. If (DA.Arg.Index != 0) { R02. If ((Last Entry > max_LE) or (Segments Left > Last Entry)) { R03. 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. R04. } R05. Decrement DA.Arg.Index by 1. R06. If (Segment List[Segments Left][DA.Arg.Index] == 0) { R07. Decrement Segments Left by 1. R08. Decrement IPv6 Hop Limit by 1. R09. Update IPv6 DA with Segment List[Segments Left] R10.1. Push the MPLS label stack for B. R10.2. Submit the packet to the MPLS engine for transmission. R11. } R12. } Else { R13. If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){ R14. 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. R15. } R16. Decrement Segments Left by 1. R17. Set DA.Arg.Index to (128/LNFL - 1). R18. } R19. Decrement IPv6 Hop Limit by 1. R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits [LBL..LBL+LNFL-1] of the Destination Address of the IPv6 header. R21.1. Push the MPLS label stack for B. R21.2. Submit the packet to the MPLS engine for transmission. S16. } When processing the upper-layer header of a packet matching a FIB entry locally instantiated as an End.BM SID with the 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. }
Acknowledgements The authors would like to thank , , , , , , , and for their insightful feedback and suggestions. The authors would also like to thank , , , , , and for their thorough review of this document.
Contributors ZTE Corporation
China liu.aihua@zte.com.cn
Alibaba
USA d.cai@alibaba-inc.com
Cisco Systems, Inc.
Canada ddukes@cisco.com
Futurewei Technologies Ltd.
USA james.n.guichard@futurewei.com
Huawei Technologies
China c.l@huawei.com
NTT Network Innovations
USA robert@raszuk.net
Cisco Systems, Inc.
India ketant.ietf@gmail.com
Bell Canada
Canada daniel.voyer@bell.ca
Broadcom
Israel shay.zadok@broadcom.com
Authors' Addresses 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
Cisco Systems, Inc.
France fclad.ietf@gmail.com