rfc9800v1.txt		rfc9800.txt
	skipping to change at line 14 ¶		skipping to change at line 14 ¶
	Updates: 8754 C. Filsfils		Updates: 8754 C. Filsfils
	Category: Standards Track Cisco Systems, Inc.		Category: Standards Track Cisco Systems, Inc.
	ISSN: 2070-1721 Z. Li		ISSN: 2070-1721 Z. Li
	Huawei Technologies		Huawei Technologies
	B. Decraene		B. Decraene
	Orange		Orange
	F. Clad, Ed.		F. Clad, Ed.
	Cisco Systems, Inc.		Cisco Systems, Inc.
	June 2025		June 2025
	Compressed SRv6 Segment List Encoding (CSID)		Compressed SRv6 Segment List Encoding
	Abstract		Abstract
	Segment Routing over IPv6 (SRv6) is the instantiation of Segment		Segment Routing over IPv6 (SRv6) is the instantiation of Segment
	Routing (SR) on the IPv6 data plane. This document specifies new		Routing (SR) on the IPv6 data plane. This document specifies new
	flavors for the SRv6 endpoint behaviors defined in RFC 8986, which		flavors for the SRv6 endpoint behaviors defined in RFC 8986, which
	enable the compression of an SRv6 segment list. Such compression		enable the compression of an SRv6 segment list. Such compression
	significantly reduces the size of the SRv6 encapsulation needed to		significantly reduces the size of the SRv6 encapsulation needed to
	steer packets over long segment lists.		steer packets over long segment lists.
	skipping to change at line 210 ¶		skipping to change at line 210 ¶
	* LBL is the Locator-Block length of the SID.		* LBL is the Locator-Block length of the SID.
	* LNL is the Locator-Node length of the SID.		* LNL is the Locator-Node length of the SID.
	* FL is the Function length of the SID.		* FL is the Function length of the SID.
	* AL is the Argument length of the SID.		* AL is the Argument length of the SID.
	In addition, the Locator-Node and Function length (LNFL) is the sum		In addition, the Locator-Node and Function length (LNFL) is the sum
	of the Locator-Node length and the Function length of the SID. It is		of the LNL and the FL of the SID. It is also referred to as the
	also referred to as the "CSID length".		"CSID length".
	2.1. Requirements Language		2.1. Requirements Language
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in		"OPTIONAL" in this document are to be interpreted as described in
	BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all		BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
	3. Basic Concepts		3. Basic Concepts
	In an SR domain, all SRv6 SIDs instantiated from the same Locator-		In an SR domain, all SRv6 SIDs instantiated from the same Locator-
	Block share the same most significant bits. In addition, when the		Block share the same most significant bits. In addition, when the
	combined length of the SRv6 SID Locator, Function, and Argument is		combined length of the SRv6 SID Locator, Function, and Argument is
	smaller than 128 bits, the least significant bits of the SID are		smaller than 128 bits, the least significant bits of the SID are
	padded with zeros. The compressed segment list encoding seeks to		padded with zeros. The compressed segment list encoding seeks to
	decrease the packet header length by avoiding the repetition of the		decrease the packet header length by avoiding the repetition of the
	same Locator-Block and reducing the use of padding bits.		same Locator-Block and reducing the use of padding bits.
	Building upon, and fully compatible with, the mechanisms specified in		Building upon, and fully compatible with the mechanisms specified in
	[RFC8754] and [RFC8986], the compressed segment list encoding		[RFC8754] and [RFC8986], the compressed segment list encoding
	leverages a SID list compression logic at the SR source node (see		leverages a SID list compression logic at the SR source node (see
	Section 6) in combination with new flavors of the SRv6 endpoint		Section 6) in combination with new flavors of the SRv6 endpoint
	behaviors that process the compressed SID list (see Section 4).		behaviors that process the compressed SID list (see Section 4).
	An SR source node constructs and compresses the SID list depending on		An SR source node constructs and compresses the SID list depending on
	the SIDs instantiated on each SR segment endpoint node that the		the SIDs instantiated on each SR segment endpoint node that the
	packet is intended to traverse, as well as its own compression		packet is intended to traverse, as well as its own compression
	capabilities. The resulting compressed SID list is a combination of		capabilities. The resulting compressed SID list is a combination of
	CSID sequences, for the SIDs that the SR source node was able to		CSID sequences, for the SIDs that the SR source node was able to
	skipping to change at line 323 ¶		skipping to change at line 323 ¶
	CSIDs, where K is computed as floor((128-LBL)/LNFL) (floor(x) is the		CSIDs, where K is computed as floor((128-LBL)/LNFL) (floor(x) is the
	greatest integer less than or equal to x [GKP94]). Each CSID		greatest integer less than or equal to x [GKP94]). Each CSID
	container for NEXT-CSID is independent, such that contiguous CSID		container for NEXT-CSID is independent, such that contiguous CSID
	containers in a CSID sequence can be considered to be separate CSID		containers in a CSID sequence can be considered to be separate CSID
	sequences.		sequences.
	When a CSID sequence compressed using the NEXT-CSID flavor comprises		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		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		have the NEXT-CSID flavor. It can be bound to any SRv6 endpoint
	behavior, including [RFC8986] behaviors and REPLACE-CSID flavor, as		behavior, including [RFC8986] behaviors and REPLACE-CSID flavor, as
	long as the updated destination address resulting from the processing		long as the updated Destination Address resulting from the processing
	of the previous CSID in the sequence is a valid form for that last		of the previous CSID in the sequence is a valid form for that last
	SID. Line S12 of the first pseudocode in Section 6.2 provides		SID. Line S12 of the first pseudocode in Section 6.2 provides
	sufficient conditions to ensure this property.		sufficient conditions to ensure this property.
	+------------------------------------------------------------------+		+------------------------------------------------------------------+
	| Locator-Block |Loc-Node| Argument |		| Locator-Block |Loc-Node| Argument |
	| |Function| |		| |Function| |
	+------------------------------------------------------------------+		+------------------------------------------------------------------+
	<----------------------> <------> <------------------------------>		<----------------------> <------> <------------------------------>
	LBL LNFL AL		LBL LNFL AL
	skipping to change at line 391 ¶		skipping to change at line 391 ¶
	| 7th CSID | 8th CSID |		| 7th CSID | 8th CSID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| 0 |		| 0 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Second CSID Container		Second CSID Container
	Figure 2: Compressed SID List of Eight NEXT-CSID Flavor SIDs with		Figure 2: Compressed SID List of Eight NEXT-CSID Flavor SIDs with
	a 48-Bit Locator-Block, 16-Bit Combined Locator-Node and		a 48-Bit Locator-Block, 16-Bit Combined Locator-Node and
	Function, and 64-Bit Argument		Function, and 64-Bit Argument
	An implementation MUST support a 32-bit Locator-Block length (LBL)		An implementation MUST support a 32-bit LBL and a 16-bit CSID length
	and a 16-bit CSID length (LNFL) for NEXT-CSID flavor SIDs, and it MAY		(LNFL) for NEXT-CSID flavor SIDs, and it MAY support any additional
	support any additional Locator-Block and CSID length.		Locator-Block and CSID length.
	The Argument length (AL) for NEXT-CSID flavor SIDs is equal to 128-		The AL for NEXT-CSID flavor SIDs is equal to 128-LBL-LNFL.
	LBL-LNFL.
	When processing an IPv6 packet that matches a Forwarding Information		When processing an IPv6 packet that matches a Forwarding Information
	Base (FIB) entry locally instantiated as a SID with the NEXT-CSID		Base (FIB) entry locally instantiated as a SID with the NEXT-CSID
	flavor, the SR segment endpoint node applies the procedure specified		flavor, the SR segment endpoint node applies the procedure specified
	in the following subsection that corresponds to the SID behavior. If		in the following subsection that corresponds to the SID behavior. If
	the SID also has the PSP, USP, or USD flavor, the procedure is		the SID also has the PSP, USP, or USD flavor, the procedure is
	modified as described in Section 4.1.7.		modified as described in Section 4.1.7.
	An SR segment endpoint node instantiating a SID of this document with		An SR segment endpoint node instantiating a SID of this document with
	the NEXT-CSID flavor MUST accept any Argument value for that SID.		the NEXT-CSID flavor MUST accept any Argument value for that SID.
	skipping to change at line 429 ¶		skipping to change at line 428 ¶
	4.1.1. End with NEXT-CSID		4.1.1. End with NEXT-CSID
	When processing an IPv6 packet that matches a FIB entry locally		When processing an IPv6 packet that matches a FIB entry locally
	instantiated as an End SID with the NEXT-CSID flavor, the procedure		instantiated as an End SID with the NEXT-CSID flavor, the procedure
	described in Section 4.1 of [RFC8986] is executed with the following		described in Section 4.1 of [RFC8986] is executed with the following
	modifications.		modifications.
	The below pseudocode is inserted between lines S01 and S02 of the SRH		The below pseudocode is inserted between lines S01 and S02 of the SRH
	processing in Section 4.1 of [RFC8986]. In addition, this pseudocode		processing in Section 4.1 of [RFC8986]. In addition, this pseudocode
	is executed before processing any extension header that is not an		is executed before processing the first header in the IPv6 extension
	SRH, a Hop-by-Hop header or a Destination Options header, or before		header chain that is not an SRH, a Hop-by-Hop header, or a
	processing the upper-layer header, whichever comes first.		Destination Options header. If the IPv6 extension header chain does
			not include any header matching this criteria, this pseudocode is
			executed before processing the upper-layer header.
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	N03. Send an ICMP Time Exceeded message to the Source Address,		N03. Send an ICMP Time Exceeded message to the Source Address,
	Code 0 (Hop limit exceeded in transit),		Code 0 (Hop limit exceeded in transit),
	interrupt packet processing and discard the packet.		interrupt packet processing and discard the packet.
	N04. }		N04. }
	N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the		N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
	Destination Address.		Destination Address.
	N06. Set the bits [(LBL+AL)..127] of the Destination Address to		N06. Set the bits [(LBL+AL)..127] of the Destination Address to
	skipping to change at line 478 ¶		skipping to change at line 479 ¶
	replacing line N08 as shown below.		replacing line N08 as shown below.
	N08. Submit the packet to the IPv6 module for transmission to the		N08. Submit the packet to the IPv6 module for transmission to the
	new destination via a member of J.		new destination via a member of J.
	| Note: the variable J is defined in Section 4.2 of [RFC8986].		| Note: the variable J is defined in Section 4.2 of [RFC8986].
	The resulting pseudocode is inserted between lines S01 and S02 of the		The resulting pseudocode is inserted between lines S01 and S02 of the
	SRH processing in Section 4.1 of [RFC8986] after applying the		SRH processing in Section 4.1 of [RFC8986] after applying the
	modification described in Section 4.2 of [RFC8986]. In addition,		modification described in Section 4.2 of [RFC8986]. In addition,
	this pseudocode is executed before processing any extension header		this pseudocode is executed before processing the first header in the
	that is not an SRH, a Hop-by-Hop header or a Destination Options		IPv6 extension header chain that is not an SRH, a Hop-by-Hop header,
	header, or before processing the upper-layer header, whichever comes		or a Destination Options header. If the IPv6 extension header chain
	first.		does not include any header matching this criteria, this pseudocode
			is executed before processing the upper-layer header.
	A rendering of the complete pseudocode is provided in Appendix A.2.		A rendering of the complete pseudocode is provided in Appendix A.2.
	4.1.3. End.T with NEXT-CSID		4.1.3. End.T with NEXT-CSID
	When processing an IPv6 packet that matches a FIB entry locally		When processing an IPv6 packet that matches a FIB entry locally
	instantiated as an End.T SID with the NEXT-CSID flavor, the procedure		instantiated as an End.T SID with the NEXT-CSID flavor, the procedure
	described in Section 4.3 of [RFC8986] is executed with the following		described in Section 4.3 of [RFC8986] is executed with the following
	modifications.		modifications.
	skipping to change at line 504 ¶		skipping to change at line 506 ¶
	N08.1. Set the packet's associated FIB table to T.		N08.1. Set the packet's associated FIB table to T.
	N08.2. Submit the packet to the egress IPv6 FIB lookup for		N08.2. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the new destination.		transmission to the new destination.
	| Note: the variable T is defined in Section 4.3 of [RFC8986].		| Note: the variable T is defined in Section 4.3 of [RFC8986].
	The resulting pseudocode is inserted between lines S01 and S02 of the		The resulting pseudocode is inserted between lines S01 and S02 of the
	SRH processing in Section 4.1 of [RFC8986] after applying the		SRH processing in Section 4.1 of [RFC8986] after applying the
	modification described in Section 4.3 of [RFC8986]. In addition,		modification described in Section 4.3 of [RFC8986]. In addition,
	this pseudocode is executed before processing any extension header		this pseudocode is executed before processing the first header in the
	that is not an SRH, a Hop-by-Hop header or a Destination Options		IPv6 extension header chain that is not an SRH, a Hop-by-Hop header,
	header, or before processing the upper-layer header, whichever comes		or a Destination Options header. If the IPv6 extension header chain
	first.		does not include any header matching this criteria, this pseudocode
			is executed before processing the upper-layer header.
	A rendering of the complete pseudocode is provided in Appendix A.3.		A rendering of the complete pseudocode is provided in Appendix A.3.
	4.1.4. End.B6.Encaps with NEXT-CSID		4.1.4. End.B6.Encaps with NEXT-CSID
	When processing an IPv6 packet that matches a FIB entry locally		When processing an IPv6 packet that matches a FIB entry locally
	instantiated as an End.B6.Encaps SID with the NEXT-CSID flavor, the		instantiated as an End.B6.Encaps SID with the NEXT-CSID flavor, the
	procedure described in Section 4.13 of [RFC8986] is executed with the		procedure described in Section 4.13 of [RFC8986] is executed with the
	following modifications.		following modifications.
	skipping to change at line 535 ¶		skipping to change at line 538 ¶
	Hop Limit, and Next Header fields.		Hop Limit, and Next Header fields.
	N08.5. Submit the packet to the egress IPv6 FIB lookup for		N08.5. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the next destination.		transmission to the next destination.
	| Note: the variables A and B, as well as the values of the		| Note: the variables A and B, as well as the values of the
	| Payload Length, Traffic Class, Flow Label, Hop Limit, and Next		| Payload Length, Traffic Class, Flow Label, Hop Limit, and Next
	| Header are defined in Section 4.13 of [RFC8986].		| Header are defined in Section 4.13 of [RFC8986].
	The resulting pseudocode is inserted between lines S01 and S02 of the		The resulting pseudocode is inserted between lines S01 and S02 of the
	SRH processing in Section 4.13 of [RFC8986]. In addition, this		SRH processing in Section 4.13 of [RFC8986]. In addition, this
	pseudocode is executed before processing any extension header that is		pseudocode is executed before processing the first header in the IPv6
	not an SRH, a Hop-by-Hop header or a Destination Options header, or		extension header chain that is not an SRH, a Hop-by-Hop header, or a
	before processing the upper-layer header, whichever comes first.		Destination Options header. If the IPv6 extension header chain does
			not include any header matching this criteria, this pseudocode is
			executed before processing the upper-layer header.
	A rendering of the complete pseudocode is provided in Appendix A.4.		A rendering of the complete pseudocode is provided in Appendix A.4.
	Similar to the base End.B6.Encaps SID defined in Section 4.13 of		Similar to the base End.B6.Encaps SID defined in Section 4.13 of
	[RFC8986], the NEXT-CSID flavor variant updates the Destination		[RFC8986], the NEXT-CSID flavor variant updates the Destination
	Address field of the inner IPv6 header to the next SID in the		Address field of the inner IPv6 header to the next SID in the
	original segment list before encapsulating the packet with the		original segment list before encapsulating the packet with the
	segment list of SR Policy B. At the endpoint of SR Policy B, 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		encapsulation is removed and the inner packet is forwarded towards
	the exposed destination address, which already contains the next SID		the exposed Destination Address, which already contains the next SID
	in the original segment list.		in the original segment list.
	4.1.5. End.B6.Encaps.Red with NEXT-CSID		4.1.5. End.B6.Encaps.Red with NEXT-CSID
	This is an optimization of the End.B6.Encaps with NEXT-CSID behavior.		This is an optimization of the End.B6.Encaps with NEXT-CSID behavior.
	When processing an IPv6 packet that matches a FIB entry locally		When processing an IPv6 packet that matches a FIB entry locally
	instantiated as an End.B6.Encaps.Red SID with the NEXT-CSID flavor,		instantiated as an End.B6.Encaps.Red SID with the NEXT-CSID flavor,
	the procedure described in Section 4.1.4 of this document is executed		the procedure described in Section 4.1.4 of this document is executed
	with the modifications in Section 4.14 of [RFC8986].		with the modifications in Section 4.14 of [RFC8986].
	skipping to change at line 576 ¶		skipping to change at line 581 ¶
	The pseudocode in Section 4.1.1 of this document is modified by		The pseudocode in Section 4.1.1 of this document is modified by
	replacing line N08 as shown below.		replacing line N08 as shown below.
	N08.1. Push the MPLS label stack for B.		N08.1. Push the MPLS label stack for B.
	N08.2. Submit the packet to the MPLS engine for transmission.		N08.2. Submit the packet to the MPLS engine for transmission.
	| Note: the variable B is defined in Section 4.15 of [RFC8986].		| Note: the variable B is defined in Section 4.15 of [RFC8986].
	The resulting pseudocode is inserted between lines S01 and S02 of the		The resulting pseudocode is inserted between lines S01 and S02 of the
	SRH processing in Section 4.15 of [RFC8986]. In addition, this		SRH processing in Section 4.15 of [RFC8986]. In addition, this
	pseudocode is executed before processing any extension header that is		pseudocode is executed before processing the first header in the IPv6
	not an SRH, a Hop-by-Hop header or a Destination Options header, or		extension header chain that is not an SRH, a Hop-by-Hop header, or a
	before processing the upper-layer header, whichever comes first.		Destination Options header. If the IPv6 extension header chain does
			not include any header matching this criteria, this pseudocode is
			executed before processing the upper-layer header.
	A rendering of the complete pseudocode is provided in Appendix A.5.		A rendering of the complete pseudocode is provided in Appendix A.5.
	4.1.7. Combination with PSP, USP, and USD Flavors		4.1.7. Combination with PSP, USP, and USD Flavors
	PSP: The PSP flavor defined in Section 4.16.1 of [RFC8986] is		PSP: The PSP flavor defined in Section 4.16.1 of [RFC8986] is
	unchanged when combined with the NEXT-CSID flavor.		unchanged when combined with the NEXT-CSID flavor.
	USP: The USP flavor defined in Section 4.16.2 of [RFC8986] is		USP: The USP flavor defined in Section 4.16.2 of [RFC8986] is
	unchanged when combined with the NEXT-CSID flavor.		unchanged when combined with the NEXT-CSID flavor.
	USD: The USP flavor defined in Section 4.16.3 of [RFC8986] is		USD: The USD flavor defined in Section 4.16.3 of [RFC8986] is
	unchanged when combined with the NEXT-CSID flavor.		unchanged when combined with the NEXT-CSID flavor.
	4.2. REPLACE-CSID Flavor		4.2. REPLACE-CSID Flavor
	A CSID sequence compressed using the mechanism of the REPLACE-CSID		A CSID sequence compressed using the mechanism of the REPLACE-CSID
	flavor starts with a CSID container in fully formed 128-bit SID		flavor starts with a CSID container in fully formed 128-bit SID
	format. The Locator-Block of this SID is the common Locator-Block		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		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		are those of the first CSID, and its Argument carries the index of
	the current CSID in the current CSID container. The Argument value		the current CSID in the current CSID container. The Argument value
	skipping to change at line 718 ¶		skipping to change at line 725 ¶
	with a 48-Bit Locator-Block, 32-Bit Combined Locator-Node and		with a 48-Bit Locator-Block, 32-Bit Combined Locator-Node and
	Function, and 48-Bit Argument		Function, and 48-Bit Argument
	This document updates [RFC8754] by allowing each entry in the SRH		This document updates [RFC8754] by allowing each entry in the SRH
	Segment List to be either an IPv6 address or a REPLACE-CSID container		Segment List to be either an IPv6 address or a REPLACE-CSID container
	in packed format. The SRv6 endpoint behaviors specified herein		in packed format. The SRv6 endpoint behaviors specified herein
	ensure that this entry is never copied as is to the IPv6 header and		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		that the Destination Address field of the IPv6 header is always a
	valid SRv6 SID conforming to [RFC9602].		valid SRv6 SID conforming to [RFC9602].
	The REPLACE-CSID flavor SIDs support any Locator-Block length (LBL),		The REPLACE-CSID flavor SIDs support any LBL, depending on the needs
	depending on the needs of the operator, as long as it does not exceed		of the operator, as long as it does not exceed 128-LNFL-
	128-LNFL-ceiling(log_2(128/LNFL)) (ceiling(x) is the least integer		ceiling(log_2(128/LNFL)) (ceiling(x) is the least integer greater
	greater than or equal to x [GKP94]), so that enough bits remain		than or equal to x [GKP94]), so that enough bits remain available for
	available for the CSID and Argument. A Locator-Block length of 48,		the CSID and Argument. An LBL of 48, 56, 64, 72, or 80 bits is
	56, 64, 72, or 80 bits is recommended for easier reading in		recommended for easier reading in operation.
	operation.
	This document defines the REPLACE-CSID flavor for 16-bit and 32-bit		This document defines the REPLACE-CSID flavor for 16-bit and 32-bit
	CSID lengths (LNFL). An implementation MUST support a 32-bit CSID		CSID lengths (LNFL). An implementation MUST support a 32-bit CSID
	length for REPLACE-CSID flavor SIDs.		length for REPLACE-CSID flavor SIDs.
	The Argument length (AL) for REPLACE-CSID flavor SIDs is equal to		The AL for REPLACE-CSID flavor SIDs is equal to 128-LBL-LNFL. The
	128-LBL-LNFL. The index value is encoded in the least significant X		index value is encoded in the least significant X bits of the
	bits of the Argument, where X is computed as ceiling(log_2(128/		Argument, where X is computed as ceiling(log_2(128/LNFL)).
	LNFL)).
	When processing an IPv6 packet that matches a FIB entry locally		When processing an IPv6 packet that matches a FIB entry locally
	instantiated as a SID with the REPLACE-CSID flavor, the SR segment		instantiated as a SID with the REPLACE-CSID flavor, the SR segment
	endpoint node applies the procedure specified in the following		endpoint node applies the procedure specified in the following
	subsection that corresponds to the SID behavior. If the SID also has		subsection that corresponds to the SID behavior. If the SID also has
	the PSP, USP, or USD flavor, the procedure is modified as described		the PSP, USP, or USD flavor, the procedure is modified as described
	in Section 4.2.8.		in Section 4.2.8.
	At a high level, at the start of a CSID sequence using the REPLACE-		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		CSID flavor, the first CSID container in fully formed 128-bit SID
	skipping to change at line 778 ¶		skipping to change at line 783 ¶
	instantiated as an End SID with the REPLACE-CSID flavor, the SRH		instantiated as an End SID with the REPLACE-CSID flavor, the SRH
	processing described in Section 4.1 of [RFC8986] is executed with the		processing described in Section 4.1 of [RFC8986] is executed with the
	following modifications.		following modifications.
	Line S02 of SRH processing in Section 4.1 of [RFC8986] is replaced as		Line S02 of SRH processing in Section 4.1 of [RFC8986] is replaced as
	follows.		follows.
	S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or		S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or
	Segment List[0][DA.Arg.Index-1] == 0)) {		Segment List[0][DA.Arg.Index-1] == 0)) {
	Lines S09 to S15 are replaced by the following pseudo code.		Lines S09 to S15 are replaced by the following pseudocode.
	R01. If (DA.Arg.Index != 0) {		R01. If (DA.Arg.Index != 0) {
	R02. If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {		R02. If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
	R03. Send an ICMP Parameter Problem to the Source Address,		R03. Send an ICMP Parameter Problem to the Source Address,
	Code 0 (Erroneous header field encountered),		Code 0 (Erroneous header field encountered),
	Pointer set to the Segments Left field,		Pointer set to the Segments Left field,
	interrupt packet processing and discard the packet.		interrupt packet processing and discard the packet.
	R04. }		R04. }
	R05. Decrement DA.Arg.Index by 1.		R05. Decrement DA.Arg.Index by 1.
	R06. If (Segment List[Segments Left][DA.Arg.Index] == 0) {		R06. If (Segment List[Segments Left][DA.Arg.Index] == 0) {
	skipping to change at line 1003 ¶		skipping to change at line 1008 ¶
	USD: The USD flavor defined in Section 4.16.3 of [RFC8986] is		USD: The USD flavor defined in Section 4.16.3 of [RFC8986] is
	unchanged when combined with the REPLACE-CSID flavor.		unchanged when combined with the REPLACE-CSID flavor.
	5. CSID Allocation		5. CSID Allocation
	The CSID value of 0 is reserved. It is used to indicate the end of a		The CSID value of 0 is reserved. It is used to indicate the end of a
	CSID container.		CSID container.
	In order to efficiently manage the CSID numbering space, a deployment		In order to efficiently manage the CSID numbering space, a deployment
	may divide it into two non-overlapping sub-spaces: a Global		may divide it into two non-overlapping sub-spaces: a GIB and a LIB.
	Identifiers Block (GIB) and a Local Identifiers Block (LIB).
	The CSID values that are allocated from the GIB have a global		The CSID values that are allocated from the GIB have a global
	semantic within the Locator-Block, while those that are allocated		semantic within the Locator-Block, while those that are allocated
	from the LIB have a local semantic on an SR segment endpoint node and		from the LIB have a local semantic on an SR segment endpoint node and
	within the scope of the Locator-Block.		within the scope of the Locator-Block.
	The concept of LIB is applicable to SRv6 and specifically to its		The concept of LIB is applicable to SRv6 and specifically to its
	NEXT-CSID and REPLACE-CSID flavors. The shorter the CSID, the more		NEXT-CSID and REPLACE-CSID flavors. The shorter the CSID, the more
	benefit the LIB brings.		benefit the LIB brings.
	skipping to change at line 1064 ¶		skipping to change at line 1068 ¶
	FIB entry that matches only the Locator and Function parts of the SID		FIB entry that matches only the Locator and Function parts of the SID
	(i.e., with a prefix length of LBL + LNL + FL).		(i.e., with a prefix length of LBL + LNL + FL).
	In addition, an SR segment endpoint node instantiating NEXT-CSID		In addition, an SR segment endpoint node instantiating NEXT-CSID
	flavor SIDs from both the GIB and LIB may install combined "Global +		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		Local" FIB entries to match a sequence of global and local CSIDs in a
	single longest-prefix match (LPM) lookup.		single longest-prefix match (LPM) lookup.
	For example, let us consider an SR segment endpoint node 10		For example, let us consider an SR segment endpoint node 10
	instantiating the following two NEXT-CSID flavor SIDs according to		instantiating the following two NEXT-CSID flavor SIDs according to
	the CSID length, Locator-Block length, and GIB/LIB recommendations in		the CSID length, LBL, and GIB/LIB recommendations in this section.
	this section.
	* The SID 2001:db8:b1:10:: bound to the End behavior with the NEXT-		* The SID 2001:db8:b1:10:: bound to the End behavior with the NEXT-
	CSID flavor is instantiated from GIB with:		CSID flavor is instantiated from a GIB with:
	- Locator-Block length (LBL) = 48 (Locator-Block value		- LBL = 48 (Locator-Block value 0x20010db800b1),
	0x20010db800b1),
	- Locator-Node length (LNL) = 16 (Locator-Node value 0x0010),		- LNL = 16 (Locator-Node value 0x0010),
	- Function length (FL) = 0, and		- FL = 0, and
	- Argument length (AL) = 64.		- AL = 64.
	* The SID 2001:db8:b1:f123:: bound to the End.X behavior for its		* 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		local IGP adjacency 123 with the NEXT-CSID flavor is instantiated
	from LIB with:		from LIB with:
	- Locator-Block length (LBL) = 48 (Locator-Block value		- LBL = 48 (Locator-Block value 0x20010db800b1),
	0x20010db800b1),
	- Locator-Node length (LNL) = 0,		- LNL = 0,
	- Function length (FL) = 16 (Function value 0xf123), and		- FL = 16 (Function value 0xf123), and
	- Argument length (AL) = 64.		- AL = 64.
	For SID 2001:db8:b1:10::, Node 10 would install the FIB entry		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.		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		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		2001:db8:b1:f123::/64 bound to the End.X SID for adjacency 123 with
	the NEXT-CSID flavor.		the NEXT-CSID flavor.
	In addition, Node 10 may also install the combined FIB entry		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		2001:db8:b1:10:f123::/80 bound to the End.X SID for adjacency 123
	with the NEXT-CSID flavor.		with the NEXT-CSID flavor.
	As another example, let us consider an SR segment endpoint node 20		As another example, let us consider an SR segment endpoint node 20
	instantiating the following two REPLACE-CSID flavor SIDs according to		instantiating the following two REPLACE-CSID flavor SIDs according to
	the CSID length, Locator-Block length, and GIB/LIB recommendations in		the CSID length, LBL, and GIB/LIB recommendations in this section.
	this section.
	* 2001:db8:b2:20:1:: from GIB with Locator-Block length (LBL) = 48,		* 2001:db8:b2:20:1:: from a GIB with LBL = 48, LNL = 16, FL = 16, AL
	Locator-Node length (LNL) = 16, Function length (FL) = 16,		= 48, and bound to the End behavior with the REPLACE-CSID flavor.
	Argument length (AL) = 48, and bound to the End behavior with the
	REPLACE-CSID flavor.
	* 2001:db8:b2:20:123:: from GIB with Locator-Block length (LBL) =		* 2001:db8:b2:20:123:: from a GIB with LBL = 48, LNL = 16, FL = 16,
	48, Locator-Node length (LNL) = 16, Function length (FL) = 16,		AL = 48, and bound to the End.X behavior for its local IGP
	Argument length (AL) = 48, and bound to the End.X behavior for its		adjacency 123 with the REPLACE-CSID flavor.
	local IGP adjacency 123 with the REPLACE-CSID flavor.
	For SID 2001:db8:b2:20:1::, Node 20 would install the FIB entry		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		2001:db8:b2:20:1::/80 bound to the End SID with the REPLACE-CSID
	flavor.		flavor.
	For SID 2001:db8:b2:20:123::, Node 20 would install the FIB entry		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		2001:db8:b2:20:123::/80 bound to the End.X SID for adjacency 123 with
	the REPLACE-CSID flavor.		the REPLACE-CSID flavor.
	6. SR Source Node		6. SR Source Node
	skipping to change at line 1140 ¶		skipping to change at line 1137 ¶
	segment list, along with their respective SRv6 endpoint behavior,		segment list, along with their respective SRv6 endpoint behavior,
	structure, and any other relevant attribute (e.g., the set of L3		structure, and any other relevant attribute (e.g., the set of L3
	adjacencies associated with an End.X SID).		adjacencies associated with an End.X SID).
	6.1. SID Validation for Compression		6.1. SID Validation for Compression
	As part of the compression process or as a preliminary step, the SR		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		source node MUST validate the SID structure of each SID of this
	document in the segment list. The SR source node does so regardless		document in the segment list. The SR source node does so regardless
	of whether the segment list is explicitly configured, locally		of whether the segment list is explicitly configured, locally
	computed, or advertised by a controller (e.g., via BGP [SR-BGP] or		computed, or advertised by a controller (e.g., via BGP
	PCEP [RFC9603]).		[BGP-SR-Policy] or PCEP [RFC9603]).
	A SID structure is valid for compression if it meets all the		A SID structure is valid for compression if it meets all the
	following conditions:		following conditions:
	* The Locator-Block length is not 0.		* The LBL is not 0.
	* The sum of the Locator-Node length and Function length is not 0.		* The sum of the LNFL is not 0.
	* The Argument length is equal to 128-LBL-LNL-FL.		* The AL is equal to 128-LBL-LNL-FL.
	When compressing a SID list, the SR source node MUST treat an invalid		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		SID structure as unknown. A SID with an unknown SID structure is not
	compressible.		compressible.
	Section 8 discusses how the SIDs of this document and their structure		Section 8 discusses how the SIDs of this document and their structure
	can be advertised to the SR source node through various control plane		can be advertised to the SR source node through various control plane
	protocols. The SID structure may also be learned through		protocols. The SID structure may also be learned through
	configuration or other management protocols. The details of such		configuration or other management protocols. The details of such
	mechanisms are outside the scope of this document.		mechanisms are outside the scope of this document.
	skipping to change at line 1232 ¶		skipping to change at line 1229 ¶
	container {		container {
	S13. Copy the Locator-Node, Function, and Argument of S to the		S13. Copy the Locator-Node, Function, and Argument of S to the
	most significant remaining Argument bits of the CSID		most significant remaining Argument bits of the CSID
	container		container
	S14. } // End If		S14. } // End If
	S15. } // End If		S15. } // End If
	S16. Push the NEXT-CSID container onto the compressed SID list		S16. Push the NEXT-CSID container onto the compressed SID list
	* When the compression method encounters a series of REPLACE-CSID		* When the compression method encounters a series of REPLACE-CSID
	flavor SIDs of the same CSID length in the uncompressed SID list,		flavor SIDs of the same CSID length in the uncompressed SID list,
	it compresses the series as per the following high-level pseudo		it compresses the series as per the following high-level
	code. A compression checking function ComCheck(F, S) is defined		pseudocode. A compression checking function ComCheck(F, S) is
	to check if two SIDs F and S share the same SID structure and		defined to check if two SIDs F and S share the same SID structure
	Locator-Block value, and if S has either no Argument or an		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)		Argument with value 0. If the check passes, then ComCheck(F,S)
	returns true.		returns true.
	S01. Initialize a REPLACE-CSID container in full SID format equal to		S01. Initialize a REPLACE-CSID container in full SID format equal to
	the first SID in the series		the first SID in the series
	S02. Push the REPLACE-CSID container onto the compressed SID list		S02. Push the REPLACE-CSID container onto the compressed SID list
	S03. Initialize a new REPLACE-CSID container in packed format if		S03. Initialize a new REPLACE-CSID container in packed format if
	there are more than one SIDs and initialize the remaining		there are more than one SIDs and initialize the remaining
	capacity of the REPLACE-CSID container to 128 bits		capacity of the REPLACE-CSID container to 128 bits
	S04. For each subsequent SID in the uncompressed SID list {		S04. For each subsequent SID in the uncompressed SID list {
	skipping to change at line 1375 ¶		skipping to change at line 1372 ¶
	3. When a REPLACE-CSID flavor CSID is present as the last SID in a		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		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		the SRH, the next element in the SID list MUST be a REPLACE-CSID
	container in packed format carrying at least one CSID.		container in packed format carrying at least one CSID.
	The SR source node determines the compression scheme of REPLACE-CSID		The SR source node determines the compression scheme of REPLACE-CSID
	flavor SIDs as follows.		flavor SIDs as follows.
	When receiving a SID advertisement for a REPLACE-CSID flavor SID with		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		LNL = 16, FL = 0, AL = 128-LBL-LNFL, and all zeros as the value of
	Argument, the SR source node marks both the SID and its locator as		the Argument, the SR source node marks both the SID and its locator
	using 16-bit compression. All other SIDs allocated from this locator		as using 16-bit compression. All other SIDs allocated from this
	with LNL=16, FL=16, AL=128-LBL-LNFL, and all zeros as the value of		locator with LNL = 16, FL = 16, AL = 128-LBL-LNFL, and all zeros as
	the Argument are also marked as using 16-bit compression. When		the value of the Argument are also marked as using 16-bit
	receiving a SID advertisement for a REPLACE-CSID flavor SID with		compression. When receiving a SID advertisement for a REPLACE-CSID
	LNFL=32, AL=128-LBL-LNFL, and all zeros as the value of the Argument,		flavor SID with LNFL = 32, AL = 128-LBL-LNFL, and all zeros as the
	the SR source node marks both the SID and its locator as using 32-bit		value of the Argument, the SR source node marks both the SID and its
	compression.		locator as using 32-bit compression.
	6.5. Upper-Layer Checksums		6.5. Upper-Layer Checksums
	The Destination Address used in the IPv6 pseudo-header (Section 8.1		The Destination Address used in the IPv6 pseudo-header (Section 8.1
	of [RFC8200]) is that of the ultimate destination.		of [RFC8200]) is that of the ultimate destination.
	At the SR source node, that address will be the Destination Address		At the SR source node, that address will be the Destination Address
	as it is expected to be received by the ultimate destination. When		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		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		address can be obtained from the last element in the uncompressed SID
	skipping to change at line 1444 ¶		skipping to change at line 1441 ¶
	Block B2/m, where B2 is an IPv6 address prefix and m is the		Block B2/m, where B2 is an IPv6 address prefix and m is the
	associated prefix length. The original and target Locator-Blocks can		associated prefix length. The original and target Locator-Blocks can
	have different prefix lengths as long as the new Destination Address		have different prefix lengths as long as the new Destination Address
	formed by combining the target Locator-Block with the Locator-Node,		formed by combining the target Locator-Block with the Locator-Node,
	Function, and Argument as described in the pseudocode of Sections		Function, and Argument as described in the pseudocode of Sections
	7.1.1 and 7.1.2 is a valid IPv6 address. The target Locator-Block is		7.1.1 and 7.1.2 is a valid IPv6 address. The target Locator-Block is
	a local property of the End.LBS SID on the SR segment endpoint node.		a local property of the End.LBS SID on the SR segment endpoint node.
	| Note: a local SID property is an attribute associated with the		| Note: a local SID property is an attribute associated with the
	| SID when it is instantiated on the SR segment endpoint node.		| SID when it is instantiated on the SR segment endpoint node.
	| When the SR segment endpoint node identifies the destination		| When the SR segment endpoint node identifies the Destination
	| address of a received packet as a locally instantiated SID, it		| Address of a received packet as a locally instantiated SID, it
	| also retrieves any local property associated with this SID.		| also retrieves any local property associated with this SID.
	| Other examples of local SID properties include the set of L3		| Other examples of local SID properties include the set of L3
	| adjacencies of an End.X SID (Section 4.1 of [RFC8986]) and the		| adjacencies of an End.X SID (Section 4.2 of [RFC8986]) and the
	| lookup table of an End.DT6 SID (Section 4.6 of [RFC8986]).		| lookup table of an End.DT6 SID (Section 4.6 of [RFC8986]).
	The means by which an SR source node learns the target Locator-Block		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		associated with an End.LBS SID are outside the scope of this
	document. As examples, it could be learned via configuration or		document. As examples, it could be learned via configuration or
	signaled by a controller.		signaled by a controller.
	7.1.1. End.LBS with NEXT-CSID		7.1.1. End.LBS with NEXT-CSID
	When processing an IPv6 packet that matches a FIB entry locally		When processing an IPv6 packet that matches a FIB entry locally
	skipping to change at line 1552 ¶		skipping to change at line 1549 ¶
	behaviors that are extended in the present document.		behaviors that are extended in the present document.
	The CSID-flavored behaviors introduced by this document are		The CSID-flavored behaviors introduced by this document are
	advertised in the same manner as their base SRv6 endpoint behaviors		advertised in the same manner as their base SRv6 endpoint behaviors
	using the SRv6 extensions for various routing protocols, such as:		using the SRv6 extensions for various routing protocols, such as:
	* IS-IS [RFC9352]		* IS-IS [RFC9352]
	* OSPFv3 [RFC9513]		* OSPFv3 [RFC9513]
	* BGP [RFC9252], [RFC9514], [SR-BGP]		* BGP [RFC9252], [RFC9514], [BGP-SR-Policy]
	* BGP-LS [BGP-LS-SR]		* BGP-LS [BGP-LS-SR-Policy]
	* PCEP [RFC9603]		* PCEP [RFC9603]
	The SR segment endpoint node MUST set the SID Argument bits to 0 when		The SR segment endpoint node MUST set the SID Argument bits to 0 when
	advertising a locally instantiated SID of this document in the		advertising a locally instantiated SID of this document in the
	routing protocol (e.g., IS-IS [RFC9352], OSPF [RFC9513], or BGP-LS		routing protocol (e.g., IS-IS [RFC9352], OSPF [RFC9513], or BGP-LS
	[RFC9514]).		[RFC9514]).
	Signaling the SRv6 SID Structure is REQUIRED for all the SIDs		Signaling the SRv6 SID Structure is REQUIRED for all the SIDs
	introduced in this document. It is used by an SR source node to		introduced in this document. It is used by an SR source node to
	compress a SID list as described in Section 6. The node initiating		compress a SID list as described in Section 6. The node initiating
	the SID advertisement MUST set the length values in the SRv6 SID		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		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		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,		/48 SRv6 SID block and a /64 Locator, and having a 16-bit Function,
	the SRv6 SID Structure advertisement carries the following values.		the SRv6 SID Structure advertisement carries the following values.
	* Locator-Block length: 48		* LBL: 48
	* Locator-Node length: 16		* LNL: 16
	* Function length: 16		* FL: 16
	* Argument length: 48 (= 128-48-16-16)		* AL: 48 (= 128-48-16-16)
	A local CSID may be advertised in the control plane individually and/		A local CSID may be advertised in the control plane individually and/
	or in combination with a global CSID instantiated on the same SR		or in combination with a global CSID instantiated on the same SR
	segment endpoint node, with the End behavior, and the same Locator-		segment endpoint node, with the End behavior, and the same Locator-
	Block and flavor as the local CSID. A combined global and local CSID		Block and flavor as the local CSID. A combined global and local CSID
	is advertised as follows:		is advertised as follows:
	* The SID Locator-Block is that shared by the global and local CSIDs		* The SID Locator-Block is that shared by the global and local CSIDs
	* The SID Locator-Node is that of global CSID		* The SID Locator-Node is that of global CSID
	* The SID Function is that of the local CSID		* The SID Function is that of the local CSID
	* The SID Argument length is equal to 128-LBL-LNL-FL and the SID		* The SID AL is equal to 128-LBL-LNL-FL and the SID Argument value
	Argument value is 0		is 0
	* All other attributes of the SID (e.g., SRv6 endpoint behavior or		* All other attributes of the SID (e.g., SRv6 endpoint behavior or
	algorithm) are those of the local CSID		algorithm) are those of the local CSID
	The local CSID-combined advertisement is needed in particular for		The combined advertisement of local CSIDs with a global CSID is
	control plane protocols mandating that the SID is a subnet of a		needed in particular for control plane protocols mandating that the
	locator advertised in the same protocol (e.g., Section 8 of [RFC9352]		SID is a subnet of a locator advertised in the same protocol (e.g.,
	and Section 9 of [RFC9513] for advertising Adjacency SIDs in IS-IS		Section 8 of [RFC9352] and Section 9 of [RFC9513] for advertising
	and OSPFv3, respectively).		Adjacency SIDs in IS-IS and OSPFv3, respectively).
	For a segment list computed by a controller and signaled to an SR		For a segment list computed by a controller and signaled to an SR
	source node (e.g., via BGP [SR-BGP] or PCEP [RFC9603]), the		source node (e.g., via BGP [BGP-SR-Policy] or PCEP [RFC9603]), the
	controller provides the ordered segment list comprising the		controller provides the ordered segment list comprising the
	uncompressed SIDs, with their respective behavior and structure, to		uncompressed SIDs, with their respective behavior and structure, to
	the SR source node. The SR source node may then compress the SID		the SR source node. The SR source node may then compress the SID
	list as described in Section 6.		list as described in Section 6.
	When a node that does not support this specification receives an		When a node receives an advertisement of a SID of this document that
	advertisement of a SID of this document, it handles it as described		it does not support, it handles the advertisement as described in the
	in the corresponding control plane specification (e.g., Sections 7.2,		corresponding control plane specification (e.g., Sections 7.2, 8.1,
	8.1, and 8.2 of [RFC9352], Sections 8, 9.1, and 9.2 of [RFC9513], and		and 8.2 of [RFC9352], Sections 8, 9.1, and 9.2 of [RFC9513], and
	Section 3.1 of [RFC9252]).		Section 3.1 of [RFC9252]).
	9. Operational Considerations		9. Operational Considerations
	9.1. Flavor, Block, and CSID Length		9.1. Flavor, Block, and CSID Length
	SRv6 is intended for use in a variety of networks that require		SRv6 is intended for use in a variety of networks that require
	different prefix lengths and SID numbering spaces. Each of the two		different prefix lengths and SID numbering spaces. Each of the two
	flavors introduced in this document comes with its own		flavors introduced in this document comes with its own
	recommendations for Locator-Block and CSID length, as specified in		recommendations for Locator-Block and CSID length, as specified in
	skipping to change at line 1643 ¶		skipping to change at line 1640 ¶
	deployment design, and network scale.		deployment design, and network scale.
	Both CSID flavors can coexist in the same SR domain, on the same SR		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,		segment endpoint node, and even in the same segment list. However,
	operators should generally avoid instantiating SIDs of different CSID		operators should generally avoid instantiating SIDs of different CSID
	flavors within the same routing domain or Locator-Block since these		flavors within the same routing domain or Locator-Block since these
	SIDs have different length and allocation recommendations (see		SIDs have different length and allocation recommendations (see
	Sections 4.1, 4.2, and 9.2). In a multi-domain deployment, different		Sections 4.1, 4.2, and 9.2). In a multi-domain deployment, different
	flavors may be used in different routing domains of the SR domain.		flavors may be used in different routing domains of the SR domain.
	A deployment should use consistent Locator-Block lengths and CSID		A deployment should use consistent LBLs and CSID lengths for all SIDs
	lengths for all SIDs within a routing domain. Heterogeneous lengths,		within a routing domain. Heterogeneous lengths, while possible, may
	while possible, may impact the compression efficiency.		impact the compression efficiency.
	The compressed segment list encoding works with various Locator-Block		The compressed segment list encoding works with various Locator-Block
	allocations. For example, each routing domain within the SR domain		allocations. For example, each routing domain within the SR domain
	can be allocated a /48 Locator-Block from a global IPv6 block		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		available to the operator or from a prefix allocated to SRv6 SIDs as
	discussed in Section 5 of [RFC9602].		discussed in Section 5 of [RFC9602].
	9.2. GIB/LIB Usage		9.2. GIB/LIB Usage
	GIB and LIB usage is a local implementation and/or configuration		GIB and LIB usage is a local implementation and/or configuration
	skipping to change at line 1711 ¶		skipping to change at line 1708 ¶
	with a longer one that comprises two or more SIDs. In that case, the		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		target SID is the last element in the SID list. This operation is
	illustrated in Appendix A.1.2 of [RFC9259].		illustrated in Appendix A.1.2 of [RFC9259].
	When pinging a SID of this document, the SR source node MUST		When pinging a SID of this document, the SR source node MUST
	construct the IPv6 packet as described in Section 6, including		construct the IPv6 packet as described in Section 6, including
	computing the ICMPv6 checksum as described in Section 6.5.		computing the ICMPv6 checksum as described in Section 6.5.
	In particular, when pinging a SID of this document with a SID list		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		comprising only the target SID, the SR source node places the SID
	with Argument value 0 in the destination address of the ICMPv6 echo		with Argument value 0 in the Destination Address of the ICMPv6 echo
	request and computes the ICMPv6 checksum using this SID as the		request and computes the ICMPv6 checksum using this SID as the
	destination address in the IPv6 pseudo-header. The Argument value 0		Destination Address in the IPv6 pseudo-header. The Argument value 0
	allows the SID SR segment endpoint node (Section 4) to identify		allows the SID SR segment endpoint node (Section 4) to identify
	itself as the ultimate destination of the packet and process the		itself as the ultimate destination of the packet and process the
	ICMPv6 payload. Therefore, any existing IPv6 ping implementation can		ICMPv6 payload. Therefore, any existing IPv6 ping implementation can
	originate ICMP echo requests to a NEXT-CSID or REPLACE-CSID flavor		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		SID with a SID list comprising only the target SID, provided that the
	user ensures that the SID Argument is 0.		user ensures that the SID Argument is 0.
	9.4. ICMP Error Processing		9.4. ICMP Error Processing
	When an IPv6 node encounters an error while processing a packet, it		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		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		source with an enclosed copy of the invoking packet. For the source
	of an invoking packet to process the ICMP error message, the ultimate		of an invoking packet to process the ICMP error message, the ultimate
	destination address of the IPv6 header may be required.		Destination Address of the IPv6 header may be required.
	Section 5.4 of [RFC8754] defines the logic that an SR source node		Section 5.4 of [RFC8754] defines the logic that an SR source node
	follows to determine the ultimate destination of an invoking packet		follows to determine the ultimate destination of an invoking packet
	containing an SRH.		containing an SRH.
	For an SR source node that supports the compressed segment list		For an SR source node that supports the compressed segment list
	encoding defined in this document, the logic to determine the		encoding defined in this document, the logic to determine the
	ultimate destination is generalized as follows.		ultimate destination is generalized as follows.
	* If the destination address of the invoking IPv6 packet matches a		* If the Destination Address of the invoking IPv6 packet matches a
	known SRv6 SID, modify the invoking IPv6 packet by applying the		known SRv6 SID, modify the invoking IPv6 packet by applying the
	SRv6 endpoint behavior associated with the matched SRv6 SID;		SRv6 endpoint behavior associated with the matched SRv6 SID;
	* Repeat until the application of the SRv6 endpoint behavior would		* Repeat until the application of the SRv6 endpoint behavior would
	result in the processing of the upper-layer header.		result in the processing of the upper-layer header.
	The destination address of the resulting IPv6 packet may be used as		The Destination Address of the resulting IPv6 packet may be used as
	the ultimate destination of the invoking IPv6 packet.		the ultimate destination of the invoking IPv6 packet.
	Since the SR source node that needs to determine the ultimate		Since the SR source node that needs to determine the ultimate
	destination is the same node that originally built the SID list in		destination is the same node that originally built the SID list in
	the invoking packet, it can perform this operation for all the SIDs		the invoking packet, it can perform this operation for all the SIDs
	in the packet.		in the packet.
	10. Applicability to Other SRv6 Endpoint Behaviors		10. Applicability to Other SRv6 Endpoint Behaviors
	Future documents may extend the applicability of the NEXT-CSID and		Future documents may extend the applicability of the NEXT-CSID and
	skipping to change at line 1999 ¶		skipping to change at line 1996 ¶
	<https://www.rfc-editor.org/info/rfc8754>.		<https://www.rfc-editor.org/info/rfc8754>.
	[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,		[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
	D., Matsushima, S., and Z. Li, "Segment Routing over IPv6		D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
	(SRv6) Network Programming", RFC 8986,		(SRv6) Network Programming", RFC 8986,
	DOI 10.17487/RFC8986, February 2021,		DOI 10.17487/RFC8986, February 2021,
	<https://www.rfc-editor.org/info/rfc8986>.		<https://www.rfc-editor.org/info/rfc8986>.
	13.2. Informative References		13.2. Informative References
	[BGP-LS-SR]		[BGP-LS-SR-Policy]
	Previdi, S., Talaulikar, K., Ed., Dong, J., Gredler, H.,		Previdi, S., Talaulikar, K., Ed., Dong, J., Gredler, H.,
	and J. Tantsura, "Advertisement of Segment Routing		and J. Tantsura, "Advertisement of Segment Routing
	Policies using BGP Link-State", Work in Progress,		Policies using BGP Link-State", Work in Progress,
	Internet-Draft, draft-ietf-idr-bgp-ls-sr-policy-17, 6		Internet-Draft, draft-ietf-idr-bgp-ls-sr-policy-17, 6
	March 2025, <https://datatracker.ietf.org/doc/html/draft-		March 2025, <https://datatracker.ietf.org/doc/html/draft-
	ietf-idr-bgp-ls-sr-policy-17>.		ietf-idr-bgp-ls-sr-policy-17>.
			[BGP-SR-Policy]
			Previdi, S., Filsfils, C., Talaulikar, K., Ed., Mattes,
			P., and D. Jain, "Advertising Segment Routing Policies in
			BGP", Work in Progress, Internet-Draft, draft-ietf-idr-sr-
			policy-safi-13, 6 February 2025,
			<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
			policy-safi-13>.
	[GKP94] Graham, R., Knuth, D., and O. Patashnik, "Concrete		[GKP94] Graham, R., Knuth, D., and O. Patashnik, "Concrete
	Mathematics: A Foundation for Computer Science",		Mathematics: A Foundation for Computer Science",
	ISBN 9780201558029, 1994.		ISBN 9780201558029, 1994.
	[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast		[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
	Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,		Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
	December 2006, <https://www.rfc-editor.org/info/rfc4786>.		December 2006, <https://www.rfc-editor.org/info/rfc4786>.
	[RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,		[RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
	B., Zhuang, S., and J. Rabadan, "BGP Overlay Services		B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
	skipping to change at line 2059 ¶		skipping to change at line 2064 ¶
	Identifiers in the IPv6 Addressing Architecture",		Identifiers in the IPv6 Addressing Architecture",
	RFC 9602, DOI 10.17487/RFC9602, October 2024,		RFC 9602, DOI 10.17487/RFC9602, October 2024,
	<https://www.rfc-editor.org/info/rfc9602>.		<https://www.rfc-editor.org/info/rfc9602>.
	[RFC9603] Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M.,		[RFC9603] Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M.,
	and Y. Zhu, "Path Computation Element Communication		and Y. Zhu, "Path Computation Element Communication
	Protocol (PCEP) Extensions for IPv6 Segment Routing",		Protocol (PCEP) Extensions for IPv6 Segment Routing",
	RFC 9603, DOI 10.17487/RFC9603, July 2024,		RFC 9603, DOI 10.17487/RFC9603, July 2024,
	<https://www.rfc-editor.org/info/rfc9603>.		<https://www.rfc-editor.org/info/rfc9603>.
	[SR-BGP] Previdi, S., Filsfils, C., Talaulikar, K., Ed., Mattes,
	P., and D. Jain, "Advertising Segment Routing Policies in
	BGP", Work in Progress, Internet-Draft, draft-ietf-idr-sr-
	policy-safi-13, 6 February 2025,
	<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
	policy-safi-13>.
	Appendix A. Complete Pseudocodes		Appendix A. Complete Pseudocodes
	The content of this section is purely informative rendering of the		The content of this section is purely informative rendering of the
	pseudocodes of [RFC8986] with the modifications in this document.		pseudocodes of [RFC8986] with the modifications in this document.
	This rendering may not be used as a reference.		This rendering may not be used as a reference.
	A.1. End with NEXT-CSID		A.1. End with NEXT-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End SID with the NEXT-CSID flavor:		instantiated as an End SID with the NEXT-CSID flavor:
	skipping to change at line 2114 ¶		skipping to change at line 2112 ¶
	with Code 0 (Erroneous header field encountered)		with Code 0 (Erroneous header field encountered)
	and Pointer set to the Segments Left field,		and Pointer set to the Segments Left field,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S11. }		S11. }
	S12. Decrement IPv6 Hop Limit by 1.		S12. Decrement IPv6 Hop Limit by 1.
	S13. Decrement Segments Left by 1.		S13. Decrement Segments Left by 1.
	S14. Update IPv6 DA with Segment List[Segments Left].		S14. Update IPv6 DA with Segment List[Segments Left].
	S15. Submit the packet to the egress IPv6 FIB lookup for		S15. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the new destination.		transmission to the new destination.
	Before processing the Upper-Layer header or any IPv6 extension header		Before processing the upper-layer header or any IPv6 extension header
	other than Hop-by-Hop or Destination Options of a packet matching a		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		FIB entry locally instantiated as an End SID with the NEXT-CSID
	flavor:		flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	N03. Send an ICMP Time Exceeded message to the Source Address,		N03. Send an ICMP Time Exceeded message to the Source Address,
	Code 0 (Hop limit exceeded in transit),		Code 0 (Hop limit exceeded in transit),
	interrupt packet processing and discard the packet.		interrupt packet processing and discard the packet.
	N04. }		N04. }
	N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the		N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
	Destination Address.		Destination Address.
	N06. Set the bits [(LBL+AL)..127] of the Destination Address to		N06. Set the bits [(LBL+AL)..127] of the Destination Address to
	zero.		zero.
	N07. Decrement IPv6 Hop Limit by 1.		N07. Decrement IPv6 Hop Limit by 1.
	N08. Submit the packet to the egress IPv6 FIB lookup for		N08. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the next destination.		transmission to the next destination.
	N09. }		N09. }
	When processing the Upper-Layer header of a packet matching a FIB		When processing the upper-layer header of a packet matching a FIB
	entry locally instantiated as an End SID with the NEXT-CSID flavor:		entry locally instantiated as an End SID with the NEXT-CSID flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.2. End.X with NEXT-CSID		A.2. End.X with NEXT-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End.X SID with the NEXT-CSID flavor:		instantiated as an End.X SID with the NEXT-CSID flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	skipping to change at line 2188 ¶		skipping to change at line 2186 ¶
	with Code 0 (Erroneous header field encountered)		with Code 0 (Erroneous header field encountered)
	and Pointer set to the Segments Left field,		and Pointer set to the Segments Left field,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S11. }		S11. }
	S12. Decrement IPv6 Hop Limit by 1.		S12. Decrement IPv6 Hop Limit by 1.
	S13. Decrement Segments Left by 1.		S13. Decrement Segments Left by 1.
	S14. Update IPv6 DA with Segment List[Segments Left].		S14. Update IPv6 DA with Segment List[Segments Left].
	S15. Submit the packet to the IPv6 module for transmission		S15. Submit the packet to the IPv6 module for transmission
	to the new destination via a member of J.		to the new destination via a member of J.
	Before processing the Upper-Layer header or any IPv6 extension header		Before processing the upper-layer header or any IPv6 extension header
	other than Hop-by-Hop or Destination Options of a packet matching a		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		FIB entry locally instantiated as an End.X SID with the NEXT-CSID
	flavor:		flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	N03. Send an ICMP Time Exceeded message to the Source Address,		N03. Send an ICMP Time Exceeded message to the Source Address,
	Code 0 (Hop limit exceeded in transit),		Code 0 (Hop limit exceeded in transit),
	interrupt packet processing and discard the packet.		interrupt packet processing and discard the packet.
	N04. }		N04. }
	N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the		N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
	Destination Address.		Destination Address.
	N06. Set the bits [(LBL+AL)..127] of the Destination Address to		N06. Set the bits [(LBL+AL)..127] of the Destination Address to
	zero.		zero.
	N07. Decrement IPv6 Hop Limit by 1.		N07. Decrement IPv6 Hop Limit by 1.
	N08. Submit the packet to the IPv6 module for transmission to the		N08. Submit the packet to the IPv6 module for transmission to the
	new destination via a member of J.		new destination via a member of J.
	N09. }		N09. }
	When processing the Upper-Layer header of a packet matching a FIB		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:		entry locally instantiated as an End.X SID with the NEXT-CSID flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.3. End.T with NEXT-CSID		A.3. End.T with NEXT-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End.T SID with the NEXT-CSID flavor:		instantiated as an End.T SID with the NEXT-CSID flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	skipping to change at line 2264 ¶		skipping to change at line 2262 ¶
	and Pointer set to the Segments Left field,		and Pointer set to the Segments Left field,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S11. }		S11. }
	S12. Decrement IPv6 Hop Limit by 1.		S12. Decrement IPv6 Hop Limit by 1.
	S13. Decrement Segments Left by 1.		S13. Decrement Segments Left by 1.
	S14. Update IPv6 DA with Segment List[Segments Left].		S14. Update IPv6 DA with Segment List[Segments Left].
	S15.1. Set the packet's associated FIB table to T.		S15.1. Set the packet's associated FIB table to T.
	S15.2. Submit the packet to the egress IPv6 FIB lookup for		S15.2. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the new destination.		transmission to the new destination.
	Before processing the Upper-Layer header or any IPv6 extension header		Before processing the upper-layer header or any IPv6 extension header
	other than Hop-by-Hop or Destination Options of a packet matching a		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		FIB entry locally instantiated as an End.T SID with the NEXT-CSID
	flavor:		flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	N03. Send an ICMP Time Exceeded message to the Source Address,		N03. Send an ICMP Time Exceeded message to the Source Address,
	Code 0 (Hop limit exceeded in transit),		Code 0 (Hop limit exceeded in transit),
	interrupt packet processing and discard the packet.		interrupt packet processing and discard the packet.
	N04. }		N04. }
	N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the		N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
	Destination Address.		Destination Address.
	N06. Set the bits [(LBL+AL)..127] of the Destination Address to		N06. Set the bits [(LBL+AL)..127] of the Destination Address to
	zero.		zero.
	N07. Decrement IPv6 Hop Limit by 1.		N07. Decrement IPv6 Hop Limit by 1.
	N08.1. Set the packet's associated FIB table to T.		N08.1. Set the packet's associated FIB table to T.
	N08.2. Submit the packet to the egress IPv6 FIB lookup for		N08.2. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the new destination.		transmission to the new destination.
	N09. }		N09. }
	When processing the Upper-Layer header of a packet matching a FIB		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:		entry locally instantiated as an End.T SID with the NEXT-CSID flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.4. End.B6.Encaps with NEXT-CSID		A.4. End.B6.Encaps with NEXT-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End.B6.Encaps SID with the NEXT-CSID flavor:		instantiated as an End.B6.Encaps SID with the NEXT-CSID flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	skipping to change at line 2349 ¶		skipping to change at line 2347 ¶
	S13. Decrement Segments Left by 1.		S13. Decrement Segments Left by 1.
	S14. Update IPv6 DA with Segment List[Segments Left].		S14. Update IPv6 DA with Segment List[Segments Left].
	S15. Push a new IPv6 header with its own SRH containing B.		S15. Push a new IPv6 header with its own SRH containing B.
	S16. Set the outer IPv6 SA to A.		S16. Set the outer IPv6 SA to A.
	S17. Set the outer IPv6 DA to the first SID of B.		S17. Set the outer IPv6 DA to the first SID of B.
	S18. Set the outer Payload Length, Traffic Class, Flow Label,		S18. Set the outer Payload Length, Traffic Class, Flow Label,
	Hop Limit, and Next Header fields.		Hop Limit, and Next Header fields.
	S19. Submit the packet to the egress IPv6 FIB lookup for		S19. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the new destination.		transmission to the new destination.
	Before processing the Upper-Layer header or any IPv6 extension header		Before processing the upper-layer header or any IPv6 extension header
	other than Hop-by-Hop or Destination Options of a packet matching a		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-		FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-
	CSID flavor:		CSID flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	N03. Send an ICMP Time Exceeded message to the Source Address,		N03. Send an ICMP Time Exceeded message to the Source Address,
	Code 0 (Hop limit exceeded in transit),		Code 0 (Hop limit exceeded in transit),
	interrupt packet processing and discard the packet.		interrupt packet processing and discard the packet.
	N04. }		N04. }
	skipping to change at line 2374 ¶		skipping to change at line 2372 ¶
	N07. Decrement IPv6 Hop Limit by 1.		N07. Decrement IPv6 Hop Limit by 1.
	N08.1. Push a new IPv6 header with its own SRH containing B.		N08.1. Push a new IPv6 header with its own SRH containing B.
	N08.2. Set the outer IPv6 SA to A.		N08.2. Set the outer IPv6 SA to A.
	N08.3. Set the outer IPv6 DA to the first SID of B.		N08.3. Set the outer IPv6 DA to the first SID of B.
	N08.4. Set the outer Payload Length, Traffic Class, Flow Label,		N08.4. Set the outer Payload Length, Traffic Class, Flow Label,
	Hop Limit, and Next Header fields.		Hop Limit, and Next Header fields.
	N08.5. Submit the packet to the egress IPv6 FIB lookup for		N08.5. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the next destination.		transmission to the next destination.
	N09. }		N09. }
	When processing the Upper-Layer header of a packet matching a FIB		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		entry locally instantiated as an End.B6.Encaps SID with the NEXT-CSID
	flavor:		flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.5. End.BM with NEXT-CSID		A.5. End.BM with NEXT-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End.BM SID with the NEXT-CSID flavor:		instantiated as an End.BM SID with the NEXT-CSID flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	skipping to change at line 2429 ¶		skipping to change at line 2427 ¶
	with Code 0 (Erroneous header field encountered)		with Code 0 (Erroneous header field encountered)
	and Pointer set to the Segments Left field,		and Pointer set to the Segments Left field,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S11. }		S11. }
	S12. Decrement IPv6 Hop Limit by 1.		S12. Decrement IPv6 Hop Limit by 1.
	S13. Decrement Segments Left by 1.		S13. Decrement Segments Left by 1.
	S14. Update IPv6 DA with Segment List[Segments Left].		S14. Update IPv6 DA with Segment List[Segments Left].
	S15. Push the MPLS label stack for B.		S15. Push the MPLS label stack for B.
	S16. Submit the packet to the MPLS engine for transmission.		S16. Submit the packet to the MPLS engine for transmission.
	Before processing the Upper-Layer header or any IPv6 extension header		Before processing the upper-layer header or any IPv6 extension header
	other than Hop-by-Hop or Destination Options of a packet matching a		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		FIB entry locally instantiated as an End.BM SID with the NEXT-CSID
	flavor:		flavor:
	N01. If (DA.Argument != 0) {		N01. If (DA.Argument != 0) {
	N02. If (IPv6 Hop Limit <= 1) {		N02. If (IPv6 Hop Limit <= 1) {
	N03. Send an ICMP Time Exceeded message to the Source Address,		N03. Send an ICMP Time Exceeded message to the Source Address,
	Code 0 (Hop limit exceeded in transit),		Code 0 (Hop limit exceeded in transit),
	interrupt packet processing and discard the packet.		interrupt packet processing and discard the packet.
	N04. }		N04. }
	N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the		N05. Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
	Destination Address.		Destination Address.
	N06. Set the bits [(LBL+AL)..127] of the Destination Address to		N06. Set the bits [(LBL+AL)..127] of the Destination Address to
	zero.		zero.
	N07. Decrement IPv6 Hop Limit by 1.		N07. Decrement IPv6 Hop Limit by 1.
	N08.1. Push the MPLS label stack for B.		N08.1. Push the MPLS label stack for B.
	N08.2. Submit the packet to the MPLS engine for transmission.		N08.2. Submit the packet to the MPLS engine for transmission.
	N09. }		N09. }
	When processing the Upper-Layer header of a packet matching a FIB		When processing the upper-layer header of a packet matching a FIB
	entry locally instantiated as an End.BM SID with the NEXT-CSID		entry locally instantiated as an End.BM SID with the NEXT-CSID
	flavor:		flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.6. End with REPLACE-CSID		A.6. End with REPLACE-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End SID with the REPLACE-CSID flavor:		instantiated as an End SID with the REPLACE-CSID flavor:
	S01. When an SRH is processed {		S01. When an SRH is processed {
	S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or		S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or
	skipping to change at line 2513 ¶		skipping to change at line 2511 ¶
	R17. Set DA.Arg.Index to (128/LNFL - 1).		R17. Set DA.Arg.Index to (128/LNFL - 1).
	R18. }		R18. }
	R19. Decrement IPv6 Hop Limit by 1.		R19. Decrement IPv6 Hop Limit by 1.
	R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits		R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits
	[LBL..LBL+LNFL-1] of the Destination Address of the IPv6		[LBL..LBL+LNFL-1] of the Destination Address of the IPv6
	header.		header.
	R21. Submit the packet to the egress IPv6 FIB lookup for		R21. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the new destination.		transmission to the new destination.
	S16. }		S16. }
	When processing the Upper-Layer header of a packet matching a FIB		When processing the upper-layer header of a packet matching a FIB
	entry locally instantiated as an End SID with the REPLACE-CSID		entry locally instantiated as an End SID with the REPLACE-CSID
	flavor:		flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.7. End.X with REPLACE-CSID		A.7. End.X with REPLACE-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End.X SID with the REPLACE-CSID flavor:		instantiated as an End.X SID with the REPLACE-CSID flavor:
	S01. When an SRH is processed {		S01. When an SRH is processed {
	S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or		S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or
	skipping to change at line 2577 ¶		skipping to change at line 2575 ¶
	R17. Set DA.Arg.Index to (128/LNFL - 1).		R17. Set DA.Arg.Index to (128/LNFL - 1).
	R18. }		R18. }
	R19. Decrement IPv6 Hop Limit by 1.		R19. Decrement IPv6 Hop Limit by 1.
	R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits		R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits
	[LBL..LBL+LNFL-1] of the Destination Address of the IPv6		[LBL..LBL+LNFL-1] of the Destination Address of the IPv6
	header.		header.
	R21. Submit the packet to the IPv6 module for transmission to the		R21. Submit the packet to the IPv6 module for transmission to the
	new destination via a member of J.		new destination via a member of J.
	S16. }		S16. }
	When processing the Upper-Layer header of a packet matching a FIB		When processing the upper-layer header of a packet matching a FIB
	entry locally instantiated as an End.X SID with the REPLACE-CSID		entry locally instantiated as an End.X SID with the REPLACE-CSID
	flavor:		flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.8. End.T with REPLACE-CSID		A.8. End.T with REPLACE-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End.T SID with the REPLACE-CSID flavor:		instantiated as an End.T SID with the REPLACE-CSID flavor:
	S01. When an SRH is processed {		S01. When an SRH is processed {
	S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or		S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or
	skipping to change at line 2643 ¶		skipping to change at line 2641 ¶
	R18. }		R18. }
	R19. Decrement IPv6 Hop Limit by 1.		R19. Decrement IPv6 Hop Limit by 1.
	R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits		R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits
	[LBL..LBL+LNFL-1] of the Destination Address of the IPv6		[LBL..LBL+LNFL-1] of the Destination Address of the IPv6
	header.		header.
	R21.1. Set the packet's associated FIB table to T.		R21.1. Set the packet's associated FIB table to T.
	R21.2. Submit the packet to the egress IPv6 FIB lookup for		R21.2. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the new destination.		transmission to the new destination.
	S16. }		S16. }
	When processing the Upper-Layer header of a packet matching a FIB		When processing the upper-layer header of a packet matching a FIB
	entry locally instantiated as an End.T SID with the REPLACE-CSID		entry locally instantiated as an End.T SID with the REPLACE-CSID
	flavor:		flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.9. End.B6.Encaps with REPLACE-CSID		A.9. End.B6.Encaps with REPLACE-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End.B6.Encaps SID with the REPLACE-CSID flavor:		instantiated as an End.B6.Encaps SID with the REPLACE-CSID flavor:
	S01. When an SRH is processed {		S01. When an SRH is processed {
	S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or		S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or
	skipping to change at line 2717 ¶		skipping to change at line 2715 ¶
	header.		header.
	R21.1. Push a new IPv6 header with its own SRH containing B.		R21.1. Push a new IPv6 header with its own SRH containing B.
	R21.2. Set the outer IPv6 SA to A.		R21.2. Set the outer IPv6 SA to A.
	R21.3. Set the outer IPv6 DA to the first SID of B.		R21.3. Set the outer IPv6 DA to the first SID of B.
	R21.4. Set the outer Payload Length, Traffic Class, Flow Label,		R21.4. Set the outer Payload Length, Traffic Class, Flow Label,
	Hop Limit, and Next Header fields.		Hop Limit, and Next Header fields.
	R21.5. Submit the packet to the egress IPv6 FIB lookup for		R21.5. Submit the packet to the egress IPv6 FIB lookup for
	transmission to the next destination.		transmission to the next destination.
	S16. }		S16. }
	When processing the Upper-Layer header of a packet matching a FIB		When processing the upper-layer header of a packet matching a FIB
	entry locally instantiated as an End.B6.Encaps SID with the REPLACE-		entry locally instantiated as an End.B6.Encaps SID with the REPLACE-
	CSID flavor:		CSID flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	A.10. End.BM with REPLACE-CSID		A.10. End.BM with REPLACE-CSID
	When processing the SRH of a packet matching a FIB entry locally		When processing the SRH of a packet matching a FIB entry locally
	instantiated as an End.BM SID with the REPLACE-CSID flavor:		instantiated as an End.BM SID with the REPLACE-CSID flavor:
	S01. When an SRH is processed {		S01. When an SRH is processed {
	S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or		S02. If (Segments Left == 0 and (DA.Arg.Index == 0 or
	skipping to change at line 2781 ¶		skipping to change at line 2779 ¶
	R17. Set DA.Arg.Index to (128/LNFL - 1).		R17. Set DA.Arg.Index to (128/LNFL - 1).
	R18. }		R18. }
	R19. Decrement IPv6 Hop Limit by 1.		R19. Decrement IPv6 Hop Limit by 1.
	R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits		R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits
	[LBL..LBL+LNFL-1] of the Destination Address of the IPv6		[LBL..LBL+LNFL-1] of the Destination Address of the IPv6
	header.		header.
	R21.1. Push the MPLS label stack for B.		R21.1. Push the MPLS label stack for B.
	R21.2. Submit the packet to the MPLS engine for transmission.		R21.2. Submit the packet to the MPLS engine for transmission.
	S16. }		S16. }
	When processing the Upper-Layer header of a packet matching a FIB		When processing the upper-layer header of a packet matching a FIB
	entry locally instantiated as an End.BM SID with the REPLACE-CSID		entry locally instantiated as an End.BM SID with the REPLACE-CSID
	flavor:		flavor:
	S01. If (Upper-Layer header type is allowed by local configuration) {		S01. If (upper-layer header type is allowed by local configuration) {
	S02. Proceed to process the Upper-Layer header		S02. Proceed to process the upper-layer header
	S03. } Else {		S03. } Else {
	S04. Send an ICMP Parameter Problem to the Source Address		S04. Send an ICMP Parameter Problem to the Source Address
	with Code 4 (SR Upper-layer Header Error)		with Code 4 (SR Upper-layer Header Error)
	and Pointer set to the offset of the Upper-Layer header,		and Pointer set to the offset of the upper-layer header,
	interrupt packet processing, and discard the packet.		interrupt packet processing, and discard the packet.
	S05. }		S05. }
	Acknowledgements		Acknowledgements
	The authors would like to thank Kamran Raza, Xing Jiang, YuanChao Su,		The authors would like to thank Kamran Raza, Xing Jiang, YuanChao Su,
	Han Li, Yisong Liu, Martin Vigoureux, Joel Halpern, and Tal Mizrahi		Han Li, Yisong Liu, Martin Vigoureux, Joel Halpern, and Tal Mizrahi
	for their insightful feedback and suggestions.		for their insightful feedback and suggestions.
	The authors would also like to thank Andrew Alston, Linda Dunbar,		The authors would also like to thank Andrew Alston, Linda Dunbar,
End of changes. 92 change blocks.
	166 lines changed or deleted		164 lines changed or added
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