Compressed SRv6 Segment List Encoding

2.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

4.1. NEXT-C-SID Flavor

A C-SID sequence using the NEXT-C-SID flavor comprises one or more C-SID containers. Each C-SID container is a fully formed 128-bit SID structured as shown in Figure 1. It carries a Locator-Block followed by a series of C-SIDs. The Locator-Node and Function of the C-SID container are those of the first C-SID, and its Argument is the contiguous series of subsequent C-SIDs. The second C-SID is encoded in the most significant bits of the C-SID container Argument, the third C-SID is encoded in the bits of the Argument that immediately follow the second C-SID, and so on. When all C-SIDs have the same length, a C-SID container can carry up to K C-SIDs, where K is computed as floor((128-LBL)/LNFL) (floor(x) is the greatest integer less than or equal to x [GKP94]). Each C-SID container for NEXT-C-SID is independent, such that contiguous C-SID containers in a C-SID sequence can be considered as separate C-SID sequences.¶

When a C-SID sequence comprises at least two C-SIDs, the last C-SID in the sequence is not required to have the NEXT-C-SID flavor. It can be bound to any behavior and flavor(s), including the REPLACE-C-SID flavor, as long as the updated destination address resulting from the processing of the previous C-SID in the sequence is a valid form for that last SID. Line S12 of the first pseudocode in Section 6.2 provides sufficient conditions to ensure this property.¶

+------------------------------------------------------------------+
|     Locator-Block      |Loc-Node|            Argument            |
|                        |Function|                                |
+------------------------------------------------------------------+
 <-------- LBL ---------> < LNFL > <------------- AL ------------->

An implementation MUST support a 32-bit Locator-Block length (LBL) and a 16-bit C-SID length (LNFL) for NEXT-C-SID flavor SIDs, and may support any other Locator-Block and C-SID length.¶

A deployment should use a consistent Locator-Block length and C-SID length for all SIDs of the SR domain. Heterogeneous lengths, while possible, may impact the compression efficiency.¶

The Argument length (AL) for NEXT-C-SID flavor SIDs is equal to 128-LBL-LNFL.¶

When processing an IPv6 packet that matches a FIB entry locally instantiated as a SID with the NEXT-C-SID flavor, the SR segment endpoint node applies the procedure specified in the following subsection that corresponds to the SID behavior. If the SID also has the PSP, USP, or USD flavor, the procedure is modified as described in Section 4.1.7.¶

An SR segment endpoint node instantiating a SID of this document with the NEXT-C-SID flavor MUST accept any Argument value for that SID.¶

At high level, for any SID with the NEXT-C-SID flavor, the SR segment endpoint node determines the next SID of the SID list as follows. If the Argument value of the active SID is non-zero, the SR segment endpoint node constructs the next SID from the active SID by copying the entire SID Argument value to the bits that immediately follow the Locator-Block, thus overwriting the active SID Locator-Node and Function with those of the next C-SID, and filling the least significant LNFL bits of the Argument with zeros. Otherwise (if the Argument value is 0), the SR segment endpoint node copies the next 128-bit Segment List entry from the SRH to the Destination Address field of the IPv6 header.¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.   Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.
N09. }

N08.   Submit the packet to the IPv6 module for transmission to the
         new destination via a member of J.

N08.1.   Set the packet's associated FIB table to T.
N08.2.   Submit the packet to the egress IPv6 FIB lookup for
           transmission to the new destination.

N08.1.   Push a new IPv6 header with its own SRH containing B.
N08.2.   Set the outer IPv6 SA to A.
N08.3.   Set the outer IPv6 DA to the first SID of B.
N08.4.   Set the outer Payload Length, Traffic Class, Flow Label,
           Hop Limit, and Next Header fields.
N08.5.   Submit the packet to the egress IPv6 FIB lookup for
           transmission to the next destination.

N08.1.   Push the MPLS label stack for B.
N08.2.   Submit the packet to the MPLS engine for transmission.

4.2. REPLACE-C-SID Flavor

A C-SID sequence using the REPLACE-C-SID flavor starts with a C-SID container in fully formed 128-bit SID format. The Locator-Block of this SID is the common Locator-Block for all the C-SIDs in the C-SID sequence, its Locator-Node and Function are those of the first C-SID, and its Argument carries the index of the current C-SID in the current C-SID container. The Argument value is initially 0. When more segments are present in the segment list, the C-SID sequence continues with one or more C-SID containers in packed format carrying the subsequent C-SIDs in the sequence. Each container in packed format is a 128-bit Segment List entry split into K "positions" of LNFL bits, where K is computed as floor(128/LNFL). If LNFL does not divide into 128 perfectly, a zero pad is added in the least significant bits of the C-SID container to fill the bits left over. The second C-SID in the C-SID sequence is encoded in the least significant bit position of the first C-SID container in packed format (position K-1), the third C-SID is encoded in position K-2, and so on.¶

The last C-SID in the C-SID sequence is not required to have the REPLACE-C-SID flavor. It can be bound to any behavior and flavor(s), including the NEXT-C-SID flavor, as long as it meets the conditions defined in Section 6.¶

The structure of a SID with the REPLACE-C-SID flavor is shown in Figure 2. The same structure is also that of the C-SID container for REPLACE-C-SID in fully formed 128-bit SID format.¶

+-------------------------------------------------------------------+
|     Locator-Block      |  Locator-Node  |        Argument         |
|                        |   + Function   |                         |
+-------------------------------------------------------------------+
 <-------- LBL ---------> <---- LNFL ----> <--------- AL ---------->

The structure of a C-SID container for REPLACE-C-SID in packed format is shown in Figure 3.¶

+-------------------------------------------------------------------+
|  Fourth C-SID  |  Third C-SID   |  Second C-SID  |  First C-SID   |
|  (position 0)  |  (position 1)  |  (position 2)  |  (position 3)  |
+-------------------------------------------------------------------+
 <---- LNFL ----> <---- LNFL ----> <---- LNFL ----> <---- LNFL ---->

The REPLACE-C-SID flavor SIDs support any Locator-Block length (LBL), depending on the needs of the operator, as long as it does not exceed 128-LNFL-ceiling(log_2(128/LNFL)) (ceiling(x) is the least integer greater than or equal to x [GKP94]), so that enough bits remain available for the C-SID and Argument. A Locator-Block length of 48, 56, 64, 72, or 80 bits is recommended for easier reading in operation.¶

This document defines the REPLACE-C-SID flavor for 16-bit and 32-bit C-SID lengths (LNFL). An implementation MUST support a 32-bit C-SID length for REPLACE-C-SID flavor SIDs.¶

A deployment should use a consistent Locator-Block length and C-SID length for all SIDs of the SR domain. Heterogeneous C-SID lengths, while possible, may impact the compression efficiency.¶

The Argument length (AL) for REPLACE-C-SID flavor SIDs is equal to 128-LBL-LNFL. The index value is encoded in the least significant X bits of the Argument, where X is computed as ceiling(log_2(128/LNFL)).¶

When processing an IPv6 packet that matches a FIB entry locally instantiated as a SID with the REPLACE-C-SID flavor, the SR segment endpoint node applies the procedure specified in the following subsection that corresponds to the SID behavior. If the SID also has the PSP, USP, or USD flavor, the procedure is modified as described in Section 4.2.8.¶

At high level, at the start of a C-SID sequence using the REPLACE-C-SID flavor, the first C-SID container in fully formed 128-bit SID format is copied to the Destination Address of the IPv6 header. Then, for any SID with the REPLACE-C-SID flavor, the SR segment endpoint node determines the next SID of the SID list as follows. When an SRH is present, the SR segment endpoint node decrements the index value in the Argument of the active SID if the index value is not 0 or, if it is 0, decrements the Segments Left value in the SRH and sets the index value in the Argument of the active SID to K-1. The updated index value indicates the position of the next C-SID within the C-SID container in packed format at the "Segment List" index "Segments Left" in the SRH. The SR segment endpoint node then constructs the next SID by copying this next C-SID to the bits that immediately follow the Locator-Block in the Destination Address field of the IPv6 header, thus overwriting the active SID Locator-Node and Function with those of the next C-SID. If no SRH is present, the SR segment endpoint node ignores the index value in the SID Argument (except End.DT2M, see Section 4.2.7) and processes the upper-layer header as per [RFC8986]. The C-SID sequence ends with a last C-SID in the last C-SID container that does not have the REPLACE-C-SID flavor, or with the special C-SID value 0, or when reaching the end of the segment list, whichever comes first.¶

S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
           Segment List[0][DA.Arg.Index-1] == 0)) {

R01. If (DA.Arg.Index != 0) {
R02.   If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
R03.     Send an ICMP Parameter Problem to the Source Address,
           Code 0 (Erroneous header field encountered),
           Pointer set to the Segments Left field,
           interrupt packet processing and discard the packet.
R04.   }
R05.   Decrement DA.Arg.Index by 1.
R06.   If (Segment List[Segments Left][DA.Arg.Index] == 0) {
R07.     Decrement Segments Left by 1.
R08.     Decrement IPv6 Hop Limit by 1.
R09.     Update IPv6 DA with Segment List[Segments Left]
R10.     Submit the packet to the egress IPv6 FIB lookup for
          transmission to the new destination.
R11.   }
R12. } Else {
R13.   If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
R14.     Send an ICMP Parameter Problem to the Source Address,
           Code 0 (Erroneous header field encountered),
           Pointer set to the Segments Left field,
           interrupt packet processing and discard the packet.
R15.   }
R16.   Decrement Segments Left by 1.
R17.   Set DA.Arg.Index to (floor(128/LNFL) - 1).
R18. }
R19. Decrement IPv6 Hop Limit by 1.
R20. Write Segment List[Segments Left][DA.Arg.Index] into the bits
       [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
       header.
R21. Submit the packet to the egress IPv6 FIB lookup for
       transmission to the new destination.

R10. Submit the packet to the IPv6 module for transmission to the
       new destination via a member of J.

R21. Submit the packet to the IPv6 module for transmission to the
       new destination via a member of J.

R10.1. Set the packet's associated FIB table to T.
R10.2. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the new destination.

R21.1. Set the packet's associated FIB table to T.
R21.2. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the new destination.

R10.1. Push a new IPv6 header with its own SRH containing B.
R10.2. Set the outer IPv6 SA to A.
R10.3. Set the outer IPv6 DA to the first SID of B.
R10.4. Set the outer Payload Length, Traffic Class, Flow Label,
         Hop Limit, and Next Header fields.
R10.5. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.

R21.1. Push a new IPv6 header with its own SRH containing B.
R21.2. Set the outer IPv6 SA to A.
R21.3. Set the outer IPv6 DA to the first SID of B.
R21.4. Set the outer Payload Length, Traffic Class, Flow Label,
         Hop Limit, and Next Header fields.
R21.5. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.

R10.1. Push the MPLS label stack for B.
R10.2. Submit the packet to the MPLS engine for transmission.

R21.1. Push the MPLS label stack for B.
R21.2. Submit the packet to the MPLS engine for transmission.

R20.1.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
             Segment List[0][DA.Arg.Index-1] == 0)) {

5.1. Global C-SID

A global C-SID is a C-SID allocated from the GIB.¶

A global C-SID identifies a segment defined at the Locator-Block level. The tuple (Locator-Block, C-SID) identifies the same segment across all nodes of the SR domain. A typical example is a prefix segment bound to the End behavior.¶

A node can have multiple global C-SIDs under the same Locator-Block (e.g., one per IGP flexible algorithm ([RFC9350])). Multiple nodes may share the same global C-SID (e.g., anycast).¶

5.2. Local C-SID

A local C-SID is a C-SID allocated from the LIB.¶

A local C-SID identifies a segment defined at the node level and within the scope of a particular Locator-Block. The tuple (Locator-Block, C-SID) identifies a different segment on each node of the SR domain. A typical example is a non-routed Adjacency segment bound to the End.X behavior.¶

Let N1 and N2 be two different physical nodes of the SR domain and I a local C-SID value, N1 may allocate value I to SID S1 and N2 may allocate the same value I to SID S2.¶

5.3. GIB/LIB Usage

GIB and LIB usage is a local implementation and/or configuration decision, however, some guidelines for determining usage for specific SID behaviors and recommendations are provided.¶

The GIB number space is shared among all SR segment endpoint nodes using SRv6 locators under a Locator-Block space. The more SIDs assigned from this space, per node, the faster it is exhausted. Therefore its use is prioritized for global segments, such as SIDs that identify a node.¶

The LIB number space is unique per node. Each node is able to fully utilize the entire LIB number space without consideration of assignments at other nodes. Therefore its use is prioritized for local segments, such as SIDs that identify services (of which there may be many) at nodes, cross-connects, or adjacencies.¶

While a longer C-SID length permits more flexibility in which SID behaviors may be assigned from the GIB, it also reduces the compression efficiency.¶

Given the previous Locator-Block and C-SID length recommendations, the following GIB/LIB usage is recommended:¶

• NEXT-C-SID:¶

  • GIB: End¶

  • LIB: End.X, End.T, End.DT4/6/46/2U/2M, End.DX4/6/2/2V (including large-scale pseudowire), End.B6.Encaps, End.B6.Encaps.Red, End.BM¶

• REPLACE-C-SID:¶

  • GIB: End, End.X, End.T, End.DT4/6/46/2U/2M, End.DX4/6/2/2V, End.B6.Encaps, End.B6.Encaps.Red, End.BM¶

  • LIB: End.DX2/2V for large-scale pseudowire¶

Any other allocation is possible but may lead to a suboptimal use of the C-SID numbering space.¶

5.4. Recommended Installation of C-SIDs in FIB

Section 4.3 of [RFC8754] defines how an SR segment endpoint node identifies a locally instantiated SRv6 SID. To ensure that any valid argument value is accepted, an SR segment endpoint node instantiating a NEXT-C-SID or REPLACE-C-SID flavor SID should install a corresponding FIB entry that matches only the Locator and Function parts of the SID (i.e., with a prefix length of LBL + LNL + FL).¶

In addition, an SR segment endpoint node instantiating NEXT-C-SID flavor SIDs from both GIB and LIB may install combined "Global + Local" FIB entries to match a sequence of global and local C-SIDs in a single longest prefix match (LPM) lookup.¶

For example, let us consider an SR segment endpoint node 10 instantiating the following two NEXT-C-SID flavor SIDs according to the C-SID length, Locator-Block length, and GIB/LIB recommendations in this section.¶

• 2001:db8:b1:10:: bound to the End behavior with the NEXT-C-SID flavor is instantiated from GIB with¶

  • Locator-Block length (LBL) = 48 (Locator-Block value 0x20010db800b1),¶

  • Locator-Node length (LNL) = 16 (Locator-Node value 0x0010),¶

  • Function length (FL) = 0, and¶

  • Argument length (AL) = 64.¶

• 2001:db8:b1:f123:: bound to the End.X behavior for its local IGP adjacency 123 with the NEXT-C-SID flavor is instantiated from LIB with¶

  • Locator-Block length (LBL) = 48 (Locator-Block value 0x20010db800b1),¶

  • Locator-Node length (LNL) = 0,¶

  • Function length (FL) = 16 (Function value 0xf123), and¶

  • Argument length (AL) = 64.¶

For SID 2001:db8:b1:10::, Node 10 would install the FIB entry 2001:db8:b1:10::/64 bound the End SID with the NEXT-C-SID flavor.¶

For SID 2001:db8:b1:f123::, Node 10 would install the FIB entry 2001:db8:b1:f123::/64 bound the End.X SID for adjacency 123 with the NEXT-C-SID flavor.¶

In addition, Node 10 may also install the combined FIB entry 2001:db8:b1:10:f123::/80 bound the End.X SID for adjacency 123 with the NEXT-C-SID flavor.¶

As another example, let us consider an SR segment endpoint node 20 instantiating the following two REPLACE-C-SID flavor SIDs according to the C-SID length, Locator-Block length, and GIB/LIB recommendations in this section.¶

• 2001:db8:b2:20:1:: from GIB with Locator-Block length (LBL) = 48, Locator-Node length (LNL) = 16, Function length (FL) = 16, Argument length (AL) = 48, and bound to the End behavior with the REPLACE-C-SID flavor.¶

• 2001:db8:b2:20:123:: from GIB with Locator-Block length (LBL) = 48, Locator-Node length (LNL) = 16, Function length (FL) = 16, Argument length (AL) = 48, and bound to the End.X behavior for its local IGP adjacency 123 with the REPLACE-C-SID flavor.¶

For SID 2001:db8:b2:20:1::, Node 20 would install the FIB entry 2001:db8:b2:20:1::/80 bound the End SID with the REPLACE-C-SID flavor.¶

For SID 2001:db8:b2:20:123::, Node 20 would install the FIB entry 2001:db8:b2:20:123::/80 bound the End.X SID for adjacency 123 with the REPLACE-C-SID flavor.¶

6.1. Segment Validation for Compression

As part of the compression process or as a preliminary step, the SR source node MUST validate the SID structure, if known, of each SID of this document in the segment list. The SR source node does so regardless of whether the segment list is explicitly configured, locally computed, or advertised by a controller (e.g., via BGP [I-D.ietf-idr-sr-policy-safi] or PCEP [I-D.ietf-pce-segment-routing-ipv6]).¶

A SID structure is valid for compression if it meets all the following conditions.¶

• The Locator-Block length is not 0.¶

• The sum of the Locator-Node length and Function length is not 0.¶

• The Argument length is equal to 128-LBL-LNL-FL.¶

When compressing a segment list, the SR source node MUST treat an invalid SID structure as unknown, and treats the SID as incompressible.¶

Section 8 discusses how the SIDs of this document and their structure can be advertised to the SR source node through various control plane protocols.¶

6.2. Segment List Compression

An SR source node MAY compress a segment list when it includes NEXT-C-SID and/or REPLACE-C-SID flavor SIDs in order to reduce the packet header length.¶

It is out of the scope of this document to describe the mechanism through which an uncompressed segment list is derived. As a general guidance for implementation or future specification, such a mechanism should aim to select the combination of SIDs that would result in the shortest compressed segment list. For example, by selecting a C-SID flavor SID over an equivalent non-C-SID flavor SID or by consistently selecting SIDs of the same C-SID flavor within each routing domain.¶

The segment list that the SR source node pushes onto the packet MUST comply with the rules in Section 6.3 and Section 6.4 and result in the same set of possible forwarding paths as the original segment list.¶

If an SR source node chooses to compress the segment list, one method is described below for illustrative purposes. Any other method producing a compressed segment list of equal or shorter length than the uncompressed segment list is compliant.¶

This method walks the uncompressed segment list and compresses each series of consecutive NEXT-C-SID flavor SIDs and each series of consecutive REPLACE-C-SID flavor SIDs.¶

• When the compression method encounters a series of one or more consecutive compressible NEXT-C-SID flavor SIDs, it compresses the series as follows. A SID with the NEXT-C-SID flavor is compressible if its structure is known to the SR source node and its Argument value is 0.¶

S01. Initialize a C-SID container equal to the first SID in the
       series, and initialize the remaining capacity of the C-SID
       container to the AL of that SID
S02. For each subsequent SID in the series {
S03.   If the current SID Locator-Block matches that of the C-SID
         container and the current SID LNFL is lower than or equal to
         the remaining capacity of the C-SID container {
S04.     Copy the current SID Locator-Node and Function to the most
           significant remaining Argument bits of the C-SID container
           and decrement the remaining capacity by LNFL
S05.   } Else {
S06.     Push the C-SID container onto the compressed segment list
S07.     Initialize a new C-SID container equal to the current SID in
           the series, and initialize the remaining capacity of the
           C-SID container to the AL of that SID
S08.   } // End If
S09. } // End For
S10. If at least one SID remains in the uncompressed segment list
       (following the series of compressible NEXT-C-SID flavor SIDs){
S11.   Set S to the next SID in the uncompressed segment list
S12.   If S is advertised with a SID structure, and the Locator-Block
         of S matches that of the C-SID container, and the sum of the
         Locator-Node, Function, and Argument length of S is lower
         than or equal to the remaining capacity of the C-SID
         container {
S13.     Copy the Locator-Node, Function, and Argument of S to the
           most significant remaining Argument bits of the C-SID
           container
S14.   } // End If
S15. } // End If
S16. Push the C-SID container onto the compressed segment list

• When the compression method encounters a series of REPLACE-C-SID flavor SIDs of the same C-SID length in the uncompressed segment list, it compresses the series as per the following high-level pseudo code. A compression checking function ComCheck(F, S) is defined to check if two SIDs F and S share the same SID structure and Locator-Block value, and if S has either no Argument or an Argument with value 0. If the check passes, then ComCheck(F,S) returns true.¶

S01. Initialize the first C-SID container in full SID format equal to
       the first SID in the series
S02. Initialize the second C-SID container in packed format if there
       are more than one SIDs, and initialize the remaining capacity
       of the C-SID container to 128 bits
S03. For each subsequent SID in the uncompressed segment list {
S04.   Set S to the current SID in the uncompressed segment list
S05.   If ComCheck(First SID, S) {
S06.     If the LNFL of S is lower than or equal to
           the remaining capacity of the C-SID container {
S07.       Copy the Locator-Node and Function of S to the least
             significant remaining bits of the C-SID container
             and decrement the remaining capacity by LNFL  // Note
S08.     } Else {
S09.       Push the C-SID container onto the compressed segment list
S10.       Initialize a new C-SID container in packed format with all
             bits set to 0
S11        Copy the Locator-Node and Function of S to the least
             significant remaining bits of the C-SID container
             and decrement the remaining capacity by LNFL  // Note
S12.     }
S13.     If S is not a REPLACE-C-SID flavor SID, then break
S14.   } Else {
S15.     Break
S16.   } // End If
S17. } // End For
S18. Push the C-SID container (if it is not empty) onto the
       compressed segment list

• In all remaining cases (i.e., when the compression method encounters a SID in the uncompressed segment list that is not handled by any of the previous subroutines), it pushes this SID as is onto the compressed segment list.¶

Regardless of how a compressed segment list is produced, the SR source node writes it in the IPv6 packet as described in Sections 4.1 and 4.1.1 of [RFC8754]. The text is reproduced below for reference.¶

A source node steers a packet into an SR Policy. If the SR Policy results in a Segment List containing a single segment, and there is no need to add information to the SRH flag or add TLV; the DA is set to the single Segment List entry, and the SRH MAY be omitted.¶

When needed, the SRH is created as follows:¶

The Next Header and Hdr Ext Len fields are set as specified in [RFC8200].¶

The Routing Type field is set to 4.¶

The DA of the packet is set with the value of the first segment.¶

The first element of the SRH Segment List is the ultimate segment. The second element is the penultimate segment, and so on.¶

The Segments Left field is set to n-1, where n is the number of elements in the SR Policy.¶

The Last Entry field is set to n-1, where n is the number of elements in the SR Policy.¶

TLVs (including HMAC) may be set according to their specification.¶

The packet is forwarded toward the packet's Destination Address (the first segment).¶

When a source does not require the entire SID list to be preserved in the SRH, a reduced SRH may be used.¶

A reduced SRH does not contain the first segment of the related SR Policy (the first segment is the one already in the DA of the IPv6 header), and the Last Entry field is set to n-2, where n is the number of elements in the SR Policy.¶

6.3. Rules for segment lists containing NEXT-C-SID flavor SIDs

1. If a Destination Option header would follow an SRH with a segment list of more than one segment compressed as a single NEXT-C-SID container, the SR source node MUST NOT omit the SRH.¶

2. When the last Segment List entry (index 0) in the SRH is a C-SID container representing more than one segment, the PSP operation is performed at the segment preceding the first segment of this C-SID container in the segment list. If the PSP behavior should instead be performed at the penultimate segment along the path, the SR source node MUST NOT compress the ultimate segment of the segment list into a C-SID container.¶

3. If a Destination Option header would follow an SRH with a last Segment List entry being a NEXT-C-SID container representing more than one segment, the SR source node MUST ensure that the PSP operation is not performed before the penultimate SR segment endpoint node along the path.¶

6.4. Rules for segment lists containing REPLACE-C-SID flavor SIDs

1. All SIDs compressed in a REPLACE-C-SID sequence MUST share the same Locator-Block and the same compression scheme.¶

2. All SIDs except the last one in a C-SID sequence for REPLACE-C-SID MUST have the REPLACE-C-SID flavor. If the last C-SID container is fully filled (i.e., the last C-SID is at position 0 in the C-SID container) and the last SID in the C-SID sequence is not the last segment in the segment list, the last SID in the C-SID sequence MUST NOT have the REPLACE-C-SID flavor.¶

3. When a REPLACE-C-SID flavor C-SID is present as the last SID in a container that is not the last Segment List entry (index 0) in the SRH, the next element in the segment list MUST be a REPLACE-C-SID container in packed format carrying at least one C-SID.¶

The SR source node determines the compression scheme of REPLACE-C-SID flavor SIDs as follows.¶

When receiving a SID advertisement for a REPLACE-C-SID flavor SID with LNL=16, FL=0, AL=128-LBL-NL-FL, and the value of the Argument is all 0, the SR source node marks both the SID and its locator as using 16-bit compression. All other SIDs allocated from this locator with LNL=16, FL=16, AL=128-LBL-NL-FL, and the value of the Argument is all 0 are also marked as using 16-bit compression. When receiving a SID advertisement for a REPLACE-C-SID flavor SID with LNFL=32, AL=128-LBL-NL-FL, and the value of the Argument is all 0, the SR source node marks both the SID and its locator as using 32-bit compression.¶

6.5. Upper-Layer Checksums

The Destination Address used in the IPv6 pseudo-header (Section 8.1 of [RFC8200]) is that of the ultimate destination.¶

At the originating node, that address will be the Destination Address as it is expected to be received by the ultimate destination. When the last element in the compressed segment list is a C-SID container, this address can be obtained from the last element in the uncompressed segment list or by repeatedly applying the segment behavior as described in Section 9.2. This applies regardless of whether an SRH is present in the IPv6 packet or omitted.¶

At the recipient(s), that address will be in the Destination Address field of the IPv6 header.¶

7.1. End.PS: Prefix Swap

The End.PS behavior is a variant of the End behavior that modifies the Locator-Block of the active C-SID sequence. This document defines the End.PS behavior with the NEXT-C-SID flavor and the End.PS behavior with the REPLACE-C-SID flavor.¶

An End.PS SID is used to transition to a new Locator-Block when the routing domain boundary is on the SR segment endpoint node.¶

Each instance of an End.PS SID is associated with a target Locator-Block B2/m, where B2 is an IPv6 address prefix and m is the associated prefix length. The target Locator-Block is a local property of the End.PS SID on the SR segment endpoint node.¶

The means by which an SR source node learns the target Locator-Block associated with an End.PS SID are outside the scope of this document. As examples, it could be learnt via configuration or signaled by a controller.¶

N05.1. Initialize an IPv6 address A equal to B2.
N05.2. Copy DA.Argument into the bits [m..(m+AL-1)] of A.
N06.   Copy A to the Destination Address of the IPv6 header.

R20.1. Initialize an IPv6 address A equal to B2.
R20.2. Write Segment List[Segments Left][DA.Arg.Index] into the bits
         [m..m+LNFL-1] of the Destination Address of the IPv6 header.
R20.3. Copy A to the Destination Address of the IPv6 header.

7.2. End.XPS: L3 Cross-Connect and Prefix Swap

The End.XPS behavior is a variant of the End.X behavior that modifies the Locator-Block of the active C-SID sequence. This document defines the End.XPS behavior with the NEXT-C-SID flavor and the End.XPS behavior with the REPLACE-C-SID flavor.¶

An End.XPS SID is used to transition to a new Locator-Block when the routing domain boundary is on a link adjacent to the SR segment endpoint node.¶

Each instance of an End.XPS SID is associated with a target Locator-Block B2/m and a set, J, of one or more L3 adjacencies. The target Locator-Block and set of adjacencies are local properties of the End.XPS SID on the SR segment endpoint node.¶

The means by which an SR source node learns the target Locator-Block associated with an End.XPS SID are outside the scope of this document. As examples, it could be learnt via configuration or signaled by a controller.¶

N05.1. Initialize an IPv6 address A equal to B2.
N05.2. Copy DA.Argument into the bits [m..(m+AL-1)] of A.
N06.   Copy A to the Destination Address of the IPv6 header.

R20.1. Initialize an IPv6 address A equal to B2.
R20.2. Write Segment List[Segments Left][DA.Arg.Index] into the bits
         [m..m+LNFL-1] of the Destination Address of the IPv6 header.
R20.3. Copy A to the Destination Address of the IPv6 header.

9.1. Pinging a SID

An SR source node may ping an SRv6 SID by sending an ICMPv6 echo request packet destined to the SRv6 SID, with or without a segment list. This operation is illustrated in Appendix A.1.2 of [RFC9259].¶

When pinging a SID of this document without a segment list, the SR source node places the SID in the destination address of the ICMPv6 echo request and MUST set the Argument of the SID to 0. The Argument value 0 allows the SID SR segment endpoint node (Section 4) to identify itself as the ultimate destination of the packet and process the ICMPv6 payload. If the SR source node sets a non-zero Argument value, the SR segment endpoint node would instead attempt to determine the next destination of the packet.¶

When pinging a SID of this document via a segment list, the SR source node MUST construct the IPv6 packet as described in Section 6 and compute the ICMPv6 checksum as described in Section 6.5.¶

9.2. ICMP Error Processing

When an IPv6 node encounters an error while processing a packet, it may report that error by sending an IPv6 error message to the packet source with an enclosed copy of the invoking packet. For the source of an invoking packet to process the ICMP error message, the ultimate destination address of the IPv6 header may be required.¶

Section 5.4 of [RFC8754] defines the logic that an SR source node follows to determine the ultimate destination of an invoking packet containing an SRH.¶

For an SR source node that supports the compressed segment list encoding defined in this document, the logic to determine the ultimate destination is generalized as follows.¶

• If the destination address of the invoking IPv6 packet matches a known SRv6 SID, modify the invoking IPv6 packet by applying the SID behavior associated with the matched SRv6 SID;¶

• Repeat until the application of the SID behavior would result in the processing of the upper-layer header.¶

The destination address of the resulting IPv6 packet may be used as the ultimate destination of the invoking IPv6 packet.¶

Since the SR source node that needs to determine the ultimate destination is the same node that originally built the segment list in the invoking packet, it is able to perform this operation for all the SIDs in the packet.¶

9.3. Upper Layer Checksum Verification on Transit Nodes

Upper layer checksums are computed by the originator of an IPv6 packet and verified by the ultimate destination(s) as it processes the upper layer protocol.¶

Middleboxes such as packet sniffers, if deployed inside the SR domain, may fail to verify the upper layer checksum of transit SRv6 traffic. Making these middleboxes SRv6 aware in general or C-SID aware in particular is out of the scope of this document.¶

10.1. Cisco Systems

Cisco Systems reported the following implementations of the SR segment endpoint node NEXT-C-SID flavor (Section 4.1) and the SR source node efficient SID-list encoding (Section 6) for NEXT-C-SID flavor SIDs. These are used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic engineering functionalities.¶

• Cisco NCS 540 Series routers running IOS XR 7.3.x or above [IMPL-CISCO-NCS540]¶

• Cisco NCS 560 Series routers running IOS XR 7.6.x or above [IMPL-CISCO-NCS560]¶

• Cisco NCS 5500 Series routers running IOS XR 7.3.x or above [IMPL-CISCO-NCS5500]¶

• Cisco NCS 5700 Series routers running IOS XR 7.5.x or above [IMPL-CISCO-NCS5700]¶

• Cisco 8000 Series routers running IOS XR 7.5.x or above [IMPL-CISCO-8000]¶

• Cisco ASR 9000 Series routers running IOS XR 7.5.x or above [IMPL-CISCO-ASR9000]¶

At the time of this report, all the implementations listed above are in production and follow the specification in the latest version of this document, including all the "MUST" and "SHOULD" clauses for the NEXT-C-SID flavor.¶

This report was last updated on January 11, 2023.¶

10.2. Huawei Technologies

Huawei Technologies reported the following implementations of the SR segment endpoint node REPLACE-C-SID flavor (Section 4.2). These are used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic engineering functionalities.¶

• Huawei ATN8XX,ATN910C,ATN980B routers running VRPV800R021C00 or above.¶

• Huawei CX600-M2 routers running VRPV800R021C00 or above.¶

• Huawei NE40E,ME60-X1X2,ME60-X3X8X16 routers running VRPV800R021C00 or above.¶

• Huawei NE5000E,NE9000 routers running VRPV800R021C00 or above.¶

• Huawei NCE-IP Controller running V1R21C00 or above.¶

At the time of this report, all the implementations listed above are in production and follow the specification in the latest version of this document, including all the "MUST" and "SHOULD" clauses for the REPLACE-C-SID flavor.¶

This report was last updated on January 11, 2023.¶

10.3. Nokia

Nokia reported the following implementations ([IMPL-NOKIA-20.10]) of the SR segment endpoint node NEXT-C-SID flavor (Section 4.1). These are used as part of its shortest path forwarding (in algorithm 0 and Flex-Algo), remote and TI-LFA repair tunnel, and Traffic Engineering functionalities.¶

• Nokia 7950 XRS 20/20e routers running SROS Release 22.10 or above¶

• Nokia 7750 SR-12e routers running SROS Release 22.10 or above¶

• Nokia 7750 SR-7/12 routers running SROS Release 22.10 or above¶

• Nokia 7750 SR-7s/14s routers running SROS Release 22.10 or above¶

• Nokia 7750 SR-1/1s/2s routers running SROS Release 22.10 or above¶

At the time of this report, all the implementations listed above are in production and follow the specification in the latest version of this document, including all the "MUST" and "SHOULD" clauses for the NEXT-C-SID flavor.¶

This report was last updated on February 3, 2023.¶

10.4. Arrcus

Arrcus reported the following implementations of the SR segment endpoint node NEXT-C-SID flavor (Section 4.1). These are used as part of its SRv6 shortest path forwarding (in algorithm 0 and Flex-Algo), TI-LFA, micro-loop avoidance and Traffic Engineering functionalities.¶

• Arrcus running on Ufi Space routers S9510-28DC, S9710-76D, S9600-30DX and S9700-23D with ArcOS v5.2.1 or above¶

• Arrcus running n Ufi Space routers S9600-72XC and S9700-53DX with ArcOS v5.1.1D or above¶

• Arrcus running on Quanta router IXA and IXAE with ArcOS v5.1.1D or above¶

At the time of this report, all the implementations listed above are in production and follow the specification in the latest version of this document, including all the "MUST" and "SHOULD" clauses for the NEXT-C-SID flavor.¶

This report was last updated on March 11, 2023.¶

10.5. Juniper Networks

Juniper Networks reported the following implementations of the SR segment endpoint node NEXT-C-SID flavor (Section 4.1). These are used as part of its SRv6 shortest path forwarding (in algorithm 0 and Flex-Algo), TI-LFA, micro-loop avoidance, and Traffic Engineering functionalities.¶

Juniper release 23.3 onwards supports this functionality.¶

At the time of this report, all the implementations listed above are in development and follow the specification in the latest version of this document, including all the "MUST" and "SHOULD" clauses for the NEXT-C-SID flavor.¶

This report was last updated on May 30, 2023.¶

10.6. Marvell

Marvell reported support in the Marvell Prestera Packet Processor for the SR segment endpoint node NEXT-C-SID flavor (Section 4.1) and REPLACE-C-SID flavor (Section 4.2).¶

This report was last updated on February 15, 2023.¶

10.7. Broadcom

Broadcom reported the following implementations of the SR segment endpoint node NEXT-C-SID flavor (Section 4.1) and REPLACE-C-SID flavor (Section 4.2). These are used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic engineering functionalities. All implementation of the following list is in general availability for customers using BCM SDK 6.5.26 or above.¶

• 88850 (Jericho2c+) series¶

• 88690 (Jericho2) series¶

• 88800 (Jericho2c) series¶

• 88480 (Qunran2a) series¶

• 88280 (Qunran2u) series¶

• 88295 (Qunran2n) series¶

• 88830 (Jericho2x) series¶

At the time of this report, all the implementations listed above are in production and follow the specification in the latest version of this document, including all the "MUST" and "SHOULD" clauses for the NEXT-C-SID and REPLACE-C-SID flavors.¶

For 78900 (Tomahawk) series-related support, please contact the Broadcom team.¶

This report was last updated on February 21, 2023.¶

10.8. ZTE Corporation

ZTE Corporation reported the following implementations of the SR segment endpoint node REPLACE-C-SID flavor (Section 4.2). These are used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic engineering functionalities.¶

• ZTE M6000-18S(BRAS), M6000-8S Plus(BRAS) routers running V5.00.10.09 or above.¶

• ZTE M6000-18S(SR), M6000-8S Plus(SR) routers running V5.00.10.80 or above.¶

• ZTE T8000-18 routers running V5.00.10.07 or above.¶

This report was last updated on March 29, 2023.¶

10.9. New H3C Technologies

New H3C Technologies reported the following implementations of the SR segment endpoint node REPLACE-C-SID flavor (Section 4.2). These are used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic engineering functionalities.¶

• H3C CR16000-F, SR8800-X routers running Version 7.1.075 or above.¶

• H3C CR18000, CR19000 routers running Version 7.1.071 or above.¶

This report was last updated on March 29, 2023.¶

10.10. Ruijie Network

Ruijie Network reported the following implementations of the SR segment endpoint node REPLACE-C-SID flavor (Section 4.2). These are used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic engineering functionalities.¶

• RUIJIE RG-N8018-R, RG-N8010-R routers running N8000-R_RGOS 12.8(3)B0801 or above.¶

This report was last updated on March 29, 2023.¶

10.11. Ciena

Ciena reported the following implementations of the SR segment endpoint node NEXT-C-SID flavor (Section 4.1). These are used as part of its shortest path forwarding (in algorithm 0 and Flex-Algo), remote and TI-LFA repair tunnel, and Traffic Engineering functionalities.¶

The following platforms support implementation of the above.¶

• Ciena 5162, 5164, 5166, 5168 routers running SAOS 10.10 or above¶

• Ciena 8110, 8112, 8190 routers running SAOS 10.10 or above¶

At the time of this report, all the implementations listed above are in production and follow the specification in the latest version of this document, including all the "MUST" and "SHOULD" clauses for the NEXT-C-SID flavor.¶

This report was last updated on February 6, 2024.¶

10.12. Centec

Centec reported the following implementations of the SR segment endpoint node REPLACE-C-SID flavor (Section 4.2). These are used as part of its SRv6 TI-LFA, micro-loop avoidance, and traffic engineering functionalities. All implementation of the following list is in general availability for customers using Centec SDK 5.6.8 or above.¶

• CTC7132 (TsingMa) Series¶

• CTC8180 (TsingMa.MX) Series¶

This report was last updated on February 14, 2024.¶

10.13. Open Source

The authors found the following open source implementations of the SR segment endpoint node NEXT-C-SID flavor (Section 4.1).¶

• The Linux kernel, version 6.1 [IMPL-OSS-LINUX]¶

• The Software for Open Networking in the Cloud (SONiC), version 202212 [IMPL-OSS-SONIC], and Switch Abstraction Interface (SAI), version 1.9.0 [IMPL-OSS-SAI]¶

• The Vector Packet Processor (VPP), version 20.05 [IMPL-OSS-VPP]¶

• A generic P4 implementation [IMPL-OSS-P4]¶

The authors found the following open source implementations of the SR segment endpoint node REPLACE-C-SID flavor (Section 4.2).¶

• ONOS and P4 Programmable Switch based [IMPL-OSS-ONOS]¶

• Open SRv6 Project [IMPL-OSS-OPEN-SRV6]¶

This section was last updated on January 11, 2023.¶

10.14. Interoperability Reports

N0 --- N1 --- N2 --- N3 --- N4 --- N5 --- N6 --- N7
      (S1)   (S2)   (S3)   (S4)   (S5)   (S6)   (S7)

13.1. SRv6 Endpoint Behaviors

This I-D. requests the IANA to update the reference of the following registrations from the "SRv6 Endpoint Behaviors" registry under the top-level "Segment Routing" registry-group (https://www.iana.org/assignments/segment-routing/) with the RFC number of this document once it is published, and transfer change control to the IETF.¶

A.1. End with NEXT-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address
           with Code 0 (Hop limit exceeded in transit),
           interrupt packet processing, and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.   Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.
N09. }
S02. If (Segments Left == 0) {
S03.   Stop processing the SRH, and proceed to process the next
         header in the packet, whose type is identified by
         the Next Header field in the routing header.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06.   Send an ICMP Time Exceeded message to the Source Address
         with Code 0 (Hop limit exceeded in transit),
         interrupt packet processing, and discard the packet.
S07. }
S08. max_LE = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
S10.   Send an ICMP Parameter Problem to the Source Address
         with Code 0 (Erroneous header field encountered)
         and Pointer set to the Segments Left field,
         interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Update IPv6 DA with Segment List[Segments Left].
S15. Submit the packet to the egress IPv6 FIB lookup for
       transmission to the new destination.

Before processing the Upper-Layer header or any IPv6 extension header other than Hop-by-Hop or Destination Option of a packet matching a FIB entry locally instantiated as an End SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.   Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.
N09. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End SID with the NEXT-C-SID flavor:¶

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

A.2. End.X with NEXT-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address
           with Code 0 (Hop limit exceeded in transit),
           interrupt packet processing, and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.   Submit the packet to the IPv6 module for transmission to the
         new destination via a member of J.
N09. }
S02. If (Segments Left == 0) {
S03.   Stop processing the SRH, and proceed to process the next
         header in the packet, whose type is identified by
         the Next Header field in the routing header.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06.   Send an ICMP Time Exceeded message to the Source Address
         with Code 0 (Hop limit exceeded in transit),
         interrupt packet processing, and discard the packet.
S07. }
S08. max_LE = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
S10.   Send an ICMP Parameter Problem to the Source Address
         with Code 0 (Erroneous header field encountered)
         and Pointer set to the Segments Left field,
         interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Update IPv6 DA with Segment List[Segments Left].
S15. Submit the packet to the IPv6 module for transmission
       to the new destination via a member of J.

Before processing the Upper-Layer header or any IPv6 extension header other than Hop-by-Hop or Destination Option of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.   Submit the packet to the IPv6 module for transmission to the
         new destination via a member of J.
N09. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.X SID with the NEXT-C-SID flavor:¶

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

A.3. End.T with NEXT-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End.T SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address
           with Code 0 (Hop limit exceeded in transit),
           interrupt packet processing, and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.1. Set the packet's associated FIB table to T.
N08.2. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the new destination.
N09. }
S02. If (Segments Left == 0) {
S03.   Stop processing the SRH, and proceed to process the next
         header in the packet, whose type is identified by
         the Next Header field in the routing header.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06.   Send an ICMP Time Exceeded message to the Source Address
         with Code 0 (Hop limit exceeded in transit),
         interrupt packet processing, and discard the packet.
S07. }
S08. max_LE = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
S10.   Send an ICMP Parameter Problem to the Source Address
         with Code 0 (Erroneous header field encountered)
         and Pointer set to the Segments Left field,
         interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Update IPv6 DA with Segment List[Segments Left].
S15.1. Set the packet's associated FIB table to T.
S15.2. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the new destination.

Before processing the Upper-Layer header or any IPv6 extension header other than Hop-by-Hop or Destination Option of a packet matching a FIB entry locally instantiated as an End.T SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.1. Set the packet's associated FIB table to T.
N08.2. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the new destination.
N09. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.T SID with the NEXT-C-SID flavor:¶

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

A.4. End.B6.Encaps with NEXT-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.1. Push a new IPv6 header with its own SRH containing B.
N08.2. Set the outer IPv6 SA to A.
N08.3. Set the outer IPv6 DA to the first SID of B.
N08.4. Set the outer Payload Length, Traffic Class, Flow Label,
         Hop Limit, and Next Header fields.
N08.5. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.
N09. }
S02. If (Segments Left == 0) {
S03.   Stop processing the SRH, and proceed to process the next
         header in the packet, whose type is identified by
         the Next Header field in the routing header.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06.   Send an ICMP Time Exceeded message to the Source Address
         with Code 0 (Hop limit exceeded in transit),
         interrupt packet processing, and discard the packet.
S07. }
S08. max_LE = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
S10.   Send an ICMP Parameter Problem to the Source Address
         with Code 0 (Erroneous header field encountered)
         and Pointer set to the Segments Left field,
         interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Update IPv6 DA with Segment List[Segments Left].
S15. Push a new IPv6 header with its own SRH containing B.
S16. Set the outer IPv6 SA to A.
S17. Set the outer IPv6 DA to the first SID of B.
S18. Set the outer Payload Length, Traffic Class, Flow Label,
       Hop Limit, and Next Header fields.
S19. Submit the packet to the egress IPv6 FIB lookup for
       transmission to the new destination.

Before processing the Upper-Layer header or any IPv6 extension header other than Hop-by-Hop or Destination Option of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.1. Push a new IPv6 header with its own SRH containing B.
N08.2. Set the outer IPv6 SA to A.
N08.3. Set the outer IPv6 DA to the first SID of B.
N08.4. Set the outer Payload Length, Traffic Class, Flow Label,
         Hop Limit, and Next Header fields.
N08.5. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.
N09. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the NEXT-C-SID flavor:¶

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

A.5. End.BM with NEXT-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End.BM SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.1. Push the MPLS label stack for B.
N08.2. Submit the packet to the MPLS engine for transmission.
N09. }
S02. If (Segments Left == 0) {
S03.   Stop processing the SRH, and proceed to process the next
         header in the packet, whose type is identified by
         the Next Header field in the routing header.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06.   Send an ICMP Time Exceeded message to the Source Address
         with Code 0 (Hop limit exceeded in transit),
         interrupt packet processing, and discard the packet.
S07. }
S08. max_LE = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
S10.   Send an ICMP Parameter Problem to the Source Address
         with Code 0 (Erroneous header field encountered)
         and Pointer set to the Segments Left field,
         interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1.
S13. Decrement Segments Left by 1.
S14. Update IPv6 DA with Segment List[Segments Left].
S15. Push the MPLS label stack for B.
S16. Submit the packet to the MPLS engine for transmission.

Before processing the Upper-Layer header or any IPv6 extension header other than Hop-by-Hop or Destination Option of a packet matching a FIB entry locally instantiated as an End.BM SID with the NEXT-C-SID flavor:¶

N01. If (DA.Argument != 0) {
N02.   If (IPv6 Hop Limit <= 1) {
N03.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
N04.   }
N05.   Copy DA.Argument into the bits [LBL..(LBL+AL-1)] of the
         Destination Address.
N06.   Set the bits [(LBL+AL)..127] of the Destination Address to
         zero.
N07.   Decrement IPv6 Hop Limit by 1.
N08.1. Push the MPLS label stack for B.
N08.2. Submit the packet to the MPLS engine for transmission.
N09. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.BM SID with the NEXT-C-SID flavor:¶

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

A.6. End with REPLACE-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End SID with the REPLACE-C-SID flavor:¶

S01. When an SRH is processed {
S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
           Segment List[0][DA.Arg.Index-1] == 0)) {
S03.     Stop processing the SRH, and proceed to process the next
           header in the packet, whose type is identified by
           the Next Header field in the routing header.
S04.   }
S05.   If (IPv6 Hop Limit <= 1) {
S06.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
S07.   }
S08.   max_LE = (Hdr Ext Len / 2) - 1
R01.   If (DA.Arg.Index != 0) {
R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
R03.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R04.     }
R05.     Decrement DA.Arg.Index by 1.
R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
R07.       Decrement Segments Left by 1.
R08.       Decrement IPv6 Hop Limit by 1.
R09.       Update IPv6 DA with Segment List[Segments Left]
R10.       Submit the packet to the egress IPv6 FIB lookup for
            transmission to the new destination.
R11.     }
R12.   } Else {
R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
R14.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R15.     }
R16.     Decrement Segments Left by 1.
R17.     Set DA.Arg.Index to (128/LNFL - 1).
R18.   }
R19.   Decrement IPv6 Hop Limit by 1.
R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
         [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
         header.
R21.   Submit the packet to the egress IPv6 FIB lookup for
         transmission to the new destination.
S16. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End SID with the REPLACE-C-SID flavor:¶

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

A.7. End.X with REPLACE-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End.X SID with the REPLACE-C-SID flavor:¶

S01. When an SRH is processed {
S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
           Segment List[0][DA.Arg.Index-1] == 0)) {
S03.     Stop processing the SRH, and proceed to process the next
           header in the packet, whose type is identified by
           the Next Header field in the routing header.
S04.   }
S05.   If (IPv6 Hop Limit <= 1) {
S06.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
S07.   }
S08.   max_LE = (Hdr Ext Len / 2) - 1
R01.   If (DA.Arg.Index != 0) {
R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
R03.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R04.     }
R05.     Decrement DA.Arg.Index by 1.
R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
R07.       Decrement Segments Left by 1.
R08.       Decrement IPv6 Hop Limit by 1.
R09.       Update IPv6 DA with Segment List[Segments Left]
R10.       Submit the packet to the IPv6 module for transmission to
             the new destination via a member of J.
R11.     }
R12.   } Else {
R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
R14.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R15.     }
R16.     Decrement Segments Left by 1.
R17.     Set DA.Arg.Index to (128/LNFL - 1).
R18.   }
R19.   Decrement IPv6 Hop Limit by 1.
R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
         [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
         header.
R21.   Submit the packet to the IPv6 module for transmission to the
         new destination via a member of J.
S16. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.X SID with the REPLACE-C-SID flavor:¶

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

A.8. End.T with REPLACE-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End.T SID with the REPLACE-C-SID flavor:¶

S01. When an SRH is processed {
S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
           Segment List[0][DA.Arg.Index-1] == 0)) {
S03.     Stop processing the SRH, and proceed to process the next
           header in the packet, whose type is identified by
           the Next Header field in the routing header.
S04.   }
S05.   If (IPv6 Hop Limit <= 1) {
S06.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
S07.   }
S08.   max_LE = (Hdr Ext Len / 2) - 1
R01.   If (DA.Arg.Index != 0) {
R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
R03.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R04.     }
R05.     Decrement DA.Arg.Index by 1.
R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
R07.       Decrement Segments Left by 1.
R08.       Decrement IPv6 Hop Limit by 1.
R09.       Update IPv6 DA with Segment List[Segments Left]
R10.1.     Set the packet's associated FIB table to T.
R10.2.     Submit the packet to the egress IPv6 FIB lookup for
             transmission to the new destination.
R11.     }
R12.   } Else {
R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
R14.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R15.     }
R16.     Decrement Segments Left by 1.
R17.     Set DA.Arg.Index to (128/LNFL - 1).
R18.   }
R19.   Decrement IPv6 Hop Limit by 1.
R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
         [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
         header.
R21.1. Set the packet's associated FIB table to T.
R21.2. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the new destination.
S16. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.T SID with the REPLACE-C-SID flavor:¶

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

A.9. End.B6.Encaps with REPLACE-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the REPLACE-C-SID flavor:¶

S01. When an SRH is processed {
S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
           Segment List[0][DA.Arg.Index-1] == 0)) {
S03.     Stop processing the SRH, and proceed to process the next
           header in the packet, whose type is identified by
           the Next Header field in the routing header.
S04.   }
S05.   If (IPv6 Hop Limit <= 1) {
S06.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
S07.   }
S08.   max_LE = (Hdr Ext Len / 2) - 1
R01.   If (DA.Arg.Index != 0) {
R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
R03.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R04.     }
R05.     Decrement DA.Arg.Index by 1.
R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
R07.       Decrement Segments Left by 1.
R08.       Decrement IPv6 Hop Limit by 1.
R09.       Update IPv6 DA with Segment List[Segments Left]
R10.1.     Push a new IPv6 header with its own SRH containing B.
R10.2.     Set the outer IPv6 SA to A.
R10.3.     Set the outer IPv6 DA to the first SID of B.
R10.4.     Set the outer Payload Length, Traffic Class, Flow Label,
             Hop Limit, and Next Header fields.
R10.5.     Submit the packet to the egress IPv6 FIB lookup for
             transmission to the next destination.
R11.     }
R12.   } Else {
R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
R14.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R15.     }
R16.     Decrement Segments Left by 1.
R17.     Set DA.Arg.Index to (128/LNFL - 1).
R18.   }
R19.   Decrement IPv6 Hop Limit by 1.
R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
         [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
         header.
R21.1. Push a new IPv6 header with its own SRH containing B.
R21.2. Set the outer IPv6 SA to A.
R21.3. Set the outer IPv6 DA to the first SID of B.
R21.4. Set the outer Payload Length, Traffic Class, Flow Label,
         Hop Limit, and Next Header fields.
R21.5. Submit the packet to the egress IPv6 FIB lookup for
         transmission to the next destination.
S16. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.B6.Encaps SID with the REPLACE-C-SID flavor:¶

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

A.10. End.BM with REPLACE-C-SID

When processing the SRH of a packet matching a FIB entry locally instantiated as an End.BM SID with the REPLACE-C-SID flavor:¶

S01. When an SRH is processed {
S02.   If (Segments Left == 0 and (DA.Arg.Index == 0 or
           Segment List[0][DA.Arg.Index-1] == 0)) {
S03.     Stop processing the SRH, and proceed to process the next
           header in the packet, whose type is identified by
           the Next Header field in the routing header.
S04.   }
S05.   If (IPv6 Hop Limit <= 1) {
S06.     Send an ICMP Time Exceeded message to the Source Address,
           Code 0 (Hop limit exceeded in transit),
           interrupt packet processing and discard the packet.
S07.   }
S08.   max_LE = (Hdr Ext Len / 2) - 1
R01.   If (DA.Arg.Index != 0) {
R02.     If ((Last Entry > max_LE) or (Segments Left > Last Entry)) {
R03.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R04.     }
R05.     Decrement DA.Arg.Index by 1.
R06.     If (Segment List[Segments Left][DA.Arg.Index] == 0) {
R07.       Decrement Segments Left by 1.
R08.       Decrement IPv6 Hop Limit by 1.
R09.       Update IPv6 DA with Segment List[Segments Left]
R10.1.     Push the MPLS label stack for B.
R10.2.     Submit the packet to the MPLS engine for transmission.
R11.     }
R12.   } Else {
R13.     If((Last Entry > max_LE) or (Segments Left > Last Entry+1)){
R14.       Send an ICMP Parameter Problem to the Source Address,
             Code 0 (Erroneous header field encountered),
             Pointer set to the Segments Left field,
             interrupt packet processing and discard the packet.
R15.     }
R16.     Decrement Segments Left by 1.
R17.     Set DA.Arg.Index to (128/LNFL - 1).
R18.   }
R19.   Decrement IPv6 Hop Limit by 1.
R20.   Write Segment List[Segments Left][DA.Arg.Index] into the bits
         [LBL..LBL+LNFL-1] of the Destination Address of the IPv6
         header.
R21.1. Push the MPLS label stack for B.
R21.2. Submit the packet to the MPLS engine for transmission.
S16. }

When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.BM SID with the REPLACE-C-SID flavor:¶

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