]> Segment Type Extensions for BGP Segment Routing (SR) Policy Cisco Systems
India ketant.ietf@gmail.com
Cisco Systems
Brussels Belgium cfilsfil@cisco.com
Huawei Technologies
Italy stefano@previdi.net
Microsoft
One Microsoft Way Redmond WA 98052 United States of America pamattes@microsoft.com
Google
dhanendra.ietf@gmail.com
RTG idr This document specifies the signaling of additional Segment Routing (SR) Segment Types for SR Policies in BGP using the SR Policy Subsequent Address Family Identifier (SAFI).
Introduction The BGP Segment Routing (SR) Policy Subsequent Address Family Identifier (SAFI) was introduced by for the advertisement of SR Policies . introduced the base SR Segment Types A and B as specified by the SR Policy Architecture . This document specifies the extensions for the advertisement of the remaining SR Segment Types defined in in the SR Policy SAFI for both SR-MPLS (see ) and Segment Routing over IPv6 (SRv6) (see and ). The extensions in this document do not impact the SR Policy operations or fault management as specified in .
Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.
Segment Type Sub-TLVs The Segment List sub-TLV encodes a single explicit path towards the endpoint as described in . The Segment List sub-TLV includes the elements of the paths (i.e., segments). A Segment sub-TLV describes a single segment in a segment list (i.e., a single element of the explicit path). defines several Segment Types for SR-MPLS and SRv6 that are listed below as a reminder:
Type A:
SR-MPLS Label
Type B:
SRv6 SID
Type C:
IPv4 Prefix with optional SR Algorithm
Type D:
IPv6 Global Prefix with optional SR Algorithm for SR-MPLS
Type E:
IPv4 Prefix with Local Interface ID
Type F:
IPv4 Addresses for link endpoints as Local, Remote pair
Type G:
IPv6 Prefix and Interface ID for link endpoints as Local, Remote pair for SR-MPLS
Type H:
IPv6 Addresses for link endpoints as Local, Remote pair for SR-MPLS
Type I:
IPv6 Global Prefix with optional SR Algorithm for SRv6
Type J:
IPv6 Prefix and Interface ID for link endpoints as Local, Remote pair for SRv6
Type K:
IPv6 Addresses for link endpoints as Local, Remote pair for SRv6
specifies Segment Type Sub-TLVs for the Segment Types A and B. The following subsections specify the sub-TLVs used for encoding each of the other Segment Types above. As specified in Sections and of , validation of an explicit path encoded by the Segment List sub-TLV is beyond the scope of BGP and performed by the Segment Routing Policy Module (SRPM) as described in . As specified in , a mix of SR-MPLS and SRv6 segments make the segment-list invalid.
Segment Type C The Type C Segment sub-TLV encodes an IPv4 node address, SR Algorithm, and an optional SR-MPLS Segment Identifier (SID). The format is as follows:
Type C Segment Sub-TLV
Where:
Type:
3
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be 10 when the SR-MPLS SID is present; else, it MUST be 6.
Flags:
1 octet of flags as defined in .
SR Algorithm:
1 octet specifying the SR Algorithm as described in when the A-Flag as defined in is set. The SR Algorithm is used by the SRPM as described in . When the A-Flag is not set, this field MUST be set to zero on transmission and MUST be ignored on receipt.
IPv4 Node Address:
A 4-octet IPv4 address representing a node.
SR-MPLS SID:
Optional. A 4-octet field containing a label, Traffic Class (TC), bottom-of-stack (S), and TTL as defined for Segment Type A .
Segment Type D The Type D Segment sub-TLV encodes an IPv6 node address, SR Algorithm, and an optional SR-MPLS SID. The format is as follows:
Type D Segment Sub-TLV
Where:
Type:
4
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be 22 when the SR-MPLS SID is present; else, it MUST be 18.
Flags:
1 octet of flags as defined in .
SR Algorithm:
1 octet specifying the SR Algorithm as described in when the A-Flag as defined in is set. The SR Algorithm is used by the SRPM as described in . When the A-Flag is not set, this field MUST be set to zero on transmission and MUST be ignored on receipt.
IPv6 Node Address:
A 16-octet IPv6 address representing a node.
SR-MPLS SID:
Optional. A 4-octet field containing a label, TC, S, and TTL as defined for Segment Type A .
Segment Type E The Type E Segment sub-TLV encodes an IPv4 node address, a local interface Identifier (Local Interface ID), and an optional SR-MPLS SID. The format is as follows:
Type E Segment Sub-TLV
Where:
Type:
5
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be 14 when the SR-MPLS SID is present; else, it MUST be 10.
Flags:
1 octet of flags as defined in .
RESERVED:
1 octet of reserved bits. This field MUST be set to zero on transmission and MUST be ignored on receipt.
Local Interface ID:
4 octets carrying the interface index of the local interface (refer to TLV 258 of ).
IPv4 Node Address:
A 4-octet IPv4 address representing a node.
SR-MPLS SID:
Optional. A 4-octet field containing a label, TC, S, and TTL as defined for Segment Type A .
Segment Type F The Type F Segment sub-TLV encodes an adjacency local address, an adjacency remote address, and an optional SR-MPLS SID. The format is as follows:
Type F Segment Sub-TLV
Where:
Type:
6
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be 14 when the SR-MPLS SID is present; else, it MUST be 10.
Flags:
1 octet of flags as defined in .
RESERVED:
1 octet of reserved bits. This field MUST be set to zero on transmission and MUST be ignored on receipt.
Local IPv4 Address:
A 4-octet IPv4 address representing the local link address of the node.
Remote IPv4 Address:
A 4-octet IPv4 address representing the link address of the neighbor node.
SR-MPLS SID:
Optional. A 4-octet field containing a label, TC, S, and TTL as defined for Segment Type A .
Segment Type G The Type G Segment sub-TLV encodes an IPv6 link-local adjacency with an IPv6 local node address, a local interface identifier (Local Interface ID), an IPv6 remote node address, a remote interface identifier (Remote Interface ID), and an optional SR-MPLS SID. The format is as follows:
Type G Segment Sub-TLV
Where:
Type:
7
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be 46 when the SR-MPLS SID is present; else, it MUST be 42.
Flags:
1 octet of flags as defined in .
RESERVED:
1 octet of reserved bits. This field MUST be set to zero on transmission and MUST be ignored on receipt.
Local Interface ID:
4 octets of interface index of local interface (refer to TLV 258 of ).
IPv6 Local Node Address:
A 16-octet IPv6 address representing the node.
Remote Interface ID:
4 octets of interface index of remote interface (refer to TLV 258 of ). The value MAY be set to zero when the local node address and interface identifiers are sufficient to describe the link.
IPv6 Remote Node Address:
A 16-octet IPv6 address. The value MAY be set to zero when the local node address and interface identifiers are sufficient to describe the link.
SR-MPLS SID:
Optional. A 4-octet field containing a label, TC, S, and TTL as defined for Segment Type A .
Segment Type H The Type H Segment sub-TLV encodes an adjacency local address, an adjacency remote address, and an optional SR-MPLS SID. The format is as follows:
Type H Segment Sub-TLV
Where:
Type:
8
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be 38 when the SR-MPLS SID is present; else, it MUST be 34.
Flags:
1 octet of flags as defined in .
RESERVED:
1 octet of reserved bits. This field MUST be set to zero on transmission and MUST be ignored on receipt.
Local IPv6 Address:
A 16-octet IPv6 address representing the local link address of the node.
Remote IPv6 Address:
A 16-octet IPv6 address representing the link address of the neighbor node.
SR-MPLS SID:
Optional. A 4-octet field containing a label, TC, S, and TTL as defined for Segment Type A .
Segment Type I The Type I Segment sub-TLV encodes an IPv6 node address, an SR Algorithm, and an optional SRv6 SID. The format is as follows:
Type I Segment Sub-TLV
Where:
Type:
14
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be one of the following:
  • 42 when both SRv6 SID and SRv6 Endpoint Behavior and SID Structure are present,
  • 34 when only SRv6 SID is present, or
  • 18 when the SRv6 SID is not present.
Flags:
1 octet of flags as defined in .
SR Algorithm:
1 octet specifying the SR Algorithm as described in when the A-Flag as defined in is set. The SR Algorithm is used by the SRPM as described in . When the A-Flag is not set, this field MUST be set to zero on transmission and MUST be ignored on receipt.
IPv6 Node Address:
A 16-octet IPv6 address representing the node.
SRv6 SID:
Optional. A 16-octet IPv6 address. The value 0 MAY be used when the controller wants to indicate the desired SRv6 Endpoint Behavior or SID Structure without specifying the SID.
SRv6 Endpoint Behavior and SID Structure:
Optional, as defined in . The SRv6 Endpoint Behavior or SID Structure MUST NOT be included when the SRv6 SID has not been included.
TLV 10 defined for the advertisement of Segment Type I in the early draft versions of has been deprecated to avoid backward-compatibility issues.
Segment Type J The Type J Segment sub-TLV encodes an IPv6 link-local adjacency with a local node address, a local interface identifier (Local Interface ID), a remote IPv6 node address, a remote interface identifier (Remote Interface ID), and an optional SRv6 SID. The format is as follows:
Type J Segment Sub-TLV
Where:
Type:
15
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be one of the following:
  • 66 when both SRv6 SID and SRv6 Endpoint Behavior and SID Structure are present,
  • 58 when only SRv6 SID is present, or
  • 42 when the SRv6 SID is not present.
Flags:
1 octet of flags as defined in .
SR Algorithm:
1 octet specifying the SR Algorithm as described in when the A-Flag as defined in is set. The SR Algorithm is used by the SRPM as described in . When the A-Flag is not set, this field MUST be set to zero on transmission and MUST be ignored on receipt.
Local Interface ID:
4 octets of interface index of local interface (refer to TLV 258 of ).
IPv6 Local Node Address:
A 16-octet IPv6 address representing the node.
Remote Interface ID:
4 octets of interface index of remote interface (refer to TLV 258 of ). The value MAY be set to zero when the local node address and interface identifiers are sufficient to describe the link.
IPv6 Remote Node Address:
A 16-octet IPv6 address. The value MAY be set to zero when the local node address and interface identifiers are sufficient to describe the link.
SRv6 SID:
Optional. A 16-octet IPv6 address. The value 0 MAY be used when the controller wants to indicate the desired SRv6 Endpoint Behavior or SID Structure without specifying the SID.
SRv6 Endpoint Behavior and SID Structure:
Optional, as defined in . The SRv6 Endpoint Behavior and SID Structure MUST NOT be included when the SRv6 SID has not been included.
TLV 11 defined for the advertisement of Segment Type J in the early draft versions of has been deprecated to avoid backward-compatibility issues.
Segment Type K The Type K Segment sub-TLV encodes an adjacency local address, an adjacency remote address, and an optional SRv6 SID. The format is as follows:
Type K Segment Sub-TLV
Where:
Type:
16
Length:
Specifies the length of the value field (i.e., not including Type and Length fields) in terms of octets. The value MUST be one of the following:
  • 58 when both SRv6 SID and SRv6 Endpoint Behavior and SID Structure are present,
  • 50 when only SRv6 SID is present, or
  • 34 when the SRv6 SID is not present.
Flags:
1 octet of flags as defined in .
SR Algorithm:
1 octet specifying the SR Algorithm as described in when the A-Flag as defined in is set. The SR Algorithm is used by the SRPM as described in . When the A-Flag is not set, this field MUST be set to zero on transmission and MUST be ignored on receipt.
Local IPv6 Address:
A 16-octet IPv6 address representing the local link address of the node.
Remote IPv6 Address:
A 16-octet IPv6 address representing the link address of the neighbor node.
SRv6 SID:
Optional. A 16-octet IPv6 address. The value 0 MAY be used when the controller wants to indicate the desired SRv6 Endpoint Behavior or SID Structure without specifying the SID.
SRv6 Endpoint Behavior and SID Structure:
Optional, as defined in . The SRv6 Endpoint Behavior and SID Structure MUST NOT be included when the SRv6 SID has not been included.
TLV 12 defined for the advertisement of Segment Type K in the early draft versions of has been deprecated to avoid backward-compatibility issues.
SR Policy Segment Flags The Segment Type sub-TLVs described above may contain the following SR Policy Segment Flags in their Flags field (see also ). This document introduces additional flags below:
SR Policy Segment Flags
Where:
V-Flag:
This is an existing flag as defined in .
A-Flag:
When set, this flag indicates the presence of the SR Algorithm id in the SR Algorithm field applicable to various Segment Types. The SR Algorithm is used by the SRPM as described in .
S-Flag:
When set, this flag indicates the presence of the SR-MPLS or SRv6 SID depending on the segment type.
B-Flag:
This is an existing flag as defined in .
The following applies to the Segment Flags:
  • The V-Flag applies to all Segment Types including those introduced by this document.
  • The A-Flag applies to Segment Types C, D, I, J, and K. The value of the A-Flag MUST be ignored for Segment Types A, B, E, F, G, and H.
  • The S-Flag applies to Segment Types C, D, E, F, G, H, I, J, and K. The value of the S-Flag MUST be ignored for Segment Types A and B.
  • The B-Flag applies to Segment Types B, I, J, and K. The value of the B-Flag MUST be ignored for Segment Types A, C, D, E, F, G, and H.
IANA Considerations
SR Policy Segment List Sub-TLVs IANA has allocated the following code points from the "SR Policy Segment List Sub-TLVs" registry under the "Border Gateway Protocol (BGP) Tunnel Encapsulation" registry group. SR Policy Segment List Code Points
Value Description Reference
3 Type C Segment sub-TLV RFC 9831
4 Type D Segment sub-TLV RFC 9831
5 Type E Segment sub-TLV RFC 9831
6 Type F Segment sub-TLV RFC 9831
7 Type G Segment sub-TLV RFC 9831
8 Type H Segment sub-TLV RFC 9831
14 Type I Segment sub-TLV RFC 9831
15 Type J Segment sub-TLV RFC 9831
16 Type K Segment sub-TLV RFC 9831
SR Policy Segment Flags IANA has allocated code points from the "SR Policy Segment Flags" registry under the "Border Gateway Protocol (BGP) Tunnel Encapsulation" registry group. SR Policy Segment Flags
Bit Description Reference
1 SR Algorithm Flag (A-Flag) RFC 9831
2 SID Specified Flag (S-Flag) RFC 9831
Security Considerations The security considerations in apply to the Segment Types defined in this document. No additional security considerations are introduced.
Manageability Considerations The operations and manageability considerations in apply to the Segment Types defined in this document. No additional operations and manageability considerations are introduced.
Normative References Advertising Segment Routing Policies in BGP Huawei Technologies Cisco Systems Cisco Systems Microsoft Google
Acknowledgments The authors would like to , , and for their comments and review. The authors would like to thank for her detailed shepherd review that helped in improving the document.