BGP Link-State extensions for Segment Routing Huawei Technologies
Rome Italy stefano@previdi.net
Cisco Systems, Inc.
India ketant@cisco.com
Cisco Systems, Inc.
Brussels Belgium cfilsfil@cisco.com
RtBrick Inc.
hannes@rtbrick.com
Huawei Technologies
Huawei Building, No. 156 Beiqing Rd. Beijing 100095 China mach.chen@huawei.com
Routing Inter-Domain Routing BGP-LS Segment Routing SID MPLS Label advertisement IS-IS OSPF OSPFv3 Segment Routing (SR) allows for a flexible definition of end-to-end paths by encoding paths as sequences of topological sub-paths, called "segments". These segments are advertised by routing protocols e.g. by the link state routing protocols (IS-IS, OSPFv2 and OSPFv3) within IGP topologies. This document defines extensions to the BGP Link-state address-family in order to carry segment routing information via BGP. 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 Routing (SR) allows for a flexible definition of end-to-end paths by combining sub-paths called "segments". A segment can represent any instruction: topological or service-based. A segment can have a local semantic to an SR node or global semantic within a domain. Within IGP topologies, an SR path is encoded as a sequence of topological sub-paths, called "IGP segments". These segments are advertised by the link-state routing protocols (IS-IS, OSPFv2 and OSPFv3). defines the Link-State IGP segments - Prefix, Node, Anycast and Adjacency segments. Prefix segments, by default, represent an ECMP-aware shortest-path to a prefix, as per the state of the IGP topology. Adjacency segments represent a hop over a specific adjacency between two nodes in the IGP. A prefix segment is typically a multi-hop path while an adjacency segment, in most of the cases, is a one-hop path. Node and anycast segments are variations of the prefix segment with their specific characteristics. When Segment Routing is enabled in an IGP domain, segments are advertised in the form of Segment Identifiers (SIDs). The IGP link-state routing protocols have been extended to advertise SIDs and other SR-related information. IGP extensions are described for: IS-IS, OSPFv2 and OSPFv3. Using these extensions, Segment Routing can be enabled within an IGP domain. Segment Routing (SR) allows advertisement of single or multi-hop paths. The flooding scope for the IGP extensions for Segment routing is IGP area-wide. Consequently, the contents of a Link State Database (LSDB) or a Traffic Engineering Database (TED) has the scope of an IGP area and therefore, by using the IGP alone it is not enough to construct segments across multiple IGP Area or AS boundaries. In order to address the need for applications that require topological visibility across IGP areas, or even across Autonomous Systems (AS), the BGP-LS address-family/sub-address-family have been defined to allow BGP to carry Link-State information. The BGP Network Layer Reachability Information (NLRI) encoding format for BGP-LS and a new BGP Path Attribute called the BGP-LS attribute are defined in . The identifying key of each Link-State object, namely a node, link, or prefix, is encoded in the NLRI and the properties of the object are encoded in the BGP-LS attribute.
denotes a typical deployment scenario. In each IGP area, one or more nodes are configured with BGP-LS. These BGP speakers form an IBGP mesh by connecting to one or more route-reflectors. This way, all BGP speakers (specifically the route-reflectors) obtain Link-State information from all IGP areas (and from other ASes from EBGP peers). An external component connects to the route-reflector to obtain this information (perhaps moderated by a policy regarding what information is or isn't advertised to the external component) as described in . This document describes extensions to BGP-LS to advertise the SR information. An external component (e.g., a controller) can collect SR information from across an SR domain (as described in ) and construct the end-to-end path (with its associated SIDs) that need to be applied to an incoming packet to achieve the desired end-to-end forwarding. SR operates within a trusted domain consisting of a single or multiple ASes managed by the same administrative entity e.g. within a single provider network.
This document defines SR extensions to BGP-LS and specifies the TLVs and sub-TLVs for advertising SR information within the BGP-LS Attribute. and lists the equivalent TLVs and sub-TLVs in IS-IS, OSPFv2 and OSPFv3 protocols. BGP-LS defines the BGP-LS NLRI that can be a Node NLRI, a Link NLRI or a Prefix NLRI. BGP-LS defines the TLVs that map link-state information to BGP-LS NLRI within the BGP-LS Attribute. This document adds additional BGP-LS Attribute TLVs in order to encode SR information. It does not introduce any changes to the encoding of the BGP-LS NLRIs.
The following Node Attribute TLVs are defined: Type Description Section 1161 SID/Label 1034 SR Capabilities 1035 SR Algorithm 1036 SR Local Block 1037 SRMS Preference These TLVs should only be added to the BGP-LS Attribute associated with the Node NLRI describing the IGP node that is originating the corresponding IGP TLV/sub-TLV described below.
The SID/Label TLV is used as a sub-TLV by the SR Capabilities () and Segment Routing Local Block (SRLB) () TLVs. This information is derived from the protocol specific advertisements. IS-IS, as defined by the SID/Label sub-TLV in section 2.3 of . OSPFv2/OSPFv3, as defined by the SID/Label sub-TLV in section 2.1 of and section 3.1 of . The TLV has the following format:
Where: Type: 1161 Length: Variable. Either 3 or 4 depending whether the value is encoded as a label or as an index/SID. SID/Label: If length is set to 3, then the 20 rightmost bits represent a label (the total TLV size is 7) and the 4 leftmost bits are set to 0. If length is set to 4, then the value represents a 32 bit SID (the total TLV size is 8).
The SR Capabilities TLV is used in order to advertise the node's SR Capabilities including its Segment Routing Global Base (SRGB) range(s). In the case of IS-IS, the capabilities also include the IPv4 and IPv6 support for the SR-MPLS forwarding plane. This information is derived from the protocol specific advertisements. IS-IS, as defined by the SR Capabilities sub-TLV in section 3.1 of . OSPFv2/OSPFv3, as defined by the SID/Label Range TLV in section 3.2 of . OSPFv3 leverages the same TLV as defined for OSPFv2. The SR Capabilities TLV has the following format:
Where: Type: 1034 Length: Variable. Minimum length is 12. Flags: 1 octet of flags as defined in section 3.1 of for IS-IS. The flags are not currently defined for OSPFv2 and OSPFv3 and MUST be set to 0 and ignored on receipt. Reserved: 1 octet that MUST be set to 0 and ignored on receipt. One or more entries, each of which have the following format: Range Size: 3 octet with a non-zero value indicating the number of labels in the range. SID/Label TLV (as defined in ) used as sub-TLV which encodes the first label in the range. Since the SID/Label TLV is used to indicate the first label of the SRGB range, only label encoding is valid under the SR Capabilities TLV.
The SR Algorithm TLV is used in order to advertise the SR Algorithms supported by the node. This information is derived from the protocol specific advertisements. IS-IS, as defined by the SR-Algorithm sub-TLV in section 3.2 of . OSPFv2/OSPFv3, as defined by the SR-Algorithm TLV in section 3.1 of . OSPFv3 leverages the same TLV as defined for OSPFv2. The SR Algorithm TLV has the following format:
Where: Type: 1035 Length: Variable. Minimum length is 1 and maximum can be 256. Algorithm: One or more fields of 1 octet each identifying the algorithm.
The SR Local Block (SRLB) TLV contains the range(s) of labels the node has reserved for local SIDs. Local SIDs are used, e.g., in IGP (IS-IS, OSPF) for Adjacency-SIDs, and may also be allocated by components other than IGP protocols. As an example, an application or a controller may instruct a node to allocate a specific local SID. Therefore, in order for such applications or controllers to know the range of local SIDs available, it is required that the node advertises its SRLB. This information is derived from the protocol specific advertisements. IS-IS, as defined by the SR Local Block sub-TLV in section 3.3 of . OSPFv2/OSPFv3, as defined by the SR Local Block TLV in section 3.3. of . OSPFv3 leverages the same TLV as defined for OSPFv2. The SRLB TLV has the following format:
Where: Type: 1036 Length: Variable. Minimum length is 12. Flags: 1 octet of flags. The flags are as defined in section 3.3 of for IS-IS. The flags are not currently defined for OSPFv2 and OSPFv3 and MUST be set to 0 and ignored on receipt. Reserved: 1 octet that MUST be set to 0 and ignored on receipt. One or more entries corresponding to sub-range(s), each of which have the following format: Range Size: 3 octet value indicating the number of labels in the range. SID/Label TLV (as defined in ) used as sub-TLV which encodes the first label in the sub-range. Since the SID/Label TLV is used to indicate the first label of the SRLB sub-range, only label encoding is valid under the SR Local Block TLV.
The Segment Routing Mapping Server (SRMS) Preference TLV is used in order to associate a preference with SRMS advertisements from a particular source. specifies the SRMS functionality along with SRMS preference of the node advertising the SRMS Prefix-to-SID Mapping ranges. This information is derived from the protocol specific advertisements. IS-IS, as defined by the SRMS Preference sub-TLV in section 3.4 of . OSPFv2/OSPFv3, as defined by the SRMS Preference TLV in section 3.4 of . OSPFv3 leverages the same TLV as defined for OSPFv2. The SRMS Preference TLV has the following format:
Where: Type: 1037 Length: 1. Preference: 1 octet carrying an unsigned 8 bit SRMS preference.
The following Link Attribute TLVs are are defined: Type Description Section 1099 Adjacency SID TLV 1100 LAN Adjacency SID TLV 1172 L2 Bundle Member TLV These TLVs should only be added to the BGP-LS Attribute associated with the Link NLRI describing the link of the IGP node that is originating the corresponding IGP TLV/sub-TLV described below.
The Adjacency SID TLV is used in order to advertise information related to an Adjacency SID. This information is derived from Adj-SID sub-TLV of IS-IS (section 2.2.1 of ), OSPFv2 (section 6.1 of ) and OSPFv3 (section 7.1 of ). The Adjacency SID TLV has the following format:
Where: Type: 1099 Length: Variable. Either 7 or 8 depending on Label or Index encoding of the SID Flags. 1 octet value which should be set as: IS-IS Adj-SID flags are defined in section 2.2.1 of . OSPFv2 Adj-SID flags are defined in section 6.1 of . OSPFv3 Adj-SID flags are defined in section 7.1 of . Weight: 1 octet carrying the weight used for load-balancing purposes. The use of weight is described in section 3.4 of . Reserved: 2 octets that MUST be set to 0 and ignored on receipt. SID/Index/Label: IS-IS: Label or index value as defined in section 2.2.1 of . OSPFv2: Label or index value as defined in section 6.1 of . OSPFv3: Label or index value as defined in section 7.1 of . The Flags and, as an extension, the SID/Index/Label fields of this TLV are interpreted according to the respective underlying IS-IS, OSPFv2 or OSPFv3 protocol. The Protocol-ID of the BGP-LS Link NLRI is used to determine the underlying protocol specification for parsing these fields.
For a LAN, normally a node only announces its adjacency to the IS-IS pseudo-node (or the equivalent OSPF Designated and Backup Designated Routers). The LAN Adjacency Segment TLV allows a node to announce adjacencies to all other nodes attached to the LAN in a single instance of the BGP-LS Link NLRI. Without this TLV, the corresponding BGP-LS link NLRI would need to be originated for each additional adjacency in order to advertise the SR TLVs for these neighbor adjacencies. This information is derived from LAN-Adj-SID sub-TLV of IS-IS (section 2.2.2 of ) and LAN Adj-SID sub-TLV of OSPFv2 (section 6.2 of ) and OSPFv3 (section 7.2 of ). The LAN Adjacency SID TLV has the following format:
Where: Type: 1100 Length: Variable. For IS-IS it would be 13 or 14 depending on Label or Index encoding of the SID. For OSPF it would be 11 or 12 depending on Label or Index encoding of the SID. Flags. 1 octet value which should be set as: IS-IS LAN Adj-SID flags are defined in section 2.2.2 of . OSPFv2 LAN Adj-SID flags are defined in section 6.2 of . OSPFv3 LAN Adj-SID flags are defined in section 7.2 of . Weight: 1 octet carrying the weight used for load-balancing purposes. The use of weight is described in section 3.4 of . Reserved: 2 octets that MUST be set to 0 and ignored on receipt. Neighbor ID: 6 octets for IS-IS for the System-ID and 4 octets for OSPF for the OSPF Router-ID of the neighbor. SID/Index/Label: IS-IS: Label or index value as defined in section 2.2.2 of . OSPFv2: Label or index value as defined in section 6.2 of . OSPFv3: Label or index value as defined in section 7.2 of . The Neighbor ID, Flags and, as an extension, the SID/Index/Label fields of this TLV are interpreted according to the respective underlying IS-IS, OSPFv2 or OSPFv3 protocol. The Protocol-ID of the BGP-LS Link NLRI is used to determine the underlying protocol specification for parsing these fields.
The L2 Bundle Member Attribute TLV identifies an L2 Bundle Member link which in turn is associated with a parent L3 link. The L3 link is described by the Link NLRI defined in and the L2 Bundle Member Attribute TLV is associated with the Link NLRI. The TLV MAY include sub-TLVs which describe attributes associated with the bundle member. The identified bundle member represents a unidirectional path from the originating router to the neighbor specified in the parent L3 Link. Multiple L2 Bundle Member Attribute TLVs MAY be associated with a Link NLRI. This information is derived from L2 Bundle Member Attributes TLV of IS-IS (section 2 of ). The equivalent functionality has not been specified as yet for OSPF. The L2 Bundle Member Attribute TLV has the following format:
Where: Type: 1172 Length: Variable. L2 Bundle Member Descriptor: 4 octets field that carries a Link Local Identifier as defined in . Link attributes for L2 Bundle Member Links are advertised as sub-TLVs of the L2 Bundle Member Attribute TLV. The sub-TLVs are identical to existing BGP-LS TLVs as identified in the table below. TLV Code Point Description Reference Document 1088 Administrative group (color) 1089 Maximum link bandwidth 1090 Max. reservable link bandwidth 1091 Unreserved bandwidth 1092 TE default metric 1093 Link protection type 1099 Adjacency Segment Identifier (Adj-SID) TLV 1100 LAN Adjacency Segment Identifier (Adj-SID) TLV 1114 Unidirectional link delay 1115 Min/Max Unidirectional link delay 1116 Unidirectional Delay Variation 1117 Unidirectional packet loss 1118 Unidirectional residual bandwidth 1119 Unidirectional available bandwidth 1120 Unidirectional bandwidth utilization
The following Prefix Attribute TLVs are defined: Type Description Section 1158 Prefix SID 1159 Range 1170 Prefix Attribute Flags 1171 Source Router Identifier 1174 (suggested) Source OSPF Router-ID These TLVs should only be added to the BGP-LS Attribute associated with the Prefix NLRI describing the prefix of the IGP node that is originating the corresponding IGP TLV/sub-TLV described below.
The Prefix SID TLV is used in order to advertise information related to a Prefix SID. This information is derived from Prefix-SID sub-TLV of IS-IS (section 2.1 of ) and the Prefix SID sub-TLV of OSPFv2 (section 5 of ) and OSPFv3 (section 6 of ). The Prefix SID TLV has the following format:
Where: Type: 1158 Length: Variable. 7 or 8 depending on Label or Index encoding of the SID Flags: 1 octet value which should be set as: IS-IS Prefix SID flags are defined in section 2.1.1 of . OSPFv2 Prefix SID flags are defined in section 5 of . OSPFv3 Prefix SID flags are defined in section 6 of . Algorithm: 1 octet value identify the algorithm. The semantics of algorithm are described in section 3.1.1 of . Reserved: 2 octets that MUST be set to 0 and ignored on receipt. SID/Index/Label: IS-IS: Label or index value as defined in section 2.1 of . OSPFv2: Label or index value as defined in section 5 of . OSPFv3: Label or index value as defined in section 6 of . The Flags and, as an extension, the SID/Index/Label fields of this TLV are interpreted according to the respective underlying IS-IS, OSPFv2 or OSPFv3 protocol. The Protocol-ID of the BGP-LS Prefix NLRI is used to determine the underlying protocol specification for parsing these fields.
The Prefix Attribute Flags TLV carries IPv4/IPv6 prefix attribute flags information. These flags are defined for OSPFv2 in section 2.1 of , for OSPFv3 in section A.4.1.1 of and for IS-IS in section 2.1 of . The Prefix Attribute Flags TLV has the following format:
Where: Type: 1170 Length: Variable. Flags: a variable length flag field (according to the length field). Flags are routing protocol specific and are to be set as below: IS-IS flags correspond to the IPv4/IPv6 Extended Reachability Attribute Flags defined in section 2.1 of . In the case of the X-flag when associated with IPv6 prefix reachability, the setting corresponds to the setting of the X-flag in the fixed format of IS-IS TLVs 236 and 237 . OSPFv2 flags correspond to the Flags field of the OSPFv2 Extended Prefix TLV defined in section 2.1 of OSPFv3 flags map to the Prefix Options field defined in section A.4.1.1 of and extended in section 3.1 of The Flags field of this TLV is interpreted according to the respective underlying IS-IS, OSPFv2 or OSPFv3 protocol. The Protocol-ID of the BGP-LS Prefix NLRI is used to determine the underlying protocol specification for parsing this field.
The Source Router Identifier TLV contains the IPv4 or IPv6 Router Identifier of the originator of the Prefix. For the IS-IS protocol this is derived from the IPv4/IPv6 Source Router ID sub-TLV as defined in section 2.2 of . For the OSPF protocol, this is derived from the Prefix Source Router Address sub-TLV as defined in section 2.2 of . The Source Router Identifier TLV has the following format:
Where: Type: 1171 Length: Variable. 4 or 16 for IPv4 and IPv6 prefix respectively. Router-ID: the IPv4 or IPv6 Router-ID in case of IS-IS and the IPv4 or IPv6 Router Address in the case of OSPF.
The Source OSPF Router-ID TLV is applicable only for the OSPF protocol and contains OSPF Router-ID of the originator of the Prefix. It is derived from the Prefix Source OSPF Router-ID sub-TLV as defined in section 2.1 of . The Source OSPF Router-ID TLV has the following format:
Where: Type: 1174 (suggested) Length: 4 OSPF Router-ID: the OSPF Router-ID of the node originating the prefix.
The Range TLV is used in order to advertise a range of prefix-to-SID mappings as part of the Segment Routing Mapping Server (SRMS) functionality , as defined in the respective underlying IGP SR extensions (section 4), (section 5) and (section 2.4). The information advertised in the Range TLV is derived from the SID/Label Binding TLV in the case of IS-IS and the OSPFv2/OSPFv3 Extended Prefix Range TLV in the case of OSPFv2/OSPFv3. A Prefix NLRI, that been advertised with a Range TLV, is considered a normal routing prefix (i.e. prefix reachability) only when there is also an IGP metric TLV (TLV 1095) associated it. Otherwise, it is considered only as the first prefix in the range for prefix-to-SID mapping advertisement. The format of the Range TLV is as follows:
 Where: Type: 1159 Length: Variable. 11 or 12 depending on Label or Index encoding of the SID Flags: 1 octet value which should be set as: IS-IS SID/Label Binding TLV flags are defined in section 2.4.1 of . OSPFv2 OSPF Extended Prefix Range TLV flags are defined in section 4 of . OSPFv3 Extended Prefix Range TLV flags are defined in section 5 of . Reserved: 1 octet that MUST be set to 0 and ignored on receipt. Range Size: 2 octets that carry the number of prefixes that are covered by the advertisement.. The Flags field of this TLV is interpreted according to the respective underlying IS-IS, OSPFv2 or OSPFv3 protocol. The Protocol-ID of the BGP-LS Prefix NLRI is used to determine the underlying protocol specification for parsing this field. The prefix-to-SID mappings are advertised using sub-TLVs as below:
 The prefix-to-SID mapping information for the BGP-LS Prefix-SID TLV (used as sub-TLV in this context) is encoded as described in .
This section illustrate the IS-IS Segment Routing Extensions TLVs and sub-TLVs mapped to the ones defined in this document. The following table, illustrates for each BGP-LS TLV, its equivalence in IS-IS. Description IS-IS TLV/sub-TLV Reference SR Capabilities SR-Capabilities sub-TLV (2) SR Algorithm SR-Algorithm sub-TLV (19) SR Local Block SR Local Block sub-TLV (22) SRMS Preference SRMS Preference sub-TLV (19) Adjacency SID Adj-SID sub-TLV (31) LAN Adjacency SID LAN-Adj-SID sub-TLV (32) Prefix SID Prefix-SID sub-TLV (3) Range SID/Label Binding TLV (149) SID/Label SID/Label sub-TLV (1) Prefix Attribute Flags Prefix Attributes Flags sub-TLV (4) Source Router Identifier IPv4/IPv6 Source Router ID sub-TLV (11/12) L2 Bundle Member Attributes L2 Bundle Member Attributes TLV (25)
This section illustrate the OSPFv2 and OSPFv3 Segment Routing Extensions TLVs and sub-TLVs mapped to the ones defined in this document. The following table, illustrates for each BGP-LS TLV, its equivalence in OSPFv2 and OSPFv3. Description OSPFv2 TLV/sub-TLV Reference SR Capabilities SID/Label Range TLV (9) SR Algorithm SR-Algorithm TLV (8) SR Local Block SR Local Block TLV (14) SRMS Preference SRMS Preference TLV (15) Adjacency SID Adj-SID sub-TLV (2) LAN Adjacency SID LAN Adj-SID sub-TLV (3) Prefix SID Prefix SID sub-TLV (2) Range OSPF Extended Prefix Range TLV (2) SID/Label SID/Label sub-TLV (1) Prefix Attribute Flags Flags of OSPFv2 Extended Prefix TLV (1) Source Router Identifier Prefix Source Router-ID sub-TLV (4) Source OSPF Router-ID Prefix Source OSPF Router-ID sub-TLV (5) Description OSPFv3 TLV/sub-TLV Reference SR Capabilities SID/Label Range TLV (9) SR Algorithm SR-Algorithm TLV (8) SR Local Block SR Local Block TLV (14) SRMS Preference SRMS Preference TLV (15) Adjacency SID Adj-SID sub-TLV (5) LAN Adjacency SID LAN Adj-SID sub-TLV (6) Prefix SID Prefix SID sub-TLV (4) Range OSPFv3 Extended Prefix Range TLV (9) SID/Label SID/Label sub-TLV (7) Prefix Attribute Flags Prefix Option Fields of Prefix TLV types 3,5,6 Source OSPF Router Identifier Prefix Source Router-ID sub-TLV (27) Source OSPF Router-ID Prefix Source OSPF Router-ID sub-TLV (28)
Early allocation of codepoints has been done by IANA for this document from the registry "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" under the "BGP-LS Parameters" registry based on . The column "IS-IS TLV/Sub-TLV" defined in the registry does not require any value and should be left empty.
This section contains the global table of all TLVs/sub-TLVs defined in this document. TLV Code Point Description Reference 1034 SR Capabilities 1035 SR Algorithm 1036 SR Local Block 1037 SRMS Preference 1099 Adjacency SID 1100 LAN Adjacency SID 1158 Prefix SID 1159 Range 1161 SID/Label 1170 Prefix Attribute Flags 1171 Source Router Identifier 1172 L2 Bundle Member Attributes 1174 (suggested) Source OSPF Router-ID
This section is structured as recommended in . The new protocol extensions introduced in this document augment the existing IGP topology information that is distributed via . Procedures and protocol extensions defined in this document do not affect the BGP protocol operations and management other than as discussed in the Manageability Considerations section of . Specifically, the malformed attribute tests for syntactic checks in the Fault Management section of now encompass the new BGP-LS Attribute TLVs defined in this document. The semantic or content checking for the TLVs specified in this document and their association with the BGP-LS NLRI types or their BGP-LS Attribute is left to the consumer of the BGP-LS information (e.g. an application or a controller) and not the BGP protocol. A consumer of the BGP-LS information retrieves this information over a BGP-LS session (refer Section 1 and 2 of ). The handling of semantic or content errors by the consumer would be dictated by the nature of its application usage and hence is beyond the scope of this document. This document only introduces new Attribute TLVs and any syntactic error in them would result in the BGP-LS Attribute being discarded with an error log. The SR information introduced in BGP-LS by this specification, may be used by BGP-LS consumer applications like a SR path computation engine (PCE) to learn the SR capabilities of the nodes in the topology and the mapping of SR segments to those nodes. This can enable the SR PCE to perform path computations based on SR for traffic engineering use-cases and to steer traffic on paths different from the underlying IGP based distributed best path computation. Errors in the encoding or decoding of the SR information may result in the unavailability of such information to the SR PCE or incorrect information being made available to it. This may result in the SR PCE not being able to perform the desired SR based optimization functionality or to perform it in an unexpected or inconsistent manner. The handling of such errors by applications like SR PCE may be implementation specific and out of scope of this document. The extensions, specified in this document, do not introduce any new configuration or monitoring aspects in BGP or BGP-LS other than as discussed in . The manageability aspects of the underlying SR features are covered by , and .
The new protocol extensions introduced in this document augment the existing IGP topology information that is distributed via . The advertisement of the SR link attribute information defined in this document presents similar risk as associated with the existing set of link attribute information as described in . The Security Considerations section of also applies to these extensions. The procedures and new TLVs defined in this document, by themselves, do not affect the BGP-LS security model discussed in . The TLVs introduced in this document are used to propagate IGP defined information (, and ). These TLVs represent the SR information associated with the IGP node, link and prefix. The IGP instances originating these TLVs are assumed to support all the required security and authentication mechanisms (as described in , and ) in order to prevent any security issue when propagating the TLVs into BGP-LS. BGP-LS SR extensions enable traffic engineering use-cases within the Segment Routing domain. SR operates within a trusted domain and its security considerations also apply to BGP-LS sessions when carrying SR information. The SR traffic engineering policies using the SIDs advertised via BGP-LS are expected to be used entirely within this trusted SR domain (e.g. between multiple AS/domains within a single provider network). Therefore, precaution is necessary to ensure that the link-state information (including SR information) advertised via BGP-LS sessions is limited to consumers in a secure manner within this trusted SR domain. BGP peering sessions for address-families other than Link-State may be setup to routers outside the SR domain. The isolation of BGP-LS peering sessions is recommended to ensure that BGP-LS topology information (including the newly added SR information) is not advertised to an external BGP peering session outside the SR domain.
The following people have substantially contributed to the editing of this document:
The authors would like to thank Jeffrey Haas, Aijun Wang, Robert Raszuk and Susan Hares for their review of this document and their comments. The authors would also like to thank Alvaro Retana for his extensive review and comments which helped correct issues and improve the document.