IS-IS Extensions to Support Routing over IPv6 Dataplane

With Segment Routing (SR) , a node steers a packet through an ordered list of instructions, called segments. Segments are identified through Segment Identifiers (SIDs). Segment Routing can be directly instantiated on the IPv6 data plane through the use of the Segment Routing Header defined in . SRv6 refers to this SR instantiation on the IPv6 dataplane. The network programming paradigm is central to SRv6. It describes how any function can be bound to a SID and how any network program can be expressed as a combination of SID's. This document specifies IS-IS extensions that allow the IS-IS protocol to encode some of these functions. Familiarity with the network programming paradigm is necessary to understand the extensions specified in this document. This document defines one new top level IS-IS TLV and several new IS- IS sub-TLVs. The SRv6 Capabilities sub-TLV announces the ability to support SRv6 and some Endpoint functions listed in Section 7 as well as advertising limitations when applying such Endpoint functions. The SRv6 Locator top level TLV announces SRv6 locators - a form of summary address for the set of topology/algorithm specific SIDs associated with a node. The SRv6 End SID sub-TLV, the SRv6 End.X SID sub-TLV, and the SRv6 LAN End.X SID sub-TLV are used to advertise which SIDs are instantiated at a node and what Endpoint function is bound to each instantiated SID.

A node indicates that it has support for SRv6 by advertising a new SRv6- capabilities sub-TLV of the router capabilities TLV [RFC7981]. The SRv6 Capabilities sub-TLV may contain optional sub-sub-TLVs. No sub-sub-TLVs are currently defined. The SRv6 Capabilities sub-TLV has the following format:

defines the means to advertise node/link specific values for Maximum SID Depths (MSD) of various types. Node MSDs are advertised in a sub-TLV of the Router Capabilities TLV . Link MSDs are advertised in a sub-TLV of TLVs 22, 23, 141, 222, and 223. This document defines the relevant SRv6 MSDs and requests MSD type assignments in the MSD Types registry created by .

The Maximum Segments Left MSD Type specifies the maximum value of the "SL" field in the SRH of a received packet before applying the Endpoint function associated with a SID.

The Maximum End Pop MSD Type specifies the maximum number of SIDs in the top SRH in an SRH stack to which the router can apply "PSP" or USP" as defined in flavors.

The Maximum T.Insert MSD Type specifies the maximum number of SIDs that can be inserted as part of the "T.insert" behavior as defined in .

The Maximum T.Encaps MSD Type specifies the maximum number of SIDs that can be included as part of the "T.Encaps" behavior as defined in .

The Maximum End D MSD Type specifies the maximum number of SIDs in an SRH when performing decapsulation associated with "End.Dx" functions (e.g., "End.DX6" and "End.DT6") as defined in .

As discussed in , an SRv6 Segment Identifier (SID) is 128 bits and represented as LOC:FUNCT where LOC (the locator portion) is the L most significant bits and FUNCT is the 128-L least significant bits. L is called the locator length and is flexible. Each operator is free to use the locator length it chooses. A node is provisioned with topology/algorithm specific locators for each of the topology/algorithm pairs supported by that node. Each locator is a covering prefix for all SIDs provisioned on that node which have the matching topology/algorithm. Locators MUST be advertised in the SRv6 Locator TLV (see Section 6.1). Forwarding entries for the locators advertised in the SRv6 Locator TLV MUST be installed in the forwarding plane of receiving SRv6 capable routers when the associated topology/algorithm is supported by the receiving node. Locators are routable and MAY also be advertised in Prefix Reachability TLVs (236 or 237). Locators associated with algorithm 0 (for all supported topologies) SHOULD be advertised in a Prefix Reachability TLV (236 or 237) so that legacy routers (i.e., routers which do NOT support SRv6) will install a forwarding entry for algorithm 0 SRv6 traffic. In cases where a locator advertisement is received in both in a Prefix Reachability TLV and an SRv6 Locator TLV, the Prefix Reachability advertisement MUST be preferred when installing entries in the forwarding plane. This is to prevent inconsistent forwarding entries on SRv6 capable/SRv6 incapable routers. SRv6 SIDs are advertised as sub-TLVs in the SRv6 Locator TLV except for SRv6 End.X SIDs/LAN End.X SIDs which are associated with a specific Neighbor/Link and are therefore advertised as sub-TLVs in TLVs 22, 23, 222, 223, and 141. SRv6 SIDs are not directly routable and MUST NOT be installed in the forwarding plane. Reachability to SRv6 SIDs depends upon the existence of a covering locator. Adherence to the rules defined in this section will assure that SRv6 SIDs associated with a supported topology/algorithm pair will be forwarded correctly, while SRv6 SIDs associated with an unsupported topology/algorithm pair will be dropped. NOTE: The drop behavior depends on the absence of a default/summary route covering a given locator. In order for forwarding to work correctly, the locator associated with SRv6 SID advertisements MUST be the longest match prefix installed in the forwarding plane for those SIDs. There are a number of ways in which this requirement could be compromised Another locator associated with a different topology/algorithm is the longest match A prefix advertisement (i.e., from TLV 236 or 237) is the longest match

The SRv6 Locator TLV is introduced to advertise SRv6 Locators and End SIDs associated with each locator. This new TLV shares the sub-TLV space defined for TLVs 135, 235, 236 and 237.

The SRv6 Locator TLV has the following format:

The SRv6 End SID sub-TLV is introduced to advertise SRv6 Segment Identifiers (SID) with Endpoint functions which do not require a particular neighbor in order to be correctly applied . SRv6 SIDs associated with a neighbor are advertised using the sub-TLVs defined in Section 6. This new sub-TLV is advertised in the SRv6 Locator TLV defined in the previous section. SRv6 End SIDs inherit the topology/algorithm from the parent locator. The SRv6 End SID sub-TLV has the following format:

The SRv6 End SID MUST be a subnet of the associated Locator. SRv6 End SIDs which are NOT a subnet of the associated locator MUST be ignored. Multiple SRv6 End SIDs MAY be associated with the same locator. In cases where the number of SRv6 End SID sub-TLVs exceeds the capacity of a single TLV, multiple Locator TLVs for the same locator MAY be advertised. For a given MTID/Locator the algorithm MUST be the same in all TLVs. If this restriction is not met all TLVs for that MTID/Locator MUST be ignored.

Certain SRv6 Endpoint functions must be associated with a particular neighbor, and in case of multiple layer 3 links to the same neighbor, with a particular link in order to be correctly applied. This document defines two new sub-TLVs of TLV 22, 23, 222, 223, and 141 - namely "SRv6 End.X SID" and "SRv6 LAN End.X SID". IS-IS Neighbor advertisements are topology specific - but not algorithm specific. End.X SIDs therefore inherit the topology from the associated neighbor advertisement, but the algorithm is specified in the individual SID. All End.X SIDs MUST be a subnet of a Locator with matching topology and algorithm which is advertised by the same node in an SRv6 Locator TLV. End.X SIDs which do not meet this requirement MUST be ignored.

This sub-TLV is used to advertise an SRv6 SID associated with a point to point adjacency. Multiple SRv6 End.X SID sub-TLVs MAY be associated with the same adjacency. The SRv6 End.X SID sub-TLV has the following format:

Note that multiple TLVs for the same neighbor may be required in order to advertise all of the SRv6 End.X SIDs associated with that neighbor.

This sub-TLV is used to advertise an SRv6 SID associated with a LAN adjacency. Since the parent TLV is advertising an adjacency to the Designated Intermediate System(DIS) for the LAN, it is necessary to include the System ID of the physical neighbor on the LAN with which the SRv6 SID is associated. Given that a large number of neighbors may exist on a given LAN a large number of SRv6 LAN END.X SID sub-TLVs may be associated with the same LAN. Note that multiple TLVs for the same DIS neighbor may be required in order to advertise all of the SRv6 End.X SIDs associated with that neighbor. The SRv6 LAN End.X SID sub-TLV has the following format:

Endpoint function types are defined in . The numerical values are defined in the "SRv6 Endpoint Types" registry defined in . This section lists the Endpoint function types which MAY be advertised by IS-IS and the SID sub-TLVs in which each type MAY appear.

This document requests allocation for the following TLVs, sub- TLVs, and sub-sub-TLVs as well updating the ISIS TLV registry and defining a new registry.

This document adds one new TLV to the IS-IS TLV Codepoints registry. Value: 27 (suggested - to be assigned by IANA) Name: SRv6 Locator This TLV shares sub-TLV space with existing "Sub-TLVs for TLVs 135, 235, 236 and 237 registry". The name of this registry needs to be changed to "Sub-TLVs for TLVs 27, 135, 235, 236 and 237 registry".

This document adds the following new sub-TLV to the (renamed) "Sub-TLVs for TLVs 27, 135, 235, 236 and 237 registry". Value: 5 (suggested - to be assigned by IANA) Name: SRv6 End SID This document requests the creation of a new IANA managed registry for sub-sub-TLVs of the SRv6 End SID sub-TLV. The registration procedure is "Expert Review" as defined in [RFC7370]. Suggested registry name is "sub-sub-TLVs for SRv6 End SID sub-TLV". No sub-sub-TLVs are defined by this document except for the reserved value. 0: Reserved 1-255: Unassigned

The revised table of sub-TLVs for the (renamed) "Sub-TLVs for TLVs 27, 135, 235, 236 and 237 registry" is shown below:

This document adds the definition of a new sub-TLV in the "Sub- TLVs for TLV 242 registry". Type: 25 (Suggested - to be assigned by IANA) Description: SRv6 Capabilities This document requests the creation of a new IANA managed registry for sub-sub-TLVs of the SRv6 Capability sub-TLV. The registration procedure is "Expert Review" as defined in [RFC7370]. Suggested registry name is "sub-sub-TLVs for SRv6 Capability sub-TLV". No sub-sub-TLVs are defined by this document except for the reserved value. 0: Reserved 1-255: Unassigned

This document adds the definition of two new sub-TLVs in the "sub- TLVs for TLV 22, 23, 25, 141, 222 and 223 registry". Type: 43 (suggested - to be assigned by IANA) Description: SRv6 End.X SID Type: 44 (suggested - to be assigned by IANA) Description: SRv6 LAN End.X SID

This document defines the following new MSD types. These types are to be defined in the IGP MSD Types registry defined in . All values are suggested values to be assigned by IANA.

Security concerns for IS-IS are addressed in [ISO10589], [RFC5304], and [RFC5310].

The following people gave a substantial contribution to the content of this document and should be considered as co-authors:

Robert Hanzl Cisco Systems Millenium Plaza Building, V Celnici 10, Prague 1, Prague, Czech Republic Email rhanzl@cisco.com Ketan Talaulikar Cisco Systems, Inc. Email: ketant@cisco.com ]]>