Cisco Systems, Inc.

Eurovea Centre, Central 3 Pribinova Street 10 Bratislava 81109 Slovakia ppsenak@cisco.com

Cisco Systems, Inc.

stefano@previdi.net

Routing Open Shortest Path First IGP MPLS, SID, IGP, OSPF, Label advertisement, Segment Routing Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF). This document describes the OSPFv3 extensions required for Segment Routing with the MPLS data plane.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents () in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• .  Introduction
• .  Terminology
• .  Segment Routing Identifiers
  • .  SID/Label Sub-TLV
• .  Segment Routing Capabilities
• .  OSPFv3 Extended Prefix Range TLV
• .  Prefix-SID Sub-TLV
• .  Adjacency Segment Identifier (Adj-SID)
  • .  Adj-SID Sub-TLV
  • .  LAN Adj-SID Sub-TLV
• .  Elements of Procedure
  • .  Intra-area Segment Routing in OSPFv3
  • .  Inter-area Segment Routing in OSPFv3
  • .  Segment Routing for External Prefixes
  • .  Advertisement of Adj-SID
    • .  Advertisement of Adj-SID on Point-to-Point Links
    • .  Adjacency SID on Broadcast or NBMA Interfaces
• .  IANA Considerations
  • .  "OSPFv3 Extended-LSA TLVs" Registry
  • .  "OSPFv3 Extended-LSA Sub-TLVs" Registry
• . TLV/Sub-TLV Error Handling
• . Security Considerations
• . References
  • .  Normative References
  • .  Informative References
• Contributors
• Authors' Addresses

Introduction Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF). Prefix segments represent an ECMP-aware shortest path to a prefix (or a node) 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 cases, is a one-hop path. SR's control plane can be applied to both IPv6 and MPLS data planes, and it does not require any additional signaling (other than IGP extensions). The IPv6 data plane is out of the scope of this specification; the OSPFv3 extension for SR with the IPv6 data plane will be specified in a separate document. When used in MPLS networks, SR paths do not require any LDP or RSVP-TE signaling. However, SR can interoperate in the presence of Label Switched Paths (LSPs) established with RSVP or LDP. This document describes the OSPFv3 extensions required for Segment Routing with the MPLS data plane. Segment Routing architecture is described in . Segment Routing use cases are described in .

Terminology This section lists some of the terminology used in this document:

ABR:

Area Border Router

Adj-SID:

Adjacency Segment Identifier

AS:

Autonomous System

ASBR:

Autonomous System Boundary Router

DR:

Designated Router

IS-IS:

Intermediate System to Intermediate System

LDP:

Label Distribution Protocol

LSP:

Label Switched Path

MPLS:

Multiprotocol Label Switching

OSPF:

Open Shortest Path First

SPF:

Shortest Path First

RSVP:

Resource Reservation Protocol

SID:

Segment Identifier

SR:

Segment Routing

SRGB:

Segment Routing Global Block

SRLB:

Segment Routing Local Block

SRMS:

Segment Routing Mapping Server

TLV:

Type Length Value

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 Identifiers Segment Routing defines various types of Segment Identifiers (SIDs): Prefix-SID, Adjacency SID, and LAN Adjacency SID.

SID/Label Sub-TLV The SID/Label sub-TLV appears in multiple TLVs or sub-TLVs defined later in this document. It is used to advertise the SID or label associated with a prefix or adjacency. The SID/Label sub-TLV has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Label (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:

• Type:

  7

  Length:

  3 or 4 octets.

  SID/Label:

  If the length is set to 3, then the 20 rightmost bits represent a label. If the length is set to 4, then the value represents a 32-bit SID.

Segment Routing Capabilities Segment Routing requires some additional router capabilities to be advertised to other routers in the area. These SR capabilities are advertised in the OSPFv3 Router Information Opaque LSA (defined in ) and specified in .

OSPFv3 Extended Prefix Range TLV In some cases, it is useful to advertise attributes for a range of prefixes in a single advertisement. The SR Mapping Server, which is described in , is an example of where SIDs for multiple prefixes can be advertised. To optimize such advertisement in case of multiple prefixes from a contiguous address range, OSPFv3 Extended Prefix Range TLV is defined. The OSPFv3 Extended Prefix Range TLV is a top-level TLV of the following LSAs defined in :

• E-Intra-Area-Prefix-LSA
• E-Inter-Area-Prefix-LSA
• E-AS-External-LSA
• E-Type-7-LSA

Multiple OSPFv3 Extended Prefix Range TLVs MAY be advertised in each LSA mentioned above. The OSPFv3 Extended Prefix Range TLV has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Prefix Length | AF | Range Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address Prefix (variable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | where:

• Type:

  9

  Length:

  Variable, in octets, depending on the sub-TLVs.

  Prefix Length:

  Length of prefix in bits.

  AF:

  Address family for the prefix.

  AF:

  0 - IPv4 unicast

  AF:

  1 - IPv6 unicast

  Range Size:

  Represents the number of prefixes that are covered by the advertisement. The Range Size MUST NOT exceed the number of prefixes that could be satisfied by the Prefix Length without including:

  • Addresses from the IPv4 multicast address range (224.0.0.0/3), if the AF is IPv4 unicast.
  • Addresses other than the IPv6 unicast addresses, if the AF is IPv6 unicast.

  Flags:

  Reserved. MUST be zero when sent and are ignored when received.

  Reserved:

  SHOULD be set to 0 on transmission and MUST be ignored on reception.

  Address Prefix:

  • For the address family IPv4 unicast, the prefix itself is encoded as a 32-bit value. The default route is represented by a prefix of length 0.
  • For the address family IPv6 unicast, the prefix is encoded as an even multiple of 32-bit words and padded with zeroed bits as necessary. This encoding consumes ((PrefixLength + 31) / 32) 32-bit words.
  • Prefix encoding for other address families is beyond the scope of this specification. Prefix encoding for other address families can be defined in future Standards Track specifications from the IETF stream.

The range represents the contiguous set of prefixes with the same prefix length as specified by the Prefix Length field. The set starts with the prefix that is specified by the Address Prefix field. The number of prefixes in the range is equal to the Range Size. If the OSPFv3 Extended Prefix Range TLVs advertising the exact same range appears in multiple LSAs of the same type, originated by the same OSPFv3 router, the LSA with the numerically smallest Instance ID MUST be used, and subsequent instances of the OSPFv3 Extended Prefix Range TLVs MUST be ignored.

Prefix-SID Sub-TLV The Prefix-SID sub-TLV is a sub-TLV of the following OSPFv3 TLVs as defined in and in :

• Intra-Area Prefix TLV
• Inter-Area Prefix TLV
• External Prefix TLV
• OSPFv3 Extended Prefix Range TLV

It MAY appear more than once in the parent TLV and has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Algorithm | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Index/Label (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:

• Type:

  4

  Length:

  7 or 8 octets, depending on the V-Flag.

  Flags:

  Single-octet field. The following flags are defined: 0 1 2 3 4 5 6 7 +--+--+--+--+--+--+--+--+ | |NP|M |E |V |L | | | +--+--+--+--+--+--+--+--+ where:

  NP-Flag:

  No-PHP (Penultimate Hop Popping) Flag. If set, then the penultimate hop MUST NOT pop the Prefix-SID before delivering packets to the node that advertised the Prefix-SID.

  M-Flag:

  Mapping Server Flag. If set, the SID was advertised by an SR Mapping Server as described in .

  E-Flag:

  Explicit Null Flag. If set, any upstream neighbor of the Prefix-SID originator MUST replace the Prefix-SID with the Explicit NULL label (0 for IPv4, 2 for IPv6) before forwarding the packet.

  V-Flag:

  Value/Index Flag. If set, then the Prefix-SID carries an absolute value. If not set, then the Prefix-SID carries an index.

  L-Flag:

  Local/Global Flag. If set, then the value/index carried by the Prefix-SID has local significance. If not set, then the value/index carried by this sub-TLV has global significance.

  Other bits:

  Reserved. These MUST be zero when sent and are ignored when received.

  Reserved:

  SHOULD be set to 0 on transmission and MUST be ignored on reception.

  Algorithm:

  Single octet identifying the algorithm the Prefix-SID is associated with as defined in the IGP Algorithm Types registry . A router receiving a Prefix-SID from a remote node and with an algorithm value that the remote node has not advertised in the SR-Algorithm TLV MUST ignore the Prefix-SID sub-TLV.

  SID/Index/Label:

  According to the V-Flag and L-Flag, it contains:

  • V-Flag is set to 0 and L-Flag is set to 0: The SID/Index/Label field is a 4-octet index defining the offset in the SID/Label space advertised by this router.
  • V-Flag is set to 1 and L-Flag is set to 1: The SID/Index/Label field is a 3-octet local label where the 20 rightmost bits are used for encoding the label value.
  • All other combinations of V-Flag and L-Flag are invalid and any SID Advertisement received with an invalid setting for V- and L-Flags MUST be ignored.

If an OSPFv3 router advertises multiple Prefix-SIDs for the same prefix, topology, and algorithm, all of them MUST be ignored. When calculating the outgoing label for the prefix, the router MUST take into account, as described below, the E-, NP-, and M-Flags advertised by the next-hop router if that router advertised the SID for the prefix. This MUST be done regardless of whether the next-hop router contributes to the best path to the prefix. The NP-Flag (No-PHP) MUST be set and the E-Flag MUST be clear for Prefix-SIDs allocated to prefixes that are propagated between areas by an ABR based on intra-area or inter-area reachability, unless the advertised prefix is directly attached to such ABR. The NP-Flag (No-PHP) MUST be set and the E-Flag MUST be clear for Prefix-SIDs allocated to redistributed prefixes, unless the redistributed prefix is directly attached to the advertising ASBR. If the NP-Flag is not set, then:

• Any upstream neighbor of the Prefix-SID originator MUST pop the Prefix-SID. This is equivalent to the penultimate hop-popping mechanism used in the MPLS data plane.
• The received E-Flag is ignored.

If the NP-Flag is set and the E-Flag is not set, then:

• Any upstream neighbor of the Prefix-SID originator MUST keep the Prefix-SID on top of the stack. This is useful when the originator of the Prefix-SID needs to stitch the incoming packet into a continuing MPLS LSP to the final destination. This could occur at an ABR (prefix propagation from one area to another) or at an ASBR (prefix propagation from one domain to another).

If both the NP-Flag and E-Flag are set, then:

• Any upstream neighbor of the Prefix-SID originator MUST replace the Prefix-SID with an Explicit NULL label. This is useful, e.g., when the originator of the Prefix-SID is the final destination for the related prefix and the originator wishes to receive the packet with the original Traffic Class field .

When the M-Flag is set, the NP-Flag and the E-Flag MUST be ignored on reception. As the Mapping Server does not specify the originator of a prefix advertisement, it is not possible to determine PHP behavior solely based on the Mapping Server Advertisement. However, PHP behavior SHOULD be done in the following cases:

• The Prefix is intra-area type and the downstream neighbor is the originator of the prefix.
• The Prefix is inter-area type and the downstream neighbor is an ABR, which is advertising prefix reachability and is setting the LA-bit in the Prefix Options as described in .
• The Prefix is external type and the downstream neighbor is an ASBR, which is advertising prefix reachability and is setting the LA-bit in the Prefix Options as described in .

When a Prefix-SID is advertised in the OSPFv3 Extended Prefix Range TLV, then the value advertised in the Prefix-SID sub-TLV is interpreted as a starting SID/Label value. Example 1: If the following router addresses (loopback addresses) need to be mapped into the corresponding Prefix-SID indexes: Router-A: 2001:DB8::1/128, Prefix-SID: Index 1 Router-B: 2001:DB8::2/128, Prefix-SID: Index 2 Router-C: 2001:DB8::3/128, Prefix-SID: Index 3 Router-D: 2001:DB8::4/128, Prefix-SID: Index 4 then the Address Prefix field in the OSPFv3 Extended Prefix Range TLV would be set to 2001:DB8::1, the Prefix Length would be set to 128, the Range Size would be set to 4, and the Index value in the Prefix-SID sub-TLV would be set to 1. Example 2: If the following prefixes need to be mapped into the corresponding Prefix-SID indexes: 2001:DB8:1::0/120, Prefix-SID: Index 51 2001:DB8:1::100/120, Prefix-SID: Index 52 2001:DB8:1::200/120, Prefix-SID: Index 53 2001:DB8:1::300/120, Prefix-SID: Index 54 2001:DB8:1::400/120, Prefix-SID: Index 55 2001:DB8:1::500/120, Prefix-SID: Index 56 2001:DB8:1::600/120, Prefix-SID: Index 57 then the Prefix field in the OSPFv3 Extended Prefix Range TLV would be set to 2001:DB8:1::0, the Prefix Length would be set to 120, the Range Size would be set to 7, and the Index value in the Prefix-SID sub-TLV would be set to 51.

Adjacency Segment Identifier (Adj-SID) An Adjacency Segment Identifier (Adj-SID) represents a router adjacency in Segment Routing.

Adj-SID Sub-TLV The Adj-SID sub-TLV is an optional sub-TLV of the Router-Link TLV as defined in . It MAY appear multiple times in the Router-Link TLV. The Adj-SID sub-TLV has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Weight | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Label/Index (variable) | +---------------------------------------------------------------+ where:

• Type:

  5

  Length:

  7 or 8 octets, depending on the V-Flag.

  Flags:

  Single-octet field containing the following flags: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |B|V|L|G|P| | +-+-+-+-+-+-+-+-+ where:

  B-Flag:

  Backup Flag. If set, the Adj-SID refers to an adjacency that is eligible for protection (e.g., using IP Fast Reroute (IPFRR) or MPLS-FRR (MPLS Fast Reroute)) as described in .

  V-Flag:

  Value/Index Flag. If set, then the Adj-SID carries an absolute value. If not set, then the Adj-SID carries an index.

  L-Flag:

  Local/Global Flag. If set, then the value/index carried by the Adj-SID has local significance. If not set, then the value/index carried by this sub-TLV has global significance.

  G-Flag:

  Group Flag. When set, the G-Flag indicates that the Adj-SID refers to a group of adjacencies (and therefore MAY be assigned to other adjacencies as well).

  P-Flag.

  Persistent Flag. When set, the P-Flag indicates that the Adj-SID is persistently allocated, i.e., the Adj-SID value remains the same across router restart and/or interface flap.

  Other bits:

  Reserved. These MUST be zero when sent and are ignored when received.

  Reserved:

  SHOULD be set to 0 on transmission and MUST be ignored on reception.

  Weight:

  Weight used for load-balancing purposes. The use of the weight is defined in .

  SID/Index/Label:

  As described in .

An SR-capable router MAY allocate an Adj-SID for each of its adjacencies and set the B-Flag when the adjacency is eligible for protection by an FRR mechanism (IP or MPLS) as described in . An SR-capable router MAY allocate more than one Adj-SID to an adjacency. An SR-capable router MAY allocate the same Adj-SID to different adjacencies. When the P-Flag is not set, the Adj-SID MAY be persistent. When the P-Flag is set, the Adj-SID MUST be persistent.

LAN Adj-SID Sub-TLV The LAN Adjacency SID is an optional sub-TLV of the Router-Link TLV. It MAY appear multiple times in the Router-Link TLV. It is used to advertise a SID/Label for an adjacency to a non-DR router on a broadcast, Non-Broadcast Multi-Access (NBMA), or hybrid network. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Weight | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Neighbor ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Label/Index (variable) | +---------------------------------------------------------------+ where:

• Type:

  6

  Length:

  11 or 12 octets, depending on the V-Flag.

  Flags:

  Same as in .

  Weight:

  Weight used for load-balancing purposes. The use of the weight is defined in .

  Reserved:

  SHOULD be set to 0 on transmission and MUST be ignored on reception.

  Neighbor ID:

  The Router ID of the neighbor for which the LAN Adjacency SID is advertised.

  SID/Index/Label:

  As described in . When the P-Flag is not set, the LAN Adjacency SID MAY be persistent. When the P-Flag is set, the LAN Adjacency SID MUST be persistent.

Elements of Procedure

Intra-area Segment Routing in OSPFv3 An OSPFv3 router that supports Segment Routing MAY advertise Prefix- SIDs for any prefix to which it is advertising reachability (e.g., a loopback IP address as described in ). A Prefix-SID can also be advertised by SR Mapping Servers (as described in ). A Mapping Server advertises Prefix-SIDs for remote prefixes that exist in the OSPFv3 routing domain. Multiple Mapping Servers can advertise Prefix-SIDs for the same prefix, in which case the same Prefix-SID MUST be advertised by all of them. The SR Mapping Server could use either area flooding scope or autonomous system flooding scope when advertising Prefix-SIDs for prefixes, based on the configuration of the SR Mapping Server. Depending on the flooding scope used, the SR Mapping Server chooses the OSPFv3 LSA type that will be used. If the area flooding scope is needed, an E-Intra-Area-Prefix-LSA is used. If autonomous system flooding scope is needed, an E-AS-External-LSA is used. When a Prefix-SID is advertised by the Mapping Server, which is indicated by the M-Flag in the Prefix-SID sub-TLV (), the route type as implied by the LSA type is ignored and the Prefix-SID is bound to the corresponding prefix independent of the route type. Advertisement of the Prefix-SID by the Mapping Server using an Inter-Area Prefix TLV, External-Prefix TLV, or Intra-Area-Prefix TLV does not itself contribute to the prefix reachability. The NU-bit MUST be set in the PrefixOptions field of the LSA, which is used by the Mapping Server to advertise SID or SID Range, which prevents the advertisement from contributing to prefix reachability. An SR Mapping Server MUST use the OSPFv3 Extended Prefix Range TLVs when advertising SIDs for prefixes. Prefixes of different route types can be combined in a single OSPFv3 Extended Prefix Range TLV advertised by an SR Mapping Server. Area-scoped OSPFv3 Extended Prefix Range TLVs are propagated between areas, similar to propagation of prefixes between areas. Same rules that are used for propagating prefixes between areas are used for the propagation of the prefix ranges.

Inter-area Segment Routing in OSPFv3 In order to support SR in a multiarea environment, OSPFv3 MUST propagate Prefix-SID information between areas. The following procedure is used to propagate Prefix-SIDs between areas. When an OSPFv3 ABR advertises an Inter-Area-Prefix-LSA from an intra-area prefix to all its connected areas, it will also include the Prefix-SID sub-TLV as described in . The Prefix-SID value will be set as follows:

• The ABR will look at its best path to the prefix in the source area and find the advertising router associated with the best path to that prefix.
• The ABR will then determine if this router advertised a Prefix-SID for the prefix and use it when advertising the Prefix-SID to other connected areas.
• If no Prefix-SID was advertised for the prefix in the source area by the router that contributes to the best path to the prefix, the originating ABR will use the Prefix-SID advertised by any other router when propagating the Prefix-SID for the prefix to other areas.

When an OSPFv3 ABR advertises an Inter-Area-Prefix-LSA from an inter-area route to all its connected areas, it will also include the Prefix-SID sub-TLV as described in . The Prefix-SID value will be set as follows:

• The ABR will look at its best path to the prefix in the backbone area and find the advertising router associated with the best path to that prefix.
• The ABR will then determine if this router advertised a Prefix-SID for the prefix and use it when advertising the Prefix-SID to other connected areas.
• If no Prefix-SID was advertised for the prefix in the backbone area by the ABR that contributes to the best path to the prefix, the originating ABR will use the Prefix-SID advertised by any other router when propagating the Prefix-SID for the prefix to other areas.

Segment Routing for External Prefixes AS-External-LSAs are flooded domain wide. When an ASBR, which supports SR, originates an E-AS-External-LSA, it SHOULD also include a Prefix-SID sub-TLV as described in . The Prefix-SID value will be set to the SID that has been reserved for that prefix. When a Not-So-Stubby Area (NSSA) ABR translates an E-NSSA-LSA into an E-AS-External-LSA, it SHOULD also advertise the Prefix-SID for the prefix. The NSSA ABR determines its best path to the prefix advertised in the translated E-NSSA-LSA and finds the advertising router associated with that path. If the advertising router has advertised a Prefix-SID for the prefix, then the NSSA ABR uses it when advertising the Prefix-SID for the E-AS-External-LSA. Otherwise, the Prefix-SID advertised by any other router will be used.

Advertisement of Adj-SID The Adjacency Segment Routing Identifier (Adj-SID) is advertised using the Adj-SID sub-TLV as described in .

Advertisement of Adj-SID on Point-to-Point Links An Adj-SID MAY be advertised for any adjacency on a point-to-point (P2P) link that is in neighbor state 2-Way or higher. If the adjacency on a P2P link transitions from the FULL state, then the Adj-SID for that adjacency MAY be removed from the area. If the adjacency transitions to a state lower than 2-Way, then the Adj-SID Advertisement MUST be withdrawn from the area.

Adjacency SID on Broadcast or NBMA Interfaces Broadcast, NBMA, or hybrid networks in OSPFv3 are represented by a star topology where the DR is the central point to which all other routers on the broadcast, NBMA, or hybrid network connect. As a result, routers on the broadcast, NBMA, or hybrid network advertise only their adjacency to the DR. Routers that do not act as DR do not form or advertise adjacencies with each other. They do, however, maintain 2-Way adjacency state with each other and are directly reachable. When Segment Routing is used, each router on the broadcast, NBMA, or hybrid network MAY advertise the Adj-SID for its adjacency to the DR using the Adj-SID sub-TLV as described in . SR-capable routers MAY also advertise a LAN Adjacency SID for other neighbors (e.g., Backup Designated Router (BDR), DR-OTHER, etc.) on the broadcast, NBMA, or hybrid network using the LAN Adj-SID sub-TLV as described in .

IANA Considerations This specification updates two existing OSPFv3 registries.

"OSPFv3 Extended-LSA TLVs" Registry The following values have been allocated: OSPFv3 Extended-LSA TLVs

Value	Description	Reference
9	OSPFv3 Extended Prefix Range TLV	This document

"OSPFv3 Extended-LSA Sub-TLVs" Registry The following values have been allocated: OSPFv3 Extended-LSA Sub-TLVs

Value	Description	Reference
4	Prefix-SID sub-TLV	This document
5	Adj-SID sub-TLV	This document
6	LAN Adj-SID sub-TLV	This document
7	SID/Label sub-TLV	This document

TLV/Sub-TLV Error Handling For any new TLVs/sub-TLVs defined in this document, if the length is invalid, the LSA in which it is advertised is considered malformed and MUST be ignored. Errors SHOULD be logged subject to rate limiting.

Security Considerations With the OSPFv3 Segment Routing extensions defined herein, OSPFv3 will now program the MPLS data plane . Previously, LDP or another label distribution mechanism was required to advertise MPLS labels and program the MPLS data plane. In general, the same types of attacks that can be carried out on the IP control plane can be carried out on the MPLS control plane resulting in traffic being misrouted in the respective data planes. However, the latter can be more difficult to detect and isolate. Existing security extensions, as described in and , apply to these Segment Routing extensions. While OSPFv3 is under a single administrative domain, there can be deployments where potential attackers have access to one or more networks in the OSPFv3 routing domain. In these deployments, stronger authentication mechanisms, such as those specified in or , SHOULD be used. Implementations MUST ensure that malformed TLVs and sub-TLVs defined in this document are detected and that they do not provide a vulnerability for attackers to crash the OSPFv3 router or routing process. Reception of a malformed TLV or sub-TLV SHOULD be counted and/or logged for further analysis. Logging of malformed TLVs and sub-TLVs SHOULD be rate limited to prevent a Denial-of-Service (DoS) attack (distributed or otherwise) from overloading the OSPFv3 control plane.

References Normative References IANA In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies the architecture for Multiprotocol Label Switching (MPLS). [STANDARDS-TRACK] This memo documents an optional type of Open Shortest Path First (OSPF) area that is somewhat humorously referred to as a "not-so-stubby" area (or NSSA). NSSAs are similar to the existing OSPF stub area configuration option but have the additional capability of importing AS external routes in a limited fashion. The OSPF NSSA Option was originally defined in RFC 1587. The functional differences between this memo and RFC 1587 are explained in Appendix F. All differences, while expanding capability, are backward-compatible in nature. Implementations of this memo and of RFC 1587 will interoperate. [STANDARDS-TRACK] The architecture for Multiprotocol Label Switching (MPLS) is described in RFC 3031. A fundamental concept in MPLS is that two Label Switching Routers (LSRs) must agree on the meaning of the labels used to forward traffic between and through them. This common understanding is achieved by using a set of procedures, called a label distribution protocol, by which one LSR informs another of label bindings it has made. This document defines a set of such procedures called LDP (for Label Distribution Protocol) by which LSRs distribute labels to support MPLS forwarding along normally routed paths. [STANDARDS-TRACK] This document describes the modifications to OSPF to support version 6 of the Internet Protocol (IPv6). The fundamental mechanisms of OSPF (flooding, Designated Router (DR) election, area support, Short Path First (SPF) calculations, etc.) remain unchanged. However, some changes have been necessary, either due to changes in protocol semantics between IPv4 and IPv6, or simply to handle the increased address size of IPv6. These modifications will necessitate incrementing the protocol version from version 2 to version 3. OSPF for IPv6 is also referred to as OSPF version 3 (OSPFv3). Changes between OSPF for IPv4, OSPF Version 2, and OSPF for IPv6 as described herein include the following. Addressing semantics have been removed from OSPF packets and the basic Link State Advertisements (LSAs). New LSAs have been created to carry IPv6 addresses and prefixes. OSPF now runs on a per-link basis rather than on a per-IP-subnet basis. Flooding scope for LSAs has been generalized. Authentication has been removed from the OSPF protocol and instead relies on IPv6's Authentication Header and Encapsulating Security Payload (ESP). Even with larger IPv6 addresses, most packets in OSPF for IPv6 are almost as compact as those in OSPF for IPv4. Most fields and packet- size limitations present in OSPF for IPv4 have been relaxed. In addition, option handling has been made more flexible. All of OSPF for IPv4's optional capabilities, including demand circuit support and Not-So-Stubby Areas (NSSAs), are also supported in OSPF for IPv6. [STANDARDS-TRACK] The early Multiprotocol Label Switching (MPLS) documents defined the form of the MPLS label stack entry. This includes a three-bit field called the "EXP field". The exact use of this field was not defined by these documents, except to state that it was to be "reserved for experimental use". Although the intended use of the EXP field was as a "Class of Service" (CoS) field, it was not named a CoS field by these early documents because the use of such a CoS field was not considered to be sufficiently defined. Today a number of standards documents define its usage as a CoS field. To avoid misunderstanding about how this field may be used, it has become increasingly necessary to rename this field. This document changes the name of the field to the "Traffic Class field" ("TC field"). In doing so, it also updates documents that define the current use of the EXP field. [STANDARDS-TRACK] This document describes a mechanism to model a broadcast network as a hybrid of broadcast and point-to-multipoint networks for purposes of OSPF operation. Neighbor discovery and maintenance as well as Link State Advertisement (LSA) database synchronization are performed using the broadcast model, but the network is represented using the point-to-multipoint model in the router-LSAs of the routers connected to it. This allows an accurate representation of the cost of communication between different routers on the network, while maintaining the network efficiency of broadcast operation. This approach is relatively simple and requires minimal changes to OSPF. This document updates both OSPFv2 (RFC 2328) and OSPFv3 (RFC 5340). [STANDARDS-TRACK] It is useful for routers in an OSPFv2 or OSPFv3 routing domain to know the capabilities of their neighbors and other routers in the routing domain. This document proposes extensions to OSPFv2 and OSPFv3 for advertising optional router capabilities. The Router Information (RI) Link State Advertisement (LSA) is defined for this purpose. In OSPFv2, the RI LSA will be implemented with an Opaque LSA type ID. In OSPFv3, the RI LSA will be implemented with a unique LSA type function code. In both protocols, the RI LSA can be advertised at any of the defined flooding scopes (link, area, or autonomous system (AS)). This document obsoletes RFC 4970 by providing a revised specification that includes support for advertisement of multiple instances of the RI LSA and a TLV for functional capabilities. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. OSPFv3 requires functional extension beyond what can readily be done with the fixed-format Link State Advertisement (LSA) as described in RFC 5340. Without LSA extension, attributes associated with OSPFv3 links and advertised IPv6 prefixes must be advertised in separate LSAs and correlated to the fixed-format LSAs. This document extends the LSA format by encoding the existing OSPFv3 LSA information in Type-Length-Value (TLV) tuples and allowing advertisement of additional information with additional TLVs. Backward-compatibility mechanisms are also described. This document updates RFC 5340, "OSPF for IPv6", and RFC 5838, "Support of Address Families in OSPFv3", by providing TLV-based encodings for the base OSPFv3 unicast support and OSPFv3 address family support. Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through an ordered list of instructions, called "segments". A segment can represent any instruction, topological or service based. A segment can have a semantic local to an SR node or global within an SR domain. SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain. SR can be directly applied to the MPLS architecture with no change to the forwarding plane. A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. SR can be applied to the IPv6 architecture, with a new type of routing header. A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header. The active segment is indicated by the Destination Address (DA) of the packet. The next active segment is indicated by a pointer in the new routing header. Informative References This document describes means and mechanisms to provide authentication/confidentiality to OSPFv3 using an IPv6 Authentication Header/Encapsulating Security Payload (AH/ESP) extension header. [STANDARDS-TRACK] Currently, OSPF for IPv6 (OSPFv3) uses IPsec as the only mechanism for authenticating protocol packets. This behavior is different from authentication mechanisms present in other routing protocols (OSPFv2, Intermediate System to Intermediate System (IS-IS), RIP, and Routing Information Protocol Next Generation (RIPng)). In some environments, it has been found that IPsec is difficult to configure and maintain and thus cannot be used. This document defines an alternative mechanism to authenticate OSPFv3 protocol packets so that OSPFv3 does not depend only upon IPsec for authentication. The OSPFv3 Authentication Trailer was originally defined in RFC 6506. This document obsoletes RFC 6506 by providing a revised definition, including clarifications and refinements of the procedures. The ability for a node to specify a forwarding path, other than the normal shortest path, that a particular packet will traverse, benefits a number of network functions. Source-based routing mechanisms have previously been specified for network protocols but have not seen widespread adoption. In this context, the term "source" means "the point at which the explicit route is imposed"; therefore, it is not limited to the originator of the packet (i.e., the node imposing the explicit route may be the ingress node of an operator's network). This document outlines various use cases, with their requirements, that need to be taken into account by the Source Packet Routing in Networking (SPRING) architecture for unicast traffic. Multicast use cases and requirements are out of scope for this document.

Contributors The following people gave a substantial contribution to the content of this document and should be considered coauthors: Clarence Filsfils Cisco Systems, Inc. Brussels Belgium Email: cfilsfil@cisco.com Hannes Gredler RtBrick Inc. Austria Email: hannes@rtbrick.com Rob Shakir Google, Inc. United States of America Email: robjs@google.com Wim Henderickx Nokia Belgium Email: wim.henderickx@nokia.com Jeff Tantsura Apstra, Inc. United States of America Email: jefftant.ietf@gmail.com Thanks to Acee Lindem for his substantial contribution to the content of this document. We would like to thank Anton Smirnov for his contribution as well.

Authors' Addresses Cisco Systems, Inc.

Eurovea Centre, Central 3 Pribinova Street 10 Bratislava 81109 Slovakia ppsenak@cisco.com

Cisco Systems, Inc.

stefano@previdi.net