| Open Shortest Path First IGP | P. Psenak, Ed. |
| Internet-Draft | S. Previdi, Ed. |
| Intended status: Standards Track | C. Filsfils |
| Expires: February 16, 2015 | Cisco Systems, Inc. |
| H. Gredler | |
| Juniper Networks, Inc. | |
| R. Shakir | |
| British Telecom | |
| W. Henderickx | |
| Alcatel-Lucent | |
| J. Tantsura | |
| Ericsson | |
| August 15, 2014 |
OSPF Extensions for Segment Routing
draft-ietf-ospf-segment-routing-extensions-02
Segment Routing (SR) allows for a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological sub-paths, called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF).
This draft describes the OSPF extensions required for Segment Routing.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 16, 2015.
Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
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Segment Routing (SR) allows for a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological sub-paths, 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 does not require any additional signalling (other than IGP extensions). For example, when used in MPLS networks, SR paths do not require any LDP or RSVP-TE signalling. However, SR can interoperate in the presence of LSPs established with RSVP or LDP.
This draft describes the OSPF extensions required for Segment Routing.
Segment Routing architecture is described in [I-D.filsfils-rtgwg-segment-routing].
Segment Routing use cases are described in [I-D.filsfils-rtgwg-segment-routing-use-cases].
Segment Routing defines various types of Segment Identifiers (SIDs): Prefix-SID, Adjacency-SID, LAN Adjacency SID and Binding SID.
For the purpose of the advertisements of various SID values, new Opaque LSAs [RFC5250] are defined in [I-D.ietf-ospf-prefix-link-attr]. These new LSAs are defined as generic containers that can be used to advertise any additional attributes associated with a prefix or link. These new Opaque LSAs are complementary to the existing LSAs and are not aimed to replace any of the existing LSAs.
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:
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 TLV has following format:
Segment Routing requires some additional router capabilities to be advertised to other routers in the area.
These SR capabilities are advertised in the Router Information Opaque LSA (defined in [RFC4970]).
The SR-Algorithm TLV is a top-level TLV of the Router Information Opaque LSA (defined in [RFC4970]).
The SR-Algorithm Sub-TLV is optional. It MAY only be advertised once in the Router Information Opaque LSA. If the SID/Label Range TLV, as defined in Section 3.2, is advertised, then SR-Algorithm TLV MUST also be advertised.
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm 1 | Algorithm... | Algorithm n | | +- -+ | | + + where:
An SR Router may use various algorithms when calculating reachability to OSPF routers or prefixes in an OSPF area. Examples of these algorithms are metric based Shortest Path First (SPF), various flavors of Constrained SPF, etc. The SR-Algorithm TLV allows a router to advertise the algorithms that the router is currently using to other routers in an OSPF area. The SR-Algorithm TLV has following format:
The RI LSA can be advertised at any of the defined opaque flooding scopes (link, area, or Autonomous System (AS)). For the purpose of the SR-Algorithm TLV propagation, area scope flooding is required.
The SID/Label Range TLV is a top-level TLV of the Router Information Opaque LSA (defined in [RFC4970]).
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Range Size | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | + + where:
The SID/Label Range TLV MAY appear multiple times and has the following format:
Initially, the only supported Sub-TLV is the SID/Label TLV as defined in Section 2.1. The SID/Label advertised in the SID/Label TLV represents the first SID/Label in the advertised range.
Multiple occurrence of the SID/Label Range TLV MAY be advertised, in order to advertise multiple ranges. In such case:
The originating router advertises following ranges:
Range 1: [100, 199]
Range 2: [1000, 1099]
Range 3: [500, 599]
The receiving routers concatenate the ranges and build the Segment Routing Global Block
(SRGB) is as follows:
SRGB = [100, 199]
[1000, 1099]
[500, 599]
The indexes span multiple ranges:
index=0 means label 100
...
index 99 means label 199
index 100 means label 1000
index 199 means label 1099
...
index 200 means label 500
...
The following example illustrates the advertisement of multiple ranges:
The RI LSA can be advertised at any of the defined flooding scopes (link, area, or autonomous system (AS)). For the purposes of the SR-Capability TLV propagation, area scope flooding is required.
In some cases it is useful to advertise attributes for the range of prefixes. Segment Routing Mapping Server, which is described in [I-D.filsfils-rtgwg-segment-routing] is an example, where we need a single advertisement to advertise SIDs for multiple prefixes from a contiguous address range.
OSPF Extended Prefix Range TLV, which is a new top level TLV of the Extended Prefix LSA described in [I-D.ietf-ospf-prefix-link-attr] is defined for this purpose.
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address Prefix (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | where:
Multiple OSPF Extended Prefix Range TLVs MAY be advertised in each OSPF Extended Prefix Opaque LSA, but all prefix ranges included in a single OSPF Extended Prefix Opaque LSA MUST have the same flooding scope. The OSPF 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Reserved | MT-ID | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Index/Label (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+
|N |NP|M |E |V |L | | |
+--+--+--+--+--+--+--+--+
where:
The Prefix SID Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV described in [I-D.ietf-ospf-prefix-link-attr] and the OSPF Extended Prefix Range TLV described in Section 4. It MAY appear more than once in the parent TLV and has the following format:
If multiple Prefix-SIDs are advertised for the same prefix, the receiving router MUST use the first encoded SID and MAY use the subsequent SIDs.
When propagating Prefix-SIDs between areas, if multiple prefix-SIDs are advertised for a prefix, an implementation SHOULD preserve the original order when advertising prefix-SIDs to other areas. This allows implementations that only support a single Prefix-SID to have a consistent view across areas.
When calculating the outgoing label for the prefix, the router MUST take into account E and P flags advertised by the next-hop router, if next-hop 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 on the Prefix-SIDs allocated to inter-area prefixes that are originated by the ABR based on intra-area or inter-area reachability between areas. When the inter-area prefix is generated based on the prefix which is directly attached to the ABR, NP-Flag SHOULD NOT be set
The NP-Flag (No-PHP) MUST be be set on the Prefix-SIDs allocated to redistributed prefixes, unless the redistributed prefix is directly attached to ASBR, in which case the NP-flag SHOULD NOT be set.
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 dataplane. In such case, MPLS EXP bits of the Prefix-SID are not preserved for the final destination (the Prefix-SID being removed). If the NP-flag is clear then the received E-flag is ignored.
If the NP-flag is set then:
When M-Flag is set, NP-flag MUST be set and E-bit MUST NOT be set.
When a Prefix-SID is advertised in an Extended Prefix Range TLV, then the value advertised in Prefix SID Sub-TLV is interpreted as a starting SID value.
Router-A: 192.0.2.1/32, Prefix-SID: Index 1
Router-B: 192.0.2.2/32, Prefix-SID: Index 2
Router-C: 192.0.2.3/32, Prefix-SID: Index 3
Router-D: 192.0.2.4/32, Prefix-SID: Index 4
Example 1: if the following router addresses (loopback addresses) need to be mapped into the corresponding Prefix SID indexes:
then the Prefix field in the Extended Prefix Range TLV would be set to 192.0.2.1, Prefix Length would be set to 32, Range Size would be set to 4 and the Index value in the Prefix-SID Sub-TLV would be set to 1.
10.1.1/24, Prefix-SID: Index 51
10.1.2/24, Prefix-SID: Index 52
10.1.3/24, Prefix-SID: Index 53
10.1.4/24, Prefix-SID: Index 54
10.1.5/24, Prefix-SID: Index 55
10.1.6/24, Prefix-SID: Index 56
10.1.7/24, Prefix-SID: Index 57
Example 2: If the following prefixes need to be mapped into the corresponding Prefix-SID indexes:
then the Prefix field in the Extended Prefix Range TLV would be set to 10.1.1.0, Prefix Length would be set to 24, Range Size would be 7 and the Index value in the Prefix-SID Sub-TLV would be set to 51.
The SID/Label Binding Sub-TLV is used to advertise a SID/Label mapping for a path to the prefix.
The SID/Label Binding TLV MAY be originated by any router in an OSPF domain. The router may advertise a SID/Label binding to a FEC along with at least a single 'nexthop style' anchor. The protocol supports more than one 'nexthop style' anchor to be attached to a SID/Label binding, which results in a simple path description language. In analogy to RSVP, the terminology for this is called an 'Explicit Route Object' (ERO). Since ERO style path notation allows anchoring SID/label bindings to both link and node IP addresses, any Label Switched Path (LSP) can be described. Additionally, SID/Label Bindings from external protocols can be easily re-advertised.
The SID/Label Binding TLV may be used for advertising SID/Label Bindings and their associated Primary and Backup paths. In a single TLV, a primary ERO Path, backup ERO Path, or both can be advertised. If a router wants to advertise multiple parallel paths, then it can generate several TLVs for the same Prefix/FEC. Each occurrence of a Binding TLV for a given FEC Prefix will add a new path.
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 | Reserved | MT-ID | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | where:
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |M| | +-+-+-+-+-+-+-+-+
The SID/Label Binding Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV described in [I-D.ietf-ospf-prefix-link-attr] and the OSPF Extended Prefix Range TLV described in Section 4. Multiple SID/Label Binding TLVs can be present in their parent TLV. The SID/Label Binding Sub-TLV has following format:
The SID/Label Binding TLV supports the following Sub-TLVs:
The ERO Metric Sub-TLV is a Sub-TLV of the SID/Label Binding TLV.
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric (4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ERO Metric Sub-TLV format
The ERO Metric Sub-TLV advertises the cost of an ERO path. It is used to compare the cost of a given source/destination path. A router SHOULD advertise the ERO Metric Sub-TLV in an advertised ERO TLV. The cost of the ERO Metric Sub-TLV SHOULD be set to the cumulative IGP or TE path cost of the advertised ERO. Since manipulation of the Metric field may attract or repel traffic to and from the advertised segment, it MAY be manually overridden.
All 'ERO' information represents an ordered set which describes the segments of a path. The first ERO Sub-TLV describes the first segment of a path. Similiarly, the last ERO Sub-TLV describes the segment closest to the egress point. If a router extends or stitches a path, it MUST prepend the new segment's path information to the ERO list. This applies equally to advertised backup EROs.
All ERO Sub-TLVs must immediately follow the (SID)/Label Sub-TLV.
All Backup ERO Sub-TLVs must immediately follow the last ERO Sub-TLV.
IPv4 ERO Sub-TLV is a Sub-TLV of the SID/Label Binding Sub-TLV.
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 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 Address (4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv4 ERO Sub-TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |L| | +-+-+-+-+-+-+-+-+
The IPv4 ERO Sub-TLV describes a path segment using IPv4 Address style encoding. Its semantics have been borrowed from [RFC3209].
The Unnumbered Interface ID ERO Sub-TLV is a Sub-TLV of the SID/Label Binding Sub-TLV.
The appearance and semantics of the 'Unnumbered Interface ID' have been borrowed from [RFC3477].
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 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
Unnumbered Interface ID ERO Sub-TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |L| | +-+-+-+-+-+-+-+-+ where:
The Unnumbered Interface-ID ERO Sub-TLV describes a path segment that includes an unnumbered interface. Unnumbered interfaces are referenced using the interface index. Interface indices are assigned local to the router and therefore not unique within a domain. All elements in an ERO path need to be unique within a domain and hence need to be disambiguated using a domain unique Router-ID.
IPv4 Prefix Backup ERO Sub-TLV is a Sub-TLV of the SID/Label Binding Sub-TLV.
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 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 Address (4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv4 Backup ERO Sub-TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |L| | +-+-+-+-+-+-+-+-+
The IPv4 Backup ERO Sub-TLV describes a path segment using IPv4 Address style of encoding. Its semantics have been borrowed from [RFC3209].
The Unnumbered Interface ID Backup ERO Sub-TLV is a Sub-TLV of the SID/Label Binding Sub-TLV.
The appearance and semantics of the 'Unnumbered Interface ID' have been borrowed from [RFC3477].
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 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unnumbered Interface ID Backup ERO Sub-TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |L| | +-+-+-+-+-+-+-+-+
The Unnumbered Interface-ID Backup ERO Sub-TLV describes a path segment that includes an unnumbered interface. Unnumbered interfaces are referenced using the interface index. Interface indices are assigned local to the router and are therefore not unique within a domain. All elements in an ERO path need to be unique within a domain and hence need to be disambiguated with specification of the domain unique Router-ID.
An Adjacency Segment Identifier (Adj-SID) represents a router adjacency in Segment Routing.
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 | Reserved | MT-ID | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Label/Index (variable) | +---------------------------------------------------------------+ where:
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |B|V|L|S| | +-+-+-+-+-+-+-+-+ where:
Adj-SID is an optional Sub-TLV of the Extended Link TLV defined in [I-D.ietf-ospf-prefix-link-attr]. It MAY appear multiple times in the Extended Link TLV. Examples where more than one Adj-SID may be used per neighbor are described in [I-D.filsfils-rtgwg-segment-routing-use-cases]. The Adj-SID Sub-TLV has the following format:
An SR capable router MAY allocate an Adj-SID for each of its adjacencies and set the B-Flag when the adjacency is protected by an FRR mechanism (IP or MPLS) as described in [I-D.filsfils-rtgwg-segment-routing-use-cases].
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 | Reserved | MT-ID | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Neighbor ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Label/Index (variable) | +---------------------------------------------------------------+ where:
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |B|V|L|S| | +-+-+-+-+-+-+-+-+ where:
LAN Adj-SID is an optional Sub-TLV of the Extended Link TLV defined in [I-D.ietf-ospf-prefix-link-attr]. It MAY appear multiple times in the Extended-Link TLV. It is used to advertise a SID/Label for an adjacency to a non-DR node on a broadcast or NBMA network.
An OSPFv2 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 Section 5).
If multiple routers advertise a Prefix-SID for the same prefix, then the Prefix-SID MUST be the same. This is required in order to allow traffic load-balancing when multiple equal cost paths to the destination exist in the network.
Prefix-SID can also be advertised by the SR Mapping Servers (as described in [I-D.filsfils-rtgwg-segment-routing-use-cases]). The Mapping Server advertises Prefix-SIDs for remote prefixes that exist in the OSPFv2 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 flooding scope of the OSPF Extended Prefix Opaque LSA that is generated by the SR Mapping Server could be either area scoped or AS scoped and is determined based on the configuration of the SR Mapping Server.
In order to support SR in a multi-area environment, OSPFv2 must propagate Prefix-SID information between areas. The following procedure is used in order to propagate Prefix SIDs between areas.
When an OSPF ABR advertises a Type-3 Summary LSA from an intra-area prefix to all its connected areas, it will also originate an Extended Prefix Opaque LSA, as described in [I-D.ietf-ospf-prefix-link-attr]. The flooding scope of the Extended Prefix Opaque LSA type will be set to area-scope. The route-type in the OSPF Extended Prefix TLV is set to inter-area. The Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID value will be set as follows:
When an OSPF ABR advertises Type-3 Summary LSAs from an inter-area route to all its connected areas it will also originate an Extended Prefix Opaque LSA, as described in [I-D.ietf-ospf-prefix-link-attr]. The flooding scope of the Extended Prefix Opaque LSA type will be set to area-scope. The route-type in OSPF Extended Prefix TLV is set to inter-area. The Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID will be set as follows:
Type-5 LSAs are flooded domain wide. When an ASBR, which supports SR, generates Type-5 LSAs, it should also originate an Extended Prefix Opaque LSAs, as described in [I-D.ietf-ospf-prefix-link-attr]. The flooding scope of the Extended Prefix Opaque LSA type is set to AS-scope. The route-type in the OSPF Extended Prefix TLV is set to external. The Prefix-SID Sub-TLV is included in this LSA and the Prefix-SID value will be set to the SID that has been reserved for that prefix.
When an NSSA ABR translates Type-7 LSAs into Type-5 LSAs, it should also advertise the Prefix-SID for the prefix. The NSSA ABR determines its best path to the prefix advertised in the translated Type-7 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 Type-5 prefix. Otherwise, the Prefix-SID advertised by any other router will be used (e.g.: a Prefix-SID coming from an SR Mapping Server as defined in [I-D.filsfils-rtgwg-segment-routing-use-cases]).
The Adjacency Segment Routing Identifier (Adj-SID) is advertised using the Adj-SID Sub-TLV as described in Section 7.
An Adj-SID MAY be advertised for any adjacency on a 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 then 2-Way, then the Adj-SID advertisement MUST be removed from the area.
Broadcast or NBMA networks in OSPF are represented by a star topology where the Designated Router (DR) is the central point to which all other routers on the broadcast or NBMA network connect. As a result, routers on the broadcast or NBMA 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 or NBMA network MAY advertise the Adj-SID for its adjacency to the DR using Adj-SID Sub-TLV as described in Section 7.1.
SR capable routers MAY also advertise an Adj-SID for other neighbors (e.g. BDR, DR-OTHER) on the broadcast or NBMA network using the LAN ADJ-SID Sub-TLV as described in Section 7.2.
This specification updates several existing OSPF registries.
o 8 (IANA Preallocated) - SR-Algorithm TLV
o 9 (IANA Preallocated) - SID/Label Range TLV
Following values are allocated:
o 2 - OSPF Extended Prefix Range TLV
Following values are allocated:
o 1 - SID/Label Sub-TLV
o 2 - Prefix SID Sub-TLV
o 3 - SID/Label Binding Sub-TLV
o 4 - IPv4 ERO Sub-TLV
o 5 - Unnumbered Interface ID ERO Sub-TLV
o 6 - IPv4 Backup ERO Sub-TLV
o 7 - Unnumbered Interface ID Backup ERO Sub-TLV
o 8 - ERO Metric Sub-TLV
Following initial values are allocated:
o 1 - SID/Label Sub-TLV
o 2 - Adj-SID Sub-TLV
o 3 - LAN Adj-SID/Label Sub-TLV
Implementations must assure that malformed TLV and Sub-TLV permutations do not result in errors which cause hard OSPF failures.
The following people gave a substantial contribution to the content of this document: Acee Lindem, Ahmed Bashandy, Martin Horneffer, Bruno Decraene, Stephane Litkowski, Igor Milojevic, Rob Shakir and Saku Ytti.
We would like to thank Anton Smirnov for his contribution.
Many thanks to Yakov Rekhter, John Drake and Shraddha Hedge for their contribution on earlier incarnations of the "Binding / MPLS Label TLV" in [I-D.gredler-ospf-label-advertisement].
Thanks to Acee Lindem for the detail review of the draft, corrections, as well as discussion about details of the encoding.
| [I-D.filsfils-rtgwg-segment-routing] | Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W., Tantsura, J. and E. Crabbe, "Segment Routing Architecture", Internet-Draft draft-filsfils-rtgwg-segment-routing-01, October 2013. |
| [I-D.filsfils-rtgwg-segment-routing-use-cases] | Filsfils, C., Francois, P., Previdi, S., Decraene, B., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W., Tantsura, J., Kini, S. and E. Crabbe, "Segment Routing Use Cases", Internet-Draft draft-filsfils-rtgwg-segment-routing-use-cases-02, October 2013. |
| [I-D.gredler-ospf-label-advertisement] | Gredler, H., Amante, S., Scholl, T. and L. Jalil, "Advertising MPLS labels in OSPF", Internet-Draft draft-gredler-ospf-label-advertisement-03, May 2013. |
| [I-D.ietf-ospf-prefix-link-attr] | Psenak, P., Gredler, H., Shakir, R., Henderickx, W., Tantsura, J. and A. Lindem, "OSPFv2 Prefix/Link Attribute Advertisement", Internet-Draft draft-ietf-ospf-prefix-link-attr-00, August 2014. |
| [I-D.minto-rsvp-lsp-egress-fast-protection] | Jeganathan, J., Gredler, H. and Y. Shen, "RSVP-TE LSP egress fast-protection", Internet-Draft draft-minto-rsvp-lsp-egress-fast-protection-03, November 2013. |