| Open Shortest Path First IGP | P. Psenak, Ed. |
| Internet-Draft | S. Previdi, Ed. |
| Intended status: Standards Track | C. Filsfils |
| Expires: January 03, 2014 | Cisco Systems, Inc. |
| H. Gredler | |
| Juniper Networks, Inc. | |
| R. Shakir | |
| British Telecom | |
| July 02, 2013 |
OSPF Extensions for Segment Routing
draft-psenak-ospf-segment-routing-extensions-01
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 necessary OSPF extensions that need to be introduced 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/.
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This Internet-Draft will expire on January 03, 2014.
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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 of the cases, is a one-hop path. SR's control-plane can be applied to both IPv6 and MPLS data-planes, and do not require any additional signaling (other than the regular IGP). For example, when used in MPLS networks, SR paths do not require any LDP or RSVP-TE signaling. Still, SR can interoperate in the presence of LSPs established with RSVP or LDP .
This draft describes the necessary OSPF extensions that need to be introduced 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 (defined in [RFC5250]) are defined. These new LSAs are defined as generic containers that can be used in order to advertise any additional attributes associated with the 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:
SID/Label TLV appears as Sub-TLV in multiple TLVs or Sub-TLVs defined later in this document. It is used to advertise SID or label associated with the prefix or adjacency. SID/Lable TLV has following format:
Segment Routing requires some additional capabilities of the router to be advertised to other routers in the area.
These SR capabilities are advertised in Router Information Opaque LSA (defined in [RFC4970]).
SR-Algorithm TLV is a TLV of 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm 1 | Algorithm... | Algorithm n | | +- -+ | | + + where:
Router may use various algorithms when calculating reachability to other nodes in area or to prefixes attached to these nodes. Examples of these algorithms are metric based Shortest Path First (SPF), various sorts of Constrained SPF, etc. SR-Algorithm TLV allows a router to advertise algorithms that router is currently using to other routers in an area. SR-Algorithm TLV has following structure:
RI LSA can be advertised at any of the defined 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 TLV of 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:
SID/Label Sub-TLV MAY appear multiple times and has following format:
Currently the only supported Sub-TLV is the SID/Label TLV as defined in Section 2.1. SID/Label advertised in SID/Label TLV represents the first SID/Label from the advertised range.
RI LSA can be advertised at any of the defined flooding scopes (link, area, or autonomous system (AS)). For the purpose of the SR-Capability TLV propagation area scope flooding is required.
A new Opaque LSA (defined in [RFC5250]) is defined in OSPFv2 in order to advertise additional prefix attributes: OSPFv2 Extended Prefix Opaque LSA.
Multiple OSPFv2 Extended Prefix Opaque LSAs can be advertised by a single router. Flooding scope of the OSPFv2 Extended Prefix Opaque LSA depends on the content inside the LSA and is in control of the originating router.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 9, 10, or 11 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque type | Instance | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +- TLVs -+ | ... |
The format of the OSPFv2 Extended Prefix Opaque LSA is as follows:
Opaque type used by OSPFv2 Extended Prefix Opaque LSA is TBA.
The format of the TLVs within the body of the LSA is the same as the format used by the Traffic Engineering Extensions to OSPF defined in [RFC3630]. The LSA payload consists of one or more nested Type/Length/Value (TLV) triplets. The format of each TLV is:
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length field defines the length of the value portion in octets. The TLV is padded to 4-octet alignment; padding is not included in the length field. Nested TLVs are also 32-bit aligned. Unrecognized types are ignored.
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Route Type | Prefix Length | AF | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address Prefix (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | where:
0 - unspecified
1 - intra-area
3 - inter-area
5 - external
7 - NSSA external
The OSPF Extended Prefix TLV is used in order to advertise additional attributes associated with the prefix. Multiple OSPF Extended Prefix TLVs MAY be carried in each OSPFv2 Extended Prefix Opaque LSA, however all prefixes included in the single OSPFv2 Extended Prefix Opaque LSA MUST have the same flooding scope. The structure of the OSPF Extended Prefix TLV is as follows:
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 | MT-ID | Algorithm | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
0 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |N|P|M| | +-+-+-+-+-+-+-+-+ where:
The Prefix SID Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV. It MAY appear more than once and has 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 ones.
PHP flag MUST NOT be set on the Prefix-SIDs allocated to inter- area prefixes that are originated by the router based on intra-area or inter-area reachability between areas.
SID/Label Binding TLV is used to advertise SID/Label mapping for a prefix or a path to the prefix. SID/Label value advertised in this TLV has local significance (to the router).
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 | MT-ID | Weight | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | where:
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |M| | +-+-+-+-+-+-+-+-+
SID/Label Binding TLV is a Sub-TLV of the OSPF Extended Prefix TLV. Multiple SID/Label Binding TLVs can be present in OSPF Extended Prefix TLV. SID/Label Binding TLV has following format:
SID/Label Binding TLV currently supports following Sub-TLVs:
All 'ERO' information represents an ordered set which describes the segments of a path. The last ERO TLV describes the segment closest to the egress point, contrary the first ERO TLV describes the first segment of a path. If a router extends or stitches a path it MUST prepend the new segments path information to the ERO list.
The above similarly applies to backup EROs.
All ERO Sub-TLVs must immediately follow the (SID)/Label Sub-TLV.
All Backup ERO TLVs must immediately follow last ERO Sub-TLV.
IPv4 ERO TLV is a Sub-TLV of the SID/Lable 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 Address (4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv4 ERO TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |L| | +-+-+-+-+-+-+-+-+
The IPv4 ERO TLV describes a path segment using IPv4 Address style of encoding. Its semantics have been borrowed from [RFC3209].
Unnumbered Interface ID ERO TLV is a Sub-TLV of the SID/Lable Binding 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 TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |L| | +-+-+-+-+-+-+-+-+ where:
The Unnumbered Interface-ID ERO TLV describes a path segment that spans over 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 TLV is a Sub-TLV of the SID/Lable 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 Address (4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv4 Backup ERO TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |L| | +-+-+-+-+-+-+-+-+
The IPv4 Backup ERO TLV describes a path segment using IPv4 Address style of encoding. Its semantics have been borrowed from [RFC3209].
Unnumbered Interface ID Backup TLV is a Sub-TLV of the SID/Lable Binding 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 TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |L| | +-+-+-+-+-+-+-+-+
The Unnumbered Interface-ID ERO TLV describes a path segment that spans over 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.
An Adjacency Segment Identifier (Adj-SID) represents a router adjacency in Segment Routing. At the current stage of Segment Routing architecture it is assumed that the Adj-SID value has local significance (to the router).
A new Opaque LSA (defined in [RFC5250] is defined in OSPFv2 in order to advertise additional link attributes: the OSPFv2 Extended Link Opaque LSA.
The OSPFv2 Extended Link Opaque LSA has an area flooding scope. Multiple OSPFv2 Extended Link Opaque LSAs can be advertised by a single router in an area.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 10 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque type | Instance | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +- TLVs -+ | ... |
The format of the OSPFv2 Extended Link Opaque LSA is as follows:
Opaque type used by OSPFv2 Extended Link Opaque LSA is TBA
The format of the TLVs within the body of LSA is the same as the format used by the Traffic Engineering Extensions to OSPF defined in [RFC3630]. The LSA payload consists of one or more nested Type/Length/Value (TLV) triplets. The format of each TLV is:
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length field defines the length of the value portion in octets. The TLV is padded to 4-octet alignment; padding is not included in the length field. Nested TLVs are also 32-bit aligned. Unrecognized types are ignored.
OSPFv2 Extended Link TLV is used in order to advertise various attributes of the link. It describes a single link and is constructed of a set of Sub-TLVs. There are no ordering requirements for the Sub-TLVs. Only one Extended Link TLV SHALL be carried in each Extended Link Opaque LSA, allowing for fine granularity changes in the topology.
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link-Type | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | where:
The Extended Link TLV has 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 | MT-ID | Weight | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | where:
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |B| | +-+-+-+-+-+-+-+-+ where:
Adj-SID is an optional Sub-TLV of the Extended Link TLV. It MAY appear multiple times in 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 structure of the Adj-SID Sub-TLV is as follows:
A SR capable router MAY allocate an Adj-SID for each of its adjacencies and set the B-Flag when the adjacency is protected by a 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 | MT-ID | Weight | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Neighbor ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) | +- -+ | | where:
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |B| | +-+-+-+-+-+-+-+-+ where:
LAN Adj-SID is an optional Sub-TLV of the Extended Link TLV. It MAY appear multiple times in Extended Link TLV. It is used to advertise SID/Label for adjacency to non-DR node on broadcast or NBMA network.
The OSPFv2 node that supports segment routing MAY advertise Prefix- SIDs for any prefix that it is advertising reachability for (e.g. loopback IP address) as described in Section 4.2.
If multiple routers advertise Prefix-SID for the same prefix, then the Prefix-SID MUST be the same. This is required in order to allow traffic load-balancing if 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 advertise Prefix-SID for remote prefixes that exist in the network. Multiple Mapping Servers can advertise Prefix-SID for the same prefix, in which case the same Prefix-SID MUST be advertised by all of them. Flooding scope of the OSPF Extended Prefix Opaque LSA that is generated by the SR Mapping Server could be either area scope or autonomous system scope and is decided 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 Section 4. 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 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 Section 4. 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 Extended Prefix Opaque LSAs, as described in Section 4. The flooding scope of the Extended Prefix Opaque LSA type is set to AS-scope. The route-type in OSPF Extended Prefix TLV is set to external. 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 a NSSA ASBR 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 such path. If such advertising router has advertised a Prefix-SID for the prefix, then the NSSA ASBR 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 5.
Adj-SID MAY be advertised for any adjacency on p2p link that is in a 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 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 all other routers on the broadcast or NBMA network connect to. As a result, routers on the broadcast or NBMA network advertise only their adjacency to DR and BDR. Routers that are neither DR nor BDR do not form and do not advertise adjacencies between them. They, however, maintain a 2-Way adjacency state between them.
When Segment Routing is used, each router on the broadcast or NBMA network MAY advertise the Adj-SID for its adjacency to DR using Adj-SID Sub-TLV as described in Section 5.3.
SR capable router MAY also advertise Adj-SID for other neighbors (e.g. BDR, DR-OTHER) on broadcast or NBMA network using the LAN ADJ-SID Sub-TLV as described in section 5.1.1.2. Section 5.4.
TBD
TBD
TBD
We would like to thank Anton Smirnov and Wim Hendricks.
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].
| [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-00, June 2013. |
| [I-D.filsfils-rtgwg-segment-routing-use-cases] | 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 Use Cases", Internet-Draft draft-filsfils-rtgwg-segment-routing-use-cases-00, June 2013. |
| [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-02, April 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. |