Network Working Group P. Psenak, Ed.
Internet-Draft S. Previdi, Ed.
Intended status: Standards Track C. Filsfils
Expires: December 30, 2013 Cisco Systems, Inc.
June 28, 2013
OSPF Extensions for Segment Routing
draft-psenak-ospf-segment-routing-extensions-00
Abstract
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.
Requirements Language
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].
Status of this Memo
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 December 30, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. OSPFv2 Segment Routing Capability TLV . . . . . . . . . . . . 3
2.1. SID Range Sub-TLV . . . . . . . . . . . . . . . . . . . . 5
2.2. Segment Routing Mirroring Context Sub-TLV (SRMC) . . . . . 5
3. Segment Routing Identifiers . . . . . . . . . . . . . . . . . 6
3.1. OSPFv2 Extended Prefix Opaque LSA type . . . . . . . . . . 7
3.1.1. OSPF Extended Prefix TLV . . . . . . . . . . . . . . . 8
3.1.2. Prefix SID Sub-TLV . . . . . . . . . . . . . . . . . . 8
3.2. Adjacency Segment Identifier (Adj-SID) . . . . . . . . . . 10
3.2.1. OSPFv2 Extended Link Opaque LSA . . . . . . . . . . . 10
3.2.2. OSPFv2 Extended Link TLV . . . . . . . . . . . . . . . 11
3.2.3. Adj-SID sub-TLV . . . . . . . . . . . . . . . . . . . 12
3.2.4. Explicit Path sub-TLV . . . . . . . . . . . . . . . . 14
4. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 15
4.1. Intra-area Segment routing in OSPFv2 . . . . . . . . . . . 15
4.2. Inter-area Segment routing in OSPFv2 . . . . . . . . . . . 16
4.3. SID for External Prefixes . . . . . . . . . . . . . . . . 17
4.4. Advertisement of Adj-SID . . . . . . . . . . . . . . . . . 17
4.4.1. Advertisement of Adj-SID on Point-to-Point Links . . . 17
4.4.2. Adjacency SID on Broadcast Interfaces . . . . . . . . 17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6. Manageability Considerations . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
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). Two
types of segments are defined, Prefix segments and Adjacency
segments. 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
[draft-filsfils-rtgwg-segment-routing-00].
Segment Routing use cases are described in
[draft-filsfils-rtgwg-segment-routing-use-cases-00].
2. OSPFv2 Segment Routing Capability TLV
Segment Routing requires each router to advertise various
capabilities associated with Segment Routing throughout the
autonomous system.
For the purposes of Segment Routing, a new TLV is defined in Router
Information Opaque LSA (defined in [RFC4970]: the Segment Routing
Capability TLV (SR-Cap TLV).
If the SR-Cap TLV appear in Router Information Opaque LSA, it MUST
appear only once and has following format:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SR Capability Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Sub-TLVs (variable) -+
| ... |
where:
Type: TBA.
Length: A 16-bit field that indicates the length of the value
portion in octets. Set to 12.
SR Capabilities Flags: 2 octets field of following flags:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|X| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
R-flag: SR Control-Plane flag. If set, then the advertising
router is capable of SR control-plane.
X-Flag: Index flag. If set, then each SID with the G-Flag set
that is advertised by this router represents an index in the
SID space also advertised by this router (using the encodings
defined below). Indexed SID values are described in
[draft-filsfils-rtgwg-segment-routing-00]. If the X-Flag is
not set, then SID values 0-63 are reserved and MUST NOT be used
by the SR control plane for either node or adjacency segments.
Any SID which is received with a value 0-63 and the X-Flag
unset MUST be ignored and the router SHOULD log an error.
Other bits: MUST be zero when sent and ignored when received.
RI LSA can be advertised at any of the defined flooding scopes (link,
area, or autonomous system (AS)). For the purpose of the SR-Cap TLV
propagation, autonomous system scope flooding is required.
Within the SR-Cap TLV, new Sub-TLVs are defined:
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SID Range Sub-TLV
Segment Routing Mirroring Context Sub-TLV (SRMC)
2.1. SID Range Sub-TLV
The SID Range Sub-TLV allows a router to advertise its SID space to
other routers in the area.
The SID Range Sub-TLV is a Sub-TLV of the SR-Cap-TLV. It MUST appear
only once and 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First SID Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last SID Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
First SID Value and Last SID Value represent the local SID range
of the router. The semantic and procedures of these values are
described in [draft-filsfils-rtgwg-segment-routing-use-cases-00].
2.2. Segment Routing Mirroring Context Sub-TLV (SRMC)
Segment Routing allows the use of a context SID that determines a
specific use case or applicability. Use cases are described in
[draft-filsfils-rtgwg-segment-routing-use-cases-00]. One of such use
cases is the service mirroring, for which the new sub-TLV is defined.
Another example of mirroring use case is described in
[I-D.minto-rsvp-lsp-egress-fast-protection].
The SRMC Sub-TLV is an optional sub-TLV of SR-Cap-TLV. It MAY appear
in SR-Cap-TLV more than once and has following format:
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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 | SRMC Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mirrored Address (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Context-SID (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: TBA.
Length: 2 octets.
SRMC Flags: 2 octets field of flags. Following flags are defined:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
R-flag: Segment Routing Identifier. If set, the Mirrored
Address contains a SID.
Other bits: MUST be zero when sent and ignored when received.
Mirrored Address: IPv4 address or SID value representing the
mirrored element.
Context-SID: 32 bit value of Segment Identifier for the Mirroring
Service.
3. Segment Routing Identifiers
Segment Routing defines two types of Segment Identifiers: Prefix-SID
and Adjacency-SID.
For the purpose of the Prefix-SID and Adjacency-SID advertisement 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.
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3.1. OSPFv2 Extended Prefix Opaque LSA type
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.
The format of the OSPFv2 Extended Prefix Opaque LSA 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 9, 10, or 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque type | Instance |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
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.
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3.1.1. OSPF Extended Prefix TLV
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Type | Prefix Length | AF | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs (variable) |
+- -+
| |
Type is TBA.
Length is variable
Route type: type of the OSPF route. Supported types are:
1 - intra-area
3 - inter-area
5 - external
7 - NSSA external
Prefix length: length of the prefix
AF:
0 - IPv4 unicast
Address Prefix: the prefix itself encoded as an even multiple of
32-bit words, padded with zeroed bits as necessary. This encoding
consumes ((PrefixLength + 31) / 32) 32-bit words. The default
route is represented by a prefix of length 0.
3.1.2. Prefix SID Sub-TLV
The Prefix SID Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV.
It may MAY appear more than once for the same prefix and has
following format:
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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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix SID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type is TBA.
Length: A 16-bit field that indicates the length of the value
portion in octets. Set to 8.
Flags: 2 octets field. The following flags are defined:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|N|P|G|M| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
N-Flag: Node-SID flag. If set, then the Prefix-SID refers to
the router identified by the prefix. Typically, the N-Flag is
set on Prefix-SIDs attached to a router loopback address. The
N-Flag is set when the Prefix-SID is a Node- SID as described
in [draft-filsfils-rtgwg-segment-routing-00].
P-Flag: no-PHP flag. If set, then the penultimate hop MUST NOT
pop the Prefix-SID before delivering the packet to the node
that advertised the Prefix-SID.
G-Flag: Global flag. When set, the SID value has global
significance which means the SID has been allocated from the
Segment Routing Global Block (SRGB) as described in
[draft-filsfils-rtgwg-segment-routing-00]. When unset, the SID
value has local (within the router) significance which means
the SID has NOT been allocated from the SRGB. When the Prefix-
SID Sub-TLV carries an IGP Prefix SID, then the G-flag MUST be
set.
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M-Flag: Mapping Server Flag. If set, the SID is advertised
from the Segment Routing Mapping Server functionality as
described in
[draft-filsfils-rtgwg-segment-routing-use-cases-00].
Other bits: MUST be zero when sent and ignored when received.
Prefix Segment Identifier (SID): 32 bits of Prefix-SID.
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.
The No-PHP flag MUST be set on the Prefix-SIDs allocated to inter-
area prefixes that are originated by the router based on intra-area
reachability.
3.2. Adjacency Segment Identifier (Adj-SID)
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). The Adj-SID value is encoded as a 32
bit number.
3.2.1. OSPFv2 Extended Link Opaque LSA
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.
The format of the OSPFv2 Extended Link Opaque LSA is as follows:
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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 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.
3.2.2. OSPFv2 Extended Link TLV
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.
The Extended Link TLV has following format:
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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:
Type is TBA.
Length is variable.
Link-Type: as defined in section A.4.2 of [RFC2328].
Link-ID: as defined in section A.4.2 of [RFC2328].
Link Data: as defined in section A.4.2 of [RFC2328].
3.2.3. Adj-SID sub-TLV
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
[draft-filsfils-rtgwg-segment-routing-use-cases-00]. The structure
of the Adj-SID Sub-TLV is as follows:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | Reserved | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Adjacency SID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor ID (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Adjacency SID (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor ID (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Adjacency SID (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type is TBA.
Length is variable.
MT-ID: Multi-Topology ID (as defined in [RFC4915].
Flags: 2 octets field of following flags:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|G|S|T| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
F-Flag: FA flag. If set, then Adj-SID refers to a Forwarding
Adjacency.
G-Flag: Global flag. When set, the SID value has global
significance which means the SID has been allocated from the
Segment Routing Global Block (SRGB) as described in
[draft-filsfils-rtgwg-segment-routing-00]. When unset, the SID
value has local (within the router) significance which means
the SID has NOT been allocated from the SRGB. When the Adj-SID
Sub-TLV carries the IGP Adjacency SID, then the G-Flag MUST be
unset.
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S-Flag: Secondary SID. If set the SID represents the secondary
SID. One case where secondary SID may be useful is the
parallel adjacency case as described in
[draft-filsfils-rtgwg-segment-routing-00].
T-Flag: Protected-flag: set if the Adj-SID refer to an
adjacency being protected (e.g.: using IPFRR or MPLS-FRR) as
described in
[draft-filsfils-rtgwg-segment-routing-use-cases-00].
Other bits: MUST be zero when originated and ignored when
received.
Adj-SID: the 32 bits of Adjacency Segment Identifier.
Neighbor ID: the Neighbor ID on broadcast segment to which the SID
belongs. Only used if more then one SID is advertised in the Adj-
SID Sub-TLV.
The first Adjacency SID value in TLV is mandatory and advertises the
Adjacency Segment Identifier for the neighbor that is identified by
the Extended Link TLV itself. Subsequent Neighbor ID/Adjacency SID
tuples are optional and represent Adj-SIDs for other neighbors on the
broadcast segment.
A SR capable router SHOULD allocate an Adj-SID for each of its
adjacencies and set the T-Flag when the adjacency is protected by a
FRR mechanism (IP or MPLS) as described in
[draft-filsfils-rtgwg-segment-routing-use-cases-00].
3.2.4. Explicit Path sub-TLV
Explicit Path Sub-TLV is a Sub-TLV of the Extended Link TLV. The
structure of the Explicit Path Sub-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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Explicit Route Hop #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Explicit Route Hop #... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
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Type is TBA.
Length is variable.
Flags: 16-bit flag field where following flags are defined:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R| |
+-+-+-+-+-+-+-+-+
where:
R-Flag: Segment Routing flag. When set, the content of the
Explicit Route Object contains SIDs.
MT-ID: the Multi-Topology ID (as defined in [RFC4915].
Explicit Route Hop: the address of the explicit route hop. IPv4
and SR hops are encoded using a 32 bit value.
4. Elements of Procedure
4.1. Intra-area Segment routing in OSPFv2
The OSPFv2 node that supports segment routing MAY advertise Prefix-
SIDs for any prefix that is advertising reachability to (e.g.
loopback IP address) as described in Section 3.1. 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
functionality (as described in
[draft-filsfils-rtgwg-segment-routing-use-cases-00]) that advertise
Prefix-SID for remote prefixes which exist in the network. Multiple
SR 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.
Prefix-SID advertisements do not contribute to the prefix
reachability.
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4.2. Inter-area Segment routing in OSPFv2
In order to support SR in a multi-area environment, OSPFv2 must
propagate SR 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
route to all its connected areas, it will also originate an Extended
Prefix Opaque LSA, as described in Section 3.1. 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:
The ABR will look at it's best path to the prefix in the source
area and find out the advertising router associated with its best
path to that prefix.
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,
then the ABR will use the Prefix-SID advertised by any other
router (e.g.: a Prefix-SID coming from an SR Mapping Server as
defined in [draft-filsfils-rtgwg-segment-routing-use-cases-00])
when propagating Prefix-SID for the prefix to other areas.
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 3.1. 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:
The ABR will look at its best path to the prefix in the source
area and find out the advertising router associated with its best
path to that prefix.
The ABR will then look if such 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 ABR that contributes to the best path to the prefix, the
originating ABR will use the Prefix-SID advertised by any other
router (e.g.: a Prefix-SID coming from an SR Mapping Server as
defined in [draft-filsfils-rtgwg-segment-routing-use-cases-00])
when propagating Prefix-SID for the prefix to other areas.
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4.3. SID for External Prefixes
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 3.1. 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
[draft-filsfils-rtgwg-segment-routing-use-cases-00]).
4.4. Advertisement of Adj-SID
The Adjacency Segment Routing Identifier (Adj-SID) is advertised
using the Adj-SID Sub-TLV as described in Section 3.2.
4.4.1. Advertisement of Adj-SID on Point-to-Point Links
The Adj-SID MUST be advertised for any adjacency on a p2p link that
is in a FULL state. 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.
4.4.2. Adjacency SID on Broadcast Interfaces
Broadcast networks in OSPF are represented by a star topology where
the Designated Router (DR) is the central point all other routers on
the broadcast network connect to. As a result, the routers on the
broadcast 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
state between them.
When Segment Routing is used, each router in the broadcast segment
must advertise the Adj-SID for each of its neighbors. To satisfy
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this requirement Adj-SID on broadcast networks are advertised as
follows:
If the router is acting as DR on a broadcast segment, the Adj-SID
MUST be advertised for each adjacency in a FULL state and MAY be
advertised for any adjacency in a state 2-way or higher. When the DR
advertises the Adj-SID Sub-TLV for the broadcast link, the first Adj-
SID in the Adj-SID sub-TLV corresponds to the adjacency of DR to
itself and as such MUST be set to zero and ignored.
If the router is acting as BDR or as DR-Other, then the Adj-SID MUST
be advertised for each adjacency in a state 2-Way or higher.
5. IANA Considerations
TBD
6. Manageability Considerations
TBD
7. Security Considerations
TBD
8. Acknowledgements
We would like to thank Anton Smirnov for his contribution to the
content of this document.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
September 2003.
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[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, June 2007.
[RFC4970] Lindem, A., Shen, N., Vasseur, JP., Aggarwal, R., and S.
Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 4970, July 2007.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, July 2008.
9.2. Informative References
[I-D.minto-rsvp-lsp-egress-fast-protection]
Jeganathan, J., Gredler, H., and Y. Shen, "RSVP-TE LSP
egress fast-protection",
draft-minto-rsvp-lsp-egress-fast-protection-02 (work in
progress), April 2013.
[draft-filsfils-rtgwg-segment-routing-00]
Previdi, S. and C. Filsfils, "Segment Routing", May 2013.
[draft-filsfils-rtgwg-segment-routing-use-cases-00]
Filsfils, C., "Segment Routing Use Cases", May 2013.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems, Inc.
Apollo Business Center
Mlynske nivy 43
Bratislava 821 09
Slovakia
Email: ppsenak@cisco.com
Stefano Previdi (editor)
Cisco Systems, Inc.
Via Del Serafico, 200
Rome 00142
Italy
Email: sprevidi@cisco.com
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Clarence Filsfils
Cisco Systems, Inc.
Brussels,
Belgium
Email: cfilsfil@cisco.com
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