Internet Engineering Task Force (IETF) P. Psenak, Ed.

Internet Engineering Task Force (IETF) P. Psenak, Ed.
Request for Comments: 8666 S. Previdi, Ed.
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 December 2019

OSPFv3 Extensions for Segment Routing

Abstract

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
https://www.rfc-editor.org/info/rfc8666.

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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
2. Terminology
3. Segment Routing Identifiers
3.1. SID/Label Sub-TLV
4. Segment Routing Capabilities
5. OSPFv3 Extended Prefix Range TLV
6. Prefix-SID Sub-TLV
7. Adjacency Segment Identifier (Adj-SID)
7.1. Adj-SID Sub-TLV
7.2. LAN Adj-SID Sub-TLV
8. Elements of Procedure
8.1. Intra-area Segment Routing in OSPFv3
8.2. Inter-area Segment Routing in OSPFv3
8.3. Segment Routing for External Prefixes
8.4. Advertisement of Adj-SID
8.4.1. Advertisement of Adj-SID on Point-to-Point Links
8.4.2. Adjacency SID on Broadcast or NBMA Interfaces
9. IANA Considerations
9.1. "OSPFv3 Extended-LSA TLVs" Registry
9.2. "OSPFv3 Extended-LSA Sub-TLVs" Registry
10. TLV/Sub-TLV Error Handling
11. Security Considerations
12. References
12.1. Normative References
12.2. Informative References
Contributors
Authors' Addresses

1. 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 [RFC8402].

Segment Routing use cases are described in [RFC7855].

2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Segment Routing Identifiers

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

3.1. 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.

4. 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 [RFC7770]) and specified in [RFC8665].

5. 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 [RFC8661], 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 [RFC8362]:

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.

6. Prefix-SID Sub-TLV

The Prefix-SID sub-TLV is a sub-TLV of the following OSPFv3 TLVs as
defined in [RFC8362] and in Section 5:

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 [RFC8661].

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 [ALGOREG].

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 [RFC8665] 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 [RFC5462].

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 [RFC8362].

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 [RFC8362].

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.

7. Adjacency Segment Identifier (Adj-SID)

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

7.1. Adj-SID Sub-TLV

The Adj-SID sub-TLV is an optional sub-TLV of the Router-Link TLV as
defined in [RFC8362]. 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 Section 3.4 of [RFC8402].

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 [RFC8402].

SID/Index/Label: As described in Section 6.

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
[RFC8402].

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.

7.2. 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 [RFC6845]
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 Section 7.1.

Weight: Weight used for load-balancing purposes. The use of the
weight is defined in [RFC8402].

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 Section 6.

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.

8. Elements of Procedure

8.1. 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 Section 6).

A Prefix-SID can also be advertised by SR Mapping Servers (as
described in [RFC8661]). 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 [RFC8362] is used. If autonomous
system flooding scope is needed, an E-AS-External-LSA [RFC8362] 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 (Section 6), 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
[RFC8362] does not itself contribute to the prefix reachability. The
NU-bit [RFC5340] 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 [RFC5340] are
used for the propagation of the prefix ranges.

8.2. 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 Section 6. 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 Section 6. 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.

8.3. 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 Section 6. The Prefix-SID value
will be set to the SID that has been reserved for that prefix.

When a Not-So-Stubby Area (NSSA) [RFC3101] 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.

8.4. Advertisement of Adj-SID

The Adjacency Segment Routing Identifier (Adj-SID) is advertised
using the Adj-SID sub-TLV as described in Section 7.

8.4.1. 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.

8.4.2. Adjacency SID on Broadcast or NBMA Interfaces

Broadcast, NBMA, or hybrid [RFC6845] 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 Section 7.1.

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 Section 7.2.

9. IANA Considerations

This specification updates two existing OSPFv3 registries.

9.1. "OSPFv3 Extended-LSA TLVs" Registry

The following values have been allocated:

+-------+----------------------------------+---------------+
| Value | Description | Reference |
+=======+==================================+===============+
| 9 | OSPFv3 Extended Prefix Range TLV | This document |
+-------+----------------------------------+---------------+

Table 1: OSPFv3 Extended-LSA TLVs

9.2. "OSPFv3 Extended-LSA Sub-TLVs" Registry

The following values have been allocated:

+-------+---------------------+---------------+
| 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 |
+-------+---------------------+---------------+

Table 2: OSPFv3 Extended-LSA Sub-TLVs

10. 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.

11. Security Considerations

With the OSPFv3 Segment Routing extensions defined herein, OSPFv3
will now program the MPLS data plane [RFC3031]. Previously, LDP
[RFC5036] 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 [RFC5340] and
[RFC8362], 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 [RFC4552] or [RFC7166], 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.

12. References

12.1. Normative References

[ALGOREG] IANA, "Interior Gateway Protocol (IGP) Parameters",
<https://www.iana.org/assignments/igp-parameters>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.

[RFC3101] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
RFC 3101, DOI 10.17487/RFC3101, January 2003,
<https://www.rfc-editor.org/info/rfc3101>.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <https://www.rfc-editor.org/info/rfc5036>.

[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.

[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
2009, <https://www.rfc-editor.org/info/rfc5462>.

[RFC6845] Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast
and Point-to-Multipoint Interface Type", RFC 6845,
DOI 10.17487/RFC6845, January 2013,
<https://www.rfc-editor.org/info/rfc6845>.

[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.

[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.

[RFC8661] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., and S. Litkowski, "Segment Routing MPLS
Interworking with LDP", RFC 8661, DOI 10.17487/RFC8661,
December 2019, <https://www.rfc-editor.org/info/rfc8661>.

[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.

12.2. Informative References

[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.

[RFC7166] Bhatia, M., Manral, V., and A. Lindem, "Supporting
Authentication Trailer for OSPFv3", RFC 7166,
DOI 10.17487/RFC7166, March 2014,
<https://www.rfc-editor.org/info/rfc7166>.

[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/info/rfc7855>.

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: [email protected]

Hannes Gredler
RtBrick Inc.
Austria

Email: [email protected]

Rob Shakir
Google, Inc.
United States of America

Email: [email protected]

Wim Henderickx
Nokia
Belgium

Email: [email protected]

Jeff Tantsura
Apstra, Inc.
United States of America

Email: [email protected]

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

Peter Psenak (editor)
Cisco Systems, Inc.
Eurovea Centre, Central 3, Pribinova Street 10
81109 Bratislava
Slovakia

Email: [email protected]

Stefano Previdi (editor)
Cisco Systems, Inc.

Email: [email protected]