wdiff rfc8665v1.txt rfc8665.txt

Internet Engineering Task Force (IETF) P. Psenak, Ed.
Request for Comments: 8665 S. Previdi, Ed.
Category: Standards Track C. Filsfils
ISSN: 2070-1721 Cisco Systems, Inc.
H. Gredler
RtBrick Inc.
R. Shakir
Google, Inc.
W. Henderickx
Nokia
J. Tantsura
Apstra, Inc.
October
December 2019

OSPF Extensions for Segment Routing

Abstract

This document describes the OSPFv2 extensions required for Segment
Routing.

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/rfc8665.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
1.1. Requirements Language
2. Segment Routing Identifiers
2.1. SID/Label Sub-TLV
3. Segment Routing Capabilities
3.1. SR-Algorithm TLV
3.2. SID/Label Range TLV
3.3. SR Local Block TLV
3.4. SRMS Preference TLV
4. OSPF Extended Prefix Range TLV
5. Prefix SID Prefix-SID Sub-TLV
6. Adjacency Segment Identifier (Adj-SID)
6.1. Adj-SID Sub-TLV
6.2. LAN Adj-SID Sub-TLV
7. Elements of Procedure
7.1. Intra-area Segment Routing in OSPFv2
7.2. Inter-area Segment Routing in OSPFv2
7.3. Segment Routing for External Prefixes
7.4. Advertisement of Adj-SID
7.4.1. Advertisement of Adj-SID on Point-to-Point Links
7.4.2. Adjacency SID on Broadcast or NBMA Interfaces
8. IANA Considerations
8.1. OSPF Router Information (RI) TLVs Registry
8.2. OSPFv2 Extended Prefix Opaque LSA TLVs Registry
8.3. OSPFv2 Extended Prefix TLV Sub-TLVs Registry
8.4. OSPFv2 Extended Link TLV Sub-TLVs Registry
8.5. IGP Algorithm Types Registry
9. TLV/Sub-TLV Error Handling
10. Security Considerations
10.
11. References
10.1.
11.1. Normative References
10.2.
11.2. Informative References
Acknowledgements
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; it is not applicable
to OSPFv2, which only supports the IPv4 address family. When used in
MPLS networks, SR paths do not require any LDP or RSVP-TE signaling.
However, SR can interoperate in the presence of LSPs established with
RSVP or LDP.

There are additional segment types, e.g., Binding Segment Identifier
(SID) defined in [RFC8402].

This document describes the OSPF extensions required for Segment
Routing.

Segment Routing architecture is described in [RFC8402].

Segment Routing use cases are described in [RFC7855].

1.1. Requirements Language

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.

2. Segment Routing Identifiers

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

Extended Prefix/Link Opaque Link State Advertisements (LSAs) defined
in [RFC7684] are used for advertisements of the various SID types.

2.1. SID/Label Sub-TLV

The SID/Label Sub-TLV appears in multiple TLVs or Sub-TLVs 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:

where:

Type: 1

Length: Variable, 3, 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.

The receiving router MUST ignore the SID/Label Sub-TLV if the
length is other than 3 or 4

3. 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 Router Information Opaque
LSA (defined in [RFC7770]). The TLVs defined below are applicable to
both OSPFv2 and OSPFv3; see also [RFC8666].

3.1. SR-Algorithm TLV

The SR-Algorithm TLV is a top-level TLV of the Router Information
Opaque LSA (defined in [RFC7770]).

The SR-Algorithm TLV is optional. It SHOULD only be advertised once
in the Router Information Opaque LSA. If the SR-Algorithm TLV is not
advertised by the node, such a node is considered as not being
segment routing
Segment Routing capable.

An SR Router can 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 currently used by the router to
other routers in an OSPF area. The SR-Algorithm TLV has the
following format:

where:

Type: 8

Length: Variable, in octets, depending on the number of
algorithms advertised

Algorithm: Single octet identifying the algorithm. The following
values are defined by this document:

0: Shortest Path First (SPF) algorithm based on link metric.
This is the standard shortest path algorithm as computed
by the OSPF protocol. Consistent with the deployed
practice for link-state protocols, Algorithm 0 permits
any node to overwrite the SPF path with a different path
based on its local policy. If the SR-Algorithm TLV is
advertised, Algorithm 0 MUST be included.

1: Strict Shortest Path First (SPF) algorithm based on link
metric. The algorithm is identical to Algorithm 0, but
Algorithm 1 requires that all nodes along the path will
honor the SPF routing decision. Local policy at the node
claiming support for Algorithm 1 MUST NOT alter the SPF
paths computed by Algorithm 1.

When multiple SR-Algorithm TLVs are received from a given router, the
receiver MUST use the first occurrence of the TLV in the Router
Information Opaque LSA. If the SR-Algorithm TLV appears in multiple
Router Information Opaque LSAs that have different flooding scopes,
the SR-
Algorithm SR-Algorithm TLV in the Router Information Opaque LSA with the
area-scoped flooding scope MUST be used. If the SR-Algorithm TLV
appears in multiple Router Information Opaque LSAs that have the same
flooding scope, the SR-Algorithm TLV in the Router Information (RI)
Opaque LSA with the numerically smallest Instance ID MUST be used and
subsequent instances of the SR-Algorithm TLV MUST be ignored.

The RI LSA can be advertised at any of the defined opaque flooding
scopes (link, area, or Autonomous System (AS)). For the purpose of
SR-Algorithm TLV advertisement, area-scoped flooding is REQUIRED.

3.2. SID/Label Range TLV

Prefix SIDs

Prefix-SIDs MAY be advertised in the form of an index as described in
Section 5. Such an index defines the offset in the SID/Label space
advertised by the router. The SID/Label Range TLV is used to
advertise such SID/Label space.

The SID/Label Range TLV is a top-level TLV of the Router Information
Opaque LSA (defined in [RFC7770]).

The SID/Label Range TLV MAY appear multiple times and has the
following format:

where:

Type: 9

Length: Variable, in octets, depending on the Sub-TLVs sub-TLVs

Range Size: 3-octet SID/label range size (i.e., the number of
SIDs or labels in the range including the first SID/label). It
MUST be greater than 0.

Reserved: SHOULD be set to 0 on transmission and MUST be ignored
on reception

Initially, the only supported Sub-TLV sub-TLV is the SID/Label Sub-TLV as
defined in Section 2.1. The SID/Label Sub-TLV MUST be included in
the SID/Label Range TLV. The SID/Label advertised in the SID/Label
Sub-TLV represents the first SID/Label in the advertised range.

Only a single SID/Label Sub-TLV MAY be advertised in the SID/Label
Range TLV. If more than one SID/Label Sub-TLV is present, the SID/
Label Range TLV MUST be ignored.

Multiple occurrences of the SID/Label Range TLV MAY be advertised in
order to advertise multiple ranges. In such a case:

* The originating router MUST encode each range into a different
SID/Label Range TLV.

* The originating router decides the order in which the set of SID/
Label Range TLVs are advertised inside the Router Information
Opaque LSA. The originating router MUST ensure the order is the
same after a graceful restart (using checkpointing, nonvolatile
storage, or any other mechanism) in order to ensure the SID/Label
range and SID index correspondence is preserved across graceful
restarts.

* The receiving router MUST adhere to the order in which the ranges
are advertised when calculating a SID/Label from a SID index.

* The originating router MUST NOT advertise overlapping ranges.

* When a router receives multiple overlapping ranges, it MUST
conform to the procedures defined in [RFC8660].

The following example illustrates the advertisement of multiple
ranges.

The originating router advertises the following ranges:

Range 1: Range Size: 100 SID/Label Sub-TLV: 100
Range 1: Range Size: 100 SID/Label Sub-TLV: 1000
Range 1: Range Size: 100 SID/Label Sub-TLV: 500

The receiving routers concatenate the ranges and build the Segment
Routing Global Block (SRGB) as follows:

SRGB = [100, 199]
[1000, 1099]
[500, 599]

The indexes span multiple ranges:

index=0

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 RI LSA can be advertised at any of the defined flooding scopes
(link, area, or autonomous system (AS)). For the purpose of SID/
Label Range TLV advertisement, area-scoped flooding is REQUIRED.

3.3. SR Local Block TLV

The SR Local Block TLV (SRLB TLV) contains the range of labels the
node has reserved for local Local SIDs. SIDs from the SRLB MAY be used for
Adjacency-SIDs
Adjacency SIDs but also by components other than the OSPF protocol.
As an example, an application or a controller can instruct the router
to allocate a specific local Local SID. Some controllers or applications
can use the control plane to discover the available set of local Local SIDs
on a particular router. In such cases, the SRLB is advertised in the
control plane. The requirement to advertise the SRLB is further
described in [RFC8660]. The SRLB TLV is used to advertise the SRLB.

The SRLB TLV is a top-level TLV of the Router Information Opaque LSA
(defined in [RFC7770]).

The SRLB TLV MAY appear multiple times in the Router Information
Opaque LSA and has the following format:

where:

Type: 14

Length: Variable, in octets, depending on the Sub-TLVs sub-TLVs

Range Size: 3-octet SID/Label range size (i.e., the number of
SIDs or labels in the range including the first SID/Label). It
MUST be greater than 0.

Reserved: SHOULD be set to 0 on transmission and MUST be ignored
on reception

Initially, the only supported Sub-TLV sub-TLV is the SID/Label Sub-TLV as
defined in Section 2.1. The SID/Label Sub-TLV MUST be included in
the SRLB TLV. The SID/Label advertised in the SID/Label Sub-TLV
represents the first SID/Label in the advertised range.

Only a single SID/Label Sub-TLV MAY be advertised in the SRLB TLV.
If more than one SID/Label Sub-TLV is present, the SRLB TLV MUST be
ignored.

The originating router MUST NOT advertise overlapping ranges.

Each time a SID from the SRLB is allocated, it SHOULD also be
reported to all components (e.g., controller or applications) in
order for these components to have an up-to-date view of the current
SRLB allocation. This is required to avoid collisions between
allocation instructions.

Within the context of OSPF, the reporting of local Local SIDs is done
through OSPF Sub-TLVs, sub-TLVs, such as the Adjacency-SID Adjacency SID (Section 6).
However, the reporting of allocated local Local SIDs can also be done
through other means and protocols, which are outside the scope of
this document.

A router advertising the SRLB TLV MAY also have other label ranges,
outside of the SRLB, used for its local allocation purposes and not
advertised in the SRLB TLV. For example, it is possible that an
Adjacency-SID
Adjacency SID is allocated using a local label that is not part of
the SRLB.

The RI LSA can be advertised at any of the defined flooding scopes
(link, area, or autonomous system (AS)). For the purpose of SRLB TLV
advertisement, area-scoped flooding is REQUIRED.

3.4. SRMS Preference TLV

The Segment Routing Mapping Server Preference TLV (SRMS Preference
TLV) is used to advertise a preference associated with the node that
acts as an SR Mapping Server. The role of an SRMS is described in
[RFC8661]. SRMS preference is defined in [RFC8661].

The SRMS Preference TLV is a top-level TLV of the Router Information
Opaque LSA (defined in [RFC7770]).

The SRMS Preference TLV MAY only be advertised once in the Router
Information Opaque LSA and has the following format:

where:

Type: 15

Length: 4 octets

Preference: 1 octet, with an SRMS preference value from 0 to 255

Reserved: SHOULD be set to 0 on transmission and MUST be ignored
on reception

When multiple SRMS Preference TLVs are received from a given router,
the receiver MUST use the first occurrence of the TLV in the Router
Information Opaque LSA. If the SRMS Preference TLV appears in
multiple Router Information Opaque LSAs that have different flooding
scopes, the SRMS Preference TLV in the Router Information Opaque LSA
with the narrowest flooding scope MUST be used. If the SRMS
Preference TLV appears in multiple Router Information Opaque LSAs
that have the same flooding scope, the SRMS Preference TLV in the
Router Information Opaque LSA with the numerically smallest Instance
ID MUST be used and subsequent instances of the SRMS Preference TLV
MUST be ignored.

The RI LSA can be advertised at any of the defined flooding scopes
(link, area, or autonomous system (AS)). For the purpose of the SRMS
Preference TLV advertisement, AS-scoped flooding SHOULD be used.
This is because SRMS servers can be located in a different area than
consumers of the SRMS advertisements. If the SRMS advertisements
from the SRMS server are only used inside the SRMS server's area,
area-scoped flooding MAY be used.

4. OSPF Extended Prefix Range TLV

In some cases, it is useful to advertise attributes for a range of
prefixes. The SR Mapping Server, which is described in [RFC8661], is
an example where we need a single advertisement to advertise SIDs for
multiple prefixes from a contiguous address range.

The OSPF Extended Prefix Range TLV, which is a top-level TLV of the
Extended Prefix LSA described in [RFC7684] is defined for this
purpose.

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | AF | Range Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs (variable) |
+- -+
| |

where:

Type: 2

Length: Variable, in octets, depending on the Sub-TLVs sub-TLVs

Prefix Length: Length of prefix in bits

AF: Address family for the prefix. Currently, the only supported
value is 0 for IPv4 unicast. The inclusion of address family
in this TLV allows for future extension.

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 the IPv4 multicast address range
(224.0.0.0/3).

Flags: Single-octet field. The following flags are defined:

0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+
|IA| | | | | | | |
+--+--+--+--+--+--+--+--+

where:

IA-Flag: Inter-Area flag. Flag. If set, advertisement is of
inter-area type. An Area Border Router (ABR) that is
advertising the OSPF Extended Prefix Range TLV between
areas MUST set this bit.

This bit is used to prevent redundant flooding of Prefix
Range TLVs between areas as follows:

An ABR only propagates an inter-area Prefix Range
advertisement from the backbone area to connected
nonbackbone areas if the advertisement is considered
to be the best one. The following rules are used to
select the best range from the set of advertisements
for the same Prefix Range:

An ABR always prefers intra-area Prefix Range
advertisements over inter-area advertisements.

An ABR does not consider inter-area Prefix Range
advertisements coming from nonbackbone areas.

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. Prefix encoding for other
address families is beyond the scope of this specification.

5. Prefix SID Prefix-SID Sub-TLV

The Prefix SID Prefix-SID Sub-TLV is a Sub-TLV sub-TLV of the OSPF Extended Prefix TLV
described in [RFC7684] 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:

where:

Type: 2

Length: 7 or 8 octets, depending on the V-flag 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. 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 Explicit Null Flag. If set, any upstream neighbor
of the Prefix-SID originator MUST replace the Prefix-SID
with the Explicit-NULL Explicit NULL label (0 for IPv4) 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

MT-ID: Multi-Topology ID (as defined in [RFC4915])

Algorithm: Single octet identifying the algorithm the Prefix-SID
is associated with as defined in Section 3.1

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 (Section 3.1) MUST ignore the Prefix-SID Sub-
TLV.

SID/Index/Label: According to the V V- and L flags, L-Flags, it contains:

V-flag

V-Flag is set to 0 and L-flag 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

V-Flag is set to 1 and L-flag 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 V-Flag and L-flag L-Flag are invalid and
any SID advertisement Advertisement received with an invalid setting for V
V- and L flags L-Flags MUST be ignored.

If an OSPF 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, E-, NP-, and M flags 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 E-Flag MUST be clear for
Prefix-SIDs allocated to inter-area prefixes that are originated by
the ABR based on intra-area or inter-area reachability between areas
unless the advertised prefix is directly attached to the ABR.

The NP-Flag (No-PHP) MUST be set and the E-flag E-Flag MUST be clear for
Prefix-SIDs allocated to redistributed prefixes, unless the
redistributed prefix is directly attached to the ASBR. Autonomous System
Boundary Router (ASBR).

If the NP-Flag is not set, then any then:

Any upstream neighbor of the Prefix-
SID Prefix-SID originator MUST pop the
Prefix-SID. This is equivalent to the
penultimate-hop-popping penultimate hop-popping
mechanism used in the MPLS data plane. If
the NP-flag is not set, then the

The received E-flag E-Flag is ignored.

If the NP-flag NP-Flag is set, then:

If set and the E-flag E-Flag is not set, then any 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 Area Border Router ABR (prefix propagation from one area to another) or
at an AS Boundary
Router ASBR (prefix propagation from one domain to another).

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

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

When the M-Flag is set, the NP-flag NP-Flag and the E-flag E-Flag MUST be ignored at 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. 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 also
generating the Extended Prefix TLV with the A-flag A-Flag set for this
prefix as described in Section 2.1 of [RFC7684].

The Prefix is external type and the downstream neighbor is an
ASBR, which is advertising prefix reachability and is also
generating the Extended Prefix TLV with the A-flag A-Flag set for this
prefix as described in Section 2.1 of [RFC7684].

When a Prefix-SID is advertised in an Extended Prefix Range TLV, then
the value advertised in the Prefix SID 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 Prefix-SID indexes:

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

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.

Example 2: If the following prefixes need to be mapped into the
corresponding Prefix-SID indexes:

192.0.2.0/30, Prefix-SID: Index 51
192.0.2.4/30, Prefix-SID: Index 52
192.0.2.8/30, Prefix-SID: Index 53
192.0.2.12/30, Prefix-SID: Index 54
192.0.2.16/30, Prefix-SID: Index 55
192.0.2.20/30, Prefix-SID: Index 56
192.0.2.24/30, Prefix-SID: Index 57

then the Prefix field in the Extended Prefix Range TLV would be set
to 192.0.2.0, Prefix Length would be set to 30, Range Size would be
7, and the Index value in the Prefix-SID Sub-TLV would be set to 51.

6. Adjacency Segment Identifier (Adj-SID)

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

6.1. Adj-SID Sub-TLV

Adj-SID is an optional sub-TLV of the Extended Link TLV defined in
[RFC7684]. It MAY appear multiple times in the Extended Link TLV.
The Adj-SID Sub-TLV has the following format:

where:

Type: 2

Length

Length: 7 or 8 octets, depending on the V flag 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 or MPLS-FRR (MPLS-Fast Reroute) as described
in Section 2.1 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. Flag. When set, the P-Flag indicates
that the Adj-SID is persistently allocated, i.e., the
Adj-SID value remains consistent 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

MT-ID: Multi-Topology ID (as defined in [RFC4915]

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

SID/Index/Label: As described in Section 5

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
Section 3.5 of [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 P-Flag is not set, the Adj-SID MAY be persistent. When the
P-flag
P-Flag is set, the Adj-SID MUST be persistent.

6.2. LAN Adj-SID Sub-TLV

The LAN Adj-SID Adjacency SID is an optional sub-TLV of the Extended Link TLV
defined in [RFC7684]. 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 (Designated Router) router on a broadcast, Non-Broadcast
Multi-Access (NBMA), or hybrid [RFC6845] network.

where:

Type: 3

Length: 11 or 12 octets, depending on the V-flag V-Flag

Flags: Same as in Section 6.1

Reserved: SHOULD be set to 0 on transmission and MUST be ignored
on reception

MT-ID: Multi-Topology ID (as defined in [RFC4915])

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

Neighbor ID: The Router ID of the neighbor for which the LAN-Adj- LAN
Adjacency SID is advertised

SID/Index/Label: As described in Section 5

When the P-flag P-Flag is not set, the Adj-SID LAN Adjacency SID MAY be persistent.
When the P-flag P-Flag is set, the Adj-SID LAN Adjacency SID MUST be persistent.

7. Elements of Procedure

7.1. Intra-area Segment Routing in OSPFv2

An OSPFv2 router that supports segment routing 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).

A Prefix-SID can also be advertised by the SR Mapping Servers (as
described in [RFC8661]). A 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.

An SR Mapping Server MUST use the OSPF Extended Prefix Range TLV when
advertising SIDs for prefixes. Prefixes of different route types can
be combined in a single OSPF Extended Prefix Range TLV advertised by
an SR Mapping Server. Because the OSPF Extended Prefix Range TLV
doesn't include a Route-Type field, as in the OSPF Extended Prefix
TLV, it is possible to include adjacent prefixes from different route
types in the OSPF Extended Prefix Range TLV.

Area-scoped OSPF Extended Prefix Range TLVs are propagated between
areas. Similar to propagation of prefixes between areas, an ABR only
propagates the OSPF Extended Prefix Range TLV that it considers to be
the best from the set it received. The rules used to pick the best
OSPF Extended Prefix Range TLV are described in Section 4.

When propagating an OSPF Extended Prefix Range TLV between areas,
ABRs MUST set the IA-Flag, that IA-Flag. This is used to prevent redundant
flooding of the OSPF Extended Prefix Range TLV between areas as
described in Section 4.

7.2. Inter-area Segment Routing in OSPFv2

In order to support SR in a multiarea environment, OSPFv2 MUST
propagate Prefix-SID information between areas. The following
procedure is used to propagate Prefix SIDs 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 OSPF
Extended Prefix Opaque LSA as described in [RFC7684]. The flooding
scope of the OSPF Extended Prefix Opaque LSA type will be set to
area-local 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 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 OSPF ABR advertises Type-3 Summary LSAs from an inter-area
route to all its connected areas, it will also originate an OSPF
Extended Prefix Opaque LSA as described in [RFC7684]. The flooding
scope of the OSPF Extended Prefix Opaque LSA type will be set to
area-local 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 Prefix-SID 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 such a 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.

7.3. Segment Routing for External Prefixes

Type-5 LSAs are flooded domain wide. When an ASBR, which supports
SR, generates Type-5 LSAs, it SHOULD also originate OSPF Extended
Prefix Opaque LSAs as described in [RFC7684]. The flooding scope of
the OSPF Extended Prefix Opaque LSA type is set to AS-wide scope.
The route type in the OSPF Extended Prefix TLV is set to external.
The Prefix-
SID 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 Not-So-Stubby Area (NSSA) [RFC3101] 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.

7.4. Advertisement of Adj-SID

The Adjacency Segment Routing Identifier (Adj-SID) is advertised
using the Adj-SID Sub-TLV as described in Section 6.

7.4.1. 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
Advertisement MUST be withdrawn from the area.

7.4.2. Adjacency SID on Broadcast or NBMA Interfaces

Broadcast, NBMA, or hybrid [RFC6845] 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, 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 6.1.

SR-capable routers MAY also advertise a LAN-Adj-SID LAN Adjacency SID for other
neighbors (e.g., Backup Designated Router, DR-OTHER, etc.) on the
broadcast, NBMA, or hybrid network using the LAN-ADJ-SID LAN Adj-SID Sub-TLV as
described in Section 6.2.

8. IANA Considerations

This specification updates several existing OSPF registries. registries and
creates a new IGP registry.

8.1. OSPF Router Information (RI) TLVs Registry

The following values have been allocated:

+-------+---------------------+---------------+
| Value | TLV Name | Reference |
+=======+=====================+===============+
| 8 | SR-Algorithm TLV | This document |
+-------+---------------------+---------------+
| 9 | SID/Label Range TLV | This document |
+-------+---------------------+---------------+
| 14 | SR Local Block TLV | This document |
+-------+---------------------+---------------+
| 15 | SRMS Preference TLV | This document |
+-------+---------------------+---------------+

Table 1: OSPF Router Information (RI) TLVs

8.2. OSPFv2 Extended Prefix Opaque LSA TLVs Registry

The following values have been allocated:

+-------+--------------------------------+---------------+
| Value | Description | Reference |
+=======+================================+===============+
| 2 | OSPF Extended Prefix Range TLV | This document |
+-------+--------------------------------+---------------+

Table 2: OSPFv2 Extended Prefix Opaque LSA TLVs

8.3. OSPFv2 Extended Prefix TLV Sub-TLVs Registry

The following values have been allocated:

+-------+--------------------+---------------+
| Value | Description | Reference |
+=======+====================+===============+
| 1 | SID/Label Sub-TLV | This document |
+-------+--------------------+---------------+
| 2 | Prefix SID Prefix-SID Sub-TLV | This document |
+-------+--------------------+---------------+

Table 3: OSPFv2 Extended Prefix TLV Sub-TLVs

8.4. OSPFv2 Extended Link TLV Sub-TLVs Registry

The following initial values have been allocated:

+-------+---------------------------+---------------+
| Value | Description | Reference |
+=======+===========================+===============+
| 1 | SID/Label Sub-TLV | This document |
+-------+---------------------------+---------------+
| 2 | Adj-SID Sub-TLV | This document |
+-------+---------------------------+---------------+
| 3 | LAN Adj-SID/Label Sub-TLV | This document |
+-------+---------------------------+---------------+

Table 4: OSPFv2 Extended Link TLV Sub-TLVs

8.5. IGP Algorithm Types Registry

IANA has set up a subregistry called "IGP Algorithm Type" under the
"Interior Gateway Protocol (IGP) Parameters" registry. The
registration policy for this registry is "Standards Action"
([RFC8126] and [RFC7120]).

Values in this registry come from the range 0-255.

The initial values in the IGP Algorithm Type registry are as follows:

Table 5: IGP Algorithm Types

9. 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. An error SHOULD be logged subject to rate
limiting.

10. Security Considerations

With the OSPFv2 segment routing Segment Routing extensions defined herein, OSPFv2
will now program the MPLS data plane [RFC3031] in addition to the IP
data plane. 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 [RFC2328] and [RFC7684]
apply to these segment routing Segment Routing extensions. While OSPF is under a
single administrative domain, there can be deployments where
potential attackers have access to one or more networks in the OSPF
routing domain. In these deployments, stronger authentication
mechanisms such as those specified in [RFC7474] SHOULD be used.

Implementations MUST assure that malformed TLVs and sub-TLVs defined
in this document are detected and do not provide a vulnerability for
attackers to crash the OSPFv2 router or routing process. Reception
of malformed TLVs or sub-TLVs 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 OSPF control plane.

10.

11. References

10.1.

11.1. Normative References

[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>.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.

[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>.

[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.

[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>.

[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.

[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.

[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>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[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>.

[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>.

[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, October December 2019,
<https://www.rfc-editor.org/info/rfc8660>.

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

10.2.

11.2. Informative References

[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>.

[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>.

[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.

[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>.

[RFC8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
October
December 2019, <https://www.rfc-editor.org/info/rfc8666>.

Acknowledgements

We would like to thank Anton Smirnov for his contribution.

Thanks to Acee Lindem for the detailed review of the document,
corrections, as well as discussion about details of the encoding.

Contributors

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.

Authors' Addresses

Peter Psenak (editor)
Cisco Systems, Inc.
Apollo Business Center, Mlynske nivy 43
821 09 Bratislava
Slovakia

Email: ppsenak@cisco.com

Stefano Previdi (editor)
Cisco Systems, Inc.
Via Del Serafico, 200
00142 Rome
Italy

Email: stefano@previdi.net

Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium

Email: cfilsfil@cisco.com

Hannes Gredler
RtBrick Inc.

Email: hannes@rtbrick.com

Rob Shakir
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America

Email: robjs@google.com

Wim Henderickx
Nokia
Copernicuslaan 50
2018 Antwerp
Belgium

Email: wim.henderickx@nokia.com

Jeff Tantsura
Apstra, Inc.

Email: jefftant.ietf@gmail.com