One document matched: draft-ietf-ospf-segment-routing-extensions-02.xml
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<rfc category="std" docName="draft-ietf-ospf-segment-routing-extensions-02"
ipr="trust200902">
<front>
<title abbrev="OSPF Extensions for Segment Routing">OSPF Extensions for
Segment Routing</title>
<author fullname="Peter Psenak" initials="P." role="editor"
surname="Psenak">
<organization>Cisco Systems, Inc.</organization>
<address>
<postal>
<street>Apollo Business Center</street>
<street>Mlynske nivy 43</street>
<city>Bratislava</city>
<code>821 09</code>
<country>Slovakia</country>
</postal>
<email>ppsenak@cisco.com</email>
</address>
</author>
<author fullname="Stefano Previdi" initials="S." role="editor"
surname="Previdi">
<organization>Cisco Systems, Inc.</organization>
<address>
<postal>
<street>Via Del Serafico, 200</street>
<city>Rome</city>
<code>00142</code>
<country>Italy</country>
</postal>
<email>sprevidi@cisco.com</email>
</address>
</author>
<author fullname="Clarence Filsfils" initials="C." surname="Filsfils">
<organization>Cisco Systems, Inc.</organization>
<address>
<postal>
<street/>
<city>Brussels</city>
<region/>
<code/>
<country>Belgium</country>
</postal>
<email>cfilsfil@cisco.com</email>
</address>
</author>
<author fullname="Hannes Gredler" initials="H." surname="Gredler">
<organization>Juniper Networks, Inc.</organization>
<address>
<postal>
<street>1194 N. Mathilda Ave.</street>
<city>Sunnyvale</city>
<code>94089</code>
<region>CA</region>
<country>US</country>
</postal>
<email>hannes@juniper.net</email>
</address>
</author>
<author fullname="Rob Shakir" initials="R." surname="Shakir">
<organization>British Telecom</organization>
<address>
<postal>
<street/>
<city>London</city>
<code/>
<country>UK</country>
</postal>
<email>rob.shakir@bt.com</email>
</address>
</author>
<author fullname="Wim Henderickx" initials="W." surname="Henderickx">
<organization>Alcatel-Lucent</organization>
<address>
<postal>
<street>Copernicuslaan 50</street>
<city>Antwerp</city>
<code>2018</code>
<country>BE</country>
</postal>
<email>wim.henderickx@alcatel-lucent.com</email>
</address>
</author>
<author fullname="Jeff Tantsura" initials="J." surname="Tantsura">
<organization>Ericsson</organization>
<address>
<postal>
<street>300 Holger Way</street>
<city>San Jose</city>
<region>CA</region>
<code>95134</code>
<country>US</country>
</postal>
<email>Jeff.Tantsura@ericsson.com</email>
</address>
</author>
<date day="15" month="August" year="2014"/>
<area>Routing</area>
<workgroup>Open Shortest Path First IGP</workgroup>
<keyword>MPLS</keyword>
<keyword>SID</keyword>
<keyword>IGP</keyword>
<keyword>OSPF</keyword>
<keyword>Label advertisement</keyword>
<keyword>Segment Routing</keyword>
<abstract>
<t>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).</t>
<t>This draft describes the OSPF extensions required for Segment Routing.</t>
</abstract>
<note title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>Segment Routing (SR) allows for a flexible definition of end-to-end
paths within IGP topologies by encoding paths as sequences of
topological sub-paths, called "segments". These segments are advertised
by the link-state routing protocols (IS-IS and OSPF). Prefix segments
represent an ecmp-aware shortest-path to a prefix (or a node), as per
the state of the IGP topology. Adjacency segments represent a hop over a
specific adjacency between two nodes in the IGP. A prefix segment is typically
a multi-hop path while an adjacency segment, in most cases, is a one-hop path. SR's
control-plane can be applied to both IPv6 and MPLS data-planes, and
does not require any additional signalling (other than IGP extensions).
For example, when used in MPLS networks, SR paths do not require any LDP
or RSVP-TE signalling. However, SR can interoperate in the presence of LSPs
established with RSVP or LDP.</t>
<t>This draft describes the OSPF extensions required for Segment Routing.</t>
<t>Segment Routing architecture is described in <xref
target="I-D.filsfils-rtgwg-segment-routing"/>.</t>
<t>Segment Routing use cases are described in <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>.</t>
</section>
<section title="Segment Routing Identifiers">
<t>Segment Routing defines various types of Segment Identifiers (SIDs):
Prefix-SID, Adjacency-SID, LAN Adjacency SID and Binding SID.</t>
<t>For the purpose of the advertisements of various SID values, new
Opaque LSAs <xref target="RFC5250"/> are defined in
<xref target="I-D.ietf-ospf-prefix-link-attr"/>. These new LSAs
are defined as generic containers that can be used to advertise any additional
attributes associated with a prefix or link. These new Opaque LSAs are
complementary to the existing LSAs and are not aimed to replace any of the existing
LSAs.</t>
<section anchor="SIDLABEL" title="SID/Label Sub-TLV">
<t>The SID/Label Sub-TLV appears in multiple TLVs or Sub-TLVs defined
later in this document. It is used to advertise the SID or label
associated with a prefix or adjacency. The SID/Label TLV has following
format:<figure>
<artwork> 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:
</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 1</t>
<t>Length: variable, 3 or 4 bytes</t>
<t>SID/Label: if length is set to 3, then the 20 rightmost bits
represent a label. If length is set to 4, then the value represents
a 32 bit SID.</t>
<t>The receiving router MUST ignore SID/Label Sub-TLV if the length
is other then 3 or 4.</t>
</list></t>
</section>
</section>
<section anchor="SRCAP" title="Segment Routing Capabilities">
<t>Segment Routing requires some additional router capabilities to be advertised
to other routers in the area.</t>
<t>These SR capabilities are advertised in the Router Information Opaque LSA
(defined in <xref target="RFC4970"/>).</t>
<section anchor="SRALGO" title="SR-Algorithm TLV">
<t>The SR-Algorithm TLV is a top-level TLV of the Router Information Opaque LSA
(defined in <xref target="RFC4970"/>).</t>
<t>The SR-Algorithm Sub-TLV is optional. It MAY only be advertised once
in the Router Information Opaque LSA. If the SID/Label Range TLV, as
defined in <xref target="SIDRANGE"/>, is advertised, then SR-Algorithm TLV MUST
also be advertised.</t>
<t> An SR Router may use various algorithms when calculating reachability
to OSPF routers or prefixes in an OSPF area. Examples of these algorithms are
metric based Shortest Path First (SPF), various flavors of Constrained SPF, etc.
The SR-Algorithm TLV allows a router to advertise the algorithms that the router
is currently using to other routers in an OSPF area. The SR-Algorithm TLV has
following format: <figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 | Algorithm... | Algorithm n | |
+- -+
| |
+ +
where:</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 8</t>
<t>Length: variable</t>
<t> Algorithm: Single octet identifying the algorithm. The following
value is defined by this document:<list style="hanging">
<t>0: IGP metric based Shortest Path Tree (SPT)</t>
</list></t>
</list></t>
<t>The RI LSA can be advertised at any of the defined opaque flooding
scopes (link, area, or Autonomous System (AS)). For the purpose of the
SR-Algorithm TLV propagation, area scope flooding is required.</t>
</section>
<section anchor="SIDRANGE" title="SID/Label Range TLV">
<t>The SID/Label Range TLV is a top-level TLV of the Router Information
Opaque LSA (defined in <xref target="RFC4970"/>).</t>
<t>The SID/Label Range TLV MAY appear multiple times and has the following
format:<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Range Size | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs (variable) |
+- -+
| |
+ +
where:</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 9</t>
<t>Length: variable</t>
<t>Range Size: 3 octets of the SID/label range</t>
</list></t>
<t>Initially, the only supported Sub-TLV is the SID/Label TLV as defined
in <xref target="SIDLABEL"/>. The SID/Label advertised in the SID/Label TLV represents
the first SID/Label in the advertised range.</t>
<t>Multiple occurrence of the SID/Label Range TLV MAY be
advertised, in order to advertise multiple ranges. In such case:<list
style="symbols">
<t>The originating router MUST encode each range into a different SID/Label
Range TLV. </t>
<t>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 same after a graceful restart
(using checkpointing, non-volatile storage or any other mechanism) in order
to assure the SID/label range and SID index correspondence is preserved
across graceful restarts.</t>
<t> The receiving router must adhere to the order in which the ranges are
advertised when calculating a SID/label from a SID index.</t>
</list></t>
<t>The following example illustrates the advertisement of multiple ranges:<figure
suppress-title="true">
<artwork>
The originating router advertises following ranges:
Range 1: [100, 199]
Range 2: [1000, 1099]
Range 3: [500, 599]
The receiving routers concatenate the ranges and build the Segment Routing Global Block
(SRGB) is as follows:
SRGB = [100, 199]
[1000, 1099]
[500, 599]
The indexes span multiple ranges:
index=0 means label 100
...
index 99 means label 199
index 100 means label 1000
index 199 means label 1099
...
index 200 means label 500
...
</artwork>
</figure></t>
<t>The RI LSA can be advertised at any of the defined flooding scopes
(link, area, or autonomous system (AS)). For the purposes of the
SR-Capability TLV propagation, area scope flooding is required.</t>
</section>
</section>
<section anchor="PFXRANGE" title="OSPF Extended Prefix Range TLV">
<t>In some cases it is useful to advertise attributes for the range of prefixes.
Segment Routing Mapping Server, which is described in
<xref target="I-D.filsfils-rtgwg-segment-routing"/> is an example, where we need
a single advertisement to advertise SIDs for multiple prefixes from a contiguous
address range.</t>
<t>OSPF Extended Prefix Range TLV, which is a new top level TLV of the Extended
Prefix LSA described in <xref target="I-D.ietf-ospf-prefix-link-attr"/> is
defined for this purpose.</t>
<t>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: <figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | AF | Range Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs (variable) |
+- -+
| |
where: </artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 2.</t>
<t>Length: variable</t>
<t>Prefix length: length of the prefix</t>
<t>AF: 0 - IPv4 unicast</t>
<t>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).</t>
<t>Address Prefix: the prefix, 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 Address Prefix
represents the first prefix in the prefix range.</t>
</list></t>
</section>
<section anchor="PREFIXSID" title="Prefix SID Sub-TLV">
<t>The Prefix SID Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV described
in <xref target="I-D.ietf-ospf-prefix-link-attr"/> and the OSPF Extended Prefix Range
TLV described in <xref target="PFXRANGE"/>. It MAY appear more than once in the
parent TLV and has the following format: <figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved | MT-ID | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID/Index/Label (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 2.</t>
<t>Length: variable</t>
<t>Flags: 1 octet field. The following flags are defined: <figure
align="center">
<artwork>
0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+
|N |NP|M |E |V |L | | |
+--+--+--+--+--+--+--+--+
where:</artwork>
</figure><list style="hanging">
<t>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 to Prefix-SIDs corresponding to a router loopback address.
The N-Flag is set when the Prefix-SID is a Node-SID, as
described in <xref
target="I-D.filsfils-rtgwg-segment-routing"/>.</t>
<t>NP-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.</t>
<t>M-Flag: Mapping Server Flag. If set, the SID is advertised
from the Segment Routing Mapping Server functionality as
described in <xref target="I-D.filsfils-rtgwg-segment-routing"/>.</t>
<t>E-Flag: Explicit-Null Flag. If set, any upstream neighbor
of the Prefix-SID originator MUST replace the Prefix-SID with
a Prefix-SID having an Explicit-NULL value (0 for IPv4) before
forwarding the packet.</t>
<t>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.</t>
<t>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.</t>
<t>Other bits: Reserved. These MUST be zero when sent and are ignored when
received.</t>
</list></t>
<t>MT-ID: Multi-Topology ID (as defined in <xref
target="RFC4915"/>).</t>
<t>Algorithm: one octet identifying the algorithm the Prefix-SID
is associated with as defined in <xref target="SRALGO"/>.</t>
<t>SID/Index/Label: according to the V and L flags, it contains
either: <list style="hanging">
<t>A 32 bit index defining the offset in the SID/Label space advertised
by this router.</t>
<t>A 24 bit label where the 20 rightmost bits are used for encoding
the label value.</t>
</list></t>
</list></t>
<t>If multiple Prefix-SIDs are advertised for the same prefix, the
receiving router MUST use the first encoded SID and MAY use the
subsequent SIDs.</t>
<t>When propagating Prefix-SIDs between areas, if multiple prefix-SIDs are
advertised for a prefix, an implementation SHOULD preserve the original order
when advertising prefix-SIDs to other areas. This allows implementations that only
support a single Prefix-SID to have a consistent view across areas.</t>
<t>When calculating the outgoing label for the prefix, the router MUST
take into account E and P flags advertised by the next-hop router, if
next-hop router advertised the SID for the prefix. This MUST be done
regardless of whether the next-hop router contributes to the best path to the
prefix.</t>
<t>The NP-Flag (No-PHP) MUST be set on the Prefix-SIDs allocated to
inter-area prefixes that are originated by the ABR based on intra-area or
inter-area reachability between areas. When the inter-area prefix
is generated based on the prefix which is directly attached to the
ABR, NP-Flag SHOULD NOT be set</t>
<t>The NP-Flag (No-PHP) MUST be be set on the Prefix-SIDs allocated to
redistributed prefixes, unless the redistributed prefix is directly
attached to ASBR, in which case the NP-flag SHOULD NOT be set.</t>
<t>If the NP-Flag is not set then any upstream neighbor of the Prefix-SID
originator MUST pop the Prefix-SID. This is equivalent to the penultimate
hop popping mechanism used in the MPLS dataplane. In such case, MPLS EXP bits
of the Prefix-SID are not preserved for the final destination (the Prefix-SID
being removed). If the NP-flag is clear then the received E-flag is ignored.</t>
<t>If the NP-flag is set then:<list style="hanging">
<t> If 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 must stitch the incoming packet into a continuing
MPLS LSP to the final destination. This could occur at an inter-area border router
(prefix propagation from one area to another) or at an inter-domain border router
(prefix propagation from one domain to another).</t>
<t>If the E-flag is set then any upstream neighbor of the Prefix-SID originator
MUST replace the Prefix-SID with a Prefix-SID having an Explicit-NULL value. 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 EXP bits.</t>
</list></t>
<t>When M-Flag is set, NP-flag MUST be set and E-bit MUST NOT be set.</t>
<t>When a Prefix-SID is advertised in an Extended Prefix Range TLV, then the value
advertised in Prefix SID Sub-TLV is interpreted as a starting SID value.</t>
<t>Example 1: if the following router addresses (loopback addresses)
need to be mapped into the corresponding Prefix SID indexes: <figure
suppress-title="true">
<artwork>
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
</artwork>
</figure></t>
<t>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.</t>
<t>Example 2: If the following prefixes need to be mapped into the
corresponding Prefix-SID indexes: <figure suppress-title="true">
<artwork>
10.1.1/24, Prefix-SID: Index 51
10.1.2/24, Prefix-SID: Index 52
10.1.3/24, Prefix-SID: Index 53
10.1.4/24, Prefix-SID: Index 54
10.1.5/24, Prefix-SID: Index 55
10.1.6/24, Prefix-SID: Index 56
10.1.7/24, Prefix-SID: Index 57
</artwork>
</figure></t>
<t>then the Prefix field in the Extended Prefix Range TLV would be set to
10.1.1.0, Prefix Length would be set to 24, Range Size would be 7 and
the Index value in the Prefix-SID Sub-TLV would be set to 51.</t>
</section>
<section anchor="SIDBINDING" title="SID/Label Binding Sub-TLV">
<t>The SID/Label Binding Sub-TLV is used to advertise a SID/Label mapping
for a path to the prefix. </t>
<t>The SID/Label Binding TLV MAY be originated by any router in an
OSPF domain. The router may advertise a SID/Label binding to a FEC
along with at least a single 'nexthop style' anchor. The protocol
supports more than one 'nexthop style' anchor to be attached to a
SID/Label binding, which results in a simple path description
language. In analogy to RSVP, the terminology for this is called an
'Explicit Route Object' (ERO). Since ERO style path notation allows anchoring
SID/label bindings to both link and node IP addresses, any
Label Switched Path (LSP) can be described. Additionally, SID/Label
Bindings from external protocols can be easily re-advertised.</t>
<t>The SID/Label Binding TLV may be used for advertising SID/Label
Bindings and their associated Primary and Backup paths. In a single
TLV, a primary ERO Path, backup ERO Path, or both can be
advertised. If a router wants to advertise multiple parallel paths,
then it can generate several TLVs for the same Prefix/FEC. Each
occurrence of a Binding TLV for a given FEC Prefix will add a new path.</t>
<t>The SID/Label Binding Sub-TLV is a Sub-TLV of the OSPF Extended Prefix TLV described
in <xref target="I-D.ietf-ospf-prefix-link-attr"/> and the OSPF Extended Prefix Range
TLV described in <xref target="PFXRANGE"/>. Multiple SID/Label Binding TLVs can
be present in their parent TLV. The SID/Label Binding Sub-TLV has following format: <figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved | MT-ID | Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs (variable) |
+- -+
| |
where: </artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 3</t>
<t>Length: variable</t>
<t>Flags: 1 octet field of following flags:<figure align="center">
<artwork>
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|M| |
+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>M-bit - When the bit is set the binding represents the
mirroring context as defined in <xref
target="I-D.minto-rsvp-lsp-egress-fast-protection"/>.</t>
</list></t>
<t>MT-ID: Multi-Topology ID (as defined in <xref
target="RFC4915"/>).</t>
<t>Weight: weight used for load-balancing purposes. The use of the
weight is defined in <xref
target="I-D.filsfils-rtgwg-segment-routing"/>.</t>
</list></t>
<t>The SID/Label Binding TLV supports the following Sub-TLVs:<list
style="hanging">
<t>SID/Label Sub-TLV as described in <xref target="SIDLABEL"/>.
This Sub-TLV MUST appear in the SID/Label Binding Sub-TLV and it
MUST only appear once.</t>
<t>ERO Metric Sub-TLV as defined in <xref
target="METRICTLV"/>.</t>
<t>ERO Sub-TLVs as defined in <xref target="EROTLV"/>.</t>
</list></t>
<!-- begin ERO Metric subTLV -->
<section anchor="METRICTLV" title="ERO Metric Sub-TLV">
<t>The ERO Metric Sub-TLV is a Sub-TLV of the SID/Label Binding TLV.</t>
<t>The ERO Metric Sub-TLV advertises the cost of an ERO path. It is
used to compare the cost of a given source/destination path. A
router SHOULD advertise the ERO Metric Sub-TLV in an advertised ERO TLV.
The cost of the ERO Metric Sub-TLV SHOULD be set to the cumulative IGP or TE
path cost of the advertised ERO. Since manipulation of the Metric field may
attract or repel traffic to and from the advertised segment, it
MAY be manually overridden. <figure
title="ERO Metric Sub-TLV format">
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>Type: TBD, suggested value 8</t>
<t>Length: Always 4</t>
<t>Metric: A 4 octet metric representing the aggregate IGP or TE path cost.</t>
</list></t>
</section>
<!-- end ERO metric subTLV -->
<section anchor="EROTLV" title="ERO Sub-TLVs">
<t>All 'ERO' information represents an ordered set which describes
the segments of a path. The first ERO Sub-TLV describes the first segment of
a path. Similiarly, the last ERO Sub-TLV describes the segment closest
to the egress point. If a router extends or stitches a path, it MUST prepend
the new segment's path information to the ERO list. This applies equally to
advertised backup EROs.</t>
<t>All ERO Sub-TLVs must immediately follow the (SID)/Label
Sub-TLV.</t>
<t>All Backup ERO Sub-TLVs must immediately follow the last ERO Sub-TLV.</t>
<section anchor="IPV4ERO" title="IPv4 ERO Sub-TLV">
<t>IPv4 ERO Sub-TLV is a Sub-TLV of the SID/Label Binding
Sub-TLV.</t>
<t>The IPv4 ERO Sub-TLV describes a path segment using IPv4
Address style encoding. Its semantics have been borrowed from
<xref target="RFC3209"/>. <figure title="IPv4 ERO Sub-TLV format">
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>Type: TBD, suggested value 4</t>
<t>Length: 8 bytes</t>
<t>Flags: 1 octet field of following flags: <figure
align="center">
<artwork>
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|L| |
+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>L-bit - If the L-bit is set, then the segment path is designated
as 'loose'. Otherwise, the segment path is designated as 'strict'.</t>
</list></t>
<t>IPv4 Address - the address of the explicit route hop.</t>
</list></t>
</section>
<section anchor="UNNERO" title="Unnumbered Interface ID ERO Sub-TLV">
<t>The Unnumbered Interface ID ERO Sub-TLV is a Sub-TLV of the
SID/Label Binding Sub-TLV.</t>
<t>The appearance and semantics of the 'Unnumbered Interface ID'
have been borrowed from <xref target="RFC3477"/>.</t>
<t>The Unnumbered Interface-ID ERO Sub-TLV describes a path
segment that includes an unnumbered interface. Unnumbered
interfaces are referenced using the interface index. Interface
indices are assigned local to the router and therefore not unique
within a domain. All elements in an ERO path need to be unique
within a domain and hence need to be disambiguated using a domain
unique Router-ID. <figure
title="Unnumbered Interface ID ERO Sub-TLV format">
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 5</t>
<t>Length: 12 bytes</t>
<t>Flags: 1 octet field of following flags:<figure
align="center">
<artwork>
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|L| |
+-+-+-+-+-+-+-+-+
where: </artwork>
</figure><list style="hanging">
<t>L-bit - If the L-bit is set, then the segment path is designated
as 'loose'. Otherwise, the segment path is designated as 'strict'.</t>
</list></t>
<t>Router-ID: Router-ID of the next-hop.</t>
<t>Interface ID: is the identifier assigned to the link by the
router specified by the Router-ID.</t>
</list></t>
</section>
<section anchor="IPV4BERO" title="IPv4 Backup ERO Sub-TLV">
<t>IPv4 Prefix Backup ERO Sub-TLV is a Sub-TLV of the SID/Label
Binding Sub-TLV.</t>
<t>The IPv4 Backup ERO Sub-TLV describes a path segment using IPv4
Address style of encoding. Its semantics have been borrowed from
<xref target="RFC3209"/>. <figure
title="IPv4 Backup ERO Sub-TLV format">
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>Type: TBD, suggested value 6</t>
<t>Length: 8 bytes</t>
<t>Flags: 1 octet field of following flags: <figure
align="center">
<artwork>
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|L| |
+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>L-bit - If the L-bit is set, then the segment path is designated
as 'loose'. Otherwise, the segment path is designated as 'strict'.</t>
</list></t>
<t>IPv4 Address - the address of the explicit route hop.</t>
</list></t>
</section>
<section anchor="UNNBERO"
title="Unnumbered Interface ID Backup ERO Sub-TLV">
<t>The Unnumbered Interface ID Backup ERO Sub-TLV is a Sub-TLV of the
SID/Label Binding Sub-TLV.</t>
<t>The appearance and semantics of the 'Unnumbered Interface ID'
have been borrowed from <xref target="RFC3477"/>.</t>
<t>The Unnumbered Interface-ID Backup ERO Sub-TLV describes a path
segment that includes an unnumbered interface. Unnumbered
interfaces are referenced using the interface index. Interface
indices are assigned local to the router and are therefore not unique
within a domain. All elements in an ERO path need to be unique
within a domain and hence need to be disambiguated with specification of the
domain unique Router-ID. <figure
title="Unnumbered Interface ID Backup ERO Sub-TLV format">
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>Type: TBD, suggested value 7</t>
<t>Length: 12 bytes</t>
<t>Flags: 1 octet field of following flags: <figure
align="center">
<artwork>
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|L| |
+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>L-bit - If the L-bit is set, then the segment path is designated
as 'loose'. Otherwise, the segment path is designated as 'strict'.</t>
</list></t>
<t>Router-ID: Router-ID of the next-hop.</t>
<t>Interface ID: is the identifier assigned to the link by the
router specified by the Router-ID.</t>
</list></t>
</section>
</section>
</section>
<section anchor="ADJSID" title="Adjacency Segment Identifier (Adj-SID)">
<t>An Adjacency Segment Identifier (Adj-SID) represents a router
adjacency in Segment Routing.</t>
<section anchor="ADJSIDSUBTLV" title="Adj-SID Sub-TLV">
<t>Adj-SID is an optional Sub-TLV of the Extended Link TLV defined in
<xref target="I-D.ietf-ospf-prefix-link-attr"/>. It MAY appear multiple times
in the Extended Link TLV. Examples where more than one Adj-SID may be used per
neighbor are described in <xref target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>.
The Adj-SID Sub-TLV has the following format: <figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved | MT-ID | Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID/Label/Index (variable) |
+---------------------------------------------------------------+
where:</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 2.</t>
<t>Length: variable.</t>
<t>Flags. 1 octet field of following flags:<figure align="center">
<artwork>
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|B|V|L|S| |
+-+-+-+-+-+-+-+-+
where:</artwork>
</figure><list style="hanging">
<t>B-Flag: Backup Flag. If set, the Adj-SID refers to an
adjacency being protected (e.g.: using IPFRR or MPLS-FRR) as
described in <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>.</t>
<t>The 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.</t>
<t>The 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.</t>
<t>The S-Flag. Set Flag. When set, the S-Flag indicates that the
Adj-SID refers to a set of adjacencies (and therefore MAY be assigned
to other adjacencies as well).</t>
<t>Other bits: Reserved. These MUST be zero when sent and are ignored when
received.</t>
</list></t>
<t>MT-ID: Multi-Topology ID (as defined in <xref
target="RFC4915"/>.</t>
<t>Weight: weight used for load-balancing purposes. The use of the
weight is defined in <xref
target="I-D.filsfils-rtgwg-segment-routing"/>.</t>
<t>SID/Index/Label: according to the V and L flags, it contains
either: <list style="hanging">
<t>A 32 bit index defining the offset in the SID/Label space advertised
by this router.</t>
<t>A 24 bit label where the 20 rightmost bits are used for encoding
the label value.</t>
</list></t>
</list></t>
<t>An SR capable router MAY allocate an Adj-SID for each of its
adjacencies and set the B-Flag when the adjacency is protected by
an FRR mechanism (IP or MPLS) as described in <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>.</t>
</section>
<section anchor="LANADJSIDSUBTLV" title="LAN Adj-SID Sub-TLV">
<t>LAN Adj-SID is an optional Sub-TLV of the Extended Link TLV defined in
<xref target="I-D.ietf-ospf-prefix-link-attr"/>. It MAY appear multiple
times in the Extended-Link TLV. It is used to advertise a SID/Label for an adjacency
to a non-DR node on a broadcast or NBMA network.
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved | MT-ID | Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID/Label/Index (variable) |
+---------------------------------------------------------------+
where:</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 3.</t>
<t>Length: variable.</t>
<t>Flags. 1 octet field of following flags:<figure align="center">
<artwork>
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|B|V|L|S| |
+-+-+-+-+-+-+-+-+
where:</artwork>
</figure><list style="hanging">
<t>B-Flag: Backup-flag: set if the LAN-Adj-SID refer to an
adjacency being protected (e.g.: using IPFRR or MPLS-FRR) as
described in <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>.</t>
<t>The 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.</t>
<t> The 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.</t>
<t>The S-Flag. Set Flag. When set, the S-Flag indicates that the
Adj-SID refers to a set of adjacencies (and therefore MAY be assigned
to other adjacencies as well).</t>
<t>Other bits: Reserved. These MUST be zero when sent and are ignored when
received.</t>
</list></t>
<t>MT-ID: Multi-Topology ID (as defined in <xref
target="RFC4915"/>.</t>
<t>Weight: weight used for load-balancing purposes. The use of the
weight is defined in <xref
target="I-D.filsfils-rtgwg-segment-routing"/>.</t>
<t>SID/Index/Label: according to the V and L flags, it contains
either: <list style="hanging">
<t>A 32 bit index defining the offset in the SID/Label space advertised
by this router.</t>
<t>A 24 bit label where the 20 rightmost bits are used for encoding
the label value.</t>
</list></t>
</list></t>
</section>
</section>
<section title="Elements of Procedure">
<section title="Intra-area Segment routing in OSPFv2 ">
<t>An OSPFv2 router that supports segment routing MAY advertise Prefix-
SIDs for any prefix to which it is advertising reachability (e.g.,
a loopback IP address as described in <xref target="PREFIXSID"/>).</t>
<t>If multiple routers advertise a Prefix-SID for the same prefix, then
the Prefix-SID MUST be the same. This is required in order to allow
traffic load-balancing when multiple equal cost paths to the destination
exist in the network.</t>
<t>Prefix-SID can also be advertised by the SR Mapping Servers (as
described in <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>). The Mapping
Server advertises Prefix-SIDs for remote prefixes that exist in the
OSPFv2 routing domain. Multiple Mapping Servers can advertise Prefix-SIDs
for the same prefix, in which case the same Prefix-SID MUST be advertised by
all of them. The flooding scope of the OSPF Extended Prefix Opaque LSA
that is generated by the SR Mapping Server could be either area scoped
or AS scoped and is determined based on the configuration of the
SR Mapping Server.</t>
</section>
<section title="Inter-area Segment routing in OSPFv2">
<t>In order to support SR in a multi-area environment, OSPFv2 must
propagate Prefix-SID information between areas. The following
procedure is used in order to propagate Prefix SIDs between areas.</t>
<t>When an OSPF ABR advertises a Type-3 Summary LSA from an intra-area
prefix to all its connected areas, it will also originate an Extended
Prefix Opaque LSA, as described in <xref target="I-D.ietf-ospf-prefix-link-attr"/>.
The flooding scope of the Extended Prefix Opaque LSA type will be set to
area-scope. The route-type in the OSPF Extended Prefix TLV is set to
inter-area. The Prefix-SID Sub-TLV will be included in this LSA and
the Prefix-SID value will be set as follows: <list style="hanging">
<t>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.</t>
<t>The ABR will then determine if such router advertised a Prefix-SID
for the prefix and use it when advertising the Prefix-SID to other
connected areas.</t>
<t>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 (e.g.: a Prefix-SID coming from an SR Mapping Server
as defined in <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>) when
propagating the Prefix-SID for the prefix to other areas.</t>
</list></t>
<t>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 <xref target="I-D.ietf-ospf-prefix-link-attr"/>.
The flooding scope of the Extended Prefix Opaque LSA type will be set to
area-scope. The route-type in OSPF Extended Prefix TLV is set to
inter-area. The Prefix-SID Sub-TLV will be included in this LSA and
the Prefix-SID will be set as follows: <list style="hanging">
<t>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.</t>
<t>The ABR will then determine if such router advertised a Prefix-SID
for the prefix and use it when advertising the Prefix-SID to other
connected areas.</t>
<t>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 <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>) when
propagating the Prefix-SID for the prefix to other areas.</t>
</list></t>
</section>
<section title="SID for External Prefixes">
<t>Type-5 LSAs are flooded domain wide. When an ASBR, which supports
SR, generates Type-5 LSAs, it should also originate an Extended Prefix
Opaque LSAs, as described in <xref target="I-D.ietf-ospf-prefix-link-attr"/>.
The flooding scope of the Extended Prefix Opaque LSA type is set to AS-scope. The
route-type in the OSPF Extended Prefix TLV is set to external. The
Prefix-SID Sub-TLV is included in this LSA and the Prefix-SID value will be set
to the SID that has been reserved for that prefix.</t>
<t>When an NSSA ABR translates Type-7 LSAs into Type-5 LSAs, it should
also advertise the Prefix-SID for the prefix. The NSSA ABR determines
its best path to the prefix advertised in the translated Type-7 LSA
and finds the advertising router associated with that path. If the
advertising router has advertised a Prefix-SID for the prefix, then
the NSSA ABR uses it when advertising the Prefix-SID for the Type-5
prefix. Otherwise, the Prefix-SID advertised by any other router will
be used (e.g.: a Prefix-SID coming from an SR Mapping Server as
defined in <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>).</t>
</section>
<section title="Advertisement of Adj-SID">
<t>The Adjacency Segment Routing Identifier (Adj-SID) is advertised
using the Adj-SID Sub-TLV as described in <xref target="ADJSID"/>.</t>
<section title="Advertisement of Adj-SID on Point-to-Point Links">
<t>An Adj-SID MAY be advertised for any adjacency on a p2p link that is
in neighbor state 2-Way or higher. If the adjacency on a p2p link
transitions from the FULL state, then the Adj-SID for that adjacency
MAY be removed from the area. If the adjacency transitions to a
state lower then 2-Way, then the Adj-SID advertisement MUST be removed from the
area.</t>
</section>
<section title="Adjacency SID on Broadcast or NBMA Interfaces">
<t>Broadcast or NBMA networks in OSPF are represented by a star
topology where the Designated Router (DR) is the central point to which all
other routers on the broadcast or NBMA network connect. As a result, routers
on the broadcast or NBMA network advertise only their adjacency to the DR.
Routers that do not act as DR do not form or advertise adjacencies with each
other. They do, however, maintain 2-Way adjacency state with each other and are
directly reachable.</t>
<t>When Segment Routing is used, each router on the broadcast or
NBMA network MAY advertise the Adj-SID for its adjacency to the DR using
Adj-SID Sub-TLV as described in <xref target="ADJSIDSUBTLV"/>.</t>
<t>SR capable routers MAY also advertise an Adj-SID for other neighbors
(e.g. BDR, DR-OTHER) on the broadcast or NBMA network using the LAN
ADJ-SID Sub-TLV as described in <xref target="LANADJSIDSUBTLV"/>.</t>
</section>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This specification updates several existing OSPF registries.</t>
<section anchor="RITLVREG" title="OSPF OSPF Router Information (RI) TLVs Registry">
<t>o 8 (IANA Preallocated) - SR-Algorithm TLV</t>
<t>o 9 (IANA Preallocated) - SID/Label Range TLV</t>
</section>
<section anchor="EPLTLVREG" title="OSPF Extended Prefix LSA TLV Registry">
<t>Following values are allocated:</t>
<t>o 2 - OSPF Extended Prefix Range TLV</t>
</section>
<section anchor="EPLSTLVREG" title="OSPF Extended Prefix LSA Sub-TLV Registry">
<t>Following values are allocated:</t>
<t>o 1 - SID/Label Sub-TLV</t>
<t>o 2 - Prefix SID Sub-TLV</t>
<t>o 3 - SID/Label Binding Sub-TLV</t>
<t>o 4 - IPv4 ERO Sub-TLV</t>
<t>o 5 - Unnumbered Interface ID ERO Sub-TLV</t>
<t>o 6 - IPv4 Backup ERO Sub-TLV</t>
<t>o 7 - Unnumbered Interface ID Backup ERO Sub-TLV</t>
<t>o 8 - ERO Metric Sub-TLV</t>
</section>
<section anchor="ELLSTLVREG" title="OSPF Extended Link LSA Sub-TLV Registry">
<t>Following initial values are allocated:</t>
<t>o 1 - SID/Label Sub-TLV</t>
<t>o 2 - Adj-SID Sub-TLV</t>
<t>o 3 - LAN Adj-SID/Label Sub-TLV</t>
</section>
</section>
<section anchor="Security" title="Security Considerations">
<t>Implementations must assure that malformed TLV and Sub-TLV
permutations do not result in errors which cause hard OSPF failures.</t>
</section>
<section anchor="Contributors" title="Contributors">
<t>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.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>We would like to thank Anton Smirnov for his contribution.</t>
<t>Many thanks to Yakov Rekhter, John Drake and Shraddha Hedge for their
contribution on earlier incarnations of the "Binding / MPLS Label TLV"
in <xref target="I-D.gredler-ospf-label-advertisement"/>.</t>
<t>Thanks to Acee Lindem for the detail review of the draft, corrections,
as well as discussion about details of the encoding.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3209.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3477.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3630.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5250.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2328.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4970.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4915.xml"?>
<?rfc ?>
</references>
<references title="Informative References">
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-ospf-prefix-link-attr.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-filsfils-rtgwg-segment-routing-01.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-filsfils-rtgwg-segment-routing-use-cases-02.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-minto-rsvp-lsp-egress-fast-protection-03.xml"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-gredler-ospf-label-advertisement-03.xml"?>
</references>
</back>
</rfc>
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