One document matched: draft-psenak-ospf-segment-routing-ospfv3-extension-02.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
<?rfc tocompact="yes"?>
<?rfc tocdepth="3"?>
<?rfc tocindent="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
<?rfc comments="yes"?>
<?rfc inline="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<rfc category="std"
docName="draft-psenak-ospf-segment-routing-ospfv3-extension-02"
ipr="trust200902">
<front>
<title abbrev="OSPFv3 Extensions for Segment Routing">OSPFv3 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="2" month="July" 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 necessary OSPFv3 extensions that need to be
introduced 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 of the
cases, is a one-hop path. SR's control-plane can be applied to both IPv6
and MPLS data-planes, and do not require any additional signaling (other
than the regular IGP). For example, when used in MPLS networks, SR paths
do not require any LDP or RSVP-TE signaling. Still, SR can interoperate
in the presence of LSPs established with RSVP or LDP .</t>
<t>This draft describes the necessary OSPFv3 extensions that need to be
introduced 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>
<section anchor="SIDLABEL" title="SID/Label sub-TLV">
<t>SID/Label sub-TLV appears in multiple TLVs or Sub-TLVs defined
later in this document. It is used to advertise SID or label
associated with the prefix or adjacency. SID/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 capabilities of the router
to be advertised to other routers in the area.</t>
<t>These SR capabilities are advertised in OSPFv3 Router Information
Opaque LSA (defined in <xref target="RFC4970"/>).</t>
<section anchor="SRALGO" title="SR-Algorithm TLV">
<t>SR-Algorithm TLV is a TLV of Router Information Opaque LSA (defined
in <xref target="RFC4970"/>).</t>
<t>Router may use various algorithms when calculating reachability to
other nodes in area or to prefixes attached to these nodes. Examples
of these algorithms are metric based Shortest Path First (SPF),
various sorts of Constrained SPF, etc. SR-Algorithm TLV allows a
router to advertise algorithms that router is currently using to other
routers in an area. SR-Algorithm TLV has following structure: <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: one octet identifying the algorithm. The following
value has been defined:<list style="hanging">
<t>0: IGP metric based SPT.</t>
</list></t>
</list></t>
<t>RI LSA can be advertised at any of the defined flooding scopes
(link, area, or autonomous system (AS)). For the purpose of the
SR-Algorithm TLV propagation area scope flooding is required.</t>
</section>
<section anchor="SIDRANGE" title="SID/Label Range TLV">
<t>The SID/Label Range TLV is a TLV of Router Information Opaque LSA
(defined in <xref target="RFC4970"/>).</t>
<t>SID/Label Sub-TLV MAY appear multiple times and 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 SID/label range</t>
</list></t>
<t>Currently the only supported Sub-TLV is the SID/Label TLV as
defined in <xref target="SIDLABEL"/>. SID/Label advertised in
SID/Label TLV represents the first SID/Label from 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 in which order the set of SID/Label Range
TLVs are advertised inside 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 guarantee the same
order before and after graceful restart.</t>
<t>Receiving router must adhere to the order in which the ranges are
advertised when calculating a SID/label from the SID index.</t>
<t>A router not supporting multiple occurrences SID/Label Range TLV MUST
take into consideration the first occurrence in the received set.</t>
</list></t>
<t>Here follows an example of 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 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>RI LSA can be advertised at any of the defined flooding scopes
(link, area, or autonomous system (AS)). For the purpose of the
SR-Capability TLV propagation area scope flooding is required.</t>
</section>
</section>
<section anchor="EXTPXLSA" title="Prefix SID Identifier ">
<t>A new extended OSPFv3 LSAs as defined in <xref
target="I-D.ietf-ospf-ospfv3-lsa-extend"/> are used to advertise SID or label values
associated with the prefix in OSPFv3.</t>
<section anchor="PREFIXSID" title="Prefix SID Sub-TLV">
<t>The Prefix SID Sub-TLV is a Sub-TLV of the following OSPFv3 TLVs as
defined in <xref target="I-D.ietf-ospf-ospfv3-lsa-extend"/>: <list
style="hanging">
<t>Intra-Area Prefix TLV</t>
<t>Inter-Area Prefix TLV</t>
<t>External Prefix TLV</t>
</list></t>
<t>It MAY appear more than once and 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 | Algorithm | Range Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|N|P|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 on Prefix-SIDs attached to a router loopback address.
The N-Flag is set when the Prefix-SID is a Node- SID as
described in <xref
target="I-D.filsfils-rtgwg-segment-routing"/>.</t>
<t>P-Flag: no-PHP flag. If set, then the penultimate hop MUST
NOT pop the Prefix-SID before delivering the packet to the
node that advertised the Prefix-SID.</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-use-cases"/>.</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>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 PrefixSID has local significance. If not set, then
the value/index carried by this subTLV has global significance.</t>
<t>Other bits: MUST be zero when sent and ignored when
received.</t>
</list></t>
<t>Algorithm: one octet identifying the algorithm the Prefix-SID
is associated with as defined in <xref target="SRALGO"/>.</t>
<t>Range Size: this field provides the ability to specify a range
of addresses and their associated Prefix SIDs. It represents a
compression scheme to distribute a continuous Prefix and their
continuous, corresponding SID/Label Block. If a single SID is
advertised then the Range Size field MUST be set to 1. For range
advertisements > 1, Range Size represents the number of
addresses that need to be mapped into a Prefix-SID.</t>
<t>SID/Index/Label: label or index value depending on the V-bit setting.
<list style="hanging">
<t>Examples: <list style="hanging">
<t>A 32 bit global index defining the offset in the SID/Label space
advertised by this router - in this case the V and L flags MUST be
unset.</t>
<t>A 24 bit local label where the 20 rightmost bits are used for
encoding the label value - in this case the V and L flags MUST be
set.</t>
</list></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 ones.</t>
<t>When propagating Prefix-SIDs between areas, if multiple prefix-SIDs are
advertised for a prefix, an implementation SHOULD preserve the original ordering,
when advertising prefix-SIDs to other areas. This allows implementations that only
use 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 next-hop router contributing to the best path to the
prefix or not.</t>
<t>P-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. In case the inter-area prefix
is generated based on the prefix which is directly attached to the
ABR, P-Flag SHOULD NOT be set</t>
<t>P-Flag (no-PHP) MUST NOT be set on the Prefix-SIDs allocated to
redistributed prefixes, unless the redistributed prefix is directly
attached to ASBR, in which case the P-Flag SHOULD NOT be set.</t>
<t>If the P-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 to the ultimate hop (the Prefix-SID
being removed). If the P-flag is unset the received E-flag is ignored.</t>
<t>If the P-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 PrefixSID 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, P-flag MUST be set and E-bit MUST NOT be set.</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::1/128, Prefix-SID: Index 1
Router-B: 192::2/128, Prefix-SID: Index 2
Router-C: 192::3/128, Prefix-SID: Index 3
Router-D: 192::4/128, Prefix-SID: Index 4
</artwork>
</figure></t>
<t>then the Address Prefix field in Intra-Area Prefix TLV, Inter-Area
Prefix TLV or External Prefix TLV is set to 192::1, Prefix Length in
these TLVs would be set to 128, Range Size in Prefix SID sub-TLV would
be set to 4 and Index value 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::0/120, Prefix-SID: Index 51
10:1:1::100/120, Prefix-SID: Index 52
10:1:1::200/120, Prefix-SID: Index 53
10:1:1::300/120, Prefix-SID: Index 54
10:1:1::400/120, Prefix-SID: Index 55
10:1:1::500/120, Prefix-SID: Index 56
10:1:1::600/120, Prefix-SID: Index 57
</artwork>
</figure></t>
<t>then the Address Prefix field in Intra-Area Prefix TLV, Inter-Area
Prefix TLV or External Prefix TLV is set to 10:1:1::0, Prefix Length
in these TLVs would be set to 120, Range Size in Prefix SID sub-TLV
would be set to 7 and Index value would be set to 51.</t>
</section>
<section anchor="SIDBINDING" title="SID/Label Binding sub-TLV">
<t>SID/Label Binding sub-TLV is used to advertise SID/Label mapping
for a path to the prefix. </t>
<t>The SID/Label Binding TLV MAY be originated by any router in an
OSPFv3 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 into 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 to
anchor SID/label bindings to both link and node IP addresses any
label switched path, can be described. Furthermore also SID/Label
Bindings from external protocols can get 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 one single
TLV either a primary ERO Path, a backup ERO Path or both are
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 with respect with a given FEC Prefix has
accumulating and not canceling semantics.</t>
<t>SID/Label Binding sub-TLV is a sub-TLV of the following OSPFv3
TLVs, as defined in <xref target="I-D.ietf-ospf-ospfv3-lsa-extend"/>: <list
style="hanging">
<t>Intra-Area Prefix TLV</t>
<t>Inter-Area Prefix TLV</t>
<t>External Prefix TLV</t>
</list></t>
<t>Multiple SID/Label Binding sub-TLVs can be present in above
mentioned TLVs. 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 | Weight | Range Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs (variable) |
+- -+
| |
where: </artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 5</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>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>Range Size: usage is the same as described in <xref
target="PREFIXSID"/></t>
</list></t>
<t>SID/Label Binding sub-TLV currently supports 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 Metric subTLV -->
<section anchor="METRICTLV" title="ERO Metric sub-TLV">
<t>ERO Metric sub-TLV is a Sub-TLV of the SID/Label Binding TLV.</t>
<t>The ERO Metric sub-TLV carries 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. 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 distract traffic from and to 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 6</t>
<t>Length: 4 bytes</t>
<t>Metric: 4 bytes</t>
</list></t>
</section>
<!-- end 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 last ERO sub-TLV describes the segment
closest to the egress point, contrary the first ERO sub-TLV
describes the first segment of a path. If a router extends or
stitches a path it MUST prepend the new segments path information to
the ERO list.</t>
<t>The above similarly applies to 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 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 of 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 7</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 value of the
attribute is 'loose.' Otherwise, the value of the
attribute is 'strict.'</t>
</list></t>
<t>IPv4 Address - the address of the explicit route hop.</t>
</list></t>
</section>
<section title="IPv6 ERO sub-TLV">
<t>IPv6 ERO sub-TLV is a sub-TLV of the SID/Label Binding
sub-TLV.</t>
<t>The IPv6 ERO sub-TLV (Type TBA) describes a path segment using
IPv6 Address style of encoding. Its semantics have been borrowed
from <xref target="RFC3209"/>. <figure
title="IPv6 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- IPv6 Address -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>Type: TBD, suggested value 8</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 value of the
attribute is 'loose.' Otherwise, the value of the
attribute is 'strict.'</t>
</list></t>
<t>IPv6 Address - the address of the explicit route hop.</t>
</list></t>
</section>
<section anchor="UNNERO" title="Unnumbered Interface ID ERO sub-TLV">
<t>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 spans over an unnumbered interface. Unnumbered
interfaces are referenced using the interface index. Interface
indices are assigned local to the router and therefore not unique
within a domain. All elements in an ERO path need to be unique
within a domain and hence need to be disambiguated using a domain
unique Router-ID. <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 9</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 value of the
attribute is 'loose.' Otherwise, the value of the
attribute is '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 10</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 value of the
attribute is 'loose.' Otherwise, the value of the
attribute is 'strict.'</t>
</list></t>
<t>IPv4 Address - the address of the explicit route hop.</t>
</list></t>
</section>
<section title="IPv6 Backup ERO sub-TLV">
<t>IPv6 ERO sub-TLV is a sub-TLV of the SID/Label Binding
sub-TLV.</t>
<t>The IPv6 Backup ERO sub-TLV describes a Backup path segment
using IPv6 Address style of encoding. Its appearance and semantics
have been borrowed from <xref target="RFC3209"/>.</t>
<t>The 'L' bit in the Flags is a one-bit attribute. If the L bit
is set, then the value of the attribute is 'loose.' Otherwise, the
value of the attribute is 'strict.' <figure
title="IPv6 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- IPv6 Address -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure> where: <list style="hanging">
<t>Type: TBD, suggested value 11</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 value of the
attribute is 'loose.' Otherwise, the value of the
attribute is 'strict.'</t>
</list></t>
<t>IPv6 Address - the address of the explicit route hop.</t>
</list></t>
</section>
<section anchor="UNNBERO"
title="Unnumbered Interface ID Backup ERO sub-TLV">
<t>Unnumbered Interface ID Backup 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 spans over an unnumbered interface. Unnumbered
interfaces are referenced using the interface index. Interface
indices are assigned local to the router and therefore not unique
within a domain. All elements in an ERO path need to be unique
within a domain and hence need to be disambiguated using a domain
unique Router-ID. <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 12</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 value of the
attribute is 'loose.' Otherwise, the value of the
attribute is '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>
<section anchor="ADJSID" title="Adjacency Segment Identifier (Adj-SID)">
<t>An Adjacency Segment Identifier (Adj-SID) represents a router
adjacency in Segment Routing. At the current stage of Segment Routing
architecture it is assumed that the Adj-SID value has local significance
(to the router).</t>
<section anchor="ADJSIDSUBTLV" title="Adj-SID sub-TLV">
<t>A new extended OSPFv3 LSAs, as defined in <xref
target="I-D.ietf-ospf-ospfv3-lsa-extend"/>, are used to advertise prefix
SID in OSPFv3</t>
<t>Adj-SID sub-TLV is an optional sub-TLV of the Router-Link TLV as
defined in <xref target="I-D.ietf-ospf-ospfv3-lsa-extend"/>. It MAY appear
multiple times in Router-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 structure
of the Adj-SID Sub-TLV is as follows: <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 | Weight | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID/Label/Index (variable) |
+---------------------------------------------------------------+
where:</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 10.</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 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 PrefixSID has local significance. If not set, then
the value/index carried by this subTLV 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: MUST be zero when originated and ignored when
received.</t>
</list></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: label or index value depending on the V-bit setting.
<list style="hanging">
<t>Examples: <list style="hanging">
<t>A 32 bit global index defining the offset in the SID/Label space
advertised by this router - in this case the V and L flags MUST be
unset.</t>
<t>A 24 bit local label where the 20 rightmost bits are used for
encoding the label value - in this case the V and L flags MUST be
set.</t>
<t>16 octet IPv6 address - in this case the V-flag MUST be set. The
L-flag MUST be set for link-local IPv6 address and MUST be unset for
IPv6 global unicast address.</t>
</list></t>
</list></t>
</list></t>
<t>A SR capable router MAY allocate an Adj-SID for each of its
adjacencies and set the B-Flag when the adjacency is protected by a
FRR mechanism (IP or MPLS) as described in <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 Router-Link TLV. It MAY
appear multiple times in Router-Link TLV. It is used to advertise
SID/Label for adjacency to non-DR node on 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 | Weight | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID/Label/Index (variable) |
+---------------------------------------------------------------+
where:</artwork>
</figure><list style="hanging">
<t>Type: TBD, suggested value 11.</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 PrefixSID has local significance. If not set, then
the value/index carried by this subTLV 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: MUST be zero when originated and ignored when
received.</t>
</list></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: label or index value depending on the V-bit setting.
<list style="hanging">
<t>Examples: <list style="hanging">
<t>A 32 bit global index defining the offset in the SID/Label space
advertised by this router - in this case the V and L flags MUST be
unset.</t>
<t>A 24 bit local label where the 20 rightmost bits are used for
encoding the label value - in this case the V and L flags MUST be
set.</t>
<t>16 octet IPv6 address - in this case the V-flag MUST be set. The
L-flag MUST be set for link-local IPv6 address and MUST be unset for
IPv6 global unicast address.</t>
</list></t>
</list></t>
</list></t>
</section>
</section>
<section title="Elements of Procedure">
<section title="Intra-area Segment routing in OSPFv3 ">
<t>The OSPFv3 node that supports segment routing MAY advertise Prefix-
SIDs for any prefix that it is advertising reachability for (e.g.
loopback IP address) as described in <xref target="PREFIXSID"/>.</t>
<t>If multiple routers advertise Prefix-SID for the same prefix, then
the Prefix-SID MUST be the same. This is required in order to allow
traffic load-balancing if multiple equal cost paths to the destination
exist in the network.</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-SID for remote prefixes that exist in the
network. Multiple Mapping Servers can advertise Prefix-SID for the
same prefix, in which case the same Prefix-SID MUST be advertised by
all of them. SR Mapping Server could use either area scope or
autonomous system flooding scope when advertising Prefix SID for
prefixes, based on the configuration of the SR Mapping Server.
Depending on the flooding scope used, SR Mapping Server chooses the
LSA that will be used. If the area flooding scope is needed,
E-Intra-Area-Prefix-LSA (<xref target="I-D.ietf-ospf-ospfv3-lsa-extend"/>)
is used. If autonomous system flooding scope is needed,
E-AS-External-LSA (<xref target="I-D.ietf-ospf-ospfv3-lsa-extend"/>) is
used.</t>
<t>When Prefix-SID is advertised by the Mapping Server, which is
indicated by the M-flag in the Prefix-SID sub-TLV (<xref
target="PREFIXSID"/>), route-type as indicated by the LSA type which
is being used for flooding is ignored. Prefix SID is bound to a
prefix, in which case route-type becomes unimportant.</t>
<t>Advertisement of the Prefix-SID by the Mapping Server using
Inter-Area Prefix TLV, External Prefix TLV or Intra-Area-Prefix TLV
(<xref target="I-D.ietf-ospf-ospfv3-lsa-extend"/>) does not itself
contribute to the prefix reachability. NU-bit 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 such advertisement to
contribute to the prefix reachability.</t>
</section>
<section title="Inter-area Segment routing in OSPFv3">
<t>In order to support SR in a multi-area environment, OSPFv3 must
propagate Prefix-SID information between areas. The following
procedure is used in order to propagate Prefix SIDs between areas.</t>
<t>When an OSPFv3 ABR advertises a Inter-Area-Prefix-LSA from an
intra-area prefix to all its connected areas, it will also include
Prefix-SID sub-TLV, as described in <xref target="PREFIXSID"/>. 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 out the advertising router associated with its best
path to that prefix.</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, then the ABR will use the Prefix-SID advertised by any
other router (e.g.: a Prefix-SID coming from an SR Mapping Server
as defined in <xref
target="I-D.filsfils-rtgwg-segment-routing-use-cases"/>) when
propagating Prefix-SID for the prefix to other areas.</t>
</list></t>
<t>When an OSPFv3 ABR advertises Inter-Area-Prefix-LSA LSAs from an
inter-area route to all its connected areas it will also include
Prefix-SID sub-TLV, as described in <xref target="PREFIXSID"/>. 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 out the advertising router associated with its best
path to that prefix.</t>
<t>The ABR will then look if such router advertised a Prefix-SID
for the prefix and use it when advertising the Prefix-SID to other
connected areas.</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 Prefix-SID for the prefix to other areas.</t>
</list></t>
</section>
<section title="SID for External Prefixes">
<t>AS-External-LSAs are flooded domain wide. When an ASBR, which
supports SR, generates AS-External-LSA, it should also include
Prefix-SID sub-TLV, as described in <xref target="PREFIXSID"/>
Prefix-SID value will be set to the SID that has been reserved for
that prefix.</t>
<t>When a NSSA ASBR translates NSSA-LSA into 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
NSSA-LSA and finds the advertising router associated with such path.
If such advertising router has advertised a Prefix-SID for the prefix,
then the NSSA ASBR uses it when advertising the Prefix-SID in
AS-External-LSA. 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>Adj-SID MAY be advertised for any adjacency on p2p link that is
in a state 2-Way or higher. If the adjacency on a p2p link
transitions from the FULL state, then the Adj-SID for that adjacency
MAY be removed from the area. If the adjacency transitions to a
state lower then 2-Way, then the Adj-SID MUST be removed from the
area.</t>
</section>
<section title="Adjacency SID on Broadcast or NBMA Interfaces">
<t>Broadcast or NBMA networks in OSPFv3 are represented by a star
topology where the Designated Router (DR) is the central point all
other routers on the broadcast or NBMA network connect to. As a
result, routers on the broadcast or NBMA network advertise only
their adjacency to DR and BDR. Routers that are neither DR nor BDR
do not form and do not advertise adjacencies between them. They,
however, maintain a 2-Way adjacency state between them.</t>
<t>When Segment Routing is used, each router on the broadcast or
NBMA network MAY advertise the Adj-SID for its adjacency to DR using
Adj-SID Sub-TLV as described in <xref target="ADJSIDSUBTLV"/>.</t>
<t>SR capable router MAY also advertise Adj-SID for other neighbors
(e.g. BDR, DR-OTHER) on broadcast or NBMA network using the LAN
ADJ-SID Sub-TLV as described in section 5.1.1.2. <xref
target="LANADJSIDSUBTLV"/>.</t>
</section>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This specification updates two existing OSPF registries.</t>
<section anchor="RILSAREG" title="OSPF Router Information (RI) TLVs Registry">
<t>o suggested value 8 - SR-Algorithm TLV</t>
<t>o suggested value 9 - SID/Label Range TLV</t>
</section>
<section anchor="EXTLSAREG" title="OSPFv3 Extend-LSA sub-TLV registry">
<t>o suggested value 1 - SID/Label sub-TLV</t>
<t>o suggested value 2 - Prefix SID sub-TLV</t>
<t>o suggested value 3 - Adj-SID sub-TLV</t>
<t>o suggested value 4 - LAN Adj-SID sub-TLV</t>
<t>o suggested value 5 - SID/Label Binding sub-TLV</t>
<t>o suggested value 6 - ERO Metric sub-TLV</t>
<t>o suggested value 7 - IPv4 ERO sub-TLV</t>
<t>o suggested value 8 - IPv6 ERO sub-TLV</t>
<t>o suggested value 9 - Unnumbered Interface ID ERO sub-TLV</t>
<t>o suggested value 10 - IPv4 Backup ERO sub-TLV</t>
<t>o suggested value 11 - IPv6 Backup ERO sub-TLV</t>
<t>o suggested value 12 - Unnumbered Interface ID Backup ERO sub-TLV</t>
</section>
</section>
<section anchor="Security" title="Security Considerations">
<t>Implementations must assure that malformed permutations of the newly
defined sub-TLvs do not result in errors which cause hard OSPFv3
failures.</t>
</section>
<section anchor="Contributors" title="Contributors">
<t>The following people gave a substantial contribution to the content
of this document: 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>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3209.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3477.xml"?>
<?rfc ?>
<?rfc ?>
<?rfc ?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.4970.xml"?>
<?rfc ?>
<?rfc ?>
<?rfc ?>
</references>
<references title="Informative References">
<?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"?>
<?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-ospf-ospfv3-lsa-extend.xml"?>
</references>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-23 03:20:13 |