IS-IS Extensions to
Support Routing over IPv6 DataplaneCisco Systems821 Alder DriveMilpitas95035CAUSAginsberg@cisco.comCisco SystemsPribinova Street 10Bratislava 81109Slovakiappsenak@cisco.comCisco SystemsBrusselsBelgiumcfilsfil@cisco.comIndividualabashandy.ietf@gmail.comOrangeIssy-les-MoulineauxFrancebruno.decraene@orange.comHuawei Technologieshuzhibo@huawei.com
Routing Area
Networking Working GroupSegment Routing (SR) allows for a flexible definition of end-to-end
paths by encoding paths as sequences of topological sub-paths, called
"segments". Segment routing architecture can be implemented over an MPLS
data plane as well as an IPv6 data plane. This draft describes the IS-IS
extensions required to support Segment Routing over an IPv6 data
plane.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
[RFC2119] [RFC8174] when, and only when, they appear in all capitals, as
shown here.With Segment Routing (SR) , a node steers a packet
through an ordered list of instructions, called segments.Segments are identified through Segment Identifiers (SIDs).Segment Routing can be directly instantiated on the IPv6 data plane
through the use of the Segment Routing Header defined in . SRv6 refers to this SR
instantiation on the IPv6 dataplane.The network programming paradigm is central to
SRv6. It describes how any function can be bound to a SID and how any
network program can be expressed as a combination of SID's.This document specifies IS-IS extensions that allow the IS-IS
protocol to encode some of these functions.Familiarity with the network programming paradigm is necessary to
understand the extensions specified in this document.This document defines one new top level IS-IS TLV and several new IS-
IS sub-TLVs.The SRv6 Capabilities sub-TLV announces the ability to support SRv6
and some Endpoint functions listed in Section 7 as well as advertising
limitations when applying such Endpoint functions.The SRv6 Locator top level TLV announces SRv6 locators - a form of
summary address for the set of topology/algorithm specific SIDs
associated with a node.The SRv6 End SID sub-TLV, the SRv6 End.X SID sub-TLV, and the SRv6
LAN End.X SID sub-TLV are used to advertise which SIDs are instantiated
at a node and what Endpoint function is bound to each instantiated
SID.A node indicates that it has support for SRv6 by advertising a new
SRv6- capabilities sub-TLV of the router capabilities TLV [RFC7981].The SRv6 Capabilities sub-TLV may contain optional sub-sub-TLVs. No
sub-sub-TLVs are currently defined.The SRv6 Capabilities sub-TLV has the following format: defines the means
to advertise node/link specific values for Maximum SID Depths (MSD) of
various types. Node MSDs are advertised in a sub-TLV of the Router
Capabilities TLV . Link MSDs are advertised in a
sub-TLV of TLVs 22, 23, 141, 222, and 223.This document defines the relevant SRv6 MSDs and requests MSD type
assignments in the MSD Types registry created by .The Maximum Segments Left MSD Type specifies the maximum value of
the "SL" field
in the SRH of a received packet before applying the Endpoint function
associated with a SID.The Maximum End Pop MSD Type specifies the maximum number of SIDs
in the top SRH in an SRH stack to which the router can apply "PSP" or
USP" as defined in flavors.The Maximum T.Insert MSD Type specifies the maximum number of SIDs
that can be inserted as part of the "T.insert" behavior as defined in
.The Maximum T.Encaps MSD Type specifies the maximum number of SIDs
that can be included as part of the "T.Encaps" behavior as defined in
.The Maximum End D MSD Type specifies the maximum number of SIDs in
an SRH when performing decapsulation associated with "End.Dx"
functions (e.g., "End.DX6" and "End.DT6") as defined in .As discussed in , an SRv6 Segment
Identifier (SID) is 128 bits and represented asLOC:FUNCTwhere LOC (the locator portion) is the L most significant bits and
FUNCT is the 128-L least significant bits. L is called the locator
length and is flexible. Each operator is free to use the locator length
it chooses.A node is provisioned with topology/algorithm specific locators for
each of the topology/algorithm pairs supported by that node. Each
locator is a covering prefix for all SIDs provisioned on that node which
have the matching topology/algorithm.Locators MUST be advertised in the SRv6 Locator TLV (see Section
6.1). Forwarding entries for the locators advertised in the SRv6 Locator
TLV MUST be installed in the forwarding plane of receiving SRv6 capable
routers when the associated topology/algorithm is supported by the
receiving node.Locators are routable and MAY also be advertised in Prefix
Reachability TLVs (236 or 237).Locators associated with algorithm 0 (for all supported topologies)
SHOULD be advertised in a Prefix Reachability TLV (236 or 237) so that
legacy routers (i.e., routers which do NOT support SRv6) will install a
forwarding entry for algorithm 0 SRv6 traffic.In cases where a locator advertisement is received in both in a
Prefix Reachability TLV and an SRv6 Locator TLV, the Prefix Reachability
advertisement MUST be preferred when installing entries in the
forwarding plane. This is to prevent inconsistent forwarding entries on
SRv6 capable/SRv6 incapable routers.SRv6 SIDs are advertised as sub-TLVs in the SRv6 Locator TLV except
for SRv6 End.X SIDs/LAN End.X SIDs which are associated with a specific
Neighbor/Link and are therefore advertised as sub-TLVs in TLVs 22, 23,
222, 223, and 141.SRv6 SIDs are not directly routable and MUST NOT be installed in the
forwarding plane. Reachability to SRv6 SIDs depends upon the existence
of a covering locator.Adherence to the rules defined in this section will assure that SRv6
SIDs associated with a supported topology/algorithm pair will be
forwarded correctly, while SRv6 SIDs associated with an unsupported
topology/algorithm pair will be dropped. NOTE: The drop behavior depends
on the absence of a default/summary route covering a given locator.In order for forwarding to work correctly, the locator associated
with SRv6 SID advertisements MUST be the longest match prefix installed
in the forwarding plane for those SIDs. There are a number of ways in
which this requirement could be compromisedAnother locator associated with a different topology/algorithm is
the longest matchA prefix advertisement (i.e., from TLV 236 or 237) is the longest
matchThe SRv6 Locator TLV is introduced to advertise SRv6 Locators and End
SIDs associated with each locator.This new TLV shares the sub-TLV space defined for TLVs 135, 235, 236
and 237.The SRv6 Locator TLV has the following format:The SRv6 End SID sub-TLV is introduced to advertise SRv6 Segment
Identifiers (SID) with Endpoint functions which do not require a
particular neighbor in order to be correctly applied . SRv6 SIDs
associated with a neighbor are advertised using the sub-TLVs defined
in Section 6.This new sub-TLV is advertised in the SRv6 Locator TLV defined in
the previous section. SRv6 End SIDs inherit the topology/algorithm
from the parent locator.The SRv6 End SID sub-TLV has the following format:The SRv6 End SID MUST be a subnet of the associated
Locator. SRv6 End SIDs which are NOT a subnet of the associated
locator MUST be ignored.Multiple SRv6 End SIDs MAY be associated with the same locator. In
cases where the number of SRv6 End SID sub-TLVs exceeds the capacity
of a single TLV, multiple Locator TLVs for the same locator MAY be
advertised. For a given MTID/Locator the algorithm MUST be the same in
all TLVs. If this restriction is not met all TLVs for that
MTID/Locator MUST be ignored.Certain SRv6 Endpoint functions must be
associated with a particular neighbor, and in case of multiple layer 3
links to the same neighbor, with a particular link in order to be
correctly applied.This document defines two new sub-TLVs of TLV 22, 23, 222, 223, and
141 - namely "SRv6 End.X SID" and "SRv6 LAN End.X SID".IS-IS Neighbor advertisements are topology specific - but not
algorithm specific. End.X SIDs therefore inherit the topology from the
associated neighbor advertisement, but the algorithm is specified in the
individual SID.All End.X SIDs MUST be a subnet of a Locator with matching topology
and algorithm which is advertised by the same node in an SRv6 Locator
TLV. End.X SIDs which do not meet this requirement MUST be ignored.This sub-TLV is used to advertise an SRv6 SID associated with a
point to point adjacency. Multiple SRv6 End.X SID sub-TLVs MAY be
associated with the same adjacency.The SRv6 End.X SID sub-TLV has the following format:Note that multiple TLVs for the same neighbor may be
required in order to advertise all of the SRv6 End.X SIDs associated
with that neighbor.This sub-TLV is used to advertise an SRv6 SID associated with a LAN
adjacency. Since the parent TLV is advertising an adjacency to the
Designated Intermediate System(DIS) for the LAN, it is necessary to
include the System ID of the physical neighbor on the LAN with which
the SRv6 SID is associated. Given that a large number of neighbors may
exist on a given LAN a large number of SRv6 LAN END.X SID sub-TLVs may
be associated with the same LAN. Note that multiple TLVs for the same
DIS neighbor may be required in order to advertise all of the SRv6
End.X SIDs associated with that neighbor.The SRv6 LAN End.X SID sub-TLV has the following format:Endpoint function types are defined in . The numerical
values are defined in the "SRv6 Endpoint Types" registry defined in
. This
section lists the Endpoint function types which MAY be advertised by
IS-IS and the SID sub-TLVs in which each type MAY appear.This document requests allocation for the following TLVs, sub- TLVs,
and sub-sub-TLVs as well updating the ISIS TLV registry and defining a
new registry.This document adds one new TLV to the IS-IS TLV Codepoints
registry.Value: 27 (suggested - to be assigned by IANA)Name: SRv6 LocatorThis TLV shares sub-TLV space with existing "Sub-TLVs for TLVs 135,
235, 236 and 237 registry". The name of this registry needs to be
changed to "Sub-TLVs for TLVs 27, 135, 235, 236 and 237 registry".This document adds the following new sub-TLV to the (renamed)
"Sub-TLVs for TLVs 27, 135, 235, 236 and 237 registry".Value: 5 (suggested - to be assigned by IANA)Name: SRv6 End SIDThis document requests the creation of a new IANA managed
registry for sub-sub-TLVs of the SRv6 End SID sub-TLV. The
registration procedure is "Expert Review" as defined in [RFC7370].
Suggested registry name is "sub-sub-TLVs for SRv6 End SID sub-TLV".
No sub-sub-TLVs are defined by this document except for the reserved
value.0: Reserved1-255: UnassignedThe revised table of sub-TLVs for the (renamed) "Sub-TLVs for
TLVs 27, 135, 235, 236 and 237 registry" is shown below:This document adds the definition of a new sub-TLV in the "Sub-
TLVs for TLV 242 registry".Type: 25 (Suggested - to be assigned by IANA)Description: SRv6 CapabilitiesThis document requests the creation of a new IANA managed registry
for sub-sub-TLVs of the SRv6 Capability sub-TLV. The registration
procedure is "Expert Review" as defined in [RFC7370]. Suggested
registry name is "sub-sub-TLVs for SRv6 Capability sub-TLV". No
sub-sub-TLVs are defined by this document except for the reserved
value.0: Reserved1-255: UnassignedThis document adds the definition of two new sub-TLVs in the "sub-
TLVs for TLV 22, 23, 25, 141, 222 and 223 registry".Type: 43 (suggested - to be assigned by IANA)Description: SRv6 End.X SIDType: 44 (suggested - to be assigned by IANA)Description: SRv6 LAN End.X SIDThis document defines the following new MSD types. These types are
to be defined in the IGP MSD Types registry defined in .All values are suggested values to be assigned by IANA.Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],
and [RFC5310].The following people gave a substantial contribution to the content
of this document and should be considered as co-authors:Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in conjunction with
the protocol for providing the connectionless-mode Network Service
(ISO 8473), ISO/IEC 10589:2002, Second Edition.