IS-IS Extensions for Advertising Router
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Routing Area
Networking Working GroupSampleThis document defines a new optional Intermediate System to
Intermediate System (IS-IS) TLV named CAPABILITY, formed of multiple
sub-TLVs, which allows a router to announce its capabilities within an
IS-IS level or the entire routing domain. This document obsoletes RFC
4971.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 RFC 2119 [RFC2119].There are several situations where it is useful for the IS-IS
[ISO10589] [RFC1195] routers to learn the capabilities of the other
routers of their IS-IS level, area, or routing domain. For the sake of
illustration, three examples related to MPLS Traffic Engineering (TE)
are described here:Mesh-group: the setting up of a mesh of TE Label Switched Paths
(LSPs) [RFC5305] requires some significant configuration effort.
[RFC4972] proposes an auto-discovery mechanism whereby every Label
Switching Router (LSR) of a mesh advertises its mesh-group
membership by means of IS-IS extensions.Point to Multipoint TE LSP (RFC4875). A specific sub-TLV
[RFC5073] allows an LSR to advertise its Point To Multipoint
capabilities ([RFC4875] and [RFC4461]).Inter-area traffic engineering: Advertisement of the IPv4 and/or
the IPv6 Traffic Engineering Router IDs.The use of IS-IS for Path Computation Element (PCE) discovery may
also be considered and will be discussed in the PCE WG.The capabilities mentioned above require the specification of new
sub-TLVs carried within the CAPABILITY TLV defined in this document.Note that the examples above are provided for the sake of
illustration. This document proposes a generic capability advertising
mechanism that is not limited to MPLS Traffic Engineering.This document defines a new optional IS-IS TLV named CAPABILITY,
formed of multiple sub-TLVs, which allows a router to announce its
capabilities within an IS-IS level or the entire routing domain. The
applications mentioned above require the specification of new sub- TLVs
carried within the CAPABILITY TLV defined in this document.Definition of these sub-TLVs is outside the scope of this
document.The IS-IS Router CAPABILITY TLV is composed of 1 octet for the type,
1 octet that specifies the number of bytes in the value field, and a
variable length value field that starts with 4 octets of Router ID,
indicating the source of the TLV, followed by 1 octet of flags.A set of optional sub-TLVs may follow the flag field. Sub-TLVs are
formatted as described in [RFC5305].Currently two bit flags are defined.S bit (0x01): If the S bit is set(1), the IS-IS Router CAPABILITY TLV
MUST be flooded across the entire routing domain. If the S bit is not
set(0), the TLV MUST NOT be leaked between levels. This bit MUST NOT be
altered during the TLV leaking.D bit (0x02): When the IS-IS Router CAPABILITY TLV is leaked from
level-2 to level-1, the D bit MUST be set. Otherwise, this bit MUST be
clear. IS-IS Router CAPABILITY TLVs with the D bit set MUST NOT be
leaked from level-1 to level-2. This is to prevent TLV looping.The Router CAPABILITY TLV is OPTIONAL. As specified in Section 3,
more than one Router CAPABILITY TLV from the same source MAY be
present.This document does not specify how an application may use the Router
CAPABILITY TLV and such specification is outside the scope of this
document.The Router ID SHOULD be identical to the value advertised in the
Traffic Engineering Router ID TLV [RFC5305]. If no Traffic Engineering
Router ID is assigned the Router ID SHOULD be identical to an IP
Interface Address [RFC1195] advertised by the originating IS. If the
originating node does not support IPv4, then the reserved value 0.0.0.0
MUST be used in the Router ID field and the IPv6 TE Router ID sub-TLV
[RFC5316] MUST be present in the TLV. Router CAPABILITY TLVs which have
a Router ID of 0.0.0.0 and do NOT have the IPv6 TE Router ID sub-TLV
present MUST NOT be used.When advertising capabilities with different flooding scopes, a
router MUST originate a minimum of two Router CAPABILITY TLVs, each TLV
carrying the set of sub-TLVs with the same flooding scope. For instance,
if a router advertises two sets of capabilities, C1 and C2, with an
area/level scope and routing domain scope respectively, C1 and C2 being
specified by their respective sub-TLV(s), the router will originate two
Router CAPABILITY TLVs:In order to prevent the use of stale CAPABILITY TLVs, a system MUST
NOT use a CAPABILITY TLV present in an LSP of a system that is not
currently reachable via Level-x paths, where "x" is the level (1 or 2)
in which the sending system advertised the TLV. This requirement applies
regardless of whether or not the sending system is the originator of the
CAPABILITY TLV.When a CAPABILITY TLV is not used, either due to lack of reachability
to the originating router or due to unusable Router ID, note that
leaking the CAPABILITY TLV is one of the uses that is prohibited under
these conditions.In IS-IS, the atomic unit of the update process is a TLV - or more
precisely, in the case of TLVs that allow multiple entries to appear in
the value field (e.g., IS-neighbors), the atomic unit is an entry in the
value field of a TLV. If an update to an entry in a TLV is advertised in
an LSP fragment different from the LSP fragment associated with the old
advertisement, the possibility exists that other systems can temporarily
have either 0 copies of a particular advertisement or 2 copies of a
particular advertisement, depending on the order in which new copies of
the LSP fragment that had the old advertisement and the fragment that
has the new advertisement arrive at other systems.Wherever possible, an implementation SHOULD advertise the update to a
CAPABILITY TLV in the same LSP fragment as the advertisement that it
replaces. Where this is not possible, the two affected LSP fragments
should be flooded as an atomic action.Systems that receive an update to an existing CAPABILITY TLV can
minimize the potential disruption associated with the update by
employing a holddown time prior to processing the update so as to allow
for the receipt of multiple LSP fragments associated with the same
update prior to beginning processing.Where a receiving system has two copies of a CAPABILITY TLV from the
same system that have conflicting information for a given sub-TLV, the
procedure used to choose which copy shall be used is undefined.Routers that do not support the Router CAPABILITY TLV MUST silently
ignore the TLV(s) and continue processing other TLVs in the same LSP.
Routers that do not support specific sub-TLVs carried within a Router
CAPABILITY TLV MUST silently ignore the unsupported sub-TLVs and
continue processing those sub-TLVs that are supported in the Router
CAPABILITY TLV. How partial support may impact the operation of the
capabilities advertised within the Router CAPABILITY TLV is outside the
scope of this document.In order for Router CAPABILITY TLVs with domain-wide scope originated
by L1 Routers to be flooded across the entire domain, at least one L1/L2
Router in every area of the domain MUST support the Router CAPABILITY
TLV.If leaking of the CAPABILITY TLV is required, the entire CAPABILITY
TLV MUST be leaked into another level without change (except for changes
to the TLV flags as noted in Section 2) even though it may contain some
sub-TLVs which are unsupported by the Router doing the leaking.Any new security issues raised by the procedures in this document
depend upon the opportunity for LSPs to be snooped and modified, the
ease/difficulty of which has not been altered. As the LSPs may now
contain additional information regarding router capabilities, this new
information would also become available to an attacker. Specifications
based on this mechanism need to describe the security considerations
around the disclosure and modification of their information. Note that
an integrity mechanism, such as the one defined in [RFC5304] or
[RFC5310], should be applied if there is high risk resulting from
modification of capability information.IANA assigned a new IS-IS TLV code-point for the newly defined IS-IS
TLV type named the IS-IS Router CAPABILITY TLV and defined in this
document. The assigned value is 242.For the original version of this document (RFC 4971) the authors
thanked Jean-Louis Le Roux, Paul Mabey, Andrew Partan, and Adrian Farrel
for their useful comments.For this new version the authors would like to thank Kris Michielsen
for calling attention to the problem associated with an IPv6 only
router.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)International Organization for
StandardizationThis document makes the following changes to RFC 4971.RFC 4971 only allowed a 32 bit Router ID in the fixed header of TLV
242. This is problematic in an IPv6-only deployment where an IPv4
address may not be available. This document specifies:The Router ID SHOULD be identical to the value advertised in the
Traffic Engineering Router ID TLV (134) if available.If no Traffic Engineering Router ID is assigned the Router ID
SHOULD be identical to an IP Interface Address [RFC1195] advertised
by the originating IS.If the originating node does not support IPv4, then the reserved
value 0.0.0.0 MUST be used in the Router ID field and the IPv6 TE
Router ID sub-TLV [RFC5316] MUST be present in the TLV.In addition, some clarifying editorial changes have been made.