Signaling MSD (Maximum SID Depth) using IS-ISNuage Networksjefftant.ietf@gmail.comHuawei Technologiesuma.chunduri@huawei.comGoogle, Incaldrin.ietf@gmail.comCisco Systemsginsberg@cisco.com
Routing
IS-IS Working GroupInternet-DraftBGP-LSSIDMSDIS-ISThis document defines a way for an IS-IS Router to advertise multiple
types of supported Maximum SID Depths (MSDs) at node and/or link
granularity. Such advertisements allow entities (e.g., centralized
controllers) to determine whether a particular SID stack can be supported
in a given network. This document only defines one type of MSD maximum
label imposition, but defines an encoding that can support other MSD
types.When Segment Routing(SR) paths are computed by a centralized
controller, it is critical that the controller learns the Maximum SID
Depth(MSD) that can be imposed at each node/link a given SR path
to insure that the SID stack depth of a computed path
doesn't exceed the number of SIDs the node is capable of imposing.PCEP SR extensions draft signals MSD in SR PCE Capability
TLV and METRIC Object. However, if PCEP is not supported/configured on
the head-end of a SR tunnel or a Binding-SID anchor node and controller
does not participate in IGP routing, it has no way to learn the MSD of
nodes and links which has been configured. BGP-LS defines a way to expose topology and associated
attributes and capabilities of the nodes in that topology to a
centralized controller. MSD signaling by BGP-LS has been defined in
. Typically,
BGP-LS is configured on a small number of nodes that do not necessarily
act as head-ends. In order for BGP-LS to signal MSD for all the nodes
and links in the network MSD is relevant, MSD capabilites should be
advertised to every IS-IS router in the network.Other types of MSD are known to be useful. For example, defines Readable Label Depth
Capability (RLDC) that is used by a head-end to insert an Entropy Label
(EL) at a depth, that could be read by transit nodes.This document defines an extension to IS-IS used to advertise one or
more types of MSD at node and/or link granularity. It also creates an
IANA registry for assigning MSD type identifiers. It also defines the Base MPLS Imposition MSD type.
In the future it is expected, that new MSD types will be defined to signal additional capabilities
e.g., entropy labels, SIDs that can be imposed through recirculation, or
SIDs associated with another dataplane e.g., IPv6.BGP-LS: Distribution of Link-State and TE Information using
Border Gateway ProtocolBMI: Base MPLS Imposition is the number of MPLS labels which can
be imposed inclusive of any service/transport labelsIS-IS: Intermediate System to Intermediate SystemMSD: Maximum SID Depth - the number of SIDs a node or a link on a
node can supportPCC: Path Computation ClientPCE: Path Computation ElementPCEP: Path Computation Element ProtocolSID: Segment IdentifierSR: Segment RoutingThe 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 BCP14 ,
when, and only when they appear in all capitals, as shown here .This memo makes use of the terms defined in .The node MSD sub-TLV is defined within the body of the
IS-IS Router Capability TLV , to carry the
provisioned SID depth of the router originating the Router Capability TLV.
Node MSD is the minimum MSD supported by the node on any interface. MSD
values may be learned via a hardware API or may be provisioned.The Type: TBD1Length: variable (minimum of 2, multiple of 2 octets) and
represents the total length of value field.Value: field consists of one or more pairs of a 1 octet MSD-Type (IANA
Registry) and 1 octet Value.Node MSD value is a number in the range of 0-255. 0 represents lack
of the ability to support SID stack of any depth; any other value
represents that of the node. This value MUST represent the lowest value
supported by any link associated with the node.This sub-TLV is optional. The scope of the advertisement is specific
to the deployment.The link MSD sub-TLV is defined for TLVs 22, 23, 141,
222, and 223 to carry the MSD of the interface associated with the link.
MSD values may be learned via a hardware API or may be provisioned.The Type: TBD2Length: variable (minimum of 2, multiple of 2 octets) and
represents the total length of value field.Value: consists of one or more pairs of a 1 octet MSD-Type (IANA
Registry) and 1 octet Value.Link MSD value is a number in the range of 0-255. 0 represents lack
of the ability to support SID stack of any depth; any other value
represents that of the link when used as an outgoing link.This sub-TLV is optional. The scope of the advertisement is specific
to the deployment.When Link MSD is present for a given MSD type, the value of the Link
MSD MUST take preference over the Node MSD.The meaning of the absence of both Node and Link MSD advertisements
for a given MSD type is specific to the MSD type. Generally it can only
be inferred that the advertising node does not support advertisement of
that MSD type. However, in some cases the lack of advertisement might
imply that the functionality associated with the MSD type is not
supported. The correct interpretation MUST be specified when an MSD type
is defined.Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
labels a node is capable of imposing, including any service/transport
labels.Absence of BMI-MSD advertisements indicates solely that the advertising
node does not support advertisement of this capability.This document requests IANA to allocate a sub-TLV type (TBD1) for the
new sub TLV proposed in of this
document from IS-IS Router Capability TLV Registry as defined by .IANA has allocated the following value through the early assignment process:This document requests IANA to allocate a sub-TLV type (TBD2) as
defined in from Sub-TLVs for TLVs 22,
23, 141, 222 and 223 registry.IANA has allocated the following value through the early assignment process:Per TLV information where Link MSD sub-TLV can be part of:This document requests creation of an IANA managed registry under
a new category of "Interior Gateway Protocol (IGP) Parameters" IANA
registries to identify MSD types as proposed in and . The
registration procedure is "Expert Review" as defined in . Suggested registry name is "MSD types". Types are an
unsigned 8 bit number. The following values are defined by this document
Security considerations, as specified by are
applicable to this documentThe following people contributed to this document:Peter PsenakEmail: ppsenak@cisco.comThe authors would like to thank Acee Lindem, Stephane Litkowski and Bruno Decraene
for their reviews and valuable comments.