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 an 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. 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 by 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. Although MSD
advertisements are associated with Segment Routing, the advertisements
MAY be present even if Segment Routing itself is not enabled.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 all service/transport/special 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 BCP
14 when, and only
when, they appear in all capitals, as shown here .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 smallest MSD supported by the node on the set of interfaces
configured for use by the advertising IGP instance. MSD values may be
learned via a hardware API or may be provisioned.Type: 23 (allocated by IANA via the early assignment process)Length: 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 configured for use by the advertising IS-IS
instance.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, 25, 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.Type: 15 (allocated by IANA via the early assignment process)Length: 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. When a Link MSD type is not
signalled but the Node MSD type is, then the Node MSD type value MUST be
considered as the MSD value for that link.In order to increase flooding efficiency, it is RECOMMENDED that
routers with homogenous link MSD values advertise just the Node MSD
value.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 all
service/transport/special 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 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 as defined in
from Sub-TLVs for TLVs 22, 23, 25,
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 document.Advertisement of the additional information defined in this document
that is false, e.g., an MSD that is incorrect, may result in a path
computation failing, having a service unavailable, or instantiation of a
path that can't be supported by the head-end (the node performing the
imposition).The following people contributed to this document:Peter PsenakEmail: ppsenak@cisco.comThe authors would like to thank Acee Lindem, Ketan Talaulikar,
Stephane Litkowski and Bruno Decraene for their reviews and valuable
comments.