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 Intermediate System to
Intermediate System (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 Segment Identifier (SID) stack depth of a computed path
doesn't exceed the number of SIDs the node is capable of imposing.Path Computation Element Protocol (PCEP) SR extensions draft signals MSD in SR Path
Computation Element (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
(Distribution of Link-State and TE Information using Border Gateway
Protocol) 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 capabilities should be
advertised by every Intermediate System to Intermediate System(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. Note that
in a non-SR MPLS network, label depth is what is defined by the MSD
advertisements.BMI: Base MPLS Imposition is the number of MPLS labels which can be
imposed inclusive of all service/transport/special labelsMSD: Maximum SID Depth - the number of SIDs a node or a link on a
node can supportThe 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 (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 and
1 octet MSD-Value.MSD-Type is a value defined in the IGP MSD Types registry created by
the IANA Section of this document.MSD-Value is a number in the range of 0-255. For all MSD-Types, 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.If there exist multiple Node MSD advertisements for the same MSD-Type
originated by the same router, the procedures defined in apply. These procedures may result in different MSD
values being used by (for example) different controllers - but this does
not create any interoperability issue.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 (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 and 1
octet MSD-Value.MSD-Type is a value defined in the MSD Types registry created by the
IANA Section of this document.MSD-Value is a number in the range of 0-255. For all MSD-Types, 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.If multiple Link MSD advertisements for the same MSD-Type and the
same link are received, the procedure used to select which copy is used
is undefined.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
signaled 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 which can be imposed, 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 the
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 "IGP MSD Types". Types are an unsigned 8 bit number.
The following values are defined by this document General guidance for the Designated Experts is as defined in 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 presence of this information also may inform an attacker of how
to induce any of the aforementioned conditions.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.