Signaling MSD (Maximum SID Depth) using OSPFNuage Networksjefftant.ietf@gmail.comHuawei Technologiesuma.chunduri@huawei.comGoogle, Incaldrin.ietf@gmail.comCisco Systemsppsenak@cisco.com
Routing
OSPF Working GroupInternet-DraftBGP-LSSIDMSD OSPFThis document defines a way for an OSPF 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 is supportable
in a given network. This document only defines one type of MSD (maximum
label imposition) - but defines an encoding which can support other MSD
types. Here the term OSPF means both OSPFv2 and OSPFv3.When Segment Routing(SR) paths are computed by a centralized
controller, it is critical that the controller learns the Maximum SID
Depth(MSD) which can be imposed at the node/link a given SR path is
applied so as 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 OSPF 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 Entropy Label
(EL) at appropriate depth, so it could be read by transit nodes.This document defines an extension to OSPF 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 one
MSD type called Base MPLS Imposition MSD. 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 labelsOSPF: Open Shortest Path FirstMSD: 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.This memo makes use of the terms defined in .A new TLV within the body of the OSPF RI Opaque LSA, called
Node MSD TLV is defined to carry the provisioned SID depth
of the router originating the RI LSA. Node MSD is the lowest MSD supported
by the node.
The Type (2 bytes) of this TLV has value of 12. Length is variable (minimum of 2, multiple of 2 octets) and represents the total length of value field. Value field consists of a 1 octet sub-type (IANA Registry) and 1 octet value. Sub-Type 1 (IANA Section), MSD and
the Value field contains maximum MSD of the router originating the RI LSA. Node Maximum MSD
is a number in the range of 0-254. 0 represents lack of the ability to impose MSD stack
of any depth; any other value represents that of the node. This value SHOULD
represent the lowest value supported by node. Other Sub-types other than defined above are reserved for future extensions. This TLV is applicable to OSPFv2 and to
OSPFv3 and is optional. The scope of the advertisement is specific to the deployment.
A new sub-TLV called Link MSD sub-TLV is defined to carry the provisioned SID depth
of the interface associated with the link. The Type (2 bytes) of this TLV:For OSPFv2, the Link level MSD value is advertised as an optional Sub-TLV
of OSPFv2 Extended Link TLV as defined in , and has value of 6.For OSPFv3, the Link level MSD value is advertised as an optional Sub-TLV of the Router-Link TLV as
defined in , and has value of 16 (Suggested value - to be assigned by IANA).
Length is variable and similar to what is defined in . Value field consists of a 1 octet sub-type (IANA Registry) and 1 octet value. Sub-Type 1 (IANA Section), MSD and
the Value field contains Link MSD of the router originating the corresponding LSA as specified for OSPFv2 and OSPFv3.
Link MSD is a number in the range of 0-254. 0 represents lack of the ability to impose MSD stack
of any depth; any other value represents that of the particular link MSD value. Other Sub-types other than defined above are reserved for future extensions. When Link MSD is present for a given MSD type, the value of the Link
MSD MUST be used in preference to 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 only that the advertising
node does not support advertisement of this capability.This document includes a request to IANA to allocate TLV type
codes for the new TLV proposed in
of this document from OSPF Router
Information (RI) TLVs Registry as defined by .
For the link MSD, we request IANA to allocate new sub-TLV codes as proposed
in from OSPFv2 Extended Link TLV Sub-TLVs registry and
from Router-Link TLV defined in OSPFv3 Extend-LSA Sub-TLV registry.
This document requests creation of a new IANA managed registry under
a new category of "Interior Gateway Protocol (IGP) Parameters" IANA
registries to identify MSD types as proposed in
, .
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:Les GinsbergEmail: ginsberg@cisco.comThe authors would like to thank Stephane Litkowski and Bruno Decraene
for their reviews and valuable comments.