| OSPF Working Group | J. Tantsura |
| Internet-Draft | Nuage Networks |
| Intended status: Standards Track | U. Chunduri |
| Expires: August 30, 2018 | Huawei Technologies |
| S. Aldrin | |
| Google, Inc | |
| P. Psenak | |
| Cisco Systems | |
| February 26, 2018 |
Signaling MSD (Maximum SID Depth) using OSPF
draft-ietf-ospf-segment-routing-msd-09
This 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.
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This Internet-Draft will expire on August 30, 2018.
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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 [I-D.ietf-pce-segment-routing] 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 [RFC7752] 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 [I-D.ietf-idr-bgp-ls-segment-routing-msd]. 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, [I-D.ietf-ospf-mpls-elc] 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 Protocol
BMI: Base MPLS Imposition is the number of MPLS labels which can be imposed inclusive of any service/transport labels
OSPF: Open Shortest Path First
MSD: Maximum SID Depth - the number of SIDs a node or a link on a node can support
PCC: Path Computation Client
PCE: Path Computation Element
PCEP: Path Computation Element Protocol
SID: Segment Identifier
SR: Segment Routing
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.
This memo makes use of the terms defined in [RFC4970].
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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type and Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
Figure 1: Node MSD TLV
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 [RFC5838] 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type and Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
Figure 2: Link MSD Sub-TLV
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 [RFC7684], 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 [I-D.ietf-ospf-ospfv3-lsa-extend], and has value of 16 (Suggested value - to be assigned by IANA).
Length is variable and similar to what is defined in Section 3.
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 Section 3 of this document from OSPF Router Information (RI) TLVs Registry as defined by [RFC4970]. For the link MSD, we request IANA to allocate new sub-TLV codes as proposed in Section 4 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 Section 3, Section 4. The registration procedure is "Expert Review" as defined in [RFC8126]. Suggested registry name is "MSD types". Types are an unsigned 8 bit number. The following values are defined by this document
Value Name Reference ----- --------------------- ------------- 0 Reserved This document 1 Base MPLS Imposition MSD This document 2-250 Unassigned This document 251-254 Experimental This document 255 Reserved This document
Figure 3: MSD Types Codepoints Registry
Security considerations, as specified by [RFC7770] are applicable to this document
The following people contributed to this document:
Les Ginsberg
Email: ginsberg@cisco.com
The authors would like to thank Stephane Litkowski and Bruno Decraene for their reviews and valuable comments.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4970] | Lindem, A., Shen, N., Vasseur, JP., Aggarwal, R. and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 4970, DOI 10.17487/RFC4970, July 2007. |
| [RFC7770] | Lindem, A., Shen, N., Vasseur, JP., Aggarwal, R. and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, February 2016. |