IS-IS MSD: Signaling Maximum SID Depth (MSD) Using IS-IS
How an IS-IS router tells controllers how many segment labels it can push, and the registry of MSD types.
Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, DOI 10.17487/RFC8491, November 2018, <https://www.rfc-editor.org/info/rfc8491>.
@misc{rfc8491,
series = {Request for Comments},
number = 8491,
howpublished = {RFC 8491},
publisher = {RFC Editor},
doi = {10.17487/RFC8491},
url = {https://www.rfc-editor.org/info/rfc8491},
author = {J. Tantsura and U. Chunduri and S. Aldrin and L. Ginsberg},
title = {{Signaling Maximum SID Depth (MSD) Using IS-IS}},
pagetotal = 10,
year = 2018,
month = nov,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Tantsura
Request for Comments: 8491 Apstra, Inc.
Category: Standards Track U. Chunduri
ISSN: 2070-1721 Huawei Technologies
S. Aldrin
Google, Inc.
L. Ginsberg
Cisco Systems
November 2018
Signaling Maximum SID Depth (MSD) Using IS-IS
Abstract
This 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 Segment ID (SID) stack can be
supported in a given network. This document only defines one type of
MSD: Base MPLS Imposition. However, it defines an encoding that can
support other MSD types. This document focuses on MSD use in a
network that is Segment Routing (SR) enabled, but MSD may also be
useful when SR is not enabled.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8491.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Node MSD Advertisement . . . . . . . . . . . . . . . . . . . 4
3. Link MSD Advertisement . . . . . . . . . . . . . . . . . . . 5
4. Procedures for Defining and Using Node and Link MSD
Advertisements . . . . . . . . . . . . . . . . . . . . . . . 6
5. Base MPLS Imposition MSD . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
When Segment Routing (SR) paths are computed by a centralized
controller, it is critical that the controller learn the Maximum SID
Depth (MSD) that can be imposed at each node/link of a given SR path.
This ensures that the Segment Identifier (SID) stack depth of a
computed path does not exceed the number of SIDs the node is capable
of imposing.
[PCEP-EXT] defines how to signal MSD in the Path Computation Element
Communication Protocol (PCEP). However, if PCEP is not supported/
configured on the head-end of an SR tunnel or a Binding-SID anchor
node, and the controller does not participate in IGP routing, it has
no way of learning the MSD of nodes and links. BGP-LS (Distribution
of Link-State and TE Information Using Border Gateway Protocol)
[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 [MSD-BGP].
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 for which MSD is relevant,
MSD capabilities SHOULD be advertised by every Intermediate System to
Intermediate System (IS-IS) router in the network.
Other types of MSDs are known to be useful. For example, [ELC-ISIS]
defines Entropy Readable Label Depth (ERLD), which is used by a head-
end to insert an Entropy Label (EL) at a depth where it can be read
by transit nodes.
This document defines an extension to IS-IS used to advertise one or
more types of MSDs at node and/or link granularity. It also creates
an IANA registry for assigning MSD-Type identifiers and 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 data plane such as IPv6.
MSD advertisements MAY be useful even if Segment Routing itself is
not enabled. For example, in a non-SR MPLS network, MSD defines the
maximum label depth.
1.1. Terminology
BMI: Base MPLS Imposition is the number of MPLS labels that can be
imposed inclusive of all service/transport/special labels.
MSD: Maximum SID Depth is the number of SIDs supported by a node or
a link on a node.
SID: Segment Identifier is defined in [RFC8402].
Label Imposition: Imposition is the act of modifying and/or adding
labels to the outgoing label stack associated with a packet.
This includes:
* replacing the label at the top of the label stack with a new
label
* pushing one or more new labels onto the label stack
The number of labels imposed is then the sum of the number of labels
that are replaced and the number of labels that are pushed. See
[RFC3031] for further details.
1.2. Requirements Language
The 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Node MSD Advertisement
The Node MSD sub-TLV is defined within the body of the IS-IS Router
CAPABILITY TLV [RFC7981] to carry the provisioned SID depth of the
router originating the IS-IS 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.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ................... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Node MSD Sub-TLV
Type: 23
Length: variable (multiple of 2 octets); represents the total length
of the Value field
Value: field consists of one or more pairs of a 1-octet MSD-Type and
1-octet MSD-Value
MSD-Type: value defined in the "IGP MSD-Types" registry created by
the IANA Considerations section of this document Section 6
MSD-Value: number in the range of 0-255 (for all MSD-Types, 0
represents the lack of ability to support a 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 [RFC7981]
apply. These procedures may result in different MSD values being
used, for example, by different controllers. This does not, however,
create any interoperability issue.
3. Link MSD Advertisement
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 signaled by the forwarding plane or may be provisioned.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ................... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Link MSD Sub-TLV
Type: 15
Length: variable (multiple of 2 octets); represents the total length
of the Value field
Value: field consists of one or more pairs of a 1-octet MSD-Type and
1-octet MSD-Value
MSD-Type: value defined in the "IGP MSD-Types" registry created by
the IANA Considerations section of this document Section 6
MSD-Value: number in the range of 0-255 (for all MSD-Types, 0
represents the lack of ability to support a SID stack of any depth;
any other value represents that of the particular link when used as
an outgoing interface.)
This sub-TLV is optional.
If multiple Link MSD advertisements for the same MSD-Type and the
same link are received, the procedure to select which copy to use is
undefined.
If the advertising router performs label imposition in the context of
the ingress interface, it is not possible to meaningfully advertise
per-link values. In such a case, only the Node MSD SHOULD be
advertised.
4. Procedures for Defining and Using Node and Link MSD Advertisements
When Link MSD is present for a given MSD-Type, the value of the Link
MSD MUST take precedence over the Node MSD. If a Link MSD-Type is
not signaled, but the Node MSD-Type is, then the Node MSD-Type value
MUST be considered to be 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. In some cases, however, 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.
5. Base MPLS Imposition MSD
Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
labels that can be imposed, including all service/transport/special
labels.
The absence of BMI-MSD advertisements indicates only that the
advertising node does not support advertisement of this capability.
6. IANA Considerations
IANA has allocated a sub-TLV type for the new sub-TLV proposed in
Section 2 of this document from the "Sub-TLVs for TLV 242 (IS-IS
Router CAPABILITY TLV)" registry as defined by [RFC7981].
IANA has allocated the following value:
Value Description Reference
----- --------------- -------------
23 Node MSD This document
Figure 3: Node MSD
IANA has allocated a sub-TLV type as defined in Section 3 from the
"Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 (Extended IS
reachability, IS Neighbor Attribute, L2 Bundle Member Attributes,
inter-AS reachability information, MT-ISN, and MT IS Neighbor
Attribute TLVs)" registry.
IANA has allocated the following value:
Value Description Reference
----- --------------- -------------
15 Link MSD This document
Figure 4: Link MSD
Per-TLV information where Link MSD sub-TLV can be part of:
TLV 22 23 25 141 222 223
--- --------------------
y y y y y y
Figure 5: TLVs Where LINK MSD Sub-TLV Can Be Present
IANA has created an IANA-managed registry titled "IGP MSD-Types"
under the "Interior Gateway Protocol (IGP) Parameters" registry to
identify MSD-Types as proposed in Sections 2 and 3. The registration
procedure is "Expert Review" as defined in [RFC8126]. 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
251-254 Experimental Use This document
255 Reserved This document
Figure 6: MSD-Types Codepoints Registry
General guidance for the designated experts is defined in [RFC7370].
7. Security Considerations
Security considerations as specified by [RFC7981] are applicable to
this document.
The advertisement of an incorrect MSD value may have negative
consequences. If the value is smaller than supported, path
computation may fail to compute a viable path. If the value is
larger than supported, an attempt to instantiate a path that can't be
supported by the head-end (the node performing the SID imposition)
may occur.
The presence of this information may also inform an attacker of how
to induce any of the aforementioned conditions.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.
[RFC7370] Ginsberg, L., "Updates to the IS-IS TLV Codepoints
Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,
<https://www.rfc-editor.org/info/rfc7370>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
8.2. Informative References
[ELC-ISIS] Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and Entropy
Readable Label Depth Using IS-IS", Work in Progress,
draft-ietf-isis-mpls-elc-06, September 2018.
[MSD-BGP] Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
"Signaling MSD (Maximum SID Depth) using Border Gateway
Protocol Link-State", Work in Progress, draft-ietf-idr-
bgp-ls-segment-routing-msd-02, August 2018.
[PCEP-EXT] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "PCEP Extensions for Segment Routing",
Work in Progress, draft-ietf-pce-segment-routing-13,
October 2018.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
Acknowledgements
The authors would like to thank Acee Lindem, Ketan Talaulikar,
Stephane Litkowski, and Bruno Decraene for their reviews and valuable
comments.
Contributors
The following people contributed to this document:
Peter Psenak
Email: ppsenak@cisco.com
Authors' Addresses
Jeff Tantsura
Apstra, Inc.
Email: jefftant.ietf@gmail.com
Uma Chunduri
Huawei Technologies
Email: uma.chunduri@huawei.com
Sam Aldrin
Google, Inc.
Email: aldrin.ietf@gmail.com
Les Ginsberg
Cisco Systems
Email: ginsberg@cisco.com
AI ELI5
On this page
§1Why Routers Advertise MSD
A centralized controller that computes Segment Routing (SR) paths must learn the Maximum SID Depth (MSD) of each node and link, so a computed SID stack is never deeper than the node can impose.
PCEP can signal MSD, but only where it is configured on the head-end. BGP-LS can export it, but usually runs on few nodes. So the RFC says MSD capabilities SHOULD be advertised by every IS-IS router in the network.
The RFC defines the IS-IS extension, creates an IANA registry for MSD-Type identifiers and defines one type, Base MPLS Imposition. MSD may also be useful when SR is not enabled: in a non-SR MPLS network it defines the maximum label depth.
The planner needs to know how many labels each router can carry. Routers announce their limit so the planner can stay within it.
§1.1Key Terms: MSD, SID and Imposition
MSD is the number of SIDs supported by a node or a link on a node. Label imposition is modifying or adding labels to a packet's outgoing label stack, by replacing the top label or pushing new ones. The number imposed is the replaced labels plus the pushed ones.
BMI, Base MPLS Imposition, is the number of MPLS labels that can be imposed, inclusive of all service, transport and special labels.
Imposition is putting stickers on a package. BMI counts every sticker of every kind.
§2The Node MSD Sub-TLV
The Node MSD sub-TLV goes in the IS-IS Router CAPABILITY TLV from RFC 7981. It carries the provisioned SID depth of the originating router. Node MSD is the smallest MSD supported by the node on the interfaces configured for the advertising IGP instance. Values may come from a hardware API or be provisioned.
It is Type 23. Length is a multiple of 2 octets. The value is one or more pairs of a 1-octet MSD-Type and a 1-octet MSD-Value. MSD-Value is 0 to 255; 0 means no ability to support a SID stack of any depth, and any other value MUST be the lowest value supported by any link configured for use by the IS-IS instance.
The sub-TLV is optional. If a router originates several Node MSD advertisements for the same MSD-Type, the procedures of RFC 7981 apply. This may result in different values being used by different controllers, which the RFC says causes no interoperability issue.
The router writes down pairs: what kind of limit, and the number. It can write several pairs.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 23 | 23 for the Node MSD sub-TLV. |
LENGTH | Length | 1 byte | Multiple of 2 octets; total length of the value field. | |
MSD-TYPE | MSD-Type | 1 byte | Kind of MSD, from the IGP MSD-Types registry. | |
MSD-VALUE | MSD-Value | 1 byte | 0 to 255; 0 means no ability to support a stack. |
§3The Link MSD Sub-TLV
The Link MSD sub-TLV carries the MSD of the interface for a link. It is defined for TLVs 22, 23, 25, 141, 222 and 223, has Type 15, and uses the same pairs of MSD-Type and MSD-Value. Values may be signaled by the forwarding plane or provisioned. A nonzero value is that of the link when used as an outgoing interface.
The sub-TLV is optional. If several Link MSD advertisements for the same MSD-Type and link are received, which copy to use is undefined.
If the router imposes labels in the context of the ingress interface, a per-link value cannot be meaningful. In that case only the Node MSD SHOULD be advertised.
A router can also give a limit for each road out of it, unless the limit depends on which road the packet came in on.
§4Combining Node and Link Values
A Link MSD for an MSD-Type MUST take precedence over the Node MSD. If only the Node MSD is signaled, it MUST be used for the link. To increase flooding efficiency, routers with the same MSD on every link are RECOMMENDED to advertise just the Node MSD.
If neither is advertised for a type, generally it only means the node does not support advertising that type. In some cases it might mean the function is not supported. The correct interpretation MUST be specified when an MSD-Type is defined.
A rule for one road beats the rule for the whole router. If every road is the same, one note is enough.
§5Base MPLS Imposition MSD
The one MSD-Type defined here is Base MPLS Imposition MSD (BMI-MSD). It signals the total number of MPLS labels that can be imposed, including all service, transport and special labels.
If BMI-MSD is not advertised, that indicates only that the node does not support advertising this capability.
This is the total count of labels of every kind a router can put on a packet.
§6IANA Registry and Assignments
IANA allocated sub-TLV 23 (Node MSD) in the registry for sub-TLVs of TLV 242, and sub-TLV 15 (Link MSD) in the registry for sub-TLVs of TLVs 22, 23, 25, 141, 222 and 223.
IANA also created the IGP MSD-Types registry. The procedure is Expert Review per RFC 8126, and types are unsigned 8-bit numbers. Value 0 is reserved, 1 is Base MPLS Imposition MSD, 2 to 250 are unassigned, 251 to 254 are for experimental use, and 255 is reserved.
There is now an official list of limit kinds with numbers. Kind 1 is the total label count.
- 0Reserved — Reserved by this document.
- 1Base MPLS Imposition MSD — Total MPLS labels that can be imposed.
- 2-250Unassigned — Available for assignment.
- 251-254Experimental Use — Experimental use.
- 255Reserved — Reserved by this document.
§7Security Considerations
The security considerations of RFC 7981 apply. A wrong MSD has consequences. A value that is too small may make path computation fail to find a viable path. A value that is too large may lead to an attempt to build a path the head-end cannot support. The information may also tell an attacker how to induce these conditions.
A wrong number can make the planner give up or ask for too much, and a bad guy who sees the numbers could take advantage.