OSPF MSD: Signaling Maximum SID Depth (MSD) Using OSPF
How an OSPF router tells controllers how deep a stack of segment identifiers it can push onto a packet.
Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, DOI 10.17487/RFC8476, December 2018, <https://www.rfc-editor.org/info/rfc8476>.
@misc{rfc8476,
series = {Request for Comments},
number = 8476,
howpublished = {RFC 8476},
publisher = {RFC Editor},
doi = {10.17487/RFC8476},
url = {https://www.rfc-editor.org/info/rfc8476},
author = {J. Tantsura and U. Chunduri and S. Aldrin and P. Psenak},
title = {{Signaling Maximum SID Depth (MSD) Using OSPF}},
pagetotal = 11,
year = 2018,
month = dec,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Tantsura
Request for Comments: 8476 Apstra, Inc.
Category: Standards Track U. Chunduri
ISSN: 2070-1721 Huawei Technologies
S. Aldrin
Google, Inc.
P. Psenak
Cisco Systems
December 2018
Signaling Maximum SID Depth (MSD) Using OSPF
Abstract
This document defines a way for an Open Shortest Path First (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 Segment Identifier (SID) stack can be supported in a given
network. This document only refers to the Signaling MSD as defined
in RFC 8491, but it defines an encoding that can support other MSD
types. Here, the term "OSPF" means both OSPFv2 and OSPFv3.
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/rfc8476.
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 ....................................................3
1.1. Terminology ................................................4
1.2. Requirements Language ......................................4
2. Node MSD Advertisement ..........................................5
3. Link MSD Sub-TLV ................................................6
4. Procedures for Defining and Using Node and Link MSD
Advertisements ..................................................7
5. IANA Considerations .............................................7
6. Security Considerations .........................................8
7. References ......................................................9
7.1. Normative References .......................................9
7.2. Informative References ....................................10
Acknowledgements ..................................................11
Contributors ......................................................11
Authors' Addresses ................................................11
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 on a given SR path.
This ensures that the Segment Identifier (SID) stack depth of a
computed path doesn't 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 BGP) [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 OSPF 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 OSPF used to advertise one or
more types of MSDs at node and/or link granularity. In the future,
it is expected that new MSD-Types will be defined to signal
additional capabilities, e.g., ELs, SIDs that can be imposed through
recirculation, or SIDs associated with another data plane such
as IPv6.
MSD advertisements MAY be useful even if SR itself is not enabled.
For example, in a non-SR MPLS network, MSD defines the maximum label
depth.
1.1. Terminology
This memo makes use of the terms defined in [RFC7770].
BGP-LS: Distribution of Link-State and TE Information Using BGP
OSPF: Open Shortest Path First
MSD: Maximum SID Depth - the number of SIDs supported by a node
or a link on a node
SID: Segment Identifier as 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.
PCEP: Path Computation Element Communication Protocol
SR: Segment Routing
LSA: Link State Advertisement
RI: Router Information
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 TLV within the body of the OSPF RI Opaque LSA [RFC7770]
is defined to carry the provisioned SID depth of the router
originating the RI LSA. 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 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value | MSD-Type... | MSD-Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Node MSD TLV
Type: 12
Length: variable (multiple of 2 octets); represents the total length
of the value field in octets.
Value: consists of one or more pairs of a 1-octet MSD-Type and
1-octet MSD-Value.
MSD-Type: one of the values defined in the "IGP MSD-Types" registry
defined in [RFC8491].
MSD-Value: a number in the range of 0-255. For all MSD-Types, 0
represents the lack of ability to impose an MSD 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 OSPF instance.
This TLV is optional and is applicable to both OSPFv2 and OSPFv3.
The scope of the advertisement is specific to the deployment.
When multiple Node MSD TLVs are received from a given router, the
receiver MUST use the first occurrence of the TLV in the Router
Information (RI) LSA. If the Node MSD TLV appears in multiple RI
LSAs that have different flooding scopes, the Node MSD TLV in the RI
LSA with the area-scoped flooding scope MUST be used. If the Node
MSD TLV appears in multiple RI LSAs that have the same flooding
scope, the Node MSD TLV in the RI LSA with the numerically smallest
Instance ID MUST be used and other instances of the Node MSD TLV MUST
be ignored. The RI LSA can be advertised at any of the defined
opaque flooding scopes (link, area, or Autonomous System (AS)). For
the purpose of Node MSD TLV advertisement, area-scoped flooding is
RECOMMENDED.
3. Link MSD Sub-TLV
The Link MSD sub-TLV is defined to carry the MSD of the interface
associated with the link. MSD values may be learned via a hardware
API or may be provisioned.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value | MSD-Type... | MSD-Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Link MSD Sub-TLV
Type:
For OSPFv2, the link-level MSD-Value is advertised as an optional
sub-TLV of the OSPFv2 Extended Link TLV as defined in [RFC7684]
and has a type of 6.
For OSPFv3, the link-level MSD-Value is advertised as an optional
sub-TLV of the E-Router-LSA TLV as defined in [RFC8362] and has a
type of 9.
Length: variable; same as defined in Section 2.
Value: consists of one or more pairs of a 1-octet MSD-Type and
1-octet MSD-Value.
MSD-Type: one of the values defined in the "IGP MSD-Types" registry
defined in [RFC8491].
The MSD-Value field contains the Link MSD of the router originating
the corresponding LSA as specified for OSPFv2 and OSPFv3. The Link
MSD is a number in the range of 0-255. For all MSD-Types, 0
represents the lack of ability to impose an MSD stack of any depth;
any other value represents that of the particular link when used as
an outgoing interface.
If this sub-TLV is advertised multiple times for the same link in
different OSPF Extended Link Opaque LSAs / E-Router-LSAs originated
by the same OSPF router, the sub-TLV in the OSPFv2 Extended Link
Opaque LSA with the smallest Opaque ID or in the OSPFv3 E-Router-LSA
with the smallest Link State ID MUST be used by receiving OSPF
routers. This situation SHOULD be logged as an error.
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. 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. Per [RFC8491], the correct interpretation
MUST be specified when an MSD-Type is defined.
5. IANA Considerations
This specification updates several existing OSPF registries.
IANA has allocated TLV type 12 from the "OSPF Router Information (RI)
TLVs" registry as defined by [RFC7770].
Value Description Reference
----- --------------- -------------
12 Node MSD This document
Figure 3: RI Node MSD
IANA has allocated sub-TLV type 6 from the "OSPFv2 Extended Link TLV
Sub-TLVs" registry.
Value Description Reference
----- --------------- -------------
6 OSPFv2 Link MSD This document
Figure 4: OSPFv2 Link MSD
IANA has allocated sub-TLV type 9 from the "OSPFv3 Extended-LSA
Sub-TLVs" registry.
Value Description Reference
----- --------------- -------------
9 OSPFv3 Link MSD This document
Figure 5: OSPFv3 Link MSD
6. Security Considerations
Security concerns for OSPF are addressed in [RFC7474], [RFC4552], and
[RFC7166]. Further security analysis for the OSPF protocol is done
in [RFC6863]. Security considerations as specified by [RFC7770],
[RFC7684], and [RFC8362] are applicable to this document.
Implementations MUST ensure that malformed TLVs and sub-TLVs defined
in this document are detected and do not provide a vulnerability for
attackers to crash the OSPF router or routing process. Reception of
malformed TLVs or sub-TLVs SHOULD be counted and/or logged for
further analysis. Logging of malformed TLVs and sub-TLVs SHOULD be
rate-limited to prevent a Denial-of-Service (DoS) attack (distributed
or otherwise) from overloading the OSPF control plane.
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.
There's no DoS risk specific to this extension, and it is not
vulnerable to replay attacks.
7. References
7.1. 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>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684,
November 2015, <https://www.rfc-editor.org/info/rfc7684>.
[RFC7770] Lindem, A., Ed., 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, <https://www.rfc-editor.org/info/rfc7770>.
[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>.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362,
April 2018, <https://www.rfc-editor.org/info/rfc8362>.
[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>.
[RFC8491] 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>.
7.2. Informative References
[ELC-ISIS] Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and Entropy
Readable Label-stack Depth Using OSPF", Work in Progress,
draft-ietf-ospf-mpls-elc-07, 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-14,
October 2018.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.
[RFC6863] Hartman, S. and D. Zhang, "Analysis of OSPF Security
According to the Keying and Authentication for Routing
Protocols (KARP) Design Guide", RFC 6863,
DOI 10.17487/RFC6863, March 2013,
<https://www.rfc-editor.org/info/rfc6863>.
[RFC7166] Bhatia, M., Manral, V., and A. Lindem, "Supporting
Authentication Trailer for OSPFv3", RFC 7166,
DOI 10.17487/RFC7166, March 2014,
<https://www.rfc-editor.org/info/rfc7166>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
[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, Tal
Mizrahi, Stephane Litkowski, and Bruno Decraene for their reviews and
valuable comments.
Contributors
The following person contributed to this document:
Les Ginsberg
Email: ginsberg@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
Peter Psenak
Cisco Systems
Email: ppsenak@cisco.com
ISO Plain Language
On this page
§1Why the Depth Has to Be Known
When Segment Routing paths are computed by a centralised controller, it is critical that the controller learns the Maximum SID Depth that can be imposed at each node or link on a given path. This ensures that the SID stack depth of a computed path does not exceed the number of SIDs the node is capable of imposing.
PCEP defines how to signal MSD in the Path Computation Element Communication Protocol. But if PCEP is not supported or configured on the head-end of an SR tunnel or a Binding-SID anchor node, and the controller does not take part in IGP routing, it has no way of learning the MSD of nodes and links. BGP-LS, the distribution of link-state and traffic engineering information using BGP, defines a way to expose topology and associated attributes and capabilities of the nodes to a centralised controller, and MSD signalling by BGP-LS has been defined separately. Typically, BGP-LS is configured on a small number of nodes that do not necessarily act as head-ends. For BGP-LS to signal MSD for all the nodes and links in the network where MSD matters, MSD capabilities should be advertised by every OSPF router in the network.
Other kinds of MSD are known to be useful. One example is Entropy Readable Label Depth, which a head-end uses to insert an entropy label at a depth where it can be read by transit nodes. This document defines an extension to OSPF used to advertise one or more types of MSD at node and link granularity, and it is expected that new MSD types will later signal additional capabilities such as entropy labels, SIDs imposed through recirculation, or SIDs for another data plane such as IPv6.
MSD advertisements may be useful even if Segment Routing itself is not enabled: in a non-SR MPLS network, MSD defines the maximum label depth.
A controller that builds a path out of numbered instructions needs to know how many instructions each router can hold. Without that number it can produce a path that no router can carry, and the failure appears only later, in the data plane. OSPF already tells routers about the topology, so it is the natural place to publish the depth as well.
§1.1Terms Used Here
The RFC uses the terms defined in RFC 7770, and adds some of its own.
- MSD, Maximum SID Depth, is the number of SIDs supported by a node or by a link on a node.
- SID is the Segment Identifier defined in RFC 8402.
- Label imposition is the act of modifying or adding labels to the outgoing label stack associated with a packet. It includes replacing the label at the top of the label stack with a new label, and pushing one or more new labels onto the label stack.
The number of labels imposed is the sum of the number of labels replaced and the number of labels pushed. The RFC also names OSPF as Open Shortest Path First, LSA as Link State Advertisement, and RI as Router Information.
Two ideas matter. Maximum SID Depth is how many identifiers a router can carry. Label imposition is what a router does when it forwards a packet: it swaps the top label for another, pushes more labels on, or both. The depth limit counts both actions together.
§2The Node MSD TLV
The Node MSD TLV sits in the body of the OSPF Router Information Opaque LSA and carries the provisioned SID depth of the router that originated the RI LSA. Node MSD is the smallest MSD supported by the node on the set of interfaces configured for use by the advertising IGP instance. The values may be learned through a hardware interface or may be provisioned by an operator.
The TLV has a type of 12. Its length is variable, a multiple of 2 octets, and represents the total length of the value field in octets. The value consists of one or more pairs, each a 1-octet MSD-Type followed by a 1-octet MSD-Value. The MSD-Type is one of the values defined in the IGP MSD-Types registry, and the MSD-Value is a number in the range 0 to 255. For all MSD types, 0 represents the lack of ability to impose a stack of any depth, and any other value represents the depth of the node; that value must represent the lowest value supported by any link configured for use by the advertising OSPF instance.
The TLV is optional and applies to both OSPFv2 and OSPFv3. The scope of the advertisement is specific to the deployment.
When several Node MSD TLVs arrive from one router, the receiver must use the first occurrence of the TLV in the Router Information LSA. If the TLV appears in several RI LSAs with different flooding scopes, the one in the area-scoped LSA must be used. If it appears in several RI LSAs with the same flooding scope, the one in the LSA with the numerically smallest Instance ID must be used and the others ignored. The RI LSA can be advertised at any of the defined opaque flooding scopes, which are link, area or Autonomous System, and area-scoped flooding is recommended for the Node MSD TLV.
The router publishes one number for itself, the depth it can manage across all of its interfaces. It sits in the Router Information LSA, which OSPF already floods. The value is a byte, so the range is 0 to 255, and zero means the router cannot carry a stack at all.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | TLV type, the value 12 for Node MSD. | |
LENGTH | Length | 2 bytes | Total length of the value field in octets, a multiple of 2. | |
MSD-TYPE | MSD-Type | 1 byte | Which kind of depth is being advertised, from the IGP MSD-Types registry. | |
MSD-VALUE | MSD-Value | 1 byte | Depth from 0 to 255; 0 means the node cannot impose a stack of any depth. |
§3The Link MSD Sub-TLV
The Link MSD sub-TLV carries the MSD of the interface associated with the link. The values may be learned through a hardware interface or provisioned.
For OSPFv2 the link-level MSD-Value is advertised as an optional sub-TLV of the OSPFv2 Extended Link TLV, with a type of 6. For OSPFv3 it is advertised as an optional sub-TLV of the E-Router-LSA TLV, with a type of 9.
The length is variable and the same as in the Node MSD TLV. The value again consists of one or more pairs of a 1-octet MSD-Type and a 1-octet MSD-Value, with the type taken from the IGP MSD-Types registry. The MSD-Value field holds the Link MSD of the router that originated the corresponding LSA, in the range 0 to 255. For all MSD types, 0 represents the lack of ability to impose a stack of any depth, and any other value represents the depth of that particular link when used as an outgoing interface.
If the sub-TLV is advertised several times for the same link in different OSPF Extended Link Opaque LSAs or E-Router-LSAs originated by the same router, the copy in the OSPFv2 Extended Link Opaque LSA with the smallest Opaque ID, or in the OSPFv3 E-Router-LSA with the smallest Link State ID, must be used by receiving routers. That situation should be logged as an error.
A link can have a different limit from the router as a whole, so the link advertisement carries its own value. The two protocol versions put it in different places, but the number means the same thing: how deep a stack that interface can push.
| Protocol version | Carried in | Sub-TLV type |
|---|---|---|
| OSPFv2 | Extended Link TLV | 6 |
| OSPFv3 | E-Router-LSA TLV | 9 |
§4Which Value to Use
When a Link MSD is present for a given MSD type, its value must take precedence over the Node MSD. When a Link MSD type is not signalled but the Node MSD type is, the Node MSD value must be used as the MSD value for that link.
In order to increase flooding efficiency, it is recommended that routers with homogeneous Link MSD values advertise just the Node MSD value, since a single advertisement then covers every link.
The meaning of the absence of both Node and Link MSD advertisements for a given MSD type is specific to that type. Generally, it can only be inferred that the advertising node does not support advertisement of that type. In some cases the lack of advertisement might instead imply that the functionality associated with the type is not supported, and the correct interpretation must be specified when an MSD type is defined.
Two rules settle any question. A link value wins over the router value, and if there is no link value, the router value applies. A router whose links all behave the same is asked to advertise only the one number, to save flooding effort.
- Link value firstWhen a Link MSD is present for a type, its value takes precedence over the Node MSD.
- Node value as fallbackWhen a link does not signal that type, the Node MSD value applies to that link.
- One number is enoughRouters whose links all have the same value are asked to advertise only the Node MSD, which floods less.
- Silence has a meaningMissing advertisements usually mean the node cannot advertise that type; the exact meaning must be defined per type.
§5Registry Allocations
The specification updates several existing OSPF registries.
IANA allocated TLV type 12 from the OSPF Router Information TLVs registry defined in RFC 7770, for the Node MSD. IANA allocated sub-TLV type 6 from the OSPFv2 Extended Link TLV Sub-TLVs registry, for the OSPFv2 Link MSD, and sub-TLV type 9 from the OSPFv3 Extended-LSA Sub-TLVs registry, for the OSPFv3 Link MSD.
Three numbers are registered: 12 for the node level TLV inside the Router Information LSA, 6 for the link value in OSPFv2, and 9 for the link value in OSPFv3.
§6Security Considerations
Security concerns for OSPF are addressed in RFC 7474, RFC 4552 and RFC 7166, with further analysis in RFC 6863, and the security considerations of RFC 7770, RFC 7684 and RFC 8362 apply here as well.
Implementations must ensure that malformed TLVs and sub-TLVs defined in this document are detected and do not give an attacker a way to crash the OSPF router or routing process. Reception of malformed TLVs or sub-TLVs should be counted or logged for further analysis, and that logging should be rate limited so that a denial-of-service attack cannot overload the OSPF control plane.
Advertising an incorrect MSD value has negative consequences. If the value is smaller than what is supported, path computation may fail to compute a viable path. If it is larger than what is supported, an attempt may occur to instantiate a path that the head-end, the node performing the SID imposition, cannot support. The presence of this information may also inform an attacker of how to induce either of those conditions.
There is no denial-of-service risk specific to this extension, and it is not vulnerable to replay attacks.
The depth a router advertises is information an attacker could use, so a wrong value has real consequences: too small and the controller cannot build a path at all, too large and it builds one the router cannot carry. Implementations must also handle malformed advertisements without crashing.