OSPF MSD: Signaling Maximum SID Depth (MSD) Using OSPF
How an OSPF router tells controllers how many segment labels 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
AI ELI5
On this page
§1Why Routers Advertise MSD
When a centralized controller computes a Segment Routing (SR) path, it needs to know the Maximum SID Depth (MSD): how many Segment Identifiers (SIDs) a node or link can impose. Otherwise the computed SID stack could be deeper than the node can handle.
PCEP can signal MSD, but only if it is configured on the head-end. BGP-LS can export MSD, but it usually runs on only a few nodes. So the RFC says MSD capabilities SHOULD be advertised by every OSPF router in the network. The term OSPF here means both OSPFv2 and OSPFv3.
MSD can also be useful without SR: in a non-SR MPLS network it defines the maximum label depth.
A planner builds routes for packets, but each router can only carry so many labels. Routers shout out their limit so the planner knows.
§1.1Key Terms: MSD, SID and Imposition
MSD is the number of SIDs supported by a node or by a link on a node. A SID is defined in RFC 8402. Label imposition means modifying or adding labels to the outgoing label stack: replacing the top label, or pushing new ones. The number of labels imposed is the replaced labels plus the pushed ones.
The RFC expects further MSD-Types to be defined later, for example for entropy labels, for SIDs imposed through recirculation, or for SIDs tied to another data plane such as IPv6. This document only refers to the Signaling MSD type defined in RFC 8491.
Imposition is how many stickers a router can put on a package. Different kinds of stickers may get their own limits later.
§2The Node MSD TLV
The Node MSD TLV sits inside the OSPF Router Information (RI) Opaque LSA from RFC 7770. It carries the SID depth of the router that originated the LSA. Node MSD is the smallest MSD supported by the node on the interfaces configured for the advertising IGP instance. Values may be learned via a hardware API or provisioned.
The TLV is Type 12. Its 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-Type comes from the IGP MSD-Types registry of RFC 8491. MSD-Value is 0 to 255; 0 means no ability to impose an MSD stack of any depth, and any other value MUST be the lowest value supported by any link configured for use by the OSPF instance.
The TLV is optional and applies to both OSPFv2 and OSPFv3.
The router writes a note: type of limit, then the number. It can write several notes in one envelope.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | 12 | 12 for the Node MSD TLV. |
LENGTH | Length | 2 bytes | 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 impose a stack. | |
MSD-TYPE-N | MSD-Type... | 1 byte | Further pairs may follow. | |
MSD-VALUE-N | MSD-Value... | 1 byte | Value for the further pair. |
§2.1Handling Duplicate Node MSD TLVs
If a receiver sees several Node MSD TLVs from one router, it MUST use the first occurrence in the RI LSA. If the TLV appears in RI LSAs of different flooding scopes, the one in the area-scoped RI LSA MUST be used. If it appears in RI LSAs of the same scope, the one in the LSA with the numerically smallest Instance ID MUST be used and the others ignored.
The RI LSA can be flooded at link, area or AS scope. For Node MSD, area-scoped flooding is RECOMMENDED.
If a router sends the same note twice, everyone follows the same rule about which copy counts, so they all agree.
§3The Link MSD Sub-TLV
The Link MSD sub-TLV carries the MSD of the interface for a link. It has the same layout as the node TLV: pairs of MSD-Type and MSD-Value, with 0 meaning no ability to impose a stack of any depth. Any other value is that of the link when used as an outgoing interface.
In OSPFv2 it is an optional sub-TLV of the Extended Link TLV from RFC 7684, with type 6. In OSPFv3 it is an optional sub-TLV of the E-Router-LSA TLV from RFC 8362, with type 9.
If the same link carries this sub-TLV in several LSAs from the same router, receivers MUST use the one 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. This situation SHOULD be logged as an error.
A router can also write a limit for each road out of it, since different roads may carry different amounts.
| Scope | Carrier | Type |
|---|---|---|
| Node (OSPFv2 and OSPFv3) | TLV in the RI Opaque LSA | 12 |
| Link, OSPFv2 | Sub-TLV of the Extended Link TLV | 6 |
| Link, OSPFv3 | Sub-TLV of the E-Router-LSA TLV | 9 |
§4Combining Node and Link Values
When a Link MSD exists for an MSD-Type, it MUST take precedence over the Node MSD. If only the Node MSD is signaled, that value MUST be used for the link.
To keep flooding efficient, routers whose links all have the same MSD are RECOMMENDED to advertise just the Node MSD.
If neither a Node nor a Link MSD is advertised for a type, usually 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, per RFC 8491.
The rule for a specific road beats the rule for the whole router. If a router has the same limit everywhere, one note is enough.
§5IANA Assignments
IANA allocated TLV type 12 (Node MSD) from the OSPF Router Information (RI) TLVs registry, sub-TLV type 6 (OSPFv2 Link MSD) from the OSPFv2 Extended Link TLV Sub-TLVs registry, and sub-TLV type 9 (OSPFv3 Link MSD) from the OSPFv3 Extended-LSA Sub-TLVs registry.
The official number list now has entries for these notes, so everyone uses the same numbers.
§6Security Considerations
The usual OSPF security mechanisms apply, plus those of RFC 7770, RFC 7684 and RFC 8362. Implementations MUST detect malformed TLVs and sub-TLVs so attackers cannot crash the OSPF router or routing process. Such events SHOULD be counted or logged, and logging SHOULD be rate-limited to avoid a denial-of-service on the control plane.
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 could also tell an attacker how to cause this. The RFC states there is no DoS risk specific to this extension and that it is not vulnerable to replay attacks.
A wrong number could make the planner fail or ask for too much. Routers must also not break when they get a garbled note.