Internet DRAFT - draft-ietf-ospf-segment-routing-msd
draft-ietf-ospf-segment-routing-msd
OSPF Working Group J. Tantsura
Internet-Draft Apstra, Inc.
Intended status: Standards Track U. Chunduri
Expires: April 20, 2019 Huawei Technologies
S. Aldrin
Google, Inc
P. Psenak
Cisco Systems
October 17, 2018
Signaling MSD (Maximum SID Depth) using OSPF
draft-ietf-ospf-segment-routing-msd-25
Abstract
This document defines a way for an Open Shortest Path First (OSPF)
Router to advertise multiple types of supported Maximum SID(Segment
Identifier) Depths (MSDs) at node and/or link granularity. Such
advertisements allow entities (e.g., centralized controllers) to
determine whether a particular SID stack can be supported in a given
network. This document defines only one type of MSD, but defines an
encoding that can support other MSD types. Here the term OSPF means
both OSPFv2 and OSPFv3.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Node MSD Advertisement . . . . . . . . . . . . . . . . . . . 4
3. Link MSD sub-TLV . . . . . . . . . . . . . . . . . . . . . . 5
4. Procedures for Defining and Using Node and Link MSD
Advertisements . . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
When Segment Routing (SR) paths are computed by a centralized
controller, it is critical that the controller learns the Maximum SID
(Segment Identifier) Depth (MSD) that can be imposed at each node/
link on a given SR path to ensure that the Segment Identifier (SID)
stack depth of a computed path doesn't exceed the number of SIDs the
node is capable of imposing.
[I-D.ietf-pce-segment-routing] 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 controller does not participate in IGP
routing, it has no way to learn 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
[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
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head-ends. In order for BGP-LS to signal MSD for all the nodes and
links in the network MSD is relevant, MSD capabilities SHOULD be
advertised by 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 an Entropy Label (EL) at
a depth that 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. 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.
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
This memo makes use of the terms defined in [RFC7770]
BGP-LS: Distribution of Link-State and TE Information using Border
Gateway Protocol
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:
o replacing the label at the top of the label stack with a new label
o pushing one or more new labels onto the label stack
The number of labels imposed is then the sum of the number of labels
which are replaced and the number of labels which are pushed. See
[RFC3031] for further details.
PCC: Path Computation Client
PCE: Path Computation Element
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PCEP: Path Computation Element Protocol
SR: Segment Routing
SID: Segment Identifier
LSA: Link state advertisement
RI: OSPF Router Information LSA
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) and represents the total
length of value field in octets.
Value: consists of one or more pairs of a 1 octet MSD-type and 1
octet MSD-Value.
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MSD-Type: one of the values defined in the IGP MSD-Types registry
defined in [I-D.ietf-isis-segment-routing-msd].
MSD-Value: a number in the range of 0-255. For all MSD-Types, 0
represents lack of the ability to impose 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 applicable to OSPFv2 and to OSPFv3 and is optional. 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 LSA. If the Node MSD TLV appears in multiple Router
Information LSAs that have different flooding scopes, the Node MSD
TLV in the Router Information LSA with the area-scoped flooding scope
MUST be used. If the Node MSD TLV appears in multiple Router
Information LSAs that have the same flooding scope, the Node MSD TLV
in the Router Information (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 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:
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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 and 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 MSD-Types registry defined
in [I-D.ietf-isis-segment-routing-msd].
MSD-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 0-255. For all MSD-Types, 0 represents lack
of the ability to impose 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 in the same OSPFv2
Extended Link Opaque LSA/E-Router-LSA, only the first instance of the
TLV MUST be used by receiving OSPF routers. This situation SHOULD be
logged as an error.
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 OSPFv2 Extended Link 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 MAY be logged as a warning.
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
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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 in
[I-D.ietf-isis-segment-routing-msd].
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-
TLV registry.
Value Description Reference
----- --------------- -------------
9 OSPFv3 Link MSD This document
Figure 5: OSPFv3 Link MSD
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6. Security Considerations
Security concerns for OSPF are addressed in [RFC7474], [RFC4552] and
[RFC7166]. Further security analysis for OSPF protocol is done in
[RFC6863]. Security considerations, as specified by [RFC7770],
[RFC7684] and [RFC8362] are applicable to this document.
Implementations MUST assure that malformed TLV and Sub-TLV 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 TLV or Sub-TLV 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 also may inform an attacker of how
to induce any of the aforementioned conditions.
There's no Denial of Service risk specific to this extension, and it
is not vulnerable to replay attacks.
7. Contributors
The following people contributed to this document:
Les Ginsberg
Email: ginsberg@cisco.com
8. Acknowledgments
The authors would like to thank Acee Lindem, Ketan Talaulikar, Tal
Mizrahi, Stephane Litkowski and Bruno Decraene for their reviews and
valuable comments.
9. References
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9.1. Normative References
[I-D.ietf-isis-segment-routing-msd]
Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling MSD (Maximum SID Depth) using IS-IS", draft-
ietf-isis-segment-routing-msd-19 (work in progress),
October 2018.
[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>.
9.2. Informative References
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[I-D.ietf-idr-bgp-ls-segment-routing-msd]
Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
"Signaling MSD (Maximum SID Depth) using Border Gateway
Protocol Link-State", draft-ietf-idr-bgp-ls-segment-
routing-msd-02 (work in progress), August 2018.
[I-D.ietf-ospf-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and Entropy
Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
mpls-elc-07 (work in progress), September 2018.
[I-D.ietf-pce-segment-routing]
Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "PCEP Extensions for Segment Routing",
draft-ietf-pce-segment-routing-12 (work in progress), June
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>.
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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
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