Internet DRAFT - draft-ietf-6man-sids
draft-ietf-6man-sids
6man S. Krishnan
Internet-Draft Cisco
Intended status: Informational 11 April 2023
Expires: 13 October 2023
Segment Identifiers in SRv6
draft-ietf-6man-sids-03
Abstract
The data plane for Segment Routing over IPv6 (SRv6) is built using
IPv6 as the underlying forwarding plane. Due to this underlying use
of IPv6, Segment Identifiers (SIDs) used by SRv6 can resemble IPv6
addresses and behave like them while exhibiting slightly different
behaviors in some situations. This document explores the
characteristics of SRv6 SIDs and to clarify the relationship of SRv6
SIDs to the IPv6 Addressing Architecture.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 13 October 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. SRv6 SIDs and the IPv6 addressing architecture . . . . . . . 3
4. Special Considerations for Compressed SIDs . . . . . . . . . 4
4.1. Applicability to other forms of compressed SIDs . . . . . 5
5. Allocation of a Global Unicast Prefix for SIDs . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Segment Routing over IPv6 (SRv6) [RFC8754] uses IPv6 as the
underlying data plane. In SRv6, SR source nodes initiate packets
with a segment identifier in the Destination Address of the IPv6
header, and SR segment endpoint nodes process a local segment present
the Destination Address of an IPv6 header. Thus Segment Identifiers
(SIDs) in SRv6 can and do appear in the Destination Address of IPv6
datagrams by design. This document explores the characteristics of
SRv6 SIDs and to clarify the relationship of SRv6 SIDs to the IPv6
Addressing Architecture [RFC4291].
2. Terminology
The following terms are used as defined in [RFC8402].
* Segment Routing (SR)
* SR Domain
* Segment
* Segment Identifier (SID)
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* SRv6
* SRv6 SID
* SR Policy.
The following terms are used as defined in [RFC8754].
* Segment Routing Header (SRH)
* SR Source Node
* Transit Node
* SR Segment Endpoint Node
* Reduced SRH
* Segments Left
* Last Entry
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.
3. SRv6 SIDs and the IPv6 addressing architecture
[RFC8754] defines the Segment List of the SRH as a contiguous array
of 128-bit IPv6 addresses, and that each of the elements in this list
are SIDs. But all of these elements are not necessarily made equal.
Some of these elements may represent a local interface as described
in Section 4.3 of [RFC8754] as "A FIB entry that represents a local
interface, not locally instantiated as an SRv6 SID". From this it
follows that not all the SIDs that appear in the SRH are SRv6 SIDs as
defined by [RFC8402].
As stated above, the non-SRv6-SID elements that appear in the SRH SID
list are simply IPv6 addresses assigned to local interfaces and they
need to conform to [RFC4291]. So, the following discussions are
applicable solely to SRv6 SIDs that are not assigned to local
interfaces.
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One of the key questions to address is how these SRv6 SIDs appearing
as IPv6 Destination Addresses are perceived and treated by "transit
nodes" (that are not required to be capable of processing a Segment
or the Segment Routing Header).
Section 3.1. of [RFC8986] describes the format of an SRv6 SID as
composed of three parts LOC:FUNCT:ARG, where a locator (LOC) is
encoded in the L most significant bits of the SID, followed by F bits
of function (FUNCT) and A bits of arguments (ARG). If L+F+A < 128,
the ARG is followed by enough zero bits to fill the 128 bit SID.
Such a SID is assigned to a node within a prefix defined as a Locator
of length L. When an SRv6 SID occurs in the IPv6 Destination Address
of an IPv6 header, only the longest match prefix corresponding to the
Locator [BCP198] is used by the transit node to forward the packet to
the node identified by the Locator.
It is clear that this format for SRv6 SIDs is not compliant with the
requirements set forth in [RFC4291] for IPv6 addresses but it is also
clear that SRv6 SIDs are not intended for assignment onto interfaces
on end hosts. They look and act similar to other mechanisms that use
IPv6 addresses with different formats such as [RFC6052] that defines
the IPv6 Addressing of IPv4/IPv6 Translators and [RFC7343] that
describes ORCHIDv2 (a cryptographic hash identifier format).
While looking at the transit nodes it becomes apparent that these
addresses are used purely for forwarding and not for packet delivery
to end hosts. Hence the relevant specification to apply here is
[BCP198] that requires implementations to support the use of variable
length prefixes in forwarding while explicitly decoupling IPv6
routing and forwarding from the IPv6 address/prefix semantics
described in [RFC4291]. Please note that [BCP198] does not override
the rules in [RFC4291], but merely limits where their impact is
observed
Furthermore, in the SRv6 specifications, all SIDs assigned within a
given Locator prefix are located inside the node identified by
Locator. Therefore there does not appear to be a conflict with
section 2.6.1 of [RFC4291] since subnet-router anycast addresses are
neither required nor useful within a node.
4. Special Considerations for Compressed SIDs
[CSID] introduces an encoding for compressed segment lists (C-SIDs),
and describes how to use a single entry in the Segment list as a
container for multiple SIDs. A node taking part in this mechanism
accomplishes this by using the ARG part [RFC8986] of the Destination
Address of the IPv6 header to derive a new Destination Address. i.e.,
the Destination Address field of the packet changes at a segment
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endpoint in a way similar to how the address changes as the result of
processing a segment in the SRH.
One key thing to note here is that the Locator Block at the beginning
of the address does not get modified by the operations needed for
supporting compressed SIDs. As we have established that the SRv6
SIDs are being treated simply as routing prefixes on transit nodes
within the SR domain this does not constitute a modification to the
IPv6 data plane on such transit nodes and any changes are restricted
to SR aware nodes.
4.1. Applicability to other forms of compressed SIDs
The spring working group is in the process of analyzing multiple
mechanisms for compressing the SRv6 SID list as described in
[I-D.ietf-spring-compression-analysis]. Even though this document
focuses on [CSID], the considerations specified in this document
might also be applicable to the other mechanisms being analyzed and
compared.
5. Allocation of a Global Unicast Prefix for SIDs
All of the SRv6 related specifications discussed above are intended
to be applicable to a contained SR Domain or between collaborating SR
Domains. Nodes either inside or outside the SR Domains that are not
SR-aware will not perform any special behavior for an SRv6 SIDs and
will treat them solely as IPv6 routing prefixes.
As an added factor of safety, it is desirable to allocate some
address space that explicitly signals that the addresses within that
space cannot be expected to comply with [RFC4291]. As described in
Section 3 above, there is precedent for mechanisms that use IPv6
addresses in a manner different from that specified in [RFC4291].
This would be useful in identifying and potentially filtering packets
at the edges of the SR Domains to make it simpler for the SR domain
to fail closed.
Future specifications are needed to describe the conventions and
guidelines for the use of this newly allocated address block. The
SRv6 operational community, which is the first intended user of this
block, is requested to come up with such conventions and guidelines
in line with their requirements.
6. IANA Considerations
IANA is requested to assign a /16 address block for the purposes
described in Section 5 out of the "Reserved by IETF" range defined in
the Internet Protocol Version 6 Address Space registry.
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7. Security Considerations
The security considerations for the use of Segment Routing [RFC8402],
SRv6 [RFC8754], and SRv6 network programming [RFC8986] apply to the
use of these addresses. The use of IPv6 tunneling mechanisms
(including SRv6) also brings up additional concerns such as those
described in [RFC6169]. The usage of the prefix allocated by this
document improves security by making it simpler to filter traffic at
the edge of the SR domains.
In case the deployments do not use this allocated prefix, additional
care needs to be exercised at network ingress and egress points so
that SRv6 packets do not leak out of SR domains and they do not
accidentally enter SR unaware domains. Similarly as stated in
Section 5.1 of [RFC8754], packets entering an SR domain from the
outside need to be configured to filter out the selected prefix if it
is different from the prefix allocated here.
8. Acknowledgments
The author would like to extend a special note of thanks to Brian
Carpenter and Erik Kline for their precisely summarized thoughts on
this topic that provided the seed of this draft. The author would
also like to thank Andrew Alston, Fred Baker, Ron Bonica, Nick
Buraglio, Bruno Decraene, Dhruv Dhody, Darren Dukes, Adrian Farrel,
Clarence Filsfils, Jim Guichard, Joel Halpern, Bob Hinden, Cheng Li,
Acee Lindem, Jen Linkova, Gyan Mishra, Robert Raszuk, Alvaro Retana,
Michael Richardson, Mark Smith, Dirk Steinberg, Ole Troan, Eduard
Vasilenko, Eric Vyncke, Jingrong Xie, and Chongfeng Xie for their
ideas and comments to improve this document.
9. References
9.1. Normative References
[BCP198] Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix
Length Recommendation for Forwarding", BCP 198, RFC 7608,
DOI 10.17487/RFC7608, July 2015,
<https://www.rfc-editor.org/info/rfc7608>.
[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>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
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[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>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[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>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
9.2. Informative References
[CSID] Cheng, W., Filsfils, C., Li, Z., Decraene, B., and F.
Clad, "Compressed SRv6 Segment List Encoding in SRH", Work
in Progress, Internet-Draft, draft-ietf-spring-srv6-srh-
compression-04, 31 March 2023,
<https://www.ietf.org/archive/id/draft-ietf-spring-srv6-
srh-compression-04.txt>.
[I-D.ietf-spring-compression-analysis]
Bonica, R., Cheng, W., Dukes, D., Henderickx, W., Li, C.,
Peng, S., and C. Xie, "Compressed SRv6 SID List Analysis",
Work in Progress, Internet-Draft, draft-ietf-spring-
compression-analysis-03, 3 April 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
compression-analysis-03>.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<https://www.rfc-editor.org/info/rfc6052>.
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[RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns with IP Tunneling", RFC 6169,
DOI 10.17487/RFC6169, April 2011,
<https://www.rfc-editor.org/info/rfc6169>.
[RFC7343] Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
Routable Cryptographic Hash Identifiers Version 2
(ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, September
2014, <https://www.rfc-editor.org/info/rfc7343>.
Author's Address
Suresh Krishnan
Cisco
Email: suresh.krishnan@gmail.com
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