Distribute SRv6 Locator by IPv6 Stateless Address Autoconfiguration
draft-cheng-spring-stateless-nd-srv6-locator-02

SPRING W. Cheng
Internet Draft R. Han
Intended status: Standards Track China Mobile
Expires: June 05, 2025 C. Lin
 Y. Qiu
 New H3C Technologies
 December 04, 2025
 Distribute SRv6 Locator by IPv6 Stateless Address Autoconfiguration
 draft-cheng-spring-stateless-nd-srv6-locator-02
Abstract
 In an SRv6 network, each SRv6 Segment Endpoint Node must be assigned
 an SRv6 locator, and segment IDs are generated within the address
 space of this SRv6 locator. This document describes a method for
 assigning SRv6 locators to SRv6 Segment Endpoint Nodes through IPv6
 stateless address autoconfiguration.
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
 Task Force (IETF), its areas, and its working groups. Note that
 other groups may also distribute working documents as Internet-
 Drafts.
 Internet-Drafts are draft documents valid for a maximum of six
 months and may be updated, replaced, or obsoleted by other documents
 at any time. It is inappropriate to use Internet-Drafts as reference
 material or to cite them other than as "work in progress."
 The list of current Internet-Drafts can be accessed at
 http://www.ietf.org/ietf/1id-abstracts.txt
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 http://www.ietf.org/shadow.html
 This Internet-Draft will expire on June 05, 2026.
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Copyright Notice
 Copyright (c) 2025 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document. Please review these documents
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 respect to this document. Code Components extracted from this
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 warranty as described in the Simplified BSD License.
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Table of Contents
 1. Introduction...................................................4
 1.1. Requirements Language.....................................5
 2. Terminology....................................................5
 3. Scenario for SRv6 Locator......................................5
 4. Extension of IPv6 Neighbor Discovery Options...................7
 5. Process of Advertising SRv6 Locator by Router Advertisement....8
 5.1. Router Behavior...........................................9
 5.2. Host Behavior............................................10
 6. IANA Considerations...........................................10
 7. Security Considerations.......................................11
 8. Acknowledgements..............................................11
 9. References....................................................11
 9.1. Normative References.....................................11
 9.2. Informative References...................................11
 Authors' Addresses...............................................12
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1. Introduction
 Segment Routing (SR) [RFC8402] allows a node to steer a packet flow
 along any path. The headend is a node where the instructions for
 source routing (i.e., segments) are written into the packet. It
 hence becomes the starting node for a specific segment routing path.
 Intermediate per-path states are eliminated thanks to source
 routing. A Segment Routing Policy (SR Policy) [RFC8402] is an
 ordered list of segments (i.e., instructions) that represent a
 source-routed policy. The headend node is said to steer a flow into
 an SR Policy. The packets steered into an SR Policy have an ordered
 list of segments associated with that SR Policy written into them.
 [RFC8402] defines an SRv6 Segment Identifier (SID) as an IPv6
 address explicitly associated with the segment. When an SRv6 SID is
 in the Destination Address field of an IPv6 header of a packet, it
 is routed through transit nodes in an IPv6 network as an IPv6
 address.
 An SRv6 SID [RFC8986] is as consisting of 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). L, the locator length, is flexible, and an operator is free
 to use the locator length of their choice. F and A may be any value
 as long as L+F+A <= 128. A locator may be represented as B:N where B
 is the SRv6 SID block (IPv6 prefix allocated for SRv6 SIDs by the
 operator) and N is the identifier of the parent node instantiating
 the SID. When the LOC part of the SRv6 SIDs is routable, it leads to
 the node, which instantiates the SID.
 The SRv6 locator can be distributed to other IPv6 nodes within the
 SRv6 domain through IGP advertisement. This allows other nodes to
 learn the locator's route. The SRv6 Segment Endpoint Node then
 allocates SIDs with various behaviors based on its locator.
 In IP network customer provider edge (CPE) devices often do not
 support an IGP protocol, which makes it impossible to advertise SRv6
 locator routes for SRv6 Segment Endpoint Nodes through IGP. In such
 scenarios, SIDs can only be configured manually on CPEs, and SRv6
 Locator routes can only be statically distributed.
 To address this issue, this document proposes a method of
 dynamically advertising SRv6 locators to SRv6 Segment Endpoint Nodes
 through IPv6 stateless address configuration method. It follows the
 existing process of IPv6 stateless address configuration,
 simplifying the allocation of SRv6 locators and route distribution.
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1.1. 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. Terminology
 This document leverages the terms defined in [RFC4861] and
 [RFC8986]. The reader is assumed to be familiar with this
 terminology.
3. Scenario for SRv6 Locator
 The application scenario for obtaining SRv6 Locator through IPv6
 stateless address autoconfiguration is similar to that of [I-D.ietf-
 spring-dhc-distribute-srv6-locator-dhcp].
 In the IP backbone network, Telecom providers can use its IP Metro
 and Backbone networks to establish connectivity between access users
 who are located in different regions.
 As shown in Figure 1, access network devices (CPE) are deployed for
 access users in different regions. This deployment assumes that all
 of the relevant components in Figure 1 are part of a single trusted
 SR domain. The CPE must be operator-managed and is only applicable
 when different arms of the same company operate their portions of
 the network separately, but must trust each other.
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 Metropolitan area network
 +---------------------------+
 | |
 +------+ +------+ | +-----+ +------+ |
 |Host1 +-----+ CPE1 +----+--+BRAS1+--------+ CR1 | |
 +------+ +------+ | +-----+ +---+--+ |
 | | |
 +---------------------+-----+
 |
 +--------+-------------+
 | |
 | Backbone Network |
 | |
 +--------+-------------+
 |
 +---------------------+-----+
 | | |
 +------+ +------+ | +-----+ +--+---+ |
 |Host2 +-----+ CPE2 +----+--+BRAS2+---------+ CR2 | |
 +------+ +------+ | +-----+ +------+ |
 +---------------------------+
 Figure 1: Telecom IPv6 Network
 CPEs for access users are connected to the local metropolitan area
 network (MAN) in various ways. CPEs are responsible for assigning
 addresses to access users, so CPEs usually apply for IPv6 subnet
 prefix through DHCPv6 or stateless address autoconfiguration from
 BRAS.
 In this network, operators hope to achieve interconnection between
 access users through End-to-End SRv6 tunnels. Taking the service
 traffic from Host1 to Host2 as an example, CPE1 is the SRv6 ingress
 node and CPE2 is the SRv6 egress node. The SRv6 locator should be
 configured on CPE. Other devices in the network learn the SRv6
 locator route of the CPE.
 At the same time, SRv6 policies needs to be configured on CPEs to
 steer the service traffic between CPEs to the specified SRv6
 forwarding path. The SRv6 policy can be manually configured
 statically or issued through the controller, and its specific
 configuration method is out of the scope of this document.
 However, in Metro network, the number of CPEs is very large and
 widely distributed geographically. Moreover, the mobility
 requirements of CPE are relatively high, and the access location of
 the same CPE often changes, so its IPv6 address cannot be fixed.
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 At present, an SRv6 locator can only be configured on each CPE
 through a controller or the Command Line Interface (CLI), which
 increases the configuration complexity.
 To solve the difficulties this document proposes a method to
 allocate SRv6 locators to CPE through IPv6 stateless address
 autoconfiguration.
4. Extension of IPv6 Neighbor Discovery Options
 The SRv6 Locator option is used to specify the SRv6 locators that
 are used for stateless address autoconfiguration.
 The terms Locator Block and Locator Node correspond to the B and N
 parts, respectively, of the SRv6 Locator that is defined in Section
 3.1 of [RFC8986].
 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 | Reserved |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Valid Lifetime |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Preferred Lifetime |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | LB-Len | LN-Len | Fun-Len | Arg-Len |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 . SRv6-Locator .
 . ( Up to 16 octets ) .
 . .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 * Type: 8-bit identifier of the type of option. The value is TBA by
 IANA.
 * Length: 8-bit unsigned integer. The length of the option
 (including the type and length fields) in units of 8 octets. The
 value 0 is invalid. Nodes MUST silently discard an ND packet that
 contains an option with length zero.
 * Reserved: 16-bit unused field. It MUST be initialized to zero by
 the sender and MUST be ignored by the receiver.
 * Valid Lifetime: 32-bit unsigned integer. The valid lifetime for
 the SRv6 locator in the option, expressed in units of seconds
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 (relative to the time the packet is sent). A value of all one
 bits (0xffffffff) represents infinity.
 * Preferred Lifetime: 32-bit unsigned integer. The preferred
 lifetime for the SRv6 locator in the option, expressed in units of
 seconds (relative to the time the packet is sent). A value of all
 one bits (0xffffffff) represents infinity. Note that the value of
 this field MUST NOT exceed the Valid Lifetime field to avoid SRv6
 locator that are no longer valid.
 * LB-Len: 8-bit unsigned integer. SRv6 SID Locator Block (LB) length
 in bits.
 * LN-Len: 8-bit unsigned integer. SRv6 SID locator Node (LN) length
 in bits.
 * Fun-Len: 8-bit unsigned integer. SRv6 SID function (FUNCT) length
 in bits.
 * Arg-Len: 8-bit unsigned integer. SRv6 SID arguments (ARG) length
 in bits.
 * SRv6-Locator: 1-16 octets. This field encodes the SRv6 Locator.
 The SRv6 Locator is encoded in the minimal number of octets for
 the given number of bits. Trailing bits MUST be set to zero and
 ignored when received.
 The option only may appear in the Router Advertisement message.
 Router Advertisement messages can include zero or more SRv6 Locator
 options. If multiple SRv6 Locators need to be advertised to the same
 device, multiple SRv6 Locator options MUST be encapsulated in the
 same Router Advertisement message. The SRv6 Locator Option should be
 padded when necessary to ensure that it end on its natural 64-bit
 boundary.
 Receivers MUST silently ignore the option if they can't recognize
 and continue processing the message.
5. Process of Advertising SRv6 Locator by Router Advertisement
 This section describes router and host behavior of adverting SRv6
 locator related to the Router Discovery portion of Neighbor
 Discovery. Router Discovery is used to locate neighboring routers as
 well as learn prefixes and configuration parameters related to
 stateless address autoconfiguration.
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 The IPv6 stateless autoconfiguration mechanism requires no manual
 configuration of hosts, minimal (if any) configuration of routers,
 and no additional servers. The stateless mechanism allows a host to
 generate its own addresses using a combination of locally available
 information and information advertised by routers. Routers advertise
 prefixes that identify the subnet(s) associated with a link, while
 hosts generate an "interface identifier" that uniquely identifies an
 interface on a subnet. An address is formed by combining the two. In
 the absence of routers, a host can only generate link-local
 addresses. However, link-local addresses are sufficient for allowing
 communication among nodes attached to the same link.
 The stateless approach is used when a site is not particularly
 concerned with the exact addresses hosts use, so long as they are
 unique and properly routable.
 Global addresses generated through stateless autoconfiguration
 mechanism are formed by appending an interface identifier to a
 prefix of appropriate length. Prefixes are obtained from Prefix
 Information options contained in Router Advertisements.
 Router Advertisements are sent periodically to the all-nodes
 multicast address. To obtain an advertisement quickly, a host sends
 out Router Solicitations as described in [RFC4861].
 When the router sends a Router Advertisement, it carries the SRv6
 locator prefix information assigned to the host in the SRv6 Locator
 Option. After receiving the Router Advertisement, the host extracts
 the SRv6 Locator Option, obtains the SRv6 Locator prefix.
 The detailed process of routers and hosts when advertising SRv6
 locators during the IPv6 stateless autoconfiguration is as follows.
5.1. Router Behavior
 The Router follows the specifications in Section 6.2 of [RFC4861] to
 send out Router Advertisement messages periodically, or in response
 to Router Solicitations.
 When receiving a Router Solicitation from a host, if the router has
 already assigned an SRv6 locator to the host, it will include the
 SRv6 Locator Option in the Router Advertisement message of the
 responding host and advertise the SRv6 locator to the host.
 In the scenario where all hosts under the advertising interface
 share the SRv6 Locator prefix, the SRv6 Locator option SHOULD be
 included in the Router Advertisement message sent periodically.
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5.2. Host Behavior
 On receipt of a Router Advertisement, the host follows the
 specifications in Section 6.3 of [RFC4861] to verify the packet. For
 valid Router Advertisement, continue to extract the SRv6 Locator
 option.
 For each SRv6 Locator option, a host does the following:
 * If the SRv6 locator is not already present in the SRv6 Locator
 List, and the SRv6 Locator Option's Valid Lifetime field is non-
 zero, create a new entry for the SRv6 locator and initialize its
 invalidation timer to the Valid Lifetime value in the SRv6 Locator
 option.
 * If the SRv6 locator is already present in the host's SRv6 Locator
 List as the result of a previously received advertisement, reset
 its invalidation timer to the Valid Lifetime value in the SRv6
 Locator option. If the new Lifetime value is zero, time-out the
 SRv6 locator immediately.
 * If the SRv6 Locator option's Valid Lifetime field is zero, and the
 SRv6 locator is not present in the host's SRv6 Locator List,
 silently ignore the option.
 Whenever the invalidation timer expires for a SRv6 Locator entry,
 that entry is discarded.
 After processing the SRv6 Locator Option, the host records the SRv6
 Locator prefix, and generates the corresponding SID based on the
 local configuration. The method of generating SID based on SRv6
 locator is out of the scope of this document.
6. IANA Considerations
 IANA is asked to assign a new value for the "IPv6 Neighbor Discovery
 Option Formats" registry under the heading "Internet Control Message
 Protocol version 6 (ICMPv6) Parameters", as follows:
 +===============+=====================+================+
 | Value | Description | Reference |
 +---------------+---------------------+----------------+
 | TBA | SRv6 Locator Option | This document |
 +---------------+---------------------+----------------+
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 Table 1
7. Security Considerations
 See Section 11 of [RFC4861] for the Neighbor Discovery security
 considerations.
8. Acknowledgements
 TBD
9. References
9.1. Normative References
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
 September 2007.
 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, May 2017.
 [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>.
 [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>.
 [I-D.ietf-spring-dhc-distribute-srv6-locator-dhcp] Cheng, W., Han,
 R., Lin, C., Qiu, Y., Zhang, G., "Distribute SRv6 Locator
 by DHCP", draft-ietf-spring-dhc-distribute-srv6-locator-
 dhcp-03 (work in progress), June 2024.
9.2. Informative References
 TBD
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Authors' Addresses
 Weiqiang Cheng
 China Mobile
 Email: chengweiqiang@chinamobile.com
 Ruibo Han
 China Mobile
 Email: hanruibo@chinamobile.com
 Changwang Lin
 New H3C Technologies
 Email: linchangwang.04414@h3c.com
 Yuanxiang Qiu
 New H3C Technologies
 Email: qiuyuanxiang@h3c.com
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