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Locator/ID Separation Protocol (LISP) Map-Versioning
RFC 9302

Document Type RFC - Proposed Standard (October 2022)
Obsoletes RFC 6834
Authors Luigi Iannone , Damien Saucez , Olivier Bonaventure
Last updated 2022-10-20
Replaces draft-iannone-6834bis
RFC stream Internet Engineering Task Force (IETF)
Formats
Reviews
OPSDIR Last Call Review Incomplete, due 2022-05-19
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Padma Pillay-Esnault
Shepherd write-up Show Last changed 2021-07-20
IESG IESG state RFC 9302 (Proposed Standard)
Action Holders
(None)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Alvaro Retana
Send notices to Padma Pillay-Esnault <padma.ietf@gmail.com>
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
RFC 9302


Internet Engineering Task Force (IETF)                        L. Iannone
Request for Comments: 9302                    Huawei Technologies France
Obsoletes: 6834                                                D. Saucez
Category: Standards Track                                          Inria
ISSN: 2070-1721                                           O. Bonaventure
                                        Universite catholique de Louvain
                                                            October 2022

          Locator/ID Separation Protocol (LISP) Map-Versioning

Abstract

   This document describes the Locator/ID Separation Protocol (LISP)
   Map-Versioning mechanism, which provides in-packet information about
   Endpoint-ID-to-Routing-Locator (EID-to-RLOC) mappings used to
   encapsulate LISP data packets.  This approach is based on associating
   a version number to EID-to-RLOC mappings and transporting such a
   version number in the LISP-specific header of LISP-encapsulated
   packets.  LISP Map-Versioning is particularly useful to inform
   communicating Ingress Tunnel Routers (ITRs) and Egress Tunnel Routers
   (ETRs) about modifications of the mappings used to encapsulate
   packets.  The mechanism is optional and transparent to
   implementations not supporting this feature, since in the LISP-
   specific header and in the Map Records, bits used for Map-Versioning
   can be safely ignored by ITRs and ETRs that do not support or do not
   want to use the mechanism.

   This document obsoletes RFC 6834, which is the initial experimental
   specifications of the mechanisms updated by this document.

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/rfc9302.

Copyright Notice

   Copyright (c) 2022 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Notation
   3.  Definitions of Terms
   4.  LISP-Specific Header and Map-Version Numbers
   5.  Map Record and Map-Version
   6.  EID-to-RLOC Map-Version Number
     6.1.  The Null Map-Version
   7.  Dealing with Map-Version Numbers
     7.1.  Handling Dest Map-Version Number
     7.2.  Handling Source Map-Version Number
   8.  Security Considerations
   9.  Deployment Considerations
   10. IANA Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Appendix A.  Benefits and Case Studies for Map-Versioning
     A.1.  Map-Versioning and Unidirectional Traffic
     A.2.  Map-Versioning and Interworking
       A.2.1.  Map-Versioning and Proxy-ITRs
       A.2.2.  Map-Versioning and LISP-NAT
       A.2.3.  Map-Versioning and Proxy-ETRs
     A.3.  RLOC Shutdown/Withdraw
   Authors' Addresses

1.  Introduction

   This document describes the Map-Versioning mechanism used to provide
   information on changes in the Endpoint-ID-to-Routing-Locator (EID-to-
   RLOC) mappings used in the Locator/ID Separation Protocol (LISP)
   [RFC9300] [RFC9301] context to perform packet encapsulation.  The
   mechanism is totally transparent to Ingress and Egress Tunnel Routers
   (xTRs) not supporting or not using such functionality.  The
   architecture of LISP is described in [RFC9299].  The reader is
   expected to be familiar with this introductory document.

   This document obsoletes [RFC6834], which is the initial experimental
   specification that describes the mechanisms updated by this document.

   The basic mechanism is to associate a Map-Version number to each LISP
   EID-to-RLOC mapping and transport such a version number in the LISP-
   specific header.  When a mapping changes, a new version number is
   assigned to the updated mapping.  A change in an EID-to-RLOC mapping
   can be a modification in the RLOCs set, such as addition of, removal
   of, or change in the priority or weight of one or more RLOCs.

   When Map-Versioning is used, LISP-encapsulated data packets contain
   the version number of the two mappings used to select the RLOCs in
   the outer header (i.e., both source and destination RLOCs).  This
   information has two uses:

   1.  Map-Versioning enables the Egress Tunnel Router (ETR) receiving
       the packet to know if the Ingress Tunnel Router (ITR) is using
       the latest mapping version for the destination EID.  If this is
       not the case, the ETR can directly send a Map-Request containing
       the updated mapping to the ITR to notify it of the latest
       version.  The ETR can also solicit the ITR to trigger a Map-
       Request to obtain the latest mapping by sending a Solicit Map-
       Request (SMR) message.  Both options are defined in [RFC9301].

   2.  Map-Versioning enables an ETR receiving the packet to know if it
       has in its EID-to-RLOC Map-Cache the latest mapping for the
       source EID.  If this is not the case, a Map-Request can be sent.

   Considerations about the deployment of LISP Map-Versioning are
   discussed in Section 9.

   The benefits of Map-Versioning in some common LISP-related use cases
   are discussed in Appendix A.

2.  Requirements Notation

   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.  Definitions of Terms

   This document uses terms already defined in the main LISP
   specifications ([RFC9300] and [RFC9301]).  Here, we define the terms
   that are specific to the Map-Versioning mechanism.  Throughout the
   whole document, big-endian bit ordering is used.

   Map-Version number:  An unsigned 12-bit integer is assigned to an
     EID-to-RLOC mapping, indicating its version number (Section 6).

   Null Map-Version:  A Map-Version number with a value of 0x000 (zero),
     which is used to signal that the Map-Version feature is not used
     and no Map-Version number is assigned to the EID-to-RLOC mapping
     (Section 6.1).

   Dest Map-Version number:  Map-Version of the mapping in the EID-to-
     RLOC Map-Cache used by the ITR to select the RLOC present in the
     'Destination Routing Locator' field of the outer IP header of LISP-
     encapsulated packets (Section 7.1).

   Source Map-Version number:  Map-Version of the mapping in the EID-to-
     RLOC Database used by the ITR to select the RLOC present in the
     'Source Routing Locator' field of the outer IP header of LISP-
     encapsulated packets (Section 7.2).

4.  LISP-Specific Header and Map-Version Numbers

   In order for the versioning approach to work, the LISP-specific
   header has to carry both the Source Map-Version number and Dest Map-
   Version number.  This is done by setting the V-bit in the LISP-
   specific header as specified in [RFC9300] and shown in the example in
   Figure 1.  All permissible combinations of the flags when the V-bit
   is set to 1 are described in [RFC9300].  Not all of the LISP-
   encapsulated packets need to carry version numbers.  When the V-bit
   is set, the LISP-specific header has the following encoding:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |N|L|E|V|I|R|K|K|  Source Map-Version   |   Dest Map-Version    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Instance ID/Locator-Status-Bits               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 1: LISP-Specific Header Example When Map-Versioning Is in Use

   Source Map-Version number (12 bits):  See Section 3.

   Dest Map-Version number (12 bits):  See Section 3.

5.  Map Record and Map-Version

   To accommodate the mechanism, the Map Records that are transported in
   Map-Request/Map-Reply/Map-Register messages need to carry the Map-
   Version number as well.  For reference, the Map Record (specified in
   [RFC9301]) is reported here as an example in Figure 2.  This memo
   does not change the operation of Map-Request/Map-Reply/Map-Register
   messages; they continue to be used as specified in [RFC9301].

        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
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |                          Record TTL                           |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   R   | Locator Count | EID mask-len  | ACT |A|      Reserved         |
   e   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   c   | Rsvd  |  Map-Version Number   |       EID-Prefix-AFI          |
   o   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   r   |                          EID-Prefix                           |
   d   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  /|    Priority   |    Weight     |  M Priority   |   M Weight    |
   | L +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | o |        Unused Flags     |L|p|R|           Loc-AFI             |
   | c +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  \|                             Locator                           |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 2: Map-Record Format Example

   Map-Version Number:  Map-Version of the mapping contained in the
     Record.  As explained in Section 6.1, this field can be zero (0),
     meaning that no Map-Version is associated to the mapping.

   This packet format is backward compatible with xTRs that do not
   support Map-Versioning, since they can simply ignore those bits.

   A Map-Server receiving a message with an unexpected Map-Version
   number, for instance an old one, MUST silently drop the message and
   an appropriate log action SHOULD be taken.

6.  EID-to-RLOC Map-Version Number

   The EID-to-RLOC Map-Version number consists of an unsigned 12-bit
   integer.  The version number is assigned on a per-mapping basis,
   meaning that different mappings have different version numbers, which
   are updated independently.  An update in the version number (i.e., a
   newer version) MUST consist of an increment of the older version
   number (the only exception is for the Null Map-Version as explained
   in at the end of Section 6.1).

   The space of version numbers has a circular order where half of the
   version numbers are considered greater (i.e., newer) than the current
   Map-Version number and the other half of the version numbers are
   considered smaller (i.e., older) than the current Map-Version number.
   This is basically a serial number on which the arithmetic described
   in [RFC1982] applies.  The ordering enables different reactions to
   "older" and "newer" Map-Version numbers, whereby "older" numbers are
   discarded and "newer" numbers trigger Map-Requests (see Section 7 for
   further details).  In a formal way, assuming that we have two version
   numbers (V1 and V2), both different from the special value Null Map-
   Version (see Section 6.1), and that the numbers are expressed on 12
   bits, the following steps MUST be performed (in the same order shown
   below) to strictly define their order:

   1.  V1 = V2 : The Map-Version numbers are the same.

   2.  V2 > V1 : if and only if

         V2 > V1 AND (V2 - V1) <= 2^(12-1)

         OR

         V1 > V2 AND (V1 - V2) > 2^(12-1)

   3.  V1 > V2 : otherwise.

   Using 12 bits and assuming a Map-Version value of 69, Map-Version
   numbers in the range [70; 69 + 2048] are greater than 69, while Map-
   Version numbers in the range [69 + 2049; (69 + 4095) mod 4096] are
   smaller than 69.

   The initial Map-Version number of a new EID-to-RLOC mapping SHOULD be
   assigned randomly, but it MUST NOT be set to the Null Map-Version
   value (0x000), because the Null Map-Version number has a special
   meaning (see Section 6.1).  Optionally, the initial Map-version
   number may be configured.

   Upon reboot, an ETR will use mappings configured in its EID-to-RLOC
   Database.  If those mappings have a Map-Version number, it will be
   used according to the mechanisms described in this document.  ETRs
   MUST NOT automatically generate and assign Map-Version numbers to
   mappings in the EID-to-RLOC Database.

6.1.  The Null Map-Version

   The value 0x000 (zero) is a special Map-Version number indicating
   that there is actually no version number associated to the EID-to-
   RLOC mapping.  Such a value is used for special purposes and is named
   the Null Map-Version number.

   Map Records that have a Null Map-Version number indicate that there
   is no Map-Version number associated with the mapping.  This means
   that LISP-encapsulated packets destined to the EID-Prefix referred to
   by the Map Record MUST NOT contain any Map-Version numbers (V-bit set
   to 0).  If an ETR receives LISP-encapsulated packets with the V-bit
   set, when the original mapping in the EID-to-RLOC Database has the
   version number set to the Null Map-Version value, then those packets
   MUST be silently dropped.

   The Null Map-Version may appear in the LISP-specific header as a
   Source Map-Version number (Section 7.2).  When the Source Map-Version
   number is set to the Null Map-Version value, it means that no map
   version information is conveyed for the source site.  This means that
   if a mapping exists for the source EID in the EID-to-RLOC Map-Cache,
   then the ETR MUST NOT compare the received Null Map-Version with the
   content of the EID-to-RLOC Map-Cache (Section 7.2).

   The fact that the 0 value has a special meaning for the Map-Version
   number implies that, when updating a Map-Version number because of a
   change in the mapping, if the next value is 0, then the Map-Version
   number MUST be incremented by 2 (i.e., set to 1 (0x001), which is the
   next valid value).

7.  Dealing with Map-Version Numbers

   The main idea of using Map-Version numbers is that whenever there is
   a change in the mapping (e.g., adding/removing RLOCs, a change in the
   weights due to Traffic Engineering policies, or a change in the
   priorities) or a LISP site realizes that one or more of its own RLOCs
   are no longer reachable from a local perspective (e.g., through IGP
   or policy changes), the LISP site updates the mapping and also
   assigns a new Map-Version number.  Only the latest Map-Version number
   has to be considered valid.  Mapping updates and their corresponding
   Map-Version Number must be managed so that a very old version number
   will not be confused as a new version number (because of the circular
   numbering space).  To this end, simple measures can be taken, like
   updating a mapping only when all active traffic is using the latest
   version, or waiting a sufficient amount of time to be sure that the
   mapping in LISP caches expires, which means waiting at least as long
   as the mapping Time To Live (TTL) (as defined in [RFC9301]).

   An ETR receiving a LISP packet with Map-Version numbers checks the
   following predicates:

   1.  The ITR that has sent the packet has an up-to-date mapping in its
       EID-to-RLOC Map-Cache for the destination EID and is performing
       encapsulation correctly.  See Section 7.1 for details.

   2.  In the case of bidirectional traffic, the mapping in the local
       ETR EID-to-RLOC Map-Cache for the source EID is up to date.  See
       Section 7.2 for details.

7.1.  Handling Dest Map-Version Number

   When an ETR receives a packet, the Dest Map-Version number relates to
   the mapping for the destination EID for which the ETR is an RLOC.
   This mapping is part of the ETR EID-to-RLOC Database.  Since the ETR
   is authoritative for the mapping, it has the correct and up-to-date
   Dest Map-Version number.  A check on this version number MUST be
   done, where the following cases can arise:

   1.  The packet arrives with the same Dest Map-Version number stored
       in the EID-to-RLOC Database.  This is the regular case.  The ITR
       sending the packet has, in its EID-to-RLOC Map-Cache, an up-to-
       date mapping.  No further actions are needed.

   2.  The packet arrives with a Dest Map-Version number newer (as
       defined in Section 6) than the one stored in the EID-to-RLOC
       Database.  Since the ETR is authoritative on the mapping, meaning
       that the Map-Version number of its mapping is the correct one,
       the packet carries a version number that is not considered valid.
       Therefore, the packet MUST be silently dropped and an appropriate
       log action SHOULD be taken.

   3.  The packet arrives with a Dest Map-Version number older (as
       defined in Section 6) than the one stored in the EID-to-RLOC
       Database.  This means that the ITR sending the packet has an old
       mapping in its EID-to-RLOC Map-Cache containing stale
       information.  The ETR MAY choose to normally process the
       encapsulated datagram according to [RFC9300]; however, the ITR
       sending the packet MUST be informed that a newer mapping is
       available, respecting rate-limitation policies described in
       [RFC9301].  This is done with a Map-Request message sent back to
       the ITR, as specified in [RFC9301].  One feature introduced by
       Map-Version numbers is the possibility of blocking traffic not
       using the latest mapping.  This can happen if an ITR is not
       updating the mapping for which the ETR is authoritative, or it
       might be some form of attack.  According to the rate-limitation
       policy defined in [RFC9301] for Map-Request messages, after 10
       retries, Map-Requests are sent every 30 seconds; if after the
       first 10 retries the Dest Map-Version number in the packets is
       not updated, the ETR SHOULD drop packets with a stale Map-Version
       number.  Operators can configure exceptions to this
       recommendation, which are outside the scope of this document.

   The rule in the third case MAY be more restrictive.  If the Record
   TTL of the previous mapping has already expired, all packets arriving
   with an old Map-Version MUST be silently dropped right away without
   issuing any Map-Request.  Such action is permitted because, if the
   new mapping with the updated version number has been unchanged for at
   least the same amount of time as the Record TTL of the older mapping,
   all the entries in the EID-to-RLOC Map-Caches of ITRs must have
   expired.  Indeed, all ITRs sending traffic should have refreshed the
   mapping according to [RFC9301].

   It is a protocol violation for LISP-encapsulated packets to contain a
   Dest Map-Version number equal to the Null Map-Version number (see
   Section 6.1).

7.2.  Handling Source Map-Version Number

   When an ETR receives a packet, the Source Map-Version number relates
   to the mapping for the source EID for which the ITR that sent the
   packet is authoritative.  If the ETR has an entry in its EID-to-RLOC
   Map-Cache for the source EID, then a check MUST be performed, and the
   following cases can arise:

   1.  The packet arrives with the same Source Map-Version number as
       that stored in the EID-to-RLOC Map-Cache.  This is the regular
       case.  The ETR has in its EID-to-RLOC Map-Cache an up-to-date
       copy of the mapping.  No further actions are needed.

   2.  The packet arrives with a Source Map-Version number newer (as
       defined in Section 6) than the one stored in the local EID-to-
       RLOC Map-Cache.  This means that the ETR has in its EID-to-RLOC
       Map-Cache a mapping that is stale and needs to be updated.  A
       Map-Request MUST be sent to get the new mapping for the source
       EID, respecting rate-limitation policies described in [RFC9301].

   3.  The packet arrives with a Source Map-Version number older (as
       defined in Section 6) than the one stored in the local EID-to-
       RLOC Map-Cache.  Note that if the mapping is already present in
       the EID-to-RLOC Map-Cache, this means that an explicit Map-
       Request has been sent and a Map-Reply has been received from an
       authoritative source.  In this situation, the packet SHOULD be
       silently dropped.  Operators can configure exceptions to this
       recommendation, which are outside the scope of this document.

   If the ETR does not have an entry in the EID-to-RLOC Map-Cache for
   the source EID, then the Source Map-Version number MUST be ignored.
   See Appendix A.1 for an example of when this situation can arise.

8.  Security Considerations

   This document builds on the specification and operation of the LISP
   control and data planes.  The Security Considerations of [RFC9300]
   and [RFC9301] apply.  As such, Map-Versioning MUST NOT be used over
   the public Internet and MUST only be used in trusted and closed
   deployments.  A thorough security analysis of LISP is documented in
   [RFC7835].

   Attackers can try to trigger a large number of Map-Requests by simply
   forging packets with random Map-Versions.  The Map-Requests are rate
   limited as described in [RFC9301].  With Map-Versioning, it is
   possible to filter packets carrying invalid version numbers before
   triggering a Map-Request, thus helping to reduce the effects of DoS
   attacks.  However, it might not be enough to really protect against a
   DDoS attack.

   The present memo includes log action to be taken upon certain events.
   It is recommended that implementations include mechanisms (which are
   beyond the scope of this document) to avoid log resource exhaustion
   attacks.

   The specifications in the present memo are relatively conservative in
   the sense that, in several cases, the packets are dropped.  Such an
   approach is the outcome of considerations made about the possible
   risks that control plane actions that are triggered by the data plane
   can be used to carry out attacks.  There exists corner cases where,
   even with an invalid Map-Version number, forwarding the packet might
   be potentially considered safe; however, system manageability has
   been given priority with respect to having to put in place more
   machinery to be able to identify legitimate traffic.

9.  Deployment Considerations

   LISP requires multiple ETRs within the same site to provide identical
   mappings for a given EID-Prefix.  Map-Versioning does not require
   additional synchronization mechanisms.  Clearly, all the ETRs have to
   reply with the same mapping, including the same Map-Version number;
   otherwise, there can be an inconsistency that creates additional
   control traffic, instabilities, and traffic disruptions.

   There are two ways Map-Versioning is helpful with respect to
   synchronization.  On the one hand, assigning version numbers to
   mappings helps in debugging, since quick checks on the consistency of
   the mappings on different ETRs can be done by looking at the Map-
   Version number.  On the other hand, Map-Versioning can be used to
   control the traffic toward ETRs that announce the latest mapping.

   As an example, let's consider the topology of Figure 3 where ITR A.1
   of Domain A is sending unidirectional traffic to Domain B, while A.2
   of Domain A exchanges bidirectional traffic with Domain B.  In
   particular, ITR A.2 sends traffic to ETR B, and ETR A.2 receives
   traffic from ITR B.

    +-----------------+              +-----------------+
    | Domain A        |              | Domain B        |
    |       +---------+              |                 |
    |       | ITR A.1 |---           |                 |
    |       +---------+    \         +---------+       |
    |                 |      ------->| ETR B   |       |
    |                 |      ------->|         |       |
    |       +---------+    /         |         |       |
    |       | ITR A.2 |---      -----| ITR B   |       |
    |       |         |       /      +---------+       |
    |       | ETR A.2 |<-----        |                 |
    |       +---------+              |                 |
    |                 |              |                 |
    +-----------------+              +-----------------+

                         Figure 3: Example Topology

   Obviously, in the case of Map-Versioning, both ITR A.1 and ITR A.2 of
   Domain A must use the same value; otherwise, the ETR of Domain B will
   start to send Map-Requests.

   The same problem can, however, arise without Map-Versioning, for
   instance, if the two ITRs of Domain A send different Locator-Status-
   Bits.  In this case, either the traffic is disrupted if ETR B does
   not verify reachability or if ETR B will start sending Map-Requests
   to confirm each change in reachability.

   So far, LISP does not provide any specific synchronization mechanism
   but assumes that synchronization is provided by configuring the
   different xTRs consistently.  The same applies for Map-Versioning.
   If in the future any synchronization mechanism is provided, Map-
   Versioning will take advantage of it automatically, since it is
   included in the Map Record format, as described in Section 5.

10.  IANA Considerations

   This document has no IANA actions.

11.  References

11.1.  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>.

   [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>.

   [RFC9300]  Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
              Cabellos, Ed., "The Locator/ID Separation Protocol
              (LISP)", RFC 9300, DOI 10.17487/RFC9300, October 2022,
              <https://www.rfc-editor.org/info/rfc9300>.

   [RFC9301]  Farinacci, D., Maino, F., Fuller, V., and A. Cabellos,
              Ed., "Locator/ID Separation Protocol (LISP) Control
              Plane", RFC 9301, DOI 10.17487/RFC9301, October 2022,
              <https://www.rfc-editor.org/info/rfc9301>.

11.2.  Informative References

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,
              <https://www.rfc-editor.org/info/rfc1982>.

   [RFC6832]  Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
              "Interworking between Locator/ID Separation Protocol
              (LISP) and Non-LISP Sites", RFC 6832,
              DOI 10.17487/RFC6832, January 2013,
              <https://www.rfc-editor.org/info/rfc6832>.

   [RFC6834]  Iannone, L., Saucez, D., and O. Bonaventure, "Locator/ID
              Separation Protocol (LISP) Map-Versioning", RFC 6834,
              DOI 10.17487/RFC6834, January 2013,
              <https://www.rfc-editor.org/info/rfc6834>.

   [RFC7835]  Saucez, D., Iannone, L., and O. Bonaventure, "Locator/ID
              Separation Protocol (LISP) Threat Analysis", RFC 7835,
              DOI 10.17487/RFC7835, April 2016,
              <https://www.rfc-editor.org/info/rfc7835>.

   [RFC9299]  Cabellos, A. and D. Saucez, Ed., "An Architectural
              Introduction to the Locator/ID Separation Protocol
              (LISP)", RFC 9299, DOI 10.17487/RFC9299, October 2022,
              <https://www.rfc-editor.org/info/rfc9299>.

Appendix A.  Benefits and Case Studies for Map-Versioning

   In the following sections, we provide more discussion on various
   aspects and uses of Map-Versioning.  Security observations are
   grouped in Section 8.

A.1.  Map-Versioning and Unidirectional Traffic

   When using Map-Versioning, the LISP-specific header carries two Map-
   Version numbers for both source and destination mappings.  This can
   raise the question on what will happen in the case of unidirectional
   flows, for instance, in the case presented in Figure 4, since the
   LISP specifications do not mandate that the ETR have a mapping from
   the source EID.

    +-----------------+            +-----------------+
    | Domain A        |            | Domain B        |
    |       +---------+            +---------+       |
    |       | ITR A   |----------->| ETR B   |       |
    |       +---------+            +---------+       |
    |                 |            |                 |
    +-----------------+            +-----------------+

           Figure 4: Unidirectional Traffic between LISP Domains

   An ITR is able to put both the source and destination version numbers
   in the LISP-specific header since the Source Map-Version number is in
   its database, while the Dest Map-Version number is in its cache.

   The ETR checks only the Dest Map-Version number, ignoring the Source
   Map-Version number as specified in the final sentence of Section 7.2.

A.2.  Map-Versioning and Interworking

   Map-Versioning is compatible with the LISP interworking between LISP
   and non-LISP sites as defined in [RFC6832].  LISP interworking
   defines three techniques to allow communication LISP sites and non-
   LISP sites, namely: Proxy-ITR, LISP-NAT, and Proxy-ETR.  The
   following text describes how Map-Versioning relates to these three
   mechanisms.

A.2.1.  Map-Versioning and Proxy-ITRs

   The purpose of the Proxy-ITR (PITR) is to encapsulate traffic
   originating in a non-LISP site in order to deliver the packet to one
   of the ETRs of the LISP site (cf. Figure 5).  This case is very
   similar to the unidirectional traffic case described in Appendix A.1;
   hence, similar rules apply.

    +----------+                             +-------------+
    | LISP     |                             | non-LISP    |
    | Domain A |                             | Domain B    |
    |  +-------+        +-----------+        |             |
    |  | ETR A |<-------| Proxy-ITR |<-------|             |
    |  +-------+        +-----------+        |             |
    |          |                             |             |
    +----------+                             +-------------+

    Figure 5: Unidirectional Traffic from Non-LISP Domain to LISP Domain

   The main difference is that a Proxy-ITR does not have any mapping,
   since it just encapsulates packets arriving from the non-LISP site,
   and thus cannot provide a Source Map-Version.  In this case, the
   Proxy-ITR will just put the Null Map-Version value as the Source Map-
   Version number, while the receiving ETR will ignore the field.

   With this setup, LISP Domain A is able to check whether the PITR is
   using the latest mapping.  In the Dest Map-Version Number of the
   LISP-specific header, the Proxy-ITR will put the version number of
   the mapping it is using for encapsulation; the ETR A can use such
   value as defined in Section 7.1.

A.2.2.  Map-Versioning and LISP-NAT

   The LISP-NAT mechanism is based on address translation from non-
   routable EIDs to routable EIDs and does not involve any form of
   encapsulation.  As such, Map-Versioning does not apply in this case.

A.2.3.  Map-Versioning and Proxy-ETRs

   The purpose of the Proxy-ETR (PETR) is to decapsulate traffic
   originating in a LISP site in order to deliver the packet to the non-
   LISP site (cf.  Figure 6).  One of the main reasons to deploy PETRs
   is to bypass Unicast Reverse Path Forwarding checks on the domain.

    +----------+                             +-------------+
    | LISP     |                             | non-LISP    |
    | Domain A |                             | Domain B    |
    |  +-------+        +-----------+        |             |
    |  | ITR A |------->| Proxy-ETR |------->|             |
    |  +-------+        +-----------+        |             |
    |          |                             |             |
    +----------+                             +-------------+

    Figure 6: Unidirectional Traffic from LISP Domain to Non-LISP Domain

   A Proxy-ETR does not have any mapping, since it just decapsulates
   packets arriving from the LISP site.  In this case, the ITR can
   interchangeably put a Map-Version value or the Null Map-Version value
   as the Dest Map-Version number, since the receiving Proxy-ETR will
   ignore the field.

   With this setup, the Proxy-ETR, by looking at the Source Map-Version
   Number, is able to check whether the mapping of the source EID has
   changed.  This is useful to perform source RLOC validation.  In the
   example above, traffic coming from the LISP domain has to be LISP
   encapsulated with a source address being an RLOC of the domain.  The
   Proxy-ETR can retrieve the mapping associated to the LISP domain and
   check if incoming LISP-encapsulated traffic is arriving from a valid
   RLOC.  A change in the RLOC-Set that can be used as source addresses
   can be signaled via the version number, with the Proxy-ETR able to
   request the latest mapping if necessary as described in Section 7.2.

A.3.  RLOC Shutdown/Withdraw

   Map-Versioning can also be used to perform a graceful shutdown or to
   withdraw a specific RLOC.  This is achieved by simply issuing a new
   mapping, with an updated Map-Version number where the specific RLOC
   to be shut down is withdrawn or announced as unreachable (via the
   R-bit in the Map Record; see [RFC9301]) but without actually turning
   it off.

   Upon updating the mapping, the RLOC will receive less and less
   traffic because remote LISP sites will request the updated mapping
   and see that it is disabled.  At least one TTL, plus a little time
   for traffic transit, after the mapping is updated, it should be safe
   to shut down the RLOC gracefully, because all sites actively using
   the mapping should have been updated.

   Note that a change in ETR for a flow can result in the reordering of
   the packet in the flow just as any other routing change could cause
   reordering.

Authors' Addresses

   Luigi Iannone
   Huawei Technologies France
   Email: luigi.iannone@huawei.com

   Damien Saucez
   Inria
   2004 route des Lucioles - BP 93
   Sophia Antipolis
   France
   Email: damien.saucez@inria.fr

   Olivier Bonaventure
   Universite catholique de Louvain
   Email: olivier.bonaventure@uclouvain.be