Locator/ID Separation Protocol (LISP) Map-Versioning
draft-ietf-lisp-6834bis-14

Network Working Group                                         L. Iannone
Internet-Draft                                Huawei Technologies France
Obsoletes: 6834 (if approved)                                  D. Saucez
Intended status: Standards Track                                   INRIA
Expires: December 23, 2022                                O. Bonaventure
                                        Universite catholique de Louvain
                                                           June 21, 2022

          Locator/ID Separation Protocol (LISP) Map-Versioning
                       draft-ietf-lisp-6834bis-14

Abstract

   This document describes the LISP (Locator/ID Separation Protocol)
   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 the transport of 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 "Locator/ID Separation Protocol
   (LISP) Map-Versioning", which is the initial experimental
   specifications of the mechanisms updated by this document.

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 December 23, 2022.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  Definitions of Terms  . . . . . . . . . . . . . . . . . . . .   4
   4.  LISP-specific Header and Map-Version Numbers  . . . . . . . .   4
   5.  Map Record and Map-Version  . . . . . . . . . . . . . . . . .   5
   6.  EID-to-RLOC Map-Version Number  . . . . . . . . . . . . . . .   6
     6.1.  The Null Map-Version  . . . . . . . . . . . . . . . . . .   7
   7.  Dealing with Map-Version Numbers  . . . . . . . . . . . . . .   7
     7.1.  Handling Destination Map-Version Number . . . . . . . . .   8
     7.2.  Handling Source Map-Version Number  . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Deployment Considerations . . . . . . . . . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Benefits and Case Studies for Map-Versioning . . . .  14
     A.1.  Map-Versioning and Unidirectional Traffic . . . . . . . .  14
     A.2.  Map-Versioning and Interworking . . . . . . . . . . . . .  14
       A.2.1.  Map-Versioning and Proxy-ITRs . . . . . . . . . . . .  14
       A.2.2.  Map-Versioning and LISP-NAT . . . . . . . . . . . . .  15
       A.2.3.  Map-Versioning and Proxy-ETRs . . . . . . . . . . . .  15
     A.3.  RLOC Shutdown/Withdraw  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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1.  Introduction

   This document describes the Map-Versioning mechanism used to provide
   information on changes in the EID-to-RLOC (Endpoint ID to Routing
   Locator) mappings used in the LISP (Locator/ID Separation Protocol
   [I-D.ietf-lisp-rfc6830bis][I-D.ietf-lisp-rfc6833bis]) context to
   perform packet encapsulation.  The mechanism is totally transparent
   to xTRs (Ingress and Egress Tunnel Routers) not supporting or not
   using such functionality.  [I-D.ietf-lisp-introduction] describes the
   architecture of the Locator/ID Separation Protocol.  It is expected
   that the reader is familiar with this introductory document.

   This document obsoletes [RFC6834], which is the initial experimental
   specifications of 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, removal, or
   change in 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.  On the one hand, it enables the ETR
   (Egress Tunnel Router) receiving the packet to know if the ITR
   (Ingress Tunnel Router) 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 it a Solicit Map-
   Request (SMR) message.  Both cases are defined in
   [I-D.ietf-lisp-rfc6833bis].  On the other hand, it enables an ETR
   receiving such a 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.

   Benefits brought by Map-Versioning in some common LISP-related use
   cases are discussed in Appendix A.

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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
   specification ([I-D.ietf-lisp-rfc6830bis],
   [I-D.ietf-lisp-rfc6833bis]).  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),
     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 [I-D.ietf-lisp-rfc6830bis] 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
   [I-D.ietf-lisp-rfc6830bis].  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:

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    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
   [I-D.ietf-lisp-rfc6833bis]) 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 [I-D.ietf-lisp-rfc6833bis].

        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.

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   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, like 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 a different version number,
   which is also updated independently.  An update in the version number
   (i.e., a newer version) MUST consist of an increment of the older
   version number (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 reacting differently to
   "older" and "newer" Map-Version number, discarding the packet in the
   former case and triggering a Map-Request in the latter (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 as
   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.

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   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

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   are not reachable anymore from a local perspective (e.g., through
   IGP, or policy changes) the LISP site updates the mapping, also
   assigning 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 sufficient time to be sure that
   mapping in LISP caches expire, which means waiting at least as much
   as the mapping Time-To-Live (as defined in
   [I-D.ietf-lisp-rfc6833bis]).

   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 Destination 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
       and 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

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       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 [I-D.ietf-lisp-rfc6830bis];
       however, the ITR sending the packet MUST be informed that a newer
       mapping is available, respecting rate-limitation policies
       described in [I-D.ietf-lisp-rfc6833bis].  This is done with a
       Map-Request message sent back to the ITR, as specified in
       [I-D.ietf-lisp-rfc6833bis].  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 rate limitation policy defined
       in [I-D.ietf-lisp-rfc6833bis] 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 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 [I-D.ietf-lisp-rfc6833bis].

   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.

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   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
       [I-D.ietf-lisp-rfc6833bis].

   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
   [I-D.ietf-lisp-rfc6830bis] and [I-D.ietf-lisp-rfc6833bis] apply and,
   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 [I-D.ietf-lisp-rfc6833bis].  With Map-
   Versioning it is possible to filter packet 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 from a DDoS attack.

   The present memo includes log action to be taken upon certains event.
   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 data-plane-triggered control-plane actions can be used to
   carry out attacks.  There exists corner cases where, even with an

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   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 leggitimate 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 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.

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   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 includes no request to IANA.

11.  References

11.1.  Normative References

   [I-D.ietf-lisp-rfc6830bis]
              Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
              Cabellos-Aparicio, "The Locator/ID Separation Protocol
              (LISP)", draft-ietf-lisp-rfc6830bis-38 (work in progress),
              May 2022.

   [I-D.ietf-lisp-rfc6833bis]
              Farinacci, D., Maino, F., Fuller, V., and A. Cabellos-
              Aparicio, "Locator/ID Separation Protocol (LISP) Control-
              Plane", draft-ietf-lisp-rfc6833bis-31 (work in progress),
              May 2022.

   [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>.

11.2.  Informative References

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   [I-D.ietf-lisp-introduction]
              Cabellos-Aparicio, A. and D. Saucez, "An Architectural
              Introduction to the Locator/ID Separation Protocol
              (LISP)", draft-ietf-lisp-introduction-15 (work in
              progress), September 2021.

   [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>.

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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 Destination 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,
   ignoring the Source Map-Version number.

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

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   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.  The Proxy ITR will put in the Dest Map-
   Version Number, of the LISP-specific header, 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.

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    +----------+                             +-------------+
    | 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 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
   withdraw of 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 [I-D.ietf-lisp-rfc6833bis]), 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 re-ordering of
   the packet in the flow just as any other routing change could cause
   re-ordering.

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Authors' Addresses

   Luigi Iannone
   Huawei Technologies France

   EMail: luigi.iannone@huawei.com

   Damien Saucez
   INRIA

   EMail: damien.saucez@inria.fr

   Olivier Bonaventure
   Universite catholique de Louvain

   EMail: olivier.bonaventure@uclouvain.be

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