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MP-BGP Extension and the Procedures for IPv4/IPv6 Mapping Advertisement
draft-ietf-idr-mpbgp-extension-4map6-02

Document Type Active Internet-Draft (idr WG)
Authors Chongfeng Xie , Guozhen Dong , Xing Li , Guoliang Han , Zhongfeng Guo
Last updated 2024-04-28
Replaces draft-xie-idr-mpbgp-extension-4map6
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draft-ietf-idr-mpbgp-extension-4map6-02
Network Working Group                                             C. Xie
Internet-Draft                                                   G. Dong
Intended status: Standards Track                           China Telecom
Expires: 31 October 2024                                           X. Li
                                       CERNET Center/Tsinghua University
                                                                  G. Han
                                                Indirection Network Inc.
                                                                  Z. Guo
                                                           Alibaba Cloud
                                                           29 April 2024

MP-BGP Extension and the Procedures for IPv4/IPv6 Mapping Advertisement
                draft-ietf-idr-mpbgp-extension-4map6-02

Abstract

   This document defines MP-BGP extension and the procedures for IPv4
   service delivery in multi-domain IPv6-only underlay networks.  It
   defines a new TLV in the BGP Tunnel Encapsulation attribute to be
   used in conjunction with the specific AFI/SAFI for IPv4 and IPv6.
   The behaviors of each type of network (IPv4 and IPv6) are also
   illustrated.  In addition, this document provides the deployment and
   operation considerations when the extension is deployed.

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).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on 31 October 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Reference Topology  . . . . . . . . . . . . . . . . . . . . .   3
   3.  MP-BGP Protocol Extension . . . . . . . . . . . . . . . . . .   5
     3.1.  NLRI Encoding for Mapping Rule Advertisement  . . . . . .   5
     3.2.  4map6 Tunnel TLV  . . . . . . . . . . . . . . . . . . . .   6
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Advertisement of Mapping Rule Update by Egress PE . . . .   8
     4.2.  Receiving Mapping Rule Update by Ingress PE . . . . . . .   9
   5.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Deployment and Operation Considerations . . . . . . . . . . .  10
     6.1.  Scalability Consideration . . . . . . . . . . . . . . . .  10
     6.2.  Route Distribution Control  . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  14
   Appendix B.  IPv6-only DCN for AI-infra fabric  . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   The document [I-D.ietf-v6ops-framework-md-ipv6only-underlay] proposes
   a framework for deploying IPv6-only as the underlay in multi-domain
   networks, in which IPv4 packets will be stateless translated or
   encapsulated into IPv6 ones for transmission across IPv6-only
   underlay domains.  To achieve this goal, this framework introduces a
   specific data structure called IPv4/IPv6 address mapping rule to
   support stateless IPv4-IPv6 packet conversion at the edge of the
   network.  For brevity, in the rest of the document, we will refer to
   the IPv4/IPv6 address mapping rule as mapping rule.  For an incoming
   IPv4 packet, the mapping rules are used by the ingress PE to generate
   corresponding IPv6 source and destination addresses from the IPv4
   source and destination address of the original IPv4 packet, and vice

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   versa.  Since the mapping rule for the destination IPv4 address can
   identify the right PE egress by providing the IPv6 mapping prefix, it
   gives the direction of IPv4 service data transmission throughout the
   IPv6-only network.  It is obvious that the exchange of the mapping
   rule corresponding to the destination IPv4 address in a packet should
   precede to the process of IPv4 data transmission in IPv6-only
   network, otherwise, the data originated from IPv4 network will be
   dropped due to the absence of the IPv6 mapping prefix corresponding
   to its destination address.

   When an ingress PE processes the incoming IPv4 packets, the mapping
   rule for the source address can be obtained locally, but for the
   mapping rule of the destination address, since it is not generated
   locally by the ingress PE, it needs corresponding methods to be
   obtained remotely.  This document defines MP-BGP extension in which
   BGP update message contains the mapping rule for IPv4 service
   delivery.  The extensions include a new TLV for 4map6 tunnel type in
   BGP Encapsulation attribute corresponding to specific AFI/SAFI
   combination and related procedures in IPv6-only networks.

   It should be noted that the approach in this document focuses on
   controlled environment, for instance, one network of single network
   operator or small network of cooperating network operators.  One
   effect of using this approach is that the IPv4 address prefix in it
   will be given one IPv6 mapping prefix and advertised in a IPv6
   mapping route.

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.  Reference Topology

   In the context of this document, multi-domain underlay network refers
   to a network system composed of multiple autonomous systems (i.e.,
   AS) interconnected, each AS can serve different scenarios.  Multi-
   domain network can be operated by one or more network operators.
   Consider the following scenarios, the network shown in figure 1 is a
   typical multi-domain IPv6-only underlay network, it is used as a
   basic scenario to illustrate the extension of the MP-BGP and its
   related procedures in this document.  The whole network comprises of
   AS1, AS2 and AS3, it provides IPv4 services communications between
   IPv4 network N1 and IPv4 network N2, which have IPv4 address block
   IPv4 A1 and A2 respectively.  It is consistent with section 6 of

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   draft [I-D.ietf-v6ops-framework-md-ipv6only-underlay].

     IPv4 A1        +-------+       +-+       +------+        IPv4 A2
  +----------+    /    AS1    \    /AS2\    /    AS3   \    +----------+
  |  IPv4    |   |+--++  +---+ |  |+--+ |  | +--+ +--+  |   |  IPv4    |
  |network N1|---||PE1|--| P1|-|--||P2|-|--|-|P3|-|PE2|-|---|network N2|
  +----------+   |+---+  +---+ |  |+--+ |  |  +--+ +--+ |   +----------+
                  \           /    \   /    \          /
                    +-------+       +-+       +------+
  Figure 1:Topology of Typical Multi-domain IPv6-only Network

   PE and P routers are network devices which constitute the IPv6-only
   underlay.  The definition of PE and P is consistent with that in
   draft [I-D.ietf-v6ops-framework-md-ipv6only-underlay].  It should be
   noted that in multi-domain network, some ASBRs are not at the edge of
   the network.  In this case, they run as P routers.  On each PE router
   that the IPv4 address prefix is reachable through, there is a locally
   configured IPv6 virtual interface (VIF) address.  The VIF address, as
   an ordinary global IPv6 /128 address, must also be injected into the
   IPv6 IGP so that it is reachable across the multi-domain transit
   core.

   The extension of MP-BGP for mapping rule processing and transmission
   across domains in this document will involve PE routers, which setup
   BGP sessions to directly exchange 4map6 routing without affecting the
   RIB and FIB of intermediate P devices.  Each PE router maintains a
   Mapping rule Database (i. e., MD) as depicted in figure 2.  The entry
   in the Mapping rule Database consists of an IPv4 address prefix, IPv4
   address prefix length, IPv6 mapping prefix of the PE, IPv6 mapping
   prefix length, address origin type and forwarding Type of the PE.  It
   should be noted that the database here is just an example, and
   developers can design the structure of database according to the
   actual situation.

   +---------+---------------+--------+-------------+-------+----------+
   | IPv4    | IPv4          | IPv6   |IPv6         |Address|Forwarding|
   | Address | Address       | Mapping|Mapping      |Origin |Type      |
   | Prefix  | Prefix Length | Prefix |Prefix Length|Type   |          |
   +---------+---------------+--------+-------------+-------+----------+
         Figure 2:Entry structure of Mapping Rule Database

   The IPv4 packet sent from IPv4 network N1 will traverse the IPv6-only
   network and reach the destination network, i.e., IPv4 network N2.
   Its source address and destination address are within IPv4 address
   block A1 and A2 respectively.  Its ingress in the IPv6-only network

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   is PE1 and the egress is PE2.  Before the data packet is transmitted,
   the mapping rules corresponding to IPv4 address block A2 should be
   transmitted from PE2 to PE1.

   This mechanism is also in line with the requirements of emerging
   scenarios such as DCN for AI infra fabric, as described in
   Appendix A.

3.  MP-BGP Protocol Extension

3.1.  NLRI Encoding for Mapping Rule Advertisement

   This document specifies a way in which BGP protocol can be used by a
   given PE to tell other PE, "If you need to send IPv4 packet whose
   destination address is within a given IPv4 address block, please send
   them to me, here's the information you need to properly transform the
   IPv4 packets into IPv6 ones".  Multiprotocol BGP (MP-BGP) [RFC4760]
   specifies that the set of usable next-hop address families is
   determined by the Address Family Identifier (AFI) and the Subsequent
   Address Family Identifier (SAFI).  [RFC8950] specifies the extensions
   to allow advertisement of IPv4 NLRI or VPN IPv4 NLRI with a next-hop
   address that belongs to the IPv6 protocol.  This document specifies
   the extensions necessary to support the transmission of mapping rule
   from any egress PE to any ingress PE within and across domains.
   Since it is based on IPv6-only routing paradigm, it leverages the
   combination of AFI and SAFI, with the value of 2 and xxx
   respectively, which identifies NLRI used for IPv4 forwarding in IPv6
   network.  In addition, to support the transmission of additional
   information of the mapping rule, it defines a new TLV for the 4map6
   Tunnel Type in the BGP Tunnel Encapsulation attribute.  With this new
   TLV, Address Origin Type and Forwarding Type can be obtained from BGP
   update by the ingress PE to properly transform the IPv4 packets.  The
   route carried in the BGP update whose MP_REACH_NLRI attribute
   contains the AFI/SAFI combinations and the 4map6 TLV specified above
   is called as IPv6 mapping route.

   The use and meaning of the fields of MP_REACH_NLRI in this case are
   as follows:

           – AFI = 2 (IPv6)

           – SAFI = xxx (4map6)

           – Length of Next Hop

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           – Network Address of Next Hop = When a BGP speaker advertises
           the 4map6 NLRI via BGP, it uses its own address as the BGP
           next hop in the MP_REACH_NLRI.

           – NLRI = Synthetic IPv6 address prefix, which is composed of
           a IPv6 mapping prefix, the original IPv4 address prefix, and
           their length.  Following section 5 of [RFC4760], the
           corresponding NLRI field for IPv4/IPv6 address mapping is
           encoded as shown in figure 3:

                     +---------------------------------------------+
                     |     NLRI Length (1 octet)                   |
                     +---------------------------------------------+
                     |     IPv6 mapping prefix length (1 octet)    |
                     +---------------------------------------------+
                     |     IPv6 mapping prefix (0 ... 16 octets)   |
                     +---------------------------------------------+
                     |     IPv4 address prefix length (1 octet)    |
                     +---------------------------------------------+
                     |     IPv4 address prefix (0 ... 4 octets)     |
                     +---------------------------------------------+
                            Figure 3:Format of NLRI Field

3.2.  4map6 Tunnel TLV

   [RFC9012]defines a BGP path attribute known as the "Tunnel
   Encapsulation attribute", which can be used with BGP UPDATEs of
   various Subsequent Address Family Identifiers (SAFIs) to provide
   information needed to create tunnels and their corresponding
   encapsulation headers.  It provides encodings for a number of tunnel
   types, along with procedures for choosing between alternate tunnels
   and routing packets into tunnels.  BGP path attribute is composed of
   a set of Type-Length-Value (TLV) encodings.  Each TLV contains
   information corresponding to a particular tunnel type.  Since IPv4
   data forwarding in [I-D.ietf-v6ops-framework-md-ipv6only-underlay]
   adopts stateless IPv4/IPv6 address mapping to generate IPv6
   addresses, and supports both encapsulation and translation, a new
   tunnel type named 4map6 is defined in this document.  Following the
   format of Tunnel Encapsulation TLV defined in section 2 of [RFC9012],
   the TLV for the 4map6 Tunnel is shown as follows,

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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Tunnel Type (2 octets)    |        Length (2 octets)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                                 |
     |   Value (Address Origin Type Sub-TLV, Forwarding Type Sub-TLV)  |
     |                                                                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 4:Format of the 4map6 Tunnel TLV

   The code of "Tunnel Type" for the 4map6 Tunnel needs to be applied
   from the IANA registry "BGP Tunnel Encapsulation Attribute Tunnel
   Types" [IANA-BGP-TUNNEL-ENCAP].

   The Value field contains Sub-TLVs.  Per [RFC9012], each Sub-TLV
   consists of three fields: A 1-octet type, a 1-octet or 2-octet length
   (depending on the type), and zero or more octets of value.  At
   present, two sub-TLVs are defined for the 4map6 Tunnel TLV: "Address
   Origin Type" and "Forwarding Type", they are arranged in sequential
   order in the Value field.  These two Sub-TLVs are specified as
   follows:

   a) The Address Origin Type Sub-TLV (Type Code yy1)

   The Address Origin Type Sub-TLV specifies the type of the origin of
   IPv4 address prefix.  The code of "Sub-TLV Type" for the Address
   Origin Type needs to be applied from the IANA registry "BGP Tunnel
   Encapsulation Attribute Sub-TLVs" [IANA-BGP-TUNNEL-ENCAP].  Its Value
   field can assume the following values:

       Value Meaning

       0 The IPv4 address prefix originates from the local configuration
       of the egress PE.

       1 The IPv4 address prefix is obtained by egress PE from external
       IPv4 networks.

   b) The Forwarding Type Sub-TLV (Type Code yy2)

   The Forwarding Type Sub-TLV identifies the IPv4/IPv6 forwarding
   capability of the egress PE.  The code of "Sub-TLV Type" for the
   Forwarding Type needs to be applied from the IANA registry "BGP
   Tunnel Encapsulation Attribute Sub-TLVs" [IANA-BGP-TUNNEL-ENCAP].
   Its Value field it can assume the following values:

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       Value Meaning

       0 Translation and encapsulation

       1 Encapsulation

       2 Translation

   As a new type of Tunnel TLV, 4map6 Tunnel follows the validation
   rules of [RFC9012].  Per section 13 of RFC9012, the final octet of a
   4map6 Tunnel TLV MUST also be the final octet of its final sub-TLV,
   i.e. the Address Origin Type Sub-TLV.  If this is not the case, the
   4map6 TLV MUST be considered to be malformed, and the "Treat-as-
   withdraw" procedure of [RFC7606] of is applied.

   Furthermore, If a Tunnel Encapsulation attribute can be parsed
   correctly but contains a 4map6 Tunnel TLV whose tunnel type is not
   recognized by a particular BGP speaker, that BGP speaker MUST NOT
   consider the attribute to be malformed.  Rather, it MUST interpret
   the attribute as if that 4map6 Tunnel TLV had not been present.  If
   the route carrying the Tunnel Encapsulation attribute is propagated
   with the attribute, the 4map6 Tunnel TLV MUST remain in the
   attribute.

   In addition, ATTR_ SET attribute(type code 128), defined in [RFC
   6368], can be used to transfer the routing information of the IPv4
   network in multi-domain IPv6-only network.

4.  Operation

4.1.  Advertisement of Mapping Rule Update by Egress PE

   When a PE router learns an IPv4 route from the locally attached IPv4
   access networks, the control plane of the PE should process it as
   follows:

   1.  Install and maintain local IPv4 route in the IPv4 routing
   database.

   2.  Generate BGP update advertisement based on the IPv4 route
   advertisement and the capability of the egress PE as follows:

       1) Set the values of AFI and SAFI in MP_REACH_NLRI to 2 and xxx
       respectively.

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       2) Following the NLRI coding method in section 3.1, generate a
       new NLRI using IPv6 mapping prefix length, IPv6 mapping prefix of
       the egress PE, IPv4 address prefix length and IPv4 address
       prefix.

       3) Generate the 4map6 Tunnel TLV based on Forwarding Type of the
       egress PE and Address Origin Type of the IPv4 address prefix.

       4) The Origin ASN, Length of AS_ Path, AS_Path in the original
       IPv4 route advertisement are copied to the corresponding fields
       of ATTR_ SET attribute respectively.

   3.  Send the BGP update advertisement generated to its IPv6 peer
   routers.

4.2.  Receiving Mapping Rule Update by Ingress PE

   When a PE router receives BGP update advertisement from other PE
   routers and uses that information to populate the local Mapping Rule
   Database, the following procedures are used to update the Mapping
   rule Database and send IPv4 routing information to its IPv4 peers.

   1.  Validate the received BGP update advertisement as 4map6 routing
   by AFI = 2 (IPv6) and SAFI = xxx (4map6).

   2.  Extract the IPv6 Mapping Prefix which is encoded in the NLRI
   field and check whether it is reachable in the IPv6 underlay network,
   that is, if a routing entry containing it can be found in the
   underlying IPv6 routing table.  If yes, then proceed to the next
   step.  Otherwise, this indicates that it is unreachable, then do not
   proceed to the next step.

   3.  Extract the IPv4 address prefix which is encoded in the NLRI
   field and lookup in Mapping rule Database, if an entry which matches
   the IPv4 address prefix is found, then,

       1) Update the entry found in the Mapping rule Database with the
       IPv6 mapping prefix, Address Origin Type and Forwarding Type
       extracted from BGP advertisement, then place that as an
       associated entry next to the IPv4 network index.

       2) Redistribute the new IPv4 route to the local IPv4 routing
       table.  Set the destination network prefix as the extracted IPv4
       address prefix, set the Next Hop as Null, and set the OUTPUT
       Interface as the 4map6 VIF on the local PE router.

     else then

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       1) Install and maintain a new entry in the Mapping rule Database
       with the extracted IPv4 address prefix and its corresponding IPv6
       mapping prefix, Forwarding Type, Address Origin Type.

       2) Redistribute the new IPv4 route to the local IPv4 routing
       table.  Set the destination network prefix as the extracted IPv4
       address prefix, set the Next Hop as Null, and set the OUTPUT
       Interface as the 4map6 VIF on the local PE router.

   As mentioned in [I-D.draft-ietf-v6ops-framework-md-ipv6only-
   underlay], multi-domain IPv6-only network supports both translation
   and encapsulation technologies for IPv4 data delivery at the
   forwarding layer.  Take the encapsulation as an example, the
   reachability to the egress endpoint of tunnel may change over time,
   directly impacting the feasibility of the IPv4 service delivery.  A
   tunnel that is not feasible at some moment may become feasible at
   later time when its egress endpoint address is reachable.  The router
   may start using the newly feasible tunnel instead of an existing one.
   This may happen for translation-based data-path as well.  How this
   decision is made is outside the scope of this document.

5.  Error Handling

   When a BGP speaker encounters an error while parsing the
   4map6-related attributes, the speaker must treat the update as a
   withdrawal of existing IPv6 mapping routes, or discard the update if
   no such routes exist.  A log entry should be raised for local
   analysis.

6.  Deployment and Operation Considerations

6.1.  Scalability Consideration

   When operators use the new extension in actual IPv6 networks, it is
   necessary to consider its impact on BGP scalability.  If there is not
   specific policy consideration during deployment, for the same IPv4
   address block, different operators may use different prefixes to map
   it, so multiple synthesized IPv6 prefixes will be generated, which
   can have a significant impact on the scale of RIB and even FIB.
   Therefore, it is recommended that only one IPv6 mapping prefix should
   be configured for each IPv4 address block in principle, and this is
   also true in multi-operator scenario.  In essence, the scalability
   issue is related to the strategy of IPv6 mapping prefix allocation.
   In section 6.3 of [I-D.ietf-v6ops-framework-md-ipv6only-underlay] ,
   two configuration mapping prefix methods are proposed, WKP (i. e.,
   Well-Known Prefix) and NSP (i. e., Network Specific Prefix), which
   can be combined to assist in solving the scale concern.  For
   different types of PE, it is recommended to use the following IPv6

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   mapping prefix configuration policy:

   1) PE for access (Type 1), this kind of PE is configured with IPv4
   address blocks, which are owned by the operator.  Generally, it does
   not have direct interconnection with external IPv4 Internet.  It is
   recommended to assigns NSP as the IPv6 mapping prefix to these
   address blocks and sets the sub-field of Address Origin Type in the
   4map6 tunnel TLV to 0.  Since the NSP is part of the operator's IPv6
   address block, it is usually not necessary to advertise a dedicated
   IPv6 route for each IPv6 mapping prefix, so that no additional
   entries are added to the FIB of the P devices.

   2) Interworking PE (Type 2), this kind of PE interworks directly with
   the external IPv4 Internet.  For the address block in the IPv4 route
   announcement received from the IPv4 Internet, it is proposed to use
   WKP to configure the IPv6 mapping prefix, and set the sub-field of
   Address Origin Type in the 4map6 tunnel TLV to 1.  With this
   configuration, in a IPv6 network with multiple interworking PE,
   regardless of which PE maps and advertisements the IPv6 mapped route,
   the NLRI for the received external IPv4 address block is the same, so
   there will be no significant impact on the scale of IPv6 network
   routing.

   In addition, implementation of 4map6 related settings and policies at
   the network edge can also be useful to ensure scalability.  For
   example, setting a policy on interworking PE to prohibit self-owned
   IPv4 address blocks from backtracking to IPv6 routing.  Interworking
   PEs may receive BGP advertisement of self-owned IPv4 address blocks
   from IPv4 Internet.  If a new IPv6 mapping route is generated using
   the mapping prefix of Interworking PEs and advertised in the IPv6
   network, it will increase the load of RIB of routers and may form a
   routing loop.  Therefore, it is necessary to configure policies on
   the PE to restrict the backtracking of IPv4 address blocks to be
   advertised in the IPv6 network.  The strategy is also effective in
   multi-operator scenario.  Moreover, Restricting the advertisement of
   BGP messages with Address Origin Type value of 1 to other operators
   can also help avoid situations that multiple operators assign
   different mapping prefixes to the same IPv4 address block.

   Nevertheless, in some cases, such as when high reliability is
   required, some IPv4 address blocks need to be configured with
   multiple IPv6 mapping prefixes.

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6.2.  Route Distribution Control

   With the approach in this document, IPv6 mapping routes should only
   be used within the closed multi-domain IPv6 network.  To prevent IPv6
   mapping routes from leaving the IPv6-only network and entering the
   IPv6 Internet, it is recommended to mark them when generating IPv6
   mapping routes at the egress PE and set policies on the receiving PE
   to prevent leakage into the IPv6 Internet.  For operators, the range
   of IPv6 mapping routes distribution can be controlled based on the
   new 4map6 SAFI, to be specific, BGP message with SAFI xxx is
   restricted from being advertised on the IPv6 Internet.  In addition,
   as the IPv6 prefix of NLRI generated through mapping is longer than
   that of regular IPv6 routes, the sum of the lengths of IP mapping
   prefix and IPv4 address prefix is generally greater than 64.  But in
   BGP routing between operators, there is a convention that the prefix
   of NLRI is required to be less than a certain length, such as 48
   bits.  If a route has a long prefix without 4map6 attribute, the
   receiving BGP router can filter it out.  Furthermore, since the NLRI
   of the mapping route is synthesized based on legal IPv4 addresses and
   does not overlap with NLRIs of other native IPv6 routes, even if it
   is leaked to the IPv6 Internet, no traffic hijacking effect will
   occur.

7.  Security Considerations

   In the early stage of deployment, it can be expected that 4map6
   extension is mainly used in small multi-domain IPv6 network with a
   few operator interconnections.  At this stage, BGP collaboration was
   established on the basis of mutual trust between operators.  In case
   of accidents or malfunctions, both parties can resolve them through
   collaborative means.  Under this premise, when one egress PE of the
   other operator sends a 4map6 BGP announcement with Address Origin
   Type value of 0, the Ingress PE can trust the information in it,
   extract the items of the IPv4 address prefix, IPv6 mapping prefix,
   address origin type and forwarding type, then store them in the local
   Mapping rule Database.  However, in the distant future, if the scope
   of 4map6 usage is further expanded, a dedicated authentication
   mechanism will be needed to verify the authenticity of 4map6
   information in BGP advertisements, preventing malicious network
   operators from using their own address prefix to map other operators'
   IPv4 address blocks, thereby turning into network hijacking behavior,
   as stated in section 8.2 of
   [I-D.ietf-v6ops-framework-md-ipv6only-underlay], " The ability to
   advertise a mapping rule adds a new means by which an attacker could
   cause traffic to be diverted from its normal path."  In this case,
   similar techniques as RPKI origin validation or IRR filtering are
   needed to help prevent this.  Applying such technologies to the
   proposed mapping mechanism would mean that BGP prefix policy would

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   need to be able to be applied to the embedded IPv4 networks, this
   security enhancement can be defined in other documents.

8.  IANA Considerations

   With this document IANA is requested to allocate the following codes,

       1) A new SAFI value (xxx) for the BGP 4map6 in "Subsequent
       Address Family Identifiers (SAFI) Parameters" registry.

       2) A code for the 4map6 Tunnel in "BGP Tunnel Encapsulation
       Attribute Tunnel Types" registry.

       3) Two codes for the "Address Origin Type" and "Forwarding Type"
       Sub-TLVs in "BGP Tunnel Encapsulation Attribute Sub-TLVs"
       registry.

   All the codes above use this document as the reference.

9.  Acknowledgement

   The authors would like to thank Jeffrey Haas, Susan Hares, Robert
   Raszuk, Changwang Lin, Yingzhen Qu, Nan Geng, Gyan Mishra, Jingrong
   Xie, Acee Lindem, Tom Petch, Richard Huang, Cheng Li, Jie Dong,
   Eduard Metz, Shunwan Zhang, Linjian Song, Weiqiang Cheng, Paolo
   Volpato, Sheng Jiang, Giuseppe Fioccola, Shuping Peng, Di Ma, Ran
   Pang, Tianran Zhou, Linda Dunbar for their review and comments.

10.  References

10.1.  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>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <https://www.rfc-editor.org/info/rfc5492>.

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   [RFC6368]  Marques, P., Raszuk, R., Patel, K., Kumaki, K., and T.
              Yamagata, "Internal BGP as the Provider/Customer Edge
              Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)",
              RFC 6368, DOI 10.17487/RFC6368, September 2011,
              <https://www.rfc-editor.org/info/rfc6368>.

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/info/rfc7606>.

   [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>.

   [RFC8950]  Litkowski, S., Agrawal, S., Ananthamurthy, K., and K.
              Patel, "Advertising IPv4 Network Layer Reachability
              Information (NLRI) with an IPv6 Next Hop", RFC 8950,
              DOI 10.17487/RFC8950, November 2020,
              <https://www.rfc-editor.org/info/rfc8950>.

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,
              <https://www.rfc-editor.org/info/rfc9012>.

10.2.  Informative References

   [I-D.ietf-v6ops-framework-md-ipv6only-underlay]
              Xie, C., Ma, C., Li, X., Mishra, G. S., Boucadair, M., and
              T. Graf, "Framework of Multi-domain IPv6-only Underlay
              Network and IPv4-as-a-Service", Work in Progress,
              Internet-Draft, draft-ietf-v6ops-framework-md-ipv6only-
              underlay-04, 4 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
              framework-md-ipv6only-underlay-04>.

   [IANA-BGP-TUNNEL-ENCAP]
              IANA, "Border Gateway Protocol (BGP) Tunnel
              Encapsulation", <https://www.iana.org/assignments/bgp-
              tunnel-encapsulation/>.

Appendix A.  Contributors

   The following people have contributed to this document:

   Congxiao Bao
   CERNET Center/Tsinghua University

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   Email: congxiao@cernet.edu.cn

   Linjian Song
   Alibaba Cloud
   Email: linjian.slj@alibaba-inc.com

   Chenhao Ma
   China Telecom
   Email: Machh@chinatelecom.cn

Appendix B.  IPv6-only DCN for AI-infra fabric

   There is enormous "East-West" traffic inside the data center network,
   which are the flows between DC devices and applications.  Upgrading
   the DCN network firstly to dual-stack, then IPv6-only is nontrivial.
   One example is building AI-infra fabric on IPv6 only fabric which
   reduce data plane encapsulation overhead, simplify forwarding chip's
   feature and improve data plane performance.

   When DCN plans to transits from dual stack to IPv6-only, it is
   impossible to be done overnight.  Considerations and plans should be
   made supporting legacy IPv4 servers and applications when the DCN is
   IPv6-only.  The IPv6-only framework proposed in this memo provide
   availability for IPv4 service when the underlay Networks upgraded to
   IPv6-only.

   As shown in Figure 6, Host 1 and Host 2 are legacy servers with only
   IPv4 capability.  Traffic between Host 1 and Host 2 are carried by
   IPv6 network in the DCN.  The access switch(ASW) have the function of
   ADPT which learns IPv4/IPv6 mapping rules and delivers the IPv4
   service in IPv6-only network.

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                            Internet
                                ^
                                |
         ^     +----------------+------------------+
         |     |         Data Center Network       |
         |     +----+-------------------------+----+
         |          |                         |
         |     +----+-------------------------+----+
         |     |                                   |
   IPv6-only   |             PSW/R1                |AS2
         |     +----+--------------------------+---+
         |          |                          |
         |          |                          |
         v     +----+---+                 +----+---+
       ------- |        |                 |        |
         ^     |ASW/PE1 |AS1              |ASW/PE2 |AS1
         |     +----+---+                 +----+---+\
   dualstack        |                          |     \
         |        +-+-+                      +-+-+   +---+
         v        | H1|IPv4              IPv4| H2|   | H3| IPv6
                  +---+                      +---+   +---+

       Figure 6:IPv6-only DCN for AI infra fabric

Authors' Addresses

   Chongfeng Xie
   China Telecom
   Beiqijia Town, Changping District
   Beijing
   102209
   China
   Email: xiechf@chinatelecom.cn

   Guozhen Dong
   China Telecom
   Beiqijia Town, Changping District
   Beijing
   102209
   China
   Email: donggz@chinatelecom.cn

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   Xing Li
   CERNET Center/Tsinghua University
   Shuangqing Road No.30, Haidian District
   Beijing
   100084
   China
   Email: xing@cernet.edu.cn

   Guoliang Han
   Indirection Network Inc.
   Email: guoliang.han@indirectionnet.com

   Zhongfeng Guo
   Alibaba Cloud
   Wangjing Qiyang Rd, Chaoyang District
   Beijing
   100102
   China
   Email: guozhongfeng.gzf@alibaba-inc.com

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