MP-BGP Extension and the Procedures for IPv4/IPv6 Mapping Advertisement
draft-xie-idr-mpbgp-extension-4map6-01

Network Working Group                                             C. Xie
Internet-Draft                                                   G. Dong
Intended status: Standards Track                           China Telecom
Expires: 12 October 2023                                           X. Li
                                                                  C. Bao
                                       CERNET Center/Tsinghua University
                                                                  G. Han
                                                Indirection Network Inc.
                                                           10 April 2023

MP-BGP Extension and the Procedures for IPv4/IPv6 Mapping Advertisement
                 draft-xie-idr-mpbgp-extension-4map6-01

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 BGP path attribute known as the "4map6" in conjunction
   with the existing AFI/SAFI in IPv6-only routing paradigm for
   transferring IPv4/IPv6 address mapping rule within and across
   IPv6-only domains.  In addition, the behavior of each type of network
   node in this extension is also illustrated.

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 12 October 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Xie, et al.              Expires 12 October 2023                [Page 1]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

   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.  Terminology and Reference Topology  . . . . . . . . . . . . .   3
   3.  MP-BGP Protocol Extension . . . . . . . . . . . . . . . . . .   5
     3.1.  NLRI Encoding for Mapping Rule Advertisement  . . . . . .   5
     3.2.  4map6 BGP Path Attribute  . . . . . . . . . . . . . . . .   6
     3.3.  Explicit Withdrawal of IPv4/IPv6 Mapping Rule . . . . . .   7
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Advertisement of Mapping Rule Update by egress PE . . . .   8
     4.2.  Receiving Mapping Rule advertisement by P router  . . . .   9
     4.3.  Receiving Mapping Rule Update by Ingress PE . . . . . . .  10
   5.  Mapping Rule Capability . . . . . . . . . . . . . . . . . . .  11
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The document [I-D./draft-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.  For brevity,
   in the rest of the document, we will refer to the IPv4/IPv6 address
   mapping rule as mapping rule.  For an incoming 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 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

Xie, et al.              Expires 12 October 2023                [Page 2]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

   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 new BGP Path Attribute known as the
   "4map6" corresponding to the NLRI and a set of related procedures.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14[RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Terminology and Reference Topology

   In the context of this document, multi-domain underlay networks refer
   to a network system composed of multiple autonomous systems (i.e.,
   AS) interconnected, each AS can serve different scenarios.  Multi-
   domain networks can be operated by one or more network operators.
   Consider the following scenarios, the network shown in figure 1 is
   typical multi-domain IPv6-only underlay networks, it is used as a
   basic scenario to illustrate the extension of the MP-BGP and its
   related procedures in this document.  The network comprises of AS1,
   AS2 and AS3, it provides IPv4 services communications between IPv4
   network 1 and IPv4 network 2, which have IPv4 address block IPv4 A1
   and A2 respectively.  It is consistent with section 6 of draft [I-
   D.ietf-v6ops-framework-md-ipv6only-underlay].

 IPv4 A1          +--------+       +-+      +--------+        IPv4 A2
  +--------+     /    AS1    \    /AS2\    /   AS3     \     +--------+
 |   IPv4   |   |+--++  +--+| |  |+--+ |  | +--+  +-+-+ |   |  IPv4    |
 | network 1|---||PE1|--|P1 |-|--||P2|-|--|-|P3|--|PE2|-|---| network 2|
  +--------+    |+---+  +--+| |  |+--+ |  | +--+  +---+ |    +--------+
                 \___________/    \___/    \___________/
  Figure 1.Topology of Typical Multi-domain IPv6-only Networks

Xie, et al.              Expires 12 October 2023                [Page 3]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

   PE and P routers are network routers 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 networks, 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 following term will be used in this document,

   • Distance metric, the distance to the egress PE in terms of the
   number of ASes.

   The extension of MP-BGP4 for mapping rule processing and transmission
   across domains in this draft will involve PE and P routers.  Each PE
   and P router maintains a Mapping rule Database (MD) as depicted in
   figure 2.  The entry in the MD database consists of an IPv4 address
   prefix, IPv4 address prefix length, IPv6 mapping prefix, IPv6 mapping
   prefix length and the distance to the egress.  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          | Distance   |
   |  Address |  Address       |  Mapping | Mapping       | to the     |
   |  Prefix  |  Prefix Length |  Prefix  | Prefix Length | Egress     |
   +----------+----------------+----------+---------------+------------+
        Figure 2: Entry of Mapping Rule Database

   The IPv4 packet sent from IPv4 network 1 will traverse the IPv6-only
   network and reach the destination network, i.e., IPv4 network 2.  Its
   ingress in the IPv6-only network is PE1 and the egress is PE2.
   Before the data packet is transmitted, the address mapping rules
   corresponding to its IPv4 destination address should be transmitted
   from PE2 to PE1.  During the mapping rule announcement and
   transmission process, it may pass through the intermediate nodes,
   such as P3, P2 and P1, and finally reach PE1.  For a given
   intermediate P node, it may receive advertisement messages of this
   mapping rule from multiple upstream intermediate nodes.  In order to
   reduce the overall quantity of advertisement message, it needs to
   select and update the local MD database, generates advertisement
   messages based on the selected mapping rule information and transmit
   them to downstream intermediate nodes.

Xie, et al.              Expires 12 October 2023                [Page 4]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

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 packet into IPv6 one".  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 1 respectively,
   which identifies Network Layer Reachability Information (NLRI) used
   for unicast forwarding in IPv6 network.  In addition, in order to
   identify that this BGP update message is used for the transmission of
   the mapping rule, it needs to contain a newly defined BGP path
   attribute type -- the 4map6 attribute.  This attribute specifies the
   IPv6 mapping prefix that may be used, as well as whatever additional
   information (if any) is needed in order to properly transform the
   IPv4 packets.  The BGP update whose MP_REACH_NLRI attribute contains
   the AFI/SAFI combinations and 4map6 BGP path attribute specified
   above is called as 4map6 routing information.  The use and meaning of
   the fields of MP_REACH_NLRI in this case are as follows:

           – AFI = 2 (IPv6)

           – SAFI = 1 (Unicast)

           – Length of Next Hop

           – 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 = Composite IPv6 address prefix, which is composed of
           a IPv6 mapping prefix, the original IPv4 address prefix, and
           the remaining bits are zero.

   The NLRI field is encoded as shown in figure 3:

Xie, et al.              Expires 12 October 2023                [Page 5]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

                     +----------------------------+
                     |       Length    1 octet    |
                     +----------------------------+
                     |       Prefix    variable   |
                     +----------------------------+
                    Figure 3: Format of NLRI Field

3.2.  4map6 BGP Path Attribute

   As a new BGP path attribute defined in this document, 4map6 attribute
   is optional and transitive, it requires IANA to assign a new BGP path
   attribute value.  The attribute is composed of a set of fields as
   below,

              +---------------------------------------------------+
              |     Length of IPv6 Mapping Prefix(1 octet)       |
              +---------------------------------------------------+
              |     Forwarding Type(1 octet)                     |
              +---------------------------------------------------+
              |     Address Origin Type(1 octet)                 |
              +---------------------------------------------------+
              |     IPv4 Original ASN (4 octets)                 |
              +---------------------------------------------------+

                    Figure 4:Encoding of the 4map6 attribute

   The use and meaning of these fields are as follows:

   a) Length of IPv6 Mapping Prefix

   This is a 1-octet field whose value expresses the length of IPv6
   mapping prefix.

   b) Forwarding Type

   This field identifies the IPv4/IPv6 forwarding capability of the
   egress PE, the data octet can assume the following values:

       Value Meaning

       0 Translation and encapsulation

       1 Encapsulation

       2 Translation

   c) Address Origin Type

Xie, et al.              Expires 12 October 2023                [Page 6]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

   The data octet can assume the following value:

       Value Meaning

       0 Local

       1 Relay

   d) IPv4 Original ASN

   This field is the copy of the Origin AS number in BGP update message
   received from IPv4 domain.The value of this field exists only when
   the value of "Address Origin Type" is 1, otherwise it is NULL.

   In addition, when the value of IPv4 Original ASN is set, 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 networks.

3.3.  Explicit Withdrawal of IPv4/IPv6 Mapping Rule

   When a PE ceases to provide egress service for a given IPv4 address
   block, it may explicitly withdraw the mapping rules associated with
   it.  Suppose a PE has announced, on a given BGP session, the mapping
   rule of a given IPv4 prefix and it now wishes to withdraw that
   mapping rule.  To do so, it may send a BGP UPDATE message with an
   MP_UNREACH_NLRI attribute.

   This encoding of MP_UNREACH_NLRI attribute is used for explicitly
   withdrawing the mapping rule for a given IPv4 prefix (on a given BGP
   session).  Note that IPv4 address prefix/IPv6 mapping prefix bindings
   that were not advertised on the given session can not be withdrawn by
   this method.

   When using an MP_UNREACH_NLRI attribute to withdraw a IPv4 route
   whose NLRI was previously specified in an MP_REACH_NLRI attribute,
   the lengths and values of the respective prefixes must match, and the
   respective AFI/SAFIs must match.  An explicit withdrawal in an AFI/
   SAFI UPDATE on a given BGP session not only withdraws the binding
   between the IPv4 address prefix and the IPv6 mapping prefix, it also
   withdraws the path to that prefix that was previously advertised in
   an UPDATE on that session.

4.  Operation

Xie, et al.              Expires 12 October 2023                [Page 7]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

4.1.  Advertisement of Mapping Rule Update by egress PE

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

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

   2.  Install and maintain new entries in the MD database.  Each entry
   should consist of the IPv4 prefix and the local IPv6 mapping prefix.

   3.  Advertise the new contents of the local MD database in the form
   of BGP update advertisement to IPv6 peer routers.  The process to
   generate IPv6 route advertisement with 4map6 attribute based on IPv4
   route advertisement messages is as follows:

       a) Set the values of AFI and SAFI in MP_REACH_NLRI to 2 and 1
       respectively;

       b) The IPv6 mapping prefix of the egress PE splices IPv4 address
       blocks in IPv4 routing advertisements to form a composite IPv6
       address prefix with the length value denoted by L1.  The
       composite IPv6 address prefix is copied to address prefix field
       of the NLRI structure in the MP_ REACH_NLRI, and the length field
       of the NLRI is set to L1, the structure of the composite IPv6
       address prefix in NLRI is shown in figure 5.  L2 is used to
       denote the length of the IPv6 mapping prefix of PE2, i.e.
       Pref6-2.  When the value of L2 is available, the field of Length
       of IPv6 Mapping Prefix in the 4map6 attribute is set to L2.

       c) The values of Origin ASN in the original IPv4 route
       advertisement is copied to the fields of IPv4 Original ASN of
       4map6 attribute, the values of Length of AS_ Path, AS_Path are
       copied to the corresponding fields of ATTR_ SET attribute
       respectively.

            |--------L2--------|
            +------------------+------------------+-------------+
            |  IPv6 Mapping    |   IPv4           |  ...0000... |
            |  Prefix of PE2   |   address prefix |             |
            +------------------+------------------+-------------+
            |-----------------L1------------------|
                 Figure 5:Structure of IPv6 prefix in NLRI

Xie, et al.              Expires 12 October 2023                [Page 8]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

4.2.  Receiving Mapping Rule advertisement by P router

   When a P router receives BGP update advertisement from neighboring P
   or PE routers and uses that information to populate the local MD
   database, the following procedures are used to update the MD database
   and send mapping rule advertisement to next equipment:

   1.  Validate the received BGP update advertisement as 4map6 routing
   information by finding the 4map6 attribute.

   2.  Extract the IPv4 address prefix which is encoded in positions L2
   to L1-1 of the NLRI field and lookup its local MD database, if an
   entry which matches the IPv4 address prefix is found, then,

           – Compare the distance metric in the 4map6 attribute of BGP
           advertisement and that of the entry found, if the former is
           less than the latter, then

               • Update the entry found in the MD database with the
               attributes of BGP advertisement by extracting the IPv6
               address prefix from the IPv6 mapping prefix field and
               place that as an associated entry next to the IPv4
               network index.

               • Advertise the updated contents of the local MD database
               in the form of MP_REACH_NLRI update information to IPv6
               peer routers.

           else then

               • Keep the entry in the MD database unchanged.

               • Advertise the contents of the local MD database in the
               form of BGP update advertisement to IPv6 peer routers.

       else then

           – Install and maintain a new entry in the MD database with
           the extracted IPv4 prefix, its corresponding IPv6 mapping
           prefix and distance metric to the egress.

           – Advertise the contents of the local MD database in the form
           of BGP update advertisement to IPv6 peer routers.

   It should be noted that this process does not change or affect the
   IPv6 FIB table of the P router.

Xie, et al.              Expires 12 October 2023                [Page 9]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

4.3.  Receiving Mapping Rule Update by Ingress PE

   When a PE router receives BGP advertisement from neighboring P or PE
   routers and uses that information to populate the local MD database
   and the BGP routing database, the following procedures are used to
   update the MD database and send IPv4 routing information to its IPv4
   peers.

   1.  Validate the received BGP update advertisement as 4map6 routing
   information by finding the 4map6 attribute.

   2.  Extract the IPv6 Mapping Prefix which is encoded in positions 0
   to L2-1 of the NLRI field and compare the obtained IPv6 Mapping
   Prefix with its own IPv6 Mapping Prefix, and if the two match,
   proceed to the next step.  Otherwise, this update will be announced
   to its other BGP Peers.

   3.  Extract the IPv4 address prefix which is encoded in positions L2
   to L1-1 of the NLRI field and lookup in the MD database, if an entry
   which matches the IPv4 address prefix is found, then,

           – Compare the distance metric in the BGP advertisement and
           that of the entry found, if the former is less than the
           latter, then

               • Update the entry found in the MD database with the
               4map6 attributes of BGP advertisement by extracting the
               IPv6 address prefix from the IPv6 mapping prefix field
               and place that as an associated entry next to the IPv4
               network index

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

           else then

               • Keep the entry in the MD unchanged.

       else then

           – Install and maintain a new entry in the MD database with
           the extracted IPv4 prefix, its corresponding IPv6 mapping
           prefix and distance metric to the egress.

Xie, et al.              Expires 12 October 2023               [Page 10]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

           – Redistribute the new 4map6 routing information to the local
           IPv4 routing table.  Set the destination network prefix as
           the extracted IPv4 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 networks support both translation
   and encapsulation technologies for IPv4 data delivery at the data
   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.  Mapping Rule Capability

   [RFC5492]defines a Capabilities Optional Parameter and processing
   rules.  The Capabilities Optional Parameter is a triple that includes
   a one-octet Capability Code, a one-octet Capability length, and a
   variable-length Capability Value.  A BGP speaker can include a
   Capabilities Optional Parameter to communicate capabilities in a BGP
   OPEN message.  A PE or P router that wishes to exchange mapping rule
   information must use the Multiprotocol Extensions Capability Code as
   defined in [RFC4760], to advertise the corresponding (AFI, SAFI)
   pair.

6.  Error Handling

   When a BGP speaker encounters an error while parsing the 4map6 path
   attribute, the speaker must treat the update as a withdrawal of
   existing routes to the included 4map6 SAFI NLRIs, or discard the
   update if no such routes exist.  A log entry should be raised for
   local analysis.

7.  IANA Considerations

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

   1)A code for 4map6 path attribute in the BGP “BGP Path Attributes”
   registry

   2)Value xx for 4map6 in the BGP "Capability Codes" registry

   All the codes above use this document as the reference.

Xie, et al.              Expires 12 October 2023               [Page 11]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

8.  Security Considerations

   This extension to MP-BGP does not change the underlying security
   issues inherent in the existing MP-BGP.

9.  References

9.1.  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>.

   [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>.

   [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>.

9.2.  Informative References

Xie, et al.              Expires 12 October 2023               [Page 12]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

   [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
              Networks and IPv4-as-a-Service", Work in Progress,
              Internet-Draft, draft-ietf-v6ops-framework-md-ipv6only-
              underlay-01, 3 February 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
              framework-md-ipv6only-underlay-01>.

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

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

   Congxiao Bao
   CERNET Center/Tsinghua University
   Shuangqing Road No.30, Haidian District
   Beijing
   100084
   China
   Email: congxiao@cernet.edu.cn

Xie, et al.              Expires 12 October 2023               [Page 13]
Internet-Draft  MP-BGP Extension for 4map6 Advertisement      April 2023

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

Xie, et al.              Expires 12 October 2023               [Page 14]