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Versions: (draft-mdt-softwire-map-deployment) 00 01 02 03 04 05 06

Network Working Group                                             Q. Sun
Internet-Draft                                             China Telecom
Intended status: Informational                                   M. Chen
Expires: December 11, 2015                                          BBIX
                                                                 G. Chen
                                                            China Mobile
                                                                 T. Tsou
                                                     Huawei Technologies
                                                            S. Perreault
                                                     Jive Communications
                                                            June 9, 2015


     Mapping of Address and Port (MAP) - Deployment Considerations
                 draft-ietf-softwire-map-deployment-06

Abstract

   This document describes when and how an operator uses the technique
   of Mapping of Address and Port (MAP) for the IPv4 residual deployment
   in the IPv6-dominant domain.

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 http://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 December 11, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Case Studies . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Deployment Consideration . . . . . . . . . . . . . . . . . . .  7
     4.1.  Building the MAP Domain  . . . . . . . . . . . . . . . . .  7
       4.1.1.  MAP Deployment Model Planning  . . . . . . . . . . . .  7
       4.1.2.  MAP Domain Planning  . . . . . . . . . . . . . . . . .  8
       4.1.3.  MAP Rule Provisioning  . . . . . . . . . . . . . . . .  8
       4.1.4.  MAP DHCPv6 server deployment consideration . . . . . .  9
       4.1.5.  PSID Consideration . . . . . . . . . . . . . . . . . . 10
       4.1.6.  Addressing and Routing . . . . . . . . . . . . . . . . 10
       4.1.7.  MAP vs. MAP-T vs. 4rd  . . . . . . . . . . . . . . . . 11
     4.2.  BR Settings  . . . . . . . . . . . . . . . . . . . . . . . 12
     4.3.  CE Settings  . . . . . . . . . . . . . . . . . . . . . . . 15
     4.4.  Supporting System  . . . . . . . . . . . . . . . . . . . . 15
   5.  MAP Address Planning . . . . . . . . . . . . . . . . . . . . . 17
     5.1.  Planning for Residual Deployment, a Step-by-step Guide . . 17
     5.2.  Remarks on Deployment Paradigms  . . . . . . . . . . . . . 19
   6.  Migration Methodology  . . . . . . . . . . . . . . . . . . . . 21
     6.1.  Roadmap for MAP-based Solution . . . . . . . . . . . . . . 21
       6.1.1.  Start from Scratch . . . . . . . . . . . . . . . . . . 21
       6.1.2.  Coexiting Phases . . . . . . . . . . . . . . . . . . . 21
       6.1.3.  Exit Strategy  . . . . . . . . . . . . . . . . . . . . 21
     6.2.  Migration Mode . . . . . . . . . . . . . . . . . . . . . . 22
       6.2.1.  Passive Transition . . . . . . . . . . . . . . . . . . 22
       6.2.2.  Active Transition  . . . . . . . . . . . . . . . . . . 22
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 25
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 27
     11.2. Informative References . . . . . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29








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1.  Introduction

   IPv4 address exhaustion has become world-wide reality and the primary
   solution in the industry is to deploy IPv6-only networking.
   Meanwhile, having access to legacy IPv4 contents and services is a
   long-term requirement, will be so until the completion of the IPv6
   transition.  It demands sharing residual IPv4 address pools for IPv4
   communications across the IPv6-only domain(s).

   Mapping of Address and Port (MAP) [I-D.ietf-softwire-map] is designed
   in response to the requirement of stateless residual deployment.  The
   term "residual deployment" refers to utilizing IPv4 addresses for
   IPv4 communications going across the IPv6 domain backbone.  MAP
   assumes the IPv6-only backbone as the prerequisite of deployment so
   that native IPv6 services and applications are fully supported and
   encouraged.  The statelessness of MAP ensures only moderate overhead
   is added to part of the network devices.

   Residual deployment with MAP is new to most operators.  This document
   is motivated to provide basic understanding on the usage of MAP,
   i.e., when and how an operator can do with MAP to meet its own
   operational requirements of IPv6 transition and its facility
   conditions, in the phase of IPv4 residual deployment.  Potential
   readers of this document are those who want to know:

   1.  What are the requirements of MAP deployment ?

   2.  What technical options needs to be considered when deploying MAP,
       and how?

   3.  How does MAP impact on the address planning for both IPv6 and
       IPv4 pools?

   4.  How does MAP impact on daily network operations and
       administrations?

   5.  How do we migrate to IPv6-only network with the help of MAP?

   Terminology of this document, unless it is intentionally specified,
   follows the definitions and abbreviations of [I-D.ietf-softwire-map].

   Unless it is specifically specified, the deployment considerations
   and guidance proposed in this document are also applied to MAP-T
   [I-D.ietf-softwire-map-t], the translation variation of MAP, and 4rd
   [I-D.ietf-softwire-4rd], the reversible translation approach that
   aims to improve end-to-end consistency of double translation.





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2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].














































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3.  Case Studies

   MAP can be deployed for large-scale carrier networks.  There are
   typically two network models for broadband access service: one is to
   use PPPoE/PPPoA authentication method while the other is to use IPoE.
   The first one is usually applied to Residential network and SOHO
   networks.  Subscribers in CPNs can access broadband network by PPP
   dial-up authentication.  BRAS is the key network element which takes
   full responsibility of IP address assignment, user authentication,
   traffic aggregation, PPP session termination, etc.  Then IP traffic
   is forwarded to Core Routers through Metro Area Network, and finally
   transited to Internet via Backbone network.  The second network
   scenario is usually applied to large enterprise networks.
   Subscribers in CPNs can access broadband network by IPoE
   authentication.  IP address is normally assigned by DHCP server, or
   static configuration.

   In either case, a Customer Edge Router(CER) could obtain a prefix via
   prefix delegation procedure, and the hosts behind CER would get its
   own IPv6 addresses within the prefix through SLAAC or DHCPv6
   statefully.  A MAP CE would also obtain a set of MAP rules from
   DHCPv6 server.

   Figure 1 depicts a generic model of stateless IPv4-over-IPv6
   communication for broadband access services.

                  +------------------------------+
                  |        MAP Domain            |
              +---+---------------+--------------|
  +--------+  +                   |              |
  |        |  +---------+      +--+--+           |
  |  Host  |--|   CER   |      |     |           |
  |        |  |(MAP CE) |======| BNG | ======+---------+   +-----------+
  +--------+  +---------+      +--|--+       |  Core   |   |   IPv4    |
  +--------+      +---------------+          | Router  |---| Internet  |
  |        |  +---|-----+      +--+--+       |(MAP BR) |   |           |
  |  Host  |--|   CER   |======|     | ======+---------+   +-----------+
  |        |  |(MAP CE) |      | BNG |           |
  +--------+  +---------+      +--+--+           |
              +                   |              |
              +-------------------+--------------+

       Figure 1: Stateless IPv4-over-IPv6 broadband access  network
                               architecture

   When deploying MAP in home network, there can be two architecture: A.
   single ISP B. multihoming with two or more ISPs, sharing one CE.  In
   the single ISP model, CE needs to communicate with only one MAP BR,



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   while in multihoming model CE has to communicate with multiple MAP
   BRs.  Figure 2 [RFC7368] illustrates a typical case, where the home
   network has multiple connections to multiple providers or multiple
   logical connections to the same provider.  In the multihoming model,
   a CE will be provisioned with multiple BMRs.  Routing information
   will also be configured for multihoming; but detail of the routing
   configuration is out of the scope of this memo.


           +-------+-------+     +-------+-------+         \
           |   Service     |     |   Service     |          \
           |  Provider A   |     |  Provider B   |           | Service
           |  Core Router  |     |  Core Router  |           | Provider
           |  ( MAP BR)    |     |  (MAP BR)     |           | network
           +-------+-------+     +-------+-------+           |
                    |                 |                     /
                    |    Customer     |                   /
                    |    Internet     |                  /
                    |   connections   |                 |
                   +---------+---------+                 \
                   |       IPv6        |                   \
                   |   Customer Edge   |                    \
                   |      Router       |                    /
                   |    ( MAP CE)      |                   /
                   +---------+---------+                  /
                             |                           /
                             |                           | End-User
     ---+------------+-------+--------+-------------+--- | network(s)
        |            |                |             |     \
   +----+-----+ +----+-----+     +----+-----+ +-----+----+ \
   |IPv6 Host | |IPv6 Host |     | IPv6 Host| |IPv6 Host | /
   |          | |          |     |          | |          |/
   +----------+ +----------+     +----------+ +----------+

                         Figure 2: MAP multihoming
















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4.  Deployment Consideration

4.1.  Building the MAP Domain

   When deploying stateless MAP in an operational network, a provider
   should firstly do MAP domain planning based on that existing network.
   According to the definition of [I-D.ietf-softwire-map], a MAP domain
   is a set of MAP CEs and BRs connected to the same virtual link.  All
   CEs in the MAP domain are provisioned with a same set of MAP rules by
   MAP DHCPv6 server [I-D.ietf-softwire-map-dhcp].  There might be
   multiple BMRs in one MAP domain, e.g. in case of multi-ISP.  A CE may
   be provisioned with multiple IPv6 prefix, which can be used to find
   the corresponding BMR via longest prefix match.  As defined in
   [I-D.ietf-softwire-map-dhcp], a BMR should be provisioned together
   with a BR IPv6 address; the CE should maintain this binding, so that
   the mapping between BMR and BR is achieved which is useful in multi-
   ISP scenario.  In in mesh mode, a longest-matching prefix lookup is
   done in the IPv4 routing table and the correct FMR is chosen.

   Basically, operator should firstly determine its own deployment
   topology for MAP domain as described in Section 4.1.1, as different
   considerations apply for different deployment models.  Next, MAP
   domain planning, MAP rule provision, addressing and routing, etc.,
   for a MAP domain should be taken into consideration, as discussed in
   the sections following Section 4.1.1.

   For the scenario where one CE is corresponding with multiple MAP
   border relays, it is possible that those MAP BRs belong to different
   MAP domains.  The CE must pick up its own MAP rules and domain
   parameters in each domain.  This is a typical case of multihoming.
   The MAP rules must have the information about BR(s) and information
   about the service types and the ISP.

4.1.1.  MAP Deployment Model Planning

   In order to do MAP domain planning, an operator should firstly make
   the decision to choose mesh or hub and spoke topology according to
   the operator's network policy.  In the hub and spoke topology, all
   traffic within the same MAP domain has to go through the BR, result
   in less optimal traffic flow; however, it simplifies CE processing
   since there is no need to do FMR lookup for each incoming packet.
   Moreover, it provides enhanced manageability as the BR can take full
   control of all the traffic.  As a result, it is reasonable to deploy
   hub and spoke topology for a network with a relatively flat
   architecture.

   In mesh topology, CE to CE traffic flows are optimized since they
   pass directly between the two nodes.  Mesh topology is recommended



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   when CE to CE traffic is high and there are not too many MAP rules,
   say fewer than 10 MAP rules, in the given domain.

4.1.2.  MAP Domain Planning

   Stateless MAP offers advantages in terms of scalability, high
   reliability, etc.  As a result, it is reasonable to plan for a larger
   MAP domain to accommodate more subscribers with fewer BRs.  Moreover,
   a larger MAP domain will also be easier for management and
   maintenance.  However, a larger MAP domain may also result in less
   optimized traffic in the hub and spoke case, where all traffic has to
   go through a remote BR.  In addition, it may result in an increased
   number of MAP rules and highly centralized address management.
   Choosing appropriate domain coverage requires the evaluation of
   tradeoffs.

   When multiple IPv4 subnets are deployed in one MAP domain, it is
   recommended to further divide the MAP domain into mutiple sub-
   domains, each with only one IPv4 subnet.  This can simplify the MAP
   domain planning.  But there can be a side effect that it will
   increase the traffic between BRs.  Different subdomains could be
   distinguished by different Rule IPv4 prefixes.  As stated previously,
   all CEs within the same MAP subdomain will have the same Rule IPv4
   prefix, Rule IPv6 prefix and PSID parameters.

4.1.3.  MAP Rule Provisioning

   In stateless MAP, Mesh or Hub and Spoke communications can be
   achieved among CEs in one MAP domain in terms of assigning
   appropriate FMR(s) to CEs.  We recommend ISP deploy the full Hub and
   Spoke topology or full mesh topology describe below to simplify the
   configuration of the DHCPv6 server.

4.1.3.1.  Full Hub and Spoke Communication among CEs

   In order to achieve the full communication in the Hub and Spoke
   topology, no FMR is assigned to CEs.  In this topology, when a CE
   sends packets to another CE in the same MAP domain via BR, or using
   the DMR as FMR, the packets must go though BR before arriving at the
   destination.  DMR is specific for MAP-T only.

4.1.3.2.  Full Mesh Communication among CEs

   By assigning all BMRs in MAP domain to each CE as FMRs, Mesh
   communications can be achieved among all CEs.  In this case, when CE
   receives an IPv4 packet, it looks up for an appropriate FMR with a
   specific Rule IPv4 prefix which has the longest match with the IPv4
   destination address.



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4.1.3.3.  Mesh or Hub/Spoke communication among some CEs

   Mesh communications among some CEs along with Hub/Spoke
   communications among some other CEs can be achieved by which
   differentiated FMRs are assigned to CEs.  For instance, as shown in
   Figure 3, since both CE1 and CE2 has rule 1 and rule2, the
   communication between CE1 and CE2 can go directly without going
   though associated BR (Mesh topology).  However, for CE1 and CE3,
   since there are no rule for each other, the communication between CE1
   and CE3 must go though BR before reaching peer each other (Hub/Spoke
   topology).


              +---------------+---------+---------+---------+
              |               |   CE1   |   CE2   |   CE3   |
              +---------------+---------+---------+---------+
              |      BMR      | rule 1  | rule 2  | rule 3  |
              +---------------+---------+---------+---------+
              |               | rule 1  | rule 1  | rule 2  |
              |     FMRs      | rule 2  | rule 2  | rule 3  |
              |               |         | rule 3  |         |
              +---------------+---------+---------+---------+

                                 Figure 3:

4.1.4.  MAP DHCPv6 server deployment consideration

   All the CEs within a MAP domain will get a set of MAP rules by DHCPv6
   server.  Each Mapping Rule keeps a record of Rule IPv6 prefix, Rule
   IPv4 prefix and Rule EA-bits length.  Section 5 would give a step by
   step example of how to calculate these parameters.

   As the MAP is stateless, the deployment of DHCPv6 server is
   independent of MAP domain planning.  So there are three possible
   cases:

   MAP domain : DHCPv6 server = 1:1  This is the ideal solution that
         each MAP domain would have its own MAP DHCPv6 server.  In this
         case, MAP DHCPv6 server only needs to configure parameters for
         the specific MAP domain.  In this model, it is easy to achieve
         the configuration in MAP and no extra configuration requirement
         is needed.

   MAP domain : DHCPv6 server = 1:N  This might happen when DHCPv6
         servers are deployed in a large MAP domain in a distributed
         manner.  In this case, all these DHCPv6 servers should be
         configured with the same set of MAP rules for the MAP domain,
         including multiple BMRs, FMRs and DMRs.



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   MAP domain : DHCPv6 server = N:1  This might happen when MAP domain
         is relatively small and a single MAP DHCPv6 server is deployed
         in the network.  In this case, multiple MAP domains should be
         distinguished based on CE's IPv6 prefix in different MAP
         domains.

4.1.5.  PSID Consideration

   If a provider would like to introduce differentiated address sharing
   ratios for different CEs, it is better to define multiple MAP sub-
   domains with different Rule IPv4 prefixes.  In this way, MAP domain
   division is only a logical method, rather than a geographical one.

   The default PSID offset(a) is chosen as 6 in [I-D.ietf-softwire-map]
   and this excludes the system ports (0-1023).  For MAP, the initial
   part of the port number (the a-bits) cannot be zero (see Appendix B
   of [I-D.ietf-softwire-map].)  As is shown in the section 3.2.4 of
   [I-D.tsou-softwire-port-set-algorithms-analysis], it is possible that
   a lower value of 'a' will give a higher sharing ratio and more than
   1024 ports are excluded as a result, e.g. 'a' = 4 will exclude ports
   0 - 4095.  The value of 'a' should be made explicitly configurable by
   operators.

   With regard to PSID format, both continuous and non-continuous port
   set can be supported in GMA algorithm.  Non-continuous port set has
   the advantage of better UPnP friendly, while continuous port set is
   the simplest way to implement.  Since PSID format should be supported
   not only in CPEs, BRs and DHCPv6 server, but also in other sustaining
   systems as well, e.g. traffic logging system, user management system,
   a provider should make the decision based on a comprehensive
   investigation on its demand and the capabilities of existing
   equipments.

   Note that some ISPs may need to offer services in a MAP domain with a
   shared address, e.g. there are hosts FTP server under CEs.  The
   service provisioning may require well-know port range (i.e. port
   range belong to 0-1023).  MAP would provide operators with an option
   to generate a port range including those in 0-1023.  Afterwards,
   operators could decide to assign it to any requesting user.  However,
   if the port-set is too small, it is not suggested to assign one with
   only the port set 0~1023 or even less.  Considerable non-well-known
   ports are surely needed.  Another easier approach is assigning a
   dedicated IPv4 address to such a CE if the demand really exists.

4.1.6.  Addressing and Routing

   In MAP addressing, it should follow the MAP rule planning in the MAP
   domain.



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   For IPv4 addressing, since the number of scattered IPv4 address
   prefixes would be equal to the number of FMR rules within a MAP
   domain, one should choose as large IPv4 address pool as possible to
   reduce the number of FMR rules.  For IPv6 address, the Rule IPv6
   prefixes should be equal to the end user IPv6 prefix in MAP domain.

   If ISP has a /24 rule IPv4 prefix with sharing ratio of 64 gives
   16000 customers, and a /16 rule IPv4 prefix supports 4 million
   customer.  If up the sharing ratio to 256, 64000 and 16 million
   customers can be supports respectively.  For the ISP who has
   scattered IPv4 address prefixes, in order to reduce the number of
   FMRs, according to needs of ports they can divide different classes.
   For instance, for the enterprise customers class which need many
   ports to use, provision them the BMR with low sharing ratio while for
   the private customers class which don't need so many ports provision
   them the BMR with high sharing ratio.

   For MAP routing, there are no IPv4 routes exported to IPv6 networks.

4.1.7.  MAP vs. MAP-T vs. 4rd

   Basically, encapsulation provides an architectural building block of
   virtual link where the underlay behavior is fully hidden, while
   translation does a delivery participating into the end-to-end
   transferring path where behaviors are exposed.  It is reflected in
   the following aspects.

   1.  Option header

   If translation or 4rd 'reversible translation' is applied, IPv4
   options at the IP layer are not translated according to
   [RFC791][RFC2460], and packets with those options MUST be dropped by
   Domain-entry nodes, and return ICMPv4 error messages to signal IPv4-
   option incompatibility.  This limitation is acceptable because there
   are a lot firewalls in current IPv4 Internet also filter IPv4 packets

   2.  ICMP

   Some IPv4 ICMP codes do not have a corresponding codes in ICMPv6, a
   detailed analysis on the double translation behavior suggest that
   some ICMPv4 messages, when they are translated to ICMPv6 and back to
   ICMPv4 across the IPv6 domain, the accuracy might be sacrificed to
   some extent.  Encapsulation keeps the full transparency of ICMPv4
   messages.

   Reversible translation approach of 4rd, however, does not translate
   ICMPv4 messages into ICMPv6 version.  Instead, it treats ICMP as same
   as a transport layer protocol data unit.  This behavior is similar to



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   the encapsulation and keeps ICMP end-to-end transparency as well.

   In either the encapsulation or translation mode, if an intermediate
   node generates an ICMPv6 error message, it should be converted into
   ICMPv4 version and returned to the source with a special source
   address and following the behavior specified in [RFC6791].  However,
   the behavior and semantics of the translation from ICMPv6 to ICMPv4
   is different among encapsulation, translation and 4rd reversible
   translation approaches.  Encapsulation treats routing error in the
   IPv6 domain as an (virtual)link error between the tunnel end points,
   while translation translate IPv6 routing error into corresponding
   IPv4 version, and 4rd, however, behaves according to whether the
   Tunnel Traffic Class option is set.  The TTL behavior also reflect
   the differences among different approaches, which is worth paying
   attention to for the operating engineers.  MAP-T translator is
   compatible with single translation approach.

   3.  PMTU and fragmentation

   Both translation mode and encapsulation mode have PMTU and
   fragmentation problem.  [RFC6145] discusses the problem in details
   for the translation, while [RFC2473] could be a reference on the
   issue in encapsulation.

4.2.  BR Settings

   1.  BR placement

   BR placement has important impacts on the operation of a MAP domain.

   A first concern should be the avoidance of "triangle routing".  In
   hub and spoke mode, all traffic will be routed through BR which may
   increase the path from the CE to an IPv4 peer.  This can be
   accomplished easily by placing the BR close to the CE, such that the
   length of the path from the CE to the BR is minimized.

   However, minimizing the CE-BR path would ignore a second concern,
   that of minimizing IPv4 operations.  An ISP deploying MAP will
   probably want to focus on IPv6 operations, while keeping IPv4
   operational expenditures to a minimum.  This would imply that the
   size of the IPv4 network that the ISP has to administer would be kept
   to a minimum.  Placing the BR near the CE means that the length of
   the IPv4 network between the BR and the IPv4 Internet would be
   longer.

   Moreover, in case where the set of CEs is geographically dispersed,
   multiple BRs would be needed, which would further enlarge the IPv4
   network that the ISP has to maintain.



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   Therefore, we offer the following guideline: BRs should be placed as
   close to the border with the IPv4 Internet as possible while keeping
   triangle routing to a minimum.  Regional POPs should probably be
   considered as potential candidates.

   Note also that MAP being stateless, asymmetric routing to/from the
   IPv4 Internet is natively supported and therefore no path-pinning
   mechanisms have to be additionally implemented.

   Anycast can be used to let the network pick BR closest to a CE for
   traffic exiting the MAP domain.  This is accomplished by provisioning
   a Default Mapping Rule containing an anycast IPv6 address or prefix.
   Operationally, this allows incremental deployment of BRs in strategic
   locations without modifying the provisioning system's configuration.
   CE's close to a newly-deployed BR will automatically start using it.
   The BR MUST participate in a dynamic IGP so that this can work
   automatically.

   2.  Reliability Considerations

   Reliability of MAP is derived in major part from its statelessness.
   This means that MAP can benefit from the usual methods of Internet
   reliability.

   Anycast, already mentioned in section 4.2.1, can be used to ensure
   reliability of traffic from CE to BR.  Since there can be only one
   Default Mapping Rule per MAP domain, traffic from CE to BR will
   always use the same destination address.  When this address is
   anycast, reliability is greatly increased.  If a BR goes down, it
   stops advertising the IPv6 anycast address, and traffic is
   automatically re-routed to other BRs; the BR should also withdraw the
   routes for traffic from BR to CE, or the upstream routers connected
   to the BR should dynamically change the routes when it detects the
   failure of a BR, otherwise there will be a routing blackhole.  For
   this mechanism to work correctly, it is crucial that the anycast
   route announcement be very closely tied to BR availability.  See
   [RFC4786] for best current practices on the operation of anycast
   services.  In practice, Equal-cost multi-path (ECMP) can be used to
   achieve active/active configuration.  Operator can also increase the
   metric for one BR to have active/standby.

   For reliability within a single link can be achieved with the help of
   a redundancy protocol such as VRRP [RFC5798].  This allows operation
   of a pair of BRs in active/standby configuration.  No state needs to
   be shared for the operation of MAP, so there is no need to keep the
   standby node in a "warm" state: as long as it is up and ready to take
   over the virtual IPv6 address, quick failover can be achieved.  This
   makes the pair behave as a single, much more reliable node, with less



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   reliance on quick routing protocol convergence for reliability.

   It is expected that production-quality MAP deployments will make use
   of both anycast and a redundancy protocol such as VRRP.

   3.  MTU/Fragmentation

   If the MTU is well-managed such that the IPv6 MTU on the CE WAN side
   interface is set so that no fragmentation occurs within the boundary
   of the MAP domain, then the Tunnel MTU can be set to the known IPv6
   MTU minus the size of the encapsulating IPv6 header (40 bytes).  For
   example, if the IPv6 MTU is known to be 1500 bytes, the Tunnel MTU
   might be set to 1460 bytes.  Without more specific information, the
   Tunnel MTU SHOULD default to 1240 bytes.

   BRs using an anycast address as source can cause problems.  If
   traffic sent by a BR with a source anycast address causes an ICMP
   error to be returned, that error packet's destination address will be
   an anycast address, meaning that a different BR might receive it.  In
   the case of a Too Big ICMP error, this could cause a path MTU
   discovery black hole.  Another possible problem could occur if
   fragmented packets from different BRs using the same anycast address
   as source happen to contain the same fragment ID.  This would break
   fragment reassembly.  Since there is still no simple way to solve it
   completely, it is recommended to increase the MTU of the IPv6 network
   so that no fragmentation and Too Big ICMP error occurs.

   In MAP domains where IPv4 addresses are not shared, IPv6 destinations
   are derived from IPv4 addresses alone.  Thus, each IPv4 packet can be
   encapsulated and decapsulated independently of each other.  The
   processing is completely stateless.

   On the other hand, in MAP domains where IPv4 addresses are shared,
   BRs and CEs may have to encapsulate or translate IPv4 packets whose
   IPv6 destinations depend on destination ports.  Precautions are
   needed, due to the fact that the destination port of a fragmented
   datagram is available only in its first fragment.  A sufficient
   precaution consists in reassembling each datagram received in
   multiple packets, and to treat it as though it would have been
   received in single packet.  This function is such that MAP is in this
   case stateful at the IP layer.  (This is common with DS-lite and
   NAT64/DNS64 which, in addition, are stateful at the transport layer.)
   At domain entrance, this ensures that all pieces of all received IPv4
   datagrams go to the right IPv6 destinations.







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4.3.  CE Settings

   1. bridging vs. routing

   In routing manner, the CE runs a standard NAT44 [RFC3022] using the
   allocated public address as external IP and ports via DHCPv6 option.
   When receiving an IPv4 packet with private source address from its
   end hosts, it performs NAT44 function by translating the source
   address into public and selecting a port from the allocated port-set.
   Then it encapsulates/translates (depending on whether MAP-E or MAP-T
   is in use) the packet with the concentrator's IPv6 address as
   destination IPv6 address, and forwards it to the concentrator.  When
   receiving an IPv6 packet from the concentrator, the initiator
   decapsulates/translates the IPv6 packet to get the IPv4 packet with
   public destination IPv4 address.  Then it performs NAPT44 function
   and translates the destination address into private one based on the
   entry in NAT state table in the CE.

   The CE is responsible for performing ALG functions (e.g., SIP, FTP),
   as well as supporting NAT Traversal mechanisms (e.g., UPnP, NAT-PMP,
   manual mapping configuration).  This is no different from the
   standard IPv4 NAT today.

   For the bridging manner, end host would run a software performing CE
   functionalities.  In this case, end host gets public address
   directly.  It is also suggested that the host run a local NAT to map
   randomly generated ports into the restricted, valid port-set.
   Another solution is to have the IP stack to only assign ports within
   the restricted, valid range to applications.  Either way the host
   guarantees that every source port number in the outgoing packets
   falls into the allocated port-set.

   2.  CE-initiated application

   CE-initiated case is applied for situations where applications run on
   CE directly.  If the application in CE use the public address
   directly, it might conflict with other CEs.  So it is highly
   suggested that CE should also run a local NAT to map a private
   address to public address in CE.  In this way, the CE IPv4 address
   passed to local applications would be conflict with other CEs.

4.4.  Supporting System

   1.  Lawful Intercept

   Sharing IPv4 addresses among multiple CEs is susceptible to issues
   related to lawful intercept.  For details, see [RFC6269] section 12.




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   2.  Traffic Logging

   It is always possible for a service provider that operates a MAP
   domain to determine the IPv6 prefix associated with a MAP IPv4
   address (and port number in case of a shared address).  This mapping
   is static, and it is therefore unnecessary to log every IPv4 address
   assignment.  However, changes in that static mapping, such as rule
   changes in the provisioning system, need to be logged in order to be
   able to know the mapping at any point in time.

   Sharing IPv4 addresses among multiple CEs is susceptible to issues
   related to traffic logging.  For details, see [RFC6269] sections 8
   and 13.1.

   3.  Geo-location aware service

   Sharing IPv4 addresses among multiple CEs is susceptible to issues
   related to geo-location.  For details, see [RFC6269] section 7.

   4.  User Management

   MAP IPv4 address assignment, and hence the IPv4 service itself, is
   tied to the IPv6 prefix lease; thus, the MAP service is also tied to
   this in terms of authorization, accounting, etc.  For example, the
   MAP address has the same lifetime as its associated IPv6 prefix.


























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5.  MAP Address Planning

   This section is purposed to provide a referential guidance to
   operators, illustrating a common method of address planning with MAP
   in IPv4 residual deployment.

5.1.  Planning for Residual Deployment, a Step-by-step Guide

   Residual deployment starts from IPv6 address planning.

   (A) IPv6 considerations

   (A1)  Determine the maximum number N of CEs to be supported, and, for
         generality, suppose N = 2^n.

         For example, we suppose n = 20.  It means there will be up to
         about one million CEs.

   (A2)  Choose the length x of IPv6 prefixes to be assigned to ordinary
         customers.

         Consider we have a /32 IPv6 block, it is not a problem for the
         IPv6 deployment with the given number of CEs.  Let x = 60,
         allowing subnets inside in each CE delegated networks.

   (A3)  Multiply N by a margin coefficient K, a power of two (K = 2 ^
         k), to take into account that:

      -  Some privileged customers may be assigned IPv6 prefixes of
         length x', shorter than x, to have larger addressing spaces
         than ordinary customers, both in IPv6 and IPv4;

      -  Due to the hierarchy of routable prefixes, many theoretically
         delegable prefixes may not be actually delegable (ref: host
         density ratio of [RFC3194]).

         In our example, let's take k = 0 for simplicity.

   (B) IPv4 considerations

   (B1)  List all (non overlapping, not yet assigned to any in-running
         networks) IPv4 prefixes {Hi} that are available for IPv4
         residual deployment.

         Suppose that we hold two blocks and not yet assigned to any
         fixed network: 192.0.2.0/24 and 198.51.100.0/24.





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   (B2)  Take enough of them, among the shortest ones, to get a total
         whose size M is a power of two (M = 2 ^ m), and includes a good
         proportion of the available IPv4 space.

         If we use both blocks, M = 2^24 + 2^24, and therefore m = 25.
         Suppose the intended sharing ratio is 8 subscribers per
         address, resulting in (65536 - 1024)/8 = 8064 ports per
         subscriber assuming that the well-known ports are excluded.
         Then the PSID length to achieve this will be log2(8) = 3 bits.
         Bearing in mind the IPv4 24 bit prefix length for each of our
         two prefixes, the EA-bit length is (32 - 24) + 3 = 11 bits.

   (B3)  For each IPv4 prefix, Hi, of length hi, choose an prefix
         extension, say Ri of length ri = m - (32 - hi).

         All these indexes must be non overlapping prefixes (e.g. 0, 10,
         110, 111 for one /10, one /11, and two /12).  In our example,
         we pick 0 for a contiguous address block while 1 for another.

         Then we have:

           H1 = 192.0.2.0/24, h1 = 24, r1 = 17 => R1 = bin(0);
           H2 = 198.51.100.0/24, h2 = 24, r2 = 17 => R2 = bin(1);

   Sometimes the IPv4 residual pool is not well aggregated and the
   contiguous address blocks may have different sizes.  For example, in
   (B1), if we have H1 = 59.112.0.0/13 and H2 = 219.120.0.0/16 as the
   IPv4 residual pool, then M = 2^19 + 2^16, and in such a case, we must
   pick m so that m = ceil(log2(M)), where "ceil(x)" means the minimum
   integer not less than x, i.e., m = 20 in this case.  Therefore r1 =
   20 - (32 - 13) = 1, while r2 = 20 - (32 - 16) = 4.  Several
   combinations are available for the R1 and R2 and one only needs to
   pay attention to avoiding overlapping when picking up the values.

   (C) After (A) and (B), derive the rule(s)

   (C1)  Derive the length c of the MAP domain IPv6 prefix, C, that will
         appear at the beginning of all delegated prefixes (c = x - (n +
         k)).

   (C2)  Take any prefix for this C of length c that starts with a RIR-
         allocated IPv6 prefix.

   (C3)  For each IPv4 prefix Hi, make the rule, in which the key is Hi
         and the value is the domain IPv6 prefix C followed by the rule
         index Ri.  Then this i-th rule's Rule IPv6 Prefix will have the
         length of (c + ri).




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         Then we can do that:

               c = 40 => C = 2001:0db8:ff00::/40
               Rule 1: Rule IPv6 Prefix = 2001:0db8:ff00::/41
               Rule 2: Rule IPv6 Prefix = 2001:0db8:ff80::/41

         If we have different lengths for the Rule IPv4 prefix (as the
         extra example discussed at the end of (B)), their Rule IPv6
         prefixes should not have the same length, as their rule index
         length is different.

         As a result, for a certain CE delegating 2001:0db8:ff98:
         7650::/60, its parameters are:

              Rule IPv6 Prefix = 2001:0db8:ff80::/41 => Rule 2
              IPv4 Suffix = bin(111 0110 0)
                                         PSID = bin(101) = 0x5
              Rule IPv4 Prefix = 198.51.100.0/24
              CE IPv4 Address = 198.51.100.236

   If different sharing ratio is demanded, we may partition CEs into
   groups and do (A) and (B) for each group, determining the PSID length
   for them separately.

5.2.  Remarks on Deployment Paradigms

   1.  IPv6 address planning in residual deployment is independent of
       the usage of the residual IPv4 addresses.  The IPv4 address pool
       for "residual deployment" contains IPv4 addresses not yet
       allocated to customers/subscribers and/or those already recalled
       from ex-customers, re-programmed into relatively well-aggregated
       blocks.

   2.  It is recommended to have the number of rule entries as less as
       possible so that the merit of statelss deployment is reflected in
       practical performances.  However, this effort is often
       constrained by the condition of an operator whether (a): it holds
       large-enough contiguous IPv4 address block(s) for the residual
       deployment, and (b): a short-enough IPv6 domain prefix so that
       the /64 delegation is easily satisfied even the EA-bits is quite
       long.  When condition (a) is not satisfied, sub-domains have to
       be defined for each relatively small but contiguous aggregated
       block; when condition (b) is not satisfied, one has to divide the
       IPv4 aggregates into smaller blocks artificially in order to
       reduce the length of EA-bits.  When we have good conditions
       fitting (a) and (b), it is NOT recommended to define short EA-
       bits with small length of IPv4 suffix (the value p) nor to
       increase the number of rule entries (also the number of sub-



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       domains) unless it really has to.

   3.  An extreme case is, when EA-bits contain the full IPv4 address
       while a full IPv4 address is assigned to a CE, i.e., o = p = 32,
       and q = 0, the MAP address format becomes almost equivalent to
       RFC6052-format [RFC6052] except the off-domain IPv4 peer's mapped
       IPv6 address.  This frees the domain to distribute rules but the
       DMR.  In such a case, IPv6 addressing is fully dependent of IPv4,
       which defers from the typical residual deployment case.  MAP is
       mainly designed for residual deployment but also applied for the
       case of legacy IPv4 networks keeping communication with the IPv4
       world over the IPv6 domain without renumbering, as long as the
       address planning doesn't matter.

   4.  Another extreme case is, when EA-bits' length becomes to zero,
       i.e., o = p = q = 0, a rule actually defines a correspondence
       between an IPv6 address and an IPv4 address (or a prefix),
       without any algorithmic correlation to each other.  Using such a
       case in practice is not prohibited by the specification, but it
       is not recommended to deploy null EA-bits in large scale as the
       concern discussed in the above Remark 2, and as it has the
       limitation that the PSID must be null (q = 0) and therefore
       multiple CEs sharing a same IPv4 address is not supported here.
       It is recommended to apply Lightweight 4over6 [I-D.ietf-softwire-
       lw4over6], if a full de-correlation between IPv6 address and IPv4
       address as well as port range is demanded.

   5.  A not-so-extreme case, p = 0, o = q, i.e., only PSID is applied
       for the EA-bits, is also a case possibly happening in practice.
       It also potentially generates a huge number of rules and
       therefore large-scale deployment of this case is not recommended
       either.

   6.  For operators who would like to utilize "some bits" of IPv6
       address to do service identification, QoS differentiation, etc.,
       it is recommended that these special-purpose bits should be
       embedded before the EA-bits so as to reduce the possibility of
       bit-conflict.  However, it requires quite shorter IPv6 aggregate
       prefix of the operator.  The bit-conflict is more likely to
       happen in this case if different domains have different Rule
       prefix lengths.  Operators with this demand should pay attention
       to the impact on the domain rule planning.









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6.  Migration Methodology

6.1.  Roadmap for MAP-based Solution

6.1.1.  Start from Scratch

   IPv6 deployment normally involves a step-wise approach where parts of
   the network should properly updated gradually.  As IPv6 deployment
   progresses it may be simpler for operators to employ a single-version
   network, since deploying both IPv4 and IPv6 in parallel would cost
   more than IPv6-only network.  Therefore switching to an IPv6-only
   network in relatively small scale will become more prevalent.
   Meanwhile, a significant part of network will still stay in IPv4 for
   long time, especially at early stage of IPv6 transition.  There may
   not be enough public or private IPv4 addresses to support end-to-end
   network communication, without segmenting the network into small
   parts with sharing one IPv4 address space.  That is a time to
   introduce MAP to bridge these IPv4 islands through IPv6 network.

6.1.2.  Coexiting Phases

   SP has various deployment strategy in the middle of transition.  It's
   foreseeable that IPv6 would likely coexist with IPv4 in a long
   period.  The MAP deployment would also fit into the coexisting mode.
   To be specific, dual-stack technology is recommended in RFC6180 as
   the simplest deployment model to advance IPv6 deployment.  MAP
   technology could get along well with native IPv6 connections and
   compatible with residual IPv4 networks.  RFC6264 described a
   incremental transition approach in order to migrate networks to IPv6-
   only.  DS-Lite is treated as a technology to accelerate the whole
   process.  MAP can also take the same role to achieve a smooth
   transition.

6.1.3.  Exit Strategy

   The benefit of IPv6-only + MAP is that all IPv6 flows would go
   directly to the Internet, no need for encapsulation or translation.
   In this way, as more content providers and service are available over
   IPv6, the utilization on MAP CE and BR goes down since fewer
   destinations require MAP progressing.  This way would advance IPv6,
   because it provides everyone incentives to use IPv6, and eventually
   the result is an pure IPv6 network with no need for IPv4.  As more
   content providers and hosts equipped with IPv6 capabilities , the MAP
   utilization goes down until it is eventually not used at all when all
   content is IPv6.  In this way, MAP has an "exit strategy".  The
   corresponding solutions will leave the network in time.





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6.2.  Migration Mode

   IPv4 Residual deployment is a interim phase during IPv6 migration.
   It would be beneficial to ISPs, if this phase is as short as possible
   since end-to-end IPv6 traversal is the really goals.  When IPv6 is
   getting more and more mature, MAP would be retired in a natural way .

6.2.1.  Passive Transition

   Passive Transition is following IPv4 retirement law.  In another
   word, MAP would always get along with IPv4, even all nodes is dual-
   stack capable.  At a later stage of IPv6 migration, MAP can also be
   served for dual-stack hosts, which is sending traffic through the
   IPv4 stack.  There is still a value for this approach because it
   could steer IPv4 traffic to IPv6 going through a MAP CE processing.
   When it comes the time ISP decide to turn off IPv4, MAP would be
   unnecessary due to IPv4 disappearance.

6.2.2.  Active Transition

   Active Transition is targeting to accelerate IPv4 exit and increase
   native IPv6 utilization.  A desirable way deploying MAP is only
   providing IPv6 traversal ability to a IPv4-only host.  However, MAP
   CE can not determine received traffic is send from a IPv4 node or a
   dual-stack node.  In the latter case, IPv6 utilization is preferred
   for the most part .  When a network evolves to a post-IPv6 era, it
   might be good for ISPs to consider to implement enforcement rules to
   help IPv6 migration.

   o  ISP could install only IPv6 record (i.e.  AAAA) in DNS server,
      which would provide users with IPv6 steering effects.  When a host
      is IPv6-capable and gets IPv6 DNS reply in advance, MAP
      functionalities would be restricted by IPv6-only record response.

   o  ISP could retrieve shared IPv4 address by increasing sharing
      ratio.  In this case, number of concurrent IPv4 sessions on MAP CE
      would be suppressed.  It would encourage native IPv6 growth in
      some extent.

   o  ISP could allocate a dedicated IPv6 prefix for MAP deployment.
      The allocation could not only facilitate the differentiation
      between MAPed traffic and native IPv6 traffic, but also clearly
      observe the change of MAP traffic.  When the traffic is reducing
      for a while, ISP could close the MAP functionalities in some
      specific area.  It would result networks to native IPv6-only
      capable.





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7.  IANA Considerations

   This specification does not require any IANA actions.
















































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8.  Security Considerations

   There are no new security considerations pertaining to this document.
















































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9.  Contributors

   The members of the MAP design team are:

      Congxiao Bao, Mohamed Boucadair, Gang Chen, Maoke Chen, Wojciech
      Dec, Xiaohong Deng, Remi Despres, Jouni Korhonen, Xing Li, Satoru
      Matsushima, Tomasz Mrugalski, Tetsuya Murakami, Jacni Qin, Qiong
      Sun, Tina Tsou, Dan Wing, Leaf Yeh, and Jan Zorz.

   Thanks to Chunfa Sun who was an active co-author of some earlier
   versions of this draft.  Thanks to Shishio Tsuchiya's valueable
   suggestion for this document.







































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10.  Acknowledgements

   Remi Despres contributed the original example of step-by-step
   deployment guidance in discussion with the authors.  Ole Troan, as
   the head of MAP Design Team, joined the discussion directly and
   contributed a lot of ideas and comments.  We also thank other members
   of the MAP Design Team for their comments and suggestions.

   Thanks to Tom Talyer, Qi Sun and Ian Farrer for their thorough review
   and helpful comments.









































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11.  References

11.1.  Normative References

   [I-D.ietf-softwire-4rd]
              Despres, R., Jiang, S., Penno, R., Lee, Y., Chen, G., and
              M. Chen, "IPv4 Residual Deployment via IPv6 - a Stateless
              Solution (4rd)", draft-ietf-softwire-4rd-10 (work in
              progress), December 2014.

   [I-D.ietf-softwire-map]
              Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, "Mapping of Address and Port
              with Encapsulation (MAP)", draft-ietf-softwire-map-13
              (work in progress), March 2015.

   [I-D.ietf-softwire-map-dhcp]
              Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
              W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
              configuration of Softwire Address and Port Mapped
              Clients", draft-ietf-softwire-map-dhcp-12 (work in
              progress), March 2015.

   [I-D.ietf-softwire-map-t]
              Li, X., Bao, C., Dec, W., Troan, O., Matsushima, S., and
              T. Murakami, "Mapping of Address and Port using
              Translation (MAP-T)", draft-ietf-softwire-map-t-08 (work
              in progress), December 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

   [RFC6791]  Li, X., Bao, C., Wing, D., Vaithianathan, R., and G.
              Huston, "Stateless Source Address Mapping for ICMPv6
              Packets", RFC 6791, November 2012.

11.2.  Informative References

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC3194]  Durand, A. and C. Huitema, "The H-Density Ratio for
              Address Assignment Efficiency An Update on the H ratio",
              RFC 3194, November 2001.




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   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010.

   [RFC7368]  Chown, T., Arkko, J., Brandt, A., Troan, O., and J. Weil,
              "IPv6 Home Networking Architecture Principles", RFC 7368,
              October 2014.












































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Authors' Addresses

   Qiong Sun
   China Telecom
   Room 708 No.118, Xizhimenneidajie
   Beijing,   100035
   P.R.China

   Phone: +86 10 5855 2923
   Email: sunqiong@ctbri.com.cn


   Maoke Chen
   BBIX, Inc.
   Tokyo Shiodome Building, Higashi-Shimbashi 1-9-1
   Minato-ku, Tokyo  105-7310
   Japan

   Email: maoke@bbix.net


   Gang Chen
   China Mobile
   28 Xuanwumenxi Ave; Xuanwu District
   Beijing
   P.R. China

   Email: chengang@chinamobile.com


   Tina Tsou
   Huawei Technologies
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Phone: +1-408-330-4424
   Email: tina.tsou.zouting@huawei.com


   Simon Perreault
   Jive Communications
   Quebec, QC
   Canada

   Email: sperreault@jive.com





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