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Independent Submission X. Deng Request for Comments: 7393 Category: Informational M. Boucadair ISSN: 2070-1721 France Telecom

                                                               Q. Zhao
                    Beijing University of Posts and Telecommunications
                                                              J. Huang
                                                               C. Zhou
                                                   Huawei Technologies
                                                         November 2014
    Using the Port Control Protocol (PCP) to Update Dynamic DNS


 This document focuses on the problems encountered when using dynamic
 DNS in address-sharing contexts (e.g., Dual-Stack Lite (DS-Lite) and
 Network Address and Protocol Translation from IPv6 Clients to IPv4
 Servers (NAT64)) during IPv6 transition.  Both issues and possible
 solutions are documented in this memo.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This is a contribution to the RFC Series, independently of any other
 RFC stream.  The RFC Editor has chosen to publish this document at
 its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Deng, et al. Informational [Page 1] RFC 7393 PCP DDNS Updates November 2014

Copyright Notice

 Copyright (c) 2014 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
 ( 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.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.1.  Problem Statement . . . . . . . . . . . . . . . . . . . .   3
   1.2.  Scope and Goals . . . . . . . . . . . . . . . . . . . . .   4
 2.  Solution Space  . . . . . . . . . . . . . . . . . . . . . . .   5
   2.1.  Locate a Service Port . . . . . . . . . . . . . . . . . .   5
   2.2.  Create Explicit Mappings for Incoming Connections . . . .   5
   2.3.  Detect Changes  . . . . . . . . . . . . . . . . . . . . .   5
 3.  Some Deployment Solutions . . . . . . . . . . . . . . . . . .   7
   3.1.  Reference Topology  . . . . . . . . . . . . . . . . . . .   7
   3.2.  For Web Service . . . . . . . . . . . . . . . . . . . . .   8
   3.3.  For Non-web Service . . . . . . . . . . . . . . . . . . .   9
 4.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
 5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
   5.2.  Informative References  . . . . . . . . . . . . . . . . .  12
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

Deng, et al. Informational [Page 2] RFC 7393 PCP DDNS Updates November 2014

1. Introduction

1.1. Problem Statement

 Dynamic DNS (DDNS) is a widely deployed service to facilitate hosting
 servers (e.g., access to a webcam, HTTP server, FTP server, etc.) at
 customers' premises.  There are a number of providers that offer a
 DDNS service, working in a client and server mode, which mostly use
 web-form-based communication.  DDNS clients are generally implemented
 in the user's router or computer; once changes are detected to its
 assigned IP address, an update message is automatically sent to the
 DDNS server.  The communication between the DDNS client and the DDNS
 server is not standardized, varying from one provider to another,
 although a few standard web-based methods of updating have emerged
 over time.
 In address-sharing contexts, well-known port numbers (e.g., port 80)
 won't be available for every user [RFC6269].  As such, the DDNS
 client will have to register the IP address and/or the external
 port(s) on which the service is listening.  Also, the DDNS client has
 to report any change of this IP address and/or the external port(s).
 It will also require the ability to configure corresponding port
 forwarding on Carrier-Grade NAT (CGN) [RFC6888] devices so that
 incoming communications initiated from the Internet can be routed to
 the appropriate server behind the CGN.
 Issues encountered in address sharing are documented in [RFC6269].
 This document focuses on the problems encountered when using dynamic
 DNS in address-sharing contexts (e.g., DS-Lite [RFC6333] and NAT64
 [RFC6146]).  The main challenges are listed below:
 Announce and discover an alternate service port:  The DDNS service
    must be able to maintain an alternative port number instead of the
    default port number.
 Allow for incoming connections:  Appropriate means to instantiate
    port mappings in the address-sharing device must be supported.
 Detect changes and trigger DDNS updates:  The DDNS client must be
    triggered by the change of the external IP address and the port
    number.  Concretely, upon change of the external IP address (and/
    or external port number), the DDNS client must refresh the DNS
    records; otherwise, the server won't be reachable from outside.
    This issue is exacerbated in the DS-Lite context because no public
    IPv4 address is assigned to the Customer Premises Equipment (CPE).

Deng, et al. Informational [Page 3] RFC 7393 PCP DDNS Updates November 2014

1.2. Scope and Goals

 This document describes some candidate solutions to resolve the
 aforementioned issues with a particular focus on DS-Lite.  These
 solutions may also be valid for other address-sharing schemes.
 This document sketches deployment considerations based on the Port
 Control Protocol (PCP) [RFC6887].  Note that DDNS may be considered
 as an implementation of the rendezvous service mentioned in
 Indeed, after creating an explicit mapping for incoming connections
 using PCP, it is necessary to inform remote hosts about the IP
 address, protocol, and port number for the incoming connection to
 reach the services hosted behind a DS-Lite CGN.  This is usually done
 in an application-specific manner.  For example, a machine hosting a
 game server might use a rendezvous server specific to that game (or
 specific to that game developer), a SIP phone would use a SIP proxy,
 a client using DNS-Based Service Discovery [RFC6763] would use DNS
 Update [RFC2136][RFC3007], etc.  PCP does not provide this rendezvous
 The rendezvous function may support IPv4, IPv6, or both.  Depending
 on that support and the application's support of IPv4 or IPv6, the
 PCP client may need an IPv4 mapping, an IPv6 mapping, or both.  An
 example illustrating how the DDNS server may implement such a service
 notification functionality if necessary is provided in Section 3.
 This document does not specify any protocol extension but instead
 focuses on the elaboration of the problem space and illustrates how
 existing tools can be reused to solve the problem for some deployment
 contexts.  Particularly, this document requires no changes to PCP or
 dynamic updates in the standard domain name system [RFC2136]; rather,
 it is an operational document to make the current DDNS service
 providers aware of the impacts and issues that IPv6 transitioning and
 IPv4 address sharing will bring to them, and it gives solutions to
 address the forthcoming issues.  The current DDNS service providers
 usually employ a web-based form to maintain DDNS service registration
 and updates.
 Generic deployment considerations for DS-Lite, including Basic
 Bridging BroadBand (B4) remote management and IPv4 connectivity
 check, can be found in [RFC6908].  This document complements
 [RFC6908] with deployment considerations related to rendezvous
 service maintenance.  Additional PCP-related deployment
 considerations are available at [PCP-DEPLOYMENT].

Deng, et al. Informational [Page 4] RFC 7393 PCP DDNS Updates November 2014

 Solutions relying on DNS-Based Service Discovery [RFC6763] or Apple's
 Back to My Mac (BTMM) Service [RFC6281] are not considered in this
 document.  Moreover, this document does not assume that DDNS service
 relies on [RFC2136].
 IPv4 addresses used in the examples are derived from the IPv4 block
 reserved for documentation in [RFC6890].  DNS name examples follow

2. Solution Space

2.1. Locate a Service Port

 As listed below, at least two solutions can be used to associate a
 port number with a service:
 1.  Use service URIs (e.g., FTP, SIP, HTTP) that embed an explicit
     port number.  Indeed, the Uniform Resource Identifier (URI)
     defined in [RFC3986] allows the port number to be carried in the
     syntax (e.g., mydomain.example:15687).
 2.  Use SRV records [RFC2782].  Unfortunately, the majority of
     browsers do not support this record type.
 The DDNS client and DDNS server are to be updated so that an
 alternate port number is signaled and stored by the DDNS server.
 Requesting remote hosts will be then notified with the IP address and
 port number to reach the server.

2.2. Create Explicit Mappings for Incoming Connections

 PCP is used to install the appropriate mapping(s) in the CGN so that
 incoming packets can be delivered to the appropriate server.

2.3. Detect Changes

 In a network as described in Figure 1, a DDNS client/PCP client can
 be running on either a CPE or the host that is hosting some services
 itself.  There are several possible ways to address the problems
 stated in Section 1.1:
 1.  If the DDNS client is enabled, the host periodically issues
     (e.g., 60 minutes) PCP MAP requests (e.g., messages 1 and 2 in
     Figure 1) with short lifetimes (e.g., 30s) for the purpose of
     inquiring an external IP address and setting.  If the purpose is
     to detect any change to the external port, the host must issue a

Deng, et al. Informational [Page 5] RFC 7393 PCP DDNS Updates November 2014

     PCP mapping to install for the internal server.  Upon change of
     the external IP address, the DDNS client updates the records
     accordingly (e.g., message 3 in Figure 1).
 2.  If the DDNS client is enabled, it checks the local mapping table
     maintained by the PCP client.  This process is repeated
     periodically (e.g., 5 minutes, 30 minutes, 60 minutes).  If there
     is no PCP mapping created by the PCP client, it issues a PCP MAP
     request (e.g., messages 1 and 2 in Figure 1) for the purpose of
     inquiring an external IP address and setting up port forwarding
     mappings for incoming connections.  Upon change of the external
     IP address, the DDNS client updates the records in the DDNS
     server, e.g., message 3 in Figure 1.
                                   |  DDNS Server    |
                                             |3. DDNS updates
                                             |  (if any)

+—————+ +—————–+ |DDNS Client |1. PCP MAP request | CGN/PCP Server | |PCP Client/IWF |——————→| (PCP mapping for|80:8080+——+ |on CPE or |2. PCP MAP response | port forwarding)|←—–|Client| |the host itself|←——————| | +——+ | |3. DDNS updates | | | | (if any) | | | |——————→| | +—————+ +—————–+

IWF = Internetworking Function

                         Figure 1: Flow Chart

Deng, et al. Informational [Page 6] RFC 7393 PCP DDNS Updates November 2014

3. Some Deployment Solutions

3.1. Reference Topology

 Figure 2 illustrates the topology used for the deployment solutions
 elaborated in the following subsections.
 +--------------+   +--------+    +---------+   +--------+   +-------+
 | Service      |   |  DDNS  |    |  CGN/   |   | PCP    |   |Servers|
 | User         |---|  Server|----|  PCP    |---| Client |---|       |
 |              |   |        |    |  Server |   | /DDNS  |   |       |
 |              |   |        |    |         |   | Client |   |       |
 +--------------+   +--------+    +---------+   +--------+   +-------+
     A user         DDNS Server       AFTR        B4(CPE)      A host
  from Internet                                  behind B4
                   Figure 2: Implementation Topology
 Figure 2 involves the following entities:
 o  Servers: Refers to the servers that are deployed in the DS-Lite
    network, or more generally, an IP address-sharing environment.
    They are usually running on a host that has been assigned with a
    private IPv4 address.  Having created a proper mapping via PCP in
    the Address Family Transition Router (AFTR), these services have
    been made available to Internet users.  The services may provide
    web, FTP, SIP, and other services though these may not be able to
    be seen as using a well-known port from the outside anymore, in
    the IP address-sharing context.
 o  B4(CPE): An endpoint of an IPv4-in-IPv6 tunnel [RFC6333].  A PCP
    client together with a DDNS client are running on it.  After a PCP
    client establishes a mapping on the AFTR, an end user may register
    its domain name and its external IPv4 address plus port number to
    its DDNS service provider (DDNS server), manually or automatically
    by a DDNS client.  Later, likewise, end users may manually
    announce or let the DDNS client automatically announce IP address
    and/or port changes to the DDNS server.
 o  AFTR: Responsible for maintaining mappings between an IPv6
    address, the internal IPv4 address plus internal port, and the
    external IPv4 address plus port [RFC6333].
 o  DDNS server: Maintains a table that associates a registered domain
    name and a registered host's external IPv4 address/port number
    pair.  When being notified of IP address and port number changes
    from a DDNS client, the DDNS server announces the updates to DNS
    servers on behalf of the end user.  [RFC2136] and [RFC3007] may be

Deng, et al. Informational [Page 7] RFC 7393 PCP DDNS Updates November 2014

    used by DDNS servers to send updates to DNS servers.  In many
    current practices, a DDNS service provider usually announces its
    own IP address as the registered domain names of end users.  When
    HTTP requests reach the DDNS server, they may employ URL
    Forwarding or HTTP 301 redirection to redirect the request to a
    proper registered end user by looking up the maintained link
 o  Service users: Refers to users who want to access services behind
    an IP address-sharing network.  They issue standard DNS requests
    to locate the services, which will lead them to a DDNS server,
    provided that the requested services have been registered to a
    DDNS service provider.  The DDNS server will then handle the rest
    in the same way as described before.

3.2. For Web Service

 Current DDNS server implementations typically assume that the end
 servers host web servers on the default 80 port.  In the DS-Lite
 context, they will have to take into account that external ports
 assigned by the AFTR may be any number other than 80, in order to
 maintain proper mapping between domain names and the external IP plus
 port.  If a proper mapping is maintained, the HTTP request would be
 redirected to the AFTR, which serves the specific end host that is
 running the servers.
 Figure 3 depicts how messages are handled in order to be delivered to
 the right server.
 Web Visitor        DDNS Server       AFTR      B4(CPE)     Web Server
                                                             behind B4
 | HTTP GET*             |              |          |               |
 |---------------------->|              |          |               |
 | ip_DDNS_server        |------------->|          |               |
 |                       | HTTP 301     |          |               |
 |                       |<-------------|          |               |
 | HTTP GET* ip_aftr:8001               |          |               |
 |------------------------------------->|                          |
 |                                      | HTTP GET* ip_websrv:8000 |
 |                                      |------------------------->|
 |                                      |                          |
 |                       HTTP response  | HTTP response            |
 |                                      |                          |
                    Figure 3: HTTP Service Messages

Deng, et al. Informational [Page 8] RFC 7393 PCP DDNS Updates November 2014

 When a web user sends out an HTTP GET message to the DDNS server
 after a standard DNS query, the DDNS server redirects the request to
 a registered web server, in this case, by responding with an HTTP 301
 message.  Then, the HTTP GET message will be sent out to the AFTR,
 which will in turn find the proper hosts behind it.  For simplicity,
 messages among AFTR, B4, and the web server behind B4 are not shown
 completely; for communications among those nodes, refer to [RFC6333].

3.3. For Non-web Service

 For non-web services, as mentioned in Section 2, other means will be
 needed to inform the users about the service information.
 [RFC6763] includes an example of a DNS-based solution that allows an
 application running in the end user's device to retrieve service-
 related information via DNS SRV/TXT records and list available
 services.  In a scenario where such an application is not applicable,
 the following provides another solution for a third party, e.g., a
 DDNS service provider, to disclose services to Internet users.
 A web portal can be used to list available services.  A DDNS server
 maintains a web portal for each user's Fully Qualified Domain Name
 (FQDN), which provides service links to users.  Figure 4 assumes
 "" is a user's FQDN provided by a DDNS service

Deng, et al. Informational [Page 9] RFC 7393 PCP DDNS Updates November 2014

 +-------------+    +-------------+    +----------+ Internet +-------+
 |DDNS Client /|    |DDNS Server /|    |DNS Server|          |Visitor|
 |  Web Server |    | Web Portal  |    |          |          |       |
 +-------------+    +-------------+    +----------+          +-------+
     |      register      |                  |                    |
     |<------------------>|                  |                    |
     | |  update DNS      |                    |
     |   | <------------->  |                    |
     |                    ||                    |
     |                    |   portal's IP    |                    |
     |              +-------------+          |                    |
     |              |update portal|          |                    |
     |              +-------------+          |  DNS resolve for   |
     |                    |                  | <----------------> |
     |                    |                  | |
     |                    |                  |  get portal's IP   |
     |                    |                  |                    |
     |                    |   visit portal of  |
     |                    | <-----------------------------------> |
     |                    |                  |                    |
     |                  visit               |
     | <--------------------------------------------------------->|
     |                    |                  |                    |
                      Figure 4: Update Web Portal
 The DDNS client registers the server's information to the DDNS
 server, including the public IP address and port obtained via PCP,
 the user's FQDN, and other necessary information.  The DDNS server
 also behaves as a portal server; it registers its IP address, port
 number, and the user's FQDN to the DNS system so that visitors can
 access the web portal.
 A DDNS server also maintains a web portal for each user's FQDN and
 updates the portal according to registered information from the DDNS
 client.  When a visitor accesses "", a DNS query
 will resolve the portal server's address and port number, and the
 visitor will see the portal and the available services.

Deng, et al. Informational [Page 10] RFC 7393 PCP DDNS Updates November 2014

   |                                                             |
   |              Portal:                     |
   |                                                             |
   |    Service1: web server                                     |
   |    Link:                          |
   |                                                             |
   |    Service2: video                                          |
   |    Link:     rtsp://                 |
   |                                                             |
   |    ......                                                   |
   |                                                             |
                 Figure 5: An Example of a Web Portal
 As shown in Figure 5, the web portal shows the service URLs that are
 available to be accessed.  Multiple services are accessible per a
 user's FQDN.
 Some applications that are not HTTP based can also be delivered using
 this solution.  When a user clicks on a link, the registered
 application in the client OS will be invoked to handle the link.  How
 this can be achieved is out of the scope of this document.

4. Security Considerations

 This document does not introduce a new protocol, nor does it specify
 protocol extensions.  Security-related considerations related to PCP
 [RFC6887] and DS-Lite [RFC6333] should be taken into account.
 The protocol between the DDNS client and DDNS server is proprietary
 in most cases; some extensions may be necessary, which is up to the
 DDNS operators.  These operators should enforce security-related
 policies in order to keep illegitimate users from altering records
 installed by legitimate users or installing fake records that would
 attract illegitimate traffic.  Means to protect the DDNS server
 against Denial of Service (DoS) should be enabled.  Note that these
 considerations are not specific to address-sharing contexts but are
 valid for DDNS services in general.

Deng, et al. Informational [Page 11] RFC 7393 PCP DDNS Updates November 2014

5. References

5.1. Normative References

 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005,
 [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
            Stack Lite Broadband Deployments Following IPv4
            Exhaustion", RFC 6333, August 2011,
 [RFC6887]  Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
            Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
            2013, <>.

5.2. Informative References

            Boucadair, M., "Port Control Protocol (PCP) Deployment
            Models", Work in Progress, draft-boucadair-pcp-deployment-
            cases-03, July 2014.
 [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
            "Dynamic Updates in the Domain Name System (DNS UPDATE)",
            RFC 2136, April 1997,
 [RFC2606]  Eastlake, D. and A. Panitz, "Reserved Top Level DNS
            Names", BCP 32, RFC 2606, June 1999,
 [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
            specifying the location of services (DNS SRV)", RFC 2782,
            February 2000, <>.
 [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
            Update", RFC 3007, November 2000,
 [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
            NAT64: Network Address and Protocol Translation from IPv6
            Clients to IPv4 Servers", RFC 6146, April 2011,

Deng, et al. Informational [Page 12] RFC 7393 PCP DDNS Updates November 2014

 [RFC6269]  Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
            Roberts, "Issues with IP Address Sharing", RFC 6269, June
            2011, <>.
 [RFC6281]  Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
            "Understanding Apple's Back to My Mac (BTMM) Service", RFC
            6281, June 2011, <>.
 [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
            Discovery", RFC 6763, February 2013,
 [RFC6888]  Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
            and H. Ashida, "Common Requirements for Carrier-Grade NATs
            (CGNs)", BCP 127, RFC 6888, April 2013,
 [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., and B. Haberman,
            "Special-Purpose IP Address Registries", BCP 153, RFC
            6890, April 2013,
 [RFC6908]  Lee, Y., Maglione, R., Williams, C., Jacquenet, C., and M.
            Boucadair, "Deployment Considerations for Dual-Stack
            Lite", RFC 6908, March 2013,


 Thanks to Stuart Cheshire for bringing up DNS-Based Service Discovery
 (SD) and [RFC6281], which covers a DNS-based SD scenario and gives an
 example of how the application is a means for a solution to address
 dynamic DNS updates; in this case, Apple's BTMM can be achieved.
 Many thanks to D. Wing, D. Thaler, and J. Abley for their comments.


 The following individuals contributed text to the document:
    Xiaohong Huang
    Beijing University of Posts and Telecommunications, China
    Yan Ma
    Beijing University of Posts and Telecommunications, China

Deng, et al. Informational [Page 13] RFC 7393 PCP DDNS Updates November 2014

Authors' Addresses

 Xiaohong Deng
 Mohamed Boucadair
 France Telecom
 Rennes  35000
 Qin Zhao
 Beijing University of Posts and Telecommunications
 James Huang
 Huawei Technologies
 Cathy Zhou
 Huawei Technologies

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