Analysis of Stateful 64 Translation

Abstract

Due to specific problems, Network Address Translation - Protocol Translation (NAT-PT) was deprecated by the IETF as a mechanism to perform IPv6-IPv4 translation. Since then, new efforts have been undertaken within IETF to standardize alternative mechanisms to perform IPv6-IPv4 translation. This document analyzes to what extent the new stateful translation mechanisms avoid the problems that caused the IETF to deprecate NAT-PT.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are 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 http://www.rfc-editor.org/info/rfc6889.

Copyright Notice

Copyright © 2013 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 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 . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.2.  Context  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Analysis of 64 Translation against Concerns of RFC 4966  . . .  4
     2.1.  Problems Impossible to Solve . . . . . . . . . . . . . . .  4
     2.2.  Problems That Can Be Solved  . . . . . . . . . . . . . . .  5
     2.3.  Problems Solved  . . . . . . . . . . . . . . . . . . . . .  7
   3.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 13

1. Introduction

1.1. Definition

This document uses stateful 64 (or 64 for short) to refer to the mechanisms defined in the following documents:

   o  IP/ICMP Translation Algorithm [RFC6145]

• Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers [RFC6146]

• DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers [RFC6147]

   o  IPv6 Addressing of IPv4/IPv6 Translators [RFC6052]
   
   o  Framework for IPv4/IPv6 Translation [RFC6144]

1.2. Context

Stateful 64 is widely seen as a major interconnection technique designed to enable communications between IPv6-only and IPv4-only networks. One of the building blocks of the stateful 64 is decoupling the DNS functionality from the protocol translation itself.

This approach is pragmatic in the sense that there is no dependency on DNS implementation for the successful NAT handling. As long as there is a function (e.g., DNS64 [RFC6147] or other means) that can construct an IPv6-embedded IPv4 address with a pre-configured IPv6 prefix, an IPv4 address and a suffix (refer to [RFC6052]), NAT64 will work just fine.

The focus of the stateful 64 is on the deployment and not the implementation details. As long as a NAT64 implementation conforms to the expected behavior, as desired in the deployment scenario, the details are not very important as mentioned in this excerpt from [RFC6146]:

A NAT64 MAY perform the steps in a different order, or MAY perform different steps, but the externally visible outcome MUST be the same as the one described in this document.

1.3. Scope

This document provides an analysis of how the proposed set of documents that specify stateful IPv6-only to IPv4-only translation and replace Network Address Translation - Protocol Translation (NAT-PT) [RFC2766] address the issues raised in [RFC4966].

As a reminder, it is worth mentioning the analysis is limited in the sense that hosts from IPv6 networks can initiate a communication to IPv4 network/Internet, but not vice versa. This corresponds to Scenarios 1 and 5 described in [RFC6144]. Hence, the scenario of servers moving to IPv6 while clients remaining IPv4 remains unaddressed. Of course, IPv6-to-IPv4 communications can also be supported if static or explicit bindings (e.g., [RFC6887]) are configured on the stateful NAT64.

Stateful 64, just like any other technique under development, has some positives and some drawbacks. The ups and downs of the proposal must be clearly understood while going forward with its future development.

The scope of this document does not include stateless translation.

2. Analysis of 64 Translation against Concerns of RFC 4966

Of the set of problems pointed out in [RFC4966], the stateful 64 addresses some of them, whereas it leaves others unaddressed.

Some issues mentioned in [RFC4966] were solved by [RFC4787], [RFC5382], and [RFC5508]. At the time when NAT-PT was published, these recommendations were not in place but they are orthogonal to the translation algorithm per se; therefore, they could be implemented with NAT-PT. On the other hand, NAT64 [RFC6146] explicitly mentions that these recommendations need to be followed and thus should be seen as a complete specification.

It is also worth pointing out that the scope of the stateful 64 is reduced when compared to NAT-PT. Following is a point-by-point analysis of the problems. This document classifies the issues listed in [RFC4966] into three categories:

1. Problems impossible to solve.

2. Problems that can be solved.

3. Problems solved.

2.1. Problems Impossible to Solve

Problems discussed in [RFC4966] that are impossible to solve:

1. Inability to redirect traffic for protocols that lack de- multiplexing capabilities or are not built on top of specific transport-layer protocols for transport address translations (Section 2.2 of [RFC4966]).

Analysis: This issue is not specific to 64 but to all NAT- based solutions.

2. Loss of information due to incompatible semantics between IPv4 and IPv6 versions of headers and protocols (Section 2.4 of [RFC4966]).

Analysis: This issue is not specific to 64 but is due to the design of IPv4 and IPv6.

3. Need for the NAT64-capable device to act as proxy for correspondent node when IPv6 node is mobile, with consequent restrictions on mobility (Section 2.7 of [RFC4966]).

Analysis: This is not specific to NAT64 but to all NAT flavors. Refer to [NAT64-HARMFUL] for an early analysis on mobility complications encountered when NAT64 is involved.

2.2. Problems That Can Be Solved

Problems discussed in [RFC4966] that can be solved:

1. Disruption of all protocols that embed IP addresses (and/or ports) in packet payloads or apply integrity mechanisms using IP addresses (and ports) (Section 2.1 of [RFC4966]).

          Analysis: In the case of FTP [RFC0959], this problem can be
          mitigated in several ways (e.g., use a FTP64 Application Layer
          Gateway (ALG) [RFC6384] or in the FTP client (e.g., [FTP64])).

In the case of SIP [RFC3261], no specific issue is induced by 64; the same techniques for NAT traversal can be used when a NAT64 is involved in the path (e.g., Interactive Connectivity Establishment (ICE) [RFC5245], maintain SIP-related NAT bindings as per Section 3.4 of [RFC5853], media latching [MIDDLEBOXES], embedded SIP ALGs, etc.). [RFC6157] provides more discussion on how to establish SIP sessions between IPv4 and IPv6 SIP user agents.

The functioning of other protocols is left for future study. Note that the traversal of NAT64 by application embedding IP address literal is not specific to NAT64 but generic to all NAT-based solutions.

2. Interaction with Stream Control Transmission Protocol (SCTP) [RFC4960] and multihoming (Section 2.6 of [RFC4966]).

Analysis: Only TCP and UDP transport protocols are within the scope of NAT64 [RFC6146]. SCTP is out of scope of this document.

3. Inability to handle multicast traffic (Section 2.8 of [RFC4966]).

Analysis: This problem is not addressed by the current 64 specifications.

4. Scalability concerns together with introduction of a single point of failure and a security attack nexus (Section 3.2 of [RFC4966]).

Analysis: This is not specific to NAT64 but to all stateful NAT flavors. The presence of a single point of failure is deployment-specific; some service providers may deploy state synchronization means while others may only rely on a distributed NAT64 model.

5. Restricted validity of translated DNS records: a translated record may be forwarded to an application that cannot use it (Section 4.2 of [RFC4966]).

Analysis: If a node on the IPv4 side forwards the address of the other endpoint to a node that cannot reach the NAT box or is not covered under the endpoint-independent constraint of NAT, then the new node will not be able to initiate a successful session.

Actually, this is not a limitation of 64 (or even NAT-PT) but a deployment context where IPv4 addresses managed by the NAT64 are not globally reachable. The same limitation can be encountered when referrals (even without any NAT in the path) include reachability information with limited reachability scope (see [REFERRAL] for more discussion about issues related to reachability scope).

6. IPsec traffic using AH (Authentication Header) [RFC4302] in both transport and tunnel modes cannot be carried through NAT-PT without terminating the security associations on the NAT-PT, due to the inclusion of IP header fields in the scope of AH's cryptographic integrity protection [RFC3715] (Section 2.1 of [RFC4966]). In addition, IPsec traffic using ESP (Encapsulating Security Payload) [RFC4303] in transport mode generally uses UDP encapsulation [RFC3948] for NAT traversal (including NAT-PT traversal) in order to avoid the problems described in [RFC3715] (Section 2.1 of [RFC4966]).

Analysis: This is not specific to NAT64 but to all NAT flavors.

7. Address selection issues when either the internal or external hosts implement both IPv4 and IPv6 (Section 4.1 of [RFC4966]).

Analysis: This is out of scope of 64 since Scenarios 1 and 5 of [RFC6144] assume IPv6-only hosts.

Therefore, this issue is not resolved and mitigation techniques outside the 64 need to be used (e.g., [ADDR-SELECT]). These techniques may allow one to offload NAT64 resources and prefer native communications that do not involve address family translation. Avoiding NAT devices in the path is encouraged for mobile nodes in order to save power consumption due to keepalive messages that are required to maintain NAT states ("always-on" services). An in-depth discussion can be found in [DNS64].

2.3. Problems Solved

Problems identified in [RFC4966] that have been solved:

1. Constraints on network topology (as it relates to DNS-ALG; see Section 3.1 of [RFC4966]).

Analysis: The severity of this issue has been mitigated by the separation of the DNS from the NAT functionality. Nevertheless, a minimal coordination may be required to ensure that the NAT64 to be crossed (the one to which the IPv4- Converted IPv6 address returned to a requesting host) must be in the path and has also sufficient resources to handle received traffic.

2. Need for additional state and/or packet reconstruction in dealing with packet fragmentation. Otherwise, implement no support for fragments (Section 2.5 of [RFC4966]).

Analysis: This issue is not specific to 64 but to all NAT- based solutions. [RFC6146] specifies how to handle fragmentation; appropriate recommendations to avoid fragmentation-related DoS (Denial-of-Service) attacks are proposed (e.g., limit resources to be dedicated to out-of- order fragments).

3. Inappropriate translation of responses to A queries from IPv6 nodes (Section 4.3 of [RFC4966]).

Analysis: DNS64 [RFC6147] does not alter A queries.

4. Address selection issues and resource consumption in a DNS-ALG with multi-addressed nodes (Section 4.4 of [RFC4966]).

Analysis: Since no DNS-ALG is required to be co-located with NAT64, there is no need to maintain temporary states in anticipation of connections. Note that explicit bindings (see Section 3 of [RFC6887]) are required to allow for communications initiated from an IPv4-only client to an IPv6- only server.

5. Limitations on DNS security capabilities when using a DNS-ALG (Section 2.5 of [RFC4966]).

Analysis: A DNSSEC validating stub resolver behind a DNS64 in server mode is not supported. Therefore, if a host wants to do its own DNSSEC validation, and it wants to use a NAT64, the host has to also perform its own DNS64 synthesis. Refer to Section 3 of [RFC6147] for more details.

6. Creation of a DoS threat relating to exhaustion of memory and address/port pool resources on the translator (Section 3.4 of [RFC4966]).

Analysis: This specific DoS concern on Page 6 of [RFC4966] is under a DNS-ALG heading in that document, and refers to NAT- PT's creation of NAT mapping state when a DNS query occurred. With the new IPv6-IPv4 translation mechanisms, DNS queries do not create any mapping state in the NAT64.

To mitigate the exhaustion of port pool issue (Section 3.4 of [RFC4966]), 64 must enforce a port limit similar to the one defined in [RFC6888].

Thus, this concern can be fully eliminated in 64.

7. Requirement for applications to use keepalive mechanisms to work around connectivity issues caused by premature timeout for session table and Binding Information Base entries (Section 2.3 of [RFC4966]).

Analysis: Since NAT64 follows some of the [RFC4787], [RFC5382], and [RFC5508] requirements, there is a high lower bound for the lifetime of sessions. In NAT-PT, this was unknown and applications needed to assume the worst case. For instance, in NAT64, the lifetime for a TCP session is approximately two hours, so not much keepalive signaling overhead is needed.

Application clients (e.g., VPN clients) are not aware of the timer configured in the NAT device. For unmanaged services, a conservative approach would be adopted by applications that issue frequent keepalive messages to be sure that an active mapping is still maintained by any involved NAT64 device even if the NAT64 complies with [RFC4787], [RFC5382], and [RFC5508].

Note that keepalive messages may be issued by applications to ensure that an active entry is maintained by a firewall, with or without a NAT in the path, which is located in the boundaries of a local domain.

8. Lack of address mapping persistence: Some applications require address retention between sessions. The user traffic will be disrupted if a different mapping is used. The use of the DNS-ALG to create address mappings with limited lifetimes means that applications must start using the address shortly after the mapping is created, as well as keep it alive once they start using it (Section 3.3 of [RFC4966]).

Analysis: In the following, address persistence is used to refer to the support of "IP address pooling" behavior of "Paired" [RFC4787].

In the context of 64, the external IPv4 address (representing the IPv6 host in the IPv4 network) is assigned by the NAT64 machinery and not the DNS64 function. Therefore, address persistence can be easily ensured by the NAT64 function (which complies with NAT recommendations [RFC4787] and [RFC5382]). Address persistence should be guaranteed for both dynamic and static bindings.

In the IPv6 side of the NAT64, the same IPv6 address is used to represent an IPv4 host; no issue about address persistence is raised in an IPv6 network.

3. Conclusions

The above analysis of the solutions provided by the stateful 64 shows that the majority of the problems that are not directly related to the decoupling of NAT and DNS remain unaddressed. Some of these problems are not specific to 64 but are generic to all NAT-based solutions.

This points to several shortcomings of stateful 64 that must be addressed if the future network deployments have to move reliably towards 64 as a solution to IPv6-IPv4 interconnection.

Some of the issues, as pointed out in [RFC4966], have possible solutions. However these solutions will require significant updates to the stateful 64, increasing its complexity.

The following table summarizes the conclusions based on the analysis of stateful 64.

   +---------------+----------+---------+----------+---------+---------+
   |     Issue     |  NAT-PT  |  Exists |  DNS ALG | Generic |  Can be |
   |               | Specific |    in   | Specific |   NAT   | solved? |
   |               |          |  NAT64  |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |   Protocols   |    No    |   Yes   |    No    |   Yes   |   Yes   |
   |   embedding   |          |         |          |         |         |
   |   addresses   |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |   Protocols   |    No    |   Yes   |    No    |   Yes   |    No   |
   | without demux |          |         |          |         |         |
   |   capability  |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   | Binding state |    No    |   Yes   |    No    |   Yes   |   Yes   |
   |     decay     |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |    Loss of    |    No    |   Yes   |    No    |    No   |    No   |
   |  information  |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   | Fragmentation |    No    |    No   |    No    |   Yes   |   Yes   |
   +---------------+----------+---------+----------+---------+---------+
   |    SCTP and   |    No    |   Yes   |    No    |   Yes   |   Yes   |
   |  Multihoming  |          |         |          |         |         |
   |  interaction  |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |     Proxy     |    No    |   Yes   |    No    |    No   |    No   |
   | correspondent |          |         |          |         |         |
   |    node for   |          |         |          |         |         |
   |     MIPv6     |          |         |          |         |         |
   |   Multicast   |    No    |   Yes   |    No    |   Yes   |   Yes   |
   +---------------+----------+---------+----------+---------+---------+
   |  IPsec tunnel |    No    |   Yes   |    No    |   Yes   |   Yes   |
   |      mode     |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |    Topology   |    Yes   |    No   |    Yes   |    No   |   Yes   |
   |  constraints  |          |         |          |         |         |
   |  with DNS-ALG |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |   Scale and   |    No    |   Yes   |    No    |   Yes   |   Yes   |
   |  Single point |          |         |          |         |         |
   |   of failure  |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |    Lack of    |    No    |   Yes   |    No    |   Yes   |   Yes   |
   |    address    |          |         |          |         |         |
   |  persistence  |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |  DoS attacks  |    No    |   Yes   |    No    |   Yes   |   Yes   |
   +---------------+----------+---------+----------+---------+---------+
   |    Address    |    Yes   |    No   |    Yes   |    No   |   Yes   |
   |   selection   |          |         |          |         |         |
   |  issues with  |          |         |          |         |         |
   |   Dual stack  |          |         |          |         |         |
   |     hosts     |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |   Non-global  |    Yes   |    No   |    Yes   |    No   |   Yes   |
   |  validity of  |          |         |          |         |         |
   | Translated RR |          |         |          |         |         |
   |    records    |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |   Incorrect   |    Yes   |    No   |    Yes   |    No   |   Yes   |
   |  translation  |          |         |          |         |         |
   |      of A     |          |         |          |         |         |
   |   responses   |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |  DNS-ALG and  |    No    |   Yes   |    No    |   Yes   |   Yes   |
   |     Multi-    |          |         |          |         |         |
   |   addressed   |          |         |          |         |         |
   |     nodes     |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+
   |     DNSSEC    |    No    |   Yes   |    No    |   Yes   |   Yes   |
   |  limitations  |          |         |          |         |         |
   +---------------+----------+---------+----------+---------+---------+

Table 1: Summary of NAT64 analysis

4. Security Considerations

This document does not specify any new protocol or architecture. It only analyzes how BEHAVE WG 64 documents mitigate concerns raised in [RFC4966] and which ones are still unaddressed.

5. Acknowledgements

Many thanks to M. Bagnulo, D. Wing, X. Li, D. Anipko, and S. Moonesamy for their review and comments.

D. Black provided the IPsec text.

6. References

6.1. Normative References

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.
   
   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.
   
   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.
   
   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.
   
   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.
   
   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.
   
   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.
   
   [RFC4966]  Aoun, C. and E. Davies, "Reasons to Move the Network
              Address Translator - Protocol Translator (NAT-PT) to
              Historic Status", RFC 4966, July 2007.
   
   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.
   
   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              April 2009.
   
   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010.
   
   [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation", RFC 6144, April 2011.
   
   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.
   
   [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.
   
   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              April 2011.
   
   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
              April 2013.

6.2. Informative References

[ADDR-SELECT]

              Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
              Address Selection Policy using DHCPv6", Work in Progress,
              April 2013.
   
   [DNS64]    Wing, D., "IPv6-only and Dual Stack Hosts on the Same
              Network with DNS64", Work in Progress, February 2011.
   
   [FTP64]    Liu, D., Beijnum, I., and Z. Cao, "FTP consideration for
              IPv4/IPv6 transition", Work in Progress, January 2012.

[MIDDLEBOXES]

              Stucker, B., Tschofenig, H., and G. Salgueiro, "Analysis
              of Middlebox Interactions for Signaling Protocol
              Communication along the Media Path", Work in Progress,
              January 2013.

[NAT64-HARMFUL]

              Haddad, W. and C. Perkins, "A Note on NAT64 Interaction
              with Mobile IPv6", Work in Progress, March 2011.
   
   [REFERRAL] Carpenter, B., Boucadair, M., Halpern, J., Jiang, S., and
              K. Moore, "A Generic Referral Object for Internet
              Entities", Work in Progress, October 2009.
   
   [RFC2766]  Tsirtsis, G. and P. Srisuresh, "Network Address
              Translation - Protocol Translation (NAT-PT)", RFC 2766,
              February 2000.
   
   [RFC3715]  Aboba, B. and W. Dixon, "IPsec-Network Address Translation
              (NAT) Compatibility Requirements", RFC 3715, March 2004.
   
   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.
   
   [RFC5853]  Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,
              A., and M. Bhatia, "Requirements from Session Initiation
              Protocol (SIP) Session Border Control (SBC) Deployments",
              RFC 5853, April 2010.
   
   [RFC6157]  Camarillo, G., El Malki, K., and V. Gurbani, "IPv6
              Transition in the Session Initiation Protocol (SIP)",
              RFC 6157, April 2011.
   
   [RFC6384]  van Beijnum, I., "An FTP Application Layer Gateway (ALG)
              for IPv6-to-IPv4 Translation", RFC 6384, October 2011.
   
   [RFC6888]  Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
              A., and H. Ashida, "Common Requirements for Carrier-Grade
              NATs (CGNs)", BCP 127, RFC 6888, April 2013.

Authors' Addresses

   Reinaldo Penno
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134
   USA

EMail:

          repenno@cisco.com
   
   Tarun Saxena
   Cisco Systems
   Cessna Business Park
   Bangalore  560103
   India

EMail:

          tasaxena@cisco.com
   
   Mohamed Boucadair
   France Telecom
   Rennes  35000
   France
   
   EMail: mohamed.boucadair@orange.com
   
   Senthil Sivakumar
   Cisco Systems
   7100-8 Kit Creek Road
   Research Triangle Park, North Carolina  27709
   USA

EMail:

          ssenthil@cisco.com