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Versions: 00 draft-ietf-tsvwg-behave-requirements-update

BEHAVE                                                          R. Penno
Internet-Draft                                                     Cisco
Intended status: Best Current Practice                      S. Perreault
Expires: December 05, 2013                                      Viagenie
                                                              S. Kamiset
                                                        Insieme Networks
                                                            M. Boucadair
                                                          France Telecom
                                                                K. Naito
                                                                     NTT
                                                           June 03, 2013


   Network Address Translation (NAT) Behavioral Requirements Updates
                draft-ietf-behave-requirements-update-00

Abstract

   This document clarifies and updates several requirements of RFC4787,
   RFC5382 and RFC5508 based on operational and development experience.
   The focus of this document is NAPT44.

Requirements Language

   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].

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 05, 2013.

Copyright Notice





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   Copyright (c) 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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  TCP Session Tracking  . . . . . . . . . . . . . . . . . . . .   3
     3.1.  TCP Transitory Connection Idle-Timeout  . . . . . . . . .   4
     3.2.  TIME_WAIT State . . . . . . . . . . . . . . . . . . . . .   4
       3.2.1.   Proposal: Apply RFC6191 and PAWS to NAT  . . . . . .   5
     3.3.   TCP RST  . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Port Overlapping behavior . . . . . . . . . . . . . . . . . .   8
   5.  Address Pooling Paired (APP)  . . . . . . . . . . . . . . . .   9
   6.  EIF Security  . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  EIF Protocol Independence . . . . . . . . . . . . . . . . . .   9
   8.  EIF Mapping Refresh . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Outbound Mapping Refresh and Error Packets  . . . . . . .  10
   9.  EIM Protocol Independence . . . . . . . . . . . . . . . . . .  10
   10. Port Parity . . . . . . . . . . . . . . . . . . . . . . . . .  10
   11. Port Randomization  . . . . . . . . . . . . . . . . . . . . .  10
   12. IP Identification (IP ID) . . . . . . . . . . . . . . . . . .  10
   13. ICMP Query Mappings Timeout . . . . . . . . . . . . . . . . .  11
   14. Hairpinning Support for ICMP Packets  . . . . . . . . . . . .  11
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   17. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     18.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13









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1.  Terminology

   The reader should be familiar with all terms defined in RFC2663
   [RFC2663],RFC4787 [RFC4787],RFC5382 [RFC5382],RFC5508 [RFC5508]

2.  Introduction

   [RFC4787], [RFC5382] and [RFC5508] greatly advanced NAT
   interoperability and conformance.  But with widespread deployment and
   evolution of NAT more development and operational experience was
   acquired some areas of the original documents need further
   clarification or updates.  This documents provides such
   clarifications and updates.

2.1.  Scope

   This document focuses solely on NAPT44 and its goal is to clarify,
   fill gaps or update requirements of [RFC4787], [RFC5382] and
   [RFC5508].  It is out of the scope of this document the creation of
   completely new requirements not associated with the documents cited
   above.  New requirements would be better served elsewhere and if they
   are CGN specific in an update to [RFC6888] [I-D.ietf-behave-lsn-
   requirements]

3.  TCP Session Tracking

   [RFC5382] specifies TCP timers associated with various connection
   states but does not specify the TCP state machine a NAPT44 should use
   as a basis to apply such timers.  The TCP state machine below,
   adapted from [RFC6146], provides guidance on how TCP session tracking
   could be implemented - it is non-normative.

                                      +-----------------------------+
                                      |                             |
                                      V                             |
                                +------+     CV4                    |
                                |CLOSED|-----SYN------+             |
                                +------+              |             |
                                   ^                  |             |
                                   |TCP_TRANS T.O.    |             |
                                   |                  V             |
                                +-------+          +-------+        |
                                | TRANS |          |V4 INIT|        |
                                +-------+          +-------+        |
                                 |    ^               |             |
                           data pkt   |               |             |
                                 |  V4 or V4 RST      |             |
                                 |  TCP_EST T.O.      |             |



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                                 V    |              SV4 SYN        |
                            +--------------+          |             |
                            | ESTABLISHED  |<---------+             |
                            +--------------+                        |
                              |           |                         |
                         CV4 FIN      SV4 FIN                       |
                              |           |                         |
                              V           V                         |
                      +---------+       +----------+                |
                      |CV4 FIN  |       | SV4 FIN  |                |
                      |   RCV   |       |    RCV   |                |
                      +---------+       +----------+                |
                              |           |                         |
                         SV4 FIN      CV4 FIN                  TCP_TRANS
                              |           |                        T.O.
                              V           V                         |
                        +----------------------+                    |
                        | CV4 FIN + SV4 FIN RCV|--------------------+
                        +----------------------+

   (postamble)


3.1.  TCP Transitory Connection Idle-Timeout

   [RFC5382]:REQ-5 The transitory connection idle-timeout is defined as
   the minimum time a TCP connection in the partially open or closing
   phases must remain idle before the NAT considers the associated
   session a candidate for removal.  But the document does not clearly
   states if these can be configured separately.  This document
   clarifies that a NAT device SHOULD provide different knobs for
   configuring the open and closing idle timeouts.  This document
   further acknowledges that most TCP flows are very short (less than 10
   seconds) [FLOWRATE][TCPWILD] and therefore a partially open timeout
   of 4 minutes might be excessive if security is a concern.  Therefore
   it MAY be configured to be less than 4 minutes in such cases.  There
   also may be cases that a timeout of 4 minutes might be excessive.
   The case and the solution are written below.

3.2.  TIME_WAIT State

   The TCP TIME_WAIT state is described in [RFC0793].  The TCP TIME_WAIT
   state needs to be kept for 2MSL before a connection is CLOSED, for
   the reasons below.







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   1: In the event that packets from a session are delayed in the in-
      between network, and delivered to the end relatively later, we
      should prevent the packets from being transferred and interpreted
      as a packet that belongs to a new session.

   2: If the remote TCP has not received the acknowledgment of its
      connection termination request, it will re-send the FIN packet
      several times.

   These points are important for the TCP to work without problems.

   [RFC5283] leaves the handling of TCP connections in TIME_WAIT state
   unspecified and mentions that TIME_WAIT state is not part of the
   transitory connection idle-timeout.  If the NAT device honors the
   TIME_WAIT state, each TCP connection and its assocaited resources is
   kept for a certain period, typically for four minutes, which consumes
   port resources.

   [RFC6191] explains that in certain situation it is necessary to
   reduce the TIME_WAIT state and defines such a mechanism using TCP
   timestamps and sequence numbers.  When a connection request is
   received with a four-tuple that is in the TIME-WAIT state, the
   connection request may be accepted if the sequence number or the
   timestamp of the incoming SYN segment is greater than the last
   sequence number seen on the previous incarnation of the connection.

   [N.E] This document specifies that a NAT device should keep TCP
   connections in TIME_WAIT state unless it implements the proposal
   below?

3.2.1.  Proposal: Apply RFC6191 and PAWS to NAT

   This section proposes to apply [RFC6191] mechanism at NAT.  This
   mechanism MAY be adopted for both clients' and remote hosts' TCP
   active close.


            client                     NAT                  remote host
              |                         |                         |
              |          FIN            |          FIN            |
              |------------------------>|------------------------>|
              |                         |                         |
              |          ACK            |          ACK            |
              |<------------------------|<------------------------|
              |          FIN            |          FIN            |
              |<------------------------|<------------------------|
              |                         |                         |
              |        ACK(TSval=A)     |          ACK            |



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              |------------------------>|------------------------>|
              |                         |  -                      |
              |                         |  |                      |
              |                         |  |                      |
              |                         |  |                      |
              |                         |  | TIME_WAIT            |
              |                         |  |  ->assassinated at x |
              |                         |  |                      |
              |                         |  |                      |
              |                         |  |                      |
              |        SYN(TSval>A)     |  x      SYN             |
              |------------------------>|------------------------>|
              |                         |  -                      |
              |                         |  |                      |
              |                         |  | SYN_SENT             |
              |                         |  |                      |
              |                         |  |                      |

   (postamble)


   Also, PAWS works to discard old duplicate packets at NAT.  A packet
   can be discarded as an old duplicate if it is received with a
   timestamp or sequence number value less than a value recently
   received on the connection.

   To make these mechanisms work, we should concern the case that there
   are several clients with nonsuccessive timestamp or sequence number
   values are connected to a NAT device (i.e.  not monotonically
   increasing among clients).  Two mechanisms to solve this mechanism
   and applying [RFC6191] and PAWS to NAT are described below.  These
   mechanisms are optional.

3.2.1.1.  Rewrite timestamp and sequence number values at NAT

   Rewrite timestamp and sequence number values of outgoings packets at
   NAT to be monotonically increasing.  This can be done by adopting
   following mechanisms at NAT.


   A: Store the newest rewritten value of timestamp and sequence number
      as the "max value at the time".

   B: NAT rewrite timestamp and sequence number values of incoming
      packets to be monotonically increasing.






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   When packets come back as replies from remote hosts, NAT rewrite
   again the timestamp and sequence number values to be the original
   values.  This can be done by adopting following mechanisms at NAT.


   C: Store the values of original timestamp and sequence number of
      packets, and rewritten values of those.

3.2.1.2.  Split an assignable number of port space to each client

   Adopt following mechanisms at NAT.


   A: Choose clients that can be assigned ports.

   B: Split assignable port numbers between clients.

   Packets from other clients which are not chosen by these mechanisms
   are rejected at NAT, unless there is unassigned port left.

3.2.1.3.  Resend the last ACK to the resended FIN

   We should concern another case to make RFC6191 work at NAT.  In case
   the remote TCP could not receive the acknowledgment of its connection
   termination request, the NAT device, on behalf of clients, resends
   the last ACK packet when it receives an FIN packet of the previous
   connection, and when the state of the previous connection is deleted
   from the NAT.  This mechanism MAY be used when clients starts closing
   process, and the remote host could not receive the last ACK.

3.2.1.4.  Remote host behavior of several implementations

   To solve the port shortage problem on the client side, the behavior
   of remote host should be compliant to [RFC6191] or the mechanism
   written in 4.2.2.13 of [RFC1122], since NAT may reuse the same 5
   tuple for a new connection.  We have investigated behaviors of OSes
   (e.g., Linux, FreeBSD, Windows, MacOS), and found that they
   implemented the server side behavior of the above two.

3.3.  TCP RST











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   [RFC5382] leaves the handling of TCP RST packets unspecified.  This
   document does not try standardize such behavior but clarifies based
   on operational experience that a NAT that receives a TCP RST for an
   active mapping and performs session tracking MAY immediately delete
   the sessions and remove any state associated with it.  If the NAT
   device that performs TCP session tracking receives a TCP RST for the
   first session that created a mapping, it MAY remove the session and
   the mapping immediately.

4.  Port Overlapping behavior

   [RFC4787] [RFC5382]: REQ-1 Current RFCs specifiy a specific port
   overlapping behavior, i.e., that the external IP:port can be reused
   for connections originating from the same internal source IP:port
   irrespective of the detination.  This is known as endpoint-
   independent mapping.  This document clarifies that this port
   overlapping behavior can be extended to connections originating from
   different interal source IP:ports as long as their destinations are
   different.  This known as EDM (Endpoint Dependent Mapping).  The
   mechanism below MAY be one optional implement to NAT.

   If destination addresses and ports are different for outgoing
   connections started by local clients, NAT MAY assign the same
   external port as the source ports for the connections.  The port
   overlapping mechanism manages mappings between external packets and
   internal packets by looking at and storing their 5-tuple (protocol,
   source address, source port, destination address, destination port) .
   This enables concurrent use of a single NAT external port for
   multiple transport sessions, which enables NAT to work correctly in
   IP address resource limited network.

   Discussions:

   [RFC4787] and [RFC5382] requires "endpoint-independent mapping" at
   NAT, and port overlapping NAT cannot meet the requirement.  This
   mechanism can degrade the transparency of NAT in that its mapping
   mechanism is endpoint-dependent and makes NAT traversal harder.
   However, if a NAT adopts endpoint-independent mapping together with
   endpoint-dependent filtering, then the actual behavior of the NAT
   will be the same as port overlapping NAT.  It should also be noted
   that a lot of existing NAT devices(e.g., SEIL, FITELnet Series)
   adopted this port overlapping mechanism.


   A: Reference URL for SEIL -> www.seil.jp

   B: Reference URL for FITELnet -> www.furukawa.co.jp/fitelnet




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   The netfilter, which is a popular packet filtering mechanism for
   Linux, also adopts port overlapping behavior.

5.  Address Pooling Paired (APP)

   [RFC4787]: REQ-2 [RFC5382]:ND Address Pooling Paired behavior for NAT
   is recommended in previous documents but behavior when a public IPv4
   run out of ports is left undefined.  This document clarifies that if
   APP is enabled new sessions from a subscriber that already has a
   mapping associated with a public IP that ran out of ports SHOULD be
   dropped.  The administrator MAY provide a knob that allows a NAT
   device to starting using ports from another public IP when the one
   that anchored the APP mapping ran out of ports.  This is trade-off
   between subscriber service continuity and APP strict enforcement.
   (NE: It is sometimes referred as 'soft-APP')

6.  EIF Security

   [RFC4787]:REQ-8 and [RFC5382]:REQ-3 End-point independent filtering
   could potentially result in security attacks from the public realm.
   In order to handle this, when possible there MUST be strict filtering
   checks in the inbound direction.  A knob SHOULD be provided to limit
   the number of inbound sessions and a knob SHOULD be provided to
   enable or disable EIF on a per application basis.  This is specially
   important in the case of Mobile networks where such attacks can
   consume radio resources and count against the user quota.

7.  EIF Protocol Independence

   [RFC4787]:REQ-8 and[RFC5382]: REQ-3 Current RFCs do not specify
   whether EIF mappings are protocol independent.  In other words, if an
   outbound TCP SYN creates a mapping, it is left undefined whether
   inbound UDP packets destined to that mapping should be forwarded.
   This document specifies that EIF mappings SHOULD be protocol
   independent in order allow inbound packets for protocols that
   multiplex TCP and UDP over the same IP: port through the NAT and also
   maintain compatibility with stateful NAT64 RFC6146 [RFC6146].  But
   the administrator MAY provide a configuration knob to make it
   protocol dependent.

8.  EIF Mapping Refresh

   [RFC4787]: REQ-6 [RFC5382]: ND The NAT mapping Refresh direction MAY
   have a "NAT Inbound refresh behavior" of "True" but it does not
   clarifies how this applies to EIF mappings.  The issue in question is
   whether inbound packets that match an EIF mapping but do not create a
   new session due to a security policy should refresh the mapping
   timer.  This document clarifies that even when a NAT device has a



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   inbound refresh behavior of TRUE, such packets SHOULD NOT refresh the
   mapping.  Otherwise a simple attack of a packet every 2 minutes can
   keep the mapping indefinitely.

8.1.  Outbound Mapping Refresh and Error Packets

   In the case of NAT outbound refresh behavior there are certain types
   of packets that should not refresh the mapping even if their
   direction is outbound.  For example, if the mapping is kept alive by
   ICMP Errors or TCP RST outbound packets sent as response to inbound
   packets, these SHOULD NOT refresh the mapping.

9.  EIM Protocol Independence

   [RFC4787] [RFC5382]: REQ-1 Current RFCs do not specify whether EIM
   are protocol independent.  In other words, if a outbound TCP SYN
   creates a mapping it is left undefined whether outbound UDP can reuse
   such mapping and create session.  On the other hand, Stateful NAT64
   [RFC6146] clearly specifies three binding information bases (TCP,
   UDP, ICMP).  This document clarifies that EIM mappings SHOULD be
   protocol dependent . A knob MAY be provided in order allow protocols
   that multiplex TCP and UDP over the same source IP and port to use a
   single mapping.

10.  Port Parity

   A NAT devices MAY disable port parity preservation for dynamic
   mappings.  Nevertheless, A NAT SHOULD support means to explicitly
   request to preserve port parity (e.g., [I-D.pcp-port-set]).

11.  Port Randomization

   A NAT SHOULD follow the recommendations specified in Section 4 of
   [RFC6056] especially: "A NAPT that does not implement port
   preservation [RFC4787] [RFC5382] SHOULD obfuscate selection of the
   ephemeral port of a packet when it is changed during translation of
   that packet.  A NAPT that does implement port preservation SHOULD
   obfuscate the ephemeral port of a packet only if the port must be
   changed as a result of the port being already in use for some other
   session.  A NAPT that performs parity preservation and that must
   change the ephemeral port during translation of a packet SHOULD
   obfuscate the ephemeral ports.  The algorithms described in this
   document could be easily adapted such that the parity is preserved
   (i.e., force the lowest order bit of the resulting port number to 0
   or 1 according to whether even or odd parity is desired)."

12.  IP Identification (IP ID)




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   A NAT SHOULD handle the Identification field of translated IPv4
   packets as specified in Section 9 of [I-D.ietf-intarea-ipv4-id-
   update].

13.  ICMP Query Mappings Timeout

   Section 3.1 of [RFC5508] says that ICMP Query Mappings are to be
   maintained by NAT device.  However, RFC doesn't discuss about the
   Query Mapping timeout values.  Section 3.2 of that RFC only discusses
   about ICMP Query Session Timeouts.  ICMP Query Mappings MAY be
   deleted once the last the session using the mapping is deleted.

14.  Hairpinning Support for ICMP Packets

   [RFC5508]:REQ-7 This requirement specifies that NAT devices enforcing
   Basic NAT MUST support traversal of hairpinned ICMP Query sessions.
   This implicitly means that address mappings from external address to
   internal address (similar to Endpoint Independent Filters) MUST be
   maintained to allow inbound ICMP Query sessions.  If an ICMP Query is
   received on an external address, NAT device can then translate to an
   internal IP.  [RFC5508]:REQ-7 This requirement specifies that all NAT
   devices (i.e., Basic NAT as well as NAPT devices) MUST support the
   traversal of hairpinned ICMP Error messages.  This too requires NAT
   devices to maintain address mappings from external IP address to
   internal IP address in addition to the ICMP Query Mappings described
   in section 3.1 of that RFC.

15.  IANA Considerations

   TBD

16.  Security Considerations

   In the case of EIF mappings due to high risk of resource crunch, a
   NAT device MAY provide a knob to limit the number of inbound sessions
   spawned from a EIF mapping.

   [TCP-Security] contains a detailed discussion of the security
   implications of TCP Timestamps and of different timestamp generation
   algorithms.

17.  Acknowledgements

   Thanks to Dan Wing, Suresh Kumar, Mayuresh Bakshi, Rajesh Mohan and
   Senthil Sivamular for review and discussions

18.  References




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18.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

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

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations", RFC
              2663, August 1999.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605, October
              2003.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 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.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056, January
              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.

   [RFC6191]  Gont, F., "Reducing the TIME-WAIT State Using TCP
              Timestamps", BCP 159, RFC 6191, April 2011.

   [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.



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18.2.  Informative References

   [FLOWRATE]
              Zhang, Y., Breslau, L., Paxson, V., and S. Shenker, "On
              the Characteristics and Origins of Internet Flow Rates", .

   [I-D.ietf-pcp-port-set]
              Sun, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou, T.,
              and S. Perreault, "Port Control Protocol (PCP) Extension
              for Port Set Allocation", draft-ietf-pcp-port-set-01 (work
              in progress), May 2013.

   [I-D.naito-nat-resource-optimizing-extension]
              Kengo, K. and A. Matsumoto, "NAT TIME_WAIT reduction",
              draft-naito-nat-resource-optimizing-extension-02 (work in
              progress), July 2012.

   [TCPWILD]  Qian, F., Subhabrata, S., Spatscheck, O., Morley Mao, Z.,
              and W. Willinger, "TCP Revisited: A Fresh Look at TCP in
              the Wild", .

Authors' Addresses

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

   Email: repenno@cisco.com


   Simon Perreault
   Viagenie
   2875 boul. Laurier, suite D2-630
   Quebec, QC  G1V 2M2
   Canada

   Email: simon.perreault@viagenie.ca


   Sarat Kamiset
   Insieme Networks
   California







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   Mohamed Boucadair
   France Telecom
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com


   Kengo Naito
   NTT
   Tokyo
   Japan

   Email: kengo@lab.ntt.co.jp




































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