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rfc:rfc3948

Network Working Group A. Huttunen Request for Comments: 3948 F-Secure Corporation Category: Standards Track B. Swander

                                                             Microsoft
                                                              V. Volpe
                                                         Cisco Systems
                                                            L. DiBurro
                                                       Nortel Networks
                                                           M. Stenberg
                                                          January 2005
               UDP Encapsulation of IPsec ESP Packets

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This protocol specification defines methods to encapsulate and
 decapsulate IP Encapsulating Security Payload (ESP) packets inside
 UDP packets for traversing Network Address Translators.  ESP
 encapsulation, as defined in this document, can be used in both IPv4
 and IPv6 scenarios.  Whenever negotiated, encapsulation is used with
 Internet Key Exchange (IKE).

Huttunen, et al. Standards Track [Page 1] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Packet Formats . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  UDP-Encapsulated ESP Header Format . . . . . . . . . . .  3
     2.2.  IKE Header Format for Port 4500  . . . . . . . . . . . .  4
     2.3.  NAT-Keepalive Packet Format  . . . . . . . . . . . . . .  4
 3.  Encapsulation and Decapsulation Procedures . . . . . . . . . .  5
     3.1.  Auxiliary Procedures . . . . . . . . . . . . . . . . . .  5
           3.1.1.  Tunnel Mode Decapsulation NAT Procedure  . . . .  5
           3.1.2.  Transport Mode Decapsulation NAT Procedure . . .  5
     3.2.  Transport Mode ESP Encapsulation . . . . . . . . . . . .  6
     3.3.  Transport Mode ESP Decapsulation . . . . . . . . . . . .  6
     3.4.  Tunnel Mode ESP Encapsulation  . . . . . . . . . . . . .  7
     3.5.  Tunnel Mode ESP Decapsulation  . . . . . . . . . . . . .  7
 4.  NAT Keepalive Procedure  . . . . . . . . . . . . . . . . . . .  7
 5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     5.1.  Tunnel Mode Conflict . . . . . . . . . . . . . . . . . .  8
     5.2.  Transport Mode Conflict  . . . . . . . . . . . . . . . .  9
 6.  IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 10
 7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
 8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . 11
 A.  Clarification of Potential NAT Multiple Client Solutions . . . 12
     Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
     Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15

1. Introduction

 This protocol specification defines methods to encapsulate and
 decapsulate ESP packets inside UDP packets for traversing Network
 Address Translators (NATs) (see [RFC3715], section 2.2, case i).  The
 UDP port numbers are the same as those used by IKE traffic, as
 defined in [RFC3947].
 The sharing of the port numbers for both IKE and UDP encapsulated ESP
 traffic was selected because it offers better scaling (only one NAT
 mapping in the NAT; no need to send separate IKE keepalives), easier
 configuration (only one port to be configured in firewalls), and
 easier implementation.
 A client's needs should determine whether transport mode or tunnel
 mode is to be supported (see [RFC3715], Section 3, "Telecommuter
 scenario").  L2TP/IPsec clients MUST support the modes as defined in
 [RFC3193].  IPsec tunnel mode clients MUST support tunnel mode.

Huttunen, et al. Standards Track [Page 2] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

 An IKE implementation supporting this protocol specification MUST NOT
 use the ESP SPI field zero for ESP packets.  This ensures that IKE
 packets and ESP packets can be distinguished from each other.
 As defined in this document, UDP encapsulation of ESP packets is
 written in terms of IPv4 headers.  There is no technical reason why
 an IPv6 header could not be used as the outer header and/or as the
 inner header.
 Because the protection of the outer IP addresses in IPsec AH is
 inherently incompatible with NAT, the IPsec AH was left out of the
 scope of this protocol specification.  This protocol also assumes
 that IKE (IKEv1 [RFC2401] or IKEv2 [IKEv2]) is used to negotiate the
 IPsec SAs.  Manual keying is not supported.
 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 [RFC2119].

2. Packet Formats

2.1. UDP-Encapsulated ESP Header Format

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Source Port            |      Destination Port         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Length              |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      ESP header [RFC2406]                     |
 ~                                                               ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The UDP header is a standard [RFC0768] header, where
 o  the Source Port and Destination Port MUST be the same as that used
    by IKE traffic,
 o  the IPv4 UDP Checksum SHOULD be transmitted as a zero value, and
 o  receivers MUST NOT depend on the UDP checksum being a zero value.
 The SPI field in the ESP header MUST NOT be a zero value.

Huttunen, et al. Standards Track [Page 3] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

2.2. IKE Header Format for Port 4500

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Source Port            |      Destination Port         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Length              |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Non-ESP Marker                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      IKE header [RFC2409]                     |
 ~                                                               ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The UDP header is a standard [RFC0768] header and is used as defined
 in [RFC3947].  This document does not set any new requirements for
 the checksum handling of an IKE packet.
 A Non-ESP Marker is 4 zero-valued bytes aligning with the SPI field
 of an ESP packet.

2.3. NAT-Keepalive Packet Format

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Source Port            |      Destination Port         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Length              |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    0xFF       |
 +-+-+-+-+-+-+-+-+
 The UDP header is a standard [RFC0768] header, where
 o  the Source Port and Destination Port MUST be the same as used by
    UDP-ESP encapsulation of Section 2.1,
 o  the IPv4 UDP Checksum SHOULD be transmitted as a zero value, and
 o  receivers MUST NOT depend upon the UDP checksum being a zero
    value.
 The sender MUST use a one-octet-long payload with the value 0xFF.
 The receiver SHOULD ignore a received NAT-keepalive packet.

Huttunen, et al. Standards Track [Page 4] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

3. Encapsulation and Decapsulation Procedures

3.1. Auxiliary Procedures

3.1.1. Tunnel Mode Decapsulation NAT Procedure

 When a tunnel mode has been used to transmit packets (see [RFC3715],
 section 3, criteria "Mode support" and "Telecommuter scenario"), the
 inner IP header can contain addresses that are not suitable for the
 current network.  This procedure defines how these addresses are to
 be converted to suitable addresses for the current network.
 Depending on local policy, one of the following MUST be done:
 1.  If a valid source IP address space has been defined in the policy
     for the encapsulated packets from the peer, check that the source
     IP address of the inner packet is valid according to the policy.
 2.  If an address has been assigned for the remote peer, check that
     the source IP address used in the inner packet is the assigned IP
     address.
 3.  NAT is performed for the packet, making it suitable for transport
     in the local network.

3.1.2. Transport Mode Decapsulation NAT Procedure

 When a transport mode has been used to transmit packets, contained
 TCP or UDP headers will have incorrect checksums due to the change of
 parts of the IP header during transit.  This procedure defines how to
 fix these checksums (see [RFC3715], section 2.1, case b).
 Depending on local policy, one of the following MUST be done:
 1.  If the protocol header after the ESP header is a TCP/UDP header
     and the peer's real source and destination IP address have been
     received according to [RFC3947], incrementally recompute the
     TCP/UDP checksum:
  • Subtract the IP source address in the received packet from the

checksum.

  • Add the real IP source address received via IKE to the

checksum (obtained from the NAT-OA)

  • Subtract the IP destination address in the received packet

from the checksum.

  • Add the real IP destination address received via IKE to the

checksum (obtained from the NAT-OA).

     Note: If the received and real address are the same for a given
     address (e.g., say the source address), the operations cancel and
     don't need to be performed.

Huttunen, et al. Standards Track [Page 5] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

 2.  If the protocol header after the ESP header is a TCP/UDP header,
     recompute the checksum field in the TCP/UDP header.
 3.  If the protocol header after the ESP header is a UDP header, set
     the checksum field to zero in the UDP header.  If the protocol
     after the ESP header is a TCP header, and if there is an option
     to flag to the stack that the TCP checksum does not need to be
     computed, then that flag MAY be used.  This SHOULD only be done
     for transport mode, and if the packet is integrity protected.
     Tunnel mode TCP checksums MUST be verified.  (This is not a
     violation to the spirit of section 4.2.2.7 in [RFC1122] because a
     checksum is being generated by the sender and verified by the
     receiver.  That checksum is the integrity over the packet
     performed by IPsec.)
 In addition an implementation MAY fix any contained protocols that
 have been broken by NAT (see [RFC3715], section 2.1, case g).

3.2. Transport Mode ESP Encapsulation

               BEFORE APPLYING ESP/UDP
          ----------------------------
    IPv4  |orig IP hdr  |     |      |
          |(any options)| TCP | Data |
          ----------------------------
               AFTER APPLYING ESP/UDP
          -------------------------------------------------------
    IPv4  |orig IP hdr  | UDP | ESP |     |      |   ESP   | ESP|
          |(any options)| Hdr | Hdr | TCP | Data | Trailer |Auth|
          -------------------------------------------------------
                                    |<----- encrypted ---->|
                              |<------ authenticated ----->|
 1.  Ordinary ESP encapsulation procedure is used.
 2.  A properly formatted UDP header is inserted where shown.
 3.  The Total Length, Protocol, and Header Checksum (for IPv4) fields
     in the IP header are edited to match the resulting IP packet.

3.3. Transport Mode ESP Decapsulation

 1.  The UDP header is removed from the packet.
 2.  The Total Length, Protocol, and Header Checksum (for IPv4) fields
     in the new IP header are edited to match the resulting IP packet.
 3.  Ordinary ESP decapsulation procedure is used.
 4.  Transport mode decapsulation NAT procedure is used.

Huttunen, et al. Standards Track [Page 6] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

3.4. Tunnel Mode ESP Encapsulation

               BEFORE APPLYING ESP/UDP
          ----------------------------
    IPv4  |orig IP hdr  |     |      |
          |(any options)| TCP | Data |
          ----------------------------
               AFTER APPLYING ESP/UDP
      --------------------------------------------------------------
 IPv4 |new h.| UDP | ESP |orig IP hdr  |     |      |   ESP   | ESP|
      |(opts)| Hdr | Hdr |(any options)| TCP | Data | Trailer |Auth|
      --------------------------------------------------------------
                         |<------------ encrypted ----------->|
                   |<------------- authenticated ------------>|
 1.  Ordinary ESP encapsulation procedure is used.
 2.  A properly formatted UDP header is inserted where shown.
 3.  The Total Length, Protocol, and Header Checksum (for IPv4) fields
 in the new IP header are edited to match the resulting IP packet.

3.5. Tunnel Mode ESP Decapsulation

 1.  The UDP header is removed from the packet.
 2.  The Total Length, Protocol, and Header Checksum (for IPv4) fields
     in the new IP header are edited to match the resulting IP packet.
 3.  Ordinary ESP decapsulation procedure is used.
 4.  Tunnel mode decapsulation NAT procedure is used.

4. NAT Keepalive Procedure

 The sole purpose of sending NAT-keepalive packets is to keep NAT
 mappings alive for the duration of a connection between the peers
 (see [RFC3715], Section 2.2, case j).  Reception of NAT-keepalive
 packets MUST NOT be used to detect whether a connection is live.
 A peer MAY send a NAT-keepalive packet if one or more phase I or
 phase II SAs exist between the peers, or if such an SA has existed at
 most N minutes earlier.  N is a locally configurable parameter with a
 default value of 5 minutes.
 A peer SHOULD send a NAT-keepalive packet if a need for it is
 detected according to [RFC3947] and if no other packet to the peer
 has been sent in M seconds.  M is a locally configurable parameter
 with a default value of 20 seconds.

Huttunen, et al. Standards Track [Page 7] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

5. Security Considerations

5.1. Tunnel Mode Conflict

 Implementors are warned that it is possible for remote peers to
 negotiate entries that overlap in an SGW (security gateway), an issue
 affecting tunnel mode (see [RFC3715], section 2.1, case e).
           +----+            \ /
           |    |-------------|----\
           +----+            / \    \
           Ari's           NAT 1     \
           Laptop                     \
          10.1.2.3                     \
           +----+            \ /        \       +----+          +----+
           |    |-------------|----------+------|    |----------|    |
           +----+            / \                +----+          +----+
           Bob's           NAT 2                  SGW           Suzy's
           Laptop                                               Server
          10.1.2.3
 Because SGW will now see two possible SAs that lead to 10.1.2.3, it
 can become confused about where to send packets coming from Suzy's
 server.  Implementors MUST devise ways of preventing this from
 occurring.
 It is RECOMMENDED that SGW either assign locally unique IP addresses
 to Ari's and Bob's laptop (by using a protocol such as DHCP over
 IPsec) or use NAT to change Ari's and Bob's laptop source IP
 addresses to these locally unique addresses before sending packets
 forward to Suzy's server.  This covers the "Scaling" criteria of
 section 3 in [RFC3715].
 Please see Appendix A.

Huttunen, et al. Standards Track [Page 8] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

5.2. Transport Mode Conflict

 Another similar issue may occur in transport mode, with 2 clients,
 Ari and Bob, behind the same NAT talking securely to the same server
 (see [RFC3715], Section 2.1, case e).
 Cliff wants to talk in the clear to the same server.
           +----+
           |    |
           +----+ \
           Ari's   \
           Laptop   \
          10.1.2.3   \
           +----+    \ /                +----+
           |    |-----+-----------------|    |
           +----+    / \                +----+
           Bob's     NAT                Server
           Laptop   /
          10.1.2.4 /
                  /
          +----+ /
          |    |/
          +----+
          Cliff's
          Laptop
         10.1.2.5
 Now, transport SAs on the server will look like this:
 To Ari: Server to NAT, <traffic desc1>, UDP encap <4500, Y>
 To Bob: Server to NAT, <traffic desc2>, UDP encap <4500, Z>
 Cliff's traffic is in the clear, so there is no SA.
 <traffic desc> is the protocol and port information.  The UDP encap
 ports are the ports used in UDP-encapsulated ESP format of section
 2.1.  Y,Z are the dynamic ports assigned by the NAT during the IKE
 negotiation.  So IKE traffic from Ari's laptop goes out on UDP
 <4500,4500>.  It reaches the server as UDP <Y,4500>, where Y is the
 dynamically assigned port.
 If the <traffic desc1> overlaps <traffic desc2>, then simple filter
 lookups may not be sufficient to determine which SA has to be used to
 send traffic.  Implementations MUST handle this situation, either by
 disallowing conflicting connections, or by other means.

Huttunen, et al. Standards Track [Page 9] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

 Assume now that Cliff wants to connect to the server in the clear.
 This is going to be difficult to configure, as the server already has
 a policy (from Server to the NAT's external address) for securing
 <traffic desc>.  For totally non-overlapping traffic descriptions,
 this is possible.
 Sample server policy could be as follows:
 To Ari: Server to NAT, All UDP, secure
 To Bob: Server to NAT, All TCP, secure
 To Cliff: Server to NAT, ALL ICMP, clear text
 Note that this policy also lets Ari and Bob send cleartext ICMP to
 the server.
 The server sees all clients behind the NAT as the same IP address, so
 setting up different policies for the same traffic descriptor is in
 principle impossible.
 A problematic example of configuration on the server is as follows:
 Server to NAT, TCP, secure (for Ari and Bob)
 Server to NAT, TCP, clear (for Cliff)
 The server cannot enforce his policy, as it is possible that
 misbehaving Bob sends traffic in the clear.  This is
 indistinguishable from when Cliff sends traffic in the clear.  So it
 is impossible to guarantee security from some clients behind a NAT,
 while allowing clear text from different clients behind the SAME NAT.
 If the server's security policy allows this, however, it can do
 best-effort security: If the client from behind the NAT initiates
 security, his connection will be secured.  If he sends in the clear,
 the server will still accept that clear text.
 For security guarantees, the above problematic scenario MUST NOT be
 allowed on servers.  For best effort security, this scenario MAY be
 used.
 Please see Appendix A.

6. IAB Considerations

 The UNSAF [RFC3424] questions are addressed by the IPsec-NAT
 compatibility requirements document [RFC3715].

Huttunen, et al. Standards Track [Page 10] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

7. Acknowledgments

 Thanks to Tero Kivinen and William Dixon, who contributed actively to
 this document.
 Thanks to Joern Sierwald, Tamir Zegman, Tatu Ylonen, and Santeri
 Paavolainen, who contributed to the early documents about NAT
 traversal.

8. References

8.1. Normative References

 [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            August 1980.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
            Internet Protocol", RFC 2401, November 1998.
 [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
            Payload (ESP)", RFC 2406, November 1998.
 [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
            (IKE)", RFC 2409, November 1998.
 [RFC3947]  Kivinen, T., "Negotiation of NAT-Traversal in the IKE",
            RFC 3947, January 2005.

8.2. Informative References

 [RFC1122]  Braden, R., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC3193]  Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth,
            "Securing L2TP using IPsec", RFC 3193, November 2001.
 [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
            Self-Address Fixing (UNSAF) Across Network Address
            Translation", RFC 3424, November 2002.
 [RFC3715]  Aboba, B. and W. Dixon, "IPsec-Network Address Translation
            (NAT) Compatibility Requirements", RFC 3715, March 2004.
 [IKEv2]    Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
            Work in Progress, October 2004.

Huttunen, et al. Standards Track [Page 11] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

Appendix A. Clarification of Potential NAT Multiple Client Solutions

 This appendix provides clarification about potential solutions to the
 problem of multiple clients behind the same NAT simultaneously
 connecting to the same destination IP address.
 Sections 5.1 and 5.2 say that you MUST avoid this problem.  As this
 is not a matter of wire protocol, but a matter local implementation,
 the mechanisms do not belong in the protocol specification itself.
 They are instead listed in this appendix.
 Choosing an option will likely depend on the scenarios for which one
 uses/supports IPsec NAT-T.  This list is not meant to be exhaustive,
 so other solutions may exist.  We first describe the generic choices
 that solve the problem for all upper-layer protocols.
 Generic choices for ESP transport mode:
 Tr1) Implement a built-in NAT (network address translation) above
 IPsec decapsulation.
 Tr2) Implement a built-in NAPT (network address port translation)
 above IPsec decapsulation.
 Tr3) An initiator may decide not to request transport mode once NAT
 is detected and may instead request a tunnel-mode SA.  This may be a
 retry after transport mode is denied by the responder, or the
 initiator may choose to propose a tunnel SA initially.  This is no
 more difficult than knowing whether to propose transport mode or
 tunnel mode without NAT.  If for some reason the responder prefers or
 requires tunnel mode for NAT traversal, it must reject the quick mode
 SA proposal for transport mode.
 Generic choices for ESP tunnel mode:
 Tn1) Same as Tr1.
 Tn2) Same as Tr2.
 Tn3) This option is possible if an initiator can be assigned an
 address through its tunnel SA, with the responder using DHCP.  The
 initiator may initially request an internal address via the
 DHCP-IPsec method, regardless of whether it knows it is behind a NAT.
 It may re-initiate an IKE quick mode negotiation for DHCP tunnel SA
 after the responder fails the quick mode SA transport mode proposal.
 This happens either when a NAT-OA payload is sent or because it

Huttunen, et al. Standards Track [Page 12] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

 discovers from NAT-D that the initiator is behind a NAT and its local
 configuration/policy will only accept a NAT connection when being
 assigned an address through DHCP-IPsec.
 There are also implementation choices that offer limited
 interoperability.  Implementors should specify which applications or
 protocols should work if these options are selected.  Note that
 neither Tr4 nor Tn4, as described below, are expected to work with
 TCP traffic.
 Limited interoperability choices for ESP transport mode:
 Tr4) Implement upper-layer protocol awareness of the inbound and
 outbound IPsec SA so that it doesn't use the source IP and the source
 port as the session identifier (e.g., an L2TP session ID mapped to
 the IPsec SA pair that doesn't use the UDP source port or the source
 IP address for peer uniqueness).
 Tr5) Implement application integration with IKE initiation so that it
 can rebind to a different source port if the IKE quick mode SA
 proposal is rejected by the responder; then it can repropose the new
 QM selector.
 Limited interoperability choices for ESP tunnel mode:
 Tn4) Same as Tr4.

Huttunen, et al. Standards Track [Page 13] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

Authors' Addresses

 Ari Huttunen
 F-Secure Corporation
 Tammasaarenkatu 7
 HELSINKI  FIN-00181
 FI
 EMail: Ari.Huttunen@F-Secure.com
 Brian Swander
 Microsoft
 One Microsoft Way
 Redmond, WA  98052
 US
 EMail: briansw@microsoft.com
 Victor Volpe
 Cisco Systems
 124 Grove Street
 Suite 205
 Franklin, MA  02038
 US
 EMail: vvolpe@cisco.com
 Larry DiBurro
 Nortel Networks
 80 Central Street
 Boxborough, MA  01719
 US
 EMail: ldiburro@nortelnetworks.com
 Markus Stenberg
 FI
 EMail: markus.stenberg@iki.fi

Huttunen, et al. Standards Track [Page 14] RFC 3948 UDP Encapsulation of IPsec ESP Packets January 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Huttunen, et al. Standards Track [Page 15]

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