
IP Security Protocol Working Group (IPSEC)                   A. Huttunen
INTERNET-DRAFT                                      F-Secure Corporation 
Category: Standards track                           W. Dixon, B. Swander 
Expires: April 2002                                            Microsoft 
                                                 T. Kivinen, M. Stenberg
                                        SSH Communications Security Corp
                                                                V. Volpe 
                                                           Cisco Systems 
                                                              L. DiBurro
                                                         Nortel Networks
                                                          2 October 2001 

                   UDP Encapsulation of IPsec Packets
                   draft-ietf-ipsec-udp-encaps-01.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
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   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
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   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

   This draft defines methods to encapsulate and decapsulate ESP 
   packets inside UDP packets for the purpose of traversing NATs.

   ESP encapsulation as defined in this document is capable of being
   used in both IPv4 and IPv6 scenarios.

   The encapsulation is used whenever negotiated using IKE, as 
   defined in [Kiv00], or another key management protocol. The 
   design choices are documented in [Dixon00].

Change Log
   Version -01 
   - removed everything related to the AH-protocol
   - added instructions on how to use the encapsulation with 
     some other key management protocol than IKE

1. Introduction

   It is up to the need of the clients whether transport mode 
   or tunnel mode is to be supported. L2TP/IPsec clients MUST support 
   transport mode, and IPsec tunnel mode clients MUST support tunnel 
   mode.

   An IKE implementation supporting this draft MUST NOT generate 
   packets where the Initiator Cookie field is all zeroes. This
   ensures that IKE packets and ESP packets can be distinguished
   from each other.

   Usage with another key management protocol is described in 
   a separate section.

   ESP encapsulation as defined in this document is capable of being
   used in both IPv4 and IPv6 scenarios.

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            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Non-IKE Marker                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Non-IKE Marker                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      ESP header [RFC 2406]                    |
~                                                               ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The UDP header is a standard [RFC 768] header, where
- Source Port and Destination Port are the same as used by IKE 
  traffic.
- Checksum is zero.

Non-IKE Marker is 8 bytes of zero aligning with the Initiator 
Cookie of an IKE 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 [RFC 768] header, where
- Source Port and Destination Port are the same as used by IKE 
  traffic.
- Checksum is zero.

The sender SHOULD use a one octet long payload with the value 0xFF.
The receiver SHOULD ignore a received NAT-keepalive packet.

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, 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:
a) 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.
b) If an address has been assigned for the remote peer, check
   that the source IP address used in the inner packet is the
   same as the IP address assigned.
c) 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 contain incorrect checksums due to the change
of parts of the IP header during transit. This procedure defines how
to fix these checksums.

Depending on local policy, one of the following MUST be done:
a) If the protocol header after the ESP header is a TCP/UDP
   header, zero the checksum field in the TCP/UDP header.
b) If the protocol header after the ESP header is a TCP/UDP
   header, recompute the checksum field in the TCP/UDP header.
c) If the protocol header after the ESP header is a TCP/UDP
   header and the peer's real source IP address has been received
   according to [Kiv00], 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

In addition an implementation MAY fix any contained protocols that
have been broken by NAT.

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 | Non-| ESP |     |      |   ESP   | ESP|
         |(any options)| Hdr | IKE | Hdr | TCP | Data | Trailer |Auth|
         -------------------------------------------------------------
                                         |<----- encrypted ---->|
                                   |<------ authenticated ----->|

1) Ordinary ESP encapsulation procedure is used.
2) A properly formatted UDP header and a Non-IKE Marker 
   are inserted where shown.
3) The Total Length, Protocol and Header Checksum fields in the
   IP header are edited to match the resulting IP packet.

3.3 Transport Mode ESP Decapsulation

1) The UDP header and the Non-IKE Marker are removed from
   the packet.
2) The Total Length, Protocol and Header Checksum 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.


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 | Non-| ESP |orig IP hdr  |     |      |   ESP   | ESP|
     |(opts)| Hdr | IKE | Hdr |(any options)| TCP | Data | Trailer |Auth|
     --------------------------------------------------------------------
                              |<------------ encrypted ----------->|
                        |<------------- authenticated ------------>|

1) Ordinary ESP encapsulation procedure is used.
2) A properly formatted UDP header and a Non-IKE Marker 
   are inserted where shown.
3) The Total Length, Protocol and Header Checksum fields in the
   new IP header are edited to match the resulting IP packet.


3.5 Tunnel Mode ESP Decapsulation

1) The UDP header and the Non-IKE Marker are removed from
   the packet.
2) The Total Length, Protocol and Header Checksum 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. Reception of NAT-keepalive packets MUST NOT be
used to detect liveness of a connection.

A peer MAY send a NAT-keepalive packet if there exists one
or more phase I or phase II SAs between the peers, or 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 to send such
packets is detected according to [Kiv00] 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.

5.  Usage with Another Key Management Protocol

5.1. Requirements

The important requirements when using the encapsulation method
with another key management protocol are:
a) It must be possible to distinguish key management packets
   from ESP packets.
b) If more than one UDP port pair is being used, all the relevant
   NAT mappings must be kept alive.

5.2. Alternative Encapsulation Method 1 - Common Port

         -----------------------------------------------------------
   IPv4  | IP hdr  | UDP | ESP | ...ESP packet...                  |
         |(options)| Hdr | Hdr |                                   |
         -----------------------------------------------------------

         -----------------------------------------------------------
   IPv4  | IP hdr  | UDP | Non-| ...key management packet...       |
         |(options)| Hdr | ESP |                                   |
         -----------------------------------------------------------

Non-ESP marker in this case is 4 bytes of zero. The same port pair
is used for both types of traffic, and the keepalive mechanism is as
defined in this document for IKE traffic. It is required that an 
implementation using this method does not use ESP SPIs that are equal
to zero.

This method is more efficient than the one defined for IKE traffic
because it makes the more frequent packets smaller.

5.2. Alternative Encapsulation Method 2 - Separate Ports

         -----------------------------------------------------------
   IPv4  | IP hdr  | UDP | ESP | ...ESP packet...                  |
         |(options)| Hdr | Hdr |                                   |
         -----------------------------------------------------------

         -----------------------------------------------------------
   IPv4  | IP hdr  | UDP |       ...key management packet...       |
         |(options)| Hdr |                                         |
         -----------------------------------------------------------

In this method the two types of traffic use different UDP ports, so
no non-something markers are needed. Both UDP ports must be kept
alive using the keepalive procedure.

Whether or not this results in better bandwidth utilization than
using a common UDP port depends on the traffic characteristics. There
is less overhead per packet, but more need for keepalive packets.

6.  Intellectual Property Rights

The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this document.
For more information consult the online list of claimed rights.

SSH Communications Security Corp has notified the working group of one
or more patents or patent applications that may be relevant to this
internet-draft. SSH Communications Security Corp has already given a
licence for those patents to the IETF. For more information consult the
online list of claimed rights.

7.  Acknowledgments

Thanks to Joern Sierwald, Tamir Zegman, Larry DiBurro, Tatu Ylonen
and Santeri Paavolainen who contributed to the previous drafts
about NAT traversal.

8.  References

[RFC 768] Postel, J., "User Datagram Protocol", August 1980

[RFC-2119] Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", March 1997

[RFC 2406] Kent, S., "IP Encapsulating Security Payload (ESP)",
November 1998

[Dixon00] Dixon, W. et. al., 
draft-ietf-ipsec-udp-encaps-justification-00.txt,
"IPSec over NAT Justification for UDP Encapsulation", June 2001

[Kiv00] Kivinen, T. et. al., draft-ietf-ipsec-nat-t-ike-00.txt,
"Negotiation of NAT-Traversal in the IKE", June 2001


9.  Authors' Addresses
 
    Ari Huttunen
    F-Secure Corporation
    Tammasaarenkatu 7
    FIN-00181 HELSINKI
    Finland
    E-mail: Ari.Huttunen@F-Secure.com

    William Dixon
    Microsoft
    One Microsoft Way
    Redmond WA 98052
    E-mail: wdixon@microsoft.com

    Brian Swander
    Microsoft
    One Microsoft Way
    Redmond WA 98052
    E-mail: briansw@microsoft.com

    Tero Kivinen
    SSH Communications Security Corp
    Fredrikinkatu 42
    FIN-00100 HELSINKI
    Finland
    E-mail: kivinen@ssh.fi

    Markus Stenberg
    SSH Communications Security Corp
    Fredrikinkatu 42
    FIN-00100 HELSINKI
    Finland
    E-mail: mstenber@ssh.com

    Victor Volpe
    Cisco Systems
    124 Grove Street
    Suite 205
    Franklin, MA 02038
    E-mail: vvolpe@cisco.com

    Larry DiBurro
    Nortel Networks
    80 Central Street       
    Boxborough, MA 01719
    ldiburro@nortelnetworks.com
