IP Security Protocol Working Group (IPSEC) A. Huttunen
INTERNET-DRAFT F-Secure Corporation
Category: Standards track W. Dixon, B. Swander
Expires: 18 December 2001 Microsoft
T. Kivinen, M. Stenberg
SSH Communications Security Corp
V. Volpe
Cisco Systems
L. DiBurro
Nortel Networks
18 June 2001
UDP Encapsulation of IPsec Packets
draft-ietf-ipsec-udp-encaps-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This draft defines methods to encapsulate and decapsulate ESP and
AH 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. AH encapsulation is defined
for IPv4 scenarios only.
The encapsulation is used whenever negotiated using IKE, as
defined in [Kiv00]. The design choices are documented in [Dixon00].
1. Introduction
UDP encapsulation of ESP packets MUST be supported. 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.
UDP encapsulation of AH MAY be supported.
An IKE implementation supporting this draft for AH use MUST NOT
generate ESP SPIs that are all zeroes.
ESP encapsulation as defined in this document is capable of being
used in both IPv4 and IPv6 scenarios. AH encapsulation is defined
for IPv4 scenarios only.
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.
2.2 UDP-encapsulated AH 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Non-ESP Marker |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Reserved | IHL | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AH header [RFC 2402] |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
Non-ESP Marker is 4 bytes of zero aligning with the SPI field
of an ESP packet.
Version is a copy of the original header IP version field.
When version is IPv4, the following fields are defined:
- Reserved field MUST be zero.
- IHL is a copy of the original header length field of the IP packet.
- Identification is a copy of the original Identification field
of the IP packet.
Version, Reserved, IHL and Identification fields are later referred to
as AH Envelope.
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/AH header is a TCP/UDP
header, zero the checksum field in the TCP/UDP header.
b) If the protocol header after the ESP/AH header is a TCP/UDP
header, recompute the checksum field in the TCP/UDP header.
c) If the protocol header after the ESP/AH 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.
3.6 Transport Mode AH Encapsulation
BEFORE APPLYING AH/UDP
----------------------------
IPv4 |orig IP hdr | | |
|(any options)| TCP | Data |
----------------------------
AFTER APPLYING AH/UDP
----------------------------------------------------------
IPv4 |orig IP hdr | UDP | Non-| Non-| AH | | | |
|(any options)| Hdr | IKE | ESP | Env. | AH | TCP | Data |
----------------------------------------------------------
|<--auth.---->| |<---auth.------->|
except for
mutable fields
1) If the Version number field in the IP header is not 4,
drop the packet, otherwise continue.
2) Ordinary AH encapsulation procedure is used.
3) A properly formatted UDP header, Non-IKE marker, Non-ESP marker
and AH Envelope are inserted where shown.
4) The AH Envelope is filled with information from the IP header.
5) The Total Length, Protocol and Header Checksum fields in the
IP header are edited to match the resulting IP packet.
3.7 Transport Mode AH Decapsulation
1) If the Version number field in the AH envelope and the outer
IP header are not both 4, drop the packet, otherwise continue.
2) The values in the AH Envelope are copied to the IP header.
3) The UDP header, Non-IKE marker, Non-ESP marker and AH Envelope
are removed from the packet.
4) The Total Length, Protocol and Header Checksum fields in the
IP header are edited to match the resulting IP packet.
5) Ordinary AH decapsulation procedure is used.
6) Transport mode decapsulation NAT procedure is used.
3.8 Tunnel Mode AH Encapsulation
BEFORE APPLYING AH/UDP
----------------------------
IPv4 |orig IP hdr | | |
|(any options)| TCP | Data |
----------------------------
AFTER APPLYING AH/UDP
------------------------------------------------------------------
IPv4 |new h. | UDP | Non-| Non-| AH | |orig IP hdr | | |
|(opts) | Hdr | IKE | ESP | Env. | AH |(any options)| TCP | Data |
------------------------------------------------------------------
|<--auth.---->| |<----authenticated------------>|
except for
mutable fields
1) If the Version number field in the IP header is not 4,
drop the packet, otherwise continue.
2) Ordinary AH encapsulation procedure is used.
3) A properly formatted UDP header, Non-IKE marker, Non-ESP marker
and AH Envelope are inserted where shown.
4) The AH Envelope is filled with information from the new IP header.
5) The Total Length, Protocol and Header Checksum fields in the
new IP header are edited to match the resulting IP packet.
3.9 Tunnel Mode AH Decapsulation
1) If the Version number field in the AH envelope and the outer
IP header are not both 4, drop the packet, otherwise continue.
2) The values in the AH Envelope are copied to the outer IP header.
3) The UDP header, Non-IKE marker, Non-ESP marker and AH Envelope
are removed from the packet.
4) The Total Length, Protocol and Header Checksum fields in the
IP header are edited to match the resulting IP packet.
5) Ordinary AH decapsulation procedure is used.
6) 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. 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.
6. Acknowledgments
Thanks to Joern Sierwald, Tamir Zegman, Larry DiBurro, Tatu Ylonen
and Santeri Paavolainen who contributed to the previous drafts
about NAT traversal.
7. 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 2402] Kent, S., "IP Authentication Header", November 1998
[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
8. 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
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