Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track August 28, 2012
Expires: March 1, 2013
A TCP transport for the Internet Key Exchange
draft-ietf-ipsecme-ike-tcp-00
Abstract
This document describes using TCP for IKE messages. This facilitates
the transport of large messages over paths where fragments are either
dropped, or where packet loss makes the use of large UDP packets
unreliable.
Status of this Memo
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This Internet-Draft will expire on March 1, 2013.
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1. Introduction
The Internet Key Exchange version 2 (IKEv2), specified in [RFC5996]
uses UDP to transport the exchange messages. Some of those messages
may be fairly large. Specifically, the messages of the IKE_AUTH
exchange can become quite large, as they may contain a chain of
certificates, an "Auth" payload (that may contain a public key
signature), CRLs, and some configuration information that is carried
in the CFG payload.
When such UDP packets exceed the path MTU, they get fragmented. This
increases the probability of packets being dropped. The
retransmission mechanisms in IKE (as described in section 2.1 of RFC
5996) takes care of that as long as packet loss is at a reasonable
level. More recently we have seen a number of service providers
dropping fragmented packets. Firewalls and NAT devices need to keep
state for each packet where some (but not all) of the fragments have
passed through. This creates a burden in terms of memory, especially
for high capacity devices such as Carrier-Grade NAT (CGN) or high
capacity firewalls.
The BEHAVE working group has an Internet Draft describing required
behavior of CGNs ([I-D.ietf-behave-lsn-requirements]). It requires
CGNs to comply with [RFC4787], which in section 11 requires NAT
devices to support fragments. However, some people deploying IKE
have found that some ISPs have begun to drop fragments in preparation
for deploying CGNs. While we all hope for a future where all devices
comply with the emerging standards, or even a future where CGNs are
not required, we have to make IKE work today.
The solution described in this document is to transport the IKE
messages over a TCP ([RFC0793]) connection rather than over UDP. IKE
packets describe their own length, so they are well-suited for
transport over a stream-based connection such as TCP. The Initiator
opens a TCP connection to the Responder's port 500, sends the
requests and receives the responses, and then closes the connection.
TCP can handle arbitrary-length messages, works well with any sized
data, and is well supported by all ISP infrastructure.
1.1. Non-Goals of this Specification
Firewall traversal is not a goal of this specification. If a
firewall has a policy to block IKE and/or IPsec, hiding the IKE
exchange in TCP is not expected to help. Some implementations hide
both IKE and IPsec in a TCP connection, usually pretending to be
HTTPS by using port 443. This has a significant impact on bandwidth
and gateway capacity, and even this is defeated by better firewalls.
SSL VPNs tunnel IP packets over TLS, but the latest firewalls are
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also TLS proxies, and are able to defeat this as well.
This document is not part of that arms race. It is only meant to
allow IKE to work When faced with broken infrastructure that drops
large IP packets.
1.2. Conventions Used in This Document
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. The Protocol
2.1. Initiator
An Initiator MAY try IKE using TCP for any request. It opens a TCP
connection from an arbitrary port to port 500 of the Responder. When
the three-way handshake completes, the Initiator MUST send the
request. If the Initiator knows that this request is the last
request needed at this time, it MAY half-close the TCP connection, or
it MAY wait until the last response has been received. When all
responses have been received, the Initiator MUST close the
connection. If the peer has closed the connection before all
requests have been transmitted or responded to, the Initiator SHOULD
either open a new TCP connection or transmit them over UDP again.
An initiator MUST accept responses sent over IKE within the same
connection, but MUST also accept responses over other transports, if
the request had been sent over them as well.
2.2. Responder
A Responder MAY accept TCP connections to port 500, and if it does,
it MUST accept IKE requests over this connection. Responses to
requests received over this connection MUST also go over this
connection. If the connection has closed before the Responder has
had a chance to respond, it MUST NOT respond over UDP, but MUST
instead wait for a retransmission over UDP or over another TCP
connection.
The responder MUST accept different requests on different transports.
Specifically, the Responder MUST NOT rely on subsequent requests
coming over the same transport. For example, it is entirely
acceptable to have the IKE_INITIAL exchange come over UDP port 500,
while the IKE_AUTH request comes over TCP, and some following
requests might come over UDP port 4500 (because NAT has been
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detected).
A responder that is configured to support IKE over TCP and receives
an IKEv2 Initial request over any other transport MUST send an
IKE_TCP_SUPPORTED notification (Section 2.5) in the Initial response.
the responder MAY send this notification even if the Initial request
was received over TCP.
If the responder has some requests of its own to send, it MUST NOT
use a connection that has been opened by a peer. Instead, it MUST
either use UDP or else open a new TCP connection to the original
Initiator's TCP port 500.
The normal flow of things is that the Initiator opens a connection
and closes its side first. The responder closes after sending the
last response where the initiator has already half-closed the
connection. If, however, a significant amount of time has passed,
and neither new requests arrive nor the connection is closed by the
initiator, the Responder MAY close or even reset the connection.
This specification makes no recommendation as to how long such a
timeout should be, but a few seconds should be enough.
2.3. Transmitter
The transmitter, whether an initiator transmitting a request or a
responder transmitting a response MUST NOT retransmit over the same
connection. TCP takes care of that. It SHOULD send the IKE header
and the IKE payloads with a single command or in rapid succession,
because the receiver might block on reading from the socket.
2.4. Receiver
The IKE header is copied from RFC 5996 below for reference:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IKE SA Initiator's SPI |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IKE SA Responder's SPI |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload | MjVer | MnVer | Exchange Type | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IKE Header Format
The receiver MUST first read in the 28 bytes that make up the IKE
header. The Responder then subtracts 28 from the length field, and
reads the resulting number of bytes. The combined message, comprised
on 28 header bytes and whatever number of payload bytes is processed
the same way as regular UDP messages. That includes retransmission
detection, with one slight difference: if a retransmitted request is
detected, the response is retransmitted as well, but using the
current TCP connection rather than whatever other transport had been
used for the original transmission of the request.
2.5. IKE_TCP_SUPPORTED Notification
This notification is sent by a responder over non-TCP transports to
inform the initiator that this specification is supported and
configured.
The Notify payload is formatted as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Protocol ID ! SPI Size !IKE_TCP_SUPPORTED Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Protocol ID (1 octet) MUST be 0.
o SPI Size (1 octet) MUST be zero, in conformance with section 3.10
of RFC 5996.
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o IKE_TCP_SUPPORTED Notify Message Type (2 octets) - MUST be xxxxx,
the value assigned for IKE_TCP_SUPPORTED. TBA by IANA.
3. Operational Considerations
Most IKE messages are relatively short. All but the IKE_AUTH
exchange in IKEv2 are comprised of short messages that fit in a
single packet on most networks. The Informational exchange could be
an exception, as it may contain arbitrary-length CFG payloads, but in
practice this is not done. It is only the IKE_AUTH exchange that has
long messages. UDP has advantages in lower latency and lower
resource consumption, so it makes sense to use UDP whenever TCP is
not required.
The requirements in Section 2.2 were written so that different
requests may be sent over different transports. The initiator can
choose the transport on a per-request basis. So one obvious policy
would be to do everything over UDP except the specific requests that
tend to become too big. This way the first messages use UDP, and the
Initiator can set up the TCP connection at the same time, eliminating
the latency penalty of using TCP. This may not always be the most
efficient policy, though. It means that the first messages sent over
TCP are relatively large ones, and TCP slow start may cause an extra
roundtrip, because the message has exceeded the transmission window.
An initiator using this policy MUST NOT go to TCP if the responder
has not indicated support by sending the IKE_TCP_SUPPORTED
notification (Section 2.5) in the Initial response.
An alternative method, that is probably easier for the Initiator to
implement, is to do an entire "mission" using the same transport. So
if TCP is needed for long messages and an IKE SA has not yet been
created, the Initiator will open a TCP connection, and perform all
2-4 requests needed to set up a child SA over the same connection.
Yet another policy would be to begin by using UDP, and at the same
time set up the TCP connection. If at any point the TCP handshake
completes, the next requests go over that connection. This method
can be used to auto-discover support of TCP on the responder. This
is easier for the user than configuring which peers support TCP, but
has the potential of wasting resources, as TCP connections may finish
the three-way handshake just when IKE over UDP has finished. The
requirements from the responder ensure that all these policies will
work.
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3.1. Liveness Check
The TCP connections described in this document are short-lived. We
do not expect them to stay for the lifetime of the SA, but to get
torn down by either side within seconds of the SA being set up.
Because of this, they are not well-suited for the transport of short
requests such as those for liveness check.
Although liveness checks MAY be sent over TCP, this is not
recommended.
4. Security Considerations
Most of the security considerations for IKE over TCP are the same as
those for UDP as in RFC 5996.
For the Responder, listening to TCP port 500 involves all the risks
of maintaining any TCP server. Precautions against DoS attacks, such
as SYN cookies are RECOMMENDED. see [RFC4987] for details.
5. IANA Considerations
IANA is requested to assign a notify message type from the status
types range (16418-40959) of the "IKEv2 Notify Message Types"
registry with name "IKE_TCP_SUPPORTED"
No IANA action is required for the TCP port, as TCP port 500 is
already allocated to "ISAKMP".
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
6.2. Informative References
[I-D.ietf-behave-lsn-requirements]
Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common requirements for Carrier Grade NATs
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(CGNs)", draft-ietf-behave-lsn-requirements-09 (work in
progress), August 2012.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
Author's Address
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 67897
Israel
Email: ynir@checkpoint.com
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