Network T. Pauly
Internet-Draft Apple Inc.
Intended status: Standards Track S. Touati
Expires: May 23, 2016 Ericsson
November 20, 2015
TCP Encapsulation of IKEv2 and IPSec Packets
draft-pauly-ipsecme-tcp-encaps-01
Abstract
This document describes a method to transport IKEv2 and IPSec packets
over a TCP connection for traversing network middleboxes that may
block IKEv2 negotiation over UDP. This method, referred to as TCP
encapsulation, involves sending all packets for tunnel establishment
as well as tunneled packets over a TCP connection.
Status of This Memo
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This Internet-Draft will expire on May 23, 2016.
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Copyright (c) 2015 IETF Trust and the persons identified as the
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Prior Work and Motivation . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 4
3. TCP-Encapsulated Header Formats . . . . . . . . . . . . . . . 4
3.1. TCP-Encapsulated IKEv2 Header Format . . . . . . . . . . 4
3.2. TCP-Encapsulated ESP Header Format . . . . . . . . . . . 5
4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Connection Establishment and Teardown . . . . . . . . . . . . 6
6. Interaction with NAT Detection Payloads . . . . . . . . . . . 7
7. Using MOBIKE with TCP encapsulation . . . . . . . . . . . . . 7
8. Considerations for Keep-alives and DPD . . . . . . . . . . . 8
9. Middlebox Considerations . . . . . . . . . . . . . . . . . . 8
10. Performance Considerations . . . . . . . . . . . . . . . . . 9
10.1. TCP-in-TCP . . . . . . . . . . . . . . . . . . . . . . . 9
10.2. Added Reliability for Unreliable Protocols . . . . . . . 9
10.3. Encryption Overhead . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
14.1. Normative References . . . . . . . . . . . . . . . . . . 10
14.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
IKEv2 [RFC7296] is a protocol for establishing IPSec tunnels, using
IKE messages over UDP for control traffic, and using ESP messages (or
ESP over UDP) for its data traffic. Many network middleboxes that
filter traffic on public hotspots block all UDP traffic, including
IKEv2 and IPSec, but allow TCP connections through since they appear
to be web traffic. Devices on these networks that need to use IPSec
(to access private enterprise networks, to route voice-over-IP calls
to carrier networks, or because of security policies) are unable to
establish IPSec tunnels. This document defines a method for
encapsulating both the IKEv2 control messages as well as the IPSec
data messages within a TCP connection.
Using TCP as a transport for IPSec packets adds a third option to the
list of traditional IPSec transports:
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1. Direct. Currently, IKEv2 negotiations begin over UDP port 500.
If no NAT is detected between the initiator and the receiver,
then subsequent IKEv2 packets are sent over UDP port 500 and
IPSec data packets are sent using ESP [RFC4303].
2. UDP Encapsulation [RFC3948]. If a NAT is detected between the
initiator and the receiver, then subsequent IKEv2 packets are
sent over UDP port 4500 with four bytes of zero at the start of
the UDP payload and ESP packets are sent out over UDP port
4500.
3. TCP Encapsulation. If both of the other two methods are not
available or appropriate, both IKEv2 negotiation packets as
well as ESP packets can be sent over a single TCP connection to
the peer. This connection can itself use TLS [RFC5246] or
other methods if needed. If the connection uses TLS, it will
also be capable of traversing a web proxy [RFC2817].
1.1. Prior Work and Motivation
Encapsulating IKEv2 connections within TCP or TLS streams is a common
approach to solve the problem of UDP packets being blocked by network
middleboxes. The goal of this document is to promote
interoperability by providing a standard method of framing IKEv2 and
ESP message within streams, and to provide guidelines for how to
configure and use TCP encapsulation.
Some previous solutions include:
Cellular Network Access Interworking Wireless LAN (IWLAN) uses IKEv2
to create secure connections to cellular carrier networks for
making voice calls and accessing other network services over
Wi-Fi networks. 3GPP has recommended that IKEv2 and ESP packets
be sent within a TLS connection to be able to establish
connections on restrictive networks.
ISAKMP over TCP Various non-standard extensions to ISAKMP have been
deployed that send IPSec traffic over TCP or TCP-like packets.
SSL VPNs Many proprietary VPN solutions use a combination of TLS and
IPSec in order to provide reliability.
1.2. 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].
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2. Configuration
One of the main reasons to use TCP encapsulation is that UDP traffic
may be entirely blocked on a network. Because of this, support for
TCP encapsulation is not specifically negotiated in the IKEv2
exchange. Instead, support for TCP encapsulation must be pre-
configured on both the initiator and the responder.
The configuration defined on each peer should include the following
parameters:
o One or more TCP ports on which the responder will listen for
incoming connections. Note that the initiator may initiate TCP
connections to the responder from any local port.
o Whether or not to use TLS for connections to a given TCP port.
The responder may expect to read encapsulated IKEv2 and ESP
packets directly from the TCP connection, or it may expect to read
them from a stream of TLS data packets. The initiator should be
pre-configured to use TLS or not when communicating with a given
port on the responder.
Since TCP encapsulation of IKEv2 and IPSec packets adds overhead and
has potential performance trade-offs compared to direct or UDP-
encapsulated tunnels (as described in Performance Considerations,
Section 7), implementations SHOULD prefer IKEv2 negotiation over UDP.
3. TCP-Encapsulated Header Formats
In order to encapsulate IKEv2 and ESP messages within a stream (which
may be raw TCP, or TLS over TCP), a 32-bit length field precedes
every message. If the first 32-bits of the message are zeros (a Non-
ESP Marker), then the contents comprise an IKEv2 message. Otherwise,
the contents comprise an ESP message.
Although a stream may be able to send very long messages using this
format, implementations SHOULD limit message lengths to typical UDP
datagram payload lengths, such as 1500 bytes. The maximum message
length is used as the effective MTU for connections that are being
encrypted using ESP, so the maximum message length will influence
characteristics of inner connections, such as the TCP Maximum Segment
Size (MSS).
3.1. TCP-Encapsulated IKEv2 Header Format
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Non-ESP Marker |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IKEv2 header [RFC7296] |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1
The IKE header is preceded by a 32-bit length field in network byte
order that specifies the length of the IKE packet within the TCP
stream. As with IKEv2 over UDP port 4500, a zeroed 32-bit Non-ESP
Marker is inserted before the start of the IKEv2 header in order to
differentiate the traffic from ESP traffic between the same addresses
and ports.
o Length (4 octets, unsigned integer) - Length of the IKE packet
including the Length Field and Non-ESP Marker.
3.2. TCP-Encapsulated ESP Header Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ESP header [RFC4303] |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2
The ESP header is preceded by a 32-bit length field in network byte
order that specifies the length of the ESP packet within the TCP
stream.
o Length (4 octets, unsigned integer) - Length of the ESP packet
including the Length Field.
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4. Applicability
TCP encapsulation is applicable only when it has been configured to
be used with specific IKEv2 peers. If a responder is configured to
use TCP encapsulation, it MUST listen on the configured port(s) in
case any peers will initiate new IKEv2 sessions. Initiators MAY use
TCP encapsulation for any IKEv2 session to a peer that is configured
to support TCP encapsulation, although it is recommended that
initiators should only use TCP encapsulation when traffic over UDP is
blocked.
Any specific IKE SA, along with its Child SAs, is either TCP
encapsulated or not. A mix of TCP and UDP encapsulation for a single
SA is not allowed. The exception to this rule is SAs that are
migrated between addresses using MOBIKE (Section 7).
5. Connection Establishment and Teardown
When the initiator decides to use TCP encapsulation for IKEv2
negotiation, the initiator will initiate a TCP connection with the
responder using the configured TCP port. If TLS is being used, it
may be negotiated at this point, although the policy for the TLS
negotiation is out of scope of this document. If a web proxy is
applied to the ports for the TCP connection, and TLS is being used,
the initiator can send an HTTP CONNECT message to establish a tunnel
through the proxy [RFC2817]
Before either initiator or responder closes the TCP connection by
sending a FIN or a RST, session teardown SHOULD be gracefully
negotiated with DELETE payloads. Once all SAs have been deleted, the
initiator of the original connection MUST close the TCP connection.
An unexpected FIN or a RST on the TCP connection may indicate either
a loss of connectivity, an attack, or some other error. If a DELETE
payload has not been sent, both sides SHOULD maintain the state for
their SAs for the standard lifetime or time-out period. The original
initiator (that is, the endpoint that initiated the TCP connection
and sent the first IKE_SA_INIT message) is responsible for re-
establishing the TCP connection if it is torn down for any unexpected
reason. Since new TCP connections may use different ports due to NAT
mappings or local port allocations changing, the responder MUST allow
packets for existing SAs to be received from new source ports.
The streams of data sent over any TCP connection used for this
protocol MUST begin with a complete IKEv2 message, complying to the
format specified in Figure 1. The stream must start with an IKE
message to allow the peer to know with which IKE session the traffic
is associated. If the session had been fully established previously,
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it is suggested to send an UPDATE_SA_ADDRESSES message when using
MOBIKE, or an INFORMATIONAL message (a keepalive) when not using
MOBIKE. If either initiator or responder receives a stream that
cannot be parsed correctly, it MUST close the TCP connection.
Multiple TCP connections between the initiator and the responder are
allowed, but not recommended. IKE and IPSec messages MUST be
processed according to the standard source identification (using the
SPI) and ordering rules. It is also possible to negotiate multiple
IKE SAs over the same TCP connection, in which case messages are de-
multiplexed using the SPI of the message.
6. Interaction with NAT Detection Payloads
When negotiating over UDP port 500, IKE_SA_INIT packets include
NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP payloads to
determine if UDP encapsulation of IPSec packets should be used.
These payloads contain SHA-1 digests of the SPIs, IP addresses, and
ports. IKE_SA_INIT packets sent on a TCP connection SHOULD include
these payloads, and SHOULD use the applicable TCP ports when creating
and checking the SHA-1 digests.
If a NAT is detected due to the SHA-1 digests not matching the
expected values, no change should be made for encapsulation of
subsequent IKEv2 or ESP packets, since TCP encapsulation inherently
supports NAT traversal. Implementations MAY use the information that
a NAT is present to influence keep-alive timer values.
7. Using MOBIKE with TCP encapsulation
When an IKEv2 session is transitioned between networks using MOBIKE
[RFC4555], the initiator of the transition may switch between using
TCP encapsulation, UDP encapsulation, or no encapsulation.
Implementations that implement both MOBIKE and TCP encapsulation MUST
support dynamically enabling and disabling TCP encapsulation as
interfaces change.
The encapsulation method of ESP packets MUST always match the
encapsulation method of the IKEv2 negotiation, which may be different
when an IKEv2 endpoint changes networks. When a MOBIKE-enabled
initiator changes networks, the UPDATE_SA_ADDRESSES notification
SHOULD be sent out first over UDP before attempting over TCP. If
there is a response to the UPDATE_SA_ADDRESSES notification sent over
UDP, then the ESP packets should be sent directly over IP or over UDP
port 4500 (depending on if a NAT was detected), regardless of if a
connection on a previous network was using TCP encapsulation.
Similarly, if the responder only responds to the UPDATE_SA_ADDRESSES
notification over TCP, then the ESP packets should be sent over the
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TCP connection, regardless of if a connection on a previous network
did not use TCP encapsulation.
8. Considerations for Keep-alives and DPD
Encapsulating IKE and IPSec inside of a TCP connection can impact the
strategy that implementations use to detect peer liveness and to
maintain middlebox mappings. In addition to mechanisms in IKE and
IPSec, TCP keepalives are available. The following mechanisms may be
employed:
o IKEv2 Informational packets [RFC7296]
o IPSec ESP NAT keep-alives [RFC3948]
o TCP NAT keep-alives [RFC1122]
o TLS keep-alives [RFC6520]
It is up to the implementation to decide which keepalives are
appropriate for TCP-encapsulated connections. NAT timeouts are
generally longer for TCP ports, but implementations should still use
some form of keep-alive when a NAT is detected. If TCP NAT keep-
alives are used, IPSec ESP NAT keep-alives may be considered
redundant and can safely be disabled.
9. Middlebox Considerations
Many security networking devices such as Firewalls or Intrusion
Prevention Systems, network optimization/acceleration devices and
Network Address Translation (NAT) devices keep the state of sessions
that transverse through them.
These devices commonly track the transport layer and/or the
application layer data to drop traffic that is anomalous or malicious
in nature.
A network device that monitors up to the application layer will
commonly expect to see HTTP traffic within a TCP socket running over
port 80, if non-HTTP traffic is seen (such as IKEv2 within TCP), this
could be dropped by the security device. In this scenario the IKEv2
and ESP packets are to be TLS encapsulated (using port 443) to ensure
that the security device permits the traffic.
In the case of a TLS proxy, where the network security device
decrypts, inspects and re-encrypts the session the same
considerations must be taken into account as for HTTP, where TLS can
be inspected. In this case the TLS proxy must allow TCP
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encapsulation of IKEv2 and IPsec otherwise the session could be
dropped.
A network device that monitors the transport layer will track the
state of TCP sessions, it is common that the security device will
perform actions such as tracking TCP sequence numbers.
10. Performance Considerations
Several aspects of TCP encapsulation for IKEv2 and IPSec packets may
negatively impact the performance of connections within the tunnel.
Implementations should be aware of these and take these into
consideration when determining when to use TCP encapsulation.
10.1. TCP-in-TCP
If the outer connection between IKEv2 peers is over TCP, inner TCP
connections may suffer effects from using TCP within TCP. In
particular, the inner TCP's round-trip-time estimation will be
affected by the burstiness of the outer TCP. This will make loss-
recovery of the inner TCP traffic less reactive and more prone to
spurious retransmission timeouts.
10.2. Added Reliability for Unreliable Protocols
Since ESP is an unreliable protocol, transmitting ESP packets over a
TCP connection will change the fundamental behavior of the packets.
Some application-level protocols that prefer packet loss to delay
(such as Voice over IP or other real-time protocols) may be
negatively impacted if their packets are retransmitted by the TCP
connection due to packet loss.
10.3. Encryption Overhead
If TLS or another encryption method is used on the TCP connection,
there may be increased processing overhead for encrypting and
decrypting. This overhead may be experienced as a decrease in
throughput on CPU-limited devices, or an increase in CPU usage or
battery consumption on other devices, therefore the initiator and
responder MUST allow the selection of NULL cipher when using TLS.
Additionally, the TLS record introduces another layer of overhead,
requiring more bytes to transmit a given IKEv2 and IPSec packet.
11. Security Considerations
IKEv2 responders that support TCP encapsulation may become vulnerable
to new Denial-of-Service (DoS) attacks that are specific to TCP, such
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as SYN-flooding attacks. Responders should be aware of this
additional attack-surface.
Attackers may be able to disrupt the TCP connection by sending
spurious RST packets. Due to this, implementations SHOULD make sure
that IKE session state persists even if the underlying TCP connection
is torn down.
If TLS is used on the encapsulating TCP connection, it should not be
considered as a security measure. The security of the IKEv2 session
is entirely derived from the IKEv2 negotiation and key establishment.
12. IANA Considerations
This memo includes no request to IANA.
TCP port 4500 is already allocated to IPSec. This port MAY be used
for the protocol described in this document, but implementations MAY
prefer to use other ports based on local policy. We foresee some
implementations using TCP port 443 to more easily pass through some
middleboxes [I-D.tschofenig-hourglass].
13. Acknowledgments
The authors would like to acknowledge the input and advice of Stuart
Cheshire, Delziel Fernandes, Yoav Nir, Christoph Paasch, Yaron
Sheffer, David Schinazi, Graham Bartlett, March Wu and Kingwel Xie.
Special thanks to Eric Kinnear for his implementation work.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <http://www.rfc-editor.org/info/rfc7296>.
14.2. Informative References
[I-D.tschofenig-hourglass]
Tschofenig, H., "The New Waist of the Hourglass", draft-
tschofenig-hourglass-00 (work in progress), July 2012.
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[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000,
<http://www.rfc-editor.org/info/rfc2817>.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, DOI 10.17487/RFC3948, January 2005,
<http://www.rfc-editor.org/info/rfc3948>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,
<http://www.rfc-editor.org/info/rfc4555>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520,
DOI 10.17487/RFC6520, February 2012,
<http://www.rfc-editor.org/info/rfc6520>.
Authors' Addresses
Tommy Pauly
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
US
Email: tpauly@apple.com
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Samy Touati
Ericsson
300 Holger Way
San Jose, California 95134
US
Email: samy.touati@ericsson.com
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