Internet Engineering Task Force (IETF) T. Pauly

Internet Engineering Task Force (IETF) T. Pauly
Request for Comments: 8229 Apple Inc.
Category: Standards Track S. Touati
ISSN: 2070-1721 Ericsson
R. Mantha
Cisco Systems
August 2017

TCP Encapsulation of IKE and IPsec Packets

Abstract

This document describes a method to transport Internet Key Exchange
Protocol (IKE) and IPsec packets over a TCP connection for traversing
network middleboxes that may block IKE negotiation over UDP. This
method, referred to as "TCP encapsulation", involves sending both IKE
packets for Security Association establishment and Encapsulating
Security Payload (ESP) packets over a TCP connection. This method is
intended to be used as a fallback option when IKE cannot be
negotiated over UDP.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8229.

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................3
1.1. Prior Work and Motivation ..................................4
1.2. Terminology and Notation ...................................5
2. Configuration ...................................................5
3. TCP-Encapsulated Header Formats .................................6
3.1. TCP-Encapsulated IKE Header Format .........................6
3.2. TCP-Encapsulated ESP Header Format .........................7
4. TCP-Encapsulated Stream Prefix ..................................7
5. Applicability ...................................................8
5.1. Recommended Fallback from UDP ..............................8
6. Connection Establishment and Teardown ...........................9
7. Interaction with NAT Detection Payloads ........................11
8. Using MOBIKE with TCP Encapsulation ............................11
9. Using IKE Message Fragmentation with TCP Encapsulation .........12
10. Considerations for Keep-Alives and Dead Peer Detection ........12
11. Middlebox Considerations ......................................12
12. Performance Considerations ....................................13
12.1. TCP-in-TCP ...............................................13
12.2. Added Reliability for Unreliable Protocols ...............14
12.3. Quality-of-Service Markings ..............................14
12.4. Maximum Segment Size .....................................14
12.5. Tunneling ECN in TCP .....................................14
13. Security Considerations .......................................15
14. IANA Considerations ...........................................16
15. References ....................................................16
15.1. Normative References .....................................16
15.2. Informative References ...................................17

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

Appendix A. Using TCP Encapsulation with TLS ......................18
Appendix B. Example Exchanges of TCP Encapsulation with TLS .......19
B.1. Establishing an IKE Session ................................19
B.2. Deleting an IKE Session ....................................21
B.3. Re-establishing an IKE Session .............................22
B.4. Using MOBIKE between UDP and TCP Encapsulation .............23
Acknowledgments ...................................................25
Authors' Addresses ................................................25

1. Introduction

The Internet Key Exchange Protocol version 2 (IKEv2) [RFC7296] is a
protocol for establishing IPsec Security Associations (SAs), using
IKE messages over UDP for control traffic, and using Encapsulating
Security Payload (ESP) [RFC4303] messages for encrypted data traffic.
Many network middleboxes that filter traffic on public hotspots block
all UDP traffic, including IKE and IPsec, but allow TCP connections
through because 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 SAs.
This document defines a method for encapsulating IKE control messages
as well as 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:

1. Direct. Currently, IKE negotiations begin over UDP port 500. If
no Network Address Translation (NAT) device is detected between
the Initiator and the Responder, then subsequent IKE packets are
sent over UDP port 500, and IPsec data packets are sent
using ESP.

2. UDP Encapsulation [RFC3948]. If a NAT is detected between the
Initiator and the Responder, then subsequent IKE 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.
Some peers default to using UDP encapsulation even when no NAT is
detected on the path, as some middleboxes do not support IP
protocols other than TCP and UDP.

3. TCP Encapsulation. If the other two methods are not available or
appropriate, IKE negotiation packets as well as ESP packets can
be sent over a single TCP connection to the peer.

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Direct use of ESP or UDP encapsulation should be preferred by
IKE implementations due to performance concerns when using
TCP encapsulation (Section 12). Most implementations should use
TCP encapsulation only on networks where negotiation over UDP has
been attempted without receiving responses from the peer or if a
network is known to not support UDP.

1.1. Prior Work and Motivation

Encapsulating IKE connections within TCP streams is a common approach
to solve the problem of UDP packets being blocked by network
middleboxes. The specific goals of this document are as follows:

o To promote interoperability by defining a standard method of
framing IKE and ESP messages within TCP streams.

o To be compatible with the current IKEv2 standard without requiring
modifications or extensions.

o To use IKE over UDP by default to avoid the overhead of other
alternatives that always rely on TCP or Transport Layer Security
(TLS) [RFC5246].

Some previous alternatives 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 the Internet Security
Association and Key Management Protocol (ISAKMP) have been
deployed that send IPsec traffic over TCP or TCP-like packets.

Secure Sockets Layer (SSL) VPNs
Many proprietary VPN solutions use a combination of TLS and IPsec
in order to provide reliability. These often run on TCP port 443.

IKEv2 over TCP
IKEv2 over TCP as described in [IKE-over-TCP] is used to avoid UDP
fragmentation.

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1.2. Terminology and Notation

This document distinguishes between the IKE peer that initiates TCP
connections to be used for TCP encapsulation and the roles of
Initiator and Responder for particular IKE messages. During the
course of IKE exchanges, the role of IKE Initiator and Responder may
swap for a given SA (as with IKE SA rekeys), while the Initiator of
the TCP connection is still responsible for tearing down the TCP
connection and re-establishing it if necessary. For this reason,
this document will use the term "TCP Originator" to indicate the IKE
peer that initiates TCP connections. The peer that receives TCP
connections will be referred to as the "TCP Responder". If an IKE SA
is rekeyed one or more times, the TCP Originator MUST remain the peer
that originally initiated the first IKE SA.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

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 IKE exchange.
Instead, support for TCP encapsulation must be pre-configured on both
the TCP Originator and the TCP Responder.

Implementations MUST support TCP encapsulation on TCP port 4500,
which is reserved for IPsec NAT traversal.

Beyond a flag indicating support for TCP encapsulation, the
configuration for each peer can include the following optional
parameters:

o Alternate TCP ports on which the specific TCP Responder listens
for incoming connections. Note that the TCP Originator may
initiate TCP connections to the TCP Responder from any local port.

o An extra framing protocol to use on top of TCP to further
encapsulate the stream of IKE and IPsec packets. See Appendix A
for a detailed discussion.

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Since TCP encapsulation of IKE and IPsec packets adds overhead and
has potential performance trade-offs compared to direct or
UDP-encapsulated SAs (as described in Section 12), implementations
SHOULD prefer ESP direct or UDP-encapsulated SAs over
TCP-encapsulated SAs when possible.

3. TCP-Encapsulated Header Formats

Like UDP encapsulation, TCP encapsulation uses the first four bytes
of a message to differentiate IKE and ESP messages. TCP
encapsulation also adds a Length field to define the boundaries of
messages within a stream. The message length is sent in a 16-bit
field that precedes every message. If the first 32 bits of the
message are zeros (a non-ESP marker), then the contents comprise an
IKE message. Otherwise, the contents comprise an ESP message.
Authentication Header (AH) messages are not supported for TCP
encapsulation.

Although a TCP stream may be able to send very long messages,
implementations SHOULD limit message lengths to typical UDP datagram
ESP payload lengths. 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).

Note that this method of encapsulation will also work for placing IKE
and ESP messages within any protocol that presents a stream
abstraction, beyond TCP.

3.1. TCP-Encapsulated IKE 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Non-ESP Marker |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IKE header [RFC7296] ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1

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The IKE header is preceded by a 16-bit Length field in network byte
order that specifies the length of the IKE message (including the
non-ESP marker) within the TCP stream. As with IKE over UDP
port 4500, a zeroed 32-bit non-ESP marker is inserted before the
start of the IKE header in order to differentiate the traffic from
ESP traffic between the same addresses and ports.

o Length (2 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 16-bit Length field in network byte
order that specifies the length of the ESP packet within the TCP
stream.

The Security Parameter Index (SPI) field [RFC7296] in the ESP header
MUST NOT be a zero value.

o Length (2 octets, unsigned integer) - Length of the ESP packet,
including the Length field.

4. TCP-Encapsulated Stream Prefix

Each stream of bytes used for IKE and IPsec encapsulation MUST begin
with a fixed sequence of six bytes as a magic value, containing the
characters "IKETCP" as ASCII values. This value is intended to
identify and validate that the TCP connection is being used for TCP
encapsulation as defined in this document, to avoid conflicts with
the prevalence of previous non-standard protocols that used TCP
port 4500. This value is only sent once, by the TCP Originator only,
at the beginning of any stream of IKE and ESP messages.

If other framing protocols are used within TCP to further encapsulate
or encrypt the stream of IKE and ESP messages, the stream prefix must
be at the start of the TCP Originator's IKE and ESP message stream

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

within the added protocol layer (Appendix A). Although some framing
protocols do support negotiating inner protocols, the stream prefix
should always be used in order for implementations to be as generic
as possible and not rely on other framing protocols on top of TCP.

0 1 2 3 4 5
+------+------+------+------+------+------+
| 0x49 | 0x4b | 0x45 | 0x54 | 0x43 | 0x50 |
+------+------+------+------+------+------+

Figure 3

5. Applicability

TCP encapsulation is applicable only when it has been configured to
be used with specific IKE 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 IKE sessions. Initiators MAY use TCP
encapsulation for any IKE 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.

Since the support of TCP encapsulation is a configured property, not
a negotiated one, it is recommended that if there are multiple IKE
endpoints representing a single peer (such as multiple machines with
different IP addresses when connecting by Fully Qualified Domain
Name, or endpoints used with IKE redirection), all of the endpoints
equally support TCP encapsulation.

If TCP encapsulation is being used for a specific IKE SA, all
messages for that IKE SA and its Child SAs MUST be sent over a TCP
connection until the SA is deleted or IKEv2 Mobility and Multihoming
(MOBIKE) is used to change the SA endpoints and/or the encapsulation
protocol. See Section 8 for more details on using MOBIKE to
transition between encapsulation modes.

5.1. Recommended Fallback from UDP

Since UDP is the preferred method of transport for IKE messages,
implementations that use TCP encapsulation should have an algorithm
for deciding when to use TCP after determining that UDP is unusable.
If an Initiator implementation has no prior knowledge about the
network it is on and the status of UDP on that network, it SHOULD
always attempt to negotiate IKE over UDP first. IKEv2 defines how to
use retransmission timers with IKE messages and, specifically,
IKE_SA_INIT messages [RFC7296]. Generally, this means that the
implementation will define a frequency of retransmission and the
maximum number of retransmissions allowed before marking the IKE SA

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as failed. An implementation can attempt negotiation over TCP once
it has hit the maximum retransmissions over UDP, or slightly before
to reduce connection setup delays. It is recommended that the
initial message over UDP be retransmitted at least once before
falling back to TCP, unless the Initiator knows beforehand that the
network is likely to block UDP.

6. Connection Establishment and Teardown

When the IKE Initiator uses TCP encapsulation, it will initiate a TCP
connection to the Responder using the configured TCP port. The first
bytes sent on the stream MUST be the stream prefix value (Section 4).
After this prefix, encapsulated IKE messages will negotiate the IKE
SA and initial Child SA [RFC7296]. After this point, both
encapsulated IKE (Figure 1) and ESP (Figure 2) messages will be sent
over the TCP connection. The TCP Responder MUST wait for the entire
stream prefix to be received on the stream before trying to parse out
any IKE or ESP messages. The stream prefix is sent only once, and
only by the TCP Originator.

In order to close an IKE session, either the Initiator or Responder
SHOULD gracefully tear down IKE SAs with DELETE payloads. Once the
SA has been deleted, the TCP Originator SHOULD close the TCP
connection if it does not intend to use the connection for another
IKE session to the TCP Responder. If the connection is left idle and
the TCP Responder needs to clean up resources, the TCP Responder MAY
close the TCP connection.

An unexpected FIN or a TCP Reset on the TCP connection may indicate 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 timeout period. The TCP
Originator 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 TCP Responder MUST allow packets for existing SAs to be
received from new source ports.

A peer MUST discard a partially received message due to a broken
connection.

Whenever the TCP Originator opens a new TCP connection to be used for
an existing IKE SA, it MUST send the stream prefix first, before any
IKE or ESP messages. This follows the same behavior as the initial
TCP connection.

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If a TCP connection is being used to resume a previous IKE session,
the TCP Responder can recognize the session using either the IKE SPI
from an encapsulated IKE message or the ESP SPI from an encapsulated
ESP message. If the session had been fully established previously,
it is suggested that the TCP Originator send an UPDATE_SA_ADDRESSES
message if MOBIKE is supported, or an informational message (a
keep-alive) otherwise.

The TCP Responder MUST NOT accept any messages for the existing IKE
session on a new incoming connection, unless that connection begins
with the stream prefix. If either the TCP Originator or TCP
Responder detects corruption on a connection that was started with a
valid stream prefix, it SHOULD close the TCP connection. The
connection can be determined to be corrupted if there are too many
subsequent messages that cannot be parsed as valid IKE messages or
ESP messages with known SPIs, or if the authentication check for an
ESP message with a known SPI fails. Implementations SHOULD NOT
tear down a connection if only a single ESP message has an unknown
SPI, since the SPI databases may be momentarily out of sync. If
there is instead a syntax issue within an IKE message, an
implementation MUST send the INVALID_SYNTAX notify payload and
tear down the IKE SA as usual, rather than tearing down the TCP
connection directly.

A TCP Originator SHOULD only open one TCP connection per IKE SA, over
which it sends all of the corresponding IKE and ESP messages. This
helps ensure that any firewall or NAT mappings allocated for the TCP
connection apply to all of the traffic associated with the IKE SA
equally.

Similarly, a TCP Responder SHOULD at any given time send packets for
an IKE SA and its Child SAs over only one TCP connection. It SHOULD
choose the TCP connection on which it last received a valid and
decryptable IKE or ESP message. In order to be considered valid for
choosing a TCP connection, an IKE message must be successfully
decrypted and authenticated, not be a retransmission of a previously
received message, and be within the expected window for IKE
message IDs. Similarly, an ESP message must pass authentication
checks and be decrypted, and must not be a replay of a previous
message.

Since a connection may be broken and a new connection re-established
by the TCP Originator without the TCP Responder being aware, a TCP
Responder SHOULD accept receiving IKE and ESP messages on both old
and new connections until the old connection is closed by the TCP
Originator. A TCP Responder MAY close a TCP connection that it
perceives as idle and extraneous (one previously used for IKE and ESP
messages that has been replaced by a new connection).

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

Multiple IKE SAs MUST NOT share a single TCP connection, unless one
is a rekey of an existing IKE SA, in which case there will
temporarily be two IKE SAs on the same TCP connection.

7. 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 as defined in [RFC7296]. IKE_SA_INIT packets sent on a TCP
connection SHOULD include these payloads with the same content as
when sending over UDP 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 IKE 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.

If a NAT is detected, implementations need to handle transport mode
TCP and UDP packet checksum fixup as defined for UDP encapsulation in
[RFC3948].

8. Using MOBIKE with TCP Encapsulation

When an IKE session that has negotiated MOBIKE [RFC4555] is
transitioning between networks, 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.

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 TCP connection, regardless of
if a connection on a previous network did not use TCP encapsulation.

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9. Using IKE Message Fragmentation with TCP Encapsulation

IKE message fragmentation [RFC7383] is not required when using TCP
encapsulation, since a TCP stream already handles the fragmentation
of its contents across packets. Since fragmentation is redundant in
this case, implementations might choose to not negotiate IKE
fragmentation. Even if fragmentation is negotiated, an
implementation SHOULD NOT send fragments when going over a TCP
connection, although it MUST support receiving fragments.

If an implementation supports both MOBIKE and IKE fragmentation, it
SHOULD negotiate IKE fragmentation over a TCP-encapsulated session in
case the session switches to UDP encapsulation on another network.

10. Considerations for Keep-Alives and Dead Peer Detection

Encapsulating IKE and IPsec inside of a TCP connection can impact the
strategy that implementations use to detect peer liveness and to
maintain middlebox port mappings. Peer liveness should be checked
using IKE informational packets [RFC7296].

In general, TCP port mappings are maintained by NATs longer than UDP
port mappings, so IPsec ESP NAT keep-alives [RFC3948] SHOULD NOT be
sent when using TCP encapsulation. Any implementation using TCP
encapsulation MUST silently drop incoming NAT keep-alive packets
and not treat them as errors. NAT keep-alive packets over a
TCP-encapsulated IPsec connection will be sent as an ESP message with
a one-octet-long payload with the value 0xFF.

Note that, depending on the configuration of TCP and TLS on the
connection, TCP keep-alives [RFC1122] and TLS keep-alives [RFC6520]
may be used. These MUST NOT be used as indications of IKE peer
liveness.

11. Middlebox Considerations

Many security networking devices, such as firewalls or intrusion
prevention systems, network optimization/acceleration devices, and
NAT devices, keep the state of sessions that traverse through them.

These devices commonly track the transport-layer and/or application-
layer data to drop traffic that is anomalous or malicious in nature.
While many of these devices will be more likely to pass
TCP-encapsulated traffic as opposed to UDP-encapsulated traffic, some
may still block or interfere with TCP-encapsulated IKE and IPsec
traffic.

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A network device that monitors the transport layer will track the
state of TCP sessions, such as TCP sequence numbers. TCP
encapsulation of IKE should therefore use standard TCP behaviors to
avoid being dropped by middleboxes.

12. Performance Considerations

Several aspects of TCP encapsulation for IKE and IPsec packets may
negatively impact the performance of connections within a tunnel-mode
IPsec SA. Implementations should be aware of these performance
impacts and take these into consideration when determining when to
use TCP encapsulation. Implementations SHOULD favor using direct ESP
or UDP encapsulation over TCP encapsulation whenever possible.

12.1. TCP-in-TCP

If the outer connection between IKE peers is over TCP, inner TCP
connections may suffer negative effects from using TCP within TCP.
Running TCP within TCP is discouraged, since the TCP algorithms
generally assume that they are running over an unreliable datagram
layer.

If the outer (tunnel) TCP connection experiences packet loss, this
loss will be hidden from any inner TCP connections, since the outer
connection will retransmit to account for the losses. Since the
outer TCP connection will deliver the inner messages in order, any
messages after a lost packet may have to wait until the loss is
recovered. This means that loss on the outer connection will be
interpreted only as delay by inner connections. The burstiness of
inner traffic can increase, since a large number of inner packets may
be delivered across the tunnel at once. The inner TCP connection may
interpret a long period of delay as a transmission problem,
triggering a retransmission timeout, which will cause spurious
retransmissions. The sending rate of the inner connection may be
unnecessarily reduced if the retransmissions are not detected as
spurious in time.

The inner TCP connection's round-trip-time estimation will be
affected by the burstiness of the outer TCP connection if there are
long delays when packets are retransmitted by the outer TCP
connection. This will make the congestion control loop of the inner
TCP traffic less reactive, potentially permanently leading to a lower
sending rate than the outer TCP would allow for.

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TCP-in-TCP can also lead to increased buffering, or bufferbloat.
This can occur when the window size of the outer TCP connection is
reduced and becomes smaller than the window sizes of the inner TCP
connections. This can lead to packets backing up in the outer TCP
connection's send buffers. In order to limit this effect, the outer
TCP connection should have limits on its send buffer size and on the
rate at which it reduces its window size.

Note that any negative effects will be shared between all flows going
through the outer TCP connection. This is of particular concern for
any latency-sensitive or real-time applications using the tunnel. If
such traffic is using a TCP-encapsulated IPsec connection, it is
recommended that the number of inner connections sharing the tunnel
be limited as much as possible.

12.2. 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.

12.3. Quality-of-Service Markings

Quality-of-Service (QoS) markings, such as the Differentiated
Services Code Point (DSCP) and Traffic Class, should be used with
care on TCP connections used for encapsulation. Individual packets
SHOULD NOT use different markings than the rest of the connection,
since packets with different priorities may be routed differently and
cause unnecessary delays in the connection.

12.4. Maximum Segment Size

A TCP connection used for IKE encapsulation SHOULD negotiate its MSS
in order to avoid unnecessary fragmentation of packets.

12.5. Tunneling ECN in TCP

Since there is not a one-to-one relationship between outer IP packets
and inner ESP/IP messages when using TCP encapsulation, the markings
for Explicit Congestion Notification (ECN) [RFC3168] cannot be simply
mapped. However, any ECN Congestion Experienced (CE) marking on
inner headers should be preserved through the tunnel.

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Implementations SHOULD follow the ECN compatibility mode for tunnel
ingress as described in [RFC6040]. In compatibility mode, the outer
tunnel TCP connection marks its packet headers as not ECN-capable.
If upon egress, the arriving outer header is marked with CE, the
implementation will drop the inner packet, since there is not a
distinct inner packet header onto which to translate the ECN
markings.

13. Security Considerations

IKE Responders that support TCP encapsulation may become vulnerable
to new Denial-of-Service (DoS) attacks that are specific to TCP, such
as SYN-flooding attacks. TCP Responders should be aware of this
additional attack surface.

TCP Responders should be careful to ensure that (1) the stream prefix
"IKETCP" uniquely identifies incoming streams as streams that use the
TCP encapsulation protocol and (2) they are not running any other
protocols on the same listening port (to avoid potential conflicts).

Attackers may be able to disrupt the TCP connection by sending
spurious TCP Reset packets. Therefore, implementations SHOULD make
sure that IKE session state persists even if the underlying TCP
connection is torn down.

If MOBIKE is being used, all of the security considerations outlined
for MOBIKE apply [RFC4555].

Similarly to MOBIKE, TCP encapsulation requires a TCP Responder to
handle changes to source address and port due to network or
connection disruption. The successful delivery of valid IKE or ESP
messages over a new TCP connection is used by the TCP Responder to
determine where to send subsequent responses. If an attacker is able
to send packets on a new TCP connection that pass the validation
checks of the TCP Responder, it can influence which path future
packets will take. For this reason, the validation of messages on
the TCP Responder must include decryption, authentication, and replay
checks.

Since TCP provides reliable, in-order delivery of ESP messages, the
ESP anti-replay window size SHOULD be set to 1. See [RFC4303] for a
complete description of the ESP anti-replay window. This increases
the protection of implementations against replay attacks.

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

14. IANA Considerations

TCP port 4500 is already allocated to IPsec for NAT traversal. This
port SHOULD be used for TCP-encapsulated IKE and ESP as described in
this document.

This document updates the reference for TCP port 4500:

Keyword Decimal Description Reference
----------- -------- ------------------- ---------
ipsec-nat-t 4500/tcp IPsec NAT-Traversal RFC 8229

Figure 4

15. References

15.1. 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>.

[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>.

[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040,
November 2010, <http://www.rfc-editor.org/info/rfc6040>.

[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>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

15.2. Informative References

[IKE-over-TCP]
Nir, Y., "A TCP transport for the Internet Key Exchange",
Work in Progress, draft-ietf-ipsecme-ike-tcp-01,
December 2012.

[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>.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.

[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>.

[RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2
(IKEv2) Message Fragmentation", RFC 7383,
DOI 10.17487/RFC7383, November 2014,
<http://www.rfc-editor.org/info/rfc7383>.

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Appendix A. Using TCP Encapsulation with TLS

This section provides recommendations on how to use TLS in addition
to TCP encapsulation.

When using TCP encapsulation, implementations may choose to use TLS
[RFC5246] on the TCP connection to be able to traverse middleboxes,
which may otherwise block the traffic.

If a web proxy is applied to the ports used for the TCP connection
and TLS is being used, the TCP Originator can send an HTTP CONNECT
message to establish an SA through the proxy [RFC2817].

The use of TLS should be configurable on the peers, and may be used
as the default when using TCP encapsulation or may be used as a
fallback when basic TCP encapsulation fails. The TCP Responder may
expect to read encapsulated IKE and ESP packets directly from the TCP
connection, or it may expect to read them from a stream of TLS data
packets. The TCP Originator should be pre-configured to use TLS
or not when communicating with a given port on the TCP Responder.

When new TCP connections are re-established due to a broken
connection, TLS must be renegotiated. TLS session resumption is
recommended to improve efficiency in this case.

The security of the IKE session is entirely derived from the IKE
negotiation and key establishment and not from the TLS session (which
in this context is only used for encapsulation purposes); therefore,
when TLS is used on the TCP connection, both the TCP Originator and
the TCP Responder SHOULD allow the NULL cipher to be selected for
performance reasons.

Implementations should be aware that the use of TLS introduces
another layer of overhead requiring more bytes to transmit a given
IKE and IPsec packet. For this reason, direct ESP, UDP
encapsulation, or TCP encapsulation without TLS should be preferred
in situations in which TLS is not required in order to traverse
middleboxes.

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

Appendix B. Example Exchanges of TCP Encapsulation with TLS

B.1. Establishing an IKE Session

Client Server
---------- ----------
1) -------------------- TCP Connection -------------------
(IP_I:Port_I -> IP_R:Port_R)
TcpSyn ---------->
<---------- TcpSyn,Ack
TcpAck ---------->

2) --------------------- TLS Session ---------------------
ClientHello ---------->
ServerHello
Certificate*
ServerKeyExchange*
<---------- ServerHelloDone
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ---------->
[ChangeCipherSpec]
<---------- Finished

3) ---------------------- Stream Prefix --------------------
"IKETCP" ---------->
4) ----------------------- IKE Session ---------------------
Length + Non-ESP Marker ---------->
IKE_SA_INIT
HDR, SAi1, KEi, Ni,
[N(NAT_DETECTION_*_IP)]
<------ Length + Non-ESP Marker
IKE_SA_INIT
HDR, SAr1, KEr, Nr,
[N(NAT_DETECTION_*_IP)]
Length + Non-ESP Marker ---------->
first IKE_AUTH
HDR, SK {IDi, [CERTREQ]
CP(CFG_REQUEST), IDr,
SAi2, TSi, TSr, ...}
<------ Length + Non-ESP Marker
first IKE_AUTH
HDR, SK {IDr, [CERT], AUTH,
EAP, SAr2, TSi, TSr}

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Length + Non-ESP Marker ---------->
IKE_AUTH + EAP
repeat 1..N times
<------ Length + Non-ESP Marker
IKE_AUTH + EAP
Length + Non-ESP Marker ---------->
final IKE_AUTH
HDR, SK {AUTH}
<------ Length + Non-ESP Marker
final IKE_AUTH
HDR, SK {AUTH, CP(CFG_REPLY),
SA, TSi, TSr, ...}
-------------- IKE and IPsec SAs Established ------------
Length + ESP Frame ---------->

Figure 5

1. The client establishes a TCP connection with the server on
port 4500 or on an alternate pre-configured port that the server
is listening on.

2. If configured to use TLS, the client initiates a TLS handshake.
During the TLS handshake, the server SHOULD NOT request the
client's certificate, since authentication is handled as part of
IKE negotiation.

3. The client sends the stream prefix for TCP-encapsulated IKE
(Section 4) traffic to signal the beginning of IKE negotiation.

4. The client and server establish an IKE connection. This example
shows EAP-based authentication, although any authentication type
may be used.

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B.2. Deleting an IKE Session

Client Server
---------- ----------
1) ----------------------- IKE Session ---------------------
Length + Non-ESP Marker ---------->
INFORMATIONAL
HDR, SK {[N,] [D,]
[CP,] ...}
<------ Length + Non-ESP Marker
INFORMATIONAL
HDR, SK {[N,] [D,]
[CP], ...}

2) --------------------- TLS Session ---------------------
close_notify ---------->
<---------- close_notify
3) -------------------- TCP Connection -------------------
TcpFin ---------->
<---------- Ack
<---------- TcpFin
Ack ---------->
-------------------- IKE SA Deleted -------------------

Figure 6

1. The client and server exchange informational messages to notify
IKE SA deletion.

2. The client and server negotiate TLS session deletion using TLS
CLOSE_NOTIFY.

3. The TCP connection is torn down.

The deletion of the IKE SA should lead to the disposal of the
underlying TLS and TCP state.

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B.3. Re-establishing an IKE Session

Client Server
---------- ----------
1) -------------------- TCP Connection -------------------
(IP_I:Port_I -> IP_R:Port_R)
TcpSyn ---------->
<---------- TcpSyn,Ack
TcpAck ---------->
2) --------------------- TLS Session ---------------------
ClientHello ---------->
<---------- ServerHello
[ChangeCipherSpec]
Finished
[ChangeCipherSpec] ---------->
Finished
3) ---------------------- Stream Prefix --------------------
"IKETCP" ---------->
4) <---------------------> IKE/ESP Flow <------------------>
Length + ESP Frame ---------->

Figure 7

1. If a previous TCP connection was broken (for example, due to a
TCP Reset), the client is responsible for re-initiating the TCP
connection. The TCP Originator's address and port (IP_I and
Port_I) may be different from the previous connection's address
and port.

2. In the ClientHello TLS message, the client SHOULD send the
session ID it received in the previous TLS handshake if
available. It is up to the server to perform either an
abbreviated handshake or a full handshake based on the session ID
match.

3. After TCP and TLS are complete, the client sends the stream
prefix for TCP-encapsulated IKE traffic (Section 4).

4. The IKE and ESP packet flow can resume. If MOBIKE is being used,
the Initiator SHOULD send an UPDATE_SA_ADDRESSES message.

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B.4. Using MOBIKE between UDP and TCP Encapsulation

Client Server
---------- ----------
(IP_I1:UDP500 -> IP_R:UDP500)
1) ----------------- IKE_SA_INIT Exchange -----------------
(IP_I1:UDP4500 -> IP_R:UDP4500)
Non-ESP Marker ----------->
Initial IKE_AUTH
HDR, SK { IDi, CERT, AUTH,
CP(CFG_REQUEST),
SAi2, TSi, TSr,
N(MOBIKE_SUPPORTED) }
<----------- Non-ESP Marker
Initial IKE_AUTH
HDR, SK { IDr, CERT, AUTH,
EAP, SAr2, TSi, TSr,
N(MOBIKE_SUPPORTED) }
<------------------ IKE SA Establishment --------------->

2) ------------ MOBIKE Attempt on New Network --------------
(IP_I2:UDP4500 -> IP_R:UDP4500)
Non-ESP Marker ----------->
INFORMATIONAL
HDR, SK { N(UPDATE_SA_ADDRESSES),
N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }

3) -------------------- TCP Connection -------------------
(IP_I2:Port_I -> IP_R:Port_R)
TcpSyn ----------->
<----------- TcpSyn,Ack
TcpAck ----------->

4) --------------------- TLS Session ---------------------
ClientHello ----------->
ServerHello
Certificate*
ServerKeyExchange*
<----------- ServerHelloDone
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ----------->
[ChangeCipherSpec]
<----------- Finished

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

5) ---------------------- Stream Prefix --------------------
"IKETCP" ---------->

6) ----------------------- IKE Session ---------------------
Length + Non-ESP Marker ----------->
INFORMATIONAL (Same as step 2)
HDR, SK { N(UPDATE_SA_ADDRESSES),
N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }

<------- Length + Non-ESP Marker
HDR, SK { N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }
7) <----------------- IKE/ESP Data Flow ------------------->

Figure 8

1. During the IKE_SA_INIT exchange, the client and server exchange
MOBIKE_SUPPORTED notify payloads to indicate support for MOBIKE.

2. The client changes its point of attachment to the network and
receives a new IP address. The client attempts to re-establish
the IKE session using the UPDATE_SA_ADDRESSES notify payload, but
the server does not respond because the network blocks UDP
traffic.

3. The client brings up a TCP connection to the server in order to
use TCP encapsulation.

4. The client initiates a TLS handshake with the server.

5. The client sends the stream prefix for TCP-encapsulated IKE
traffic (Section 4).

6. The client sends the UPDATE_SA_ADDRESSES notify payload on the
TCP-encapsulated connection. Note that this IKE message is the
same as the one sent over UDP in step 2; it should have the same
message ID and contents.

7. The IKE and ESP packet flow can resume.

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RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

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, Byju Pularikkal, March Wu,
Kingwel Xie, Valery Smyslov, Jun Hu, and Tero Kivinen. Special
thanks to Eric Kinnear for his implementation work.

Authors' Addresses

Tommy Pauly
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
United States of America

Email: [email protected]

Samy Touati
Ericsson
2755 Augustine
Santa Clara, California 95054
United States of America

Email: [email protected]

Ravi Mantha
Cisco Systems
SEZ, Embassy Tech Village
Panathur, Bangalore 560 037
India

Email: [email protected]

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