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rfc:rfc8229

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.

Pauly, et al. Standards Track [Page 1] 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

Pauly, et al. Standards Track [Page 2] 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.

Pauly, et al. Standards Track [Page 3] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

 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.

Pauly, et al. Standards Track [Page 4] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

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.

Pauly, et al. Standards Track [Page 5] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

 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

Pauly, et al. Standards Track [Page 6] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

 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

Pauly, et al. Standards Track [Page 7] 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

Pauly, et al. Standards Track [Page 8] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

 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.

Pauly, et al. Standards Track [Page 9] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

 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).

Pauly, et al. Standards Track [Page 10] 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.

Pauly, et al. Standards Track [Page 11] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

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.

Pauly, et al. Standards Track [Page 12] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

 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.

Pauly, et al. Standards Track [Page 13] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

 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.

Pauly, et al. Standards Track [Page 14] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

 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.

Pauly, et al. Standards Track [Page 15] 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>.

Pauly, et al. Standards Track [Page 16] 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>.

Pauly, et al. Standards Track [Page 17] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

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.

Pauly, et al. Standards Track [Page 18] 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}

Pauly, et al. Standards Track [Page 19] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

       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.

Pauly, et al. Standards Track [Page 20] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

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.

Pauly, et al. Standards Track [Page 21] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

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.

Pauly, et al. Standards Track [Page 22] RFC 8229 TCP Encapsulation of IKE and IPsec Packets August 2017

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

Pauly, et al. Standards Track [Page 23] 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.

Pauly, et al. Standards Track [Page 24] 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: tpauly@apple.com
 Samy Touati
 Ericsson
 2755 Augustine
 Santa Clara, California  95054
 United States of America
 Email: samy.touati@ericsson.com
 Ravi Mantha
 Cisco Systems
 SEZ, Embassy Tech Village
 Panathur, Bangalore  560 037
 India
 Email: ramantha@cisco.com

Pauly, et al. Standards Track [Page 25]

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