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RFC 8229 - TCP Encapsulation of IKE and IPsec Packets

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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


   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

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

   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.

   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

   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

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",
   "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

   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.

   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

   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

   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

   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

   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

   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

   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.

   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

   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

   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).

   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
   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

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.

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

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

   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

   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.

   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.

   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

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

   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.

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,

   [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,

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,

   [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>.

15.2.  Informative References

              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,

   [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
              HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000,

   [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,

   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [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,

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,

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

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               ---------->
                                   <----------     ServerHelloDone
         Finished                  ---------->
                                   <----------            Finished

     3)  ---------------------- Stream Prefix --------------------
         "IKETCP"                  ---------->
     4)  ----------------------- IKE Session ---------------------
         Length + Non-ESP Marker   ---------->
         HDR, SAi1, KEi, Ni,
                                   <------ Length + Non-ESP Marker
                                               HDR, SAr1, KEr, Nr,
         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}

         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.

B.2.  Deleting an IKE Session

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

   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.

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]        ---------->
     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

   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.

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,
         SAi2, TSi, TSr,
                                  <-----------      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           ----------->

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

     4)  ---------------------  TLS Session  ---------------------
         ClientHello              ----------->
                                  <-----------     ServerHelloDone
         Finished                 ----------->
                                  <-----------            Finished

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

     6)  ----------------------- IKE Session ---------------------
         Length + Non-ESP Marker  ----------->
         INFORMATIONAL (Same as step 2)

                                  <------- 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

   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.


   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
   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

   Email: ramantha@cisco.com


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