Network                                                         T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                               S. Touati
Expires: September 13, December 1, 2017                                       Ericsson
                                                               R. Mantha
                                                           Cisco Systems
                                                          March 12,
                                                            May 30, 2017

               TCP Encapsulation of IKE and IPsec Packets


   This document describes a method to transport 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 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

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   This Internet-Draft will expire on September 13, December 1, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   3
     1.1.  Prior Work and Motivation . . . . . . . . . . . . . . . .   3
     1.2.  Terminology and Notation  . . . . . . . . . . . . . . . .   4
   2.  Configuration . . . . . . . . . . . . . . . . . . . . . . . .   4   5
   3.  TCP-Encapsulated Header Formats . . . . . . . . . . . . . . .   5
     3.1.  TCP-Encapsulated IKE Header Format  . . . . . . . . . . .   6
     3.2.  TCP-Encapsulated ESP Header Format  . . . . . . . . . . .   6
   4.  TCP-Encapsulated Stream Prefix  . . . . . . . . . . . . . . .   7
   5.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Recommended Fallback from UDP . . . . . . . . . . . . . .   8
   6.  Connection Establishment and Teardown . . . . . . . . . . . .   8
   7.  Interaction with NAT Detection Payloads . . . . . . . . . . .  10
   8.  Using MOBIKE with TCP encapsulation . . . . . . . . . . . . .  10
   9.  Using IKE Message Fragmentation with TCP encapsulation  . . .  11
   10. Considerations for Keep-alives and DPD  . . . . . . . . . . .  11
   11. Middlebox Considerations  . . . . . . . . . . . . . . . . . .  11  12
   12. Performance Considerations  . . . . . . . . . . . . . . . . .  12
     12.1.  TCP-in-TCP . . . . . . . . . . . . . . . . . . . . . . .  12
     12.2.  Added Reliability for Unreliable Protocols . . . . . . .  12  13
     12.3.  Quality of Service Markings  . . . . . . . . . . . . . .  12  13
     12.4.  Maximum Segment Size . . . . . . . . . . . . . . . . . .  12  13
     12.5.  Tunnelling ECN in TCP  . . . . . . . . . . . . . . . . .  14
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  13  14
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13  15
   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14  15
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14  15
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  14  15
     16.2.  Informative References . . . . . . . . . . . . . . . . .  14  16
   Appendix A.  Using TCP encapsulation with TLS . . . . . . . . . .  15  17
   Appendix B.  Example exchanges of TCP Encapsulation with TLS  . .  16  17
     B.1.  Establishing an IKE session . . . . . . . . . . . . . . .  16  17
     B.2.  Deleting an IKE session . . . . . . . . . . . . . . . . .  17  19
     B.3.  Re-establishing an IKE session  . . . . . . . . . . . . .  18  20
     B.4.  Using MOBIKE between UDP and TCP Encapsulation  . . . . .  19  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21  23

1.  Introduction

   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) 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 since they appear to be web traffic.  Devices
   on these networks that need to use IPsec (to access private
   enterprise networks, to route voice-over-IP calls to carrier
   networks, or because of security policies) are unable to establish
   IPsec SAs.  This document defines a method for encapsulating both the
   IKE control messages as well as the IPsec data messages within a TCP

   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 NAT 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 [RFC4303].

   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 are detected on the path as some middleboxes do not
         support IP protocols other than TCP and UDP.

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

   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 goal of this document is to promote
   interoperability by providing a standard method of framing IKE and
   ESP message within streams, and to provide guidelines for how to
   configure and use TCP encapsulation.

   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 ISAKMP have been
         deployed that send IPsec traffic over TCP or TCP-like packets.

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

   IKEv2 over TCP  IKEv2 over TCP as described in
         [I-D.nir-ipsecme-ike-tcp] is used to avoid UDP fragmentation.

   The goal of this specification is to provide a standardized method
   for using TCP streams to transport IPsec that is compatible with the
   current IKE standard, and avoids the overhead of other alternatives
   that always rely on TCP or TLS.

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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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.

   The configuration defined

   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 should can include the following optional

   o  One or more  Alternate TCP ports on which the specific TCP Responder will listen listens
      for incoming connections.  Note that the TCP Originator may
      initiate TCP connections to the TCP Responder from any local port.  The
      ports on which the TCP Responder listens will likely be based on
      the ports commonly allowed on restricted networks.

   o  Optionally, an  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.

   This document leaves the selection of TCP ports up to
   implementations.  It is suggested to use TCP port 4500, which is
   allocated for IPsec NAT Traversal.

   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 Performance Considerations,
   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 SPI field 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 allows peers value is intended to
   differentiate this protocol from other protocols
   identify and validate that may be run over the same TCP port.  Since TCP encapsulated IPsec connection is not assigned to a
   specific port, being used for TCP Responders may be able
   encapsulation as defined in this document, to receive multiple
   protocols on the same port.  The bytes of the stream prefix do not
   overlap avoid conflicts with
   the valid start prevalence of any other known stream protocol. 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 MOBIKE is used to change the SA
   endpoints and/or 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 negotiate IKE over UDP first.  IKEv2 defines how to
   use retransmission timers with IKE messages, and IKE_SA_INIT messages
   specifically [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 is 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 RST on the TCP connection may indicate either
   a loss of connectivity, an attack, or some other error.  If a DELETE
   payload has not been sent, both sides SHOULD maintain the state for
   their SAs for the standard lifetime or time-out period.  The 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
   keepalive) 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
   receives detects corruption on a stream connection that cannot be parsed correctly (for example, if
   the TCP Originator stream is missing the was started with a
   valid stream prefix, or message
   frames it SHOULD close the TCP connection.  The
   connection can be determined as corrupted if there are not parsable too many
   subsequent messages that cannot be parsed as valid IKE messages or
   ESP messages), it MUST close messages with known SPIs, or if the
   TCP connection. 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.

   An 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, 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).

   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 DPD

   Encapsulating IKE and IPsec inside of a TCP connection can impact the
   strategy that implementations use to detect peer liveness and to
   maintain middlebox 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 with a length value of 1
   byte, whose value is 0xFF Figure 2.

   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
   Network Address Translation (NAT) devices keep the state of sessions
   that traverse through them.

   These devices commonly track the transport layer and/or the
   application layer data to drop traffic that is anomalous or malicious
   in nature.

   A network device that monitors up to the application layer  While many of these devices will
   commonly expect be more likely to see HTTP traffic within a TCP socket running over
   port 80, if non-HTTP pass
   TCP-encapsulated traffic is seen (such as TCP encapsulated IKE),
   this could be dropped by the security device. 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 effects from using TCP within TCP.  In
   particular,  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's TCP connection's round-trip-time estimation will be
   affected by the burstiness of the outer TCP. TCP connection if there are
   long delays when packets are retransmitted by the outer TCP
   connection.  This will make loss-
   recovery the congestion control loop of the inner
   TCP traffic less reactive 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 more prone becomes smaller than the window sizes of the inner TCP
   connections.  This can lead to
   spurious retransmission timeouts. 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 DSCP and Traffic Class,
   should be used with care on TCP connections used for encapsulation.
   Individual packets 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
   maximum segment size (MSS) in order to avoid unnecessary
   fragmentation of packets.

12.5.  Tunnelling 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 messages should be preserved through the tunnel.

   Implementations SHOULD follow the ECN compatibility mode as described
   in [RFC6040].  In compatibility mode, the outer TCP connection SHOULD
   mark its packets as not ECN-capable, and MUST NOT use different clear any ECN
   markings than the rest of
   the connection, since on inner packets.  Note that outer packets with different priorities may be routed
   differently and cause unnecessary delays in ECN marked
   even though the connection.

12.4.  Maximum Segment Size

   A TCP outer connection used for IKE encapsulation SHOULD did not negotiate its
   maximum segment size (MSS) support for ECN.
   If an implementation receives such an outer packet, it MAY propagate
   the markings as described in order to avoid unnecessary
   fragmentation of the Default Tunnel Egress Behaviour
   [RFC6040] for any inner packet contained within a single outer TCP
   packet, or simply apply the rules as if the outer packet were Not-ECT
   if the inner packet spans multiple outer packets.

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 the stream prefix
   "IKETCP" uniquely identifies incoming streams using as ones that use the
   TCP encapsulation
   protocol.  The prefix was chosen to protocol, and they are not overlap with the start of running any
   known valid protocol over TCP, but implementations should make sure
   to validate this assumption in order to avoid unexpected processing
   of TCP connections. other
   protocols on the same listening port that could conflict with this.

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

   If 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 changing of 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 take.  For this reason, the validation of messages on the TCP
   Responder must include decryption, authentication, and replay checks.

   Since TCP provides a 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

   This memo includes no request to IANA.

   TCP port 4500 is already allocated to IPsec. IPsec for NAT Traversal.  This
   port MAY SHOULD be used for the protocol TCP encapsulated IKE and ESP as described in
   this document, but implementations MAY
   prefer to use other ports based on local policy. document.

   This document updates the reference for TCP port 4500:

         Keyword       Decimal    Description          Reference
         -------       -------    -----------          ---------
         ipsec-nat-t   4500/tcp   IPsec NAT-Traversal  [RFC-this-rfc]

                                 Figure 4

15.  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.

16.  References

16.1.  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, <>.

   [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, <>.

16.2.  Informative References

              Nir, Y., "A TCP transport for the Internet Key Exchange",
              draft-nir-ipsecme-ike-tcp-01 (work in progress), July

   [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 the support of how to use TLS with the 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 middle-boxes,
   which may otherwise block non-
   HTTP 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 else be 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 re-negotiated.  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
   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:TCP443 or TCP4500) 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 4 5

   1.    Client establishes a TCP connection with the server on port 443
         4500, or 4500. an alternate pre-configured port that the server is
         listening on.

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

   3.    Client send the Stream Prefix for TCP encapsulated IKE
         Section 4 traffic to signal the beginning of IKE negotiation.

   4.    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 5 6

   1.    Client and server exchange INFORMATIONAL messages to notify IKE
         SA deletion.

   2.    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:TCP443 or TCP4500) 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 6 7

   1.    If a previous TCP connection was broken (for example, due to a
         RST), 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 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 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 UPDATE_SA_ADDRESSES.

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:TCP443 or TCP4500) 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 7 8

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

   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 and 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, and should have the
         same message ID and contents.

   7.    The IKE and ESP packet flow can resume.

Authors' Addresses

   Tommy Pauly
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014


   Samy Touati
   2755 Augustine
   Santa Clara, California  95054


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