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 for its negotiation, 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.

In order to close an IKE session, either the initiator or responder SHOULD gracefully tear down IKE SAs with DELETE payloads. Once all SAs have been deleted, the initiator of the original connection MUST close the TCP connection.

An unexpected FIN or a RST on the TCP connection may indicate either a loss of connectivity, an attack, or some other error. If a DELETE payload has not been sent, both sides SHOULD maintain the state for their SAs for the standard lifetime or time-out period. The original initiator (that is, the endpoint that initiated the TCP connection and sent the first IKE_SA_INIT message) is responsible for re-establishing the TCP connection if it is torn down for any unexpected reason. Since new TCP connections may use different ports due to NAT mappings or local port allocations changing, the responder MUST allow packets for existing SAs to be received from new source ports.

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

The streams of data sent over any TCP connection used for this protocol MUST begin with the stream prefix value followed by a complete message, which is either an encapsulated IKE or ESP message. If the connection is being used to resume a previous IKE session, the 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 initiator send an UPDATE_SA_ADDRESSES message if MOBIKE is supported, or an INFORMATIONAL message (a keepalive) otherwise. If either initiator or responder receives a stream that cannot be parsed correctly, it MUST close the TCP connection.

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

A 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. Since a connection may be broken and a new connection re-established by the initiator without the responder being aware, a responder SHOULD accept receiving IKE and ESP messages on a new connection. It will then use that connection for all subsequent responses. A 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 SHOULD NOT share a single TCP connection.

7. Interaction with NAT Detection Payloads

When negotiating over UDP port 500, IKE_SA_INIT packets include NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP payloads to determine if UDP encapsulation of IPsec packets should be used. These payloads contain SHA-1 digests of the SPIs, IP addresses, and ports. IKE_SA_INIT packets sent on a TCP connection SHOULD include these payloads, and SHOULD use the applicable TCP ports when creating and checking the SHA-1 digests.

If a NAT is detected due to the SHA-1 digests not matching the expected values, no change should be made for encapsulation of subsequent 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.

8. Using MOBIKE with TCP encapsulation

When an IKE session is transitioned between networks using MOBIKE [RFC4555], the initiator of the transition may switch between using TCP encapsulation, UDP encapsulation, or no encapsulation. Implementations that implement both MOBIKE and TCP encapsulation MUST support dynamically enabling and disabling TCP encapsulation as interfaces change.

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 MAY choose to not fragment when going over a TCP connection.

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 longers 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 liveness.

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 will commonly expect to see HTTP traffic within a TCP socket running over port 80, if non-HTTP traffic is seen (such as TCP encapsulated IKE), this could be dropped by the security device.

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 the tunnel. Implementations should be aware of these and take these into consideration when determining when to use TCP encapsulation.

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, the inner TCP's round-trip-time estimation will be affected by the burstiness of the outer TCP. This will make loss-recovery of the inner TCP traffic less reactive and more prone to spurious retransmission timeouts.

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 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.

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. Responders should be aware of this additional attack-surface.

Responders should be careful to ensure that the stream prefix "IKETCP" uniquely identifies streams using the TCP encapsulation protocol. The prefix was chosen to not overlap with the start of any known valid protocol over TCP, but implementations should make sure to validate this assumption in order to avoid unexpected processing of TCP connections.

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.

14. IANA Considerations

This memo includes no request to IANA.

TCP port 4500 is already allocated to IPsec. This port MAY be used for the protocol described in this document, but implementations MAY prefer to use other ports based on local policy.

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 and Kingwel Xie. Special thanks to Eric Kinnear for his implementation work.

16. References

16.1. Normative References

16.2. Informative References

Appendix A. Using TCP encapsulation with TLS

This section provides recommendations on the support of TLS with the TCP encapsulation.

When using TCP encapsulation, implementations may choose to use TLS [RFC5246], to be able to traverse middle-boxes, which may block non HTTP traffic.

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

The use of TLS should be configurable on the peers. The 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 initiator should be pre-configured to use TLS or not when communicating with a given port on the 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 initiator and 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 middle-boxes.

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)
      TcpSyn                    ---------->
                                <----------          TcpSyn,Ack
      TcpAck                    ---------->

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

  3)  ---------------------- Stream Prefix --------------------
      "IKETCP"                  ---------->
  4)  ----------------------- IKE Session ---------------------
      IKE_SA_INIT               ---------->
      HDR, SAi1, KEi, Ni,
      [N(NAT_DETECTION_*_IP)]
                                <----------         IKE_SA_INIT
                                            HDR, SAr1, KEr, Nr,
                                         [N(NAT_DETECTION_*_IP)]
      first IKE_AUTH            ---------->
      HDR, SK {IDi, [CERTREQ]
      CP(CFG_REQUEST), IDr,
      SAi2, TSi, TSr, ...}
                                <----------      first IKE_AUTH
                                    HDR, SK {IDr, [CERT], AUTH,
                                           EAP, SAr2, TSi, TSr}
      EAP                       ---------->
      repeat 1..N times
                                <----------                 EAP
      final IKE_AUTH            ---------->
      HDR, SK {AUTH}
                                <----------      final IKE_AUTH
                                  HDR, SK {AUTH, CP(CFG_REPLY),
                                             SA, TSi, TSr, ...}
      ----------------- IKE Tunnel Established ----------------
                

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

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

                Client                              Server
              ----------                          ----------
  1)  --------------------  TCP Connection  -------------------
      (IP_I:Port_I  -> IP_R:TCP443 or TCP4500)
      TcpSyn                    ---------->
                                <----------          TcpSyn,Ack
      TcpAck                    ---------->
  2)  ---------------------  TLS Session  ---------------------
      ClientHello               ---------->
                                <----------         ServerHello
                                             [ChangeCipherSpec]
                                                       Finished
      [ChangeCipherSpec]        ---------->
      Finished
  3)  ---------------------- Stream Prefix --------------------
      "IKETCP"                  ---------->
  4)  <---------------------> IKE/ESP flow <------------------>

    

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

  2)  ------------ MOBIKE Attempt on new network --------------
      (IP_I2:UDP4500 -> IP_R:UDP4500)
      INFORMATIONAL            ----------->
      HDR, SK { N(UPDATE_SA_ADDRESSES),
      N(NAT_DETECTION_SOURCE_IP),
      N(NAT_DETECTION_DESTINATION_IP) }


  3)  --------------------  TCP Connection  -------------------
      (IP_I2:PORT_I -> IP_R:TCP443 or TCP4500)
      TcpSyn                   ----------->
                               <-----------          TcpSyn,Ack
      TcpAck                   ----------->

  4)  ---------------------  TLS Session  ---------------------
      ClientHello              ----------->
                                                    ServerHello
                                                   Certificate*
                                             ServerKeyExchange*
                               <-----------     ServerHelloDone
      ClientKeyExchange
      CertificateVerify*
      [ChangeCipherSpec]
      Finished                 ----------->
                                             [ChangeCipherSpec]
                               <-----------            Finished
  5)  ---------------------- Stream Prefix --------------------
      "IKETCP"                  ---------->

  6)  ----------------------- IKE Session ---------------------
      INFORMATIONAL            ----------->
      HDR, SK { N(UPDATE_SA_ADDRESSES),
      N(NAT_DETECTION_SOURCE_IP),
      N(NAT_DETECTION_DESTINATION_IP) }

                               <-----------       INFORMATIONAL
                          HDR, SK { N(NAT_DETECTION_SOURCE_IP),
                              N(NAT_DETECTION_DESTINATION_IP) }
  7)  <----------------- IKE/ESP data flow ------------------->