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Internet Engineering Task Force (IETF)                          T. PaulyRequest for Comments: 8229                                    Apple Inc.Category: Standards Track                                      S. TouatiISSN: 2070-1721                                                 Ericsson                                                               R. Mantha                                                           Cisco Systems                                                             August 2017TCP Encapsulation of IKE and IPsec PacketsAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8229.Pauly, et al.                Standards Track                    [Page 1]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017Copyright Notice   Copyright (c) 2017 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................31.1. Prior Work and Motivation ..................................41.2. Terminology and Notation ...................................52. Configuration ...................................................53. TCP-Encapsulated Header Formats .................................63.1. TCP-Encapsulated IKE Header Format .........................63.2. TCP-Encapsulated ESP Header Format .........................74. TCP-Encapsulated Stream Prefix ..................................75. Applicability ...................................................85.1. Recommended Fallback from UDP ..............................86. Connection Establishment and Teardown ...........................97. Interaction with NAT Detection Payloads ........................118. Using MOBIKE with TCP Encapsulation ............................119. Using IKE Message Fragmentation with TCP Encapsulation .........1210. Considerations for Keep-Alives and Dead Peer Detection ........1211. Middlebox Considerations ......................................1212. Performance Considerations ....................................1312.1. TCP-in-TCP ...............................................1312.2. Added Reliability for Unreliable Protocols ...............1412.3. Quality-of-Service Markings ..............................1412.4. Maximum Segment Size .....................................1412.5. Tunneling ECN in TCP .....................................1413. Security Considerations .......................................1514. IANA Considerations ...........................................1615. References ....................................................1615.1. Normative References .....................................1615.2. Informative References ...................................17Pauly, et al.                Standards Track                    [Page 2]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017Appendix A. Using TCP Encapsulation with TLS ......................18Appendix B. Example Exchanges of TCP Encapsulation with TLS .......19B.1. Establishing an IKE Session ................................19B.2. Deleting an IKE Session ....................................21B.3. Re-establishing an IKE Session .............................22B.4. Using MOBIKE between UDP and TCP Encapsulation .............23   Acknowledgments ...................................................25   Authors' Addresses ................................................251.  Introduction   The Internet Key Exchange Protocol version 2 (IKEv2) [RFC7296] is a   protocol for establishing IPsec Security Associations (SAs), using   IKE messages over UDP for control traffic, and using Encapsulating   Security Payload (ESP) [RFC4303] messages for encrypted data traffic.   Many network middleboxes that filter traffic on public hotspots block   all UDP traffic, including IKE and IPsec, but allow TCP connections   through because they appear to be web traffic.  Devices on these   networks that need to use IPsec (to access private enterprise   networks, to route Voice over IP calls to carrier networks, or   because of security policies) are unable to establish IPsec SAs.   This document defines a method for encapsulating IKE control messages   as well as IPsec data messages within a TCP connection.   Using TCP as a transport for IPsec packets adds a third option to the   list of traditional IPsec transports:   1.  Direct.  Currently, IKE negotiations begin over UDP port 500.  If       no Network Address Translation (NAT) device is detected between       the Initiator and the Responder, then subsequent IKE packets are       sent over UDP port 500, and IPsec data packets are sent       using ESP.   2.  UDP Encapsulation [RFC3948].  If a NAT is detected between the       Initiator and the Responder, then subsequent IKE packets are sent       over UDP port 4500 with four bytes of zero at the start of the       UDP payload, and ESP packets are sent out over UDP port 4500.       Some peers default to using UDP encapsulation even when no NAT is       detected on the path, as some middleboxes do not support IP       protocols other than TCP and UDP.   3.  TCP Encapsulation.  If the other two methods are not available or       appropriate, IKE negotiation packets as well as ESP packets can       be sent over a single TCP connection to the peer.Pauly, et al.                Standards Track                    [Page 3]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   Direct use of ESP or UDP encapsulation should be preferred by   IKE implementations due to performance concerns when using   TCP encapsulation (Section 12).  Most implementations should use   TCP encapsulation only on networks where negotiation over UDP has   been attempted without receiving responses from the peer or if a   network is known to not support UDP.1.1.  Prior Work and Motivation   Encapsulating IKE connections within TCP streams is a common approach   to solve the problem of UDP packets being blocked by network   middleboxes.  The specific goals of this document are as follows:   o  To promote interoperability by defining a standard method of      framing IKE and ESP messages within TCP streams.   o  To be compatible with the current IKEv2 standard without requiring      modifications or extensions.   o  To use IKE over UDP by default to avoid the overhead of other      alternatives that always rely on TCP or Transport Layer Security      (TLS) [RFC5246].   Some previous alternatives include:   Cellular Network Access      Interworking Wireless LAN (IWLAN) uses IKEv2 to create secure      connections to cellular carrier networks for making voice calls      and accessing other network services over Wi-Fi networks. 3GPP has      recommended that IKEv2 and ESP packets be sent within a TLS      connection to be able to establish connections on restrictive      networks.   ISAKMP over TCP      Various non-standard extensions to the Internet Security      Association and Key Management Protocol (ISAKMP) have been      deployed that send IPsec traffic over TCP or TCP-like packets.   Secure Sockets Layer (SSL) VPNs      Many proprietary VPN solutions use a combination of TLS and IPsec      in order to provide reliability.  These often run on TCP port 443.   IKEv2 over TCP      IKEv2 over TCP as described in [IKE-over-TCP] is used to avoid UDP      fragmentation.Pauly, et al.                Standards Track                    [Page 4]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 20171.2.  Terminology and Notation   This document distinguishes between the IKE peer that initiates TCP   connections to be used for TCP encapsulation and the roles of   Initiator and Responder for particular IKE messages.  During the   course of IKE exchanges, the role of IKE Initiator and Responder may   swap for a given SA (as with IKE SA rekeys), while the Initiator of   the TCP connection is still responsible for tearing down the TCP   connection and re-establishing it if necessary.  For this reason,   this document will use the term "TCP Originator" to indicate the IKE   peer that initiates TCP connections.  The peer that receives TCP   connections will be referred to as the "TCP Responder".  If an IKE SA   is rekeyed one or more times, the TCP Originator MUST remain the peer   that originally initiated the first IKE SA.   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.2.  Configuration   One of the main reasons to use TCP encapsulation is that UDP traffic   may be entirely blocked on a network.  Because of this, support for   TCP encapsulation is not specifically negotiated in the IKE exchange.   Instead, support for TCP encapsulation must be pre-configured on both   the TCP Originator and the TCP Responder.   Implementations MUST support TCP encapsulation on TCP port 4500,   which is reserved for IPsec NAT traversal.   Beyond a flag indicating support for TCP encapsulation, the   configuration for each peer can include the following optional   parameters:   o  Alternate TCP ports on which the specific TCP Responder listens      for incoming connections.  Note that the TCP Originator may      initiate TCP connections to the TCP Responder from any local port.   o  An extra framing protocol to use on top of TCP to further      encapsulate the stream of IKE and IPsec packets.  SeeAppendix A      for a detailed discussion.Pauly, et al.                Standards Track                    [Page 5]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   Since TCP encapsulation of IKE and IPsec packets adds overhead and   has potential performance trade-offs compared to direct or   UDP-encapsulated SAs (as described inSection 12), implementations   SHOULD prefer ESP direct or UDP-encapsulated SAs over   TCP-encapsulated SAs when possible.3.  TCP-Encapsulated Header Formats   Like UDP encapsulation, TCP encapsulation uses the first four bytes   of a message to differentiate IKE and ESP messages.  TCP   encapsulation also adds a Length field to define the boundaries of   messages within a stream.  The message length is sent in a 16-bit   field that precedes every message.  If the first 32 bits of the   message are zeros (a non-ESP marker), then the contents comprise an   IKE message.  Otherwise, the contents comprise an ESP message.   Authentication Header (AH) messages are not supported for TCP   encapsulation.   Although a TCP stream may be able to send very long messages,   implementations SHOULD limit message lengths to typical UDP datagram   ESP payload lengths.  The maximum message length is used as the   effective MTU for connections that are being encrypted using ESP, so   the maximum message length will influence characteristics of inner   connections, such as the TCP Maximum Segment Size (MSS).   Note that this method of encapsulation will also work for placing IKE   and ESP messages within any protocol that presents a stream   abstraction, beyond TCP.3.1.  TCP-Encapsulated IKE Header Format                        1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |            Length             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         Non-ESP Marker                        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   ~                      IKE header [RFC7296]                     ~   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                 Figure 1Pauly, et al.                Standards Track                    [Page 6]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   The IKE header is preceded by a 16-bit Length field in network byte   order that specifies the length of the IKE message (including the   non-ESP marker) within the TCP stream.  As with IKE over UDP   port 4500, a zeroed 32-bit non-ESP marker is inserted before the   start of the IKE header in order to differentiate the traffic from   ESP traffic between the same addresses and ports.   o  Length (2 octets, unsigned integer) - Length of the IKE packet,      including the Length field and non-ESP marker.3.2.  TCP-Encapsulated ESP Header Format                        1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |            Length             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   ~                     ESP header [RFC4303]                      ~   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                 Figure 2   The ESP header is preceded by a 16-bit Length field in network byte   order that specifies the length of the ESP packet within the TCP   stream.   The Security Parameter Index (SPI) field [RFC7296] in the ESP header   MUST NOT be a zero value.   o  Length (2 octets, unsigned integer) - Length of the ESP packet,      including the Length field.4.  TCP-Encapsulated Stream Prefix   Each stream of bytes used for IKE and IPsec encapsulation MUST begin   with a fixed sequence of six bytes as a magic value, containing the   characters "IKETCP" as ASCII values.  This value is intended to   identify and validate that the TCP connection is being used for TCP   encapsulation as defined in this document, to avoid conflicts with   the prevalence of previous non-standard protocols that used TCP   port 4500.  This value is only sent once, by the TCP Originator only,   at the beginning of any stream of IKE and ESP messages.   If other framing protocols are used within TCP to further encapsulate   or encrypt the stream of IKE and ESP messages, the stream prefix must   be at the start of the TCP Originator's IKE and ESP message streamPauly, et al.                Standards Track                    [Page 7]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   within the added protocol layer (Appendix A).  Although some framing   protocols do support negotiating inner protocols, the stream prefix   should always be used in order for implementations to be as generic   as possible and not rely on other framing protocols on top of TCP.                0      1      2      3      4      5               +------+------+------+------+------+------+               | 0x49 | 0x4b | 0x45 | 0x54 | 0x43 | 0x50 |               +------+------+------+------+------+------+                                 Figure 35.  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.  SeeSection 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 SAPauly, et al.                Standards Track                    [Page 8]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   as failed.  An implementation can attempt negotiation over TCP once   it has hit the maximum retransmissions over UDP, or slightly before   to reduce connection setup delays.  It is recommended that the   initial message over UDP be retransmitted at least once before   falling back to TCP, unless the Initiator knows beforehand that the   network is likely to block UDP.6.  Connection Establishment and Teardown   When the IKE Initiator uses TCP encapsulation, it will initiate a TCP   connection to the Responder using the configured TCP port.  The first   bytes sent on the stream MUST be the stream prefix value (Section 4).   After this prefix, encapsulated IKE messages will negotiate the IKE   SA and initial Child SA [RFC7296].  After this point, both   encapsulated IKE (Figure 1) and ESP (Figure 2) messages will be sent   over the TCP connection.  The TCP Responder MUST wait for the entire   stream prefix to be received on the stream before trying to parse out   any IKE or ESP messages.  The stream prefix is sent only once, and   only by the TCP Originator.   In order to close an IKE session, either the Initiator or Responder   SHOULD gracefully tear down IKE SAs with DELETE payloads.  Once the   SA has been deleted, the TCP Originator SHOULD close the TCP   connection if it does not intend to use the connection for another   IKE session to the TCP Responder.  If the connection is left idle and   the TCP Responder needs to clean up resources, the TCP Responder MAY   close the TCP connection.   An unexpected FIN or a TCP Reset on the TCP connection may indicate a   loss of connectivity, an attack, or some other error.  If a DELETE   payload has not been sent, both sides SHOULD maintain the state for   their SAs for the standard lifetime or timeout period.  The TCP   Originator is responsible for re-establishing the TCP connection if   it is torn down for any unexpected reason.  Since new TCP connections   may use different ports due to NAT mappings or local port allocations   changing, the TCP Responder MUST allow packets for existing SAs to be   received from new source ports.   A peer MUST discard a partially received message due to a broken   connection.   Whenever the TCP Originator opens a new TCP connection to be used for   an existing IKE SA, it MUST send the stream prefix first, before any   IKE or ESP messages.  This follows the same behavior as the initial   TCP connection.Pauly, et al.                Standards Track                    [Page 9]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   If a TCP connection is being used to resume a previous IKE session,   the TCP Responder can recognize the session using either the IKE SPI   from an encapsulated IKE message or the ESP SPI from an encapsulated   ESP message.  If the session had been fully established previously,   it is suggested that the TCP Originator send an UPDATE_SA_ADDRESSES   message if MOBIKE is supported, or an informational message (a   keep-alive) otherwise.   The TCP Responder MUST NOT accept any messages for the existing IKE   session on a new incoming connection, unless that connection begins   with the stream prefix.  If either the TCP Originator or TCP   Responder detects corruption on a connection that was started with a   valid stream prefix, it SHOULD close the TCP connection.  The   connection can be determined to be corrupted if there are too many   subsequent messages that cannot be parsed as valid IKE messages or   ESP messages with known SPIs, or if the authentication check for an   ESP message with a known SPI fails.  Implementations SHOULD NOT   tear down a connection if only a single ESP message has an unknown   SPI, since the SPI databases may be momentarily out of sync.  If   there is instead a syntax issue within an IKE message, an   implementation MUST send the INVALID_SYNTAX notify payload and   tear down the IKE SA as usual, rather than tearing down the TCP   connection directly.   A TCP Originator SHOULD only open one TCP connection per IKE SA, over   which it sends all of the corresponding IKE and ESP messages.  This   helps ensure that any firewall or NAT mappings allocated for the TCP   connection apply to all of the traffic associated with the IKE SA   equally.   Similarly, a TCP Responder SHOULD at any given time send packets for   an IKE SA and its Child SAs over only one TCP connection.  It SHOULD   choose the TCP connection on which it last received a valid and   decryptable IKE or ESP message.  In order to be considered valid for   choosing a TCP connection, an IKE message must be successfully   decrypted and authenticated, not be a retransmission of a previously   received message, and be within the expected window for IKE   message IDs.  Similarly, an ESP message must pass authentication   checks and be decrypted, and must not be a replay of a previous   message.   Since a connection may be broken and a new connection re-established   by the TCP Originator without the TCP Responder being aware, a TCP   Responder SHOULD accept receiving IKE and ESP messages on both old   and new connections until the old connection is closed by the TCP   Originator.  A TCP Responder MAY close a TCP connection that it   perceives as idle and extraneous (one previously used for IKE and ESP   messages that has been replaced by a new connection).Pauly, et al.                Standards Track                   [Page 10]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   Multiple IKE SAs MUST NOT share a single TCP connection, unless one   is a rekey of an existing IKE SA, in which case there will   temporarily be two IKE SAs on the same TCP connection.7.  Interaction with NAT Detection Payloads   When negotiating over UDP port 500, IKE_SA_INIT packets include   NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP payloads to   determine if UDP encapsulation of IPsec packets should be used.   These payloads contain SHA-1 digests of the SPIs, IP addresses, and   ports as defined in [RFC7296].  IKE_SA_INIT packets sent on a TCP   connection SHOULD include these payloads with the same content as   when sending over UDP and SHOULD use the applicable TCP ports when   creating and checking the SHA-1 digests.   If a NAT is detected due to the SHA-1 digests not matching the   expected values, no change should be made for encapsulation of   subsequent IKE or ESP packets, since TCP encapsulation inherently   supports NAT traversal.  Implementations MAY use the information that   a NAT is present to influence keep-alive timer values.   If a NAT is detected, implementations need to handle transport mode   TCP and UDP packet checksum fixup as defined for UDP encapsulation in   [RFC3948].8.  Using MOBIKE with TCP Encapsulation   When an IKE session that has negotiated MOBIKE [RFC4555] is   transitioning between networks, the Initiator of the transition may   switch between using TCP encapsulation, UDP encapsulation, or no   encapsulation.  Implementations that implement both MOBIKE and TCP   encapsulation MUST support dynamically enabling and disabling TCP   encapsulation as interfaces change.   When a MOBIKE-enabled Initiator changes networks, the   UPDATE_SA_ADDRESSES notification SHOULD be sent out first over UDP   before attempting over TCP.  If there is a response to the   UPDATE_SA_ADDRESSES notification sent over UDP, then the ESP packets   should be sent directly over IP or over UDP port 4500 (depending on   if a NAT was detected), regardless of if a connection on a previous   network was using TCP encapsulation.  Similarly, if the Responder   only responds to the UPDATE_SA_ADDRESSES notification over TCP, then   the ESP packets should be sent over the TCP connection, regardless of   if a connection on a previous network did not use TCP encapsulation.Pauly, et al.                Standards Track                   [Page 11]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 20179.  Using IKE Message Fragmentation with TCP Encapsulation   IKE message fragmentation [RFC7383] is not required when using TCP   encapsulation, since a TCP stream already handles the fragmentation   of its contents across packets.  Since fragmentation is redundant in   this case, implementations might choose to not negotiate IKE   fragmentation.  Even if fragmentation is negotiated, an   implementation SHOULD NOT send fragments when going over a TCP   connection, although it MUST support receiving fragments.   If an implementation supports both MOBIKE and IKE fragmentation, it   SHOULD negotiate IKE fragmentation over a TCP-encapsulated session in   case the session switches to UDP encapsulation on another network.10.  Considerations for Keep-Alives and Dead Peer Detection   Encapsulating IKE and IPsec inside of a TCP connection can impact the   strategy that implementations use to detect peer liveness and to   maintain middlebox port mappings.  Peer liveness should be checked   using IKE informational packets [RFC7296].   In general, TCP port mappings are maintained by NATs longer than UDP   port mappings, so IPsec ESP NAT keep-alives [RFC3948] SHOULD NOT be   sent when using TCP encapsulation.  Any implementation using TCP   encapsulation MUST silently drop incoming NAT keep-alive packets   and not treat them as errors.  NAT keep-alive packets over a   TCP-encapsulated IPsec connection will be sent as an ESP message with   a one-octet-long payload with the value 0xFF.   Note that, depending on the configuration of TCP and TLS on the   connection, TCP keep-alives [RFC1122] and TLS keep-alives [RFC6520]   may be used.  These MUST NOT be used as indications of IKE peer   liveness.11.  Middlebox Considerations   Many security networking devices, such as firewalls or intrusion   prevention systems, network optimization/acceleration devices, and   NAT devices, keep the state of sessions that traverse through them.   These devices commonly track the transport-layer and/or application-   layer data to drop traffic that is anomalous or malicious in nature.   While many of these devices will be more likely to pass   TCP-encapsulated traffic as opposed to UDP-encapsulated traffic, some   may still block or interfere with TCP-encapsulated IKE and IPsec   traffic.Pauly, et al.                Standards Track                   [Page 12]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   A network device that monitors the transport layer will track the   state of TCP sessions, such as TCP sequence numbers.  TCP   encapsulation of IKE should therefore use standard TCP behaviors to   avoid being dropped by middleboxes.12.  Performance Considerations   Several aspects of TCP encapsulation for IKE and IPsec packets may   negatively impact the performance of connections within a tunnel-mode   IPsec SA.  Implementations should be aware of these performance   impacts and take these into consideration when determining when to   use TCP encapsulation.  Implementations SHOULD favor using direct ESP   or UDP encapsulation over TCP encapsulation whenever possible.12.1.  TCP-in-TCP   If the outer connection between IKE peers is over TCP, inner TCP   connections may suffer negative effects from using TCP within TCP.   Running TCP within TCP is discouraged, since the TCP algorithms   generally assume that they are running over an unreliable datagram   layer.   If the outer (tunnel) TCP connection experiences packet loss, this   loss will be hidden from any inner TCP connections, since the outer   connection will retransmit to account for the losses.  Since the   outer TCP connection will deliver the inner messages in order, any   messages after a lost packet may have to wait until the loss is   recovered.  This means that loss on the outer connection will be   interpreted only as delay by inner connections.  The burstiness of   inner traffic can increase, since a large number of inner packets may   be delivered across the tunnel at once.  The inner TCP connection may   interpret a long period of delay as a transmission problem,   triggering a retransmission timeout, which will cause spurious   retransmissions.  The sending rate of the inner connection may be   unnecessarily reduced if the retransmissions are not detected as   spurious in time.   The inner TCP connection's round-trip-time estimation will be   affected by the burstiness of the outer TCP connection if there are   long delays when packets are retransmitted by the outer TCP   connection.  This will make the congestion control loop of the inner   TCP traffic less reactive, potentially permanently leading to a lower   sending rate than the outer TCP would allow for.Pauly, et al.                Standards Track                   [Page 13]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   TCP-in-TCP can also lead to increased buffering, or bufferbloat.   This can occur when the window size of the outer TCP connection is   reduced and becomes smaller than the window sizes of the inner TCP   connections.  This can lead to packets backing up in the outer TCP   connection's send buffers.  In order to limit this effect, the outer   TCP connection should have limits on its send buffer size and on the   rate at which it reduces its window size.   Note that any negative effects will be shared between all flows going   through the outer TCP connection.  This is of particular concern for   any latency-sensitive or real-time applications using the tunnel.  If   such traffic is using a TCP-encapsulated IPsec connection, it is   recommended that the number of inner connections sharing the tunnel   be limited as much as possible.12.2.  Added Reliability for Unreliable Protocols   Since ESP is an unreliable protocol, transmitting ESP packets over a   TCP connection will change the fundamental behavior of the packets.   Some application-level protocols that prefer packet loss to delay   (such as Voice over IP or other real-time protocols) may be   negatively impacted if their packets are retransmitted by the TCP   connection due to packet loss.12.3.  Quality-of-Service Markings   Quality-of-Service (QoS) markings, such as the Differentiated   Services Code Point (DSCP) and Traffic Class, should be used with   care on TCP connections used for encapsulation.  Individual packets   SHOULD NOT use different markings than the rest of the connection,   since packets with different priorities may be routed differently and   cause unnecessary delays in the connection.12.4.  Maximum Segment Size   A TCP connection used for IKE encapsulation SHOULD negotiate its MSS   in order to avoid unnecessary fragmentation of packets.12.5.  Tunneling ECN in TCP   Since there is not a one-to-one relationship between outer IP packets   and inner ESP/IP messages when using TCP encapsulation, the markings   for Explicit Congestion Notification (ECN) [RFC3168] cannot be simply   mapped.  However, any ECN Congestion Experienced (CE) marking on   inner headers should be preserved through the tunnel.Pauly, et al.                Standards Track                   [Page 14]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017   Implementations SHOULD follow the ECN compatibility mode for tunnel   ingress as described in [RFC6040].  In compatibility mode, the outer   tunnel TCP connection marks its packet headers as not ECN-capable.   If upon egress, the arriving outer header is marked with CE, the   implementation will drop the inner packet, since there is not a   distinct inner packet header onto which to translate the ECN   markings.13.  Security Considerations   IKE Responders that support TCP encapsulation may become vulnerable   to new Denial-of-Service (DoS) attacks that are specific to TCP, such   as SYN-flooding attacks.  TCP Responders should be aware of this   additional attack surface.   TCP Responders should be careful to ensure that (1) the stream prefix   "IKETCP" uniquely identifies incoming streams as streams that use the   TCP encapsulation protocol and (2) they are not running any other   protocols on the same listening port (to avoid potential conflicts).   Attackers may be able to disrupt the TCP connection by sending   spurious TCP Reset packets.  Therefore, implementations SHOULD make   sure that IKE session state persists even if the underlying TCP   connection is torn down.   If MOBIKE is being used, all of the security considerations outlined   for MOBIKE apply [RFC4555].   Similarly to MOBIKE, TCP encapsulation requires a TCP Responder to   handle changes to source address and port due to network or   connection disruption.  The successful delivery of valid IKE or ESP   messages over a new TCP connection is used by the TCP Responder to   determine where to send subsequent responses.  If an attacker is able   to send packets on a new TCP connection that pass the validation   checks of the TCP Responder, it can influence which path future   packets will take.  For this reason, the validation of messages on   the TCP Responder must include decryption, authentication, and replay   checks.   Since TCP provides reliable, in-order delivery of ESP messages, the   ESP anti-replay window size SHOULD be set to 1.  See [RFC4303] for a   complete description of the ESP anti-replay window.  This increases   the protection of implementations against replay attacks.Pauly, et al.                Standards Track                   [Page 15]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 201714.  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-TraversalRFC 8229                                 Figure 415.  References15.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.              Stenberg, "UDP Encapsulation of IPsec ESP Packets",RFC 3948, DOI 10.17487/RFC3948, January 2005,              <http://www.rfc-editor.org/info/rfc3948>.   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",RFC 4303, DOI 10.17487/RFC4303, December 2005,              <http://www.rfc-editor.org/info/rfc4303>.   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion              Notification",RFC 6040, DOI 10.17487/RFC6040,              November 2010, <http://www.rfc-editor.org/info/rfc6040>.   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.              Kivinen, "Internet Key Exchange Protocol Version 2              (IKEv2)", STD 79,RFC 7296, DOI 10.17487/RFC7296,              October 2014, <http://www.rfc-editor.org/info/rfc7296>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <http://www.rfc-editor.org/info/rfc8174>.Pauly, et al.                Standards Track                   [Page 16]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 201715.2.  Informative References   [IKE-over-TCP]              Nir, Y.,"A TCP transport for the Internet Key Exchange",              Work in Progress,draft-ietf-ipsecme-ike-tcp-01,              December 2012.   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -              Communication Layers", STD 3,RFC 1122,              DOI 10.17487/RFC1122, October 1989,              <http://www.rfc-editor.org/info/rfc1122>.   [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within              HTTP/1.1",RFC 2817, DOI 10.17487/RFC2817, May 2000,              <http://www.rfc-editor.org/info/rfc2817>.   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition              of Explicit Congestion Notification (ECN) to IP",RFC 3168, DOI 10.17487/RFC3168, September 2001,              <http://www.rfc-editor.org/info/rfc3168>.   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol              (MOBIKE)",RFC 4555, DOI 10.17487/RFC4555, June 2006,              <http://www.rfc-editor.org/info/rfc4555>.   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security              (TLS) Protocol Version 1.2",RFC 5246,              DOI 10.17487/RFC5246, August 2008,              <http://www.rfc-editor.org/info/rfc5246>.   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport              Layer Security (TLS) and Datagram Transport Layer Security              (DTLS) Heartbeat Extension",RFC 6520,              DOI 10.17487/RFC6520, February 2012,              <http://www.rfc-editor.org/info/rfc6520>.   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2              (IKEv2) Message Fragmentation",RFC 7383,              DOI 10.17487/RFC7383, November 2014,              <http://www.rfc-editor.org/info/rfc7383>.Pauly, et al.                Standards Track                   [Page 17]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017Appendix A.  Using TCP Encapsulation with TLS   This section provides recommendations on how to use TLS in addition   to TCP encapsulation.   When using TCP encapsulation, implementations may choose to use TLS   [RFC5246] on the TCP connection to be able to traverse middleboxes,   which may otherwise block the traffic.   If a web proxy is applied to the ports used for the TCP connection   and TLS is being used, the TCP Originator can send an HTTP CONNECT   message to establish an SA through the proxy [RFC2817].   The use of TLS should be configurable on the peers, and may be used   as the default when using TCP encapsulation or may be used as a   fallback when basic TCP encapsulation fails.  The TCP Responder may   expect to read encapsulated IKE and ESP packets directly from the TCP   connection, or it may expect to read them from a stream of TLS data   packets.  The TCP Originator should be pre-configured to use TLS   or not when communicating with a given port on the TCP Responder.   When new TCP connections are re-established due to a broken   connection, TLS must be renegotiated.  TLS session resumption is   recommended to improve efficiency in this case.   The security of the IKE session is entirely derived from the IKE   negotiation and key establishment and not from the TLS session (which   in this context is only used for encapsulation purposes); therefore,   when TLS is used on the TCP connection, both the TCP Originator and   the TCP Responder SHOULD allow the NULL cipher to be selected for   performance reasons.   Implementations should be aware that the use of TLS introduces   another layer of overhead requiring more bytes to transmit a given   IKE and IPsec packet.  For this reason, direct ESP, UDP   encapsulation, or TCP encapsulation without TLS should be preferred   in situations in which TLS is not required in order to traverse   middleboxes.Pauly, et al.                Standards Track                   [Page 18]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017Appendix B.  Example Exchanges of TCP Encapsulation with TLSB.1.  Establishing an IKE Session                   Client                              Server                 ----------                          ----------     1)  --------------------  TCP Connection  -------------------         (IP_I:Port_I  -> IP_R:Port_R)         TcpSyn                    ---------->                                   <----------          TcpSyn,Ack         TcpAck                    ---------->     2)  ---------------------  TLS Session  ---------------------         ClientHello               ---------->                                                       ServerHello                                                      Certificate*                                                ServerKeyExchange*                                   <----------     ServerHelloDone         ClientKeyExchange         CertificateVerify*         [ChangeCipherSpec]         Finished                  ---------->                                                [ChangeCipherSpec]                                   <----------            Finished     3)  ---------------------- Stream Prefix --------------------         "IKETCP"                  ---------->     4)  ----------------------- IKE Session ---------------------         Length + Non-ESP Marker   ---------->         IKE_SA_INIT         HDR, SAi1, KEi, Ni,         [N(NAT_DETECTION_*_IP)]                                   <------ Length + Non-ESP Marker                                                       IKE_SA_INIT                                               HDR, SAr1, KEr, Nr,                                           [N(NAT_DETECTION_*_IP)]         Length + Non-ESP Marker   ---------->         first IKE_AUTH         HDR, SK {IDi, [CERTREQ]         CP(CFG_REQUEST), IDr,         SAi2, TSi, TSr, ...}                                   <------ Length + Non-ESP Marker                                                    first IKE_AUTH                                       HDR, SK {IDr, [CERT], AUTH,                                              EAP, SAr2, TSi, TSr}Pauly, et al.                Standards Track                   [Page 19]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017         Length + Non-ESP Marker   ---------->         IKE_AUTH + EAP         repeat 1..N times                                   <------ Length + Non-ESP Marker                                                    IKE_AUTH + EAP         Length + Non-ESP Marker   ---------->         final IKE_AUTH         HDR, SK {AUTH}                                   <------ Length + Non-ESP Marker                                                    final IKE_AUTH                                     HDR, SK {AUTH, CP(CFG_REPLY),                                                SA, TSi, TSr, ...}         -------------- IKE and IPsec SAs Established ------------         Length + ESP Frame        ---------->                                 Figure 5   1.  The client establishes a TCP connection with the server on       port 4500 or on an alternate pre-configured port that the server       is listening on.   2.  If configured to use TLS, the client initiates a TLS handshake.       During the TLS handshake, the server SHOULD NOT request the       client's certificate, since authentication is handled as part of       IKE negotiation.   3.  The client sends the stream prefix for TCP-encapsulated IKE       (Section 4) traffic to signal the beginning of IKE negotiation.   4.  The client and server establish an IKE connection.  This example       shows EAP-based authentication, although any authentication type       may be used.Pauly, et al.                Standards Track                   [Page 20]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017B.2.  Deleting an IKE Session                   Client                              Server                 ----------                          ----------     1)  ----------------------- IKE Session ---------------------         Length + Non-ESP Marker   ---------->         INFORMATIONAL         HDR, SK {[N,] [D,]                [CP,] ...}                                   <------ Length + Non-ESP Marker                                                     INFORMATIONAL                                                HDR, SK {[N,] [D,]                                                        [CP], ...}     2)  ---------------------  TLS Session  ---------------------         close_notify              ---------->                                   <----------        close_notify     3)  --------------------  TCP Connection  -------------------         TcpFin                    ---------->                                   <----------                 Ack                                   <----------              TcpFin         Ack                       ---------->         --------------------  IKE SA Deleted  -------------------                                 Figure 6   1.  The client and server exchange informational messages to notify       IKE SA deletion.   2.  The client and server negotiate TLS session deletion using TLS       CLOSE_NOTIFY.   3.  The TCP connection is torn down.   The deletion of the IKE SA should lead to the disposal of the   underlying TLS and TCP state.Pauly, et al.                Standards Track                   [Page 21]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017B.3.  Re-establishing an IKE Session                   Client                              Server                 ----------                          ----------     1)  --------------------  TCP Connection  -------------------         (IP_I:Port_I  -> IP_R:Port_R)         TcpSyn                    ---------->                                   <----------          TcpSyn,Ack         TcpAck                    ---------->     2)  ---------------------  TLS Session  ---------------------         ClientHello               ---------->                                   <----------         ServerHello                                                [ChangeCipherSpec]                                                          Finished         [ChangeCipherSpec]        ---------->         Finished     3)  ---------------------- Stream Prefix --------------------         "IKETCP"                  ---------->     4)  <---------------------> IKE/ESP Flow <------------------>         Length + ESP Frame        ---------->                                 Figure 7   1.  If a previous TCP connection was broken (for example, due to a       TCP Reset), the client is responsible for re-initiating the TCP       connection.  The TCP Originator's address and port (IP_I and       Port_I) may be different from the previous connection's address       and port.   2.  In the ClientHello TLS message, the client SHOULD send the       session ID it received in the previous TLS handshake if       available.  It is up to the server to perform either an       abbreviated handshake or a full handshake based on the session ID       match.   3.  After TCP and TLS are complete, the client sends the stream       prefix for TCP-encapsulated IKE traffic (Section 4).   4.  The IKE and ESP packet flow can resume.  If MOBIKE is being used,       the Initiator SHOULD send an UPDATE_SA_ADDRESSES message.Pauly, et al.                Standards Track                   [Page 22]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017B.4.  Using MOBIKE between UDP and TCP Encapsulation                     Client                              Server                   ----------                          ----------         (IP_I1:UDP500 -> IP_R:UDP500)     1)  ----------------- IKE_SA_INIT Exchange -----------------         (IP_I1:UDP4500 -> IP_R:UDP4500)         Non-ESP Marker           ----------->         Initial IKE_AUTH         HDR, SK { IDi, CERT, AUTH,         CP(CFG_REQUEST),         SAi2, TSi, TSr,         N(MOBIKE_SUPPORTED) }                                  <-----------      Non-ESP Marker                                                  Initial IKE_AUTH                                        HDR, SK { IDr, CERT, AUTH,                                              EAP, SAr2, TSi, TSr,                                             N(MOBIKE_SUPPORTED) }         <------------------ IKE SA Establishment --------------->     2)  ------------ MOBIKE Attempt on New Network --------------         (IP_I2:UDP4500 -> IP_R:UDP4500)         Non-ESP Marker           ----------->         INFORMATIONAL         HDR, SK { N(UPDATE_SA_ADDRESSES),         N(NAT_DETECTION_SOURCE_IP),         N(NAT_DETECTION_DESTINATION_IP) }     3)  --------------------  TCP Connection  -------------------         (IP_I2:Port_I -> IP_R:Port_R)         TcpSyn                   ----------->                                  <-----------          TcpSyn,Ack         TcpAck                   ----------->     4)  ---------------------  TLS Session  ---------------------         ClientHello              ----------->                                                       ServerHello                                                      Certificate*                                                ServerKeyExchange*                                  <-----------     ServerHelloDone         ClientKeyExchange         CertificateVerify*         [ChangeCipherSpec]         Finished                 ----------->                                                [ChangeCipherSpec]                                  <-----------            FinishedPauly, et al.                Standards Track                   [Page 23]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017     5)  ---------------------- Stream Prefix --------------------         "IKETCP"                  ---------->     6)  ----------------------- IKE Session ---------------------         Length + Non-ESP Marker  ----------->         INFORMATIONAL (Same as step 2)         HDR, SK { N(UPDATE_SA_ADDRESSES),         N(NAT_DETECTION_SOURCE_IP),         N(NAT_DETECTION_DESTINATION_IP) }                                  <------- Length + Non-ESP Marker                             HDR, SK { N(NAT_DETECTION_SOURCE_IP),                                 N(NAT_DETECTION_DESTINATION_IP) }     7)  <----------------- IKE/ESP Data Flow ------------------->                                 Figure 8   1.  During the IKE_SA_INIT exchange, the client and server exchange       MOBIKE_SUPPORTED notify payloads to indicate support for MOBIKE.   2.  The client changes its point of attachment to the network and       receives a new IP address.  The client attempts to re-establish       the IKE session using the UPDATE_SA_ADDRESSES notify payload, but       the server does not respond because the network blocks UDP       traffic.   3.  The client brings up a TCP connection to the server in order to       use TCP encapsulation.   4.  The client initiates a TLS handshake with the server.   5.  The client sends the stream prefix for TCP-encapsulated IKE       traffic (Section 4).   6.  The client sends the UPDATE_SA_ADDRESSES notify payload on the       TCP-encapsulated connection.  Note that this IKE message is the       same as the one sent over UDP in step 2; it should have the same       message ID and contents.   7.  The IKE and ESP packet flow can resume.Pauly, et al.                Standards Track                   [Page 24]

RFC 8229       TCP Encapsulation of IKE and IPsec Packets    August 2017Acknowledgments   The authors would like to acknowledge the input and advice of Stuart   Cheshire, Delziel Fernandes, Yoav Nir, Christoph Paasch, Yaron   Sheffer, David Schinazi, Graham Bartlett, Byju Pularikkal, March Wu,   Kingwel Xie, Valery Smyslov, Jun Hu, and Tero Kivinen.  Special   thanks to Eric Kinnear for his implementation work.Authors' Addresses   Tommy Pauly   Apple Inc.   1 Infinite Loop   Cupertino, California  95014   United States of America   Email: tpauly@apple.com   Samy Touati   Ericsson   2755 Augustine   Santa Clara, California  95054   United States of America   Email: samy.touati@ericsson.com   Ravi Mantha   Cisco Systems   SEZ, Embassy Tech Village   Panathur, Bangalore  560 037   India   Email: ramantha@cisco.comPauly, et al.                Standards Track                   [Page 25]