Network T. Pauly Internet-Draft Apple Inc. Intended status: Standards Track S. Touati Expires: August 31, 2017 Ericsson R. Mantha Cisco Systems February 27, 2017 TCP Encapsulation of IKE and IPsec Packets draft-ietf-ipsecme-tcp-encaps-08 Abstract 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on August 31, 2017. Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Pauly, et al. Expires August 31, 2017 [Page 1] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Prior Work and Motivation . . . . . . . . . . . . . . . . 3 1.2. Terminology and Notation . . . . . . . . . . . . . . . . 4 2. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 4 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. Performance Considerations . . . . . . . . . . . . . . . . . 12 12.1. TCP-in-TCP . . . . . . . . . . . . . . . . . . . . . . . 12 12.2. Added Reliability for Unreliable Protocols . . . . . . . 12 12.3. Quality of Service Markings . . . . . . . . . . . . . . 12 12.4. Maximum Segment Size . . . . . . . . . . . . . . . . . . 12 13. Security Considerations . . . . . . . . . . . . . . . . . . . 13 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 16.1. Normative References . . . . . . . . . . . . . . . . . . 14 16.2. Informative References . . . . . . . . . . . . . . . . . 14 Appendix A. Using TCP encapsulation with TLS . . . . . . . . . . 15 Appendix B. Example exchanges of TCP Encapsulation with TLS . . 16 B.1. Establishing an IKE session . . . . . . . . . . . . . . . 16 B.2. Deleting an IKE session . . . . . . . . . . . . . . . . . 17 B.3. Re-establishing an IKE session . . . . . . . . . . . . . 18 B.4. Using MOBIKE between UDP and TCP Encapsulation . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 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 Pauly, et al. Expires August 31, 2017 [Page 2] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 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 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 peer. Direct use of ESP or UDP Encapsulation should be preferred by IKE implementations due to performance concerns when using TCP Encapsulation [Section 12]. Most implementations should use TCP Encapsulation only on networks where negotiation over UDP has been attempted without receiving responses from the peer, or if a network is known to not support UDP. 1.1. Prior Work and Motivation Encapsulating IKE connections within TCP streams is a common approach to solve the problem of UDP packets being blocked by network middleboxes. The 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: Pauly, et al. Expires August 31, 2017 [Page 3] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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. 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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. Pauly, et al. Expires August 31, 2017 [Page 4] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 The configuration defined on each peer should include the following parameters: o One or more TCP ports on which the TCP Responder will listen 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 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 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. Pauly, et al. Expires August 31, 2017 [Page 5] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 stream. 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. Pauly, et al. Expires August 31, 2017 [Page 6] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 to differentiate this protocol from other protocols that may be run over the same TCP port. Since TCP encapsulated IPsec is not assigned to a specific port, TCP Responders may be able to receive multiple protocols on the same port. The bytes of the stream prefix do not overlap with the valid start of any other known stream protocol. 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 Pauly, et al. Expires August 31, 2017 [Page 7] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 Pauly, et al. Expires August 31, 2017 [Page 8] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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. 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. If either TCP Originator or TCP Responder receives a stream that cannot be parsed correctly (for example, if the TCP Originator stream is missing the stream prefix, or message frames are not parsable as IKE or ESP messages), it MUST close the TCP connection. 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 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, 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 Pauly, et al. Expires August 31, 2017 [Page 9] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 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. If a NAT is detected, implementations need to handle transport mode TCP and UDP packet checksum fixup as defined for UDP encapsulation [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. Expires August 31, 2017 [Page 10] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 9. Using IKE Message Fragmentation with TCP encapsulation IKE Message Fragmentation [RFC7383] is not required when using TCP encapsulation, since a TCP stream already handles the fragmentation of its contents across packets. Since fragmentation is redundant in this case, implementations might choose to not negotiate IKE fragmentation. Even if fragmentation is negotiated, an implementation SHOULD NOT send fragments when going over a TCP connection, although it MUST support receiving fragments. If an implementation supports both MOBIKE and IKE fragmentation, it SHOULD negotiate IKE fragmentation over a TCP encapsulated session in case the session switches to UDP encapsulation on another network. 10. Considerations for Keep-alives and 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 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 Pauly, et al. Expires August 31, 2017 [Page 11] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 a tunnel-mode IPsec SA. 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. Pauly, et al. Expires August 31, 2017 [Page 12] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 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. 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 [RFC4303] SHOULD be set to 1. 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. This port MAY be used for the protocol described in this document, but implementations MAY prefer to use other ports based on local policy. Pauly, et al. Expires August 31, 2017 [Page 13] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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, . [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 [I-D.nir-ipsecme-ike-tcp] Nir, Y., "A TCP transport for the Internet Key Exchange", draft-nir-ipsecme-ike-tcp-01 (work in progress), July 2012. [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989, . [RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000, . Pauly, et al. Expires August 31, 2017 [Page 14] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 [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 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 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. 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 Pauly, et al. Expires August 31, 2017 [Page 15] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 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 --------------------- 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 Pauly, et al. Expires August 31, 2017 [Page 16] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 1. Client establishes a TCP connection with the server on port 443 or 4500. 2. Client initiates TLS handshake. During TLS handshake, the server SHOULD NOT request the 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 Pauly, et al. Expires August 31, 2017 [Page 17] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 5 1. Client and server exchange INFORMATIONAL messages to notify IKE SA deletion. 2. 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. B.3. Re-establishing an IKE session Pauly, et al. Expires August 31, 2017 [Page 18] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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 <------------------> Length + ESP frame ----------> Figure 6 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 -----------> Pauly, et al. Expires August 31, 2017 [Page 19] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 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: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 --------------------- 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) } Pauly, et al. Expires August 31, 2017 [Page 20] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 <------- Length + Non-ESP Marker HDR, SK { N(NAT_DETECTION_SOURCE_IP), N(NAT_DETECTION_DESTINATION_IP) } 7) <----------------- IKE/ESP data flow -------------------> Figure 7 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 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 US Email: tpauly@apple.com Pauly, et al. Expires August 31, 2017 [Page 21] Internet-Draft TCP Encapsulation of IKE and IPsec Packets February 2017 Samy Touati Ericsson 2755 Augustine Santa Clara, California 95054 US Email: samy.touati@ericsson.com Ravi Mantha Cisco Systems SEZ, Embassy Tech Village Panathur, Bangalore 560 037 India Email: ramantha@cisco.com Pauly, et al. Expires August 31, 2017 [Page 22]