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Clarifications and Implementation Guidelines for using TCP Encapsulation in IKEv2

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Author Valery Smyslov
Last updated 2018-12-17 (Latest revision 2018-09-07)
Replaces draft-smyslov-ipsec-tcp-guidelines
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Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Informational                         December 18, 2018
Expires: June 21, 2019

Clarifications and Implementation Guidelines for using TCP Encapsulation
                                in IKEv2


   The Internet Key Exchange Protocol version 2 (IKEv2) defined in
   [RFC7296] uses UDP transport for its messages.  [RFC8229] specifies a
   way to encapsulate IKEv2 and ESP (Encapsulating Security Payload)
   messages in TCP, thus making possible to use them in network
   environments that block UDP traffic.  However, some nuances of using
   TCP in IKEv2 are not covered by that specification.  This document
   provides clarifications and implementation guidelines for [RFC8229].

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 June 21, 2019.

Copyright Notice

   Copyright (c) 2018 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
   ( 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

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   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
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   3
   3.  TCP Encapsulation Format  . . . . . . . . . . . . . . . . . .   3
   4.  Falling back from UDP to TCP  . . . . . . . . . . . . . . . .   3
   5.  Retransmissions . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Using Cookies and Puzzles . . . . . . . . . . . . . . . . . .   4
   7.  Error Handling in the IKE_SA_INIT . . . . . . . . . . . . . .   5
   8.  Interaction with IKEv2 Extensions . . . . . . . . . . . . . .   6
     8.1.  MOBIKE Protocol . . . . . . . . . . . . . . . . . . . . .   6
     8.2.  IKEv2 Redirect  . . . . . . . . . . . . . . . . . . . . .   7
     8.3.  IKEv2 Session Resumption  . . . . . . . . . . . . . . . .   7
     8.4.  IKEv2 Protocol Support for High Availability  . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Internet Key Exchange version 2 (IKEv2) as it is defined in
   [RFC7296] uses UDP as a transport protocol.  As time passed the
   network environment has been evolved and sometimes this evolution has
   resulted in situations when UDP messages are dropped by network
   infrastructure.  This may happen either by incapability of network
   devices to properly handle them (e.g. non-initial fragments of UDP
   messages) of by deliberate configuration of network devices that
   blocks UDP traffic.

   Several standard solutions have been developed to deal with such
   situations.  In particular, [RFC7383] defines a way to avoid IP
   fragmentation of large IKE messages and [RFC8229] specifies a way to
   transfer IKEv2 and ESP (Encapsulated Security Payload) messages over
   a stream protocol like TCP.  This document focuses on the latter
   specification and its goal is to give implementers guidelines how to
   properly use reliable connection-oriented stream transport in IKEv2.

   Since originally IKEv2 relied on unreliable transport, it was
   designed to deal with this unreliability.  IKEv2 has its own
   retransmission timers, replay detection logic etc.  Using reliable
   transport makes many of such things unnecessary.  On the other hand,

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   connection-oriented transport require IKEv2 to keep the connection
   alive and to restore it in case it is broken, the tasks that were not
   needed before.  [RFC8229] gives recommendations how peers must behave
   in different situations to keep the connection.  However,
   implementation experience has revealed that not all situations are
   covered in [RFC8229], that may lead to interoperability problems or
   to suboptimal performance.  This memo gives implementers more
   guidelines how to use reliable stream transport in IKEv2 in
   situations, which are not covered in [RFC8229].

2.  Terminology and Notation

   This document shares the terminology with [RFC8229].  In particular,
   it uses terms "TCP Originator" and "TCP Responder" to refer to the
   parties that initiate or responded to the TCP connection created for
   the initial IKE SA (in a possible series of successive rekeys).  More
   details are given in Section 1.2 of [RFC8229].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  TCP Encapsulation Format

   Section 3 of [RFC8229] describes how IKE and ESP packets are
   encapsulated in TCP stream.  For this purpose every IKE or ESP packet
   is prepended with 16-bit Length field.  However, the text in the
   first para of the section is not very explicit on what the Length
   field means - whether it indicates only the length of the following
   IKE or ESP message or the length of field itself is also counted.
   The following text in the same section clarifies it - the value of
   the Length field includes the length of the field itself (2 octets).
   It means that values 0 and 1 must never appear there.  The receiver
   MUST treat these values in the Length field as fatal error and MUST
   close TCP session in this case.

4.  Falling back from UDP to TCP

   Section 5.1 of [RFC8229] describes how the fallback from UDP to TCP
   must be handled.  It is recommended, that in the absence of prior
   knowledge, implementations first try to use UDP and then fall back
   TCP if no reply is received within some period of time after several
   retransmissions.  In this case a new IKE_SA_INIT exchange MUST be
   initiated with new initiator's SPI and with recalculated content of
   NAT_DETECTION_SOURCE_IP notification.

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5.  Retransmissions

   Section 2.1 of [RFC7296] describes how IKEv2 deals with unreliability
   of UDP protocol.  In brief, exchange initiator is responsible for
   retransmissions and must retransmit requests message until response
   message is received.  If no reply is received after several
   retransmissions, the SA is deleted.  The responder never retransmits
   but must resend the response message in case it receives
   retransmitted request.

   When IKEv2 uses reliable transport protocol, most of these rules
   become unnecessary.  Since [RFC8229] doesn't provide clear guidance
   on using retransmissions in case of TCP encapsulation, this memo
   gives the following rules.

   o  the exchange initiator SHOULD NOT retransmit request message; if
      no response is received within some reasonable period of time, the
      IKE SA is deleted

   o  if TCP connection is broken and then restored while the exchange
      initiator is waiting for the response, the initiator MUST
      retransmit the request and continue to wait for the response

   o  the exchange responder acts as described in Section 2.1 of
      [RFC7296], i.e. using TCP encapsulation doesn't change the
      responder's behavior

6.  Using Cookies and Puzzles

   IKEv2 provides a DoS attack protection mechanism called Cookie, which
   is described in Section 2.6 of [RFC7296].  [RFC8019] extends this
   mechanism for protection against DDoS attacks by means of Client
   Puzzles.  Both mechanisms allow the responder to keep no state until
   the initiator proves its IP address is real (and solves puzzle in the
   latter case).

   [RFC8229] gives no guidance on how these mechanisms should be used in
   case of TCP encapsulation.  However, the connection-oriented nature
   of TCP brings additional considerations for using these mechanisms.
   In general, Cookie provides less value in case of TCP encapsulation,
   because when the responder receives the IKE_SA_INIT request the TCP
   session has already been established, so the initiator's IP address
   has been verified.  Moreover, TCP Responder creates state as far as
   the SYN packer is received (unless SYN Cookies described in [RFC4987]
   are employed), that violates the stateless nature of IKEv2 Cookies.
   So, it makes little sense to send Cookie request in this situation,
   unless the responder in concerned with the possibility of TCP
   Sequence Number attacks (see [RFC6528] for details).  On the other

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   hand, Puzzles still remain useful and their use requires using

   The following considerations are applicable for using Cookie and
   Puzzle mechanisms in case of TCP encapsulation.

   o  the exchange responder SHOULD NOT request Cookie unless the
      responder has good reason to do it (like a concern of the
      possibility of TCP Sequence Number attacks or Puzzle request is
      sent in the same message)

   o  if the responder chooses to send Cookie request (possibly along
      with Puzzle request), then the TCP connection that the IKE_SA_INIT
      request message was received over SHOULD be closed, so that the
      responder remains stateless at least until the Cookie (or Puzzle
      Solution) is returned

      *  note, that if this TCP connection is closed, then the responder
         MUST NOT include the initiator's TCP port into the Cookie
         calculation (*), since the Cookie will be returned over a new
         TCP connection with a different port

   o  the exchange initiator acts as described in Section 2.6 of
      [RFC7296] and Section 7 of [RFC8019], i.e. using TCP encapsulation
      doesn't change the initiator's behavior

   (*) Examples of Cookie calculation methods are given in Section 2.6
   of [RFC7296] and in Section of [RFC8019] and they don't
   include transport protocol ports.  However these examples are given
   for illustrative purposes, since Cookie generation algorithm is a
   local matter and some implementations might include port numbers,
   that won't work with TCP encapsulation.

7.  Error Handling in the IKE_SA_INIT

   Section 2.21.1 of [RFC7296] describes how error notifications should
   be handled in the IKE_SA_INIT exchange.  In particular, it is advised
   that the initiator should not act immediately after receiving error
   notification and should instead wait some time for valid response,
   since the IKE_SA_INIT messages are completely unauthenticated.  This
   advice has little sense in case of TCP encapsulation.  If the
   initiator receives the response message over TCP, then either this
   message is genuine and was sent by the peer, or the TCP session was
   hijacked and the message is forged, but in this case no genuine
   messages from the responder will be received.

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   So, in case of TCP encapsulation the initiator SHOULD NOT wait for
   additional messages in case it receives error notification from the
   responder in the IKE_SA_INIT exchange.

8.  Interaction with IKEv2 Extensions

8.1.  MOBIKE Protocol

   MOBIKE protocol, that allows IKEv2 SA to migrate between IP
   addresses, is defined in [RFC4555], and [RFC4621] further clarifies
   the details of the protocol.  Section 8 of [RFC8229] describes how
   interaction between MOBIKE and TCP encapsulation.  This memo provides
   clarifications and additional recommendations for using MOBIKE in
   case of TCP encapsulation.

   [RFC8229] recommends, that in case of IP address change, the
   initiator first initiates the INFORMATIONAL exchange containing
   UPDATE_SA_ADDRESSES notification using UDP transport and if no
   response is received then send this notification over TCP transport.
   However, this recommendation lacks some details.  In particular, it
   is not clear whether falling back from UDP to TCP requires initiating
   a new INFORMATIONAL exchange or not.

   From MOBIKE point of view UDP and TCP transports can be seen as two
   different network attachments, so falling back from the former to the
   latter is very similar to changing peer's IP address.  For that
   reason, the initiator first initiates the INFORMATIONAL exchange over
   UDP, and if no response is received within some time after several
   retransmissions, then the initiator changes transport from UDP to TCP
   in this very exchange.  New INFORMATIONAL exchange MUST NOT be
   started in this situation.

   It means that after switching to TCP the content of the
   NAT_DETECTION_SOURCE_IP notification will in most cases be incorrect
   (since UDP and TCP source ports will most probably be different), and
   the peer will falsely think that there is a NAT in between.  This
   should not cause problems because in this case all traffic will be
   encapsulated in TCP anyway, and TCP encapsulation is the same with
   regardless of NAT presence.

   MOBIKE protocol defined the NO_NATS_ALLOWED notification that can be
   used to detect the presence of NAT between peer and to refuse to
   communicate in this situation.  In case of TCP the NO_NATS_ALLOWED
   notification SHOULD be ignored because TCP generally has no problems
   with NAT boxes.

   Section 3.7 of [RFC4555] describes an additional optional step in the
   process of changing IP addresses called Return Routability Check.  It

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   is performed by the responder in order to be sure that the new
   initiator's address is in fact routable.  In case of TCP
   encapsulation this check has little value, since TCP handshake proves
   routability of the TCP Originator's address.  So, in case of TCP
   encapsulation the Return Routability Check SHOULD NOT be performed.

8.2.  IKEv2 Redirect

   Redirect mechanism for IKEv2 is defined in [RFC5685].  This mechanism
   allows security gateways to redirect clients to another gateway
   either during IKE SA establishment or after it is set up.  If a
   client is connecting to a security gateway using TCP transport and
   then is being redirected to another security gateway, then the client
   must disregard the current transport.  In other words, the client
   MUST again first try UDP and then fall back to TCP while establishing
   a new IKE SA, regardless of the transport of the SA the redirect
   notification was received over (unless the client's configuration
   instructs it to instantly use TCP for the gateway it is redirected

8.3.  IKEv2 Session Resumption

   Session resumption for IKEv2 is defined in [RFC5723].  Once IKE SA is
   established the server creates a resumption ticket where information
   about this SA is stored, and transfers this ticket to the client.
   The ticket may be later used to resume the IKE SA if it is deleted.
   In the event of resumption the client presents the ticket in a new
   exchange, called IKE_SESSION_RESUME.  For the new SA some parameters
   are taken from the ticket and some are re-negotiated (more details
   are given in Section 5 of [RFC5723]).  If TCP encapsulation was used
   in an old SA, then the client SHOULD resume this SA using TCP,
   without first trying to connect over UDP.

8.4.  IKEv2 Protocol Support for High Availability

   [RFC6311] defines a support for High Availability in IKEv2.  The core
   idea is that in case of cluster failover a new active node
   immediately initiates the special INFORMATION exchange containing the
   IKEV2_MESSAGE_ID_SYNC notification, which instructs the client to
   skip some number of Message IDs that might not be synchronized yet
   between nodes at the time of failover.

   The problem is that TCP states are much harder to synchronize than
   IKE states - it requires access to TCP/IP stack internals, which is
   not always available for IKE/IPsec implementations.  If a cluster
   implementation doesn't synchronize TCP states between nodes, then
   after failover event the new active node will not have any TCP
   connection with the client, so the node cannot initiate the

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   INFORMATIONAL exchange as required by [RFC6311].  Since the cluster
   usually acts as TCP Responder, the new active node cannot re-
   establish TCP connection, since only the TCP Originator can do it.
   And for the client the situation of cluster failover may remain
   unknown for long time if it has no IKE or ESP traffic to send.  Once
   the client sends any ESP or IKEv2 packet, the cluster node will reply
   with TCP RST and the client (as TCP Originator) will restore the TCP
   connection so that the node will be able to initiate the
   INFORMATIONAL exchange informing the client about the cluster

   This memo makes the following recommendation: if support for High
   Availability in IKEv2 is negotiated and TCP transport is used and a
   client is TCP Originator, then the client SHOULD periodically send
   IKEv2 messages (e.g. by initiating liveness check exchange) whenever
   there is no any IKEv2 or ESP traffic.  This differs from the
   recommendations given in Section 2.4 of [RFC7296] in the following:
   the liveness check should be periodically performed even if the
   client has nothing to send over ESP.  The frequency of sending such
   messages should be high enough to allow quick detection and restoring
   of broken TCP connection.

9.  Security Considerations

   Security considerations concerning using TCP encapsulation in IKEv2
   and ESP are given in [RFC8229].  This memo doesn't provide additional
   security considerations.

10.  Acknowledgements

   Author would like to thank Tommy Pauly and Tero Kivinen for their
   valuable comments.

11.  References

11.1.  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, <https://www.rfc-

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

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   [RFC5685]  Devarapalli, V. and K. Weniger, "Redirect Mechanism for
              the Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5685, DOI 10.17487/RFC5685, November 2009,

   [RFC5723]  Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
              Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
              DOI 10.17487/RFC5723, January 2010, <https://www.rfc-

   [RFC6311]  Singh, R., Ed., Kalyani, G., Nir, Y., Sheffer, Y., and D.
              Zhang, "Protocol Support for High Availability of IKEv2/
              IPsec", RFC 6311, DOI 10.17487/RFC6311, July 2011,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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, <>.

   [RFC8019]  Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
              Protocol Version 2 (IKEv2) Implementations from
              Distributed Denial-of-Service Attacks", RFC 8019,
              DOI 10.17487/RFC8019, November 2016, <https://www.rfc-

   [RFC8229]  Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
              of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
              August 2017, <>.

11.2.  Informative References

   [RFC4621]  Kivinen, T. and H. Tschofenig, "Design of the IKEv2
              Mobility and Multihoming (MOBIKE) Protocol", RFC 4621,
              DOI 10.17487/RFC4621, August 2006, <https://www.rfc-

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,

   [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
              2012, <>.

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

Author's Address

   Valery Smyslov
   PO Box 81
   Moscow (Zelenograd)  124460

   Phone: +7 495 276 0211

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