Clarifications and Implementation Guidelines for using TCP Encapsulation in IKEv2
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Network Working Group V. Smyslov Internet-Draft ELVIS-PLUS Intended status: Informational June 18, 2019 Expires: December 20, 2019 Clarifications and Implementation Guidelines for using TCP Encapsulation in IKEv2 draft-smyslov-ipsecme-tcp-guidelines-02 Abstract 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 working documents as Internet-Drafts. The list of current Internet- Drafts is at https://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 December 20, 2019. Copyright Notice Copyright (c) 2019 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 (https://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 Smyslov Expires December 20, 2019 [Page 1] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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 . . . . . . . . . . . . . . . . 4 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 . . . . . . . . . . . . . . . . . . 9 11.2. Informative References . . . . . . . . . . . . . . . . . 10 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, Smyslov Expires December 20, 2019 [Page 2] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "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. Note, that since TCP header is longer than UDP header, and TCP encapsulation also requires prepending of 16-bit Length field, some very long ESP and IKE messages that could be sent over UDP cannot be encapsulated in TCP, because their total length after encapsulation would exceed 65535 and thus could not be represented in Length field. Smyslov Expires December 20, 2019 [Page 3] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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. 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 Smyslov Expires December 20, 2019 [Page 4] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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 hand, Puzzles still remain useful and their use requires using Cookies. 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 184.108.40.206 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 Smyslov Expires December 20, 2019 [Page 5] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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. 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. Smyslov Expires December 20, 2019 [Page 6] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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 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 to). 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 Smyslov Expires December 20, 2019 [Page 7] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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 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 failover. 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 Smyslov Expires December 20, 2019 [Page 8] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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-editor.org/info/rfc2119>. [RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol (MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006, <https://www.rfc-editor.org/info/rfc4555>. [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, <https://www.rfc-editor.org/info/rfc5685>. [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-editor.org/info/rfc5723>. [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, <https://www.rfc-editor.org/info/rfc6311>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [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, <https://www.rfc-editor.org/info/rfc7296>. [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-editor.org/info/rfc8019>. [RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229, August 2017, <https://www.rfc-editor.org/info/rfc8229>. Smyslov Expires December 20, 2019 [Page 9] Internet-Draft IKEv2 TCP Encapsulation Guidelines June 2019 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-editor.org/info/rfc4621>. [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007, <https://www.rfc-editor.org/info/rfc4987>. [RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February 2012, <https://www.rfc-editor.org/info/rfc6528>. [RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2 (IKEv2) Message Fragmentation", RFC 7383, DOI 10.17487/RFC7383, November 2014, <https://www.rfc-editor.org/info/rfc7383>. Author's Address Valery Smyslov ELVIS-PLUS PO Box 81 Moscow (Zelenograd) 124460 RU Phone: +7 495 276 0211 Email: firstname.lastname@example.org Smyslov Expires December 20, 2019 [Page 10]