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Internet Engineering Task Force (IETF)                     R. SeggelmannRequest for Comments: 6520                                     M. TuexenCategory: Standards Track               Muenster Univ. of Appl. SciencesISSN: 2070-1721                                              M. Williams                                                   GWhiz Arts & Sciences                                                           February 2012Transport Layer Security (TLS) andDatagram Transport Layer Security (DTLS) Heartbeat ExtensionAbstract   This document describes the Heartbeat Extension for the Transport   Layer Security (TLS) and Datagram Transport Layer Security (DTLS)   protocols.   The Heartbeat Extension provides a new protocol for TLS/DTLS allowing   the usage of keep-alive functionality without performing a   renegotiation and a basis for path MTU (PMTU) discovery for DTLS.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 5741.   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/rfc6520.Seggelmann, et al.           Standards Track                    [Page 1]

RFC 6520              TLS/DTLS Heartbeat Extension         February 2012Copyright Notice   Copyright (c) 2012 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  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Heartbeat Hello Extension . . . . . . . . . . . . . . . . . . .33.  Heartbeat Protocol  . . . . . . . . . . . . . . . . . . . . . .44.  Heartbeat Request and Response Messages . . . . . . . . . . . .55.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . .66.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .77.  Security Considerations . . . . . . . . . . . . . . . . . . . .78.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . .79.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .71.  Introduction1.1.  Overview   This document describes the Heartbeat Extension for the Transport   Layer Security (TLS) and Datagram Transport Layer Security (DTLS)   protocols, as defined in [RFC5246] and [RFC6347] and their   adaptations to specific transport protocols described in [RFC3436],   [RFC5238], and [RFC6083].   DTLS is designed to secure traffic running on top of unreliable   transport protocols.  Usually, such protocols have no session   management.  The only mechanism available at the DTLS layer to figure   out if a peer is still alive is a costly renegotiation, particularly   when the application uses unidirectional traffic.  Furthermore, DTLS   needs to perform path MTU (PMTU) discovery but has no specific   message type to realize it without affecting the transfer of user   messages.Seggelmann, et al.           Standards Track                    [Page 2]

RFC 6520              TLS/DTLS Heartbeat Extension         February 2012   TLS is based on reliable protocols, but there is not necessarily a   feature available to keep the connection alive without continuous   data transfer.   The Heartbeat Extension as described in this document overcomes these   limitations.  The user can use the new HeartbeatRequest message,   which has to be answered by the peer with a HeartbeartResponse   immediately.  To perform PMTU discovery, HeartbeatRequest messages   containing padding can be used as probe packets, as described in   [RFC4821].1.2.  Conventions   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 [RFC2119].2.  Heartbeat Hello Extension   The support of Heartbeats is indicated with Hello Extensions.  A peer   cannot only indicate that its implementation supports Heartbeats, it   can also choose whether it is willing to receive HeartbeatRequest   messages and respond with HeartbeatResponse messages or only willing   to send HeartbeatRequest messages.  The former is indicated by using   peer_allowed_to_send as the HeartbeatMode; the latter is indicated by   using peer_not_allowed_to_send as the Heartbeat mode.  This decision   can be changed with every renegotiation.  HeartbeatRequest messages   MUST NOT be sent to a peer indicating peer_not_allowed_to_send.  If   an endpoint that has indicated peer_not_allowed_to_send receives a   HeartbeatRequest message, the endpoint SHOULD drop the message   silently and MAY send an unexpected_message Alert message.   The format of the Heartbeat Hello Extension is defined by:   enum {      peer_allowed_to_send(1),      peer_not_allowed_to_send(2),      (255)   } HeartbeatMode;   struct {      HeartbeatMode mode;   } HeartbeatExtension;   Upon reception of an unknown mode, an error Alert message using   illegal_parameter as its AlertDescription MUST be sent in response.Seggelmann, et al.           Standards Track                    [Page 3]

RFC 6520              TLS/DTLS Heartbeat Extension         February 20123.  Heartbeat Protocol   The Heartbeat protocol is a new protocol running on top of the Record   Layer.  The protocol itself consists of two message types:   HeartbeatRequest and HeartbeatResponse.   enum {      heartbeat_request(1),      heartbeat_response(2),      (255)   } HeartbeatMessageType;   A HeartbeatRequest message can arrive almost at any time during the   lifetime of a connection.  Whenever a HeartbeatRequest message is   received, it SHOULD be answered with a corresponding   HeartbeatResponse message.   However, a HeartbeatRequest message SHOULD NOT be sent during   handshakes.  If a handshake is initiated while a HeartbeatRequest is   still in flight, the sending peer MUST stop the DTLS retransmission   timer for it.  The receiving peer SHOULD discard the message   silently, if it arrives during the handshake.  In case of DTLS,   HeartbeatRequest messages from older epochs SHOULD be discarded.   There MUST NOT be more than one HeartbeatRequest message in flight at   a time.  A HeartbeatRequest message is considered to be in flight   until the corresponding HeartbeatResponse message is received, or   until the retransmit timer expires.   When using an unreliable transport protocol like the Datagram   Congestion Control Protocol (DCCP) or UDP, HeartbeatRequest messages   MUST be retransmitted using the simple timeout and retransmission   scheme DTLS uses for flights as described inSection 4.2.4 of   [RFC6347].  In particular, after a number of retransmissions without   receiving a corresponding HeartbeatResponse message having the   expected payload, the DTLS connection SHOULD be terminated.  The   threshold used for this SHOULD be the same as for DTLS handshake   messages.  Please note that after the timer supervising a   HeartbeatRequest messages expires, this message is no longer   considered in flight.  Therefore, the HeartbeatRequest message is   eligible for retransmission.  The retransmission scheme, in   combination with the restriction that only one HeartbeatRequest is   allowed to be in flight, ensures that congestion control is handled   appropriately in case of the transport protocol not providing one,   like in the case of DTLS over UDP.Seggelmann, et al.           Standards Track                    [Page 4]

RFC 6520              TLS/DTLS Heartbeat Extension         February 2012   When using a reliable transport protocol like the Stream Control   Transmission Protocol (SCTP) or TCP, HeartbeatRequest messages only   need to be sent once.  The transport layer will handle   retransmissions.  If no corresponding HeartbeatResponse message has   been received after some amount of time, the DTLS/TLS connection MAY   be terminated by the application that initiated the sending of the   HeartbeatRequest message.4.  Heartbeat Request and Response Messages   The Heartbeat protocol messages consist of their type and an   arbitrary payload and padding.   struct {      HeartbeatMessageType type;      uint16 payload_length;      opaque payload[HeartbeatMessage.payload_length];      opaque padding[padding_length];   } HeartbeatMessage;   The total length of a HeartbeatMessage MUST NOT exceed 2^14 or   max_fragment_length when negotiated as defined in [RFC6066].   type:  The message type, either heartbeat_request or      heartbeat_response.   payload_length:  The length of the payload.   payload:  The payload consists of arbitrary content.   padding:  The padding is random content that MUST be ignored by the      receiver.  The length of a HeartbeatMessage is TLSPlaintext.length      for TLS and DTLSPlaintext.length for DTLS.  Furthermore, the      length of the type field is 1 byte, and the length of the      payload_length is 2.  Therefore, the padding_length is      TLSPlaintext.length - payload_length - 3 for TLS and      DTLSPlaintext.length - payload_length - 3 for DTLS.  The      padding_length MUST be at least 16.   The sender of a HeartbeatMessage MUST use a random padding of at   least 16 bytes.  The padding of a received HeartbeatMessage message   MUST be ignored.   If the payload_length of a received HeartbeatMessage is too large,   the received HeartbeatMessage MUST be discarded silently.Seggelmann, et al.           Standards Track                    [Page 5]

RFC 6520              TLS/DTLS Heartbeat Extension         February 2012   When a HeartbeatRequest message is received and sending a   HeartbeatResponse is not prohibited as described elsewhere in this   document, the receiver MUST send a corresponding HeartbeatResponse   message carrying an exact copy of the payload of the received   HeartbeatRequest.   If a received HeartbeatResponse message does not contain the expected   payload, the message MUST be discarded silently.  If it does contain   the expected payload, the retransmission timer MUST be stopped.5.  Use Cases   Each endpoint sends HeartbeatRequest messages at a rate and with the   padding required for the particular use case.  The endpoint should   not expect its peer to send HeartbeatRequests.  The directions are   independent.5.1.  Path MTU Discovery   DTLS performs path MTU discovery as described inSection 4.1.1.1 of   [RFC6347].  A detailed description of how to perform path MTU   discovery is given in [RFC4821].  The necessary probe packets are the   HeartbeatRequest messages.   This method of using HeartbeatRequest messages for DTLS is similar to   the one for the Stream Control Transmission Protocol (SCTP) using the   padding chunk (PAD-chunk) defined in [RFC4820].5.2.  Liveliness Check   Sending HeartbeatRequest messages allows the sender to make sure that   it can reach the peer and the peer is alive.  Even in the case of   TLS/TCP, this allows a check at a much higher rate than the TCP keep-   alive feature would allow.   Besides making sure that the peer is still reachable, sending   HeartbeatRequest messages refreshes the NAT state of all involved   NATs.   HeartbeatRequest messages SHOULD only be sent after an idle period   that is at least multiple round-trip times long.  This idle period   SHOULD be configurable up to a period of multiple minutes and down to   a period of one second.  A default value for the idle period SHOULD   be configurable, but it SHOULD also be tunable on a per-peer basis.Seggelmann, et al.           Standards Track                    [Page 6]

RFC 6520              TLS/DTLS Heartbeat Extension         February 20126.  IANA Considerations   IANA has assigned the heartbeat content type (24) from the "TLS   ContentType Registry" as specified in [RFC5246].  The reference is toRFC 6520.   IANA has created and now maintains a new registry for Heartbeat   Message Types.  The message types are numbers in the range from 0 to   255 (decimal).  IANA has assigned the heartbeat_request (1) and the   heartbeat_response (2) message types.  The values 0 and 255 should be   reserved.  This registry uses the Expert Review policy as described   in [RFC5226].  The reference is toRFC 6520.   IANA has assigned the heartbeat extension type (15) from the TLS   "ExtensionType Values" registry as specified in [RFC5246].  The   reference is toRFC 6520.   IANA has created and now maintains a new registry for Heartbeat   Modes.  The modes are numbers in the range from 0 to 255 (decimal).   IANA has assigned the peer_allowed_to_send (1) and the   peer_not_allowed_to_send (2) modes.  The values 0 and 255 should be   reserved.  This registry uses the Expert Review policy as described   in [RFC5226].  The reference is toRFC 6520.7.  Security Considerations   The security considerations of [RFC5246] and [RFC6347] apply to this   document.  This document does not introduce any new security   considerations.8.  Acknowledgments   The authors wish to thank Pasi Eronen, Adrian Farrel, Stephen   Farrell, Adam Langley, Nikos Mavrogiannopoulos, Tom Petch, Eric   Rescorla, Peter Saint-Andre, and Juho Vaehae-Herttua for their   invaluable comments.9.  References9.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an              IANA Considerations Section in RFCs",BCP 26,RFC 5226,              May 2008.Seggelmann, et al.           Standards Track                    [Page 7]

RFC 6520              TLS/DTLS Heartbeat Extension         February 2012   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security              (TLS) Protocol Version 1.2",RFC 5246, August 2008.   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:              Extension Definitions",RFC 6066, January 2011.   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer              Security Version 1.2",RFC 6347, January 2012.9.2.  Informative References   [RFC3436]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport              Layer Security over Stream Control Transmission Protocol",RFC 3436, December 2002.   [RFC4820]  Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and              Parameter for the Stream Control Transmission Protocol              (SCTP)",RFC 4820, March 2007.   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU              Discovery",RFC 4821, March 2007.   [RFC5238]  Phelan, T., "Datagram Transport Layer Security (DTLS) over              the Datagram Congestion Control Protocol (DCCP)",RFC 5238, May 2008.   [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram              Transport Layer Security (DTLS) for Stream Control              Transmission Protocol (SCTP)",RFC 6083, January 2011.Seggelmann, et al.           Standards Track                    [Page 8]

RFC 6520              TLS/DTLS Heartbeat Extension         February 2012Authors' Addresses   Robin Seggelmann   Muenster University of Applied Sciences   Stegerwaldstr. 39   48565 Steinfurt   DE   EMail: seggelmann@fh-muenster.de   Michael Tuexen   Muenster University of Applied Sciences   Stegerwaldstr. 39   48565 Steinfurt   DE   EMail: tuexen@fh-muenster.de   Michael Glenn Williams   GWhiz Arts & Sciences   2885 Denise Court   Newbury Park, CA, 91320   USA   EMail: michael.glenn.williams@gmail.comSeggelmann, et al.           Standards Track                    [Page 9]