Transport Layer Security (TLS) and

Datagram Transport Layer Security (DTLS) Heartbeat Extension

Abstract

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 in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6520.

Copyright Notice

Copyright © 2012 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 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
   2.  Heartbeat Hello Extension . . . . . . . . . . . . . . . . . . . 3
   3.  Heartbeat Protocol  . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Heartbeat Request and Response Messages . . . . . . . . . . . . 5
   5.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1. Introduction

1.1. 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.

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.

3. 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 in Section 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.

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.

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 in Section 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.

6. IANA Considerations

IANA has assigned the heartbeat content type (24) from the "TLS ContentType Registry" as specified in [RFC5246]. The reference is to RFC 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 to RFC 6520.

IANA has assigned the heartbeat extension type (15) from the TLS "ExtensionType Values" registry as specified in [RFC5246]. The reference is to RFC 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 to RFC 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. References

9.1. 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.
   
   [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.

Authors' 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.com