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IETF RFC 6520
Last modified on Wednesday, February 8th, 2012
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Internet Engineering Task Force (IETF) R. Seggelmann
Request for Comments: 6520 M. Tuexen
Category: Standards Track Muenster Univ. of Appl. Sciences
ISSN: 2070-1721 M. Williams
GWhiz Arts & Sciences
February 2012
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/RFC 6520.
Seggelmann, et al. Standards Track PAGE 1 
RFC 6520 TLS/DTLS Heartbeat Extension February 2012
Copyright Notice
Copyright (c) 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 [RFC 5246] and [RFC 6347] and their
adaptations to specific transport protocols described in [RFC 3436],
[RFC 5238], and [RFC 6083].
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
[RFC 4821].
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 [RFC 2119].
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 2012
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
[RFC 6347]. 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 [RFC 6066].
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 in Section 4.1.1.1 of
[RFC 6347]. A detailed description of how to perform path MTU
discovery is given in [RFC 4821]. 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 [RFC 4820].
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 2012
6. IANA Considerations
IANA has assigned the heartbeat content type (24) from the "TLS
ContentType Registry" as specified in [RFC 5246]. 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 [RFC 5226]. The reference is to RFC 6520.
IANA has assigned the heartbeat extension type (15) from the TLS
"ExtensionType Values" registry as specified in [RFC 5246]. 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 [RFC 5226]. The reference is to RFC 6520.
7. Security Considerations
The security considerations of [RFC 5246] and [RFC 6347] 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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 5226] 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
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC 6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC 6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
9.2. Informative References
[RFC 3436] Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
Layer Security over Stream Control Transmission Protocol",
RFC 3436, December 2002.
[RFC 4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, March 2007.
[RFC 4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC 5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over
the Datagram Congestion Control Protocol (DCCP)",
RFC 5238, May 2008.
[RFC 6083] 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 2012
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
Seggelmann, et al. Standards Track PAGE 9
RFC TOTAL SIZE: 16858 bytes
PUBLICATION DATE: Wednesday, February 8th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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