One document matched: draft-ietf-dccp-dtls-04.txt
Differences from draft-ietf-dccp-dtls-03.txt
INTERNET-DRAFT DTLS over DCCP December 21, 2007
DTLS over DCCP
Internet Draft T. Phelan
Document: draft-ietf-dccp-dtls-04.txt Sonus Networks
Expires: June 2008 December 21, 2007
Intended status: Proposed Standard
Datagram Transport Layer Security (DTLS) over the Datagram
Congestion Control Protocol (DCCP)
Status of this Memo
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This Internet-Draft will expire on September 30, 2007.
Abstract
This document specifies the use of Datagram Transport Layer Security
(DTLS) over the Datagram Congestion Control Protocol (DCCP). DTLS
provides communications privacy for datagram protocols and allows
client/server applications to communicate in a way that is designed
to prevent eavesdropping, tampering, or message forgery. DCCP is a
transport protocol that provides a congestion-controlled unreliable
datagram service.
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Table of Contents
1. Introduction...................................................3
2. Terminology....................................................3
3. DTLS over DCCP.................................................3
3.1 DCCP and DTLS Sequence Numbers.............................3
3.2 DCCP and DTLS Connection Handshakes........................4
3.3 Effects of DCCP Congestion Control.........................5
3.4 DTLS Sessions and DCCP Connections.........................6
3.5 PMTU Discovery.............................................7
3.6 DCCP Service Codes.........................................7
3.7 New Versions of DTLS.......................................8
4. Security Considerations........................................8
5. IANA Considerations............................................8
6. References.....................................................9
6.1 Normative References.......................................9
6.2 Informative References.....................................9
7. Author's Address...............................................9
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1. Introduction
This document specifies how to use Datagram Transport Layer Security
(DTLS), as specified in [RFC4347], over the Datagram Congestion
Control Protocol (DCCP), as specified in [RFC4340].
DTLS is an extension of Transport Layer Security (TLS, [RFC4346])
that modifies TLS for use with the unreliable transport protocol UDP.
TLS is a protocol that allows client/server applications to
communicate in a way that is designed to prevent eavesdropping,
tampering and message forgery. DTLS can be viewed as TLS-plus-
adaptations-for-unreliability.
DCCP provides an unreliable transport service, similar to UDP, but
with adaptive congestion control, similar to TCP and SCTP. DCCP can
be viewed equally well as either UDP-plus-congestion-control or TCP-
minus-reliability (although, unlike TCP, DCCP offers multiple
congestion control algorithms).
The combination of DTLS and DCCP will offer transport security
capabilities to DCCP users similar to those available for TCP, UDP
and SCTP.
2. Terminology
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].
3. DTLS over DCCP
The approach here is very straightforward -- DTLS records are
transmitted in the Application Data fields of DCCP-Data and DCCP-
DataAck packets (in the rest of the document assume that "DCCP-Data
packet" means "DCCP-Data or DCCP-DataAck packet"). Multiple DTLS
records MAY be sent in one DCCP-Data packet, as long as the resulting
packet is within the Path Maximum Transfer Unit (PMTU) currently in
force, if the Don't Fragment (DF) bit is being used, or within the
current DCCP maximum packet size if the DF bit is not being used (see
section 3.5 for more information on PMTU Discovery). A single DTLS
record MUST be fully contained in a single DCCP-Data packet; it MUST
NOT be split over multiple packets.
3.1 DCCP and DTLS Sequence Numbers
Both DCCP and DTLS use sequence numbers in their packets/records.
These sequence numbers serve somewhat, but not completely,
overlapping functions. Consequently, there is no connection between
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the sequence number of a DCCP packet and the sequence number in a
DTLS record contained in that packet and no connection between
sequence number-related features such as DCCP synchronization and
DTLS anti-replay protection.
3.2 DCCP and DTLS Connection Handshakes
Unlike UDP, DCCP is connection-oriented, and has a connection
handshake procedure that precedes the transmission of DCCP-Data and
DCCP-DataAck packets. DTLS is also connection-oriented, and has a
handshake procedure of its own that must precede the transmission of
actual application information. Using the rule of mapping DTLS
records to DCCP-Data and DCCP-DataAck packets in section 3, above,
the two handshakes are forced to happen in series, with the DCCP
handshake first, followed by the DTLS handshake. This is how TLS
over TCP works.
However, the DCCP handshake packets DCCP-Request and DCCP-Response
have Application Data fields and can carry user data during the DCCP
handshake, and this creates the opportunity to perform the handshakes
partially in parallel. DTLS client implementations MAY choose to
transmit the ClientHello message in the DCCP-Request packet. DTLS
server implementations MAY choose to respond to a ClientHello message
received in a DCCP-Request packet with a HelloVerifyRequest message,
if denial of service countermeasures are to be used, or a
ServerHelloDone message otherwise, in the DCCP-Response packet. DTLS
servers MAY also choose to delay the response until the handshake
completes and then send the response in a DCCP-Data packet.
DTLS handshake messages can be quite large, theoretically up to 2^24-
1 bytes and in practice often many kilobytes. Subsequently, unlike
other DTLS messages, the handshake messages may be fragmented over
multiple DTLS records. If the size of the ClientHello is too large
to transmit in its entirety in a DCCP-Request packet the ClientHello
MUST be sent in DCCP-Data packets after the DCCP handshake is
complete. Similarly, if the server response to a ClientHello is too
large to transmit in its entirety in a DCCP-Response packet, it MUST
be sent in DCCP-Data packets after the DCCP handshake is complete.
Transmission of subsequent DTLS handshake messages MUST wait for the
completion of the DCCP handshake and use DCCP-Data packets.
Note that even though the DCCP handshake is a reliable process
(handshake messages are retransmitted as required if messages are
lost), the transfer of Application Data in DCCP-Request and DCCP-
Response packets is not necessarily reliable. For example, DCCP
Server implementations are free to discard Application Data received
in DCCP-Request packets. And if DCCP-Request or DCCP-Response
packets need to be retransmitted, the DCCP implementation may choose
to not include the Application Data present in the initial message.
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Since the DTLS handshake is also a reliable process, it will
interoperate across the data delivery unreliability of DCCP (after
all, one of the basic functions of DTLS is to work over unreliable
transport). If ClientHello messages or the HelloVerifyRequest or
ServerHelloDone messages are lost, the ClientHello message will be
retransmitted by DTLS.
This is regardless of whether the messages were sent in DCCP-
Response/Request packets or DCCP-Data packets. However, the only way
for DTLS to retransmit a ClientHello message that was originally
transmitted in a DCCP-Request packet (and it or the response was lost
somehow) is to wait for the DCCP handshake to complete and then send
the ClientHello in a DCCP-Data packet. This is due to the
characteristic of DCCP that the next opportunity to send data after
sending data in a DCCP-Request is only after the connection handshake
completes.
DCCP and DTLS use similar strategies for retransmitting handshake
messages. If there is no response to the original request (DCCP-
Request or ClientHello respectively) within normally 1 second, the
message is retransmitted. The timer is then doubled and the process
repeated until a response is received, or a maximum time is exceeded.
Therefore, if the ClientHello message is sent in a DCCP-Request
packet, and the DCCP-Request or DCCP-Response message is lost, the
DCCP and DTLS handshakes could be timing out on similar schedules.
The DCCP-Request packets will be retransmitted on timeout, but the
ClientHello packet cannot be retransmitted until the DCCP handshake
completes (there is no possibility of adding new Application Data to
a DCCP-Request retransmission). In order to avoid multiple
retransmissions queuing up before the first retransmission can be
sent, DTLS over DCCP MUST wait until the completion of the DCCP
handshake before restarting its retransmission timer.
3.3 Effects of DCCP Congestion Control
Given the large potential sizes of the DTLS handshake messages, it is
possible that DCCP congestion control could throttle the transmission
of the DTLS handshake to the point that the transfer cannot complete
before the DTLS timeout and retransmission procedures take effect.
Adding retransmitted messages to a congested situation might only
make matters worse and delay connection establishment.
Note that a DTLS over UDP application transmitting handshake data
into this same network situation will not necessarily receive better
throughput, and might actually see worse effective throughput.
Without the pacing of slow-start and congestion control, a UDP
application might be making congestion worse and lowering the
effective throughput it receives.
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As stated in [RFC4347], "mishandling of the [retransmission] timer
can lead to serious congestion problems". This remains as true for
DTLS over DCCP as it is for DTLS over UDP.
DTLS over DCCP implementations SHOULD take steps to avoid
retransmitting a request that has been queued but not yet actually
transmitted by DCCP, when the underlying DCCP implementation can
provide this information. For example, DTLS could delay starting the
retransmission timer until DCCP indicates the message has been
transferred from DCCP to the IP layer.
In addition to the retransmission issues, if the throughput needs of
the actual application data differ from the needs of the DTLS
handshake, it is possible that the handshake transference could leave
the DCCP congestion control in a state that is not immediately
suitable for the application data that will follow. For example,
DCCP CCID2 ([RFC4341]) congestion control uses an Additive Increase
Multiplicative Decrease (AIMD) algorithm similar to TCP congestion
control. If it is used then it is possible that transference of a
large handshake could cause a multiplicative decrease that would not
have happened with the application data. The application might then
be throttled while waiting for additive increase to return throughput
to acceptable levels.
Applications where this might be a problem should consider using DCCP
CCID3 ([RFC4342]). CCID3 implements TCP-Friendly Rate Control (TFRC,
[RFC3448])). TFRC varies the allowed throughput more slowly than
AIMD and might avoid the discontinuities possible with CCID2.
3.4 DTLS Sessions and DCCP Connections
There is no necessary relationship between the life of a DTLS session
and the life of a DCCP connection. Often the session and connection
lives start and stop together (DCCP connection establishment
immediately followed by DTLS session establishment, DTLS session
termination immediately followed by DCCP connection termination), but
this is not the only possibility.
A single DTLS session may span multiple DCCP connections using the
DTLS session resumption features. The session resumption feature of
DTLS is widely used and this situation is likely to occur frequently.
It is even possible to resume a DTLS session over a different
transport.
A DCCP connection has no knowledge of the type of application data it
is transferring. It could conceivably contain multiple DTLS sessions,
in series or even in parallel, while simultaneously transferring non-
DTLS data. In practice this could be difficult to demultiplex at the
application/DTLS level and support for this is likely to be rare to
nonexistent. [RFC4347] does not specifically exclude multiple DTLS
sessions simultaneously sharing the same underlying transport.
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A special case of this DCCP connection flexibility is an application
that starts up transferring non-DTLS data, and then switches to DTLS
after some time. This is likely to be useful and has implications
for the choice of DCCP Service Code. See section 3.6 for more
information on this.
3.5 PMTU Discovery
Each DTLS record must fit within a single DCCP-Data packet. DCCP
packets are normally transmitted with the DF (Don't Fragment) bit set
for IPv4, and of course all IPv6 packets are unfragmentable in the
network. Because of this, DCCP performs Path Maximum Transmission
Unit (PMTU) Discovery.
DTLS also normally uses the DF bit and performs PMTU Discovery on its
own, using an algorithm that is strongly similar to the one used by
DCCP. A DTLS over DCCP implementation MAY use the DCCP-managed value
for PMTU and not perform PMTU Discovery on its own. Alternatively, a
DTLS over DCCP implementation MAY choose to use its own PMTU
Discovery calculations, as specified in [RFC4347], but MUST NOT use a
value greater than the value determined by DCCP.
DTLS implementations normally allow applications to reset the PMTU
estimate back to the initial state. When that happens, DTLS over
DCCP implementations SHOULD also reset the DCCP PMTU estimation.
DTLS implementations also sometimes allow applications to control the
use of the DF bit (when running over IPv4). DTLS over DCCP
implementations SHOULD control the use of the DF bit by DCCP in
concert with the application's indications, when the DCCP
implementation supports this. Note that DCCP implementations are not
required to support sending packets with the DF bit not set.
Note that the DCCP Maximum Packet Size (MPS in [RFC4340]) is bounded
by the current congestion control state (Congestion Control Maximum
Packet Size, CCMPS in [RFC4340]). Even when the DF bit is not set
and DCCP packets may then be fragmented, the MPS may be less than the
65,535 bytes normally used in UDP. It is also possible for the DCCP
CMPS, and thus the MPS, to vary over time as congestion conditions
change. DTLS over DCCP implementations MUST NOT use a DTLS record
size that is greater than the DCCP MPS currently in force.
3.6 DCCP Service Codes
The DCCP connection handshake includes a field called Service Code
that is intended to describe "the application-level service to which
the client application wants to connect". Further, "Service Codes
are intended to provide information about which application protocol
a connection intends to use, thus aiding middleboxes and reducing
reliance on globally well-known ports" [RFC4340].
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It is expected that many middleboxes will give different privileges
to applications running DTLS over DCCP versus just DCCP. Therefore,
applications that use DTLS over DCCP sometimes and just DCCP other
times MUST register and use different Service Codes for each mode of
operation. Applications that use both DCCP and DTLS over DCCP MAY
choose to listen for incoming connections on the same DCCP port and
distinguish the mode of the request by the offered Service Code.
Some applications may start out using DCCP without DTLS, and then
optionally switch to using DTLS over the same connection. Since
there is no way to change the Service Code for a connection after it
is established, these applications will use one Service Code.
3.7 New Versions of DTLS
As DTLS matures, revisions to and updates for [RFC4347] can be
expected. DTLS includes mechanisms for identifying the version in
use and presumably future versions will either include backward
compatibility modes or at least not allow connections between
dissimilar versions. Since DTLS over DCCP simply encapsulates the
DTLS records transparently, these changes should not affect this
document and the methods of this document should apply to future
versions of DTLS.
Therefore, in the absence of a revision to this document, it is
assumed to apply to all future versions of DTLS. This document will
only be revised if a revision to DTLS makes a revision to the
encapsulation necessary.
It is RECOMMENDED that an application migrating to a new version of
DTLS keep the same DCCP Service Code used for the old version and
allow DTLS to provide the version negotiation support. If the
application developers feel that the new version of DTLS provides
significant new capabilities to the application that will change the
behavior of middleboxes, they MAY use a new Service Code.
4. Security Considerations
Security considerations for DTLS are specified in [RFC4347] and for
DCCP in [RFC4340]. The combination of DTLS and DCCP introduces no
new security considerations.
5. IANA Considerations
There are no IANA actions required for this document.
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6. References
6.1 Normative References
[RFC4347] Rescorla, E., "Datagram Transport Layer Security", RFC
4347, April 2006.
[RFC4340] Kohler, E., Handley, M., Floyd, S., "Datagram Congestion
Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
6.2 Informative References
[RFC4341] Floyd, S., Kohler, E., "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., Padhye, J., " Profile for Datagram
Congestion Control Protocol (DCCP) Congestion Control ID
3: TCP-Friendly Rate Control (TFRC)", RFC 4342, March
2006.
[RFC3448] Handley, M., Floyd, S., Padhye, J., Widmer, J., " TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003.
7. Author's Address
Tom Phelan
Sonus Networks
7 Technology Park Dr.
Westford, MA USA 01886
Phone: 978-614-8456
Email: tphelan@sonusnet.com
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