Internet DRAFT - draft-rescorla-tls-dtls13
draft-rescorla-tls-dtls13
TLS E. Rescorla
Internet-Draft RTFM, Inc.
Obsoletes: 6347 (if approved) H. Tschofenig
Intended status: Standards Track ARM Limited
Expires: September 14, 2017 N. Modadugu
Google, Inc.
March 13, 2017
The Datagram Transport Layer Security (DTLS) Protocol Version 1.3
draft-rescorla-tls-dtls13-01
Abstract
This document specifies Version 1.3 of the Datagram Transport Layer
Security (DTLS) protocol. DTLS 1.3 allows client/server applications
to communicate over the Internet in a way that is designed to prevent
eavesdropping, tampering, and message forgery.
The DTLS 1.3 protocol is intentionally based on the Transport Layer
Security (TLS) 1.3 protocol and provides equivalent security
guarantees. Datagram semantics of the underlying transport are
preserved by the DTLS protocol.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. DTLS Design Rational and Overview . . . . . . . . . . . . . . 5
3.1. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Reordering . . . . . . . . . . . . . . . . . . . . . 6
3.1.2. Message Size . . . . . . . . . . . . . . . . . . . . 6
3.2. Replay Detection . . . . . . . . . . . . . . . . . . . . 7
4. The DTLS Record Layer . . . . . . . . . . . . . . . . . . . . 7
4.1. Sequence Number Handling . . . . . . . . . . . . . . . . 8
4.2. Transport Layer Mapping . . . . . . . . . . . . . . . . . 9
4.3. PMTU Issues . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Record Payload Protection . . . . . . . . . . . . . . . . 11
4.4.1. Anti-Replay . . . . . . . . . . . . . . . . . . . . . 11
4.4.2. Handling Invalid Records . . . . . . . . . . . . . . 12
5. The DTLS Handshake Protocol . . . . . . . . . . . . . . . . . 12
5.1. Denial-of-Service Countermeasures . . . . . . . . . . . . 13
5.2. DTLS Handshake Message Format . . . . . . . . . . . . . . 16
5.3. ACK Message . . . . . . . . . . . . . . . . . . . . . . . 20
5.4. Handshake Message Fragmentation and Reassembly . . . . . 20
5.5. Timeout and Retransmission . . . . . . . . . . . . . . . 21
5.5.1. State Machine . . . . . . . . . . . . . . . . . . . . 25
5.5.2. Timer Values . . . . . . . . . . . . . . . . . . . . 28
5.6. CertificateVerify and Finished Messages . . . . . . . . . 28
5.7. Alert Messages . . . . . . . . . . . . . . . . . . . . . 28
5.8. Establishing New Associations with Existing Parameters . 29
5.9. Epoch Values and Rekeying . . . . . . . . . . . . . . . . 29
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6. Application Data Protocol . . . . . . . . . . . . . . . . . . 32
7. Security Considerations . . . . . . . . . . . . . . . . . . . 32
8. Changes to DTLS 1.2 . . . . . . . . . . . . . . . . . . . . . 32
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1. Normative References . . . . . . . . . . . . . . . . . . 33
10.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 35
Appendix B. Working Group Information . . . . . . . . . . . . . 35
Appendix C. Contributors . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH
The source for this draft is maintained in GitHub. Suggested changes
should be submitted as pull requests at https://github.com/tlswg/
dtls13-spec. Instructions are on that page as well. Editorial
changes can be managed in GitHub, but any substantive change should
be discussed on the TLS mailing list.
The primary goal of the TLS protocol is to provide privacy and data
integrity between two communicating peers. The TLS protocol is
composed of two layers: the TLS Record Protocol and the TLS Handshake
Protocol. However, TLS must run over a reliable transport channel -
typically TCP [RFC0793].
There are applications that utilize UDP as a transport and to offer
communication security protection for those applications the Datagram
Transport Layer Security (DTLS) protocol has been designed. DTLS is
deliberately designed to be as similar to TLS as possible, both to
minimize new security invention and to maximize the amount of code
and infrastructure reuse.
DTLS 1.0 was originally defined as a delta from TLS 1.1 and DTLS 1.2
was defined as a series of deltas to TLS 1.2. There is no DTLS 1.1;
that version number was skipped in order to harmonize version numbers
with TLS. This specification describes the most current version of
the DTLS protocol aligning with the efforts around TLS 1.3.
Implementations that speak both DTLS 1.2 and DTLS 1.3 can
interoperate with those that speak only DTLS 1.2 (using DTLS 1.2 of
course), just as TLS 1.3 implementations can interoperate with TLS
1.2 (see Appendix D of [I-D.ietf-tls-tls13] for details). While
backwards compatibility with DTLS 1.0 is possible the use of DTLS 1.0
is not recommended as explained in Section 3.1.2 of RFC 7525
[RFC7525].
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2. Conventions and Terminology
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 RFC
2119 [RFC2119].
The following terms are used:
- client: The endpoint initiating the TLS connection.
- connection: A transport-layer connection between two endpoints.
- endpoint: Either the client or server of the connection.
- handshake: An initial negotiation between client and server that
establishes the parameters of their transactions.
- peer: An endpoint. When discussing a particular endpoint, "peer"
refers to the endpoint that is remote to the primary subject of
discussion.
- receiver: An endpoint that is receiving records.
- sender: An endpoint that is transmitting records.
- session: An association between a client and a server resulting
from a handshake.
- server: The endpoint which did not initiate the TLS connection.
The reader is assumed to be familiar with the TLS 1.3 specification
since this document defined as a delta from TLS 1.3.
Figures in this document illustrate various combinations of the DTLS
protocol exchanges and the symbols have the following meaning:
- '+' indicates noteworthy extensions sent in the previously noted
message.
- '*' indicates optional or situation-dependent messages/extensions
that are not always sent.
- '{}' indicates messages protected using keys derived from a
[sender]_handshake_traffic_secret.
- '[]' indicates messages protected using keysderived from
traffic_secret_N.
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3. DTLS Design Rational and Overview
The basic design philosophy of DTLS is to construct "TLS over
datagram transport". Datagram transport does not require or provide
reliable or in-order delivery of data. The DTLS protocol preserves
this property for application data. Applications such as media
streaming, Internet telephony, and online gaming use datagram
transport for communication due to the delay-sensitive nature of
transported data. The behavior of such applications is unchanged
when the DTLS protocol is used to secure communication, since the
DTLS protocol does not compensate for lost or re-ordered data
traffic.
TLS cannot be used directly in datagram environments for the
following five reasons:
1. TLS does not allow independent decryption of individual records.
Because the integrity check indirectly depends on a sequence
number, if record N is not received, then the integrity check on
record N+1 will be based on the wrong sequence number and thus
will fail. DTLS solves this problem by adding explicit sequence
numbers.
2. The TLS handshake is a lock-step cryptographic handshake.
Messages must be transmitted and received in a defined order; any
other order is an error. Clearly, this is incompatible with
reordering and message loss.
3. Not all TLS 1.3 handshake messages (such as the NewSessionTicket
message) are acknowledged. Hence, a new acknowledgement message
has to be added to detect message loss.
4. Handshake messages are potentially larger than any given
datagram, thus creating the problem of IP fragmentation.
5. Datagram transport protocols, like UDP, are more vulnerable to
denial of service attacks and require a return-routability check
with the help of cookies to be integrated into the handshake. A
detailed discussion of countermeasures can be found in
Section 5.1.
3.1. Packet Loss
DTLS uses a simple retransmission timer to handle packet loss.
Figure 1 demonstrates the basic concept, using the first phase of the
DTLS handshake:
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Client Server
------ ------
ClientHello ------>
X<-- HelloRetryRequest
(lost)
[Timer Expires]
ClientHello ------>
(retransmit)
Figure 1: DTLS Retransmission Example.
Once the client has transmitted the ClientHello message, it expects
to see a HelloRetryRequest from the server. However, if the server's
message is lost, the client knows that either the ClientHello or the
HelloRetryRequest has been lost and retransmits. When the server
receives the retransmission, it knows to retransmit.
The server also maintains a retransmission timer and retransmits when
that timer expires.
Note that timeout and retransmission do not apply to the
HelloRetryRequest since this would require creating state on the
server. The HelloRetryRequest is designed to be small enough that it
will not itself be fragmented, thus avoiding concerns about
interleaving multiple HelloRetryRequests.
3.1.1. Reordering
In DTLS, each handshake message is assigned a specific sequence
number within that handshake. When a peer receives a handshake
message, it can quickly determine whether that message is the next
message it expects. If it is, then it processes it. If not, it
queues it for future handling once all previous messages have been
received.
3.1.2. Message Size
TLS and DTLS handshake messages can be quite large (in theory up to
2^24-1 bytes, in practice many kilobytes). By contrast, UDP
datagrams are often limited to less than 1500 bytes if IP
fragmentation is not desired. In order to compensate for this
limitation, each DTLS handshake message may be fragmented over
several DTLS records, each of which is intended to fit in a single IP
datagram. Each DTLS handshake message contains both a fragment
offset and a fragment length. Thus, a recipient in possession of all
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bytes of a handshake message can reassemble the original unfragmented
message.
3.2. Replay Detection
DTLS optionally supports record replay detection. The technique used
is the same as in IPsec AH/ESP, by maintaining a bitmap window of
received records. Records that are too old to fit in the window and
records that have previously been received are silently discarded.
The replay detection feature is optional, since packet duplication is
not always malicious, but can also occur due to routing errors.
Applications may conceivably detect duplicate packets and accordingly
modify their data transmission strategy.
4. The DTLS Record Layer
The DTLS record layer is similar to that of TLS 1.3 unless noted
otherwise. The only change is the inclusion of an explicit epoch and
sequence number in the record. This sequence number allows the
recipient to correctly verify the TLS MAC. The DTLS record format is
shown below:
struct {
opaque content[DTLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} DTLSInnerPlaintext;
struct {
ContentType opaque_type = 23; /* application_data */
ProtocolVersion legacy_record_version = {254,253); // DTLSv1.2
uint16 epoch; // DTLS-related field
uint48 sequence_number; // DTLS-related field
uint16 length;
opaque encrypted_record[length];
} DTLSCiphertext;
type: The content type of the record.
legacy_record_version: This field is redundant and it is treated in
the same way as specified in the TLS 1.3 specification. The DTLS
version 1.2 version number is reused, namely { 254, 253 }. This
field is deprecated and MUST be ignored.
epoch: A counter value that is incremented on every cipher state
change.
sequence_number: The sequence number for this record.
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length: Identical to the length field in a TLS 1.3 record.
encrypted_record: Identical to the encrypted_record field in a TLS
1.3 record.
4.1. Sequence Number Handling
DTLS uses an explicit sequence number, rather than an implicit one,
carried in the sequence_number field of the record. Sequence numbers
are maintained separately for each epoch, with each sequence_number
initially being 0 for each epoch. For instance, if a handshake
message from epoch 0 is retransmitted, it might have a sequence
number after a message from epoch 1, even if the message from epoch 1
was transmitted first. Note that some care needs to be taken during
the handshake to ensure that retransmitted messages use the right
epoch and keying material.
The epoch number is initially zero and is incremented each time
keying material changes and a sender aims to rekey. More details are
provided in Section 5.9. In order to ensure that any given sequence/
epoch pair is unique, implementations MUST NOT allow the same epoch
value to be reused within two times the TCP maximum segment lifetime.
Note that because DTLS records may be reordered, a record from epoch
1 may be received after epoch 2 has begun. In general,
implementations SHOULD discard packets from earlier epochs, but if
packet loss causes noticeable problems they MAY choose to retain
keying material from previous epochs for up to the default MSL
specified for TCP [RFC0793] to allow for packet reordering. (Note
that the intention here is that implementers use the current guidance
from the IETF for MSL, not that they attempt to interrogate the MSL
that the system TCP stack is using.) Until the handshake has
completed, implementations MUST accept packets from the old epoch.
Conversely, it is possible for records that are protected by the
newly negotiated context to be received prior to the completion of a
handshake. For instance, the server may send its Finished message
and then start transmitting data. Implementations MAY either buffer
or discard such packets, though when DTLS is used over reliable
transports (e.g., SCTP), they SHOULD be buffered and processed once
the handshake completes. Note that TLS's restrictions on when
packets may be sent still apply, and the receiver treats the packets
as if they were sent in the right order. In particular, it is still
impermissible to send data prior to completion of the first
handshake.
Implementations MUST either abandon an association or re-key prior to
allowing the sequence number to wrap.
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Implementations MUST NOT allow the epoch to wrap, but instead MUST
establish a new association, terminating the old association.
4.2. Transport Layer Mapping
Each DTLS record MUST fit within a single datagram. In order to
avoid IP fragmentation, clients of the DTLS record layer SHOULD
attempt to size records so that they fit within any PMTU estimates
obtained from the record layer.
Note that unlike IPsec, DTLS records do not contain any association
identifiers. Applications must arrange to multiplex between
associations. With UDP, the host/port number is used to look up the
appropriate security association for incoming records.
Multiple DTLS records may be placed in a single datagram. They are
simply encoded consecutively. The DTLS record framing is sufficient
to determine the boundaries. Note, however, that the first byte of
the datagram payload must be the beginning of a record. Records may
not span datagrams.
Some transports, such as DCCP [RFC4340], provide their own sequence
numbers. When carried over those transports, both the DTLS and the
transport sequence numbers will be present. Although this introduces
a small amount of inefficiency, the transport layer and DTLS sequence
numbers serve different purposes; therefore, for conceptual
simplicity, it is superior to use both sequence numbers.
Some transports provide congestion control for traffic carried over
them. If the congestion window is sufficiently narrow, DTLS
handshake retransmissions may be held rather than transmitted
immediately, potentially leading to timeouts and spurious
retransmission. When DTLS is used over such transports, care should
be taken not to overrun the likely congestion window. [RFC5238]
defines a mapping of DTLS to DCCP that takes these issues into
account.
4.3. PMTU Issues
In general, DTLS's philosophy is to leave PMTU discovery to the
application. However, DTLS cannot completely ignore PMTU for three
reasons:
- The DTLS record framing expands the datagram size, thus lowering
the effective PMTU from the application's perspective.
- In some implementations, the application may not directly talk to
the network, in which case the DTLS stack may absorb ICMP
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[RFC1191] "Datagram Too Big" indications or ICMPv6 [RFC4443]
"Packet Too Big" indications.
- The DTLS handshake messages can exceed the PMTU.
In order to deal with the first two issues, the DTLS record layer
SHOULD behave as described below.
If PMTU estimates are available from the underlying transport
protocol, they should be made available to upper layer protocols. In
particular:
- For DTLS over UDP, the upper layer protocol SHOULD be allowed to
obtain the PMTU estimate maintained in the IP layer.
- For DTLS over DCCP, the upper layer protocol SHOULD be allowed to
obtain the current estimate of the PMTU.
- For DTLS over TCP or SCTP, which automatically fragment and
reassemble datagrams, there is no PMTU limitation. However, the
upper layer protocol MUST NOT write any record that exceeds the
maximum record size of 2^14 bytes.
The DTLS record layer SHOULD allow the upper layer protocol to
discover the amount of record expansion expected by the DTLS
processing.
If there is a transport protocol indication (either via ICMP or via a
refusal to send the datagram as in Section 14 of [RFC4340]), then the
DTLS record layer MUST inform the upper layer protocol of the error.
The DTLS record layer SHOULD NOT interfere with upper layer protocols
performing PMTU discovery, whether via [RFC1191] or [RFC4821]
mechanisms. In particular:
- Where allowed by the underlying transport protocol, the upper
layer protocol SHOULD be allowed to set the state of the DF bit
(in IPv4) or prohibit local fragmentation (in IPv6).
- If the underlying transport protocol allows the application to
request PMTU probing (e.g., DCCP), the DTLS record layer should
honor this request.
The final issue is the DTLS handshake protocol. From the perspective
of the DTLS record layer, this is merely another upper layer
protocol. However, DTLS handshakes occur infrequently and involve
only a few round trips; therefore, the handshake protocol PMTU
handling places a premium on rapid completion over accurate PMTU
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discovery. In order to allow connections under these circumstances,
DTLS implementations SHOULD follow the following rules:
- If the DTLS record layer informs the DTLS handshake layer that a
message is too big, it SHOULD immediately attempt to fragment it,
using any existing information about the PMTU.
- If repeated retransmissions do not result in a response, and the
PMTU is unknown, subsequent retransmissions SHOULD back off to a
smaller record size, fragmenting the handshake message as
appropriate. This standard does not specify an exact number of
retransmits to attempt before backing off, but 2-3 seems
appropriate.
4.4. Record Payload Protection
Like TLS, DTLS transmits data as a series of protected records. The
rest of this section describes the details of that format.
4.4.1. Anti-Replay
DTLS records contain a sequence number to provide replay protection.
Sequence number verification SHOULD be performed using the following
sliding window procedure, borrowed from Section 3.4.3 of [RFC4303].
The receiver packet counter for this session MUST be initialized to
zero when the session is established. For each received record, the
receiver MUST verify that the record contains a sequence number that
does not duplicate the sequence number of any other record received
during the life of this session. This SHOULD be the first check
applied to a packet after it has been matched to a session, to speed
rejection of duplicate records.
Duplicates are rejected through the use of a sliding receive window.
(How the window is implemented is a local matter, but the following
text describes the functionality that the implementation must
exhibit.) A minimum window size of 32 MUST be supported, but a
window size of 64 is preferred and SHOULD be employed as the default.
Another window size (larger than the minimum) MAY be chosen by the
receiver. (The receiver does not notify the sender of the window
size.)
The "right" edge of the window represents the highest validated
sequence number value received on this session. Records that contain
sequence numbers lower than the "left" edge of the window are
rejected. Packets falling within the window are checked against a
list of received packets within the window. An efficient means for
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performing this check, based on the use of a bit mask, is described
in Section 3.4.3 of [RFC4303].
If the received record falls within the window and is new, or if the
packet is to the right of the window, then the receiver proceeds to
MAC verification. If the MAC validation fails, the receiver MUST
discard the received record as invalid. The receive window is
updated only if the MAC verification succeeds.
4.4.2. Handling Invalid Records
Unlike TLS, DTLS is resilient in the face of invalid records (e.g.,
invalid formatting, length, MAC, etc.). In general, invalid records
SHOULD be silently discarded, thus preserving the association;
however, an error MAY be logged for diagnostic purposes.
Implementations which choose to generate an alert instead, MUST
generate error alerts to avoid attacks where the attacker repeatedly
probes the implementation to see how it responds to various types of
error. Note that if DTLS is run over UDP, then any implementation
which does this will be extremely susceptible to denial-of-service
(DoS) attacks because UDP forgery is so easy. Thus, this practice is
NOT RECOMMENDED for such transports.
If DTLS is being carried over a transport that is resistant to
forgery (e.g., SCTP with SCTP-AUTH), then it is safer to send alerts
because an attacker will have difficulty forging a datagram that will
not be rejected by the transport layer.
5. The DTLS Handshake Protocol
DTLS 1.3 re-uses the TLS 1.3 handshake messages and flows, with the
following changes:
1. To handle message loss, reordering, and fragmentation
modifications to the handshake header are necessary.
2. Retransmission timers are introduced to handle message loss.
3. The TLS 1.3 KeyUpdate message is not used in DTLS 1.3 for re-
keying.
4. A new ACK message has been added for reliable message delivery of
certain handshake messages.
Note that TLS 1.3 already supports a cookie extension, which used to
prevent denial-of-service attacks. This DoS prevention mechanism is
described in more detail below since UDP-based protocols are more
vulnerable to amplification attacks than a connection-oriented
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transport like TCP that performs return-routability checks as part of
the connection establishment.
With these exceptions, the DTLS message formats, flows, and logic are
the same as those of TLS 1.3.
5.1. Denial-of-Service Countermeasures
Datagram security protocols are extremely susceptible to a variety of
DoS attacks. Two attacks are of particular concern:
1. An attacker can consume excessive resources on the server by
transmitting a series of handshake initiation requests, causing
the server to allocate state and potentially to perform expensive
cryptographic operations.
2. An attacker can use the server as an amplifier by sending
connection initiation messages with a forged source of the
victim. The server then sends its response to the victim
machine, thus flooding it. Depending on the selected ciphersuite
this response message can be quite large, as it is the case for a
Certificate message.
In order to counter both of these attacks, DTLS borrows the stateless
cookie technique used by Photuris [RFC2522] and IKE [RFC5996]. When
the client sends its ClientHello message to the server, the server
MAY respond with a HelloRetryRequest message. The HelloRetryRequest
message, as well as the cookie extension, is defined in TLS 1.3. The
HelloRetryRequest message contains a stateless cookie generated using
the technique of [RFC2522]. The client MUST retransmit the
ClientHello with the cookie added as an extension. The server then
verifies the cookie and proceeds with the handshake only if it is
valid. This mechanism forces the attacker/client to be able to
receive the cookie, which makes DoS attacks with spoofed IP addresses
difficult. This mechanism does not provide any defence against DoS
attacks mounted from valid IP addresses.
The DTLS 1.3 specification changes the way how cookies are exchanged
compared to DTLS 1.2. DTLS 1.3 re-uses the HelloRetryRequest message
and conveys the cookie to the client via an extension. The client
receiving the cookie uses the same extension to place the cookie
subsequently into a ClientHello message.
DTLS 1.2 on the other hand used a separate message, namely the
HelloVerifyRequest, to pass a cookie to the client and did not
utilize the extension mechanism. For backwards compatibility reason
the cookie field in the ClientHello is present in DTLS 1.3 but is
ignored by a DTLS 1.3 compliant server implementation.
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The exchange is shown in Figure 2. Note that the figure focuses on
the cookie exchange; all other extensions are omitted.
Client Server
------ ------
ClientHello ------>
<----- HelloRetryRequest
+ cookie
ClientHello ------>
+ cookie
[Rest of handshake]
Figure 2: DTLS Exchange with HelloRetryRequest contain the Cookie
Extension
The cookie extension is defined in Section 4.2.2 of
[I-D.ietf-tls-tls13]. When sending the initial ClientHello, the
client does not have a cookie yet. In this case, the cookie
extension is omitted and the legacy_cookie field in the ClientHello
message SHOULD be set to a zero length vector (i.e., a single zero
byte length field) and MUST be ignored by a server negotiating DTLS
1.3.
When responding to a HelloRetryRequest, the client MUST create a new
ClientHello message following the description in Section 4.1.2 of
[I-D.ietf-tls-tls13].
The server SHOULD use information received in the ClientHello to
generate its cookie, such as version, random, ciphersuites. The
server MUST use the same version number in the HelloRetryRequest that
it would use when sending a ServerHello. Upon receipt of the
ServerHello, the client MUST verify that the server version values
match and MUST terminate the connection with an "illegal_parameter"
alert otherwise.
If the HelloRetryRequest message is used, the initial ClientHello and
the HelloRetryRequest are included in the calculation of the
handshake_messages (for the CertificateVerify message) and
verify_data (for the Finished message). However, the computation of
the message hash for the HelloRetryRequest is done according to the
description in Section 4.4.1 of [I-D.ietf-tls-tls13].
The handshake transcript is not reset with the second ClientHello and
a stateless server-cookie implementation requires the transcript of
the HelloRetryRequest to be stored in the cookie or the internal
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state of the hash algorithm, since only the hash of the transcript is
required for the handshake to complete.
When the second ClientHello is received, the server can verify that
the cookie is valid and that the client can receive packets at the
given IP address.
One potential attack on this scheme is for the attacker to collect a
number of cookies from different addresses and then reuse them to
attack the server. The server can defend against this attack by
changing the secret value frequently, thus invalidating those
cookies. If the server wishes that legitimate clients be able to
handshake through the transition (e.g., they received a cookie with
Secret 1 and then sent the second ClientHello after the server has
changed to Secret 2), the server can have a limited window during
which it accepts both secrets. [RFC5996] suggests adding a key
identifier to cookies to detect this case. An alternative approach
is simply to try verifying with both secrets. It is RECOMMENDED that
servers implement a key rotation scheme that allows the server to
manage keys with overlapping lifetime.
Alternatively, the server can store timestamps in the cookie and
reject those cookies that were not generated within a certain amount
of time.
DTLS servers SHOULD perform a cookie exchange whenever a new
handshake is being performed. If the server is being operated in an
environment where amplification is not a problem, the server MAY be
configured not to perform a cookie exchange. The default SHOULD be
that the exchange is performed, however. In addition, the server MAY
choose not to do a cookie exchange when a session is resumed.
Clients MUST be prepared to do a cookie exchange with every
handshake.
If a server receives a ClientHello with an invalid cookie, it MUST
NOT respond with a HelloRetryRequest. Restarting the handshake from
scratch, without a cookie, allows the client to recover from a
situation where it obtained a cookie that cannot be verified by the
server. As described in Section 4.1.4 of
[I-D.ietf-tls-tls13],clients SHOULD also abort the handshake with an
"unexpected_message" alert in response to any second
HelloRetryRequest which was sent in the same connection (i.e., where
the ClientHello was itself in response to a HelloRetryRequest).
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5.2. DTLS Handshake Message Format
In order to support message loss, reordering, and message
fragmentation, DTLS modifies the TLS 1.3 handshake header:
enum {
hello_request_RESERVED(0),
client_hello(1),
server_hello(2),
hello_verify_request_RESERVED(3),
new_session_ticket(4),
end_of_early_data(5),
hello_retry_request(6),
encrypted_extensions(8),
certificate(11),
server_key_exchange_RESERVED(12),
certificate_request(13),
server_hello_done_RESERVED(14),
certificate_verify(15),
client_key_exchange_RESERVED(16),
finished(20),
key_update_RESERVED(24),
ack([[TBD RFC Editor -- Proposal: 25]]),
message_hash(254),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
uint16 message_seq; /* DTLS-required field */
uint24 fragment_offset; /* DTLS-required field */
uint24 fragment_length; /* DTLS-required field */
select (HandshakeType) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case end_of_early_data: EndOfEarlyData;
case hello_retry_request: HelloRetryRequest;
case encrypted_extensions: EncryptedExtensions;
case certificate_request: CertificateRequest;
case certificate: Certificate;
case certificate_verify: CertificateVerify;
case finished: Finished;
case new_session_ticket: NewSessionTicket;
case key_update: KeyUpdate; /* reserved */
case ack: ACK; /* DTLS-required field */
} body;
} Handshake;
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In addition to the handshake messages that are deprecated by the TLS
1.3 specification DTLS 1.3 furthermore deprecates the
HelloVerifyRequest message originally defined in DTLS 1.0. DTLS
1.3-compliant implements MUST NOT use the HelloVerifyRequest to
execute a return-routability check. A dual-stack DTLS 1.2/DTLS 1.3
client MUST, however, be prepared to interact with a DTLS 1.2 server.
A DTLS 1.3 MUST NOT use the KeyUpdate message to change keying
material used for the protection of traffic data. Instead the epoch
field is used, which is explained in Section 5.9.
The format of the ClientHello used by a DTLS 1.3 client differs from
the TLS 1.3 ClientHello format as shown below.
uint16 ProtocolVersion;
opaque Random[32];
uint8 CipherSuite[2]; /* Cryptographic suite selector */
struct {
ProtocolVersion legacy_version = { 254,253 }; // DTLSv1.2
Random random;
opaque legacy_session_id<0..32>;
opaque legacy_cookie<0..2^8-1>; // DTLS
CipherSuite cipher_suites<2..2^16-2>;
opaque legacy_compression_methods<1..2^8-1>;
Extension extensions<0..2^16-1>;
} ClientHello;
legacy_version: In previous versions of DTLS, this field was used
for version negotiation and represented the highest version number
supported by the client. Experience has shown that many servers
do not properly implement version negotiation, leading to "version
intolerance" in which the server rejects an otherwise acceptable
ClientHello with a version number higher than it supports. In
DTLS 1.3, the client indicates its version preferences in the
"supported_versions" extension (see Section 4.2.1 of
[I-D.ietf-tls-tls13]) and the legacy_version field MUST be set to
{254, 253}, which was the version number for DTLS 1.2.
random: Same as for TLS 1.3
legacy_session_id: Same as for TLS 1.3
legacy_cookie: A DTLS 1.3-only client MUST set the legacy_cookie
field to zero length.
cipher_suites: Same as for TLS 1.3
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legacy_compression_methods: Same as for TLS 1.3
extensions: Same as for TLS 1.3
The first message each side transmits in each handshake always has
message_seq = 0. Whenever a new message is generated, the
message_seq value is incremented by one. When a message is
retransmitted, the old message_seq value is re-used, i.e., not
incremented.
Here is an example:
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Client Server
------ ------
ClientHello
(message_seq=0)
-------->
X<---- HelloRetryRequest
(lost) (message_seq=0)
[Timer Expires]
ClientHello
(message_seq=0)
(retransmit) -------->
<-------- HelloRetryRequest
(message_seq=0)
ClientHello -------->
(message_seq=1)
+cookie
<-------- ServerHello
(message_seq=1)
EncryptedExtensions
(message_seq=2)
Certificate
(message_seq=3)
CertificateVerify
(message_seq=4)
Finished
(message_seq=5)
Certificate -------->
(message_seq=2)
CertificateVerify
(message_seq=3)
Finished
(message_seq=4)
<-------- Ack
(message_seq=6)
Figure 3: Example DTLS Exchange illustrating Message Loss
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From the perspective of the DTLS record layer, the retransmission is
a new record. This record will have a new
DTLSPlaintext.sequence_number value.
DTLS implementations maintain (at least notionally) a
next_receive_seq counter. This counter is initially set to zero.
When a message is received, if its sequence number matches
next_receive_seq, next_receive_seq is incremented and the message is
processed. If the sequence number is less than next_receive_seq, the
message MUST be discarded. If the sequence number is greater than
next_receive_seq, the implementation SHOULD queue the message but MAY
discard it. (This is a simple space/bandwidth tradeoff).
5.3. ACK Message
struct {} ACK;
The ACK handshake message is used by an endpoint to respond to a
message where the TLS 1.3 handshake does not foresee such return
message. With the use of the ACK message the sender is able to
determine whether a transmitted request has been lost and needs to be
retransmitted. Since the ACK message does not contain any
correlation information the sender MUST only have one such message
outstanding at a time.
The ACK message uses a handshake content type and is encrypted under
the appropriate application traffic key. [[OPEN ISSUE: It seems odd
to have the ACK that responds to CFIN encrypted under the application
key. Also, what do you do about ACKs that have to deal with key
changes.]]
5.4. Handshake Message Fragmentation and Reassembly
Each DTLS message MUST fit within a single transport layer datagram.
However, handshake messages are potentially bigger than the maximum
record size. Therefore, DTLS provides a mechanism for fragmenting a
handshake message over a number of records, each of which can be
transmitted separately, thus avoiding IP fragmentation.
When transmitting the handshake message, the sender divides the
message into a series of N contiguous data ranges. These ranges MUST
NOT be larger than the maximum handshake fragment size and MUST
jointly contain the entire handshake message. The ranges MUST NOT
overlap. The sender then creates N handshake messages, all with the
same message_seq value as the original handshake message. Each new
message is labeled with the fragment_offset (the number of bytes
contained in previous fragments) and the fragment_length (the length
of this fragment). The length field in all messages is the same as
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the length field of the original message. An unfragmented message is
a degenerate case with fragment_offset=0 and fragment_length=length.
When a DTLS implementation receives a handshake message fragment, it
MUST buffer it until it has the entire handshake message. DTLS
implementations MUST be able to handle overlapping fragment ranges.
This allows senders to retransmit handshake messages with smaller
fragment sizes if the PMTU estimate changes.
Note that as with TLS, multiple handshake messages may be placed in
the same DTLS record, provided that there is room and that they are
part of the same flight. Thus, there are two acceptable ways to pack
two DTLS messages into the same datagram: in the same record or in
separate records.
5.5. Timeout and Retransmission
DTLS messages are grouped into a series of message flights, according
to the diagrams below. Although each flight of messages may consist
of a number of messages, they should be viewed as monolithic for the
purpose of timeout and retransmission.
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Client Server
ClientHello +----------+
+ key_share* | Flight 1 |
+ pre_shared_key* --------> +----------+
+----------+
<-------- HelloRetryRequest | Flight 2 |
+ cookie +----------+
ClientHello +----------+
+ key_share* | Flight 3 |
+ pre_shared_key* --------> +----------+
+ cookie
ServerHello
+ key_share*
+ pre_shared_key* +----------+
{EncryptedExtensions} | Flight 4 |
{CertificateRequest*} +----------+
{Certificate*}
{CertificateVerify*}
<-------- {Finished}
[Application Data*]
{Certificate*} +----------+
{CertificateVerify*} | Flight 5 |
{Finished} --------> +----------+
[Application Data]
+----------+
<-------- [Ack] | Flight 6 |
[Application Data*] +----------+
[Application Data] <-------> [Application Data]
Figure 4: Message Flights for full DTLS Handshake (with Cookie
Exchange)
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ClientHello +----------+
+ pre_shared_key | Flight 1 |
+ key_share* --------> +----------+
ServerHello
+ pre_shared_key +----------+
+ key_share* | Flight 2 |
{EncryptedExtensions} +----------+
<-------- {Finished}
[Application Data*]
+----------+
{Finished} --------> | Flight 3 |
[Application Data*] +----------+
+----------+
<-------- [Ack] | Flight 4 |
[Application Data*] +----------+
[Application Data] <-------> [Application Data]
Figure 5: Message Flights for Resumption and PSK Handshake (without
Cookie Exchange)
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Client Server
ClientHello
+ early_data
+ psk_key_exchange_modes +----------+
+ key_share* | Flight 1 |
+ pre_shared_key +----------+
(Application Data*) -------->
ServerHello
+ pre_shared_key
+ key_share* +----------+
{EncryptedExtensions} | Flight 2 |
{Finished} +----------+
<-------- [Application Data*]
+----------+
(EndOfEarlyData) | Flight 3 |
{Finished} --------> +----------+
[Application Data*]
+----------+
<-------- [Ack] | Flight 4 |
[Application Data*] +----------+
[Application Data] <-------> [Application Data]
Figure 6: Message Flights for the Zero-RTT Handshake
Client Server
+----------+
<-------- [NewSessionTicket] | Flight 1 |
+----------+
+----------+
[Ack] --------> | Flight 2 |
+----------+
Figure 7: Message Flights for New Session Ticket Message
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Client Server
+----------+
<-------- [CertificateRequest] | Flight 1 |
+----------+
[Certificate] +----------+
[CertificateVerify] | Flight 2 |
[Finished] --------> +----------+
Figure 8: Message Flights for Post-Handshake Authentication (Success)
Client Server
+----------+
<-------- [CertificateRequest] | Flight 1 |
+----------+
[Certificate] +----------+
[Finished] --------> | Flight 2 |
+----------+
Figure 9: Message Flights for Post-Handshake Authentication (Decline)
Note: The application data sent by the client is not included in the
timeout and retransmission calculation.
5.5.1. State Machine
DTLS uses a simple timeout and retransmission scheme with the state
machine shown in Figure 10. Because DTLS clients send the first
message (ClientHello), they start in the PREPARING state. DTLS
servers start in the WAITING state, but with empty buffers and no
retransmit timer.
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+-----------+
| PREPARING |
+---> | |
| | |
| +-----------+
| |
| | Buffer next flight
| |
| \|/
| +-----------+
| | |
| | SENDING |<------------------+
| | | |
| +-----------+ |
Receive | | |
next | | Send flight |
flight | +--------+ |
| | | Set retransmit timer |
| | \|/ |
| | +-----------+ |
| | | | |
+--)--| WAITING |-------------------+
| | | | Timer expires |
| | +-----------+ |
| | | |
| | | |
| | +------------------------+
| | Read retransmit
Receive | |
last | |
flight | |
| |
\|/\|/
+-----------+
| |
| FINISHED |
| |
+-----------+
| /|\
| |
| |
+---+
Server read retransmit
Retransmit ACK
Figure 10: DTLS Timeout and Retransmission State Machine
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The state machine has three basic states.
In the PREPARING state, the implementation does whatever computations
are necessary to prepare the next flight of messages. It then
buffers them up for transmission (emptying the buffer first) and
enters the SENDING state.
In the SENDING state, the implementation transmits the buffered
flight of messages. Once the messages have been sent, the
implementation then enters the FINISHED state if this is the last
flight in the handshake. Or, if the implementation expects to
receive more messages, it sets a retransmit timer and then enters the
WAITING state.
There are three ways to exit the WAITING state:
1. The retransmit timer expires: the implementation transitions to
the SENDING state, where it retransmits the flight, resets the
retransmit timer, and returns to the WAITING state.
2. The implementation reads a retransmitted flight from the peer:
the implementation transitions to the SENDING state, where it
retransmits the flight, resets the retransmit timer, and returns
to the WAITING state. The rationale here is that the receipt of
a duplicate message is the likely result of timer expiry on the
peer and therefore suggests that part of one's previous flight
was lost.
3. The implementation receives the next flight of messages: if this
is the final flight of messages, the implementation transitions
to FINISHED. If the implementation needs to send a new flight,
it transitions to the PREPARING state. Partial reads (whether
partial messages or only some of the messages in the flight) do
not cause state transitions or timer resets.
Because DTLS clients send the first message (ClientHello), they
start in the PREPARING state. DTLS servers start in the WAITING
state, but with empty buffers and no retransmit timer.
In addition, for at least twice the default Maximum Segment
Lifetime (MSL) defined for [RFC0793], when in the FINISHED state,
the server MUST respond to retransmission of the client's second
flight with a retransmit of its ACK.
Note that because of packet loss, it is possible for one side to
be sending application data even though the other side has not
received the first side's Finished message. Implementations MUST
either discard or buffer all application data packets for the new
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epoch until they have received the Finished message for that
epoch. Implementations MAY treat receipt of application data
with a new epoch prior to receipt of the corresponding Finished
message as evidence of reordering or packet loss and retransmit
their final flight immediately, shortcutting the retransmission
timer.
5.5.2. Timer Values
Though timer values are the choice of the implementation, mishandling
of the timer can lead to serious congestion problems; for example, if
many instances of a DTLS time out early and retransmit too quickly on
a congested link. Implementations SHOULD use an initial timer value
of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double
the value at each retransmission, up to no less than the RFC 6298
maximum of 60 seconds. Application specific profiles, such as those
used for the Internet of Things environment, may recommend longer
timer values. Note that we recommend a 100 msec timer rather than
the 3-second RFC 6298 default in order to improve latency for time-
sensitive applications. Because DTLS only uses retransmission for
handshake and not dataflow, the effect on congestion should be
minimal.
Implementations SHOULD retain the current timer value until a
transmission without loss occurs, at which time the value may be
reset to the initial value. After a long period of idleness, no less
than 10 times the current timer value, implementations may reset the
timer to the initial value. One situation where this might occur is
when a rehandshake is used after substantial data transfer.
5.6. CertificateVerify and Finished Messages
CertificateVerify and Finished messages have the same format as in
TLS 1.3. Hash calculations include entire handshake messages,
including DTLS-specific fields: message_seq, fragment_offset, and
fragment_length. However, in order to remove sensitivity to
handshake message fragmentation, the CertificateVerify and the
Finished messages MUST be computed as if each handshake message had
been sent as a single fragment following the algorithm described in
Section 4.4.3 and Section 4.4.4 of [I-D.ietf-tls-tls13],
respectively.
5.7. Alert Messages
Note that Alert messages are not retransmitted at all, even when they
occur in the context of a handshake. However, a DTLS implementation
which would ordinarily issue an alert SHOULD generate a new alert
message if the offending record is received again (e.g., as a
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retransmitted handshake message). Implementations SHOULD detect when
a peer is persistently sending bad messages and terminate the local
connection state after such misbehavior is detected.
5.8. Establishing New Associations with Existing Parameters
If a DTLS client-server pair is configured in such a way that
repeated connections happen on the same host/port quartet, then it is
possible that a client will silently abandon one connection and then
initiate another with the same parameters (e.g., after a reboot).
This will appear to the server as a new handshake with epoch=0. In
cases where a server believes it has an existing association on a
given host/port quartet and it receives an epoch=0 ClientHello, it
SHOULD proceed with a new handshake but MUST NOT destroy the existing
association until the client has demonstrated reachability either by
completing a cookie exchange or by completing a complete handshake
including delivering a verifiable Finished message. After a correct
Finished message is received, the server MUST abandon the previous
association to avoid confusion between two valid associations with
overlapping epochs. The reachability requirement prevents off-path/
blind attackers from destroying associations merely by sending forged
ClientHellos.
5.9. Epoch Values and Rekeying
A recipient of a DTLS message needs to select the correct keying
material in order to process an incoming message. With the
possibility of message loss and re-order an identifier is needed to
determine which cipher state has been used to protect the record
payload. The epoch value fulfills this role in DTLS. In addition to
the key derivation steps described in Section 7 of
[I-D.ietf-tls-tls13] triggered by the states during the handshake a
sender may want to rekey at any time during the lifetime of the
connection and has to have a way to indicate that it is updating its
sending cryptographic keys.
This version of DTLS assigns dedicated epoch values to messages in
the protocol exchange to allow identification of the correct cipher
state:
- epoch value (0) is used with unencrypted messages. There are
three unencrypted messages in DTLS, namely ClientHello,
ServerHello, and HelloRetryRequest.
- epoch value (1) is used for messages protected using keys derived
from early_traffic_secret. This includes early data sent by the
client and the EndOfEarlyData message.
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- epoch value (2) is used for messages protected using keys derived
from the handshake_traffic_secret. Messages transmitted during
the initial handshake, such as EncryptedExtensions,
CertificateRequest, Certificate, CertificateVerify, and Finished
belong to this category. Note, however, post-handshake are
protected under the appropriate application traffic key and are
not included in this category.
- epoch value (3) is used for payloads protected using keys derived
from the initial traffic_secret_0. This may include handshake
messages, such as post-handshake messages (e.g., a
NewSessionTicket message).
- epoch value (4 to 2^16-1) is used for payloads protected using
keys from the traffic_secret_N (N>0).
Using these reserved epoch values a receiver knows what cipher state
has been used to encrypt and integrity protect a message.
Implementations that receive a payload with an epoch value for which
no corresponding cipher state can be determined MUST generate a
"unexpected_message" alert. For example, client incorrectly uses
epoch value 5 when sending early application data in a 0-RTT
exchange. A server will not be able to compute the appropriate keys
and will therefore have to respond with an alert.
Increasing the epoch value by a sender (starting with value 4
upwards) corresponds semantically to rekeying using the KeyUpdate
message in TLS 1.3. Instead of utilizing an dedicated message in
DTLS 1.3 the sender uses an increase in the epoch value to signal
rekeying. Hence, a sender that decides to increment the epoch value
MUST send all its traffic using the next generation of keys, computed
as described in Section 7.2 of [I-D.ietf-tls-tls13]. Upon receiving
a payload with such a new epoch value, the receiver MUST update their
receiving keys and if they have not already updated their sending
state up to or past the then current receiving generation MUST send
messages with the new epoch value prior to sending any other
messages. For epoch values lower than 4 the key schedule described
in Section 7.1 of [I-D.ietf-tls-tls13] is applicable. As a
difference to the functionality of the KeyUpdate in TLS 1.3 the
sender forces the receiver to increase the epoch value for outgoing
data as well.
Note that epoch values do not wrap. If a DTLS implementation would
need to wrap the epoch value, it MUST terminate the connection.
The traffic key calculation is described in Section 7.3 of
[I-D.ietf-tls-tls13].
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Figure 11 illustrates the epoch values in an example DTLS handshake.
Client Server
------ ------
ClientHello
(epoch=0)
-------->
<-------- HelloRetryRequest
(epoch=0)
ClientHello -------->
(epoch=0)
<-------- ServerHello
(epoch=0)
{EncryptedExtensions}
(epoch=2)
{Certificate}
(epoch=2)
{CertificateVerify}
(epoch=2)
{Finished}
(epoch=2)
{Certificate} -------->
(epoch=2)
{CertificateVerify}
(epoch=2)
{Finished}
(epoch=2)
<-------- [Ack]
(epoch=3)
[Application Data] -------->
(epoch=3)
<-------- [Application Data]
(epoch=3)
Some time later ...
(Post-Handshake Message Exchange)
<-------- [NewSessionTicket]
(epoch=3)
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[Ack] -------->
(epoch=3)
Some time later ...
(Rekeying)
<-------- [Application Data]
(epoch=4)
[Application Data] -------->
(epoch=4)
Figure 11: Example DTLS Exchange with Epoch Information
6. Application Data Protocol
Application data messages are carried by the record layer and are
fragmented and encrypted based on the current connection state. The
messages are treated as transparent data to the record layer.
7. Security Considerations
Security issues are discussed primarily in [I-D.ietf-tls-tls13].
The primary additional security consideration raised by DTLS is that
of denial of service. DTLS includes a cookie exchange designed to
protect against denial of service. However, implementations that do
not use this cookie exchange are still vulnerable to DoS. In
particular, DTLS servers that do not use the cookie exchange may be
used as attack amplifiers even if they themselves are not
experiencing DoS. Therefore, DTLS servers SHOULD use the cookie
exchange unless there is good reason to believe that amplification is
not a threat in their environment. Clients MUST be prepared to do a
cookie exchange with every handshake.
Unlike TLS implementations, DTLS implementations SHOULD NOT respond
to invalid records by terminating the connection.
8. Changes to DTLS 1.2
Since TLS 1.3 introduce a large number of changes to TLS 1.2, the
list of changes from DTLS 1.2 to DTLS 1.3 is equally large. For this
reason this section focuses on the most important changes only.
- New handshake pattern, which leads to a shorter message exchange
- Support for AEAD-only ciphers
- HelloRetryRequest of TLS 1.3 used instead of HelloVerifyRequest
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- More flexible ciphersuite negotiation
- New session resumption mechanism
- PSK authentication redefined
- New key derivation hierarchy utilizing a new key derivation
construct
- Removed support for weaker and older cryptographic algorithms
- Improved version negotation
9. IANA Considerations
IANA is requested to allocate a new value in the TLS HandshakeType
Registry for the ACK message defined in Section 5.3.
10. References
10.1. Normative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-19 (work in progress),
March 2017.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<http://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
DOI 10.17487/RFC4443, March 2006,
<http://www.rfc-editor.org/info/rfc4443>.
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[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<http://www.rfc-editor.org/info/rfc6298>.
10.2. Informative References
[RFC2522] Karn, P. and W. Simpson, "Photuris: Session-Key Management
Protocol", RFC 2522, DOI 10.17487/RFC2522, March 1999,
<http://www.rfc-editor.org/info/rfc2522>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<http://www.rfc-editor.org/info/rfc4340>.
[RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over
the Datagram Congestion Control Protocol (DCCP)",
RFC 5238, DOI 10.17487/RFC5238, May 2008,
<http://www.rfc-editor.org/info/rfc5238>.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, DOI 10.17487/RFC5996, September 2010,
<http://www.rfc-editor.org/info/rfc5996>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.
10.3. URIs
[1] mailto:tls@ietf.org
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Appendix A. History
RFC EDITOR: PLEASE REMOVE THE THIS SECTION
draft-01 - Alignment with version -19 of the TLS 1.3 specification
draft-00
- Initial version using TLS 1.3 as a baseline.
- Use of epoch values instead of KeyUpdate message
- Use of cookie extension instead of cookie field in ClientHello and
HelloVerifyRequest messages
- Added ACK message
- Text about sequence number handling
Appendix B. Working Group Information
The discussion list for the IETF TLS working group is located at the
e-mail address tls@ietf.org [1]. Information on the group and
information on how to subscribe to the list is at
https://www1.ietf.org/mailman/listinfo/tls
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/tls/current/index.html
Appendix C. Contributors
Many people have contributed to previous DTLS versions and they are
acknowledged in prior versions of DTLS specifications.
For this version of the document we would like to thank:
* Ilari Liusvaara
Independent
ilariliusvaara@welho.com
* Martin Thomson
Mozilla
martin.thomson@gmail.com
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Authors' Addresses
Eric Rescorla
RTFM, Inc.
EMail: ekr@rtfm.com
Hannes Tschofenig
ARM Limited
EMail: hannes.tschofenig@arm.com
Nagendra Modadugu
Google, Inc.
EMail: nagendra@cs.stanford.edu
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