TLS E. Rescorla
Internet-Draft RTFM, Inc.
Obsoletes: 6347 (if approved) H. Tschofenig
Intended status: Standards Track Arm Limited
Expires: January 9, 2020 N. Modadugu
Google, Inc.
July 08, 2019
The Datagram Transport Layer Security (DTLS) Protocol Version 1.3
draft-ietf-tls-dtls13-32
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 with the exception of order protection/non-replayability.
Datagram semantics of the underlying transport are preserved by the
DTLS protocol.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 9, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. DTLS Design Rationale and Overview . . . . . . . . . . . . . 5
3.1. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Reordering . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Message Size . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Replay Detection . . . . . . . . . . . . . . . . . . . . 7
4. The DTLS Record Layer . . . . . . . . . . . . . . . . . . . . 7
4.1. Determining the Header Format . . . . . . . . . . . . . . 11
4.2. Sequence Number and Epoch . . . . . . . . . . . . . . . . 11
4.2.1. Processing Guidelines . . . . . . . . . . . . . . . . 11
4.2.2. Reconstructing the Sequence Number and Epoch . . . . 12
4.2.3. Sequence Number Encryption . . . . . . . . . . . . . 13
4.3. Transport Layer Mapping . . . . . . . . . . . . . . . . . 14
4.4. PMTU Issues . . . . . . . . . . . . . . . . . . . . . . . 14
4.5. Record Payload Protection . . . . . . . . . . . . . . . . 16
4.5.1. Anti-Replay . . . . . . . . . . . . . . . . . . . . . 16
4.5.2. Handling Invalid Records . . . . . . . . . . . . . . 17
5. The DTLS Handshake Protocol . . . . . . . . . . . . . . . . . 17
5.1. Denial-of-Service Countermeasures . . . . . . . . . . . . 18
5.2. DTLS Handshake Message Format . . . . . . . . . . . . . . 21
5.3. ClientHello Message . . . . . . . . . . . . . . . . . . . 22
5.4. Handshake Message Fragmentation and Reassembly . . . . . 23
5.5. End Of Early Data . . . . . . . . . . . . . . . . . . . . 24
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5.6. DTLS Handshake Flights . . . . . . . . . . . . . . . . . 24
5.7. Timeout and Retransmission . . . . . . . . . . . . . . . 28
5.7.1. State Machine . . . . . . . . . . . . . . . . . . . . 28
5.7.2. Timer Values . . . . . . . . . . . . . . . . . . . . 30
5.8. CertificateVerify and Finished Messages . . . . . . . . . 31
5.9. Alert Messages . . . . . . . . . . . . . . . . . . . . . 31
5.10. Establishing New Associations with Existing Parameters . 31
6. Example of Handshake with Timeout and Retransmission . . . . 32
6.1. Epoch Values and Rekeying . . . . . . . . . . . . . . . . 34
7. ACK Message . . . . . . . . . . . . . . . . . . . . . . . . . 36
7.1. Sending ACKs . . . . . . . . . . . . . . . . . . . . . . 37
7.2. Receiving ACKs . . . . . . . . . . . . . . . . . . . . . 38
8. Key Updates . . . . . . . . . . . . . . . . . . . . . . . . . 38
9. Connection ID Updates . . . . . . . . . . . . . . . . . . . . 38
9.1. Connection ID Example . . . . . . . . . . . . . . . . . . 40
10. Application Data Protocol . . . . . . . . . . . . . . . . . . 42
11. Security Considerations . . . . . . . . . . . . . . . . . . . 42
12. Changes to DTLS 1.2 . . . . . . . . . . . . . . . . . . . . . 43
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 44
14.1. Normative References . . . . . . . . . . . . . . . . . . 44
14.2. Informative References . . . . . . . . . . . . . . . . . 45
Appendix A. Protocol Data Structures and Constant Values . . . . 47
A.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 47
A.2. Handshake Protocol . . . . . . . . . . . . . . . . . . . 47
A.3. ACKs . . . . . . . . . . . . . . . . . . . . . . . . . . 49
A.4. Connection ID Management . . . . . . . . . . . . . . . . 49
Appendix B. History . . . . . . . . . . . . . . . . . . . . . . 49
Appendix C. Working Group Information . . . . . . . . . . . . . 50
Appendix D. Contributors . . . . . . . . . . . . . . . . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 51
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].
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There are applications that use UDP [RFC0768] 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 [RFC4347] was originally defined as a delta from TLS 1.1
[RFC4346] and DTLS 1.2 [RFC6347] was defined as a series of deltas to
TLS 1.2 [RFC5246]. 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
based on TLS 1.3 [TLS13].
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 [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].
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The following terms are used:
- client: The endpoint initiating the DTLS 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.
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- session: An association between a client and a server resulting
from a handshake.
- server: The endpoint which did not initiate the DTLS connection.
- CID: Connection ID
The reader is assumed to be familiar with the TLS 1.3 specification
since this document is defined as a delta from TLS 1.3. As in TLS
1.3 the HelloRetryRequest has the same format as a ServerHello
message but for convenience we use the term HelloRetryRequest
throughout this document as if it were a distinct message.
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 keys derived from
traffic_secret_N.
3. DTLS Design Rationale and Overview
The basic design philosophy of DTLS is to construct "TLS over
datagram transport". Datagram transport does not require nor 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 reordered data traffic.
TLS cannot be used directly in datagram environments for the
following five reasons:
1. TLS relies on an implicit sequence number on records. If a
record is not received, then the recipient will use the wrong
sequence number when attempting to remove record protection from
subsequent records. DTLS solves this problem by adding sequence
numbers.
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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. DTLS handshake messages are also
assigned sequence numbers to enable reassembly in the correct
order in case datagrams are lost or reordered.
3. During the handshake, messages are implicitly acknowledged by
other handshake messages, but the last flight of messages and
post-handshake messages (such as the NewSessionTicket message) do
not result in any direct response that would allow the sender to
detect loss. DTLS adds an acknowledgment message to enable
better loss recovery.
4. Handshake messages are potentially larger than can be contained
in a single datagram. DTLS adds fields to handshake messages to
support fragmentation and reassembly.
5. Datagram transport protocols, like UDP, are susceptible to
abusive behavior effecting denial of service attacks against
nonparticipants. DTLS adds a return-routability check that uses
the TLS HelloRetryRequest message (see Section 5.1 for details).
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:
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 or a ServerHello from the server.
However, if the server's message is lost, the client knows that
either the ClientHello or the response from the server has been lost
and retransmits. When the server receives the retransmission, it
knows to retransmit.
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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.
3.2. Reordering
In DTLS, each handshake message is assigned a specific sequence
number. 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.
3.3. 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
packet. Each DTLS handshake message contains both a fragment offset
and a fragment length. Thus, a recipient in possession of all bytes
of a handshake message can reassemble the original unfragmented
message.
3.4. 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 different from the TLS 1.3 record layer.
1. The DTLSCiphertext structure omits the superfluous version number
and type fields.
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2. DTLS adds an epoch and sequence number to the TLS record header.
This sequence number allows the recipient to correctly verify the
DTLS MAC. However, the number of bits used for the epoch and
sequence number fields in the DTLSCiphertext structure have been
reduced from those in previous versions.
3. The DTLSCiphertext structure has a variable length header.
Note that the DTLS 1.3 record layer is different from the DTLS 1.2
record layer.
DTLSPlaintext records are used to send unprotected records and
DTLSCiphertext records are used to send protected records.
The DTLS record formats are shown below. Unless explicitly stated
the meaning of the fields is unchanged from previous TLS / DTLS
versions.
struct {
ContentType type;
ProtocolVersion legacy_record_version;
uint16 epoch = 0 // DTLS field
uint48 sequence_number; // DTLS field
uint16 length;
opaque fragment[DTLSPlaintext.length];
} DTLSPlaintext;
struct {
opaque content[DTLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} DTLSInnerPlaintext;
struct {
opaque unified_hdr[variable];
opaque encrypted_record[length];
} DTLSCiphertext;
Figure 2: DTLS 1.3 Record Format
unified_hdr: The unified_hdr is a field of variable length, as shown
in Figure 3.
encrypted_record: Identical to the encrypted_record field in a TLS
1.3 record.
The DTLSCiphertext header is tightly bit-packed, as shown below:
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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0|0|1|C|S|L|E E|
+-+-+-+-+-+-+-+-+
| Connection ID | Legend:
| (if any, |
/ length as / C - Connection ID (CID) present
| negotiated) | S - Sequence number length
+-+-+-+-+-+-+-+-+ L - Length present
| 8 or 16 bit | E - Epoch
|Sequence Number|
+-+-+-+-+-+-+-+-+
| 16 bit Length |
| (if present) |
+-+-+-+-+-+-+-+-+
Figure 3: DTLS 1.3 CipherText Header
Fixed Bits: The three high bits of the first byte of the
DTLSCiphertext header are set to 001.
C: The C bit (0x10) is set if the Connection ID is present.
S: The S bit (0x08) indicates the size of the sequence number. 0
means an 8-bit sequence number, 1 means 16-bit.
L: The L bit (0x04) is set if the length is present.
E: The two low bits (0x03) include the low order two bits of the
epoch.
Connection ID: Variable length CID. The CID concept is described in
[DTLS-CID]. An example can be found in Section 9.1.
Sequence Number: The low order 8 or 16 bits of the record sequence
number. This value is 16 bits if the S bit is set to 1, and 8
bits if the S bit is 0.
Length: Identical to the length field in a TLS 1.3 record.
As with previous versions of DTLS, multiple DTLSPlaintext and
DTLSCiphertext records can be included in the same underlying
transport datagram.
Figure 4 illustrates different record layer header types.
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0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Content Type | |0|0|1|1|1|1|E E| |0|0|1|0|0|0|E E|
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| 16 bit | | 16 bit | |8-bit Seq. No. |
| Version | |Sequence Number| +-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ | |
| 16 bit | | | | Encrypted |
| Epoch | / Connection ID / / Record /
+-+-+-+-+-+-+-+-+ | | | |
| | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | | 16 bit |
| 48 bit | | Length | DTLSCiphertext
|Sequence Number| +-+-+-+-+-+-+-+-+ Structure
| | | | (minimal)
| | | Encrypted |
+-+-+-+-+-+-+-+-+ / Record /
| 16 bit | | |
| Length | +-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+
| | DTLSCiphertext
| | Structure
/ Fragment / (full)
| |
+-+-+-+-+-+-+-+-+
DTLSPlaintext
Structure
Figure 4: Header Examples
The length field MAY be omitted by clearing the L bit, which means
that the record consumes the entire rest of the datagram in the lower
level transport. In this case it is not possible to have multiple
DTLSCiphertext format records without length fields in the same
datagram.
Omitting the length field MUST only be used for data which is
protected with one of the application_traffic_secret values, and not
for messages protected with either [sender]_handshake_traffic_sercret
or [sender]_early_traffic_secret values. When using an
[sender]_application_traffic_secret for message protection,
Implementations MAY include the length field at their discretion.
When expanded, the epoch and sequence number can be combined into an
unpacked RecordNumber structure, as shown below:
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struct {
uint16 epoch;
uint48 sequence_number;
} RecordNumber;
This 64-bit value is used in the ACK message as well as in the
"record_sequence_number" input to the AEAD function.
The entire header value shown above is used as it appears on the wire
as the additional data value for the AEAD function. Note that this
design is different from the additional data calculation for DTLS 1.2
and for DTLS 1.2 with Connection ID.
4.1. Determining the Header Format
Implementations can distinguish the two header formats by examining
the first byte:
- If the first byte is alert(21), handshake(22), or ack(proposed,
25), the record MUST be interpreted as a DTLSPlaintext record.
- If the first byte is any other value, then receivers MUST check to
see if the leading bits of the first byte are 001. If so, the
implementation MUST process the record as DTLSCiphertext; the true
content type will be inside the protected portion.
- Otherwise, the record MUST be rejected as if it had failed
deprotection, as described in Section 4.5.2.
4.2. Sequence Number and Epoch
DTLS uses an explicit or partly 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.
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 6.1.
4.2.1. Processing Guidelines
Because DTLS records could be reordered, a record from epoch M may be
received after epoch N (where N > M) has begun. In general,
implementations SHOULD discard packets from earlier epochs, but if
packet loss causes noticeable problems implementations 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
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that the intention here is that implementers use the current guidance
from the IETF for MSL, as specified in [RFC0793] or successors not
that they attempt to interrogate the MSL that the system TCP stack is
using.)
Conversely, it is possible for records that are protected with the
new epoch 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 [RFC4960]), 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.
Implementations MUST send retransmissions of lost messages using the
same epoch and keying material as the original transmission.
Implementations MUST either abandon an association or re-key prior to
allowing the sequence number to wrap.
Implementations MUST NOT allow the epoch to wrap, but instead MUST
establish a new association, terminating the old association.
4.2.2. Reconstructing the Sequence Number and Epoch
When receiving protected DTLS records message, the recipient does not
have a full epoch or sequence number value and so there is some
opportunity for ambiguity. Because the full epoch and sequence
number are used to compute the per-record nonce, failure to
reconstruct these values leads to failure to deprotect the record,
and so implementations MAY use a mechanism of their choice to
determine the full values. This section provides an algorithm which
is comparatively simple and which implementations are RECOMMENDED to
follow.
If the epoch bits match those of the current epoch, then
implementations SHOULD reconstruct the sequence number by computing
the full sequence number which is numerically closest to one plus the
sequence number of the highest successfully deprotected record.
During the handshake phase, the epoch bits unambiguously indicate the
correct key to use. After the handshake is complete, if the epoch
bits do not match those from the current epoch implementations SHOULD
use the most recent past epoch which has matching bits, and then
reconstruct the sequence number as described above.
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4.2.3. Sequence Number Encryption
In DTLS 1.3, when records are encrypted, record sequence numbers are
also encrypted. The basic pattern is that the underlying encryption
algorithm used with the AEAD algorithm is used to generate a mask
which is then XORed with the sequence number.
When the AEAD is based on AES, then the Mask is generated by
computing AES-ECB on the first 16 bytes of the ciphertext:
Mask = AES-ECB(sn_key, Ciphertext[0..15])
When the AEAD is based on ChaCha20, then the mask is generated by
treating the first 12 bytes of the ciphertext as the Nonce and the
next 4 bytes as the counter, passing them to the ChaCha20 block
function (Section 2.3 of [CHACHA]):
Mask = ChaCha20(sn_key, Ciphertext[0..12], Ciphertext[13..15])
The sn_key is computed as follows:
[sender]_sn_key = HKDF-Expand-Label(Secret, "sn" , "", key_length)
[sender] denotes the sending side. The Secret value to be used is
described in Section 7.3 of [TLS13].
The encrypted sequence number is computed by XORing the leading bytes
of the Mask with the sequence number. Decryption is accomplished by
the same process.
This procedure requires the ciphertext length be at least 16 bytes.
Receivers MUST reject shorter records as if they had failed
deprotection, as described in Section 4.5.2. Senders MUST pad short
plaintexts out (using the conventional record padding mechanism) in
order to make a suitable-length ciphertext. Note most of the DTLS
AEAD algorithms have a 16-byte authentication tag and need no
padding. However, some algorithms such as TLS_AES_128_CCM_8_SHA256
have a shorter authentication tag and may require padding for short
inputs.
Note that sequence number encryption is only applied to the
DTLSCiphertext structure and not to the DTLSPlaintext structure,
which also contains a sequence number.
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4.3. Transport Layer Mapping
DTLS messages MAY be fragmented into multiple DTLS records. 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.
Multiple DTLS records MAY be placed in a single datagram. Records
are encoded consecutively. The length field from DTLS records
containing that field can be used to determine the boundaries between
records. The final record in a datagram can omit the length field.
The first byte of the datagram payload MUST be the beginning of a
record. Records MUST NOT span datagrams.
DTLS records, as defined in this document, do not contain any
association identifiers and 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.
However, the CID extension defined in [DTLS-CID] adds an association
identifier to DTLS records.
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.
4.4. 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.
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- In some implementations, the application may not directly talk to
the network, in which case the DTLS stack may absorb ICMP
[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.
Note that DTLS does not defend against spoofed ICMP messages;
implementations SHOULD ignore any such messages that indicate PMTUs
below the IPv4 and IPv6 minimums of 576 and 1280 bytes respectively
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.
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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
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.
4.5. 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.
4.5.1. Anti-Replay
Each DTLS record contains 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 received packet counter for a session MUST be initialized to zero
when that 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 lifetime of the session. This check SHOULD happen after
deprotecting the packet; otherwise the packet discard might itself
serve as a timing channel for the record number. Note that
decompressing the records number is still a potential timing channel
for the record number, though a less powerful one than whether it was
deprotected.
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.) The receiver SHOULD pick a window large enough to handle
any plausible reordering, which depends on the data rate. (The
receiver does not notify the sender of the window size.)
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The "right" edge of the window represents the highest validated
sequence number value received on the 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
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 packet is new.
The window MUST NOT be updated until the packet has been deprotected
successfully.
4.5.2. 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, both to increase the reliability
of DTLS service and to avoid the risk of spoofing attacks sending
traffic to unrelated third parties.
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. A new ACK content type has been added for reliable message
delivery of handshake messages.
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Note that TLS 1.3 already supports a cookie extension, which is 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
transport like TCP that performs return-routability checks as part of
the connection establishment.
DTLS implementations do not use the TLS 1.3 "compatibility mode"
described in Section D.4 of [TLS13]. DTLS servers MUST NOT echo the
"session_id" value from the client and endpoints MUST NOT send
ChangeCipherSpec messages.
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 parameters
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 [RFC7296]. 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 defense 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
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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 reasons, the cookie field in the
ClientHello is present in DTLS 1.3 but is ignored by a DTLS 1.3
compliant server implementation.
The exchange is shown in Figure 5. 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 5: DTLS exchange with HelloRetryRequest containing the
"cookie" extension
The cookie extension is defined in Section 4.2.2 of [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
[TLS13].
If the HelloRetryRequest message is used, the initial ClientHello and
the HelloRetryRequest are included in the calculation of the
transcript hash. The computation of the message hash for the
HelloRetryRequest is done according to the description in
Section 4.4.1 of [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. If the client's apparent IP address is embedded in
the cookie, this prevents an attacker from generating an acceptable
ClientHello apparently from another user.
One potential attack on this scheme is for the attacker to collect a
number of cookies from different addresses where it controls
endpoints 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 to allow
legitimate clients to handshake through the transition (e.g., a
client 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.
[RFC7296] 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 cookies that were generated outside a certain interval 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 terminate the handshake with an "illegal_parameter" alert. This
allows the client to restart the connection from scratch without a
cookie.
As described in Section 4.1.4 of [TLS13], clients MUST 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 {
client_hello(1),
server_hello(2),
new_session_ticket(4),
end_of_early_data(5),
encrypted_extensions(8),
certificate(11),
certificate_request(13),
certificate_verify(15),
finished(20),
key_update(24),
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 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;
} body;
} Handshake;
The first message each side transmits in each association 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. From the perspective of the DTLS record layer, the
retransmission is a new record. This record will have a new
DTLSPlaintext.sequence_number value.
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DTLS implementations maintain (at least notionally) a
next_receive_seq counter. This counter is initially set to zero.
When a handshake message is received, if its message_seq value
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).
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.
5.3. ClientHello Message
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<8..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 [TLS13]) and
the legacy_version field MUST be set to {254, 253}, which was the
version number for DTLS 1.2. The version fields for DTLS 1.0 and
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DTLS 1.2 are 0xfeff and 0xfefd (to match the wire versions) but
the version field for DTLS 1.3 is 0x0304.
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. If a DTLS 1.3 ClientHello is received with
any other value in this field, the server MUST abort the handshake
with an "illegal_parameter" alert.
cipher_suites: Same as for TLS 1.3.
legacy_compression_methods: Same as for TLS 1.3.
extensions: Same as for TLS 1.3.
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. 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
the length field of the original message. An unfragmented message is
a degenerate case with fragment_offset=0 and fragment_length=length.
Each range MUST be delivered in a single packet.
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.
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5.5. End Of Early Data
The DTLS 1.3 handshake has one important difference from the TLS 1.3
handshake: the EndOfEarlyData message is omitted both from the wire
and the handshake transcript: because DTLS records have epochs,
EndOfEarlyData is not necessary to determine when the early data is
complete, and because DTLS is lossy, attackers can trivially mount
the deletion attacks that EndOfEarlyData prevents in TLS. Servers
SHOULD aggressively age out the epoch 1 keys upon receiving the first
epoch 2 record and SHOULD NOT accept epoch 1 data after the first
epoch 3 record is received. (See Section 6.1 for the definitions of
each epoch.)
5.6. DTLS Handshake Flights
DTLS messages are grouped into a series of message flights, according
to the diagrams below.
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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 6: Message flights for a 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 7: 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*]
+----------+
{Finished} --------> | Flight 3 |
[Application Data*] +----------+
+----------+
<-------- [ACK] | Flight 4 |
[Application Data*] +----------+
[Application Data] <-------> [Application Data]
Figure 8: Message flights for the Zero-RTT handshake
Client Server
+----------+
<-------- [NewSessionTicket] | Flight 1 |
+----------+
+----------+
[ACK] --------> | Flight 2 |
+----------+
Figure 9: Message flights for the new session ticket message
Note: The application data sent by the client is not included in the
timeout and retransmission calculation.
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5.7. Timeout and Retransmission
5.7.1. 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.
+-----------+
| PREPARING |
+----------> | |
| | |
| +-----------+
| |
| | Buffer next flight
| |
| \|/
| +-----------+
| | |
| | SENDING |<------------------+
| | | |
| +-----------+ |
Receive | | |
next | | Send flight or partial |
flight | | flight |
| +---------------+ |
| | | Set retransmit timer |
| | \|/ |
| | +-----------+ |
| | | | |
+--)---------| WAITING |-------------------+
| | +----->| | Timer expires |
| | | +-----------+ |
| | | | | | |
| | | | | | |
| | +----------+ | +--------------------+
| | Receive record | Read retransmit or ACK
Receive | | Send ACK |
last | | |
flight | | | Receive ACK
| | | for last flight
\|/\|/ |
|
+-----------+ |
| | <---------+
| FINISHED |
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| |
+-----------+
| /|\
| |
| |
+---+
Server read retransmit
Retransmit ACK
Figure 10: DTLS timeout and retransmission state machine
The state machine has four basic states: PREPARING, SENDING, WAITING,
and FINISHED.
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. If the implementation has received one or more
ACKs (see Section 7) from the peer, then it SHOULD omit any messages
or message fragments which have already been ACKed. 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 four 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 an ACK from the peer: upon receiving an
ACK for a partial flight (as mentioned in Section 7.1), the
implementation transitions to the SENDING state, where it
retransmits the unacked portion of the flight, resets the
retransmit timer, and returns to the WAITING state. Upon
receiving an ACK for a complete flight, the implementation
cancels all retransmissions and either remains in WAITING, or, if
the ACK was for the final flight, transitions to FINISHED.
3. 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
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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.
4. The implementation receives some or all 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) may also trigger the implementation to send an ACK, as
described in Section 7.1.
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 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.
5.7.2. 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 a 100 msec timer is recommended 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.
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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.
5.8. 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 [TLS13], respectively.
5.9. 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
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.
5.10. 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.
Note: it is not always possible to distinguish which association a
given packet is from. For instance, if the client performs a
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handshake, abandons the connection, and then immediately starts a new
handshake, it may not be possible to tell which connection a given
protected record is for. In these cases, trial decryption MAY be
necessary, though implementations could also use some sort of CID,
such as the one specified in [I-D.ietf-tls-dtls-connection-id].
6. Example of Handshake with Timeout and Retransmission
The following is an example of a handshake with lost packets and
retransmissions.
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Client Server
------ ------
Record 0 -------->
ClientHello
(message_seq=0)
+cookie
X<----- Record 0
(lost) ServerHello
(message_seq=1)
EncryptedExtensions
(message_seq=2)
Certificate
(message_seq=3)
<-------- Record 1
CertificateVerify
(message_seq=4)
Finished
(message_seq=5)
Record 1 -------->
ACK [1]
<-------- Record 2
ServerHello
(message_seq=1)
EncryptedExtensions
(message_seq=2)
Certificate
(message_seq=3)
Record 2 -------->
Certificate
(message_seq=2)
CertificateVerify
(message_seq=3)
Finished
(message_seq=4)
<-------- Record 3
ACK [2]
Figure 11: Example DTLS exchange illustrating message loss
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6.1. 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 [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 client_early_traffic_secret. Note this epoch is skipped if
the client does not offer early data.
- epoch value (2) is used for messages protected using keys derived
from [sender]_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 [sender]_application_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 [sender]_application_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.
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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 [TLS13].
Figure 12 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 ...
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(Post-Handshake Message Exchange)
<-------- [NewSessionTicket]
(epoch=3)
[ACK] -------->
(epoch=3)
Some time later ...
(Rekeying)
<-------- [Application Data]
(epoch=4)
[Application Data] -------->
(epoch=4)
Figure 12: Example DTLS exchange with epoch information
7. ACK Message
The ACK message is used by an endpoint to indicate handshake-
containing the TLS records it has received from the other side. ACK
is not a handshake message but is rather a separate content type,
with code point TBD (proposed, 25). This avoids having ACK being
added to the handshake transcript. Note that ACKs can still be sent
in the same UDP datagram as handshake records.
struct {
RecordNumber record_numbers<0..2^16-1>;
} ACK;
record_numbers: a list of the records containing handshake messages
in the current flight which the endpoint has received, in
numerically increasing order. ACKs only cover the current
outstanding flight (this is possible because DTLS is generally a
lockstep protocol). Thus, an ACK from the server would not cover
both the ClientHello and the client's Certificate.
Implementations can accomplish this by clearing their ACK list
upon receiving the start of the next flight.
ACK records MUST be sent with an epoch that is equal to or higher
than the record which is being acknowledged. Implementations SHOULD
simply use the current key.
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7.1. Sending ACKs
When an implementation receives a partial flight, it SHOULD generate
an ACK that covers the messages from that flight which it has
received so far. Implementations have some discretion about when to
generate ACKs, but it is RECOMMENDED that they do so under two
circumstances:
- When they receive a message or fragment which is out of order,
either because it is not the next expected message or because it
is not the next piece of the current message. Implementations
MUST NOT send ACKs for handshake messages which they discard as
out-of-order, because otherwise those messages will not be
retransmitted.
- When they have received part of a flight and do not immediately
receive the rest of the flight (which may be in the same UDP
datagram). A reasonable approach here is to set a timer for 1/4
the current retransmit timer value when the first record in the
flight is received and then send an ACK when that timer expires.
In addition, implementations MUST send ACKs upon receiving all of any
flight which they do not respond to with their own messages.
Specifically, this means the client's final flight of the main
handshake, the server's transmission of the NewSessionTicket, and
KeyUpdate messages. ACKs SHOULD NOT be sent for other complete
flights because they are implicitly acknowledged by the receipt of
the next flight, which generally immediately follows the flight.
Each NewSessionTicket or KeyUpdate is an individual flight; in
particular, a KeyUpdate sent in response to a KeyUpdate with
update_requested does not implicitly acknowledge that message.
Implementations MAY acknowledge the records corresponding to each
transmission of that flight or simply acknowledge the most recent
one.
ACKs MUST NOT be sent for other records of any content type other
than handshake or for records which cannot be unprotected.
Note that in some cases it may be necessary to send an ACK which does
not contain any record numbers. For instance, a client might receive
an EncryptedExtensions message prior to receiving a ServerHello.
Because it cannot decrypt the EncryptedExtensions, it cannot safely
acknowledge it (as it might be damaged). If the client does not send
an ACK, the server will eventually retransmit its first flight, but
this might take far longer than the actual round trip time between
client and server. Having the client send an empty ACK shortcuts
this process.
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7.2. Receiving ACKs
When an implementation receives an ACK, it SHOULD record that the
messages or message fragments sent in the records being ACKed were
received and omit them from any future retransmissions. Upon receipt
of an ACK for only some messages from a flight, an implementation
SHOULD retransmit the remaining messages or fragments. Note that
this requires implementations to track which messages appear in which
records. Once all the messages in a flight have been acknowledged,
the implementation MUST cancel all retransmissions of that flight.
As noted above, the receipt of any packet responding to a given
flight MUST be taken as an implicit acknowledgement for the entire
flight.
8. Key Updates
As with TLS 1.3, DTLS 1.3 implementations send a KeyUpdate message to
indicate that they are updating their sending keys. As with other
handshake messages with no built-in response, KeyUpdates MUST be
acknowledged. In order to facilitate epoch reconstruction
Section 4.2.2 implementations MUST NOT send with the new keys or send
a new KeyUpdate until the previous KeyUpdate has been acknowledged
(this avoids having too many epochs in active use).
Due to loss and/or re-ordering, DTLS 1.3 implementations may receive
a record with an older epoch than the current one (the requirements
above preclude receiving a newer record). They SHOULD attempt to
process those records with that epoch (see Section 4.2.2 for
information on determining the correct epoch), but MAY opt to discard
such out-of-epoch records.
Although KeyUpdate MUST be acknowledged, it is possible for the ACK
to be lost, in which case the sender of the KeyUpdate will retransmit
it. Implementations MUST retain the ability to ACK the KeyUpdate for
up to 2MSL. It is RECOMMENDED that they do so by retaining the pre-
update keying material, but they MAY do so by responding to messages
which appear to be out-of-epoch with a canned ACK message; in this
case, implementations SHOULD rate limit how often they send such
ACKs.
9. Connection ID Updates
If the client and server have negotiated the "connection_id"
extension [DTLS-CID], either side can send a new CID which it wishes
the other side to use in a NewConnectionId message.
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enum {
cid_immediate(0), cid_spare(1), (255)
} ConnectionIdUsage;
opaque ConnectionId<0..2^8-1>;
struct {
ConnectionIds cids<0..2^16-1>;
ConnectionIdUsage usage;
} NewConnectionId;
cid Indicates the set of CIDs which the sender wishes the peer to
use.
usage Indicates whether the new CIDs should be used immediately or
are spare. If usage is set to "cid_immediate", then one of the
new CID MUST be used immediately for all future records. If it is
set to "cid_spare", then either existing or new CID MAY be used.
Endpoints SHOULD use receiver-provided CIDs in the order they were
provided. Endpoints MUST NOT have more than one NewConnectionId
message outstanding.
If the client and server have negotiated the "connection_id"
extension, either side can request a new CID using the
RequestConnectionId message.
struct {
uint8 num_cids;
} RequestConnectionId;
num_cids The number of CIDs desired.
Endpoints SHOULD respond to RequestConnectionId by sending a
NewConnectionId with usage "cid_spare" containing num_cid CIDs soon
as possible. Endpoints MUST NOT send a RequestConnectionId message
when an existing request is still unfulfilled; this implies that
endpoints needs to request new CIDs well in advance. An endpoint MAY
ignore requests, which it considers excessive (though they MUST be
acknowledged as usual).
Endpoints MUST NOT send either of these messages if they did not
negotiate a CID. If an implementation receives these messages when
CIDs were not negotiated, it MUST abort the connection with an
unexpected_message alert.
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9.1. Connection ID Example
Below is an example exchange for DTLS 1.3 using a single CID in each
direction.
Note: The connection_id extension is defined in [DTLS-CID], which is
used in ClientHello and ServerHello messages.
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Client Server
------ ------
ClientHello
(connection_id=5)
-------->
<-------- HelloRetryRequest
(cookie)
ClientHello -------->
(connection_id=5)
+cookie
<-------- ServerHello
(connection_id=100)
EncryptedExtensions
(cid=5)
Certificate
(cid=5)
CertificateVerify
(cid=5)
Finished
(cid=5)
Certificate -------->
(cid=100)
CertificateVerify
(cid=100)
Finished
(cid=100)
<-------- Ack
(cid=5)
Application Data ========>
(cid=100)
<======== Application Data
(cid=5)
Figure 13: Example DTLS 1.3 Exchange with CIDs
If no CID is negotiated, then the receiver MUST reject any records it
receives that contain a CID.
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10. 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.
11. Security Considerations
Security issues are discussed primarily in [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.
DTLS implementations MUST NOT update their sending address in
response to packets from a different address unless they first
perform some reachability test; no such test is defined in this
specification. Even with such a test, An on-path adversary can also
black-hole traffic or create a reflection attack against third
parties because a DTLS peer has no means to distinguish a genuine
address update event (for example, due to a NAT rebinding) from one
that is malicious. This attack is of concern when there is a large
asymmetry of request/response message sizes.
With the exception of order protection and non-replayability, the
security guarantees for DTLS 1.3 are the same as TLS 1.3. While TLS
always provides order protection and non-replayability, DTLS does not
provide order protection and may not provide replay protection.
Unlike TLS implementations, DTLS implementations SHOULD NOT respond
to invalid records by terminating the connection.
If implementations process out-of-epoch records as recommended in
Section 8, then this creates a denial of service risk since an
adversary could inject packets with fake epoch values, forcing the
recipient to compute the next-generation application_traffic_secret
using the HKDF-Expand-Label construct to only find out that the
message was does not pass the AEAD cipher processing. The impact of
this attack is small since the HKDF-Expand-Label only performs
symmetric key hashing operations. Implementations which are
concerned about this form of attack can discard out-of-epoch records.
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The security and privacy properties of the CID for DTLS 1.3 builds on
top of what is described in [DTLS-CID]. There are, however, several
improvements:
- The use of the Post-Handshake message allows the client and the
server to update their CIDs and those values are exchanged with
confidentiality protection.
- With multi-homing, an adversary is able to correlate the
communication interaction over the two paths, which adds further
privacy concerns. In order to prevent this, implementations
SHOULD attempt to use fresh CIDs whenever they change local
addresses or ports (though this is not always possible to detect).
The RequestConnectionId message can be used by a peer to ask for
new CIDs to ensure that a pool of suitable CIDs is available.
- Switching CID based on certain events, or even regularly, helps
against tracking by on-path adversaries but the sequence numbers
can still allow linkability. For this reason this specification
defines an algorithm for encrypting sequence numbers, see
Section 4.2.3. Note that sequence number encryption is used for
all encrypted DTLS 1.3 records irrespectively of the use of a CID.
- DTLS 1.3 encrypts handshake messages much earlier than in previous
DTLS versions. Therefore, less information identifying the DTLS
client, such as the client certificate, is available to an on-path
adversary.
12. Changes to DTLS 1.2
Since TLS 1.3 introduces 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
- Only AEAD ciphers are supported. Additional data calculation has
been simplified.
- Removed support for weaker and older cryptographic algorithms
- HelloRetryRequest of TLS 1.3 used instead of HelloVerifyRequest
- More flexible ciphersuite negotiation
- New session resumption mechanism
- PSK authentication redefined
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- New key derivation hierarchy utilizing a new key derivation
construct
- Improved version negotiation
- Optimized record layer encoding and thereby its size
- Added CID functionality
- Sequence numbers are encrypted.
13. IANA Considerations
IANA is requested to allocate a new value in the "TLS ContentType"
registry for the ACK message, defined in Section 7, with content type
25. IANA is requested to reserve the content type range 32-63 so
that content types in this range are not allocated.
IANA is requested to allocate two values in the "TLS Handshake Type"
registry, defined in [TLS13], for RequestConnectionId (TBD), and
NewConnectionId (TBD), as defined in this document.
14. References
14.1. Normative References
[CHACHA] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
<https://www.rfc-editor.org/info/rfc8439>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://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,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://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,
<https://www.rfc-editor.org/info/rfc6298>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
14.2. Informative References
[DTLS-CID]
Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
Identifiers for DTLS 1.2", draft-ietf-tls-dtls-connection-
id-06 (work in progress), July 2019.
[I-D.ietf-tls-dtls-connection-id]
Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
Identifiers for DTLS 1.2", draft-ietf-tls-dtls-connection-
id-06 (work in progress), July 2019.
[RFC2522] Karn, P. and W. Simpson, "Photuris: Session-Key Management
Protocol", RFC 2522, DOI 10.17487/RFC2522, March 1999,
<https://www.rfc-editor.org/info/rfc2522>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://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,
<https://www.rfc-editor.org/info/rfc4340>.
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[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006,
<https://www.rfc-editor.org/info/rfc4346>.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
<https://www.rfc-editor.org/info/rfc4347>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over
the Datagram Congestion Control Protocol (DCCP)",
RFC 5238, DOI 10.17487/RFC5238, May 2008,
<https://www.rfc-editor.org/info/rfc5238>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[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, <https://www.rfc-editor.org/info/rfc7525>.
14.3. URIs
[1] mailto:tls@ietf.org
[2] https://www1.ietf.org/mailman/listinfo/tls
[3] https://www.ietf.org/mail-archive/web/tls/current/index.html
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Appendix A. Protocol Data Structures and Constant Values
This section provides the normative protocol types and constants
definitions.
A.1. Record Layer
struct {
ContentType type;
ProtocolVersion legacy_record_version;
uint16 epoch = 0 // DTLS field
uint48 sequence_number; // DTLS field
uint16 length;
opaque fragment[DTLSPlaintext.length];
} DTLSPlaintext;
struct {
opaque content[DTLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} DTLSInnerPlaintext;
struct {
opaque unified_hdr[variable];
opaque encrypted_record[length];
} DTLSCiphertext;
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0|0|1|C|S|L|E E|
+-+-+-+-+-+-+-+-+
| Connection ID | Legend:
| (if any, |
/ length as / C - Connection ID (CID) present
| negotiated) | S - Sequence number length
+-+-+-+-+-+-+-+-+ L - Length present
| 8 or 16 bit | E - Epoch
|Sequence Number|
+-+-+-+-+-+-+-+-+
| 16 bit Length |
| (if present) |
+-+-+-+-+-+-+-+-+
A.2. Handshake Protocol
enum {
hello_request_RESERVED(0),
client_hello(1),
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server_hello(2),
hello_verify_request_RESERVED(3),
new_session_ticket(4),
end_of_early_data(5),
hello_retry_request_RESERVED(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(24),
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 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;
} body;
} Handshake;
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
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CipherSuite cipher_suites<2..2^16-2>;
opaque legacy_compression_methods<1..2^8-1>;
Extension extensions<8..2^16-1>;
} ClientHello;
A.3. ACKs
struct {
RecordNumber record_numbers<0..2^16-1>;
} ACK;
A.4. Connection ID Management
enum {
cid_immediate(0), cid_spare(1), (255)
} ConnectionIdUsage;
opaque ConnectionId<0..2^8-1>;
struct {
ConnectionIds cids<0..2^16-1>;
ConnectionIdUsage usage;
} NewConnectionId;
struct {
uint8 num_cids;
} RequestConnectionId;
Appendix B. History
RFC EDITOR: PLEASE REMOVE THE THIS SECTION
IETF Drafts
draft-32: - Editorial improvements and clarifications.
draft-31: - Editorial improvements in text and figures. - Added
normative reference to ChaCha20 and Poly1305.
draft-30: - Changed record format - Added text about end of early
data - Changed format of the Connection ID Update message - Added
Appendix A "Protocol Data Structures and Constant Values"
draft-29: - Added support for sequence number encryption - Update to
new record format - Emphasize that compatibility mode isn't used.
draft-28: - Version bump to align with TLS 1.3 pre-RFC version.
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draft-27: - Incorporated unified header format. - Added support for
CIDs.
draft-04 - 26: - Submissions to align with TLS 1.3 draft versions
draft-03 - Only update keys after KeyUpdate is ACKed.
draft-02 - Shorten the protected record header and introduce an
ultra-short version of the record header. - Reintroduce KeyUpdate,
which works properly now that we have ACK. - Clarify the ACK rules.
draft-01 - Restructured the ACK to contain a list of packets and also
be a record rather than a handshake message.
draft-00 - First IETF Draft
Personal Drafts 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 C. 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 [2]
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/tls/current/index.html [3]
Appendix D. Contributors
Many people have contributed to previous DTLS versions and they are
acknowledged in prior versions of DTLS specifications or in the
referenced specifications. The sequence number encryption concept is
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taken from the QUIC specification. We would like to thank the
authors of the QUIC specification for their work.
In addition, we would like to thank:
* David Benjamin
Google
davidben@google.com
* Thomas Fossati
Nokia
thomas.fossati@nokia.com
* Tobias Gondrom
Huawei
tobias.gondrom@gondrom.org
* Ilari Liusvaara
Independent
ilariliusvaara@welho.com
* Martin Thomson
Mozilla
martin.thomson@gmail.com
* Christopher A. Wood
Apple Inc.
cawood@apple.com
* Yin Xinxing
Huawei
yinxinxing@huawei.com
Authors' Addresses
Eric Rescorla
RTFM, Inc.
EMail: ekr@rtfm.com
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@arm.com
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Nagendra Modadugu
Google, Inc.
EMail: nagendra@cs.stanford.edu
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