wdiff rfc9147v1.txt rfc9147.txt

Internet Engineering Task Force (IETF) E. Rescorla
Request for Comments: 9147 RTFM, Inc. Mozilla
Obsoletes: 6347 H. Tschofenig
Category: Standards Track Arm Limited
ISSN: 2070-1721 N. Modadugu
Google, Inc.
August 2021
March 2022

The Datagram Transport Layer Security (DTLS) Protocol Version 1.3

Abstract

This document specifies Version 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. non-replayability. Datagram
semantics of the underlying transport are preserved by the DTLS
protocol.

This document obsoletes RFC 6347.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9147.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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in the Simplified Revised BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

Table of Contents

1. Introduction
2. Conventions and Terminology
3. DTLS Design Rationale and Overview
3.1. Packet Loss
3.2. Reordering
3.3. Fragmentation
3.4. Replay Detection
4. The DTLS Record Layer
4.1. Demultiplexing DTLS Records
4.2. Sequence Number and Epoch
4.2.1. Processing Guidelines
4.2.2. Reconstructing the Sequence Number and Epoch
4.2.3. Record Number Encryption
4.3. Transport Layer Mapping
4.4. PMTU Issues
4.5. Record Payload Protection
4.5.1. Anti-Replay
4.5.2. Handling Invalid Records
4.5.3. AEAD Limits
5. The DTLS Handshake Protocol
5.1. Denial-of-Service Countermeasures
5.2. DTLS Handshake Message Format
5.3. ClientHello Message
5.4. ServerHello Message
5.5. Handshake Message Fragmentation and Reassembly
5.6. EndOfEarlyData Message
5.7. DTLS Handshake Flights
5.8. Timeout and Retransmission
5.8.1. State Machine
5.8.2. Timer Values
5.8.3. Large Flight Sizes
5.8.4. State Machine Duplication for Post-Handshake Messages
5.9. CertificateVerify and Finished Messages
5.10. Cryptographic Label Prefix
5.11.
5.10. Alert Messages
5.12.
5.11. Establishing New Associations with Existing Parameters
6. Example of Handshake with Timeout and Retransmission
6.1. Epoch Values and Rekeying
7. ACK Message
7.1. Sending ACKs
7.2. Receiving ACKs
7.3. Design Rationale
8. Key Updates
9. Connection ID Updates
9.1. Connection ID Example
10. Application Data Protocol
11. Security Considerations
12. Changes since DTLS 1.2
13. Updates Affecting DTLS 1.2
14. IANA Considerations
15. References
15.1. Normative References
15.2. Informative References
Appendix A. Protocol Data Structures and Constant Values
A.1. Record Layer
A.2. Handshake Protocol
A.3. ACKs
A.4. Connection ID Management
Appendix B. Analysis of Limits on CCM Usage
B.1. Confidentiality Limits
B.2. Integrity Limits
B.3. Limits for AEAD_AES_128_CCM_8
Appendix C. Implementation Pitfalls
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

The primary goal of the TLS protocol is to establish an
authenticated, confidentiality- and integrity-protected channel
between two communicating peers. The TLS protocol is composed of two
layers: the TLS Record Protocol record protocol and the TLS Handshake Protocol. handshake protocol.
However, TLS must run over a reliable transport channel -- typically
TCP [RFC0793].

There are applications that use UDP [RFC0768] as a transport, transport and to
offer communication security protection for those applications, the
Datagram Transport Layer Security (DTLS) protocol has been developed. developed
to offer communication security protection for those applications.
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
as a delta from TLS 1.3 [TLS13]. It obsoletes DTLS 1.2.

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 [RFC7525] and
[DEPRECATE]. [RFC7525]. [DEPRECATE]
forbids the use of DTLS 1.0.

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.

association: Shared state between two endpoints established with a
DTLS handshake.

connection: Synonym for association.

endpoint: Either the client or server of the connection.

epoch: One set of cryptographic keys used for encryption and
decryption.

handshake: An initial negotiation between client and server that
establishes the parameters of the connection.

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.

server: The endpoint that did not initiate the DTLS connection.

CID: Connection ID.

MSL: Maximum Segment Lifetime.

The reader is assumed to be familiar with [TLS13]. 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.

DTLS 1.3 uses network byte order (big-endian) format for encoding
messages based on the encoding format defined in [TLS13] and earlier
(D)TLS specifications.

The reader is also assumed to be familiar with [RFC9146], as this
document applies the CID functionality to DTLS 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 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, neither requires nor does
it provide,
provides 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.
Note that while low-latency streaming and gaming use DTLS to protect
data (e.g., for protection of a WebRTC data channel), telephony
utilizes DTLS for key establishment and the Secure Real-time
Transport Protocol (SRTP) for protection of data [RFC5763].

TLS cannot be used directly over datagram transports for the
following five four 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 to records.

2. The TLS handshake is a lock-step cryptographic protocol.
Messages must be transmitted and received in a defined order; any
other order is an error. The DTLS handshake includes message
sequence numbers to enable fragmented message reassembly and in-
order delivery in case datagrams are lost or reordered.

3. During the handshake, messages are implicitly acknowledged by
other handshake messages. Some 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
acknowledgement 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.

4. Datagram transport protocols, like UDP, protocols are susceptible to abusive behavior
effecting denial-of-service (DoS) attacks against
nonparticipants. DTLS adds a return-routability check and DTLS
1.3 uses the TLS 1.3 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 timer expires, the client knows that either the
ClientHello or the response from the server has been lost, which
causes the client to retransmit the ClientHello. When the server
receives the retransmission, it knows to retransmit its
HelloRetryRequest or ServerHello.

The server also maintains a retransmission timer for messages it
sends (other than HelloRetryRequests) HelloRetryRequest) and retransmits when that timer
expires. Not applying retransmissions to the HelloRetryRequest
avoids the need to create 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.

For more details detail on timeouts and retransmission, see Section 5.8.

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. Fragmentation

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
UDP datagram (see Section 4.4 for guidance). 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 1.3 record layer is different from the TLS 1.3 record layer
and also different from the DTLS 1.2 record layer.

1. The DTLSCiphertext structure omits the superfluous version number
and type fields.

2. DTLS adds an epoch and sequence number to the TLS record header.
This sequence number allows the recipient to correctly decrypt
and verify the DTLS Message Authentication Code (MAC). records. However, the number of bits used for
the epoch and sequence number fields in the DTLSCiphertext
structure has been reduced from those in previous versions.

3. The DTLS epoch serialized in DTLSPlaintext is 2 octets long for
compatibility with DTLS 1.2. However, this value is set as the
least significant 2 octets of the connection epoch, which is an 8
octet counter incremented on every KeyUpdate. See Section 4.2
for details. The sequence number is set to be the low order 48
bits of the 64 bit sequence number. Plaintext records MUST NOT
be sent with sequence numbers that would exceed 2^48-1, so the
upper 16 bits will always be 0.

4. The DTLSCiphertext structure has a variable length variable-length header.

DTLSPlaintext records are used to send unprotected records, records and
DTLSCiphertext records are used to send protected records.

The DTLS record formats are shown below. Unless explicitly stated, stated
the meaning of the fields is unchanged from previous TLS/DTLS
versions.

struct {
ContentType type;
ProtocolVersion legacy_record_version;
uint16 epoch = 0
uint48 sequence_number;
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 Formats

legacy_record_version: This value MUST be set to {254, 253} for all
records other than the initial ClientHello (i.e., one not
generated after a HelloRetryRequest), where it may also be {254,
255} for compatibility purposes. It MUST be ignored for all
purposes. See [TLS13], Appendix D.1 for the rationale for this.

epoch: The least significant 2 bytes of the connection epoch value.

unified_hdr: The unified header (unified_hdr) is a structure of
variable length, as shown in Figure 3.

encrypted_record: The AEAD-encrypted encrypted form ("AEAD" stands for
"Authenticated Encryption with Associated Data") of the serialized
DTLSInnerPlaintext structure.

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 Unified Header

Fixed Bits: The three high bits of the first byte of the unified
header are set to 001. This ensures that the value will fit
within the DTLS region when multiplexing is performed as described
in [RFC7983]. It also ensures that distinguishing encrypted DTLS
1.3 records from encrypted DTLS 1.2 records is possible when they
are carried on the same host/port quartet; such multiplexing is
only possible when CIDs [RFC9146] are in use, in which case DTLS
1.2 records will have the content type tls12_cid (25).

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. Implementations
MAY mix sequence numbers of different lengths on the same
connection.

L: The L bit (0x04) is set if the length is present.

E: The two low bits (0x03) include the low order low-order two bits of the
epoch.

Connection ID: Variable length Variable-length CID. The CID functionality is
described in [RFC9146]. An example can be found in Section 9.1.

Sequence Number: The low order 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 headers.

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 | | | |8-bit |8 bit Seq. No. |
| Version | / Connection ID / +-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+ | | | |
| 16 bit | +-+-+-+-+-+-+-+-+ | Encrypted |
| Epoch | | 16 bit | / Record /
+-+-+-+-+-+-+-+-+ |Sequence Number| | |
| | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | | 16 bit |
| 48 bit | | Length | DTLSCiphertext
|Sequence Number| +-+-+-+-+-+-+-+-+ Structure
| | | | (minimal)
| | | Encrypted |
+-+-+-+-+-+-+-+-+ / Record /
| 16 bit | | |
| Length | +-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+
| | DTLSCiphertext
| | Structure
/ Fragment / (full)
| |
+-+-+-+-+-+-+-+-+

DTLSPlaintext
Structure

Figure 4: DTLS 1.3 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 the last
record in a datagram. Implementations MAY mix records with and
without length fields on the same connection.

If a Connection ID is negotiated, then it MUST be contained in all
datagrams. Sending implementations MUST NOT mix records from
multiple DTLS associations in the same datagram. If the second or
later record has a connection ID that which does not correspond to the
same association used for previous records, the rest of the datagram
MUST be discarded.

When expanded, the epoch and sequence number can be combined into an
unpacked RecordNumber structure, as shown below:

struct {
uint16
uint64 epoch;
uint48
uint64 sequence_number;
} RecordNumber;

This 64-bit 128-bit value is used in the ACK message as well as in the
"record_sequence_number" input to the AEAD Authenticated Encryption with
Associated Data (AEAD) function. The entire header value shown in
Figure 4 (but prior to record number encryption; see Section 4.2.3)
is used as the additional data value for the AEAD function. For
instance, if the minimal variant is used, the AAD Associated Data (AD) is
2 octets long. Note that this design is different from the
additional data calculation for DTLS 1.2 and for DTLS 1.2 with
Connection ID. IDs. In DTLS 1.3 the 64-bit sequence_number is used as
the sequence number for the AEAD computation; unlike DTLS 1.2, the
epoch is not included.

4.1. Demultiplexing DTLS Records

DTLS 1.3 uses 1.3's header format is more complicated to demux than DTLS 1.2,
which always carried the content type as the first byte. As
described in Figure 5, the first byte determines how an incoming DTLS
record is demultiplexed. The first 3 bits of the first byte
distinguish a variable length DTLS 1.3 encrypted record format from record types used in
previous DTLS versions and plaintext DTLS 1.3 record types. Hence,
the demultiplexing
process is hence more complex, since more header formats need range 32 (0b0010 0000) to 63 (0b0011 1111) needs to be
distinguished. excluded
from future allocations by IANA to avoid problems while
demultiplexing; see Section 14. Implementations can demultiplex DTLS
1.3 records by examining the first byte as follows:

* If the first byte is alert(21), handshake(22), or ack(proposed,
26), 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.

Figure 5 shows this demultiplexing procedure graphically, taking DTLS
1.3 and earlier versions of DTLS into account.

Figure 5: Demultiplexing DTLS 1.2 and DTLS 1.3 Records

Note: The optimized DTLS header format shown in Figure 3, which
does not carry the Content Type in the Unified Header format,
requires a different demultiplexing strategy compared to what was
used in previous DTLS versions where the Content Type was conveyed
in every record. As described in Figure 5, the first byte
determines how an incoming DTLS record is demultiplexed. The
first 3 bits of the first byte distinguish a DTLS 1.3 encrypted
record from record types used in previous DTLS versions and
plaintext DTLS 1.3 record types. Hence, the range 32 (0b0010
0000) to 63 (0b0011 1111) needs to be excluded from future
allocations by IANA to avoid problems while demultiplexing; see
Section 14.

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. Implementations
SHOULD discard records from earlier epochs but 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, 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
records, though when DTLS is used over reliable transports (e.g., the
Stream Control Transmission Protocol (SCTP)
SCTP [RFC4960]), they SHOULD be buffered and processed once the
handshake completes. Note that TLS's restrictions on when records
may be sent still apply, and the receiver treats the records 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 rekey 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, the recipient does not have a
full epoch or sequence number value in the record and so there is
some opportunity for ambiguity. Because the full epoch and sequence number are is
used to compute the per-record nonce, nonce and the epoch determines the
keys, 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 that which is comparatively simple and that 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 that which is numerically closest to one plus the
sequence number of the highest successfully deprotected record in the
current epoch.

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 that which has matching bits, and
then reconstruct the sequence number for that epoch as described
above.

4.2.3. Record 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
that
which is then XORed with the sequence number.

When the AEAD is based on AES, then the Mask 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 4 bytes of the ciphertext as the block counter and
the next 12 bytes as the nonce, passing them to the ChaCha20 block
function (Section 2.3 of [CHACHA]):

Mask = ChaCha20(sn_key, Ciphertext[0..3], Ciphertext[4..15])

The sn_key is computed as follows:

[sender]_sn_key = HKDF-Expand-Label(Secret, "sn", "", key_length)

[sender] denotes the sending side. The per-epoch Secret value to be
used is described in Section 7.3 of [TLS13]. Note that a new key is
used for each epoch: because the epoch is sent in the clear, this
does not result in ambiguity.

The encrypted sequence number is computed by XORing the leading bytes
of the Mask mask with the on-the-wire representation of the sequence
number. Decryption is accomplished by the same process.

This procedure requires the ciphertext length to 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 that most of the
DTLS AEAD algorithms have a 16-byte 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.

Future cipher suites, which are not based on AES or ChaCha20, MUST
define their own record sequence number encryption in order to be
used with DTLS.

Note that sequence number encryption is only applied to the
DTLSCiphertext structure and not to the DTLSPlaintext structure,
which even
though it also contains a sequence number.

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 Path MTU (PMTU) estimates
obtained from the record layer. For more information about PMTU
issues, see Section 4.4.

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 without CIDs 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 without CIDs.

Some transports, such as the Datagram Congestion Control Protocol
(DCCP) 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 the 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
"Datagram Too Big" indications [RFC1191] or ICMPv6 "Packet Too
Big" indications [RFC4443].

* 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 fragments fragment and
reassembles
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 also allow the upper layer protocol to
discover the amount of record expansion expected by the DTLS
processing; alternately, it MAY report PMTU estimates minus the
estimated expansion from the transport layer and DTLS record framing.

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.

If there is a transport protocol indication that the PMTU was
exceeded (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] and [RFC4821] for
IPv4 or via [RFC8201] for IPv6. In particular:

* Where allowed by the underlying transport protocol, the upper
layer protocol SHOULD be allowed to set the state of the Don't
Fragment (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
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, the handshake layer SHOULD immediately attempt
to fragment the message, 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 specification 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]. Because each epoch resets the sequence number space, a
separate sliding window is needed for each epoch.

The received record counter for an epoch MUST be initialized to zero
when that epoch is first used. 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
in that epoch during the lifetime of the association. This check
SHOULD happen after deprotecting the record; otherwise, the record
discard might itself serve as a timing channel for the record number.
Note that computing the full record number from the partial is still
a potential timing channel for the record number, though a less
powerful one than whether the record 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.)

The "right" edge of the window represents the highest validated
sequence number value received in the epoch. Records that contain
sequence numbers lower than the "left" edge of the window are
rejected. Records falling within the window are checked against a
list of received records 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 record is to the right of the
window, then the record is new.

The window MUST NOT be updated due to a received record until the that
record 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 that which choose to instead generate an alert instead MUST
generate fatal alerts to avoid attacks where the attacker repeatedly
probes the implementation to see how it responds to various types of
errors.
error. Note that if DTLS is run over UDP, then any implementation
that
which does this will be extremely susceptible to DoS attacks because
UDP forgery is so easy. Thus, generating fatal alerts 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.

Note that because invalid records are rejected at a layer lower than
the handshake state machine, they do not affect pending
retransmission timers.

4.5.3. AEAD Limits

Section 5.5 of [TLS13] defines limits on the number of records that
can be protected using the same keys. These limits are specific to
an AEAD algorithm and apply equally to DTLS. Implementations SHOULD
NOT protect more records than allowed by the limit specified for the
negotiated AEAD. Implementations SHOULD initiate a key update before
reaching this limit.

[TLS13] does not specify a limit for AEAD_AES_128_CCM, but the
analysis in Appendix B shows that a limit of 2^23 packets can be used
to obtain the same confidentiality protection as the limits specified
in TLS.

The usage limits defined in TLS 1.3 exist for protection against
attacks on confidentiality and apply to successful applications of
AEAD protection. The integrity protections in authenticated
encryption also depend on limiting the number of attempts to forge
packets. TLS achieves this by closing connections after any record
fails an authentication check. In comparison, DTLS ignores any
packet that cannot be authenticated, allowing multiple forgery
attempts.

Implementations MUST count the number of received packets that fail
authentication with each key. If the number of packets that fail
authentication exceeds a limit that is specific to the AEAD in use,
an implementation SHOULD immediately close the connection.
Implementations SHOULD initiate a key update with update_requested
before reaching this limit. Once a key update has been initiated,
the previous keys can be dropped when the limit is reached rather
than closing the connection. Applying a limit reduces the
probability that an attacker is able to successfully forge a packet;
see [AEBounds] and [ROBUST].

For AEAD_AES_128_GCM, AEAD_AES_256_GCM, and AEAD_CHACHA20_POLY1305,
the limit on the number of records that fail authentication is 2^36.
Note that the analysis in [AEBounds] supports a higher limit for
AEAD_AES_128_GCM and AEAD_AES_256_GCM, but this specification
recommends a lower limit. For AEAD_AES_128_CCM, the limit on the
number of records that fail authentication is 2^23.5; see Appendix B.

The AEAD_AES_128_CCM_8 AEAD, as used in TLS_AES_128_CCM_8_SHA256,
does not have a limit on the number of records that fail
authentication that both limits the probability of forgery by the
same amount and does not expose implementations to the risk of denial
of service; see Appendix B.3. Therefore, TLS_AES_128_CCM_8_SHA256
MUST NOT be used in DTLS without additional safeguards against
forgery. Implementations MUST set usage limits for
AEAD_AES_128_CCM_8 based on an understanding of any additional
forgery protections that are used.

Any TLS cipher suite that is specified for use with DTLS MUST define
limits on the use of the associated AEAD function that preserves
margins for both confidentiality and integrity. That is, limits MUST
be specified for the number of packets that can be authenticated and
for the number of packets that can fail authentication before a key
update is required. Providing a reference to any analysis upon which
values are based -- and any assumptions used in that analysis --
allows limits to be adapted to varying usage conditions.

5. The DTLS Handshake Protocol

DTLS 1.3 reuses 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.

Note that

In addition, DTLS reuses TLS 1.3 already supports a cookie extension, which is used 1.3's "cookie" extension to prevent denial-of-service attacks. provide a
return-routability check as part of connection establishment. This
is an important DoS prevention mechanism
is described in more detail below, since for UDP-based protocols are more
vulnerable to amplification attacks than a connection-oriented
transport like protocols,
unlike TCP-based protocols, for which TCP that performs return-routability checks establishes return-
routability as part of the connection establishment.

DTLS implementations do not use the TLS 1.3 "compatibility mode"
described in Appendix D.4 of [TLS13]. DTLS servers MUST NOT echo the
"legacy_session_id" value from the client, 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 address that
belongs to a 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 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 the Internet Key
Exchange (IKE) protocol 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 "cookie" extension, is defined in TLS 1.3.
The HelloRetryRequest message contains a stateless cookie (see
[TLS13], Section 4.2.2). The client MUST send a new 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 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 how cookies are exchanged compared
to DTLS 1.2. DTLS 1.3 reuses 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 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 6. 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 6: DTLS Exchange with HelloRetryRequest Containing the
"cookie" Extension

The cookie "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 "cookie" extension is omitted and the
legacy_cookie field in the ClientHello message MUST be set to a zero-
length vector (i.e., a zero-valued single byte length field).

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 content or
hash of the initial ClientHello (and HelloRetryRequest) to be stored
in the cookie. The initial ClientHello is included in the handshake
transcript as a synthetic "message_hash" message, so only the hash
value is needed for the handshake to complete, though the complete
HelloRetryRequest contents are needed.

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 key rotation
scheme that allows the server to manage keys with overlapping
lifetimes.

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, e.g., where ICE
[RFC8445] has been used to establish bidirectional connectivity, 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 or, more generically, when the DTLS handshake uses
a PSK-based key exchange and the IP address matches one associated
with the Pre-Shared Key
(PSK). PSK. Servers that which process 0-RTT requests and send 0.5-RTT
responses without a cookie exchange risk being used in an
amplification attack if the size of outgoing messages greatly exceeds
the size of those that are received. A server SHOULD limit the
amount of data it sends toward a client address to three times the
amount of data sent by the client before it verifies that the client
is able to receive data at that address. A client address is valid
after a cookie exchange or handshake completion. Clients MUST be
prepared to do a cookie exchange with every handshake. Note that
cookies are only valid for the existing handshake and cannot be
stored for future handshakes.

If a server receives a ClientHello with an invalid cookie, it MUST
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 that which was sent in the same connection (i.e.,
where the ClientHello was itself in response to a HelloRetryRequest).

DTLS clients that which do not want to receive a Connection ID SHOULD
still offer the "connection_id" extension [RFC9146] unless there is
an application profile to the contrary. This permits a server that which
wants to receive a CID to negotiate one.

5.2. DTLS Handshake Message Format

In order

DTLS uses the same Handshake messages as TLS 1.3. However, prior to
transmission they are converted to DTLSHandshake messages, which
contain extra data needed to support message loss, reordering, and
message
fragmentation, DTLS modifies the TLS 1.3 handshake header: fragmentation.

enum {
client_hello(1),
server_hello(2),
new_session_ticket(4),
end_of_early_data(5),
encrypted_extensions(8),
request_connection_id(9), /* New */
new_connection_id(10), /* New */
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 (msg_type) {
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;
case request_connection_id: RequestConnectionId;
case new_connection_id: NewConnectionId;
} body;
} Handshake; DTLSHandshake;

In DTLS 1.3, the message transcript is computed over the original TLS
1.3-style Handshake messages without the message_seq,
fragment_offset, and fragment_length values. Note that this is a
change from DTLS 1.2 where those values were included in the
transcript.

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 reused, 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.

Note: In DTLS 1.2, the message_seq was reset to zero in case of a
rehandshake (i.e., renegotiation). On the surface, a rehandshake
in DTLS 1.2 shares similarities with a post-handshake message
exchange in DTLS 1.3. However, in DTLS 1.3 the message_seq is not
reset, to allow distinguishing a retransmission from a previously
sent post-handshake message from a newly sent post-handshake
message.

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 trade-off).

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 implementations 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 supported_versions entries for
DTLS 1.0 and DTLS 1.2 are 0xfeff and 0xfefd (to match the wire
versions). The value 0xfefc is used to indicate DTLS 1.3.

random: Same as for TLS 1.3, except that the downgrade sentinels
described in Section 4.1.3 of [TLS13] when TLS 1.2 and TLS 1.1 and
below are negotiated apply to DTLS 1.2 and DTLS 1.0, respectively.

legacy_session_id: Versions of TLS and DTLS before version 1.3
supported a "session resumption" feature, which has been merged
with pre-shared keys (PSK) in version 1.3. A client that which has a
cached session ID set by a pre-DTLS 1.3 server SHOULD set this
field to that value. Otherwise, it MUST be set as a zero-length
vector (i.e., a zero-valued single byte length field).

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; only suites with DTLS-OK=Y may
be used.

legacy_compression_methods: Same as for TLS 1.3.

extensions: Same as for TLS 1.3.

5.4. ServerHello Message

The DTLS 1.3 ServerHello message is the same as the TLS 1.3
ServerHello message, except that the legacy_version field is set to
0xfefd, indicating DTLS 1.2.

5.5. Handshake Message Fragmentation and Reassembly

As described in Section 4.3, one or more handshake messages may be
carried in a single datagram. However, handshake messages are
potentially bigger than the size allowed by the underlying datagram
transport. DTLS provides a mechanism for fragmenting a handshake
message over a number of records, each of which can be transmitted in
separate datagrams, 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 DTLSHandshake messages, all
with the same message_seq value as the original handshake DTLSHandshake
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 handshake message fragment that is
placed into a record MUST be delivered in a single UDP datagram.

When a DTLS implementation receives a handshake message fragment
corresponding to the next expected handshake message sequence number,
it MUST buffer process it, either by buffering it until it has the entire
handshake message or by processing any in-order portions of the
message. The transcript consists of complete TLS Handshake messages
(reassembled as necessary). Note that this requires removing the
message_seq, fragment_offset, and fragment_length fields to create
the Handshake structure.

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. Senders MUST
NOT change handshake message bytes upon retransmission. Receivers
MAY check that retransmitted bytes are identical and SHOULD abort the
handshake with an "illegal_parameter" alert if the value of a byte
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 handshake messages into the same datagram: in the same
record or in separate records.

5.6. EndOfEarlyData Message

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 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 NOT accept records from epoch 1 indefinitely once they are
able to process records from epoch 3. Though reordering of IP
packets can result in records from epoch 1 arriving after records
from epoch 3, this is not likely to persist for very long relative to
the round trip time. Servers could discard epoch 1 keys after the
first epoch 3 data arrives, or retain keys for processing epoch 1
data for a short period. (See Section 6.1 for the definitions of
each epoch.)

5.7. DTLS Handshake Flights

DTLS handshake messages are grouped into a series of message flights.
A flight starts with the handshake message transmission of one peer
and ends with the expected response from the other peer. Table 1
contains a complete list of message combinations that constitute
flights.

+======+========+========+===================================+
| Note | Client | Server | Handshake Messages |
+======+========+========+===================================+
| | x | | ClientHello |
+------+--------+--------+-----------------------------------+
| | | x | HelloRetryRequest |
+------+--------+--------+-----------------------------------+
| | | x | ServerHello, EncryptedExtensions, |
| | | | CertificateRequest, Certificate, |
| | | | CertificateVerify, Finished |
+------+--------+--------+-----------------------------------+
| 1 | x | | Certificate, CertificateVerify, |
| | | | Finished |
+------+--------+--------+-----------------------------------+
| 1 | | x | NewSessionTicket |
+------+--------+--------+-----------------------------------+

Table 1: Flight Handshake Message Combinations

Remarks:

* Table 1 does not highlight any of the optional messages.

* Regarding note (1): When a handshake flight is sent without any
expected response, as is the case with the client's final flight
or with the NewSessionTicket message, the flight must be
acknowledged with an ACK message.

Below are several example message exchanges illustrating the flight
concept. The notational conventions from [TLS13] are used.

Client Server

+--------+
ClientHello | Flight |
--------> +--------+

+--------+
<-------- HelloRetryRequest | Flight |
+ cookie +--------+

+--------+
ClientHello | Flight |
+ cookie --------> +--------+

ServerHello
{EncryptedExtensions} +--------+
{CertificateRequest*} | Flight |
{Certificate*} +--------+
{CertificateVerify*}
{Finished}
<-------- [Application Data*]

{Certificate*} +--------+
{CertificateVerify*} | Flight |
{Finished} --------> +--------+
[Application Data]
+--------+
<-------- [ACK] | Flight |
[Application Data*] +--------+

[Application Data] <-------> [Application Data]

Figure 7: Message Flights for a Full DTLS Handshake (with Cookie
Exchange)

ClientHello +--------+
+ pre_shared_key | Flight |
+ psk_key_exchange_modes +--------+
+ key_share* -------->

ServerHello
+ pre_shared_key +--------+
+ key_share* | Flight |
{EncryptedExtensions} +--------+
<-------- {Finished}
[Application Data*]
+--------+
{Finished} --------> | Flight |
[Application Data*] +--------+

+--------+
<-------- [ACK] | Flight |
[Application Data*] +--------+

[Application Data] <-------> [Application Data]

Figure 8: Message Flights for Resumption and PSK Handshake
(without Cookie Exchange)

Client Server

ClientHello
+ early_data
+ psk_key_exchange_modes +--------+
+ key_share* | Flight |
+ pre_shared_key +--------+
(Application Data*) -------->

ServerHello
+ pre_shared_key
+ key_share* +--------+
{EncryptedExtensions} | Flight |
{Finished} +--------+
<-------- [Application Data*]

+--------+
{Finished} --------> | Flight |
[Application Data*] +--------+

+--------+
<-------- [ACK] | Flight |
[Application Data*] +--------+

[Application Data] <-------> [Application Data]

Figure 9: Message Flights for the Zero-RTT Handshake

Client Server

+--------+
<-------- [NewSessionTicket] | Flight |
+--------+

+--------+
[ACK] --------> | Flight |
+--------+

Figure 10: Message Flights for the NewSessionTicket Message

KeyUpdate, NewConnectionId, and RequestConnectionId follow a pattern similar
pattern to NewSessionTicket: a single message sent by one side
followed by an ACK by the other.

5.8. Timeout and Retransmission

5.8.1. State Machine

DTLS uses a simple timeout and retransmission scheme with the state
machine shown in Figure 11.

+-----------+
| 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 | (Maybe Send ACK) |
last | |
flight | | Receive ACK
| | for last flight
\|/ |
|
+-----------+ |
| | <---------+
| FINISHED |
| |
+-----------+
| /|\
| |
| |
+---+

Server read retransmit
Retransmit ACK

Figure 11: 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 transmission 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 that which have already been ACKed. acknowledged. Once the
messages have been sent, the implementation then sets a retransmit
timer and 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, adjusts and
re-arms the retransmit timer (see Section 5.8.2), 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 unacknowledged portion of the flight, adjusts and re-
arms
re-arms 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: peer
when none of the messages that it sent in response to that flight
have been acknowledged: the implementation transitions to the
SENDING state, where it retransmits the flight, adjusts and re-arms re-
arms 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.

4. The implementation receives some or all of 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) 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 MSL defined for
[RFC0793], when in the FINISHED state, the server MUST respond to
retransmission of the client's final 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 records for epoch 3 and above
until they have received the Finished message from the peer.
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.

5.8.2. Timer Values

The configuration of timer settings varies with implementations, and
certain deployment environments require timer value adjustments.
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.

Unless implementations have deployment-specific and/or external
information about the round trip time, implementations SHOULD use an
initial timer value of 1000 ms and double the value at each
retransmission, up to no less than 60 seconds (the maximum as
specified in RFC 6298 [RFC6298]). Application-specific profiles MAY
recommend shorter or longer timer values. For instance:

* Profiles for specific deployment environments, such as in low-
power, multi-hop mesh scenarios as used in some Internet of Things
(IoT) networks, MAY specify longer timeouts. See [IOT-PROFILE]
for more information about one such DTLS 1.3 IoT profile.

* Real-time protocols MAY specify shorter timeouts. It is
RECOMMENDED that for DTLS-SRTP [RFC5764], a default timeout of 400
ms be used; because customer experience degrades with one-way
latencies of greater than 200 ms, real-time deployments are less
likely to have long latencies.

In settings where there is external information (for instance, from
an Interactive Connectivity Establishment (ICE) handshake [RFC8445], ICE [RFC8445] handshake, or from previous connections to the same
server) about the RTT, implementations SHOULD use 1.5 times that RTT
estimate as the retransmit timer.

Implementations SHOULD retain the current timer value until a message
is transmitted and acknowledged without having to be retransmitted,
at which time the value SHOULD be adjusted to 1.5 times the measured
round trip time for that message. After a long period of idleness,
no less than 10 times the current timer value, implementations MAY
reset the timer to the initial value.

Note that because retransmission is for the handshake and not
dataflow, the effect on congestion of shorter timeouts is smaller
than in generic protocols such as TCP or QUIC. Experience with DTLS
1.2, which uses a simpler "retransmit everything on timeout"
approach, has not shown serious congestion problems in practice.

5.8.3. Large Flight Sizes

DTLS does not have any built-in congestion control or rate control;
in general, this is not an issue because messages tend to be small.
However, in principle, some messages -- especially Certificate -- can
be quite large. If all the messages in a large flight are sent at
once, this can result in network congestion. A better strategy is to
send out only part of the flight, sending more when messages are
acknowledged. Several extensions have been standardized to reduce
the size of the certificate Certificate message -- for example, the cached
information "cached_info"
extension [RFC7924]; certificate compression [RFC8879]; and
[RFC6066], which defines the "client_certificate_url" extension
allowing DTLS clients to send a sequence of Uniform Resource Locators
(URLs) instead of the client certificate.

DTLS stacks SHOULD NOT send more than 10 records in a single
transmission.

5.8.4. State Machine Duplication for Post-Handshake Messages

DTLS 1.3 makes use of the following categories of post-handshake
messages:

1. NewSessionTicket

2. KeyUpdate

3. NewConnectionId

4. RequestConnectionId

5. Post-handshake client authentication

Messages of each category can be sent independently, and reliability
is established via independent state machines, each of which behaves
as described in Section 5.8.1. For example, if a server sends a
NewSessionTicket and a CertificateRequest message, two independent
state machines will be created.

As explained in the corresponding sections, sending

Sending multiple instances of messages of a given category without
having completed earlier transmissions is allowed for some
categories, but not for others. Specifically, a server MAY send
multiple NewSessionTicket messages at once without awaiting ACKs for
earlier NewSessionTicket messages first. Likewise, a server MAY send
multiple CertificateRequest messages at once without having completed
earlier client authentication requests before. In contrast,
implementations MUST NOT send KeyUpdate, NewConnectionId, or
RequestConnectionId messages if an earlier message of the same type
has not yet been acknowledged.

Note: Except for post-handshake client authentication, which
involves handshake messages in both directions, post-handshake
messages are single-flight, and their respective state machines on
the sender side reduce to waiting for an ACK and retransmitting
the original message. In particular, note that a
RequestConnectionId message does not force the receiver to send a
NewConnectionId message in reply, and both messages are therefore
treated independently.

Creating and correctly updating multiple state machines require requires
feedback from the handshake logic to the state machine layer,
indicating which message belongs to which state machine. For
example, if a server sends multiple CertificateRequest messages and
receives a Certificate message in response, the corresponding state
machine can only be determined after inspecting the
certificate_request_context field. Similarly, a server sending a
single CertificateRequest and receiving a NewConnectionId message in
response can only decide that the NewConnectionId message should be
treated through an independent state machine after inspecting the
handshake message type.

5.9. 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 Finished
messages MUST be computed as if each handshake message had been sent
as a single fragment following the algorithms described in
Sections 4.4.3 and 4.4.4 of [TLS13], respectively.

5.10. Cryptographic Label Prefix

Section 7.1 of [TLS13] specifies that HKDF-Expand-Label use uses a label
prefix of "tls13 ". For DTLS 1.3, that label SHALL be "dtls13".
This ensures key separation between DTLS 1.3 and TLS 1.3. Note that
there is no trailing space; this is necessary in order to keep the
overall label size inside of one hash iteration because "DTLS" is one
letter longer than "TLS".

5.11.

5.10. Alert Messages

Note that Alert alert messages are not retransmitted at all, even when they
occur in the context of a handshake. However, a DTLS implementation
that
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. Note that
alerts are not reliably transmitted; implementations SHOULD NOT
depend on receiving alerts in order to signal errors or connection
closure.

5.12.

Any data received with an epoch/sequence number pair after that of a
valid received closure alert MUST be ignored. Note: this is a change
from TLS 1.3 which depends on the order of receipt rather than the
epoch and sequence number.

5.11. 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 record is from. For instance, if the client performs a
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 use CIDs to avoid
the 5-tuple-based ambiguity.

6. Example of Handshake with Timeout and Retransmission

The following is an example of a handshake with lost packets and
retransmissions. Note that the client sends an empty ACK message
because it can only acknowledge Record 2 sent by the server once it
has processed messages in Record 0 needed to establish epoch 2 keys,
which are needed to encrypt or decrypt messages found in Record 2.
Section 7 provides the necessary background details for this
interaction. Note: For simplicity, we are not resetting record
numbers in this diagram, so "Record 1" is really "Epoch 2, Record 0,
etc.". 0",
etc.

Client Server
------ ------

Record 0 -------->
ClientHello
(message_seq=0)

X<----- Record 0
(lost) ServerHello
(message_seq=0)
Record 1
EncryptedExtensions
(message_seq=1)
Certificate
(message_seq=2)

<-------- Record 2
CertificateVerify
(message_seq=3)
Finished
(message_seq=4)

Record 1 -------->
ACK []

<-------- Record 3
ServerHello
(message_seq=0)
EncryptedExtensions
(message_seq=1)
Certificate
(message_seq=2)

<-------- Record 4
CertificateVerify
(message_seq=3)
Finished
(message_seq=4)

Record 2 -------->
Certificate
(message_seq=1)
CertificateVerify
(message_seq=2)
Finished
(message_seq=3)

<-------- Record 5
ACK [2]

Figure 12: Example DTLS Exchange Illustrating Message Loss

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 reordering, 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 TLS 1.3-defined key derivation steps (see Section 7 of [TLS13]),
a sender may want to rekey at any time during the lifetime of the
connection. It therefore needs 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 that 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, that post-handshake
messages 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 values (4 to 2^16-1) 2^64-1) are 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 record with an epoch value for which
no corresponding cipher state can be determined SHOULD handle it as a
record that which fails deprotection.

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 13 illustrates the epoch values in an example DTLS handshake.

Client Server
------ ------

Record 0
ClientHello
(epoch=0)
-------->
Record 0
<-------- HelloRetryRequest
(epoch=0)
Record 1
ClientHello -------->
(epoch=0)
Record 1
<-------- ServerHello
(epoch=0)
{EncryptedExtensions}
(epoch=2)
{Certificate}
(epoch=2)
{CertificateVerify}
(epoch=2)
{Finished}
(epoch=2)
Record 2
{Certificate} -------->
(epoch=2)
{CertificateVerify}
(epoch=2)
{Finished}
(epoch=2)
Record 2
<-------- [ACK]
(epoch=3)
Record 3
[Application Data] -------->
(epoch=3)
Record 3
<-------- [Application Data]
(epoch=3)

Some time later ...
(Post-Handshake Message Exchange)
Record 4
<-------- [NewSessionTicket]
(epoch=3)
Record 4
[ACK] -------->
(epoch=3)

Some time later ...
(Rekeying)
Record 5
<-------- [Application Data]
(epoch=4)
Record 5
[Application Data] -------->
(epoch=4)

Figure 13: Example DTLS Exchange with Epoch Information

7. ACK Message

The ACK message is used by an endpoint to indicate which handshake
records it has received and processed from the other side. ACK is
not a handshake message but is rather a separate content type, with
code point 26. 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 that which the endpoint has received and either
processed or buffered, in numerically increasing order.

Implementations MUST NOT acknowledge records containing handshake
messages or fragments that which have not been processed or buffered.
Otherwise, deadlock can ensue. As an example, implementations MUST
NOT send ACKs for handshake messages that which they discard because they
are not the next expected message.

During the handshake, ACKs only cover the current outstanding flight
(this is possible because DTLS is generally a lockstep lock-step protocol).
In particular, receiving a message from a handshake flight implicitly
acknowledges all messages from the previous flight(s). Accordingly,
an ACK from the server would not cover both the ClientHello and the
client's Certificate, Certificate message, because the ClientHello and client
Certificate are in different flights. Implementations can accomplish
this by clearing their ACK list upon receiving the start of the next
flight.

After the handshake,

For post-handshake messages, ACKs SHOULD be sent once for each
received and processed handshake record (potentially subject to some
delay) and MAY cover more than one flight. This includes records
containing messages that which are discarded because a previous copy has
been received.

During the handshake, ACK records MUST be sent with an epoch that which is
equal to or higher than the record that which is being acknowledged. Note
that some care is required when processing flights spanning multiple
epochs. For instance, if the client receives only the ServerHello
and Certificate and wishes to ACK them in a single record, it must do
so in epoch 2, as it is required to use an epoch greater than or
equal to 2 and cannot yet send with any greater epoch.
Implementations SHOULD simply use the highest current sending epoch,
which will generally be the highest available. After the handshake,
implementations MUST use the highest available sending epoch.

7.1. Sending ACKs

When an implementation detects a disruption in the receipt of the
current incoming flight, it SHOULD generate an ACK that covers the
messages from the current flight that flight which it has received and processed so far.
Implementations have some discretion about which events to treat as
signs of disruption, but it is RECOMMENDED that they generate ACKs
under two circumstances:

* When they receive a message or fragment that 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.

* 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). "Immediately" is hard to define. One approach 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. Note: The 1/4 value here is somewhat
arbitrary. Given that the round trip estimates in the DTLS
handshake are generally very rough (or the default), any value
will be an approximation, and there is an inherent compromise due
to competition between retransmission due to over-aggressive
ACKing and over-aggressive timeout-based retransmission. As a
comparison point, QUIC's loss-based recovery algorithms
([RFC9002], Section 6.1.2) work out to a delay of about 1/3 of the
retransmit timer.

In general, flights MUST be ACKed unless they are implicitly
acknowledged. In the present specification, the following flights
are implicitly acknowledged by the receipt of the next flight, which
generally immediately follows the flight:

1. Handshake flights other than the client's final flight of the
main handshake.

2. The server's post-handshake CertificateRequest.

ACKs SHOULD NOT be sent for these flights unless the responding
flight cannot be generated immediately. All other flights MUST be
ACKed. In this case, implementations MAY send explicit ACKs for the
complete received flight even though it will eventually also be
implicitly acknowledged through the responding flight. A notable
example for this is the case of client authentication in constrained
environments, where generating the CertificateVerify message can take
considerable time on the client. All other flights MUST be ACKed. Implementations MAY acknowledge the
records corresponding to each transmission of each flight or simply
acknowledge the most recent one. In general, implementations SHOULD
ACK as many received packets as can fit into the ACK record, as this
provides the most complete information and thus reduces the chance of
spurious retransmission; if space is limited, implementations SHOULD
favor including records that which have not yet been acknowledged.

Note: While some post-handshake messages follow a request/response
pattern, this does not necessarily imply receipt. For example, a
KeyUpdate sent in response to a KeyUpdate with request_update set
to "update_requested" does not implicitly acknowledge the earlier
KeyUpdate message because the two KeyUpdate messages might have
crossed in flight.

ACKs MUST NOT be sent for other records of any content type other than
handshake or for records that which cannot be unprotected. deprotected.

Note that in some cases it may be necessary to send an ACK that 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.

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 that leaves it with only some messages from a flight having
been acknowledged, an implementation SHOULD retransmit the
unacknowledged 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.
Implementations MUST treat a record as having been acknowledged if it
appears in any ACK; this prevents spurious retransmission in cases
where a flight is very large and the receiver is forced to elide
acknowledgements for records that which have already been ACKed. As noted
above, the receipt of any record responding to a given flight MUST be
taken as an implicit acknowledgement for the entire flight to which
it is responding.

7.3. Design Rationale

ACK messages are used in two circumstances, namely:

* On sign of disruption, or lack of progress, progress; and

* To indicate complete receipt of the last flight in a handshake.

In the first case, the use of the ACK message is optional optional, because
the peer will retransmit in any case and therefore the ACK just
allows for selective or early retransmission, as opposed to the timeout-
based
timeout-based whole flight retransmission in previous versions of
DTLS. When DTLS 1.3 is used in deployments with lossy networks, such
as low-
power, low-power, long-range radio networks as well as low-power mesh
networks, the use of ACKs is recommended.

The use of the ACK for the second case is mandatory for the proper
functioning of the protocol. For instance, the ACK message sent by
the client in Figure 13 acknowledges receipt and processing of record Record
4 (containing the NewSessionTicket message), and if it is not sent,
the server will continue retransmission of the NewSessionTicket
indefinitely until its maximum retransmission count is reached.

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 records 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 reordering, 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.

Due to the possibility of an ACK message for a KeyUpdate being lost
and thereby preventing the sender of the KeyUpdate from updating its
keying material, receivers MUST retain the pre-update keying material
until receipt and successful decryption of a message using the new
keys.

Figure 14 shows an example exchange illustrating that a successful ACK
processing updates the keys of the KeyUpdate message sender, which is
reflected in the change of epoch values.

Client Server

/-------------------------------------------\
| |
| Initial Handshake |
\-------------------------------------------/

[Application Data] -------->
(epoch=3)

<-------- [Application Data]
(epoch=3)

/-------------------------------------------\
| |
| Some time later ... |
\-------------------------------------------/

[Application Data] -------->
(epoch=3)

[KeyUpdate]
(+ update_requested -------->
(epoch 3)

<-------- [Application Data]
(epoch=3)

[Ack]

[ACK]
<-------- (epoch=3)

[Application Data]
(epoch=4) -------->

<-------- [KeyUpdate]
(epoch=3)

[Ack]

[ACK] -------->
(epoch=4)

<-------- [Application Data]
(epoch=4)

Figure 14: Example DTLS Key Update

With a 128-bit key as in AES-128, rekeying 2^64 times has a high
probability of key reuse within a given connection. Note that even
if the key repeats, the IV is also independently generated. In order
to provide an extra margin of security, sending implementations MUST
NOT allow the epoch to exceed 2^48-1. In order to allow this value
to be changed later, receiving implementations MUST NOT enforce this
rule. If a sending implementation receives a KeyUpdate with
request_update set to "update_requested", it MUST NOT send its own
KeyUpdate if that would cause it to exceed these limits and SHOULD
instead ignore the "update_requested" flag. Note: this overrides the
requirement in TLS 1.3 to always send a KeyUpdate in response to
"update_requested".

9. Connection ID Updates

If the client and server have negotiated the "connection_id"
extension [RFC9146], either side can send a new CID that it wishes
the other side to use in a NewConnectionId message.

enum {
cid_immediate(0), cid_spare(1), (255)
} ConnectionIdUsage;

opaque ConnectionId<0..2^8-1>;

struct {
ConnectionIds
ConnectionId cids<0..2^16-1>;
ConnectionIdUsage usage;
} NewConnectionId;

cids: Indicates the set of CIDs that 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 CIDs MUST be used immediately for all future records. If it
is set to "cid_spare", then either an existing or new CIDs CID MAY be
used.

Endpoints SHOULD use receiver-provided CIDs in the order they were
provided. Implementations that which receive more spare CIDs than they
wish to maintain MAY simply discard any extra CIDs. Endpoints MUST
NOT have more than one NewConnectionId message outstanding.

Implementations that which either did not negotiate the "connection_id"
extension or which have negotiated receiving an empty CID MUST NOT
send NewConnectionId. Implementations MUST NOT send
RequestConnectionId when sending an empty Connection ID.
Implementations that which detect a violation of these rules MUST
terminate the connection with an "unexpected_message" alert.

Implementations SHOULD use a new CID whenever sending on a new path
and SHOULD request new CIDs for this purpose if path changes are
anticipated.

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_cids CIDs as
soon as possible. Endpoints MUST NOT send a RequestConnectionId
message when an existing request is still unfulfilled; this implies
that endpoints need to request new CIDs well in advance. An endpoint
MAY handle requests that which it considers excessive by responding with a
NewConnectionId message containing fewer than num_cids CIDs,
including no CIDs at all. Endpoints MAY handle an excessive number
of RequestConnectionId messages by terminating the connection using a
"too_many_cids_requested" (alert number 52) alert.

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.

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 "connection_id" extension, which is used in ClientHello
and ServerHello messages, is defined in [RFC9146].

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 ACK
(cid=5)

Application Data ========>
(cid=100)
<======== Application Data
(cid=5)

Figure 15: 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.

10. Application Data Protocol

Application data messages are carried by the record layer and are
split into records 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 by excessive resource consumption. 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.

Some key properties required of the cookie for the cookie-exchange
mechanism to be functional are described in Section 3.3 of [RFC2522]:

* The cookie MUST depend on the client's address.

* It MUST NOT be possible for anyone other than the issuing entity
to generate cookies that are accepted as valid by that entity.
This typically entails an integrity check based on a secret key.

* Cookie generation and verification are triggered by
unauthenticated parties, and as such their resource consumption
needs to be restrained in order to avoid having the cookie-
exchange mechanism itself serve as a DoS vector.

Although the cookie must allow the server to produce the right
handshake transcript, it SHOULD be constructed so that knowledge of
the cookie is insufficient to reproduce the ClientHello contents.
Otherwise, this may create problems with future extensions such as
those described in [TLS-ESNI].
Encrypted Client Hello [TLS-ECH].

When cookies are generated using a keyed authentication mechanism, it
should be possible to rotate the associated secret key, so that
temporary compromise of the key does not permanently compromise the
integrity of the cookie-exchange mechanism. Though this secret is
not as high-value as, for example, e.g., a session-ticket-encryption key, rotating
the cookie-generation key on a similar timescale would ensure that
the key-rotation key rotation functionality is exercised regularly and thus in
working order.

The cookie exchange provides address validation during the initial
handshake. DTLS with Connection IDs allows for endpoint addresses to
change during the association; any such updated addresses are not
covered by the cookie exchange during the handshake. DTLS
implementations MUST NOT update the address they send to in response
to packets from a different address unless they first perform some
reachability test; no such test is defined in this specification. specification and
a future specification would need to specify a complete procedure for
how and when to update addresses. Even with such a test, an active
on-path adversary can also black-
hole 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.

TLS 1.3 requires replay protection for 0-RTT data (or rather, for
connections that use 0-RTT data; see Section 8 of [TLS13]). DTLS
provides an optional per-record replay-protection mechanism, since
datagram protocols are inherently subject to message reordering and
replay. These two replay-protection mechanisms are orthogonal, and
neither mechanism meets the requirements for the other.

DTLS 1.3's handshake transcript does not include the new DTLS fields,
which makes it have the same format as TLS 1.3. However, the DTLS
1.3 and TLS 1.3 transcripts are disjoint because they use different
version numbers. Additionally, the DTLS 1.3 key schedule uses a
different label and so will produce different keys for the same
transcript.

The security and privacy properties of the CID for DTLS 1.3 build on
top of what is described for DTLS 1.2 in [RFC9146]. There are,
however, several differences:

* In both versions of DTLS, extension negotiation is used to agree
on the use of the CID feature and the CID values. In both
versions, the CID is carried in the DTLS record header (if
negotiated). However, the way the CID is included in the record
header differs between the two versions.

* 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.

* The ability to use multiple CIDs allows for improved privacy
properties in multihomed scenarios. When only a single CID is in
use on multiple paths from such a host, an adversary can correlate
the communication interaction across 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.

* The mechanism for encrypting sequence numbers (Section 4.2.3)
prevents trivial tracking by on-path adversaries that attempt to
correlate the pattern of sequence numbers received on different
paths; such tracking could occur even when different CIDs are used
on each path, in the absence of sequence number encryption.
Switching CIDs based on certain events, or even regularly, helps
against tracking by on-path adversaries. Note that sequence
number encryption is used for all encrypted DTLS 1.3 records
irrespective of whether a CID is used or not. Unlike the sequence
number, the epoch is not encrypted because it acts as a key
identifier, which may improve correlation of packets from a single
connection across different network paths.

* 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 since DTLS 1.2

Since TLS 1.3 introduces a large number of changes with respect 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 cipher suite negotiation.

* New session resumption mechanism.

* PSK authentication redefined.

* 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. Updates Affecting DTLS 1.2

This document defines several changes that optionally affect
implementations of DTLS 1.2, including those that which do not also
support DTLS 1.3.

* A version downgrade protection mechanism as described in [TLS13],
Section 4.1.3 and applying to DTLS as described in Section 5.3.

* The updates described in [TLS13], Section 3. 1.3.

* The new compliance requirements described in [TLS13], Section 9.3.

14. IANA Considerations

IANA has allocated the content type value 26 in the "TLS ContentType"
registry for the ACK message, defined in Section 7. The value for
the "DTLS-OK" column is "Y". IANA has reserved the content type
range 32-63 so that content types in this range are not allocated.

IANA has allocated value 52 for the "the too_many_cids_requested" "too_many_cids_requested" alert
in the "TLS Alerts" registry. The value for the "DTLS-OK" column is
"Y".

IANA has allocated two values in the "TLS HandshakeType" registry,
defined in [TLS13], for RequestConnectionId request_connection_id (9) and NewConnectionId
new_connection_id (10), as defined in this document. The value for
the "DTLS-OK" column is "Y".

IANA has added this RFC as a reference to the "TLS Cipher Suites"
registry along with the following Note:

| Any TLS cipher suite that is specified for use with DTLS MUST
| define limits on the use of the associated AEAD function that
| preserves margins for both confidentiality and integrity, as
| specified in RFC 9147, Section TODO. 4.5.3 of RFC 9147.

15. References

15.1. 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>.

[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>.

[RFC9146] Rescorla, E., Ed., Tschofenig, H., Ed., Fossati, T., and
A. Kraus, "Connection Identifiers Identifier for DTLS 1.2", RFC 9146,
DOI 10.17487/RFC9146, August 2021, March 2022,
<https://www.rfc-editor.org/info/rfc9146>.

[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>.

15.2. Informative References

[AEAD-LIMITS]
Günther, F., Thomson, M., and C.A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-03, 12 July 2021,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-03>.

[AEBounds] Luykx, A. and K. Paterson, "Limits on Authenticated
Encryption Use in TLS", 8 March 2016, 28 August 2017,
<https://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.

[CCM-ANALYSIS]
Jonsson, J., "On the Security of CTR + CBC-MAC", Selected
Areas in Cryptography pp. 76-93,
DOI 10.1007/3-540-36492-7_7, February 2003,
<https://doi.org/10.1007/3-540-36492-7_7>.

[DEPRECATE]
Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.

[IOT-PROFILE]
Tschofenig, H. and T. Fossati, "TLS/DTLS 1.3 Profiles for
the Internet of Things", Work in Progress, Internet-Draft,
draft-ietf-uta-tls13-iot-profile-02, 12 July 2021,
draft-ietf-uta-tls13-iot-profile-04, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-uta-
tls13-iot-profile-02>.
tls13-iot-profile-04>.

[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>.

[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>.

[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <https://www.rfc-editor.org/info/rfc5763>.

[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010,
<https://www.rfc-editor.org/info/rfc5764>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[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>.

[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<https://www.rfc-editor.org/info/rfc7924>.

[RFC7983] Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
Updates for Secure Real-time Transport Protocol (SRTP)
Extension for Datagram Transport Layer Security (DTLS)",
RFC 7983, DOI 10.17487/RFC7983, September 2016,
<https://www.rfc-editor.org/info/rfc7983>.

[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.

[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.

[RFC8879] Ghedini, A. and V. Vasiliev, "TLS Certificate
Compression", RFC 8879, DOI 10.17487/RFC8879, December
2020, <https://www.rfc-editor.org/info/rfc8879>.

[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[RFC9002] Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
May 2021, <https://www.rfc-editor.org/info/rfc9002>.

[ROBUST] Fischlin, M., Günther, F., and C. Janson, "Robust
Channels: Handling Unreliable Networks in the Record
Layers of QUIC and DTLS 1.3", received 15 June 2020, last
revised 22 February 2021,
<https://eprint.iacr.org/2020/718>.

[TLS-ESNI]

[TLS-ECH] Rescorla, E., Oku, K., Sullivan, N., and C.A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-11, 7 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-11>.

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
uint48 sequence_number;
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;

struct {
uint16
uint64 epoch;
uint48
uint64 sequence_number;
} RecordNumber;

A.2. Handshake Protocol

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_RESERVED(6),
encrypted_extensions(8),
request_connection_id(9), /* New */
new_connection_id(10), /* New */
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),
certificate_url_RESERVED(21),
certificate_status_RESERVED(22),
supplemental_data_RESERVED(23),
key_update(24),
message_hash(254),
(255)
} HandshakeType;

uint16 ProtocolVersion;
opaque Random[32];

uint8 CipherSuite[2]; /* Cryptographic suite selector */

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
ConnectionId cids<0..2^16-1>;
ConnectionIdUsage usage;
} NewConnectionId;

struct {
uint8 num_cids;
} RequestConnectionId;

Appendix B. Analysis of Limits on CCM Usage

TLS [TLS13] and [AEBounds] do not specify limits on key usage for
AEAD_AES_128_CCM. However, any AEAD that is used with DTLS requires
limits on use that ensure that both confidentiality and integrity are
preserved. This section documents that analysis for
AEAD_AES_128_CCM.

[CCM-ANALYSIS] is used as the basis of this analysis. The results of
that analysis are used to derive usage limits that are based on those
chosen in [TLS13].

This analysis uses symbols for multiplication (*), division (/), and
exponentiation (^), plus parentheses for establishing precedence.
The following symbols are also used:

t: The size of the authentication tag in bits. For this cipher, t
is 128.

n: The size of the block function in bits. For this cipher, n is
128.

l: The number of blocks in each packet (see below).

q: The number of genuine packets created and protected by endpoints.
This value is the bound on the number of packets that can be
protected before updating keys.

v: The number of forged packets that endpoints will accept. This
value is the bound on the number of forged packets that an
endpoint can reject before updating keys.

The analysis of AEAD_AES_128_CCM relies on a count of the number of
block operations involved in producing each message. For simplicity,
and to match the analysis of other AEAD functions in [AEBounds], this
analysis assumes a packet length of 2^10 blocks and a packet size
limit of 2^14 bytes.

For AEAD_AES_128_CCM, the total number of block cipher operations is
the sum of of: the length of the associated data in blocks, the length
of the ciphertext in blocks, and the length of the plaintext in
blocks, plus 1. In this analysis, this is simplified to a value of
twice the maximum length of a record in blocks (that is, "2l 2l = 2^11"). 2^11).
This simplification is based on the associated data being limited to
one block.

B.1. Confidentiality Limits

For confidentiality, Theorem 2 in [CCM-ANALYSIS] establishes that an
attacker gains a distinguishing advantage over an ideal pseudorandom
permutation (PRP) of no more than:

(2l * q)^2 / 2^n

For a target advantage in a single-key setting of 2^-60, which
matches that used by [TLS13], TLS 1.3, as summarized in [AEAD-LIMITS], this
results in the relation:

q <= 2^23

That is, endpoints cannot protect more than 2^23 packets with the
same set of keys without causing an attacker to gain a larger
advantage than the target of 2^-60.

B.2. Integrity Limits

For integrity, Theorem 1 in [CCM-ANALYSIS] establishes that an
attacker gains an advantage over an ideal PRP of no more than:

v / 2^t + (2l * (v + q))^2 / 2^n

The goal is to limit this advantage to 2^-57, to match the target in
[TLS13].
TLS 1.3, as summarized in [AEAD-LIMITS]. As "t" t and "n" n are both 128,
the first term is negligible relative to the second, so that term can
be removed without a significant effect on the result. This produces
the relation:

v + q <= 2^24.5

Using the previously established value of 2^23 for "q" q and rounding,
this leads to an upper limit on "v" v of 2^23.5. That is, endpoints
cannot attempt to authenticate more than 2^23.5 packets with the same
set of keys without causing an attacker to gain a larger advantage
than the target of 2^-57.

B.3. Limits for AEAD_AES_128_CCM_8

The TLS_AES_128_CCM_8_SHA256 cipher suite uses the AEAD_AES_128_CCM_8
function, which uses a short authentication tag (that is, t=64).

The confidentiality limits of AEAD_AES_128_CCM_8 are the same as
those for AEAD_AES_128_CCM, as this does not depend on the tag
length; see Appendix B.1.

The shorter tag length of 64 bits means that the simplification used
in Appendix B.2 does not apply to AEAD_AES_128_CCM_8. If the goal is
to preserve the same margins as other cipher suites, then the limit
on forgeries is largely dictated by the first term of the advantage
formula:

v <= 2^7

As this represents attempts to that fail authentication, applying this
limit might be feasible in some environments. However, applying this
limit in an implementation intended for general use exposes
connections to an inexpensive denial-of-service attack.

This analysis supports the view that TLS_AES_128_CCM_8_SHA256 is not
suitable for general use. Specifically, TLS_AES_128_CCM_8_SHA256
cannot be used without additional measures to prevent forgery of
records, or to mitigate the effect of forgeries. This might require
understanding the constraints that exist in a particular deployment
or application. For instance, it might be possible to set a
different target for the advantage an attacker gains based on an
understanding of the constraints imposed on a specific usage of DTLS.

Appendix C. Implementation Pitfalls

In addition to the aspects of TLS that have been a source of
interoperability and security problems (Appendix C.3 of [TLS13]),
DTLS presents a few new potential sources of issues, noted here.

* Do you correctly handle messages received from multiple epochs
during a key transition? This includes locating the correct key
as well as performing replay detection, if enabled.

* Do you retransmit handshake messages that are not (implicitly or
explicitly) acknowledged (Section 5.8)?

* Do you correctly handle handshake message fragments received,
including when they are out of order?

* Do you correctly handle handshake messages received out of order?
This may include either buffering or discarding them.

* Do you limit how much data you send to a peer before its address
is validated?

* Do you verify that the explicit record length is contained within
the datagram in which it is contained?

Acknowledgements

We would like to thank Jonathan Hammell, Bernard Aboba, and Andy
Cunningham for their review comments.

Additionally, we would like to thank the IESG members for their
review comments: Martin Duke, Erik Kline, Francesca Palombini, Lars
Eggert, Zaheduzzaman Sarker, John Scudder, Éric Vyncke, Robert
Wilton, Roman Danyliw, Benjamin Kaduk, Murray Kucherawy, Martin
Vigoureux, and Alvaro Retana.

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
taken from the QUIC specification. We would like to thank the
authors of the QUIC specification for their work. Felix Günther and
Martin Thomson contributed the analysis in Appendix B.

In addition, we would like to thank:

Hanno Becker
Arm Limited
Email: Hanno.Becker@arm.com

David Benjamin
Google
Email: davidben@google.com

Thomas Fossati
Arm Limited
Email: thomas.fossati@arm.com

Tobias Gondrom
Huawei
Email: tobias.gondrom@gondrom.org

Felix Günther
ETH Zurich
Email: mail@felixguenther.info

Benjamin Kaduk
Akamai Technologies
Email: kaduk@mit.edu

Ilari Liusvaara
Independent
Email: ilariliusvaara@welho.com

Martin Thomson
Mozilla
Email: martin.thomson@gmail.com

Christopher A. Wood
Apple Inc.
Cloudflare
Email: cawood@apple.com caw@heapingbits.net

Yin Xinxing
Huawei
Email: yinxinxing@huawei.com

Hanno Becker
Arm Limited

Email: Hanno.Becker@arm.com

The sequence number encryption concept is taken from QUIC [RFC9000].
We would like to thank the authors of RFC 9000 for their work. Felix
Günther and Martin Thomson contributed the analysis in Appendix B.
We would like to thank Jonathan Hammell, Bernard Aboba, and Andy
Cunningham for their review comments.

Authors' Addresses

Eric Rescorla
RTFM, Inc.
Mozilla
Email: ekr@rtfm.com

Hannes Tschofenig
Arm Limited
Email: hannes.tschofenig@arm.com

Nagendra Modadugu
Google, Inc.
Email: nagendra@cs.stanford.edu