| TLS | E. Rescorla |
| Internet-Draft | RTFM, Inc. |
| Obsoletes: 6347 (if approved) | H. Tschofenig |
| Intended status: Standards Track | ARM Limited |
| Expires: June 2, 2018 | N. Modadugu |
| Google, Inc. | |
| November 29, 2017 |
The Datagram Transport Layer Security (DTLS) Protocol Version 1.3
draft-ietf-tls-dtls13-07
This document specifies Version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.
The DTLS 1.3 protocol is intentionally based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 2, 2018.
Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
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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.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH
The source for this draft is maintained in GitHub. Suggested changes should be submitted as pull requests at https://github.com/tlswg/dtls13-spec. Instructions are on that page as well. Editorial changes can be managed in GitHub, but any substantive change should be discussed on the TLS mailing list.
The primary goal of the TLS protocol is to provide privacy and data integrity between two communicating peers. The TLS protocol is composed of two layers: the TLS Record Protocol and the TLS Handshake Protocol. However, TLS must run over a reliable transport channel – typically TCP [RFC0793].
There are applications that utilize UDP [RFC0768] as a transport and to offer communication security protection for those applications the Datagram Transport Layer Security (DTLS) protocol has been designed. DTLS is deliberately designed to be as similar to TLS as possible, both to minimize new security invention and to maximize the amount of code and infrastructure reuse.
DTLS 1.0 [RFC4347] was originally defined as a delta from TLS 1.1 [RFC4346] and DTLS 1.2 [RFC6347] was defined as a series of deltas to TLS 1.2 [RFC5246]. There is no DTLS 1.1; that version number was skipped in order to harmonize version numbers with TLS. This specification describes the most current version of the DTLS protocol aligning with the efforts around TLS 1.3 [I-D.ietf-tls-tls13].
Implementations that speak both DTLS 1.2 and DTLS 1.3 can interoperate with those that speak only DTLS 1.2 (using DTLS 1.2 of course), just as TLS 1.3 implementations can interoperate with TLS 1.2 (see Appendix D of [I-D.ietf-tls-tls13] for details). While backwards compatibility with DTLS 1.0 is possible the use of DTLS 1.0 is not recommended as explained in Section 3.1.2 of RFC 7525 [RFC7525].
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119 [RFC2119].
The following terms are used:
The reader is assumed to be familiar with the TLS 1.3 specification since this document defined as a delta from TLS 1.3.
Figures in this document illustrate various combinations of the DTLS protocol exchanges and the symbols have the following meaning:
The basic design philosophy of DTLS is to construct “TLS over datagram transport”. Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or re-ordered data traffic.
TLS cannot be used directly in datagram environments for the following five reasons:
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 from the server. However, if the server’s message is lost, the client knows that either the ClientHello or the HelloRetryRequest has been lost and retransmits. When the server receives the retransmission, it knows to retransmit.
The server also maintains a retransmission timer and retransmits when that timer expires.
Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests.
In DTLS, each handshake message is assigned a specific sequence number within that handshake. When a peer receives a handshake message, it can quickly determine whether that message is the next message it expects. If it is, then it processes it. If not, it queues it for future handling once all previous messages have been received.
TLS and DTLS handshake messages can be quite large (in theory up to 2^24-1 bytes, in practice many kilobytes). By contrast, UDP datagrams are often limited to less than 1500 bytes if IP fragmentation is not desired. In order to compensate for this limitation, each DTLS handshake message may be fragmented over several DTLS records, each of which is intended to fit in a single IP datagram. Each DTLS handshake message contains both a fragment offset and a fragment length. Thus, a recipient in possession of all bytes of a handshake message can reassemble the original unfragmented message.
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.
The DTLS record layer is similar to that of TLS 1.3. There are three major changes:
The DTLS record formats are shown below. DTLSPlaintext records are used to send unprotected records and DTLSCiphertext or DTLSShortCiphertext are used to send protected records.
struct {
ContentType type;
ProtocolVersion version;
uint16 epoch = 0 // DTLS field
uint48 sequence_number; // DTLS field
uint16 length;
opaque fragment[DTLSPlaintext.length];
} DTLSPlaintext;
struct {
opaque content[DTLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} DTLSInnerPlaintext;
struct {
ContentType opaque_type = 23; /* application_data */
uint32 epoch_and_sequence;
uint16 length;
opaque encrypted_record[length];
} DTLSCiphertext;
As with previous versions of DTLS, multiple DTLSPlaintext and DTLSCiphertext records can be included in the same underlying transport datagram.
The short DTLS header format is:
struct {
uint16 short_epoch_and_sequence; // 001ESSSS SSSSSSSS
opaque encrypted_record[remainder_of_datagram];
} DTLSShortCiphertext;
The short_epoch_and_sequence document contains the epoch and sequence packed into a 16 bit integer as follows:
In this format, the length field is omitted and therefore the record consumes the entire rest of the datagram in the lower level transport. It is not possible to have multiple DTLSShortCiphertext format records in the same datagram.
DTLSShortCiphertext MUST only be used for data which is protected with one of the application_traffic_secret values, and not for either handshake or early data. When using an application_traffic_secret for message protection, Implementations MAY use either DTLSCiphertext or DTLSShortCiphertext at their discretion.
DTLS uses an explicit sequence number, rather than an implicit one, carried in the sequence_number field of the record. Sequence numbers are maintained separately for each epoch, with each sequence_number initially being 0 for each epoch. For instance, if a handshake message from epoch 0 is retransmitted, it might have a sequence number after a message from epoch 1, even if the message from epoch 1 was transmitted first. Note that some care needs to be taken during the handshake to ensure that retransmitted messages use the right epoch and keying material.
The epoch number is initially zero and is incremented each time keying material changes and a sender aims to rekey. More details are provided in Section 6.1. In order to ensure that any given sequence/epoch pair is unique, implementations MUST NOT allow the same epoch value to be reused within two times the TCP maximum segment lifetime (MSL).
Note that because DTLS records may be reordered, a record from epoch 1 may be received after epoch 2 has begun. In general, implementations SHOULD discard packets from earlier epochs, but if packet loss causes noticeable problems they MAY choose to retain keying material from previous epochs for up to the default MSL specified for TCP [RFC0793] to allow for packet reordering. (Note that the intention here is that implementers use the current guidance from the IETF for MSL, not that they attempt to interrogate the MSL that the system TCP stack is using.) Until the handshake has completed, implementations MUST accept packets from the old epoch.
Conversely, it is possible for records that are protected by the newly negotiated context to be received prior to the completion of a handshake. For instance, the server may send its Finished message and then start transmitting data. Implementations MAY either buffer or discard such packets, though when DTLS is used over reliable transports (e.g., SCTP), they SHOULD be buffered and processed once the handshake completes. Note that TLS’s restrictions on when packets may be sent still apply, and the receiver treats the packets as if they were sent in the right order. In particular, it is still impermissible to send data prior to completion of the first handshake.
Implementations MUST either abandon an association or re-key prior to allowing the sequence number to wrap.
Implementations MUST NOT allow the epoch to wrap, but instead MUST establish a new association, terminating the old association.
Implementations can distinguish the three header formats by examining the first byte, which in the DTLSPlaintext and DTLSCiphertext header represents the content type. If the first byte is alert(21), handshake(22), or ack(25), the record MUST be interpreted as a DTLSPlaintext record. If the first byte is application_data(23) then the record MUST be interpreted handled as DTLSCiphertext; the true content type will be inside the protected portion.
If the first byte is any other other value, then receivers MUST check to see if the leading bits of the first byte are 001. If so, they MUST process the record as DTLSShortCiphertext. Otherwise, the record MUST be rejected as if it had failed deprotection.
When receiving protected DTLS records message, the recipient does not have a full epoch or sequence number value and so there is some opportunity for ambiguity. Because the full epoch and sequence number are used to compute the per-record nonce, failure to reconstruct these values leads to failure to deprotect the record, and so implementations MAY use a mechanism of their choice to determine the full values. This section provides an algorithm which is comparatively simple and which implementations are RECOMMENDED to follow.
If the epoch bits match those of the current epoch, then implementations SHOULD reconstruct the sequence number by computing the full sequence number which is numerically closest to one plus the sequence number of the highest successfully deprotected record.
If the epoch bits do not match those from the current epoch, then the record is either from a previous epoch or from a future epoch. Implementations SHOULD use the epoch value which would produce a sequence number which is numerically closest to what would be reconstructed for that epoch, as determined by the algorithm in the paragraph above.
Note: the DTLSShortCiphertext format does not allow for easy reconstruction of sequence numbers if ~2000 datagrams in sequence are lost. Implementations which may encounter this situation SHOULD use the DTLSCiphertext format.
Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer.
Note that unlike IPsec, DTLS records do not contain any association identifiers. Applications must arrange to multiplex between associations. With UDP, the host/port number is used to look up the appropriate security association for incoming records.
Multiple DTLS records may be placed in a single datagram. They are simply encoded consecutively. The DTLS record framing is sufficient to determine the boundaries. Note, however, that the first byte of the datagram payload must be the beginning of a record. Records may not span datagrams.
Some transports, such as DCCP [RFC4340], provide their own sequence numbers. When carried over those transports, both the DTLS and the transport sequence numbers will be present. Although this introduces a small amount of inefficiency, the transport layer and DTLS sequence numbers serve different purposes; therefore, for conceptual simplicity, it is superior to use both sequence numbers.
Some transports provide congestion control for traffic carried over them. If the congestion window is sufficiently narrow, DTLS handshake retransmissions may be held rather than transmitted immediately, potentially leading to timeouts and spurious retransmission. When DTLS is used over such transports, care should be taken not to overrun the likely congestion window. [RFC5238] defines a mapping of DTLS to DCCP that takes these issues into account.
In general, DTLS’s philosophy is to leave PMTU discovery to the application. However, DTLS cannot completely ignore PMTU for three reasons:
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:
The DTLS record layer SHOULD allow the upper layer protocol to discover the amount of record expansion expected by the DTLS processing.
If there is a transport protocol indication (either via ICMP or via a refusal to send the datagram as in Section 14 of [RFC4340]), then the DTLS record layer MUST inform the upper layer protocol of the error.
The DTLS record layer SHOULD NOT interfere with upper layer protocols performing PMTU discovery, whether via [RFC1191] or [RFC4821] mechanisms. In particular:
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:
Like TLS, DTLS transmits data as a series of protected records. The rest of this section describes the details of that format.
DTLS records contain a sequence number to provide replay protection. Sequence number verification SHOULD be performed using the following sliding window procedure, borrowed from Section 3.4.3 of [RFC4303].
The receiver packet counter for this session MUST be initialized to zero when the session is established. For each received record, the receiver MUST verify that the record contains a sequence number that does not duplicate the sequence number of any other record received during the life of this session. This SHOULD be the first check applied to a packet after it has been matched to a session, to speed rejection of duplicate records.
Duplicates are rejected through the use of a sliding receive window. (How the window is implemented is a local matter, but the following text describes the functionality that the implementation must exhibit.) A minimum window size of 32 MUST be supported, but a window size of 64 is preferred and SHOULD be employed as the default. Another window size (larger than the minimum) MAY be chosen by the receiver. (The receiver does not notify the sender of the window size.)
The “right” edge of the window represents the highest validated sequence number value received on this session. Records that contain sequence numbers lower than the “left” edge of the window are rejected. Packets falling within the window are checked against a list of received packets within the window. An efficient means for performing this check, based on the use of a bit mask, is described in Section 3.4.3 of [RFC4303].
If the received record falls within the window and is new, or if the packet is to the right of the window, then the receiver proceeds to MAC verification. If the MAC validation fails, the receiver MUST discard the received record as invalid. The receive window is updated only if the MAC verification succeeds.
Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties.
If DTLS is being carried over a transport that is resistant to forgery (e.g., SCTP with SCTP-AUTH), then it is safer to send alerts because an attacker will have difficulty forging a datagram that will not be rejected by the transport layer.
DTLS 1.3 re-uses the TLS 1.3 handshake messages and flows, with the following changes:
Note that TLS 1.3 already supports a cookie extension, which used to prevent denial-of-service attacks. This DoS prevention mechanism is described in more detail below since UDP-based protocols are more vulnerable to amplification attacks than a connection-oriented transport like TCP that performs return-routability checks as part of the connection establishment.
With these exceptions, the DTLS message formats, flows, and logic are the same as those of TLS 1.3.
Datagram security protocols are extremely susceptible to a variety of DoS attacks. Two attacks are of particular concern:
In order to counter both of these attacks, DTLS borrows the stateless cookie technique used by Photuris [RFC2522] and IKE [RFC5996]. When the client sends its ClientHello message to the server, the server MAY respond with a HelloRetryRequest message. The HelloRetryRequest message, as well as the cookie extension, is defined in TLS 1.3. The HelloRetryRequest message contains a stateless cookie generated using the technique of [RFC2522]. The client MUST retransmit the ClientHello with the cookie added as an extension. The server then verifies the cookie and proceeds with the handshake only if it is valid. This mechanism forces the attacker/client to be able to receive the cookie, which makes DoS attacks with spoofed IP addresses difficult. This mechanism does not provide any defence against DoS attacks mounted from valid IP addresses.
The DTLS 1.3 specification changes the way how cookies are exchanged compared to DTLS 1.2. DTLS 1.3 re-uses the HelloRetryRequest message and conveys the cookie to the client via an extension. The client receiving the cookie uses the same extension to place the cookie subsequently into a ClientHello message. DTLS 1.2 on the other hand used a separate message, namely the HelloVerifyRequest, to pass a cookie to the client and did not utilize the extension mechanism. For backwards compatibility reason the cookie field in the ClientHello is present in DTLS 1.3 but is ignored by a DTLS 1.3 compliant server implementation.
The exchange is shown in Figure 2. Note that the figure focuses on the cookie exchange; all other extensions are omitted.
Client Server
------ ------
ClientHello ------>
<----- HelloRetryRequest
+ cookie
ClientHello ------>
+ cookie
[Rest of handshake]
Figure 2: DTLS Exchange with HelloRetryRequest contain the Cookie Extension
The cookie extension is defined in Section 4.2.2 of [I-D.ietf-tls-tls13]. When sending the initial ClientHello, the client does not have a cookie yet. In this case, the cookie extension is omitted and the legacy_cookie field in the ClientHello message SHOULD be set to a zero length vector (i.e., a single zero byte length field) and MUST be ignored by a server negotiating DTLS 1.3.
When responding to a HelloRetryRequest, the client MUST create a new ClientHello message following the description in Section 4.1.2 of [I-D.ietf-tls-tls13].
If the HelloRetryRequest message is used, the initial ClientHello and the HelloRetryRequest are included in the calculation of the handshake_messages (for the CertificateVerify message) and verify_data (for the Finished message). However, the computation of the message hash for the HelloRetryRequest is done according to the description in Section 4.4.1 of [I-D.ietf-tls-tls13].
The handshake transcript is not reset with the second ClientHello and a stateless server-cookie implementation requires the transcript of the HelloRetryRequest to be stored in the cookie or the internal state of the hash algorithm, since only the hash of the transcript is required for the handshake to complete.
When the second ClientHello is received, the server can verify that the cookie is valid and that the client can receive packets at the given IP address. If the client’s apparent IP address is embedded in the cookie, this prevents an attacker from generating an acceptable ClientHello apparently from another user.
One potential attack on this scheme is for the attacker to collect a number of cookies from different addresses where it controls endpoints and then reuse them to attack the server. The server can defend against this attack by changing the secret value frequently, thus invalidating those cookies. If the server wishes that legitimate clients be able to handshake through the transition (e.g., they received a cookie with Secret 1 and then sent the second ClientHello after the server has changed to Secret 2), the server can have a limited window during which it accepts both secrets. [RFC5996] suggests adding a key identifier to cookies to detect this case. An alternative approach is simply to try verifying with both secrets. It is RECOMMENDED that servers implement a key rotation scheme that allows the server to manage keys with overlapping lifetime.
Alternatively, the server can store timestamps in the cookie and reject those cookies that were not generated within a certain amount of time.
DTLS servers SHOULD perform a cookie exchange whenever a new handshake is being performed. If the server is being operated in an environment where amplification is not a problem, the server MAY be configured not to perform a cookie exchange. The default SHOULD be that the exchange is performed, however. In addition, the server MAY choose not to do a cookie exchange when a session is resumed. Clients MUST be prepared to do a cookie exchange with every handshake.
If a server receives a ClientHello with an invalid cookie, it MUST NOT respond with a HelloRetryRequest. Restarting the handshake from scratch, without a cookie, allows the client to recover from a situation where it obtained a cookie that cannot be verified by the server. As described in Section 4.1.4 of [I-D.ietf-tls-tls13],clients SHOULD also abort the handshake with an “unexpected_message” alert in response to any second HelloRetryRequest which was sent in the same connection (i.e., where the ClientHello was itself in response to a HelloRetryRequest).
In order to support message loss, reordering, and message fragmentation, DTLS modifies the TLS 1.3 handshake header:
enum {
hello_request_RESERVED(0),
client_hello(1),
server_hello(2),
hello_verify_request_RESERVED(3),
new_session_ticket(4),
end_of_early_data(5),
hello_retry_request(6),
encrypted_extensions(8),
certificate(11),
server_key_exchange_RESERVED(12),
certificate_request(13),
server_hello_done_RESERVED(14),
certificate_verify(15),
client_key_exchange_RESERVED(16),
finished(20),
key_update(24),
message_hash(254),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
uint16 message_seq; /* DTLS-required field */
uint24 fragment_offset; /* DTLS-required field */
uint24 fragment_length; /* DTLS-required field */
select (HandshakeType) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case end_of_early_data: EndOfEarlyData;
case hello_retry_request: HelloRetryRequest;
case encrypted_extensions: EncryptedExtensions;
case certificate_request: CertificateRequest;
case certificate: Certificate;
case certificate_verify: CertificateVerify;
case finished: Finished;
case new_session_ticket: NewSessionTicket;
case key_update: KeyUpdate;
} body;
} Handshake;
The first message each side transmits in each association always has message_seq = 0. Whenever a new message is generated, the message_seq value is incremented by one. When a message is retransmitted, the old message_seq value is re-used, i.e., not incremented. From the perspective of the DTLS record layer, the retransmission is a new record. This record will have a new DTLSPlaintext.sequence_number value.
DTLS implementations maintain (at least notionally) a next_receive_seq counter. This counter is initially set to zero. When a handshake message is received, if its message_seq value matches next_receive_seq, next_receive_seq is incremented and the message is processed. If the sequence number is less than next_receive_seq, the message MUST be discarded. If the sequence number is greater than next_receive_seq, the implementation SHOULD queue the message but MAY discard it. (This is a simple space/bandwidth tradeoff).
In addition to the handshake messages that are deprecated by the TLS 1.3 specification DTLS 1.3 furthermore deprecates the HelloVerifyRequest message originally defined in DTLS 1.0. DTLS 1.3-compliant implements MUST NOT use the HelloVerifyRequest to execute a return-routability check. A dual-stack DTLS 1.2/DTLS 1.3 client MUST, however, be prepared to interact with a DTLS 1.2 server.
The format of the ClientHello used by a DTLS 1.3 client differs from the TLS 1.3 ClientHello format as shown below.
uint16 ProtocolVersion;
opaque Random[32];
uint8 CipherSuite[2]; /* Cryptographic suite selector */
struct {
ProtocolVersion legacy_version = { 254,253 }; // DTLSv1.2
Random random;
opaque legacy_session_id<0..32>;
opaque legacy_cookie<0..2^8-1>; // DTLS
CipherSuite cipher_suites<2..2^16-2>;
opaque legacy_compression_methods<1..2^8-1>;
Extension extensions<0..2^16-1>;
} ClientHello;
Each DTLS message MUST fit within a single transport layer datagram. However, handshake messages are potentially bigger than the maximum record size. Therefore, DTLS provides a mechanism for fragmenting a handshake message over a number of records, each of which can be transmitted separately, thus avoiding IP fragmentation.
When transmitting the handshake message, the sender divides the message into a series of N contiguous data ranges. These ranges MUST NOT be larger than the maximum handshake fragment size and MUST jointly contain the entire handshake message. The ranges MUST NOT overlap. The sender then creates N handshake messages, all with the same message_seq value as the original handshake message. Each new message is labeled with the fragment_offset (the number of bytes contained in previous fragments) and the fragment_length (the length of this fragment). The length field in all messages is the same as the length field of the original message. An unfragmented message is a degenerate case with fragment_offset=0 and fragment_length=length.
When a DTLS implementation receives a handshake message fragment, it MUST buffer it until it has the entire handshake message. DTLS implementations MUST be able to handle overlapping fragment ranges. This allows senders to retransmit handshake messages with smaller fragment sizes if the PMTU estimate changes.
Note that as with TLS, multiple handshake messages may be placed in the same DTLS record, provided that there is room and that they are part of the same flight. Thus, there are two acceptable ways to pack two DTLS messages into the same datagram: in the same record or in separate records.
DTLS messages are grouped into a series of message flights, according to the diagrams below.
Client Server
ClientHello +----------+
+ key_share* | Flight 1 |
+ pre_shared_key* --------> +----------+
+----------+
<-------- HelloRetryRequest | Flight 2 |
+ cookie +----------+
ClientHello +----------+
+ key_share* | Flight 3 |
+ pre_shared_key* --------> +----------+
+ cookie
ServerHello
+ key_share*
+ pre_shared_key* +----------+
{EncryptedExtensions} | Flight 4 |
{CertificateRequest*} +----------+
{Certificate*}
{CertificateVerify*}
<-------- {Finished}
[Application Data*]
{Certificate*} +----------+
{CertificateVerify*} | Flight 5 |
{Finished} --------> +----------+
[Application Data]
+----------+
<-------- [Ack] | Flight 6 |
[Application Data*] +----------+
[Application Data] <-------> [Application Data]
Figure 3: Message Flights for full DTLS Handshake (with Cookie Exchange)
ClientHello +----------+
+ pre_shared_key | Flight 1 |
+ key_share* --------> +----------+
ServerHello
+ pre_shared_key +----------+
+ key_share* | Flight 2 |
{EncryptedExtensions} +----------+
<-------- {Finished}
[Application Data*]
+----------+
{Finished} --------> | Flight 3 |
[Application Data*] +----------+
+----------+
<-------- [Ack] | Flight 4 |
[Application Data*] +----------+
[Application Data] <-------> [Application Data]
Figure 4: Message Flights for Resumption and PSK Handshake (without Cookie Exchange)
Client Server
ClientHello
+ early_data
+ psk_key_exchange_modes +----------+
+ key_share* | Flight 1 |
+ pre_shared_key +----------+
(Application Data*) -------->
ServerHello
+ pre_shared_key
+ key_share* +----------+
{EncryptedExtensions} | Flight 2 |
{Finished} +----------+
<-------- [Application Data*]
+----------+
(EndOfEarlyData) | Flight 3 |
{Finished} --------> +----------+
[Application Data*]
+----------+
<-------- [Ack] | Flight 4 |
[Application Data*] +----------+
[Application Data] <-------> [Application Data]
Figure 5: Message Flights for the Zero-RTT Handshake
Client Server
+----------+
<-------- [NewSessionTicket] | Flight 1 |
+----------+
+----------+
[Ack] --------> | Flight 2 |
+----------+
Figure 6: Message Flights for New Session Ticket Message
Note: The application data sent by the client is not included in the timeout and retransmission calculation.
DTLS uses a simple timeout and retransmission scheme with the state machine shown in Figure 7. Because DTLS clients send the first message (ClientHello), they start in the PREPARING state. DTLS servers start in the WAITING state, but with empty buffers and no retransmit timer.
+-----------+
| PREPARING |
+----------> | |
| | |
| +-----------+
| |
| | Buffer next flight
| |
| \|/
| +-----------+
| | |
| | SENDING |<------------------+
| | | |
| +-----------+ |
Receive | | |
next | | Send flight or partial |
flight | | flight |
| +---------------+ |
| | | Set retransmit timer |
| | \|/ |
| | +-----------+ |
| | | | |
+--)---------| WAITING |-------------------+
| | +----->| | Timer expires |
| | | +-----------+ |
| | | | | | |
| | | | | | |
| | +----------+ | +--------------------+
| | Receive record | Read retransmit or ACK
Receive | | Send ACK |
last | | |
flight | | | Receive ACK
| | | for last flight
\|/\|/ |
|
+-----------+ |
| | <---------+
| FINISHED |
| |
+-----------+
| /|\
| |
| |
+---+
Server read retransmit
Retransmit ACK
Figure 7: DTLS Timeout and Retransmission State Machine
The state machine has three basic states.
In the PREPARING state, the implementation does whatever computations are necessary to prepare the next flight of messages. It then buffers them up for transmission (emptying the buffer first) and enters the SENDING state.
In the SENDING state, the implementation transmits the buffered flight of messages. If the implementation has received one or more ACKs Section 7 from the peer, then it SHOULD omit any messages or message fragments which have already been ACKed. Once the messages have been sent, the implementation then enters the FINISHED state if this is the last flight in the handshake. Or, if the implementation expects to receive more messages, it sets a retransmit timer and then enters the WAITING state.
There are four ways to exit the WAITING state:
Because DTLS clients send the first message (ClientHello), they start in the PREPARING state. DTLS servers start in the WAITING state, but with empty buffers and no retransmit timer.
In addition, for at least twice the default Maximum Segment Lifetime (MSL) defined for [RFC0793], when in the FINISHED state, the server MUST respond to retransmission of the client’s second flight with a retransmit of its ACK.
Note that because of packet loss, it is possible for one side to be sending application data even though the other side has not received the first side’s Finished message. Implementations MUST either discard or buffer all application data packets for the new epoch until they have received the Finished message for that epoch. Implementations MAY treat receipt of application data with a new epoch prior to receipt of the corresponding Finished message as evidence of reordering or packet loss and retransmit their final flight immediately, shortcutting the retransmission timer.
Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems; for example, if many instances of a DTLS time out early and retransmit too quickly on a congested link. Implementations SHOULD use an initial timer value of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double the value at each retransmission, up to no less than the RFC 6298 maximum of 60 seconds. Application specific profiles, such as those used for the Internet of Things environment, may recommend longer timer values. Note that we recommend a 100 msec timer rather than the 3-second RFC 6298 default in order to improve latency for time-sensitive applications. Because DTLS only uses retransmission for handshake and not dataflow, the effect on congestion should be minimal.
Implementations SHOULD retain the current timer value until a transmission without loss occurs, at which time the value may be reset to the initial value. After a long period of idleness, no less than 10 times the current timer value, implementations may reset the timer to the initial value. One situation where this might occur is when a rehandshake is used after substantial data transfer.
CertificateVerify and Finished messages have the same format as in TLS 1.3. Hash calculations include entire handshake messages, including DTLS-specific fields: message_seq, fragment_offset, and fragment_length. However, in order to remove sensitivity to handshake message fragmentation, the CertificateVerify and the Finished messages MUST be computed as if each handshake message had been sent as a single fragment following the algorithm described in Section 4.4.3 and Section 4.4.4 of [I-D.ietf-tls-tls13], respectively.
Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected.
If a DTLS client-server pair is configured in such a way that repeated connections happen on the same host/port quartet, then it is possible that a client will silently abandon one connection and then initiate another with the same parameters (e.g., after a reboot). This will appear to the server as a new handshake with epoch=0. In cases where a server believes it has an existing association on a given host/port quartet and it receives an epoch=0 ClientHello, it SHOULD proceed with a new handshake but MUST NOT destroy the existing association until the client has demonstrated reachability either by completing a cookie exchange or by completing a complete handshake including delivering a verifiable Finished message. After a correct Finished message is received, the server MUST abandon the previous association to avoid confusion between two valid associations with overlapping epochs. The reachability requirement prevents off-path/blind attackers from destroying associations merely by sending forged ClientHellos.
Note: it is not always possible to distinguish which association a given packet is from. For instance, if the client performs a handshake, abandons the connection, and then immediately starts a new handshake, it may not be possible to tell which connection a given protected record is for. In these cases, trial decryption MAY be necessary, though implementations could also use some sort of connection identifier, such as the one specified in [I-D.rescorla-tls-dtls-connection-id].
The following is an example of a handshake with lost packets and retransmissions.
Client Server
------ ------
Record 0 -------->
ClientHello
(message_seq=0)
+cookie
X<----- Record 0
(lost) ServerHello
(message_seq=1)
EncryptedExtensions
(message_seq=2)
Certificate
(message_seq=3)
<-------- Record 1
CertificateVerify
(message_seq=4)
Finished
(message_seq=5)
Record 1 -------->
ACK [1]
<-------- Record 2
ServerHello
(message_seq=1)
EncryptedExtensions
(message_seq=2)
Certificate
(message_seq=3)
Record 2 -------->
Certificate
(message_seq=2)
CertificateVerify
(message_seq=3)
Finished
(message_seq=4)
<-------- Record 3
ACK [2]
Figure 8: Example DTLS Exchange illustrating Message Loss
A recipient of a DTLS message needs to select the correct keying material in order to process an incoming message. With the possibility of message loss and re-order an identifier is needed to determine which cipher state has been used to protect the record payload. The epoch value fulfills this role in DTLS. In addition to the key derivation steps described in Section 7 of [I-D.ietf-tls-tls13] triggered by the states during the handshake a sender may want to rekey at any time during the lifetime of the connection and has to have a way to indicate that it is updating its sending cryptographic keys.
This version of DTLS assigns dedicated epoch values to messages in the protocol exchange to allow identification of the correct cipher state:
Using these reserved epoch values a receiver knows what cipher state has been used to encrypt and integrity protect a message. Implementations that receive a payload with an epoch value for which no corresponding cipher state can be determined MUST generate a “unexpected_message” alert. For example, client incorrectly uses epoch value 5 when sending early application data in a 0-RTT exchange. A server will not be able to compute the appropriate keys and will therefore have to respond with an alert.
Note that epoch values do not wrap. If a DTLS implementation would need to wrap the epoch value, it MUST terminate the connection.
The traffic key calculation is described in Section 7.3 of [I-D.ietf-tls-tls13].
Figure 9 illustrates the epoch values in an example DTLS handshake.
Client Server
------ ------
ClientHello
(epoch=0)
-------->
<-------- HelloRetryRequest
(epoch=0)
ClientHello -------->
(epoch=0)
<-------- ServerHello
(epoch=0)
{EncryptedExtensions}
(epoch=2)
{Certificate}
(epoch=2)
{CertificateVerify}
(epoch=2)
{Finished}
(epoch=2)
{Certificate} -------->
(epoch=2)
{CertificateVerify}
(epoch=2)
{Finished}
(epoch=2)
<-------- [Ack]
(epoch=3)
[Application Data] -------->
(epoch=3)
<-------- [Application Data]
(epoch=3)
Some time later ...
(Post-Handshake Message Exchange)
<-------- [NewSessionTicket]
(epoch=3)
[Ack] -------->
(epoch=3)
Some time later ...
(Rekeying)
<-------- [Application Data]
(epoch=4)
[Application Data] -------->
(epoch=4)
Figure 9: Example DTLS Exchange with Epoch Information
The ACK message is used by an endpoint to indicate handshake-containing the TLS records it has received from the other side. ACK is not a handshake message but is rather a separate content type, with code point TBD (proposed, 25). This avoids it consuming space in the handshake message sequence. Note that ACKs can still be piggybacked on the same UDP datagram as handshake records.
struct {
uint64 record_numbers<0..2^16-1>;
} ACK;
ACK records MUST be sent with an epoch that is equal to or higher than the record which is being acknowledged. Implementations SHOULD simply use the current key.
When an implementation receives a partial flight, it SHOULD generate an ACK that covers the messages from that flight which it has received so far. Implementations have some discretion about when to generate ACKs, but it is RECOMMENDED that they do so under two circumstances:
In addition, implementations MUST send ACKs upon receiving all of any flight which they do not respond to with their own messages. Specifically, this means the client’s final flight of the main handshake, the server’s transmission of the NewSessionTicket, and KeyUpdate messages. ACKs SHOULD NOT be sent for other complete flights because they are implicitly acknowledged by the receipt of the next flight, which generally immediately follows the flight. Each NewSessionTicket or KeyUpdate is an individual flight; in particular, a KeyUpdate sent in response to a KeyUpdate with update_requested does not implicitly acknowledge that message. Implementations MAY ACK the records corresponding to each transmission of that flight or simply ACK the most recent one.
ACKs MUST NOT be sent for other records of any content type other than handshake or for records which cannot be unprotected.
Note that in some cases it may be necessary to send an ACK which does not contain any record numbers. For instance, a client might receive an EncryptedExtensions message prior to receiving a ServerHello. Because it cannot decrypt the EncryptedExtensions, it cannot safely ACK 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.
When an implementation receives an ACK, it SHOULD record that the messages or message fragments sent in the records being ACKed were received and omit them from any future retransmissions. Upon receipt of an ACK for only some messages from a flight, an implementation SHOULD retransmit the remaining messages or fragments. Note that this requires implementations to track which messages appear in which records. Once all the messages in a flight have been acknowledged, the implementation MUST cancel all retransmissions of that flight. As noted above, the receipt of any packet responding to a given flight MUST be taken as an implicit ACK for the entire flight.
DTLS 1.3 implementations MUST send a KeyUpdate message prior to updating the keys they are using to protect application data traffic. As with other handshake messages with no built-in response, KeyUpdates MUST be acknowledged. In order to facilitate epoch reconstruction Section 4.1.2 implementations MUST NOT send a new KeyUpdate until the previous KeyUpdate has been acknowledged (this avoids having too many epochs in active use).
Due to loss and/or re-ordering, DTLS 1.3 implementations may receive a record with a different epoch than the current one. They SHOULD attempt to process those records with that epoch (see Section 4.1.2 for information on determining the correct epoch), but MAY opt to discard such out-of-epoch records. Implementations SHOULD NOT discard the keys for their current epoch prior to receiving a KeyUpdate.
Although KeyUpdate MUST be ACKed, it is possible for the ACK to be lost, in which case the sender of the KeyUpdate will retransmit it. Implementations MUST retain the ability to ACK the KeyUpdate for up to 2MSL. It is RECOMMENDED that they do so by retaining the pre-update keying material, but they MAY do so by responding to messages which appear to be out-of-epoch with a canned ACK message; in this case, implementations SHOULD rate limit how often they send such ACKs.
Application data messages are carried by the record layer and are fragmented and encrypted based on the current connection state. The messages are treated as transparent data to the record layer.
Security issues are discussed primarily in [I-D.ietf-tls-tls13].
The primary additional security consideration raised by DTLS is that of denial of service. DTLS includes a cookie exchange designed to protect against denial of service. However, implementations that do not use this cookie exchange are still vulnerable to DoS. In particular, DTLS servers that do not use the cookie exchange may be used as attack amplifiers even if they themselves are not experiencing DoS. Therefore, DTLS servers SHOULD use the cookie exchange unless there is good reason to believe that amplification is not a threat in their environment. Clients MUST be prepared to do a cookie exchange with every handshake.
Unlike TLS implementations, DTLS implementations SHOULD NOT respond to invalid records by terminating the connection.
If implementations process out-of-epoch records as recommended in Section 8, then this creates a denial of service risk since an adversary could inject packets with fake epoch values, forcing the recipient to compute the next-generation application_traffic_secret using the HKDF-Expand-Label construct to only find out that the message was does not pass the AEAD cipher processing. The impact of this attack is small since the HKDF-Expand-Label only performs symmetric key hashing operations. Implementations which are concerned about this form of attack can discard out-of-epoch records.
Since TLS 1.3 introduces a large number of changes to TLS 1.2, the list of changes from DTLS 1.2 to DTLS 1.3 is equally large. For this reason this section focuses on the most important changes only.
IANA is requested to allocate a new value in the TLS ContentType Registry for the ACK message defined in Section 7, with content type 25.
| [I-D.ietf-tls-tls13] | Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-21, July 2017. |
| [RFC0768] | Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980. |
| [RFC0793] | Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981. |
| [RFC1191] | Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4443] | Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006. |
| [RFC4821] | Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007. |
| [RFC6298] | Paxson, V., Allman, M., Chu, J. and M. Sargent, "Computing TCP's Retransmission Timer", RFC 6298, DOI 10.17487/RFC6298, June 2011. |
RFC EDITOR: PLEASE REMOVE THE THIS SECTION
IETF Drafts draft-03 - Only update keys after KeyUpdate is ACKed.
draft-02 - Shorten the protected record header and introduce an ultra-short version of the record header. - Reintroduce KeyUpdate, which works properly now that we have ACK. - Clarify the ACK rules.
draft-01 - Restructured the ACK to contain a list of packets and also be a record rather than a handshake message.
draft-00 - First IETF Draft
Personal Drafts draft-01 - Alignment with version -19 of the TLS 1.3 specification
draft-00
The discussion list for the IETF TLS working group is located at the e-mail address tls@ietf.org. Information on the group and information on how to subscribe to the list is at https://www1.ietf.org/mailman/listinfo/tls
Archives of the list can be found at: https://www.ietf.org/mail-archive/web/tls/current/index.html
Many people have contributed to previous DTLS versions and they are acknowledged in prior versions of DTLS specifications.
For this version of the document we would like to thank:
* Ilari Liusvaara Independent ilariliusvaara@welho.com
* Martin Thomson Mozilla martin.thomson@gmail.com