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RFC 6347 - 4.1. Record Layer

4. Differences from TLS​

As mentioned in Section 3, DTLS is intentionally very similar to TLS. Therefore, instead of presenting DTLS as a new protocol, we present it as a series of deltas from TLS 1.2 [TLS12]. Where we do not explicitly call out differences, DTLS is the same as in [TLS12].

4.1. Record Layer​

The DTLS record layer is extremely similar to that of TLS 1.2. The only change is the inclusion of an explicit sequence number in the record. This sequence number allows the recipient to correctly verify the TLS MAC. The DTLS record format is shown below:

struct {
     ContentType type;
     ProtocolVersion version;
     uint16 epoch;                                    // New field
     uint48 sequence_number;                          // New field
     uint16 length;
     opaque fragment[DTLSPlaintext.length];
   } DTLSPlaintext;

type

Equivalent to the type field in a TLS 1.2 record.

version

The version of the protocol being employed. This document describes DTLS version 1.2, which uses the version { 254, 253 }. The version value of 254.253 is the 1's complement of DTLS version 1.2. This maximal spacing between TLS and DTLS version numbers ensures that records from the two protocols can be easily distinguished. It should be noted that future on-the-wire version numbers of DTLS are decreasing in value (while the true version number is increasing in value.)

epoch

A counter value that is incremented on every cipher state change.

sequence_number

The sequence number for this record.

length

Identical to the length field in a TLS 1.2 record. As in TLS 1.2, the length should not exceed 2^14.

fragment

Identical to the fragment field of a TLS 1.2 record.

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.

If several handshakes are performed in close succession, there might be multiple records on the wire with the same sequence number but from different cipher states. The epoch field allows recipients to distinguish such packets. The epoch number is initially zero and is incremented each time a ChangeCipherSpec message is sent. 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. In practice, TLS implementations rarely rehandshake; therefore, we do not expect this to be a problem.

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 [TCP] to allow for packet reordering. (Note that the intention here is that implementors 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.

Note that in the special case of a rehandshake on an existing association, it is safe to process a data packet immediately, even if the ChangeCipherSpec or Finished messages have not yet been received provided that either the rehandshake resumes the existing session or that it uses exactly the same security parameters as the existing association. In any other case, the implementation MUST wait for the receipt of the Finished message to prevent downgrade attack.

As in TLS, implementations MUST either abandon an association or rehandshake prior to allowing the sequence number to wrap.

Similarly, implementations MUST NOT allow the epoch to wrap, but instead MUST establish a new association, terminating the old association as described in Section 4.2.8. In practice, implementations rarely rehandshake repeatedly on the same channel, so this is not likely to be an issue.

4.1.1. Transport Layer Mapping​

Each DTLS record MUST fit within a single datagram. In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer.

Note that unlike IPsec, DTLS records do not contain any association identifiers. Applications must arrange to multiplex between associations. With UDP, this is presumably done with the host/port number.

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 [DCCP] 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. In the future, extensions to DTLS may be specified that allow the use of only one set of sequence numbers for deployment in constrained environments.

Some transports, such as DCCP, 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. [DCCPDTLS] defines a mapping of DTLS to DCCP that takes these issues into account.

4.1.1.1. PMTU Issues​

In general, DTLS's philosophy is to leave PMTU discovery to the application. However, DTLS cannot completely ignore PMTU for three reasons:

  • The DTLS record framing expands the datagram size, thus lowering the effective PMTU from the application's perspective.

  • In some implementations, the application may not directly talk to the network, in which case the DTLS stack may absorb ICMP [RFC1191] "Datagram Too Big" indications or ICMPv6 [RFC4443] "Packet Too Big" indications.

  • The DTLS handshake messages can exceed the PMTU.

In order to deal with the first two issues, the DTLS record layer SHOULD behave as described below.

If PMTU estimates are available from the underlying transport protocol, they should be made available to upper layer protocols. In particular:

  • For DTLS over UDP, the upper layer protocol SHOULD be allowed to obtain the PMTU estimate maintained in the IP layer.

  • For DTLS over DCCP, the upper layer protocol SHOULD be allowed to obtain the current estimate of the PMTU.

  • For DTLS over TCP or SCTP, which automatically fragment and reassemble datagrams, there is no PMTU limitation. However, the upper layer protocol MUST NOT write any record that exceeds the maximum record size of 2^14 bytes.

The DTLS record layer SHOULD allow the upper layer protocol to discover the amount of record expansion expected by the DTLS processing. Note that this number is only an estimate because of block padding and the potential use of DTLS compression.

If there is a transport protocol indication (either via ICMP or via a refusal to send the datagram as in Section 14 of [DCCP]), then the DTLS record layer MUST inform the upper layer protocol of the error.

The DTLS record layer SHOULD NOT interfere with upper layer protocols performing PMTU discovery, whether via [RFC1191] or [RFC4821] mechanisms. In particular:

  • Where allowed by the underlying transport protocol, the upper layer protocol SHOULD be allowed to set the state of the DF bit (in IPv4) or prohibit local fragmentation (in IPv6).

  • If the underlying transport protocol allows the application to request PMTU probing (e.g., DCCP), the DTLS record layer should honor this request.

The final issue is the DTLS handshake protocol. From the perspective of the DTLS record layer, this is merely another upper layer protocol. However, DTLS handshakes occur infrequently and involve only a few round trips; therefore, the handshake protocol PMTU handling places a premium on rapid completion over accurate PMTU discovery. In order to allow connections under these circumstances, DTLS implementations SHOULD follow the following rules:

  • If the DTLS record layer informs the DTLS handshake layer that a message is too big, it SHOULD immediately attempt to fragment it, using any existing information about the PMTU.

  • If repeated retransmissions do not result in a response, and the PMTU is unknown, subsequent retransmissions SHOULD back off to a smaller record size, fragmenting the handshake message as appropriate. This standard does not specify an exact number of retransmits to attempt before backing off, but 2-3 seems appropriate.

4.1.2. 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.

4.1.2.1. MAC​

The DTLS MAC is the same as that of TLS 1.2. However, rather than using TLS's implicit sequence number, the sequence number used to compute the MAC is the 64-bit value formed by concatenating the epoch and the sequence number in the order they appear on the wire. Note that the DTLS epoch + sequence number is the same length as the TLS sequence number.

TLS MAC calculation is parameterized on the protocol version number, which, in the case of DTLS, is the on-the-wire version, i.e., {254, 253} for DTLS 1.2.

Note that one important difference between DTLS and TLS MAC handling is that in TLS, MAC errors must result in connection termination. In DTLS, the receiving implementation MAY simply discard the offending record and continue with the connection. This change is possible because DTLS records are not dependent on each other in the way that TLS records are.

In general, DTLS implementations SHOULD silently discard records with bad MACs or that are otherwise invalid. They MAY log an error. If a DTLS implementation chooses to generate an alert when it receives a message with an invalid MAC, it MUST generate a bad_record_mac alert with level fatal and terminate its connection state. Note that because errors do not cause connection termination, DTLS stacks are more efficient error type oracles than TLS stacks. Thus, it is especially important that the advice in Section 6.2.3.2 of [TLS12] be followed.

4.1.2.2. Null or Standard Stream Cipher​

The DTLS NULL cipher is performed exactly as the TLS 1.2 NULL cipher.

The only stream cipher described in TLS 1.2 is RC4, which cannot be randomly accessed. RC4 MUST NOT be used with DTLS.

4.1.2.3. Block Cipher​

DTLS block cipher encryption and decryption are performed exactly as with TLS 1.2.

4.1.2.4. AEAD Ciphers​

TLS 1.2 introduced authenticated encryption with additional data (AEAD) cipher suites. The existing AEAD cipher suites, defined in [ECCGCM] and [RSAGCM], can be used with DTLS exactly as with TLS 1.2.

4.1.2.5. New Cipher Suites​

Upon registration, new TLS cipher suites MUST indicate whether they are suitable for DTLS usage and what, if any, adaptations must be made (see Section 7 for IANA considerations).

4.1.2.6. Anti-Replay​

DTLS records contain a sequence number to provide replay protection. Sequence number verification SHOULD be performed using the following sliding window procedure, borrowed from Section 3.4.3 of [ESP].

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 [ESP].

If the received record falls within the window and is new, or if the packet is to the right of the window, then the receiver proceeds to MAC verification. If the MAC validation fails, the receiver MUST discard the received record as invalid. The receive window is updated only if the MAC verification succeeds.

4.1.2.7. Handling Invalid Records​

Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. Implementations which choose to generate an alert instead, MUST generate fatal level alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports.

If DTLS is being carried over a transport that is resistant to forgery (e.g., SCTP with SCTP-AUTH), then it is safer to send alerts because an attacker will have difficulty forging a datagram that will not be rejected by the transport layer.

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