The Datagram Transport Layer Security (DTLS) Protocol Version 1.3RTFM, Inc.ekr@rtfm.comARM Limitedhannes.tschofenig@arm.comGoogle, Inc.nagendra@cs.stanford.edu
Security
TLSInternet-DraftThis 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.RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPHThe 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 .There are applications that utilize UDP as a transport and to offer communication
security protection for those applications the Datagram Transport Layer
Security (DTLS) protocol has been designed. DTLS is deliberately designed to be
as similar to TLS as possible, both to minimize new security invention and to
maximize the amount of code and infrastructure reuse.DTLS 1.0 was originally defined as a delta from TLS 1.1 and
DTLS 1.2 was defined as a series of deltas to TLS 1.2 . There
is no DTLS 1.1; that version number was skipped in order to harmonize version numbers
with TLS. This specification describes the most current version of the DTLS protocol
aligning with the efforts around TLS 1.3 .Implementations that speak both DTLS 1.2 and DTLS 1.3 can interoperate with those
that speak only DTLS 1.2 (using DTLS 1.2 of course), just as TLS 1.3 implementations
can interoperate with TLS 1.2 (see Appendix D of 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 .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 .The following terms are used:client: The endpoint initiating the DTLS connection.connection: A transport-layer connection between two endpoints.endpoint: Either the client or server of the connection.handshake: An initial negotiation between client and server that establishes
the parameters of their transactions.peer: An endpoint. When discussing a particular endpoint, “peer” refers to
the endpoint that is remote to the primary subject of discussion.receiver: An endpoint that is receiving records.sender: An endpoint that is transmitting records.session: An association between a client and a server resulting from a handshake.server: The endpoint which did not initiate the DTLS connection.The reader is assumed to be familiar with the TLS 1.3 specification since this
document defined as a delta from TLS 1.3.Figures in this document illustrate various combinations of the DTLS protocol exchanges and the symbols have the following meaning:’+’ indicates noteworthy extensions sent in the previously noted message.‘*’ indicates optional or situation-dependent messages/extensions that are not always sent.’{}’ indicates messages protected using keys derived from a [sender]_handshake_traffic_secret.’[]’ indicates messages protected using keys derived from traffic_secret_N.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:TLS does not allow independent decryption of individual records.
Because the integrity check indirectly depends on a sequence number,
if record N is not received, then the integrity check
on record N+1 will be based on the wrong sequence number and
thus will fail. DTLS solves this problem by adding explicit
sequence numbers.The TLS handshake is a lock-step cryptographic handshake.
Messages must be transmitted and received in a defined order;
any other order is an error.
This is incompatible with reordering and message loss.Not all TLS 1.3 handshake messages (such as the NewSessionTicket message)
are acknowledged. Hence, a new acknowledgement message has to be added
to detect message loss.Handshake messages are potentially larger than any given datagram,
thus creating the problem of IP fragmentation.Datagram transport protocols, like UDP, are susceptible to abusive behavior
effecting denial of
service attacks against nonparticipants,
and require a return-routability check with the help of
cookies to be integrated into the handshake. A detailed discussion of
countermeasures can be found in .DTLS uses a simple retransmission timer to handle packet loss.
demonstrates the basic concept, using the first
phase of the DTLS handshake: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 DTLSCiphertext structure omits the superfluous version number fieldDTLS adds an explicit epoch and sequence number
in the record header. This sequence number allows the recipient to correctly
verify the DTLS MAC.DTLS adds a short header format (DTLSShortCiphertext) that can be
used to reduce overhead once the handshake is complete.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.
The content type of the record.
The low order two bits of the epoch and the low order 30 bits of
the sequence number, laid out as a 32 bit integer.
The first 2 bits hold the low order bits from the epoch and the
remaining 30 bits hold the low order bits from the sequence number
(see for how to use this value).
Identical to the length field in a TLS 1.3 record.
Identical to the encrypted_record 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.The short DTLS header format is:The short_epoch_and_sequence document contains the epoch and sequence
packed into a 16 bit integer as follows:The first three bits are set to 001 in order to allow multiplexing
between DTLS and VoIP protocols (STUN, RTP/RTCP, etc.)
and distinguish the short from long header formats.The fourth bit is the low order bit of the epoch value.The remaining bits contain the low order 12 bits of the sequence
number.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 . 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 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 , 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.
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: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 or ICMPv6
“Packet Too Big” indications.The DTLS handshake messages can exceed the PMTU.In order to deal with the first two issues, the DTLS record layer
SHOULD behave as described below.If PMTU estimates are available from the underlying transport
protocol, they should be made available to upper layer
protocols. In particular:For DTLS over UDP, the upper layer protocol SHOULD be allowed to
obtain the PMTU estimate maintained in the IP layer.For DTLS over DCCP, the upper layer protocol SHOULD be allowed to
obtain the current estimate of the PMTU.For DTLS over TCP or SCTP, which automatically fragment and
reassemble datagrams, there is no PMTU limitation. However, the
upper layer protocol MUST NOT write any record that exceeds the
maximum record size of 2^14 bytes.The DTLS record layer SHOULD allow the upper layer protocol to
discover the amount of record expansion expected by the DTLS
processing.If there is a transport protocol indication (either via ICMP or via a
refusal to send the datagram as in Section 14 of ), 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 or
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.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 .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 .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:To handle message loss, reordering, and fragmentation modifications to
the handshake header are necessary.Retransmission timers are introduced to handle message loss.A new ACK content type has been added for reliable message delivery of handshake messages.Note that TLS 1.3 already supports a cookie extension, which used to
prevent denial-of-service attacks. This DoS prevention mechanism is
described in more detail below since UDP-based protocols are more vulnerable
to amplification attacks than a connection-oriented 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: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.An attacker can use the server as an amplifier by sending
connection initiation messages with a forged source of the
victim. The server then sends its response to the victim
machine, thus flooding it. Depending on the selected
ciphersuite this response message can be quite large, as it
is the case for a Certificate message.In order to counter both of these attacks, DTLS borrows the stateless
cookie technique used by Photuris and IKE . 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 . 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 . Note that
the figure focuses on the cookie exchange; all other extensions
are omitted.The cookie extension is defined in Section 4.2.2 of . 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 .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 .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. 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 ,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: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.
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 ) and the
legacy_version field MUST be set to {254, 253}, which was the version
number for DTLS 1.2.
Same as for TLS 1.3
Same as for TLS 1.3
A DTLS 1.3-only client MUST set the legacy_cookie field to zero length.
Same as for TLS 1.3
Same as for TLS 1.3
Same as for TLS 1.3Each 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.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 .
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.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 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:The retransmit timer expires: the implementation transitions to
the SENDING state, where it retransmits the flight, resets the
retransmit timer, and returns to the WAITING state.The implementation reads a ACK from the peer: upon receiving
an ACK for a partial flight (as mentioned in ,
the implementation transitions
to the SENDING state, where it retransmits the unacked portion
of the flight, resets the retransmit timer, and returns to the
WAITING state. Upon receiving an ACK for a complete flight,
the implementation cancels all retransmissions and either
remains in WAITING, or, if the ACK was for the final flight,
transitions to FINISHED.The implementation reads a retransmitted flight from the peer: the
implementation transitions to the SENDING state, where it
retransmits the flight, resets the retransmit timer, and returns
to the WAITING state. The rationale here is that the receipt of a
duplicate message is the likely result of timer expiry on the peer
and therefore suggests that part of one’s previous flight was
lost.The implementation receives some or all next flight of messages: if
this is the final flight of messages, the implementation
transitions to FINISHED. If the implementation needs to send a new
flight, it transitions to the PREPARING state. Partial reads
(whether partial messages or only some of the messages in the
flight) may also trigger the implementation to send an ACK, as
described in .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 , 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 ) 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 , 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
.The following is an example of a handshake with lost packets and
retransmissions.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 triggered by the states during the handshake
a sender may want to rekey at any time during
the lifetime of the connection and has to have a way to indicate that it is
updating its sending cryptographic keys.This version of DTLS assigns dedicated epoch values to messages in the
protocol exchange to allow identification of the correct cipher state:epoch value (0) is used with unencrypted messages. There are
three unencrypted messages in DTLS, namely ClientHello, ServerHello,
and HelloRetryRequest.epoch value (1) is used for messages protected using keys derived
from early_traffic_secret. This includes early data sent by the
client and the EndOfEarlyData message.epoch value (2) is used for messages protected using keys derived
from the handshake_traffic_secret. Messages transmitted during
the initial handshake, such as EncryptedExtensions,
CertificateRequest, Certificate, CertificateVerify, and Finished
belong to this category. Note, however, post-handshake are
protected under the appropriate application traffic key and are not included in this category.epoch value (3) is used for payloads protected using keys derived
from the initial traffic_secret_0. This may include handshake
messages, such as post-handshake messages (e.g., a
NewSessionTicket message).epoch value (4 to 2^16-1) is used for payloads protected using keys from the traffic_secret_N (N>0).Using these reserved epoch values a receiver knows what cipher state
has been used to encrypt and integrity protect a
message. Implementations that receive a payload with an epoch value
for which no corresponding cipher state can be determined MUST
generate a “unexpected_message” alert. For example, client incorrectly
uses epoch value 5 when sending early application data in a 0-RTT
exchange. A server will not be able to compute the appropriate keys
and will therefore have to respond with an alert.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 . illustrates the epoch values in an example DTLS handshake.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.
a list of the records containing handshake messages in the current
flight which the endpoint has received, in numerically increasing
order. ACKs only cover the current outstanding flight (this is
possible because DTLS is generally a lockstep protocol). Thus, an ACK
from the server would not cover both the ClientHello and the client’s
Certificate. Implementations can accomplish this by clearing their ACK
list upon receiving the start of the next flight.ACK records MUST be sent with an epoch that is equal to or higher
than the record which is being acknowledged. Implementations SHOULD
simply use the current key.When an implementation receives a partial flight, it SHOULD generate
an ACK that covers the messages from that flight which it has
received so far. Implementations have some discretion about when
to generate ACKs, but it is RECOMMENDED that they do so under
two circumstances:When they receive a message or fragment which is out of order,
either because it is not the next expected message or because
it is not the next piece of the current message. Implementations
MUST NOT send ACKs for handshake messages which they discard
as out-of-order, because otherwise those messages will not be
retransmitted.When they have received part of a flight and do not immediately
receive the rest of the flight (which may be in the same UDP
datagram). A reasonable approach here is to
set a timer for 1/4 the current retransmit timer value when
the first record in the flight is received and then send an
ACK when that timer expires.In addition, implementations MUST send ACKs upon receiving
all of any flight which they do not respond to with their
own messages. Specifically, this means the client’s final
flight of the main handshake, the server’s transmission
of the NewSessionTicket, and KeyUpdate messages.
ACKs SHOULD NOT be sent for other
complete flights because they are implicitly acknowledged
by the receipt of the next flight, which generally
immediately follows the flight. Each NewSessionTicket
or KeyUpdate is an individual flight; in particular, a KeyUpdate
sent in response to a KeyUpdate with update_requested does not
implicitly acknowledge that message. Implementations MAY
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 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 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 .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
, 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.New handshake pattern, which leads to a shorter message exchangeSupport for AEAD-only ciphersHelloRetryRequest of TLS 1.3 used instead of HelloVerifyRequestMore flexible ciphersuite negotiationNew session resumption mechanismPSK authentication redefinedNew key derivation hierarchy utilizing a new key derivation constructRemoved support for weaker and older cryptographic algorithmsImproved version negotationIANA is requested to allocate a new value in the TLS ContentType Registry for the ACK message defined in , with content type 25.User Datagram ProtocolKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Path MTU discoveryThis memo describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. It specifies a small change to the way routers generate one type of ICMP message. For a path that passes through a router that has not been so changed, this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by current practice. [STANDARDS-TRACK]Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) SpecificationThis document describes the format of a set of control messages used in ICMPv6 (Internet Control Message Protocol). ICMPv6 is the Internet Control Message Protocol for Internet Protocol version 6 (IPv6). [STANDARDS-TRACK]Packetization Layer Path MTU DiscoveryThis document describes a robust method for Path MTU Discovery (PMTUD) that relies on TCP or some other Packetization Layer to probe an Internet path with progressively larger packets. This method is described as an extension to RFC 1191 and RFC 1981, which specify ICMP-based Path MTU Discovery for IP versions 4 and 6, respectively. [STANDARDS-TRACK]Transmission Control ProtocolComputing TCP's Retransmission TimerThis document defines the standard algorithm that Transmission Control Protocol (TCP) senders are required to use to compute and manage their retransmission timer. It expands on the discussion in Section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST. This document obsoletes RFC 2988. [STANDARDS-TRACK]The Transport Layer Security (TLS) Protocol Version 1.3This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.Internet Key Exchange Protocol Version 2 (IKEv2)This document describes version 2 of the Internet Key Exchange (IKE) protocol. IKE is a component of IPsec used for performing mutual authentication and establishing and maintaining Security Associations (SAs). This document replaces and updates RFC 4306, and includes all of the clarifications from RFC 4718. [STANDARDS-TRACK]Photuris: Session-Key Management ProtocolThis document defines the basic protocol mechanisms. This document defines an Experimental Protocol for the Internet community.IP Encapsulating Security Payload (ESP)This document describes an updated version of the Encapsulating Security Payload (ESP) protocol, which is designed to provide a mix of security services in IPv4 and IPv6. ESP is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality. This document obsoletes RFC 2406 (November 1998). [STANDARDS-TRACK]Datagram Congestion Control Protocol (DCCP)The Datagram Congestion Control Protocol (DCCP) is a transport protocol that provides bidirectional unicast connections of congestion-controlled unreliable datagrams. DCCP is suitable for applications that transfer fairly large amounts of data and that can benefit from control over the tradeoff between timeliness and reliability. [STANDARDS-TRACK]The Transport Layer Security (TLS) Protocol Version 1.1This document specifies Version 1.1 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]Datagram Transport Layer SecurityThis document specifies Version 1.0 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. [STANDARDS-TRACK]Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP)This document specifies the use of Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP). DTLS provides communications privacy for applications that use datagram transport protocols and allows client/server applications to communicate in a way that is designed to prevent eavesdropping and detect tampering or message forgery. DCCP is a transport protocol that provides a congestion-controlled unreliable datagram service. [STANDARDS-TRACK]The Transport Layer Security (TLS) Protocol Version 1.2This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]Datagram Transport Layer Security Version 1.2This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document updates DTLS 1.0 to work with TLS version 1.2. [STANDARDS-TRACK]Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, including attacks on its most commonly used cipher suites and their modes of operation. This document provides recommendations for improving the security of deployed services that use TLS and DTLS. The recommendations are applicable to the majority of use cases.Multiplexing Scheme Updates for Secure Real-time Transport Protocol (SRTP) Extension for Datagram Transport Layer Security (DTLS)This document defines how Datagram Transport Layer Security (DTLS), Real-time Transport Protocol (RTP), RTP Control Protocol (RTCP), Session Traversal Utilities for NAT (STUN), Traversal Using Relays around NAT (TURN), and ZRTP packets are multiplexed on a single receiving socket. It overrides the guidance from RFC 5764 ("SRTP Extension for DTLS"), which suffered from four issues described and fixed in this document.This document updates RFC 5764.The Datagram Transport Layer Security (DTLS) Connection IdentifierThis document specifies the "Connection ID" concept for the Datagram Transport Layer Security (DTLS) protocol, version 1.2 and version 1.3. A Connection ID is an identifier carried in the record layer header that gives the recipient additional information for selecting the appropriate security association. In "classical" DTLS, selecting a security association of an incoming DTLS record is accomplished with the help of the 5-tuple. If the source IP address and/or source port changes during the lifetime of an ongoing DTLS session then the receiver will be unable to locate the correct security context.RFC EDITOR: PLEASE REMOVE THE THIS SECTIONIETF 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 DraftPersonal Drafts
draft-01
- Alignment with version -19 of the TLS 1.3 specificationdraft-00Initial version using TLS 1.3 as a baseline.Use of epoch values instead of KeyUpdate messageUse of cookie extension instead of cookie field in
ClientHello and HelloVerifyRequest messagesAdded ACK messageText about sequence number handlingThe 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/tlsArchives of the list can be found at:
https://www.ietf.org/mail-archive/web/tls/current/index.htmlMany 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: