| Internet-Draft | DTLS 1.2 Connection ID | November 2020 |
| Rescorla, et al. | Expires 6 May 2021 | [Page] |
This document specifies the Connection ID (CID) construct for the Datagram Transport Layer Security (DTLS) protocol version 1.2.¶
A CID 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.¶
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The Datagram Transport Layer Security (DTLS) [RFC6347] protocol was designed for securing connection-less transports, like UDP. DTLS, like TLS, starts with a handshake, which can be computationally demanding (particularly when public key cryptography is used). After a successful handshake, symmetric key cryptography is used to apply data origin authentication, integrity and confidentiality protection. This two-step approach allows endpoints to amortize the cost of the initial handshake across subsequent application data protection. Ideally, the second phase where application data is protected lasts over a long period of time since the established keys will only need to be updated once the key lifetime expires.¶
In DTLS as specified in RFC 6347, the IP address and port of the peer are used to identify the DTLS association. Unfortunately, in some cases, such as NAT rebinding, these values are insufficient. This is a particular issue in the Internet of Things when devices enter extended sleep periods to increase their battery lifetime. The NAT rebinding leads to connection failure, with the resulting cost of a new handshake.¶
This document defines an extension to DTLS 1.2 to add a CID to the DTLS record layer. The presence of the CID is negotiated via a DTLS extension.¶
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].¶
This document defines the "connection_id" extension, which is used in ClientHello and ServerHello messages.¶
The extension type is specified as follows.¶
enum {
connection_id(TBD1), (65535)
} ExtensionType;
¶
The extension_data field of this extension, when included in the ClientHello, MUST contain the ConnectionId structure. This structure contains the CID value the client wishes the server to use when sending messages to the client. A zero-length CID value indicates that the client is prepared to send with a CID but does not wish the server to use one when sending. Alternatively, this can be interpreted as the client wishes the server to use a zero-length CID; the result is the same.¶
struct {
opaque cid<0..2^8-1>;
} ConnectionId;
¶
A server willing to use CIDs will respond with a "connection_id" extension in the ServerHello, containing the CID it wishes the client to use when sending messages towards it. A zero-length value indicates that the server will send with the client's CID but does not wish the client to include a CID (or again, alternately, to use a zero-length CID).¶
Because each party sends the value in the "connection_id" extension it wants to receive as a CID in encrypted records, it is possible for an endpoint to use a globally constant length for such connection identifiers. This can in turn ease parsing and connection lookup, for example by having the length in question be a compile-time constant. Such implementations MUST still be able to send CIDs of different length to other parties. Implementations that want to use variable-length CIDs are responsible for constructing the CID in such a way that its length can be determined on reception. Note that there is no CID length information included in the record itself.¶
In DTLS 1.2, CIDs are exchanged at the beginning of the DTLS session only. There is no dedicated "CID update" message that allows new CIDs to be established mid-session, because DTLS 1.2 in general does not allow TLS 1.3-style post-handshake messages that do not themselves begin other handshakes. When a DTLS session is resumed or renegotiated, the "connection_id" extension is negotiated afresh.¶
If DTLS peers have not negotiated the use of CIDs then the RFC 6347-defined record format and content type MUST be used.¶
If DTLS peers have negotiated the use of a CIDs using the ClientHello and the ServerHello messages then the peers need to take the following steps.¶
The DTLS peers determine whether incoming and outgoing messages need to use the new record format, i.e., the record format containing the CID. The new record format with the the tls12_cid content type is only used once encryption is enabled. Plaintext payloads never use the new record type and the CID content type.¶
For sending, if a zero-length CID has been negotiated then the RFC 6347-defined record format and content type MUST be used (see Section 4.1 of [RFC6347]) else the new record layer format with the tls12_cid content type defined in Figure 3 MUST be used.¶
When transmitting a datagram with the tls12_cid content type, the new MAC computation defined in Section 5 MUST be used.¶
For receiving, if the tls12_cid content type is set, then the CID is used to look up the connection and the security association. If the tls12_cid content type is not set, then the connection and security association is looked up by the 5-tuple and a check MUST be made to determine whether the expected CID value is indeed zero length. If the check fails, then the datagram MUST be dropped.¶
When receiving a datagram with the tls12_cid content type, the new MAC computation defined in Section 5 MUST be used. When receiving a datagram with the RFC 6347-defined record format the MAC calculation defined in Section 4.1.2 of [RFC6347] MUST be used.¶
This specification defines the DTLS 1.2 record layer format and [I-D.ietf-tls-dtls13] specifies how to carry the CID in DTLS 1.3.¶
To allow a receiver to determine whether a record has a CID or not, connections which have negotiated this extension use a distinguished record type tls12_cid(TBD2). Use of this content type has the following three implications:¶
Plaintext records are not impacted by this extension. Hence, the format of the DTLSPlaintext structure is left unchanged, as shown in Figure 1.¶
struct {
ContentType type;
ProtocolVersion version;
uint16 epoch;
uint48 sequence_number;
uint16 length;
opaque fragment[DTLSPlaintext.length];
} DTLSPlaintext;
When CIDs are being used, the content to be sent is first wrapped along with its content type and optional padding into a DTLSInnerPlaintext structure. This newly introduced structure is shown in Figure 2. The DTLSInnerPlaintext byte sequence is then encrypted. To create the DTLSCiphertext structure shown in Figure 3 the CID is added.¶
struct {
opaque content[length];
ContentType real_type;
uint8 zeros[length_of_padding];
} DTLSInnerPlaintext;
struct {
ContentType outer_type = tls12_cid;
ProtocolVersion version;
uint16 epoch;
uint48 sequence_number;
opaque cid[cid_length]; // New field
uint16 length;
opaque enc_content[DTLSCiphertext.length];
} DTLSCiphertext;
All other fields are as defined in RFC 6347.¶
Several types of ciphers have been defined for use with TLS and DTLS and the MAC calculations for those ciphers differ slightly.¶
This specification modifies the MAC calculation as defined in [RFC6347] and [RFC7366], as well as the definition of the additional data used with AEAD ciphers provided in [RFC6347], for records with content type tls12_cid. The modified algorithm MUST NOT be applied to records that do not carry a CID, i.e., records with content type other than tls12_cid.¶
The following fields are defined in this document; all other fields are as defined in the cited documents.¶
The length (in bytes) of the serialised DTLSInnerPlaintext (two-byte integer).¶
The length MUST NOT exceed 2^14.¶
Note "+" denotes concatenation.¶
The following MAC algorithm applies to block ciphers that do not use the with Encrypt-then-MAC processing described in [RFC7366].¶
MAC(MAC_write_key, seq_num +
tls12_cid +
DTLSCiphertext.version +
cid +
cid_length +
length_of_DTLSInnerPlaintext +
DTLSInnerPlaintext.content +
DTLSInnerPlaintext.real_type +
DTLSInnerPlaintext.zeros
)
¶
The following MAC algorithm applies to block ciphers that use the with Encrypt-then-MAC processing described in [RFC7366].¶
MAC(MAC_write_key, seq_num +
tls12_cid +
DTLSCipherText.version +
cid +
cid_length +
length of (IV + DTLSCiphertext.enc_content) +
IV +
DTLSCiphertext.enc_content);
¶
For ciphers utilizing authenticated encryption with additional data the following modification is made to the additional data calculation.¶
additional_data = seq_num +
tls12_cid +
DTLSCipherText.version +
cid +
cid_length +
length_of_DTLSInnerPlaintext;
¶
When a record with a CID is received that has a source address different than the one currently associated with the DTLS connection, the receiver MUST NOT replace the address it uses for sending records to its peer with the source address specified in the received datagram unless the following three conditions are met:¶
The conditions above are necessary to protect against attacks that use datagrams with spoofed addresses or replayed datagrams to trigger attacks. Note that there is no requirement for use of the anti-replay window mechanism defined in Section 4.1.2.6 of DTLS 1.2. Both solutions, the "anti-replay window" or "newer" algorithm, will prevent address updates from replay attacks while the latter will only apply to peer address updates and the former applies to any application layer traffic.¶
Note that datagrams that pass the DTLS cryptographic verification procedures but do not trigger a change of peer address are still valid DTLS records and are still to be passed to the application.¶
Application protocols that implement protection against these attacks depend on being aware of changes in peer addresses so that they can engage the necessary mechanisms. When delivered such an event, an application layer-specific address validation mechanism can be triggered, for example one that is based on successful exchange of a minimal amount of ping-pong traffic with the peer. Alternatively, an DTLS-specific mechanism may be used, as described in [I-D.tschofenig-tls-dtls-rrc].¶
DTLS implementations MUST silently discard records with bad MACs or that are otherwise invalid.¶
Figure 4 shows an example exchange where a CID is used uni-directionally from the client to the server. To indicate that a zero-length CID is present in the "connection_id" extension we use the notation 'connection_id=empty'.¶
Client Server
------ ------
ClientHello -------->
(connection_id=empty)
<-------- HelloVerifyRequest
(cookie)
ClientHello -------->
(connection_id=empty)
(cookie)
ServerHello
(connection_id=100)
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
<CID=100>
[ChangeCipherSpec]
<-------- Finished
Application Data ========>
<CID=100>
<======== Application Data
Legend:
<...> indicates that a connection id is used in the record layer
(...) indicates an extension
[...] indicates a payload other than a handshake message
Note: In the example exchange the CID is included in the record layer once encryption is enabled. In DTLS 1.2 only one handshake message is encrypted, namely the Finished message. Since the example shows how to use the CID for payloads sent from the client to the server, only the record layer payloads containing the Finished message or application data include a CID.¶
The CID replaces the previously used 5-tuple and, as such, introduces an identifier that remains persistent during the lifetime of a DTLS connection. Every identifier introduces the risk of linkability, as explained in [RFC6973].¶
An on-path adversary observing the DTLS protocol exchanges between the DTLS client and the DTLS server is able to link the observed payloads to all subsequent payloads carrying the same ID pair (for bi-directional communication). Without multi-homing or mobility, the use of the CID exposes the same information as the 5-tuple.¶
With multi-homing, a passive attacker is able to correlate the communication interaction over the two paths and the sequence number makes it possible to correlate packets across CID changes. The lack of a CID update mechanism in DTLS 1.2 makes this extension unsuitable for mobility scenarios where correlation must be considered. Deployments that use DTLS in multi-homing environments and are concerned about this aspects SHOULD refuse to use CIDs in DTLS 1.2 and switch to DTLS 1.3 where a CID update mechanism is provided and sequence number encryption is available.¶
The specification introduces record padding for the CID-enhanced record layer, which is a privacy feature not available with the original DTLS 1.2 specification. Padding allows to inflate the size of the ciphertext making traffic analysis more difficult. More details about record padding can be found in Section 5.4 and Appendix E.3 of RFC 8446.¶
Finally, endpoints can use the CID to attach arbitrary per-connection metadata to each record they receive on a given connection. This may be used as a mechanism to communicate per-connection information to on-path observers. There is no straightforward way to address this concern with CIDs that contain arbitrary values. Implementations concerned about this aspect SHOULD refuse to use CIDs.¶
An on-path adversary can create reflection attacks against third parties because a DTLS peer has no means to distinguish a genuine address update event (for example, due to a NAT rebinding) from one that is malicious. This attack is of concern when there is a large asymmetry of request/response message sizes.¶
Additionally, an attacker able to observe the data traffic exchanged between two DTLS peers is able to replay datagrams with modified IP address/port numbers.¶
The topic of peer address updates is discussed in Section 6.¶
IANA is requested to allocate an entry to the existing TLS "ExtensionType Values" registry, defined in [RFC5246], for connection_id(TBD1) as described in the table below. IANA is requested to add an extra column to the TLS ExtensionType Values registry to indicate whether an extension is only applicable to DTLS and to include this document as an additional reference for the registry.¶
Value Extension Name TLS 1.3 DTLS Only Recommended Reference -------------------------------------------------------------------- TBD1 connection_id CH, SH Y N [[This doc]]¶
Note: The value "N" in the Recommended column is set because this extension is intended only for specific use cases. This document describes the behavior of this extension for DTLS 1.2 only; it is not applicable to TLS, and its usage for DTLS 1.3 is described in [I-D.ietf-tls-dtls13].¶
IANA is requested to allocate tls12_cid(TBD2) in the "TLS ContentType Registry". The tls12_cid ContentType is only applicable to DTLS 1.2.¶
RFC EDITOR: PLEASE REMOVE THE THIS SECTION¶
draft-ietf-tls-dtls-connection-id-08¶
draft-ietf-tls-dtls-connection-id-07¶
draft-ietf-tls-dtls-connection-id-06¶
draft-ietf-tls-dtls-connection-id-05¶
draft-ietf-tls-dtls-connection-id-04¶
draft-ietf-tls-dtls-connection-id-03¶
draft-ietf-tls-dtls-connection-id-02¶
draft-ietf-tls-dtls-connection-id-01¶
draft-ietf-tls-dtls-connection-id-00¶
draft-rescorla-tls-dtls-connection-id-00¶
RFC EDITOR: PLEASE REMOVE THE THIS SECTION¶
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 this specification and we would like to thank the following individuals for their contributions:¶
* Yin Xinxing Huawei yinxinxing@huawei.com¶
* Nikos Mavrogiannopoulos RedHat nmav@redhat.com¶
* Tobias Gondrom tobias.gondrom@gondrom.org¶
Additionally, we would like to thank the Connection ID task force team members:¶
The task force team discussed various design ideas, including cryptographically generated session ids using hash chains and public key encryption, but dismissed them due to their inefficiency. The approach described in this specification is the simplest possible design that works given the limitations of DTLS 1.2. DTLS 1.3 provides better privacy features and developers are encouraged to switch to the new version of DTLS.¶
Finally, we want to thank the IETF TLS working group chairs, Chris Wood, Joseph Salowey, and Sean Turner, for their patience, support and feedback.¶
We would like to thank Achim Kraus for his review comments and implementation feedback.¶