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Table of Contents

1. Introduction

The Datagram Transport Layer Security (DTLS) protocol [RFC6347] was designed for securing data sent over datagram transports (e.g., 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 Connection ID (CID) to the DTLS record layer. The presence of the CID is negotiated via a DTLS extension.¶

Adding a CID to the ciphertext record format presents an opportunity to make other changes to the record format. In keeping with the best practices established by TLS 1.3, the type of the record is encrypted, and a mechanism is provided for adding padding to obfuscate the plaintext length.¶

2. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This document assumes familiarity with DTLS 1.2 [RFC6347]. The presentation language used in this document is described in Section 3 of [RFC8446].¶

3. The "connection_id" Extension

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(54), (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 using a CID but does not wish the server to use one when sending.¶

  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 using the client's CID but does not wish the client to include a CID when sending.¶

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 deployment-specific 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 lengths to other parties. Since the CID length information is not included in the record itself, 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.¶

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, or a zero-length CID has been advertised for a given direction, then the record format and content type defined in RFC 6347 MUST be used to send in the indicated direction(s).¶

If DTLS peers have negotiated the use of a non-zero-length CID for a given direction, then once encryption is enabled, they MUST send with the record format defined in Figure 3 (see Section 4) with the new Message Authentication Code (MAC) computation defined in Section 5 and the content type tls12_cid. Plaintext payloads never use the new record format or the CID content type.¶

When 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 the security association are looked up by the 5-tuple and a check MUST be made to determine whether a non-zero-length CID is expected. If a non-zero-length CID is expected for the retrieved association, then the datagram MUST be treated as invalid, as described in Section 4.1.2.1 of [RFC6347].¶

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 record format defined in RFC 6347, the MAC calculation defined in Section 4.1.2 of [RFC6347] MUST be used.¶

4. Record Layer Extensions

This specification defines the CID-enhanced record layer format for DTLS 1.2, and [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 that have negotiated this extension use a distinguished record type tls12_cid(25). The use of this content type has the following three implications:¶

• The CID field is present and contains one or more bytes.¶
• The MAC calculation follows the process described in Section 5.¶
• The real content type is inside the encryption envelope, as described below.¶

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.¶

     struct {
         opaque content[length];
         ContentType real_type;
         uint8 zeros[length_of_padding];
     } DTLSInnerPlaintext;

content:

Corresponds to the fragment of a given length.¶

real_type:

The content type describing the cleartext payload.¶

zeros:

An arbitrary-length run of zero-valued bytes may appear in the cleartext after the type field. This provides an opportunity for senders to pad any DTLS record by a chosen amount as long as the total stays within record size limits. See Section 5.4 of [RFC8446] for more details. (Note that the term TLSInnerPlaintext in RFC 8446 refers to DTLSInnerPlaintext in this specification.)¶

The DTLSInnerPlaintext byte sequence is then encrypted. To create the DTLSCiphertext structure shown in Figure 3, the CID is added.¶

     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;

outer_type:

The outer content type of a DTLSCiphertext record carrying a CID is always set to tls12_cid(25). The real content type of the record is found in DTLSInnerPlaintext.real_type after decryption.¶

cid:

The CID value, cid_length bytes long, as agreed at the time the extension has been negotiated. Recall that each peer chooses the CID value it will receive and use to identify the connection, so an implementation can choose to always receive CIDs of a fixed length. If, however, an implementation chooses to receive CIDs of different lengths, the assigned CID values must be self-delineating, since there is no other mechanism available to determine what connection (and thus, what CID length) is in use.¶

enc_content:

The encrypted form of the serialized DTLSInnerPlaintext structure.¶

All other fields are as defined in RFC 6347.¶

5. Record Payload Protection

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 Authenticated Encryption with Associated Data (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.¶

cid:

Value of the negotiated CID (variable length).¶

cid_length:

The length (in bytes) of the negotiated CID (one-byte integer).¶

length_of_DTLSInnerPlaintext:

The length (in bytes) of the serialized DTLSInnerPlaintext (two-byte integer). The length MUST NOT exceed 2^14.¶

seq_num_placeholder:

8 bytes of 0xff.¶

Note that "+" denotes concatenation.¶

    MAC(MAC_write_key,
        seq_num_placeholder +
        tls12_cid +
        cid_length +
        tls12_cid +
        DTLSCiphertext.version +
        epoch +
        sequence_number +
        cid +
        length_of_DTLSInnerPlaintext +
        DTLSInnerPlaintext.content +
        DTLSInnerPlaintext.real_type +
        DTLSInnerPlaintext.zeros
    );

    MAC(MAC_write_key,
        seq_num_placeholder +
        tls12_cid +
        cid_length +
        tls12_cid +
        DTLSCiphertext.version +
        epoch +
        sequence_number +
        cid +
        DTLSCiphertext.length +
        IV +
        ENC(content + padding + padding_length)
    );

    additional_data = seq_num_placeholder +
                      tls12_cid +
                      cid_length +
                      tls12_cid +
                      DTLSCiphertext.version +
                      epoch +
                      sequence_number +
                      cid +
                      length_of_DTLSInnerPlaintext;

6. Peer Address Update

When a record with a CID is received that has a source address different from 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 received datagram has been cryptographically verified using the DTLS record layer processing procedures.¶
• The received datagram is "newer" (in terms of both epoch and sequence number) than the newest datagram received. Reordered datagrams that are sent prior to a change in a peer address might otherwise cause a valid address change to be reverted. This also limits the ability of an attacker to use replayed datagrams to force a spurious address change, which could result in denial of service. An attacker might be able to succeed in changing a peer address if they are able to rewrite source addresses and if replayed packets are able to arrive before any original.¶
• There is a strategy for ensuring that the new peer address is able to receive and process DTLS records. No strategy is mandated by this specification, but see note (*) below.¶

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 the use of the anti-replay window mechanism defined in Section 4.1.2.6 of [RFC6347]. 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.¶

(*) Note: Application protocols that implement protection against spoofed addresses depend on being aware of changes in peer addresses so that they can engage the necessary mechanisms. When delivered such an event, an address validation mechanism specific to the application layer can be triggered -- for example, one that is based on successful exchange of a minimal amount of ping-pong traffic with the peer. Alternatively, a DTLS-specific mechanism may be used, as described in [DTLS-RRC].¶

DTLS implementations MUST silently discard records with bad MACs or that are otherwise invalid.¶

7. Example

Figure 4 shows an example exchange where a CID is used unidirectionally 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.¶

8. Privacy Considerations

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 bidirectional communication). Without multihoming or mobility, the use of the CID exposes the same information as the 5-tuple.¶

With multihoming, a passive attacker is able to correlate the communication interaction over the two paths. 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 multihoming environments and are concerned about these 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.¶

This 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 the size of the ciphertext to be inflated, making traffic analysis more difficult. More details about record padding can be found in Section 5.4 and Appendix E.3 of [RFC8446].¶

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.¶

9. Security Considerations

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 particular concern when the request is small and the response large. See Section 6 for more on address updates.¶

Additionally, an attacker able to observe the data traffic exchanged between two DTLS peers is able to replay datagrams with modified IP addresses / port numbers.¶

The topic of peer address updates is discussed in Section 6.¶

10. IANA Considerations

This document implements three IANA updates.¶

Acknowledgements

We would like to thank Hanno Becker, Martin Duke, Lars Eggert, Ben Kaduk, Warren Kumari, Francesca Palombini, Tom Petch, John Scudder, Sean Turner, Éric Vyncke, and Robert Wilton for their review comments.¶

Finally, we want to thank the IETF TLS Working Group chairs, Chris Wood, Joseph Salowey, and Sean Turner, for their patience, support, and feedback.¶

Contributors

Many people have contributed to this specification, and we would like to thank the following individuals for their contributions:¶

Additionally, we would like to thank the Connection ID task force team members:¶

• Martin Thomson (Mozilla)¶

• Christian Huitema (Private Octopus Inc.)¶

• Jana Iyengar (Google)¶

• Daniel Kahn Gillmor (ACLU)¶

• Patrick McManus (Mozilla)¶

• Ian Swett (Google)¶

• Mark Nottingham (Fastly)¶

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.¶

Authors' Addresses