INTERNET-DRAFT E. Rescorla Obsoletes (if approved): RFC 4347 RTFM, Inc. Intended Status: Proposed Standard N. Modadugu Stanford University October 7, 2009 (Expires April 2010) Datagram Transport Layer Security version 1.2 Status of This Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Rescorla & Modadugu Standards Track [Page 1] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This 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. Legal This documents and the information contained therein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Table of Contents 1. Introduction 3 1.1. Requirements Terminology 4 2. Usage Model 4 3. Overview of DTLS 5 3.1. Loss-Insensitive Messaging 5 3.2. Providing Reliability for Handshake 5 3.2.1. Packet Loss 6 3.2.2. Reordering 6 3.2.3. Message Size 6 3.3. Replay Detection 7 4. Differences from TLS 7 4.1. Record Layer 7 4.1.1. Transport Layer Mapping 9 4.1.1.1. PMTU Issues 10 4.1.2. Record Payload Protection 11 4.1.2.1. MAC 11 4.1.2.2. Null or Standard Stream Cipher 12 4.1.2.3. Block Cipher 12 Rescorla & Modadugu Standards Track [Page 2] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 4.1.2.3. AEAD Ciphers 12 4.1.2.5. New Cipher Suites 12 4.1.2.6. Anti-replay 13 4.1.2.7. Handling Invalid Records 13 4.2. The DTLS Handshake Protocol 14 4.2.1. Denial of Service Countermeasures 14 4.2.2. Handshake Message Format 17 4.2.3. Message Fragmentation and Reassembly 18 4.2.4. Timeout and Retransmission 19 4.2.4.1. Timer Values 23 4.2.5. ChangeCipherSpec 23 4.2.6. CertificateVerify and Finished Messages 23 4.2.7. Alert Messages 23 4.3. Summary of new syntax 24 4.3.1. Record Layer 25 4.3.2. Handshake Protocol 25 5. Security Considerations 26 6. Acknowledgements 27 7. IANA Considerations 27 8. References 27 8.1. Normative References 27 8.2. Informative References 28 1. Introduction TLS [TLS] is the most widely deployed protocol for securing network traffic. It is widely used for protecting Web traffic and for e-mail protocols such as IMAP [IMAP] and POP [POP]. The primary advantage of TLS is that it provides a transparent connection-oriented channel. Thus, it is easy to secure an application protocol by inserting TLS between the application layer and the transport layer. However, TLS must run over a reliable transport channel -- typically TCP [TCP]. It therefore cannot be used to secure unreliable datagram traffic. However, an increasing number of application layer protocols have been designed that use UDP transport. In particular protocols such as the Session Initiation Protocol (SIP) [SIP] and electronic gaming protocols are increasingly popular. (Note that SIP can run over both TCP and UDP, but that there are situations in which UDP is preferable). Currently, designers of these applications are faced with a number of unsatisfactory choices. First, they can use IPsec [RFC4301]. However, for a number of reasons detailed in [WHYIPSEC], this is only suitable for some applications. Second, they can design a custom application layer security protocol. Unfortunately, although application layer security protocols generally provide superior security properties (e.g., end-to-end security in the case of S/MIME), they typically requires a large amount of effort to Rescorla & Modadugu Standards Track [Page 3] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 design -- in contrast to the relatively small amount of effort required to run the protocol over TLS. In many cases, the most desirable way to secure client/server applications would be to use TLS; however, the requirement for datagram semantics automatically prohibits use of TLS. This memo describes a protocol for this purpose: Datagram Transport Layer Security (DTLS). 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 [DTLS1] was originally defined as a delta from [TLS11]. This document introduces a new version of DTLS, DTLS 1.2, which is defined as a series of deltas to TLS 1.2 [TLS12] There is no DTLS 1.1. That version number was skipped in order to harmonize version numbers with TLS. This version also clarifies some confusing points in the DTLS 1.0 specification. 1.1. Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [REQ]. 2. Usage Model The DTLS protocol is designed to secure data between communicating applications. It is designed to run in application space, without requiring any kernel modifications. Datagram transport does not require or provide reliable or in-order delivery of data. The DTLS protocol preserves this property for payload 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. Rescorla & Modadugu Standards Track [Page 4] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 3. Overview of DTLS The basic design philosophy of DTLS is to construct "TLS over datagram transport." The reason that TLS cannot be used directly in datagram environments is simply that packets may be lost or reordered. TLS has no internal facilities to handle this kind of unreliability, and therefore TLS implementations break when rehosted on datagram transport. The purpose of DTLS is to make only the minimal changes to TLS required to fix this problem. To the greatest extent possible, DTLS is identical to TLS. Whenever we need to invent new mechanisms, we attempt to do so in such a way that preserves the style of TLS. Unreliability creates problems for TLS at two levels: 1. TLS's traffic encryption layer does not allow independent decryption of individual records. If record N is not received, then record N+1 cannot be decrypted. 2. The TLS handshake layer assumes that handshake messages are delivered reliably and breaks if those messages are lost. The rest of this section describes the approach that DTLS uses to solve these problems. 3.1. Loss-Insensitive Messaging In TLS's traffic encryption layer (called the TLS Record Layer), records are not independent. There are two kinds of inter-record dependency: 1. Cryptographic context (stream cipher key stream) is retained between records. 2. Anti-replay and message reordering protection are provided by a MAC that includes a sequence number, but the sequence numbers are implicit in the records. DTLS solves the first problem by banning stream ciphers. DTLS solves the second problem by adding explicit sequence numbers. 3.2. Providing Reliability for Handshake The TLS handshake is a lockstep cryptographic handshake. Messages must be transmitted and received in a defined order, and any other order is an error. Clearly, this is incompatible with reordering and message loss. In addition, TLS handshake messages are potentially larger than any given datagram, thus creating the problem of Rescorla & Modadugu Standards Track [Page 5] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 fragmentation. DTLS must provide fixes for both of these problems. 3.2.1. Packet Loss DTLS uses a simple retransmission timer to handle packet loss. The following figure demonstrates the basic concept, using the first phase of the DTLS handshake: Client Server ------ ------ ClientHello ------> X<-- HelloVerifyRequest (lost) [Timer Expires] ClientHello ------> (retransmit) Once the client has transmitted the ClientHello message, it expects to see a HelloVerifyRequest from the server. However, if the server's message is lost the client knows that either the ClientHello or the HelloVerifyRequest 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: timeout and retransmission do not apply to the HelloVerifyRequest, because this requires creating state on the server. 3.2.2. Reordering 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 up for future handling once all previous messages have been received. 3.2.3. Message Size 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 <1500 bytes if fragmentation is not desired. In order to compensate for this limitation, each DTLS handshake message may be fragmented over several DTLS records. Each Rescorla & Modadugu Standards Track [Page 6] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 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. 3.3. Replay Detection 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. 4. Differences from TLS As mentioned in Section 3, DTLS is intentionally very similar to TLS. Therefore, instead of presenting DTLS as a new protocol, we present it as a series of deltas from TLS 1.2 [TLS12]. Where we do not explicitly call out differences, DTLS is the same as in [TLS12]. 4.1. Record Layer The DTLS record layer is extremely similar to that of TLS 1.2. The only change is the inclusion of an explicit sequence number in the record. This sequence number allows the recipient to correctly verify the TLS MAC. The DTLS record format is shown below: struct { ContentType type; ProtocolVersion version; uint16 epoch; // New field uint48 sequence_number; // New field uint16 length; opaque fragment[DTLSPlaintext.length]; } DTLSPlaintext; type Equivalent to the type field in a TLS 1.2 record. version The version of the protocol being employed. This document describes DTLS Version 1.2, which uses the version { 254, 253 }. The version value of 254.253 is the 1's complement of DTLS Version 1.2. This maximal spacing between TLS and DTLS version numbers ensures that records from the two protocols can be easily distinguished. It should be noted that future on-the-wire Rescorla & Modadugu Standards Track [Page 7] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 version numbers of DTLS are decreasing in value (while the true version number is increasing in value.) epoch A counter value that is incremented on every cipher state change. sequence_number The sequence number for this record. length Identical to the length field in a TLS 1.2 record. As in TLS 1.2, the length should not exceed 2^14. fragment Identical to the fragment field of a TLS 1.2 record. DTLS uses an explicit sequence number, rather than an implicit one, carried in the sequence_number field of the record. Sequence numbers are maintained separately for each epoch, with each sequence_number initially being 0 for each epoch. For instance, if a handshake message from epoch 0 is retransmitted, it might have a sequence number after a message from epoch 1, even if the message from epoch 1 was transmitted first. Note that some care needs to be taken during the handshake to ensure that retransmitted messages use the right epoch and keying material. If several handshakes are performed in close succession, there might be multiple records on the wire with the same sequence number but from different cipher states. The epoch field allows recipients to distinguish such packets. The epoch number is initially zero and is incremented each time the ChangeCipherSpec messages is sent. In order to ensure that any given sequence/epoch pair is unique, implementations MUST NOT allow the same epoch value to be reused within two times the TCP maximum segment lifetime. In practice, TLS implementations rarely rehandshake and we therefore do not expect this to be a problem. Note that because DTLS records may be reordered, a record from epoch 1 may be received after epoch 2 has begun. In general, implementations SHOULD discard packets from earlier epochs, but if packet loss causes noticeable problems MAY choose to retain keying material from previous epochs for up to 120 seconds (the default TCP MSL) to allow for packet reordering. 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 Rescorla & Modadugu Standards Track [Page 8] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 handshake. For instance, the server may send its Finished and then start transmitting data. Implementations MAY either buffer or discard such packets, though when DTLS is used over reliable transports (e.g., SCTP), they SHOULD be buffered and processed once the handshake completes. Note that TLS's restrictions on when packets may be sent still apply, and the receiver treats the packets as if they were sent in the right order. In particular, it is still impermissible to send data prior to completion of the first handshake. Note that in the special case of a rehandshake on an existing association, it is safe to process a data packet immediately even if the CSS or Finished has not yet been received provided that either the rehandshake resumes the existing session or that it uses exactly the same security parameters as the existing association. In an other case, the implementation MUST wait for the receipt of the Finished to prevent downgrade attack. 4.1.1. Transport Layer Mapping Each DTLS record MUST fit within a single datagram. In order to avoid fragmentation, that clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. Note that unlike IPsec, DTLS records do not contain any association identifiers. Applications must arrange to multiplex between associations. With UDP, this is presumably done with host/port number. Multiple DTLS records may be placed in a single datagram. They are simply encoded consecutively. The DTLS record framing is sufficient to determine the boundaries. Note, however, that the first byte of the datagram payload must be the beginning of a record. Records may not span datagrams. Some transports, such as DCCP [DCCP] provide their own sequence numbers. When carried over those transports, both the DTLS and the transport sequence numbers will be present. Although this introduces a small amount of inefficiency, the transport layer and DTLS sequence numbers serve different purposes, and therefore for conceptual simplicity it is superior to use both sequence numbers. In the future, extensions to DTLS may be specified that allow the use of only one set of sequence numbers for deployment in constrained environments. Rescorla & Modadugu Standards Track [Page 9] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 Some transports, such as DCCP, provide congestion control for traffic carried over them. If the congestion window is sufficiently narrow, DTLS handshake retransmissions may be held rather than transmitted immediately, potentially leading to timeouts and spurious retransmission. When DTLS is used over such transports, care should be taken not to overrun the likely congestion window. [DCCPDTLS] defines a mapping of DTLS to DCCP that takes these issues into account. 4.1.1.1. PMTU Issues In general, DTLS's philosophy is to leave PMTU discovery to the application. However, DTLS cannot completely ignore PMTU for three reasons: - The DTLS record framing expands the datagram size, thus lowering the effective PMTU from the application's perspective. - In some implementations the application may not directly talk to the network, in which case the DTLS stack may absorb ICMP [RFC1191] Datagram Too Big indications. - The DTLS handshake messages can exceed the PMTU. In order to deal with the first two issues, the DTLS record layer SHOULD behave as described below. If PMTU estimates are available from the underlying transport protocol, they should be made available to upper layer protocols. In particular: - For DTLS over UDP, the upper layer protocol SHOULD be allowed to obtain the PMTU estimate maintained in the IP layer. - For DTLS over DCCP, the upper layer protocol SHOULD be allowed to obtain the current estimate of the PMTU. - For DTLS over TCP or SCTP, which automatically fragment and reassemble datagrams, there is no PMTU limitation. However, the upper layer protocol MUST NOT write any record that exceeds the maximum record size of 2^14 bytes. The DTLS record layer SHOULD allow the upper layer protocol to discover the amount of record expansion expected by the DTLS processing. Note that this number is only an estimate because of block padding and the potential use of DTLS compression. Rescorla & Modadugu Standards Track [Page 10] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 If there is a transport protocol indication (either via ICMP or via a refusal to send the datagram as in DCCP Section 14), then DTLS record layer should inform the upper layer protocol of the error. The DTLS record layer SHOULD not interfere with upper layer protocols performing PMTU discovery, whether via [RFC1191] or [RFC4821] mechanisms. In particular: - Where allowed by the underlying transport protocol, the upper layer protocol SHOULD be allowed to set the state of the DF bit (in IPv4) or prohibit local fragmentation (in IPv6). - If the underlying transport protocol allows the application to request PMTU probing (e.g., DCCP), the DTLS record layer should honor this request. The final issue is the DTLS handshake protocol. From the perspective of the DTLS record layer, this is merely another upper layer protocol. However, DTLS handshakes occur infrequently and involve only a few round trips, and therefore the handshake protocol PMTU handling places a premium on rapid completion over accurate PMTU discovery. In order to allow connections under these circumstances, DTLS implementations SHOULD follow the following rules: - If the DTLS record layer informs the DTLS handshake layer that a message is too big, it SHOULD immediately attempt to fragment it, using any existing information about the PMTU. - If repeated retransmissions do not result in a response, and the PMTU is unknown, subsequent retransmissions SHOULD back off to a smaller record size, fragmenting the handshake message as appropriate. This standard does not specify an exact number of retransmits to attempt before backing off, but 2-3 seems appropriate. 4.1.2. Record Payload Protection Like TLS, DTLS transmits data as a series of protected records. The rest of this section describes the details of that format. 4.1.2.1. MAC The DTLS MAC is the same as that of TLS 1.2. However, rather than using TLS's implicit sequence number, the sequence number used to compute the MAC is the 64-bit value formed by concatenating the epoch and the sequence number in the order they appear on the wire. Note Rescorla & Modadugu Standards Track [Page 11] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 that the DTLS epoch + sequence number is the same length as the TLS sequence number. TLS MAC calculation is parameterized on the protocol version number, which, in the case of DTLS, is the on-the-wire version, i.e., {254, 253} for DTLS 1.2. Note that one important difference between DTLS and TLS MAC handling is that in TLS MAC errors must result in connection termination. In DTLS, the receiving implementation MAY simply discard the offending record and continue with the connection. This change is possible because DTLS records are not dependent on each other in the way that TLS records are. In general, DTLS implementations SHOULD silently discard records with bad MACs or that are otherwise invalid. If a DTLS implementation chooses to generate an alert when it receives a message with an invalid MAC, it MUST generate a bad_record_mac alert with level fatal and terminate its connection state. 4.1.2.2. Null or Standard Stream Cipher The DTLS NULL cipher is performed exactly as the TLS 1.2 NULL cipher. The only stream cipher described in TLS 1.2 is RC4, which cannot be randomly accessed. RC4 MUST NOT be used with DTLS. 4.1.2.3. Block Cipher DTLS block cipher encryption and decryption are performed exactly as with TLS 1.2. 4.1.2.3. AEAD Ciphers TLS 1.2 introduced authenticated encryption with additional data (AEAD) cipher suites. The existing AEAD cipher suites, defined in [ECCGCM] and [RSAGCM] can be used with DTLS exactly as with TLS 1.2. 4.1.2.5. New Cipher Suites Upon registration, new TLS cipher suites MUST indicate whether they are suitable for DTLS usage and what, if any, adaptations must be made. Rescorla & Modadugu Standards Track [Page 12] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 4.1.2.6. Anti-replay DTLS records contain a sequence number to provide replay protection. Sequence number verification SHOULD be performed using the following sliding window procedure, borrowed from Section 3.4.3 of [ESP]. The receiver packet counter for this session MUST be initialized to zero when the session is established. For each received record, the receiver MUST verify that the record contains a Sequence Number that does not duplicate the Sequence Number of any other record received during the life of this session. This SHOULD be the first check applied to a packet after it has been matched to a session, to speed rejection of duplicate records. Duplicates are rejected through the use of a sliding receive window. (How the window is implemented is a local matter, but the following text describes the functionality that the implementation must exhibit.) A minimum window size of 32 MUST be supported, but a window size of 64 is preferred and SHOULD be employed as the default. Another window size (larger than the minimum) MAY be chosen by the receiver. (The receiver does not notify the sender of the window size.) The "right" edge of the window represents the highest validated Sequence Number value received on this session. Records that contain Sequence Numbers lower than the "left" edge of the window are rejected. Packets falling within the window are checked against a list of received packets within the window. An efficient means for performing this check, based on the use of a bit mask, is described in Section 3.4.3 of [ESP]. If the received record falls within the window and is new, or if the packet is to the right of the window, then the receiver proceeds to MAC verification. If the MAC validation fails, the receiver MUST discard the received record as invalid. The receive window is updated only if the MAC verification succeeds. 4.1.2.7. Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.) In general, invalid records SHOULD be silently discarded, thus preserving the association. Implementations which choose to generate an alert instead, MUST generate fatal level alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to DoS attacks because UDP forgery is so easy. Thus, this practice is NOT Rescorla & Modadugu Standards Track [Page 13] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 RECOMMENDED for such transports. If DTLS is being carried over a transport which is resistant to forget (e.g., SCTP with SCTP-AUTH), then it is safer to send alerts because an attacker will have difficulty forging a datagram which will not be rejected by the transport layer. 4.2. The DTLS Handshake Protocol DTLS uses all of the same handshake messages and flows as TLS, with three principal changes: 1. A stateless cookie exchange has been added to prevent denial of service attacks. 2. Modifications to the handshake header to handle message loss, reordering, and fragmentation. 3. Retransmission timers to handle message loss. With these exceptions, the DTLS message formats, flows, and logic are the same as those of TLS 1.2. 4.2.1. Denial of Service Countermeasures Datagram security protocols are extremely susceptible to a variety of denial of service (DoS) attacks. Two attacks are of particular concern: 1. 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. 2. 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 next message (in DTLS, a Certificate message, which can be quite large) to the victim machine, thus flooding it. In order to counter both of these attacks, DTLS borrows the stateless cookie technique used by Photuris [PHOTURIS] and IKE [IKEv2]. When the client sends its ClientHello message to the server, the server MAY respond with a HelloVerifyRequest message. This message contains a stateless cookie generated using the technique of [PHOTURIS]. The client MUST retransmit the ClientHello with the cookie added. The server then verifies the cookie and proceeds with the handshake only Rescorla & Modadugu Standards Track [Page 14] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 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 defense against DoS attacks mounted from valid IP addresses. The exchange is shown below: Client Server ------ ------ ClientHello ------> <----- HelloVerifyRequest (contains cookie) ClientHello ------> (with cookie) [Rest of handshake] DTLS therefore modifies the ClientHello message to add the cookie value. struct { ProtocolVersion client_version; Random random; SessionID session_id; opaque cookie<0..2^8-1>; // New field CipherSuite cipher_suites<2..2^16-1>; CompressionMethod compression_methods<1..2^8-1>; } ClientHello; When sending the first ClientHello, the client does not have a cookie yet; in this case, the Cookie field is left empty (zero length). The definition of HelloVerifyRequest is as follows: struct { ProtocolVersion server_version; opaque cookie<0..2^8-1>; } HelloVerifyRequest; The HelloVerifyRequest message type is hello_verify_request(3). The server_version field is defined as in TLS. When responding to a HelloVerifyRequest the client MUST use the same parameter values (version, random, session_id, cipher_suites, Rescorla & Modadugu Standards Track [Page 15] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 compression_method) as it did in the original ClientHello. The server SHOULD use those values to generate its cookie and verify that they are correct upon cookie receipt. The server MUST use the same version number in the HelloVerifyRequest that it would use when sending a ServerHello. Upon receipt of the ServerHello, the client MUST verify that the server version values match. Note: this specification increases the cookie size limit to 255 bytes for greater future flexibility. The limit remains 32 for previous versions of DTLS. The DTLS server SHOULD generate cookies in such a way that they can be verified without retaining any per-client state on the server. One technique is to have a randomly generated secret and generate cookies as: Cookie = HMAC(Secret, Client-IP, Client-Parameters) 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. One potential attack on this scheme is for the attacker to collect a number of cookies from different addresses 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. [IKEv2] suggests adding a version number to cookies to detect this case. An alternative approach is simply to try verifying with both secrets. 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 HelloVerifyRequest is used, the initial ClientHello and HelloVerifyRequest are not included in the calculation of the handshake_messages (for the CertificateVerify message) and verify_data (for the Finished message). If a server receives a ClientHello with an invalid cookie, it SHOULD treat it the same as a ClientHello with no cookie. This avoids Rescorla & Modadugu Standards Track [Page 16] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 race/deadlock conditions if the client somehow gets a bad cookie (e.g., because the server changes its cookie signing key). Note to implementors: this may results in clients receiving multiple HelloVerifyRequest messages with different cookies. Clients SHOULD handle this by sending a new HelloVerify in response to the new HelloVerifyRequest. 4.2.2. Handshake Message Format In order to support message loss, reordering, and fragmentation, DTLS modifies the TLS 1.2 handshake header: struct { HandshakeType msg_type; uint24 length; uint16 message_seq; // New field uint24 fragment_offset; // New field uint24 fragment_length; // New field select (HandshakeType) { case hello_request: HelloRequest; case client_hello: ClientHello; case hello_verify_request: HelloVerifyRequest; // New type case server_hello: ServerHello; case certificate:Certificate; case server_key_exchange: ServerKeyExchange; case certificate_request: CertificateRequest; case server_hello_done:ServerHelloDone; case certificate_verify: CertificateVerify; case client_key_exchange: ClientKeyExchange; case finished: Finished; } body; } Handshake; The first message each side transmits in each handshake always has message_seq = 0. Whenever each new message is generated, the message_seq value is incremented by one. When a message is retransmitted, the same message_seq value is used. For example: Client Server ------ ------ ClientHello (seq=0) ------> X<-- HelloVerifyRequest (seq=0) (lost) [Timer Expires] Rescorla & Modadugu Standards Track [Page 17] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 ClientHello (seq=0) ------> (retransmit) <------ HelloVerifyRequest (seq=0) ClientHello (seq=1) ------> (with cookie) <------ ServerHello (seq=1) <------ Certificate (seq=2) <------ ServerHelloDone (seq=3) [Rest of handshake] Note, however, that 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 message is received, if its sequence number 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). 4.2.3. Message Fragmentation and Reassembly As noted in Section 4.1.1, each DTLS message MUST fit within a single transport layer datagram. However, handshake messages are potentially bigger than the maximum record size. Therefore, DTLS provides a mechanism for fragmenting a handshake message over a number of records. 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 SHOULD 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 labelled 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 Rescorla & Modadugu Standards Track [Page 18] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 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. 4.2.4. Timeout and Retransmission DTLS messages are grouped into a series of message flights, according to the diagrams below. Although each flight of messages may consist of a number of messages, they should be viewed as monolithic for the purpose of timeout and retransmission. Rescorla & Modadugu Standards Track [Page 19] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 Client Server ------ ------ ClientHello --------> Flight 1 <------- HelloVerifyRequest Flight 2 ClientHello --------> Flight 3 ServerHello \ Certificate* \ ServerKeyExchange* Flight 4 CertificateRequest* / <-------- ServerHelloDone / Certificate* \ ClientKeyExchange \ CertificateVerify* Flight 5 [ChangeCipherSpec] / Finished --------> / [ChangeCipherSpec] \ Flight 6 <-------- Finished / Figure 1. Message flights for full handshake Client Server ------ ------ ClientHello --------> Flight 1 ServerHello \ [ChangeCipherSpec] Flight 2 <-------- Finished / [ChangeCipherSpec] \Flight 3 Finished --------> / Figure 2. Message flights for session-resuming handshake (no cookie exchange) DTLS uses a simple timeout and retransmission scheme with the following state machine. 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. Rescorla & Modadugu Standards Track [Page 20] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 +-----------+ | PREPARING | +---> | | <--------------------+ | | | | | +-----------+ | | | | | | | | | Buffer next flight | | | | | \|/ | | +-----------+ | | | | | | | SENDING |<------------------+ | | | | | | Send | +-----------+ | | HelloRequest Receive | | | | next | | Send flight | | or flight | +--------+ | | | | | Set retransmit timer | | Receive | | \|/ | | HelloRequest | | +-----------+ | | Send | | | | | | ClientHello +--)--| WAITING |-------------------+ | | | | | Timer expires | | | | +-----------+ | | | | | | | | | | | | | | +------------------------+ | | | Read retransmit | Receive | | | last | | | flight | | | | | | \|/\|/ | | +-----------+ | | | | | FINISHED | -------------------------------+ | | +-----------+ | /|\ | | | | +---+ Read retransmit Retransmit last flight Rescorla & Modadugu Standards Track [Page 21] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 Figure 3. DTLS timeout and retransmission state machine The state machine has three basic states. In the PREPARING state the implementation does whatever computations are necessary to prepare the next flight of messages. It then buffers them up for transmission (emptying the buffer first) and enters the SENDING state. In the SENDING state, the implementation transmits the buffered flight of messages. 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 three ways to exit the WAITING state: 1. 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. 2. 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. 3. The implementation receives the 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) do not cause state transitions or timer resets. 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. When the server desires a rehandshake, it transitions from the FINISHED state to the PREPARING state to transmit the HelloRequest. When the client receives a HelloRequest it transitions from FINISHED to PREPARING to transmit the ClientHello. In addition, for at least 2MSL, when in the FINISHED state, the node which transmits the last flight (the server in an ordinary handshake or the client in a resumed handshake) MUST respond to a retransmit of the peer's last Rescorla & Modadugu Standards Track [Page 22] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 flight with a retransmit of the last flight. This avoids deadlock conditions if the last flight gets lost. This requirement applies to DTLS 1.0 as well, and though not explicit in [DTLS1] but was always required for the state machine to function correctly. 4.2.4.1. Timer Values 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 1 second (the minimum defined in RFC 2988 [RFC2988]) and double the value at each retransmission, up to no less than the RFC 2988 maximum of 60 seconds. Note that we recommend a 1-second timer rather than the 3-second RFC 2988 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. 4.2.5. ChangeCipherSpec As with TLS, the ChangeCipherSpec message is not technically a handshake message but MUST be treated as part of the same flight as the associated Finished message for the purposes of timeout and retransmission. 4.2.6. CertificateVerify and Finished Messages CertificateVerify and Finished messages have the same format as in TLS. Hash calculations include entire handshake messages, including DTLS specific fields: message_seq, fragment_offset and fragment_length. However, in order to remove sensitivity to fragmentation, the Finished MAC MUST be computed as if each handshake message had been sent as a single fragment. Note that in cases where the cookie exchange is used, the initial ClientHello and HelloVerifyRequest MUST NOT be included in the CertificateVerify or Finished MAC computations. 4.2.7. Alert Messages Rescorla & Modadugu Standards Track [Page 23] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 Note that Alert messages are not retransmitted at all, even when they occur in the context of a handshake. However, a DTLS implementation 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. 4.3. Summary of new syntax This section includes specifications for the data structures that have changed between TLS 1.2 and DTLS. Rescorla & Modadugu Standards Track [Page 24] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 4.3.1. Record Layer struct { ContentType type; ProtocolVersion version; uint16 epoch; // New field uint48 sequence_number; // New field uint16 length; opaque fragment[DTLSPlaintext.length]; } DTLSPlaintext; struct { ContentType type; ProtocolVersion version; uint16 epoch; // New field uint48 sequence_number; // New field uint16 length; opaque fragment[DTLSCompressed.length]; } DTLSCompressed; struct { ContentType type; ProtocolVersion version; uint16 epoch; // New field uint48 sequence_number; // New field uint16 length; select (CipherSpec.cipher_type) { case block: GenericBlockCipher; case aead: GenericAEADCipher; } fragment; } DTLSCiphertext; 4.3.2. Handshake Protocol enum { hello_request(0), client_hello(1), server_hello(2), hello_verify_request(3), // New field certificate(11), server_key_exchange (12), certificate_request(13), server_hello_done(14), certificate_verify(15), client_key_exchange(16), finished(20), (255) } HandshakeType; struct { HandshakeType msg_type; uint24 length; Rescorla & Modadugu Standards Track [Page 25] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 uint16 message_seq; // New field uint24 fragment_offset; // New field uint24 fragment_length; // New field select (HandshakeType) { case hello_request: HelloRequest; case client_hello: ClientHello; case server_hello: ServerHello; case hello_verify_request: HelloVerifyRequest; // New field case certificate:Certificate; case server_key_exchange: ServerKeyExchange; case certificate_request: CertificateRequest; case server_hello_done:ServerHelloDone; case certificate_verify: CertificateVerify; case client_key_exchange: ClientKeyExchange; case finished: Finished; } body; } Handshake; struct { ProtocolVersion client_version; Random random; SessionID session_id; opaque cookie<0..2^8-1>; // New field CipherSuite cipher_suites<2..2^16-1>; CompressionMethod compression_methods<1..2^8-1>; } ClientHello; struct { ProtocolVersion server_version; opaque cookie<0..2^8-1>; } HelloVerifyRequest; 5. Security Considerations This document describes a variant of TLS 1.2 and therefore most of the security considerations are the same as those of TLS 1.2 [TLS12], described in Appendices D, E, and F. 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 which do not use this cookie exchange are still vulnerable to DoS. In particular, DTLS servers which 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. Rescorla & Modadugu Standards Track [Page 26] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 6. Acknowledgements The authors would like to thank Dan Boneh, Eu-Jin Goh, Russ Housley, Constantine Sapuntzakis, and Hovav Shacham for discussions and comments on the design of DTLS. Thanks to the anonymous NDSS reviewers of our original NDSS paper on DTLS [DTLS] for their comments. Also, thanks to Steve Kent for feedback that helped clarify many points. The section on PMTU was cribbed from the DCCP specification [DCCP]. Pasi Eronen provided a detailed review of this specification. Helpful comments on the document were also received from Mark Allman, Jari Arkko, Mohamed Badra, Joel Halpern, Ted Hardie, Allison Mankin, Robin Seggelman and Michael Tuexen. 7. IANA Considerations This document uses the same identifier space as TLS [TLS12], so no new IANA registries are required. When new identifiers are assigned for TLS, authors MUST specify whether they are suitable for DTLS. This document defines a new handshake message, hello_verify_request, whose value has been allocated from the TLS HandshakeType registry defined in [TLS12]. The value "3" has been assigned by the IANA. 8. References 8.1. Normative References [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. [RFC4821] Mathis, M., and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. [RSAGCM] Salowey, J., Choudhury, A., and D. McGrew, "AES-GCM Cipher Suites for TLS", RFC 5288, August 2008. [TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. Rescorla & Modadugu Standards Track [Page 27] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 [TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, May 2008. 8.2. Informative References [DCCP] Kohler, E., Handley, M., Floyd, S., Padhye, J., "Datagram Congestion Control Protocol", Work in Progress, 10 March 2005. [DCCPDTLS] T. Phelan, "Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP)", RFC 5238, May 2008. [DTLS] Modadugu, N., Rescorla, E., "The Design and Implementation of Datagram TLS", Proceedings of ISOC NDSS 2004, February 2004. [DTLS1] Rescorla, E., and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, April 2006. [ECCGCM] E. Rescorla, "TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode", RFC 5289, August 2008. [ESP] S. Kent "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [IKEv2] C. Kaufman (ed), "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [PHOTURIS] Karn, P. and W. Simpson, "Photuris: Session-Key Management Protocol", RFC 2522, March 1999. [POP] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [REQ] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Rescorla & Modadugu Standards Track [Page 28] draft-ietf-tls-rfc4347-bis-00 DTLS June 2008 [SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [TLS11] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. [WHYIPSEC] Bellovin, S., "Guidelines for Mandating the Use of IPsec", Work in Progress, October 2003. Authors' Addresses Eric Rescorla RTFM, Inc. 2064 Edgewood Drive Palo Alto, CA 94303 EMail: ekr@rtfm.com Nagendra Modadugu Computer Science Department Stanford University 353 Serra Mall Stanford, CA 94305 EMail: nagendra@cs.stanford.edu Rescorla & Modadugu Standards Track [Page 29]