TLS Working Group                                              M. Badra
Internet-Draft                                         LIMOS Laboratory
Intended status: Standards Track                              I. Hajjeh
Expires:  April 27, 2008                                      ESRGroups
                                                       October 25, 2007

                           MTLS: TLS Multiplexing

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   The Transport Layer Security (TLS) standard provides connection
   security with mutual authentication, data confidentiality and
   integrity, key generation and distribution, and security parameters
   negotiation. However, missing from the protocol is a way to
   multiplex application data over a single TLS session.

   This document defines MTLS, a new TLS sub-protocol running over TLS
   (or DTLS) Record protocol. The MTLS design provides application
   multiplexing over a single TLS (or DTLS) session. Therefore, instead
   of associating a TLS connection with each application, MTLS allows

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   several applications to protect their exchanges over a single TLS

1. Introduction

   HTTP over TLS [HTTPTLS], POP over TLS and IMAP over TLS [POPTLS] are
   examples of securing, respectively HTTP, POP and IMAP data exchanges
   using the TLS protocol [TLS].

   TLS ([TLS], [DTLS]) is the most deployed security protocol for
   securing exchanges, for authenticating entities and for generating
   and distributing cryptographic keys. However, what is missing from
   the protocol is the way to multiplex application data over the same
   TLS session.

   Actually, TLS (or DTLS) clients and servers MUST establish a TLS (or
   DTLS) session for each application they want to run over a transport
   layer. However, some applications may agree or be configured to use
   the same security policies or parameters (e.g. authentication method
   and cipher_suite) and then to share a single TLS session to protect
   their exchanges. In this way, this document extends TLS to allow
   application multiplexing over TLS.

   The document motivations included:

   o   TLS is application protocol-independent. Higher-level protocol
       can operate on top of the TLS protocol transparently.

   o   TLS is a protocol of a modular nature. Since TLS is developed in
       four independent protocols, the approach defined in this
       document can be added by extending the TLS protocol and with a
       total reuse of pre-existing TLS infrastructures and

   o   It provides a secure VPN tunnel over a transport layer. Unlike
       "ssh-connection" [SSHCON], MTLS can run over unreliable
        transport protocols, such as UDP.

   o   Establishing a single session for a number of applications
       -instead of establishing a session per application- reduces
       resource consumption, latency and messages flow that are
       associated with executing simultaneous TLS sessions.

   o   TLS can not forbid an intruder to analyze the traffic and cannot
       protect data from inference. Thus, the intruder can know the
       type of application data transmitted through the TLS session.
       However, the extension defined in this document allows, by its
       design, data protection against inference.

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1.2. Requirements language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [KEYWORDS].

2. TLS multiplexing overview and considerations

   This document defines a new TLS sub-protocol called Multiplexing TLS
   (MTLS) to handle data multiplexing, and it specifies the content
   type mtls(TBA). It extends also TLS with a new extension type (TBA)
   allowing the negotiation of data multiplexing features.

2.1. Handshake

   This document defines an extension of type "data_multiplexing". The
   "extension_data" field of this extension is zero-length.

   Based on the TLS Extensions [TLSEXT], a client and a server can, in
   an ordinary TLS handshake, negotiate the future use of MTLS. If the
   client does attempt to initiate a TLS connection using MTLS with a
   server that does not support it, it will be automatically alerted.
   For servers aware of MTLS but not wishing to use it, it will
   gracefully revert to an ordinary TLS handshake or stop the

   The negotiation usually starts with the client determining whether
   the server is capable of and willing to use MTLS or not. In order to
   allow a TLS client to negotiate the application multiplexing
   functionality, a new extension type SHOULD be added to the Extended
   Client and Extended Server Hello messages.

   If the server is able of and willing to use the "data_multiplexing"
   extension, it MUST reply with an empty extension of the same type.
   Once the Handshake is complete, the client and the server SHOULD
   establish and manage many application channels using the
   requests/responses defined below.

2.1.1. Opening and closing connections

   Once the Handshake is complete, both the client and the server can
   start data multiplexing using a set of requests/responses defined
   below. All requests/requests will pass through MTLS layer and are
   formatted into MTLS packets, depending on each request/response.

   The sender MAY request the opening of many channels. For each
   channel, the MTLS layer generates and sends the following request:

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       struct {
              uint8 type;
              opaque sender_channel_id[2];
              uint32 sender_window_length;
              uint32 sender_max_packet_length;
              opaque source_address_machine<4..7>;
              opaque source_port[2];
              opaque destination_address_machine<4..7>;
              opaque destination_port[2];
           } RequestEstablishmentChannel;

   The field "type" specifies the MTLS packet type (types are
   summarized below), the "max_packet_length" and the
   "sender_channel_id" are used as described below. The
   "source_address_machine" MAY carry either the numeric IP address or
   the domain name of the host from where the application originates
   the data multiplexing request and the "port" is the port on the host
   from where the connection originated.

   The sender initializes its "window_length" with the data length (in
   octets), specifying how many bytes the receiver can maximally send
   on the channel before receiving a new window length (available free
   space). Each end of the channel establishes a "receive buffer" and a
   "send buffer".

   The sender initializes its "max_packet_length" with the data length
   (in octets), specifying the maximal packet's length in octets the
   receiver can send on the channel.

   The "destination_address_machine" and "destination_port" specify the
   TCP/IP host and port where the recipient should connect the channel.
   The "destination_address_machine" MAY be either a domain name or a
   numeric IP address.

   The receiver decides whether it can open the channel, and replies
   with one of the following messages:

       struct {
              uint8 type;
              opaque sender_channel_id[2];
              opaque receiver_channel_id[2];
              uint32 receiver_window_length;
              uint32 max_packet_length;
           } RequestEstablishmentSuccess;

       struct {
              uint8 type;
              opaque sender_channel_id[2];
              opaque error<0..2^16>;
           } RequestEchecChannel;

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   The field "error" conveys a description of the error.

   If an error occurs at the MTLS layer, the established secure session
   is still valid and no alert of any type is sent by the TLS Record.

   Each MTLS channel has its identifier computed as:

             channel_id = sender_channel_id" + "receiver_channel_id

   Where "+" indicates concatenation.

   The following packet MAY be sent to notify the receiver that the
   sender will not send any more data on this channel and that any data
   received after a closure request will be ignored. The sender of the
   closure request MAY close its "receive buffer" without waiting for
   the receiver's response. However, the receiver MUST respond with a
   confirmation of the closure and close down the channel immediately,
   discarding any pending writes.

       struct {
              uint8 type;
              opaque channel_id[4];
           } CloseChannel;

       struct {
              uint8 type;
              opaque channel_id[4];
           } ConfirmationCloseChannel;

2.2. MTLS sub-protocol

   The structure of the MTLS packet is described below. The
   "sender_channel_id" and "receiver_channel_id" are the same gererated
   during the connection establishment. The length conveys the data
   length of the current packet.

   Each entity maintains its "max_packet_length" (that is originally
   initialized during the connection establishment) to a value not
   bigger than the maximum size of this entity's "receive buffer". For
   each received packet, the entity MUST subtract the packet's length
   from the "max_packet_length". The result is always positive since
   the packet's length is always less than or equal to the current

   The free space of the "receive buffer" MAY increase in length.
   Consequently, the entity MUST inform the other end about this
   increase, allowing the other entity to send packet with length
   bigger than the old "max_packet_length" but smaller or equal than
   the new value.

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   The entity MAY indicate this increase by sending an Acknowledgment
   packet. The Acknowledgment packet carries the available free space
   ("free_space" field in octets) the receiver of that packet can send
   on the channel before receiving a new window length.

   If the length of the "receive buffer" does not change,
   Acknowledgment packet will never be sent.

   In the case where the "receive buffer" of an entity fills up, the
   other entity MUST wait for an Acknowledgment packet before sending
   any more MTLSPlaintext packets.

      struct {
              uint8 type;
              opaque channel_id[4];
              uint32 length;
              opaque data[MTLSPlaintext.length];
            } MTLSPlaintext;

      struct {
              uint8 type;
              opaque channel_id[4];
              uint32 free_space;
           } Acknowledgment;

   The TLS Record Layer receives data from MTLS, supposes it as
   uninterpreted data and applies the fragmentation and the
   cryptographic operations on it, as defined in [TLS]. The type is set
   to mtls(TBA).

   Note: multiple MTLS fragments MAY be coalesced into a single
   TLSPlaintext record.

   Received data is decrypted, verified, decompressed, and reassembled,
   then delivered to MTLS sub-protocol. Next, the MTLS sends data to
   the appropriate application using the channel identifier and the
   length value.

      enum {
            change_cipher_spec(20), alert(21), handshake(22),
            application_data(23), mtls(TBA), (255)
          } ContentType;

2.3. MTLS Message Types

   Additional message types can be supported by MTLS.

                RequestEstablishmentChannel        0x01
                RequestEstablishmentSuccess        0x02
                RequestEchecChannel                0x03
                CloseChannel                       0x04

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                ConfirmationCloseChannel           0x05
                MTLSPlaintext                      0x06
                Acknowledgment                     0x07

3. Security Considerations

   Security issues are discussed throughout this document, and in
   [TLS], [DTLS] and [TLSEXT] documents.

   If a fatal error related to any channel or a connection of an
   arbitrary application occurs, the secure session MUST NOT be
   resumed. This is logic since the Record protocol does not
   distinguish between the MTLS channels. However, if an error occurs
   at the MTLS layer, both parties immediately close the related
   channels, but not the TLS session (no alert of any type is sent by
   the TLS Record).

4. IANA Considerations

   This section provides guidance to the IANA regarding registration of
   values related to the TLS protocol.

   There are name spaces that require registration: the mtls content
   type, the data_multiplexing extension, and the MTLS message types.

5. References

5.1. Normative References

   [TLS]      Dierks, T., Rescorla, E., "The TLS Protocol Version 1.1",
              RFC 4346, April 200P.

   [TLSEXT]   Blake-Wilson, S., et. al., "Transport Layer Security
             (TLS) Extensions", RFC 4346, April 2006.

   [DTLS]     Rescorla, E., Modadugu, N., "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

5.2. Informative References

   [HTTPTLS]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [POPTLS]   Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
              2595, June 1999.

   [SSHCON]   Lonvick, C., "SSH Connection Protocol", RFC 4254, January

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Author's Addresses

   Mohamad Badra
   LIMOS Laboratory - UMR6158, CNRS
   France                    Email:

   Ibrahim Hajjeh
   ESRGroups, Security WG
   France                    Email:

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