Internet Engineering Task Force                                M. Badra
INTERNET DRAFT                                               ENST Paris
                                                              I. Hajjeh
Expires: December 2006                                    June 15, 2006

                           MTLS: TLS Multiplexing


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   Copyright (C) The Internet Society (2006). All Rights Reserved.


   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

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

   TLS ([TLS], [TLSv1.1] and [DTLS]) is the most deployed security
   protocol for securing exchanges, 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", "SHALL", "SHOULD", and "MAY", in this document
   are to be interpreted as described in RFC-2119.

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(TBD). It extends also TLS with a new extension type
   allowing the negotiation of data multiplexing features.

2.1 Handshake

   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.

   This document defines an extension of type
   "application_layer_protocol". The "extension_data" field of this
   extension contains a "data_multiplexing", where:

       Struct {
              ApplicationLayerProtocol alp_list<0..2^22-1>;
           } data_multiplexing;

       struct {
              SenderChannelID sender_channel_id;
              ReceiverChannelID receiver_channel_id;
              uint32 max_packet_length;
              ApplicationpProtocolName apn;
           } ApplicationLayerProtocol;

       opaque SenderChannelID [2];
       opaque ReceiverChannelID [2];
       Opaque ApplicationpProtocolName<1..2^4>;

   Each channel has its identifier, which is composed of two parts
   (sender_channel_id and receiver_channel_id) generated respectively
   by the sender and the receiver. During the Handshake phase, the

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   sender generates the sender_channel_id's value and initializes the
   receiver_channel_id to empty field, in which the receiver replies
   with a generated receiver_channel_id.

   The sender (respectively the receiver) initializes its
   max_packet_length with the data length (in octets), specifying how
   many bytes the receiver (respectively the sender) can maximally send
   on the channel. Each end of the channel establishes a 'receive
   buffer' and a 'send buffer'.

   How the negotiation of options within an extension is handled is up
   to the definition of that extension. Implementations of this
   document MAY allow the server to respond with the intersection
   between what the client and the server support. However, the server
   MAY reply with all the applications it supports, but in this case
   the server MUST support at least one application requested by the
   client. The sender_channel_id, receiver_channel_id and the
   max_packet_size MUST be omitted from the server response for each
   application not requested by the client.

   Note: if the server (receiver) agrees, the client (sender) SHOULD
   establish a single TLS (respectively DTLS) session for all
   applications it wishes to run over a single TLS session. In this
   case, the sender SHOULD send a data multiplexing extension
   containing "ALL" as ApplicationpProtocolName value. The
   sender_channel_id, the receiver_channel_id and the max_packet_length
   fields SHOULD be omitted. If the receiver is able to negotiate such
   a session, it will reply with a list of applications it can accept
   to run through a single TLS session. The receiver_channel_id, the
   sender_channel_id and the max_packet_length fields SHOULD be

   However, the client MAY indicate to the server its support of the
   data multiplexing extension without determining the application
   types it wishes to multiplex. In this case, the client sends an
   empty data multiplexing extension. 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, the two entities 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.

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   The sender MAY request the opening of many channels. For each
   request, the MTLS layer MUST generate and send the following

       struct {
              uint8 type;
              SenderChannelID sender_channel_id;
              uint32 max_packet_length;// of the sender of this packet
              ApplicationpProtocolName apn;
           } RequestEstablishmentChannel;

   The field "type" specifies the MTLS packet type (types are
   summarized below), max_packet_length and the sender_channel_id are
   used as previously described.

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

       struct {
              uint8 type;
              SenderChannelID sender_channel_id;
              ReceiverChannelID receiver_channel_id;
              uint32 max_packet_length;
           } RequestEstablishmentSuccess;

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

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

   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;
              SenderChannelID sender_channel_id;
              ReceiverChannelID receiver_channel_id;
           } CloseChannel;

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       struct {
              uint8 type;
              SenderChannelID sender_channel_id;
              ReceiverChannelID receiver_channel_id;
           } ConfirmationCloseChannel;

2.2 MTLS sub-protocol

   The structure of the MTLS packet is described below. The channel_id
   value depends on the originator of the packet; for received
   (respectively submitted) packets, it conveys the sender_channel_id
   (respectively receiver_channel_id). The length conveys the data
   length of the current packet.

   Each entity maintains its max_packet_length that is originally
   initialized (during the channel establishment or during the
   handshake phase) 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 max_packet_length.

   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.

   The entity MAY indicate this increase using either data or
   Acknowledgment packets. In the first case, the entity MUST set the
   max_packet_length_changed to 1 and extra_length set to the extra
   free space. In the second case, the entity only needs to send the
   length of the extra free space.

   If the length of the 'receive buffer' does not change,
   Acknowledgment packet will never be sent. However, the entity MAY
   send data packet but in this case, it MUST set the
   max_packet_length_changed to 0 and MUST consequently remove the
   extra_length field from the packet header.

   In the case where the 'receive buffer' of an entity fills up, the
   other entity MUST wait for an Acknowledgment or a data packet with
   packet_length_changed set to 1, before sending any more
   MTLSPlaintext packets.

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     struct {
              uint8 type;
              uint16 channel_id;
              uint8 max_packet_length_changed;
              uint32 extra_length; // omitted if the value of the
                                   // max_packet_length_changed is 0
              uint32 length;
              opaque data[MTLSPlaintext.length];
           } MTLSPlaintext;

     struct {
              uint8 type;
              uint16 channel_id; // of the receiver of that packet
              uint32 extra_length;
           } 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].

   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.

2.3 MTLS Message Types

                RequestEstablishmentChannel        0x01
                RequestEstablishmentSuccess        0x02
                RequestEchecChannel                0x03
                CloseChannel                       0x04
                ConfirmationCloseChannel           0x05
                MTLSPlaintext                      0x06
                Acknowledgment                     0x07

Security Considerations

   Security issues are discussed throughout this document, and in
   [TLS], [TLSv1.1], [DTLS] and [TLSEXT] documents.

   If a fatal error related to a channel or a connection of an
   arbitrary application occurs, the secure session MUST NOT be

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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.


   [TLS]      Dierks, T., et. al., "The TLS Protocol Version 1.0", RFC
              2246, January 1999.

   [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.

   [TLSv1.1]  Dierks, T., Rescorla, E., "The TLS Protocol Version 1.1",
              RFC 4346, April 200P.

   [SMTPTLS]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              TLS", RFC 2487, January 1999.

   [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

Author's Addresses

   Mohamad Badra
   ENST Paris
   France                    Email:

   Ibrahim Hajjeh
   ESRGroups, Security WG
   France                    Email:

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