[Search] [txt|pdf|pdfized|bibtex] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: 00 01 02                                          Experimental
Internet Engineering Task Force                             Eddie Kohler
INTERNET-DRAFT                                                      UCLA
draft-kohler-dccp-mobility-02.txt                           25 June 2006
Expires: 25 December 2006


  Generalized Connections in the Datagram Congestion Control Protocol


Status of this Memo

    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
    have been or will be disclosed, and any of which he or she becomes
    aware will be disclosed, in accordance with Section 6 of BCP 79.

    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.

    This Internet-Draft will expire on 25 December 2006.

Abstract

    This document describes a mechanism by which the set of addresses
    bound to an application-level Datagram Congestion Control Protocol
    (DCCP) connection [RFC4340] can change over that connection's
    lifetime.  The essential abstraction is the Generalized Connection,
    which combines multiple distinct transport-level DCCP connections
    into a single application-level entity.






Kohler                                                          [Page 1]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


Table of Contents

    1. Introduction ...................................................3
    2. Conventions and Terminology ....................................3
    3. Requirements ...................................................3
    4. Protocol .......................................................4
       4.1. Overview ..................................................4
       4.2. Gencon Option .............................................5
       4.3. Initiate Gencon Message ...................................6
       4.4. Approve Gencon Message ....................................8
       4.5. Attach Gencon Message .....................................9
       4.6. Challenge Gencon Message .................................10
       4.7. Confirm Gencon Message ...................................12
       4.8. Detach Gencon Message ....................................13
       4.9. Prefer Gencon Message ....................................14
       4.10. Gencon Reset Code .......................................14
       4.11. Unexpected Options ......................................15
    5. Using Generalized Connections .................................17
    6. Crypto Suites .................................................18
    7. Security Considerations .......................................18
    8. IANA Considerations ...........................................19
    9. Thanks ........................................................19
    Normative References .............................................19
    Informative References ...........................................19
    Authors' Addresses ...............................................20
    Full Copyright Statement .........................................20
    Intellectual Property ............................................20
























Kohler                                                          [Page 2]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


1.  Introduction

    The Datagram Congestion Control Protocol (DCCP) as currently
    specified [RFC4340] does not support address rebinding.  Each DCCP
    connection is associated with exactly two network-level addresses
    over its lifetime, one per endpoint.  However, there are good
    arguments for supporting address rebinding, such as mobility and
    multihoming, at the transport layer, not least required interactions
    with congestion control.  DCCP is an unreliable protocol; its
    application-level semantics allow packet reordering, loss, and
    duplication.  This allows DCCP to address the address rebinding
    problem in a particularly simple way.  Multiple transport-level DCCP
    connections, with different sequence number spaces, congestion
    control state, and so forth, are simply aggregated in the relevant
    endpoints into a single application-level entity, called a
    Generalized Connection.  Little coordination is required among a
    generalized connection's components.

2.  Conventions and 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 [RFC2119].

    All multi-byte numerical quantities in DCCP, such as port numbers,
    Sequence Numbers, and arguments to options, are transmitted in
    network byte order (most significant byte first).

    Random numbers in DCCP Generalized Connections are used for their
    security properties and SHOULD be chosen according to the guidelines
    in [RFC4086].

3.  Requirements

    The generalized connection mechanism was designed to fulfill the
    following requirements and non-requirements.

    o  An endpoint does not need to announce a new address before moving
       to that address.

       RATIONALE: Mobile hosts may not know a new address until a move
       completes; and by that time, the old address may not be usable.
       Some multihomed hosts can know each of their addresses, but
       announcing addresses before using them does not prevent all
       attacks; see, for example, the "address squatting" attacks in
       [ANC04].





Kohler                                              Section 3.  [Page 3]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    o  Move requests must be safe against hijacking.  Even attackers
       that can snoop on part or all of data traffic must not be able to
       move a connection.  However, move requests need not be safe
       against man-in-the-middle attackers with control over which
       packets get delivered (such as routers).

       RATIONALE: Moving a connection is in some ways the worst possible
       attack: An attacker takes over a user's identity, without the
       user becoming aware of the attack or being able to stop the
       attack.  We must prevent this.  However, an endpoint with full
       control over the path could carry out this kind of attack even
       without mobility support.  Therefore, we choose to allow a DCCP
       mobility mechanism to be vulnerable to attackers that can snoop a
       packet sent by the mobile host, then prevent that snooped packet
       from being delivered to the stationary host.

    o  Mobility must not create new, large opportunities for denial-of-
       service attacks.

    o  Endpoints must be able to move freely between different NAT
       domains using the mobility mechanism.

    o  Simultaneous moves need not be supported.

    o  Cryptography is allowed.

    It is difficult, perhaps impossible, to fulfill both the NAT
    traversal and hijacking prevention requirements.  Natural mechanisms
    for preventing hijacking, such as cryptographically signing the
    packet's network headers, fail in the presence of NATs, which change
    those headers.  NATs essentially hijack connections by definition;
    we want to allow that, but prevent malicious hijacking.  The
    solution below represents one attempt.

4.  Protocol

4.1.  Overview

    A generalized connection groups one or more transport connections,
    called component connections, into a single application-level
    entity.  To move addresses, a host attaches a new component
    connection, then detaches the old component connection.  To
    implement multihoming, a host maintains multiple component
    connections with different endpoint addresses.

    The multiple component connections that make up a generalized
    connection are treated independently at the transport level.  They
    maintain independent sequence number spaces and congestion control



Kohler                                            Section 4.1.  [Page 4]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    state, for example.  The application, however, sees only one socket,
    which corresponds to the generalized connection.  Data received on
    any component connection is sent to that socket, and data sent via
    the socket may be transmitted over any component connection.

    The first connection handshake in a generalized connection must
    register the intention to set up a generalized connection.  The
    generalized connection's identifier is then agreed upon by the two
    endpoints (assuming they both support generalized connections).
    Thereafter, new component connections specify the intended
    generalized connection in their handshakes.  A public-key
    cryptographic protocol prevents connection hijacking by passive
    attackers.  However, attackers who can prevent packets from being
    delivered, or alter packets in flight, can hijack the connection, as
    is also possible in the absence of this extension.

    (1) Connection initiation with preparation for generalized
        connections:
        A  -->  DCCP-Request with Initiate Gencon   -->  B
        A  <--  DCCP-Response with Approve Gencon   <--  B

    (2) Adding a new component to the generalized connection:
        A' -->  DCCP-Request with Attach Gencon     -->  B
        A' <--  DCCP-Response with Challenge Gencon <--  B
        A' -->  DCCP-Ack with Confirm Gencon        -->  B

    (3) Removing a component from the generalized connection:
        A  -->  DCCP-Sync with Detach Gencon        -->  B
        A  <--  DCCP-SyncAck                        <--  B

    (4) Marking a component as preferred for data transfer:
        A  -->  any packet with Prefer Gencon       -->  B


4.2.  Gencon Option

    The Gencon option, for Generalized Connection, is used to implement
    the subprotocols that create and update generalized connections.
    These subprotocols may contain messages that exceed the 253-byte
    option length limit.  Therefore, the payloads from all of a packet's
    Gencon options are concatenated to form a single Gencon message.

    Gencon messages follow a common format, as follows.








Kohler                                            Section 4.2.  [Page 5]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    +--------+------ ... ------+------ ... ------+-------- ...
    | GCType |    Gencon ID    |  Component ID   | Payload
    +--------+------ ... ------+------ ... ------+-------- ...
                  (8 bytes)         (4 bytes)


    Gencon Type (GCType): 8 bits
        Defines the Gencon message type.  Seven types are currently
        defined, as follows:

           GCType  Meaning       Payload
           ------  -------       -------
             0     Initiate      Public Key
             1     Approve       Public Key
             2     Attach        Optional Nonce
             3     Challenge     Nonce and Optional Token
             4     Confirm       Token
             5     Detach        Token
             6     Prefer        -
           7-255   Reserved

                            Table 1: Gencon Types


    Gencon ID: 64 bits (8 bytes)
        The Gencon ID uniquely identifies the generalized connection at
        both endpoints, and is used to identify new component
        connections for an existing generalized connection.  To ensure
        uniqueness at both endpoints, the Gencon ID is defined in two
        parts: the client defines the upper 32 bits in its Initiate
        Gencon message, which is sent on the first DCCP-Request, and the
        server adds the lower 32 bits in its Approve Gencon message,
        which is sent on the corresponding DCCP-Response.  Neither the
        upper nor the lower 32 bits of the Gencon ID may equal zero.

    Component ID: 32 bits (4 bytes)
        The Component ID identifies a component connection within a
        generalized connection.  It MUST NOT equal zero.

4.3.  Initiate Gencon Message

    The client sends an Initiate Gencon message on the DCCP-Request
    packet that initiates a new generalized connection.  This message
    contains half of the Gencon ID that will define the generalized
    connection, and the client's public key that will be used to
    validate later Gencon messages.  The server's DCCP-Response packet
    will include an Approve Gencon message to complete the handshake.




Kohler                                            Section 4.3.  [Page 6]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    The Initiate Gencon message has the following format:

    +--------+----- ... -----+----- ... -----+
    |00000000|   Gencon ID   | Component ID  | (continued)
    +--------+----- ... -----+----- ... -----+
     GCType=0    (8 bytes)       (4 bytes)

        +--------+--------+--------+--------+------ ... ------+
        |   Crypto Suite  |     Length      |       Key       | ...
        +--------+--------+--------+--------+------ ... ------+
                                              (Length-4 bytes)



    Gencon ID
        The client specifies the upper 32 bits (4 bytes) of the
        generalized connection's Gencon ID in its Initiate Gencon
        message.  These bits MUST NOT equal zero, while the lower 32
        bits MUST equal zero.  They also MUST NOT equal the upper 32
        bits of the Gencon ID of any other generalized connection
        currently active at the server.  Furthermore, they SHOULD be
        chosen so as to minimize duplication: that is, no recently-
        active generalized connection from this endpoint should have had
        the same upper 32 bits.

    Component ID
        This field is chosen by the client to identify this component
        connection.  It MUST NOT equal zero, and its most significant
        bit MUST equal zero; thus, values 1-2147483647 are acceptable.
        One is a perfectly reasonable choice for this first component
        connection.

    Crypto Suite: 16 bits
        Defines the public-key cryptographic protocol to be used by
        other Gencon messages to validate future component connections.
        The Crypto Suite defines how certain other fields, such as Key,
        are interpreted.

    Length: 16 bits
        Equals the byte length of the following Key, plus four.

    Key: variable length
        The client's public key in the specified Crypto Suite.  The
        receiving endpoint uses this public key only for this
        generalized connection, although an endpoint MAY reuse a public
        key on multiple generalized connections.  There is no way to
        change this key on an existing generalized connection; if a
        client suspects that a generalized connection's key has been



Kohler                                            Section 4.3.  [Page 7]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


        compromised, the client should shut down all corresponding
        generalized connections.  The format of this field depends on
        the value of Crypto Suite.

    Remainder of message
        The Initiate message MAY contain more than one Crypto
        Suite/Length/Key triple, allowing the receiving endpoint to
        select a Crypto Suite it understands.

    The Crypto Suite/Key pair resembles a DCCP feature.  We do not use
    the existing DCCP feature negotiation mechanism for several reasons:
    (1) Crypto Suite/Key negotiation can take place only on connection
    initiation.  (2) Keys in some Crypto Suites may exceed the 255-byte
    feature length limitation.

4.4.  Approve Gencon Message

    On receiving a DCCP-Request containing an acceptable Initiate Gencon
    message, the server responds with a DCCP-Response packet containing
    an Approve Gencon message.  This serves three purposes: it
    acknowledges the creation of a generalized connection, it completes
    the specification of the Gencon ID, and it defines the server's
    public key.

    The Approve message has the following format:

    +--------+----- ... -----+----- ... -----+
    |00000001|   Gencon ID   | Component ID  | (continued)
    +--------+----- ... -----+----- ... -----+
     GCType=1    (8 bytes)       (4 bytes)

        +--------+--------+--------+--------
        |   Crypto Suite  |      Key ...
        +--------+--------+--------+--------



    Gencon ID
        The upper 32 bits of this field MUST equal the value specified
        by the client in its Initiate Gencon message.  The lower 32 bits
        are newly provided by the server, and MUST NOT equal zero.  The
        result -- a 64-bit number, neither of whose halves equals zero
        -- is the connection's complete Gencon ID.  The server MUST
        choose these lower 32 bits so that no other currently-active
        connection with the same endpoint has the same Gencon ID.
        Furthermore, it SHOULD choose these bits so as to minimize
        duplication, ensuring that new connections receive Gencon IDs
        that have not been seen before.



Kohler                                            Section 4.4.  [Page 8]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    Component ID
        MUST equal the value specified by the client in its Initiate
        message.

    Crypto Suite
        MUST equal one of the Crypto Suites specified by the client in
        its Initiate message.

    Key The server's public key in the specified Crypto Suite.  See the
        comments above on the client's public key.  No Length field is
        necessary as this Gencon message will contain exactly one Key.

4.5.  Attach Gencon Message

    To add a component connection to a generalized connection, one of
    the endpoint hosts opens a new DCCP connection to the other in the
    conventional way -- that is, using a DCCP-Request packet.  This
    DCCP-Request packet contains an Attach Gencon message with the
    generalized connection's Gencon ID.  This initial message is
    unverified; a protocol consisting of a Challenge Gencon message,
    sent on the DCCP-Response, and a Confirm Gencon message, sent on the
    DCCP-Ack, verifies that the mobile endpoint host's private key
    approves of the move.

    Note that the "client" of the new component connection need not be
    the same endpoint as the "client" of the original component
    connection.  The original server is the endpoint that received the
    original DCCP-Request containing an Initiate Gencon message; to add
    a new component connection, it sends a DCCP-Request Gencon message
    to the original client, and is thus the client for the new component
    connection.  Alternatively, the server could convince the client to
    open a new connection using application messages of some kind.

    The Attach message has the following format:

    +--------+----- ... -----+----- ... -----+----- ... -----+
    |00000010|   Gencon ID   | Component ID  |    C-Nonce    |
    +--------+----- ... -----+----- ... -----+----- ... -----+
     GCType=2    (8 bytes)       (4 bytes)       (8 bytes)


    Gencon ID
        The Gencon ID of the generalized connection.

    Component ID
        The Component ID of this new component connection.  This value
        is chosen by the component client; it MUST NOT have been used
        for any previous successful component connection, and MUST NOT



Kohler                                            Section 4.5.  [Page 9]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


        equal zero.  The value of the most significant bit is set based
        on whether the component client (the endpoint sending this DCCP-
        Request) is the original client (the endpoint that sent the
        first DCCP-Request in the generalized connection), according to
        this table:

        Component Client    Component ID MSB     Component ID Range
        ----------------    ----------------     ------------------
        Original Client            0                1-2147483647
        Original Server            1            2147483648-4294967295


        This restriction ensures that the endpoints will not pick the
        same Component ID if they try to attach new component
        connections simultaneously.

    C-Nonce: 64 bits
        Nonce fields are used as challenges to verify that the other
        protocol endpoint knows the expected private key.  The special
        value zero indicates the lack of a nonce.  Nonce values MUST be
        chosen apparently randomly, and MUST NOT be reused on the same
        generalized connection ID.  (That is, given an endpoint and a
        Gencon ID, the multiset of Nonce values contained in Gencon
        messages with that endpoint and Gencon ID must contain no
        duplicates, except possibly for zero.)  Thus, attackers should
        not be able to predict the next nonce an endpoint will use.
        Endpoints can obtain apparently-random Nonce values either with
        sources of true randomness (as long as values are not reused),
        through encryption operations (for example, using a block cipher
        to encrypt a value that increases by one per Nonce), or possibly
        in other ways.  If an endpoint uses encryption, it MUST include
        a random salt generated specifically for the generalized
        connection, ensuring that attackers cannot predict the next
        Nonce even given the value of the current Nonce.  Random numbers

        The component client MAY include an 8-byte Nonce, called the C-
        Nonce, in the Attach Gencon message.  This expresses a desire to
        verify that the component server is valid, i.e. knows the
        expected private key.

4.6.  Challenge Gencon Message

    The DCCP-Response packet sent in response to a new component
    connection -- that is, to a DCCP-Request packet containing an Attach
    Gencon message -- MUST contain a Challenge Gencon message.  This
    message is effectively an Init Cookie; the component server MUST NOT
    accept the new component connection until the Challenge is correctly
    confirmed with a Confirm Gencon message.



Kohler                                           Section 4.6.  [Page 10]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    The Challenge Gencon message has the following format:

    +--------+----- ... -----+----- ... -----+
    |00000011|   Gencon ID   | Component ID  | (continued)
    +--------+----- ... -----+----- ... -----+
     GCType=3    (8 bytes)       (4 bytes)

        +----- ... -----+--------+--------
        |    S-Nonce    |      Token ...
        +----- ... -----+--------+--------
            (8 bytes)        (optional)


    Gencon ID
        The Gencon ID of the generalized connection.

    Component ID
        The Component ID of the new component connection; MUST equal the
        Component ID from the corresponding Attach Gencon message.

    S-Nonce
        This Nonce is used to verify that the component client knows the
        relevant private key.  It follows the restrictions described
        above, and MUST NOT be zero.

    Token: variable length
        If the Attach Gencon message contained a nonzero C-Nonce, then
        the component server MUST include a signed Token in its
        Challenge Gencon message.  This Token will prove to the
        component client that the component server knows the relevant
        private key.  The format for the Token depends on the specified
        Crypto Suite.  Most Crypto Suites will construct Tokens using
        the following general procedure.

        To create a Token in response to a particular Nonce, an endpoint
        constructs the following 40-byte Generic Token Message
        structure:














Kohler                                           Section 4.6.  [Page 11]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


        Bytes +--------+--------+--------+--------+
          0   | GCType |00000000| Sequence Number .   3
              +--------+--------+--------+--------+
          4   .     Sequence Number (low bits)    |   7
              +--------+--------+--------+--------+
          8   |00000000|00000000|   Ack. Number   .  11
              +--------+--------+--------+--------+
         12   . Acknowledgement Number (low bits) |  15
              +--------+--------+--------+--------+
         16   |       Gencon ID (high bits)       .  19
              +--------+--------+--------+--------+
         20   .       Gencon ID (low bits)        |  23
              +--------+--------+--------+--------+
         24   |           Component ID            |  27
              +--------+--------+--------+--------+
         28   |           Component ID            |  31
              +--------+--------+--------+--------+
         32   |         Nonce (high bits)         .  35
              +--------+--------+--------+--------+
         36   .         Nonce (low bits)          |  39
              +--------+--------+--------+--------+


        GCType is the Gencon Type of the Gencon message containing the
        Token.  Sequence Number is the Sequence Number of the packet
        that will contain this message.  Acknowledgement Number is the
        Acknowledgement Number of the packet that will contain this
        message, which must equal the Sequence Number of the packet that
        contained the Nonce.  Note that the Component ID is included
        twice.

        This structure is then signed using the local endpoint's private
        key.  The signature is the Token.  The other endpoint can
        decrypt the Token using the corresponding public key; if the
        signature matches, then some party that knows the relevant
        private key approved of those contents.

        The Nonce in a Challenge Gencon message's Token shall equal the
        C-Nonce included in the corresponding Attach Gencon message.

4.7.  Confirm Gencon Message

    After receiving the DCCP-Request containing a Challenge Gencon
    message, the component client MUST send a DCCP-Ack packet containing
    a Confirm Gencon message in response.  This message contains a token
    that the component server will use to verify the client's identity.





Kohler                                           Section 4.7.  [Page 12]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    The Confirm message has the following format:

    +--------+----- ... -----+----- ... -----+--------+--------
    |00000100|   Gencon ID   | Component ID  |      Token ...
    +--------+----- ... -----+----- ... -----+--------+--------
     GCType=4    (8 bytes)       (4 bytes)       (variable)


    Gencon ID
        The Gencon ID of the generalized connection.

    Component ID
        The Component ID of the new component connection; MUST equal the
        Component ID from the corresponding Attach and Challenge Gencon
        messages.

    Token
        A Token created in response to the S-Nonce from the Challenge
        Gencon message.

4.8.  Detach Gencon Message

    A component connection may be closed in the usual way, via DCCP-
    CloseReq, DCCP-Close, and DCCP-Reset packets.  Sometimes, however,
    an endpoint loses control of a component connection before getting a
    chance to close it; this may particularly happen in mobile hosts.
    The Detach Gencon message allows an endpoint to close a different
    component connection by Component ID.  The receiver treats the named
    component connection(s) as if they had received a sequence-valid
    DCCP-Reset message, with Reset Reason 1, "Closed".  Detach Gencon
    messages MUST be sent only on DCCP-Sync, DCCP-CloseReq, DCCP-Close,
    and DCCP-Reset packets.  Since a packet can contain at most one
    Gencon message, one component connection can be detached per packet
    (unless the Component ID is set to zero, in which case all component
    connections are detached except the current component connection).

    The Detach message has the following format:

    +--------+----- ... -----+----- ... -----+--------+--------
    |00000101|   Gencon ID   | Component ID  |      Token ...
    +--------+----- ... -----+----- ... -----+--------+--------
     GCType=5    (8 bytes)       (4 bytes)       (variable)


    Gencon ID
        The Gencon ID of the generalized connection.





Kohler                                           Section 4.8.  [Page 13]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    Component ID
        The Component ID of the component connection that should be
        closed.  This MUST NOT equal the Component ID of the component
        connection on which the message appears.  The special value of 0
        indicates that all component connections of the given
        generalized connection should be shut down EXCEPT for the
        component connection on which the message appears.

    Token
        A Token, as above.  The Nonce field used to construct the Token
        equals zero.

4.9.  Prefer Gencon Message

    The Prefer Gencon message asks the receiving endpoint to send its
    data over the named component connection.  This allows an endpoint
    to manage how it receives data in case that, for example, different
    simultaneously-active component connections have different costs or
    varying reliability.  This information is treated as a hint; the
    receiving endpoint need not actually change how it sends data.

    The Prefer message has the following format:

    +--------+----- ... -----+----- ... -----+--------+--------
    |00000110|   Gencon ID   | Component ID  |      Token ...
    +--------+----- ... -----+----- ... -----+--------+--------
     GCType=6    (8 bytes)       (4 bytes)       (variable)


    Gencon ID
        The Gencon ID of the generalized connection.

    Component ID
        The Component ID of the component connection that the receiving
        endpoint should use to send data.

    Token
        A Token, as above.  The Nonce field used to construct the Token
        equals zero.

4.10.  Gencon Reset Code

    DCCP Reset Code 12 is allocated for resets relating to Generalized
    Connections.  The Data 1 field corresponds to the Gencon Type of the
    Gencon message that caused the reset.  The Data 2 & 3 fields default
    to zero, or are set as described below.





Kohler                                          Section 4.10.  [Page 14]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


4.11.  Unexpected Options

    Endpoints MUST handle invalid, unexpected, and otherwise malformed
    Gencon options in the following way.

    o  A Gencon message is Mandatory when any of its component Gencon
       options is marked Mandatory.  If a Mandatory Gencon message is
       "ignored" according to the following list, then the receiving
       endpoint MUST reset the connection using Reset Code 6, "Mandatory
       Failure", as described in [RFC4340], Section 5.8.2.

    o  Any Gencon message that does not meet basic formatting
       requirements, such as message length, MUST be ignored.

    o  Any Gencon message with unrecognized Gencon Type MUST be ignored.

    o  An Initiate Gencon message received on any packet other than a
       DCCP-Request MUST be ignored.

    o  An Initiate Gencon message whose Gencon ID and/or Component ID do
       not meet the specified requirements MUST be ignored.

    o  An Initiate Gencon message whose Crypto Suites are not
       understood, whose Length values are invalid (less than four or
       too large for the Gencon message) or inappropriate for the
       corresponding Crypto Suite, or whose corresponding Key does not
       meet the requirements of the selected Crypto Suite, MUST result
       in connection reset.  The receiver will reset the connection with
       Reset Code 12. The Data 2 & 3 fields of the DCCP-Reset packet are
       set to the problematic Crypto Suite.

    o  An Approve Gencon message received on any packet other than a
       DCCP-Response MUST be ignored.

    o  An Approve Gencon message received on a DCCP-Response, where the
       corresponding DCCP-Request did not contain an Initiate Gencon
       message, MUST be ignored.

    o  An Approve Gencon message whose Gencon ID and/or Component ID do
       not meet the specified requirements, or whose Crypto Suite does
       not equal some Crypto Suite from its client's Initiate message,
       or whose Key does not meet the requirements of the Crypto Suite,
       MUST be ignored.

    o  An Attach Gencon message received on any packet other than a
       DCCP-Request MUST be ignored.





Kohler                                          Section 4.11.  [Page 15]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    o  An Attach Gencon message whose Gencon ID does not correspond to a
       current connection, or whose Component ID is zero, or whose
       Component ID was used by a previous successful component
       connection on this generalized connection, MUST be ignored.  (A
       component connection is "successful" once a suitable Confirm
       Gencon message has been received.)

    o  A Challenge Gencon message received on any packet other than a
       DCCP-Response MUST be ignored.

    o  A Challenge Gencon message received on a DCCP-Response, where the
       corresponding DCCP-Request did not contain a Challenge Gencon
       message, MUST be ignored.

    o  A Challenge Gencon message whose Gencon ID and/or Component ID do
       not correspond to the Attach message's, or whose Nonce is zero,
       MUST be ignored.

    o  A Challenge Gencon message sent in response to an Attach message
       with a Nonce, whose Token is missing or invalid, MUST be ignored.

    o  A Confirm Gencon message received on any packet other than a
       packet whose Acknowledgement Number equals that of a DCCP-
       Response packet that contained a Challenge message, MUST be
       ignored.

    o  A Confirm Gencon message whose Gencon ID and/or Component ID do
       not correspond to the Attach message's, or whose Token is missing
       or invalid, MUST be ignored.

    o  A Detach Gencon message received on any packet other than a DCCP-
       Sync, DCCP-CloseReq, DCCP-Close, or DCCP-Reset packet MUST be
       ignored.

    o  A Detach Gencon message whose Gencon ID does not correspond to
       the expected Gencon ID, or whose nonzero Component ID does not
       equal that of a currently active component connection, MUST be
       ignored.

    o  A Detach Gencon message whose Component ID equals that of the
       component connection on which the message appears MUST be
       ignored.

    o  A Detach Gencon message whose Token is missing or invalid MUST be
       ignored.

    o  A Prefer Gencon message whose Gencon ID does not correspond to
       the expected Gencon ID, or whose Component ID does not equal that



Kohler                                          Section 4.11.  [Page 16]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


       of a currently active component connection, MUST be ignored.

    o  A Prefer Gencon message whose Token is missing or invalid MUST be
       ignored.

5.  Using Generalized Connections

    A client that expects to use address rebinding, or that wants to
    support servers that use address rebinding, SHOULD send an Initiate
    Gencon message on its DCCP-Request.  If the server responds with a
    valid Approve Gencon message, then the connection is Gencon-enabled;
    and as a consequence, the application's data stream may be
    associated with more than one active connection.  This section
    describes how that association is managed.

    A generalized connection lasts as long as it has at least one
    associated component connection.  When a generalized connection's
    last component connection is closed (moves to TIMEWAIT or CLOSED
    state), either through an explicit termination handshake or because
    of a timeout, the application-level connection MUST also be closed.
    APIs that report a reason for connection closure SHOULD use the
    reason associated with the last-closed component connection; if more
    than one connection closes at the same time, the choice is
    arbitrary.

    When a generalized connection has multiple components, the endpoint
    must decide which connection to use to send data.  Unless there is
    some external basis for choice, such as cost, the endpoint SHOULD
    send its data on the last connection for which it received a valid
    Prefer message.

    An endpoint SHOULD NOT use generalized connections simply to improve
    its throughput with parallel connections.  There SHOULD be a
    substantive difference between the component connections, such as
    different network access technologies or failure dependencies.  An
    endpoint that suspects that its partner is "cheating" with
    generalized connections MAY reset one or more component connections
    with Reset Code 11, "Aggression Penalty".

    When multiple component connections are sending data, that data MUST
    be enqueued for the application in the order it is received.  There
    is no way, other than simple delay, to enforce ordering among data
    received on different component connections.  This is intentional;
    application-level sequence numbers are easily layered on top of
    DCCP, and lack of sequencing may discourage aggressive use of
    parallel component connections.





Kohler                                             Section 5.  [Page 17]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


6.  Crypto Suites

    One Crypto Suite is currently recognized for use with Generalized
    Connections.  Crypto Suite 1, RSA-SHA512, implies the RSASSA-
    PKCS1-v1_5 signature scheme described in [RFC3447], Section 9.2.1,
    using the SHA-512 hash function.  Key values are represented in
    ASN.1 using the RSA public key syntax described in [RFC3447],
    Appendix A.1.1.  Only the encoded RSAPublicKey sequence is
    transmitted, not the rsaEncryption OID.  Valid Keys MUST have
    modulus "n" greater than or equal to 2^1536, and publicExponent "e"
    greater than or equal to 65537.  Token values are signatures output
    from the RSASSA-PKCS1-V1_5-SIGN function defined in [RFC3447],
    Section 8.2.1, where the EMSA-PKCS1-v1_5-ENCODE function uses the
    SHA-512 hash function and the SHA-512 DigestInfo value.  The message
    signed to create the Token equals the Generic Token Message
    construct described in Section 4.6.

7.  Security Considerations

    The base DCCP protocol is intended to protect against some classes
    of attacks, and explicitly declares itself vulnerable to other
    classes of attacks.  Specifically,
        Attackers cannot hijack a DCCP connection (close the connection
        unexpectedly, or cause attacker data to be accepted by an
        endpoint as if it came from the sender) unless they can guess
        valid sequence numbers.  Thus, as long as endpoints choose
        initial sequence numbers well, a DCCP attacker must snoop on
        data packets to get any reasonable probability of success.
        Sequence number validity checks provide this guarantee.  See
        Section 18 of [RFC4340].

    The address rebinding support described in this document preserves
    this security model for existing connection features.  Generalized
    connections, however, enlarge the possible semantics of DCCP
    interactions.  This section describes the security consequences of
    the Gencon mechanism, as applied to those new semantics.

    A "full hijacking" attack is defined as an attack where, using
    mobility and multihoming support, an attacker transparently adds
    itself as an endpoint to a generalized connection.  Any attacker
    that resides on the path, and can control the delivery of messages,
    thus faking the ownership of an endpoint's IP address, can execute a
    full hijacking attack; this is true in unextended DCCP, and
    multihoming and mobility support does not change this fact.  DCCP
    multihoming and mobility support aims to provide the following
    security guarantee for other attackers: An attacker cannot
    successfully execute a full hijacking attack unless it (1) can snoop
    the channel, and (2) knows an endpoint's private key.



Kohler                                             Section 7.  [Page 18]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


    Assume that the attacker does not know an endpoint's private key.
    Such an attacker cannot generate correct Tokens, and in particular,
    it cannot generate the Gencon Confirm Token required to complete a
    connection handshake.  Thus, the only way for it to execute a full
    hijacking attack is by replaying a previous Gencon Confirm message.
    That message must have the same Gencon ID as the connection to be
    hijacked.  The current connection must not have successfully used
    the replayed message's Component ID.  The attacker must use the same
    Sequence Number as in the replayed message, and convince the other
    endpoint to use the same Sequence Number for its packets (ensuring
    that the Acknowledgement Number in the replayed message is correct).
    All of this is difficult, but not impossible.  However, the attacker
    must also arrange for the other endpoint to use the same Nonce as in
    the previously replayed message, and the protocol explicitly forbids
    this.

8.  IANA Considerations

    IANA is requested to reserve the value 45 for the Gencon option from
    the DCCP Option Types registry, and to create a new registry for
    DCCP Gencon Types, populated initially with the values in Table 1.

9.  Thanks

    Thanks to Pasi Sarolahti for comments that inspired revisions to the
    specification.

Normative References

     [RFC2119]      Bradner, S., "Key words for use in RFCs to indicate
                    requirement levels", BCP 14, RFC 2119, March 1997.

     [RFC3447]      Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                    Standards (PKCS) #1: RSA Cryptography Specifications
                    Version 2.1", RFC 3447, February 2003.

     [RFC4086]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                    "Randomness Requirements for Security", BCP 106, RFC
                    4086, June 2005.

     [RFC4340]      Kohler, E., Handley, M., and S. Floyd, "Datagram
                    Congestion Control Protocol", RFC 4340, March 2006.

Informative References

     [ANC04]        Tuomas Aura, Pekka Nikander, and Gonzalo Camarillo.
                    Effects of Mobility and Multihoming on Transport-
                    Protocol Security.  2004 IEEE Symposium Security and



Kohler                                                         [Page 19]


INTERNET-DRAFT          Expires: 25 December 2006              June 2006


                    Privacy.

Authors' Addresses

    Eddie Kohler <kohler@cs.ucla.edu>
    4531C Boelter Hall
    UCLA Computer Science Department
    Los Angeles, CA 90095
    USA


Full Copyright Statement

    Copyright (C) The Internet Society (2006).  This document is subject
    to the rights, licenses and restrictions contained in BCP 78, and
    except as set forth therein, the authors retain all their rights.

    This document and the information contained herein are provided on
    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY 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 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
    WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed
    to pertain to the implementation or use of the technology described
    in this document or the extent to which any license under such
    rights might or might not be available; nor does it represent that
    it has made any independent effort to identify any such rights.
    Information on the procedures with respect to rights in RFC
    documents can be found in BCP 78 and BCP 79.

    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use
    of such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository
    at http://www.ietf.org/ipr.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights that may cover technology that may be required to implement
    this standard.  Please address the information to the IETF at ietf-
    ipr@ietf.org.



Kohler                                                         [Page 20]