Filename: xxx-new-create-and-extend-cells.txt
Title: Adding new, extensible CREATE, EXTEND, and related cells
Author: Robert Ransom
Created: 2010-12-27
Status: Open

Overview and Motivation:

  In Tor's current circuit protocol, every field, including the 'onion
  skin', in the EXTEND relay cell has a fixed meaning and length.
  This prevents us from extending the current EXTEND cell to support
  IPv6 relays, efficient UDP-based link protocols, larger 'onion
  keys', new circuit-extension handshake protocols, or larger
  identity-key fingerprints.  We will need to support all of these
  extensions in the near future.  This proposal specifies a
  replacement EXTEND2 cell and related cells that provide more room
  for future extension.

Design:

  FIXME - allocate command ID numbers (non-RELAY commands for CREATE2 and CREATED2; RELAY commands for EXTEND2 and EXTENDED2)                                        

  The CREATE2 cell contains the following payload:

        Handshake type length                 [1 byte]
        Handshake type                        [variable]
        Handshake data length                 [2 bytes]
        Handshake data                        [variable]

  The relay payload for an EXTEND2 relay cell contains the following
  payload:

        Link target specifier type length     [1 byte]
        Link target specifier type            [variable]
        Link target specifier length          [2 bytes]
        Link target specifier                 [variable]
        Handshake type length                 [1 byte]
        Handshake type                        [variable]
        Handshake data length                 [2 bytes]
        Handshake data                        [variable]

  The CREATED2 cell and EXTENDED2 relay cell contain the following
  payload:

        Handshake data length                 [2 bytes]
        Handshake data                        [variable]

  All four cell types are padded to 512-byte cells.

  When a relay X receives an EXTEND2 relay cell:

  * X finds or opens a link to the relay Y using the link target
    specifier in the EXTEND2 relay cell; if X fails to open a link, it
    replies with a TRUNCATED relay cell. (FIXME: what do we do now?)

  * X copies the handshake type and data into a CREATE2 cell and sends
    it along the link to Y.

  * If the handshake data is valid, Y replies by sending a CREATED2
    cell along the link to X; otherwise, Y replies with a TRUNCATED
    relay cell. (XXX: we currently use a DESTROY cell?)

  * X copies the contents of the CREATED2 cell into an EXTENDED2 relay
    cell and sends it along the circuit to the OP.


  A link target specifier of type “legacy” contains the following
  data:

        Relay IP address (FIXME: byte order?) [4 bytes]
        Relay OR port (FIXME: byte order?)    [2 bytes]
        Relay identity key SHA-1 digest       [20 bytes]

  These values are processed as section 5.1 of tor-spec.txt specifies
  for the current EXTEND relay cell.


  The first (client->relay) message in a handshake of type “legacy”
  contains the following data:

        ‘Onion skin’ (as in CREATE cell)      [DH_LEN+KEY_LEN+PK_PAD_LEN bytes]

  This value is generated and processed as sections 5.1 and 5.2 of
  tor-spec.txt specify for the current CREATE cell.

  The second (relay->client) message in a handshake of type “legacy”
  contains the following data:

        Relay DH public key                   [DH_LEN bytes]
        KH (see section 5.2 of tor-spec.txt)  [HASH_LEN bytes]

  These values are generated and processed as sections 5.1 and 5.2 of
  tor-spec.txt specify for the current CREATED cell.

  After successfully completing a handshake of type “legacy”, the
  client and relay use the current relay cryptography protocol.

Bugs:

  This specification does not accommodate:

  * circuit-extension handshakes requiring more than one round

    No circuit-extension handshake should ever require more than one
    round (i.e. more than one message from the client and one reply
    from the relay).  We can easily extend the protocol to handle
    this, but we will never need to.

  * circuit-extension handshakes in which either message cannot fit in
    a single 512-byte cell along with the other required fields

    This can be handled by specifying a dummy handshake type whose
    data (sent from the client) consists of another handshake type and
    the beginning of the data required by that handshake type, and
    then using several (newly defined) HANDSHAKE_COMPLETION relay
    cells sent in each direction to transport the remaining handshake
    data.

    The specification of a HANDSHAKE_COMPLETION relay cell and its
    associated dummy handshake type can safely be postponed until we
    develop a circuit-extension handshake protocol that would require
    it.

  * link target specifiers that cause EXTEND2 cells to exceed 512
    bytes

    This can be handled by specifying a LONG_COMMAND relay cell type
    that can be used to transport a large ‘virtual cell’ in multiple
    512-byte cells.

    The specification of a LONG_COMMAND relay cell can safely be
    postponed until we develop a link target specifier, a RELAY_BEGIN2
    relay cell and stream target specifier, or some other relay cell
    type that would require it.