[tor-dev] Proposal 214: Allow 4-byte circuit IDs in a new link protocol

[Nick Mathewson]

Filename: 214-longer-circids.txt
Title: Allow 4-byte circuit IDs in a new link protocol
Author: Nick Mathewson
Created: 6 Nov 2012
Status: Open


0. Overview

   Relays are running out of circuit IDs.  It's time to make the field
   bigger.

1. Background and Motivation

   Long ago, we thought that 65535 circuit IDs would be enough for anybody.
   It wasn't.  But our cell format in link protocols is still:

    Cell [512 bytes]
      CircuitID [2 bytes]
      Command [1 byte]
      Payload [509 bytes]

    Variable-length cell [Length+5 bytes]
       CircID   [4 bytes]
       Command  [1 byte]
       Length   [2 bytes]
       Payload  [Length bytes]

   This means that a relay can run out of circuit IDs pretty easily.

2. Design

   I propose a new link cell format for relays that support it.  It should
   be:

    Cell [514 bytes]
       CircuitID [4 bytes]
       Command [1 byte]
       Payload [509 bytes]

    Variable cell (Length+7 bytes)
       CircID   [4 bytes]
       Command  [1 byte]
       Length   [2 bytes]
       Payload  [Length bytes]

   We need to keep the payload size in fixed-length cells to its current
   value, since otherwise the relay protocol won't work.

   This new cell format should be used only when the link protocol is 4.
   (To negotiation link protocol 4, both sides need to use the "v3"
   handshake, and include "4" in their version cells.  If version 4 or
   later is negotiated, this is the cell format to use.)

2.1. Better allocation of circuitID space

   In the current Tor design, circuit ID allocation is determined by
   whose RSA public key has the lower modulus.  How ridiculous!
   Instead, I propose that when the version 4 link protocol is in use,
   the connection initiator use the low half of the circuit ID space,
   and the responder use the low half of the circuit ID space.

3. Discussion

   * Why 4 bytes?

     Because 3 would result in an odd cell size, and 8 seems like
     overkill.

   * Will this be distinguishable from the v3 protocol?

     Yes. Anybody who knows they're seeing the Tor protocol can probably
     tell by the TLS record sizes which version of the protocol is in
     use.  Probably not a huge deal though; which approximate range of
     versions of Tor a client or server is running is not something
     we've done much to hide in the past.

   * Why a new link protocol and not a new cell type?

     Because pretty much every cell has a meaningful circuit ID.

   * Okay, why a new link protocol and not a new _set of_ cell types?

     Because it's a bad idea to mix short and long circIDs on the same
     channel.  (That would leak which cells go with what kind of
     circuits ID, potentially.)

   * How hard is this to implement?

     I wasn't sure, so I coded it up.  I've got a probably-buggy
     implementation in branch "wide_cird_ids" in my public repository.
     Be afraid!  More testing is needed!
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