[tor-dev] Proposal 184: Miscellaneous changes for a v3 Tor link protocol

[Nick Mathewson]

On Thu, Sep 22, 2011 at 7:55 PM, Roger Dingledine <arma at mit.edu> wrote:
> On Tue, Sep 20, 2011 at 03:33:42PM -0400, Nick Mathewson wrote:
>> Design: Indicating variable-length cells.
>>
>>   Beginning with the v3 link protocol, we specify that all cell
>>   types in the range 128..255 indicate variable-length cells.
>>   Cell types in the range 0..127 are still used for 512-byte
>>   cells, except that the VERSIONS cell type (7) also indicates a
>>   variable-length cell (for backward compatibility).
>>
>>   As before, all Tor instances must ignore cells with types that
>>   they don't recognize.
>
> Sounds good.
>
>> Design: Variable-length padding.
>>
>>   We add a new variable-length cell type, "VPADDING", to be used for
>>   padding.  All Tor instances may send a DROP cell at any point that
>>   a VERSIONS cell is not required; a VPADDING cell's body may be any
>>   length; the body of a VPADDING cell MAY have any content.  Upon
>>   receiving a VPADDING cell, the recipient should drop it, as with a
>>   PADDING cell.
>
> Also sounds fine. But to clarify, did you mean to talk about a DROP cell
> in one of those sentences? I think you meant to say VPADDING there?

Yup; will fix.

> Also to be clear, there's no way to send a variable-length padding
> cell that's less than 5 bytes, right? I don't imagine that will bite us
> immediately, but we should call it out as a known constraint.

Correct. I'll add a note.

> By "the body of a vpadding cell may have any content", did you have in
> mind to randomize it to help protect against future TLS gotchas? Or just
> to leave us the option to do so in the future?

Both, but I think "randomize it now" is the answer.  I'll add a note.

-- 
Nick