[tor-dev] Proposal 319: RELAY_FRAGMENT cells

[Nick Mathewson <nickm@xxxxxxxxxxxxxx>]

```
Filename: 319-wide-everything.md
Title: RELAY_FRAGMENT cells
Author: Nick Mathewson
Created: 11 May 2020
Status: Open
```

(This proposal is part of the Walking Onions spec project.)

# Introduction

Proposal 249 described a system for `CREATE` cells to become wider, in order to
accommodate hybrid crypto.  And in order to send those cell bodies across
circuits, it described a way to split `CREATE` cells into multiple `EXTEND`
cells.

But there are other cell types that can need to be wider too. For
example, `INTRODUCE` and `RENDEZVOUS` cells also contain key material
used for a handshake: if handshakes need to grow larger, then so do
these cells.

This proposal describes an encoding for arbitrary "wide" relay cells,
that can be used to send a wide variant of anything.

To be clear, although this proposal describes a way that all relay
cells can become "wide", I do not propose that wide cells should
actually be _allowed_ for all relay cell types.

# Proposal

We add a new relay cell type: `RELAY_FRAGMENT`.  This cell type contains part
of another relay cell.  A `RELAY_FRAGEMENT` cell can either introduce a new
fragmented cell, or can continue one that is already in progress.

The format of a RELAY_FRAGMENT body is one of the following:

    // First body in a series
    struct fragment_begin {
       // What relay_command is in use for the underlying cell?
       u8 relay_command;
       // What will the total length of the cell be once it is reassembled?
       u16 total_len;
       // Bytes for the cell body
       u8 body[];
    }

    // all other cells.
    struct fragment_continued {
       // More bytes for the cell body.
       u8 body[];
    }

To send a fragmented cell, first a party sends a RELAY_FRAGMENT cell
containing a "fragment_begin" payload.  This payload describes the total
length of the cell, the relay command

Fragmented cells other than the last one in sequence MUST be sent full of
as much data as possible.  Parties SHOULD close a circuit if they receive a
non-full fragmented cell that is not the last fragment in a sequence.

Fragmented cells MUST NOT be interleaved with other relay cells on a circuit,
other than cells used for flow control. (Currently, this is only SENDME
cells.)  If any party receives any cell on a circuit, other than a flow
control cell or a RELAY_FRAGEMENT cell, before the fragmented cell is
complete, than it SHOULD close the circuit.

Parties MUST NOT send extra data in fragmented cells beyond the amount given
in the first 'total_len' field.

Not every relay command may be sent in a fragmented cell.  In this proposal,
we allow the following cell types to be fragmented: EXTEND2, EXTENDED2,
INTRODUCE1, INTRODUCE2, RENDEZVOUS.  Any party receiving a command that they
believe should not be fragmented should close the circuit.

Not all lengths up to 65535 are valid lengths for a fragmented cell.  Any
length under 499 bytes SHOULD cause the circuit to close, since that could
fit into a non-fragmented RELAY cell.  Parties SHOULD enforce maximum lengths
for cell types that they understand.

All `RELAY_FRAGMENT` cells for the fragmented cell must have the
same Stream ID.  (For those cells allowed above, the Stream ID is
always zero.)  Implementations SHOULD close a circuit if they
receive fragments with mismatched Stream ID.

# Onion service concerns.

We allocate a new extension for use in the ESTABLISH_INTRO by onion services,
to indicate that they can receive a wide INTRODUCE2 cell.  This extension
contains:

        struct wide_intro2_ok {
          u16 max_len;
        }

We allocate a new extension for use in the `ESTABLISH_RENDEZVOUS`
cell, to indicate acceptance of wide `RENDEZVOUS2` cells.  This
extension contains:

        struct wide_rend2_ok {
          u16 max_len;
        }

(Note that `ESTABLISH_RENDEZVOUS` cells do not currently have a an
extension mechanism.  They should be extended to use the same
extension format as `ESTABLISH_INTRO` cells, with extensions placed
after the rendezvous cookie.)

# Handling RELAY_EARLY

The first fragment of each EXTEND cell should be tagged with `RELAY_EARLY`.
The remaining fragments should not.  Relays should accept `EXTEND` cells if and
only if their _first_ fragment is tagged with `RELAY_EARLY`.

> Rationale: We could allow any fragment to be tagged, but that would give
> hostile guards an opportunity to move RELAY_EARLY tags around and build a
> covert channel.  But if we later move to a relay encryption method that
> lets us authenticate RELAY_EARLY, we could then require only that _any_
> fragment has RELAY_EARLY set.

# Compatibility

This proposal will require the allocation of a new 'Relay' protocol version,
to indicate understanding of the RELAY_FRAGMENTED command.
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