``` 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.