``` Filename: 340-packed-and-fragmented.md Title: Packed and fragmented relay messages Author: Nick Mathewson Created: 31 May 2022 Status: Open ```

# Introduction

Tor sends long-distance messages on circuits via _relay cells_. The current relay cell format allows one _relay message_ (e.g., "BEGIN" or "DATA" or "END") per relay cell. We want to relax this 1:1 requirement, between messages and cells, for two reasons:

* To support relay messages that are longer than the current 498-byte limit. Applications would include wider handshake messages for postquantum crypto, UDP messages, and SNIP transfer in walking onions.

* To transmit small messages more efficiently. Several message types (notably `SENDME`, `XON`, `XOFF`, and several types from [proposal 329](./329-traffic-splitting.txt)) are much smaller than the relay cell size, and could be sent comparatively often.

In this proposal, we describe a way to decouple relay cells from relay messages. Relay messages can now be packed into multiple cells or split across multiple cells.

This proposal combines ideas from [proposal 319](./319-wide-everything.md) (fragmentation) and [proposal 325](./325-packed-relay-cells.md) (packed cells). It requires [ntor v3](./332-ntor-v3-with-extra-data.md) and prepares for [next-generation relay cryptography](./308-counter-galois-onion).

## A preliminary change: Relay encryption, version 1.5

We are fairly sure that, whatever we do for our next batch of relay cryptography, we will want to increase the size of the data used to authenticate relay cells to 128 bits. (Currently it uses a 4-byte tag plus 2 bytes of zeros.)

To avoid proliferating formats, I'm going to suggest that we make the other changes in this proposal changes concurrently with a change in our relay cryptography, so that we do not have too many incompatible cell formats going on at the same time.

The new format for a decrypted relay _cell_ will be:

recognized [2 bytes] digest [14 bytes] body [509 - 16 = 493 bytes]

"Digest" and "recognized" are computed as before; the only difference is that they occur _before_ the rest of the cell, and that "digest" is truncated to 14 bytes instead of 4.

If we are lucky, we won't have to build this encryption at all, and we can just move to some version of GCM-UIV or other RPRP that reserves 16 bytes for an authentication tag or similar cryptographic object.

## New relay message packing

Relay _messages_ now follow the following format:

Header command u8 length u16 stream_id u16 Body data u8[length]

We require that "command" is never 0.

Messages can be split across relay cells; multiple messages can occur in a single relay cell. We enforce the following rules:

* Headers may not be split across cells. * If a 0 byte follows any relay message, there are no more messages in that cell. * A relay cell may not be "empty": it must have at least some part of some message.

Unless specified elsewhere, **all** message types may be packed, and **all** message types may be fragmented.

Every command has an associated maximum length for its messages. If not specified elsewhere, the maximum length for every message is 498 bytes (for legacy reasons).

Receivers MUST validate that headers are well-formed and have valid lengths while handling the cell in which the header is encoded. If the header is invalid, the receiver must destroy the circuit.

### Some examples

## Negotiation

This message format requires a new `Relay` subprotocol version to indicate support. If a client wants to indicate support for this format, it sends the following extension as part of its `ntor3` handshake:

RELAY_PROTOCOL version u8

The `version` field is the `Relay` subprotocol version that the client wants to use. The relay must send back the same extension in its ntor3 handshake to acknowledge support.

## Migration

We add a consensus parameter, "streamed-relay-messages", with default value 0, minimum value 0, and maximum value 1.

If this value is 0, then clients will not (by default) negotiate this relay protocol. If it is 1, then clients will negotiate it when relays support it.

For testing, clients can override this setting. Once enough relays support this proposal, we'll change the consensus parameter to 1. Later, we'll change the default to 1 as well.

## Packing decisions

We specify the following greedy algorithm for making decisions about fragmentation and packing. Other algorithms are possible, but this one is fairly simple, and using it will help avoid distinguishability issues:

Whenever a client or relay is about to send a cell that would leave at least 32 bytes unused in a relay cell, it checks to see whether there is any pending data to be sent in the same circuit (in a data cell). If there is, then it adds a DATA message to the end of the current cell, with as much data as possible. Otherwise, the client sends the cell with no packed data.

## Onion services

Negotiating this for onion services will happen in a separate proposal; it is not a current priority, since there is nothing sent over rendezvous circuits that we currently need to fragment or pack.

## Miscellany

### Handling `RELAY_EARLY`

The `RELAY_EARLY` status for a command is determined based on the relay cell in which the command's _header_ appeared.

### Handling `SENDME`s

SENDME messages may not be fragmented; the body and the command must appear in the same cell. (This is necessary so authenticated sendmes can have a reasonable implementation.)

### An exception for `DATA`.

Data messages may not be fragmented. (There is never a reason to do this.)

### Extending message-length maxima

For now, the maximum length for every message is 498 bytes, except as follows:

- `DATAGRAM` messages (see proposal 339) have a maximum body length of 1967 bytes. (This works out to four relay cells, and accommodates most reasonable MTU choices)

Any increase in maximum length for any other message type requires a new Relay subprotocol version. (For example, if we later want to allow EXTEND2 messages to be 2000 bytes long, we need to add a new proposal saying so, and reserving a new subprotocol version.)

# Appendix: Example cells

Here is an example of the simplest case: one message, sent in one relay cell. Here it is a BEGIN message.

``` Cell 1: Cell header circid .. [4 bytes] command RELAY [1 byte] relay cell header recognized 0 [2 bytes] digest (...) [14 bytes] message header: command BEGIN [1 byte] length 23 [2 bytes] stream_id (...) [2 bytes] message body "www.torproject.org:443\0" [23 bytes] end-of-messages marker 0 [1 byte] padding up to end of cell random [464 bytes]

```

Here's an example with fragmentation only: a large EXTEND2 message split across two relay cells.

``` Cell 1: Cell header circid .. [4 bytes] command RELAY_EARLY [1 byte] relay cell header recognized 0 [2 bytes] digest (...) [14 bytes] message header: command EXTEND [1 byte] length 800 [2 bytes] stream_id 0 [2 bytes] message body (extend body, part 1) [488 bytes]

Cell 2: Cell header circid .. [4 bytes] command RELAY [1 byte] relay cell header recognized 0 [2 bytes] digest (...) [14 bytes] message body, continued (extend body, part 2) [312 bytes] end-of-messages marker 0 [1 byte] padding up to end of cell random [180 bytes]

```

Here is an example with packing only: A begin_dir message and a data message in the same cell.

``` Cell 1: Cell header circid .. [4 bytes] command RELAY [1 byte] relay cell header recognized 0 [2 bytes] digest (...) [14 bytes] message header: command BEGIN_DIR [1 byte] length 0 [2 bytes] stream_id 32 [2 bytes] message body: (empty) --- [0 bytes] message header: command DATA [1 byte] length 25 [2 bytes] stream_id 32 [2 bytes] message body: "HTTP/1.0 GET /tor/foo\r\n\r\n" [25 bytes] end-of-messages marker 0 [1 byte] padding up to end of cell random [457 bytes]

```

Here is an example with packing and fragmentation: a large DATAGRAM cell, a SENDME cell, and an XON cell. (Note that this sequence of cells would not actually be generated by the algorithm described in "Packing decisions" above; this is only an example of what parties need to accept.)

``` Cell 1: Cell header circid .. [4 bytes] command RELAY [1 byte] relay cell header recognized 0 [2 bytes] digest (...) [14 bytes] message header: command DATAGRAM [1 byte] length 1200 [2 bytes] stream_id 99 [2 bytes] message body (datagram body, part 1) [488 bytes]

Cell 2: Cell header circid .. [4 bytes] command RELAY [1 byte] relay cell header recognized 0 [2 bytes] digest (...) [14 bytes] message body, continued (datagram body, part 2) [493 bytes]

Cell 3: Cell header circid .. [4 bytes] command RELAY [1 byte] relay cell header recognized 0 [2 bytes] digest (...) [14 bytes] message body, continued (datagram body, part 3) [219 bytes] (488+493+219=1200) message header: command SENDME [1 byte] length 23 [2 bytes] stream_id 0 [2 bytes] message body: version 1 [1 byte] datalen 20 [2 bytes] data (digest to ack) [20 bytes] message header: command XON [1 byte] length 1 [2 bytes] stream_id 50 [2 bytes] message body: version 1 [1 byte] end-of-messages marker 0 [1 byte] padding up to end of cell random [239 bytes] ```