Filename: 340-packed-and-fragmented.md
Title: Packed and fragmented relay messages
Author: Nick Mathewson
Created: 31 May 2022
Status: Open
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:
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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) 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 (fragmentation) and proposal 325 (packed cells). It requires ntor v3 and prepares for next-generation relay cryptography.
Additionally, this proposal has been revised to incorporate another protocol change, and move StreamId from the relay cell header into a new, optional header.
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]
The recognized and digest fields 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.
The body MUST contain exactly 493 bytes as cells have a fixed size.
We define this new format for a relay message which has to fit within one relay cell. However, the body can be split accross many relay cells:
Message Header
command u8
length u16
Message Routing Header (optional)
stream_id u16
Message Body
data u8[length]
One big change from the current tor protocol is something that has become
optional: we have moved stream_id into a separate inner header that only
appears sometimes named the Message Routing Header. The command value tells us
if the header is to be expected or not.
The following message types take required stream IDs: BEGIN, DATA, END,
CONNECTED, RESOLVE, RESOLVED, and BEGIN_DIR, XON, XOFF.
The following message types from proposal 339 (UDP) take required stream IDs:
CONNECT_UDP, CONNECTED_UDP and DATAGRAM.
No other message types take stream IDs. The stream_id field, when present,
MUST NOT be zero.
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 a message body, there are no more messages.
- 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 the cell header and the message header are
well-formed and have valid lengths while handling the cell in which the header
is encoded. If any of them is invalid, the circuit MUST be destroyed.
An unrecognized command is considered invalid and thus MUST result in the
circuit being immediately destroyed.
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:
EXT_FIELD_TYPE:
[03] -- Packed and Fragmented Cell Request
This field is zero payload length. Its presence signifies that the client wants to use packed and fragmented cells on the circuit.
The Exit side ntorv3 response payload is encoded as:
EXT_FIELD_TYPE:
[04] -- Packed and Fragmented Cell Response
Again, the extension presence indicates to the client that the Exit has acknowledged the feature and is ready to use it. If the extension is not present, the client MUST not use the packed and fragmented feature even though the Exit has advertised the correct protover.
The client MUST reject the handshake and thus close the circuit if:
- The response extension is seen for a non-ntorv3 handshake.
- The response extension is seen but no request was made initially.
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.
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.
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.
The RELAY_EARLY status for a command is determined based on the relay
cell in which the command's header appeared.
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.)
Data messages may not be fragmented. (There is never a reason to do this.)
For now, the maximum length for every message body is 493 bytes, except as follows:
DATAGRAMmessages (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.)
Here is an example of the simplest case: one message, sent in one relay cell:
Cell 1:
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]
message routing header:
stream_id 42 [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]
Total of 514 bytes which is the absolute maximum cell size.
Here's an example with fragmentation only: a large EXTEND2 message split across two relay cells.
Cell 1:
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]
message body:
(extend body, part 1) [490 bytes]
Cell 2:
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) [310 bytes] (310+490=800)
end-of-messages marker:
0 [1 byte]
padding up to end of cell:
random [182 bytes]
Each cells are 514 bytes for a message body totalling 800 bytes.
Here is an example with packing only: A BEGIN_DIR message and a data message
in the same cell.
Cell 1:
header:
circid .. [4 bytes]
command RELAY [1 byte]
relay cell header:
recognized 0 [2 bytes]
digest (...) [14 bytes]
# First relay message
message header:
command BEGIN_DIR [1 byte]
length 0 [2 bytes]
message routing header:
stream_id 32 [2 bytes]
# Second relay message
message header:
command DATA [1 byte]
length 25 [2 bytes]
message routing header:
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:
header:
circid .. [4 bytes]
command RELAY [1 byte]
relay cell header:
recognized 0 [2 bytes]
digest (...) [14 bytes]
# First message
message header:
command DATAGRAM [1 byte]
length 1200 [2 bytes]
message routing header:
stream_id 99 [2 bytes]
message body:
(datagram body, part 1) [488 bytes]
Cell 2:
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:
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)
# Second message
message header:
command SENDME [1 byte]
length 23 [2 bytes]
message body:
version 1 [1 byte]
datalen 20 [2 bytes]
data (digest to ack) [20 bytes]
# Third message
message header:
command XON [1 byte]
length 1 [2 bytes]
message routing header:
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 [241 bytes]