In working on circumvention protocols, I have repeatedly felt the need for a piece that is missing from our current designs. This document summarizes the problems I perceive, and how I propose to solve them.

In short, I think that every circumvention transport should incorporate some kind of session/reliability protocol—even the ones built on reliable channels that seemingly don't need it. It solves all kinds of problems related to performance and robustness. By session/reliability protocol, I mean something that offers a reliable stream abstraction, with sequence numbers and retransmissions, like QUIC or SCTP. Instead of a raw unstructured data stream, the obfuscation layer will carry encoded datagrams that are provided by the session/reliability layer.

When I say that circumvention transports should incorporate something like QUIC, for example, I don't mean that QUIC UDP packets are what we should send on the wire. No—I am not proposing a new outer layer, but an additional inner layer. We take the datagrams provided by the session/reliability layer, and encode them as appropriate for whatever obfuscation layer we happen to be using. So with meek, for example, instead of sending an unstructured blob of data in each HTTP request/response, we would send a handful of QUIC packets, encoded into the HTTP body. The receiving side would decode the packets and feed them into a local QUIC engine, which would reassemble them and output the original stream. A way to think about it is that the the sequencing/reliability layer is the "TCP" to the obfuscation layer's "IP". The obfuscation layer just needs to deliver chunks of data, on a best-effort basis, without getting blocked by a censor. The sequencing/reliability layer builds a reliable data stream atop that foundation.

I believe this design can improve existing transports, as well as enable new transports that are now possible now, such as those built on unreliable channels. Here is a list of selected problems with existing or potential transports, and how a sequencing/reliability layer helps solve them:

• Problem: Censors can disrupt obfs4 by terminating long-lived TCP connections, as Iran did in 2013, killing connections after 60 seconds.
  • This problem exists because the obfs4 session is coupled with the TCP connection. The obfs4 session begins and ends exactly when the TCP connection does. We need an additional layer of abstraction, a virtual session that exists independently of any particular TCP connection. That way, if a TCP connection is terminated, it doesn't destroy all the state of the obfs4 session—you can open another TCP connection and resume where you left off, without needing to re-bootstrap Tor or your VPN or whatever was using the channel.
• Problem: The performance of meek is limited because it is half-duplex: it never sends and receives at the same time. This is because, while the bytes in a single HTTP request arrive in order, the ordering of multiple simultaneous requests is not guaranteed. Therefore, the client sends a request, then waits for the server's response before sending another, resulting in a delay of an RTT between successive sends.
  • The session/reliability layer provides sequence numbers and reordering. Both sides can send data whenever is convenient, or as needed for traffic shaping, and any unordered data will be put back in order at the other end. A client could even split its traffic over two or more CDNs, with different latency characteristics, and know that the server will buffer and reorder the encoded packets to correctly recover the data stream.
• Problem: A Snowflake client can only use one proxy at a time, and that proxy may be slow or unreliable. Finding a working proxy is slow because each non-working one must time out in succession before trying another one.
  • The problem exists because even though each WebRTC DataChannel is reliable (DataChannel uses SCTP internally), there's no ordering between multiple simultaneous DataChannels on separate Snowflake proxies. Furthermore, if and when a proxy goes offline, we cannot tell whether the last piece of data we sent was received by the bridge or not—the SCTP ACK information is not accessible to us higher in the stack—so even if we reconnect to the bridge through another proxy, we don't know whether we need to retransmit the last piece of data or not. All we can do is tear down the entire session and start it up again from scratch. As in the obfs4 case, this problem is solved by having an independent virtual session that persists across transient WebRTC sessions. An added bonus is the opportunity to use more than one proxy at once, to increase bandwidth or as a hedge against one of them disappearing.
• Problem: DNS over HTTPS is an unreliable channel: it is reliable TCP up to the DoH server, but after that, recursive resolutions are plain old unreliable UDP. And as with meek, the ordering of simultaneous DNS-over-HTTPS requests is not guaranteed.
  • Solved by retransmission in the session layer. There's no DNS pluggable transport yet, but I think some kind of retransmission layer will be a requirement for it. Existing DNS tunnel software uses various ad-hoc sequencing/retransmission protocols. I think that a proper user-space reliability layer is the "right" way to do it.
• Problem: Shadowsocks opens a separate encrypted TCP connection for every connection made to the proxy. If a web page loads resources from 5 third parties, then the Shadowsocks client makes 5 parallel connections to the proxy.
  • This problem is really about multiplexing, not session/reliability, but several candidate session/reliability protocols additionally offer multiplexing, for example streams in QUIC, streams in SCTP, or smux for KCP. Tor does not have this problem, because Tor already is a multiplexing protocol, with multiple virtual circuits and streams in one TCP/TLS connection. But every system could benefit from adding multiplexing at some level. Shadowsocks, for example, could open up one long-lived connection, and each new connection to the proxy would only open up a new stream inside the long-lived connection. And if the long-lived connection were killed, all the stream state would still exist at both endpoints and could be resumed on a new connection.

As an illustration of what I'm proposing, here's the protocol layering of meek (which sends chunks of the Tor TLS stream inside HTTP bodies), and where the new session/reliability layer would be inserted. Tor can remain oblivious to what's happening: just as before it didn't "know" that it was being carried over HTTP, it doesn't now need to know that it is being carried over QUIC-in-HTTP (for example).

[TLS]
[HTTP]
[session/reliability layer] ⇐ 🆕
[Tor]
[application data]

I've done a little survey and identified some suitable candidate protocols that also seem to have good Go packages:

• QUIC with quic-go
• KCP with kcp-go
• SCTP with pion/sctp

I plan to evaluate at least these three candidates and develop some small proofs of concept. The overall goal of my proposal is to liberate the circumvention context from particular network connections and IP addresses.

Related work

The need for a session and sequencing layer has been felt—and dealt with—repeatedly in many different projects. It has not yet, I think, been treated systematically or recognized as a common need. Systems typically implement some form of TCP-like SEQ and ACK numbers. The ones that don't, are usually built on the assumption of one long-lived TCP connection, and therefore are really using the operating system's sequencing and reliability functions behind the scenes.

Here are are few examples:

• Code Talker Tunnel (a.k.a. SkypeMorph) uses SEQ and ACK numbers and mentions selective ACK as a possible extension. I think it uses the UDP 4-tuple to distinguish sessions, but I'm not sure.
• OSS used SEQ and ACK numbers and a random ID to distinguish sessions.
• I wasted time in the early development of meek grappling with sequencing, before punting by strictly serializing requests, sacrificing performance for simplicity. meek uses an X-Session-Id HTTP header to distinguish sessions.
• DNS tunnels all tend to do their own idiosyncratic thing. dnscat2, one of the better-thought-out ones, uses explicit SEQ and ACK numbers.

My position is that SEQ/ACK schemes are subtle enough and independent enough that they should be treated as a separate layer, not as an underspecified and undertested component of some specific system.

Psiphon can use obfuscated QUIC as a transport. It's directly using QUIC UDP on the wire, except that each UDP datagram is additionally obfuscated before being sent. You can view my proposal as an extension of this design: instead of always sending QUIC packets as single UDP datagrams, we allow them to be encoded/encapsulated into a variety of carriers.

MASQUE tunnels over HTTPS and can use QUIC, but is not really an example of the kind of design I'm talking about. It leverages the multiplexing provided by HTTP/2 (over TLS/TCP) or HTTP/3 (over QUIC/UDP). In HTTP/2 mode it does not introduce its own session or reliability layer (instead using that of the underlying TCP connection); and in HTTP/3 mode it directly exposes the QUIC packets on the network as UDP datagrams, instead of encapsulating them as an inner layer. That is, it's using QUIC as a carrier for HTTP, rather than HTTP as a carrier for QUIC. The main similarity I spot in the MASQUE draft is the envisioned connection migration which frees the circumvention session from specific endpoint IP addresses.

Mike Perry wrote a detailed summary of considerations for migrating Tor to use end-to-end QUIC between the client and the exit. What Mike describes is similar to what is proposed here—especially the subtlety regarding protocol layering. The idea is not to use QUIC hop-by-hop, replacing the TLS/TCP that is used today, but to encapsulate QUIC packets end-to-end, and use some other unreliable protocol to carry them hop-by-hop between relays. Tor would not be using QUIC as the network transport, but would use features of the QUIC protocol.

Anticipated questions

• Q: Why don't VPNs like Wireguard have to worry about all this?
  • A: Because they are implemented in kernel space, not user space, they are, in effect, using the operating system's own sequencing and reliability features. Wireguard just sees IP packets; it's the kernel's responsibility to notice, for example, when a TCP segment need to be retransmitted, and retransmit it. We should do in user space what kernel-space VPNs have been doing all along!
• Q: You're proposing, in some cases, to run a reliable protocol inside another reliable protocol (e.g. QUIC-in-obfs4-in-TCP). What about the reputed inefficiency TCP-in-TCP?
  • A: Short answer: don't worry about it. I think it would be premature optimization to consider at this point. The fact that the need for a session/reliability layer has been felt for so long by so many systems indicates that we should start experimenting, at least. There's contradictory information online as to whether TCP-in-TCP is as bad as they say, and anyway there are all kinds of performance settings we may tweak if it turns out to be a problem. But again: let's avoid premature optimization and not allow imagined obstacles to prevent us from trying.
• Q: QUIC is not just a reliability protocol; it also has its own authentication and encryption based on TLS. Do we need that extra complexity if the underlying protocol (e.g. Tor) is already encrypted and authenticated independently?
  • A: The transport and TLS parts of QUIC are specified separately (draft-ietf-quic-transport and draft-ietf-quic-tls), so in principle they are separable and we could just use the transport part without encryption or authentication, as if it were SCTP or some other plaintext protocol. In practice, quic-go assumes right in the API that you'll be using TLS, so separating them may be more trouble than it's worth. Let's start with simple layering that is clearly correct, and only later start breaking abstraction for better performance if we need to.
• Q: What about traffic fingerprinting? If you simply obfuscate and send each QUIC packet as it is produced, you will leak traffic features through their size and timing, especially when you consider retransmissions and ACKs.
  • A: Don't do that, then. There's no essential reason why the traffic pattern of the obfuscation layer needs to match that of the sequencing/reliability layer. Part of the job of the obfuscation layer is to erase such traffic features, if doing so is a security requirement. That implies, at least, not simply sending each entire QUIC packet as soon as it is produced, but padding, splitting, and delaying as appropriate. An ideal traffic scheduler would be independent of the underlying stream—its implementation would not even know how much actual data is queued to send at any time. But it's likely we don't have to be quite that rigorous in implementation, at least at this point.