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SCION SDKv0.8.1

Tunnels

The HTTP/3 client guide sends an HTTP/3 request over SCION. This page is for the traffic that is not an HTTP/3 request: a protocol client of your own, a library that takes a java.net.Socket, or an OkHttp stack you already have. All of them travel through a CONNECT tunnel on the same client.

The page builds on the guide. It uses the client the guide builds, the same test network, and the same sample app.

What a tunnel is​

openTunnel asks the client for a byte stream to a host:port. The client resolves host to a SCION address, as it does for the host of a request URL, and opens an HTTP/3 connection to the server at that address. That server is the gateway. It accepts a CONNECT request for the host:port, opens a TCP connection to that port, and passes bytes between the two connections. The tunnel is your end of that byte stream. It travels on the HTTP/3 connection your requests use, and it has no framing of its own.

The gateway decides which hosts and ports it forwards a tunnel to. The client verifies the gateway's certificate against host, as it does for a request URL.

How a SCION host is named​

A host on SCION publishes its SCION address in DNS as a TXT record, in the TXT-based SCION address resolution (TSAR) format. The client's own resolver reads that record. The platform's resolver, which InetAddress and OkHttp's default Dns use, reads A and AAAA records only. A host that exists on SCION alone has none, so a platform lookup fails for it. Nothing on this page needs a platform lookup: every API below takes the host name and hands it to the client, which resolves it. A dnsOverride replaces the TSAR lookup for one host, for a tunnel as for a request.

The test network's server is its own gateway with two behaviours. A tunnel to localhost is an echo: every byte written into it comes back. A tunnel to http.invalid reaches an HTTP/1.1 server inside the tunnel.

The byte stream​

openTunnel returns a ScionHttp3Tunnel. The sample writes a message into the echo and reads back what comes out:

Tunnels.kt
/** Sends [message] through a tunnel to the echo and reads back what comes out. */
suspend fun echo(message: String): String =
    client.openTunnel(echoHost, port).use { tunnel ->
        tunnel.write(message.encodeToByteArray())
        // The peer sees the end of the stream. The echo ends its side in turn.
        tunnel.shutdownWrite()

        val received = ByteArrayOutputStream()
        while (true) {
            val chunk = tunnel.read(4096)
            if (chunk.isEmpty()) break
            received.write(chunk)
        }
        received.toString(Charsets.UTF_8.name())
    }

Four things about the stream matter:

  • The two directions are independent. One coroutine can read while another writes.
  • shutdownWrite ends your direction only. The peer sees the end of the stream, and your reads stay open until the peer ends its direction. A read then returns an empty array.
  • One read returns at most one frame. A payload the transport split arrives over several reads, so read in a loop until you have what you expect.
  • Cancellation closes the tunnel. A read or a write that is cancelled mid-flight may have lost the bytes it just read or written part of its data. The stream is not usable after it.

Close the tunnel when you are done with it. Closing resets the stream. A call still in flight ends with TunnelClosed.

A java.net.Socket​

ScionTunnelSocket is a java.net.Socket whose bytes travel through a tunnel. It is for a protocol library that takes a socket and knows nothing about SCION:

Tunnels.kt
suspend fun echoThroughSocket(message: String): String =
    withContext(Dispatchers.IO) {
        ScionTunnelSocket(client).use { socket ->
            socket.connect(InetSocketAddress.createUnresolved(echoHost, port))
            socket.getOutputStream().write(message.encodeToByteArray())
            socket.shutdownOutput()
            socket.getInputStream().readBytes().decodeToString()
        }
    }

connect opens the tunnel to the endpoint's host name and port. Use InetSocketAddress.createUnresolved, so that no platform lookup happens. That lookup fails for a host that exists on SCION alone, as How a SCION host is named explains. A resolved address works too: the socket takes the host name from it and ignores the IP address.

The socket keeps the Socket contract that a library expects:

  • Its streams block the calling thread. Do not use one on the main thread.
  • soTimeout bounds a read and a passed deadline arrives as SocketTimeoutException. The socket stays usable after it.
  • shutdownOutput is shutdownWrite on the tunnel.
  • close from another thread ends a blocked read with SocketException.
  • The socket options, such as tcpNoDelay and keepAlive, can be set and read, but have no effect. There is no TCP connection to apply them to.

A tunnel failure arrives as an IOException.

OkHttp​

ScionTunnelSocketFactory is a javax.net.SocketFactory that creates tunnel sockets over one client. OkHttp takes it and needs one more setting:

Tunnels.kt
private val okHttp =
    OkHttpClient
        .Builder()
        .socketFactory(ScionTunnelSocketFactory(client))
        // OkHttp resolves the host before it asks for a socket. A host that exists on SCION
        // alone has no A record.
        .dns(
            object : Dns {
                override fun lookup(hostname: String): List<InetAddress> {
                    if (hostname != HTTP_HOST) throw UnknownHostException(hostname)
                    return listOf(InetAddress.getByAddress(hostname, byteArrayOf(0, 0, 0, 0)))
                }
            },
        ).build()

/** `GET /hello` with OkHttp, as HTTP/1.1 inside a tunnel. */
suspend fun fetchWithOkHttp(): HelloScion.Reply =
    withContext(Dispatchers.IO) {
        val request = Request.Builder().url("http://$HTTP_HOST:$port/hello").build()
        okHttp.newCall(request).execute().use { response ->
            HelloScion.Reply(response.code, response.body?.string().orEmpty())
        }
    }

OkHttp resolves the host of the URL through its Dns before it asks the factory for a socket and then connects the socket to the address it got. The default Dns is the platform's resolver, which fails for a host that exists on SCION alone, as How a SCION host is named explains. The Dns above returns a placeholder address that keeps the host name. The socket takes the name from it and the tunnel carries the name to the gateway, which is where the SCION lookup happens.

The Dns refuses every host that is not meant for SCION. The two settings belong together: this factory never connects to the placeholder, but a platform socket would connect to 0.0.0.0 on the device itself. Set both on one OkHttp client. A client derived with newBuilder() inherits both.

Every connection in OkHttp's pool is one tunnel. OkHttp opens one when it needs one and reuses it for the next request to the same host, as it does with a TCP connection. Requests over the tunnel are HTTP/1.1. The library sends an https URL over TLS through the tunnel with OkHttp's own TLS stack and trust settings.

The sample uses an http URL to the test network's plain HTTP/1.1 host. The library's e2e tests fetch an https URL through the same socket with OkHttp's TLS stack over the tunnel. The sample's OkHttp client, its socket factory, and the client under them are built once and kept.

Errors​

The tunnel arms of ScionHttp3Exception add to the arms the guide lists:

  • TunnelRefused is a CONNECT the gateway answered with a status other than 2xx. It carries the status. isRetryable is true for a 5xx status and false for a 4xx status.
  • TunnelReset is a tunnel the peer reset while it was open.
  • TunnelDisconnected is a tunnel that ended because the connection under it went away, for example when the network changed. Open a new one.
  • TunnelClosed is a call on a tunnel your app already closed, or a write after shutdownWrite.

On a ScionTunnelSocket, and so in OkHttp, all of them arrive as an IOException. Its cause is the ScionHttp3Exception.

Try it​

Start the test network as the guide describes, run the sample app, and press one of the three tunnel buttons. Echo through a tunnel and Echo through a socket show the message the app sent, back from the echo. Fetch with OkHttp shows:

200

world

That is GET /hello, as HTTP/1.1 inside a CONNECT tunnel, over SCION.

Where to go next​