This is done for simplification. FIN implies PSH, RST doesn't have
any meaning with PSH, and SYN|PSH only makes sense in the context
of TCP Fast Open, in the context of which, any data in the original
SYN already implies PSH.
This commit completely reworks packet dispatch in TCP sockets,
and brings significant improvements to processing as well.
In particular:
* Challenge ACKs now do not reset retransmit timer; instead,
TcpSocket::process directly returns a TcpRepr without altering
any internal state at all.
* Retransmit and close (aka TIME-WAIT) timers are unified
and restructured into a enum that actually matches semantics
of the timers.
* If a packet cannot be emitted, no internal state is changed.
* The dispatch of RST packets in case of connection abort
is brought in line with dispatch of all other packets.
* Packet dispatch now follows a series of steps with clean
separation of concerns, like packet processing:
1. If we should retransmit, update state to assume that
all in-flight packets are lost.
2. Prepare the packet that would be sent next, considering
the in-flight packets, if any.
3. Check if the packet contains anything new, or it's the same
as the one already in flight. If it is, bail.
4. Finalize and try to actually transmit the packet.
If we can't do that, bail.
5. Update the internal state to reflect that the packet
we've just sent is in flight.
We checked for frames too short before, but frames too long are
troublesome too, since e.g. TCP and UDP do not carry an explicit
payload length in their headers.
The use of this type has several drawbacks:
* It does not allow distinguishing between different error
conditions. In fact, we wrongly conflated some of them
before this commit.
* It does not allow propagation via ? and requires manual use
of map_err, which is especially tiresome for downstream code.
* It prevents us from expanding the set of error conditions
even if right now we have only one.
* It prevents us from blanket using Result<T> everywhere
(a nitpick at most).
Instead, use Result<T, Error> everywhere, and differentiate error
conditions where applicable.
Various parts of smoltcp require an arrow of time; a monotonically
increasing timestamp. Most obviously this is TCP sockets, but
the tracer and the pcap writer devices also benefit from having
timestamps. There are a few ways this could be implemented:
1. using a static Cell, global for the entire smoltcp crate;
2. using a static method on Device;
3. using an instance method on Device;
4. passing the current timestamp into *Interface::poll.
The first two options are undesirable because they create a notion
of global clock, and interfere e.g. with mocking.
The third option is undesirable because not all devices are
inherently tied to a particular clock, e.g. a loopback device isn't.
Therefore, the timestamp is injected into both sockets and devices
through the *Interface::poll method.
whitequark
David Coffill