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.
The changes in this commit affect the following scenario:
* Remote end sends octets 1..2, they are delivered and buffered
on local end;
* Remote end sends octets 3..4, they are lost;
* Remote end sends octets 5..6, they are delivered but cannot
be buffered on local end because we don't perform reassembly.
Before this commit, we would silently drop the segment with octets
5..6, relying on retransmission timer on the remote end. This works,
but can result in severe decrease in throughput. After this commit,
we send a duplicate ACK, which may trigger fast retransmit, if
implemented by the congestion control algorithm on the remote end.
This is a form of an uninitialized read bug; although safe it caused
panics. In short, transmit buffers received from the network stack
should be considered uninitialized (in practice they will often
contain previously transmitted packets or parts thereof). Wrapping
them with the only method we had (e.g. Ipv4Packet) treated the buffer
as if it contained a valid incoming packet, which can easily fail
with Error::Truncated.
This commit splits every `fn new(buffer: T) -> Result<Self, Error>`
method on a `Packet` into three smaller ones:
* `fn check_len(&self) -> Result<(), Error>`, purely a validator;
* `fn new(T) -> Self`, purely a wrapper;
* `fn new_checked(T) -> Result<Self, Error>`, a validating wrapper.
This makes it easy to process ingress packets (using `new_checked`),
egress packets (using `new`), and, if needed, maintain the invariants
at any point during packet construction (using `check_len`).
Fixes#17.
whitequark
Egor Karavaev