tcp: add Delayed ACK

[Dirbaio]

This adds Delayed ACK to TCP sockets. By default is on at 10ms. User can set another delay or turn it off, per-socket.

Some highlights of tests against a Linux stack:

Echo of a single character

Delayed ACK saves us from sending a packet, allowing the ACK to be send as part of the echo response.

Before:

2109	153.035063600	192.168.69.100	192.168.69.1	TCP	55	46560 → 6970 [PSH, ACK] Seq=2 Ack=2 Win=64256 Len=1
2110	153.035360221	192.168.69.1	192.168.69.100	TCP	54	6970 → 46560 [ACK] Seq=2 Ack=3 Win=63 Len=0
2111	153.035494863	192.168.69.1	192.168.69.100	TCP	55	6970 → 46560 [PSH, ACK] Seq=2 Ack=3 Win=64 Len=1
2112	153.056838360	192.168.69.100	192.168.69.1	TCP	54	46560 → 6970 [ACK] Seq=3 Ack=3 Win=64256 Len=0

After:

2123	303.804961760	192.168.69.100	192.168.69.1	TCP	55	46578 → 6970 [PSH, ACK] Seq=1 Ack=1 Win=64256 Len=1
2124	303.805361855	192.168.69.1	192.168.69.100	TCP	55	6970 → 46578 [PSH, ACK] Seq=1 Ack=2 Win=64 Len=1
2125	303.826386620	192.168.69.100	192.168.69.1	TCP	54	46578 → 6970 [ACK] Seq=2 Ack=2 Win=64256 Len=0

Big transfer

Here the ACK is sent every 2 received packets. Unfortunately it doesn't get coalesced with the window update, because every 2 packets we must send an immediate non-delayed ack.

Before:
2081	80.302120849	192.168.69.100	192.168.69.1	TCP	1078	47952 → 1235 [PSH, ACK] Seq=687105 Ack=1 Win=64256 Len=1024
2082	80.302160968	192.168.69.1	192.168.69.100	TCP	  54	1235 → 47952 [ACK] Seq=1 Ack=688129 Win=1024 Len=0
2083	80.302171172	192.168.69.1	192.168.69.100	TCP	  54	[TCP Window Update] 1235 → 47952 [ACK] Seq=1 Ack=688129 Win=2048 Len=0
2084	80.322883121	192.168.69.100	192.168.69.1	TCP	1078	47952 → 1235 [ACK] Seq=688129 Ack=1 Win=64256 Len=1024
2085	80.322924012	192.168.69.1	192.168.69.100	TCP	  54	1235 → 47952 [ACK] Seq=1 Ack=689153 Win=1024 Len=0
2086	80.322934248	192.168.69.1	192.168.69.100	TCP	  54	[TCP Window Update] 1235 → 47952 [ACK] Seq=1 Ack=689153 Win=2048 Len=0

After:
3169	348.618900317	192.168.69.100	192.168.69.1	TCP	1078	47976 → 1235 [ACK] Seq=530433 Ack=1 Win=64256 Len=1024
3170	348.627494172	192.168.69.100	192.168.69.1	TCP	1078	[TCP Window Full] 47976 → 1235 [PSH, ACK] Seq=531457 Ack=1 Win=64256 Len=1024
3171	348.627556721	192.168.69.1	192.168.69.100	TCP	54	1235 → 47976 [ACK] Seq=1 Ack=532481 Win=1024 Len=0
3172	348.627573109	192.168.69.1	192.168.69.100	TCP	54	[TCP Window Update] 1235 → 47976 [ACK] Seq=1 Ack=532481 Win=2048 Len=0

[Dirbaio]

Clippy is failing but it's not on my code...

• What do we do about this?
• How is it possible, if it passes on master?

[Dirbaio]

It broke spontaneously because CI runs clippy on latest stable, and 1.49 is just out which added some warnings. Maybe it's not a good idea to enforce "No warnings on latest stable"

[crawford]

@Dirbaio I figured that might happen. I've opened #407 to pin to 1.49.0 so that we can deal with the new lints on our own schedule.

[whitequark]

LGTM. I did my best to double-check the new logic, but my experience here is limited.