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I lead the Windows TCP team. We blogged about recent TCP advancements which is very relevant: https://techcommunity.microsoft.com/t5/networking-blog/algor...

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A couple of questions:

* What are the reasons for disabling TCP timestamps by default? (If you can answer) will they be eventually enabled by default? (The reason I'm asking is that Linux uses TS field as storage for syncookies, and without it will drop WScale and SACK options greatly degrading Windows TCP perf in case of a synflood.[1])

* I've noticed "Pacing Profile : off" in the `netsh interface tcp show global` output. Is that the same as tcp pacing in fq qdisc[2]? (If you can answer) will it be eventually enabled by default?

[1] https://elixir.bootlin.com/linux/v5.13-rc2/source/net/ipv4/s... [2] https://man7.org/linux/man-pages/man8/tc-fq.8.html

Windows historically defaulted to accepting timestamps when negotiated by the peer but didn't initiate the negotiation. There are benefits to timestamps and one downside (12 bytes overhead per packet). Re. syncookies, that's an interesting problem but under a severe syn attack, degraded performance is not going to be the biggest worry for the server. We might turn them on but for the other benefits, no committed plans. Re. pacing profile, no that's pacing implemented at the TCP layer itself (unlike fq disc) and is an experimental knob off by default.

re. syncookies: Linux by default starts issuing syncookies when listening socket's backlog overflows, so it may be accidentally triggered even by a small connection spike. (This, of course, is not an excuse for a service misconfiguration but it is quite common: somaxconn on Linux before 5.4 used to be 128 and many services use the default.)

re: pacing: Awesome!! I would guess it is similar to Linux "internal implementation for pacing"[1]. Looking forward to it eventually graduating form being experimental! As a datapoint: enabling pacing on our Edge hosts (circa 2017) resulted in ~17% reduction in packet loss (w/ CUBIC) and even fully eliminated queue drops on our shallow-buffered routers. There were a couple of roadbumps (e.g. "tcp: do not pace pure ack packets"[2]) but Eric Dumazet fixed all of them very quickly.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin... [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

Thanks for the heads up. We will investigate to see what fraction of connections end up losing these options. Pacing TCP is certainly on our roadmap. Our QUIC implementation MsQuic paces by default already.

Do you have any details on how or when Microsoft will roll out quick in Windows? Will it work by just specifying the quick protocol when creating a socket like with tcp?

I have a question, why is it that when opening two sockets on Windows and connecting them through TCP, there is about a 40% difference in transfer rate when sending from socket A to B, compared to sending from B to A?

That's not expected. Are you using loopback sockets or are these sockets on different endpoints? Is this unidirectional or bidirectional traffic, i.e. are you doing both transfers from A to B and B to A simultaneously?

I see the issue both on the same computer, so loopback, as well as on two computers on the same LAN. I tested with unidirectional traffic. Pretty easy to test for yourself. The problem appears to be with the TCP protocol implementation. For UDP transmission speed is the same both ways.

Are equivalents to linux' BQL/AQL, fq_codel, TCP_NOTSENT_LOWAT in the pipeline?

I cannot comment on queuing disciplines and limits in future products. Re. TCP_NOTSENT_LOWAT, you may want to look at the Ideal Send Backlog API that allows an application to have just more than BDP queued to keep the performance at the maximum throughput while minimizing the amount of data queued: https://docs.microsoft.com/en-us/windows/win32/winsock/sio-i...

Interesting. ISB does seem less ergonomic for blocking IO but it makes sense for overlapped requests.

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