[Bug 3505] New: SSH_MSG_CHANNEL_WINDOW_ADJUST bottleneck

[bugzilla-daemon at mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=3505

            Bug ID: 3505
           Summary: SSH_MSG_CHANNEL_WINDOW_ADJUST bottleneck
           Product: Portable OpenSSH
           Version: -current
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: P5
         Component: Miscellaneous
          Assignee: unassigned-bugs at mindrot.org
          Reporter: mindrot at dimebar.com

tl;dr: Should we increase the maximum channel window size to allow
higher throughput for WAN data?


https://github.com/openssh/openssh-portable/commit/395ecc2bdeefd86a31562dd4145f370b816814bd
committed on 2007-06-12 is still the current #define for
CHAN_TCP_WINDOW_DEFAULT.

64*32*1024 = 2 MiB, but in 2022, I predict that networks are faster on
average than in 2007.

Some examples for 2 MiB BDP:
- 200ms @   84 mbps
- 100ms @  168 mbps
-  50ms @  336 mbps
-  25ms @  671 mbps
-  12ms @ 1398 mbps
-   6ms @ 2796 mbps
-   3ms @ 5592 mbps

Not everyone runs networks which exceed these parameters, but we do on
high-latency WANs.

The problem I'm seeing is the artificial bandwidth restriction
resulting from CHAN_TCP_WINDOW_DEFAULT being supplied to channel_new's
local_window_max.

It doesn't matter how much memory is given over to TCP buffers; because
OpenSSH (and scp, and rsync) receivers limit the channel window to
2MiB, an artificial speed limit exists.

After 15 years of
#define CHAN_TCP_WINDOW_DEFAULT (64*CHAN_TCP_PACKET_DEFAULT)

is it time for an increase? If not now, when? If not an increase,
perhaps a config option?

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