[Cerowrt-devel] rsync over ssh

Dave Taht dave.taht at gmail.com
Wed Jan 29 17:18:08 EST 2014

On Wed, Jan 29, 2014 at 1:27 PM, Michael Richardson <mcr at sandelman.ca> wrote:
> I often run rsync over ssh to transfer files.
> I suspect that they are being classified wrong, and are competing with my
> interactive ssh sessions.  I experience classic bufferbloat issues with a
> congested uplink when typing.  (But, DNS and web is unaffected, it seems)

0) you should measure your delay and post...

1) I switched to mosh for most of my interactive sessions that I used
to use ssh for. Helps.

2) there are patches to rsync to use a different cc algo and diffserv
marking, but those
only work over native tcp.


> I notice in ssh_config(5) man page that it says:
>      IPQoS   Specifies the IPv4 type-of-service or DSCP class for connections.  Accepted values are ``af11'',
>              ``af12'', ``af13'', ``af21'', ``af22'', ``af23'', ``af31'', ``af32'', ``af33'', ``af41'', ``af42'',
>              ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'', ``cs4'', ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'',
>              ``throughput'', ``reliability'', or a numeric value.  This option may take one or two arguments, sepa-
>              rated by whitespace.  If one argument is specified, it is used as the packet class unconditionally.  If
>              two values are specified, the first is automatically selected for interactive sessions and the second
>              for non-interactive sessions.  The default is ``lowdelay'' for interactive sessions and ``throughput''
>              for non-interactive sessions.

3) I don't think rsync currently tries to set anything when using ssh
as a transport. Perhaps
those patches can be improved to pass it down to the ssh call?

> I can see no evidence that my IPv6 packets are being changed in anyway.

A lot of systems don't have the correct IPV6_TCLASS call.

> On IPv4, I see:
> 16:25:50.744290 IP (tos 0x8, ttl 64, id 62723, offset 0, flags [DF], proto TCP (6), length 52)
> > Flags [F.], cksum 0x5735 (correct), seq 623906, ack 3314, win 110, options [nop,nop,TS val 1178738167 ecr 2623900673], length 0
> I'm not sure that "tos 0x8" is particularly useful. It's DSCP 1, and I think
> it's actually being marked TOS-like.

Your rsync over ssh should get marked background (CS1) somehow, and
ssh interactive, interactive.

openssh has different behavior than dropbear. Dropbear only recently
added some support
for saner classification.

> Will any value here have any affect on default 3.10 qos.sh script?
> --
> ]               Never tell me the odds!                 | ipv6 mesh networks [
> ]   Michael Richardson, Sandelman Software Works        | network architect  [
> ]     mcr at sandelman.ca  http://www.sandelman.ca/        |   ruby on rails    [
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Dave Täht

Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html

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