TOS byte for bulk transfers

Catalin Patulea cat at vv.carleton.ca
Sat Nov 23 17:52:02 WST 2013


I noticed that dropbear sets IPTOS_LOWDELAY on all sockets:
https://secure.ucc.asn.au/hg/dropbear/file/14342451d3df/dbutil.c#l190

This is great for interactive sessions, but not ideal for bulk
transfer sessions like scp or sftp. Many networks ignore the TOS byte,
but on my local network I respect it because I trust my devices and
wish to prioritize some of them (SIP phone).

The problem that I ran into was that an sftp upload slowed all the
rest of my Internet traffic to a crawl because it was prioritized.
Ideally I would like dropbear to not set that TOS byte for bulk
transfers.

The definition of "bulk transfer" seems a bit hard to pin down. Would
any "subsystem" request cause the connection to be considered bulk?
That covers sftp but what about scp. Would bulk sessions also disable
TCP_NODELAY? What about sshfs mounts (sftp subsystem) where file
operations may happen as a result of interactive user actions (low
latency is desirable)?

Is this the right place to solve this problem? Should I be fixing this
at the network layer in some way?


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