Ian McDonald wrote:

On 6/26/07, OBATA Noboru [off-list ref] wrote:

quoted

From: OBATA Noboru <redacted>

Make TCP_RTO_MAX a variable, and allow a user to change it via a
new sysctl entry /proc/sys/net/ipv4/tcp_rto_max.  A user can
then guarantee TCP retransmission to be more controllable, say,
at least once per 10 seconds, by setting it to 10.  This is
quite helpful on failover-capable network devices, such as an
active-backup bonding device.  On such devices, it is desirable
that TCP retransmits a packet shortly after the failover, which
is what I would like to do with this patch.  Please see
Background and Problem below for rationale in detail.

RFC2988 says this:
  (2.4) Whenever RTO is computed, if it is less than 1 second then the
        RTO SHOULD be rounded up to 1 second.

        Traditionally, TCP implementations use coarse grain clocks to
        measure the RTT and trigger the RTO, which imposes a large
        minimum value on the RTO.  Research suggests that a large
        minimum RTO is needed to keep TCP conservative and avoid
        spurious retransmissions [AP99].  Therefore, this
        specification requires a large minimum RTO as a conservative
        approach, while at the same time acknowledging that at some
        future point, research may show that a smaller minimum RTO is
        acceptable or superior.

  (2.5) A maximum value MAY be placed on RTO provided it is at least 60
        seconds.

Your code doesn't seem to meet requirements of section 2.5 as your
minimum is 1 second.

(At the risk of having another Emily Litella moment entering a 
discussion late...)

I thought that those sorts of things were generally referring to the 
_default_ setting?

I think if you're trying to solve the bonding issue then you should
solve that issue, not hack the TCP implementation as that opens it up
to abuse in other ways.

FWIW, other stacks have a "tcp_rexmit_interval_max" without too much 
trouble:

$ ndd -h tcp_rexmit_interval_max

tcp_rexmit_interval_max:

     Upper limit for computed round trip time-out. [1,7200000]
     Default: 60000 (1 minute)

[Interesting to me that the default happens to be the aforementioned 60 
seconds :) ]

In the abstract, if we wanted a quick recovery in TCP from a link 
failover, I suppose it could be possible for a machine-local link 
failover if the link-failover code could then call back up into TCP to 
say "Yo, TCP, any connections you had going over this link/path/route 
should probably go ahead and try retransmitting now rather than later."

Of course, that does seem rather more complicated than having the 
administrator set an upper bound on the RTO, and wouldn't deal with 
non-machine-local link failover.

rick jones