[brussels-dev] shrinking the ndd tunable list

[Peter Memishian]

 > > Sure, it's a workaround (it actually only clobbers it for apps that
 > > haven't explicitly requested a TTL).
 > 
 > but that would cause most well-behaved apps to be affected by
 > the workaround for the problem app

Most apps are not very sensitive to the TTL so the affect is negligible in
practice.  Again, this is a knob that is tweaked in the field in an act of
desperation.  I think it'd be fine if it was only an mdb tunable, though I
recall we got feedback that some customers are allergic to workarounds
that involve mdb.

 > Looking at the code, it seems like ips_ip_def_ttl is used for all
 > the icmp errors. This then begs the question, "so what is icmp_def_ttl?"- 
 > that's the ttl that can be set by both IP_TTL and by ndd for raw sockets
 > Then there's tcp_ttl: that seems to be used for creating the
 > header template for all tcp packets on a conn. All the ulp ttls seem to 
 > follow a similar pattern. And all of them can be over-ridden by the
 > IP_TTL sockopt.

I don't know why there are so many of these knobs; I suspect the source
revision history will tell some interesting stories :-)

 > If all we want is a sledge-hammer to force ttls, can we not achieve
 > this by at most one ndd tunable that sets the ip ttl (instead of
 > having so many ulp ttls!)?

Probably, though there is utility in having the unicast, multicast, and
broadcast tunables remain separate.

-- 
meem