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Re: Ping flooding

  • From: Vern Paxson
  • Date: Fri Jul 12 13:09:25 1996

> Maybe in a perfect world, but given that all ISPs are not created equal, it 
> is usually the case that the two paths don't have the same latency and packet
> loss characteristics. 

That's the point I'm trying to make.  In the real world these don't actually
matter, in terms of meaning that asymmetric routing itself presents a TCP
performance problem.

The two directions of a symmetric path can also have different packet loss
characteristics, due to different levels of congestion.  So this is not
a problem specific to asymmetric routing.  (It's also usually not a problem
for bulk-transfer TCP because the acks take so little bandwidth compared to
the data, and loss of an ack is not as serious as loss of a data packet
because acks are cumulative.)

That the two directions of an asymmetric path have different latencies
doesn't in most ways matter either.  TCP's performance is determined by
the RTT - whether it can fill the pipe, whether it times out and retransmits
correctly - and not by the unidirectional prop time.  The only major real-world
application I came across that has trouble with asymmetric routing is NTP.
It has robust algorithms to avoid being fooled by asymmetric paths, but if
two NTP communities are only connected by an asymmetric path, then they will
indeed keep incorrect time with respect to one another (assuming they're not
separately synchronized, which of course is how you solve this problem in

Asymmetric routing is also a network measurement and troubleshooting pain -
I don't mean to discount that.

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