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What Is A Duplex
Mismatch?
A duplex mismatch is a common cause of end-to-end performance
problems. It is often manifested by a connection that will work
at low speed, but drops packets when high-speed flows are attempted
(usually with some traffic flowing in the opposite direction).
It is due to a failure in the protocol used to negotiate whether
an Ethernet connection can have traffic flowing only in one
direction at a time (half-duplex) or in both directions at the
same time (full-duplex). It is apparent that a mismatch occurs
when one machine is set at full-duplex and another at half-duplex.
What is not so obvious is that:
When auto-negotiate meets auto-negotiate, the transmission rate
is full-duplex;
When half-duplex meets auto-negotiate, the rate is half-duplex.
In both cases, no problems occur. However, when auto-negotiate
meets full-duplex, auto-negotiate converts to half-duplex and
not, as you would expect, to full-duplex -– the auto-negotiation
capability is programmed to assume half-duplex unless it meets
another machine set at auto-negotiate.
If, for any reason, one side believes it is full-duplex, and
the other half-duplex, the side that is full-duplex may send
packets when the other side is not expecting them, and those
packets are dropped. To avoid these problems, a common campus
network recommendation is to always force both sides to auto-negotiate (because most new devices come preset to auto-negotiate).
Back
to Case Study
t
is a cakebox? It is a small, inexpensive PC
running Linux. It is configured such that, when you plug it
into a DHCP-enabled Ethernet port and give it power, it registers
its presence with an LDAP server where it logs its current
IP address so you can “find” it. You can then
connect to it remotely and run a series of network utilities
(like Iperf, traceroute, and pchar). Cakeboxes were developed
by Internet2 to test H.323 video conferencing capabilities
and have been used for a variety of other end-to-end performance
tests. Instructions on “building” a cakebox are
available to member institutions upon request.
DHCP-enabled Ethernet port and give it power, it registers
its presence with an LDAP server where it logs its current
IP address so you can “find” it. You can then
connect to it remotely and run a series of network utilities
(like Iperf, traceroute, and pchar). Cakeboxes were developed
by Internet2 to test H.323 video conferencing capabilities
and have been used for a variety of other end-to-end performance
tests. Instructions on “building” a cakebox are
available to member institutions upon request.
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