[gPXE] any negative implications of setting TCP window size to(65536 - 4) ?

[Glenn Brown]

The Ethernet spec. prohibits "significant" levels our out-of-order 
delivery, so out-of-order packets are not likely to be the problem 
within Reni's Ethernet LAN.

Most Ethernet switches drop internally when packets arrive faster than 
they can be sent and buffers overflow.  (This is true even if hardware 
flow control is enabled on the links.)  Reni's 100mbit links are 
trivially overwhelmed by a gbit sender.  In such fast->slow cases, you 
should expect to see periodic retransmissions as part of TCP's normal 
congestion detection, which repeatedly ramps up the send rate linearly 
until a timeout occurs (because a packet was dropped) and then slows by 
a factor of 2 and retransmits all unacked packets.  It would be normal 
to see <=75mbit/s throughput in the gbit->100mbit direction, even with 
no other traffic on the network.  It would only be normal to see no 
retransmissions if the TCP send window was small enough to never 
overflow the buffering at the 1gbit->100mbit transition.

It sounds to me like Reni's network has <16K buffering at the 
1gbit->100mbit transition.

--Glenn