[Winpcap-users] Transmitting Packets During Receive

[jklein at rockwellcollins.com]

Howdy,

I have requirements to create a Traffic Generator that pushes/receives raw 
Ethernet Packets with a special EtherType. WinPcap seems to be the perfect 
tool for the job... But I have run into the following issue.

My initial attempt had me create one pcap_t with a filter for the specific 
EtherType that was not sourced from my IP address.

I call pcap_sendpacket on one thread and pcap_next_ex with a 20ms timeout 
on another thread.

At first everything looked great.. Until I noticed what as I increased my 
transmit, things began to go bad. I noticed that as I increased my 
transmit, a LOT of my packets didn't make it to the destination. I sniffed 
the ethernet with a second PC and noticed that every time any packet was 
received on the transmitting PC, I didn't see any of my transmitted 
packets for several hundred milliseconds.

It seems that there is a period of time that when a packet is received, 
that calling pcap_sendpacket is stopped?

I thought that maybe this was due to having one pcap_t used by both the 
transmit and receive threads.. So I opened two separate pcap_t and only 
assigned the filter to the one that received packets. I got the exact same 
behavior..

I then completely removed the receiving thread and did not set any filter. 
All I was doing is calling pcap_sendpacket.. Still the same behavior!?!

So again, every time my PC receives a packet, there is several hundred 
milliseconds where pcap_sendpacket is unresponsive.. What would cause 
this?

Thanks,
John
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