We encounter UDP packet loss. We are aware of the unreliable nature of UDP
but the way we loss packets is rather strange. Assume 2 PCs linked with a
cross Ethernet cable. One PC having an UDP testserver and the other an UDP
client.
Besides the UDP client we have also a tcpdump -nn -x -i eth0 > dumpfile
running. Looking at the packets (every packet has a increasing number) in
the UDP client we encounter sometimes (appears to be random…) packet loss.
Sometimes +/-50 successive packets. Even seen losses of more than 150
packets in a row. Actually we never found a loss of one packet in a row, up
till now.
When searching the dumpfile will learn that the packets do arrive at the pc!
Oke so they are lost somewhere in the stack. Why?
Some context of our UDP testserver:
The UDP server streams as fast as possible (although send socket call is
synchronize) e.g. 50 Mega bit of data and will sleep the rest of the second
before it send the next chunk of packets (50 Mbit in total). The packet size
is 1500 minus UDP /IP header lengths. Both PCs have a NIC that is 100 Mbit.
Every packet has a tracking number in the payload that is increased for
every packet.
Does anybody know where/why they are lost? Is there a max of the outstanding
packets in the kernel? If yes how do I tweak this max. How do I detect if
the stack desides to throw away some of the UDP packets (statistics).
At the moment the server side is not the problem but this is not proven yet.
Examining the dumpfile we found the also packet loss but not the
packetnumbers we found in the UDP client. So this implies that the udpdump
is not recording every packet. Is this right?
Current settings in proc/sys/net/core
hot_list_length = 128
Lo_cong = 100
Message_burst = 50
Message_cost = 5
Mod_cong = 290
Netdev_max_backlog = 300
No_cong = 20
No_cong_tresh = 20
Opt_memmax = 10240
[r/w]mem_[default/max] = 65535
Grep from the web
hot_list_length, maximum number of skb-heads to be cached, default 128 What
does this mean?
optmem_max, maximum amount of option memory buffers. What does this mean?
net.core.rmem_max, maximum receive socket buffer size, default 65535 What
does this mean? Do I suffer from it while sending /receiving packets with
the size of 1500bytes
All related information is appreciated
Thanx in advance Andre
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