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some info on packet loss and performance

Community Veteran
Posts: 1,990
Thanks: 5
Registered: ‎11-12-2013

some info on packet loss and performance

Quick reasoning behind why BT wholesale supposedbly acceptable 3% packet loss is stupid.
Each packet transmitted is the size of the mtu value.
If we assume a mtu of 1500 bytes making mss 1460 bytes, and a tcp window size of 64240 bytes, then there is over 400 packets in a block of data that would only require a average packet loss of just 0.25% to affect throughput.
If a packet is dropped in the block of data then it has to be retransmitted, however its not just the single packet retransmitted its the entire block that is covered by the acknowledgement.  If selective acks are been used then only a portion of the block has to be retransmitted so in other words selective acks provide a degree of resistance to packet loss, but not all senders utilise selective acks as selective acks are not a all win, they add cpu utilisation and make a content provider more vulnerable to DDOS.  If selective acks are not been used then the entire block has to be retransmitted.
so eg. if a single 1500byte packet is dropped and selective acks is not been used, the entire 64k has to be retransmitted just for one single dropped packet.
Of course now days on modern operating systems with fast connections tcp window sizes are often bigger than 64240 bytes, eg. windows vista/7/8 will only utilise window sizes under 65k if the user overides the autotuning system setting it to disabled or if the sender has capped to such a value.  eg. with a 256k window size a packet loss of 0.08% is enough, and it gets even worse as window sizes increase.
This is why during congestion tcp will reduce window sizes as smaller windows sizes equals less retransmissions.
3% packet loss been acceptable is absolute bonkers.
1 REPLY
Community Veteran
Posts: 6,824
Thanks: 1
Registered: ‎27-10-2012

Re: some info on packet loss and performance

I think the guy at BT Wholesale that said 3% packet loss is not a fault probably has no understanding of networking and the impact of packet loss. I am surprised that no one from BT Wholesale issued a further statement.