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Re: [lwip-users] TCP bandwidth limited by rate of ACKs

From: Simon Goldschmidt
Subject: Re: [lwip-users] TCP bandwidth limited by rate of ACKs
Date: Mon, 10 Oct 2011 16:30:56 +0200

Mason <address@hidden> wrote:
> I send 58,400,000 bytes from the PC to the STB.
> This takes 14.25 seconds, i.e. an average of 33 Mbit/s
> I was expecting to reach 80+ Mbit/s, so I captured the conversation
> with Wireshark, and I noticed that the sender is being throttled
> because the receiver (the STB running lwip) is not sending ACKs
> fast enough.

>From reading the wireshark capture, it seems that the receiver simply is not 
>fast enough. It correctly sends an ACK for every 2nd data packet - no 
>retransmissions involved mean no data dropped. And no data dropped normally 
>means no configuration error in lwIP. It's just processing 2 data packets 
>while in the same time the sender sends three packets.

Where does your expectation of 80+ Mbit/s come from? If the hardware is really 
fast enough, you seem to have a bottleneck somewhere in your code, supposedly 
either somewhere around your netif driver or in your OS threading? If your 
netif driver uses PBUF_POOL for rx packets, you might want to check that 
PBUF_POOL_BUFSIZE is large enough to hold a complete frame.

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