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Re: [lwip-users] pbuf chain and packet queue

From: Simon Goldschmidt
Subject: Re: [lwip-users] pbuf chain and packet queue
Date: Wed, 27 Jan 2010 15:11:12 +0100

Bernhard \'Gustl\' Bauer wrote:
> Hi,
> my pbufs are large enough to contain a complete packet, so pbuf chains 
> should not occur. While hunting an error I discovered a pbuf with these 
> settings:
> pbuf->next    !=NULL
> pbuf->len     = 568
> pbuf->tot_len = 4948
> My TCP_MSS is 1460. So 4948 = 3 * TCP_MSS + 568
> Is this a pbuf chain, or a packet queue?
> The description in pbuf.c indicates the first. But in the pcap file that 
> is attached I have identified them as no. 196, 75, 77, and 79. So it 
> must be a packet queue, mustn't ist? But then the description in pbuf.c 
> is wrong.

It might be a hybrid: A pbuf chain made up of multiple packets. If enabled, 
lwIP queues out-of-sequence packets and chains the pbufs together passing them 
to your application as one when the data gets in-sequence due to missing 
packets being received.

So if you have the OOSEQ code turned on, your application must still handle 
pbuf chains correctly even if your netif driver doesn't create them.

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