RE: [lwip-users] low level output question

[John Kennedy]

So my quandary is this, I have a breakpoint set in low_level_output.  When I 
send a ping I hit the breakpoint and the pbuf lwip is trying to send is an ARP 
response packet, the pbuf has len == tot_len = 1518 although the ARP response 
packet is actually only 60 bytes?


 
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John Kennedy


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-----Original Message-----
From: address@hidden [mailto:address@hidden
Sent: Friday, May 08, 2009 11:51 AM
To: Mailing list for lwIP users
Subject: Re: [lwip-users] low level output question

When allocating a pbuf, the end of it is defined by the length passed to
pbuf_alloc, while the start is defined by the layer: pbuf_alloc
allocates a pbuf (or a chain) of length + layer offset. E.g. if you call
pbuf_alloc(PBUF_IP, 10, PBUF_RAM), a pbuf with a total length of 10 + 20
+ 16 = 46 bytes (your length + IP header + ethernet header), while the
initial payload pointer points to 36 bytes after the actual reserved
memory. This can later be adjusted by calling pbuf_header.

However, a pbuf cannot be lengthend at the end*: you have to allocate a
new pbuf and chain the two to one packet. Doing that, tot_len of all
pbufs in the chain will grow by 'len' of the newly chained pbuf.
Theoretically, this could of course result in chains with tot_len > MTU
+ ethernet header, but a driver is not required to send this: the core
code makes sure this does not happen!

Simon


John Kennedy wrote:
> Ok, so what determines the length of a pbuf when it's allocated and what 
> determines if pbufs are chained or not, and if chained will tot_len ever 
> exceed the max Ethernet frame size?
> Specifically, when sending an IP packet does lwip allocate a pbuf big enough 
> for a max Ethernet frame then set tot_len to the actual packet length?



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