How to avoid fragmentation

How to avoid fragmentation

[Gervasio Bernal]

Hi all!!

I'm developing a new target extension and I need to add some extra data
and headers in TCP/UDP packets. But I'm having a problem: fragmentation.
In the header developed by me I add a new checksum field to verify the
integrity of that header. This works fine while there is no
fragmentation. If the packet is fragmented I can't recalculate that
checksum.

Does someone know how to correct or avoid this? Or have experience with
fragmentation?

Thanks a lot!

Re: How to avoid fragmentation

[Pablo Neira]

Gervasio Bernal wrote:
> I'm developing a new target extension and I need to add some extra data
> and headers in TCP/UDP packets. But I'm having a problem: fragmentation.
> In the header developed by me I add a new checksum field to verify the
> integrity of that header. This works fine while there is no
> fragmentation. If the packet is fragmented I can't recalculate that
> checksum.
> 
> Does someone know how to correct or avoid this? Or have experience with
> fragmentation?

Hm, still missing some information. You mean that you can't recalculate 
the checksum because the checksum part is missing, don't you? Use 
skb_header_pointer(...) to work with the headers, it's fragment-aware.

--
Pablo

Re: How to avoid fragmentation

[Pablo Neira]

Hi,

Gervasio Bernal wrote:
> I will show you an example:
> 
> +---------------+	
> |    IP		|	
> |    header 	|	
> |---------------|
> |    TCP	|
> |    header 	|
> |---------------|
> |  My header	|
> |---------------|
> |		|
> |   USER DATA	|
> |		|
> +---------------+
> 
> I calculte the USER DATA checksum and put it in a field of My header.
> If the packet is larger than MTU, it will be fragmented and the checksum
> calculated in My header will be incorrect when I recieve it in the other
> endpoint of the communication.
> 
> There is some form to avoid the fragmentation?

In this case, have a look at skb_checksum()

-- 
Pablo