[Click] set_ip_header semantics

[Bart Braem]

Hi Eddie,

On 31 Oct 2008, at 01:37, Eddie Kohler wrote:

> Bart Braem wrote:
>> Students were doing exercise where they had to change an IP header  
>> of  a packet. What they did was use ip_header to get the current   
>> ip_header, take a copy of that struct, modify it and then set it  
>> with set_ip_header. And that did not work.
>> We were also confused in the beginning, until we realised the   
>> semantics of set_ip_header: it only sets the pointers and it does  
>> not  copy anything. In fact, do we understand correctly that the  
>> set_ip_header should be treated as an annotation function and that  
>> the  pointer set with it should lie within the packet boundaries?
>
> That's exactly correct.  In fact, since commit  
> f03d536cc8392f673fb90da298ebbacb77eb3ca1, we assert that the pointer  
> is in range.
>

I did not notice that patch, the assertion certainly makes sense  
there. Thanks!


>> We suggest to update the network header methods to better reflect   
>> these semantics.
>> We could introduce naming like set_ip_header_anno.
>> But we also suggest to change the call itself and use an offset   
>> parameter instead of a pointer to the data. This way confusion is   
>> impossible because it is clear that the network_header methods work  
>> on in-packet data.
>> What do you think?
>
> I see what you mean, but don't you think the assertion is enough?   
> Perhaps a name change to _anno() would be useful, but  
> set_ip_header() is very well established by this point.  My tendency  
> is to leave things as they are.


In my opinion a name change would be better, but I do understand that  
then also similar functions for all other headers would have to be  
changed as well to stay consistent. Which is quite a large job, but I  
am willing to create the patch myself.
It will be a large job, but it can only improve the internal  
consistency and the accessibility of Click to new users.

Regards,
Bart
-- 
Bart Braem
PATS research group - IBBT
Dept. of Mathematics and Computer Sciences
University of Antwerp
Campus Middelheim, G3.30
Middelheimlaan 1
B-2020 Antwerpen, Belgium
Phone: +32 (0)3 265.32.91
Fax: +32 (0)3 265.37.77
Web: www.pats.ua.ac.be