[Click] header annotations, variable meanings, etc.

[Eddie Kohler]

Hi Nick,

Check out, for example, elements/ip/setipdhcp.cc (the smaction() method).

You need to create a WritablePacket using the uniqueify() method, then you can 
simply modify its data.

    WritablePacket *wp = p->uniqueify();
    if (!wp)
        // not enough memory to create a new packet
        return 0;
    wp->data()[0] = 'W';  // or whatever

Eddie


Nicholas Murphy wrote:
> Eddie-
> 
> Thanks for your responses.  One quick followup: what is the appropriate 
> way to actually make modifications to the packet if modifying the 
> annotations doesn't actually change the packet?
> 
> Thanks,
> Nick
> 
> On Dec 9, 2006, at 7:09 PM, Eddie Kohler wrote:
> 
>> Hi Nicholas,
>>
>> Nicholas Murphy wrote:
>>> A few random (probably newbie) questions:
>>> 1) So, you seem to need to run packets through something like  
>>> CheckIPHeader in order to fill out the packet header annotations,  
>>> correct?  Do these annotations persist across elements that use  
>>> "uniquify", things like Tee, etc.?
>>
>> Yes.
>>
>>> 2) Are transport-layer annotations only filled out once you run  
>>> through something like IPClassifier?
>>
>> The CheckIPHeader annotation sets both the start-network-header 
>> annotation and the end-network-header==start-transport-header annotation.
>>
>>> 3) Am I right in thinking that variables like "network_length()"  
>>> represent the length from the beginning of the network header to the  
>>> end of the packet?
>>
>> Yes!
>>
>>> 4) Am I right in assuming there will never be any layering  
>>> information (e.g., ethernet footer) after the data?
>>
>> In practice yes, although this depends on your network layer.
>>
>>
>>> 5) Does changing the annotation variables (e.g., 
>>> ip_header()->ip_len)  actually modify the packet itself?
>>
>> Nope!
>>
>> Eddie
>>
>>> Hopefully these are all easy yes/no's. :)
>>> Thanks,
>>> Nick
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