[Click] A new question

[Beyers Cronje]

Hi Khan,

It's fairly simple. Follow the following steps:

1. Get a pointer to a WritablePacket. I.e. in your push function
'WritablePacket *p = p_in->uniqueify();'
2. Get a pointer to the IP Header. I.e. 'click_ip *iph = p->ip_header();'
3. Make changes to ip header using the pointer you got in step 2.
4. Similar to make changes to the TCP Header get a pointer to the TCP
Header. I.e. 'click_tcp *tcph = p->tcp_header();'
5. Make changes to the tcp header using the pointer you got in step 4.
6. By far the easiest way to update the IP and TCP header checksums is to
use SetTCPChecksum and SetIPChecksum elements.  See
http://read.cs.ucla.edu/click/elements/settcpchecksum and
http://read.cs.ucla.edu/click/elements/setipchecksum

Notes. Before you can apply step 1 above you have to set the packet's header
annotations. This is done using either MarkIPHeader or CheckIPHeader
elements.

There are quite a few examples of elements making changes to the header,
look through the source. For one have a look at 'elements/ip/iprewriter.cc'

Beyers

FromDevice(eth0) -> MarkIPHeader(14) -> YourElement -> SetIPHeader

On 8/6/07, Muddassir Khan <muddassir.khan at gmail.com> wrote:
>
>
> hello
>
> how are u doing? i dont think u would remeber me but you mailed me a very
> nice reply to some very basic questions.
> it helped me alot and i think i understand most of the concepts.
>
> now i have a few detailed questions. they might still sound very naive . i
> hope i am able to explain them.
> as i remeber u said ur expertise lies in TCP/IP
> so here are the questions:
>
> i want to make an element whihc changes the header of IP and TCP of a
> packet, it woul dbe a push elemet , so Packet * is pushed in and pushed out.
>
> 1. I was looking at the Packet class in CLICK and i couldnt find any
> function like  " set_tcp_header() " there was one for set_ether_header and
> also for ip and ip6.
>
> if i had to change something in the IP header or TCP header, how can i set
> the packet header ( as i remeber we can only write to Writable Packet or we
> can use the set_ether_header() functions)
>
> this brings me to teh next question
>
> how can i calculate and set the new checksum for  tcp adn ip headers n
> ether cheksum
>
>
> i hope i was clear in explaingin my questions.
>
> take care
>
> Khan
>
>
>
> On 5/25/07, Beyers Cronje <bcronje at gmail.com > wrote:
> >
> > Hi,
> >
> > The following response is my personal experiences and opinions as a
> > happy Click user :)
> >
> > i have similar problem as rest of the beginners, the problem is that
> > > CLICK
> > > have evolved so much that for a starter it gives a huge setback when u
> > > look
> > > at the amount of code written.
> >
> >
> > First off I would recommend that anyone who wants to start off using
> > Click must have some basic network knowledge, ie OSI Layers, IP Addressing,
> > subnetting, routing, TCP/UDP etc. Without this I think you'll have a hard
> > time using Click.
> >
> > Before you even start looking at the code you need to understand Click's
> > architecture and how Click packet processing works. Eddie's thesis document
> > http://pdos.csail.mit.edu/papers/click:kohler-phd/thesis.pdf does a
> > great job of this without even touching on any code at all. Once you have a
> > basic understanding of the architecture and packet passing through elements
> > you can start to play with small Click configs, ie
> >
> > FromDevice(eth0) -> Counter -> Discard;
> >
> > Understand how packets are pushed or pulled from one element to the
> > next. I.e. a simple config to receive packets on eth0 and transmitting
> > them out on eth1 would look like this:
> >
> > FromDevice(eth0) -> Queue -> ToDevice(eth1);
> >
> > The above example, packets are pushed from FromDevice to the Queue. The
> > Queue element is sort of like a bridge between the push and pull paths.
> > FromDevice pushes packes into the Queue where it's stored, while ToDevice
> > pulls packets from the Queue out eth1.
> > It's crucial to understand PUSH and PULL operation and element input and
> > output ports in Click. Refer to the thesis doc above for more information.
> >
> > Use Print elements to visualize things. I.e:
> >
> > FromDevice(eth0) -> Print(FromDevicePushToQueue) -> Queue ->
> > Print(ToDevicePullFromQueue) -> ToDevice(eth1);
> > or
> > tee :: Tee(2);
> > FromDevice(eth0) -> tee;
> > tee[0] -> Print(FirstCopy) -> Discard;
> > tee[1] -> Print(SecondCopy) -> Discard;
> >
> > Only after you understand basic Click operation I would recommend
> > looking at the code. Personally some of the beauty of Click is the
> > modularity. It enables you to break things down in small isolated elements.
> > In other words there is no need to look at ALL of the code written for the
> > entire Click distribution to start learning. Start small, look at the
> > Discard, Tee, Counter etc elements. Look at the basic functions required for
> > an element - Configure(), Push()/Pull()/Simple_Action() etc.
> > Write a simple element that prints out the Source IP address of a packet
> > using click_chatter() and grow from there.
> >
> > I have been trying to study  a project which uses click framework, and
> > > for
> > > that testing purpose i tried to use the debugger mode, and it was
> > > amazing at
> > > the way the command was being shifted to so many files, ( which btw i
> > > didnt
> > > get much)
> >
> >
> > Stay away from a debugger if you want to learn a framework. You'll see A
> > LOT of jumping around in any reasonable size software project, especially so
> > for C++ or any OOP project. Debuggers are meant for debugging...
> > Rather isolate a specific element in your project and use click_chatter
> > with lots of output, or if you have to use a debugger make use of the build
> > in Step-Over/Into/Out etc features.
> >
> > FromDump is also a good tool. Use ethereal or Wireshark to capture
> > packets. Then use FromDump to 'replay' the captured packets into a Click
> > config. This way you are assured that everytime all packets will be
> > controlled and the same.
> >
> > i just want to ask if anyone has ever transferred a bitmap or .wav or
> > > any
> > > video file using click? i found one file in the element/userlevel/
> > > folder
> > > names fromfile.hh fromfile.c , but amazingly it hasn't been derived
> > > from
> > > element class, there was no documentation available for it on the
> > > click
> > > website which clearly shows that it was not the original click file.
> >
> >
> > Click is a Packet Processing Engine and NOT a transport protocol or
> > protocol stack. You can do a lot of things with Click, even implement a
> > protocol stack yourself, but it remains a packet processing engine. User
> > FromDump instead for your purposes.
> > Also remember, the very first question in the Click FAQ document is
> > surprisingly:
> >
> > Is Click experimental software? Yes.
> >
> > In other words I'm sure FromFile has/had some experimental use at some
> > stage :)
> >
> >
> > i tried looking into the fromdevice and ratedsource elements but
> > > couldn't
> > > find much help in them either.
> >
> >
> > FromDevice, PollDevice, RatedSource etc are some of the more complex
> > elements. They sit at the start of the PUSH path and implement more advanced
> > features of Click such as Tasks and Timers.
> >
> > as from the previous mails i find that not much help is being offered to
> > >
> > > people who are not that good at programming, as this group is meant
> > > for the
> > > testers of click, but if someone does reply and help me out , it would
> > > be
> > > very kind of him/her.
> >
> >
> > Why dont you post specific questions regarding the project you are
> > studying. What does the project do? Post some example click config files or
> > code segments you dont understand. Note though that my personal interests
> > and research is more focused on TCP/IP and Ethernet, so dont expect much
> > input from myself on Wireless networking with Click :)
> >
> > i hope by doing this small experiement with someone's help i would be
> > > able
> > > to understand the working of CLICK and so would help me alot in
> > > working on
> > > it for future.
> >
> >
> > I hope this helped a bit. Once you get the basics right you'll see Click
> > is a breeze.
> >
> >
> > Beyers
> >
>
>