Re: Need some clarification or help

["Jee J.Z." <jz105@york.ac.uk>]

Hi Christian,


> > This sounds to me like that you need two transport layers to do this at
the
> > receivers. Otherwise the encapsulated TCP packets will be forwarded as
the
> > payload of UDP packets to upper layers. The receivers even don't know
they
> > were actually TCP packets, and upper layers will get confused what these
> > payloads mean. Am I missing something obvious?
>
> What I have in mind is:
> On the receiver the TCP packet, which is part of the payload of the
> received ipq-captured UDP packet, shall be decapsulated and then injected
> to the kernel (instead of the received UDP packet). Therefore only one
> transport layer is envolved.

Ok, sorry for my misunderstanding. I think this way should work. Just for
curiosity, since multiple receivers will send out their own ACKs for TCP
packets, how does the TCP sender handle these duplicated ACKs?

> As I mentioned in my first mail, this method works very well with
> encapsulated UDP and ICMP packets, but not with TCP, although a tcpdump
> running on the receiver displays the injected IP/TCP packet, but with some
> additional bytes at the end. In other words tcpdump seems to display an
> IP/TCP packet, which according to the length field of IP shall have only
> 64 bytes but really consists of 80 bytes (numbers are only examples).

Does tcpdump show whether the checksum is correct or not? Ethereal does.

> My assumption is, that these surplus bytes beyond the end, lead to the
> problem (maybe a checksum error because of those bytes not belonging to
> the TCP packet.) I do not see why during injection the surplus bytes
> have not been deleted, although the verdict call is parameterized with
> the new length.

Is the length of surplus bytes constant? I tried injecting more bytes
before, and the system does not think these additional bytes as a part of
the original packet; instead, the system thinks it as packet suffix and does
not include it when calculating checksum. However, things might be
different -- just for your information.

Jee

> Regards
> Christian
>
>
> >
> > Cheers,
> > Jee
> >
> > > >
> > > > > Here is what I wish to do:
> > > > > For the transmission of IP packets (UDP, ICMP, TCP) between two
hosts
> > > > > I want to send these packets through a UDP tunnel.
> > > >
> > > > Tunnelling is a very different matter from converting UDP packets
into
> > TCP
> > > > packets, and should be eminently feasible.
> > >
> > > As I tried to explain, I do NOT want to convert UDP to TCP, I only
want to
> > > transport TCP packets as UDP payload.
> > >
> > > I would be glad if you can comment the method described above.
> > >
> > > Regards
> > > Christian
> > >
> > > -- 
> > > Christian Riechmann    E-Mail: riechmann@fgan.de
> > > c/o FGAN/FKIE          Tel: (+49) 228/9435 345,378
> > > Neuenahrer Strasse 20  Fax: (+49) 228/9435 685
> > > D-53343 Wachtberg, Germany
> > >
> > >
> >
> >
>
> -- 
> Christian Riechmann    E-Mail: riechmann@fgan.de
> c/o FGAN/FKIE          Tel: (+49) 228/9435 345,378
> Neuenahrer Strasse 20  Fax: (+49) 228/9435 685
> D-53343 Wachtberg, Germany
>