2.4 and ip fragmentation question

2.4 and ip fragmentation question

[Stephan von Krawczynski]

Hello,

is ip fragmentation thought to work with multiple fragmented packets all with
same ID field, same source and destination address? Or can one consider this
situation as generally unsolvable and broken by application?

Regards,
Stephan

Re: 2.4 and ip fragmentation question (background info)

[Stephan von Krawczynski]

On Wed, 31 Dec 2003 12:23:25 +0100
Stephan von Krawczynski <skraw@ithnet.com> wrote:

> Hello,
> 
> is ip fragmentation thought to work with multiple fragmented packets all with
> same ID field, same source and destination address? Or can one consider this
> situation as generally unsolvable and broken by application?
> 
> Regards,
> Stephan

As this question obviously sounded significantly stupid enough not to be
answered I may point you to this code in 2.4 include/net/ip.h:

static inline void ip_select_ident(struct iphdr *iph, struct dst_entry *dst,
struct sock *sk)
{
        if (iph->frag_off&__constant_htons(IP_DF)) {
                /* This is only to work around buggy Windows95/2000
                 * VJ compression implementations.  If the ID field
                 * does not change, they drop every other packet in
                 * a TCP stream using header compression.
                 */
                iph->id = ((sk && sk->daddr) ? htons(sk->protinfo.af_inet.id++)
: 0);
        } else
                __ip_select_ident(iph, dst);
}


As you all know this sets the ID field inside the ip-header. Interestingly it
depends on frag_off and sk->daddr field.
I ran into an application (formerly for 2.2 kernel) where the author (!=me)
obviously was unaware of this dependency and initialised these fields after
calling ip_select_ident. The outcome was that everything runs normal during low
traffic, but when more packets were transferred it looked like a increasing
amount of packets got "0" as ID, because iph->frag_off was not initialised
correctly and the skbs were of course not zeroed. Still this would have been no
problem if these packets weren't fragmented.
What I saw was that packets got corrupted during high load (because
fragmentation obviously vomitted on the high rate of "ID=0" packets), but all
was perfectly well during low load.

Should the author have read some doc where it is clearly stated that
ip_select_ident needs a more or less completely initialised ip header to work
as expected? (other way round see my original question...)

Regards,
Stephan