From: Eric Dumazet <eric.dumazet@gmail•com>
To: thomas yang <lampsu@gmail•com>
Cc: netdev@vger•kernel.org
Subject: Re: IP header identification field is zero, why?
Date: Wed, 07 Oct 2009 16:55:28 +0200 [thread overview]
Message-ID: <4ACCABE0.2070804@gmail.com> (raw)
In-Reply-To: <f4f837ab0910070704k7a193be9i285573b423b96ba2@mail.gmail.com>
thomas yang a écrit :
> This is captured on my PC (Fedora 11, Linux）
>
> [root@localhost ~]# tcpdump -i eth1 icmp -n -x
> tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
> listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
> 17:02:39.025882 IP 192.168.1.64 > 192.168.1.1: ICMP echo request, id
> 25096, seq 1, length 64
> 0x0000: 4500 0054 0000 4000 4001 b717 c0a8 0140
> ......
> 17:02:39.027866 IP 192.168.1.64 > 192.168.1.1: ICMP echo request, id
> 25096, seq 2, length 64
> 0x0000: 4500 0054 0000 4000
>
> The IP header 'identification' field is zero, why?
>
> I wrote a simple UDP server and UDP client programs, and captured some
> packets, the IP identification is also zero.
>
> Should the host increase this field for each packet it sends?
>
>
> I captured some TCP packets, all of the IP identification are
> different, not zero.
>
Very good questions, this bothered me too.
ping sends "echo request" datagrams with DF set (Dont Fragment),
and ID=0, this is a user program building a packet from scratch.
When linux replies with a "echo reply", DF is not set and an ID is included
in the answer, increasing at each packet.
About your UDP tests, DF is automatically set, and
I believe ID on DF frames is generated only for connected sockets.
cf include/net/ip.h
static inline void ip_select_ident(struct iphdr *iph, struct dst_entry *dst, struct sock *sk)
{
if (iph->frag_off & 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 && inet_sk(sk)->daddr) ?
htons(inet_sk(sk)->id++) : 0;
} else
__ip_select_ident(iph, dst, 0);
}
next prev parent reply other threads:[~2009-10-07 14:56 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-10-07 14:04 IP header identification field is zero, why? thomas yang
2009-10-07 14:55 ` Eric Dumazet [this message]
2009-10-08 9:08 ` thomas yang
2009-10-08 9:18 ` Eric Dumazet
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