On 09/15/14 03:10, 李凌 wrote:
Hello,everyone!
It is my pleasure to write here for you.
I've got some problems with the wireshark that how the software confirm
if the tcp packet is out-of-order or not.
I captured a pcap file named 'example.pcap',in this file No.507, No.508
,No.509 make me confused：
（because the pcap file is too large ，it is more than 7MB，so I have to
export the right packets as plain text named No507-No509.txt ）
507 IP_ID:15689 TCP_SEQ:727452
508 IP_ID:15690 TCP_SEQ:669373------out of order
509 IP_ID:15691 TCP_SEQ:670825------TCP retransmission
No.508 Packet has a IP header ID that is 15690 which is bigger than
No.507.This means the server sended No.508 packet after No.507
packet,and wireshark captured them the same way .So,as I known ,No.508
may be a retransmission instead of out-of-order packet.However,
wireshark tags a out-of-order flag on No.508 which makes me confused,Is
there any rule I don't get? I got nothing on the Internet about this
question ，could you please help me？
Yeah, I have never really understood what "out of order" really means.
In my mind a packet would either be a retransmission (something which
should have come before the last packet we've seen) or it's ahead of the
last packet we've seen (indicating we've missed one or more packets).
The idea behind "out of order" is, I believe, to indicate when something
is too far "out of order" to be a retransmission or a simple hole in
the sequence (i.e., a couple of missed packets). Here's the logic
Wireshark uses to determine if a TCP segment is out of order:
/* If the segment came relativly close since the segment with the highest
* seen sequence number and it doesn't look like a retransmission
* then it is an OUT-OF-ORDER segment.
*/
t=(pinfo->fd->abs_ts.secs-tcpd->fwd->nextseqtime.secs)*1000000000;
t=t+(pinfo->fd->abs_ts.nsecs)-tcpd->fwd->nextseqtime.nsecs;
if( t < ooo_thres
&& tcpd->fwd->nextseq != seq + seglen ) {
if(!tcpd->ta) {
tcp_analyze_get_acked_struct(pinfo->fd->num, seq, ack, TRUE, tcpd);
}
tcpd->ta->flags|=TCP_A_OUT_OF_ORDER;
goto finished_checking_retransmission_type;
}
