Wireshark-dev: Re: [Wireshark-dev] Reassembling splitted PPP packets
From: philippe alarcon <[email protected]>
Date: Fri, 6 Mar 2009 16:16:56 +0100
Hello,

As far as I have seen in WireShark sources, it is able to dissect PPP packets,
and a PPP dissector is embedded.
Nevertheless the example of packet will help to understand
how it is managed by WireShark.

Regards
Philippe




Date: Fri, 6 Mar 2009 05:51:49 -0800
From: [email protected]
To: [email protected]
Subject: Re: [Wireshark-dev] Reassembling splitted PPP packets

Hello,
 
you are right, every PPP header has a length of 7 byte and I have one byte which indicates the total length of my PPP header.
Header_Size (size of all PPP header, a multiple of 7)
Msg_ID (2byte)
Freq_ID (2byte)
Start_Pos (1byte)
End_Pos (1byte)
Flag (1byte)
 ... (more PPP header)
MUX_Packet (begin of multiplexer packet)
 
and you are right - once again :). There is a mistake in tmpOffset - it should be reset to 7
tmpOffset = i * tmpOffset+1;
for i = 1, tmpOffset = 7+1 = 8
-> tmpOffset = 8;

for i = 2, tmpOffset = 2 x 7 + 1 = 15
-> tmpOffset = 7;

for i = 3, tmpOffset = 3 x 7 + 1 = 22
-> tmpOffset = 7;
 
Concerning PPP dissection: Do I have to call a special dissector or will wireshark do it?
 
I´ll send you an example of a packet on monday - today I´m not in the office...
 
Thanks
Chris

Von: philippe alarcon <[email protected]>
An: wireshark-dev <[email protected]>
Gesendet: Donnerstag, den 5. März 2009, 14:51:51 Uhr
Betreff: Re: [Wireshark-dev] Reassembling splitted PPP packets

Hello Chris,

Could you send us an example of stored packets within a pcap file ?

Then regarding your code, I think there could be a problem how
your header offset is managed (tmpOffset variable).

I have understood that the packet begins with several headers,
each header has a length of 7 octets.

tmpOffset is updated after each extracted field,
and for one header, tmpOffset = tmpOffset + 7.
Correct ?

Then when beginning the following loop, tmpOffset is updated as the following :
tmpOffset = i * tmpOffset+1;
for i = 0, tmpOffset = 0
for i = 1, tmpOffset = 7+1 = 8
for i = 2, tmpOffset = 2 x (8 + 7 + 1) = 32
for i = 3, tmpOffset = 2 x (32 + 7 + 1) = 80


Regards
Philippe

> Date: Thu, 5 Mar 2009 05:02:45 -0800
> From: [email protected]
> To: [email protected]
> Subject: [Wireshark-dev] Reassembling splitted PPP packets
>
>
> Hej,
>
> I´ve written a dissector for a multiplexer-protocol. The payload of these multiplexer packets could be PPP packets, most of these packets will be splitted to several mux packets.
> I´ve tried to reassemble these PPP packets (reading that article 9.4.1. How to reassemble split UDP packets), but it doesn´t work...
> To get the necessary data I´ve added a new header to my multiplexer packet so I have the information about the fragments.
>
> What am I doing wrong?
>
> //Check if there is a PPP packet inside
> if (sizeMuxPPPHeader > 0){
> guint16 tmpOffset = 1;
> guint16 tmpOffsetBegin = 1;
> guint16 tmpOffsetEnd = 1;
>
> //There could be more than one PPP packet in the multiplexer packet
> for (i = 0; i < sizeMuxPPPHeader/7; i++){
>
> tvbuff_t* new_tvb = NULL;
> fragment_data *frag_msg = NULL;
> guint16 msg_seqid; //ID of the message
> guint16 msg_num; //Sequence number
>
> guint8 msg_start; //Start position of PPP packet
> guint8 msg_end; //End of PPP packet
> guint8 msg_flag; //Flag of packet
>
> tmpOffset = i * tmpOffset+1;
>
> //Get the necessary data
> msg_seqid = tvb_get_ntohs(tvb, tmpOffset); tmpOffset += 2;
> msg_num = tvb_get_ntohs(tvb, tmpOffset); tmpOffset += 2;
> msg_start = tvb_get_guint8(tvb, tmpOffset); tmpOffset += 1;
> msg_end = tvb_get_guint8(tvb, tmpOffset); tmpOffset += 1;
> msg_flag = tvb_get_guint8(tvb, tmpOffset); tmpOffset += 1;
>
> //Calculate the offset
> tmpOffsetBegin = sizeMuxPPPHeader + 1 + msg_start;
> tmpOffsetEnd = sizeMuxPPPHeader + 1 + msg_end;
>
> pinfo->fragmented = TRUE;
> frag_msg = fragment_add_seq_check(tvb, tmpOffsetBegin, pinfo,
> msg_seqid, /* ID for fragments belonging together */
> msg_fragment_table, /* list of message fragments */
> msg_reassembled_table, /* list of reassembled messages */
> msg_num, /* fragment sequence number */
> tmpOffsetEnd, /* fragment length - to the end */
> msg_flag); /* More fragments? */
>
>
> new_tvb = process_reassembled_data(tvb, tmpOffsetBegin, pinfo,
> "Reassembled Message", frag_msg, &msg_frag_items,
> NULL, mux27010_tree);
>
> if (frag_msg) { /* Reassembled */
> if (check_col(pinfo->cinfo, COL_INFO))
> col_append_str(pinfo->cinfo, COL_INFO,
> " (Message Reassembled)");
> } else { /* Not last packet of reassembled Short Message */
> if (check_col(pinfo->cinfo, COL_INFO))
> col_append_fstr(pinfo->cinfo, COL_INFO,
> " (Message fragment %u)", msg_num);
> }
> if (new_tvb) { /* take it all */
> next_tvb = new_tvb;
> } else { /* make a new subset */
> next_tvb = tvb_new_subset(tvb, tmpOffsetBegin, -1, -1);
> }
>
> Regards, Chris
>
>
>
>
>
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