Libipq and c programming (how to read data in packet)

MiniComa · 07-14-2012, 08:40 AM

Hello,

I am working with video packets and know that MPEG4 packets begin with hex values 00 00 01 B6. I want to know if I am working with MPEG 4 or not. How can I read information from packet to equal these values? m->payload gives only payload of packet without any headers.

Code:

 case IPQM_PACKET:
 {
// take packets from buffer here
  ipq_packet_msg_t *m = ipq_get_packet(buf);

   int x;
  if(value in m->payload[x] == value) ??
{
  printf("MPEG packet");
x++;
}
 
 // and put them back here
status = ipq_set_verdict(h, m->packet_id,NF_ACCEPT, 0, NULL);
    
  break;
     }

Full code: http://pastebin.com/raw.php?i=c2UgZkEj

MC

MiniComa · 07-15-2012, 06:27 AM

Tried memcmp() function but this didnt give me wanted results.

firstfire · 07-15-2012, 06:39 AM

Hi.

According to this man page you can read packet payload using code like

Code:

     int i;
     if( m->data_len > 0 ){ 
         for(i=0; i < m->data_len; i++)
             printf("%02x ", m->payload[i]);
         printf("\n");
     }

I think this payload contains all data except for IP headers.

Are we talking about MPEG4 streaming or transferring of mp4 file over HTTP? What do you mean by "video packet" ?

AFAICT you should decode the payload data depending on the protocol stack used. These four bytes may identify MPEG4 packet only when you know their expected position in the stream, which depends on the protocol stack involved. For example BMP file can also contain this sequence of bytes and only if you know that the file is transferred over (e.g.) TCP/HTTP protocol you may find the expected position of magic numbers.

From your description I suspect that something like MPEG packetized stream is used in your case, but it is hard to say how is it transmitted over IP.

P.S. According to this page libipq has been deprecated in favour of libnetfilter_queue.

MiniComa · 07-15-2012, 07:16 AM

Hi,

Thank you firstfire for replying.

I am using vlc for UDP streaming. All the information is encapsulated in MPEG TS. I use iptables to get all the UDP packets from server to my buffer first. I think that libipq remove all headers (IP,UDP,TCP) and now payload contains MPEG TS header and data. Video packet = mpeg ts + data.

As I work with Ubuntu and execute code, I got error: "Segmentation fault (core dumped)" instantly. I will try to debug it and see where is the problem.

EDIT: I was wrong. This is part of payload of one packet (no headers removed at all

):
IP header [45 00 05 40 87 9b 00 00 80] UDP [11] 2b 96 SRC adr [c0 a8 00 65] c0 a8 00 c6 04 0e 04 d2 05 2c 5c 0f 47 00 e7 1c cc 9b 75
15 9f 70 4d 3c 0f 1e cc cc cd 45 49 a4 64 50 e4 aa e4 6....
I know that I send MPEG 4 but cant find value 00 00 01 B6. So this value can be incorrect.

EDIT 2: I do not use .mp4 container. Information in first post was misleading.

EDIT 3: 45 00 05 40 87 95 00 00 80 11 2b 9c c0 a8 00 65 c0 a8 00 c6 04 0e 04 d2 05 2c ee f3 MPEG TS sync byte 47 00 e7 and e7 = 1110 0111. From MPEG Transport stream
it say that there is only adaptation field and no payload. So the problem can be here.

MiniComa · 07-15-2012, 08:22 AM

I have taken packets where there are only payload but havent find wanted value.

firstfire · 07-15-2012, 01:27 PM

Quote:

Originally Posted by MiniComa

EDIT: I was wrong. This is part of payload of one packet (no headers removed at all

):
IP header [45 00 05 40 87 9b 00 00 80] UDP [11] 2b 96 SRC adr [c0 a8 00 65] c0 a8 00 c6 04 0e 04 d2 05 2c 5c 0f 47 00 e7 1c cc 9b 75
15 9f 70 4d 3c 0f 1e cc cc cd 45 49 a4 64 50 e4 aa e4 6....
I know that I send MPEG 4 but cant find value 00 00 01 B6. So this value can be incorrect.

EDIT 3: 45 00 05 40 87 95 00 00 80 11 2b 9c c0 a8 00 65 c0 a8 00 c6 04 0e 04 d2 05 2c ee f3 MPEG TS sync byte 47 00 e7 and e7 = 1110 0111. From MPEG Transport stream
it say that there is only adaptation field and no payload. So the problem can be here.

As you already figured out, 0x47 looks very much like sync byte. Also 0x040e = 1038 and 0x04d2=1234 are probably source and destination ports. 4 byte Transport Stream Header can be decoded as

Code:

47                  sync byte
00 = 0b00000000
       ^^^^
       |||First 5 bits of packet ID (PID) = 0
       ||Normal priority
       |Not start of a PES or PSI
       No transport error

e7 = 231            Second part of Packet ID. Complete PID = 231.

1c = 0b00011100
       ^^^^^
        | |Continuity counter = 0b1100 = 12
        | Adaptation field exist = 01 = payload only
        Not scrambled

So, my analysis differs from yours in that you have payload only, no adaptation field. And we should note that this packet is not start of Packetized elementary stream (PES), because

Code:

<the byte after sync byte, i.e. 0x00> & (1<<6) == 0

Maybe it is PES that starts with 00 00 01 <stream ID> can be used to detect MPEG4. So you probably should try to find that PES starting packets (for which the above condition is no met) and analyze them.

EDIT: This pdf file near page 49 may be useful.

MiniComa · 07-16-2012, 05:57 AM

Firstfire,

You are right. I have made too fast calculations and counted that PID only consists of 5 bits instead of 13. I will try to read pdf file first.

Edit: This pdf is useful but the main idea of what I want to do is go further and identify not only MPEG packets but I, B, P frames. I have found this information in ISO/IEC 14496-2:2001:

vop_start_code: This is the bit string ‘000001B6’ in hexadecimal. It marks the start of a video object plane.

vop_coding_type: The vop_coding_type identifies whether a VOP is an intra-coded VOP (I), predictive-coded
VOP (P), bidirectionally predictive-coded VOP (B) or sprite coded VOP (S). The meaning of vop_coding_type is
defined in Table 6-20.

Table 6-20 -- Meaning of vop_coding_type
vop_coding_type coding method
00 intra-coded (I)
01 predictive-coded (P)
10 bidirectionally-predictive-coded (B)
11 sprite (S)

Did I interpret this information incorrectly?

firstfire · 07-16-2012, 08:07 AM

Hi.

Well, I'm not really familiar with this matters.. but here is a discussion which looks related.

mithun_linux · 12-13-2012, 03:08 AM

Hi i am also working on the same thing of transport stream..im stuck there..i coudnt read the contents of the .ts file which i m having with me ...can u please post the code here so that it ll be helpfull for me to view the code and i can proceed wit tat file...??

MiniComa · 12-13-2012, 12:33 PM

Hi. Try this code: http://pastebin.com/uqQdyb3P

mithun_linux · 12-19-2012, 12:59 AM

Hi i tried that code but it is not working .. i dont see anywer to access a file which we need to process with.

mithun_linux · 12-27-2012, 03:24 AM

Can any one of you please post the code for reading the "xxx.ts" file and printing its hexdump please? i Dont know how to read it