• C

Retrieving Multicast Packet Contents

I have a question concerning the retrieval of a multicast packet's contents.  I need to translate the network packets that I have
been getting via multicast protocol.  Each packet has a binary representation of the message header and includes the payload
in compressed RLC format.  Its structure is as follows:

sequence number - four bytes (binary)
timestamp - four bytes (binary)
message body length - two bytes (binary)

message - RLC -compressed

Knowing this structure and that my packet is received as follows

recvStringLen = recvfrom(sock, recvString, MAXRECVSTRING, 0, NULL, 0)

How do I obtain the header and body contents?  I believe the RLC compression is thru
zlib.  How do I go about uncompressing the payload?

I am writing the program in C using Microsoft Visual C++.
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sunnycoderConnect With a Mentor Commented:
Hi jtrades,

struct mcpacket {
       unsigned int seqno;   //This assumes 4 byte ints which hold true for most machines. If available, uint32 fits better.
       unsigned int timestamp;
       char body[1];
struct mcpacket * pktptr;
recvStringLen = recvfrom(sock, recvString, MAXRECVSTRING, 0, NULL, 0);
pktptr = (struct mcpacket *)recvString;

//Now &(pktptr->body) is the beginning of your compressed body and it has recvStringLen - 8 bytes .. Of course this is assuming that you read whole packet in one call ...

>I believe the RLC compression is thruzlib.  How do I go about uncompressing the payload?
Are you sure about this? I am not much into compression but if you believe this is through zlib, then this might be the API you are looking for
 int uncompress (Bytef *dest, uLongf *destLen, const Bytef *source, uLong sourceLen);

You have pointer to source and sourceLen ... destLen and dest pointer would be the length and poiner to your output buffer where uncompressed data would be stored.

jtradesAuthor Commented:
The code provided by sunnycoder did not work that well.  Here's the code that I am currently using.

// Obtain packet
recvfrom (hSock, fromBuffer, PACKET_SIZE, 0, (struct sockaddr *)&fromAddr, &fromSize);


// fromBuffer was passed into a function (as packet) where its contents were gathered as follows
unsigned char byte1, byte2, byte3, byte4;                  // Temporary holding place for requested byte of data
int sequence=0;                                                            // Current packet's sequence number
int timestamp=0;                                                      // Current packet's timestamp
void *UnCompDataBuff = NULL;                                    // Temporary holding place for uncompressed message
uLongf UnCompSize;                                                      // Educated guess at uncompressed message size
int body_length=0;      
char compressed_message[MAX_COMPRESSED_MSG_SIZE];      // Compressed body message
char uncompressed_message[MAX_UNCOMPRESSED_MSG_SIZE];      // Uncompressed body message

// First four bytes should be sequence number
byte1 = packet[current_byte++];
byte2 = packet[current_byte++];
byte3 = packet[current_byte++];
byte4 = packet[current_byte++];

sequence = pack (byte1,  byte2, byte3, byte4, TRUE);

// Second four bytes should be timestamp
byte1 = packet[current_byte++];
byte2 = packet[current_byte++];
byte3 = packet[current_byte++];
byte4 = packet[current_byte++];

timestamp = pack (byte1,  byte2, byte3, byte4, TRUE);

// Next two bytes should be body length of message
byte1 = packet[current_byte++];
byte2 = packet[current_byte++];

body_length = pack (byte1,  byte2, NULL, NULL, FALSE);

// Copy over body message                  
for (i=0; i<body_length && j<MAX_SIZE; i++) {
   compressed_message[j] = packet[current_byte++];

// Uncompress body message (compressed RLC message)
UnCompDataBuff = malloc(UnCompSize);
uncompress((Bytef*)UnCompDataBuff, &UnCompSize, (const Bytef*)compressed_message, body_length);

   strncpy (uncompressed_message, (char *)UnCompDataBuff, UnCompSize);
   uncompressed_message[UnCompSize] = '\0';

free (UnCompDataBuff);
jtradesAuthor Commented:
Forgot to add the pack function code ->

pack (unsigned char a, unsigned char b, unsigned char c, unsigned char d, int packFour)
      // Function is used to pack two or four characters into an int.  It uses shift operations
      // to do the packing byte by byte.

      int p = a;

      p = (p << 8) | b;

      if (packFour) {
            p = (p << 8) | c;
            p = (p << 8) | d;

      return p;

} // pack
jtradesAuthor Commented:
To the administrator, I answered most of the question myself.  However, you may reward sunnycoder 100 points for providing the link to the compression code.
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