RTP标头C编程

Question

I create UDP packets (insert IP and UDP headers) and send across the UDP socket. I want to add a dummy RTP header to the packet that I create. I created the RTP structure and inserting the RTP header as below:

rtph->cc = 4
rtph->x = 1
rtph->p = 1
rtph->version = 2
rtph->pt = 7
rtph->m = 1
rtph->seq = 0
rtph->ts = random()
rtph->ssrc = 0

When I capture in the wireshasrk I get Unknown RTP Version 3.

Any help is appreciated

Answer 1

If you define your own struct in C for RTP you must take the byte order into account.

typedef struct _RTPHeader
{
  //first byte
#if G_BYTE_ORDER == G_LITTLE_ENDIAN
  unsigned int         CC:4;        /* CC field */
  unsigned int         X:1;         /* X field */
  unsigned int         P:1;         /* padding flag */
  unsigned int         version:2;
#elif G_BYTE_ORDER == G_BIG_ENDIAN
  unsigned int         version:2;
  unsigned int         P:1;         /* padding flag */
  unsigned int         X:1;         /* X field */
  unsigned int         CC:4;        /* CC field*/
#else
#error "G_BYTE_ORDER should be big or little endian."
#endif
  //second byte
#if G_BYTE_ORDER == G_LITTLE_ENDIAN
  unsigned int         PT:7;     /* PT field */
  unsigned int         M:1;       /* M field */
#elif G_BYTE_ORDER == G_BIG_ENDIAN
  unsigned int         M:1;         /* M field */
  unsigned int         PT:7;       /* PT field */
#else
#error "G_BYTE_ORDER should be big or little endian."
#endif
  guint16              seq_num;      /* length of the recovery */
  guint32              TS;                   /* Timestamp */
  guint32              ssrc;
} RTPHeader; //12 bytes

The struct must be 12 bytes long, so you need force the compiler to pack it into a 12bytes long struct and not padding it. Also I would add a static assert to check at compile time weather the struct really 12 bytes long, so:

#ifdef __WIN32__

#define PACKED
#pragma pack(push,1)

#else
#define PACKED __attribute__ ((__packed__))
#endif

//---------------------- STATIC ASSERT ----------------------------------
//Source: http://www.pixelbeat.org/programming/gcc/static_assert.html
#define ASSERT_CONCAT_(a, b) a##b
#define ASSERT_CONCAT(a, b) ASSERT_CONCAT_(a, b)
/* These can't be used after statements in c89. */
#ifdef __COUNTER__
#define STATIC_ASSERT(e,m) \
    ;enum { ASSERT_CONCAT(static_assert_, __COUNTER__) = 1/(!!(e)) }
#else
  /* This can't be used twice on the same line so ensure if using in headers
   * that the headers are not included twice (by wrapping in #ifndef...#endif)
   * Note it doesn't cause an issue when used on same line of separate modules
   * compiled with gcc -combine -fwhole-program.  */
#define STATIC_ASSERT(e,m) \
    ;enum { ASSERT_CONCAT(assert_line_, __LINE__) = 1/(!!(e)) }
#endif

typedef struct _RTPHeader
    {
      //first byte
    #if G_BYTE_ORDER == G_LITTLE_ENDIAN
      unsigned int         CC:4;        /* CC field */
      unsigned int         X:1;         /* X field */
      unsigned int         P:1;         /* padding flag */
      unsigned int         version:2;
    #elif G_BYTE_ORDER == G_BIG_ENDIAN
      unsigned int         version:2;
      unsigned int         P:1;         /* padding flag */
      unsigned int         X:1;         /* X field */
      unsigned int         CC:4;        /* CC field*/
    #else
    #error "G_BYTE_ORDER should be big or little endian."
    #endif
      //second byte
    #if G_BYTE_ORDER == G_LITTLE_ENDIAN
      unsigned int         PT:7;     /* PT field */
      unsigned int         M:1;       /* M field */
    #elif G_BYTE_ORDER == G_BIG_ENDIAN
      unsigned int         M:1;         /* M field */
      unsigned int         PT:7;       /* PT field */
    #else
    #error "G_BYTE_ORDER should be big or little endian."
    #endif
      guint16              seq_num;      /* length of the recovery */
      guint32              TS;                   /* Timestamp */
      guint32              ssrc;
    } RTPHeader; //12 bytes

STATIC_ASSERT (sizeof (RTPHeader) == 12, "RTPHeader size doesn't seem to be cool.");

#ifdef __WIN32__
#pragma pack(pop)
#undef PACKED

#else
#undef PACKED
#endif

The version number must be 2 and then if you write to the bytes with this struct, like this:

char buffer[1400];
RTPHeader *rtph = (RTPHeader*) buffer;
rtph->version = 2;

then theoretically it would be ok.