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A. Algorithms Connected: An Internet Encyclopedia
A. Algorithms

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A. Algorithms

A. Algorithms

We provide examples of C code for aspects of RTP sender and receiver algorithms. There may be other implementation methods that are faster in particular operating environments or have other advantages. These implementation notes are for informational purposes only and are meant to clarify the RTP specification.

The following definitions are used for all examples; for clarity and brevity, the structure definitions are only valid for 32-bit big- endian (most significant octet first) architectures. Bit fields are assumed to be packed tightly in big-endian bit order, with no additional padding. Modifications would be required to construct a portable implementation.

   /*
    * rtp.h  --  RTP header file (RFC XXXX)
    */
   #include <sys/types.h>

   /*
    * The type definitions below are valid for 32-bit architectures and
    * may have to be adjusted for 16- or 64-bit architectures.
    */
   typedef unsigned char  u_int8;
   typedef unsigned short u_int16;
   typedef unsigned int   u_int32;
   typedef          short int16;

   /*
    * Current protocol version.
    */
   #define RTP_VERSION    2


   #define RTP_SEQ_MOD (1<<16)
   #define RTP_MAX_SDES 255      /* maximum text length for SDES */


   typedef enum {
       RTCP_SR   = 200,
       RTCP_RR   = 201,
       RTCP_SDES = 202,
       RTCP_BYE  = 203,
       RTCP_APP  = 204
   } rtcp_type_t;

   typedef enum {
       RTCP_SDES_END   = 0,
       RTCP_SDES_CNAME = 1,
       RTCP_SDES_NAME  = 2,
       RTCP_SDES_EMAIL = 3,
       RTCP_SDES_PHONE = 4,
       RTCP_SDES_LOC   = 5,
       RTCP_SDES_TOOL  = 6,
       RTCP_SDES_NOTE  = 7,
       RTCP_SDES_PRIV  = 8
   } rtcp_sdes_type_t;

   /*
    * RTP data header
    */
   typedef struct {
       unsigned int version:2;   /* protocol version */
       unsigned int p:1;         /* padding flag */
       unsigned int x:1;         /* header extension flag */
       unsigned int cc:4;        /* CSRC count */
       unsigned int m:1;         /* marker bit */
       unsigned int pt:7;        /* payload type */
       u_int16 seq;              /* sequence number */
       u_int32 ts;               /* timestamp */
       u_int32 ssrc;             /* synchronization source */
       u_int32 csrc[1];          /* optional CSRC list */
   } rtp_hdr_t;

   /*
    * RTCP common header word
    */
   typedef struct {
       unsigned int version:2;   /* protocol version */
       unsigned int p:1;         /* padding flag */
       unsigned int count:5;     /* varies by packet type */
       unsigned int pt:8;        /* RTCP packet type */
       u_int16 length;           /* pkt len in words, w/o this word */
   } rtcp_common_t;

   /*
    * Big-endian mask for version, padding bit and packet type pair
    */
   #define RTCP_VALID_MASK (0xc000 | 0x2000 | 0xfe)
   #define RTCP_VALID_VALUE ((RTP_VERSION << 14) | RTCP_SR)

   /*
    * Reception report block
    */
   typedef struct {
       u_int32 ssrc;             /* data source being reported */
       unsigned int fraction:8;  /* fraction lost since last SR/RR */
       int lost:24;              /* cumul. no. pkts lost (signed!) */
       u_int32 last_seq;         /* extended last seq. no. received */
       u_int32 jitter;           /* interarrival jitter */
       u_int32 lsr;              /* last SR packet from this source */
       u_int32 dlsr;             /* delay since last SR packet */
   } rtcp_rr_t;

   /*
    * SDES item
    */
   typedef struct {
       u_int8 type;              /* type of item (rtcp_sdes_type_t) */
       u_int8 length;            /* length of item (in octets) */
       char data[1];             /* text, not null-terminated */
   } rtcp_sdes_item_t;

   /*
    * One RTCP packet
    */
   typedef struct {
       rtcp_common_t common;     /* common header */
       union {
           /* sender report (SR) */
           struct {
               u_int32 ssrc;     /* sender generating this report */
               u_int32 ntp_sec;  /* NTP timestamp */
               u_int32 ntp_frac;
               u_int32 rtp_ts;   /* RTP timestamp */
               u_int32 psent;    /* packets sent */
               u_int32 osent;    /* octets sent */
               rtcp_rr_t rr[1];  /* variable-length list */
           } sr;

           /* reception report (RR) */
           struct {
               u_int32 ssrc;     /* receiver generating this report */
               rtcp_rr_t rr[1];  /* variable-length list */
           } rr;

           /* source description (SDES) */
           struct rtcp_sdes {
               u_int32 src;      /* first SSRC/CSRC */
               rtcp_sdes_item_t item[1]; /* list of SDES items */
           } sdes;

           /* BYE */
           struct {
               u_int32 src[1];   /* list of sources */
               /* can't express trailing text for reason */
           } bye;
       } r;
   } rtcp_t;


   typedef struct rtcp_sdes rtcp_sdes_t;


   /*
    * Per-source state information
    */
   typedef struct {
       u_int16 max_seq;        /* highest seq. number seen */
       u_int32 cycles;         /* shifted count of seq. number cycles */
       u_int32 base_seq;       /* base seq number */
       u_int32 bad_seq;        /* last 'bad' seq number + 1 */
       u_int32 probation;      /* sequ. packets till source is valid */
       u_int32 received;       /* packets received */
       u_int32 expected_prior; /* packet expected at last interval */
       u_int32 received_prior; /* packet received at last interval */
       u_int32 transit;        /* relative trans time for prev pkt */
       u_int32 jitter;         /* estimated jitter */
       /* ... */
   } source;


Next: A.1 RTP Data Header Validity Checks

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A. Algorithms

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