/* 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;
A.1 RTP Data Header Validity Checks
An RTP receiver should check the validity of the RTP header on
incoming packets since they might be encrypted or might be from a
different application that happens to be misaddressed. Similarly, if
encryption is enabled, the header validity check is needed to verify
that incoming packets have been correctly decrypted, although a
failure of the header validity check (e.g., unknown payload type) may
not necessarily indicate decryption failure.
Only weak validity checks are possible on an RTP data packet from a
source that has not been heard before:
o RTP version field must equal 2.
o The payload type must be known, in particular it must not be
equal to SR or RR.
o If the P bit is set, then the last octet of the packet must
contain a valid octet count, in particular, less than the total
packet length minus the header size.
o The X bit must be zero if the profile does not specify that
the header extension mechanism may be used. Otherwise, the
Schulzrinne, et al Standards Track [Page 59]
RFC 1889 RTP January 1996
extension length field must be less than the total packet size
minus the fixed header length and padding.
o The length of the packet must be consistent with CC and
payload type (if payloads have a known length).
The last three checks are somewhat complex and not always possible,
leaving only the first two which total just a few bits. If the SSRC
identifier in the packet is one that has been received before, then
the packet is probably valid and checking if the sequence number is
in the expected range provides further validation. If the SSRC
identifier has not been seen before, then data packets carrying that
identifier may be considered invalid until a small number of them
arrive with consecutive sequence numbers.
