includes a timestamp which also indicates a sequence number for the first video frame contained in the rtp data packet since the timestamp is based on a constant frequency clock e g 90 khz for the h 263 protocol the timestamps of successive frames should be one clock period or a multiple of one clock period apart if there is a gap in the timestamps ascertained from the rtp header then it is known that a rtp packet has been lost by using the information in the rtp header the payload handler 37 can determine how many packets have been lost the payload handler 37 receives the rtp packet and converts it into an extended bitstream having the form of bitstream 40 in fig3 if a packet is missing then the packet_lost entry of the flags field is set in the bitstream information stream to indicate to the codec 36 that that packet has been lost assuming that the next packet is received the bit offset field of the bitstream information stream will indicate the next valid bitstream packet in the standard compressed bitstream 41 the payload handler 37 also fills in the appropriate information for the packet in the bitstream information stream and bitstream information trailer to be used by the codec 36 to reconstruct lost macroblocks.