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Negative Acknowledgements

Packets lost are detected using the RTP sequence number. After a packet loss, the receiver will resynchronize on the next GOB. However, as H.261 uses differential encoding, parts of the images may remain blurred as long as all corresponding MBs are not encoded in INTRA mode; i.e. absolute encoding without relation to previous frame. There are several ways to put it right.

The fastest way is to request a refreshment. As all GOB belonging to a given video image carry the same time stamp, the receiver can determine a list of GOBs which were really received for that time stamp, and thus identify the "missing blocks". Requesting a specific reinitialization of these missing blocks is more efficient than requesting a complete reinitialization of the image through the "Full Intra Request" item. But this method can cause congestion in case of numerous participants. Another method consists to periodically force INTRA encoding each MB. The INTRA refreshment rate can be raised in order to speed the recovering when the loss rate measured is important.

The format of the NACK option is as follow:

0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |F|  NACK=97    |  Length = 3   |           reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     FGOBL     |     LGOBL     |         reserved        |SIZE |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      timestamp (seconds)      |     timestamp (fraction)      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The different fields have the following values:

_____________________________________________________
   | F         |   Last option bit, as defined by RTP.  |
   |___________|________________________________________|
   | NACK      |   Negative Acknowledgement (97)        |
   |___________|________________________________________|
   | Length    |   Three 32 bits word.                  |
   |___________|________________________________________|
   | FGOBL     |   First GOB Lost:                      |
   |           |   Identifies the first GOB lost number.|
   |___________|________________________________________|
   | LGOBL     |   Last GOB Lost:                       |
   |           |   Identifies the last GOB lost number. |
   |___________|________________________________________|
   | R         |   Reserved.                            |
   |___________|________________________________________|
   | SIZE      |   Repeat the format indicator of the   |
   |           |   received image, including the number |
   |           |   of the SCIF subimage if present.     |
   |___________|________________________________________|
   | Timestamp |   The RTP timestamp of the             |
   |           |   original image                       |
   |___________|________________________________________|