Suppression of ACK/NACK Explosion for Reliable Multicast Protocol

Suppression of ACK/NACK Explosion for Reliable Multicast Protocol

   Disclosed is a protocol to realize reliable transmission
of data from one server to multiple clients using IP Multicast.

   Using IP Multicast, data packets sent from a server
reaches at a lot of clients almost at a time, and each client sends
an ACK or a NACK to the server.  As a result, a lot of ACKs and NACKs
are returned to the server at a time if a conventional multicast
protocol is used.  This situation is called "ACK/NACK explosion".  If
the "ACK/NACK explosion" occurred, the server can not correctly
receive ACK/NACK from clients.  This results in transmission failure.

   The protocol disclosed here suppresses "ACK/NACK explosion"
by multicasting a content of a NACK sent from one of clients to
other clients.  In this protocol, a NACK consists of a set of IDs
assigned to failed packets; i.e. a client informs the server of IDs
of packets it fails to receive.  On receiving NACKs, the server
re-transmits only the packets indicated in NACKs.

   This protocol consists from the following sequence:
  1.  A server transmits a series of packets for a data to
       clients using IP Multicast.
  2.  Each client receives the series of packets.
  3.  I) Only if a client fails to receive a series of packet,
          the client sends a NACK after a randomized waiting time
          elapsed.
     II) When the server receives NACKs from clients, it
          m...