The original MDP code release was developed between 1995-1997 and was based upon the existing IMM MBone tool. The current MDPv2 effort is focused on improving the quality and structure of the reference prototype code base as well as integrating advanced protocol concepts. Several technical ideas for integrating FEC and parity-based repairing into reliable multicasting were developed at NRL in 1996. Development began on the MDPv2 implementation in 1997 as a vehicle to demonstrate and prototype their ideas of parity-based repairing for reliable multicasting in a real data/file transfer application. Valuable design lessons have been learned in this initial integration effort and the present protocol design provides a hybrid parity and explicit-repair recovery capability for general multicast object transport reliability. Greatest round trip time (GRTT) measurements are used between the source and the group members to adaptively control a number of key protocol timers and repair windows to maintain high data transfer rates and efficiency. The design can optionally operate in EMCON (where receivers are completely silent) modes of operation.

The original MBONE Image Multicaster (imm) application was designed and implemented in 1993 as a reliable multicast tool for disseminating compressed imagery files to a group of multicast receivers. It was operated and tested within the MBONE for periodic bulk transfer of satellite imagery files using UDP/IP multicast transport as defined in RFC 1112. Besides imagery dissemination support, the IMM reliable multicast framework is useful and has been demonstrated as a multicast bulk file transfer protocol for more general data types. Get the 1993 white paper "Reliable File Transfer in the Multicast Domain" by Winston Dang for a short description of the original protocol framework.

The collaborative MDP effort during 1995-1997 developed improvements to the IMM toolkit for more general reliable multicast file transfer use. As a result, a design framework description was created and used the term Multicast Dissemination Protocol (MDP) to refer to the protocol aspects underlying IMM which provide reliable multicast file transfer. Please see the Internet Draft for more information.

The current MDPv2 effort extends this framework to include a number of more advanced protocol techniques and provides a more general software implementation.

The MDP Framework

MDP is a protocol framework that implements reliable multicasting for bulk files and other objects using application layer framing concepts. The MDP framework can basically be described as rate controlled data transmission with Selective ARQ recovery with repair cycles designed to achieve scalability in a multicast setting using random back-off NACKing with NACK suppression mechanisms. MDP also includes options for positive acknowledgement (ACK) of received data.

Basic MDP Protocol Features

  • Efficient one-to-many bulk data multicast dissemination
  • Use of selective negative acknowledgement (NACK) receiver-based protocol
  • NACK backoff to avoid receiver message implosion
  • Aggregation of control messaging for bandwidth efficiency
  • Good convergence in high error rate conditions
  • On-demand or timed dissemination of files or directories
  • Optional positive receipts from selected receivers
  • Good properties for asymmetric and tactical operation

MDPv2 extends this framework by including optional parity-based repair using forward error correction (FEC) coding techniques. Some analyses of the benefits of this hybrid approach is presented in "Erasure-based Coding for Reliable Multicast Retransmission".

MDPv2 Protocol Extensions

  • Parity-based repair mechanism for scalability with uncorrelated receiver loss.
  • Improved repair cycle timing based on automated group round-trip timing
  • Highly scalable implementation (state kept is independent of group size)
  • Support for EMCON (silent clients) modes of file transmission
  • Potential for support of non-real-time and real-time reliable and robust streaming
  • Better properties for asymmetric and tactical operation
  • Tunable protocol parameters for adaptation to extreme network environments

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