RFC 1458 (rfc1458) - Page 3 of 19

Requirements for Multicast Protocols

RFC 1458          Requirements for Multicast Protocols          May 1993


   Traditional imaging applications involve images on the order of 512
   by 512 pixels.  In contrast, a single image used for remote sensing
   can have tens of thousands of pixels on a side.  Multiplying the data
   volume associated with remotely sensed images by even a small number
   of users clearly motivates moving beyond the current suite of
   reliable protocols.

   Basic image communication applications involve distribution of
   individual images to multiple users for both individual and
   collaborative analyses, and network efficiency requires the use of
   multicast protocols.  Areas where multicasting offers significant
   advantages include real-time image acquisition and dissemination,
   distribution of annotated image-based reports, and image
   conferencing.  Images are viewed on a heterogeneous set of
   workstations with differing processing and display capabilities,
   traveling over a heterogeneous network with bandwidths varying by up
   to six orders of magnitude between the initial down link and the
   slowest end user.

2.2 Requirements

   Multicast protocols used for image communications must address
   several requirements.  Setting up a multicast group first requires
   assigning a multicast group address.  All multicast traffic is then
   delivered to this address, which implies that all members of the
   group must be listening for traffic with this address.

   Within an image communications architecture such as that used for the
   ImNet program, diversity and adaptability can be accommodated by
   trading quality of service (i.e., image quality) with speed of
   transmission.  Multicast support for quality-speed trades can be
   realized either through the use of different multicast groups, where
   each group receives a different image quality, or through the use of
   a single hierarchical stream with routers (or users) extracting
   relevant portions.

   Due to the current inability of routers to support selective
   transmission of partial streams, a multiple stream approach is being
   used within ImNet.  Efficient operation using a multiple stream
   approach requires that users be able to switch streams very quickly,
   and that streams with no listeners not be disseminated.
   Consequently, rapid configuration of multicast groups and rapid
   switching between multicast groups switching is essential.

   Inevitably, network congestion or buffer overruns result in packet
   loss. A full range of transport reliability is required within an
   image communications framework. For some applications such as image
   conferencing, packet loss does not present a problem as dropped mouse



Braudes & Zabele