RFC 2689 (rfc2689) - Page 3 of 14

Providing Integrated Services over Low-bitrate Links

RFC 2689       Integrated Services over Low-bitrate Links September 1999


2.  Design Considerations

   The main design goal for an architecture that addresses real-time
   multimedia flows over low-bitrate links is that of minimizing the
   end-to-end delay.  More specifically, the worst case delay (after
   removing possible outliers, which are equivalent to packet losses
   from an application point of view) is what determines the playout
   points selected by the applications and thus the delay actually
   perceived by the user.

   In addition, any such architecture should obviously undertake every
   attempt to maximize the bandwidth actually available to media data;
   overheads must be minimized.

   An important component of the integrated services architecture is the
   provision of reservations for real-time flows.  One of the problems
   that systems on low-bitrate links (routers or hosts) face when
   performing admission control for such reservations is that they must
   translate the bandwidth requested in the reservation to the one
   actually consumed on the link.  Methods such as data compression
   and/or header compression can reduce the requirements on the link,
   but admission control can only make use of the reduced requirements
   in its calculations if it has enough information about the data
   stream to know how effective the compression will be.  One goal of
   the architecture therefore is to provide the integrated services
   admission control with this information.  A beneficial side effect
   may be to allow the systems to perform better compression than would
   be possible without this information.  This may make it worthwhile to
   provide this information even when it is not intended to make a
   reservation for a real-time flow.

3.  The Need for a Concerted Approach

   Many technical approaches come to mind for addressing these problems,
   in particular a new form of low-delay encapsulation to address delay
   and header compression methods to address overhead.  This section
   shows that these techniques should be combined to solve the problem.

3.1.  Real-Time Encapsulation

   The purpose of defining a real-time link-layer encapsulation protocol
   is to be able to introduce newly arrived real-time packets into the
   link-layer data stream without having to wait for the currently
   transmitted (possibly large) packet to end.  Obviously, a real-time
   encapsulation must be part of any complete solution as the problem of
   delays induced by large frames on the link can only be solved on this
   layer.




Bormann                      Informational