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Thinking in Java. Edycja polska. Wydanie IV

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RFC 2386           A Framework for QoS-based Routing         August 1998


   Alternate path routing [ZES97] deals with this issue by using probe
   messages to find a path with sufficient resources. Path QoS
   Computation (PQC) method, proposed in [GOA97], propagates bandwidth
   allocation information in RSVP PATH messages. A router receiving the
   PATH message gets an indication of the resource allocation only on
   those links in the path to itself from the source.  Allocation for
   the same flow on other remote branches of the multicast tree is not
   available. Thus, the PQC method may not be sufficient to find
   feasible QoS-accommodating paths to all receivers.

3.5  Granularity of Routing Decision

   Routing in the Internet is currently based only on the destination
   address of a packet.  Many multicast routing protocols require
   routing based on the source AND destination of a packet. The
   Integrated Services architecture and RSVP allow QoS determination for
   an individual flow between a source and a destination. This set of
   routing granularities presents a problem for QoS routing solutions.

   If routing based only on destination address is considered, then an
   intermediate router will route all flows between different sources
   and a given destination along the same path. This is acceptable if
   the path has adequate capacity but a problem arises if there are
   multiple flows to a destination that exceed the capacity of the link.

   One version of QOSPF [ZSSC97] determines QoS routes based on source
   and destination address.  This implies that all traffic between a
   given source and destination, regardless of the flow, will travel
   down the same route.  Again, the route must have capacity for all the
   QoS traffic for the source/destination pair.  The amount of routing
   state also increases since the routing tables must include
   source/destination pairs instead of just the destination.

   The best granularity is found when routing is based on individual
   flows but this incurs a tremendous cost in terms of the routing
   state.  Each QoS flow can be routed separately between any source and
   destination. PQC [GOA97] and alternate path routing [ZES97], are
   examples of solutions which operate at the flow level.

   Both source/destination and flow-based routing may be susceptible to
   packet looping under hop-by-hop forwarding. Suppose a node along a
   flow or source/destination-based path loses the state information for
   the flow.  Also suppose that the flow-based route is different from
   the regular destination-based route. The potential then exists for a
   routing loop to form when the node forwards a packet belonging to the
   flow using its destination-based routing table to a node that occurs

Crawley, et. al.             Informational                      [Page 8]