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RFC 2386           A Framework for QoS-based Routing         August 1998


   earlier on the flow-based path. This is because the latter node may
   use its flow-based routing table to forward the packet again to the
   former and this can go on indefinitely.

3.6   Metrics and Path Computation

3.6.1 Metric Selection and Representation

   There are some considerations in defining suitable link and node
   metrics [WC96]. First, the metrics must represent the basic network
   properties of interest. Such metrics include residual bandwidth,
   delay and jitter.  Since the flow QoS requirements have to be mapped
   onto path metrics, the metrics define the types of QoS guarantees the
   network can support.  Alternatively, QoS-based routing cannot support
   QoS requirements that cannot be meaningfully mapped onto a reasonable
   combination of path metrics.  Second, path computation based on a
   metric or a combination of metrics must not be too complex as to
   render them impractical. In this regard, it is worthwhile to note
   that path computation based on certain combinations of metrics (e.g.,
   delay and jitter) is theoretically hard. Thus, the allowable
   combinations of metrics must be determined while taking into account
   the complexity of computing paths based on these metrics and the QoS
   needs of flows. A common strategy to allow flexible combinations of
   metrics while at the same time reduce the path computation complexity
   is to utilize "sequential filtering". Under this approach, a
   combination of metrics is ordered in some fashion, reflecting the
   importance of different metrics (e.g., cost followed by delay, etc.).
   Paths based on the primary metric are computed first (using a simple
   algorithm, e.g., shortest path) and a subset of them are eliminated
   based on the secondary metric and so forth until a single path is
   found. This is an approximation technique and it trades off global
   optimality for path computation simplicity (The filtering technique
   may be simpler, depending on the set of metrics used. For example,
   with bandwidth and cost as metrics, it is possible to first eliminate
   the set of links that do not have the requested bandwidth and then
   compute the least cost path using the remaining links.)

   Now, once suitable link and node metrics are defined, a uniform
   representation of them is required across independent domains -
   employing possibly different routing schemes - in order to derive
   path metrics consistently (path metrics are obtained by the
   composition of link and node metrics). Encoding of the maximum,
   minimum, range, and granularity of the metrics are needed. Also, the
   definitions of comparison and accumulation operators are required. In
   addition, suitable triggers must be defined for indicating a
   significant change from a minor change.  The former will cause a
   routing update to be generated. The stability of the QoS routes would


Crawley, et. al.             Informational                      [Page 9]