1. Introduction

   Reliable multicast transport is a desirable technology for the
   efficient and reliable distribution of data to a group on the
   Internet.  The complexities of group communication paradigms
   necessitate different protocol types and instantiations to meet the
   range of performance and scalability requirements of different
   potential reliable multicast applications and users [3].  This
   document addresses the creation of negative-acknowledgment (NACK)-
   oriented reliable multicast (NORM) protocols.  While different
   protocol instantiations may be required to meet specific application
   and network architecture demands [4], there are a number of
   fundamental components that may be common to these different
   instantiations.  This document describes the framework and common
   "building block" components relevant to multicast protocols based
   primarily on NACK operation for reliable transport.  While this
   document discusses a large set of reliable multicast components and
   issues relevant to NORM protocol design, it specifically addresses in
   detail the following building blocks which are not addressed in other
   IETF documents:

      1) NORM sender transmission strategies,

      2) NACK-oriented repair process with timer-based feedback
         suppression, and

      3) Round-trip timing for adapting NORM timers.

   The potential relationships to other reliable multicast transport
   building blocks (Forward Error Correction (FEC), congestion control)
   and general issues with NORM protocols are also discussed.  This
   document is a product of the IETF RMT WG and follows the guidelines
   provided in RFC 3269 [5].  The key words "MUST", "MUST NOT",
   "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
   "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in BCP 14, RFC 2119 [1].