RMT Working Group                                  R.Kermode/Motorola
Internet Engineering Task Force                    L.Vicisano/Cisco
INTERNET-DRAFT draft-ietf-rmt-author-guidelines-02.txt 20 November 2001
Expires 20 May 2002

Author Guidelines for RMT Building Blocks and Protocol Instantiation documents


                          Status of this Memo

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This document provides general guidelines to assist the authors of
reliable multicast transport (RMT) building block and protocol
instantiation definitions. The purpose of these guidelines is to ensure
that any building block and protocol instantiation definitions produced
contain sufficient information to fully explain their operation and use.
In addition these guidelines provide directions to specify modular and
clearly defined RMT building blocks and protocol instantiations that can
be refined and augmented to safely create new protocols for use in new
scenarios for which any existing protocols were not designed.

Copyright Notice

Copyright (C) The Internet Society (2001).  All Rights Reserved.

Draft                  RMT Draft Author Guidelines      20 November 2001

1.  Introduction

Reliable Multicast Transport (RMT) protocols can be constructed in a
variety of ways, some of which will work better for certain situations
then others. It is believed that the requirements space for reliable
multicast transport is sufficiently diverse that no one protocol can
meet all the requirements [RFC2887].  However, it is also believed that
there is sufficient commonality between the various approaches that it
should be possible to define a number of building blocks [RFC3048] from
which the various RMT protocols can be constructed.

One key benefit of this approach is that the same building block can be
used multiple times in different protocol instantiations.  Another key
benefit is that  building blocks may be upgraded as experience and
understanding is gained. For this operation to be possible the building
block needs to be clearly defined in terms of what it does, how
interacts with other building blocks and how fits into the overall
architecture of a protocol instantiation. This description should also
be sufficiently detailed so that those wishing to improve upon a
particular building block or protocol instantiation can do with a full
understanding of the design decisions and tradeoffs were made earlier.

The building block approach presents also some dangers that must be well
understood in order to avoid potential specification flaws.

The most important danger is related to inappropriate usage of building
blocks. Although any effort should be made in order to produce a modular
and reusable specification of building blocks, for practical reasons
this goal is not always fully achievable. This results in the
specification of building blocks whose applicability is context
dependent, which in turn creates the potential for the risk of co-
dependence incompatibilities between buildibg blocks. An example of such
an incompatibility would be situation where the combinations of building
blocks A and B works, the combination of building blocks B and C works,
however the combination of building blocks A, B, and C does not work.

In order to avoid misusage of and incompatibilities between building
blocks, any external dependency must be highlighted in the building
block specification. Furthermore, the specification must contain a
precise applicability statement for the building block.  Conversely, any
protocol instantiation specification must state how any building block
being used in it meets the protocol instantiation's applicability
requirements. These guidelines are not intended to replace the common
practice of Internet specioficaiton writing, but to augment in a manner

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that better fits the RMT framework.

1.1.  Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in RFC 2119 [RFC2119].

2.  The Guidelines

This document provides guidelines for authors of the two main kinds of
RMT drafts; building block drafts and protocol instantiation drafts. The
guidelines for each are as follows.

2.1.  Building Block Draft Guidelines

All RMT Building block drafts MUST contain sections that cover the

2.1.1.  Rationale

Individual building blocks SHOULD be reusable within multiple protocols
and MUST provide functionality not present within other building blocks.
If a building block is currently used in a single protocol
instantiation, then it MUST specify some functionality that is likely to
be reused in another (future) protocol instantiation.

The rational section of a building block draft must clearly define why
the particular level of granularity for the functional decomposition
that resulted in the building block was chosen. If the granularity is
too small it is highly likely that the building blocks will be trivial,
and therefore require excessive additional effort to realize a working
protocol. Conversely, if the level of granularity is too large building
blocks will only usable within a single protocol instantiation. This
section MUST show that level of granularity is appropriate so that
neither problem occurs.

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2.1.2.  Functionality

The functionality section within a building block draft MUST describe
all algorithms and functions contained within the building block. In
addition, the external interfaces for accessing these algorithms and
functions must be fully specified so that the building block can be
combined with other building blocks and any additional functionality
specified within a protocol instantiation draft to realize a working

2.1.3.  Applicability Statement

One of the most important sections of a building block draft will be the
Applicability Statement. The purpose of this section is to provide
sufficient details about the intended use of the building block so that
potential authors of protocol instantiation will be able to use the
building block in conformance to its applicability constraints.  Also
the Applicability Statement section will enable future building blocks
drafts authors to quickly be able to determine whether or not their
particular need can be met with an existing building block. For this to
be possible the Applicability Statement MUST describe

o  Intended scenarios for the building block's use

o  The building blocks known failure modes, why they occur, and how they
   can be detected.

o  A list of environmental considerations that includes but is not
   limited to whether the building block requires multi-source multicast
   or can be used in single-source only multicast networks, satellite
   networks, asymmetric networks, and wireless networks.

o  A list of potential areas of conflict or incompatibilities with other
   building blocks

2.1.4.  Packet-Header Fields

If a building block implements a functionality whose realization
requires an exchange of protocol messages between multiple agents, then
the building block specification MUST state what kind of information is
required and how this is exchanged. This includes detailed description
the data format and various communication requirements, such as timing
constraints, and network requirement (e.g. multicast vs. unicast

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Typically the data format specification is at the level of "generic
header fields" without a full bit-level header specification. Generic
header fields MAY specify additional requirement, such as representation
precision or preferred position within the packet header (this last
constraint might be dictated by efficiency concerns).

A building block specification MAY specify "abstract messages" that
carry particular information for exclusive use within the building
block, however, more frequently, it will rely on the protocol messages
specified in the protocol instantiation to carry the information it

The building block that provides Generic Router Assist functionality is
and exception to the rule state above. For efficiency reason, this
building block may fully specify header fields and position of these
fields within the packet-header.

2.1.5.  Requirements from other Building Blocks

Each building block will encapsulate a specific well defined piece of
functionality that is common to multiple protocol instantiations.
However, this does not mean that building block definitions will be
generated in isolation from other building blocks. For example, a
congestion control building block will have specific requirements
regarding loss notification from either a NACK or ACK building block.
The "Requirements from other Building Blocks" section is included to
capture these requirements so that the authors of related building
blocks can determine what functionality they need to provide in order to
use a particular building block.

Specifically, the "Requirements from other Building Blocks section" MUST
provide a complete and exhaustive enumeration of all the requirements
that will be made upon other building blocks in order for the building
block being specified to operate in its intended manner. Requirements
that SHOULD be enumerated include but are not limited to

o  Event generation for and responses to other building blocks

o  Message ordering relative to messages from other building blocks.

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2.1.6.  Security Considerations

Protocol instantiations have the ultimate responsibility of addressing
security requirements, in conformance to RFC2357.  Security
consideration may not be applicable to generic building blocks other
than a specific "security" building block.  Some building blocks,
however, may raise special security issues, either due to the nature of
communication required by the building block or due to the intended
usage of the building block in a protocol instantiation.  When special
security issues are present in a building block, its specification MUST
address them explicitly.

An example of this might be a building block that involves exchange of
data that is particularly sensitive to security attacks.

2.1.7.  Codepoint Considerations

Certain Building Blocks will specify general frameworks for describing
functionality while leaving the detail open for implementation specific
algorithms. One example of such a building block is the Forward Error
Correction (FEC) building block which describes how the framing aspects
for FEC message fragments but not the algorithms used to generate the
redundant data.

2.1.8.  Summary Checklist

   _ Provide justification for the building block's existence
   _ Provide rationale for the building block's granularity

   _ Functionality contained within the building block
   _ External interfaces

Applicability Statement
   _ Intended usage
   _ Failure modes (including means of detection if known)
   _ Environmental considerations
   _ Incompatibilities / Conflicts with other building blocks

Packet Header Fields
   _ Specification of logical packet-header fields (*)
   _ Abstract messages specifications (*)

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Requirements from other building blocks;
   _ Mandatory needs from other building blocks

Security Considerations
   _ Specify as much as possible (w.r.t. procedures, algorithms and
     data encoding), without affecting the general applicability of
     the building block.

(*) May not be applicable to some building blocks.

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2.2.  Protocol Instantiation Draft Guidelines

Protocol Instantiation drafts have one purpose: to specify how one can
combine multiple building blocks to construct a new fully specified
working protocol. To that end RMT Protocol Instantiation drafts MUST
contain the following four sections.

2.2.1.  Applicability Statement

The applicability statement's purpose is to frame the design space in
which the fully realized protocol will operate and to thereby enable
subsequent would be RMT protocol designers to determine whether or not
an existing protocol already meets their needs. For this to be possible
the applicability statement MUST adhere to the following guidelines

1)  The target application space for which the protocol is intended MUST
    be clearly identified.  For example; is the protocol to be used for
    real-time delivery or not, or non-real time file transfer.

2)  The target scale, in terms of maximum number of receivers per
    session, for which the protocol is intended MUST be clearly
    specified. If the protocol has an architectural limitation resulting
    from the optimization of another feature, say per packet
    acknowledgment, this SHOULD be included.

3)  The applicability statement MUST identify the intended environment's
    for the protocols use AND list any environments in which the
    protocol should not be used. Example environments that should be
    considered include asymmetric networks, wireless networks, and
    satellite networks.

4)  Finally, all protocols have inherent weaknesses that stem from the
    optimization for a specific feature. These weaknesses can manifest
    in spectacular failure modes when certain conditions occur. When
    known these conditions and the nature of how the subsequent failure
    can be detected MUST be included in the applicability statement.

2.2.2.  Architecture Definition

Protocol Instantiations define how to combine one or more building
blocks to create a working protocol. The Architecture Definition lays
out the framework for how this take place.  For this framework to be
complete, it MUST contain the following information:

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1)  An overview of the major facets of the protocols operation.

2)  Full enumeration and overview of which Building Blocks with explicit
    references to their documents that define them.

3)  An overview of how the aforementioned building blocks are to be

4)  A discussion of the design tradeoffs made in the selection of the
    chosen architecture.

2.2.3.  Conformance Statement

The conformance statement below MUST be included and adhered to

    "This Protocol Instantiation document, in conjunction with the
     following Building Block documents identified in [list of
     relevant building block references] completely specifies a
     working reliable multicast transport protocol that conforms
     to the requirements described in RFC 2357."

Protocol instantiation document authors are specifically reminded that
RFC2357 requires that any RMT protocol put forward for standardization
with the IETF is required to protect the network in as much as is
possible. This does not mean that RMT protocols will be held to a higher
standard than unicast transport protocols, merely that they should be
designed to perform at least as well as unicast transport protocols when
it comes to the possibility of protocol failure.

2.2.4.  Functionality Definition

Building Block documents will be incomplete in that they will specify an
abstract framework of a building block's functionality.  Complete
algorithmic specifications for each building block along with any
additional functionality MUST provided within the Protocol Instantiation
document's functionality definition.  Furthermore, this description must
show that each building block is used in accordance with its respective
applicability statement.  Finally the functionality description must
provide a description of the abstract programming interface for
interfacing the protocol instantiation with the applications that will
use it.

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2.2.5.  Packet Formats

Once all the functionality has been fully defined, the Protocol
Instantiation document must defined the packet formats that will be used
by the protocol. Each message part and the rules for their concatenation
MUST be specified for both IPv4 [RFC791] and IPv6 [RFC2460]. Support for
IPSEC [RFC2401] MUST be explicitly shown.

In recognition of the fact that protocols will evolve and that IP
protocol numbers are a scarce resource, protocol instantiations MUST
initially define packet formats for use over UDP [RFC768].  Whether or
not a particular Relibale Multicast Transport protocol instantiaion
becomes sufficiently popular to warrant its own protocol number is an
issue which will be deferred until such time that the protocol has been
sufficiently widely deployed and understood.

2.2.6.  Summary Checklist

Applicability Statement
   _ Target application space
   _ Target scale
   _ Intended environment
   _ Weaknesses and known failure modes

Architecture Definition
   _ Operational overview
   _ Building blocks used
   _ Details on how building blocks are joined

Conformance Statement
   _ Inclusion of mandatory paragraph

Functionality Definition
   _ Building block algorithmic specification
   _ Addition functionality specification
   _ Compliance with building block applicability statements
   _ Abstract program interface

Packet Formats
   _ IPv4 message parts
   _ IPv6 message parts
   _ IPSEC support
   _ Message ordering

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3.  IANA Considerations

There are no explicit IANA considerations for this document.

4.  Acknowledgments

This document represents an overview of the mandatory elements required
for the specification of building blocks and protocol instantiations
within the the for RMT working group.  The requirements presented are a
summarization of discussions held between the RMT Working Group chairs
and the participants in the IRTF Reliable Multicast Research Group.
Although the name of these participants are too numerous to list here,
the Working Group chairs would like to thank everyone who has
participated in these discussions for their contributions.

5.  References

     J. Postel, "User Datagram Protocol", RFC768, August 1980.

     J. Postel, "Darpa Internet Protocol Specification", ST5, RFC791,
     September 1981.

     S. Kent, R. Atkinson, "Security Architecture for the Internet
     Protocol", RFC2401, November 1998.

     S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
     Specification", RFC2460, December 1998.

     M. Handley, B.Whetten, R. Kermode, S.Floyd, L. Vicisano, and M.
     Luby, "The Reliable Multicast Design Space for Bulk Data Transfer,"
     Internet Draft, Internet Engineering Task Force, August 2000.

     B.Whetten, L. Vicisano, R. Kermode, M. Handley, S.Floyd, and M.
     Luby, "Reliable Multicast Transport Building Blocks for One-to-Many
     Bulk-Data Transfer," Internet  Draft, Internet Engineering Task
     Force, January 2001.

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6.  Authors' Addresses

Roger Kermode
Motorola Australian Research Centre
Locked Bag 5028
Botany  NSW  1455,

Lorenzo Vicisano
Cisco Systems,
170 West Tasman Dr.
San Jose, CA 95134, USA

7.  Full Copyright Statement

Copyright (C) The Internet Society (2000).  All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works. However, this document itself
may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
languages other than English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an "AS

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Table of Contents

1 Introduction ....................................................    2
1.1 Terminology ...................................................    3
2 The Guidelines ..................................................    3
2.1 Building Block Draft Guidelines ...............................    3
2.1.1 Rationale ...................................................    3
2.1.2 Functionality ...............................................    4
2.1.3 Applicability Statement .....................................    4
2.1.4 Packet-Header Fields ........................................    4
2.1.5 Requirements from other Building Blocks .....................    5
2.1.6 Security Considerations .....................................    6
2.1.7 Codepoint Considerations ....................................    6
2.1.8 Summary Checklist ...........................................    6
2.2 Protocol Instantiation Draft Guidelines .......................    8
2.2.1 Applicability Statement .....................................    8
2.2.2 Architecture Definition .....................................    8
2.2.3 Conformance Statement .......................................    9
2.2.4 Functionality Definition ....................................    9
2.2.5 Packet Formats ..............................................   10
2.2.6 Summary Checklist ...........................................   10
3 IANA Considerations .............................................   11
4 Acknowledgments .................................................   11
5 References ......................................................   11
6 Authors' Addresses ..............................................   12
7 Full Copyright Statement ........................................   12

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