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Keywords: definitions, operation
Network Working Group R. Kermode
Request for Comments: 3269 Motorola
Category: Informational L. Vicisano
Cisco
April 2002
Author Guidelines for Reliable Multicast Transport (RMT) Building Blocks
and Protocol Instantiation documents
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
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.
Table of Contents
1 Introduction ................................................... 2
1.1 Terminology .................................................. 3
2 The Guidelines ................................................. 3
2.1 Building Block Document 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 .................................... 5
2.1.7 Codepoint Considerations ................................... 6
2.1.8 Summary Checklist .......................................... 6
2.2 Protocol Instantiation Document Guidelines ................... 7
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2.2.1 Applicability Statement .................................... 7
2.2.2 Architecture Definition .................................... 7
2.2.3 Conformance Statement ...................................... 8
2.2.4 Functionality Definition ................................... 8
2.2.5 Packet Formats ............................................. 9
2.2.6 Summary Checklist .......................................... 9
3 IANA Considerations ............................................ 9
4 Acknowledgements ............................................... 10
5 References ..................................................... 10
6 Authors' Addresses ............................................. 11
7 Full Copyright Statement ....................................... 12
1. Introduction
Reliable Multicast Transport (RMT) protocols can be constructed in a
variety of ways, some of which will work better for certain
situations than 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 it interacts with other building blocks, and how it 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 so
with a full understanding of the design decisions and tradeoffs that
were made earlier.
The building block approach also presents 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 efforts 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 building blocks. An
example of such an incompatibility would be situation where the
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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 specification writing, but to
augment them in a manner that better fits the RMT framework.
1.1. Terminology
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 RFC 2119 [RFC2119].
2. The Guidelines
This document provides guidelines for authors of the two main kinds
of RMT documents; building block documents and protocol instantiation
documents. The guidelines for each are as follows.
2.1. Building Block Document Guidelines
All RMT Building block documents MUST contain sections that cover the
following.
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 rationale section of a building block document must clearly
define why the particular level of granularity for the functional
decomposition resulted in that building block being 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 be usable within a single
protocol instantiation. The rationale section MUST show that the
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 document 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 document to
realize a working protocol.
2.1.3. Applicability Statement
One of the most important sections of a building block document 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 instantiations will be
able to use the building block in conformance to its applicability
constraints. Also the Applicability Statement section will enable
future building block document authors to quickly 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 block's 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 the exchanged occurs. This includes
detailed description of the data format and various communication
requirements, such as timing constraints, and network requirements
(e.g., multicast vs. unicast delivery).
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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 requirements, 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 needs.
The building block that provides Generic Router Assist functionality
is an exception to the rule stated above. For efficiency reasons,
this building block may fully specify header fields and positions of
these fields within the packet-header.
2.1.5. Requirements from other Building Blocks
Each building block will specify a 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.
2.1.6. Security Considerations
Protocol instantiations have the ultimate responsibility of
addressing security requirements, in conformance to RFC 2357.
Security considerations may not be applicable to generic building
blocks other than a specific "security" building block. Some
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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 the framing aspects for FEC message fragments but not the
algorithms used to generate the redundant data.
2.1.8. Summary Checklist
Rationale
_ Provide justification for the building block's existence
_ Provide rationale for the building block's granularity
Functionality
_ 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 (*)
Requirements from other building blocks;
_ Mandatory needs from other building blocks
Security Considerations
_ Specify as much as possible (with respect to 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 Document Guidelines
Protocol Instantiation documents 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
documents 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 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, such as per
packet acknowledgment, this SHOULD be included.
3) The applicability statement MUST identify the intended
environments for the protocol's 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 protocol's operation.
2) Full enumeration and overview of which Building Blocks are used
with explicit references to their documents that define them.
3) An overview of how the aforementioned building blocks are to be
joined.
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 RFC 2357 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 be 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 define 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 Reliable Multicast Transport protocol
instantiation 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
3. IANA Considerations
There are no explicit IANA considerations for this document.
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4. Acknowledgements
This document represents an overview of the mandatory elements
required for the specification of building blocks and protocol
instantiations within the 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
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC791] Postel, J., "Darpa Internet Protocol Specification", STD 5,
RFC 791, September 1981.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC2460, December 1998.
[RFC2887] Handley, M., Floyd, S., Whetten, B., Kermode, R., Vicisano,
L. and M. Luby, "The Reliable Multicast Design Space for
Bulk Data Transfer", RFC 2887, August 2000.
[RFC3048] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd,
S. and M. Luby, "Reliable Multicast Transport Building
Blocks for One-to-Many Bulk-Data Transfer", RFC 3048,
January 2001.
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6. Authors' Addresses
Roger Kermode
Motorola Australian Research Centre
Locked Bag 5028
Botany NSW 1455,
Australia.
EMail: Roger.Kermode@motorola.com
Lorenzo Vicisano
Cisco Systems,
170 West Tasman Dr.
San Jose, CA 95134, USA
EMail: lorenzo@cisco.com
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7. Full Copyright Statement
Copyright (C) The Internet Society (2002). 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 Standards process must be
followed, or as required to translate it into 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 IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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