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Keywords: NACK, Negative-acknowledgement, Oriented Reliable Multicast, NORM
Network Working Group B. Adamson
Request for Comments: 3941 NRL
Category: Experimental C. Bormann
Universitaet Bremen TZI
M. Handley
UCL
J. Macker
NRL
November 2004
Negative-Acknowledgment (NACK)-Oriented Reliable Multicast (NORM)
Building Blocks
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document discusses the creation of negative-acknowledgment
(NACK)-oriented reliable multicast (NORM) protocols. The rationale
for NORM goals and assumptions are presented. Technical challenges
for NACK-oriented (and in some cases general) reliable multicast
protocol operation are identified. These goals and challenges are
resolved into a set of functional "building blocks" that address
different aspects of NORM protocol operation. It is anticipated that
these building blocks will be useful in generating different
instantiations of reliable multicast protocols.
Adamson, et al. Experimental [Page 1]
RFC 3941 NORM Building Blocks November 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Rationale. . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Delivery Service Model . . . . . . . . . . . . . . . . . 4
2.2. Group Membership Dynamics . . . . . . . . . . . . . . . . 5
2.3. Sender/Receiver Relationships . . . . . . . . . . . . . . 5
2.4. Group Size Scalability. . . . . . . . . . . . . . . . . . 6
2.5. Data Delivery Performance . . . . . . . . . . . . . . . . 6
2.6. Network Environments. . . . . . . . . . . . . . . . . . . 6
2.7. Router/Intermediate System Assistance . . . . . . . . . . 7
3. Functionality. . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. NORM Sender Transmission. . . . . . . . . . . . . . . . . 10
3.2. NORM Repair Process . . . . . . . . . . . . . . . . . . . 11
3.2.1. Receiver NACK Process Initiation . . . . . . . . . 11
3.2.2. NACK Suppression . . . . . . . . . . . . . . . . . 13
3.2.3. NACK Content . . . . . . . . . . . . . . . . . . . 17
3.2.3.1. NACK and FEC Repair Strategies. . . . . . 17
3.2.3.2. NACK Content Format . . . . . . . . . . . 20
3.2.4. Sender Repair Response . . . . . . . . . . . . . . 21
3.3. NORM Receiver Join Policies and Procedures. . . . . . . . 23
3.4. Reliable Multicast Member Identification. . . . . . . . . 24
3.5. Data Content Identification . . . . . . . . . . . . . . . 24
3.6. Forward Error Correction (FEC). . . . . . . . . . . . . . 26
3.7. Round-trip Timing Collection. . . . . . . . . . . . . . . 27
3.7.1. One-to-Many Sender GRTT Measurement. . . . . . . . 27
3.7.2. One-to-Many Receiver RTT Measurement . . . . . . . 29
3.7.3. Many-to-Many RTT Measurement . . . . . . . . . . . 29
3.7.4. Sender GRTT Advertisement. . . . . . . . . . . . . 30
3.8. Group Size Determination/Estimation . . . . . . . . . . . 31
3.9. Congestion Control Operation. . . . . . . . . . . . . . . 31
3.10 Router/Intermediate System Assistance . . . . . . . . . . 31
3.11 NORM Applicability. . . . . . . . . . . . . . . . . . . . 31
4. Security Considerations. . . . . . . . . . . . . . . . . . . . 32
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.1. Normative References. . . . . . . . . . . . . . . . . . . 33
6.2. Informative References. . . . . . . . . . . . . . . . . . 33
7. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 35
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 36
Adamson, et al. Experimental [Page 2]
RFC 3941 NORM Building Blocks November 2004
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].
Statement of Intent
This memo contains part of the definitions necessary to fully specify
a Reliable Multicast Transport protocol in accordance with RFC 2357.
As per RFC 2357, the use of any reliable multicast protocol in the
Internet requires an adequate congestion control scheme.
While waiting for such a scheme to be available, or for an existing
scheme to be proven adequate, the Reliable Multicast Transport
working group (RMT) publishes this Request for Comments in the
"Experimental" category.
Adamson, et al. Experimental [Page 3]
RFC 3941 NORM Building Blocks November 2004
It is the intent of RMT to re-submit this specification as an IETF
Proposed Standard as soon as the above condition is met.
2. Rationale
Each potential protocol instantiation using the building blocks
presented here (and in other applicable building block documents)
will have specific criteria that may influence individual protocol
design. To support the development of applicable building blocks, it
is useful to identify and summarize driving general protocol design
goals and assumptions. These are areas that each protocol
instantiation will need to address in detail. Each building block
description in this document will include a discussion of the impact
of these design criteria. The categories of design criteria
considered here include:
1) Delivery Service Model,
2) Group Membership Dynamics,
3) Sender/receiver relationships,
4) Group Size Scalability,
5) Data Delivery Performance,
6) Network Environments, and
7) Router/Intermediate System Interactions.
All of these areas are at least briefly discussed. Additionally,
other reliable multicast transport building block documents such as
[9] have been created to address areas outside of the scope of this
document. NORM protocol instantiations may depend upon these other
building blocks as well as the ones presented here. This document
focuses on areas that are unique to NORM but may be used in concert
with the other building block areas. In some cases, a building block
may be able address a wide range of assumptions, while in other cases
there will be trade-offs required to meet different application needs
or operating environments. Where necessary, building block features
are designed to be parametric to meet different requirements. Of
course, an underlying goal will be to minimize design complexity and
to at least recommend default values for any such parameters that
meet a general purpose "bulk data transfer" requirement in a typical
Internet environment.
2.1. Delivery Service Model
The implicit goal of a reliable multicast transport protocol is the
reliable delivery of data among a group of members communicating
using IP multicast datagram service. However, the specific service
the application is attempting to provide can impact design decisions.
A most basic service model for reliable multicast transport is that
of "bulk transfer" which is a primary focus of this and other related
Adamson, et al. Experimental [Page 4]
RFC 3941 NORM Building Blocks November 2004
RMT working group documents. However, the same principles in
protocol design may also be applied to other services models, e.g.,
more interactive exchanges of small messages such as with white-
boarding or text chat. Within these different models there are
issues such as the sender's ability to cache transmitted data (or
state referencing it) for retransmission or repair. The needs for
ordering and/or causality in the sequence of transmissions and
receptions among members in the group may be different depending upon
data content. The group communication paradigm differs significantly
from the point-to-point model in that, depending upon the data
content type, some receivers may complete reception of a portion of
data content and be able to act upon it before other members have
received the content. This may be acceptable (or even desirable) for
some applications but not for others. These varying requirements
drive the need for a number of different protocol instantiation
designs. A significant challenge in developing generally useful
building block mechanisms is accommodating even a limited range of
these capabilities without defining specific application-level
details.
2.2. Group Membership Dynamics
One area where group communication can differ from point-to-point
communications is that even if the composition of the group changes,
the "thread" of communication can still exist. This contrasts with
the point-to-point communication model where, if either of the two
parties leave, the communication process (exchange of data) is
terminated (or at least paused). Depending upon application goals,
senders and receivers participating in a reliable multicast transport
"session" may be able to join late, leave, and/or potentially rejoin
while the ongoing group communication "thread" still remains
functional and useful. Also note that this can impact protocol
message content. If "late joiners" are supported, some amount of
additional information may be placed in message headers to
accommodate this functionality. Alternatively, the information may
be sent in its own message (on demand or intermittently) if the
impact to the overhead of typical message transmissions is deemed too
great. Group dynamics can also impact other protocol mechanisms such
as NACK timing, congestion control operation, etc.
2.3. Sender/Receiver Relationships
The relationship of senders and receivers among group members
requires consideration. In some applications, there may be a single
sender multicasting to a group of receivers. In other cases, there
may be more than one sender or the potential for everyone in the
group to be a sender _and_ receiver of data may exist.
Adamson, et al. Experimental [Page 5]
RFC 3941 NORM Building Blocks November 2004
2.4. Group Size Scalability
Native IP multicast [2] may scale to extremely large group sizes. It
may be desirable for some applications to scale along with the
multicast infrastructure's ability to scale. In its simplest form,
there are limits to the group size to which a NACK-oriented protocol
can apply without NACK implosion problems. Research suggests that
NORM group sizes on the order of tens of thousands of receivers may
operate with modest feedback to the sender using probabilistic,
timer-based suppression techniques [7]. However, the potential for
router assistance and/or other NACK suppression heuristics may enable
these protocols to scale to very large group sizes. In large scale
cases, it may be prohibitive for members to maintain state on all
other members (in particular, other receivers) in the group. The
impact of group size needs to be considered in the development of
applicable building blocks.
2.5. Data Delivery Performance
There is a trade-off between scalability and data delivery latency
when designing NACK-oriented protocols. If probabilistic, timer-
based NACK suppression is to be used, there will be some delays built
into the NACK process to allow suppression to occur and for the
sender of data to identify appropriate content for efficient repair
transmission. For example, backoff timeouts can be used to ensure
efficient NACK suppression and repair transmission, but this comes at
a cost of increased delivery latency and increased buffering
requirements for both senders and receivers. The building blocks
SHOULD allow applications to establish bounds for data delivery
performance. Note that application designers must be aware of the
scalability trade-off that is made when such bounds are applied.
2.6. Network Environments
The Internet Protocol has historically assumed a role of providing
service across heterogeneous network topologies. It is desirable
that a reliable multicast protocol be capable of effectively
operating across a wide range of the networks to which general
purpose IP service applies. The bandwidth available on the links
between the members of a single group today may vary between low
numbers of kbit/s for wireless links and multiple Gbit/s for high
speed LAN connections, with varying degrees of contention from other
flows. Recently, a number of asymmetric network services including
56K/ADSL modems, CATV Internet service, satellite and other wireless
communication services have begun to proliferate. Many of these are
inherently broadcast media with potentially large "fan-out" to which
IP multicast service is highly applicable. Additionally, policy
and/or technical issues may result in topologies where multicast
Adamson, et al. Experimental [Page 6]
RFC 3941 NORM Building Blocks November 2004
connectivity is limited to a single source multicast (SSM) model from
a specific source [8]. Receivers in the group may be restricted to
unicast feedback for NACKs and other messages. Consideration must be
given, in building block development and protocol design, to the
nature of the underlying networks.
2.7. Router/Intermediate System Assistance
While intermediate assistance from devices/systems with direct
knowledge of the underlying network topology may be used to leverage
the performance and scalability of reliable multicast protocols,
there will continue to be a number of instances where this is not
available or practical. Any building block components for NACK-
oriented reliable multicast SHALL be capable of operating without
such assistance. However, it is RECOMMENDED that such protocols also
consider utilizing these features when available.
3. Functionality
The previous section has presented the role of protocol building
blocks and some of the criteria that may affect NORM building block
identification/design. This section describes different building
block areas applicable to NORM protocols. Some of these areas are
specific to NACK-oriented protocols. Detailed descriptions of such
areas are provided. In other cases, the areas (e.g., node
identifiers, forward error correction (FEC), etc.) may be applicable
to other forms of reliable multicast. In those cases, the discussion
below describes requirements placed on those other general building
block areas from the standpoint of NACK-oriented reliable multicast.
Where applicable, other building block documents are referenced for
possible contribution to NORM protocols.
For each building block, a notional "interface description" is
provided to illustrate any dependencies of one building block
component upon another or upon other protocol parameters. A building
block component may require some form of "input" from another
building block component or other source to perform its function.
Any "inputs" required by a building block component and/or any
resultant "output" provided will be defined and described in each
building block component's interface description. Note that the set
of building blocks presented here do not fully satisfy each other's
"input" and "output" needs. In some cases, "inputs" for the building
blocks here must come from other building blocks external to this
document (e.g., congestion control or FEC). In other cases NORM
building block "inputs" must be satisfied by the specific protocol
instantiation or implementation (e.g., application data and control).
Adamson, et al. Experimental [Page 7]
RFC 3941 NORM Building Blocks November 2004
The following building block components relevant to NORM are
identified:
(NORM-Specific)
1) NORM Sender Transmission
2) NORM Repair Process
3) NORM Receiver Join Policies
(General Purpose)
4) Node (member) Identification
5) Data Content Identification
6) Forward Error Correction (FEC)
7) Round-trip Timing Collection
8) Group Size Determination/Estimation
9) Congestion Control Operation
10) Router/Intermediate System Assistance
11) Ancillary Protocol Mechanisms
Figure 1 provides a pictorial overview of these building block areas
and some of their relationships. For example, the content of the
data messages that a sender initially transmits depends upon the
"Node Identification", "Data Content Identification", and "FEC"
components while the rate of message transmission will generally
depend upon the "Congestion Control" component. Subsequently, the
receivers' response to these transmissions (e.g., NACKing for repair)
will depend upon the data message content and inputs from other
building block components. Finally, the sender's processing of
receiver responses will feed back into its transmission strategy.
Adamson, et al. Experimental [Page 8]
RFC 3941 NORM Building Blocks November 2004
Application Data and Control
|
v
.---------------------. .-----------------------.
| Node Identification |----------->| Sender Transmission |<------.
`---------------------' _.-' `-----------------------' |
.---------------------. _.-' .' | .--------------. |
| Data Identification |--' .'' | | Join Policy | |
`---------------------' .' ' v `--------------' |
.---------------------. .' ' .------------------------. |
.->| Congestion Control |-' ' | Receiver NACK | |
| `---------------------' .' | Repair Process | |
| .---------------------. .' | .------------------. | |
| | FEC |'. | | NACK Initiation | | |
| `---------------------'` `._ | `------------------' | |
| .---------------------. ``. `-._ | .------------------. | |
`--| RTT Collection |._` ` `->| | NACK Content | | |
`---------------------' .`- ` | `------------------' | |
.---------------------. \ `-`._ | .------------------. | |
| Group Size Est. |---.-`---`->| | NACK Suppression | | |
`---------------------'`. ` ` | `------------------' | |
.---------------------. ` ` ` `------------------------' |
| Other | ` ` ` | .-----------------. |
`---------------------' ` ` ` | |Router Assistance| |
`. ` ` v `-----------------' |
`.`' .-------------------------. |
`>| Sender NACK Processing |_____/
| and Repair Response |
`-------------------------'
^ ^
| |
.-----------------------------.
| (Security) |
`-----------------------------'
Fig. 1 - NORM Building Block Framework
The components on the left side of this figure are areas that may be
applicable beyond NORM. The most significant of these components are
discussed in other building block documents such as [9]. A brief
description of these areas and their role in the NORM protocol is
given below. The components on the right are seen as specific to
NORM protocols, most notably the NACK repair process. These areas
are discussed in detail below. Some other components (e.g.,
"Security") impact many aspects of the protocol, and others such as
"Router Assistance" may be more transparent to the core protocol
processing. The sections below describe the "NORM Sender
Adamson, et al. Experimental [Page 9]
RFC 3941 NORM Building Blocks November 2004
Transmission", "NORM Repair Process", and "RTT Collection" building
blocks in detail. The relationships to and among the other building
block areas are also discussed, focusing on issues applicable to NORM
protocol design. Where applicable, specific technical
recommendations are made for mechanisms that will properly satisfy
the goals of NORM transport for the Internet.
3.1. NORM Sender Transmission
NORM senders will transmit data content to the multicast session.
The data content will be application dependent. The sender will
transmit data content at a rate, and with message sizes, determined
by application and/or network architecture requirements. Any FEC
encoding of sender transmissions SHOULD conform with the guidelines
of [9]. When congestion control mechanisms are needed (REQUIRED for
general Internet operation), NORM transmission SHALL be controlled by
the congestion control mechanism. In any case, it is RECOMMENDED
that all data transmissions from NORM senders be subject to rate
limitations determined by the application or congestion control
algorithm. The sender's transmissions SHOULD make good utilization
of the available capacity (which may be limited by the application
and/or by congestion control). As a result, it is expected there
will be overlap and multiplexing of new data content transmission
with repair content. Other factors related to application operation
may determine sender transmission formats and methods. For example,
some consideration needs to be given to the sender's behavior during
intermittent idle periods when it has no data to transmit.
In addition to data content, other sender messages or commands may be
employed as part of protocol operation. These messages may occur
outside of the scope of application data transfer. In NORM
protocols, reliability of such protocol messages may be attempted by
redundant transmission when positive acknowledgement is prohibitive
due to group size scalability concerns. Note that protocol design
SHOULD provide mechanisms for dealing with cases where such messages
are not received by the group. As an example, a command message
might be redundantly transmitted by a sender to indicate that it is
temporarily (or permanently) halting transmission. At this time, it
may be appropriate for receivers to respond with NACKs for any
outstanding repairs they require following the rules of the NORM NACK
procedure. For efficiency, the sender should allow sufficient time
between the redundant transmissions to receive any NACK-oriented
responses from the receivers to this command.
In general, when there is any resultant NACK or other feedback
operation, the timing of redundant transmission of control messages
issued by a sender and other NORM protocol timeouts should be
dependent upon the group greatest round trip timing (GRTT) estimate
Adamson, et al. Experimental [Page 10]
RFC 3941 NORM Building Blocks November 2004
and any expected resultant NACK or other feedback operation. The
NORM GRTT is an estimate of the worst-case round-trip timing from a
sender to any receivers in the group. It is assumed that the GRTT
interval is a conservative estimate of the maximum span (with respect
to delay) of the multicast group across a network topology with
respect to given sender. NORM instantiations SHOULD be able to
dynamically adapt to a wide range of multicast network topologies.
Sender Transmission Interface Description
Inputs:
1) Application data and control
2) Sender node identifier
3) Data identifiers
4) Segmentation and FEC parameters
5) Transmission rate
6) Application controls
7) Receiver feedback messages (e.g., NACKs)
Outputs:
1) Controlled transmission of messages with headers uniquely
identifying data or repair content within the context of the
NORM session.
2) Commands indicating sender's status or other transport
control actions to be taken.
3.2. NORM Repair Process
A critical component of NORM protocols is the NACK repair process.
This includes the receiver's role in detecting and requesting repair
needs, and the sender's response to such requests. There are four
primary elements of the NORM repair process:
1) Receiver NACK process initiation,
3) NACK suppression,
2) NACK message content,
4) Sender NACK processing and response.
3.2.1. Receiver NACK Process Initiation
The NORM NACK process (cycle) will be initiated by receivers that
detect a need for repair transmissions from a specific sender to
achieve reliable reception. When FEC is applied, a receiver should
Adamson, et al. Experimental [Page 11]
RFC 3941 NORM Building Blocks November 2004
initiate the NACK process only when it is known its repair
requirements exceed the amount of pending FEC transmission for a
given coding block of data content. This can be determined at the
end of the current transmission block (if it is indicated) or upon
the start of reception of a subsequent coding block or transmission
object. This implies the NORM data content is marked to identify its
FEC block number and that ordinal relationship is preserved in order
of transmission.
Alternatively, if the sender's transmission advertises the quantity
of repair packets it is already planning to send for a block, the
receiver may be able to initiate the NACK processor earlier.
Allowing receivers to initiate NACK cycles at any time they detect
their repair needs have exceeded pending repair transmissions may
result in slightly quicker repair cycles. However, it may be useful
to limit NACK process initiation to specific events such as at the
end-of-transmission of an FEC coding block or upon detection of
subsequent coding blocks. This can allow receivers to aggregate NACK
content into a smaller number of NACK messages and provide some
implicit loose synchronization among the receiver set to help
facilitate effective probabilistic suppression of NACK feedback. The
receiver MUST maintain a history of data content received from the
sender to determine its current repair needs. When FEC is employed,
it is expected that the history will correspond to a record of
pending or partially-received coding blocks.
For probabilistic, timer-base suppression of feedback, the NACK cycle
should begin with receivers observing backoff timeouts. In
conjunction with initiating this backoff timeout, it is important
that the receivers record the current position in the sender's
transmission sequence at which they initiate the NACK cycle. When
the suppression backoff timeout expires, the receivers should only
consider their repair needs up to this recorded transmission position
in making the decision to transmit or suppress a NACK. Without this
restriction, suppression is greatly reduced as additional content is
received from the sender during the time a NACK message propagates
across the network to the sender and other receivers.
Receiver NACK Process Initiation Interface Description
Inputs:
1) Sender data content with sequencing identifiers from sender
transmissions.
2) History of content received from sender.
Adamson, et al. Experimental [Page 12]
RFC 3941 NORM Building Blocks November 2004
Outputs:
1) NACK process initiation decision
2) Recorded sender transmission sequence position.
3.2.2. NACK Suppression
An effective NORM feedback suppression mechanism is the use of random
backoff timeouts prior to NACK transmission by receivers requiring
repairs [10]. Upon expiration of the backoff timeout, a receiver
will request repairs unless its pending repair needs have been
completely superseded by NACK messages heard from other receivers
(when receivers are multicasting NACKs) or from some indicator from
the sender. When receivers are unicasting NACK messages, the sender
may facilitate NACK suppression by forwarding a representation of
NACK content it has received to the group at large or provide some
other indicator of the repair information it will be subsequently
transmitting.
For effective and scalable suppression performance, the backoff
timeout periods used by receivers should be independently, randomly
picked by receivers with a truncated exponential distribution [6].
This results in the majority of the receiver set holding off
transmission of NACK messages under the assumption that the smaller
number of "early NACKers" will supersede the repair needs of the
remainder of the group. The mean of the distribution should be
determined as a function of the current estimate of sender<->group
GRTT and a group size estimate that is determined by other mechanisms
within the protocol or preset by the multicast application.
A simple algorithm can be constructed to generate random backoff
timeouts with the appropriate distribution. Additionally, the
algorithm may be designed to optimize the backoff distribution given
the number of receivers (R) potentially generating feedback. This
"optimization" minimizes the number of feedback messages (e.g., NACK)
in the worst-case situation where all receivers generate a NACK. The
maximum backoff timeout (T_maxBackoff) can be set to control reliable
delivery latency versus volume of feedback traffic. A larger value
of T_maxBackoff will result in a lower density of feedback traffic
for a given repair cycle. A smaller value of T_maxBackoff results in
shorter latency which also reduces the buffering requirements of
senders and receivers for reliable transport.
Given the receiver group size (R), and maximum allowed backoff
timeout (T_maxBackoff), random backoff timeouts (t') with a truncated
exponential distribution can be picked with the following algorithm:
Adamson, et al. Experimental [Page 13]
RFC 3941 NORM Building Blocks November 2004
1) Establish an optimal mean (L) for the exponential backoff based on
the group size:
L = ln(R) + 1
2) Pick a random number (x) from a uniform distribution over a range
of:
L L L
-------------------- to -------------------- + ----------
T_maxBackoff*(exp(L)-1) T_maxBackoff*(exp(L)-1) T_maxBackoff
3) Transform this random variate to generate the desired random
backoff time (t') with the following equation:
t' = T_maxBackoff/L * ln(x * (exp(L) - 1) * (T_maxBackoff/L))
This C language function can be used to generate an appropriate
random backoff time interval:
double RandomBackoff(double maxTime, double groupSize)
{
double lambda = log(groupSize) + 1;
double x = UniformRand(lambda/maxTime) +
lambda / (maxTime*(exp(lambda)-1));
return ((maxTime/lambda) *
log(x*(exp(lambda)-1)*(maxTime/lambda)));
} // end RandomBackoff()
where UniformRand(double max) returns random numbers with a uniform
distribution from the range of 0..max. For example, based on the
POSIX "rand()" function, the following C code can be used:
double UniformRand(double max)
{
return (max * ((double)rand()/(double)RAND_MAX));
}
The number of expected NACK messages generated (N) within the first
round trip time for a single feedback event is approximately:
N = exp(1.2 * L / (2*T_maxBackoff/GRTT))
Thus the maximum backoff time can be adjusted to tradeoff worst-case
NACK feedback volume versus latency. This is derived from [6] and
assumes T_maxBackoff >= GRTT, and L is the mean of the distribution
optimized for the given group size as shown in the algorithm above.
Adamson, et al. Experimental [Page 14]
RFC 3941 NORM Building Blocks November 2004
Note that other mechanisms within the protocol may work to reduce
redundant NACK generation further. It is suggested that T_maxBackoff
be selected as an integer multiple of the sender's current advertised
GRTT estimate such that:
T_maxBackoff = K * GRTT ;where K >= 1
For general Internet operation, a default value of K=4 is RECOMMENDED
for operation with multicast (to the group at large) NACK delivery
and a value of K=6 for unicast NACK delivery. Alternate values may
be used to for buffer utilization, reliable delivery latency and
group size scalability tradeoffs.
Given that (K*GRTT) is the maximum backoff time used by the receivers
to initiate NACK transmission, other timeout periods related to the
NACK repair process can be scaled accordingly. One of those timeouts
is the amount of time a receiver should wait after generating a NACK
message before allowing itself to initiate another NACK
backoff/transmission cycle (T_rcvrHoldoff). This delay should be
sufficient for the sender to respond to the received NACK with repair
messages. An appropriate value depends upon the amount of time for
the NACK to reach the sender and the sender to provide a repair
response. This MUST include any amount of sender NACK aggregation
period during which possible multiple NACKs are accumulated to
determine an efficient repair response. These timeouts are further
discussed in the section below on "Sender NACK Processing and Repair
Response".
There are also secondary measures that can be applied to improve the
performance of feedback suppression. For example, the sender's data
content transmissions can follow an ordinal sequence of transmission.
When repairs for data content occur, the receiver can note that the
sender has "rewound" its data content transmission position by
observing the data object, FEC block number, and FEC symbol
identifiers. Receivers SHOULD limit transmission of NACKs to only
when the sender's current transmission position exceeds the point to
which the receiver has incomplete reception. This reduces premature
requests for repair of data the sender may be planning to provide in
response to other receiver requests. This mechanism can be very
effective for protocol convergence in high loss conditions when
transmissions of NACKs from other receivers (or indicators from the
sender) are lost. Another mechanism (particularly applicable when
FEC is used) is for the sender to embed an indication of impending
repair transmissions in current packets sent. For example, the
indication may be as simple as an advertisement of the number of FEC
packets to be sent for the current applicable coding block.
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Finally, some consideration might be given to using the NACKing
history of receivers to weight their selection of NACK backoff
timeout intervals. For example, if a receiver has historically been
experiencing the greatest degree of loss, it may promote itself to
statistically NACK sooner than other receivers. Note this requires
there is correlation over successive intervals of time in the loss
experienced by a receiver. Such correlation MAY not be present in
multicast networks. This adjustment of backoff timeout selection may
require the creation of an "early NACK" slot for these historical
NACKers. This additional slot in the NACK backoff window will result
in a longer repair cycle process that may not be desirable for some
applications. The resolution of these trade-offs may be dependent
upon the protocol's target application set or network.
After the random backoff timeout has expired, the receiver will make
a decision on whether to generate a NACK repair request or not (i.e.,
it has been suppressed). The NACK will be suppressed when any of the
following conditions has occurred:
1) The accumulated state of NACKs heard from other receivers (or
forwarding of this state by the sender) is equal to or supersedes
the repair needs of the local receiver. Note that the local
receiver should consider its repair needs only up to the sender
transmission position recorded at the NACK cycle initiation (when
the backoff timer was activated).
2) The sender's data content transmission position "rewinds" to a
point ordinally less than that of the lowest sequence position of
the local receiver's repair needs. (This detection of sender
"rewind" indicates the sender has already responded to other
receiver repair needs of which the local receiver may not have
been aware). This "rewind" event can occur any time between 1)
when the NACK cycle was initiated with the backoff timeout
activation and 2) the current moment when the backoff timeout has
expired to suppress the NACK. Another NACK cycle must be
initiated by the receiver when the sender's transmission sequence
position exceeds the receiver's lowest ordinal repair point. Note
it is possible that the local receiver may have had its repair
needs satisfied as a result of the sender's response to the repair
needs of other receivers and no further NACKing is required.
If these conditions have not occurred and the receiver still has
pending repair needs, a NACK message is generated and transmitted.
The NACK should consist of an accumulation of repair needs from the
receiver's lowest ordinal repair point up to the current sender
transmission sequence position. A single NACK message should be
generated and the NACK message content should be truncated if it
exceeds the payload size of single protocol message. When such NACK
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payload limits occur, the NACK content SHOULD contain requests for
the ordinally lowest repair content needed from the sender.
NACK Suppression Interface Description
Inputs:
1) NACK process initiation decision.
2) Recorded sender transmission sequence position.
3) Sender GRTT.
4) Sender group size estimate.
5) Application-defined bound on backoff timeout period.
6) NACKs from other receivers.
7) Pending repair indication from sender (may be forwarded
NACKs).
8) Current sender transmission sequence position.
Outputs:
1) Yes/no decision to generate NACK message upon backoff timer
expiration.
3.2.3. NACK Content
The content of NACK messages generated by reliable multicast
receivers will include information detailing their current repair
needs. The specific information depends on the use and type of FEC
in the NORM repair process. The identification of repair needs is
dependent upon the data content identification (See Section 3.5
below). At the highest level the NACK content will identify the
sender to which the NACK is addressed and the data transport object
(or stream) within the sender's transmission that needs repair. For
the indicated transport entity, the NACK content will then identify
the specific FEC coding blocks and/or symbols it requires to
reconstruct the complete transmitted data. This content may consist
of FEC block erasure counts and/or explicit indication of missing
blocks or symbols (segments) of data and FEC content. It should also
be noted that NORM can be effectively instantiated without a
requirement for reliable NACK delivery using the techniques discussed
here.
3.2.3.1. NACK and FEC Repair Strategies
Where FEC-based repair is used, the NACK message content will
minimally need to identify the coding block(s) for which repair is
needed and a count of erasures (missing packets) for the coding
block. An exact count of erasures implies the FEC algorithm is
capable of repairing _any_ loss combination within the coding block.
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This count may need to be adjusted for some FEC algorithms.
Considering that multiple repair rounds may be required to
successfully complete repair, an erasure count also implies that the
quantity of unique FEC parity packets the server has available to
transmit is essentially unlimited (i.e., the server will always be
able to provide new, unique, previously unsent parity packets in
response to any subsequent repair requests for the same coding
block). Alternatively, the sender may "round-robin" transmit through
its available set of FEC symbols for a given coding block, and
eventually affect repair. For a most efficient repair strategy, the
NACK content will need to also _explicitly_ identify which symbols
(information and/or parity) the receiver requires to successfully
reconstruct the content of the coding block. This will be
particularly true of small to medium size block FEC codes (e.g., Reed
Solomon) that are capable of provided a limited number of parity
symbols per FEC coding block.
When FEC is not used as part of the repair process, or the protocol
instantiation is required to provide reliability even when the sender
has transmitted all available parity for a given coding block (or the
sender's ability to buffer transmission history is exceeded by the
delay*bandwidth*loss characteristics of the network topology), the
NACK content will need to contain _explicit_ coding block and/or
segment loss information so that the sender can provide appropriate
repair packets and/or data retransmissions. Explicit loss
information in NACK content may also potentially serve other
purposes. For example, it may be useful for decorrelating loss
characteristics among a group of receivers to help differentiate
candidate congestion control bottlenecks among the receiver set.
When FEC is used and NACK content is designed to contain explicit
repair requests, there is a strategy where the receivers can NACK for
specific content that will help facilitate NACK suppression and
repair efficiency. The assumptions for this strategy are that sender
may potentially exhaust its supply of new, unique parity packets
available for a given coding block and be required to explicitly
retransmit some data or parity symbols to complete reliable transfer.
Another assumption is that an FEC algorithm where any parity packet
can fill any erasure within the coding block (e.g., Reed Solomon) is
used. The goal of this strategy is to make maximum use of the
available parity and provide the minimal amount of data and repair
transmissions during reliable transfer of data content to the group.
When systematic FEC codes are used, the sender transmits the data
content of the coding block (and optionally some quantity of parity
packets) in its initial transmission. Note that a systematic FEC
coding block is considered to be logically made up of the contiguous
set of data vectors plus parity vectors for the given FEC algorithm
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used. For example, a coding scheme that provides for 64 data symbols
and 32 parity symbols per coding block would contain FEC symbol
identifiers in the range of 0 to 95.
Receivers then can construct NACK messages requesting sufficient
content to satisfy their repair needs. For example, if the receiver
has three erasures in a given received coding block, it will request
transmission of the three lowest ordinal parity vectors in the coding
block. In our example coding scheme from the previous paragraph, the
receiver would explicitly request parity symbols 64 to 66 to fill its
three erasures for the coding block. Note that if the receiver's
loss for the coding block exceeds the available parity quantity
(i.e., greater than 32 missing symbols in our example), the receiver
will be required to construct a NACK requesting all (32) of the
available parity symbols plus some additional portions of its missing
data symbols in order to reconstruct the block. If this is done
consistently across the receiver group, the resulting NACKs will
comprise a minimal set of sender transmissions to satisfy their
repair needs.
In summary, the rule is to request the lower ordinal portion of the
parity content for the FEC coding block to satisfy the erasure repair
needs on the first NACK cycle. If the available number of parity
symbols is insufficient, the receiver will also request the subset of
ordinally highest missing data symbols to cover what the parity
symbols will not fill. Note this strategy assumes FEC codes such as
Reed-Solomon for which a single parity symbol can repair any erased
symbol. This strategy would need minor modification to take into
account the possibly limited repair capability of other FEC types.
On subsequent NACK repair cycles where the receiver may have received
some portion of its previously requested repair content, the receiver
will use the same strategy, but only NACK for the set of parity
and/or data symbols it has not yet received. Optionally, the
receivers could also provide a count of erasures as a convenience to
the sender or intermediate systems assisting NACK operation.
After receipt and accumulation of NACK messages during the
aggregation period, the sender can begin transmission of fresh
(previously untransmitted) parity symbols for the coding block based
on the highest receiver erasure count _if_ it has a sufficient
quantity of parity symbols that were _not_ previously transmitted.
Otherwise, the sender MUST resort to transmitting the explicit set of
repair vectors requested. With this approach, the sender needs to
maintain very little state on requests it has received from the group
without need for synchronization of repair requests from the group.
Since all receivers use the same consistent algorithm to express
their explicit repair needs, NACK suppression among receivers is
simplified over the course of multiple repair cycles. The receivers
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can simply compare NACKs heard from other receivers against their own
calculated repair needs to determine whether they should transmit or
suppress their pending NACK messages.
3.2.3.2. NACK Content Format
The format of NACK content will depend on the protocol's data service
model and the format of data content identification the protocol
uses. This NACK format also depends upon the type of FEC encoding
(if any) used. Figure 2 illustrates a logical, hierarchical
transmission content identification scheme, denoting that the notion
of objects (or streams) and/or FEC blocking is optional at the
protocol instantiation's discretion. Note that the identification of
objects is with respect to a given sender. It is recommended that
transport data content identification is done within the context of a
sender in a given session. Since the notion of session "streams" and
"blocks" is optional, the framework degenerates to that of typical
transport data segmentation and reassembly in its simplest form.
Session_
\_
Sender_
\_
[Object/Stream(s)]_
\_
[FEC Blocks]_
\_
Symbols
Fig. 2: NORM Data Content Identification Hierarchy
The format of NACK messages should meet the following goals:
1) Able to identify transport data unit transmissions required to
repair a portion of the received content, whether it is an entire
missing object/stream (or range), entire FEC coding block(s), or
sets of symbols,
2) Be simple to process for NACK aggregation and suppression,
3) Be capable of including NACKs for multiple objects, FEC coding
blocks and/or symbols in a single message, and
4) Have a reasonably compact format.
If the NORM transport object/stream is identified with an <objectId>
and the FEC symbol being transmitted is identified with an
<fecPayloadId>, the concatenation of <objectId::fecPayloadId>
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comprises a basic transport protocol data unit (TPDU) identifier for
symbols from a given source. NACK content can be composed of lists
and/or ranges of these TPDU identifiers to build up NACK messages to
describe the receivers repair needs. If no hierarchical object
delineation or FEC blocking is used, the TPDU is a simple linear
representation of the data symbols transmitted by the sender. When
the TPDU represents a hierarchy for purposes of object/stream
delineation and/or FEC blocking, the NACK content unit may require
flags to indicate which portion of the TPDU is applicable. For
example, if an entire "object" (or range of objects) is missing in
the received data, the receiver will not necessarily know the
appropriate range of <sourceBlockNumbers> or <encodingSymbolIds> for
which to request repair and thus requires some mechanism to request
repair (or retransmission) of the entire unit represented by an
<objectId>. The same is true if entire FEC coding blocks represented
by one or a range of <sourceBlockNumbers> have been lost.
NACK Content Interface Description
Inputs:
1) Sender identification.
2) Sender data identification.
3) Sender FEC Object Transmission Information.
4) Recorded sender transmission sequence position.
5) Current sender transmission sequence position. History of
repair needs for this sender.
Outputs:
1) NACK message with repair requests.
3.2.4. Sender Repair Response
Upon reception of a repair request from a receiver in the group, the
sender will initiate a repair response procedure. The sender may
wish to delay transmission of repair content until it has had
sufficient time to accumulate potentially multiple NACKs from the
receiver set. This allows the sender to determine the most efficient
repair strategy for a given transport stream/object or FEC coding
block. Depending upon the approach used, some protocols may find it
beneficial for the sender to provide an indicator of pending repair
transmissions as part of its current transmitted message content.
This can aid some NACK suppression mechanisms. The amount of time to
perform this NACK aggregation should be sufficient to allow for the
maximum receiver NACK backoff window ("T_maxBackoff" from Section
3.2.2) and propagation of NACK messages from the receivers to the
sender. Note the maximum transmission delay of a message from a
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receiver to the sender may be approximately (1*GRTT) in the case of
very asymmetric network topology with respect to transmission delay.
Thus, if the maximum receiver NACK backoff time is T_maxBackoff =
K*GRTT, the sender NACK aggregation period should be equal to at
least:
T_sndrAggregate = T_maxBackoff + 1*GRTT = (K+1)*GRTT
Immediately after the sender NACK aggregation period, the sender will
begin transmitting repair content determined from the aggregate NACK
state and continue with any new transmission. Also, at this time,
the sender should observe a "holdoff" period where it constrains
itself from initiating a new NACK aggregation period to allow
propagation of the new transmission sequence position due to the
repair response to the receiver group. To allow for worst case
asymmetry, this "holdoff" time should be:
T_sndrHoldoff = 1*GRTT
Recall that the receivers will also employ a "holdoff" timeout after
generating a NACK message to allow time for the sender's response.
Given a sender <T_sndrAggregate> plus <T_sndrHoldoff> time of
(K+1)*GRTT, the receivers should use holdoff timeouts of:
T_rcvrHoldoff = T_sndrAggregate + T_sndrHoldoff = (K+2)*GRTT
This allows for a worst-case propagation time of the receiver's NACK
to the sender, the sender's aggregation time and propagation of the
sender's response back to the receiver. Additionally, in the case of
unicast feedback from the receiver set, it may be useful for the
sender to forward (via multicast) a representation of its aggregated
NACK content to the group to allow for NACK suppression when there is
not multicast connectivity among the receiver set.
At the expiration of the <T_sndrAggregate> timeout, the sender will
begin transmitting repair messages according to the accumulated
content of NACKs received. There are some guidelines with regards to
FEC-based repair and the ordering of the repair response from the
sender that can improve reliable multicast efficiency:
1) When FEC is used, it is beneficial that the sender transmit
previously untransmitted parity content as repair messages
whenever possible. This maximizes the receiving nodes' ability
to reconstruct the entire transmitted content from their
individual subsets of received messages.
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2) The transmitted object and/or stream data and repair content
should be indexed with monotonically increasing sequence numbers
(within a reasonably large ordinal space). If the sender observes
the discipline of transmitting repair for the earliest content
(e.g., ordinally lowest FEC blocks) first, the receivers can use a
strategy of withholding repair requests for later content until
the sender once again returns to that point in the object/stream
transmission sequence. This can increase overall message
efficiency among the group and help work to keep repair cycles
relatively synchronized without dependence upon strict time
synchronization among the sender and receivers. This also helps
minimize the buffering requirements of receivers and senders and
reduces redundant transmission of data to the group at large.
Sender Repair Response Interface Description
Inputs:
1) Receiver NACK messages
2) Group timing information
Outputs
1) Repair messages (FEC and/or Data content retransmission)
2) Advertisement of current pending repair transmissions when
unicast receiver feedback is detected.
3.3. NORM Receiver Join Policies and Procedures
Consideration should be given to the policies and procedures by which
new receivers join a group (perhaps where reliable transmission is
already in progress) and begin requesting repair. If receiver joins
are unconstrained, the dynamics of group membership may impede the
application's ability to meet its goals for forward progression of
data transmission. Policies limiting the opportunities when
receivers begin participating in the NACK process may be used to
achieve the desired behavior. For example, it may be beneficial for
receivers to attempt reliable reception from a newly-heard sender
only upon non-repair transmissions of data in the first FEC block of
an object or logical portion of a stream. The sender may also
implement policies limiting the receivers from which it will accept
NACK requests, but this may be prohibitive for scalability reasons in
some situations. Alternatively, it may be desirable to have a looser
transport synchronization policy and rely upon session management
mechanisms to limit group dynamics that can cause poor performance,
in some types of bulk transfer applications (or for potential
interactive reliable multicast applications).
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Group Join Policy Interface Description
Inputs:
1) Current object/stream data/repair content and sequencing
identifiers from sender transmissions.
Outputs:
1) Receiver yes/no decision to begin receiving and NACKing for
reliable reception of data
3.4. Reliable Multicast Member Identification
In a NORM protocol (or other multicast protocols) where there is the
potential for multiple sources of data, it is necessary to provide
some mechanism to uniquely identify the sources (and possibly some or
all receivers in some cases) within the group. Identity based on
arriving packet source addresses is insufficient for several reasons.
These reasons include routing changes for hosts with multiple
interfaces that result in different packet source addresses for a
given host over time, network address translation (NAT) or firewall
devices, or other transport/network bridging approaches. As a
result, some type of unique source identifier <sourceId> field should
be present in packets transmitted by reliable multicast session
members.
3.5. Data Content Identification
The data and repair content transmitted by a NORM sender requires
some form of identification in the protocol header fields. This
identification is required to facilitate the reliable NACK-oriented
repair process. These identifiers will also be used in NACK messages
generated. This building block document assumes two very general
types of data that may comprise bulk transfer session content. One
type is static, discrete objects of finite size and the other is
continuous non-finite streams. A given application may wish to
reliably multicast data content using either one or both of these
paradigms. While it may be possible for some applications to further
generalize this model and provide mechanisms to encapsulate static
objects as content embedded within a stream, there are advantages in
many applications to provide distinct support for static bulk objects
and messages with the context of a reliable multicast session. These
applications may include content caching servers, file transfer, or
collaborative tools with bulk content. Applications with
requirements for these static object types can then take advantage of
transport layer mechanisms (i.e., segmentation/reassembly, caching,
integrated forward error correction coding, etc.) rather than being
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required to provide their own mechanisms for these functions at the
application layer.
As noted, some applications may alternatively desire to transmit bulk
content in the form of one or more streams of non-finite size.
Example streams include continuous quasi-real-time message broadcasts
(e.g., stock ticker) or some content types that are part of
collaborative tools or other applications. And, as indicated above,
some applications may wish to encapsulate other bulk content (e.g.,
files) into one or more streams within a multicast session.
The components described within this building block document are
envisioned to be applicable to both of these models with the
potential for a mix of both types within a single multicast session.
To support this requirement, the normal data content identification
should include a field to uniquely identify the object or stream
<objectId> within some reasonable temporal or ordinal interval. Note
that it is _not_ expected that this data content identification will
be globally unique. It is expected that the object/stream identifier
will be unique with respect to a given sender within the reliable
multicast session and during the time that sender is supporting a
specific transport instance of that object or stream.
Since the "bulk" object/stream content usually requires segmentation,
some form of segment identification must also be provided. This
segment identifier will be relative to any object or stream
identifier that has been provided. Thus, in some cases, NORM
protocol instantiations may be able to receive transmissions and
request repair for multiple streams and one or more sets of static
objects in parallel. For protocol instantiations employing FEC the
segment identification portion of the data content identifier may
consist of a logical concatenation of a coding block identifier
<sourceBlockNumber> and an identifier for the specific data or parity
symbol <encodingSymbolId> of the code block. The FEC Building Block
document [9] provides a standard message format for identifying FEC
transmission content. NORM protocol instantiations using FEC SHOULD
follow that document's guidelines.
Additionally, flags to determine the usage of the content identifier
fields (e.g., stream vs. object) may be applicable. Flags may also
serve other purposes in data content identification. It is expected
that any flags defined will be dependent upon individual protocol
instantiations.
In summary, the following data content identification fields may be
required for NORM protocol data content messages:
1) Source node identifier (<sourceId>)
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2) Object/Stream identifier (<objectId>), if applicable.
3) FEC Block identifier (<sourceBlockNumber>), if applicable.
4) FEC Symbol identifier (<encodingSymbolId>)
5) Flags to differentiate interpretation of identifier fields or
identifier structure that implicitly indicates usage.
6) Additional FEC transmission content fields per FEC Building Block
These fields have been identified because any generated NACK messages
will use these identifiers in requesting repair or retransmission of
data. NORM protocols that use these data content fields should also
be compatible with support for intermediate system assistance to
reliable multicast transport operation when available.
3.6. Forward Error Correction (FEC)
Multiple forward error correction (FEC) approaches have been
identified that can provide great performance enhancements to the
repair process of NACK-oriented and other reliable multicast
protocols [11], [12], [13]. NORM protocols can reap additional
benefits since FEC-based repair does not _generally_ require explicit
knowledge of repair content within the bounds of its coding block
size (in symbols). In NORM, parity repair packets generated will
generally be transmitted only in response to NACK repair requests
from receiving nodes. However, there are benefits in some network
environments for transmitting some predetermined quantity of FEC
repair packets multiplexed with the regular data symbol transmissions
[14]. This can reduce the amount of NACK traffic generated with
relatively little overhead cost when group sizes are very large or
the network connectivity has a large delay*bandwidth product with
some nominal level of expected packet loss. While the application of
FEC is not unique to NORM, these sorts of requirements may dictate
the types of algorithms and protocol approaches that are applicable.
A specific issue related to the use of FEC with NORM is the mechanism
used to identify the portion(s) of transmitted data content to which
specific FEC packets are applicable. It is expected that FEC
algorithms will be based on generating a set of parity repair packets
for a corresponding block of transmitted data packets. Since data
content packets are uniquely identified by the concatenation of
<sourceId::objectId::sourceBlockNumber::encodingSymbolId> during
transport, it is expected that FEC packets will be identified in a
similar manner. The FEC Building Block document [9] provides
detailed recommendations concerning application of FEC and standard
formats for related reliable multicast protocol messages.
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3.7. Round-trip Timing Collection
The measurement of packet propagation round-trip time (RTT) among
members of the group is required to support timer-based NACK
suppression algorithms, timing of sender commands or certain repair
functions, and congestion control operation. The nature of the
round-trip information collected is dependent upon the type of
interaction among the members of the group. In the case where only
"one-to-many" transmission is required, it may be that only the
sender require RTT knowledge of the greatest RTT (GRTT) among the
receiver set and/or RTT knowledge of only a portion of the group.
Here, the GRTT information might be collected in a reasonably
scalable manner. For congestion control operation, it is possible
that RTT information may be required by each receiver in the group.
In this case, an alternative RTT collection scheme may be utilized
where receivers collect individual RTT measurements with respect to
the sender and advertise them to the group or sender. Where it is
likely that exchange of reliable multicast data will occur among the
group on a "many-to-many" basis, there are alternative measurement
techniques that might be employed for increased efficiency [15]. And
in some cases, there might be absolute time synchronization among
hosts that may simplify RTT measurement. There are trade-offs in
multicast congestion control design that require further
consideration before a universal recommendation on RTT (or GRTT)
measurement can be specified. Regardless of how the RTT information
is collected (and more specifically GRTT) with respect to congestion
control or other requirements, the sender will need to advertise its
current GRTT estimate to the group for various timeouts used by
receivers.
3.7.1. One-to-Many Sender GRTT Measurement
The goal of this form of RTT measurement is for the sender to learn
the GRTT among the receivers who are actively participating in NORM
operation. The set of receivers participating in this process may be
the entire group or some subset of the group determined from another
mechanism within the protocol instantiation. An approach to collect
this GRTT information follows.
The sender periodically polls the group with a message (independent
or "piggy-backed" with other transmissions) containing a <sendTime>
timestamp relative to an internal clock at the sender. Upon
reception of this message, the receivers will record this <sendTime>
timestamp and the time (referenced to their own clocks) at which it
was received <recvTime>. When the receiver provides feedback to the
sender (either explicitly or as part of other feedback messages
depending upon protocol instantiation specification), it will
construct a "response" using the formula:
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grttResponse = sendTime + (currentTime - recvTime)
where the <sendTime> is the timestamp from the last probe message
received from the source and the (<currentTime> - <recvTime>) is the
amount of time differential since that request was received until the
receiver generated the response.
The sender processes each receiver response by calculating a current
RTT measurement for the receiver from whom the response was received
using the following formula:
RTT_rcvr = currentTime - grttResponse
During the each periodic GRTT probing interval, the source keeps the
peak round trip timing measurement (RTT_peak) from the set of
responses it has received. A conservative estimate of GRTT is kept
to maximize the efficiency of redundant NACK suppression and repair
aggregation. The update to the source's ongoing estimate of GRTT is
done observing the following rules:
1) If a receiver's response round trip time (RTT_rcvr) is greater
than the current GRTT estimate, the GRTT is immediately updated to
this new peak value:
GRTT = RTT_rcvr
2) At the end of the response collection period (i.e., the GRTT probe
interval), if the recorded "peak" response RTT_peak) is less than
the current GRTT estimate, the GRTT is updated to:
GRTT = MAX(0.9*GRTT, RTT_peak)
3) If no feedback is received, the sender GRTT estimate remains
unchanged.
4) At the end of the response collection period, the peak tracking
value (RTT_peak) is reset to ZERO for subsequent peak detection.
The GRTT collection period (i.e., period of probe transmission) could
be fixed at a value on the order of that expected for group
membership and/or network topology dynamics. For robustness, more
rapid probing could be used at protocol startup before settling to a
less frequent, steady-state interval. Optionally, an algorithm may
be developed to adjust the GRTT collection period dynamically in
response to the current GRTT estimate (or variations in it) and to an
estimation of packet loss. The overhead of probing messages could
then be reduced when the GRTT estimate is stable and unchanging, but
be adjusted to track more dynamically during periods of variation
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RFC 3941 NORM Building Blocks November 2004
with correspondingly shorter GRTT collection periods. GRTT
collection may also be coupled with collection of other information
for congestion control purposes.
In summary, although NORM repair cycle timeouts are based on GRTT, it
should be noted that convergent operation of the protocol does not
_strictly_ depend on highly accurate GRTT estimation. The current
mechanism has proved sufficient in simulations and in the
environments where NORM-like protocols have been deployed to date.
The estimate provided by the algorithm tracks the peak envelope of
actual GRTT (including operating system effect as well as network
delays) even in relatively high loss connectivity. The steady-state
probing/update interval may potentially be varied to accommodate
different levels of expected network dynamics in different
environments.
3.7.2. One-to-Many Receiver RTT Measurement
In this approach, receivers send messages with timestamps to the
sender. To control the volume of these receiver-generated messages,
a suppression mechanism similar to that described for NACK
suppression my be used. The "age" of receivers' RTT measurement
should be kept by receivers and used as a metric in competing for
feedback opportunities in the suppression scheme. For example,
receiver who have not made any RTT measurement or whose RTT
measurement has aged most should have precedence over other
receivers. In turn the sender may have limited capacity to provide
an "echo" of the receiver timestamps back to the group, and it could
use this RTT "age" metric to determine which receivers get
precedence. The sender can determine the GRTT as described in 3.7.1
if it provides sender timestamps to the group. Alternatively,
receivers who note their RTT is greater than the sender GRTT can
compete in the feedback opportunity/suppression scheme to provide the
sender and group with this information.
3.7.3. Many-to-Many RTT Measurement
For reliable multicast sessions that involve multiple senders, it may
be useful to have RTT measurements occur on a true "many-to-many"
basis rather than have each sender independently tracking RTT. Some
protocol efficiency can be gained when receivers can infer an
approximation of their RTT with respect to a sender based on RTT
information they have on another sender and that other sender's RTT
with respect to the new sender of interest. For example, for
receiver "a" and sender's "b" and "c", it is likely that:
RTT(a<->b) <= RTT(a<->c)) + RTT(b<->c)
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RFC 3941 NORM Building Blocks November 2004
Further refinement of this estimate can be conducted if RTT
information is available to a node concerning its own RTT to a small
subset of other group members and RTT information among those other
group members it learns during protocol operation.
3.7.4. Sender GRTT Advertisement
To facilitate deterministic NORM protocol operation, the sender
should robustly advertise its current estimation of GRTT to the
receiver set. Common, robust knowledge of the sender's current
operating GRTT estimate among the group will allow the protocol to
progress in its most efficient manner. The sender's GRTT estimate
can be robustly advertised to the group by simply embedding the
estimate into all pertinent messages transmitted by the sender. The
overhead of this can be made quite small by quantizing (compressing)
the GRTT estimate to a single byte of information. The following C-
language functions allows this to be done over a wide range (RTT_MIN
through RTT_MAX) of GRTT values while maintaining a greater range of
precision for small GRTT values and less precision for large values.
Values of 1.0e-06 seconds and 1000 seconds are RECOMMENDED for
RTT_MIN and RTT_MAX respectively. NORM applications may wish to
place an additional, smaller upper limit on the GRTT advertised by
senders to meet application data delivery latency constraints at the
expense of greater feedback volume in some network environments.
unsigned char QuantizeGrtt(double grtt)
{
if (grtt > RTT_MAX)
grtt = RTT_MAX;
else if (grtt < RTT_MIN)
grtt = RTT_MIN;
if (grtt < (33*RTT_MIN))
return ((unsigned char)(grtt / RTT_MIN) - 1);
else
return ((unsigned char)(ceil(255.0-
(13.0 * log(RTT_MAX/grtt)))));
}
double UnquantizeRtt(unsigned char qrtt)
{
return ((qrtt <= 31) ?
(((double)(qrtt+1))*(double)RTT_MIN) :
(RTT_MAX/exp(((double)(255-qrtt))/(double)13.0)));
}
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RFC 3941 NORM Building Blocks November 2004
Note that this function is useful for quantizing GRTT times in the
range of 1 microsecond to 1000 seconds. Of course, NORM protocol
implementations may wish to further constrain advertised GRTT
estimates (e.g., limit the maximum value) for practical reasons.
3.8. Group Size Determination/Estimation
When NORM protocol operation includes mechanisms that excite feedback
from the group at large (e.g., congestion control), it may be
possible to roughly estimate the group size based on the number of
feedback messages received with respect to the distribution of the
probabilistic suppression mechanism used. Note the timer-based
suppression mechanism described in this document does not require a
very accurate estimate of group size to perform adequately. Thus, a
rough estimate, particularly if conservatively managed, may suffice.
Group size may also be determined administratively. In absence of a
group size determination mechanism a default group size value of
10,000 is RECOMMENDED for reasonable management of feedback given the
scalability of expected NORM usage.
3.9. Congestion Control Operation
Congestion control that fairly shares available network capacity
with other reliable multicast and TCP instantiations is REQUIRED for
general Internet operation. The TCP-Friendly Multicast Congestion
Control (TFMCC) [16] or Pragmatic General Multicast Congestion
Control (PGMCC) techniques [17] may be applied to NORM operation to
meet this requirement.
3.10. Router/Intermediate System Assistance
NACK-oriented protocols may benefit from general purpose router
assistance. In particular, additional NACK suppression where routers
or intermediate systems can aggregate NACK content (or filter
duplicate NACK content) from receivers as it is relayed toward the
sender could enhance NORM group size scalability. For NORM protocols
using FEC, it is possible that intermediate systems may be able to
filter FEC repair messages to provide an intelligent "subcast" of
repair content to different legs of the multicast topology depending
on the repair needs learned from previous receiver NACKs. Both of
these types of assist functions would require router interpretation
of transport data unit content identifiers and flags.
3.11. NORM Applicability
The NORM building block applies to protocols wishing to employ
negative acknowledgement to achieve reliable data transfer. Properly
designed negative-acknowledgement (NACK)-oriented reliable multicast
Adamson, et al. Experimental [Page 31]
RFC 3941 NORM Building Blocks November 2004
(NORM) protocols offer scalability advantages for applications and/or
network topologies where, for various reasons, it is prohibitive to
construct a higher order delivery infrastructure above the basic
Layer 3 IP multicast service (e.g., unicast or hybrid
unicast/multicast data distribution trees). Additionally, the
scalability property of NACK-oriented protocols [18], [19] is
applicable where broad "fan-out" is expected for a single network hop
(e.g., cable-TV data delivery, satellite, or other broadcast
communication services). Furthermore, the simplicity of a protocol
based on "flat" group-wide multicast distribution may offer
advantages for a broad range of distributed services or dynamic
networks and applications. NORM protocols can make use of reciprocal
(among senders and receivers) multicast communication under the Any-
Source Multicast (ASM) model defined in RFC 1112 [2], and are capable
of scalable operation in asymmetric topologies such as Single-Source
Multicast (SSM) [8] where there may only be unicast routing service
from the receivers to the sender(s).
NORM operation is compatible with transport layer forward error
correction coding techniques as described in [13] and congestion
control mechanisms such as those described in [16] and [17]. A
principal limitation of NORM operation involves group size
scalability when network capacity for receiver feedback is very
limited. NORM operation is also governed by implementation buffering
constraints. Buffering greater than that required for typical
point-to-point reliable transport (e.g., TCP) is recommended to allow
for disparity in the receiver group connectivity and to allow for the
feedback delays required to attain group size scalability.
4. Security Considerations
NORM protocols are expected to be subject to the same sort of
security vulnerabilities as other IP and IP multicast protocols.
NORM is compatible with IP security (IPsec) authentication mechanisms
[20] that are RECOMMENDED for protection against session intrusion
and denial of service attacks. A particular threat for NACK based
protocols is that of NACK replay attacks that would prevent a NORM
sender from making forward progress in transmission. Any standard
IPsec mechanisms that can provide protection against such replay
attacks are RECOMMENDED for use. Additionally, NORM protocol
instantiations SHOULD consider providing support for their own NACK
replay attack protection when network layer mechanisms are not
available. The IETF Multicast Security (msec) Working Group is also
developing solutions which may be applicable to NORM in the future.
Adamson, et al. Experimental [Page 32]
RFC 3941 NORM Building Blocks November 2004
5. Acknowledgements (and these are not Negative)
The authors would like to thank Rick Jones, and Joerg Widmer for
their valuable comments on this document. The authors would also
like to thank the RMT working group chairs, Roger Kermode and Lorenzo
Vicisano, for their support in development of this specification, and
Sally Floyd for her early inputs into this document.
6. References
6.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, August 1989.
6.2. Informative References
[3] Mankin, A., Romanow, A., Bradner, S., and V. Paxson, "IETF
Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.
[4] Clark, D. and D. Tennenhouse, "Architectural Considerations for
a New Generation of Protocols". In Proc. ACM SIGCOMM, pages
201--208, September 1990.
[5] Kermode, R. and L. Vicisano, "Author Guidelines for Reliable
Multicast Transport (RMT) Building Blocks and Protocol
Instantiation documents", RFC 3269, April 2002.
[6] Nonnenmacher, J. and E. Biersack, "Optimal Multicast Feedback,"
in IEEE Infocom, San Francisco, California, p. 964, March/April
1998.
[7] Macker, J. and R. Adamson, "Quantitative Prediction of Nack
Oriented Reliable Multicast (NORM) Feedback", Proc. IEEE MILCOM
2002, October 2002.
[8] Holbrook, H., "A Channel Model for Multicast", Ph.D.
Dissertation, Stanford University, Department of Computer
Science, Stanford, California, August 2001.
[9] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
J. Crowcroft, "Forward Error Correction (FEC) Building Block",
RFC 3452, December 2002.
Adamson, et al. Experimental [Page 33]
RFC 3941 NORM Building Blocks November 2004
[10] Floyd, S., Jacobson, V., McCanne, S., Liu, C., and L. Zhang. "A
Reliable Multicast Framework for Light-weight Sessions and
Application Level Framing", Proc. ACM SIGCOMM, August 1995.
[11] Metzner, J., "An Improved Broadcast Retransmission Protocol",
IEEE Transactions on Communications, Vol. Com-32, No.6, June
1984.
[12] Macker, J., "Reliable Multicast Transport and Integrated
Erasure-based Forward Error Correction", Proc. IEEE MILCOM 97,
October 1997.
[13] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
J. Crowcroft, "The Use of Forward Error Correction (FEC) in
Reliable Multicast", RFC 3453, December 2002.
[14] Gossink, D. and J. Macker, "Reliable Multicast and Integrated
Parity Retransmission with Channel Estimation", IEEE GLOBECOM
98'.
[15] Ozdemir, V., Muthukrishnan, S., and I. Rhee, "Scalable, Low-
Overhead Network Delay Estimation", NCSU/AT&T White Paper,
February 1999.
[16] Widmer, J. and M. Handley, "Extending Equation-Based Congestion
Control to Multicast Applications", Proc ACM SIGCOMM 2001, San
Diego, August 2001.
[17] Rizzo, L., "pgmcc: A TCP-Friendly Single-Rate Multicast
Congestion Control Scheme", Proc ACM SIGCOMM 2000, Stockholm,
August 2000.
[18] Pingali, S., Towsley, D., and J. Kurose, "A Comparison of
Sender-Initiated and Receiver-Initiated Reliable Multicast
Protocols". In Proc. INFOCOM, San Francisco, CA, October 1993.
[19] B.N. Levine, J.J. Garcia-Luna-Aceves, "A Comparison of Known
Classes of Reliable Multicast Protocols", Proc. International
Conference on Network Protocols (ICNP-96), Columbus, Ohio, Oct
29--Nov 1, 1996.
[20] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
Adamson, et al. Experimental [Page 34]
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7. Authors' Addresses
Brian Adamson
Naval Research Laboratory
Washington, DC 20375
EMail: adamson@itd.nrl.navy.mil
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen, Germany
EMail: cabo@tzi.org
Mark Handley
Department of Computer Science
University College London
Gower Street
London
WC1E 6BT
UK
EMail: M.Handley@cs.ucl.ac.uk
Joe Macker
Naval Research Laboratory
Washington, DC 20375
EMail: macker@itd.nrl.navy.mil
Adamson, et al. Experimental [Page 35]
RFC 3941 NORM Building Blocks November 2004
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Adamson, et al. Experimental [Page 36]