RMT Working Group B. Adamson/NRL
INTERNET-DRAFT C. Bormann/Tellique
draft-ietf-rmt-bb-norm-07 M. Handley/ACIRI
Expires: October 2003 J. Macker/NRL
October 2003
NACK-Oriented Reliable Multicast (NORM) Building Blocks
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document discusses the creation the 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.
1.0 Introduction
Reliable multicast transport is a desirable technology for the
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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 [1]. 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) 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.
2.0 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.
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All of these areas are at least briefly discussed. Additionally,
other reliable multicast transport building block documents, [13],
[14], and [17] 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 RMT
working group documents. However, the same principals in protocol
design may also be applied to other services models (e.g. more
interactive exchanges of small messages such as weith 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
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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.
2.4 Group Size Scalability
Native IP multicast [5] 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 probablistic, timer-
based suppression techniques[15]. However, the potential for router
assistance 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 probablistic, 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
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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 "fanouts" to which
IP multicast service is highly applicable. Additionally, policy
and/or technical issues may result in topologies where multicast
connectivity is limited to a single source multicast (SSM) model from
a specific source [16]. 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 be
consider utilizing these features when available.
3.0 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
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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
components's interface description. Note that the set of building
blocks presented here do not fully satisify 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).
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 pictoral overview of these building block areas
and some of their relationships. For example, the content of the data
messages that sender initially transmits depends upon the "Node
Identification", "Data Content Identification", and "FEC" components
whil 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.
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Finally, the sender's processing of receiver responses will feed back
into its transmission strategy.
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 signficant of these components, FEC
and Congestion Control, are discussed in other building block
documents [13], [14], and [17]. 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.
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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 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
[13]. 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 scalablity 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
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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
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
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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.
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Receiver NACK Process Initiation Interface Description:
Inputs:
1) Sender data content with sequencing identifiers from sender
transmissions.
2) History of content received from sender.
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[6]. 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 [7].
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
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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:
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
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NACK feedback volume versus latency. This is derived from [7] 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.
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 utliization, 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 advertisment of the number of FEC packets to be sent for the
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current applicable coding block.
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.
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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
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
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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. This count
may need to be adjusted for some FEC algorithms. Considering that
multiple repair rounds may be required to successfully complete
repair, and erasure count also 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
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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
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
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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 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)
is 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
Figure 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. FEC
erasure counts may also be desirable.
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4) Have a reasonably compact format, and
5) Be capable of working with the Generic Router Assist (GRA)
building block.
If the NORM transport object/stream is identified with an <objectId>
and the FEC symbol being transmitted is identified with and
<fecPayloadId>, the concatenation of <objectId::fecPayloadId>
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.
5) 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
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upon the approach used, some protocols may find it beneficial for the
sender to provide an indicator of pending repair transmissions as part
of the 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 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 aggregration 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
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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.
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 witholding 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)
1) 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
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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).
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
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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 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-realtime message broadcasts
(e.g., stock ticker) or some content types that are part of
collaborative tools or other more complex 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 draft 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 identifer for the specific data or parity symbol
<endcodingSymbolId> of the code block. The FEC Building Block
document [13] 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
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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>)
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
[9],[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 [8]. 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 which 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
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transport, it is expected that FEC packets will be identified in a
similar manner. The FEC Building Block specification [13] provides
detailed recommendations concerning application of FEC and standard
formats for related reliable multicast protocol messages.
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 [12]. 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
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<recvTime>. When the receiver provides feedback to the sender (either
explicitly or as part of other feedback messages depending upon
protocol instantiaon specification), it will construct a "response"
using the formula:
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 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
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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 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. It should be
noted that the TFMCC Congestion Control building block described a
similar approach to receiver RTT measurement as part of its congestion
control operation [14].
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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)
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-lanquage 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.
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unsigned char QuantizeGrtt(double grtt)
{
if (grtt > RTT_MAX)
grtt = RTT_MAX;
else if (grtt < 1.0e-06)
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)));
}
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 [14] or PGMCC specification [17] may be applied to NORM
operation to meet this requirement.
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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 toplogy depending on
the repair needs learned from previous receiver NACKs. Both of these
types of Generic Router Assist (GRA) functions would require router
interpretation of transport data unit content identifiers and flags.
Additionally, the GRA router should observe NACK repair process
timeouts based on the NORM sender's GRTT advertisement.
4.0 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
(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 [2, 3] is applicable
where broad "fanout" is expected for a single network hop (e.g.,
cable-TV data delivery, satellite, or other broadcast communication
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 [5], and are capable of
scalable operation in asymmetric topologies such as Single-Source
Multicast (SSM) [16] 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 [18] and congestion
control mechanisms described in [14] and [17]. A principle 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
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required to attain group size scalability.
5.0 Security Considerations
NORM protocols are expected to be subject to same sort of security
vulnerabilities as other IP and IP multicast protocols. NORM is
compatible with IP security (IPSEC) authentication mechanisms [19]
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.
6.0 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.
7.0 References
[1] A. Mankin, A. Romanow, S. Bradner, V. Paxson, "IETF
Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.
[2] S. Pingali, D. Towsley, J. Kurose, "A Comparison of Sender-
Initiated and Receiver-Initiated Reliable Multicast
Protocols". In Proc. INFOCOM, San Francisco, CA, October
1993.
[3] 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.
[4] D. Clark, D. Tennenhouse, "Architectural Considerations for
a New Generation of Protocols". In Proc. ACM SIGCOMM,
pages 201--208, September 1990.
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[5] S. Deering, "Host Extensions for IP Multicasting". Internet
RFC1112, August 1989.
[6] S. Floyd, V. Jacobson, S. McCanne, C. Liu, and L. Zhang.
"A Reliable Multicast Framework for Light-weight Sessions
and Application Level Framing", Proc. ACM SIGCOMM, August
1995.
[7] J. Nonnenmacher and E. W. Biersack, "Optimal Multicast
Feedback," in IEEE Infocom , (San Francisco, California), p.
964, March/April 1998.
[8] D. Gossink, J. Macker, "Reliable Multicast and Integrated
Parity Retransmission with Channel Estimation", IEEE
GLOBECOM 98'.
[9] J. Metzner, "An Improved Broadcast Retransmission Protocol",
IEEE Transactions on Communications, Vol. Com-32, No.6,
June 1984.
[10] J. Macker, "Integrated Erasure-Based Coding for Reliable
Multicast Transmission", IRTF Meeting presentation, March
1997.
[11] J. Macker, "Reliable Multicast Transport and Integrated
Erasure-based Forward Error Correction", Proc. IEEE MILCOM
97, October 1997.
[12] V. Ozdemir, S. Muthukrishnan, I. Rhee, "Scalable, Low-
Overhead Network Delay Estimation", NCSU/AT&T White Paper,
February 1999.
[13] M. Luby, L. Vicisano, J. Gemmell, L. Rizzo, M. Handley, and
J. Crowcroft, "Forward Error Correction (FEC) Building
BLock", RFC 3452, December 2002.
[14] J. Widmer, M. Handley, "TCP-Friendly Multicast Congestion
Control (TFMCC) Protocol Specification", Internet Draft
draft-ietf-rmt-bb-tfmcc-01.txt, November 2002, work in
progress. Citation for informational purposes only.
[15] J. Macker, R. Adamson, "Quantitative Prediction of Nack
Oriented Reliable Multicast (NORM) Feedback", Proc. IEEE
MILCOM 2002, October 2002.
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[16] Holbrook, H. W., "A Channel Model for Multicast", Ph.D.
Dissertation, Stanford University, Department of Computer
Science, Stanford, California, August 2001.
[17] Rizzo, L., Vicisano, L, Handley, M, "PGMCC Single Rate
Multicast Congestion Control Protocol Specification",
Internet Draft draft-ietf-rmt-bb-pgmcc-01.txt, June 2002,
work in progress. Citation for informational purposes only.
[18] 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.
[19] S. Kent and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
8.0 Authors' Addresses
Brian Adamson
adamson@itd.nrl.navy.mil
Naval Research Laboratory
Washington, DC, USA, 20375
Carsten Bormann
cabo@tellique.de
Tellique Kommunikationstechnik GmbH
Gustav-Meyer-Allee 25 Geb ude 12
D-13355 Berlin, Germany
Mark Handley
mjh@aciri.org
1947 Center Street, Suite 600
Berkeley, CA 94704
Joe Macker
macker@itd.nrl.navy.mil
Naval Research Laboratory
Washington, DC, USA, 20375
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