RMT Working Group B. Adamson/NRL
INTERNET-DRAFT C. Bormann/Tellique
draft-ietf-rmt-pi-norm-06 M. Handley/ACIRI
Expires: September 2003 J. Macker/NRL
March 2003
NACK-Oriented Reliable Multicast Protocol (NORM)
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 describes the messages and procedures of the Negative-
acknowledgement (NACK) Oriented Reliable Multicast (NORM) protocol.
This protocol is designed to provide end-to-end reliable transport of
bulk data objects or streams over generic IP multicast routing and
forwarding services. NORM uses a selective, negative acknowledgement
mechanism for transport reliability and offers additional protocol
mechanisms to conduct reliable multicast sessions with limited "a
priori" coordination among senders and receivers. A congestion
control scheme is specified to allow the NORM protocol fairly share
available network bandwidth with other transport protocols such as
Transmission Control Protocol (TCP). It is capable of operating with
both reciprocal multicast routing among senders and receivers and with
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asymmetric connectivity (possibly a unicast return path) from the
senders to receivers. The protocol offers a number of features to
allow different types of applications or possibly other higher level
transport protocols to utilize its service in different ways. The
protocol leverages the use of FEC-based repair and other IETF reliable
multicast transport (RMT) building blocks in its design.
1.0 Introduction and Applicability
The Negative-acknowledgement (NACK) Oriented Reliable Multicast (NORM)
protocol is designed to provide reliable transport of data from one or
more sender(s) to a group of receivers over an IP multicast network.
The primary design goals of NORM are to provide efficient, scalable,
and robust bulk data (e.g. computer files, transmission of persistent
data) transfer across possibly heterogeneous IP networks and
topologies. The NORM protocol design provides support for distributed
multicast session participation with minimal coordination among
senders and receivers. NORM allows senders and receivers to
dynamically join and leave multicast sessions at will with minimal
overhead for control information and timing synchronization among
participants. To accommodate this capability, NORM protocol message
headers contain some common information allowing receivers to easily
synchronize to senders throughout the lifetime of a reliable multicast
session. NORM is designed to be self-adapting to a wide range of
dynamic network conditions with little or no pre-configuration. The
protocol is purposely designed to be tolerant of inaccurate timing
estimations or lossy conditions that may occur many networks including
mobile and wireless. The protocol is also designed to exhibit
convergence and efficient operation even in situations of heavy packet
loss and large queueing or transmission delays.
This document is a product of the IETF RMT WG and follows the
guidelines provided in RFC 3269 [1]. 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 [2].
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1.1 NORM Delivery Service Model
A NORM protocol instance (NormSession) is defined within the context
of participants communicating connectionless (e.g. Internet Protocol
(IP) or User Datagram Protocol (UDP)) packets over a network using
pre-determined addresses and host port numbers. Generally, the
participants exchange packets using an IP multicast group address, but
unicast transport may also be established or applied as an adjunct to
multicast delivery. In the case of multicast, the participating
NormNodes will communicate using a common IP multicast group address
and port number that has been chosen via means outside the context of
the given NormSession. Other IETF data format and protocol standards
exist that may be applied to describe and convey the required "a
priori" information for a specific NormSession (e.g. Session
Description Protocol (SDP) [5], Session Announcement Protocol (SAP)
[6], etc).
The NORM protocol design is principally driven with the assumption of
a single sender transmitting bulk data content to a group of
receivers. However, the protocol MAY operate with multiple senders
within the context of a single NormSession. In initial
implementations of this protocol, it is anticipated that multiple
senders will transmit independently of one another and receivers will
maintain state as necessary for each independent sender. However, in
future versions of NORM, it is possible that some aspects of protocol
operation (e.g. round-trip time collection) may provide for alternate
modes allowing more efficient performance for applications requiring
multiple senders.
NORM provides for three types of bulk data content objects
(NormObjects) to be reliably transported. These types include:
1) static computer memory data content (NORM_OBJECT_DATA type),
2) computer storage files (NORM_OBJECT_FILE type), and
3) non-finite streams of continuous data content
(NORM_OBJECT_STREAM type).
The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is
simply to provide a "hint" to receivers in NormSessions serving
multiple types of content as to what type of storage should be
allocated for received content (i.e. memory or file storage). Other
than that distinction, the two are identical, providing for reliable
transport of finite (but potentially very large) units of content.
These static data and file services are anticipated to be useful for
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multicast-based cache applications with the ability to reliably
provide transmission of large quantities of static data. Other types
of static data/file delivery services might make use of these
transport object types, too. The use of the NORM_OBJECT_STREAM type
is at the application's discretion and could be used to carry static
data or file content also. The NORM reliable stream service opens up
additional possibilities such as serialized reliable messaging or
other unbounded, perhaps dynamically produced content. The
NORM_OBJECT_STREAM provides for reliable transport analogous to that
of the Transmission Control Protocol (TCP), although NORM receivers
will be able to begin receiving stream content at any point in time.
The applicability of this feature will depend upon the application.
The NORM protocol also allows for a small amount of "out-of-band" data
(sent as NORM_INFO messages) to be attached to the data content
objects transmitted by the sender. This readily-available "out-of-
band" data allows multicast receivers to quickly and efficiently
determine the nature of the corresponding data, file, or stream bulk
content being transmitted. This allows application-level control of
the receiver node's participation in the current transport activity.
This also allows the protocol to be flexible with minimal pre-
coordination among senders and receivers. The NORM_INFO content is
designed to be atomic in that its size MUST fit into the payload
portion of a single NORM message.
NORM does _not_ provide for global or application-level identification
of data content within in its message headers. Note the NORM_INFO
out-of-band data mechanism could be leveraged by the application for
this purpose if desired, or identification could alternatively be
embedded within the data content. NORM does identify transmitted
content (NormObjects) with transport identifiers that are applicable
only while the sender is transmitting and/or repairing the given
object. These transport data content identifiers (NormTransportIds)
are assigned in a monotonically increasing fashion by each NORM sender
during the course of a NormSession. Each sender maintains its
NormTransportId assignments independently so that individual
NormObjects may be uniquely identified during transport with the
concatenation of the sender session-unique identifier (NormNodeId) and
the assigned NormTransportId. The NormTransportIds are assigned from
a large, but fixed, numeric space in increasing order and may be
reassigned during long-lived sessions. The NORM protocol provides
mechanisms so that the sender application may terminate transmission
of data content and inform the group of this in an efficient manner.
Other similar protocol control mechanisms (e.g. session termination,
receiver synchronization, etc) are specified so that reliable
multicast application variants may construct different, complete bulk
transfer communication models to meet their goals.
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In summary, the NORM protocol's goal is to provide reliable transport
of different types of data content (including potentially mixed
types). The senders enqueue and transmit bulk content in the form of
static data or files and/or non-finite, ongoing stream types. The
sender will provide for repair transmission of this content in
response to NACK messages received from the receiver group.
Mechanisms for "out-of-band" information and other transport control
mechanisms are specified for use by applications to form complete
reliable multicast solutions for different purposes.
1.2 NORM Scalability
Group communication scalability requirements lead to adaptation of
negative acknowledgement (NACK) based protocol schemes when feedback
for reliability is required [7]. NORM is a protocol centered around
the use of selective NACKs to request repairs of missing data. NORM
provides for the use of packet-level forward error correction (FEC)
techniques for efficient multicast repair and optional proactive
transmission robustness[8]. FEC-based repair can be used to greatly
reduce the quantity of reliable multicast repair requests and repair
transmissions[9]. The principal factor in NORM scalability is the
volume of feedback traffic generated by the receiver set to facilitate
reliability and congestion control. NORM uses probabilistic
suppression of redundant feedback based on exponentially distributed
random backoff timers. The performance of this type of suppression
relative to other techniques is described in [10]. NORM dynamically
measures the group's roundtrip timing status to set its suppression
and other protocol timers. This allows NORM to scale well while
maintaining reliable data delivery transport with low latency relative
to the network topology over which it is operating. Feedback messages
can be either multicast to the group at large or sent via unicast
routing to the sender. In the case of unicast feedback, the sender
"advertises" the feedback state to the group to facilitate feedback
suppression. In typical Internet environments, it is expected that
the NORM protocol will readily scale to group sizes on the order of
tens of thousands of receivers. A study of the quantity of feedback
for this type of protocol is described in [11]. NORM is able to
operate with a smaller amount of feedback than a single TCP
connection, even with relatively large numbers of receivers. Thus,
depending upon the network topology, it is possible that NORM may
scale to larger group sizes. With respect to computer resource usage,
the NORM protocol does _not require that state be kept on all
receivers in the group. NORM senders maintain state only for
receivers providing explicit congestion control feedback. NORM
receivers must maintain state for for each active sender. This may
constrain the number of simultaneous senders in some uses of NORM.
1.3 NORM Environmental Requirements and Considerations
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All of the environmental requirements and considerations that apply to
the RMT FEC Building Block and the the RMT TCP-Friendly Multicast
Congestion Control (TFMCC) Building Block also apply to NORM. When
the RMT GRA Building Block is used with NORM, its environmental
requirements and considerations SHALL also apply.
The NORM protocol SHALL be capable of operating in an end-to-end
fashion with no assistance from intermediate systems beyond basic IP
multicast group management, routing, and forwarding services. The
NORM protocol SHOULD be compatible with techniques like Generic Router
Assist (GRA) [12] for performance benefits when applicable. While the
techniques utilized in NORM are principally applicable to "flat" end-
to-end IP multicast multicast topologies, they could also be applied
in the sub-levels of hierarchical (e.g. tree-based) multicast
distribution if so desired. NORM can make use of reciprocal (among
senders and receivers) multicast communication under the Any-Source
Multicast (ASM) model defined in RFC 1112 [13], but SHALL also be
capable of scalable operation in asymmetric topologies such as Source
Specific Multicast (SSM) [14] where there may only be unicast routing
service from the receivers to the sender(s).
NORM is compatible with IPv4 and IPv6. Additionally, NORM may be used
with networks employing Network Address Translation (NAT) providing
the NAT device supports IP multicast and/or can cache UDP traffic
source port numbers for remapping feedback traffic from receivers to
the sender(s).
2.0 NORM Architecture Definition
A NormSession is comprised of participants (NormNodes) acting as
senders and/or receivers. NORM senders transmit data content in the
form of NormObjects to the session destination address and the NORM
receivers attempt to reliably receive the transmitted content using
negative acknowledgments to request repair. Each NormNode within a
NormSession is assumed to have a preselected unique 32-bit identifier
(NormNodeId). NormNodes MUST have uniquely assigned identifiers
within a single NormSession to distinquish between possible multiple
senders and to distinguish feedback information from different
receivers. There are two reserved NormNodeId values. A value of
0x00000000 is considered an invalid NormNodeId value and a value of
0xffffffff is a "wildcard" NormNodeId. Whilte, the protocol does not
preclude multiple sender nodes concurrently transmitting within the
context of a single NORM session (i.e. many- to-many operation), any
type of interactive coordination among NORM senders is assumed to be
controlled by the application or higher protocol layer. There are
some optional mechanisms specified in this document which can be
leveraged for such application layer coordination.
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As previusly noted, NORM allows for reliable transmission of three
different basic types of data content. The first type is
NORM_OBJECT_DATA which is used for static, persistent blocks of data
content maintained in the sender's application memory storage. The
second type is NORM_OBJECT_FILE which corresponds to data stored in
the sender's non-volatile file system. The NORM_OBJECT_DATA and
NORM_OBJECT_FILE types both represent "NormObjects" of finite but
potentially very large size. The third type of data content is
NORM_OBJECT_STREAM which corresponds to an ongoing transmission of
undefined length. This is analogous to the reliable streaming content
provide by TCP for unicast data transport. The format of the stream
content is application-defined and may be byte or message oriented.
The NORM protocol provides for "flushing" of the stream to expedite
delivery or possible enforce application message boundaries. NORM
protocol implementations may offer either (or both) in-order delivery
of the stream data to the receive application or out-of-order (more
immediate) delivery of received segments of the stream to the receiver
application. In either case, NORM sender and receiver implementations
provide buffering to facilitate repair of the stream as it is
transported. All NormObjects are logically segmented into FEC coding
blocks and segments for transmission by the sender.
NormObjects and associated transmission segments are temporarily yet
uniquely identified within the NormSession context using the given
sender's NormNodeId and a temporarily unique NormObjectTransportId.
These data content identifiers are sender-assigned and applicable and
valid only during a NormObject's actual _transport_ (i.e. for as long
as the sender is transmitting and providing repair of the indicated
NormObject). For a long-lived session, the NormObjectTransportId
field can wrap and previously-used identifiers may be re-used. Note
that globally unique identification of transported data content is not
provided by NORM and, if required, must be managed by the NORM
application. Individual NormObject segments are further identified
with FEC coding block and symbol (segment) indentifiers. This is
discussed in detail later in this document.
2.1 NORM Protocol Operation Overview
A NORM sender primarily generates messages of type NORM_DATA that
carry the NormObject data content segments and related FEC parity-
based repair segments for the bulk data/file or stream objects being
transferred. By default, FEC segments are sent only in response to
receiver repair requests (NACKs) and thus normally impose no
additional transmission overhead. However, the NORM implementation
MAY be optionally configured to proactively transmit some amount of
FEC segments along with the data content to potentially enhance
performance (e.g., improved delay) at the cost of additional overhead
with initial data transmission. This configuration may be sensible
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for certain network conditions and can allow for robust, asymmetric
multicast (e.g., unidirectional routing, satellite, cable) [19] with
reduced receiver feedback, or, in some cases, no feedback.
A sender message of type NORM_INFO is also defined and is used to
carry any optional "out-of-band" context information for a given
transport object. A single NORM_INFO message can be associated with a
NormObject. Because of its atomic nature, missing NORM_INFO messages
can be NACKed and repaired with a slightly lower delay process than
NORM's general FEC-encoded data content. NORM_INFO may serve special
purposes for some bulk transfer, reliable multicast applications where
receivers join the group mid-stream and need to ascertain contextual
information on the current content being transmitted. The NACK
process for NORM_INFO will be described later.
The sender also generates messages of type NORM_CMD to assist in
certain protocol operations such as congestion control, end-of-
transmission flushing, round trip time estimation, receiver
synchronization, and optional positive acknowledgement requests or
application defined commands. The transmission of NORM_CMD messages
from the sender is accomplished by one of three different processes.
These are: single, best effort unreliable transmission of the command;
repeated redundant transmissions of the command; and positively-
acknowledged commands. The transmission technique used for a given
command depends upon the function of the command. Several core
commands are defined for basic protocol operation. Additionally,
implementations MAY wish to consider providing the OPTIONAL
application-defined commands that can take advantage of the
transmission methodologies available for commands. This allows for
application-level session management mechanisms which can make use of
information available to the underlying NORM protocol engine (e.g.
round-trip timing, transmission rate, etc).
NORM receivers generate messages of type NORM_NACK or NORM_ACK in
response to transmissions of data and commands from a sender. The
NORM_NACK messages are generated to request repair of detected data
transmission losses. Receivers generally detect losses by tracking
the sequence of transmission from a sender. Sequencing information is
embedded in the transmitted data packets and end-of-transmission
commands from the sender. NORM_ACK messages are generated in response
to certain commands transmitted by the sender. In the general (and
most scalable) protocol mode, NORM_ACK messages are sent only in
response to congestion control commands from the sender. The feedback
volume of these congestion control NORM_ACK messages is controlled
using the same timer-based probabilistic suppression techniques as for
NORM_NACK messages to avoid feedback implosion. In order to meet
potential application requirements for positive acknowledgement from
receivers, other NORM_ACK messages are defined and available for use.
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All sender and receiver transmissions are subject to rate control
governed by a peak transmission rate set for each participant by the
application. This can be used to limit the quantity of multicast data
transmitted by the group. When NORM's congestion control algorithm is
enabled the rate for senders is automatically adjusted. In some
networks, it may be desirable to establish minimum and maximum bounds
for the rate adjustment depending upon the application even when
dynamic congestion control is enabled. However, in the case of the
general Internet, congestion control policy SHALL be observed which is
compatible with coexistent TCP flows.
2.2 NORM Protocol Building Blocks
The operation of the NORM protocol is based upon the concepts
presented in the Nack-Oriented Reliable Multicast (NORM) Building
Block document[15]. This includes the basic NORM architecture and the
data transmission, repair, and feedback strategies discussed in that
document. NORM also makes use of Forward Error Correction encoding
techiques for repair messaging and optional transmission robustness as
described in [16]. NORM uses the FEC Payload ID as specified by the
FEC Building Block Document[17]. Additionally, for congestion
control, the NORM protocol specifies a mechanism based on the TCP-
Friendly Multicast Congestion Control (TFMCC) Building Block described
in [18].
2.3 NORM Design Tradeoffs
While the various features of NORM are designed to provide some
measure of general purpose utility, it is important to emphasize the
understanding that "no one size fits all" in the reliable multicast
transport arena. There are numerous engineering tradeoffs involved in
reliable multicast transport design and this requires an increased
awareness of application and network architecture considerations.
Performance requirements affecting design can include: group size,
heterogeneity (e.g., capacity and/or delay), asymmetric delivery, data
ordering, delivery delay, group dynamics, mobility, congestion
control, and transport across low capacity connections. NORM contains
various parameters to accommodate many of these differing
requirements. The NORM protocol and its mechanisms MAY be applied in
multicast applications outside of bulk data transfer, but there is an
assumed model of bulk transfer transport service that drives the
trade-offs that determine the scalability and performance described in
this document.
The ability of NORM to provide reliable data delivery is also governed
by any buffer constraints of the sender and receiver applications.
NORM protocol implementations SHOULD be designed to operate with the
greatest efficiency and robustness possible within application-defined
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buffer constraints. Buffer requirements for reliability, as always,
are a function of the delay-bandwidth product of the network topology.
NORM performs best with additional buffering as compared to typical
point-to-point transport NORM feedback suppression based upon
randomly-delayed transmissions from the receiver set. There are
definitive tradeoffs between buffer utilization, group size
scalability, and efficiency of performance. Large buffer sizes allow
the NORM protocol to perform most efficiently in large delay-bandwidth
topologies and allow for longer feedback suppression backoff timeouts.
This yields improved group size scalability. NORM can operate with
reduced buffering but at a cost of decreased efficiency (lower
relative goodput) and reduced group size scalability.
3.0 Conformance Statement
This Protocol Instantiation document, in conjunction with the
following Building Block documents identified in [15], [16], [17], and
[18] completely specifies a working reliable multicast transport
protocol that conforms to the requirements described in RFC 2357 [3].
4.0 NORM Message Formats
As mentioned in Section 2.1, there are two primary classes of NORM
messages: sender messages and receiver messages. NORM_CMD, NORM_INFO,
and NORM_DATA message types are generated by senders of data content,
and NORM_NACK and NORM_ACK messages generated by receivers within a
NormSession. An auxillary message type of NORM_REPORT is also
provided for experimental purposes. This section described the
message formats used by the NORM protocol. These messages and their
fields are referenced in the detailed functional description of the
NORM protocol given in Section 5.0. Individual NORM messages are
designed to be compatible with the MTU limitations of encapsulating
Internet protocols including IPv4, IPv6, and UDP. The current NORM
protocol specification assumes UDP encapsulation and leverages the
transport features of UDP. The NORM messages are independent of
network addresses and can be used in IPv4 and IPv6 networks.
4.1 NORM Common Message Header
There are some common message fields contained in all NORM message
types. All NORM protocol messages begin with a common header with
information fields as follows:
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NORM Common Message Header Format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
The "version" field is a 8-bit value indicating the protocol version
number. Currently, NORM implementations SHOULD ignore received
messages with a different protocol version number than their own. This
number is intended to indicate and distinguish upgrades of the
protocol which may be non-interoperable.
The message "type" field is a 8-bit value indicating the NORM protocol
message type. These types are defined as follows:
Message Value
NORM_INFO 1
NORM_DATA 2
NORM_CMD 3
NORM_NACK 4
NORM_ACK 5
NORM_REPORT 6
The "sequence" field is a 16-bit value that is set by the message
originator as a monotonically increasing number incremented with each
NORM message transmitted to the session's destination address. The
"sequence" field SHOULD not be incremented for messages not sent to
the session group address (e.g. unicast NACKs or unicast ACKs). This
value can be monitored by receiving nodes to detect packet losses in
the transmission from a sender. Note that this value is NOT used in
the NORM protocol to detect missing reliable data content and does NOT
identify the application data or FEC payload that may be attached.
This sequence number is intended for use in estimating raw packet loss
for congestion control purposes. The size of this field is intended
to be sufficient to allow detection of a reasonable range of packet
loss within the delay-bandwidth product of expected network
connections.
The "source_id" field is a 32-bit value identifying the node that sent
the message. A participant's NORM node identifier (NormNodeId) can be
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set according to the application needs but unique identifiers must be
assigned within a single NormSession. In some cases, use of the host
IP address or a hash of it can suffice, but alternative methodologies
for assignment and potential collision resolution of node identifiers
within a multicast session need to be considered. For example, the
"source identifier" mechanism defined in the Real-Time Protocol (RTP)
specification [20] may be applicable to use for NORM node identifiers.
At this point in time, the protocol makes no assumptions about how
these unique identifiers are actually assigned.
4.2 NORM Sender Messages
NORM sender messages include the NORM_DATA type, the NORM_INFO type,
and the NORM_CMD type. NORM_DATA and NORM_INFO messages contain
application data content while NORM_CMD messages for various protocol
functions.
4.2.1 NORM_DATA Message
The NORM_DATA message is expected to be the predominant type
transmitted by NORM senders. These messages are used to encapsulate
segmented data content for objects of type NORM_OBJECT_DATA,
NORM_OBJECT_FILE, and NORM_OBJECT_STREAM. NORM_DATA messages may
contain original or FEC-encoded application data content. The payload
size of these messages SHALL be limited to a maximum of the sender's
NormSegmentSize. A sender's NormSegmentSize is assumed to be constant
for the duration of a given sender's term of participation in the
session. The NormSegmentSize is expected to be configurable by the
sender application prior to session participation as needed for
network topology maximum transmission unit (MTU) considerations. For
IPv6, MTU discovery may be leveraged at session startup
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NORM_DATA Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 2 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | grtt | gsize | fec_id = 129 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_instance_id | fec_num_parity |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_max_block_len | segment_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id | object_size (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_size (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_len | fec_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_len* | offset (msb)* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| offset (lsb)* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data* |
*Note: The "payload_len" and "offset" fields for NORM_DATA messages
containing parity information are actually values computed from FEC
encoding of the "payload_len" and "offset" fields of the data segments
of the applicable coding block. So, for parity segments, these do
_not_ represent actual values. Parity packets can be identified as
packets where "fec_symbol_id >= fec_block_len".
The "version", "type", "sequence", and "source_id" fields form the
NORM Common Message Header asdescribed in Section 4.1.
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The "flags" field contains a number of different binary flags
providing information and hints regarding how the receiver should
handle the identified object. Defined flags in this field include:
+---------------------+-------+------------------------------------------+
| Flag | Value | Purpose |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_REPAIR | 0x01 | Indicates message is a repair |
| | | transmission |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_EXPLICIT | 0x02 | Indicates a repair segment intended |
| | | which meets a specific receiver erasure, |
| | | as compared to parity segments provided |
| | | by the sender for general purpose (with |
| | | respect to an FEC coding block) erasure |
| | | filling. |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_INFO | 0x04 | Indicates availability of NORM_INFO for |
| | | object |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_UNRELIABLE | 0x08 | Indicates that repair transmissions for |
| | | the specified object will be |
| | | unavailable. (One-shot, best effort |
| | | transmission) |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_FILE | 0x10 | Indicates object is "file-based" data |
| | | (hint to use disk storage for reception) |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_STREAM | 0x20 | Indicates object is of type |
| | | NORM_OBJECT_STREAM. |
+---------------------+-------+------------------------------------------+
The NORM_FLAG_REPAIR flag is set when the associated message is a
repair transmission. This information can be used by receivers to
help observe a join policy where it is desired that newly joining
receivers only begin participating in the NACK process upon receipt of
new (non-repair) data content. The NORM_FLAG_EXPLICIT flag is used to
mark repair messages sent when the data sender has exhausted its
ability to provide "fresh" (previously untransmitted) parity segments
as repair. This flag may be used by intermediate systems implementing
Generic Router Assist (GRA) functionality to control subcasting of
repair content to different legs of a reliable multicast topology with
disparate repair needs. The NORM_FLAG_INFO flag is set only when there
optional NORM_INFO content is available for the associated object.
Thus, receivers will NACK for retransmission of NORM_INFO only when it
is available. The NORM_FLAG_UNRELIABLE flag is set when the sender
wishes to transmit an object with only "best effort" delivery and will
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not supply repair transmissions for the object. The NORM_FLAG_FILE
flag can be set as a "hint" from the sender that the associated object
should be stored in non-volatile storage. The NORM_FLAG_STREAM flag
is set when the identified object is of type NORM_OBJECT_STREAM.
The "grtt" field contains a non-linear quantized representation of the
sender's current estimate of group round-trip time (GRTT) (This is
also referred to as R_max in the TFMCC Building Block [18]). This
value is used to control timing of the NACK repair process and other
aspects of protocol operation as described in this document. The
algorithm for encoding and decoding this field is described in the RMT
NORM Building Block document[15].
The "gsize" field contains a representation of the sender's current
estimate of group size. This value is used to control feedback
suppression mechanisms within the protocol for more optimized
performance for different group sizes. The 8-bit "gsize" field
consists of 4 bits of mantissa in the 4 most significant bits and 4
bits of base 10 exponent (order of magnitude) information in the 4
least significant bits. For example, to represent an approximate
group size of 100 (or 1e02), the value of the upper 4 bits is 0x01 (to
represent the mantissa of 1) and the lower 4 bits value would be 0x02
for an 8-bit representation of "0x12". As another example, a group
size of 9000 (9e03) would be represented by the value 0x93. The group
size does not need to be represented with a high degree of precision
to appropriately scale backoff timers, etc.
The "fec_id" field corresponds to the FEC Encoding Identifier
described in the FEC Building Block document [17]. Note the packet
format illustrated above assumes "Small Block Systematic Codes" that
corresponds to an FEC Encoding Identifier equal to 129. The other
"fec_" fields may be interpreted or sized differently to supportother
FEC Encoding Identifier types in the future.
The "fec_instance_id" corresponds to the "FEC Instance ID" of the FEC
Object Transmission Informatiom given in the FEC Building Block
document[17]. The "fec_instance_id" SHALL be a value corresponding to
the particular type of Small Block Systematic Code being used (e.g.
Reed-Solomon GF(2^8), Reed-Solomon GF(2^16), etc). The standardized
assignment of FEC Instance ID values is described in [17].
The "fec_num_parity" corresponds to the "maximum number of of encoding
symbols that can be generated for any source block" as described in
for FEC Object Transmission Information for Small Block Systematic
Codes in the FEC Building Block document [17]. For example, Reed-
Solomon codes may be arbitrarily shortened to create different code
variations for a given block length. In the case of Reed-Solomon
(GF(2^8) and GF(2^16) codes, this value indicates the maximum number
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of parity segments available from the sender for the coding blocks.
This field MAY be interpreted differently for other systematic codes
as they are defined.
The "fec_max_block_len" indicates the current maximum number of user
data segments per FEC coding block to be used by the sender during the
session. This allows receivers to allocate appropriate buffer space
for buffering blocks transmitted by the sender.
The "segment_size" field indicates the sender's current setting for
maximum message payload content (in bytes). This allows receivers to
allocate appropriate buffering resources and to determine other
information in order to properly process received data messaging.
The "object_transport_id" field is a monotonically and incrementally
increasing value assigned by a sender to the object being transmitted.
Transmissions and repair requests related to that object use the same
"object_transport_id" value. For sessions of very long or indefinite
duration, the "object_transport_id" field may be repeated, but it is
presumed that the 16-bit field size provides an adequate enough
sequence space to prevent temporary object confusion amongst receivers
and sources (i.e. receivers SHOULD re-synchronize with a server when
receiving object sequence identifiers sufficiently out-of-range with
the current state kept for a given source). During the course of its
transmission within a NORM session, an object is uniquely identified
by the concatenation of the sender "node_id" and the given
"object_transport_id". Note that NORM_INFO messages associated with
the identified object carry the same "object_transport_id" value.
The 48-bit "object_size" field indicates the total size of the object
(in bytes) for the static object types of NORM_OBJECT_FILE and
NORM_OBJECT_DATA. This information is used by receivers to determine
storage requirements and/or allocate storage for the received object.
Receivers with insufficient storage capability may wish to forego
reliable reception (i.e. not NACK for) of the indicated object. In
the case of objects of type NORM_OBJECT_STREAM, the "object_size"
field is used to by the sender to indicate the size of its stream
buffer to the receiver group. In turn, the receivers SHOULD use this
information to allocate a stream buffer for reception of corresponding
size.
The "fec_block_number", "fec_block_len", and "fec_symbol_id" fields
correspond to the "Source Block Number", "Source Block Length, and
"Encoding Symbol ID" fields of the FEC Payload ID format given by the
FEC Building Block document[17]. The "fec_block_number" identifies
the coding block's relative position with a NormObject. Note that,
for NormObjects of type NORM_OBJECT_STREAM, the "fec_block_number" may
wrap for very long lived sessions. The "fec_block_len" indicates the
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number of user data segments in the identified coding block. Given
the "fec_block_len" (Source block length) information of how many
symbols of application data is contained in the block, the receiver
can determine whether the attached segment is data or parity content
and treat it appropriately. The "fec_symbol_id" identifies which
specific symbol (segment) within the coding block the attached payload
conveys. Depending upon the value of the "fec_symbol_id" and the
associated "fec_block_len" and "fec_num_parity" parameters for the
block, the symbol (segment) referenced may be a user data or an FEC
parity segment. For systematic codes, symbols numbered 0 through
(fec_block_len-1) contain application data while segments numbered
(fec_block_len) through (fec_block_len+fec_num_parity-1) contain the
parity symbols calculated for the block.
The concatenation of
object_tranport_id::fec_block_number::fec_symbol_id can be viewed as a
unique transport data unit (TPDU) identifier for the attached segment
with respect to the NORM sender.
The "payload_len" and "offset" fields are used to identify the
relative position and quantity of the content of the message payload.
For senders employing systematic FEC encoding, these fields will
correspond to actual length and offset values for NORM_DATA messages
which contain original data content. For NORM_DATA messages
containing calculated parity content, these fields will actually
contain values computed by FEC encoding of the "payload_len" and
"offset" values of the NORM_DATA segments of the corresponding FEC
coding block. Thus, the "payload_len" and "offset" values of missing
data content can be determined when decoding an FEC coding block.
The "payload_data" field contains original data or computed parity
content of the identified segment. The maximum length of this field
corresponds to the sender's NormSegmentSize. The length of this field
for messages containing parity content will always be of the length
NormSegmentSize. When encoding data segments of varying sizes, the
FEC encoder SHALL assume zero value padding for data segments with
length less than the NormSegmentSize. The receiver will use the
"payload_len" information to properly retrieve received data content
and deliver it to the application.
4.2.2 NORM_INFO Message
The NORM_INFO message is used to convey OPTIONAL, application-defined,
"out-of-band" context information for transmitted NormObjects. An
example NORM_INFO use for bulk file transfer is to place MIME type
information for the associated file, data, or stream object into the
NORM_INFO payload. Receivers may use the NORM_INFO content to make a
decision as whether to participate in reliable reception of the
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associated object. Each NormObject can have an independent unit of
NORM_INFO associated with it. NORM_DATA messages contain a flag to
indicate the availability of NORM_INFO for a given NormObject. NORM
receivers may NACK for retransmission of NORM_INFO when they have not
received it for a given NormObject. The size of the NORM_INFO content
is limited to that of a single NormSegmentSize for the given sender.
This atomic nature allows the NORM_INFO to be rapidly and efficiently
repaired within the NORM reliable transmission process.
When NORM_INFO content is available for a NormObject, the
NORM_FLAG_INFO flag SHALL be set in NORM_DATA messages for the
corresponding "object_transport_id" and the NORM_INFO message shall be
transmitted as the first message for the NormObject.
NORM_INFO Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 1 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | grtt | gsize | fec_id = 129 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_encoding_name | fec_num_parity |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_max_block_len | segment_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id | object_size (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_size (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data |
The "version", "type", "sequence", and "source_id" fields form the
NORM Common Message Header asdescribed in Section 4.1.
The "flags", "grtt", "gsize", "fec_id", "fec_encoding_name",
"fec_num_parity", "fec_max_block_len", "segment_size",
"object_transport_id", and "object_size" fields carry the same
information and serve the same purpose as with NORM_DATA messages.
These values allow the receiver to prepare appropriate buffering, etc,
for further transmissions from the sender when NORM_INFO is the first
message received.
The NORM_INFO "payload_data" field contains sender application-defined
content which can be used by receiver applications for various
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purposes as described above.
4.2.3 NORM_CMD Message
NORM_CMD messages are transmitted by senders to perform a number of
different protocol functions. This includes functions such as round-
trip timing collection, congestion control functions, synchronization
of sender/receiver repair "windows", and notification of sender
status. A core set of NORM_CMD messages is enumerated. Additionally,
a range of command types remain available for potential application-
specific use. Some NORM_CMD types may have dynamic content attached.
Any attached content will be limited to maximum length of the sender
NormSegmentSize to retain the atomic nature of commands. All NORM_CMD
message begins with a common set of fields, after the usual NORM
message common header. The standard NORM_CMD fields are:
NORM_CMD Standard Fields
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt | gsize | flavor | ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "version", "type", "sequence", and "source_id" fields form the
NORM Common Message Header as described in Section 4.1.
The "grtt" and "gsize" fields provide the same information and serve
the same purpose as with NORM_DATA and NORM_INFO messages. The
"flavor" field indicates the type of command to follow. The remainder
of the NORM_CMD message is dependent upon the command type ("flavor").
The command flavors include:
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+----------------------+--------------+----------------------------------+
| Command | Flavor Value | Purpose |
+----------------------+--------------+----------------------------------+
|NORM_CMD(FLUSH) | 1 | Used to indicate sender |
| | | temporary or permanent end-of- |
| | | transmission. (Assists in |
| | | robustly initiating outstanding |
| | | repair requests from receivers). |
+----------------------+--------------+----------------------------------+
|NORM_CMD(SQUELCH) | 2 | Used to advertise sender's |
| | | current repair window in |
| | | response to out-of-range NACKs |
| | | from receivers. |
+----------------------+--------------+----------------------------------+
|NORM_CMD(ACK_REQ) | 3 | Used to request positive |
| | | acknowledgement from a list of |
| | | receivers. |
+----------------------+--------------+----------------------------------+
|NORM_CMD(REPAIR_ADV) | 4 | USed to advertise sender's |
| | | aggregated repair state for |
| | | suppression of unicast receiver |
| | | feedback. |
+----------------------+--------------+----------------------------------+
|NORM_CMD(CC) | 5 | Used for GRTT measurement and |
| | | explicitly collection of |
| | | congestion control feedback. |
+----------------------+--------------+----------------------------------+
|NORM_CMD(APPLICATION) | 6 | Used for application-defined |
| | | purposes which may need to |
| | | temporarily preempt data |
| | | transmission. |
+----------------------+--------------+----------------------------------+
NORM_CMD(FLUSH) Message
The NORM_CMD(FLUSH) command is sent when the sender reaches the end of
all data content and pending repairs it has queued for transmission.
This command is repeated once per 2*GRTT to excite the receiver set
for any outstanding repair requests up to and including the
transmission point indicated within the NORM_CMD(FLUSH) message. The
number of repeats is equal to NORM_ROBUST_FACTOR. The greater the
NORM_ROBUST_FACTOR, the greater the probability that all applicable
receivers will be excited for repair requests (NACKs) _and_ that the
corresponding NACKs are delivered to the sender. If a NORM_NACK
message interrupts its flush process, the sender will re-initiate the
flush process when any resulting repair transmissions are completed.
Note that receivers also employ a timeout mechanism to self-initiate
NACKing when no messages are received from a sender. This inactivity
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timeout is related to 2*GRTT*NORM_ROBUST_FACTOR and will be discussed
more later. With a sufficient NORM_ROBUST_FACTOR value, data content
is delivered with a high assurance of reliability. The penalty of a
large NORM_ROBUST_FACTOR value is potentially excess sender
NORM_CMD(FLUSH) transmissions and a longer timeout for receivers to
self-initiate the terminal NACK process.
For finite-size transport objects such NORM_OBJECT_DATA and
NORM_OBJECT_FILE, the flush process (if there are no further pending
transmissions) will occur at the end of these objects and thus any FEC
repair information is available for repairs in response to repair
requests elicited by the flush command. However, for
NORM_OBJECT_STREAM, the flush may occur at any time, including in the
middle of an FEC coding block if systematic FEC codes are emplyed. In
this case, the sender will not yet be able to provide FEC parity
content as repair for the concurrent coding block and will be limited
to explicitly repairing stream data content for that block.
Applications that anticipate frequent flushing of stream content
SHOULD be judicious in the selection of the FEC coding block size
(i.e. do not use a very large coding block size if frequent flushing
occurs). For example, a reliable multicast application transmitting
an on-going series of intermittent, relatively small messaging content
will need to trade-off using the NORM_OBJECT_DATA paradigm versus the
NORM_OBJECT_STREAM paradigm with an appropriate FEC coding block size.
This is analogous to application trade-offs for other transport
protocols such as the selection of different TCP modes of operation
such as "no delay", etc.
NORM_CMD(FLUSH) Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt | gsize | flavor = 1 | flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id | fec_block_number (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) | fec_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In addition to the NORM common message header and standard NORM_CMD
fields, the NORM_CMD(FLUSH) message contains fields to identify the
current status and logical transmit position of the sender.
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The "flags" field contains sender status information. A single
NORM_CMD(FLUSH) flag is currently defined:
NORM_FLUSH_FLAG_EOT = 0x01
When the NORM_FLUSH_FLAG_EOT flag is set, this indicates the sender is
preparing to terminate transmission and will no longer provide
response to repair requests. This allows the receiver set to
gracefully reach closure of operation with this sender and free any
resources that are no longer needed.
The "object_transport_id", "fec_block_number", and "fec_symbol_id"
fields indicate the sender's current logical "transmit position".
These fields are interpreted in the same manner as the fields of the
same names in the NORM_DATA message type. Upon receipt of the the
NORM_CMD(FLUSH), receivers are expected to check their completion
state _through_ (including) this transmission position. If receivers
have outstanding repair needs in this range, they SHALL initiate the
NORM NACK Repair Process as described in Section 5.3. If receivers
have no outstanding repair needs, no response is generated.
For NORM_OBJECT_STREAM objects, receivers MUST request "explicit-only"
repair of the identified "fec_block_number" if the given
"fec_symbol_id" is less than the sender's "fec_max_block_len - 1".
This condition indicates the sender has not yet completed encoding the
corresponding FEC block and parity content is not yet available. An
"explicit-only" repair request consists of NACK content for the
applicable "fec_block_number" which does not include any requests for
parity-based repair. This allows NORM sender applications to "flush"
an ongoing stream of transmission when needed, even if in the middle
of an FEC block. Once the sender resumes stream transmission and
passes the end of the pending coding block, subsequent NACKs from
receivers SHALL request parity-based repair as normal. Note that the
use of a systematic FEC code is assumed here. Normal receiver NACK
inititation and construction is discussed in detail in Section 5.3.
NORM_CMD(SQUELCH) Message
The NORM_CMD(SQUELCH) command is transmitted in response to invalid
NORM_NACK content received by the sender. Invalid NORM_NACK content
consists of repair requests for NormObjects for which the sender is
unable or unwilling to provide repair. This includes repair requests
for outdated objects, aborted objects, or those objects which the
sender previously transmitted marked with the NORM_FLAG_UNRELIABLE
flag. This command indicates to receivers what content is available
for repair, thus serving as a description of the sender's current
"repair window". Receivers SHALL not generate repair requests for
content identified as invalid by a NORM_CMD(SQUELCH).
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The NORM_CMD(SQUELCH) command is sent once per 2*GRTT at the most.
The NORM_CMD(SQUELCH) advertises the current "repair window" of the
sender by identifying the earliest (lowest) transmission point for
which it will provide repair, along with an encoded list of objects
from that point forward that are no longer valid for repair. This
mechanism allows the sender application to cancel or abort
transmission and/or repair of previously enqueued objects. The list
also contains the identifiers for any objects within the repair window
which were sent with the NORM_FLAG_UNRELIABLE flag set. In normal
conditions, it is expected the NORM_CMD(SQUELCH) will be needed
infrequently, and generally only to provide a reference repair window
for receivers who have fallen "out-of-sync" with the sender due to
extremely poor network conditions.
The starting point of the invalid NormObject list begins with the
lowest invalid NormTransportId greater than the current "repair
window" start from the invalid NACK(s) that prompted the generation of
the squelch. The length of the list is limited by the sender's
NormSegmentSize. This allows the receivers to learn the status of the
sender's applicable object repair window with minimal transmission of
NORM_CMD(SQUELCH) commands. The format of the NORM_CMD(SQUELCH)
message is:
NORM_CMD(SQUELCH) Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt | gsize | flavor = 2 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id | fec_block_number (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) | fec_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| invalid_object_list ... |
In addition to the NORM common message header and standard NORM_CMD
fields, the NORM_CMD(SQUELCH) message contains fields to identify the
earliest logical transmit position of the sender's current repair
window and an "invalid object list" beginning with the index of the
logically earliest invalid repair request from the offending NACK
message which initiated the squelch transmission.
The "object_transport_id", "fec_block_number", and "fec_symbol_id"
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fields are concatenated to indicate the beginning of the sender's
current repair window (i.e. the logically earliest point in its
transmission history for which the sender can provide repair). This
serves as an advertisement of a "synchronization point" for receivers
to request repair. Note, that while the "fec_symbol_id" is provided
here, the sender's repair window will generally be incremented on an
FEC coding block basis and the "fec_symbol_id" will be zero.
The "invalid_object_list" is a list of 16-bit NormTransportIds that,
although they are within the sender's current repair window, are no
longer available for repair from the sender. For example, a sender
application may dequeue an out-of-date object even though it is still
within the repair window. The total size of the "invalid_object_list"
content is implied by the packets payload length and is limited to a
maximum of the NormSegmentSize of the sender. Thus, for very large
repair windows, it is possible that a single NORM_CMD(SQUELCH) message
may not be capable of listing the entire set of invalid objects in the
repair window. In this case, the sender SHALL ensure that the list
begins with a NormObjectId that is greater than or equal to the lowest
ordinal invalid NormObjectId from the NACK message(s) that prompted
the NORM_CMD(SQUELCH) generation. The NormObjectIds in the
"invalid_object_list" must be greater than the "object_transport_id"
marking the beginning of the sender's repair window. This insures
convergence of the squelch process, even if multiple invalid NACK/
squelch iterations are required. This explicit description of invalid
content within the sender's current window allows the sender
application (most notably for discrete "object" based transport) to
arbitrarily invalidate (i.e. dequeue) portions of enqueued content
(e.g. certain objects) for which it no longer wishes to provide
reliable transport.
NORM_CMD(REPAIR_ADV) Message
The NORM_CMD(REPAIR_ADV) message is used by the sender to "advertise"
its aggregated repair state from accumulated NORM_NACK messages
accumulated during a repair cycle and/or congestion control feedback
received. This message is sent only when the sender has received
NORM_NACK and/or NORM_ACK(RTT) (when congestion control is enabled)
messages via unicast transmission instead of multicast. By "echoing"
this information to the receiver set, suppression of feedback can be
achieved even when receivers are unicasting that feedback instead of
multicasting it among the group[11].
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NORM_CMD(REPAIR_ADV) Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt | gsize | flavor = 4 | flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_adv_content ... |
The "grtt", "gsize" and "flavor" fields serve the same purpose as in
other NORM_CMD messages.
The "flags" field provide information on the NORM_CMD(REPAIR_ADV)
content. There is currently one NORM_CMD(REPAIR_ADV) flag defined:
NORM_REPAIR_ADV_FLAG_LIMIT = 0x01
This flag is set by the sender when it is unable to fits its full
current repair state into a single NormSegmentSize. If this flag is
set, receivers should limit their NACKing to generating NACKs only up
through the maximum ordinal transmission position
(objectId::fecBlockId::fecSymbolId) included in the
"repair_adv_content".
When congestion control operation is enabled, the "cc_flags",
"cc_rtt", and "cc_rate" fields contain values for the receiver with
the lowest calculated congestion control rate from which feedback was
received since the last NORM_CMD(REPAIR_ADV) transmission. These
fields are used by receivers to suppress rounds of congestion control
feedback. The definition of these fields is given in the description
of the NORM_CMD(CC) message below.
The "repair_adv_content" is in exactly the same form as the
"nack_content" of NORM_NACK messages and can be processed by receivers
for suppression purposes in the same manner with the exception of the
condition when the NORM_REPAIR_ADV_FLAG_LIMIT is set.
NORM_CMD(CC) Message
The NORM_CMD(CC) messages contains fields to enable sender->receiver
group greatest round-trip time (GRTT) measurement and provide
congestion control information to the group. The NORM_CMD(CC) message
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is usually transmitted as part of NORM congestion control operation.
If NORM is operated in a private network with congestion control
operation disabled, the NORM_CMD(CC) message is then used to
facilitate GRTT measurement by the sender.
NORM_CMD(CC) Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt | gsize | flavor = 5 | flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_rate | cc_sequence | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_list ... |
The NORM common message header and standard NORM_CMD fields serve
their usual purposes.
The "flags" field is used to indicate NORM_CMD(CC) options. Currently
a single NORM_CMD(CC) flag is defined:
NORM_CC_FLAG_ENABLE = 0x01
When set, this indicates the sender has enabled congestion control
feedback collection, and receivers should respond observing the
procedures describe in Section 5.5.2, "NORM Congestion Control
Operation". When this flag is cleared (i.e. congestion control
feedback collection is disabled), this indicates the sender is not
observing congestion control operation and the NORM_CMD(CC) message is
being used only to provide a reference timestamp for GRTT measurement
via receiver NORM_NACK feedback.
The "send_time" field is a timestamp indicating the time that the
NORM_CMD(CC) message was transmitted. This consists of a 64-bit field
containing 32-bits with the time in seconds ("sent_time_sec") and
32-bits with the time in microseconds ("send_time_usec") since some
reference time the source maintains (usually 00:00:00, 1 January
1970). The byte ordering of the fields is "Big Endian" network order.
Receivers use this timestamp adjusted by the amount of delay from when
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they received the NORM_CMD(CC) message to when they respond for the
"grtt_response" portion of NORM_ACK and NORM_NACK messages generated.
This allows the sender to evaluate the round-trip time to different
receivers for congestion control and other (e.g. GRTT determination)
purposes.
The "send_rate" field indicates the sender's current transmission rate
in bytes per second. The 16-bit "send_rate" field consists of 12 bits
of mantissa in the most significant byte and 4 bits of base 10
exponent (order of magnitude) information in the least significant
byte. The 12-bit mantissa portion of the field is scaled such that a
floating point value of 0.0 corresponds to 0 and a floating point
value of 10.0 corresponds to 4096. Thus:
value = (int) (mantissa * 4096.0 / 10.0 + 0.5)
For example, to represent a transmission rate of 256kbps (3.2e+04
bytes per second), the lower 4 bits of the 16-bit field contain a
value of 0x04 to represent the exponent while the upper 12 bits
contain a value of 0x51f as determined from the equation given above:
value = (int)((3.2 * 4096.0 / 10.0) + 0.5) = 1311 = 0x51f
To decode the "send_rate" field, the following equation can be used:
sendRate = <upper12bits> * 10.0 / 4096.0 * power(10.0, <lower4bits>)
Note the maximum transmission rate representable by this scheme is
approximately 9.99e+15 bytes per second.
The "cc_sequence" field is a sequence number applied by the sender to
congestion control command messages. The greatest received
"cc_sequence" value is recorded by receivers and fed back to the
sender in any NORM_ACK or NORM_NACK messages generated by the
receivers for that sender.
The "reserved" field is for potential future use and should be set to
zero in this version of the NORM protocol.
The "cc_node_list" consists of a list of NormNodeIds and their
associated congestion control status. This includes the current
limiting receiver (CLR) node, any potential limiting receiver (PLR)
nodes which have been identified, and some number of receivers for
which congestion control status is being provided, most notably
including the receivers' current RTT measurement. The length of the
"cc_node_list" provides for at least the CLR and one other receiver,
but may be configurable for more timely feedback to the group. The
list length can be inferred from the length of the NORM_CMD(CC)
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message.
Each item in the "cc_node_list" is in the following format:
Congestion Control Node List Item Fields
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "cc_node_id" is the NormNodeId of the receiver which the item
represents.
The "cc_flags" field contains flags indicating the congestion control
status of the indicated receiver. The following flags are defined:
+-------------------+-------+------------------------------------------+
| Flag | Value | Purpose |
+-------------------+-------+------------------------------------------+
|NORM_CC_FLAG_CLR | 0x01 | Receiver is the current limiting |
| | | receiver (CLR) |
+-------------------+-------+------------------------------------------+
|NORM_CC_FLAG_PLR | 0x02 | Receiver is a potential limiting |
| | | receiver (PLR) |
+-------------------+-------+------------------------------------------+
|NORM_CC_FLAG_RTT | 0x04 | Receiver has measured RTT with respect |
| | | to sender |
+-------------------+-------+------------------------------------------+
|NORM_CC_FLAG_START | 0x08 | Sender/receiver is in "slow start" phase |
| | | of congestion control operation (i.e. |
| | | The receiver has not yet detected any |
| | | packet loss and the "cc_rate" field is a |
| | | function of the receiver's measured |
| | | receive rate). |
+-------------------+-------+------------------------------------------+
|NORM_CC_FLAG_LEAVE | 0x10 | Receiver is imminently leaving the |
| | | session and its feedback should not be |
| | | considered in congestion control |
| | | operation. |
+-------------------+-------+------------------------------------------+
The "cc_rtt" contains a quantized representation of the receiver's
individual sender<->receiver RTT as measured by the sender. This
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field is valid only if the NORM_FLAG_RTT flag is set in the "cc_flags"
field. This one byte field is a quantized representation of the RTT
using the algorithm described in the NORM Building Block document
[15].
The "cc_rate" field contains a representation of the receiver's
current calculated (during steady-state congestion control operation)
or twice its measured (during the "slow start" phase) congestion
control rate. This field is encoded and decoded using the same
technique as described for the NORM_CMD(CC) "send_rate" field.
NORM_CMD(ACK_REQ) Message
The NORM_CMD(ACK_REQ) message is used by the sender to request
acknowledgement from a specified list of receivers. This message is
used in providing a lightweight positive acknowledgement mechanism
that is OPTIONAL for use by the reliable multicast application. The
NORM protocol defines a specific acknowledgement mechanism to
determine that watermark points in the reliable transmission have been
achieved by specific receivers. Addtionally, a range of
acknowledgement request types is provided for use at the application's
discretion. Provision for application-defined, positively-
acknowledged commands allows the application to automatically take
advantage of transmission and round-trip timing information available
to the NORM protocol. The details of the NORM positive
acknowledgement process including transmission of the
NORM_CMD(ACK_REQ) messages and the receiver response (NORM_ACK) are
described in Section 5.5.3. The format of the NORM_CMD(ACK_REQ)
message is:
NORM_CMD(ACK_REQ) Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt | gsize | flavor = 3 | ack_type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ack_req_content |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ack_req_content (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list ... |
The NORM common message header and standard NORM_CMD fields serve
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their usual purposes.
The "ack_type" field indicates type of acknowledemgent being requested
and thus implies rules for how the receiver will treat this request.
The following "ack_type" values are defined and are also used in
NORM_ACK messages described later:
+----------------------+--------+----------------------------------+
| ACK Type | Value | Purpose |
+----------------------+--------+----------------------------------+
|NORM_ACK(WATERMARK) | 1 | Used to request acknowledgement |
| | | of reliable reception of |
| | | watermark transmission point. |
+----------------------+--------+----------------------------------+
|NORM_ACK(CC) | 2 | Used to identify NORM_ACK |
| | | messages sent for congestion |
| | | control only. |
+----------------------+--------+----------------------------------+
|NORM_ACK(RESERVED) | 3-15 | Reserved for possible future |
| | | NORM protocol use. |
+----------------------+--------+----------------------------------+
|NORM_ACK(APPLICATION) | 16-255 | Used at application's |
| | | discretion. |
+----------------------+--------+----------------------------------+
The "ack_req_content" field consists of 8 bytes which is interpreted
differently for different "ack_type" values.
The "acking_node_list" field is a list of NormNodeIds. The listed
NormNodes are expected to explicitly respond to the acknowledgement
request according the rules for the type of acknowledgment requested
and the NORM Positive Acknowledgment procedure described in Section
5.3.3.
The NORM_ACK(WATERMARK) type indicates the sender wishes to receive
acknowledgement from receivers in the "acking_node_list" who have
achieved completion of reception through a specific "watermark point"
in terms of a logical transmission position. This "watermark point"
is given in the "ack_req_content" field.
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The format of the NORM_CMD(ACK_REQ(WATERMARK)) message is:
NORM_CMD(ACK_REQ(WATERMARK)) Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt | gsize | flavor = 3 | ack_flavor = 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id | fec_block_number (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) | fec_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list ... |
The NORM common message header and standard NORM_CMD fields serve
their usual purposes. The "ack_flavor" is set to a value of one.
The "object_transport_id", "fec_block_number", and "fec_symbol_id" are
used to identify the watermark point for which positive
acknowledgement is requested. This watermark point is similar to the
transmission position given in NORM_CMD(FLUSH) messages. Furthermore,
NORM receivers (whether or not they are included in the
"acking_node_list") SHALL treat the ACK_REQ(WATERMARK) command as
equivalent to a NORM_CMD(FLUSH) command and appropriately initiate
NACK repair cycles in response to any detected missing data up through
the indicated watermark point.
The "acking_node_list" field contains the NormNodeIds of the current
NORM receivers which should positive acknowledge (NORM_ACK) this
request. The packet payload length implies the length of the
"acking_node_list" and its length is limited to the NormSegmentSize.
The individual NormNodeId items are listed in network (Big Endian)
order. If a receiver is included in the "acking_node_list" and it has
no repair needs up through the watermark point, it SHALL schedule
transmission of a NORM_ACK message as described in Section 5.5.3.
The NORM_ACK(CC) type is provided only for when receivers generate
NORM_ACK messages in response to NORM_CMD(CC) messages for congestion
control operation. There is no corresponding NORM_CMD(ACK_REQ(CC))
message.
The NORM_ACK(RESERVED) range of types is provided for possible future
NORM protocol use.
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The NORM_ACK(APPLICATION) range of types is provided so that NORM
applications may implement application-defined, positively-
acknowledged commands which are able to leverage internal transmission
and round-trip timing information available to the NORM protocol
implementation. The interpretation of the "ack_req_content" is
application-defined in this case.
NORM_CMD(APPLICATION) Message
This command allows the NORM application to robustly transmit
application-defined commands. The command message preempts any
ongoing data transmission and is repeated NORM_ROBUST_FACTOR times at
a rate of once per 2*GRTT. This rate of repetition allows the
application to collect any response (if that is the application's
purpose for the command) before it is repeated. Possible responses
might include initiation of data transmission , NORM_CMD(APPLICATION)
messages, or even application-defined, positively-acknowledge commands
from other NormSession participants. The transmission of these
commands will preempt data transmission when they are scheduled and
may be multiplexed with ongoing data transmission. This type of
robustly transmitted command allows NORM applications to define a
complete set of session control mechanisms with less state than the
transfer of FEC encoded reliable content requires while taking
advantage of NORM transmission and round-trip timing information.
NORM_CMD(APPLICATION) Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt | gsize | flavor = 6 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| application defined content ... |
The NORM common message header and NORM_CMD fields are interpreted as
previously described.
The "application-defined content" contains information in a format at
the discretion of the application. The size of this payload is
limited a maximum of the sender's NormSegmentSize setting.
4.3 Receiver Messages
The NORM message types generated by pariticipating receivers consist
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of NORM_NACK and NORM_ACK message types. NORM_NACK messages are sent
to request repair of missing data content from sender transmission and
NORM_ACK messages are generated in response to certain sender commands
including NORM_CMD(CC) and NORM_CMD(ACK_REQ).
4.3.1 NORM_NACK Message
The principal purpose of NORM_NACK messages is for receivers to
request repair of sender content via selective, negative
acknowledgement upon detection of incomplete data. NORM_NACK messages
will be transmitted according to the rules of NORM_NACK generation and
suppression described in Section 5.3. The content of these messages
is in a format that can potentially be used by compatible intermediate
systems [12] to provide assistance in promoting protocol scalability
and efficiency when available. NORM_NACK messages also contain
additional fields to provide feedback to the sender(s) for purposes of
round-trip timing collection and congestion control.
The payload of NORM_NACK messages contains one or more repair requests
for different objects or portions of those objects. The NORM_NACK
message format is as follows:
NORM_NACK Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_rate | cc_sequence | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nack_content ... |
The NORM common message header fields serve their usual purposes.
The "server_id" field identifies the NORM sender to which the
NORM_NACK message is destined.
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The "grtt_response" fields contain an adjusted version of the
timestamp from the most recently received NORM_CMD(CC) message for the
indicated NORM sender. The format of the "grtt_response" is the same
as the "send_time" field of the NORM_CMD(CC). The "grtt_response"
value is _relative_ to the "send_time" the source provided with a
corresponding NORM_CMD(CC) command. The receiver adjusts the source's
NORM_CMD(CC) "send_time" timestamp by adding the time differential
from when the receiver received the NORM_CMD(CC) to when the
NORM_NACK is transmitted to calculate the value in the "grtt_response"
field. This is the "receive_to_response_differential" value used in
the following formula:
"grtt_response" = NORM_CMD(CC) "send_time" + receive_to_response_differential
The receiver SHALL set the "grtt_response" to a ZERO value, to
indicate that the it has not yet received a NORM_CMD(CC) message from
the indicated sender and that the sender should ignore the
"grtt_response" in this message.
The "cc_flags" field contains bits representing the receiver's state
with respect to congestion control operation. The possible values for
the "cc_flags" field are those specified for the NORM_CMD(CC) message
node list item flags.
The "cc_rtt" field SHALL be set to a default maximum value and the
NORM_CC_FLAG_RTT flag SHALL be cleared when the receiver has not yet
received RTT measurement information. When the receiver has received
RTT measurement information, it shall set the "cc_rtt" value
accordingly and set the NORM_CC_FLAG_RTT flag in the "cc_flags" field.
The "cc_loss" field is the receiver's current packet loss fraction
estimate for the indicated source. The loss fraction is a value from
0.0 to 1.0 corresponding to a range of zero to 100 percent packet
loss. The 16-bit "cc_loss" value is calculated by the following
formula:
"cc_loss" = decimal_loss_fraction * 65535.0
The "cc_rate" field represents the receivers current local congestion
control rate. During "slow start", when the receiver has detected no
loss, this value is set to twice the actual rate it has measured from
the corresponding sender and the NORM_CC_FLAG_START is set in the
"cc_flags' field. Otherwise, the receiver calculates a congestion
control rate based on its loss measurement and RTT measurement
information (even if default) for the "cc_rate" field.
The "cc_sequence" field contains the current greatest "cc_sequence"
number of received NORM_CMD(CC) messages from the corresponding
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sender. This information can assist the sender in congestion control
operation by providing an indicator of how current ("fresh") the
receiver's round-trip measurement reference time is and whether the
receiver has been successfully receiving recent congestion control
probes. For example, if it is apparent the receiver has not been
receiving recent congestion control probes (and thus possibly other
messages from the sender), the sender may choose to take congestion
avoidance measures.
The "reserved" field is for potential future NORM use and SHALL be
set to ZERO for this version of the protocol.
The "nack_content" of the NORM_NACK message specifies the repair needs
of the receiver with respect to the NORM sender indicated by the
"server_id" field. The receiver constructs repair requests based on
the NORM_DATA and/or NORM_INFO segments it requires from the sender in
order to complete reliable reception. A single repair request
consists of a list of items, ranges, and/or FEC coding block erasure
counts for needed NORM_DATA and/or NORM_INFO content. Multiple repair
requests may be concatenated within the "nack_content" field of a
NORM_NACK message. Note that a single repair request can possibly
include multiple "items", "ranges", or "erasure_counts". In turn, the
"nack_content" field may contain multiple repair request. A single
repair request has the following format:
NACK Repair Request Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form | flags | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id | fec_block_number (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) | fec_symbol_id or erasure_count|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
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The "form" field indicates currently whether the repair request
content that follows is a list of NORM_NACK_ITEMS, NORM_NACK_RANGES,
or NORM_NACK_ERASURES. Possible values for the "form" field include:
Form Value
NORM_NACK_ITEMS 1
NORM_NACK_RANGES 2
NORM_NACK_ERASURES 3
When the repair request consists of individual NORM_NACK_ITEMS, each
concatenation of object_transport_id::fec_block_number::fec_symbol_id
identifies an individual repair need. When the repair request "form"
is NORM_NACK_RANGES, the inclusive range of sender information needed
by the receive is given in pairs of
object_transport_id::fec_block_number::fec_symbol_id. When the repair
request form is NORM_NACK_ERASURES, each
object_transport_id::fec_block_number::erasure_count concatenation
listed indicates the receiver's FEC erasure count for the identified
object and FEC encoding block.
The "flags" field is currently used to indicate if the NACK content
applies to NORM_DATA content, NORM_INFO content, or both. Thus,
defined flags in this field include:
+------------------+-------+------------------------------------------+
| Flag | Value | Purpose |
+------------------+-------+------------------------------------------+
|NORM_NACK_SEGMENT | 0x01 | Indicates the listed segment(s) are |
| | | required as repair. |
+------------------+-------+------------------------------------------+
|NORM_NACK_BLOCK | 0x02 | Indicates the entire listed block(s) are |
| | | required as repair. |
+------------------+-------+------------------------------------------+
|NORM_NACK_INFO | 0x04 | Indicates the object's NORM_INFO is |
| | | required as repair. |
+------------------+-------+------------------------------------------+
|NORM_NACK_OBJECT | 0x08 | Indicates the entire listed object(s) |
| | | are required as repair. |
+------------------+-------+------------------------------------------+
When the NORM_FLAG_SEGMENT flag is set, the "object_transport_id",
"fec_block_number" and "fec_symbol_id" fields are concatenated to
determine which sets or ranges of individual NORM_DATA segments are
needed to repair complete content at this receiver. When the
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NORM_FLAG_BLOCK flag is set, this indicates the receiver is completely
missing the indicated coding block(s) and requires transmissions
sufficient to repair the indicated block(s) in their entirety. In
this case the "fec_symbol_id" repair request fields are ignored. When
the NORM_NACK_INFO flag is set, this indicates the receiver is missing
the NORM_INFO segment for the indicated "object_transport_id". Note
the NORM_NACK_INFO may be set in combination with the NORM_NACK_BLOCK
or NORM_NACK_SEGMENT flags, or may be set alone. When the
NORM_NACK_OBJECT flag is set, this indicates the receiver is missing
the entire NormTransportObject referenced by the
"object_transport_id". This also implicitly requests any available
NORM_INFO for the NormObject, if applicable. The "fec_block_number"
and "fec_symbol_id" fields are ignored when the flag NORM_NACK_OBJECT
is set.
The "length" field is given (in bytes) to indicate the length of the
list of repair request items or ranges. Multiple lists of repair
request items and/or ranges may be concatenated together within a
single NORM_NACK message.
The "object_transport_id", "fec_block_number, and "fec_symbol_id"
fields comprise repair request list items to be interpreted according
to the repair request "form" and "flags" fields. As noted, when the
"form" is NORM_NACK_RANGES, pairs of
object_transport_id::fec_block_number::erasure_count define each
repair request list item.
NORM_NACK Content Examples:
In Example 1, a list of individual NORM_NACK_ITEM repair requests is
given. In Example 2, a list of NORM_NACK_RANGE requests _and_ a
single NORM_NACK_ITEM request are concatenated to illustrate the
possible content of a NORM_NACK message. Note that FEC coding block
erasure counts are provided in each case. The erasure counts are not
really necessary since the sender can easily determine the erasure
count while processing the NACK content. However, the erasure count
option may be useful for operation with Generic Router Assist (GRA).
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Example 1:
NORM_NACK content for: Object 12, Coding Block 3, Segments 2,5,8
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 3 | flags = 0x01 | length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 12 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number(lsb) = 3 | erasure_count = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x01 | length = 24 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 12 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) = 3 | fec_symbol_id = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 12 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) = 3 | fec_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 12 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) = 3 | fec_symbol_id = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Example 2:
NORM_NACK content for: Object 18 Coding Block 6, Segments 5, 6, 7, 8, 9, 10; and Object 19 NORM_INFO and Coding Block 1, segment 3
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 3 | flags = 0x01 | length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 18 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) = 6 | erasure_count = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 2 | flags = 0x01 | length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 18 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) = 6 | fec_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 18 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) = 6 | fec_symbol_id = 10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 3 | flags = 0x05 | length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 19 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) = 1 | erasure_count = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x05 | length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id = 19 | fec_block_number (msb) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) = 1 | fec_symbol_id = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.3.2 NORM_ACK Message
The NORM_ACK message is primarily used as part of NORM congestion
control operation and round-trip timing measurement. The generation
of NORM_ACK messages for round-trip timing and congestion-control
operation is described in Sections 5.5.1 and 5.5.2, respecctively.
Some applications may benefit from some limited form of positive
acknowledgement for certain functions. A simple, scalable positive
acknowledgement scheme is defined in Section 5.5.3 which can be
leveraged by protocol implementations when appropriate.
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NORM_ACK Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | type = 3 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_rate | cc_sequence | ack_type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ack_content ... |
The NORM common message header fields serve their usual purposes.
The "server_id", "grtt_response", "cc_flags", "cc_rtt", "cc_loss",
"cc_rate", and "cc_sequence" fields serve the same purpose as the
corresponding fields in NORM_NACK messages.
The "ack_type" field indicates the nature of the NORM_ACK message.
This directly corresponds to the "ack_type" field of the
NORM_CMD(ACK_REQ) message.
The "ack_content" format is a function of the "ack_type". The
NORM_ACK(CC) message has no attached content. Only the NORM_ACK
header applies. In the case of NORM_ACK(WATERMARK), a specific
"ack_content" format is defined:
NORM_ACK(WATERMARK) Ack Content
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_transport_id | fec_block_number (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_block_number (lsb) | fec_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "object_transport_id", "fec_block_number", and "fec_symbol_id" are
used by the receiver to acknowledge a NORM_CMD(ACK_REQ(WATERMARK))
transmitted by the sender identified by the "server_id" field.
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The "ack_content" of NORM_ACK messages for application-defined
"ack_type" values is specific to the application but is limited in
size to a maximum the NormSegmentSize of the sender referenced by the
"server_id".
4.4 General Messages
4.4.1 NORM_REPORT
This is an optional message generated by NORM participants. This
message could be used for periodic performance reports from receivers
in experimental NORM implementations. The format of this message is
currently undefined. Experimental NORM implementations may define
NORM_REPORT formats as needed for test purposes.
5.0 Functionality Definition
This section describes the detailed interactions of senders and
receivers participating in a NORM session. A simple synopsis of
protocol operation is given in the following items.
1) The sender periodically transmits NORM_CMD(CC) messages as
needed to initialize and collect roundtrip timing and
congestion control feedback from the receiver set.
2) The sender transmits an ordinal set of NormObjects segmented
in the form of NORM_DATA (and optional NORM_INFO) messages
labeled with NormTransportIds and logically identified with
FEC encoding block numbers and symbol identifiers.
3) As receivers detect missing content from the sender, they
initiate repair requests with NORM_NACK messages. Note the
receivers track the sender's most recent
object_transport_id::fec_block_number::fec_symbol_id
transmit position and NACK _only_ for content ordinally
prior to that transmit position. The receivers use random
backoff timeouts before generating NORM_NACK messages and
wait an appropriate amount of time before repeating the
NORM_NACK if their repair request is not satisified.
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4) The sender aggregates repair requests from the receiver set
and logically "rewinds" to send appropriate repair messages.
The sender sends repairs for the earliest ordinal transmit
position first and maintains this ordinal repair
transmission sequence. Previously untransmitted FEC parity
content for the applicable FEC coding block is used for
repair transmissions to the greatest extent possible. If
the sender exhausts its available FEC parity content on
multiple repair cycles for the same coding block, it resorts
to an explicit repair strategy (again using parity content)
to complete repairs. (The use of explicit repair is
expected to be an exception in general protocol operation,
but the possibility does exist for extreme conditions). The
sender immediately assumes transmission of new content once
it has sent pending repair transmissions.
5) The sender transmits NORM_CMD(FLUSH) messages when it
reaches the end of newly available transmit content.
Receivers respond to the NORM_CMD(FLUSH) messages with
NORM_NACK transmissions (following the same suppression
backoff timeout strategy as for data) if they require
further repair.
6) The sender transmission rate is subject to rate control
limits determined by congestion control. Each sender in a
NormSession maintains its own independent congestion control
state. Receivers provide congestion control feedback in
NORM_NACK and NORM_ACK messages. This feedback is
controlled using suppression mechanism similar to that for
NORM_NACK messages.
While the overall concept of the protocol is relatively simple, there
are details to each of these aspects that need to be addressed for
successful, robust, and scalable operation.
5.1 NORM Sender Initialization and Transmission
Upon startup, the NORM sender immediately begins sending NORM_CMD(CC)
messages to collect GRTT and other information from the potential
group. If congestion control operation is enabled the
NORM_CC_FLAG_ENABLE MUST be set. Congestion control operation SHALL
be observed at all times when operating in the general Internet. Even
if congestion control operation is disabled at the sender, it may be
desirable to set the the NORM_CC_FLAG_ENABLE to proactively collect
feedback from the receivers to have input to GRTT measurement prior to
NACK initiation.
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In some cases, applications may wish for the sender to also proceed
with data transmission immediately. In other cases, the sender may
wish to defer data transmission until it has received some feedback or
request from the receiver set indicating that receivers are indeed
present. Note, in some applications (e.g. web push), this indication
may come out-of-band with respect to the multicast session via other
means. The periodic transmission of NORM_CMD(CC) messages may precede
actual data transmission in order to have initial GRTT measurement.
The NORM protocol sender message headers contain all information
necessary to configure receivers for subsequent reliable reception.
This includes FEC coding parameters, the sender NormSegmentSize, and
other information. Additionally, applications may leverage the use of
NORM_INFO messages associated with the session data objects in the
session to provide application-specific context information for the
session and data being transmitted.
The NORM sender begins segmenting application-enqueued data into
NORM_DATA segments and transmitting it to the group. The rate of
transmission is controlled via the congestion control mechanisms
described in this document or at a fixed rate if desired for closed
network operations. The receivers participating in the multicast
group provide feedback to the sender as needed. When the sender
reaches the end of data it has enqueued for transmission or any
pending repairs, it transmits a series of NORM_CMD(FLUSH) messages at
a rate of one per 2*GRTT. Receivers may respond to these
NORM_CMD(FLUSH) messages with additional repair requests. A protocol
parameter "NORM_ROBUST_FACTOR" determines the number of flush messages
sent. If receivers request repair, the repair is provided and
flushing occurs again at the end of repair transmission.
5.2 NORM Receiver Initialization and Reception
The NORM protocol is designed such that receivers may join and leave
the group at will. However, some applications may be constrained such
that receivers need to be members of the group prior to start of data
transmission. NORM applications may use different policies to
constrain the impact of new receivers joining the group in the middle
of a session. For example, a useful implementation policy is for new
receivers joining the group to restrain requesting repair of transport
objects in progress. The NORM sender implementation may wish to
impose additional constraints to limit the ability of receivers to
disrupt reliable multicast performance by joining, leaving, and
rejoining the group often. Different receiver "join policies" may be
appropriate for different applications and/or scenarios. A default
policy of allowing receivers to request repair only for coding blocks
with a NormTransportId and FEC coding block number greater than or
equal to the first non-repair NORM_DATA or NORM_INFO message received
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upon joining the group is RECOMMENDED for general purpose operation.
5.3 NORM Receiver NACK Procedure
When the receiver detects it is missing data from a sender's NORM
transmissions, it initiates its NACKing procedure. The NACKing
procedure SHALL be initiated _only_ at NormObject boundaries, FEC
coding block boundaries, or upon receipt of a NORM_CMD(FLUSH) or
NORM_CMD(ACK_REQ(WATERMARK)) message.
The NACKing procedure begins with a random backoff timeout. The
duration of the backoff timeout is chosen using the "RandomBackoff"
algorithm described in the NORM Building Block document [15] using
(K*GRTTsender) for the "maxTime" parameter and the sender advertised
group size (GSIZEsender) as the "groupSize" parameter. The backoff
factor "K" MUST be greater than one to provide for feedback
suppression. A value of K = 4 is RECOMMENDED for the Any Source
Multicast (ASM) model while a value of K = 6 is RECOMMENDED for Single
Source Multicast (SSM) operation. Thus:
T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender)
During this backoff time, the receiver accumulates external pending
repair state from NORM_NACK messages and NORM_CMD(REPAIR_ADV) messages
received. At the end of the backoff time, the receiver SHALL generate
a NORM_NACK message only if the following conditions are met:
1) The sender's current transmit position (in terms of
object_transport_id::fec_block_number::fec_symbol_id)
exceeds the earliest repair position of the receiver.
2) The repair state accumulated from NORM_NACK and
NORM_CMD(REPAIR_ADV) messages do not equal or supersede the
receiver's repair needs.
If these conditions are met, the receiver immediately generates a
NORM_NACK message when the backoff timeout expires.
The content of the NORM_NACK message contains repair request content
beginning with lowest ordinal repair position for the receiver up to
the most recently heard ordinal transmission position for the sender.
If the size of the NORM_NACK content exceeds the NormSegmentSize, the
NACK content is limited to that point so that the receiver only
generates a single NORM_NACK message per NACK cycle for a given
sender.
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For each partially-received FEC coding block requiring repair, the
receiver SHALL, on its _first_ repair attempt for the block, request
the parity portion of the FEC coding block beginning with the lowest
ordinal _parity_ "fec_symbol_id" and request the number of symbols
corresponding to its data segment erasure count for the block. On
_subsequent_ repair cycles for the same coding block, the receiver
SHALL request only those repair symbols from the first set it has not
yet received up to the remaining erasure count for that applicable
coding block. Note that the sender may have provided other additional
parity segments for other receivers that could also be used to satisfy
the local receiver's erasure-filling needs. In the case where the
erasure count for a partially-received FEC coding block exceeds the
maximum number of parity symbols available from the sender for the
block (as indicated by the NORM_DATA "fec_num_parity" field), the
receiver SHALL request all available parity segments and the ordinally
highest missing data segments required to satisfy its erasure needs
for the block. The goal of this strategy is for the overall receiver
set to request a lowest common denominator set of repair symbols for a
given FEC coding block. This allows the sender to construct the most
efficient repair transmission segment set and enables effective NACK
suppression among the receivers even with uncorrelated packet loss.
This approach also requires no synchronization among the receiver set
in their repair requests for the sender.
For FEC coding blocks or NormObjects missing in their entirety, the
NORM receiver constructs repair requests with NORM_NACK_BLOCK or
NORM_NACK_OBJECT flags set as appropriate. The request for
retransmission of NORM_INFO is accomplished by setting the
NORM_NACK_INFO flag in a corresponding repair request.
5.4 NORM Sender NACK Processing and Repair Transmission
The principle goal of the sender is to make forward progress in the
transmission of data its application has enqueued. However, the
sender must occasionally "rewind" to satisfy the repair needs of
receivers who have NACKed. Aggregation of multiple NACKs is used to
determine an optimal repair strategy when a NACK event occurs. Since
receivers initiate the NACK process on coding block or object
boundaries, there is some loose degree of synchronization of the
repair process.
5.4.1 NORM Sender Repair State Aggregation
When a sender is in its normal state of transmitting new data and
receives a NACK, it begins a procedure to accumulate NACK repair state
from NORM_NACK messages before beginning repair transmissions. Note
that this period of aggregating repair state does _not_ interfere with
its ongoing transmission of new data.
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The period of time during which the sender aggregates NORM_NACK
messages is equal to K*GRTT where "K" is the same backoff scaling
value used by the receivers and "GRTT" is the sender's current
estimate of the group's greatest round-trip time. When this period
ends, the sender "rewinds" by incorporating the accumulated repair
state into its pending transmission state and begins transmitting
repair messages, then continues with new transmissions of any enqueued
data. Also, at this point in time, the sender begins a "holdoff"
timeout of 1*GRTT during which time the sender constrains itself from
initiating a new repair aggregation cycle, even if NORM_NACK messages
arrive. If additional NORM_NACK messages are received during this
hold-off period, the sender will immediately incorporate these "late
messages" into its pending transmission state ONLY if the NACK content
is ordinally greater than the sender's current transmission position.
This "holdoff" time allows worst case time for the sender to propagate
its current transmission sequence position to the group, thus avoiding
redundant repair transmissions. After the holdoff timeout expires, a
new NACK accumulation period can be begun (upon arrival of a NACK) in
concert with the pending repair and new data transmission. The sender
repeats the same process of incorporating accumulated repair state
into its transmission plan during the the new aggregation period and
subsequently "rewinding" to transmit the lowest ordinal repair data.
5.4.2 NORM Sender FEC Repair Transmission Strategy
The NORM sender should leverage transmission of FEC parity content for
repair to the greatest extent possible. Recall that the receivers use
a strategy to request a lowest common denominator of explicit repair
(including parity content) in the formation of their NORM_NACK
messages. Before falling back to explicitly satisfying all of the
different receivers' repair needs, the sender can make use of the
general erasure-filling capability of FEC-generated parity segments.
The sender can determine the maximum erasure filling needs for
individual FEC coding blocks from the NORM_NACK messages received
during the repair aggregation period. Then, if the sender has a
sufficient number (less than or equal to the maximum erasure count) of
previously unsent parity segments available for the applicable coding
blocks, the sender can transmit these in lieu of the specific packets
the receiver set has requested. Only after exhausting its supply of
"fresh" (unsent) parity segments for a given coding block should the
sender resort to explicit transmission of the receiver set's repair
needs. In general, if a sufficiently powerful FEC code is used, the
need for explicit repair will be an exception, and the fulfillment of
reliable multicast can be accomplished quite efficiently. However,
the ability to resort to explicit repair allows the protocol to be
reliable under even very extreme circumstances.
NORM_DATA messages sent as repair transmissions are flagged with the
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NORM_FLAG_REPAIR flag. This allows receivers to obey any policies
that limit new receivers from joining the reliable transmission on
repair transmissions.
To facilitate operation with Generic Router Assist (GRA) [12], the
sender can additionally flag NORM_DATA transmissions sent as explicit
repair with the NORM_FLAG_EXPLICIT flag. The GRA router needs to only
subcast a sufficient count of non-explicit parity repairs to satisfy
the sub-tree's erasure filling needs for a given FEC coding block.
When the sender has resorted to explicit repair, the GRA router will
subcast all of the explicit repair packets to those portions of the
routing tree still requiring repair for a given coding block. (Note
the GRA router will be required to conduct repair state accumulation
for sub-routes in a manner similar to the sender's repair state
accumulation in order to have sufficient information to perform the
subcasting. Additionally, the GRA router can perform additional
NORM_NACK suppression/aggregation as it conducts this repair state
accumulation for NORM repair cycles).
5.4.3 NORM Sender NORM_CMD(SQUELCH) Generation
If the sender receives a NORM_NACK message for repair of data it is no
longer supporting, the sender generates a NORM_CMD(SQUELCH) message to
advertise its repair window and squelch any receivers from additional
NACKing of invalid data. The transmission rate of NORM_CMD(SQUELCH)
messages is limited to once per 2*GRTT. The "invalid_object_list" (if
applicable) of the NORM_CMD(SQUELCH) message SHALL begin with the
lowest "object_transport_id" from the invalid NORM_NACK messages
received since the last NORM_CMD(SQUELCH) transmission. Lower ordinal
invalid "object_transport_ids" should be included only while the
NORM_CMD(SQUELCH) payload is less than the sender's NormSegmentSize
parameter.
5.4.4 NORM Sender NORM_CMD(REPAIR_ADV) Generation
When a NORM sender receives NORM_NACK messages from receivers via
unicast transmission, it uses NORM_CMD(REPAIR_ADV) messages to
advertise its accumulated repair state to the receiver set since the
receiver set is not directly sharing their repair needs via multicast
communication. The NORM_CMD(REPAIR_ADV) message is multicast to the
receiver set by the sender. The payload portion of this message has
content in the same format as the NORM_NACK receiver message payload.
Receivers are then able to perform feedback suppression in the same
manner as with NORM_NACK messages directly received from other
receivers. Note the sender does not merely retransmit NACK content it
receives, but instead transmits a representation of its aggregated
repair state. The transmission of NORM_CMD(REPAIR_ADV) messages are
subject to the sender transmit rate limit and NormSegmentSize
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limitation. When the NORM_CMD(REPAIR_ADV) message is of maximum size,
receivers SHALL consider the maximum ordinal transmission position
value embedded in the message as the senders "current" transmission
position and suppress requests for ordinally higher repair. For
congestion control operation, the NORM_CMD(REPAIR_ADV) fields of
"cc_flags", "cc_rtt", and "cc_rate" contain the "worst case" values
received for each field since the last NORM_CMD(REPAIR_ADV)
transmission. This means the minimum received "cc_rate" and the set
of "cc_flag" values resulting in the most suppression (i.e. the
NORM_CC_FLAG_RTT flag is unset if _any_ congestion control feedback
was received with that flag unset since the last NORM_CMD(REPAIR_ADV)
transmission).
5.5 Additional NORM Protocol Mechanisms
In addition to the principal function of data content transmission and
repair, there are some other protocol mechanisms that help NORM to
adapt to network conditions and play fairly with other coexistent
protocols.
5.5.1 NORM Greatest Round-trip Time (GRTT) Collection
For NORM receivers to appropriately scale backoff timeouts and the
senders to use proper corresponding timeouts, the participants must
agree on a common timeout basis. Each NORM sender monitors the round-
trip time of active receivers and determines the group greatest round-
trip time (GRTT). The sender advertises this GRTT estimate in every
message it transmits so that receivers have this value available for
scaling their timers. To measure the current GRTT, the sender
periodically sends NORM_CMD(CC) messages which contain a locally
generated timestamp. Receivers are expected to record this timestamp
along with the time the NORM_CMD(CC) message is received. Then, when
the receivers generate feedback messages to the sender, an adjusted
version of the sender timestamp is embedded in the feedback message
(NORM_NACK or NORM_ACK). The adjustment adds the amount of time the
receiver held the timestamp before generating its response. Upon
receipt of this adjusted timestamp, the sender is able to calculate
the round-trip time to that receiver.
The round-trip time for each receiver is fed into an algorithm that
weights and smooths the values for a conservative estimate of the
GRTT. The algorithm and methodology is described in the NORM Building
Block document [11] in the section entitled "One-to-Many Sender GRTT
Measurement". A conservative estimate helps feedback suppression at a
small cost in overall protocol repair delay. The sender's current
estimate of GRTT is advertised in the "grtt" field found in all NORM
sender messages. The advertised GRTT is also limited to be at least
as big as the nominal inter-packet transmission time given the
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sender's current transmission rate. The reason for this additional
limit is to keep the receiver somewhat "event driven" by making sure
the sender has had adequate time to generate any response to repair
requests from receivers given transmit rate limitations due to
congestion control or configuration.
When the NORM_CC_FLAG_ENABLE is set in NORM_CMD(CC) messages, the
receivers respond to NORM_CMD(CC) messages as described in Section
5.5.2, "NORM Congestion Control Operation". The NORM_CMD(CC) messages
are periodically generated by the sender as described for congestion
control operation. This provides for active, but controlled, feedback
from the group in the form of NORM_ACK messages and can provide GRTT
feedback even if no NORM_NACK messages are being sent. If operating
without congestion control in a closed network, the NORM_CMD(CC)
messages may be sent periodically with the NORM_CC_FLAG_ENABLE flag
cleared. In this case, receivers will only provide GRTT measurement
feedback when NORM_NACK messages are generated as no NORM_ACK messages
are generated in response to the NORM_CMD(CC). In this case, the
NORM_CMD(CC) messages may be sent less frequently, as little as once
per minute, to conserve network capacity. Note that the
NORM_CC_FLAG_ENABLE can also be set to actively solicit RTT feedback
from the receiver group per congestion control operation even though
the sender may not be observing congestion control rate adjustment.
NORM operation without congestion control should only be considered in
closed networks.
5.5.2 NORM Congestion Control Operation
This section describes congestion control operation for the NORM
protocol. The supporting NORM message formats and approach described
here are an adaptation of the equation-based TCP-Friendly Multicast
Congestion Control (TFMCC) approach described in [18] and [21]. With
this TFMCC-based approach, the transmission rate of NORM senders is
controlled in a rate-based manner as opposed to window-based
congestion control algorithms as in TCP. However, it is possible that
the NORM protocol message set may alternatively be used to support a
window-based multicast congestion control scheme such as PGMCC [22].
The details of that alternative may be described separately or in a
future revision of this document. In either case (rate-based TFMCC or
window-based PGMCC), successful control of sender transmission depends
upon collection of sender->receiver packet loss estimates and
sender<->receiver RTT to identify the congestion control bottleneck
path(s) within the multicast topology and adjust the sender rate
accordingly. The receiver with loss and RTT estimates that correspond
to the lowest result transmission rate is identified as the "current
limiting receiver" (CLR).
The steady-state sender transmission rate, to be "friendly" with
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competing TCP flows is calculated as:
S
Rsender = ---------------------------------------------------------------
tRTT * (sqrt((2/3)*p) + 12 * sqrt((3/8)*p) * p * (1 + 32*(p^2)))
where
S = Nominal transmitted packet size. (The "nominal" packet size
is determined by the sender as an exponentially weighted
moving average (EWMA) of transmitted packet sizes to account
for variable message sizes).
tRTT = The RTT estimate of the current "current limiting receiver"
(CLR).
p = The loss event fraction of the CLR.
To support congestion control feedback collection and operation, the
NORM sender periodically transmits NORM_CMD(CC) command messages. The
GRTT is determined from congestion control feedback included in NACKs
and ACKs from the receiver set. The NORM_CMD(CC) messages are
multiplexed with NORM data and repair transmissions and serve several
purposes:
1) Stimulate explicit feedback from the general receiver set to
collect congestion control information.
2) Communicate state to the receiver set on the sender's
current congestion control status including details of the
CLR.
3) Initiate rapid (immediate) feedback from the CLR in order to
closely track the dynamics of congestion control for that
"worst path" in the sender->receiver multicast topology.
The format of the NORM_CMD(CC) message is describe in Section 4.2.3 of
this document. The NORM_CMD(CC) message contains information to allow
for determination of sender<->receiver RTTs, to inform the group of
the congestion control CLR, and to provide feedback of individual RTT
information to receivers in the group. The NORM_CMD(CC) also provides
for exciting feedback from a set of potential limiting receiver (PLR)
nodes that may be determined administratively or possibly
algorithmically based on congestion control feedback. The details of
PLR selection are not discussed in this document.
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5.5.2.1 NORM_CMD(CC) Transmission
The NORM_CMD(CC) message is tranmitted periodically by the sender
along with its normal data transmission. Note that the repeated
transmission of NORM_CMD(CC) messages may be initiated some time
before transmission of user data content at session startup. This may
be done to collect some estimation of the current state of the
multicast topology with respect to group and individual RTT and
congestion control state.
A NORM_CMD(CC) message is immediately transmitted at sender startup.
The interval of subsequent NORM_CMD(CC) message transmission is
determined as follows:
1) By default, the interval is set according to the current
sender GRTT estimate. A startup GRTT of 0.5 seconds is
recommended when no feedback has yet been received from the
group.
2) If a CLR has been identified (based on previous receiver
feedback), the interval is the sender<->receiver RTT for the
CLR.
3) Additionally, if the interval of nominal data message
transmission is greater than the GRTT or CLR RTT interval,
the NORM_CMD(CC) interval is set to this greater value.
This ensures that the transmission of this control message
is not done to the exclusion of user data transmission.
The NORM_CMD(CC) "cc_sequence" field is incremented with each
transmission of a NORM_CMD(CC) command. The greatest "cc_sequence"
recently received by receivers is included in their feedback to the
sender. This allows the sender to determine the "age" of feedback to
assist in congestion avoidance.
The sender advertises its current transmission rate in the "send_rate"
field of the NORM_CMD(CC) message. This rate information is used by
receivers to bias the timing of explicit feedback and to initialize
loss estimation during congestion control startup or restart.
The "cc_node_list" contains a list of entries identifying receivers
and their current congestion control state (status "flags", "rtt" and
"loss" estimates). The list may be empty if the sender has not yet
received any feedback from the group. If the sender has received
feedback, the list will minimally contain an entry identifying the
CLR. A NORM_CC_FLAG_CLR flag value is provided for the "cc_flags"
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field to identify the CLR entry. It is recommended that the CLR entry
be the first in the list for implementation efficiency. Additional
entries in the list are used to provide sender-measured individual RTT
estimates to receivers in the group. The number of additional entries
in this list is dependent upon the percentage of control traffic the
sender application is willing to send with respect to user data
message transmissions. More entries in the list may allow the sender
to be more responsive to congestion control dynamics. The length of
the list may be dynamically determined according to the current
transmission rate and scheduling of NORM_CMD(CC) messages. The
maximum length of the list corresponds to the sender's
"NormSegmentSize" parameter for the session. The inclusion of
additional entries in the list based on receiver feedback are
prioritized with following rules:
1) Receivers that have not yet been provided RTT feedback get
first priority. Of these, those with the greatest loss
fraction receive precedence for list inclusion.
2) Secondly, receivers that have previously been provided RTT
are included with receivers yielding the lowest calculated
congestion rate getting precedence.
There are also "cc_flag" values in addition to NORM_CC_FLAG_CLR that
are used for other congestion control functions. The NORM_CC_FLAG_CLR
flag value is used to mark additional receivers from which the sender
would like to have immediate, non-suppressed feedback. These may be
receivers which the sender algorithmically identified as potential,
future CLRs or which have been pre-configured as potential congestion
control points in the network. The NORM_CC_FLAG_RTT indicates the
validity of the "cc_rtt" field for the associated receiver node.
Normally, this flag will be set since the receivers in the list will
typically be receivers from which the sender has received feedback.
However, in the case that the NORM sender has been pre-configured with
a set of PLR nodes, feedback from those receivers may not yet have
been collected and thus the "cc_rtt" and "cc_rate" fields do not
contain valid values.
5.5.2.2 NORM_CMD(CC) Feedback Response
Receivers explicitly respond to NORM_CMD(CC) messages in the form of a
NORM_ACK(RTT) message. Receivers that are are marked as CLR or PLR
nodes in the NORM_CMD(CC) "cc_node_list" immeditately provide feedback
in the form of a NORM_ACK to this message. When a NORM_CMD(CC) is
received, non-CLR or non-PLR nodes initiate random feedback backoff
timeouts similar to that used when the receiver initiates a repair
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cycle (see Section 5.3) in response to detection of data loss. The
goal of the congestion control feedback is to determine the receivers
with the lowest congestion control rates. As described in [21], the
receiver congestion control feedback (ACK) timeouts can be biased in
favor of lower rate receivers (while maintaining effective feedback
suppression). Such biasing is not necessarily possible with
suppression of NORM_NACK messages since previous data and repair loss
history may not be correlated with the current data loss.
The backoff timeout for the congestion control response is picked and
biased as follows:
T_backoff = y*r*(K*GRTTsender) + (1 - y)*RandomBackoff(K*GRTTsender, GSIZEsender)
where
"y" is the fraction of (K*GRTT) used to offset the backoff with
respect to the sender's current transmission rate. A value of y =
0.25 is recommended.
"r" is adjusted ratio of the local receiver's calculated rate to the
sender's current rate. During steady-state congestion control
operation, "r" is determined as:
r = (MAX(MIN((Rcalc / Rsender), 0.9), 0.5) - 0.5) / 0.4
During the "slow start" phase of congestion control operation, "r" is
determined simply as:
r = Rrecv / Rsender
where "Rrecv" is the measured received rate. The receiver places a
value equal to two times this "Rrecv" rate in the "cc_rate" field of
its NORM_NACK or NORM_ACK feedback messages during the "slow start"
phase of congestion control operation. If the sender chooses this
rate as its congestion control rate, this prevents the sender from
overshooting an appropriate rate by more than a factor of two during
this "slow start" period when receivers have experienced no loss.
The RandomBackoff() algorithm provides a truncated exponentially
distributed random number and is described in the NORM Building Block
document [11]. The same backoff factor "K" used with the GRTT as for
NORM_NACK suppression. As previously noted, a value of K = 4 is
generally recommended for ASM operation and K = 6 for SSM operation.
A receiver SHALL cancel the backoff timeout and thus its pending
transmission of a NORM_ACK(RTT) message under the following
conditions:
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1) The receiver provides another feedback message (NORM_NACK or
NORM_ACK) before the congestion control feedback timeout
expires,
2) A "suppressing" NORM_ACK(RTT) message is heard from another
receiver or via a NORM_CMD(REPAIR_ADV) message from the
sender. The local receiver's feedback is canceled if the
rate of the competing feedback (Rfb) is sufficiently close
to or less than the local receiver's calculated rate
(Rcalc). The local receiver's feedback is canceled when:
Rcalc > (0.9 * Rfb)
According to [21], this bias of suppression is recommended to help
ensure that the receiver with the lowest rate reports, while still
maintaining a low volume of feedback from the receiver set.
When the backoff timer expires, the receiver generates a NORM_ACK(RTT)
message to provide feedback to the sender and group. This message may
be multicast to the group for most effective suppression in ASM
topologies or unicast to the sender depending upon how the NORM
protocol is deployed and configured. In the congestion control
feedback fields of any NORM_ACK or NORM_NACK messages, receivers will
include an adjusted version of the sender timestamp from the most
recently received NORM_CMD(CC) message and the greatest "cc_sequence"
received. The receiver SHALL also set any applicable "cc_flags", its
current "cc_rate", and its "cc_rtt" if known. The sender can use the
receiver-provided previous "cc_rtt" value to smooth its RTT estimate
when it is valid. As noted in [18], a smoothing constant of 0.5 is
recommended for regular receivers and 0.9 for CLR (and PLR) receivers
from which more rapid feedback is received.
During "slow start" (when the receiver has not yet detected loss from
the sender), the receiver uses a value equal to two times its measured
rate from the sender in the "cc_rate" field. For steady-state
congestion control operation, the receiver "cc_rate" value is based on
the equation based value using its current loss event estimate and
sender<->receiver RTT information.
After a congestion control feedback message is generated or when the
feedback is suppressed, the receiver begins a "holdoff" timeout period
during which it will restrain itself from initiating another feedback
cycle, even if NORM_CMD(CC) messages are received from the sender
(unless the receive becomes marked as a CLR or PLR node). The value
of this holdoff timeout period is:
T_holdoff = (K*GRTT)
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Thus, non-CLR receivers are constrained to providing explicit
congestion control feedback once per K*GRTT intervals. Note, however,
that as the session progresses, different receivers will be responding
to different NORM_CMD(CC) messages and there will be relatively
continuous feedback of congestion control information while the sender
is active.
5.5.2.3 Congestion Control Rate Adjustment
During steady-state operation, the sender will directly adjust its
transmission rate to the rate indicated by the feedback from its
currently selected CLR according to any limitations described in [18].
As noted there, the estimation of parameters (loss and RTT) for the
CLR will generally constrain the rate changes possible within
acceptable bounds. For rate increases, the sender SHALL observe a
maximum rate of increase of one packet per RTT at all times during
steady-state operation. Note that the sender SHALL maintain a
smoothed RTT estimate for the CLR upon new feedback from the CLR
where:
RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew
"RTT_clrNew" is the new RTT calculated from the timestamp in the
feedback message received from the CLR. The RTTclr is initialized to
RTT_clrNew on the first feedback message received. Note that the same
procedure is observed by the sender for PLR receivers and that if a
PLR is "promoted" to PLR status, the smoothed estimate can be
continued.
There are some additional periods besides steady-state operation which
need to be considered in this protocol operation. These periods aare:
1) during session startup,
2) when no feedback is received from the CLR, and
3) when the sender has a break in data transmission.
During session startup, the congestion control operation SHALL
observe a "slow start" procedure to quickly approach its fair
bandwidth share. An initial sender startup rate is assumed where:
Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.
The rate is increased only when feedback is received from the
receiver set. The "slow start" phase proceeds until any receiver
provides feedback indicating that loss has occurred. Rate increase
during "slow start" is applied as:
Rnew = Rrecv_min
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where "Rrecv_min" is the minimum reported receiver rate in the
"cc_rate" field of congestion control feedback messages received
from the group. Note that during "slow start", receivers use two
times their measured rate from the sender in the "cc_rate" field of
their feedback. Rate increase adjustment is limited to once per
GRTT during slow start.
If the CLR or any receiver intends to leave the group, it will set
the NORM_CC_FLAG_LEAVE in its congestion control feedback message
as an indication that the sender should not select it as the CLR.
When the CLR changes to a lower rate receiver, the sender should
immediately adjust to the new lower rate. The sender is limited to
increasing its rate at one additional packet per RTT towards a new,
higher CLR rate.
The sender should also track the "age" of the feedback it has
received from the CLR by comparing its current "cc_sequence" value
(Ssender) to the last "cc_sequence" value received from the CLR
(Sclr). As the "age" of the CLR feedback increases with no new
feedback, the sender SHALL begin reducing its rate once per CLR RTT
as a congestion avoidance measure.
The following algorithm is used to determine the decrease in sender
rate (Rsender bytes/sec) as the CLR feedback, unexpectedly,
excessively ages:
Age = Ssender - Sclr;
rate1 = MAX((Rsender - NormSegmentSize), 0.0); // bytes per sec
rate2 = Rsender * 0.5
if (Age > 4)
Rsender = MIN(rate1, rate2);
else if (Age > 2)
Rsender = MAX(rate1, rate2);
This rate reduction occurs limited to the lower bound on NORM
transmission rate. After NORM_ROBUST_FACTOR consecutive
NORM_CMD(CC) rounds without any feedback from the CLR, the sender
SHOULD assume the CLR has left the group and pick the receiver with
the next lowest rate as the new CLR. Note this assumes that the
sender does not have explicit knowledge that the CLR intentionally
left the group. After such a CLR timeout, the sender will be
transmitting with a minimal rate and should return to slow start as
described here for a break in data transmission.
When the sender has a break in its data transmission, it can
continue to probe the group with NORM_CMD(CC) messages to maintain
RTT collection from the group. This will enable the sender to
quickly determine an appropriate CLR upon data transmission
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restart. However, the sender should exponentially reduce its
target rate to be used for transmission restart as time since the
break elapses. The target rate SHOULD be recalculated once per CLR
RTT as:
Rsender = Rsender * 0.5;
Upon restart, the sender should set the NORM_FLAG_START flag in its
NORM_CMD(CC) messages and the group should observer "slow start"
congestion control procedures until any receiver experiences a new
loss event.
5.5.3 NORM Positive Acknowledgment Procedure
NORM provides an option for the source application to request
positive acknowledgment (ACK) of NORM_CMD(ACK_REQ) messages from
members of the group. There are a few types of specific
acknowledgement requests that are defined for the NORM protocol and
a range of acknowledgment request types which left to be defined by
the application. One predefined acknowledgement type is the
NORM_ACK(WATERMARK) that is used to determine if receivers have
acheived completion of reliable reception up through an identified
transmission point with respect to the sender's logical sequence of
transmission. The NORM_ACK(WATERMARK) acknowledgement may be used
to assist in application flow control when the sender has
information on a portion of the receiver set. Another predefined
acknowledgement type is NORM_ACK(CC), which is used to explicitly
provide congestion control feedback in response to NORM_CMD(CC)
messages transmitted by the sender. Note the NORM_ACK(CC) response
does NOT follow the positive acknowledgement procedure described
here. The NORM_CMD(ACK_REQ) and NORM_ACK messages contain an
"ack_type" field to identify the type of acknowledgement requested
and provided. A range of "ack_type" values is provided for
application-defined use. While the application initiates the
acknowledgement request and interprets application-defined
"ack_type" values, the acknowledgment request and response is
conducted with the following procedure.
The NORM positive acknowledgement procedure uses polling by the
sender to query the receiver group for response. Note this polling
procedure is not intended to scale to very large receiver groups,
but could be used in large group setting to query a critical subset
of the group. The NORM_CMD(ACK_REQ) message is used for polling
and contains a list of NormNodeIds for receivers that should
respond to the command. The list of receivers providing
acknowledgement is determined by the source application with "a
priori" knowledge of participating nodes or via some other
application-level mechanism.
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The ACK process is initiated by the sender who generates
NORM_CMD(ACK_REQ) messages in periodic "rounds". For
NORM_ACK(WATERMARK), these requests contain the
"object_transport_id", "fec_block_number", and "fec_symbol_id"
denoting the watermark transmission point. For application-defined
requests, the "ack_req_content" field of the NORM_CMD(ACK_REQ) is
set and interpreted by the sender and receiver applications,
respectively. In response to the NORM_CMD(ACK_REQ), the listed
receivers randomly spread NORM_ACK messages uniformly in time over
a window of (1*GRTT). These NORM_ACK messages are typically
unicast to the sender.
The ACK process is self-limiting and avoids ACK implosion in that:
1) Only a single NORM_CMD(ACK_REQ) message is generated once
per (2*GRTT), and
2) The size of the "acking_node_list" of NormNodeIds from which
ACK is requested is limited to a maximum of the sender
NormSegmentSize setting per round of the positive
acknowledgement process.
Because the size of the included list is limited to the sender's
NormSegmentSize setting, multiple NORM_CMD(ACK_REQ) rounds may be
required to achieve responses from all receivers specified. The
content of the attached NormNodeId list will be dynamically updated
as this process progresses and ACKs are received from the specified
receiver set. Thus, as the sender receives responses from
receivers, it eliminates them from the subsequent NORM_CMD(ACK_REQ)
message payload list and adds in any pending receiver NormNodeIds
keeping within the NormSegmentSize limitation of the list size.
Each receiver is queried a maximum number of times
(NORM_ROBUST_FACTOR, by default). Receivers not responding within
this number of repeated requests are removed from the payload list
to make potential room for other receivers pending acknowledgement.
The transmission of the NORM_CMD(ACK_REQ) is repeated until no
further responses are required or until the repeat threshold is
exceeded for all pending receivers. The transmission of
NORM_CMD(ACK_REQ) messages to conduct the positive acknowledgment
process is multiplexed with ongoing sender data transmissions.
However, the positive acknowledgment process may be interrupted in
response to negative acknowledgement repair requests (NACKs)
received from receivers during the acknowledgment period. The ACK
process is resumed once any pending repairs have been transmitted.
In the case of NORM_CMD(ACK_REQ(WATERMARK)) commands, receivers
will not ACK until they have received complete transmission of all
data up to and including the watermark transmission point. All
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receivers SHALL interpret the watermark point provided in the
request in the same manner as the transmission point given in
NORM_CMD(FLUSH) messages and NACK for repairs if needed.
5.5.4 Group Size Estimation
NORM sender messages contain a "gsize" field that is a
representation of the group size and is used in scaling random
backoff timer ranges. The use of the group size estimate within
the NORM protocol does not require a precise estimation and works
reasonably well if the estimate is within an order of magnitude of
the actual group size. By default, the NORM sender group size
estimate may be administratively configured. Also given the
expected scalability of the NORM protocol for general use, a
default value of 10,000 is recommended for use as the group size
estimate.
It is possible that group size may be algorithmically approximated
from the volume of congestion control feedback messages which
follow the exponentially weighted random backoff. However, the
specification of such an algorithm is currently beyond the scope of
this document.
5.5.5 Operation with Generic Router Assist (GRA)
NORM packet formats will be extended to allow for operation with
GRA reliable multicast functions. Additional NACK suppression and
selective sub-casting of repair transmissions in the network will
be possible with GRA. (Section 5.4.2 discusses some NORM
mechanisms related to this). Additional details will be provide in
future versions of this document as GRA specifications mature.
6.0 Security Considerations
The same security considerations that apply to the NORM, FEC, and
TFMCC building blocks also apply to the NORM protocol. In
addition to vulnerabilities that any IP and IP multicast protocol
implementation may be generally subject to, the NACK based feedback
of NORM may be exploited by replay attacks which force the NORM
sender to unnecessarily transmit repair information. This MAY be
addressed by network layer IP security implementations that guard
against this potential security exploitation. It is RECOMMENDED
that such IP security mechanisms be used when available. Another
possible approach is for NORM senders to use the "sequence" field
from the NORM Common Message Header to detect replay attacks. This
can be accomplished if the sender is willing to maintain state on
receivers which are NACKing. A cache of receiver state may provide
some protection against replay attacks. Note that the "sequence"
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field should be incremented by NormNodes with independent values
for "sender" messages versus "receiver" messages so that the
congestion control loss estimation function of the "sequence" field
can be preserved for sender messages when receiver messages are
unicast to the sender.
While NORM does leverage FEC-based repair for scalability, this
does not alone guarantee integrity of received data. Application-
level integrity-checking of data content is highly RECOMMENDED.
The NORM protocol is compatible with the use of the IP security
(IPSEC) architecture described in [23].
7.0 Suggested Use
The present NORM protocol is seen as useful tool for the reliable
data transfer over generic IP multicast services. It is not the
intention of the authors to suggest it is suitable for supporting
all envisioned multicast reliability requirements. NORM provides a
simple and flexible framework for multicast applications with a
degree of concern for network traffic implosion and protocol
overhead efficiency. NORM-like protocols have been successfully
demonstrated within the MBone for bulk data dissemination
applications, including weather satellite compressed imagery
updates servicing a large group of receivers and a generic web
content reliable "push" application.
In addition, this framework approach has some design features
making it attractive for bulk transfer in asymmetric and wireless
internetwork applications. NORM is capable of successfully
operating independent of network structure and in environments with
high packet loss, delay, and misordering. Hybrid
proactive/reactive FEC-based repairing improve protocol performance
in some multicast scenarios. A sender-only repair approach often
makes additional engineering sense in asymmetric networks. NORM's
unicast feedback capability may be suitable for use in asymmetric
networks or in networks where only unidirectional multicast
routing/delivery service exists. Asymmetric architectures
supporting multicast delivery are likely to make up an important
portion of the future Internet structure (e.g., DBS/cable/PSTN
hybrids) and efficient, reliable bulk data transfer will be an
important capability for servicing large groups of subscribed
receivers.
8.0 References
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[1] Kermode, R., Vicisano, L., "Author Guidelines for Reliable
Multicast Transport (RMT) Building Blocks and Protocol
Instantiation documents", RFC 3269, April 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd
S. and Luby, M., "Reliable Multicast Transport Building
Blocks for One-to-Many Bulk-Data Transfer", RFC 3048,
January 2001.
[5] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[6] Handley, M., Perkins, C. and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[7] 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.
[8] 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.
[9] J. Macker, R. Adamson, "The Multicast Dissemination Protocol
(MDP) Toolkit", Proc. IEEE MILCOM 99, October 1999.
[10] J. Nonnenmacher and E. Biersack, "Optimal Multicast
Feedback", Proc. IEEE INFOCOMM, p. 964, March/April 1998.
[11] J. Macker, R. Adamson, "Quantitative Prediction of Nack
Oriented Reliable Multicast (NORM) Feedback", Proc. IEEE
MILCOM 2002, October 2002.
[12] T. Speakman, L. Vicisano, "Reliable Multicast Transport
Building Block Generic Roouter Assist - Signalling Protocol
Specification", Internet Draft draft-ietf-rmt-bb-gra-
signalling-01.txt, January 2003, work in progress. Citation
for informational purposes only.
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[13] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC 1112, August 1989.
[14] Holbrook, H. W., "A Channel Model for Multicast", Ph.D.
Dissertation, Stanford University, Department of Computer
Science, Stanford, California, August 2001.
[15] B. Adamson, C. Bormann, M. Handley, and J. Macker, "NACK-
Oriented Reliable Multicast (NORM) Protocol Building
Blocks", Internet Draft draft-ietf-rmt-bb-norm-05.txt, March
2003, work in progress. Citation for informational purposes
only.
[16] M. Luby, L. Vicisano, J. Gemmell, L. Rizzo, M. Handley, and
J. Crowcroft, "The Use of Forward Error Correction (FEC) in
Reliable Multicast", RFC 3453, December 2002.
[17] M. Luby, L. Vicisano, J. Gemmell, L. Rizzo, M. Handley, and
J. Crowcroft, "Forward Error Correction (FEC) Building
BLock", RFC 3452, December 2002.
[18] 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.
[19] D. Gossink, J. Macker, "Reliable Multicast and Integrated
Parity Retransmission with Channel Estimation", IEEE
GLOBECOMM 98', September 1998.
[20] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC 1889,
January 1996.
[21] J. Widmer and M. Handley, "Extending Equation-Based
Congestion Control to Multicast Applications", Proc ACM
SIGCOMM 2001, San Diego, August 2001.
[22] L. Rizzo, "pgmcc: A TCP-Friendly Single-Rate Multicast
Congestion Control Scheme", Proc ACM SIGCOMM 2000,
Stockholm, August 2000.
[23] S. Kent and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
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7.0 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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