rfc3940
Network Working Group B. Adamson
Request for Comments: 3940 NRL
Category: Experimental C. Bormann
Universitaet Bremen TZI
M. Handley
UCL
J. Macker
NRL
November 2004
Negative-acknowledgment (NACK)-Oriented
Reliable Multicast (NORM) Protocol
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes the messages and procedures of the Negative-
acknowledgment (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 acknowledgment
mechanism for transport reliability and offers additional protocol
mechanisms to allow for operation with minimal "a priori"
coordination among senders and receivers. A congestion control
scheme is specified to allow the NORM protocol to 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 asymmetric connectivity (possibly a unicast return path) between
the senders and 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.
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Table of Contents
1. Introduction and Applicability. . . . . . . . . . . . . . . . 3
1.1. NORM Delivery Service Model. . . . . . . . . . . . . . . 4
1.2. NORM Scalability . . . . . . . . . . . . . . . . . . . . 6
1.3. Environmental Requirements and Considerations. . . . . . 7
2. Architecture Definition . . . . . . . . . . . . . . . . . . . 7
2.1. Protocol Operation Overview. . . . . . . . . . . . . . . 9
2.2. Protocol Building Blocks . . . . . . . . . . . . . . . . 10
2.3. Design Tradeoffs . . . . . . . . . . . . . . . . . . . . 11
3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 12
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. NORM Common Message Header and Extensions. . . . . . . . 14
4.2. Sender Messages. . . . . . . . . . . . . . . . . . . . . 16
4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . . 16
4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . . 24
4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . . 26
4.3. Receiver Messages. . . . . . . . . . . . . . . . . . . . 43
4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . . 43
4.3.2. NORM_ACK Message. . . . . . . . . . . . . . . . . 50
4.4. General Purpose Messages . . . . . . . . . . . . . . . . 52
4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . . 52
5. Detailed Protocol Operation . . . . . . . . . . . . . . . . . 52
5.1. Sender Initialization and Transmission . . . . . . . . . 54
5.1.1. Object Segmentation Algorithm . . . . . . . . . . 55
5.2. Receiver Initialization and Reception. . . . . . . . . . 57
5.3. Receiver NACK Procedure. . . . . . . . . . . . . . . . . 57
5.4. Sender NACK Processing and Response. . . . . . . . . . . 59
5.4.1. Sender Repair State Aggregation . . . . . . . . . 60
5.4.2. Sender FEC Repair Transmission Strategy . . . . . 61
5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . . 62
5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation. . . . . . 62
5.5. Additional Protocol Mechanisms . . . . . . . . . . . . . 63
5.5.1. Greatest Round-trip Time Collection . . . . . . . 63
5.5.2. NORM Congestion Control Operation . . . . . . . . 64
5.5.3. NORM Positive Acknowledgment Procedure. . . . . . 72
5.5.4. Group Size Estimate . . . . . . . . . . . . . . . 74
6. Security Considerations . . . . . . . . . . . . . . . . . . . 75
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 75
8. Suggested Use . . . . . . . . . . . . . . . . . . . . . . . . 75
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 76
10. References. . . . . . . . . . . . . . . . . . . . . . . . . . 76
10.1. Normative References. . . . . . . . . . . . . . . . . . 76
10.2. Informative References. . . . . . . . . . . . . . . . . 77
11. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 79
Full Copyright Statement. . . . . . . . . . . . . . . . . . . 80
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1. Introduction and Applicability
The Negative-acknowledgment (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 in
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 queuing 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].
Statement of Intent
This memo contains part of the definitions necessary to fully specify
a Reliable Multicast Transport protocol in accordance with RFC 2357.
As per RFC 2357, the use of any reliable multicast protocol in the
Internet requires an adequate congestion control scheme.
While waiting for such a scheme to be available, or for an existing
scheme to be proven adequate, the Reliable Multicast Transport
working group (RMT) publishes this Request for Comments in the
"Experimental" category.
It is the intent of RMT to re-submit this specification as an IETF
Proposed Standard as soon as the above condition is met.
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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) [7], Session Announcement Protocol
(SAP) [8], etc.).
The NORM protocol design is principally driven by 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
independent of one another and receivers will maintain state as
necessary for each 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
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
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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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To summarize, the NORM protocol provides 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. NORM senders
provide for repair transmission of data and/or FEC 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 acknowledgment (NACK) based protocol schemes when feedback
for reliability is required [9]. 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 [10]. FEC-based repair can be used to
greatly reduce the quantity of reliable multicast repair requests and
repair transmissions [11] in a NACK-oriented protocol. 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 [12]. 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 [13].
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 each active sender. This may
constrain the number of simultaneous senders in some uses of NORM.
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1.3. Environmental Requirements and Considerations
All of the environmental requirements and considerations that apply
to the RMT NORM Building Block [4] and the RMT FEC Building Block [5]
also apply to the NORM protocol.
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. While
the techniques utilized in NORM are principally applicable to "flat"
end-to-end IP 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 [3], 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. 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 distinguish 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. While 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 that can be
leveraged for such application layer coordination.
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As previously 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 stream service
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 possibly 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
symbols for transmission by the sender. In NORM, an FEC encoding
symbol directly corresponds to the payload of NORM_DATA messages or
"segment". Note that when systematic FEC codes are used, the payload
of NORM_DATA messages sent for the first portion of a FEC encoding
block are source symbols (actual segments of original user data),
while the remaining symbols for the block consist of parity symbols
generated by FEC encoding. These parity symbols are generally sent
in response to repair requests, but some number may be sent
proactively at the end each encoding block to increase the robustness
of transmission. When non-systematic FEC codes are used, all symbols
sent consist of FEC encoding parity content. In this case, the
receiver must receive a sufficient number of symbols to reconstruct
(via FEC decoding) the original user data for the given block. In
this document, the terms "symbol" and "segment" are used
interchangeably.
Transmitted NormObjects are temporarily yet uniquely identified
within the NormSession context using the given sender's NormNodeId,
NormInstanceId, and a temporary NormObjectTransportId. Depending
upon the implementation, individual NORM senders may manage their
NormInstanceIds independently, or a common NormInstanceId may be
agreed upon for all participating nodes within a session if needed as
a session identifier. NORM NormObjectTransportId 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
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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. The
individual segments or symbols of the NormObject are further
identified with FEC payload identifiers which include coding block
and symbol identifiers. These are discussed in detail later in this
document.
2.1. Protocol Operation Overview
A NORM sender primarily generates messages of type NORM_DATA. These
messages carry original data segments or FEC symbols and repair
segments/symbols for the bulk data/file or stream NormObjects being
transferred. By default, redundant FEC symbols are sent only in
response to receiver repair requests (NACKs) and thus normally little
or no additional transmission overhead is imposed due to FEC
encoding. However, the NORM implementation MAY be optionally
configured to proactively transmit some amount of redundant FEC
symbols along with the original content to potentially enhance
performance (e.g., improved delay) at the cost of additional
transmission overhead. This option may be sensible for certain
network conditions and can allow for robust, asymmetric multicast
(e.g., unidirectional routing, satellite, cable) [15] 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 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.
When the NORM_INFO message type is used, its transmission should
precede transmission of any NORM_DATA message for the associated
NormObject.
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 acknowledgment requests or
application defined commands. The transmission of NORM_CMD messages
from the sender is accomplished by one of three different procedures.
These procedures are: single, best effort unreliable transmission of
the command; repeated redundant transmissions of the command; and
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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 that 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
acknowledgment from receivers, other NORM_ACK messages are defined
and available for use. 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 that is compatible with coexistent
TCP flows.
2.2. Protocol Building Blocks
The operation of the NORM protocol is based primarily upon the
concepts presented in the Nack-Oriented Reliable Multicast (NORM)
Building Block document [4]. This includes the basic NORM
architecture and the data transmission, repair, and feedback
strategies discussed in that document. Additional reliable multicast
building blocks are applied in creating the full NORM protocol
instantiation [16]. NORM also makes use of Forward Error Correction
encoding techniques for repair messaging and optional transmission
robustness as described in [10]. NORM uses the FEC Payload ID as
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specified by the FEC Building Block Document [5]. Additionally, for
congestion control, this document includes a baseline congestion
control mechanism (NORM-CC) based on the TCP-Friendly Multicast
Congestion Control (TFMCC) scheme described in [19].
2.3. 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 buffer constraints. Buffer requirements for
reliability, as always, are a function of the delay-bandwidth product
of the network topology. NORM performs best when allowed more
buffering resources than typical point-to-point transport protocols.
This is because NORM feedback suppression is based upon randomly-
delayed transmissions from the receiver set, rather than immediately
transmitted feedback. 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.
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3. Conformance Statement
This Protocol Instantiation document, in conjunction with the RMT
Building Block documents of [4] and [5], completely specifies a
working reliable multicast transport protocol that conforms to the
requirements described in RFC 2357 [17].
This document specifies the following message types and mechanisms
which are REQUIRED in complying NORM protocol implementations:
+--------------------+-----------------------------------------------+
| Message Type | Purpose |
+--------------------+-----------------------------------------------+
|NORM_DATA | Sender message for application data |
| | transmission. Implementations must support |
| | at least one of the NORM_OBJECT_DATA, |
| | NORM_OBJECT_FILE, or NORM_OBJECT_STREAM |
| | delivery services. The use of the NORM FEC |
| | Object Transmission Information header |
| | extension is OPTIONAL with NORM_DATA |
| | messages. |
+--------------------+-----------------------------------------------+
|NORM_CMD(FLUSH) | Sender command to excite receivers for repair |
| | requests in lieu of ongoing NORM_DATA |
| | transmissions. Note the use of the |
| | NORM_CMD(FLUSH) for positive acknowledgment |
| | of data receipt is OPTIONAL. |
+--------------------+-----------------------------------------------+
|NORM_CMD(SQUELCH) | Sender command to advertise its current valid |
| | repair window in response to invalid requests |
| | for repair. |
+--------------------+-----------------------------------------------+
|NORM_CMD(REPAIR_ADV)| Sender command to advertise current repair |
| | (and congestion control state) to group when |
| | unicast feedback messages are detected. Used |
| | to control/suppress excessive receiver |
| | feedback in asymmetric multicast topologies. |
+--------------------+-----------------------------------------------+
|NORM_CMD(CC) | Sender command used in collection of round |
| | trip timing and congestion control status |
| | from group (this may be OPTIONAL if |
| | alternative congestion control mechanism and |
| | round trip timing collection is used). |
+--------------------+-----------------------------------------------+
|NORM_NACK | Receiver message used to request repair of |
| | missing transmitted content. |
+--------------------+-----------------------------------------------+
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+--------------------+-----------------------------------------------+
|NORM_ACK | Receiver message used to proactively provide |
| | feedback for congestion control purposes. |
| | Also used with the OPTIONAL NORM Positive |
| | Acknowledgment Process. |
+--------------------+-----------------------------------------------+
This document also describes the following message types and
associated mechanisms which are OPTIONAL for complying NORM protocol
implementations:
+----------------------+----------------------------------------------+
| Message Type | Purpose |
+----------------------+----------------------------------------------+
|NORM_INFO | Sender message for providing ancillary |
| | context information associated with NORM |
| | transport objects. The use of the NORM FEC |
| | Object Transmission Information header |
| | extension is OPTIONAL with NORM_INFO |
| | messages. |
+----------------------+----------------------------------------------+
|NORM_CMD(EOT) | Sender command to indicate it has reached |
| | end-of-transmission and will no longer |
| | respond to repair requests. |
+----------------------+----------------------------------------------+
|NORM_CMD(ACK_REQ) | Sender command to support application- |
| | defined, positively acknowledged commands |
| | sent outside of the context of the bulk data |
| | content being transmitted. The NORM Positive|
| | Acknowledgment Procedure associated with this|
| | message type is OPTIONAL. |
+----------------------+----------------------------------------------+
|NORM_CMD(APPLICATION) | Sender command containing application-defined|
| | commands sent outside of the context of the |
| | bulk data content being transmitted. |
+----------------------+----------------------------------------------+
|NORM_REPORT | Optional message type reserved for |
| | experimental implementations of the NORM |
| | protocol. |
+----------------------+----------------------------------------------+
4. 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 auxiliary message type of
Adamson, et al. Experimental [Page 13]
RFC 3940 NORM Protocol November 2004
NORM_REPORT is also provided for experimental purposes. This section
describes 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. 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 and Extensions
There are some common message fields contained in all NORM message
types. Additionally, a header extension mechanism is defined to
expand the functionality of the NORM protocol without revision to
this document. All NORM protocol messages begin with a common header
with information fields as follows:
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 | hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Common Message Header Format
The "version" field is a 4-bit value indicating the protocol version
number. NORM implementations SHOULD ignore received messages with
version numbers different from their own. This number is intended to
indicate and distinguish upgrades of the protocol which may be non-
interoperable. The NORM version number for this specification is 1.
The message "type" field is a 4-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
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RFC 3940 NORM Protocol November 2004
The 8-bit "hdr_len" field indicates the number of 32-bit words that
comprise the given message's header portion. This is used to
facilitate header extensions that may be applied. The presence of
header extensions are implied when the "hdr_len" value is greater
than the base value for the given message "type".
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 a given destination address. A
"sequence" field number space SHOULD be maintained for messages sent
to the NormSession group address. This value can be monitored by
receiving nodes to detect packet losses in the transmission from a
sender and used in estimating raw packet loss for congestion control
purposes. 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. With message
authentication, the "sequence" field may also be leveraged for
protection from message "replay" attacks, particularly of NORM_NACK
or other feedback messages. In this case, the receiver node should
maintain a monotonically increasing "sequence" field space for each
destination to which it transmits (this may be multiple destinations
when unicast feedback is used). 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 set according to 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 [18] 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.
NORM Header Extensions
When header extensions are applied, they follow the message type's
base header and precede any payload portion. There are two formats
for header extensions, both of which begin with an 8-bit "het"
(header extension type) field. One format is provided for variable-
length extensions with "het" values in the range from 0 through 127.
The other format is for fixed length (one 32-bit word) extensions
with "het" values in the range from 128 through 255. These formats
are given here:
Adamson, et al. Experimental [Page 15]
RFC 3940 NORM Protocol November 2004
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het <=127 | hel | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Header Extension Content |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Variable Length Header Extension 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het >=128 | reserved | Header Extension Content |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Fixed Length (32-bit) Header Extension Format
The "Header Extension Content" portion of these header extension
format is defined for each header extension type defined for NORM
messages. Some header extensions are defined within this document
for NORM baseline FEC and congestion control operations.
4.2. 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 are used for various
protocol control 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 format of NORM_DATA messages is comprised of three logical
portions: 1) a fixed-format NORM_DATA header portion, 2) a FEC
Payload ID portion with a format dependent upon the FEC encoding
used, and 3) a payload portion containing source or encoded
application data content. Note for objects of type
NORM_OBJECT_STREAM, the payload portion contains additional fields
used to appropriately recover stream content. NORM implementations
MAY also extend the NORM_DATA header to include a FEC Object
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RFC 3940 NORM Protocol November 2004
Transmission Information (EXT_FTI) header extension. This allows
NORM receivers to automatically allocate resources and properly
perform FEC decoding without the need for pre-configuration or out-
of-band information.
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| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header_extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_reserved* | payload_len* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_offset* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data* |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_DATA Message Format
*NOTE: The "payload_reserved", "payload_len" and "payload_offset"
fields are present only for objects of type NORM_OBJECT_STREAM. The
"payload_len" and "payload_offset" fields allow senders to
arbitrarily vary the size of NORM_DATA payload segments for streams.
This allows applications to flush transmitted streams as needed to
meet unique streaming requirements. For objects of types
NORM_OBJECT_FILE and NORM_OBJECT_DATA, these fields are unnecessary
since the receiver can calculate the payload length and offset
information from the "fec_payload_id" using the algorithm described
in Section 5.1.1. The "payload_reserved" field is kept for
anticipated future NORM stream control functions. When systematic
FEC codes (e.g., "fec_id" = 129) are used, the "payload_len" and
"payload_offset" fields contain actual length and offset values for
the encapsulated application data segment for those NORM_DATA
messages containing source data symbols. In NORM_DATA messages that
contain parity information, these fields are not actual length or
Adamson, et al. Experimental [Page 17]
RFC 3940 NORM Protocol November 2004
offset values, but instead are values computed from FEC encoding the
"payload_len" and "payload_offset" fields of the _source_ data
symbols of the corresponding applicable coding block.
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of the NORM_DATA "type" field is 2. The NORM_DATA _base_
"hdr_len" value is 4 (32-bit words) plus the size of the
"fec_payload_id" field. The "fec_payload_id" field size depends upon
the FEC encoding used for the referenced NormObject. The "fec_id"
field is used to indicate the FEC coding type. For example, when
small block, systematic codes are used, a "fec_id" value of 129 is
indicated and the size of the "fec_payload_id" is two 32-bit words.
In this case the NORM_DATA base "hdr_len" value is 6. The cumulative
size of any header extensions applied is added into the "hdr_len"
field.
The "instance_id" field contains a value generated by the sender to
uniquely identify its current instance of participation in the
NormSession. This allows receivers to detect when senders have
perhaps left and rejoined a session in progress. When a sender
(identified by its "source_id") is detected to have a new
"instance_id", the NORM receivers SHOULD drop their previous state on
the sender and begin reception anew.
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 [19]). 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 [4].
The "backoff" field value is used by receivers to determine the
maximum backoff timer value used in the timer-based NORM NACK
feedback suppression. This 4-bit field supports values from 0-15
which is multiplied by the sender GRTT to determine the maximum
backoff timeout. The "backoff" field informs the receiver set of the
sender's backoff factor parameter "Ksender". Recommended values and
their use are described in the NORM receiver NACK procedure
description in Section 5.3. The "gsize" field contains a
representation of the sender's current estimate of group size. This
4-bit field can roughly represent values from ten to 500 million
where the most significant bit value of 0 or 1 represents a mantissa
of 1 or 5, respectively and the three least significant bits
incremented by one represent a base 10 exponent (order of magnitude).
For examples, a field value of "0x0" represents 1.0e+01 (10), a value
of "0x8" represents 5.0e+01 (50), a value of "0x1" represents 1.0e+02
Adamson, et al. Experimental [Page 18]
RFC 3940 NORM Protocol November 2004
(100), and a value of "0xf" represents 5.0e+08. For NORM feedback
suppression purposes, the group size does not need to be represented
with a high degree of precision. The group size may even be
estimated somewhat conservatively (i.e., overestimated) to maintain
low levels of feedback traffic. A default group size estimate of
10,000 ("gsize" = 0x4) is recommended for general purpose reliable
multicast applications using the NORM protocol.
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 to |
| | | meet 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. |
+--------------------+-------+-----------------------------------------+
|NORM_FLAG_MSG_START | 0x40 | Marks the first segment of application |
| | | messages embedded in |
| | | NORM_OBJECT_STREAMs. |
+--------------------+-------+-----------------------------------------+
NORM_FLAG_REPAIR 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
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RFC 3940 NORM Protocol November 2004
of new (non-repair) data content. NORM_FLAG_EXPLICIT 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 could possibly be used by intermediate systems
implementing functionality to control sub-casting of repair content
to different legs of a reliable multicast topology with disparate
repair needs. NORM_FLAG_INFO is set only when optional NORM_INFO
content is actually available for the associated object. Thus,
receivers will NACK for retransmission of NORM_INFO only when it is
available for a given object. NORM_FLAG_UNRELIABLE is set when the
sender wishes to transmit an object with only "best effort" delivery
and will not supply repair transmissions for the object. NORM
receivers SHOULD NOT execute repair requests for objects marked with
the NORM_FLAG_UNRELIABLE flag. Note that receivers may inadvertently
request repair of such objects when all segments (or info content)
for those objects are not received (i.e., a gap in the
"object_transport_id" sequence is noted). In this case, the sender
should invoke the NORM_CMD(SQUELCH) process as described in Section
4.2.3. NORM_FLAG_FILE can be set as a "hint" from the sender that
the associated object should be stored in non-volatile storage.
NORM_FLAG_STREAM is set when the identified object is of type
NORM_OBJECT_STREAM. When NORM_FLAG_STREAM is set, the
NORM_FLAG_MSG_START can be optionally used to mark the first data
segments of application-layer messages transported within the NORM
stream. This allows NORM receiver applications to "synchronize" with
NORM senders and to be able to properly interpret application layer
data when joining a NORM session already in progress. In practice,
the NORM implementation MAY set this flag for the segment transmitted
following an explicit "flush" of the stream by the application.
The "fec_id" field corresponds to the FEC Encoding Identifier
described in the FEC Building Block document [5]. The "fec_id" value
implies the format of the "fec_payload_id" field and, coupled with
FEC Object Transmission Information, the procedures to decode FEC
encoded content. Small block, systematic codes ("fec_id" = 129) are
expected to be used for most NORM purposes and the NORM_OBJECT_STREAM
requires systematic FEC codes for most efficient performance.
The "object_transport_id" field is a monotonically and incrementally
increasing value assigned by the sender to NormObjects 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 avoid 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
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RFC 3940 NORM Protocol November 2004
course of its transmission within a NORM session, an object is
uniquely identified by the concatenation of the sender "source_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 "fec_payload_id" identifies the attached NORM_DATA "payload"
content. The size and format of the "fec_payload_id" field depends
upon the FEC type indicated by the "fec_id" field. These formats are
given in the FEC Building Block document [5] and any subsequent
extensions of that document. As an example, the format of the
"fec_payload_id" format small block, systematic codes ("fec_id" =
129) given here:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_len | encoding_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Small Block, Systematic Code ("fec_id" = 129) "fec_payload_id" Format
The FEC payload identifier "source_block_number", "source_block_len",
and "encoding_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 IETF FEC Building Block document
[5]. The "source_block_number" identifies the coding block's
relative position with a NormObject. Note that, for NormObjects of
type NORM_OBJECT_STREAM, the "source_block_number" may wrap for very
long lived sessions. The "source_block_len" indicates the number of
user data segments in the identified coding block. Given the
"source_block_len" information of how many symbols of application
data are contained in the block, the receiver can determine whether
the attached segment is data or parity content and treat it
appropriately. The "encoding_symbol_id" identifies which specific
symbol (segment) within the coding block the attached payload
conveys. Depending upon the value of the "encoding_symbol_id" and
the associated "source_block_len" parameters for the block, the
symbol (segment) referenced may be a user data or an FEC parity
segment. For systematic codes, encoding symbols numbered less than
the source_block_len contain original application data while segments
greater than or equal to source_block_len contain parity symbols
calculated for the block. The concatenation of
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RFC 3940 NORM Protocol November 2004
object_transport_id::fec_payload_id can be viewed as a unique
transport protocol data unit identifier for the attached segment with
respect to the NORM sender's instance within a session.
Additional FEC Object Transmission Information (as described in the
FEC Building Block document [5]) is required to properly receive and
decode NORM transport objects. This information MAY be provided as
out-of-band session information. However, in some cases, it may be
useful for the sender to include this information "in band" to
facilitate receiver operation with minimal preconfiguration. For
this purpose, the NORM FEC Object Transmission Information Header
Extension (EXT_FTI) is defined. This header extension MAY be applied
to NORM_DATA and NORM_INFO messages to provide this necessary
information. The exact format of the extension depends upon the FEC
code in use, but in general it SHOULD contain any required details on
the FEC code in use (e.g., FEC Instance ID, etc.) and the byte size
of the associated NormObject (For the NORM_OBJECT_STREAM type, this
size corresponds to the stream buffer size maintained by the NORM
sender). As an example, the format of the EXT_FTI for small block
systematic codes ("fec_id" = 129) is given here:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 64 | hel = 4 | object_length (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_length (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_instance_id | segment_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_max_block_len | fec_num_parity |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FEC Object Transmission Information Header Extension (EXT_FTI) for
Small Block Systematic Codes ("fec_id" = 129)
The header extension type "het" field value for this header extension
is 64. The header extension length "hel" depends upon the format of
the FTI for FEC code type identified by the "fec_id" field. In this
example (for "fec_id" = 129), the "hel" field value is 4.
The 48-bit "object_length" 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_length"
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RFC 3940 NORM Protocol November 2004
field is used 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_instance_id" corresponds to the "FEC Instance ID" described
in the FEC Building Block document [5]. In this case, 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 [5]. 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 "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 "fec_num_parity" corresponds to the "maximum number 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 [5]. 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
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 payload portion of NORM_DATA messages includes source data or FEC
encoded application content.
The "payload_reserved", "payload_len" and "payload_offset" fields are
present ONLY for transport objects of type NORM_OBJECT_STREAM. These
fields indicate the size and relative position (within the stream) of
the application content represented by the message payload. For
senders employing systematic FEC encoding, these fields contain
_actual_ length and offset values (in bytes) for the payload of
messages which contain original data source symbols. For NORM_DATA
messages containing calculated parity content, these fields will
actually contain values computed by FEC encoding of the "payload_len"
and "payload_offset" values of the NORM_DATA data segments of the
corresponding FEC coding block. Thus, the "payload_len" and
"payload_offset" values of missing data content can be determined
upon decoding a FEC coding block. Note that these fields do NOT
Adamson, et al. Experimental [Page 23]
RFC 3940 NORM Protocol November 2004
contribute to the value of the NORM_DATA "hdr_len" field. These
fields are NOT present when the "flags" portion of the NORM_DATA
message indicate the transport object if of type NORM_OBJECT_FILE or
NORM_OBJECT_DATA. In this case, the length and offset information
can be calculated from the "fec_payload_id" using the methodology
described in Section 5.1.1. Note that for long-lived streams, the
"payload_offset" field can wrap.
The "payload_data" field contains the original application source or
parity content for the symbol identified by the "fec_payload_id".
The length of this field SHALL be limited to a maximum of the
sender's NormSegmentSize bytes as given in the FTI for the object.
Note the length of this field for messages containing parity content
will always be of 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.
It is RECOMMENDED that a sender's NormSegmentSize generally be
constant for the duration of a given sender's term of participation
in the session, but may possibly vary on a per-object basis. 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 possibly leveraged at session startup to perform
this configuration. The "payload_data" content may be delivered
directly to the application for source symbols (when systematic FEC
encoding is used) or upon decoding of the FEC block. For
NORM_OBJECT_FILE and NORM_OBJECT_STREAM objects, the data segment
length and offset can be calculated using the algorithm described in
Section 5.1.1. For NORM_OBJECT_STREAM objects, the length and offset
is obtained from the segment's corresponding "payload_len" and
"payload_offset" fields.
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 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
Adamson, et al. Experimental [Page 24]
RFC 3940 NORM Protocol November 2004
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.
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| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header_extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_INFO Message Format
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of "hdr_len" field when no header extensions are present is 4.
The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and
"object_transport_id" 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.
As with NORM_DATA messages, the NORM FTI Header Extension (EXT_FTI)
may be optionally applied to NORM_INFO messages. To conserve
protocol overhead, some NORM implementations may wish to apply the
EXT_FTI when used to NORM_INFO messages only and not to NORM_DATA
messages.
Adamson, et al. Experimental [Page 25]
RFC 3940 NORM Protocol November 2004
The NORM_INFO "payload_data" field contains sender application-
defined content which can be used by receiver applications for
various purposes as described above.
4.2.3. NORM_CMD Messages
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 messages begin with a common set of fields,
after the usual NORM message common header. The standard NORM_CMD
fields are:
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| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor | |
+-+-+-+-+-+-+-+-+ NORM_CMD Content +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD Standard Fields
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of the "hdr_len" field for NORM_CMD messages without header
extensions present depends upon the "flavor" field.
The "instance_id", "grtt", "backoff", 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"). NORM command flavors include:
Adamson, et al. Experimental [Page 26]
RFC 3940 NORM Protocol November 2004
+----------------------+-------------+---------------------------------+
| Command |Flavor Value | Purpose |
+----------------------+-------------+---------------------------------+
|NORM_CMD(FLUSH) | 1 | Used to indicate sender |
| | | temporary end-of-transmission. |
| | | (Assists in robustly initiating |
| | | outstanding repair requests from|
| | | receivers). May also be |
| | | optionally used to collect |
| | | positive acknowledgment of |
| | | reliable reception from subset |
| | | of receivers. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(EOT) | 2 | Used to indicate sender |
| | | permanent end-of-transmission. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(SQUELCH) | 3 | Used to advertise sender's |
| | | current repair window in |
| | | response to out-of-range NACKs |
| | | from receivers. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(CC) | 4 | Used for GRTT measurement and |
| | | collection of congestion control|
| | | feedback. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(REPAIR_ADV) | 5 | Used to advertise sender's |
| | | aggregated repair/feedback state|
| | | for suppression of unicast |
| | | feedback from receivers. |
+----------------------+-------------+---------------------------------+
|NORM_CMD(ACK_REQ) | 6 | Used to request application- |
| | | defined positive acknowledgment |
| | | from a list of receivers |
| | | (OPTIONAL). |
+----------------------+-------------+---------------------------------+
|NORM_CMD(APPLICATION) | 7 | Used for application-defined |
| | | purposes which may need to |
| | | temporarily preempt data |
| | | transmission (OPTIONAL). |
+----------------------+-------------+---------------------------------+
4.2.3.1. 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 may indicate a temporary or permanent end of data
transmission, but the sender is still willing to respond to repair
requests. This command is repeated once per 2*GRTT to excite the
Adamson, et al. Experimental [Page 27]
RFC 3940 NORM Protocol November 2004
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 unless a list of
receivers from which explicit positive acknowledgment is expected
("acking_node_list") is given. In that case, the "acking_node_list"
is updated as acknowledgments are received and the NORM_CMD(FLUSH) is
repeated according to the mechanism described in Section 5.5.3. The
greater the NORM_ROBUST_FACTOR, the greater the probability that all
applicable receivers will be excited for acknowledgment or repair
requests (NACKs) _and_ that the corresponding NACKs are delivered to
the sender. If a NORM_NACK message interrupts the flush process, the
sender will re-initiate the flush process after any resulting repair
transmissions are completed.
Note that receivers also employ a timeout mechanism to self-initiate
NACKing (if there are outstanding repair needs) when no messages of
any type are received from a sender. This inactivity 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 as NORM_OBJECT_DATA and
NORM_OBJECT_FILE, the flush process (if there are no further pending
objects) occurs at the end of these objects. Thus, FEC repair
information is always 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 employed.
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.
Adamson, et al. Experimental [Page 28]
RFC 3940 NORM Protocol November 2004
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| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 1 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(FLUSH) Message Format
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.
The "fec_id" field indicates the FEC type used for the flushing
"object_transport_id" and implies the size and format of the
"fec_payload_id" field. Note the "hdr_len" value for the
NORM_CMD(FLUSH) message is 4 plus the size of the "fec_payload_id"
field when no header extensions are present.
The "object_transport_id" and "fec_payload_id" fields indicate the
sender's current logical "transmit position". These fields are
interpreted in the same manner as in the NORM_DATA message type.
Upon receipt of 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 to the NORM_CMD(FLUSH) is generated.
For NORM_OBJECT_STREAM objects using systematic FEC codes, receivers
MUST request "explicit-only" repair of the identified
"source_block_number" if the given "encoding_symbol_id" is less than
the "source_block_len". 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 "source_block_number" which does not
include any requests for parity-based repair. This allows NORM
Adamson, et al. Experimental [Page 29]
RFC 3940 NORM Protocol November 2004
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 usual. Note that the use of a systematic FEC code is
assumed here. Normal receiver NACK initiation and construction is
discussed in detail in Section 5.3. The OPTIONAL "acking_node_list"
field contains a list of NormNodeIds for receivers from which the
sender is requesting explicit positive acknowledgment of reception up
through the transmission point identified by the
"object_transport_id" and "fec_payload_id" fields. The length of the
list can be inferred from the length of the received NORM_CMD(FLUSH)
message. When the "acking_node_list" is present, the lightweight
positive acknowledgment process described in Section 5.5.3 SHALL be
observed.
4.2.3.2. NORM_CMD(EOT) Message
The NORM_CMD(EOT) command is sent when the sender reaches permanent
end-of-transmission with respect to the NormSession and will not
respond to further repair requests. This allows receivers to
gracefully reach closure of operation with this sender (without
requiring any timeout) and free any resources that are no longer
needed. The NORM_CMD(EOT) command SHOULD be sent with the same
robust mechanism as used for NORM_CMD(FLUSH) commands to provide a
high assurance of reception by the receiver set.
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| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 2 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(EOT) Message Format
The value of the "hdr_len" field for NORM_CMD(EOT) messages without
header extensions present is 4. The "reserved" field is reserved for
future use and MUST be set to an all ZERO value. Receivers MUST
ignore the "reserved" field.
Adamson, et al. Experimental [Page 30]
RFC 3940 NORM Protocol November 2004
4.2.3.3. NORM_CMD(SQUELCH) Message
The NORM_CMD(SQUELCH) command is transmitted in response to outdated
or 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).
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 specific previously enqueued objects.
The list also contains the identifiers for any objects within the
repair window that 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:
Adamson, et al. Experimental [Page 31]
RFC 3940 NORM Protocol November 2004
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 3 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| invalid_object_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(SQUELCH) Message Format
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" and "fec_payload_id" 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). The "fec_id" field
implies the size and format of the "fec_payload_id" field. This
serves as an advertisement of a "synchronization point" for receivers
to request repair. Note, that while an "encoding_symbol_id" may be
included in the "fec_payload_id" field, the sender's repair window
SHOULD be aligned on FEC coding block boundaries and thus the
"encoding_symbol_id" SHOULD be zero.
The "invalid_object_list" is a list of 16-bit NormTransportIds that,
although they are within the range of 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 can be determined from the packet's
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,
Adamson, et al. Experimental [Page 32]
RFC 3940 NORM Protocol November 2004
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.
4.2.3.4. NORM_CMD(CC) Message
The NORM_CMD(CC) messages contains fields to enable sender-to-
receiver group greatest round-trip time (GRTT) measurement and to
excite the group for congestion control feedback. A baseline NORM
congestion control scheme (NORM-CC), based on the TCP-Friendly
Multicast Congestion Control (TFMCC) scheme of [19] is described in
Section 5.5.2 of this document. The NORM_CMD(CC) message is usually
transmitted as part of NORM-CC congestion control operation. A NORM
header extension is defined below to be used with the NORM_CMD(CC)
message to support NORM-CC operation. Different header extensions
may be defined for the NORM_CMD(CC) (and/or other NORM messages as
needed) to support alternative congestion control schemes in the
future. If NORM is operated in a private network with congestion
control operation disabled, the NORM_CMD(CC) message is then used for
GRTT measurement only and may optionally be sent less frequently than
with congestion control operation.
Adamson, et al. Experimental [Page 33]
RFC 3940 NORM Protocol November 2004
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| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 4 | reserved | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_list (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(CC) Message Format
The NORM common message header and standard NORM_CMD fields serve
their usual purposes.
The "reserved" field is for potential future use and should be set to
ZERO in this version of the NORM protocol.
The "cc_sequence" field is a sequence number applied by the sender.
For NORM-CC operation, it is used to provide functionality equivalent
to the "feedback round number" (fb_nr)described in [19]. The most
recently received "cc_sequence" value is recorded by receivers and
can be fed back to the sender in congestion control feedback
generated by the receivers for that sender. The "cc_sequence" number
can also be used in NORM implementations to assess how recently a
receiver has received NORM_CMD(CC) probes from the sender. This can
be useful instrumentation for complex or experimental multicast
routing environments.
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 ("send_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
Adamson, et al. Experimental [Page 34]
RFC 3940 NORM Protocol November 2004
from the time they received the NORM_CMD(CC) message to the time of
their response as the "grtt_response" portion of NORM_ACK and
NORM_NACK messages generated. This allows the sender to evaluate
round-trip times to different receivers for congestion control and
other (e.g., GRTT determination) purposes.
To facilitate the baseline NORM-CC scheme described in Section 5.5.2,
a NORM-CC Rate header extension (EXT_RATE) is defined to inform the
group of the sender's current transmission rate. This is used along
with the loss detection "sequence" field of all NORM sender messages
and the NORM_CMD(CC) GRTT collection process to support NORM-CC
congestion control operation. The format of this header extension is
as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 128 | reserved | send_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM-CC Rate Header Extension Format (EXT_RATE)
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 portion and 4 bits of
base 10 exponent (order of magnitude) information in the least
significant portion. 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:
send_rate = (((int)(Value_mantissa * 4096.0 / 10.0 + 0.5)) << 4) |
Value_exponent;
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:
send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4;
= (0x51f << 4) | 0x4
= 0x51f4
To decode the "send_rate" field, the following equation can be used:
value = (send_rate >> 4) * 10.0 / 4096.0 *
power(10.0, (send_rate & x000f))
Adamson, et al. Experimental [Page 35]
RFC 3940 NORM Protocol November 2004
Note the maximum transmission rate that can be represented by this
scheme is approximately 9.99e+15 bytes per second.
When this extension is present, a "cc_node_list" may be attached as
the payload of the NORM_CMD(CC) message. The presence of this header
extension also implies that NORM receivers should respond according
to the procedures described in Section 5.5.2. 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 that 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 maximum 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) message.
Each item in the "cc_node_list" is in the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Congestion Control Node List Item Format
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:
Adamson, et al. Experimental [Page 36]
RFC 3940 NORM Protocol November 2004
+------------------+-------+------------------------------------------+
| Flag | Value | Purpose |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_CLR | 0x01 | Receiver is the current limiting |
| | | receiver (CLR). |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_PLR | 0x02 | Receiver is a potential limiting |
| | | receiver (PLR). |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_RTT | 0x04 | Receiver has measured RTT with respect |
| | | to sender. |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_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 |
| | | the receiver's actual measured receive |
| | | rate). |
+------------------+-------+------------------------------------------+
|NORM_FLAG_CC_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 RTT as
measured by the sender with respect to the indicated receiver. This
field is valid only if the NORM_FLAG_CC_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 [4]. 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.
4.2.3.5. NORM_CMD(REPAIR_ADV) Message
The NORM_CMD(REPAIR_ADV) message is used by the sender to "advertise"
its aggregated repair state from 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(CC) (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 [13].
Adamson, et al. Experimental [Page 37]
RFC 3940 NORM Protocol November 2004
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| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 5 | flags | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_adv_payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(REPAIR_ADV) Message Format
The "instance_id", "grtt", "backoff", "gsize", and "flavor" fields
serve the same purpose as in other NORM_CMD messages. The value of
the "hdr_len" field when no extensions are present is 4.
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 fit its full
current repair state into a single NormSegmentSize. If this flag is
set, receivers should limit their NACK response to generating NACK
content only up through the maximum ordinal transmission position
(objectId::fecPayloadId) included in the "repair_adv_content".
When congestion control operation is enabled, a header extension may
be applied to the NORM_CMD(REPAIR_ADV) representing the most limiting
(in terms of congestion control feedback suppression) congestion
control response. This allows the NORM_CMD(REPAIR_ADV) message to
suppress receiver congestion control responses as well as NACK
feedback messages. The field is defined as a header extension so
that alternative congestion control schemes may be used with NORM
without revision to this document. A NORM-CC Feedback Header
Extension (EXT_CC) is defined to encapsulate congestion control
feedback within NORM_NACK, NORM_ACK, and NORM_CMD(REPAIR_ADV)
messages. If another congestion control technique (e.g., Pragmatic
General Multicast Congestion Control (PGMCC) [20]) is used within a
Adamson, et al. Experimental [Page 38]
RFC 3940 NORM Protocol November 2004
NORM implementation, an additional header extension MAY need to be
defined to encapsulate any required feedback content. The NORM-CC
Feedback Header Extension format is:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 3 | hel = 3 | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_rate | cc_reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM-CC Feedback Header Extension (EXT_CC) Format
The "cc_sequence" field contains the current greatest "cc_sequence"
value receivers have received in NORM_CMD(CC) messages from the
sender. This information assists 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. For NORM_CMD(REPAIR_ADV) messages, the sender
SHALL set the "cc_sequence" field value to the value set in the last
NORM_CMD(CC) message sent.
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. These fields are used by receivers in
controlling (suppressing as necessary) their congestion control
feedback. For NORM_CMD(REPAIR_ADV) messages, the NORM_FLAG_CC_RTT
should be set only when all feedback messages received by the sender
have the flag set. Similarly, the NORM_FLAG_CC_CLR or
NORM_FLAG_CC_PLR should be set only when no feedback has been
received from non-CLR or non-PLR receivers. And the
NORM_FLAG_CC_LEAVE should be set only when all feedback messages the
sender has received have this flag set. These heuristics for setting
the flags in NORM_CMD(REPAIR_ADV) ensure the most effective
suppression of receivers providing unicast feedback messages.
The "cc_rtt" field SHALL be set to a default maximum value and the
NORM_FLAG_CC_RTT flag SHALL be cleared when no receiver has yet
received RTT measurement information. When a receiver has received
RTT measurement information, it shall set the "cc_rtt" value
accordingly and set the NORM_FLAG_CC_RTT flag in the "cc_flags"
field.
Adamson, et al. Experimental [Page 39]
RFC 3940 NORM Protocol November 2004
For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_rtt"
field value to the largest non-CLR/non-PLR RTT it has measured from
receivers for the current feedback round.
The "cc_loss" field represents 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
For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
field value to the largest non-CLR/non-PLR loss estimate it has
received from receivers for the current feedback round.
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_FLAG_CC_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. For
NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
field value to the lowest non-CLR/non-PLR "cc_rate" report it has
received from receivers for the current feedback round.
The "cc_reserved" field is reserved for future NORM protocol use.
Currently, senders SHALL set this field to ZERO, and receivers SHALL
ignore the content of this field.
The "repair_adv_payload" 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.
4.2.3.6. NORM_CMD(ACK_REQ) Message
The NORM_CMD(ACK_REQ) message is used by the sender to request
acknowledgment from a specified list of receivers. This message is
used in providing a lightweight positive acknowledgment mechanism
that is OPTIONAL for use by the reliable multicast application. A
range of acknowledgment 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
Adamson, et al. Experimental [Page 40]
RFC 3940 NORM Protocol November 2004
positive acknowledgment 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:
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| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 6 | reserved | ack_type | ack_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(ACK_REQ) Message F
