Network Working Group B. Adamson
Internet-Draft Naval Research Laboratory
Obsoletes: 3940 (if approved) C. Bormann
Intended status: Standards Track Universitaet Bremen TZI
Expires: October 26, 2009 M. Handley
University College London
J. Macker
Naval Research Laboratory
April 24, 2009
NACK-Oriented Reliable Multicast Protocol
draft-ietf-rmt-pi-norm-revised-11
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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.
This document obsoletes RFC 3940.
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Table of Contents
1. Introduction and Applicability . . . . . . . . . . . . . . . . 5
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 6
1.2. NORM Data Delivery Service Model . . . . . . . . . . . . . 6
1.3. NORM Scalability . . . . . . . . . . . . . . . . . . . . . 8
1.4. Environmental Requirements and Considerations . . . . . . 9
2. Architecture Definition . . . . . . . . . . . . . . . . . . . 9
2.1. Protocol Operation Overview . . . . . . . . . . . . . . . 11
2.2. Protocol Building Blocks . . . . . . . . . . . . . . . . . 13
2.3. Design Tradeoffs . . . . . . . . . . . . . . . . . . . . . 13
3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 14
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 16
4.1. NORM Common Message Header and Extensions . . . . . . . . 16
4.2. Sender Messages . . . . . . . . . . . . . . . . . . . . . 19
4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . . . . 19
4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . . . . 29
4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . . . . 30
4.3. Receiver Messages . . . . . . . . . . . . . . . . . . . . 48
4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . . . . 48
4.3.2. NORM_ACK Message . . . . . . . . . . . . . . . . . . . 54
4.4. General Purpose Messages . . . . . . . . . . . . . . . . . 56
4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . . . . 56
5. Detailed Protocol Operation . . . . . . . . . . . . . . . . . 56
5.1. Sender Initialization and Transmission . . . . . . . . . . 58
5.1.1. Object Segmentation Algorithm . . . . . . . . . . . . 59
5.2. Receiver Initialization and Reception . . . . . . . . . . 60
5.3. Receiver NACK Procedure . . . . . . . . . . . . . . . . . 60
5.4. Sender NACK Processing and Response . . . . . . . . . . . 62
5.4.1. Sender Repair State Aggregation . . . . . . . . . . . 63
5.4.2. Sender FEC Repair Transmission Strategy . . . . . . . 64
5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . . . . 65
5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 65
5.5. Additional Protocol Mechanisms . . . . . . . . . . . . . . 66
5.5.1. Greatest Round-trip Time Collection . . . . . . . . . 66
5.5.2. NORM Congestion Control Operation . . . . . . . . . . 67
5.5.3. NORM Positive Acknowledgment Procedure . . . . . . . . 75
5.5.4. Group Size Estimate . . . . . . . . . . . . . . . . . 77
6. Security Considerations . . . . . . . . . . . . . . . . . . . 77
6.1. Baseline Secure NORM Operation . . . . . . . . . . . . . . 79
6.1.1. IPsec Approach . . . . . . . . . . . . . . . . . . . . 79
6.1.2. IPsec Requirements . . . . . . . . . . . . . . . . . . 82
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 83
7.1. Explicit IANA Assignment Guidelines . . . . . . . . . . . 83
7.1.1. NORM Header Extension Types . . . . . . . . . . . . . 83
7.1.2. NORM Stream Control Codes . . . . . . . . . . . . . . 84
7.1.3. NORM_CMD Message Sub-types . . . . . . . . . . . . . . 85
8. Suggested Use . . . . . . . . . . . . . . . . . . . . . . . . 86
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9. Changes from RFC3940 . . . . . . . . . . . . . . . . . . . . . 86
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 87
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 87
11.1. Normative References . . . . . . . . . . . . . . . . . . . 87
11.2. Informative References . . . . . . . . . . . . . . . . . . 88
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 90
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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 obsoletes the Experimental RFC 3940
specification.
This document is a product of the IETF RMT working group and follows
the guidelines provided in the Author Guidelines for Reliable
Multicast Transport (RMT) Building Blocks and Protocol Instantiation
documents [RFC3269].
Statement of Intent
This memo contains the definitions necessary to fully specify a
Reliable Multicast Transport protocol in accordance with the criteria
of IETF Criteria for Evaluating Reliable Multicast Transport and
Application Protocols [RFC2357]. The NORM specification described in
this document was previously published in the "Experimental Category"
[RFC3940]. It was the stated intent of the RMT working group to re-
submit this specifications as an IETF Proposed Standard in due
course. This Proposed Standard specification is thus based on RFC
3940 and has been updated according to accumulated experience and
growing protocol maturity since the publication of RFC 3940. Said
experience applies both to this specification itself and to
congestion control strategies related to the use of this
specification. The differences between RFC 3940 and this document
are listed in Section 9.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. NORM Data 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) [RFC4566], Session Announcement
Protocol (SAP) [RFC2974], 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.
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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
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
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transfer communication models to meet their goals.
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.
1.3. NORM Scalability
Group communication scalability requirements lead to adaptation of
negative acknowledgment (NACK) based protocol schemes when feedback
for reliability is required [RmComparison]. 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 [RFC3453]. FEC-based
repair can be used to greatly reduce the quantity of reliable
multicast repair requests and repair transmissions [MdpToolkit] 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
[McastFeedback]. NORM dynamically measures the group's round-trip
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 relays 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
[NormFeedback]. 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
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state only for receivers providing explicit congestion control
feedback. However, NORM receivers must maintain state for each
active sender. This may constrain the number of simultaneous senders
in some uses of NORM.
1.4. Environmental Requirements and Considerations
All of the environmental requirements and considerations that apply
to the Multicast NACK Building Block [RFC5401], FEC Building Block
[RFC5052], and TCP-Friendly Multicast Congestion Control (TFMCC)
Building Block [RFC4654] 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 Host Extensions for IP Multicasting [RFC1112],
but SHALL also be capable of scalable operation in asymmetric
topologies such as Source-Specific Multicast (SSM) [RFC4607] 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 "wild card" NormNodeId. While the protocol does
not preclude multiple sender nodes concurrently transmitting within
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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.
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.
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
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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
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) [FecHybrid] 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
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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 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.). A notable
distinction between "NORM_DATA" message and some "NORM_CMD" message
transmissions is that typically a receiver will need to allocate
resources to manage reliable reception when "NORM_DATA" messages are
received. However some "NORM_CMD" messages may be completely atomic
and no specific state may need to be kept. Thus, for session
management or other purposes it is possible that even participants
acting principally as data receivers MAY transmit "NORM_CMD"
messages. However, it is RECOMMENDED that this is not done within
the context of the NORM multicast session unless congestion control
is addressed. For example, many receiver nodes transmitting
"NORM_CMD" messages simultaneously can cause congestion for the
destination(s).
All sender 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.
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
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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.
2.2. Protocol Building Blocks
The operation of the NORM protocol is based primarily upon the
concepts presented in the Multicast NACK Building Block [RFC5401]
document. This includes the basic NORM architecture and the data
transmission, repair, and feedback strategies discussed in that
document. The reliable multicast building block approach, as
described in Reliable Multicast Transport Building Blocks for One-to-
Many Bulk-Data Transfer [RFC3048], is applied in creating the full
NORM protocol instantiation. NORM also makes use of the parity-based
encoding techniques for repair messaging and optional transmission
robustness as described in The Use of Forward Error Correction (FEC)
in Reliable Multicast [RFC3453]. NORM uses the FEC Payload ID as
specified by the FEC Building Block document [RFC5052].
Additionally, for congestion control, this document fully specifies a
baseline congestion control mechanism (NORM-CC) based on the TCP-
Friendly Multicast Congestion Control (TFMCC) scheme[TfmccPaper],
[RFC4654].
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 trade-offs 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
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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 trade-offs between buffer
utilization, group size scalability, and efficiency of performance.
Large buffer sizes allow the NORM protocol to perform most
efficiently in large delay-bandwidth topologies and allow for longer
feedback suppression backoff timeouts. This yields improved group
size scalability. NORM can operate with reduced buffering but at a
cost of decreased efficiency (lower relative goodput) and reduced
group size scalability.
3. Conformance Statement
This RMT Protocol Instantiation document, in conjunction with the
Multicast Negative-Acknowledgment (NACK) [RFC5401] and Forward Error
Correction (FEC) [RFC5052] Building Blocks, completely specifies a
working reliable multicast transport protocol that conforms to the
requirements described in RFC 2357.
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. |
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| "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. |
| "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. |
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| "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. Sender messages SHOULD be governed
by congestion control for Internet use. For session management or
other purposes, receivers may wish to employ "NORM_CMD" message
transmissions. The principal rationale for distinguishing sender and
receiver messages is that receivers will typically need to allocate
resources to support reliable reception from sender(s) and NORM
sender messages are subject to congestion control. NORM receivers
MAY employ the "NORM_CMD" message type for application-defined
purposes but it is RECOMMENDED that congestion control and feedback
implosion issues be addressed. Additionally, an auxiliary message
type of "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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 |
+---------------+-------+
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. The "sequence" field serves two separate purposes,
depending upon the message type:
1. NORM senders MUST set the "sequence" field of sender messages
("NORM_INFO", "NORM_DATA", and "NORM_CMD") so that receivers can
monitor the "sequence" value to maintain an estimate of packet
loss that can be used for congestion control purposes (See
Section 5.5.2 for a detailed description of NORM Congestion
Control operation). A monotonically-increasing sequence number
space MUST be maintained to mark NORM sender messages in this
way. Note that this "sequence" number is explicitly NOT used in
NORM as part of its reliability procedures. The NORM object and
FEC payload identifiers are used to detect missing content for
reliable transfer purposes.
2. NORM receivers SHOULD set the "sequence" field to support
protection from message replay attacks of "NORM_NACK" or
"NORM_NACK" messages. Note that, depending upon configuration,
NORM feedback messages may be sent to the session multicast
address or unicast address[es] of the active NORM sender[s].
Thus, a separate, monotonically-increasing sequence number space
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MUST be maintained for each destination address to which the NORM
receiver is transmitting feedback messages.
Note that these two separate purposes necessitate the maintenance of
separate sequence spaces to support the functions described here.
And, in the case of NORM receivers, additional sequence spaces are
needed when feedback messages are sent to the sender unicast
address[es] instead of the session address.
The "source_id" field is a 32-bit value that uniquely identifies the
node that sent the message within the context of a single
NormSession. This value is termed the NORM node identifier
(NormNodeId) and unique NormNodeId 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 SHOULD be considered. For example, the
techniques for managing the 32-bit "synchronization source" (SSRC)
identifiers defined in the Real-Time Protocol (RTP) specification
[RFC3550] are applicable for use with NORM node identifiers. In most
deployments of the NORM protocol to date, the NormNodeId assignments
are administratively configured.
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:
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
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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 the header extension is
defined for each extension type. 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
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.
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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_len* | payload_msg_start* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_offset* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data* |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_DATA Message Format
*IMPORTANT NOTE: The "payload_len", "payload_msg_start" and
"payload_offset" fields are present ONLY for objects of type
"NORM_OBJECT_STREAM". These fields, as with the entire payload, are
subject to any FEC encoding used. Thus, when systematic FEC codes
are used, these values may be directly interpreted for packets
containing source symbols only while packets containing FEC parity
content require decoding before these fields can be interpreted.
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 (i.e. 4 32-bit words) plus the size of the
"fec_payload_id" field. The "fec_payload_id" field size depends upon
the FEC encoding type referenced by the "fec_id" field. 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
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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, or at least treat this
"instance" as a new, separate sender.
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 [TfmccPaper]). This value is used
to control timing of the NACK repair process and other aspects of
protocol operation as described in this document. Normally, the
advertised "grtt" value will correspond to what the sender has
measured based on feedback from the group, but, at low transmission
rates, the advertised "grtt" SHALL be set to "MAX(grttMeasured,
NormSegmentSize/senderRate)" where the "NormSegmentSize" is sender's
segment size in bytes and the "senderRate" is the sender's current
transmission rate in bytes per second. The algorithm for encoding
and decoding this field is described in the Multicast NACK Building
Block [RFC5401].
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 receivers 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 (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" =
0x3) is recommended for general purpose reliable multicast
applications using the NORM protocol.
The "flags" field contains a number of different binary flags
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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_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
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" (not previously transmitted) 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
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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". The presence of "NORM_FLAG_STREAM" overrides
that of "NORM_FLAG_FILE" with respect to interpretation of object
size and the format of "NORM_DATA" messages.
The "fec_id" field corresponds to the FEC Encoding Identifier
described in the FEC Building Block document [RFC5052]. 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
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 descriptions of specific FEC schemes such as those
described in the FEC Basic Schemes [RFC5445] specification or in
other FEC Schemes. As an example, the format of the "fec_payload_id"
format for Small Block, Systematic codes ("fec_id" = 129) from theFEC
Basic Schemes [RFC5445] specification is given here:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example: FEC Payload Id Format for 'fec_id' = 129
In this example FEC payload identifier, the "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 for Small Block Systematic FEC
Schemes identified by a "fec_id" value of 129 as specified by the FEC
Basic Schemes [RFC5445] specification. 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. Some applications may
dynamically "shorten" code blocks when the pending information
content is not predictable (e.g. real-time message streams). In that
case, the "source_block_len" value given for an "encoding_symbol_id"
that contains FEC parity content SHALL take precedence over the
"source_block_len" value provided for any packets containing source
symbols. Also, the "source_block_len" value given for an ordinally
higher "encoding_symbol_id" SHALL take precedence over the
"source_block_len" given for prior encoding symbols. The reason for
this is that the sender may only know the maximum source block length
at the time is transmitting source symbols, but then subsequently
"shorten" the code and then provide that last source symbol and/or
encoding symbols with FEC parity content. 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 "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.
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Additional FEC Object Transmission Information (FTI) (as described in
the FEC Building Block [RFC5052]) 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 pre-configuration. 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 format of the EXT_FTI consists of two parts, a
general part that contains the size of the associated transport
object and a portion that depends upon the FEC scheme being used.
The "fec_id" field in "NORM_DATA" and "NORM_INFO" messages identifies
the FEC scheme. The format of the EXT_FTI general part 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_size (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_size (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Scheme specific content ... |
EXT_FTI Header Extension General Portion Format
The header extension type "het" field value for the EXT_FTI header
extension is 64. The header extension length "hel" value depends
upon the format of the FTI for encoding type identified by the
"fec_id" field.
The 48-bit "object_size" field indicates the total length of the
object (in bytes) for the static object types of "NORM_OBJECT_FILE"
and "NORM_OBJECT_DATA". This information is used by receivers to
determine storage requirements and/or allocate storage for the
received object. Receivers with insufficient storage capability may
wish to forego reliable reception (i.e., not NACK for) of the
indicated object. In the case of objects of type
"NORM_OBJECT_STREAM", the "object_size" field is used by the sender
to advertise 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.
As noted, the 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.). As an example, the format
of the EXT_FTI for small block systematic codes ("fec_id" = 129) is
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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_size (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_size (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_instance_id | segment_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_max_block_len | fec_num_parity |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example: EXT_FTI Header Extension Format for 'fec_id' = 129
In this example (for "fec_id" = 129), the "hel" field value is 4.
The size of the EXT_FTI header extension may be different for other
FEC schemes.
The 48-bit "object_size" serves the purpose described previously.
The "fec_instance_id" corresponds to the "FEC Instance ID" described
in the FEC Building Block [RFC5052]. In this case, the
"fec_instance_id" is 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 RFC 5052.
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.
Typically, FEC parity symbol segments will be of this size.
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 [RFC5052]. 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
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as they are defined.
The payload portion of "NORM_DATA" messages includes source data or
FEC encoded application content. The content of this payload depends
upon the FEC scheme being employed, and support for streaming using
the "NORM_OBJECT_STREAM" type, when applicable, necessitates some
additional content in the payload.
The "payload_len", "payload_msg_start", and "payload_offset" fields
are present ONLY for transport objects of type "NORM_OBJECT_STREAM".
These 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 REQUIRED block partitioning algorithm
described in the FEC Building Block [RFC5052]. When systematic FEC
codes (e.g., "fec_id" = 129) are used, the "payload_len",
"payload_msg_start", and "payload_offset" fields contain actual
payload_data length, message start index (or stream control code),
and byte offset values for the associated application stream data
segment (the remainder of the "payload_data" field content) for those
"NORM_DATA" messages containing source data symbols. In "NORM_DATA"
messages that contain FEC parity content, these fields do not contain
values that can be directly interpreted, but instead are values
computed from FEC encoding the "payload_len", "payload_msg_start",
and "payload_offset" fields for the source data segments of the
corresponding coding block. The actual "payload_msg_start",
"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 contribute to the value of the "NORM_DATA" "hdr_len"
field. These fields are present only when the "flags" portion of the
"NORM_DATA" message indicate the transport object is of type
"NORM_OBJECT_STREAM".
The "payload_len" value, when non-zero, indicates the length (in
bytes) of the source content contained in the associated
"payload_data" field. However, when the "payload_len" value is equal
to ZERO, this indicates that the "payload_msg_start" field should be
alternatively interpreted as a "stream_control_code". The only
"stream_control_code" value defined is "NORM_STREAM_END = 0". The
"NORM_STREAM_END" code indicates that the sender is terminating
transmission of stream content at the corresponding position in the
stream and the receiver should not expect content (or NACK for any
content) following that position in the stream. It is expected that
additional specifications may extend the functionality of the NORM
stream transport mode by defining additional stream control codes.
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These control codes are delivered to the recipient application
reliably, in-order with respect to the streamed application data
content.
The "payload_msg_start" field serves one of two exclusive purposes.
When the "payload_len" value is non-zero, the "payload_msg_start"
field, when also set to a non-zero value, indicates that the
associated "payload_data" content contains an application-defined
message boundary (start-of-message). When such a message boundary is
indicated, the first byte of an application-defined message, with
respect to the "payload_data" field, will be found at an offset of
"payload_msg_start - 1" bytes. Thus, if a "NORM_DATA" payload for a
"NORM_OBJECT_STREAM" contains the start of an application message at
the first byte of the "payload_data" field, the value of the
"payload_msg_start" field will be '1'. NORM implementations SHOULD
provide sender stream applications with a capability to mark message
boundaries in this manner. Similarly, the NORM receiver
implementation SHOULD enable the application to recover such message
boundary information. This enables NORM receivers to "synchronize"
reliable reception of transmitted message stream content in a
meaningful way (i.e., meaningful to the application) at any time,
whether joining a session already in progress, or departing the
session and returning. Note that if the value of the
"payload_msg_start" field is ZERO, no message boundary is present.
The "payload_msg_start" value will always be less than or equal to
the "payload_len" value except for the special case of "payload_len =
0", that indicates the "payload_msg_start" field should instead be
interpreted as a "stream_control_code"
The "payload_offset" field indicates the relative byte position (from
the sender stream transmission start) of the source content contained
in the "payload_data" field. Note that for long-lived streams, the
"payload_offset" field may 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,
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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 block partitioning
algorithm described in the FEC Building Block [RFC5052]. For
"NORM_OBJECT_STREAM" objects, the length and offset is obtained from
the segment's corresponding embedded "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 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.
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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.
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.
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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:
+-------------------------+--------+--------------------------------+
| Command | Flavor | 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. |
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| "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
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.
A default value of "NORM_ROBUST_FACTOR" equal to 20 is RECOMMENDED.
If a "NORM_NACK" message interrupts the flush process, the sender
SHALL re-initiate the flush process after any resulting repair
transmissions are completed.
Note that receivers also employ a timeout mechanism to self-initiate
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NACKing (if there are outstanding repair needs) when no messages of
any type are received from a sender. This inactivity timeout is
related to the "NORM_CMD(FLUSH)" and "NORM_ROBUST_FACTOR" and is
specified in Section 5.3. Receivers SHALL self-initiate the NACK
repair process when the inactivity timeout has expired for a specific
sender and the receiver has pending repairs needed from that sender.
With a sufficiently large "NORM_ROBUST_FACTOR" value, data content is
delivered with a high assurance of reliability. The penalty of a
large "NORM_ROBUST_FACTOR" value is the potential transmission of
excess "NORM_CMD(FLUSH)" messages and a longer inactivity timeout for
receivers to self-initiate a 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 for the concurrent coding block and will be
limited to explicitly repairing the stream with source 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 messages 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.
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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
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. 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
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NACK content for the applicable "source_block_number" which does not
include any requests for parity-based repair. This allows NORM
sender applications to "flush" an ongoing stream of transmission when
needed, even if in the middle of an FEC block. Once the sender
resumes stream transmission and passes the end of the pending coding
block, subsequent NACKs from receivers SHALL request parity-based
repair as usual. Note that the use of a systematic FEC code is
assumed here. It should also be noted that a sender has the option
of arbitrarily shortening a given code block when such an application
"flush" occurs. In this case, the receiver will request explicit
repair, but the sender MAY provide FEC-based repair (parity segments)
in response. These parity segments MUST contain the corrected
"source_block_len" for the shortened block and that
"source_block_len" associated with segments containing parity content
SHALL override the previously advertised "source_block_len".
Similarly, the "source_block_len" associated with the highest ordinal
"encoding_symbol_id" shall take precedence over prior symbols when a
difference (e.g., due to code shortening at the sender) occurs.
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.
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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.
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
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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:
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 = 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 "NORM_CMD(SQUELCH)" transmission. The
value of the "hdr_len" field when no extensions are present is 4 plus
the size of the "fec_payload_id" field that is dependent upon the FEC
scheme identified by the "fec_id" field.
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.
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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, 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 ordinally 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 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 RFC 4654 is fully
specified 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 network where
resources are explicitly dedicated to the NORM session and therefore
congestion control operation is disabled, the "NORM_CMD(CC)" message
is then used soley for GRTT measurement and may optionally be sent
less frequently than with congestion control operation.
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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 value of the "hdr_len" field when no
header extensions are present is 6.
The "reserved" field is for potential future use and MUST be set to
ZERO in this version of the NORM protocol and its baseline NORM-CC
congestion control scheme. It may be possible that alternative
congestion control schemes may use the "NORM_CMD(CC)" message defined
here and leverage the "reserved" field for scheme-specific purposes.
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 RFC 4654. 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")
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and 32-bits with the time in microseconds ("send_time_usec") since
some reference time the source maintains (usually 00:00:00, 1 January
1970). The byte ordering of the fields is "Big Endian" network
order. Receivers use this timestamp adjusted by the amount of delay
from 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 integer exponent (E) information in the least significant
portion. The 12-bit mantissa portion of the field is scaled such
that a base 10 mantissa (M) floating point value of 0.0 corresponds
to 0 and a value of 10.0 corresponds to 4096 in the upper 12 bits of
the 16-bit "send_rate" field. Thus:
send_rate = (((int)(M * 4096.0 / 10.0 + 0.5)) << 4) | E;
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 (E) while the upper 12 bits
contain a value of 0x51f (M) 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) * (10/4096) * power(10, (send_rate & x000f))
Note the maximum transmission rate that can be represented by this
scheme is approximately 9.99e+15 bytes per second.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "cc_node_id" is the NormNodeId of the receiver which the item
represents.
The "cc_flags" field contains flags indicating the congestion control
status of the indicated receiver. The following flags are defined:
+----------------------+-------+------------------------------------+
| Flag | Value | Purpose |
+----------------------+-------+------------------------------------+
| "NORM_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). |
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| "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 Multicast NACK
Building Block [RFC5401].
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
relaying 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 [NormFeedback].
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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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) [PgmccPaper]) is used
within a NORM implementation, an additional header extension MAY need
to be defined encapsulate any required feedback content. The NORM-CC
Feedback Header Extension format 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 3 | hel = 3 | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_rate | cc_reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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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. 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" = floor(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
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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
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 Format
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The NORM common message header and standard "NORM_CMD" fields serve
their usual purposes. The value of the "hdr_len" field for
"NORM_CMD(ACK_REQ)" messages with no header extension present is 4.
The "ack_type" field indicates the type of acknowledgment being
requested and thus implies rules for how the receiver will treat this
request. The following "ack_type" values are defined and are also
used in "NORM_ACK" messages described later:
+------------------------+--------+---------------------------------+
| ACK Type | Value | Purpose |
+------------------------+--------+---------------------------------+
| "NORM_ACK_CC" | 1 | Used to identify "NORM_ACK" |
| | | messages sent in response to |
| | | "NORM_CMD(CC)" messages. |
| "NORM_ACK_FLUSH" | 2 | Used to identify "NORM_ACK" |
| | | messages sent in response to |
| | | "NORM_CMD(FLUSH)" messages. |
| "NORM_ACK_RESERVED" | 3-15 | Reserved for possible future |
| | | NORM protocol use. |
| "NORM_ACK_APPLICATION" | 16-255 | Used at application's |
| | | discretion. |
+------------------------+--------+---------------------------------+
The "NORM_ACK_CC" value is provided for use only in "NORM_ACKs"
generated in response to the "NORM_CMD(CC)" messages used in
congestion control operation. Similarly, the "NORM_ACK_FLUSH" is
provided for use only in "NORM_ACKs" generated in response to
applicable "NORM_CMD(FLUSH)" messages. "NORM_CMD"(ACK_REQ) messages
with "ack_type" of "NORM_ACK_CC" or "NORM_ACK_FLUSH" SHALL NOT be
generated by the sender.
The "NORM_ACK_RESERVED" range of "ack_type" values is provided for
possible future NORM protocol use.
The "NORM_ACK_APPLICATION" range of "ack_type" values is provided so
that NORM applications may implement application-defined, positively-
acknowledged commands that are able to leverage internal transmission
and round-trip timing information available to the NORM protocol
implementation.
The "ack_id" provides a sequenced identifier for the given
"NORM_CMD(ACK_REQ)" message. This "ack_id" is returned in "NORM_ACK"
messages generated by the receivers so that the sender may associate
the response with its corresponding request.
The "reserved" field is reserved for possible future protocol use and
SHALL be set to ZERO by senders and ignored by receivers.
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The "acking_node_list" field contains the NormNodeIds of the current
NORM receivers that are desired to provide positive acknowledge
("NORM_ACK") to this request. The packet payload length implies the
length of the "acking_node_list" and its length is limited to the
sender NormSegmentSize. The individual NormNodeId items are listed
in network (Big Endian) byte order. If a receiver's NormNodeId is
included in the "acking_node_list", it SHALL schedule transmission of
a "NORM_ACK" message as described in Section 5.5.3.
4.2.3.7. NORM_CMD(APPLICATION) Message
This command allows the NORM application to robustly transmit
application-defined commands. The command message preempts any
ongoing data transmission and is repeated up to "NORM_ROBUST_FACTOR"
times at a rate of once per "2*GRTT". This rate of repetition allows
the application to observe any response (if that is the application's
purpose for the command) before it is repeated. Possible responses
may include initiation of data transmission, other
"NORM_CMD(APPLICATION)" messages, or even application-defined,
positively-acknowledge commands from other NormSession participants.
The transmission of these commands will preempt data transmission
when they are scheduled and may be multiplexed with ongoing data
transmission. This type of robustly transmitted command allows NORM
applications to define a complete set of session control mechanisms
with less state than the transfer of FEC encoded reliable content
requires while taking advantage of NORM transmission and round-trip
timing information.
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 = 7 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application-Defined Content |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_CMD(APPLICATION) Message Format
The NORM common message header and "NORM_CMD" fields are interpreted
as previously described. The value of the "NORM_CMD(APPLICATION)"
"hdr_len" field when no header extensions are present is 4.
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The "Application-Defined Content" area contains information in a
format at the discretion of the application. The size of this
payload SHALL be limited to a maximum of the sender's NormSegmentSize
setting. Upon reception, the NORM protocol implementation SHALL
deliver the content to the receiver application. Note that any
detection of duplicate reception of a "NORM_CMD(APPLICATION)" message
is the responsibility of the application.
4.3. Receiver Messages
The NORM message types generated by participating receivers consist
of the "NORM_NACK" and "NORM_ACK" message types. "NORM_NACK"
messages are sent to request repair of missing data content from
sender transmission and "NORM_ACK" messages are generated in response
to certain sender commands including "NORM_CMD(CC)" and
"NORM_CMD(ACK_REQ)".
4.3.1. NORM_NACK Message
The principal purpose of "NORM_NACK" messages is for receivers to
request repair of sender content via selective, negative
acknowledgment upon detection of incomplete data. "NORM_NACK"
messages will be transmitted according to the rules of "NORM_NACK"
generation and suppression described in Section 5.3. "NORM_NACK"
messages also contain additional fields to provide feedback to the
sender(s) for purposes of round-trip timing collection and congestion
control.
The payload of "NORM_NACK" messages contains one or more repair
requests for different objects or portions of those objects. The
"NORM_NACK" message format is as follows:
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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=4| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nack_payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_NACK Message Format
The NORM common message header fields serve their usual purposes.
The value of the "hdr_len" field for "NORM_NACK" messages without
header extensions present is 6.
The "server_id" field identifies the NORM sender to which the
"NORM_NACK" message is destined.
The "instance_id" field contains the current session identifier given
by the sender identified by the "server_id" field in its sender
messages. The sender SHOULD ignore feedback messages which contain
an invalid "instance_id" value.
The "grtt_response" fields contain an adjusted version of the
timestamp from the most recently received "NORM_CMD(CC)" message for
the indicated NORM sender. The format of the "grtt_response" is the
same as the "send_time" field of the "NORM_CMD(CC)". The
"grtt_response" value is relative to the "send_time" the source
provided with a corresponding "NORM_CMD(CC)" command. The receiver
adjusts the source's "NORM_CMD(CC)" "send_time" timestamp by adding
the time delta from when the receiver received the "NORM_CMD(CC)" to
when the "NORM_NACK" is transmitted in response to calculate the
value in the "grtt_response" field. This is the
"receive_to_response_delta" value used in the following formula:
grtt_response = NORM_CMD(CC) send_time + receive_to_response_delta
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The receiver SHALL set the "grtt_response" to a ZERO value, to
indicate that it has not yet received a "NORM_CMD(CC)" message from
the indicated sender and that the sender should ignore the
"grtt_response" in this message.
For NORM-CC operation, the NORM-CC Feedback Header Extension, as
described in the "NORM_CMD(REPAIR_ADV}" message description, is added
to "NORM_NACK" messages to provide feedback on the receivers current
state with respect to congestion control operation. Note that
alternative header extensions for congestion control feedback may be
defined for alternative congestion control schemes for NORM use in
the future.
The "reserved" field is for potential future NORM use and SHALL be
set to ZERO for this version of the protocol.
The "nack_payload" of the "NORM_NACK" message specifies the repair
needs of the receiver with respect to the NORM sender indicated by
the "server_id" field. The receiver constructs repair requests based
on the "NORM_DATA" and/or "NORM_INFO" segments it requires from the
sender in order to complete reliable reception up to the sender's
transmission position at the moment the receiver initiates the NACK
Procedure as described in Section 5.3. A single NORM Repair Request
consists of a list of items, ranges, and/or FEC coding block erasure
counts for needed "NORM_DATA" and/or "NORM_INFO" content. Multiple
repair requests may be concatenated within the "nack_payload" field
of a "NORM_NACK" message. Note that a single NORM Repair Request can
possibly include multiple "items", "ranges", or "erasure_counts". In
turn, the "nack_payload" field MAY contain multiple repair requests.
A single NORM Repair Request has 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form | flags | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_request_items |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Repair Request Format
The "form" field indicates the type of repair request items given in
the "repair_request_items" list. Possible values for the "form"
field include:
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+----------------------+-------+
| Form | Value |
+----------------------+-------+
| "NORM_NACK_ITEMS" | 1 |
| "NORM_NACK_RANGES" | 2 |
| "NORM_NACK_ERASURES" | 3 |
+----------------------+-------+
A "form" value of "NORM_NACK_ITEMS" indicates each repair request
item in the "repair_request_items" list is to be treated as an
individual request. A value of "NORM_NACK_RANGES" indicates that the
"repair_request_items" list consists of pairs of repair request items
that correspond to inclusive ranges of repair needs. And the
"NORM_NACK_ERASURES" "form" indicates that the repair request items
are to be treated individually and that the "encoding_symbol_id"
portion of the "fec_payload_id" field of the repair request item (see
below) is to be interpreted as an erasure count for the FEC coding
block identified by the repair request item's "source_block_number".
The "flags" field is currently used to indicate the level of data
content for which the repair request items apply (i.e., an individual
segment, entire FEC coding block, or entire transport object).
Possible flag values include:
+---------------------+-------+-------------------------------------+
| Flag | Value | Purpose |
+---------------------+-------+-------------------------------------+
| "NORM_NACK_SEGMENT" | 0x01 | Indicates the listed segment(s) or |
| | | range of segments are required as |
| | | repair. |
| "NORM_NACK_BLOCK" | 0x02 | Indicates the listed block(s) or |
| | | range of blocks in entirety are |
| | | required as repair. |
| "NORM_NACK_INFO" | 0x04 | Indicates that "NORM_INFO" is |
| | | required as repair for the listed |
| | | object(s). |
| "NORM_NACK_OBJECT" | 0x08 | Indicates the listed object(s) or |
| | | range of objects in entirety are |
| | | required as repair. |
+---------------------+-------+-------------------------------------+
When the "NORM_NACK_SEGMENT" flag is set, the "object_transport_id"
and "fec_payload_id" fields are used to determine which sets or
ranges of individual "NORM_DATA" segments are needed to repair
content at the receiver. When the "NORM_NACK_BLOCK" flag is set,
this indicates the receiver is completely missing the indicated
coding block(s) and requires transmissions sufficient to repair the
indicated block(s) in their entirety. When the "NORM_NACK_INFO" flag
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is set, this indicates the receiver is missing the "NORM_INFO"
segment for the indicated "object_transport_id". Note the
"NORM_NACK_INFO" may be set in combination with the "NORM_NACK_BLOCK"
or "NORM_NACK_SEGMENT" flags, or may be set alone. When the
"NORM_NACK_OBJECT" flag is set, this indicates the receiver is
missing the entire NormTransportObject referenced by the
"object_transport_id". This also implicitly requests any available
"NORM_INFO" for the NormObject, if applicable. The "fec_payload_id"
field is ignored when the flag "NORM_NACK_OBJECT" is set.
The "length" field value is the length in bytes of the
"repair_request_items" field.
The "repair_request_items" field consists of a list of individual or
range pairs of transport data unit identifiers 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id | reserved | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Repair Request Item Format
The "fec_id" indicates the FEC type and can be used to determine the
format of the "fec_payload_id" field. The "reserved" field is kept
for possible future use and SHALL be set to a ZERO value and ignored
by NORM nodes processing NACK content.
The "object_transport_id" corresponds to the NormObject for which
repair is being requested and the "fec_payload_id" identifies the
specific FEC coding block and/or segment being requested. When the
"NORM_NACK_OBJECT" flag is set, the value of the "fec_payload_id"
field is ignored. When the "NORM_NACK_BLOCK" flag is set, only the
FEC code block identifier portion of the "fec_payload_id" is to be
interpreted.
The format of the "fec_payload_id" field depends upon the "fec_id"
field value.
When the receiver's repair needs dictate that different forms (mixed
ranges and/or individual items) or types (mixed specific segments
and/or blocks or objects in entirety) are required to complete
reliable transmission, multiple NORM Repair Requests with different
"form" and or "flags" values can be concatenated within a single
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"NORM_NACK" message. Additionally, NORM receivers SHALL construct
"NORM_NACK" messages with their repair requests in ordinal order with
respect to "object_transport_id" and "fec_payload_id" values. The
"nack_payload" size SHALL NOT exceed the NormSegmentSize for the
sender to which the "NORM_NACK" is destined.
NORM_NACK Content Examples:
In these examples, a small block, systematic FEC code ("fec_id" =
129) is assumed with a user data block length of 32 segments. In
Example 1, a list of individual "NORM_NACK_ITEMS" repair requests is
given. In Example 2, a list of "NORM_NACK_RANGES" requests AND a
single "NORM_NACK_ITEMS" request are concatenated to illustrate the
possible content of a "NORM_NACK" message. Note that FEC coding
block erasure counts could also be provided in each case. However,
the erasure counts are not really necessary since the sender can
easily determine the erasure count while processing the NACK content.
However, the erasure count option may be useful for operation with
other FEC codes or for intermediate system purposes.
Example 1: "NORM_NACK" "nack_payload" for: Object 12, Coding Block 3,
Segments 2,5,and 8
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x01 | length = 36 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Example 2: "NORM_NACK" "nack_payload" for: Object 18, Coding Block 6,
Segments 5, 6, 7, 8, 9, 10; and Object 19 "NORM_INFO" and Coding
Block 1, segment 3
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 2 | flags = 0x01 | length = 24 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x05 | length = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 19 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.3.2. NORM_ACK Message
The "NORM_ACK" message is intended to be used primarily as part of
NORM congestion control operation and round-trip timing measurement.
As mentioned in the "NORM_CMD(ACK_REQ)" message description, the
acknowledgment type "NORM_ACK_CC" is provided for this purpose. The
generation of "NORM_ACK(CC)" messages for round-trip timing
estimation and congestion-control operation is described in
Section 5.5.1 and Section 5.5.2, respectively. However, some
multicast applications may benefit from some limited form of positive
acknowledgment for certain functions. A simple, scalable positive
acknowledgment scheme is defined in Section 5.5.3 that can be
leveraged by protocol implementations when appropriate. The
"NORM_CMD(FLUSH)" may be used for OPTIONAL collection of positive
acknowledgment of reliable reception to a certain "watermark"
transmission point from specific receivers using this mechanism. The
"NORM_ACK" type "NORM_ACK_FLUSH" is provided for this purpose and the
format of the "nack_payload" for this acknowledgment type is given
below. Beyond that, a range of application-defined "ack_type" values
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is provided for use at the NORM application's discretion.
Implementations making use of application-defined positive
acknowledgments may also make use the "nack_payload" as needed,
observing the constraint that the "nack_payload" field size be
limited to a maximum of the NormSegmentSize for the sender to which
the "NORM_ACK" is destined.
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=5| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | ack_type | ack_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ack_payload (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM_ACK Message Format
The NORM common message header fields serve their usual purposes.
The value of the "hdr_len" field when no header extensions are
present is 6.
The "server_id", "instance_id", and "grtt_response" fields serve the
same purpose as the corresponding fields in "NORM_NACK" messages.
And header extensions may be applied to support congestion control
feedback or other functions in the same manner.
The "ack_type" field indicates the nature of the "NORM_ACK" message.
This directly corresponds to the "ack_type" field of the
"NORM_CMD(ACK_REQ)" message to which this acknowledgment applies.
The "ack_id" field serves as a sequence number so that the sender can
verify that a "NORM_ACK" message received actually applies to a
current acknowledgment request. The "ack_id" field is not used in
the case of the "NORM_ACK_CC" and "NORM_ACK_FLUSH" acknowledgment
types.
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The "ack_payload" format is a function of the "ack_type". The
"NORM_ACK_CC" message has no attached content. Only the "NORM_ACK"
header applies. In the case of "NORM_ACK_FLUSH", a specific
"ack_payload" format is defined:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id | reserved | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "object_transport_id" and "fec_payload_id" are used by the
receiver to acknowledge applicable "NORM_CMD(FLUSH)" messages
transmitted by the sender identified by the "server_id" field.
The "ack_payload" of "NORM_ACK" messages for application-defined
"ack_type" values is specific to the application but is limited in
size to a maximum the NormSegmentSize of the sender referenced by the
"server_id".
4.4. General Purpose Messages
Some additional message formats are defined for general purpose in
NORM multicast sessions whether the participant is acting as a sender
and/or receiver within the group.
4.4.1. NORM_REPORT Message
This is an OPTIONAL message generated by NORM participants. This
message may be used for periodic performance reports from receivers
in experimental NORM implementations. The format of this message is
currently undefined. Experimental NORM implementations may define
"NORM_REPORT" formats as needed for test purposes. These report
messages SHOULD be disabled for interoperability testing between
different compliant NORM implementations.
5. Detailed Protocol Operation
This section describes the detailed interactions of senders and
receivers participating in a NORM session. A simple synopsis of
protocol operation is given here:
1. The sender periodically transmits "NORM_CMD(CC)" messages as
needed to initialize and collect round-trip timing and congestion
control feedback from the receiver set.
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2. The sender transmits an ordinal set of NormObjects segmented in
the form of "NORM_DATA" messages labeled with NormTransportIds
and logically identified with FEC encoding block numbers and
symbol identifiers. "NORM_INFO" messages may optionally precede
the transmission of data content for NORM transport objects.
3. As receivers detect missing content from the sender, they
initiate repair requests with "NORM_NACK" messages. Note the
receivers track the sender's most recent objectId::fecPayloadId
transmit position and NACK ONLY for content ordinally prior to
that transmit position. The receivers schedule random backoff
timeouts before generating "NORM_NACK" messages and wait an
appropriate amount of time before repeating the "NORM_NACK" if
their repair request is not satisfied.
4. The sender aggregates repair requests from the receivers and
logically "rewinds" its transmit position to send appropriate
repair messages. The sender sends repairs for the earliest
ordinal transmit position first and maintains this ordinal repair
transmission sequence. FEC parity content not previously
transmitted for the applicable FEC coding block is used for
repair transmissions to the greatest extent possible. If the
sender exhausts its available FEC parity content on multiple
repair cycles for the same coding block, it resorts to an
explicit repair strategy (possibly using parity content) to
complete repairs. (The use of explicit repair is expected to be
an exception in general protocol operation, but the possibility
does exist for extreme conditions). The sender immediately
assumes transmission of new content once it has sent pending
repairs.
5. The sender transmits "NORM_CMD(FLUSH)" messages when it reaches
the end of enqueued transmit content and pending repairs.
Receivers respond to the "NORM_CMD(FLUSH)" messages with
"NORM_NACK" transmissions (following the same suppression backoff
timeout strategy as for data) if they require further repair.
6. The sender transmissions are subject to rate control limits
determined by congestion control mechanisms. In the baseline
NORM-CC operation, each sender in a NormSession maintains its own
independent congestion control state. Receivers provide
congestion control feedback in "NORM_NACK" and "NORM_ACK"
messages. "NORM_ACK" feedback for congestion control purposes is
governed using a suppression mechanism similar to that for
"NORM_NACK" messages.
While this overall concept is relatively simple, there are details to
each of these aspects that need to be addressed for successful,
efficient, robust, and scalable NORM protocol operation.
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5.1. Sender Initialization and Transmission
Upon startup, the NORM sender immediately begins sending
"NORM_CMD(CC)" messages to collect round trip timing and other
information from the potential group. If NORM-CC congestion control
operation is enabled, the NORM-CC Rate header extension MUST be
included in these messages. Congestion control operation SHALL be
observed at all times when not operating using dedicated resources,
like in the general Internet. Even if congestion control operation
is disabled at the sender, it may be desirable to use the
"NORM_CMD(CC)" messaging to collect feedback from the group using the
baseline NORM-CC feedback mechanisms. This proactive feedback
collection can be used to establish a GRTT estimate prior to data
transmission and potential NACK operation.
In some cases, applications may wish for the sender to also proceed
with data transmission immediately. In other cases, the sender may
wish to defer data transmission until it has received some feedback
or request from the receiver set indicating that receivers are indeed
present. Note, in some applications (e.g., web push), this
indication may come out-of-band with respect to the multicast session
via other means. As noted, the periodic transmission of
"NORM_CMD(CC)" messages may precede actual data transmission in order
to have an initial GRTT estimate.
With inclusion of the OPTIONAL NORM FEC Object Transmission
Information Header Extension (EXT_FTI), the NORM protocol sender
message headers can contain all information necessary to prepare
receivers for subsequent reliable reception. This includes FEC
coding parameters, the sender NormSegmentSize, and other information.
If this header extension is not used, it is presumed that receivers
have received the FEC Object Transmission Information via other
means. Additionally, applications may leverage the use of
"NORM_INFO" messages associated with the session data objects in the
session to provide application-specific context information for the
session and data being transmitted. These mechanisms allow for
operation with minimal pre-coordination among the senders and
receivers.
The NORM sender begins segmenting application-enqueued data into
"NORM_DATA" segments and transmitting it to the group. For objects
of type "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE", the segmentation
algorithm described in FEC Building Block [RFC5052] is RECOMMENDED.
For objects of type "NORM_OBJECT_STREAM", segmentation will typically
be into uniform FEC coding block sizes, with individual segment sizes
controlled by the application. In most cases, the application and
NORM implementation SHOULD strive to produce full-sized
("NormSegmentSize") segments when possible. The rate of transmission
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is controlled via congestion control mechanisms or is a fixed rate if
desired for closed network operations. The receivers participating
in the multicast group provide feedback to the sender as needed.
When the sender reaches the end of data it has enqueued for
transmission or any pending repairs, it transmits a series of
"NORM_CMD(FLUSH)" messages at a rate of one per "2*GRTT". Receivers
may respond to these "NORM_CMD(FLUSH)" messages with additional
repair requests. A protocol parameter ""NORM_ROBUST_FACTOR""
determines the number of flush messages sent. If receivers request
repair, the repair is provided and flushing occurs again at the end
of repair transmission. The sender may attach an OPTIONAL
"acking_node_list" to "NORM_CMD(FLUSH)" containing the NormNodeIds
for receivers from which it expects explicit positive acknowledgment
of reception. The "NORM_CMD(FLUSH)" message may be also used for
this optional function any time prior to the end of data enqueued for
transmission with the "NORM_CMD(FLUSH)" messages multiplexed with
ongoing data transmissions. The OPTIONAL NORM positive
acknowledgment procedure is described in Section 5.5.3.
5.1.1. Object Segmentation Algorithm
NORM senders and receivers MUST use a common algorithm for logically
segmenting transport data into FEC encoding blocks and symbols so
that appropriate NACKs can be constructed to request repair of
missing data. NORM FEC coding blocks are comprised of multi-byte
symbols (segments) that are transmitted in the payload of "NORM_DATA"
messages. Each "NORM_DATA" message will contain one or more source
or encoding symbol(s) identified by the "fec_payload_id" field and
the NormSegmentSize sender parameter defines the maximum size (in
bytes) of the "payload_data" field containing the content (a
"segment"). The FEC encoding type and associated parameters govern
the source block size (number of source symbols per coding block,
etc.). NORM senders and receivers use these FEC parameters, along
with the NormSegmentSize and transport object size to compute the
source block structure for transport objects. These parameters are
provided in the FEC Object Transmission Information for each object.
The block partitioning algorithm described in the FEC Building Block
[RFC5052] is RECOMMENDED for use to compute a source block structure
such that all source blocks are as close to being equal length as
possible. This helps avoid the performance disadvantages of "short"
FEC blocks. Note this algorithm applies only to the statically-sized
"NORM_OBJECT_DATA" and "NORM_OBJECT_FILE" transport object types
where the object size is fixed and predetermined. For
"NORM_OBJECT_STREAM" objects, the object is segmented according to
the maximum source block length given in the FEC Transmission
Information, unless the FEC Payload ID indicates an alternative size
for a given block.
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5.2. Receiver Initialization and Reception
The NORM protocol is designed such that receivers may join and leave
the group at will. However, some applications may be constrained
such that receivers need to be members of the group prior to start of
data transmission. NORM applications may use different policies to
constrain the impact of new receivers joining the group in the middle
of a session. For example, a useful implementation policy is for new
receivers joining the group to limit or avoid repair requests for
transport objects already in progress. The NORM sender
implementation may wish to impose additional constraints to limit the
ability of receivers to disrupt reliable multicast performance by
joining, leaving, and rejoining the group often. Different receiver
"join policies" may be appropriate for different applications and/or
scenarios. For general purpose operation, a default policy where
receivers are allowed to request repair only for coding blocks with a
NormTransportId and FEC coding block number greater than or equal to
the first non-repair "NORM_DATA" or "NORM_INFO" message received upon
joining the group is RECOMMENDED. For objects of type
"NORM_OBJECT_STREAM" it is RECOMMENDED that the join policy constrain
receivers to start reliable reception at the current FEC coding block
for which non-repair content is received.
For typical operation, it is expected that NORM receivers will join a
specified multicast group and/or listen on an specific port number
for sender transmissions. As the NORM receiver receives "NORM_DATA"
messages it will provide content to its application as appropriate.
5.3. Receiver NACK Procedure
When the receiver detects it is missing data from a sender's NORM
transmissions, it initiates its NACKing procedure. The NACKing
procedure SHALL be initiated ONLY at FEC coding block boundaries,
NormObject boundaries, upon receipt of a "NORM_CMD(FLUSH)" message,
or upon an "inactivity" timeout when "NORM_DATA" or "NORM_INFO"
transmissions are no longer received from a previously active sender.
The RECOMMENDED value of such an inactivity timeout is:
T_inactivity = NORM_ROBUST_FACTOR * 2 * GRTTSender
where the ""GRTTsender"" value corresponds to the GRTT estimate
advertised in the "grtt" field of NORM sender messages. A minimum
""T_inactivity"" value of 1 second is RECOMMENDED. The NORM receiver
SHOULD reset this inactivity timer and repeat NACK initiation upon
timeout for up to "NORM_ROBUST_FACTOR" times or more depending upon
the application's need for persistence by its receivers. It is also
important that receivers rescale the ""T_inactivity"" timeout as the
sender's advertised GRTT changes.
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The NACKing procedure begins with a random backoff timeout. The
duration of the backoff timeout is chosen using the "RandomBackoff"
algorithm described in the Multicast NACK Building Block [RFC5401]
using ("Ksender*GRTTsender") for the "maxTime" parameter and the
sender advertised group size ("GSIZEsender") as the "groupSize"
parameter. NORM senders provide values for "GRTTsender", "Ksender"
and "GSIZEsender" via the "grtt", "backoff", and "gsize" fields of
transmitted messages. The "GRTTsender" value is determined by the
sender based on feedback it has received from the group while the
"Ksender" and "GSIZEsender" values may determined by application
requirements and expectations or ancillary information. The backoff
factor ""Ksender"" MUST be greater than "one" to provide for
effective feedback suppression. A value of "K = 4" is RECOMMENDED
for the Any Source Multicast (ASM) model while a value of "K = 6" is
RECOMMENDED for Single Source Multicast (SSM) operation.
Thus:
T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender)
To avoid the possibility of NACK implosion in the case of sender or
network failure during SSM operation, the receiver SHALL
automatically suppress its NACK and immediately enter the "holdoff"
period described below when "T_backoff" is greater than
"(Ksender-1)*GRTTsender". Otherwise, the backoff period is entered
and the receiver MUST accumulate external pending repair state from
"NORM_NACK" messages and "NORM_CMD(REPAIR_ADV)" messages received.
At the end of the backoff time, the receiver SHALL generate a
"NORM_NACK" message only if the following conditions are met:
1. The sender's current transmit position (in terms of objectId::
fecPayloadId) exceeds the earliest repair position of the
receiver.
2. The repair state accumulated from "NORM_NACK" and
"NORM_CMD(REPAIR_ADV)" messages do not equal or supersede the
receiver's repair needs up to the sender transmission position at
the time the NACK procedure (backoff timeout) was initiated.
If these conditions are met, the receiver immediately generates a
"NORM_NACK" message when the backoff timeout expires. Otherwise, the
receiver's NACK is considered to be "suppressed" and the message is
not sent. At this time, the receiver begins a "holdoff" period
during which it constrains itself to not re-initiate the NACKing
process. The purpose of this timeout is to allow the sender worst-
case time to respond to the repair needs before the receiver requests
repair again. The value of this "holdoff" timeout ("T_rcvrHoldoff")
as described in [RFC5401] is:
T_rcvrHoldoff =(Ksender+2)*GRTTsender
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The "NORM_NACK" message contains repair request content beginning
with lowest ordinal repair position of the receiver up through the
coding block prior to the most recently heard ordinal transmission
position for the sender. If the size of the "NORM_NACK" content
exceeds the sender's NormSegmentSize, the NACK content is truncated
so that the receiver only generates a single "NORM_NACK" message per
NACK cycle for a given sender. In summary, a single NACK message is
generated containing the receiver's lowest ordinal repair needs.
For each partially-received FEC coding block requiring repair, the
receiver SHALL, on its FIRST repair attempt for the block, request
the parity portion of the FEC coding block beginning with the lowest
ordinal parity "encoding_symbol_id" (i.e., "encoding_symbol_id" =
"source_block_len") and request the number of FEC symbols
corresponding to its data segment erasure count for the block. On
subsequent repair cycles for the same coding block, the receiver
SHALL request only those repair symbols from the first set it has not
yet received up to the remaining erasure count for that applicable
coding block. Note that the sender may have provided other
different, additional parity segments for other receivers that could
also be used to satisfy the local receiver's erasure-filling needs.
In the case where the erasure count for a partially-received FEC
coding block exceeds the maximum number of parity symbols available
from the sender for the block (as indicated by the "NORM_DATA"
"fec_num_parity" field), the receiver SHALL request all available
parity segments plus the ordinally highest missing data segments
required to satisfy its total erasure needs for the block. The goal
of this strategy is for the overall receiver set to request a lowest
common denominator set of repair symbols for a given FEC coding
block. This allows the sender to construct the most efficient repair
transmission segment set and enables effective NACK suppression among
the receivers even with uncorrelated packet loss. This approach also
requires no synchronization among the receiver set in their repair
requests for the sender.
For FEC coding blocks or NormObjects missed in their entirety, the
NORM receiver constructs repair requests with "NORM_NACK_BLOCK" or
"NORM_NACK_OBJECT" flags set as appropriate. The request for
retransmission of "NORM_INFO" is accomplished by setting the
"NORM_NACK_INFO" flag in a corresponding repair request.
5.4. Sender NACK Processing and Response
The principle goal of the sender is to make forward progress in the
transmission of data its application has enqueued. However, the
sender must occasionally "rewind" its logical transmission point to
satisfy the repair needs of receivers who have NACKed. Aggregation
of multiple NACKs is used to determine an optimal repair strategy
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when a NACK event occurs. Since receivers initiate the NACK process
on coding block or object boundaries, there is some loose degree of
synchronization of the repair process even when receivers experience
uncorrelated data loss.
5.4.1. Sender Repair State Aggregation
When a sender is in its normal state of transmitting new data and
receives a NACK, it begins a procedure to accumulate NACK repair
state from "NORM_NACK" messages before beginning repair
transmissions. Note that this period of aggregating repair state
does NOT interfere with its ongoing transmission of new data.
As described in [RFC5401], the period of time during which the sender
aggregates "NORM_NACK" messages is equal to:
T_sndrAggregate = (Ksender+1)*GRTT
where ""Ksender"" is the same backoff scaling value used by the
receivers, and "GRTT" is the sender's current estimate of the group's
greatest round-trip time. Note that for NORM unicast sessions the
""T_sndrAggregate"" time can be set to ZERO since there is only one
receiver. Similarly, the ""Ksender"" value should be set to ZERO for
NORM unicast sessions to minimize repair latency.
When this period ends, the sender "rewinds" by incorporating the
accumulated repair state into its pending transmission state and
begins transmitting repair messages. After pending repair
transmissions are completed, the sender continues with new
transmissions of any enqueued data. Also, at this point in time, the
sender begins a "holdoff" timeout during which time the sender
constrains itself from initiating a new repair aggregation cycle,
even if "NORM_NACK" messages arrive. As described in [RFC5401], the
value of this sender "holdoff" period is:
T_sndrHoldoff = (1*GRTT)
If additional "NORM_NACK" messages are received during this sender
"holdoff" period, the sender will immediately incorporate these late-
arriving messages into its pending transmission state ONLY if the
NACK content is ordinally greater than the sender's current
transmission position. This "holdoff" time allows worst case time
for the sender to propagate its current transmission sequence
position to the group, thus avoiding redundant repair transmissions.
After the holdoff timeout expires, a new NACK accumulation period can
be begun (upon arrival of a NACK) in concert with the pending repair
and new data transmission. Recall that receivers are not to initiate
the NACK repair process until the sender's logical transmission
position exceeds the lowest ordinal position of their repair needs.
With the new NACK aggregation period, the sender repeats the same
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process of incorporating accumulated repair state into its
transmission plan and subsequently "rewinding" to transmit the lowest
ordinal repair data when the aggregation period expires. Again, this
is conducted in concert with ongoing new data and/or pending repair
transmissions.
5.4.2. Sender FEC Repair Transmission Strategy
The NORM sender should leverage transmission of FEC parity content
for repair to the greatest extent possible. Recall that the
receivers use a strategy to request a lowest common denominator of
explicit repair (including parity content) in the formation of their
"NORM_NACK" messages. Before falling back to explicitly satisfying
different receivers' repair needs, the sender can make use of the
general erasure-filling capability of FEC-generated parity segments.
The sender can determine the maximum erasure filling needs for
individual FEC coding blocks from the "NORM_NACK" messages received
during the repair aggregation period. Then, if the sender has a
sufficient number (less than or equal to the maximum erasure count)
of previously unsent parity segments available for the applicable
coding blocks, the sender can transmit these in lieu of the specific
packets the receiver set has requested. Only after exhausting its
supply of "fresh" (unsent) parity segments for a given coding block
should the sender resort to explicit transmission of the receiver
set's repair needs. In general, if a sufficiently powerful FEC code
is used, the need for explicit repair will be an exception, and the
fulfillment of reliable multicast can be accomplished quite
efficiently. However, the ability to resort to explicit repair
allows the protocol to be reliable under even very extreme
circumstances.
"NORM_DATA" messages sent as repair transmissions SHALL be flagged
with the "NORM_FLAG_REPAIR" flag. This allows receivers to obey any
policies that limit new receivers from joining the reliable
transmission when only repair transmissions have been received.
Additionally, the sender SHOULD additionally flag "NORM_DATA"
transmissions sent as explicit repair with the "NORM_FLAG_EXPLICIT"
flag.
Although NORM end system receivers do not make use of the
"NORM_FLAG_EXPLICIT" flag, this message transmission status could be
leveraged by intermediate systems wishing to "assist" NORM protocol
performance. If such systems are properly positioned with respect to
reciprocal reverse-path multicast routing, they need to sub-cast only
a sufficient count of non-explicit parity repairs to satisfy a
multicast routing sub-tree's erasure filling needs for a given FEC
coding block. When the sender has resorted to explicit repair, then
the intermediate systems should sub-cast all of the explicit repair
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packets to those portions of the routing tree still requiring repair
for a given coding block. Note the intermediate systems will be
required to conduct repair state accumulation for sub-routes in a
manner similar to the sender's repair state accumulation in order to
have sufficient information to perform the sub-casting.
Additionally, the intermediate systems could perform additional
"NORM_NACK" suppression/aggregation as it conducts this repair state
accumulation for NORM repair cycles. The detail of this type of
operation are beyond the scope of this document, but this information
is provided for possible future consideration.
5.4.3. Sender NORM_CMD(SQUELCH) Generation
If the sender receives a "NORM_NACK" message for repair of data it is
no longer supporting, the sender generates a "NORM_CMD(SQUELCH)"
message to advertise its repair window and squelch any receivers from
additional NACKing of invalid data. The transmission rate of
"NORM_CMD(SQUELCH)" messages is limited to once per "2*GRTT". The
"invalid_object_list" (if applicable) of the "NORM_CMD(SQUELCH)"
message SHALL begin with the lowest "object_transport_id" from the
invalid "NORM_NACK" messages received since the last
"NORM_CMD(SQUELCH)" transmission. Lower ordinal invalid
"object_transport_ids" should be included only while the
"NORM_CMD(SQUELCH)" payload is less than the sender's NormSegmentSize
parameter.
5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation
When a NORM sender receives "NORM_NACK" messages from receivers via
unicast transmission, it uses "NORM_CMD(REPAIR_ADV)" messages to
advertise its accumulated repair state to the receiver set since the
receiver set is not directly sharing their repair needs via multicast
communication. A NORM sender implementation MAY use a separate port
number from the NormSession port number as the source port for its
transmissions. Thus NORM receivers can direct any unicast feedback
messages to this sender port number that is distinct from the NORM
session (or destination) port number. Then, the NORM sender
implementation can discriminate unicast feedback messages from
multicast feedback messages when there is a mix of multicast and
unicast feedback receivers. The "NORM_CMD(REPAIR_ADV)" message is
multicast to the receiver set by the sender. The payload portion of
this message has content in the same format as the "NORM_NACK"
receiver message payload. Receivers are then able to perform
feedback suppression in the same manner as with "NORM_NACK" messages
directly received from other receivers. Note the sender does not
merely retransmit NACK content it receives, but instead transmits a
representation of its aggregated repair state. The transmission of
"NORM_CMD(REPAIR_ADV)" messages are subject to the sender transmit
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rate limit and NormSegmentSize limitation. When the
"NORM_CMD(REPAIR_ADV)" message is of maximum size, receivers SHALL
consider the maximum ordinal transmission position value embedded in
the message as the senders current transmission position and
implicitly suppress requests for ordinally higher repair. For
congestion control operation, the sender may also need to provide
information so that dynamic congestion control feedback can be
suppressed as needed among receivers. This document specifies the
NORM-CC Feedback Header Extension that is applied for baseline
NORM-CC operation. If other congestion control mechanisms are used
within a NORM implementation, other header extensions may be defined.
Whatever content format is used for this purpose should ensure that
maximum possible suppression state is conveyed to the receiver set.
5.5. Additional Protocol Mechanisms
In addition to the principal function of data content transmission
and repair, there are some other protocol mechanisms that help NORM
to adapt to network conditions and play fairly with other coexistent
protocols.
5.5.1. Greatest Round-trip Time Collection
For NORM receivers to appropriately scale backoff timeouts and the
senders to use proper corresponding timeouts, the participants must
agree on a common timeout basis. Each NORM sender monitors the
round-trip time of active receivers and determines the group greatest
round-trip time (GRTT). The sender advertises this GRTT estimate in
every message it transmits so that receivers have this value
available for scaling their timers. To measure the current GRTT, the
sender periodically sends "NORM_CMD(CC)" messages that contain a
locally generated timestamp. Receivers are expected to record this
timestamp along with the time the "NORM_CMD(CC)" message is received.
Then, when the receivers generate feedback messages to the sender, an
adjusted version of the sender timestamp is embedded in the feedback
message ("NORM_NACK" or "NORM_ACK"). The adjustment adds the amount
of time the receiver held the timestamp before generating its
response. Upon receipt of this adjusted timestamp, the sender is
able to calculate the round-trip time to that receiver.
The round-trip time for each receiver is fed into an algorithm that
weights and smoothes the values for a conservative estimate of the
GRTT. The algorithm and methodology are described in the Multicast
NACK Building Block [RFC5401] in the section entitled "One-to-Many
Sender GRTT Measurement". A conservative estimate helps guarantee
feedback suppression at a small cost in overall protocol repair
delay. The sender's current estimate of GRTT is advertised in the
"grtt" field found in all NORM sender messages. The advertised GRTT
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is also limited to a minimum of the nominal inter-packet transmission
time given the sender's current transmission rate and system clock
granularity. The reason for this additional limit is to keep the
receiver somewhat event-driven by making sure the sender has had
adequate time to generate any response to repair requests from
receivers given transmit rate limitations due to congestion control
or configuration.
When the NORM-CC Rate header extension is present in "NORM_CMD(CC)"
messages, the receivers respond to "NORM_CMD(CC)" messages as
described in Section 5.5.2, "NORM Congestion Control Operation". The
"NORM_CMD(CC)" messages are periodically generated by the sender as
described for congestion control operation. This provides for
proactive, but controlled, feedback from the group in the form of
"NORM_ACK" messages. This provides for GRTT feedback even if no
"NORM_NACK" messages are being sent. If operating without congestion
control in a closed network, the "NORM_CMD(CC)" messages may be sent
periodically without the NORM-CC Rate header extension. In this
case, receivers will only provide GRTT measurement feedback when
"NORM_NACK" messages are generated since no "NORM_ACK" messages are
generated. In this case, the "NORM_CMD(CC)" messages may be sent
less frequently, perhaps as little as once per minute, to conserve
network capacity. Note that the NORM-CC Rate header extension may
also be used to proactively solicit RTT feedback from the receiver
group per congestion control operation even though the sender may not
be conducting congestion control rate adjustment. NORM operation
without congestion control should be considered only in closed
networks.
5.5.2. NORM Congestion Control Operation
This section describes baseline congestion control operation for the
NORM protocol (NORM-CC). The supporting NORM message formats and
approach described here are an adaptation of the equation-based TCP-
Friendly Multicast Congestion Control (TFMCC) approach[RFC4654].
This congestion control scheme is REQUIRED for operation within the
general Internet unless the NORM implementation is adapted to use
another IETF-sanctioned reliable multicast congestion control
mechanism. With this TFMCC-based approach, the transmissions of NORM
senders are controlled in a rate-based manner as opposed to window-
based congestion control algorithms as in TCP. However, it is
possible that the NORM protocol message set may alternatively be used
to support a window-based multicast congestion control scheme such as
PGMCC. The details of that alternative may be described separately
or in a future revision of this document. In either case (rate-based
TFMCC or window-based PGMCC), successful control of sender
transmission depends upon collection of sender-to-receiver packet
loss estimates and RTTs to identify the congestion control bottleneck
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path(s) within the multicast topology and adjust the sender rate
accordingly. The receiver with loss and RTT estimates that
correspond to the lowest resulting calculated transmission rate is
identified as the "current limiting receiver" (CLR). In the case of
a tie (where candidate CLRs are within 10% of the same calculated
rate), the receiver with the largest RTT value SHOULD be designated
as the CLR.
As described in [TcpModel], a steady-state sender transmission rate,
to be "friendly" with competing TCP flows can be calculated as:
S
Rsender = ----------------------------------------------------------
tRTT*(sqrt((2/3)*p) + 12*sqrt((3/8)*p) * p * (1 + 32*(p^2)))
where
"S" = nominal transmitted packet size. (In NORM, the "nominal"
packet size can be determined by the sender as an exponentially
weighted moving average (EWMA) of transmitted packet sizes to account
for variable message sizes).
"tRTT" = RTT estimate of the current "current limiting receiver"
(CLR).
"p" = loss event fraction of the CLR.
To support congestion control feedback collection and operation, the
NORM sender periodically transmits "NORM_CMD(CC)" command messages.
"NORM_CMD(CC)" messages are multiplexed with NORM data and repair
transmissions and serve several purposes:
1. Stimulate explicit feedback from the general receiver set to
collect congestion control information.
2. Communicate state to the receiver set on the sender's current
congestion control status including details of the CLR.
3. Initiate rapid (immediate) feedback from the CLR in order to
closely track the dynamics of congestion control for that current
worst path in the group multicast topology.
The format of the "NORM_CMD(CC)" message is described in
Section 4.2.3 of this document. The "NORM_CMD(CC)" message contains
information to allow measurement of RTTs, to inform the group of the
congestion control CLR, and to provide feedback of individual RTT
measurements to the receivers in the group. The "NORM_CMD(CC)" also
provides for exciting feedback from OPTIONAL "potential limiting
receiver" (PLR) nodes that may be determined administratively or
possibly algorithmically based on congestion control feedback. PLR
nodes are receivers that have been identified to have potential for
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(perhaps soon) becoming the CLR and thus immediate, up-to-date
feedback is beneficial for congestion control performance. The PLR
list may be populated with a small number of receivers the sender
identifies as approaching the CLR loss and delay conditions based on
feedback from the group.
5.5.2.1. NORM_CMD(CC) Transmission
The "NORM_CMD(CC)" message is transmitted periodically by the sender
along with its normal data transmission. Note that the repeated
transmission of "NORM_CMD(CC)" messages may be initiated some time
before transmission of user data content at session startup. This
may be done to collect some estimation of the current state of the
multicast topology with respect to group and individual RTT and
congestion control state.
A "NORM_CMD(CC)" message is immediately transmitted at sender
startup. The interval of subsequent "NORM_CMD(CC)" message
transmission is determined as follows:
1. By default, the interval is set according to the current sender
GRTT estimate. A startup GRTT of 0.5 seconds is recommended when
no feedback has yet been received from the group.
2. Until a CLR has been identified (based on previous receiver
feedback) or when no data transmission is pending, the
"NORM_CMD(CC)" interval is doubled up from its current interval
to a maximum of once per 30 seconds. This results in a low duty
cycle for "NORM_CMD(CC)" probing when no CLR is identified or
there is no pending data to transmit.
3. When a CLR has been identified (based on receiver feedback) and
data transmission is pending, the probing interval is set to the
RTT between the sender and the CLR ("RTT_clr").
4. Additionally, when the data transmission rate is low with respect
to the "RTT_clr" interval used for probing, the implementation
should ensure that no more than one "NORM_CMD(CC)" message is
sent per "NORM_DATA" message when there is data pending
transmission. This ensures that the transmission of this control
message is not done to the exclusion of user data transmission.
The "NORM_CMD(CC)" "cc_sequence" field is incremented with each
transmission of a "NORM_CMD(CC)" command. The greatest "cc_sequence"
recently received by receivers is included in their feedback to the
sender. This allows the sender to determine the age of feedback to
assist in congestion avoidance.
The NORM-CC Rate Header Extension is applied to the "NORM_CMD(CC)"
message and the sender advertises its current transmission rate in
the "send_rate" field. The rate information is used by receivers to
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initialize loss estimation during congestion control startup or
restart.
The "cc_node_list" contains a list of entries identifying receivers
and their current congestion control state (status "flags", "rtt" and
"loss" estimates). The list may be empty if the sender has not yet
received any feedback from the group. If the sender has received
feedback, the list will minimally contain an entry identifying the
CLR. A "NORM_FLAG_CC_CLR" flag value is provided for the "cc_flags"
field to identify the CLR entry. It is RECOMMENDED that the CLR
entry be the first in the list for implementation efficiency.
Additional entries in the list are used to provide sender-measured
individual RTT estimates to receivers in the group. The number of
additional entries in this list is dependent upon the percentage of
control traffic the sender application is willing to send with
respect to user data message transmissions. More entries in the list
may allow the sender to be more responsive to congestion control
dynamics. The length of the list may be dynamically determined
according to the current transmission rate and scheduling of
"NORM_CMD(CC)" messages. The maximum length of the list corresponds
to the sender's NormSegmentSize parameter for the session. The
inclusion of additional entries in the list based on receiver
feedback are prioritized with following rules:
1. Receivers that have not yet been provided a RTT measurement get
first priority. Of these, those with the greatest loss fraction
receive precedence for list inclusion.
2. Secondly, receivers that have previously been provided a RTT
measurement are included with receivers yielding the lowest
calculated congestion rate getting precedence.
There are "cc_flag" values in addition to "NORM_FLAG_CC_CLR" that are
used for other congestion control functions. The "NORM_FLAG_CC_PLR"
flag value is used to mark additional receivers from that the sender
would like to have immediate, non-suppressed feedback. These may be
receivers that the sender algorithmically identified as potential
future CLRs or that have been pre-configured as potential congestion
control points in the network. The "NORM_FLAG_CC_RTT" indicates the
validity of the "cc_rtt" field for the associated receiver node.
Normally, this flag will be set since the receivers in the list will
typically be receivers from which the sender has received feedback.
However, in the case that the NORM sender has been pre-configured
with a set of PLR nodes, feedback from those receivers may not yet
have been collected and thus the "cc_rtt" field does not contain a
valid value when this flag is not set. Similarly, a value of ZERO
for the "cc_rate" field here should be treated as an invalid value
and be ignored for the purposes of feedback suppression, etc.
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5.5.2.2. NORM_CMD(CC) Feedback Response
Receivers explicitly respond to "NORM_CMD(CC)" messages in the form
of a "NORM_ACK(RTT)" message. The goal of the congestion control
feedback is to determine the receivers with the lowest congestion
control rates. Receivers that are marked as CLR or PLR nodes in the
"NORM_CMD(CC)" "cc_node_list" immediately provide feedback in the
form of a "NORM_ACK" to this message. When a "NORM_CMD(CC)" is
received, non-CLR or non-PLR nodes initiate random feedback backoff
timeouts similar to that used when the receiver initiates a repair
cycle (see Section 5.3) in response to detection of data loss. The
backoff timeout for the congestion control response is generated as
follows:
T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender)
The ""RandomBackoff()"" algorithm provides a truncated exponentially
distributed random number and is described in the Multicast NACK
Building Block [RFC5401]. The same backoff factor "K = Ksender" MAY
be used as with "NORM_NACK" suppression. However, in cases where the
application purposefully specifies a very small "Ksender" backoff
factor to minimize the NACK repair process latency (trading off group
size scalability), it is RECOMMENDED that a larger backoff factor for
congestion control feedback is maintained, since there may often be a
larger volume of congestion control feedback than NACKs in many cases
and some congestion control feedback latency may be tolerable where
reliable delivery latency is not. As previously noted, a backoff
factor value of "K = 4" is generally recommended for ASM operation
and "K = 6" for SSM operation. A receiver SHALL cancel the backoff
timeout and thus its pending transmission of a "NORM_ACK(RTT)"
message under the following conditions:
1. The receiver generates another feedback message ("NORM_NACK" or
other "NORM_ACK") before the congestion control feedback timeout
expires (these messages will convey the current congestion
control feedback information),
2. A "NORM_CMD(CC)" or other receiver feedback with an ordinally
greater "cc_sequence" field value is received before the
congestion control feedback timeout expires (this is similar to
the TFMCC feedback round number),
3. When the "T_backoff" is greater than "1*GRTTsender". This
prevents NACK implosion in the event of sender or network
failure,
4. "Suppressing" congestion control feedback is heard from another
receiver (in a "NORM_ACK" or "NORM_NACK") or via a
"NORM_CMD(REPAIR_ADV)" message from the sender. The local
receiver's feedback is "suppressed" if the rate of the competing
feedback ("Rfb") is sufficiently close to or less than the local
receiver's calculated rate ("Rcalc"). The local receiver's
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feedback is canceled when "Rcalc > (0.9 * Rfb)". Also note
receivers that have not yet received an RTT measurement from the
sender are suppressed only by other receivers that have not yet
measured RTT. Additionally, receivers whose RTT estimate has
aged considerably (i.e., they haven't been included in the
"NORM_CMD(CC)" "cc_node_list" in a long time) may wish to compete
as a receiver with no prior RTT measurement after some long term
expiration period.
When the backoff timer expires, the receiver SHALL generate a
"NORM_ACK(RTT)" message to provide feedback to the sender and group.
This message may be multicast to the group for most effective
suppression in ASM topologies or unicast to the sender depending upon
how the NORM protocol is deployed and configured.
Whenever any feedback is generated (including this "NORM_ACK(RTT)"
message), receivers include an adjusted version of the sender
timestamp from the most recently received "NORM_CMD(CC)" message and
the "cc_sequence" value from that command in the applicable
"NORM_ACK" or "NORM_NACK" message fields. For NORM-CC operation, any
generated feedback message SHALL also contain the NORM-CC Feedback
header extension. The receiver provides its current "cc_rate"
estimate, "cc_loss" estimate, "cc_rtt" if known, and any applicable
"cc_flags" via this header extension.
During slow start (when the receiver has not yet detected loss from
the sender), the receiver uses a value equal to two times its
measured rate from the sender in the "cc_rate" field. For steady-
state congestion control operation, the receiver "cc_rate" value is
from the equation-based value using its current loss event estimate
and sender<->receiver RTT information. (The GRTT is used when the
receiver has not yet measured its individual RTT).
The "cc_loss" field value reflects the receiver's current loss event
estimate with respect to the sender in question.
When the receiver has a valid individual RTT measurement, it SHALL
include this value in the "cc_rtt" field. The "NORM_FLAG_CC_RTT"
MUST be set when the "cc_rtt" field is valid.
After a congestion control feedback message is generated or when the
feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
period during which it will restrain itself from providing congestion
control feedback, even if "NORM_CMD(CC)" messages are received from
the sender (unless the receive becomes marked as a CLR or PLR node).
The value of this holdoff timeout ("T_ccHoldoff") period is:
T_ccHoldoff = (K*GRTT)
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Thus, non-CLR receivers are constrained to providing explicit
congestion control feedback once per "K*GRTT" intervals. Note,
however, that as the session progresses, different receivers will be
responding to different "NORM_CMD(CC)" messages and there will be
relatively continuous feedback of congestion control information
while the sender is active.
5.5.2.3. Congestion Control Rate Adjustment
During steady-state operation, the sender will directly adjust its
transmission rate to the rate indicated by the feedback from its
currently selected CLR. As noted in [TfmccPaper], the estimation of
parameters (loss and RTT) for the CLR will generally constrain the
rate changes possible within acceptable bounds. For rate increases,
the sender SHALL observe a maximum rate of increase of one packet per
RTT at all times during steady-state operation.
The sender processes congestion control feedback from the receivers
and selects the CLR based on the lowest rate receiver. Receiver
rates are either determined directly from the slow start "cc_rate"
provided by the receiver in the NORM-CC Feedback header extension or
by performing the equation-based calculation using individual RTT and
loss estimates ("cc_loss") as feedback is received.
The sender can calculate a current RTT for a receiver ("RTT_rcvrNew")
using the "grtt_response" timestamp included in feedback messages.
When the "cc_rtt" value in a response is not valid, the sender simply
uses this "RTT_rcvrNew" value as the receiver's current RTT
("RTT_rcvr"). For non-CLR and non-PLR receivers, the sender can use
the "cc_rtt" value provided in the NORM-CC Feedback header extension
as the receiver's previous RTT measurement ("RTT_rcvrPrev") to smooth
according to:
RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew
For CLR receivers where feedback is received more regularly, the
sender SHOULD maintain a more smoothed RTT estimate upon new feedback
from the CLR where:
RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew
""RTT_clrNew"" is the new RTT calculated from the timestamp in the
feedback message received from the CLR. The "RTT_clr" is initialized
to "RTT_clrNew" on the first feedback message received. Note that
the same procedure is observed by the sender for PLR receivers and
that if a PLR is "promoted" to CLR status, the smoothed estimate can
be continued.
There are some additional periods besides steady-state operation that
need to be considered in NORM-CC operation. These periods are:
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1. during session startup,
2. when no feedback is received from the CLR, and
3. when the sender has a break in data transmission.
During session startup, the congestion control operation SHALL
observe a "slow start" procedure to quickly approach its fair
bandwidth share. An initial sender startup rate is assumed where:
Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.
The rate is increased only when feedback is received from the
receiver set. The "slow start" phase proceeds until any receiver
provides feedback indicating that loss has occurred. Rate increase
during slow start is applied as:
Rnew = Rrecv_min
where "Rrecv_min" is the minimum reported receiver rate in the
"cc_rate" field of congestion control feedback messages received from
the group. Note that during slow start, receivers use two times
their measured rate from the sender in the "cc_rate" field of their
feedback. Rate increase adjustment is limited to once per GRTT
during slow start.
If the CLR or any receiver intends to leave the group, it will set
the "NORM_FLAG_CC_LEAVE" in its congestion control feedback message
as an indication that the sender should not select it as the CLR.
When the CLR changes to a lower rate receiver, the sender should
immediately adjust to the new lower rate. The sender is limited to
increasing its rate at one additional packet per RTT towards any new,
higher CLR rate.
The sender should also track the age of the feedback it has received
from the CLR by comparing its current "cc_sequence" value
("Seq_sender") to the last "cc_sequence" value received from the CLR
("Seq_clr"). As the age of the CLR feedback increases with no new
feedback, the sender SHALL begin reducing its rate once per "RTT_clr"
as a congestion avoidance measure. The following algorithm is used
to determine the decrease in sender rate (Rsender bytes/sec) as the
CLR feedback, unexpectedly, excessively ages:
Age = Seq_sender - Seq_clr;
if (Age > 4) Rsender = Rsender * 0.5;
This rate reduction is limited to the lower bound on NORM
transmission rate. After "NORM_ROBUST_FACTOR" consecutive
"NORM_CMD(CC)" rounds without any feedback from the CLR, the sender
SHOULD assume the CLR has left the group and pick the receiver with
the next lowest rate as the new CLR. Note this assumes that the
sender does not have explicit knowledge that the CLR intentionally
left the group. If no receiver feedback is received, the sender MAY
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wish to withhold further transmissions of "NORM_DATA" segments and
maintain "NORM_CMD(CC)" transmissions only until feedback is
detected. After such a CLR timeout, the sender will be transmitting
with a minimal rate and should return to slow start as described here
for a break in data transmission.
When the sender has a break in its data transmission, it can continue
to probe the group with "NORM_CMD(CC)" messages to maintain RTT
collection from the group. This will enable the sender to quickly
determine an appropriate CLR upon data transmission restart.
However, the sender should exponentially reduce its target rate to be
used for transmission restart as time since the break elapses. The
target rate SHOULD be recalculated once per "RTT_clr" as:
Rsender = Rsender * 0.5;
If the minimum NORM rate is reached, the sender should set the
"NORM_FLAG_START" flag in its "NORM_CMD(CC)" messages upon restart
and the group should observer slow start congestion control
procedures until any receiver experiences a new loss event.
5.5.3. NORM Positive Acknowledgment Procedure
NORM provides options for the source application to request positive
acknowledgment (ACK) of "NORM_CMD(FLUSH)" and "NORM_CMD(ACK_REQ)"
messages from members of the group. There are some specific
acknowledgment requests defined for the NORM protocol and a range of
acknowledgment request types that are left to be defined by the
application. One predefined acknowledgment type is the
"NORM_ACK_FLUSH" type. This acknowledgment is used to determine if
receivers have achieved completion of reliable reception up through a
specific logical transmission point with respect to the sender's
sequence of transmission. The "NORM_ACK_FLUSH" acknowledgment may be
used to assist in application flow control when the sender has
information on a portion of the receiver set. Another predefined
acknowledgment type is "NORM_ACK(CC)", which is used to explicitly
provide congestion control feedback in response to "NORM_CMD(CC)"
messages transmitted by the sender for NORM-CC operation. Note the
"NORM_ACK(CC)" response does NOT follow the positive acknowledgment
procedure described here. The "NORM_CMD(ACK_REQ)" and "NORM_ACK"
messages contain an "ack_type" field to identify the type of
acknowledgment requested and provided. A range of "ack_type" values
is provided for application-defined use. While the application is
responsible for initiating the acknowledgment request and interprets
application-defined "ack_type" values, the acknowledgment procedure
SHOULD be conducted within the protocol implementation to take
advantage of timing and transmission scheduling information available
to the NORM transport.
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The NORM positive acknowledgment procedure uses polling by the sender
to query the receiver group for response. Note this polling
procedure is not intended to scale to very large receiver groups, but
could be used in large group setting to query a critical subset of
the group. Either the "NORM_CMD(ACK_REQ)", or when applicable, the
"NORM_CMD(FLUSH)" message is used for polling and contains a list of
NormNodeIds for receivers that should respond to the command. The
list of receivers providing acknowledgment is determined by the
source application with a priori knowledge of participating nodes or
via some other application-level mechanism.
The ACK process is initiated by the sender that generates
"NORM_CMD(FLUSH)" or "NORM_CMD(ACK_REQ)" messages in periodic rounds.
For "NORM_ACK_FLUSH" requests, the "NORM_CMD(FLUSH)" contain a
"object_transport_id" and "fec_payload_id" denoting the watermark
transmission point for which acknowledgment is requested. This
watermark transmission point is echoed in the corresponding fields of
the "NORM_ACK(FLUSH)" message sent by the receiver in response.
"NORM_CMD(ACK_REQ)" messages contain an "ack_id" field which is
similarly echoed in response so that the sender may match the
response to the appropriate request.
In response to the "NORM_CMD(ACK_REQ)", the listed receivers randomly
spread "NORM_ACK" messages uniformly in time over a window of
(1*GRTT). These "NORM_ACK" messages are typically unicast to the
sender. (Note that "NORM_ACK(CC)" messages SHALL be multicast or
unicast in the same manner as "NORM_NACK" messages).
The ACK process is self-limiting and avoids ACK implosion in that:
1. Only a single "NORM_CMD(ACK_REQ)" message is generated once per
(2*GRTT), and,
2. The size of the "acking_node_list" of NormNodeIds from which
acknowledgment is requested is limited to a maximum of the sender
NormSegmentSize setting per round of the positive acknowledgment
process.
Because the size of the included list is limited to the sender's
NormSegmentSize setting, multiple "NORM_CMD(ACK_REQ)" rounds may be
required to achieve responses from all receivers specified. The
content of the attached NormNodeId list will be dynamically updated
as this process progresses and "NORM_ACK" responses are received from
the specified receiver set. As the sender receives valid responses
(i.e., matching watermark point or "ack_id") from receivers, it SHALL
eliminate those receivers from the subsequent "NORM_CMD(ACK_REQ)"
message "acking_node_list" and add in any pending receiver
NormNodeIds while keeping within the NormSegmentSize limitation of
the list size. Each receiver is queried a maximum number of times
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("NORM_ROBUST_FACTOR", by default). Receivers not responding within
this number of repeated requests are removed from the payload list to
make room for other potential receivers pending acknowledgment. The
transmission of the "NORM_CMD(ACK_REQ)" is repeated until no further
responses are required or until the repeat threshold is exceeded for
all pending receivers. The transmission of "NORM_CMD(ACK_REQ)" or
"NORM_CMD(FLUSH)" messages to conduct the positive acknowledgment
process is multiplexed with ongoing sender data transmissions.
However, the "NORM_CMD(FLUSH)" positive acknowledgment process may be
interrupted in response to negative acknowledgment repair requests
(NACKs) received from receivers during the acknowledgment period.
The "NORM_CMD(FLUSH)" positive acknowledgment process is restarted
for receivers pending acknowledgment once any the repairs have been
transmitted.
In the case of "NORM_CMD(FLUSH)" commands with an attached
"acking_node_list", receivers will not ACK until they have received
complete transmission of all data up to and including the given
watermark transmission point. All receivers SHALL interpret the
watermark point provided in the request NACK for repairs if needed as
for "NORM_CMD(FLUSH)" commands with no attached "acking_node_list".
5.5.4. Group Size Estimate
NORM sender messages contain a "gsize" field that is a representation
of the group size and is used in scaling random backoff timer ranges.
The use of the group size estimate within the NORM protocol does not
require a precise estimation and works reasonably well if the
estimate is within an order of magnitude of the actual group size.
By default, the NORM sender group size estimate may be
administratively configured. Also, given the expected scalability of
the NORM protocol for general use, a default value of 10,000 is
RECOMMENDED for use as the group size estimate.
It is possible that group size may be algorithmically approximated
from the volume of congestion control feedback messages which follow
the exponentially weighted random backoff. However, the
specification of such an algorithm is currently beyond the scope of
this document.
6. Security Considerations
The same security considerations that apply to the Multicast NACK
[RFC5401], TFMCC [RFC4654], and FEC [RFC5052] Building Blocks also
apply to the NORM protocol. In addition to the vulnerabilities that
any IP and IP multicast protocol implementation may be generally
subject to, the NACK-based feedback of NORM may be exploited by
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replay attacks which force the NORM sender to unnecessarily transmit
repair information. This MAY be addressed by network layer IP
security implementations that guard against this potential security
exploitation or alternatively with a security mechanism that uses the
"EXT_AUTH" header extension for similar purposes. Such security
mechanisms SHOULD be deployed and used when available.
The NORM protocol is compatible with the use of IP security (IPsec)
[RFC4301] and the IPsec Encapsulating Security Payload (ESP) protocol
or Authentication Header (AF) extension can be used to secure IP
packets transmitted by NORM participants. A baseline approach to
secure NORM operation using IPsec is described below. Compliant
implementations of this specification are REQUIRED to be compatible
with IPsec usage as described in Section 6.1.
Additionally, the "EXT_AUTH" header extension (HET = 1) is defined
for use by security mechanisms to provide an alternative form of
authentication and/or encryption of NORM messages. The format of
this header extension and its processing is outside the scope of this
document and is to be communicated out-of-band as part of the session
description. It is possible that an EXT_AUTH implementation of MAY
also provide for encryption of NORM message payloads as well as
authentication. The use of this approach as compared to IPsec can
allow for header compression techniques to be applied jointly to IP
and NORM protocol headers. In cases where security analysis deems
that encryption of NORM protocol header content is beneficial or
necessary, the aforementioned use of IPsec ESP may be more
appropriate. If EXT_AUTH is present, whatever packet authentication
checks that can be performed immediately upon reception of the packet
MUST be performed before accepting the packet and performing any
congestion control-related action on it. Some packet authentication
schemes impose a delay of several seconds between when a packet is
received and when the packet can be fully authenticated. Any
congestion control related action that is appropriate MUST NOT be
postponed by any such full packet authentication.
Consideration MUST also be given to the potential for replay-attacks
that would transplant authenticated packets from one NORM session to
another to disrupt service. To avoid this potential, unique keys
SHOULD be assigned on a per-session basis or NORM sender nodes SHOULD
be configured to use unique "instance_id" identifiers that are
managed as part of the security association for the sessions.
It should be noted that NORM implementations can use the "sequence"
field from the NORM Common Message Header to detect replay attacks.
This can be accomplished if the NORM sender maintains state on
receivers which are NACKing. A cache of such receiver state can be
used to provide protection against NACK replay attacks. NORM
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receivers MUST also maintain similar state for protection against
possible replay of other receiver messages in ASM operation as well.
For example, a receiver could be suppressed from providing NACK or
congestion control feedback by replay of certain receiver messages.
For these reasons, authentication of NORM messages (e.g., via IPsec)
SHOULD be applied for protection against similar attacks that use
fabricated messages. Also, encryption of messages to provide
confidentiality of application data and protect privacy of users MAY
also be applied using IPsec or similar mechanisms.
When applicable security measures are used, automated key management
mechanisms such as those described in the Group Domain of
Interpretation (GDOI) [RFC3547], Multimedia Internet KEYing (MIKEY)
[RFC3830] or Group Secure Association Key Management Protocol
(GSAKMP) [RFC4535] specifications SHOULD be applied.
It is also important to note that while NORM does leverage FEC-based
repair for scalability, this alone does not guarantee integrity of
received data. Application-level integrity-checking of received data
content is highly RECOMMENDED.
6.1. Baseline Secure NORM Operation
This section describes a baseline mode of secure NORM protocol
operation based on application of the IPsec security protocol. This
approach is documented here to provide a reference, interoperable
secure mode of operation. However, additional approaches to NORM
security, including other forms of IPsec application, MAY be
specified in the future. For example, the use of the EXT_AUTH header
extension could enable NORM-specific authentication or security
encapsulation headers similar to those of IPsec to be specified and
inserted into the NORM protocol message headers. This would allow
header compression techniques to be applied to IP and NORM protocol
headers when needed in a similar fashion to that of RTP [RFC3550] and
as preserved in the specification for Secure Real Time Protocol
(SRTP) [RFC3711].
The baseline approach described is applicable to NORM operation
configured for SSM (or SSM-like) operation where there is a single
sender and the receivers are providing unicast feedback. This form
of NORM operation allows for IPsec to be used with a manageable
number of security associations (SA).
6.1.1. IPsec Approach
For NORM one-to-many SSM operation with unicast feedback from
receivers, each node SHALL be configured with two transport mode
IPsec security associations and corresponding Security Policy
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Database (SPD) entries. One entry will be used for sender-to-group
multicast packet authentication and optionally encryption while the
other entry will be used to provide security for the unicast feedback
messaging from the receiver(s) to the sender.
The NORM sender SHALL use an IPsec SA configured for ESP protocol
[RFC4303] operation with the option for data origination
authentication enabled. It is also RECOMMENDED that this IPsec ESP
SA be also configured to provide confidentiality protection for IP
packets containing NORM protocol messages. This is suggested to make
the realization of complex replay attacks much more difficult. The
encryption key for this SA SHALL be preplaced at the sender and
receiver(s) prior to NORM protocol operation. Use of automated key
management is RECOMMENDED as a rekey SHALL be required prior to
expiration of the sequence space for the SA. This is necessary so
that receivers may use the built-in IPsec replay attack protection
possible for an IPsec SA with a single source (the NORM sender).
Thus the receivers SHALL enable replay attack protection for this SA
used to secure NORM sender traffic. An IPsec SPD entry MUST be
configured to process outbound packets to the session (destination)
address and UDP port number of the applicable (NormSession).
The NORM receiver(s) MUST be configured with the SA and SPD entry to
properly process the IPsec-secured packets from the sender. The NORM
receiver(s) SHALL also use a common, second IPsec SA (common Security
Parameter Index (SPI) and encryption key) configured for ESP
operation with the option for data origination authentication
enabled. Similar to the NORM sender, is RECOMMENDED this IPsec ESP
SA be also configured to provide confidentiality protection for IP
packets containing NORM protocol messages. The receivers MUST have
an IPsec SPD entry configured to process outbound NORM/UDP packets
directed to the NORM sender source address and port number using this
second SA. As noted for NORM unicast feedback, the sender's
transmission port number SHOULD be selected to be distinct from the
multicast session port number to allow discrimination between unicast
and multicast feedback messages when access to the IP destination
address is not possible (e.g., a user-space NORM implementation).
For processing of packets from receivers, the NORM sender SHALL be
configured with this common, second SA (and the corresponding SPD
entry needed) in order to properly process messages from the
receiver.
Multiple receivers using a common IPsec SA for traffic directed to
the NORM sender (i.e., many-to-one) typically prevents the use of
built-in IPsec replay attack protection by the NORM sender with
current IPsec implementations. Thus the built-in IPsec replay attack
protection for this second SA at the sender MUST be disabled unless
the particular IPsec implementation manages its replay protection on
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a per-source basis. So, to support a fully secure mode of operation,
the NORM sender implementation MUST provide replay attack protection
based upon the "sequence" field of NORM protocol messages from
receivers. This can be accomplished with high assurance of security,
even with the limited size (16-bits) of this field, because
1. NORM receiver NACK and non-CLR ACK feedback messages are sparse.
2. The more frequent "NORM_ACK" feedback from CLR or PLR nodes are
only a small set of receivers for which the sender must keep more
persistent replay attack state.
3. "NORM_NACK" feedback messages that precede the sender's current
repair window do not significantly impact protocol operation
(generation of "NORM_CMD(SQUELCH)" is limited) and could be in
fact ignored. This means the sender can prune any replay attack
state for receivers that precede the current repair window.
4. "NORM_ACK" messages correspond to either a specific sender
"ack_id", the sender "cc_sequence" for ACKs sent in response to
"NORM_CMD(CC)", or the sender's current repair window in the case
of ACKs sent in response to "NORM_CMD(FLUSH)". Thus, the sender
can prune any replay attack state for receivers that precede the
current applicable sequence or repair window space.
Note that use of ESP confidentiality for secure NORM protocol
operation makes it more difficult for adversaries to conduct any form
of replay attacks. Additionally, it should be noted that a NORM
sender implementation with access to the full ESP protocol header
could also use the ESP sequence information to make replay attack
protection even more robust, by maintaining per-source sequence
state. The design of this baseline security approach for NORM
intentionally places any more complex processing state or processing
(e.g. replay attack protection given multiple receivers) at the NORM
sender since NORM receiver implementations may need to have a more
light-weight realization in many cases.
This baseline approach can be used for NORM protocol sessions with
multiple senders if the SA pairs described are established for each
sender. For small-sized groups, it is even possible that many-to-
many (ASM) IPsec configuration could be achieved where each
participant uses a unique SA (with a unique SPI). This does not
scale to larger group sizes given the complex set of SA and SPD
entries each participant would need to maintain.
It is anticipated in early deployments of this baseline approach to
NORM security that key management will be conducted out-of-band with
respect to NORM protocol operation. In the case of one-to-many NORM
operation, it is possible that receivers may retrieve keying
information from a central server as needed or otherwise conduct
group key updates with a similar centralized approach. However, it
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may be possible with some key management schemes for rekey messages
to be transmitted to the group as a message or transport object
within the NORM reliable transfer session. Similarly, for group-wise
communication sessions it is possible that potential group
participants may request keying and/or rekeying as part of NORM
communications. Additional specification is necessary to define an
in-band key management scheme for NORM sessions perhaps using the
mechanisms of the automated group key management specifications cited
in this document.
6.1.2. IPsec Requirements
In order to implement this secure mode of NORM protocol operation,
the following IPsec capabilities are required.
6.1.2.1. Selectors
The implementation MUST be able to use the source address,
destination address, protocol (UDP), and UDP port numbers as
selectors in the SPD.
6.1.2.2. Mode
IPsec in transport mode MUST be supported. The use of IPsec
[RFC4301] processing for secure NORM traffic MUST be configured such
that unauthenticated packets are not received by the NORM protocol
implementation.
6.1.2.3. Key Management
An automated key management scheme for group key distribution and
rekeying such as GDOI [RFC3547], GSAKMP [RFC4535], or MIKEY [RFC3830]
is RECOMMENDED for use. Relatively short-lived NORM sessions MAY be
able to use Manual Keying with a single, preplaced key, particularly
if Extended Sequence Numbering (ESN) [RFC4303] is available in the
IPsec implementation used. It should also be noted that it may be
possible for key update messages (e.g., the GDOI GROUPKEY-PUSH
message) to be included as part of the NORM application reliable data
transmission if appropriate interfaces are available between the NORM
application and the key management daemon.
6.1.2.4. Security Policy
Receivers MUST accept protocol messages only from the designated,
authorized sender(s). It is expected that appropriate key management
will provide encryption keys only to receivers authorized to
participate in a designated session. The approach outlined here
allows receiver sets to be controlled on a per-sender basis.
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6.1.2.5. Authentication and Encryption
Large NORM group sizes will necessitate some form of key management
that does rely upon shared secrets. The GDOI and GSAKMP protocols
mentioned here allow for certificate-based authentication. It is
RECOMMENDED these certificates use IP addresses for authentication
although it may alternatively possible to have authentication
associated with pre-assigned NormNodeId values. However, it is
likely that available group key management implementations will not
be NORM-specific.
6.1.2.6. Availability
The IPsec requirements profile outlined here is commonly available on
many potential NORM hosts. The principal issue is that configuration
and operation of IPsec typically requires privileged user
authorization. Automated key management implementations are
typically configured with the privileges necessary to effect system
IPsec configuration needed.
7. IANA Considerations
Values of NORM Header Extension Types, Stream Control Codes, and
"NORM_CMD" message sub-types are subject to IANA registration. They
are in the registry named "Reliable Multicast Transport (RMT) NORM
Protocol Parameters" located at time of publication at:
http:///www.iana.org/assignments/norm-parameters
It should be also noted that reliable multicast building block
components used by this specification also have their respective IANA
considerations and those documents should be consulted accordingly.
In particular, the FEC Building Block used by NORM does require IANA
registration of the FEC codecs used. The registration instructions
for FEC codecs are provided in RFC 5052.
7.1. Explicit IANA Assignment Guidelines
This document introduces three namespaces that are registered for the
NORM Header Extension Types, Stream Control Codes and "NORM_CMD"
Message Sub-types. This section describes explicit IANA assignment
guidelines for each of these.
7.1.1. NORM Header Extension Types
This document defines a namespace for NORM Header Extension Types
named:
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"ietf:rmt:norm:extension"
The NORM Header Extension Type field is an 8-bit value. The values
of this field identify extended header content that allows the
protocol functionality to be expanded to include additional features
and operating modes. The values that can be assigned within the
"ietf:rmt:norm:extension" namespace are numeric indexes in the range
{0, 255}, boundaries included. Values in the range {0,127} indicate
variable length extended header fields while values in the range
{128,255} indicate extensions of a fixed 4-byte length. This
specification registers the following NORM Header Extension Types:
+-------+------------+--------------------+
| Value | Name | Reference |
+-------+------------+--------------------+
| 1 | "EXT_AUTH" | This specification |
| 3 | "EXT_CC" | This specification |
| 64 | "EXT_FTI" | This specification |
| 128 | "EXT_RATE" | This specification |
+-------+------------+--------------------+
Requests for assignment of additional NORM Header Extension Type
values are granted on a "Specification Required" basis as defined by
IANA Guidelines [RFC5226]. Any such header extension specifications
MUST include a description of protocol actions to be taken when the
extension type is encountered by a protocol implementation not
supporting that specific option. For example, it may be possible for
protocol implementations to ignore unknown header extensions in many
cases.
7.1.2. NORM Stream Control Codes
This document defines a namespace for NORM Stream Control Codes
named:
"ietf:rmt:norm:streamControlCode"
NORM Stream Control Codes are 16-bit values that may be inserted
within a "NORM_OBJECT_STREAM" delivery object to convey sequenced,
out-of-band (with respect to the stream data) control signaling
applicable to the referenced stream object. These control codes are
to be delivered to the application or protocol implementation with
reliable delivery, in-order with respect to the their inserted
position within the stream. This specification registers the
following NORM Stream Control Code:
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+-------+-------------------+--------------------+
| Value | Name | Reference |
+-------+-------------------+--------------------+
| 0 | "NORM_STREAM_END" | This specification |
+-------+-------------------+--------------------+
Additional NORM Stream Control Code value assignment requests are
granted on a "Specification Required" basis as defined by IANA
Guidelines [RFC5226]. The full 16-bit space outside of the value
assigned in this specification are available for future assignment.
Note that in addition to describing the control code's expected
interpretation, such specifications MUST include a description of
protocol actions to be taken when the control code is encountered by
a protocol implementation not supporting that specific option.
7.1.3. NORM_CMD Message Sub-types
This document defines a namespace for "NORM_CMD" Message Sub-types
named:
"ietf:rmt:norm:command"
The "NORM_CMD" sub-type field is an 8-bit value with valid values in
the range of 1-255. Note the value 0 is reserved to indicate an
invalid "NORM_CMD" message sub-type. The current specification
defines a number of "NORM_CMD" message sub-types that senders can use
to signal the receivers in various aspects of NORM protocol
operation. This specification registers the following "NORM_CMD"
Message Sub-types:
+-------+-------------------------+--------------------+
| Value | Name | Reference |
+-------+-------------------------+--------------------+
| 0 | reserved | This specification |
| 1 | "NORM_CMD(FLUSH)" | This specification |
| 2 | "NORM_CMD(EOT)" | This specification |
| 3 | "NORM_CMD(SQUELCH)" | This specification |
| 4 | "NORM_CMD(CC)" | This specification |
| 5 | "NORM_CMD(REPAIR_ADV)" | This specification |
| 6 | "NORM_CMD(ACK_REQ)" | This specification |
| 7 | "NORM_CMD(APPLICATION)" | This specification |
+-------+-------------------------+--------------------+
Future specifications extending NORM may wish to define additional
"NORM_CMD" messages to enhance protocol functionality. "NORM_CMD"
message sub-type value assignment requests are granted on a
"Specification Required" basis as defined by IANA Guidelines
[RFC5226]. Note that in addition to describing the command sub-
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type's expected interpretation, specifications MUST include a
description of protocol actions to be taken when the command is
encountered by a protocol implementation not supporting that specific
option.
Note that this specification already provides for an "application-
defined" "NORM_CMD" message sub-type that may be used at the
discretion of individual applications using NORM for transport.
These "application-defined" commands may be suitable for many
application-specific purposes and do not require standards action.
In any case, such additional messages SHALL be subject to the same
congestion control constraints as the existing NORM sender message
set.
8. Suggested Use
The present NORM protocol is seen as useful tool for the reliable
data transfer over generic IP multicast services. It is not the
intention of the authors to suggest it is suitable for supporting all
envisioned multicast reliability requirements. NORM provides a
simple and flexible framework for multicast applications with a
degree of concern for network traffic implosion and protocol overhead
efficiency. NORM-like protocols have been successfully demonstrated
within the MBone for bulk data dissemination applications, including
weather satellite compressed imagery updates servicing a large group
of receivers and a generic web content reliable "push" application.
In addition, this framework approach has some design features making
it attractive for bulk transfer in asymmetric and wireless
internetwork applications. NORM is capable of successfully operating
independent of network structure and in environments with high packet
loss, delay, and out-of-order delivery. Hybrid proactive/reactive
FEC-based repairing improve protocol performance in some multicast
scenarios. A sender-only repair approach often makes additional
engineering sense in asymmetric networks. NORM's unicast feedback
capability may be suitable for use in asymmetric networks or in
networks where only unidirectional multicast routing/delivery service
exists. Asymmetric architectures supporting multicast delivery are
likely to make up an important portion of the future Internet
structure (e.g., DBS/cable/PSTN hybrids) and efficient, reliable bulk
data transfer will be an important capability for servicing large
groups of subscribed receivers.
9. Changes from RFC3940
This section lists the changes between the Experimental version of
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this specification, RFC 3940, and this version:
1. Removal of the "NORM_FLAG_MSG_START" for "NORM_OBJECT_STREAM",
replacing it with the "payload_msg_start" field in the FEC-
encoded preamble of the "NORM_OBJECT_STREAM NORM_DATA" payload,
2. Definition of IANA namespace for header extension assignment,
3. Removal of file blocking scheme description that is now specified
in the FEC Building Block document [RFC5052],
4. Removal of restriction of NORM receiver feedback message rate to
local NORM sender rate (This caused congestion control failures
in high speed operation. The extremely low feedback rate of the
NORM protocol as compared to TCP avoids any resultant impact to
the network as shown in [Mdpcc]),
5. Correction of errors in some message format descriptions, and
6. Correction of inconsistency in specification of the inactivity
timeout.
7. Addition of IPsec secure mode description with IPsec
requirements.
8. Addition of the EXT_AUTH header extension definition.
9. Clarification of interpretation of "Source Block Length" when FEC
codes are arbitrarily shortened by the sender.
10. Acknowledgments
(and these are not Negative)
The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
Toni Paila, Michael Luby, and Joerg Widmer for their valuable input
and comments on this document. The authors would also like to thank
the RMT working group chairs, Roger Kermode and Lorenzo Vicisano, for
their support in development of this specification, and Sally Floyd
for her early input into this document.
11. References
11.1. Normative References
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
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[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
[RFC4654] Widmer, J. and M. Handley, "TCP-Friendly Multicast
Congestion Control (TFMCC): Protocol Specification",
RFC 4654, August 2006.
[RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052, August 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5401] Adamson, B., Bormann, C., Handley, M., and J. Macker,
"Multicast Negative-Acknowledgment (NACK) Building
Blocks", RFC 5401, November 2008.
11.2. Informative References
[FecHybrid]
Gossink, D. and J. Macker, "Reliable Multicast and
Integrated Parity Retransmission with Channel Estimation",
IEEE Globecomm , 1998.
[McastFeedback]
Nonnenmacher, J. and E. Biersack, "Optimal Multicast
Feedback", IEEE INFOCOM, p. 964, March/April 1998.
[MdpToolkit]
Macker, J. and B. Adamson, "The Multicast Dissemination
Protocol (MDP) Toolkit", Proc. IEEE MILCOM , October 1999.
[Mdpcc] Adamson, B. and J. Macker, "A TCP-Friendly, Rate-based
Mechanism for NACK-Oriented Reliable Multicast Congestion
Control", Proc. IEEE GLOBECOMM , November 2001.
[NormFeedback]
Adamson, B. and J. Macker, "Quantitative Prediction of
NACK-Oriented Reliable Multicast (NORM) Feedback", IEEE
MILCOM , October 2002.
[PgmccPaper]
Rizzo, L., "pgmcc: A TCP-Friendly Single-Rate Multicast
Congestion Control Scheme", ACM SIGCOMM , August 2000.
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[RFC2357] Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF
Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC3048] Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
Floyd, S., and M. Luby, "Reliable Multicast Transport
Building Blocks for One-to-Many Bulk-Data Transfer",
RFC 3048, January 2001.
[RFC3269] Kermode, R. and L. Vicisano, "Author Guidelines for
Reliable Multicast Transport (RMT) Building Blocks and
Protocol Instantiation documents", RFC 3269, April 2002.
[RFC3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley,
M., and J. Crowcroft, "The Use of Forward Error Correction
(FEC) in Reliable Multicast", RFC 3453, December 2002.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC3940] Adamson, B., Bormann, C., Handley, M., and J. Macker,
"Negative-acknowledgment (NACK)-Oriented Reliable
Multicast (NORM) Protocol", RFC 3940, November 2004.
[RFC4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, June 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, March 2009.
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[RmComparison]
Pingali, S., Towsley, D., and J. Kurose, "A Comparison of
Sender-Initiated and Receiver-Initiated Reliable Multicast
Protocols", Proc. INFOCOMM, San Francisco CA,
October 1993.
[TcpModel]
Padhye, J., Firoiu, V., Towsley, D., and J. Kurose,
"Modeling TCP Throughput: A Simple Model and its Empirical
Validation", ACM SIGCOMM , 1998.
[TfmccPaper]
Widmer, J. and M. Handley, "Extending Equation-Based
Congestion Control to Multicast Applications", ACM
SIGCOMM , August 2001.
Authors' Addresses
Brian Adamson
Naval Research Laboratory
Washington, DC 20375
USA
Email: adamson@itd.nrl.navy.mil
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen
Germany
Email: cabo@tzi.org
Mark Handley
University College London
Gower Street
London WC1E 6BT
UK
Email: M.Handley@cs.ucl.ac.uk
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Joe Macker
Naval Research Laboratory
Washington, DC 20375
USA
Email: macker@itd.nrl.navy.mil
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