RMT Working Group Brian Whetten
Internet Engineering Task Force Consultant
Internet Draft Dah Ming Chiu
Document: draft-ietf-rmt-bb-track-02.txt Miriam Kadansky
November 2002 Sun Microsystems
Expires May 2003 Seok Joo Koh
ETRI
Gursel Taskale
TIBCO
Reliable Multicast Transport Building Block for TRACK
<draft-ietf-rmt-bb-track-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document describes the TRACK Building Block. It contains
functions relating to positive acknowledgments and hierarchical tree
construction and maintenance. It is primarily meant to be used as
part of the TRACK Protocol Instantiation. It is also designed to be
useful as part of overlay multicast systems that wish to offer
efficient confirmed delivery of multicast messages.
Conventions used in this document
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.
1. Introduction
Whetten Expires - May 2003 [Page 1]
TRACK BUILDING BLOCK November 2002
One of the protocol instantiations the RMT WG is chartered to create
is a TRee-based ACKnowledgement protocol (TRACK). Rather than create
a set of monolithic protocol specifications, the RMT WG has chosen to
break the reliable multicast protocols into Building Blocks (BB) and
Protocol Instantiations (PI). A Building Block is a specification of
the algorithms of a single component, with an abstract interface to
other BBs and PIs. A PI combines a set of BBs, adds in the
additional required functionality not specified in any BB, and
specifies the specific instantiation of the protocol. For more
information, see the Reliable Multicast Transport Building Blocks and
Reliable Multicast Design Space documents [2][3].
As specified in [2], there are two primary reliability requirements
for a transport protocol, ensuring goodput, and confirming delivery
to the Sender. The NORM and ALC PIs are responsible solely for
ensuring goodput. TRACK is designed to offer application level
confirmed delivery, aggregation of control traffic and Receiver
statistics, local recovery, automatic tree building, and enhanced
flow and congestion control.
Whereas the NORM and ALC PIs run only over other building blocks, the
TRACK PI has a more difficult integration task. To run in
conjunction with NORM, it must either re-implement the functionality
in the NORM PI, or integrate directly with the NORM PI. In addition,
in order to have reasonable commercial applicability, TRACK needs to
be able to run over other protocols in addition to NORM. To meet
both of these challenges, the TRACK PI is designed to integrate with
other transport layer protocols, including NORM, PGM [20], ALC [19],
UDP, or an overlay multicast system. In order to accomplish this,
there can be multiple TRACK PIs, one for each transport protocol it
is specified to integrate with. The vast majority of the protocol
functionality exists in this document, the TRACK BB, which in turn
references the automatic tree building block [16]. For more details
on the specific functionality of TRACK, please see the reference
TRACK PI[21].
TRACK is organized around a Data Channel and a Control Channel. The
Data Channel is responsible for multicast data from the Sender to all
other nodes in a TRACK session. In order to integrate with NORM and
other goodput-ensuring transport protocols, these protocols are used
as the Data Channel for a given Data Session. This Data Channel MAY
also provide congestion control. Otherwise, congestion control MUST
be provided by the TRACK PI, through using the TFMCC or other
approved congestion control building block.
This document describes the TRACK Building Block. It contains
functions relating to positive acknowledgments and hierarchical tree
construction and maintenance. While named as a building block, this
Whetten Expires - May 2003 [Page 2]
TRACK BUILDING BLOCK November 2002
document describes more functionality than the PI documents. With
the exception of congestion control, almost all of the functionality
is encapsulated in this document or the BBs it references. The TRACK
PIs are then primarily responsible for instantiating packet formats
in conjunction with the other transport protocols it uses as its Data
Channel.
The TRACK BB assumes that there is an Automatic Tree Building BB [16]
which provides the list of parents (known as Service Nodes within the
Tree BB) each node should join to. If Receivers are used that may
also serve as Repair Heads, the TRACK BB assumes the Auto Tree BB is
also responsible for selecting the role of each Receiver as either
Receiver or Repair Head. However, the TRACK BB may specify that a
particular node may not operate as a Repair Head.
The TRACK BB also assumes that a separate session advertisement
protocol notifies the Receivers as to when to join a session, the
data multicast address for the session, and the control parameters
for the session. This functionality MAY be provided in a TRACK PI
document.
The TRACK BB provides the following detailed functionality.
... .Hierarchical Session Creation and Maintenance. This set of
functionality is responsible for creating and maintaining (but not
configuring) a hierarchical tree of Repair Heads and Receivers.
- Bind. When a child knows the parent it wishes to join to for
a given Data Session, it binds to that parent.
- Unbind. When a child wishes to leave a Data Session, either
because the session is over or because the application is
finished with the session, it initiates an unbind operation
with its parent.
- Eject. A parent can also force a child to unbind. This
happens if the parent needs to leave the session, if the child
is not behaving correctly, or if the parent wants to move the
child to another parent as part of tree configuration
maintenance.
- Fault Detection. In order to verify liveness, parents and
children send regular heartbeat messages between themselves.
The Sender also sends regular null data messages to the group,
if it has no data to send.
- Fault Recovery. When a child detects that its parent is no
longer reachable, it may switch to another parent. When a
parent detects that one of its children is no longer
reachable, it removes that child from its membership list and
reports this up the tree to the Sender of the Data Session.
- Distributed Membership. Each Parent is responsible for
maintaining a local list of the children attached to it.
Whetten Expires - May 2003 [Page 3]
TRACK BUILDING BLOCK November 2002
- Data Sessions. This functionality is responsible for the reliable,
ordered transmission of a set of data messages, which together
constitute a Data Session. These are initially transmitted using
another transport protocol, the Data Channel Protocol, which has
primary responsibility for ensuring goodput and congestion control.
- Data Transmission. The Sender takes sequenced data messages
from the application, and passes them to the Data Channel
Protocol for multicast transmission. It delays passing them
to the Data Channel Protocol if it is presently flow
controlled.
- Flow Control and Buffer Management. Receivers and Repair
Heads MAY maintain a set of buffers that are at least as large
as the Senders transmission window. The Receivers pass their
reception status up to the Sender as part of their TRACK
messages. This MAY be used to advance the buffer windows at
each node and limit the Senders window advancement to the
speed of the slowest Receiver.
- Retransmission Requests. While primary responsibility for
goodput rests with the Data Channel Protocol, Receivers MAY
request retransmission of lost messages from their parents.
- Local Recovery. Repair heads keep track of retransmission
requests from their children, and provide repairs to them. If
a Repair Head cannot fulfill a retransmission request, it
forwards it up the tree.
- End of Stream. When a Data Session is completed, this is
signaled as an End of Stream condition.
...TRACK Generation and Aggregation. This set of functionality is
responsible for periodically generating TRACK messages from all
Receivers and aggregating them at Repair Heads. These messages
provide updated flow control window information, roundtrip time
measurements, and congestion control statistics. They OPTIONALLY
acknowledge receipt of data, OPTIONALLY report missing messages,
and OPTIONALLY provide group statistics. The algorithms include:
- TRACK Timing. In order to avoid ACK implosion, the Receivers
and Repair Heads use timing algorithms to control the speed at
which TRACK messages are sent.
- TRACK Aggregation. In order to provide the highest levels of
scalability and reliability, interior tree nodes provide
aggregation of control traffic flowing up the tree. The
aggregated feedback information includes that used for end-to-
end confirmed delivery, flow control, congestion control, and
group membership monitoring and management.
- Statistics Request. A Sender may prompt Receivers to generate
and report a set of statistics back to the Sender. These
statistics are self-describing data types, and may be defined
by either the TRACK PI or the application.
Whetten Expires - May 2003 [Page 4]
TRACK BUILDING BLOCK November 2002
- Statistics Aggregation. In addition to the predefined
aggregation types, aggregation of self-describing data may
also be performed on Receiver statistics flowing up the tree.
. Application Level Confirmed Delivery. Senders can issue requests
for application level confirmation of data up to a given message.
Receivers reply to this request, and the confirmations are reliably
forwarded up the tree.
- Distributed RTT Calculations. One of the primary challenges of
congestion control is efficient RTT calculations. TRACK provides
two methods to perform these calculations.
- Sender Per-Message RTT Calculations. On demand, a Sender
stamps outgoing messages with a timestamp. As each TRACK is
passed up the tree, the amount of dally time spent waiting at
each node is accumulated. The lowest measurements are passed
up the tree, and the dally time is subtracted from the
original measurement.
- Local Per-Level RTT Calculations. Each parent measures the
local RTT to each of its children as part of the keep-alive
messages used for failure detection.
2. Applicability Statement
The primary objective of TRACK is to provide additional functionality
in conjunction with a receiver reliable protocol. These functions
MAY include application layer reliability, enhanced congestion
control, flow control, statistics reporting, local recovery, and
automatic tree building. It is designed to do this while still
offering scalability in the range of 10,000s of Receivers per Data
Session. The primary corresponding design tradeoffs are additional
complexity, and lower isolation of nodes in the face of network and
host failures.
There is a fundamental tradeoff between reliability and real-time
performance in the face of failures. There are two primary types of
single layer reliability that have been proposed to deal with this:
Sender reliable and Receiver reliable delivery. Sender reliable
delivery is similar to TCP, where the Sender knows the identity of
the Receivers in a Data Session, and is notified when any of them
fails to receive all the data messages. Receiver reliable delivery
limits knowledge of group membership and failures to only the actual
Receivers. Senders do not have any knowledge of the membership of a
group, and do not require Receivers to explicitly join or leave a
Data Session. Receiver reliable protocols scale better in the face
of networks that have frequent failures, and have very high isolation
of failures between Receivers. This TRACK BB provides Sender
Whetten Expires - May 2003 [Page 5]
TRACK BUILDING BLOCK November 2002
reliable delivery, typically in conjunction with a Receiver reliable
system.
This BB is specified according to the guidelines in [21]. It
specifies all communication between entities in terms of messages,
rather than packets. A message is an abstract communication unit,
which may be part of, or all of, a given packet. It does not have a
specific format, although it does contain a list of fields, some of
which may be optional, and some of which may have fixed lengths
associated with them. It is up to each protocol instantiation to
combine the set of messages in this BB, with those in other
components, and create the actual set of packet formats that will be
used.
As mentioned in the introduction, this BB assumes the existence of a
separate Auto Tree Configuration BB. It also assumes that Data
Sessions are advertised to all Receivers as part of an external BB or
other component.
Except where noted, this applicability statement is applicable both
to the TRACK BB and to the TRACK PIs.
2.1 Application Types
TRACK is designed to support a wide range of applications that
require one to many bulk data transfer and application layer
confirmed delivery. Examples of applications that fit into the one-
to-many data dissemination model are: real time financial news and
market data distribution, electronic software distribution, audio
video streaming, distance learning, software updates and server
replication.
Historically, financial applications have had the most stringent
reliability requirements, while audio video streaming have had the
least stringent. For applications that do not require this level of
reliability, or that demand the lowest levels of latency and the
highest levels of failure isolation, TRACK may be less applicable.
TRACK is designed to work in conjunction with a receiver reliable
protocol such as NORM, to allow applications to select this tradeoff
on a dynamic basis.
2.2 Network Infrastructure
TRACK is designed to work over almost all multicast and broadcast
capable network infrastructures. It is specifically designed to be
able to support both asymmetrical and single source multicast
environments.
Whetten Expires - May 2003 [Page 6]
TRACK BUILDING BLOCK November 2002
Asymmetric networks with very low upbound bandwidth and a very low
loss Data Channel may be better served solely through NACK based
protocols, particularly if high reliability is not required. A good
example is some satellite networks.
Networks that have very high loss rates, and regularly experience
partial network partitions, router flapping, or other persistent
faults, may be better served through NACK only protocols. Some
wireless networks fall in to this category.
2.3 Private and Public Networks
TRACK is designed to work in private networks, controlled networks
and in the public Internet. A controlled network typically has a
single administrative domain, has more homogenous network bandwidth,
and is more easily managed and controlled. These networks have the
fewest barriers to IP multicast deployment and the most immediate
need for reliable multicast services. Deployment in the Internet
requires a protocol to span multiple administrative domains, over
vastly heterogeneous networks.
2.4 Manual vs. Automatic Controls
Some networks can take advantage of manual or centralized tools for
configuring and controlling the usage of a reliable multicast group.
In the public Internet the tools have to span multiple administrative
domains where policies may be inconsistent. Hence, it is preferable
to design tools that are fully distributed and automatic. To address
these requirements, TRACK provides automatic configuration, but can
also support manual configuration options.
2.5 Heterogeneous Networks
While the majority of controlled networks are symmetrical and support
many-to-many multicast, in designing a protocol for the Internet, we
must deal with virtually all major network types. These include
asymmetrical networks, satellite networks, networks where only a
single node may send to a multicast group, and wireless networks.
TRACK takes this into account by not requiring any many-to-many
multicast services. TRACK does not assume that the topology used for
sending control messages has any congruence to the topology of the
multicast address used for sending data messages.
2.6 Use of Network Infrastructure
TRACK is designed to run in either single level or hierarchical
configurations. In a single level, there is no need for specialized
network infrastructure. In hierarchical configurations, special
Whetten Expires - May 2003 [Page 7]
TRACK BUILDING BLOCK November 2002
nodes called Repair Heads are defined, which may run either as part
of a distributed application, or as part of dedicated server
software. TRACK does not specifically support or require Generic
Router Assist or other router level assist.
2.7 Deployment Constraints
The two primary tradeoffs TRACK has, for the functionality it
provides, are additional complexity, and decreased failure isolation.
Hence, if target applications are to be deployed in networks with
high rates of persistent failures, and isolation of failed Receivers
from affecting other Receivers is of high importance, TRACK may not
be appropriate. Similarly, if simplicity is paramount, TRACK may not
be appropriate.
2.8 Target Scalability
The target scalability of TRACK is tens of thousands of simultaneous
Receivers per Data Session. Dedicated Repair Heads are targeted to
be able to support thousands of simultaneous Data Sessions.
2.9 Known Failure Modes
If a hierarchical Control Tree is misconfigured, so that loop-free,
contiguous connection is not provided, failure will occur. This
failure is designed to occur gracefully, at the initialization of a
Data Session.
If the configuration parameters on control traffic are poorly chosen
on an asymmetrical network, where there is much less control channel
bandwidth available than data channel bandwidth, there may be a very
high rate of control traffic. This control traffic is not
dynamically congestion controlled like the data traffic, and so could
potentially cause congestion collapse.
This potential control channel overload could be exacerbated by an
application that makes overly heavy use of the application level
confirmation or statistics gathering functions.
2.10 Potential Conflicts With Other Components
None are known of at this time.
3. Architecture Definition
3.1 TRACK Entities
3.1.1 Node Types
Whetten Expires - May 2003 [Page 8]
TRACK BUILDING BLOCK November 2002
TRACK divides the operation of the protocol into three major
entities: Sender, Receiver, and Repair Head. The Repair Head
corresponds to the Service Node described in the Tree Building draft.
It is assumed that Senders and Receivers typically run as part of an
application on an end host client. Repair Heads MAY be components in
the network infrastructure, managed by different network managers as
part of different administrative domains, or MAY run on an end host
client, in which case they function as both Receivers and Repair
Heads. Absent of any automatic tree configuration, it is assumed
that the Infrastructure Repair Heads have relatively static
configurations, which consist of a list of nearby possible Repair
Heads. Senders and Receivers, on the other hand, are transient
entities, which typically only exist for the duration of a single
Data Session. In addition to these core components, applications that
use TRACK are expected to interface with other services that reside
in other network entities, such as multicast address allocation,
session advertisement, network management consoles, DHCP, DNS,
overlay networking, application level multicast, and multicast key
management.
3.1.2 Multicast Group Address
A Multicast Group Address is a logical address that is used to
address a set of TRACK nodes. It is RECOMMENDED to consist of a pair
consisting of an IP multicast address and a UDP port number. In this
case, it may optionally have a Time To Live (TTL) value, although
this value MUST only be used for providing a global scope to a Data
Session, and not for scoping of local retransmissions. Data Multicast
Addresses are Multicast Group Addresses.
TRACK MAY be used with an overlay multicast or application layer
multicast system. In this case, a Multicast Group Address MAY have a
different format. The TRACK PI is responsible for specifying the
format of a Multicast Group Address.
3.1.3 Data Session
A Data Session is the unit of reliable delivery of TRACK. It
consists of a sequence of sequentially numbered Data messages, which
are sent by a single Sender over a single Data Multicast Address.
They are delivered reliably, with acknowledgements and
retransmissions occurring over the Control Tree. A Data Session ID
uniquely identifies it. A given Data Session is received by a set of
zero or more Receivers, and a set of zero or more Repair Heads. One
or more Data Sessions MAY share the same Data Multicast Address
(although this is NOT RECOMMENDED). Each TRACK node can
simultaneously participate in multiple Data Sessions. A Receiver
Whetten Expires - May 2003 [Page 9]
TRACK BUILDING BLOCK November 2002
MUST join all the Data Multicast Addresses and Control Trees
corresponding to the Data Sessions it wishes to receive.
3.1.4 Data Channel
A Data Session is multicast over a Data Channel. The Data Channel is
responsible for efficiently delivering the Data messages to the
members of a Data Session, and providing statistical reliability
guarantees on this delivery. It does this by employing a Data
Channel Protocol, such as NORM, ALC, PGM, or Overlay Multicast.
TRACK is then responsible for providing application level, Sender
based reliability, by confirming delivery to all Receivers, and
optionally retransmitting lost messages that did not get correctly
delivered by the Data Channel. A common scenario would be to use
TRACK to provide application level confirmation of delivery, and
recover from persistent failures in the network that are beyond the
scope of the Data Channel Protocol.
3.1.5 Data Channel Protocol
This is the transport protocol used by a TRACK PI to ensure goodput
and statistical reliability on a Data Channel.
3.1.6 Data Multicast Address
This is the Multicast Group Address used by the Data Channel
Protocol, to efficiently deliver Data messages to all Receivers and
Repair Heads. All Data Multicast Addresses used by TRACK are assumed
to be unidirectional and only support a single Sender.
3.1.7 Control Tree
A Control Tree is a hierarchical communication path used to send
control information from a set of Receivers, through zero or more
Repair Heads (RHs), to a Sender. Information from lower nodes is
aggregated as the information is relayed to higher nodes closer to
the Sender. Each Data Session uses a Control Tree. It is acceptable
to have a degenerate Control Tree with no Repair Heads, which
connects all of the Receivers directly to the Sender.
Each RH in the Control Tree uses a separate Local Control Channel for
communicating with its children. It is RECOMMENDED that each Local
Control Channel correspond to a separate Multicast Group Address.
Optionally, these RH multicast addresses MAY be the same as the Data
Multicast Address.
3.1.8 Local Control Channel
Whetten Expires - May 2003 [Page 10]
TRACK BUILDING BLOCK November 2002
A Local Control Channel is a unidirectional multicast path from a
Repair Head or Sender to its children. It uses a Multicast Group
Address for this communication.
3.1.9 Host ID
With the widespread deployment of network address translators,
creating a short globally unique ID for a host is a challenge. By
default, TRACK uses a 48 bit long Host ID field, filled with the low-
order 48 bits of the MD5 signature of the DNS name of the source. A
TRACK PI, to match up with the goodput-ensuring protocol that TRACK
PI uses as its Data Channel Protocol, MAY redefine the length and
contents of this identifier.
3.1.10 Data Session ID
A Data Session ID is a globally unique identifier for a Data Session.
It may either be selected by the Data Channel Protocol (i.e. NORM) or
by TRACK. By default, it is the combination of the Host ID for the
Sender, combined with the 16 bit port number used for the Data
Session at the Sender. This identifier is included in every TRACK
message.
3.1.11 Child ID
All members in a TRACK Data Session, besides the Sender, are
identified by the combination of their Host ID, and the port number
with which they send IP packets to their parent.
3.1.12 Message Sequence Numbers
A Message Sequence Number is a 32 bit number in the range from 1
through 2^32 û 1, which is used to specify the sequential order of a
Data message in a Data Stream. A Sender node assigns consecutive
Sequence Numbers to the Data messages provided by the Sender
application. By default, zero is reserved to indicate that the Data
Session has not yet started. A TRACK PI MAY redefine this. Message
Sequence Numbers may wrap around, and so Sequence Number arithmetic
MUST be used to compare any two Sequence Numbers.
3.1.13 Data Queue
A Data Queue is a buffer, maintained by a Sender or a Repair Head,
for transmission and retransmission of the Data messages provided by
the Sender application. New Data messages are added to the Data
Queue as they arrive from the sending application, up to a specified
buffer limit. The admission rate of messages to the network is
controlled by the flow and congestion control algorithms. Once a
Whetten Expires - May 2003 [Page 11]
TRACK BUILDING BLOCK November 2002
message has been received by the Receivers of a Data Stream, it may
be deleted from the buffer.
At the Sender, A TRACK PI may integrate the Data Queue with the
buffer used by the Data Channel Protocol.
3.2 Basic Operation of the Protocol
For each Data Session, TRACK provides sequenced, reliable delivery of
data from a single Sender to up to tens of thousands of Receivers. A
TRACK Data Session consists of a network that has exactly one Sender
node, zero or more Receiver nodes and zero or more Repair Heads.
The figure below illustrates a TRACK Data Session with multiple
Repair Heads.
Before a Data Session starts, a session advertisement MUST be
received by all members of the Data Session, notifying them to join
the group, and the appropriate configuration information for the Data
Session. This MAY be provided directly by the application, by an
external service, or by the TRACK PI.
A Sender joins the Control Tree and a Data Channel Protocol. It
multicasts Data messages on the Data Multicast Address, using the
Data Channel Protocol. All of the nodes in the session subscribe to
the Data Multicast Address and join the Data Channel Protocol.
There is no assumption of congruence between the topology of the Data
Multicast Address and the topology of the Control Tree.
-------> SD (Sender node)----->|
^^^ |
/ | \ Control |
TRACKs / | \ Tree |
/ | \ |
/ | \ (Repair |
/ | \ Head |
/ | \ nodes) v
RH RH RH <------------|
^^ ^^^ ^^ | Data
/ | / | \ | \ | Channel
/ | / | \ | \ |
/ | / | \ | \ v
R R R R R R R <---------
(Receiver Nodes)
A Receiver joins the appropriate Data Channel Protocol, and the Data
Multicast Address used by that protocol, in order to receive Data. A
Receiver periodically informs its parent about the messages that it
Whetten Expires - May 2003 [Page 12]
TRACK BUILDING BLOCK November 2002
has received by unicasting a TRACK message to the parent. It MAY
also request retransmission of lost messages in this TRACK. Each
parent node aggregates the TRACKs from its child nodes and (if it is
not the Sender) unicasts a single aggregated TRACK to its parent.
The Sender and each Repair Head have a multicast Local Control
Channel to their children. This is used for transmitting Heartbeat
messages that inform their child nodes that the parent node is still
functioning. This channel is also used to perform local
retransmission of lost Data messages to just these children. TRACK
MUST still provide correct operation even if multicast addresses are
reused across multiple Data Sessions or multiple Local Control
Channels. It is NOT RECOMMENDED to use the same multicast address
for multiple Local Control Channels serving any given Data Session.
The communication path forms a loop from the Sender to the Receivers,
through the Repair Heads back to the Sender. Original data (ODATA),
Retransmission (RDATA) and NullData messages regularly exercise the
downward data direction. Heartbeat messages exercise the downward
control direction. TRACK messages regularly exercise the Control
Tree in the upward direction. This combination constantly checks
that all of the nodes in the tree are still functioning correctly,
and initiates fault recovery when required.
This hierarchical infrastructure allows TRACK to provide a number of
functions in a scaleable way. Application level confirmation of
delivery and statistics aggregation both operate in a request-reply
mode. A sender issues a request for application level confirmation
or statistics reporting, and the receivers report back the
appropriate information in their TRACK messages. This information is
aggregated by the Repair Heads, and passed back up to the Sender.
Since TRACK messages are not delivered with the reliability of data
messages, Receivers and Repair Heads transmit this information
redundantly.
TRACK also gathers control information that is useful for improving
the performance of flow and congestion control algorithms, including
scaleable round trip time measurements.
Normally, goodput in ensured by lower level protocols, such as the
NACKs and FEC algorithms in NORM and PGM. However, TRACKs MAY also
include optional retransmission requests, in the form of selective
bitmaps indicating which messages need to be retransmitted. The RH
is then responsible for retransmitting these messages on the Local
Control Channel to its children.
3.3 Component Relationships
Whetten Expires - May 2003 [Page 13]
TRACK BUILDING BLOCK November 2002
TRACK is primarily designed to run in conjunction with another
transport protocol that is responsible for ensuring goodput. It is
RECOMMENDED that this Data Channel Protocol also be responsible for
congestion control, although the TRACK PI MAY provide this congestion
control function instead, and MAY pass the congestion control
statistics it collects to the Data Channel Protocol, in order to
enhance the performance of the congestion control algorithms.
The primary Data Channel Protocol that TRACK is designed to work with
is NORM. In this case, the NORM PI is responsible for interfacing
with the NACK BB, the FEC BB, the Generic Router Assist BB, and the
appropriate congestion control BB.
TRACK then adds additional functionality that complements this
receiver-reliable protocol, such as application level confirmed
delivery, retransmission in the face of persistent failures,
statistics aggregation, and collection of extra information for
congestion control.
The TRACK BB is responsible for specifying all of the TRACK-specific
functionality. It interfaces with the Automatic Tree Building Block.
The TRACK PI is then responsible for instantiating a complete
protocol that includes all of the other components. It is expected
that there will be multiple TRACK PIs, one for each Data Channel
Protocol that it is specified to work with.
The following figure illustrates this, for the case where NORM is the
Data Channel Protocol.
+----------+
| |
| TRACK |
| PI |
| |
+----------+
/ \
/ \
/ \
+---------+ +---------+
| | | |
| TRACK | | NORM | Data Channel
| BB | | PI | Protocol
| | | |
+---------+ +---------+
| |
| |
| |
+---------+ +-----------------------+
| | | |
Whetten Expires - May 2003 [Page 14]
TRACK BUILDING BLOCK November 2002
| Tree | | FEC, CC, GRA, NACK |
| BB | | Building Blocks |
| | | |
+---------+ +-----------------------+
For more details on integration, please see the example TRACK PI over
UDP [17].
4. TRACK Functionality
4.1 Hierarchical Session Creation and Maintenance
4.1.1 Overview of Tree Configuration
Before a Data Session starts reliably delivering data, the tree for
the Data Session needs to be created. This process binds each
Receiver to either a Repair Head or the Sender, and binds the
participating Repair Heads into a loop-free tree structure with the
Sender as the root of the tree. This process requires tree
configuration knowledge, which can be provided with some combination
of manual and/or automatic configuration. The algorithms for
automatic tree configuration are part of the Automatic Tree
Configuration BB. They return to each node the address of the parent
it should bind to, as well as zero or more backup parents to use if
the primary parent fails.
In addition to receiving the tree configuration information, the
Receivers all receive a Session Advertisement message from the
Senders, informing them of the Data Multicast Address and other
session configuration information. This advertisement may contain
other relevant session information such as whether or not Repair
Heads should be used, whether manual or automatic tree configuration
should be used, the time at which the session will start, and other
protocol settings. This advertisement is created as part of either
the TRACK PI or as part of an external service. In this way, the
Sender enforces a set of uniform session configuration parameters on
all members of the session.
As described in the automatic tree configuration BB, the general
algorithm for a given node in tree creation is as follows.
1) Get advertisement that a session is starting
2) Get a list of neighbor candidates using the getSNs Tree BB
interface, and OPTIONALLY contact them
3) Select best neighbor as parent in a loop free manner
4) Bind to parent
5) Optionally, later rebind to another parent
Whetten Expires - May 2003 [Page 15]
TRACK BUILDING BLOCK November 2002
When a child finishes step 4, it is up to automatic tree
configuration to, if necessary, continue building the tree in order
to connect the node back to the Sender. After the session is
created, children can unbind from their parents and bind again to new
parents. This happens when faults occur, or as part of a tree
optimization process. Steps 1 through 3 are external to the TRACK
BB. Step 4 is performed as part of session creation. Step 5 is
performed as part of session maintenance in conjunction with
automatic tree building, as either an Unbind or Eject, combined with
another Bind operation.
Once steps 1 through 3 are completed, Receivers join the Data
Multicast Address, and attempt to Bind to either the Sender or a
local Repair Head. A Receiver will attempt to bind to the first node
in the tree configuration list returned by step 3, and if this fails,
it will move to the next one. A Receiver only binds to a single
Repair Head or Sender, at a time, for each Data Session.
The automatic tree building BB ensures that the tree is formed
without loops. As part of this, when a Repair Head has a Receiver
attempt to bBnd to it for a given Data Session, it may not at first
be able to accept the connection, until it is able to join the tree
itself. Because of this, a Receiver will sometimes have to
repeatedly attempt to Bind to a given parent before succeeding.
Once the Sender initiates tree building, it is also free to start
sending Data messages on the Data Multicast Address. Repair Heads
and Receivers may start receiving these messages, but may not request
retransmission or deliver data to the application until they receive
confirmation that they have successfully bound to the tree.
4.1.2 Bind
4.1.2.1 Input Parameters
In order to join a Data Session and Bind to the tree, the following
nodes need the following parameters.
A Repair Head requires the following parameters.
- Session: the unique identifier for the Data Session to join,
received from the session advertisement algorithm specified in the
PI.
- ParentAddress: the address and port of the parent node to which
the node should connect, received from the Auto Tree BB.
Whetten Expires - May 2003 [Page 16]
TRACK BUILDING BLOCK November 2002
- UDPListenPort: the number of the port on which the node will
listen for its childrens control messages. This parameter is
configured by the application.
- RepairAddr: the multicast address, UDP port, and TTL on which this
node sends control messages to its children. This parameter is
configured by the application.
A Sender requires the above parameters, except for the ParentAddress.
A Receiver requires the above parameters, except for the
UDPListenPort and RepairAddr.
4.1.2.2 Bind Algorithm
A Bind operation happens when a child wishes to join a parent in the
distribution tree for a given Data Session. The Receivers initiate
the first Bind protocols to their parents, which then cause recursive
binding by each parent, up to the Sender. Each Receiver sends a
separate BindRequest message for each of the streams that it would
like to join. At the discretion of the PI, multiple BindRequest
messages may be bundled together in a single message.
A node sends a BindRequest message to its automatically selected or
manually configured parent node. The parent node sends either a
BindConfirm message or a BindReject message. Reception of a
BindConfirm message terminates the algorithm successfully, while
receipt of a BindReject message causes the node to either retry the
same parent or restart the Bind algorithm with its next parent
candidate (depending on the BindReject reason code), or if it has
none, to declare a REJECTED_BY_PARENT error. Once the node is
accepted by a Repair head, it informs the Tree BB using the setSN
interface.
Reliability is achieved through the use of a standard request-
response protocol. At the beginning of the algorithm, the child
initializes TimeMaxBindResponse to the constant
TIMEOUT_PARENT_RESPONSE and initializes NumBindResponseFailures to 0.
Every time it sends a BindRequest message, it waits
TimeMaxBindResponse for a response from the parent node. If no
response is received, the node doubles its value for
TimeMaxBindResponse, but limits TimeMaxBindResponse to be no larger
than MAX_TIMEOUT_PARENT_RESPONSE. It also
increments NumBindResponseFailures, and retransmits the BindRequest
message. If NumBindResponseFailures reaches NUM_MAX_PARENT_ATTEMPTS,
it reports a PARENT_UNREACHABLE error.
When a parent receives a BindRequest message, it first consults the
automatic tree building BB for approval (using the acceptChild Tree
BB interface), for instance to ensure that accepting the BindRequest
Whetten Expires - May 2003 [Page 17]
TRACK BUILDING BLOCK November 2002
will not cause a loop in the tree. Then the parent checks to be sure
that it does not have more than MaxChildren children already bound to
it for this session. If it can accept the child, it sends back a
BindConfirm message. Otherwise, it sends the node a BindReject
message. Then the parent checks to see if it is already a member of
this Data Session. If it is not yet a member of this session, it
attempts to join the tree itself.
The BindConfirm message contains the lowest Sequence Number that the
Repair Head has available. If this number is 0, then the Repair Head
has all of the data available from the start of the session.
Otherwise, the requesting node is attempting a late join, and can
only use this Repair Head if late join was allowed by the PI. If
late join is not allowed, the node may try another Repair Head, or
give up.
Similarly, if a failure recovery occurs, when a node tries to bind to
a new Repair Head, it must follow the same rules as for a late join.
See Fault Recovery, below.
4.1.3 Unbind
A child may decide to leave a Data Session for the following reasons.
1) It detects that the Data Session is finished. 2) The application
requests to leave the Data Session. 3) It is not able to keep up
with the data rate of the Data Session. When any of these conditions
occurs, it initiates an Unbind process.
An Unbind is, like the Bind function, a simple request-reply
protocol. Unlike the Bind function, it only has a single response,
UnbindConfirm. With this exception, the Unbind operation uses the
same state variables and reliability algorithms as the Bind function.
When a child receives an UnbindConfirm message from its parent, it
reports a LEFT_DATA_SESSION_GRACEFULLY event. If it does not receive
this message after NUM_MAX_PARENT_ATTEMPTS, then it reports a
LEFT_DATA_SESSION_ABNORMALLY event. Unbinds are reported to the Tree
BB using the lostSN interface.
4.1.4 Eject
A parent may decide to remove one or more of its children from a data
stream for the following reasons. 1) The parent needs to leave the
group due to application reasons. 2) The Repair Head detects an
unrecoverable failure with either its parent or the Sender. 3) The
parent detects that the child is not able to keep up with the speed
of the data stream. 4) The parent is not able to handle the load of
its children and needs some of them to move to another parent. In
the first two cases, the parent needs to multicast the advertisement
Whetten Expires - May 2003 [Page 18]
TRACK BUILDING BLOCK November 2002
of the termination of one or more Data Sessions to all of its
children. In the second two cases, it needs to send one or more
unicast notifications to one or more of its children.
Consequently, an Eject can be done either with a repeated multicast
advertisement message to all children, or a set of unicast request-
reply messages to the subset of children that it needs to go to.
For the multicast version of Eject, the parent sends a multicast
UnbindRequest message to all of its children for a given Data
Session, on its Local Multicast Channel. It is only necessary to
provide statistical reliability on this message, since children will
detect the parents failure even if the message is not received.
Therefore, the UnbindRequest message is sent
FAILURE_DETECTION_REDUNDANCY times.
For the unicast version of Eject, the parent sends a unicast
UnbindRequest message to all of its children. Each of them responds
with an EjectConfirm. Reliability is ensured through the same
request-reply mechanism as the Bind operation.
Ejections are reported to the Tree BB using the removeChild
interface.
4.1.5 Fault Detection
There are three cases where fault detection is needed. 1) Detection
(by a child) that a parent has failed. 2) Detection (by a parent)
that a child has failed. 3) Detection (by either a Repair Head or
Receiver) that a Sender has failed.
In order to be scaleable and efficient, fault detection is primarily
accomplished by periodic keep-alive messages, combined with the
existing TRACK messages. Nodes expect to see keep-alive messages
every set period of time. If more than a fixed number of periods go
by, and no keep-alive messages of a given type are received, the node
declares a preliminary failure. The detecting node may then ping the
potentially failed node before declaring it failed, or it can just
declare it failed.
Failures are detected through three keep-alive messages: Heartbeat,
TRACK, and NullData. The Heartbeat message is multicast periodically
from a parent to its children on its Local Control Channel. NullData
messages are multicast by a Sender on the Data Control Channel when
it has no data to send. TRACK messages are generated periodically,
even if no data is being sent to a Data Session, as described in
section 7.2.
Whetten Expires - May 2003 [Page 19]
TRACK BUILDING BLOCK November 2002
Heartbeat messages are multicast every HeartbeatPeriod seconds, from
a parent to its children. Every time that a parent sends a
Retransmission message or a Heartbeat message (as well as at
initialization time), it resets a timer for HeartbeatPeriod seconds.
If the timer goes off, a Heartbeat is sent. The HeatbeatPeriod is
dynamically computed as follows:
interval = AckWindow / MessageRate
HeartbeatPeriod = 2 * interval
Global configuration parameters ConstantHeartbeatPeriod and
MinimumHeartbeatPeriod can be used to either set HeartbeatPeriod to a
constant, or give HeartbeatPeriod a lower bound, globally.
Similarly, a NullData message is multicast by the Sender to all Data
Session members, every NULL_DATA_PERIOD. The NullData timer is set
to NULL_DATA_PERIOD, and is reset every time that a Data or NullData
message is sent by the Sender.
The key parameter for failure detection is the global tree parameter
FAILURE_DETECTION_REDUNDANCY. The higher the value for this
parameter, the more keep-alive messages that must be missed before a
failure is declared.
A major goal of failure detection is for children to detect parent
failures fast enough that there is a high probability they can rejoin
the stream at another parent, before flow control has advanced the
buffer window to a point where the child can not recover all lost
messages in the stream. In order to attempt to do this, children
detect a failure of a parent if FAILURE_DETECTION_REDUNDANCY *
HeartbeatPeriod time goes by without any heartbeats. As part of
buffer window advancement, all parents MAY choose to buffer all
messages for a minimum of FAILURE_DETECTION_REDUNDANCY * 2 *
HeartbeatPeriod seconds, which gives children a period of time to
find a new parent before the buffers are freed. Children report
parent failures to the Tree BB using the lostSN interface.
A parent detects a preliminary failure of one of its children if it
does not receive any TRACK messages from that child in
FAILURE_DETECTION_REDUNDANCY * TrackTimeout seconds (see discussion
of how TrackTimeout is computed below). Because a failed child can
slow down the groups progress, it is very important that a parent
resolve the childs status quickly. Once a parent declares a
preliminary failure of a child, it issues a set of up to
FAILURE_DETECTION_REDUNDANCY Heartbeat messages that are unicast (or
multicast) to the failed Receiver(s). These messages are spaced
apart by 2*LocalRTT, where LocalRTT is the round trip time that has
been measured to the child in question (see below for description of
Whetten Expires - May 2003 [Page 20]
TRACK BUILDING BLOCK November 2002
how LocalRTT is measured). These Heartbeat messages contain a
ChildrenList field that contains the children who are requested to
send a TRACK immediately.
Whenever a child receives a Heartbeat message where the child is
identified in the ChildrenList field, it immediately sends a TRACK to
its parent. If a parent does not receive a TRACK message from a
child after waiting a period of 2*LocalRTT after the last Heartbeat
message to that child, it declares the child failed, and removes it
from the parents child membership list. It informs the Tree BB using
the removeChild interface.
A child or a Repair Head detects the failure of a Sender if it does
not receive a Data or NullData message from a Sender in
FAILURE_DETECTION_REDUNDANCY * NULL_DATA_PERIOD.
Note that the more Receivers there are in a tree, and the higher the
loss rate, the larger FAILURE_DETECTION_REDUNDANCY must be, in order
to give the same probability that erroneous failures wont be
declared.
4.1.6 Fault Notification
When a parent detects the failure of a child, it adds a failure
notification field to the next TRANSMISSION_REDUNDANCY TRACK messages
that it sends up the tree. It sends this notification multiple times
because TRACKs are not delivered reliably. A failure notification
field includes the failure code, as well as a list of one or more
failed nodes. Failure notifications are aggregated up the tree and
delivered to the Sender. A failure notification is not a definitive
report of a node failure, as the child may have detected a
communication failure with its parent and moved to a different Repair
Head.
4.1.7 Fault Recovery
The Fault Recovery algorithms require a list of one or more addresses
of alternate parents that can be bound to, and that still provide
loop free operation.
If a child detects the failure of its parent, it then re-runs the
Bind operation to a new parent candidate, in order to rejoin the
tree. A node may perform a late join, i.e. binding with a Repair
Head which cannot provide all the necessary repair data, only if
allowed by the PI.
4.1.8 Distributed Membership.
Whetten Expires - May 2003 [Page 21]
TRACK BUILDING BLOCK November 2002
Each Repair Head is responsible for maintaining a set of state
variables on the status of its children. Unlike the Generic Router
Assist, this is hard state, that only is removed when a child leaves
that Repair Head gracefully, or after the Repair Head detects that a
child has failed. These variables MUST include, but are not
necessarily limited to, the following:
- ChildID. This is the two byte identifier assigned to the Child by
the Repair Head. This uniquely identifies this Child to this
Repair Head, but has no meaning outside that scope.
- GlobalChildIdentifier. This is the globally unique identifier for
this Child.
- ChildRTT. This is the weighted average of the local RTT to this
Child.
- LastTRACK. This is the contents of the last TRACK message sent
from this Child, if any, not including options.
- LastApplicationLevelConfirmation. This is the contents of the last
Application Level Confirmation sent from this Child, if any.
- Last Statistics. This is the contents of the last Statistics
message sent from this Child, if any.
- ChildLiveness. This is a set of variables that keep track of the
liveness of each child. This includes the last time a TRACK
message was received from this child, as well as the number of
Heartbeat messages that have been directed at it, and the time at
which the last Heartbeat message was sent to the child. Please see
Fault Detection, above, for more details.
4.2 Data Sessions.
4.2.1 Data Transmission and Retransmission
Data is multicast by a Sender on the Data Multicast Address via the
Data Channel Protocol. The Data Channel Protocol is responsible for
taking care of as many retransmissions as possible, and for ensuring
the goodput of the Data Session. TRACK is then responsible for
providing OPTIONAL flow control and application level reliability.
The mechanics of an application level confirmation of delivery are
handled by TRACK, including keeping track of the distributed
membership list of receivers and aggregating acknowledgements up the
Control Tree. Please see below for more details on flow control and
application level confirmation.
A common scenario for handling recovery of lost messages is to allow
the Data Channel Protocol to provide statistical reliability, and
then allow TRACK to provide retransmissions for more persistent
failure cases, such as if a Receiver is not able to receive any Data
messages for a few minutes.
Whetten Expires - May 2003 [Page 22]
TRACK BUILDING BLOCK November 2002
Retransmissions of data messages may be multicast by the Sender on
the Data Multicast Address or be multicast on a Local Control Channel
by a Repair Head.
A Repair Head joins all of the Data Multicast Addresses that any of
its descendants have joined. A Repair Head is responsible for
receiving and buffering all data messages using the reliability
semantics configured for a stream. As a simple to implement option,
a Repair Head MAY also function as a Receiver, and pass these data
messages to an attached application.
For additional fault tolerance, a Receiver MAY subscribe to the
multicast address associated with the Local Control Channel of one or
more Repair Heads in addition to the multicast address of its parent.
In this case it does not bind to this Repair Head or Sender, but will
process Retransmission messages sent to this address. If the
Receivers Repair Head fails and it transfers to another Repair Head,
this minimizes the number of data messages it needs to recover after
binding to the new Repair Head.
4.2.2 Local Retransmission
If a Repair Head or Sender determines from its child nodes TRACK
messages that a Data message was missed, the Repair Head retransmits
the Data message. The Repair Head or Sender multicasts the
Retransmission message on its multicast Local Control Channel. In
the event that a Repair Head receives a retransmission and knows that
its children need this repair, it re-multicasts the retransmission to
its children.
The scope of retransmission (the multicast TTL) is considered part of
the Control Channels multicast address, and is derived during tree
configuration.
A Repair Head maintains the following state for each of its children,
for the purpose of providing repair service to the local group:
- HighestConsecutivelyReceived. A Sequence Number indicating all
Data messages up to this number (inclusive) that have been received
by a given child.
- MissingMessages. A data structure to keep track of the reception
status of the Data messages with Sequence Number higher than
HighestConsecutivelyReceived.
The minimum HighestConsecutivelyReceived value of all its children is
kept as the variable LocalStable.
Whetten Expires - May 2003 [Page 23]
TRACK BUILDING BLOCK November 2002
A Repair Head also maintains a retransmission buffer. The size of the
retransmission buffer MUST be greater than the maximum value of a
Senders transmission window. The retransmission buffer MUST keep all
the Data messages received by the Repair Head with Sequence Number
higher than LocalStable, optionally some messages with Sequence
Number lower than LocalStable if there is room (beyond the maximum
value of Senders transmission window). The latter messages are kept
in the retransmission buffer in case a Receiver from another group
losses its parent and needs to join this group.
As TRACK messages are received, the Repair Head updates the above
state variables.
To perform local repair, a Repair Head implements a retransmission
queue with memory. Each lost message is entered into the
retransmission queue in increasing order according to its Sequence
Number. If the same Data message has already been retransmitted
recently (recognized due to the queues memory) it is delayed by the
local group RTT (see roundtrip time measurement) before
retransmission.
Retransmissions MAY NOT be sent at a faster rate than the current
TransmissionRate advertised by the Sender.
4.2.3 Flow and Rate Control
TRACK offers the ability to limit the rate of Data traffic, through
both flow control and rate limits.
When a Receiver sends a TRACK to its parent, the HighestAllowed field
provides information on the status of the Receivers flow control
window. The value of HighestAllowed is computed as follows:
HighestAllowed = seqnum + ReceiverWindow
Where seqnum is the highest Sequence Number of consecutively received
data messages at the Receiver. The size of the ReceiverWindow may
either be based on a parameter local to the Receiver or be a global
parameter.
If flow control is enabled for a given Data Session, then a Sender
MUST NOT send any Data messages to the Data Channel Protocol that are
higher than the current value for HighestAllowed that it has. On
startup, HighestAllowed is initialized to ReceiverWindow.
In addition, the Sender application MAY provide minimum and maximum
rate limits. Unless overridden by the Data Channel Protocol, a
Sender will not offer Data messages to the Data Channel Protocol at
lower than MinimumDataRate (except possibly during short periods of
Whetten Expires - May 2003 [Page 24]
TRACK BUILDING BLOCK November 2002
time when certain slow Receivers are being ejected), or higher than
MaximumDataRate. If a Receiver is not able to keep up with the
minimum rate for a period of time, it SHOULD leave the group
promptly. Receivers that leave the group MAY attempt to rejoin the
group at a later time, but SHOULD NOT attempt an immediate
reconnection.
4.2.4 Reliability Window
The Sender and each Repair Head maintain a window of messages for
possible retransmission. As messages are acknowledged by all of its
children, they are released from the parents retransmission buffer,
as described in 4.2.2. In addition, there are two global parameters
that can affect when a parent releases a data message from the
retransmission buffer -- MinHoldTime, and MaxHoldTime.
MinHoldTime specifies a minimum length of time a message must be held
for retransmission from when it was received. This parameter is
useful to handle scenarios where one or more children have been
disconnected from their parent, and have to reconnect to another.
If, for example, MinHoldTime is set to FAILURE_DETECTION_REDUNDANCY *
2 * ConstantHeartbeatPeriod, then there is a high likelihood that any
child will be able to recover any lost messages after reconnecting to
another parent.
The Sender continually advertises to the members of the Data Session
both edges of its retransmission window. The higher value is the
SeqNum field in each Data or NullData message, which specifies the
highest Sequence Number of any data message sent. The trailing edge
of the window is advertised in the HighestReleased field. This
specifies the largest Sequence Number of any message sent that has
subsequently been released from the Senders retransmission window.
If both values are the same then the window is presently empty. Zero
is not a legitimate value for a data Sequence Number, so if either
field has a value of zero, then no messages have yet reached that
state. All Sequence Number fields use Sequence Number arithmetic so
that a Data Session can continue after exhausting the Sequence Number
space.
When a member of a Data Session receives an advertisement of a new
HighestReleased value, it stores this, and is no longer allowed to
ask for retransmission for any messages up to and including the
HighestReleased value. If it has any outstanding missing messages
that are less than or equal to HighestReleased, it MAY move forward
and continue delivering the next data messages in the stream. It
also SHOULD report an error for the messages that are no longer
recoverable.
Whetten Expires - May 2003 [Page 25]
TRACK BUILDING BLOCK November 2002
MaxHoldTime specifies the maximum length of time a message may be
held for retransmission. This parameter is set at the Sender which
uses it to set the HighestReleased field in data message headers.
This is particularly useful for real-time, semi-reliable streams such
as live video, where retransmissions are only useful for up to a few
seconds. When combined with Unordered delivery semantics, and
application-level jitter control at the Receivers, this provides Time
Bounded Reliability. MaxHoldTime MUST always be larger than
MinHoldTime.
4.2.5 Ordering Semantics
TRACK offers two flavors of ordering semantics: Ordered or Unordered.
One of these is selected on a per session basis as part of the
Session Configuration Parameters.
Unordered service provides a reliable stream of messages, without
duplicates, and delivers them to the application in the order
received. This allows the lowest latency delivery for time sensitive
applications. It may also be used by applications that wish to
provide its own jitter control.
Ordered service provides TCP semantics on delivery. All messages are
delivered in the order sent, without duplicates.
4.2.6 Retransmission Requests.
A Receiver detects that it has missed one or more Data messages by
gaps in the sequence numbers of received messages. Each Receiver
keeps track of HighestSequenceNumber, the highest sequence number
known of for a Data Session, as observed from Data, RData, and
NullData messages. Any sequence numbers between HighestReleased and
HighestSequenceNumber that have not been received are assumed to be
missing.
When a Receiver detects missing messages it MAY send off a request
for retransmission, if local retransmission is enabled. It does this
by sending a Retransmission Request message. The timing of this
request is described below.
4.2.7 End Of Stream.
When an application signals that a Data Session is complete, the
Sender advertises this to its children by setting the End of Session
option on the last Data Message in the Data Session, as well as all
subsequent retransmissions of that Data Message, and all subsequent
Null Data messages.
Whetten Expires - May 2003 [Page 26]
TRACK BUILDING BLOCK November 2002
The Sender SHOULD NOT leave the Data Session until it has a report
from the TRACK reports that all group members have left the Data
Session, or it has waited a period of at least
FAILURE_DETECTION_REDUNDANCY * TrackTimeout seconds.
4.3 Control Traffic Generation and Aggregation.
One of the largest challenges for scaleable reliable multicast
protocols has been that of controlling the potential explosion of
control traffic. There is a fundamental tradeoff between the latency
with which losses can be detected and repaired, and the amount of
control traffic generated by the protocol.
TRACK messages are the primary form of control traffic in this BB.
They are sent from Receivers and Repair Heads to their parents.
TRACK messages may be sent for the following purposes:
- to request retransmission of messages
- to advance the Senders transmission window for flow control
purposes
- to deliver application level confirmation of data reception
- to propagate other relevant feedback information up through the
session (such as RTT and loss reports, for congestion control)
4.3.1 TRACK Generation with the Rotating TRACK Algorithm
Each Receiver sends a TRACK message to its parent once per AckWindow
of data messages received. A Receiver uses an offset from the
boundary of each AckWindow to send its TRACK, in order to reduce
burstiness of control traffic at the parents. Each parent has a
maximum number of children, MaxChildren. When a child binds to the
parent, the parent assigns a locally unique ChildID to that child,
between 0 and MaxChildren-1.
Each child in a tree generates a TRACK message at least once every
AckWindow of data messages, when the most recent data messages
Sequence Number, modulo AckWindow, is equal to MemberID. If the
message that would have triggered a given TRACK for a given node is
missed, the node will generate the TRACK as soon as it learns that it
has missed the message, typically through receipt of a higher
numbered data message.
Together, AckWindow and MaxChildren determine the maximum ratio of
control messages to data messages seen by each parent, given a
constant load of data messages.
In each data message, the Sender advertises the current MessageRate
(measured in messages per second) it is sending data at. This rate
is generated by the congestion control algorithms in use at the
Sender.
Whetten Expires - May 2003 [Page 27]
TRACK BUILDING BLOCK November 2002
At the time a node sends a regular TRACK, it also computes a
TRACKTimeout value:
interval = AckWindow / MessageRate
TRACKTimeout = 2 * interval
If no TRACKs are sent within TRACKTimeout interval, a TRACK is
generated, and TRACKTimeout is increased by a factor of 2, up to a
value of MAX_TRACK_TIMEOUT.
This timer mechanism is used by a Receiver to ensure timely repair of
lost messages and regular feedback propagation up the tree even when
the Sender is not sending data continuously. This mechanism
complements the AckWindow-based regular TRACK generation mechanism.
4.3.2 TRACK Aggregation.
There are many reasons for providing feedback from all the Receivers
to the Sender in an aggregated form. The major ones are listed
below:
1) End-to-end delivery confirmation. This confirmation tells the
Sender that all the Receivers (in the entire tree) have received data
messages up to a certain Sequence Number. This is carried in an
Application Level Confirmation message.
2) Flow control. The aggregated information is carried in the field
HighestAllowed. It tells the Sender the highest Sequence Number that
all the Receivers (in the entire tree) are prepared to receive.
3) Congestion control feedback. Information about the state of the
tree can be passed up to help control the congestion control
algorithms for the group.
4) Counting current membership in the group. This information is
carried in the field SubTreeCount. This lets the Sender know the
number of Receivers currently connected to the repair tree.
5) Measuring the round-trip time from the Sender to the "worst"
Receiver.
A Repair Head maintains state for each child. Each time a TRACK
(from a child) is received, the corresponding states for that child
are updated based on the information in the TRACK message. When a
Repair Head sends a TRACK message to its parent, the following fields
of its TRACK message are derived from the aggregation of the
Whetten Expires - May 2003 [Page 28]
TRACK BUILDING BLOCK November 2002
corresponding states for its children. The following rules describe
how the aggregation is performed:
- WorstLossRate. Take the maximum value of the WorstLossRate from
all Children.
- SubTreeCount. Take the sum of the SubTreeCount from all Children.
- HighestAllowed. Take the minimum of the HighestAllowed value from
all children.
- WorstEdgeThroughput. Take the minimum value of the
WorstEdgeThroughput field from all Children.
- UnicastCost. Take the sum of the UnicastCost from all Children.
- MulticastCost. Take the sum of the MulticastCost from all
Children.
- SenderDallyTime: take the minimum value, for all of the children,
of (childs reported SenderDallyTime + childs local dally time).
- FailureCount: take the sum of the FailureCount for all Children.
- FailureList: concatenate the FailureList fields for all Children,
up to a maximum list size of MaxFailureListSize.
Note, the SenderTimeStamp, ParentTimestamp, and ParentDallyTime
fields are not aggregated. The Sender will derive the roundtrip time
to the worst Receiver by doing its local aggregation for
SenderDallyTime and then compute:
RTT = currentTime û SenderTimeStamp û SenderDallyTime.
Application level confirmations (ALCs) are handled as follows. For a
set of ALC requests from receivers, the ones with the highest value
for HighConfirmationSequenceNumber are considered, and all others are
discarded.
For the ConfirmationStatus field, the following rules apply. Note
that ConfirmationStatus of SomeReceiversAcknowledge can correspond to
a ConfirmationCount of zero.
If all children report AllReceiversAcknowledge Then
ConfirmationStatus = AllReceiversAcknowlege
Else If at least one child reports (ListOfFailures OR
FailuresExceedMaximumListSize) Then
If the count of all reported failures >
MaximumFailureListSize Then
ConfimationStatus = FailuresExceedMaximumListSize
Else
ConfirmationStatus = ListOfFailures
Else
ConfirmationStatus = SomeReceiversAcknowledge
The ConfirmationCount field is equal to the sum of the
ConfimationCount for the aggregated ALC reports of all Children. The
PendingCount field is equal to the sum of the PendingCount fields of
all Children. The FailureList field is the concatenation of the
Whetten Expires - May 2003 [Page 29]
TRACK BUILDING BLOCK November 2002
FailureList fields of all aggregated ALC reports of all children, up
to a maximum length of MaximumFailureListSize.
In addition to these fields with fixed aggregation rules, TRACK
supports a set of user defined aggregation statistics. These
statistics are self describing in terms of their data type and
aggregation method. Statistics reports are numbered, and only the
most recent statistics report request is aggregated to the Sender.
Statistics are aggregated over the set of Child statistics reports
that have been received with that number. Aggregation methods
include minimum, maximum, sum, product, and concatenation.
4.3.3 Statistics Reporting.
A Sender can request a list of aggregated statistics from all
Receivers in the group. There are a set of predefined statistics,
such as loss rate and average throughput. There is also the capacity
to request a set of other TRACK statistics, as well as application
defined statistics.
The format of each statistic is self-describing, both in terms of
data type, size, and aggregation method. A Sender reliably sends out
a statistics request by attaching it as an option to a Data message.
When a Receiver gets a request for a statistic, it fills in the data
fields, and forwards it up the tree in the next TRACK message. Since
TRACKs are not reliable, multiple copies are sent in a total of
NumReplies consecutive TRACK messages from each Receiver. Each
statistics report is aggregated according to the method described in
the statistic, and the result is delivered to the Sender.
Most aggregation options have fixed length no matter how many
Receivers there are. The one exception is concatenation, which
creates a list of values from some or all Receivers, up to a length
of MaximumStatisticsListSize entries. It is NOT RECOMMENDED to use
this to create group-wide lists, unless the groups size is carefully
controlled.
4.4 Application Level Confirmed Delivery.
Flow control and the reliability window are concerned with goodput,
of delivering data with a high probability that it is delivered at
all Receivers. However, neither mechanism provides explicit
confirmation to the Sender as to the list of recipients for each
message. Application level confirmed delivery allows applications to
determine the set of applications that have received a set of data
messages.
There are three primary factors that determine the reliability
semantics of a message: the senders knowledge of the Receiver list,
Whetten Expires - May 2003 [Page 30]
TRACK BUILDING BLOCK November 2002
the application level actions that must be performed in order to
consider a message delivered, and the response to persistent failure
conditions at Receivers. For example, an extremely strong
distributed guarantee would consist of the following. First, the
full Receiver membership list is known at the Sender, and verified to
make sure no Receivers have left the group. Second, the application
at each Receiver must write the Data to persistent store before it
can be acknowledged. Third, Receivers are given a very long period
of time - say one hour û to recover all lost Data messages, before
they are ejected from the Data Session. In the meantime,
transmission of Data messages is flow controlled by the slowest
receivers.
A weaker form of reliability would include the following. First,
that the Sender gets a count of Receivers, and otherwise depends on
the distributed group membership algorithms to maintain the
membership list. Second, that Data messages are considered reliably
delivered as soon as the application receives the Data from TRACK.
Third, that Retransmissions are limited to only 30 seconds, and
Receivers must choose to leave the Data Session or continue with
missing Data messages, if a failure takes longer than this period to
recover from.
TRACK provides the functionality to easily implement a wide range of
application level confirmation semantics, based on how these three
items are configured. It is the applications responsibility to then
select the configurations it desires for a given Data Session.
4.4.1 Application Level Confirmation Mechanisms
The primary mechanism for application level confirmation (ALC) of
delivery is the ALC report. To check for ALC of delivery, a Sender
issues a Application Level Confirmation Request, by attaching this
message as an option to a Data message, and reliably transmitting it
to all Receivers. Each ALC Request includes a specified level of
reliability, a reply redundancy factor, and the range of Data message
sequence numbers that the ALC Confirmation covers.
When a Receiver gets an ALC Request, it checks to see if the
application has delivered the specified range of Data Messages,
including both the Low Confirmation Sequence Number and the High
Confirmation Sequence Number. When it sends the next TRACK out, it
sets the ConfirmationStatus field to either SomeReceiversAcknowledge
if it is still pending confirmation, AllReceiversAcknowledge if it
has application level confirmation, ListOfFailures if it has a
failure and MaximumFailureListSize > 0, or
FailuresExceedsMaximumListSize otherwise. It also sets the
ConfirmCount to 1 if it has a confirmation, and PendingCount to 1 if
Whetten Expires - May 2003 [Page 31]
TRACK BUILDING BLOCK November 2002
it is still pending. If the Immediate ACK bit is set in the ALC
Request, the Receiver generates an ACK immediately.
One example of how an application can implicitly signal confirmation
of delivery is through the freeing of buffers passed to it by the
transport. The API could specify that whenever an application has
freed up a buffer containing one or more data messages, then these
messages are considered acknowledged by the application.
Alternatively, the application could be required to explicitly
acknowledge each message.
4.5 Distributed RTT Calculations.
This TRACK BB provides two algorithms for distributed RTT
calculations ù LocalRTT measurements and SenderRTT measurements.
LocalRTT measurements are only between a parent and its children.
SenderRTT measurements are end-to-end RTT measurements, measuring the
RTT to the worst Receiver as selected by the congestion control
algorithms.
The SenderRTT is useful for congestion control. It can be used to set
the data rate based on the TCP response function, which is being
proposed for the congestion control building blocks.
The LocalRTT can be used to (a) quickly detect faulty children (as
described under fault detection) or (b) avoid sending unnecessary
retransmissions (as described in the local repair algorithm).
In the case of LocalRTT measurements, a parent initiates measurement
by including a ParentTimestamp field in a Heartbeat message sent to
its children. When a child receives a Heartbeat message with this
field set, it notes the time of receipt using its local system clock,
and stores this with the message as HeartbeatReceiveTime. When the
child next generates a TRACK, just before sending it, it measures its
system clock again as TRACKSendTime, and calculates the
LocalDallyTime.
LocalDallyTime = TRACKSendTime û HeartbeatReceiveTime.
The child includes this value, along with the ParentTimestamp field,
as fields in the next TRACK message sent. Every heartbeat message
that is multicast to all children SHOULD include a ParentTimestamp
field.
The SenderRTT algorithm is similar. A Sender initiates the process
by including a SenderTimestamp field in a data message. When a
Receiver gets a message with this field set, it keeps track of the
DataReceiveTime for that message, and when it generates the next
TRACK message, includes the SenderTimestamp and SenderDallyTime
Whetten Expires - May 2003 [Page 32]
TRACK BUILDING BLOCK November 2002
value. These values are aggregated by Repair Heads, as described
above.
Each node only keeps track of the most recent value for
{SenderTimestamp, DataReceiveTime} and {ParentTimestamp,
HeartbeatReceiveTime}, replacing any older values any time that a new
message is received with these values set. As long as it has non-
zero values to report, each node sends up both a {SenderTimestamp,
SenderDallyTime} and a {ParentTimestamp, LocalDallyTime} set of
fields in each TRACK message generated.
Unless redefined by the TRACK PI, these RTT measurements are averaged
using an exponentially weighted moving average, where the first RTT
measurement, RTT_measurement, initializes the average RTT_average,
and then each successive measurement is averaged in according to the
following formula. The RECOMMENDED value for alpha is 1/8.
RTT_average = RTT_measurement * alpha + RTT_average (1-alpha)
4.6 SNMP Support
The Repair Heads and the Sender are designed to interact with SNMP
management tools. This allows network managers to easily monitor and
control the sessions being transmitted. All TRACK nodes MAY have
SNMP MIBs defined in a separate document. SNMP support is OPTIONAL
for Receiver nodes, but is RECOMMENDED for all other nodes.
4.7 Late Join Semantics
TRACK offers three flavors of late join support:
a) No Recovery
A Receiver binds to a Repair Head after the session has started
and agrees to the reliability service starting from the Sequence
Number in the current data message received from the Sender.
b) Continuation
This semantic is used when a Receiver has lost its Repair Head
and needs to re-affiliate. In this case, the Receiver must
indicate the oldest Sequence Number it needs to repair in order
to continue the reliability service it had from the previous
Repair Head. The binding occurs if this is possible.
c) No Late Join
For some applications, it is important that a Receiver receives
either all data or no data (e.g. software distribution). In this
case option (c) is used.
These are specified by the LateJoinSemantics session parameter, and
enforced by a Parent when a Child attempts to bind to it.
5. Message Types
Whetten Expires - May 2003 [Page 33]
TRACK BUILDING BLOCK November 2002
The following table summarizes the messages and their fields used by
the TRACK BB. All messages contain the session identifier. For more
details, please see the sample TRACK PI [17].
+--------------------------------------------------------------------+
Message From To Mcast? Fields
+--------------------------------------------------------------------+
BindRequest Child Parent no Scope, Level, Role, Rejoin
BindSequenceNumber, SubTreeCount
+--------------------------------------------------------------------+
BindConfirm Parent Child no RepairAddr, BindSequenceNumber
LowestAvailableRepair
Level, ChildIndex, Role
+--------------------------------------------------------------------+
BindReject Parent Child no Reason, BindSequenceNumber
+--------------------------------------------------------------------+
UnbindRequest Child Parent no Reason, ChildIndex
+--------------------------------------------------------------------+
UnbindConfirm Parent Child no
+--------------------------------------------------------------------+
EjectRequest Parent Child either Reason, AlternateParent
+--------------------------------------------------------------------+
EjectConfirm Child Parent no
+--------------------------------------------------------------------+
Heartbeat Parent Child either Level, ParentTimestamp
ChildrenList, SeqNum
HighestReleased
+--------------------------------------------------------------------+
NullData, Sender all yes SenderTimeStamp, DataLength
OData HighestReleased, SeqNum
EndOfStream, TransmissionRate
+--------------------------------------------------------------------+
Rdata Parent Child yes SenderTimeStamp, DataLength
HighestReleased, SeqNum
EndOfStream, TransmissionRate
+--------------------------------------------------------------------+
Track Child Parent no BitMask, SubTreeCount
Slowest, HighestAllowed
Whetten Expires - May 2003 [Page 34]
TRACK BUILDING BLOCK November 2002
ParentThere, ParentTimeStamp
ParentDallyTime, SenderTimeStamp
SenderDallyTime, CongestionControl
FailureList
+--------------------------------------------------------------------+
ALCRequest Sender Receiver yes Immediate, Reliability
NumReplies, SeqNumRange
+--------------------------------------------------------------------+
ALCReply Child Parent yes SeqNumRange, ConfirmStatus
ConfirmCount, PendingCount
FailedChildren
+--------------------------------------------------------------------+
StatsRequest Sender Receiver yes Immediate, StatsSeqNum
NumReplies, StatsList
+--------------------------------------------------------------------+
StatsReply Child Parent yes StatsSeqNum, StatsList
+--------------------------------------------------------------------+
Whetten Expires - May 2003 [Page 35]
TRACK BUILDING BLOCK November 2002
The various fields of the messages are described as follows:
- BindSequenceNumber: This is a monotonically increasing sequence
number for each bind request from a given Receiver for a given Data
Session.
- Scope: an integer to indicate how far a repair message travels.
This is optional.
- Rejoin: a flag as to whether this Receiver was previously a member
of this Data Session.
- Level: an integer that indicates the level in the repair tree.
This value is used to keep loops in the tree from forming, in
addition to indicating the distance from the Sender. Any changes in
a nodes level are passed down to the Tree BB using the
treeLevelUpdate interface.
- Role: This indicates if the bind requestor is a Receiver or Repair
Head.
- SubTreeCount: This is an integer indicating the current number of
Receivers below the node.
- RepairAddr: This field in the BindConfirm message is used to tell
the Receiver which multicast address the Repair Head will be sending
retransmissions on. If this field is null, then the Receiver should
expect retransmissions to be sent on the Senders data multicast
address.
- AlternateParent: This is an optional field that specifies another
parent a Child may attempt to bind to.
- SeqNum: an integer indicating the Sequence Number of a data message
within a given Data Session. For a Heartbeat, it is the highest
sequence number the parent knows about.
- ChildIndex: This is an integer the Repair Head assigns to a
particular child. The child Receiver uses this value to implement
the rotating TRACK Generation algorithm.
- LowestRepairAvailable: This is the lowest sequence number that a
Repair Head will provide repairs for.
- Reason: a code indicating the reason for the BindReject,
UnbindRequest, or EjectRequest message.
Whetten Expires - May 2003 [Page 36]
TRACK BUILDING BLOCK November 2002
- ParentTimestamp: This field is included in Heartbeat messages to
signal the need to do a local RTT measurement from a parent. It is
the time when the parent sent the message.
- ChildrenList: This field contains the identifiers for a list of
children. As part of the keepalive message, this field together with
the SeqNum field is used to urge those listed Receivers to send a
TRACK (for the provided SeqNum). The Repair Head sending this must
have been missing the regular TRACKs from these children for an
extended period of time.
- SenderTimestamp: This field is included in Data messages to signal
the need to do a roundtrip time measurement from the Sender, through
the tree, and back to the Sender. It is the time (measured by the
Senders local clock) when it sent the message.
- ApplicationSynch: a Sequence Number signaling a request for
confirmed delivery by the application.
- EndOfStream: indicates that this message is the end of the data for
this session.
- TransmissionRate: This field is used by the Sender to tell the
Receivers its sending rate, in messages per second. It is part of
the data or nulldata messages.
- HighestReleased: This field contains a Sequence Number,
corresponding to the trailing edge of the Senders retransmission
window. It is used (as part of the data, nulldata or retransmission
headers) to inform the Receivers that they should no longer attempt
to recover those messages with a smaller (or same) Sequence Number.
- HighestAllowed: a Sequence Number, used for flow control from the
Receivers. It signals the highest
Sequence Number the Sender is allowed to send that will not overrun
the Receivers buffer pools.
- BitMask: an array of 1s and 0s. Together with a Sequence Number it
is used to indicate lost data messages. If the ith element is a 1,
it indicates the message SeqNum+i is lost.
- Slowest: This field contains a field that characterizes the slowest
Receiver in the subtree beneath (and including) the node sending the
TRACK. This is used to provide information for the congestion
control BB.
- SenderDallyTime: This field is associated with a SenderTimestamp
field. It contains the sum of the waiting time that should be
subtracted from the RTT measurement at the Sender.
Whetten Expires - May 2003 [Page 37]
TRACK BUILDING BLOCK November 2002
- ParentDallyTime: This is the same as the SenderDallyTime, but is
associated with a ParentTimestamp instead of a SenderTimestamp.
- DataLength: This is the length of the Data payload.
- CongestionControl: This includes any additional congestion control
variables for aggregation, such as WorstLossRate,
WorstEdgeThroughput, UnicastCost, and MulticastCost.
- ApplicationConfirms: This is the SeqNum value for which delivery
has been confirmed by all children at or below this parent.
- FailedChildren: This is a list of all children that have recently
been dropped from the repair tree.
- Immediate: If set to 1, a Receiver should immediately send a TRACK
on receipt of this packet.
- Reliability: The level of reliability required in order to consider
the set of data packets reliably delivered.
- NumReplies: The number of consecutive TRACK messages that should be
sent with this message attached
- SeqNumRange: The set of data messages that the ALC request applies
to.
- ConfirmStatus: The acknowledgement status of the Receivers in the
subtree up to the node that sends this message.
- ConfirmCount: The number of Receivers in the subtree up to the node
that sends this message, that have acknowledged the ALC request.
- PendingCount: The number of Receivers in this subtree that are
still pending in their decision as to acknowledging this ALC request.
- StatsSeqNum: The number of this request for statistics.
- StatsList: The list of statistics to be filled in by Receivers, and
aggregated by the control tree.
6. Global Configuration Variables, Constants, and Reason Codes
6.1 Global Configuration Variables
These are variables that control the Data Session and are advertised to
all participants. Some of them MAY instead be configured as constants.
Whetten Expires - May 2003 [Page 38]
TRACK BUILDING BLOCK November 2002
- TimeMaxBindResponse: the time, in seconds, to wait for a response
to a BindRequest. Initial value is TIMEOUT_PARENT_RESPONSE
(recommended value is 3). Maximum value is
MAX_TIMEOUT_PARENT_RESPONSE.
- MaxChildren: The maximum number of children a Repair Head is
allowed to handle. Recommended value: 32.
- ConstantHeartbeatPeriod: Instead of dynamically calculating the
HeartbeatPeriod, a constant period may be used instead. Recommended
value: 3 seconds.
- MinimumHeartbeatPeriod: The minimum value for the dynamically
calculated HeartbeatPeriod. Recommended value: 1 second.
- MinHoldTime: The minimum amount of time a Repair Head holds on to
data messages.
- MaxHoldTime: The maximum amount of time a Repair Head holds on to
data messages.
- AckWindow: The number of messages seen before a Receiver issues an
acknowledgement. Recommended value: 32.
- LateJoinSemantics: The options available to a Receiver who wishes
to join a Data Session that is already in progress.
- MaximumFailureListSize: The maximum number of entries that can be
in a failure list. This MUST be small enough that the FailureList
does not ever cause a TRACK to exceed the size of a maximum UDP
packet. Recommended value: 800.
- MaximumStatisticsListSize: The maximum number of entries that can
be in a statistics list. This MUST be small enough that the
FailureList does not ever cause a TRACK to exceed the size of a
maximum UDP packet. Recommended value: 100.
- MaximumDataRate: The maximum admission rate for data messages from
the application to the Data Channel Protocol.
- MinimumDataRate: The minimum admission rate for data messages from
the application to the Data Channel Protocol.
6.2 Constants
- NUM_MAX_PARENT_ATTEMPTS: The number of times to try to bind to a
Repair Head before declaring a PARENT_UNREACHABLE error. Recommended
value is 5.
Whetten Expires - May 2003 [Page 39]
TRACK BUILDING BLOCK November 2002
- TIMEOUT_PARENT_RESPONSE: The minimum value, in seconds, between
attempts to contact a parent. Recommended value is 1 second.
- MAX_TIMEOUT_PARENT_RESPONSE: The maximum value, in seconds,
between attempts to contact a parent. Recommended value is 16.
- NULL_DATA_PERIOD: The time between transmission of NullData
Messages. Recommended value is 1.
- FAILURE_DETECTION_REDUNDANCY: The number of times a message is sent
without receiving a response before declaring an error. Recommended
value is 3.
- MAX_TRACK_TIMEOUT: The maximum value for TRACKTimeout. Recommended
value is 5 seconds.
- TRANSMISSION_REDUNDANCY: The number of times a failure notification
is redundantly sent up the tree in a TRACK message. Recommended
value is 3.
6.3 Reason Codes
- BindReject reason codes
- LOOP_DETECTED
- MAX_CHILDREN_EXCEEDED
- UnbindRequest reason codes
- SESSION_DONE
- APPLICATION_REQUEST
- RECEIVER_TOO_SLOW
- EjectRequest reason codes
- PARENT_LEAVING
- PARENT_FAILURE
- CHILD_TOO_SLOW
- PARENT_OVERLOADED
7. Security
As specified in [12], the primary security requirement for a TRACK
protocol is protection of the transport infrastructure. This is
accomplished through the use of lightweight group authentication of
the control and, optionally, the data messages sent to the group.
These algorithms use IPsec and shared symmetric keys. For TRACK,
[12] recommends that there be one shared key for the Data Session and
one for each Local Control Channel. These keys are distributed
through a separate key manager component, which may be either
centralized or distributed. Each member of the group is responsible
Whetten Expires - May 2003 [Page 40]
TRACK BUILDING BLOCK November 2002
for contacting the key manager, establishing a pair-wise security
association with the key manager, and obtaining the appropriate keys.
The exact algorithms for this BB are presently the subject of
research within the IRTF Secure Multicast Group (SMuG) and
standardization within the Multicast Security working group.
8. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Whetten, B., et. al. "Reliable Multicast Transport Building
Blocks for One-to-Many Bulk-Data Transfer." RFC 3048, January
2001.
[3] Handley, M., et. al. "The Reliable Multicast Design Space for
Bulk Data Transfer." RFC 2887, August 2000.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[5] Whetten, B., Taskale, G. "Overview of the Reliable Multicast
Transport Protocol II (RMTP-II)." IEEE Networking, Special Issue
on Multicast, February 2000.
[6] Nonnenmacher, J., Biersack, E. "Reliable Multicast: Where to
use Forward Error Correction", Proc. 5th. Workshop on Protocols
for High Speed Networks, Sophia Antipolis, France, Oct. 1996.
[7] Nonnenmacher, J., et. al. "Parity-Based Loss Recovery for
Reliable Multicast Transmission", In Proc. of ACM SIGCOMM 97,
Cannes, France, September 1997.
[8] Rizzo, L. "Effective erasure codes for reliable computer
communications protocols", DEIT Technical Report LR-970115.
[9] Nonnenmacher, J., Biersack, E. "Optimal Multicast Feedback",
Proc. IEEE INFOCOM 1998, March 1998.
[10] Whetten, B., Conlan, J. "A Rate Based Congestion Control
Scheme for Reliable Multicast", GlobalCast Communications
Technical White Paper, November 1998.
http://www.talarian.com/rmtp-ii
[11] Padhye, J., et. al. "Modeling TCP Throughput: A Simple Model
and its Empirical Validation". University of Massachusetts
Technical Report CMPSCI TR 98-008.
Whetten Expires - May 2003 [Page 41]
TRACK BUILDING BLOCK November 2002
[12] Hardjorno, T., Whetten, B. "Security Requirements for TRACK"
draft-ietf-rmt-pi-track-security-00.txt, June 2000. Work in
Progress.
[13] Golestani, J., "Fundamental Observations on Multicast
Congestion Control in the Internet", Bell Labs, Lucent Technology,
paper presented at the July 1998 RMRG meeting.
[14] Kadansky, M., D. Chiu, J. Wesley, J. Provino, "Tree-based
Reliable Multicast (TRAM)", draft-kadansky-tram-02.txt, Work in
Progress.
[15] Whetten, B., M. Basavaiah, S. Paul, T. Montgomery, "RMTP-II
Specification", draft-whetten-rmtp-ii-00.txt, April 8, 1998. Work
in Progress.
[16] Kadansky, M., Chiu, D. M., Whetten, B., Levine, B. N., Taskale,
G., Cain, B., Thaler, D., Koh, s. J., "Reliable Multicast
Transport Building Block: Tree Auto-Configuration", draft-ietf-
rmt-bb-tree-config-02.txt, March 2, 2001. Work in Progress.
[17] Whetten, B. et. al., "TRACK Protocol Instantiation Over UDP",
draft-ietf-rmt-track-pi-udp-00.txt, November 2002.
[18] Adamson, B., et. al., "NACK Oriented Reliable Multicast
Protocol (NORM), draft-ietf-rmt-pi-norm-02.txt, July 2001. Work
in Progress.
[19] Vicisano, L., et. al., "Asynchronous Layered Coding - A
scalable reliable multicast protocol", draft-ietf-rmt-pi-alc-
02.txt, July 2001. Work in Progress.
[20] Speakman, T., et. al., "Pragmatic General Multicast (PGM)",
draft-speakman-pgm-spec-06.txt, Feb 2001. Work in Progress.
[21] Kermode, R., Vicisano, L., "Author Guidelines for RMT Building
Blocks and Protocol Instantiation Documents", RFC 3269.
10. Acknowledgements
We would like to thank the follow people: Sanjoy Paul, Seok Joo Koh,
Supratik Bhattacharyya, Joe Wesley, and Joe Provino.
11. Authors Addresses
Brian Whetten
890 Sea Island Lane
Foster City, CA 94404
b2@whetten.net
Whetten Expires - May 2003 [Page 42]
TRACK BUILDING BLOCK November 2002
Dah Ming Chiu
Sun Microsystems Laboratories
1 Network Drive
Burlington, MA 01803
dahming.chiu@sun.com
Miriam Kadansky
Sun Microsystems Laboratories
1 Network Drive
Burlington, MA 01803
miriam.kadansky@sun.com
Seok Joo Koh
sjkoh@pec.etri.re.kr
Gursel Taskale
TIBCO Corporation
gursel@tibco.com
Full Copyright Statement
"Copyright (C) The Internet Society (2000). All Rights Reserved. This
document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Whetten Expires - May 2003 [Page 43]
TRACK BUILDING BLOCK November 2002
Whetten Expires - May 2003 [Page 44]