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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Group (Multicast) Communication Group (Multicast) Communication in Wide Area Networks in Wide Area Networks
Mostafa Ammar Don TowsleyMostafa Ammar Don TowsleyCollege of Computing Dept. of Computer Science
Georgia Tech University of Massachusetts
Atlanta, GA Amherst, MA
[email protected] [email protected]
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Definition
MulticastMulticast: is the act of sending a message to multiple receivers using a single local “transmit” operation
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
A Spectrum of Paradigms
UnicastUnicast BroadcastBroadcast
MulticastMulticast
Send toSend tooneone
Send toSend toAllAll
Send to someSend to some
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
The Layering of Multicast
MulticastMulticastbybyUnicastUnicast
MulticastMulticastbybyBroadcastBroadcast
MulticastMulticastbybyMulticastMulticast
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
The Many Uses of Multicasting
Teleconferencing Distributed Games Software/File Distribution Video Distribution Replicated Database Updates
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Application Models
Point-to-Multipoint: Single Source, Multiple Receivers
Multipoint-to-Multipoint: Multiple Sources, Multiple Receivers
Sources are receivers Sources are not receivers
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Scope of Tutorial Support for multicast communication in
– Transport– Network– Link Layer
Also important: enforcing reception semantics across receivers (ordering, atomicity) -- in distributed computing literature
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Support Challenges
Overhead for network layer support Scalability Dealing with heterogeneity
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Outline
IP Multicast and the Mbone Multicast Routing QoS and Real-Time support Reliable Multicast Transport Multicast Flow Control ATM and IP/ATM Multicast Summary
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IP Multicast and the Mbone
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Server-Oriented Multicast source sets up one-to-many multicast group each source responsible for its own group examples:
– ATM (explicit connection to each receiver)
– ST-II (receivers listed in setup message)
for connection-oriented services(packet header size!)
discourages dynamic groups
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Receiver-Oriented Multicast
Deering, 1991
senders need not be members
groups may be of any size
no topological restrictions on membership
membership dynamic and autonomous
host groups may be transient or permanent
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IP Multicast host-group model network-level; same packet format, different
address routers do all of the work special IP addresses: 224.0.0.0
- 239.255.255.255 28 bits 268 million groups (plus scope) ttl value limits distribution
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Forwarding
1. check incoming interface: discard if not on shortest path to source
2. forward to all outgoing interfaces
3. don’t forward if interface has been pruned
4. prunes time out every minute
5. routers may send grafts upstream
Routing done by DVMRP
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IP Multicast (cont.)
Basic idea is to flood and prune
R
R
router packet
S
noreceiver
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IP Multicast (cont.)Prune branches where no members and branches not
on shortest paths
R
R
2nd packet
S
prune
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IP Multicast (cont.)Add new user via grafting; departure via pruning
R
R
S
graft
R
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
MboneMBONE
tunnel endpoint
IP router
WS
Mbone = multicast backbone
virtual network overlaying Internet
needed until mcast capable routers deployed
IP in IP encapsulation limited capacity,
resilience
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Mbone Protocols IP UDP: best effort RTP: real-time
transport RSVP: resource
reservation protocol SDP/SAP: session
description, announcement protocols
physical layer
IP
UDP TCP
SAP HTTP SMTP
SDP
session directory
RSVP RTP, RTCP
conf. control
audio videosharedtools
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Session Protocols
session description protocol (SDP)– used to describe (not necessarily) mcast session
» name, purpose
» start time, duration
» media (type, transport protocol, format)
» how to receive media
session announcement protocol (SAP)– mcast protocol for SDP
– periodic transmission to known mcast address
– frequency depends on other announcements and scope
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Session Directory
used to allocate multicast addresses to sessions– birthday problem, N addresses support
sessions using random address allocation with negligible collision probability
– random allocation currently used in popular session directory tools sd (LBL), sdr UCL)
advertises multicast sessions– uses SDP
1 / N
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Example: sdr
Title: don-temp4.psCreator: XV Version 3.00 Rev: 3/30/93 - by John BradleyCreationDate:
Title: don-temp3.psCreator: XV Version 3.00 Rev: 3/30/93 - by John BradleyCreationDate:
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Mbone Applications
freewarevic, nv (video), vat, nevot (audio), wb (whiteboard)
IVS (teleconferencing)
commercial IP/TV - teleconferencing (Precept)
Most group applications use IP unicastE.g., CuSeeMe builds own mcast tree
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
MBone Behavior
Motivation: quantitative understanding of session dynamics,
and loss behavior
session dynamics:– sizes, durations of sessions
– join/leave behavior
loss behavior:– loss rates
– spatial and temporal correlation
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Mbone Session Dynamics
Goal: to characterize the dynamics of Mbone sessions -- Join/Leave Behavior
capture data using MlistenMlisten toolhttp://www.cc.gatech.edu/computing/Telecomm/mbone/
data pre-processed to account for– Mbone un-reliability,– abnormal usage
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Typical Sessions
long-lived (Shuttle)
short-lived (Seminars)
Title: temp.dviCreator: dvipsk 5.526a Copyright 1986, 1993 Radical Eye SoftwareCreationDate:
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Session Stats
Statistics for interarrival of rcvrs rcvr membership
duration
Title: temp.dviCreator: dvipsk 5.526a Copyright 1986, 1993 Radical Eye SoftwareCreationDate:
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Tree Size
multicast vs unicast tree cost
sensitivity of multicast tree to source location
Mbone vs Internet tree
Title: temp.dviCreator: dvipsk 5.526a Copyright 1986, 1993 Radical Eye SoftwareCreationDate:
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Audio/Video
audio receivers
vs
video receivers
Title: temp.dviCreator: dvipsk 5.526a Copyright 1986, 1993 Radical Eye SoftwareCreationDate:
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Mbone Loss Behavior
Goal: to characterize loss behavior on Mbone metrics
– loss rates, spatial and temporal correlation methodology
– rcvr processes on ~25 MBone hosts
– rcvrs listen to WRN (& other sources) which sends audio at 80ms intervals; record mcast packet receptions
– off-line analysis of trace data
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Measurements
Machine Name Locationalpa Georgia Techanhur, spiff Swedish Inst. Computer Sci.artemis, atlas Inst. Blaise Pascal, Parisbagpipe, ocarina U. Kentuckycedar U. Texascollage, zip EIT, Californiadixie UC Irvineedgar U. Washingtonerlang, trantor U. Massachusettsexcalibur USCfloat U. Virginiaganef UCLAlaw UC Berkeleypax INRIA, Sophia Antipolistove U. Marylandursa, lupus GMD Fokus, Berlinwillow U. Arizona
Date Source # rcvrs trace length (pkts)
9/19/95 WRN 8 17K9/20/95 UCB 9 20K10/30/95 WRN 10 57K11/1/95 WRN 9 41K11/13/95 WRN 9 40K11/14/95 WRN 8 30K11/28/95 WRN 7 20K12/4/95 WRN 8 45K12/11/95 WRN 9 70K12/16/95 WRN 7 50K12/18/95 WRN 7 69K4/19/96 RFV 11 45K4/24/96 UCB 12 93K5/8/96 RFV 10 148K
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Where does MBone loss occur?
4/16/96 dataset across data sets:
backbone loss small compared to overall loss
no loss in end-systems
Title: /tmp/xfig-export003787Creator: fig2devCreationDate: Sat May 11 23:53:21 1996
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Simultaneous Packet Loss
4/19/96 dataset: 47% pkts lost somewhere
similar results across datasets models of packet loss:
– star: end-end loss independent
– full topology: measured per link loss independent
– modified star: source-to-backbone plus star
Title: (Maya:Chart View)Creator: DeltaGraph Professional 3.0CreationDate:
Q: distribution of number of rcvr’s losing pkt?
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Temporal Loss Correlation
Q: do losses occur singly or in “bursts”
12/11/95 dataset, rcvr loss vs time– occasional long
periods of 100% loss
– spatial correlation
Title: Creator: gnuplotCreationDate:
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Burst Loss Length Distribution
sample distribution from “alps”– 12/1/95 dataset
generally isolated losses with occasional longer bursts
Title: Creator: gnuplotCreationDate:
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Temporal Loss Correlation
12/11/95 dataset(above)– similar results over other datasets
“most” loss bursts are 1 packets long but often significant loss in long bursts
Machine % Loss avg. burstlen
medianlength
75percntile
99percntile
length oflongest
% loss in longbursts
alps 5.93 1.21 1 1 3 179 4.3anhur 5.15 1.065 1 1 2 4 0cedar 14.22 1.333 1 1 8 14 0collage 9.08 1.155 1 1 3 175 2.75erlang 10.41 1.921 1 1 4 2518 34.6float 10.44 1.129 1 1 3 7 0law 12.09 1.698 1 1 3 2518 29.8pax 16.98 1.557 1 1 3 2603 21.9tove 5.46 1.097 1 1 3 10 0
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
MBone Measurements Summary
identifiable backbone loss small wrt overall loss spatial loss correlation: limited temporal loss correlation:
– most loss bursts have length one
– significant loss occurs in long bursts
reference: ftp://gaia.cs.umass.edu/pub/Yajn96:loss.ps lessons for multicast protocols/applications? loss/delay correlation? sensitivity to period, probe size, 1-many, many-many
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Routing
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Theoretical Basis
The Steiner Tree Problem is NP-Complete– Graph G = (V, E)– Positive Edge Weights W(e)– R a subset of V– B positive integer bound
Is there a subtree of G that includes all R with Is there a subtree of G that includes all R with cost no more than B?cost no more than B?
– Heuristic less than twice optimum
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Principles of Multicast Routing
Addressing
– List Addressing » Not Scalable
– Group Addressing» Less Control
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Principles of Multicast Routing
Reuse of unicast routing infrastructure
– Desirable
– Too Constraining Multicast routing overhead
– Needs to be minimized
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Principles of Multicast Routing
Evaluation
– Bandwidth Usage
– Delay -- Average, Maximum, Variance
– Concentration
– Router/Switch overhead
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Basic Multicast Routing Protocols
SourceSource
A B C
D
E
Problem:
Given a source and a set of destinations,
Route same packet to at least (or exactly) this set of destinations
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Basic Multicast Routing Protocols
Multicast by BroadcastMulticast by Broadcast
– Filter above network layer
– Natural in Broadcast Networks (Satellite, Bridged LANs)
– Use Flooding in PSN– Bandwidth inefficient, Security concerns
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Basic Multicast Routing Protocols
Separately addressed packetsSeparately addressed packets
SourceSource
A B C
D
Eto Ato B
to C
to D
to E
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Basic Multicast Routing Protocols
Multidestination AddressingMultidestination AddressingAA
BB
CC
DD
EE
SourceSource
to (A,B,C,D,E)to (A,B,C,D,E)
to (A,B)to (A,B)
to (C,D,E)to (C,D,E)
to (A,B)to (A,B)
to (D,C)to (D,C)
to (B)to (B)
to (C)to (C)
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Spanning Tree ForwardingSpanning Tree Forwarding– Shared or Source-Based
Basic Multicast Routing Protocols
AABB
CC
DD
EE
SourceSource
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Basic Multicast Routing Protocols
Reverse Path ForwardingReverse Path Forwarding
– Dalal and Metcalfe
– Basis for DVMRP (the original Mbone routing protocol)
– Main advantage: Use of existing unicast routing infratsrutcure
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Reverse Path Forwarding
A Broadcast Protocol Group addressing used Routers/Switches Forward based on source
of multicast packet
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Reverse Path Forwarding
Flood if packet arrives from Source on link that router would use to send packets to source
Otherwise Discard Rule avoids flooding loops Uses Shortest Path Tree from destinations
to source (reverse tree)
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Reverse Path Forwarding
SS xx
yy
zz
wwDestinationsDestinations
toto useuse
SS yy
Routing TableRouting Table
to Groupto Group
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Reverse Path Forwarding
AABB
CC
DD
EE
SourceSource
Shortest Path TreeShortest Path Treeto Sourceto Source
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Shared Tree VS Source-Based Tree
RPF routes over source-based treesource-based tree– Good delay properties– Per source and group overhead
Spanning Tree Forwarding uses shared treeshared tree– Per group overhead– Higher delays– More Traffic Concentration
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Routing Performance Evaluation
Simulation, Analysis and Experimentation Need
– Network Models (Waxman, GT-ITM, Tiers)– Application Models (Mlisten)– Traffic Models– Data Loss Models
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Internet Multicast Routing
Group Addressing
– Class D IP addresses Link Layer Multicast Two Protocol Functions
– Group Management» IGMP
– Route Establishment» DVMRP, MOSPF, CBT, PIM
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Link Layer Multicast
Example Ethernet:– Ethernet multicast addresses– Algorithmic mapping between IP mcast
address and Ethernet mcast address To join group:
– Recognize IP mcast address– Interface recognizes link layer mcast address– Inform local router using IGMP
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Internet Group Management Protocol
Used by end-systemend-system to declare membership in particular multicast group to nearest router(s)router(s)
– Version 1: Timed-out Leave
– Vesrion 2: Fast (Explicit Leave)
– Version 3: Per-Source Join
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IGMPv1
Joining Host send IGMP Report Leaving Host does nothing Router periodically polls hosts on subnet
using IGMP Query Hosts respond to Query in a randomized
fashion
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IGMPv2
ADDS:– Group Specific Queries– Leave Group Message
Host sends Leave Group message if it was the one to respond to most recent query
Router receiving Leave Group msg queries group.
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IGMPv3
Unclear Status?? ADDS:
– Group-Source Specific Queries, Reports and Leaves
Inclusion/Exclusion of sources
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Routing Protocols
Source -based Tree Protocols:– DVMRP– MOSPF– PIM-DM
Shared-Tree Protocols– CBT– PIM-SM
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
DVMRP
Distance Vector Multicast Routing Protocol
– An enhancement of Reverse Path Forwarding that :» Uses Distance Vector Routing Packets for
building tree» Prunes broadcast tree links that are not used
(non-membership reports)» Allows for Broadcast links (LANs)
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Forwarding in DVMRP
1. check incoming interface: discard if not on shortest path to source
2. forward to all outgoing interfaces
3. don’t forward if interface has been pruned
4. prunes time out every minute
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
DVMRP Forwarding (cont.)
Basic idea is to flood and prune
R
R
router
S
noreceiver
packet
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
DVMRP Forwarding (cont.)Prune branches where no members and branches not
on shortest paths
R
R
S
prune
2nd packet
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
DVMRP Forwarding (cont.)Add new user via grafting; departure via pruning
R
R
S
R
Report
graft
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Overheard on Mbone Mailing List!
“Help, we are unable to send prunes” Response:
“Well, have you tried to send plums? Raisins or grapes? ……
Perhaps your multicast implementation does not support fruit at all?”
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast OSPF
Link-State Multicast Routing Routers maintain topology DBs Group-Membership-LSA broadcast by
routers to advertise links with members Routers compute and cache pruned SPTs
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Protocol Independent Multicast
Motivation:
– DVMRP good for dense group membership
– Need shared/source-based tree flexibility
– Independence from Unicast Routing Two PIM modes:
– Dense Mode (approx. DVMRP)
– Sparse Mode
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
PIM- Dense Mode
Independent from underlying unicast routing
Slight efficiency cost Contains protocol mechanisms to:
– detect leaf routers– avoid packet duplicates
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
PIM - Sparse Mode
Rendezvous Point (Core): Receivers Meet Sources
Reception through RP connection = Shared Tree
Establish Path to Source = Source-Based Tree
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
PIM - Sparse Mode
ReceiverReceiver
SourceSource
Rendez-VousRendez-Vous
RegisterRegister
JoinJoinSourceSourceJoinJoin
PrunePrune
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PIM - Sparse Mode
ReceiverReceiver
SourceSource
Rendez-VousRendez-Vous
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Core-Based Trees
A shared-tree protocol One node on shared tree is Core Core Sender sends to Core Core forwards over multicast tree
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Core-Based Trees
AABB
CC
DD
EE
SourceSource
Core-based treeCore-based tree
CoreCore
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
PIM and CBT Issues
Unidirectional VS Bidirectional Shared Trees
Core/RP Placement and Selection Multiple Cores/RPS Dynamic Cores/RPs
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Real-Time Multicasting and Quality-of-Service (QoS)
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
The Problem
sender receiver timet0 t1 t2
pkts
gen
erat
ed (
rece
ived
)
How to get smooth, continuous playout adaptation by appl.
– playout delay adj.– loss concealment
perf. guarantees by network
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Requirements for Real-Time Applications
transport protocol must provide timing information
call admission/reservation protocols needed to – determine availability of resources for particular
performance requirement– reserve (allocate) resources
support for multicast– heterogeneity, scalability
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
RTP: Real-Time Transport Protocol
timing info. for playout reordering information loss detection for quality estimation, recovery synchronization
– network jitter– clock drift– intermedia (lip sync)
QoS feedback source identification
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
RTP: Real-Time Transport Protocol
Schulzrinne, et al. RFC 1889 product of IETF Audio Video Transport Working Group
(AVT WG) goals
– lightweight, interoperability
– easy integration with application
– mechanism - not policy
– scalability - unicast, multipoint 2 - 1000s participants
– separation of control/data
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
RTCP: RTP Control Protocol
provides monitoring capablities– quality of routes– state of participants (talker indication)
feedback to application– QoS feedback - adjust sender rate
scalability- randomized control traffic; rate decreases as number increases
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Performance Guarantees to Application
deterministic guarantees– absolute guarantees on loss, delay or jitter
statistical guarantees– probabilistic guarantees– cell loss probability < – prob. pkt delay exceeds D is less than
different services for different performance requirements
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Example: Internet Services
Guaranteed: no loss, upper bound on delay– invoked by specifying traffic (TSpec)
Controlled load: negligible losses, like unloaded network => delay-adaptive applications– invoked by specifying traffic (Tspec)
Best Effort: traditional service
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Guaranteed Service
user specifies traffic (TSpec) – token bucket spec. (r - token rate,
b - bucket depth in bytes)
– p - peak rate– m - minimum policed unit– M - max. packet size
and desired service (Rspec) – desired bandwidth R > r– slack term S
b
r
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Guaranteed Service (cont.)
delay is decreasing function of R for weighted fair queueing
user does not provide delay bound; delay bound controlled by choice of R,S
call admission, scheduling policy unspecified– service oriented towards WFQ
b M R p R p r prop delay / / .
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Controlled Load Service
user sees unloaded network– buffer loss on order of loss due to noise, faults, etc.
– delays should be mostly = propagation delay + processing costs
TSpec is same as for guaranteed service; no RSpec
call admission, scheduling, buffer management unspecified
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Call Setup
contract between network and application– network guarantees performance
– application guarantees traffic behavior» peak rate
» average rate
» burst size
approaches– one pass
– two pass
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
One Pass sender or receiver oriented source (rcvr) sends resource
reservation to rcvrs (source) cannot specify/guarantee
QoS soft state possible
– periodic refreshes
e.g., RSVP one pass w. advertisement
rsvtn
source
rcvr1
rcvr2
rsvtn
source
rcvr1
rcvr2
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Two Pass sender/rcvr initiated forward phase: check for
resources at each link reverse phase
– inform routers if call admitted– reserve resources
reserve max resources resource reclamation phase can
be added passive two-phase - notify
originator
source
rcvr1
rcvr2
fwd phaserev phase
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
RSVP: ReSerVation Protocol
Zhang, etal. 1993 receivers control reservations
– consistent with IP multicast
– good scalability
– supports heterogeneity
separate resource reservation from usage– packet filters control usage
soft state– end system periodically refresh state
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
RSVP Operation
rcvr joins group via IGMP source sends PATH messages to rcvrs rcvrs send RESV messages back to source(s) reservations can be lowered,
merged between senders (audio) one pass => rcvr does not know final QoS => one pass with advertising
R
R
R D
D
Sdata (mcast)
PATHRESV
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Other Issues
policing/traffic shaping– leaky bucket
– Generic Cell Rate Algorithm (GCRA)
» peak cell rate, mean cell rate, cell delay variation tolerance
interaction with routing what does a guarantee really mean? pricing performance evaluation
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Grand-Unified Multicast (GUM)
Putting it all together A protocol for inter-domain multicast routing Bidirectional Shared Tree for inter-domain
routing Receiver domains can utilize choice of
protocol Very New -- Still needs digesting
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Summary
Multicast Routing is a well researched problem.
Challenge now is– deployment– inter-operability– management
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Reliable Multicast
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Problem
How to transfer data reliably from source to R receivers
scalability: 10s -- 100s -- 1000s -- 10000s -- 100000s of receivers
heterogeneity feedback implosion problem
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Feedback Implosion Problem
. . .
AC
K
ACK
ACK
ACK
ACK
AC
K AC
K
sender
rcvrs
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Issues
level of reliability– full reliability (data)
– semi-reliability (video)
ordering– no ordering
– ordering per sender
– full ordering (distr. computing)
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Approaches
shift responsibilities to receivers feedback suppression multiple multicast groups local recovery server-based recovery forward error correction (FEC)
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Applications
application requirements– low latency
» file transfer (finite duration)
» DIS, teleconferencing
– high throughputs - streaming applications (billing, etc.)
application characteristics– one-many: one sender, all other participants rcvrs
(streaming appl. teleconferencing)
– many-many: all participants send and receive (DIS)
Page 102
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Sender Oriented Reliable Mcast
Sender: mcasts all (re)transmissions
selective repeat
use of timeouts for loss detection
ACK table
Rcvr: ACKs received pkts
Note: group membership important
X
sender
receivers
AC
K AC
K
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Vanilla Rcvr Oriented Reliable Mcast
Sender: mcasts (re)transmissions
selective repeat
responds to NAKs
Rcvr: upon detecting pkt loss
sends pt-pt NAK
timers to detect lost retransmission
Note: easy to allow joins/leaves
X
sender
receivers
NA
K
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Sender vs. Receiver (cont.)
Significant performance improvement shifting burden to receivers for 1-many; not as great for many-many
0
20
40
60
80
100
120
140
160
0 100 200 300 400 500 600 700 800 900 1000
No. Receivers
p=0.01
p=0.05
p=0.10
p=0.25
One-to-Many Comparison
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
0 100 200 300 400 500 600 700 800 900 1000
No. Receivers
p=0.01
p=0.05
p=0.10
p=0.25
Many-to-Many Comparison
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Feedback Suppression randomly delay NAKs multicast to all receivers
+ reduce bandwidth
- additional complexity at receivers (timers, etc)
- increase latencies (timers)X
sender
NAK
X
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Performance of Feedback Suppression
Additional thruput improvement for 1-many; costly for many-many
0
5
10
15
20
25
0 100 200 300 400 500 600 700 800 9001000No. Receivers
p=0.01
p=0.05
p=0.10
p=0.25
One-to-Many Comparison
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
0 100 200 300 400 500 600 700 800 9001000
No. Receivers
p=0.01
p=0.05
p=0.10
p=0.25
Many-to-Many Comparison
Page 107
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multiple Multicast Groups
mcast group per pkt– all rcvrs belong to one group for original transmissions
– rcvr losing pkt j joins group for j
– additional performance improvement for small no. groups (Kasera etal 1997)
receivers divided into destination groups– identify rcvrs with different capabilities
– place similar rcvrs into same group
– additional improvement (Ammar, Wu 1992)
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Server-based Reliable Multicast
server server
sender
receivers
first transmisions mcast to all receivers and servers
each receiver assigned to server
servers perform loss recovery servers can be subset of rcvrs
or provided by network can have more than 2 levels
LBRM (Cheriton)
Page 109
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Benefits of Adding Servers 2 levels, 4 rcvrs/server equal losses on each link rcvr oriented, NAK
suppression at all levels
Comments: clear performance benefits how to configure
– static/dynamic
– many-many0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 100 200 300 400 500 600 700 800 900 1000
No. Receivers
p=0.01
p=0.05
p=0.10
p=0.25
One-to-Many
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Local Recovery
Lost packets recovered from nearby receivers deterministic methods
– impose tree structure on rcvrs with sender as root– rcvr goes to upstream node on tree
RMTP (Lucent)
self-organizing methods– rcvrs elect nearby rcvr to act as retransmitter using
scoped multicast and random delays (SRM)
hybrid methods
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Issues with Server- and Local Based Recovery
how to configure tree what constitutes a local group how to permit joins/leaves how to adapt to time-varying network
conditions
no definitive resolution
Page 112
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Some Examples of Protocols
Page 113
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
RAMP (TASC)
Reliable Adaptive Multicast Protocol supports sender and rcvr oriented reliability mixture of reliable/unreliable senders/rcvrs
supported late joins and leaves supported rate-based flow control
Page 114
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
RMTP (Lucent)
Reliable Multicast Transport Protocol imposes a tree structure on rcvrs corresponding to
multicast routing tree leaves send periodic status msgs to upstream nodes nodes inside tree
– provide repairs to downstream nodes– send aggregate status msgs upstream
late-joins supported thru 2-level cache rate- and window-based flow control
Page 115
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
SRM (LBL)
Scalable Reliable Multicast framework integrated with application rcvr-oriented using NAK suppression and self-
organizing local recovery supports late-joins and leaves as built in wb, uses rate-based flow control has been used with 100s of participants over the
Internet
Page 116
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Other Examples Single Connection Emulation (SCE, Georgia Tech)
– sender oriented, offers semantics of TCP (sender ordering, etc)
– late-joins are not supported.
ISIS (Cornell, Isis Dist. Systems)– general purpose toolkit for providing reliable data transfer
to dist. applications– sender and rcvr oriented– wide range of ordering semantics supported– late-joins and leaves supported
Page 117
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Other Examples
Local Group based Multicast Protocol (LGMP, Karlsruhe)– self-organizing local- recovery based scheme
Xpress Transport Protocol (XTP)– sender-oriented extension of unicast protocol
Page 118
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Forward Error Correction (FEC)
Add redundancy in order to reduce need to recover from losses (e.g., Reed Solomon codes)
(k,n) code– for every k data pkts, construct n-k parity pkts
– can recover all data pkts if no more than n-k losses
+ reduce loss probability- greater overheads at end-hosts
Q: can FEC reduce network resource utilization?
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Potential Benefits of FEC
D1D3 D2
D1
D1D2
D2
D3
D3
D1D3 D2
D1
D1D2
D2
D3
D3
P
P
P
Initial Transmission
Data Retransmission
Parity Retransmission
One parity pkt can recover different data pkts at different rcvrs
P=D1 D2 D3
X
X
X
Page 120
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
An Integrated Approach using (k,n) Erasure Codes transmit packets in blocks
of k send redundancy packets
in response to NAKs
Assumptions: independent losses, p
significant performance gain achievable with small no. of parity pkts, n-k
1
1.5
2
2.5
3
3.5
4
1 10 100 1000 10000 100000 1000000
No. Receivers
no FEC
(7,8)
(7,9)
(7,10)
(7,inf)
p=0.01
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
End-Host Overheads
Software encoding/decoding rates for1KB pkt size (Rizzo)
=> FEC in SW is feasible for many
applications
B
B
BB
BB B B B B
J
J
JJ J J J J J J
HH H H H H H H H H
F
F
FF
F F F F F F
Ñ
ÑÑ
Ñ Ñ Ñ Ñ Ñ Ñ ÑÉ
É É É É É É É É É0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0 10 20 30 40 50 60 70 80 90 100
Percentage Redundancy
B k=10, enc
J k=20, enc
H k=100, enc
F k=10, dec
Ñ k=20, dec
É k=100, dec
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
End-Host Thruput Comparison: No FEC vs FEC
No FEC, rcvr oriented with NAK suppression
FEC, rcvr oriented with NAK suppression– SW encoding at sender
– HW encoding at sender
SW FEC thruput determined by sender
HW FEC thruput determined by rcvr
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1x100 1x101 1x102 1x103 1x104 1x105 1x106
No. Receivers
no FEC
SW FEC
HW FEC
Thruput Comparison (p=0.01, k=20)
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Performance Evaluation - no. transmissions to get pkt j to all rcvrs
- no. transmsissions of pkt j needed for rcvr i
well understood for spatially and temporally independent losses
spatial correlation: computationally expensive for general topologies see Towsley85, Tripathi94, Nonnenmacher97
temporal correlation: has not been touched on (see Nonnenmacher etal 97 for one treatment)
MjM
i R
Mj j i max ,
1
Mj i,
Page 124
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Summary
reliable mcast is a hot topic unresolved issues
– proper integration of different ideas wrt different applications
– integration with flow control
– interaction with group memebrship
– notion of semi-reliability
Page 125
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Multicast Applications
Page 126
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Problem Match transmission rates to
– Network capacity
– Receiver “consumption” rates
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Flow Control Challenges
Accommodating heterogeneity among receivers and paths leading to them
Preserving fairness among
– receivers of same flow
– distinct flows Scalability of feedback
Page 128
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Flow Control Solutions
Loss-Tolerant Applications (e.g., Video)
– Information content per unit time can be preserved at lower data rates
Applications demanding data integrity
– lower data rates => lower information content per unit time
GoalGoal: Co-Existence with TCP?
Page 129
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Video Flow Control
Scalable Feedback Control (Bolot, Turletti and Wakeman)
– Receivers measure loss rates– Randomly generated feedback– Source estimates receivers’ state and adjusts
video rate by changing compression parameters Problem: fairness among receivers
Page 130
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Video Flow Control
Improving fairness using Destination Set Grouping (Cheung, Li, Ammar)
– Send replicated video streams at different rates– Receivers can control rate of each stream
within limits– Receivers can move among streams
Fairness at the expense of increased bandwidth consumption
Page 131
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Video Flow Control
DSG and Single Stream Comparison
Single GroupSingle Group
ReceiversReceivers
Intra-StreamProtocol
DSGDSG
High
Med.
Low
Intra-StreamProtocol
Inter-StreamProtocol
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Video Flow Control
Receiver-Driven Layered Multicast (McCanne, Jacobson and Vetterli)
– Single video stream subdivided into layers– Receivers add and drop layers depending on
congestion– Challenge: Distributed Consensus, Layer
Synchronization
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Video Flow Control
Receiver-driven Layered Multicast
Source
Receivers
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Video Flow Control
Receiver-Driven Layered Multicast– Drop Layer: indicated by loss– Add Layer: – No such indication– Use join experiments with shared learning
» Reluctance to join layers that failed» Inform others via multicast of failed
experiments
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multicast Video Flow Control
Layered Video Multicast with Retransmissions (LVMR) (Li, Paul, Ammar)– Uses agents within network to maintain
information about join experiments– Reduces overhead of shared learning
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
Less Understood/Mature Area Some Possibilities:
– Window flow control (a la TCP)» Not Scalable, Not Fair (across receivers)
– Explicit rate control (e.g., ATM ABR)» Scalable but still not fair
– Multiple Multicast Groups
Page 137
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
The Single Connection Emulation (SCE) Architecture (Window Flow Control)
ApplicationApplication
TCPTCP
SCESCE
IPIP
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
ATM Available Bit Rate
Source
Receivers
Explicit Rate Messages
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
ATM Available Bit Rate
– Consolidation Algorithm
– Consolidation Noise
– Transient Time
– Volume of Feedback
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Flow Control for Reliable Multicast
Multiple Multicast Groups– Simulcasting or Destination Set Splitting
(Ammar, Wu and Cheung, Ammar)
– Data Partitioning (Bhattacharyya, Towsley, Kurose, Nagarajan)
– Cumulative Layering (Vicisano)
Page 141
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
Simulcasting:
– Similar to DSG protocol for Video
– Send multiple (uncoordinated streams) at different rates
– Each stream carries all data
– Receivers join appropriate stream - one at a time
Page 142
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
Data Partitioning:
– Send multiple streams at different rates
– Synchronous start time for receivers
– Partition data among streams with replication among streams allowed
– Schedule data for optimum completion time
Page 143
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
Data Partitioning Example
A D C BFlow 1rate = 1
B CFlow 2rate = 1
CD
Flow 3rate = 2
R1 R2 R3
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
Data Partitioning– Can achieve ideal completion time with as many
channels as receivers– Can achieve close to ideal with a few channels– Same completion time as simulcast but less
bandwidth consumed– Can improve by dynamic rate adjustment– Requires coordination and scheduling among
channels
Page 145
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
Cumulative Layering
– Multiple data streams at different rates
– Each stream contains entire data
– Receivers join asynchronously -- Streams transmit for indefinite duration
– Schedule to minimize reception time
– (Scheme allows for FEC encoding)
Page 146
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
Channel 1Rate = 1 A B C D
Channel 2Rate = 1 C D A B
Channel 3Rate = 2
B
DAC
BD
AC
Receiver 1, BW = 1Receiver 2, BW = 2
Receiver 3, BW = 4
Page 147
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Flow Control for Reliable Multicast
Cumulative Layering
– Can achieve minimum reception time with asynchronous receivers
– Schedulability requires some parameter relationships
– Synchronization among channels needed
Page 148
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Summary
Flow control for multicast communication is a hardhard problem:– Scalability– Heterogeneity– Added dynamic dimension (receivers and their
join behavior)– Plenty of room for innovation!
Page 149
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ATM and IP over ATM Multicast
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ATM Multicast
ATM: Connection-Oriented (VC) Multicast connection establishment
– UNI 3.0/3.1» Source-Controlled
» Full Knowledge of Receivers required
– UNI 4.0» Leaf-Initiated Join (LIJ)
» Group identified by (Source, Tree ID)
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
IP Multicast over ATM
Problem (for UNI3.x): Mapping Mapping
IP’s connectionless, receiver-controlled model
to
ATM’s connection-oriented, source-controlled model
Page 152
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
The MARS Architecture
MARS: Multicast Address Resolution Server
Stores mapping between
IP group address
and
Unicast addresses of ATM endpoints
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
MARS -- The Mesh Approach
IP receivers joining group register their ATM addresses with the MARS
IP source consults MARS for list of ATM addresses
IP source opens multipoint connection to ATM end-points
Receivers joining/leaving need to inform MARS
Page 154
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
MARS -- The VC Mesh
ATM Cluster
Source
Receivers
ATM MultipointConnection
MARS
MARS Request
Reply
Page 155
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
MARS -- The Multicast Server
The VC Mesh approach requires one multipoint VC per source per group
Alternative:– Provide a multicast server (MCS) per group– One multipoint VC from MCS to receivers– Source sends to MCS– MARS now returns ATM address of MCS
Page 156
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
MARS -- The Multicast Server
Receivers
ATM Cluster
Source
MARS
MARS Request
Reply
MulticastServer
Page 157
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
VC Mesh VS MCS
VC Mesh– Higher VC consumption– Higher signaling overhead– Better Delay – Less Vulnerability
MCS : Requires Reflection Suppression Comparison similar to Shared Tree VS
Source-based Tree in multicast routing
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VC Mesh VS MCS
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9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
The MARS for UNI 4.0
LIJ is closer to IP model than UNI3.x MARS is still needed to map IP group
address to the (source, tree ID) pair
Page 160
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Multipoint-to-Multipoint Support in ATM
How to support multiple sources on same multipoint VC and shared tree
Problem: Interleaving of Cells from different AAL PDUs within switch
Solution:– No problem for AAL 3/4– For AAL 5 : SEAM and SMART
Page 161
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Simple and Efficient ATM Multicasting (SEAM)
Proposes a Core-Based Tree Approach Solves cell interleaving by “cut through”
switching – Forward cells belonging to same packet
together and– Buffer other cells for same VC
Page 162
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Shared Many-to-Many ATM Reservations (SMART)
Many-to-Many connection over a single VCC
Addresses– Cell-interleaving– On-demand bandwidth sharing (a la RSVP)
Solution: Take Turns (essentially) Signalling Protocol
Page 163
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Summary
ATM and Internet multicast mechanisms are incompatible
Convergence approaches have been defined ATM multicast still has problems:
– many-to-many– QoS– routing
Page 164
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Summary
Page 165
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Topics Covered in Tutorial
overview of multicast at– network layer (routing, congestion control)
– transport layer (reliability, flow control)
– session layer (Internet centric view)
examples taken from– Internet, MBone
– ATM
Page 166
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Other Topics
application issues– stronger semantics: ordering requirements,
atomicity, etc. (Isis, Horus, Berman et al, Cornell)
session semantics group membership performance evaluation:
how to evaluate large networks supporting large numbers of multicast applications
Page 167
9/14/97 SIGCOMM97 Tutorial, Copyright Ammar and Towsley
Other Topics
Control Issues: control issues related to TCP have
generated 100s of papers and are still not resolved
control aspects of multicast add at least one additional layer of complexity– a very fruitful area for research and
development
Page 168
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