active reliable multicast how it works, how it can be used on computational grids
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ACTIVE RELIABLE MULTICAST
HOW IT WORKS, HOW IT CAN BE USED ON COMPUTATIONAL GRIDS
Congduc PHAM
SUN's "Gourmandise Cérébrale"SUN Labs Europe, Thursday, February 14th, 2002
http://www.ens-lyon.fr/LIP/RESAM
2
Outline
Introduction How it works How it can be used on computational
grids
multica
st!
multicast!multicast!Everybody's talking
about multicast! Really annoying ! Why would I need
multicast for by the way?
multicast!
multicast!
multicast!
multicast!
multicast!
multicast!
multicast!
multicast!
multicast!
mu
ltic
ast!
multicast!alone
multicast!
4
high-speed www video-conferencing video-on-demand interactive TV programs remote archival systems tele-medecine, white board high-performance computing, grids virtual reality, immersion systems distributed interactive
simulations/gaming…
Challenges for the Internet
Think about…
5
From unicast…
Problem Sending same
data to many receivers via unicast is inefficient
Example Popular WWW
sites become serious bottlenecks
Sender
data
datadata
data
Receiver Receiver Receiver
datadata
6
…to multicast on the Internet.Sender
Not n-unicast from the sender perspective
Efficient one to many data distribution
Towards low latence, high bandwidth
data
datadata
data
Receiver Receiver Receiver
7
User perspective of the Internet
from UREC, http://www.urec.fr
8
What it is in reality…
from UREC, http://www.urec.fr
9
Links: the basic element in networks
Backbone links optical fibers 10 to 160 GBits/s with DWDM techniques
End-user access V.90 56Kbits/s modem on twisted pair 512Kbits/s to 2Mbits/s with xDSL modem 1Mbits/s to 10Mbits/s Cable-modem 64Kbits/s to 1930Kbits/s ISDN access 9.6Kbits/s (GSM) to 2Mbits/s (UMTS) 155Mbits/s to 1Gbits/s SDH
10
Routers: key elements of internetworking
Routers run routing protocols and build routing
table, receive data packets and perform
relaying, may have to consider Quality of Service
constraints for scheduling packets, are highly optimized for packet
forwarding functions.
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The Wild Wild Web
important data
heterogeneity,link failures,
congested routerspacket loss, packet drop,bit errors…
?
12
At the routing level management of the group address (IGMP) dynamic nature of the group membership construction of the multicast tree (DVMRP,
PIM, CBT…) multicast packet forwarding
At the transport level reliability, loss recovery strategies flow control congestion avoidance
Multicast difficulties
13
Reliable multicast
What is the problem of loss recovery? feedback (ACK or NACK) implosion replies/repairs duplications difficult adaptability to dynamic
membership changes Design goals
reduces recovery latencies reduces the feedback traffic improves recovery isolation
How does it work?
Active Reliable Multicast
15
What is active networking?
Programmable nodes/routers Customized computations on packets Standardized execution environment
and programming interface No killer applications, only a different
way to offer high-value services, in an elegant manner
However, adds extra processing cost
16
Motivations behind active networking
user applications can implement, and deploy customized services and protocols
specific data filtering criteria (DIS, HLA) fast collective and gather operations…
globally better performances by reducing the amount of traffic
high throughput low end-to-end latency
17
Active networks implementations
Discrete approach (operator's approach) Adds dynamic deployment features in
nodes/routers New services can be downloaded into
router's kernel Integrated approach
Adds executable code to data packets Capsule = data + code Granularity set to the packets
18
DataData
The discrete approach
Separates the injection of programs from the processing of packets
active code A1
active code A2
A1A2
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The integrated approach
User packets carry code to be applied on the data part of the packet
High flexibility to define new services
data code
data datacode
data
datadata
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An active router
IP packet
IP packet
Filter Action
Forwardingtable
Routingagent
IP input processing IP output processing
IP packet
Packet scheduler
IP output processing
IP packet
Packet scheduler
some layer for executing code.Let's call it Active Layer
AL packet
21
Solutions for Reliable Multicast
Traditional end-to-end retransmission schemes scoped retransmission with the TTL
fields receiver-based local NACK suppression
Active contributions cache of data to allow local recoveries feedback aggregation subcast …
22
A step toward active services: LBRM
23
Active local recovery
routers perform cache of data packets repair packets are sent by routers,
when available
data1data2data3data4data5
datadatadata5
NACK4data4
data1data2data3data4data5
data1data2data3data5
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Global NACKs suppression
NACK4NACK4
NACK4
NACK4data4
NACK4
only one NACK is forwarded to the source
25
Local NACKs suppression
data
NACK
NACK
NACK
NACK
NACK
26
Active subcast features
Send repair packet only to the relevant set of receivers
NACK4
NACK4
NACK4
NACK4
data
4
data4
data4
data4
data4
data4
data4data4
data
4
data4
data4
How can it be
used?
Active Reliable Multicast
Computational gridsThe DyRAM frameworkSome simulation resultsConclusions and
perspectives
GRID?
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What is a computational grid?
application user
from Dorian Arnold: Netsolve Happenings
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Distributed & interactive simulations:DIS, HLA,Training.
Some grid applicationsAstrophysics:Black holes, neutron stars, supernovae
Mechanics:Fluid dynamic,CAD, simulation.
Chemistry&biology:Molecular simulations, Genomic simulations.
W ide- ar ea int er act ive simulat ions
IN T E R N E T
human in t he loopfl ight s imulat or
bat t le fi eld simulat ion
displaycomput er - basedsub- mar ine simulat or
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Data replications
Code & data transfers, interactive job submissions
Data communications for distributed applications (collective & gather operations, sync. barrier)
Databases, directories services
Data replications
Code & data transfers, interactive job submissions
Data communications for distributed applications (collective & gather operations, sync. barrier)
Databases, directories services
Reliable multicast: a big win for grids
Multicast address group 224.2.0.1
224.2.0.1
SDSC IBM SP1024 procs5x12x17 =1020
NCSA Origin Array256+128+1285x12x(4+2+2) =480
CPlant cluster256 nodes
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From reliable multicast to Nobel prize!
We see something,but too weak.
Please simulateto enhance signal!
Resource Broker:7 sites OK, but need to send data fast…
Resource Broker:LANL is best match…
but down for the moment
OK! Resource EstimatorSays need 5TB, 2TF.Where can I do this?
From President@earth.org
Congratulations, you have done a great job, it's the discovery of the century!!
The phenomenon was short but we manage to react quickly. This would have not been possible without efficient multicast facilities to enable quick reaction and fast distribution of data.
Nobel Prize is on the way :-)
From President@earth.org
Congratulations, you have done a great job, it's the discovery of the century!!
The phenomenon was short but we manage to react quickly. This would have not been possible without efficient multicast facilities to enable quick reaction and fast distribution of data.
Nobel Prize is on the way :-)
32
Multicast communications on grids
Dynamic groups are very difficult to handle with the reliability constraint
Mixture of high-throughput (data replication) and low latencies (distributed applications) needs
The application under consideration can have a great impact on the protocol design (i.e. local recoveries)
A one protocol-fits-all solution is difficult!
33
The DyRAM framework (M. Maimour)
Receiver-based: use of NACKs. No cache in routers, receivers
perform local recoveries… …which are based on a tree structure
constructed on a per-packet basis. Routers play an active role. Low-overhead active services Focus on low latency Load balancing features
core networkGbits rate
1000 Base FX
active routeractive router
active router
active router
active router
Server
100 Base TX
where to put activecomponents?
35
Related works on local recovery
SRM any receiver in the neighborhood
RMTP, TMTP, LMS, PGM, TRAM a designated receiver
LBRM a logging server
36
Active services in DyRAM
Designed to provide low latencies Session initialization Early packet loss detection NACK aggregation Subcast of repair packets Dynamic replier election
37
DyRAM and IP multicast
Relies on IP multicast but has few interactions
Runs its own simple session protocol to gather additional topological information at the DyRAM level to enhance the group anonymity imposed by IP multicast
DyRAM: session initialization
IP multicastIP multicast
IP multicast
DyRAMDyRAM
IP multicast
IP multicast
DyRAMDyRAMINIT
INITINIT
INIT
INIT
Reply @ Reply @
Reply @
R1
R2R3R4
R5 R6 R7
Reply @D1
Total Replies=5@R1,vif 1@R2,vif 2@R3,vif 2@R4,vif 2@D1,vif 0
Reply @ Reply @
Total Replies=3
@R5,vif 1@R6,vif 1@R7,vif 0
0
12
1 0
D1
D0
39
How and where losses can occur
Packet losses occur mainly in edge routers
In this case, all downstream links would most likely be affected by a packet loss
On medium speed LAN, when a packet has been sent on the wire all computers will usually be able to receive it
On very high-speed LAN, computers can be the bottleneck
40
DyRAM: early packet loss detection
The repair latency can be reduced if the lost packet could be requested as soon as possible
DyRAM realizes this functionality by enabling some routers to detect losses and therefore to generate NACKs towards the source
This loss detection service should be located near the source, but not too near!
41
DyRAM: replier election
A receiver is elected to be a replier for each lost packet
Several recovery trees at a given time Load balancing can be taken into
account, several optimizations possible
Uses the topological information gathered during the session initialization
DyRAM: replier election
IP multicastIP multicast
IP multicast
DyRAMDyRAM
IP multicast
IP multicast
DyRAMDyRAM
R1
R2R3R4
R5 R6 R7
@R5,vif 1@R6,vif 1@R7,vif 0
0
12
1 0
NAK 2,@ NAK 2,@
NAK 2,@
NAK 2 from R1
NAK 2 from R2
NAK 2 from R3
NAK 2 from R4
NAK 2
Repair 2
Repair 2
Repair 2
Repair 2
D0
D1
@R1,vif 1@R2,vif 2@R3,vif 2@R4,vif 2@D1,vif 0
NAK 2
NAK 2
43
DyRAM: subcasting
Tries to solve the exposure problem Using the NACK pattern to select
relevant links can not avoid exposure Use of IP addresses is more costly
but allows for an exact matching Several optimizations possible,
including a dynamic selection of the appropriate mechanism
44
Routers’ soft state
The NACK State (NS) structure which maintains for each lost packet,
seq : the sequence number of the requested packet.
rank : the number of NACK received. subList : List of the links from which
similar NACKs arrived (or IP addresses).
45
Routers’ soft state (cont.)
The Track List (TL) structure which maintains for each multicast session,
lastOrdered : the sequence number of the last received packet in order
lastReceived : the sequence number of the last received data packet
lostList : a bit vector that keeps track of received packet
Reduces the replier election delay.
core networkGbits rate
100 Base TX
active routeractive router
active router
active router
active router
1000 Base FX
sourcesource
The backbone is fast, very fast (DWDM, 10Gbits/s not uncommun), so nothing else than fast forwarding functions.
The active router associated to the source can perform early processing on packets. For instance our DyRAM protocol uses subcast and loss detection facilities in order to reduce the end-to-end latency.
A hierarchy of active routers can be used for processing specific functions at different layers of the hierarchy. For instance, having an active router at the nearest location from the source/destination could performs very efficient NACK packets suppression
Any receiver can be designated as a replier for a loss
packet.The election is performed by the
associated upstream active router on a per-
packet basis. Therefore several
loss recovery trees can co-exist in
parallel at a given time.
DyRAM can increases performances by associating a dedicated active router to a pool of computing resources.
One benefit of active networking is to unload the source from heavy retransmission overheads.
DyRAM overview
47
Some simulation results
Network model and used metrics Local recovery from the receivers DyRAM vs. ARM DyRAM combined with cache at
routers
48
Network model
10 MBytes file transfer
49
Metrics
Load at the source : the number of the retransmissions from the source.
Load at the network : the consumed bandwidth.
Completion time per packet (latency).
50
Local recovery from the receivers (1)
Local recoveries reduces the load at the source (especially for high loss rates and a large number of the receivers).
p=0.25#grp: 6…24
4 receivers/group
51
Local recovery from the receivers (2)
As the groups size increases, doing the recoveries from the receivers greatly reduces the bandwidth consumption
48 receivers distributed in g groups #grp: 2…24
52
Local recovery from the receivers (3)
Local recoveries reduces the end-to-end delay (per packet)
#grp: 6…24
4 receivers/group
p=0.25
53
DyRAM vs ARM
ARM performs better than DyRAM only for very low loss rates and with considerable caching requirements
54
DyRAM with cache at the routers (1)
When DyRAM benefits from the cache at the routers in addition to the recovery from the receivers, it always performs better than ARM.
p=0.25
ARM without cache
55
DyRAM with cache at the routers (2)
When DyRAM benefits from the cache at the routers in addition to the recovery from the receivers, it always performs better than ARM.
p=0.25
ARM without cache
56
DyRAM: early loss detection
p=0.25 p=0.5
#grp: 6…244 receivers/group
57
Conclusions
Reliability on large-scale multicast session in difficult. Active services can provide efficient solutions for avoiding implosion and exposure.
The main design goals for DyRAM is to reduce the end-to-end delays (recovery for instance) to enable large distributed applications on computational grids.
58
References
D. L. Tennehouse, J. M. Smith, W. D. Sincoskie, D. J. Wetherall, and G. J. Winden. A survey of active network research. IEEE Communications Magazine, pages 80--86, January 1997.
L. Wei, H. Lehman, S. J. Garland, and D. L. Tennenhouse. Active reliable multicast. IEEE INFOCOM'98, March 1998.
M. Maimour, C. Pham. A Throughput Analysis of Reliable Multicast Protocols in an Active Networking Environment. IEEE ISCC'2001, Hammanet, Tunisia.
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