presentation slides prepared by ramakrishnan.v lms: a router assisted scheme for reliable multicast...
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Presentation slides prepared by Ramakrishnan.V
LMS: A Router Assisted Scheme for Reliable Multicast
Christos Papadopoulos, University of Southern California
Guru Parulkar, Washington University
George Varghese, University of California at San Diego
Introduction
Internet architecture is largely responsible for the undisputed success of the Internet today.
Unfortunately, the end-to-end model which was so successful in the unicast case has proven much harder to apply to multicast.
LMS facilitates very efficient solutions (compared to pure end-to-end schemes) to problems like scalable reliable multicast.
Difficulties - Reliable Multicast
Implosion
Host Exposure
Recovery latency
Adaptability to dynamicity
Key Feature of LMS (Light-Weight Multicast Service)
Separation of forwarding and error control Forwarding (by routers) Error control (by receivers – end to end)
No packet storing or processing at routers.
Core Ideas
Each router selects a replier (surrogate). Routers steer requests to repliers. Routers help repliers multicast replies to loss
sub-tree. LMS restricts the scope of forwarding.
LMS achieves the efficiency of the heavy-weight model, but without the weight.
LMS Model
R
Control messages
Router storespackets, receivesNACKs and sendsretransmissions
Heavy-weight model
R
Router chooses a receiveras a surrogate.
Router relays messagesfrom surrogate to theSub-tree.
Receiver actingas a surrogate
LMS
Router steers all controlmessages to surrogate.
LMS: Concepts
Replier Provides Retransmission Receiver volunteered to
answer requests Turning point
Where requests start to move downstream
Directed Multicast Multicast to a sub-tree
SS
RR
RR
AA AA AA AA
AA
PP
PP
PP Replier
Replier link
Replier link
Selecting a Replier (surrogate)
If the router has two or more downstream links it selects one as the replier link.
If the router has only one downstream link that becomes the replier link by default.
If the source is directly attached to the router the source becomes the replier.
Replier Selection (contd…)
Receivers express desire by piggy backing information on the join request.
Receivers communicate a cost of their appropriateness as repliers.
Selection based on the advertised cost.
Steering Messages to Repliers
Each request is multicast, which keeps receiver actions simple.
Hop-by-Hop forwarding requires routers to examine each request, which is done via the IP Router Alert option, included in every request.
Request Handling at the Routers
LMS avoids request implosion because each router allows only one request to escape upstream - the one coming from the replier link.
LMS: Request forwarding
Multicast to the group.
If a request reaches a turning point, it’s forwarded towards the replier.
No request suppression or merging, but scope of requests is limited.
SS
R2R2
R1R1
AA AA AA AA
AA
P2P2
P1P1
LMS: Reply forwarding At turning point,
<router_addr:link_id> is added into request packet.
Replier includes it into its retransmission packet.
Routers need not remember anything about requests as they pass through.
Routers are not even aware that these recovery messages.
SS
R2R2
R1R1
AA AA AA AA
AA
P2P2
P1P1
turning point
LMS Concepts review
Concepts Replier selection. Steering of requests to repliers. Establishing turning points. Directed multicast.
These concepts work together to enable receivers to construct an efficient recovery mechanism.
Problem: Exposure
Loss at the replier link may result in duplicates Mitigation: Using the cost field to select a replier that
advertises the least loss.
Simulations
Using 3 Topologies Binary Trees Random Topology Transit-stub topology
Using 100’s of nodes and tens of topologies.
Compare with PGM and SRM.
Simulation Parameters
Loss at the source Test control of NACK implosion.
Loss at the receiver Test control of Exposure.
Loss at each Link Working in random loss scenario.
Simulation Summary
LMS performance improves as the group gets larger because more helpers.
LMS and PGM to perform much better than SRM.
Comparing LMS and PGM LMS is much simpler to implement, and
performance is on par with PGM. Significantly lower recovery latency, while
trading very little in terms of exposure.
Selecting Repliers in a LAN
So far we have assumed that there was only one receiver at each link.
Receivers on a LAN use a simple election mechanism to elect a replier and therefore make a LAN appear as having a single receiver.
Proxy Directed Multicast
The request may arrive after the buffers at the replier have been purged.
Note that once a request passes the turning point it contains enough information to uniquely identify the sub-tree that requires the retransmission.
Replier can forward the request to another member (LMS again).
Incremental Deployment
Use Source Path Messages (SPMs)to create an overlay.
Incremental deployment has an effect on exposure.
Other Applications - ANYCAST
Grouping servers in a well-known multicast group. Servers tell the routers to advertise the existence of a
replier (server) in all links which ensures that routers find the nearest server in any direction.