Satellite Multicast for Web Applications

Satellite Multicast for Web Applications

Hilmar Linder

[email protected]

University of Salzburg/Austria

SatCAST H. Linder - unisal 2

Overview

• Introduction• IP multicast• Reliable Multicast:

– Building Blocks– The RRMP Protocol

• Summary• Questions


Why IP multicast ?

• Need for efficient delivery to multiple destinations across inter/intranets

• Broadcast: – Send a copy to every machine on the net– Simple, but inefficient– All nodes “must” process the packet even if they don’t

care– Wastes more CPU cycles of slower machines

(“broadcast radiation”)– Network loops lead to “broadcast storms”


Why IP multicast ? (contd)

• Replicated Unicast:– Sender sends a copy to each receiver in turn– Receivers need to register or sender must be pre-

configured– Sender is focal point of all control traffic– Latency = time between the first and last receiver

getting a copy {can be large if transmission times are large}


Why IP multicast ? (contd)

• Application-layer relays:– A “relay” node or set of nodes does the replicated

unicast function instead of the source– Multiple relays can handle “groups” of receivers and

reduce number of packets per multicast => efficiency– Manager has to manually configure names of receivers

in relays etc => too much administrative burden

• Alternative: build replication/multicast engine at the network layer


Multicast Protocol Architecture

Internet Protocol

Reliable MulticastTCP

Middleware

Networking Infrastructure

IP Multicast

Applications

Unreliable

Reliable


IP multicast concepts

• Message sent to multicast “group” (of receivers)– Senders need not be group members– Each group has a “group address” – Class D Address– Use “group address” instead of destination address in

IP packet sent to group– Groups can have any size– End-stations (receivers) can join/leave groups at will


IP multicast concepts (contd)

• Packets are not unnecessarily duplicated or delivered to destinations outside the group– Distribution tree for delivery/distribution of packets– Packets forwarded “away” from the source– Only one copy of a packet appears on any subnet – Packets delivered only to “interested” receivers =>

multicast delivery tree changes dynamically– Network has to actively discover paths between senders

and receivers– Non-member nodes even on a single subnet do not

receive packets (unlike subnet-specific broadcast)


IP multicast concepts (contd)

• Group membership on a single subnet is achieved through IGMP (and ICMPv6 in IPv6)

• Tree is built by multicast routing protocols.– Algorithms based on:

• Flooding

• Spanning trees

• Reverse-path broadcasting

• Core based trees


Multicast Applications

• News/sports/stock/weather updates• Distance learning• Routing updates (OSPF, RIP etc)• Pointcast-type “push” apps• Videoconferencing, shared whiteboards• Distributed interactive gaming or simulations• Email distribution lists• Voice-over-IP• Database replication


Multicast Application Characteristics

• Number of (simultaneous) senders to the group– 1XN versus NxM

• The size of the groups– Number of members (receivers)– Geographic extent

• The lifetime of the group• Level of human interactivity

– Lecture mode versus interactive– Data-only (e.g. database replication) versus multimedia

• Number of aggregate packets/second• The peak/average bandwidth used by source


Real-time Non-real-time

Multimedia

Data-only

• Video server• Video conferencing• Internet audio• Multimedia Events

• Stock quotes • News feeds • White boarding • Interactive gaming

• Replication: • Video & web servers • Kiosks • Content delivery • Intranet & Internet

• Data delivery • Server-server • Server-desktop • DB replication • SW distribution

• Requires reliability

What applications need Reliable Multicast?


Multicast Protocol Architecture

Internet Protocol

Reliable MulticastTCP

Middleware

Networking Infrastructure

Multicast

Applications

Unreliable

Reliable


Goal

• Fully reliable multicast:– All packets delivered to all receivers

• Can’t we just extend TCP?

SS RRData

ACKs


Data Reliability

SS

AA BB CC DD

Data


Data Reliability

SS

AA BB CC DD

DataACKs


Data Reliability

SS

AA BB CC DD

“ACK Implosion”


Challenges for Reliable Multicast

• Scalability to the number of receivers:– Heterogeneity of receivers– Feedback implosion– Congestion control

• How to achieve reliability:– Automatic Repeat Request (ARQ)– Forward Error Correction (FEC)


Building Blocks

• Elements from unicast:– Loss detection– Loss recovery: ARQ versus FEC

• Additional elements for multicast– Mechanisms for control message Implosion Avoidance– Mechanisms to deal with heterogeneous receivers


Where to place the loss detection

• Receiver-based (NAK)– distributed over receivers– 1 NAK per packet (for all receivers)

• Sender-based (ACK)– centralized– 1 ACK per receiver and packet– needs a table of per-receiver ACK


Feedback Processing

• Assume R receivers, independent packet loss probability

• Feedback per packet:– average number of ACKs: R - p*R– average number of NAKs: p*R

-> more ACKs than NAKs

• Processing: higher throughput for receiver-based loss detection

• Reliability needs ACK: No NAK does not mean successful reception!


Feedback suppression

• At Receivers:– choose a random timer in [0,T] due to a exponential

distribution and listen for other’s feedback– On reception of feedback: cancel timer– On timer expiration: multicast feedback, so that other

receivers can see it

AA BB

SS

NAK

(suppressed)


Loss Recovery

• FEC versus ARQ: ARQ is the only way to guarantee 100% reliability

• Who retransmits:– Sender, other receiver, network node

• How to retransmit:– Unicast or Multicast

• What is retransmitted– Originals or Parities


Forward Error Correction based on (n,k) Encoding

Original packetsOriginal packets1 2 k

1 2 k Original packetsOriginal packets

DecodeDecode

1 2 k k+1k+2 n

Encode (copy Encode (copy 1st k)1st k)

Take any kTake any k


Hybrid ARQ Example

AA BB

SS

1

2


Hybrid ARQ Example (contd)

AA BB

SS

1 1

2 2



AA BB

SS

1 2



AA BB

SS

NAK(1 packet lost)

(suppressed)

1 2



AA BB

SS

= + Retransmission of Parity

1

1

2

2



AA BB

SS

1 2

PP



AA BB

SS

1 2P P



AA BB

SS

++= =

1 122 P P



AA BB

SS

Two losses recovered with a single retransmission...221 1


Hybrid ARQ Algorithm

• Hybrid ARQ Algorithm:– Sender send k original packets– Receiver detects that k-l packets have been received

and sends a NAK(l)– Sender receives NAK(L1), NAK(L2), .., NAK(Ln) from the

receivers and sends Lmax = max {L1, L2, .., Ln} parity packets.

• A single parity packet can repair the loss of different data packets at different receivers


Performance Measure

• The mean number of transmission E[M] per packet affects:– The bandwidth needed– The total transfer latency– The processing rate at the sender and receiver

• With FEC, E[M] can be reduced dramatically• Processing costs for FEC need to be considered

– How fast is encoding/decoding– Throughput of a protocol based on FEC


Scenario specific selection of Mechanisms

• FEC is of particular benefit in the following scenarios:– Large groups– No feedback– Heterogeneous RTTs– Limited buffer

• ARQ is of particular benefit in the following scenarios:– Heterogeneous loss– Loss in shared links of the multicast tree dominates– Small groups– Non-interactive applications


Restricted Reliable Multicast Protocol (RRMP)

• Error Detection and Correction Module– FEC only

– Hybrid ARQ

– Proactive ARQ

• Transport Module:– Bandwidth Management

– Flow Control

– RTT estimation

– Group Management

• Statistics Module


Proactive Error Control

• Parities are sent pro-actively along with data– avoid retransmission– reduce latency

AA BB

SS

1 1

2 2

AA BB

SS

++= =

1 122 P P


Mercure Network Simulation

QLK

PEK

ARE

NBO

BKKOCO

Connections btw. A Sites

Sender in PEK, Receiver in NBO Packet Loss Rate: 5


Summary

• There is no “one-fits-all” reliable multicast protocol

• Application requirements influence the decision for the “best” multicast protocol

• Standardization of RM “building blocks” is currently ongoing

• Further research for “Internet compatibility” of RM protocols is necessary


Questions ??

Documents

Satellite Multicast for Web Applications