IP Multicast Channels: EXPRESS Support for Large-scale Single-source Applications

Authors: Hugh W. Holbrook and David R. Cheriton

Presenter: Mridul Sharma

Contents

• Introduction

• IP Multicast Channels

• ECMP

• Multi-source Multicast Applications

• Cost and Scalability

• Costing Overhead and Proactive Counting

• Conclusion

Focus

Provide explicit support for large-scale multicast applications by extending the IP Multicast service model to support multicast channels

IP Multicast: Group Model

• Hosts aggregated into groups with single address

• Good for multicast discovery & small scale meetings over the internet

Problems

• Strained for very large scale multicast applications such as Internet TV– Violates common ISP billing models– Provides no indication of group size– No restriction on allowed senders– World-wide unique multicast address– Scaling IP multicast routing for conventional

group semantics remains an issue

IP Multicast Channels

• A multicast channel is a datagram delivery service identified by a tuple (S, E) where S is the sender’s source address and E is a channel destination address.

• Only the source host S may send to E.

Channel vs. Group Addressing

S S

(S,E)

G

Single-source IP Multicast Addresses

• 224 class D addresses allocated by IANA

• Routers identify a channel multicast datagram by its destination address

• Same service interface as IP Multicast for packet transmission to, and reception on, a channel

222.0.0.0 239.255.255.255

IP Multicast addresses

Single-source multicastAddresses (232.*.*.*)

EXPRESS Service Interface Extensions

• Source service interface– Count = CountQuery(channel, countId,

timeout)

– channelKey(channel, K(S, E) )

• Subscriber service interface– Result = newSubscription(channel [, K(S, E) ]),

– Count(channel, countId, count)

Advantages

• Source– 224 channels per source– Address management is simplified– Authenticated subscription option– CountQuery mechanism (number of subscribers

or subscriber vote)

Advantages (Contd.)

• Subscriber– Receives traffic only from the source it

designates– Ability to provide feedback

Advantages (Contd.)

• ISP– Provides basis for charging– Counting facility increases revenue– EXPRESS is relatively simple to implement

and manage

EXPRESS Count Management Protocol

• A single common management protocol

• Maintains both the distribution tree and supports source-directed counting and voting

• RPF is used to route subscriptions and unsubscriptions towards the source

ECMP

• Generic Counting Operation– CountQuery– Count– CountResponse– A router can initiate a query without source co-

operation

ECMP (Contd.)

• Distribution Tree Maintenance– New subscription– Unsubsciption– Router can use either TCP or UDP mode for

ECMP

ECMP: Subscription

ECMP (Contd.)

• Neighbor Discovery– Periodic CountQuery message– countId: neighbors; all channels

• EXPRESS Packet Forwarding– Forwarding Information Base entries at each

router– Forwarding procedure is nearly identical to IP

Multicast

ECMP (Contd.)

• Authenticated ECMP vs. End-to-end Encryption– Authentication provides restricted access while

encryption provides confidentiality

ECMP Advantages

• Simple integrated protocol– Supports subscription, multicast channel

maintenance and counting

• No change in host OS if it supports IP Multicast

• Multicast traffic travel only along paths from source to subscribers

Multi-source Multicast Applications

• Multiple channels, one per source– Applicable when new source is going to transmit for

extended period of time

• Several sources sharing a channel using higher level relaying through the channel’s source host – Supported by middleware layer for session

management

The Session Relay Approach

Advantages of SR Approach

• Appropriate placement of SRs to minimize communication is under application control

• Applications can have additional backup SRs for fault tolerance, placement, switching over etc– “Hot” and “cold” standby

• SR can provide application-specific functionality

Session Relaying

• As an ISP Service

• For other applications

• Cost/ Performance

Cost and Scalability

• Cost of router FIB memory for channels

• Cost of management-level router state

• Cost of maintaining this state

Counting Overhead & Proactive Counting

• Counting Overhead– Small for large-scale channels if approximated

over long time periods– Excessive use of counting is expensive

• Proactive Counting– Receivers and routers proactively send count

messages upstream

Related Work

• Service Models and Routing

• Accounting

• Counting

Conclusions

• Straightforward extension to the conventional IP multicast

• Simple implementation

• Additional capabilities like access control, accounting and local-to-host multicast address allocation

• Almost single source and truly multi source multicast applications can be implemented