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 2 24 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 IP Multicast addresses Single-source multicast Addresses (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 –2 24 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