Express

IP Multicast Channels: EXPRESS Support for Large-Scale Single-Source Applications

Computer Science 6390 – Advanced Computer Networks Holbrook and Cheriton Dr. Jorge A. Cobb SIGCOMM’99

IP Any Source Multicast 

IP Multicast (RFC 1112) or ASM – Any Source Multicast • Hosts aggregated into groups with single address

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ASM 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 (run out of addresses)

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EXPRESS 

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

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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.

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Only the source host S may send to (S,E).

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Channel vs. Group Addressing S

S’

S

(S,E)

G

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Single-source IP Multicast Addresses

222.0.0.0

IP Multicast addresses

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224 class D addresses allocated by IANA

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A source S can freely pick any of these to make a “channel” (S,E)

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Sources just send data addressed to (S,E)

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Receivers have to inform their routers they want to join (S,E) (more on this later)

239.255.255.255

Single-source multicast Addresses (232.*.*.*)

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Is it a channel or a group? 

Is a multicast packet received addressed to a group (regular IP multicast) or an IP channel?

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Look at the destination

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If destination ∈ (232.*.*.*) it is for a channel

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If destination ∉ (232.*.*.*) it is for a regular IP group

• Channel = (S,E), S = src IP address, E = dest IP addr • I.e., the source helps identify the “group” • Group G = dest IP addr • I.e. the source does not identify the “group” 6

EXPRESS “Counting” Messages 

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There are three message types • CountQuery(chanel,countId,timeout) • Count(chanel,countId,count,[K(S;E)]) • CountResponse(chanel,countId,status) Counts may be initiated by a source or an intermediate router • Initiator sends a CountQuery to all its children, which propagates down the tree • Receiving hosts respond with a Count message • Router acknowledges (or rejects) the Count message of a child with a CountResponse • Once a router receives a Count from all children, it sends a count to its parent.

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EXPRESS Service Interface Extensions 

Source service interface

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Count = CountQuery(channel, countId, timeout) channelKey(channel, K(S, E) )

Subscriber service interface

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Result = newSubscription(channel [, K(S, E) ]),

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An unsolicited count message is sent towards the source in order to join the channel • countID in this case is subscriberID.

Count(channel, countId, count)

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used by subscriber to reply to CountQuery

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Advantages 

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Source • 224 channels per source • Address management is simplified • Authenticated subscription option • CountQuery mechanism (number of subscribers or subscriber vote) Subscriber • Receives traffic only from the source it designates • Ability to provide feedback ISP • Provides basis for charging • Counting facility increases revenue • EXPRESS is relatively simple to implement and manage 9

EXPRESS Count Management Protocol    

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A single common management protocol Maintains the distribution tree and supports source-directed counting and voting RPF is used to route subscriptions and unsubscriptions towards the source Generic Counting Operation • CountQuery (propagates from source to leaves) • Count (response of child to parent’s CountQuery) • CountResponse (parent accepts/rejects child’s count message) • A router can initiate a query without source co-operation Distribution Tree Maintenance • New subscription • Unsubsciption • Router can use either TCP or UDP mode for ECMP

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ECMP (Contd.) 

Neighbor/host discovery/refresh

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neighbors: all express routers reply all-channels: all hosts reply with a count message

Forwarding Information Base entries at each router Forwarding procedure is nearly identical to IP Multicast

Authenticated ECMP and End-to-end Encryption

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EXPRESS Packet Forwarding

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Periodic CountQuery message countId:

Authentication provides restricted access while encryption provides confidentiality

Advantages

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Simple integrated protocol

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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 11

Multi-source Multicast Applications 

Multiple channels, one per source • Applicable when new source is going to transmit for extended period of time

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Otherwise, several short-term sources share a channel (SR,E) • Data is relayed via a higher level (application) relay through the channel (SR,E), SR = session relay host • Relaying is done via encapsulation from temporary source to SR.

Receiver A wants to send a small message over the channel

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EXPRESS Conclusions 

Extension to the conventional IP multicast

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Simple implementation

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

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

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