Network Protocols: Design and Analysis-Multicast Overview

Network Protocols:

Design and Analysis

Polly Huang EE NTU

http://cc.ee.ntu.edu.tw/~phuang phuang@cc.ee.ntu.edu.tw

Why Study Multicast?

• want to send info to a group of people

– allows you to send one packet and let the network make copies to everyone

• supports anonymous addressing

– don’t have to keep track of individual users

– don’t worry about changes in group membership – but:

• may not get particular users

• all users may not need all info sent to the group

Multicast Goals

• Efficient data distribution

– send only one copy of pkt over each link, not n

• Anonymous group addressing

– ex. to get a phone number, you call the operator —any operator, not just Lilly

Unicast vs. Multicast

Src Src

compare amount of ban dwidth consumed

Unicast vs. Multicast

Src Src

Unicast vs. Multicast

Src Src

Unicast vs. Multicast

Src _Src

Unicast vs. Multicast

Src Src

in this case, multicast requ ires about half as many pk t-hops compared to unicas t

Applications of multicast

• multiplayer games

– use proxy servers/app-level “multicast” – or just use centralized server

• distributed applications:

– ex. query distributed/replicated database (user-to-application) – inside an distributed application

• teleconference • broadcasting

• sensor networks or other physically distributed applications

Multicast:

Anonymous Addressing

• Applications:

– broadcast (don’t care about users), some distrib uted apps

• Special case: anycast

– find me the nearest receiver in the multicast gro up

Multicast:

Bandwidth Reduction

– broadcast and teleconference

• but some caveats

– reliability? how do we do failure/loss recover in multicast

– different users with different start times? broadcast works best if e veryone’s syncrhonization (option: just start people in the middle, or do caching, multiple versions of stream [ex. using layered/partia l encoding]…)

Common Problems in Multicast

• scalability

– number of sources – number of receivers

– geographic/network distance (sparse vs. dense groups)

• message implosion

Common Techniques in Mcast

• soft state

– rather than reliable send and ACK, send periodically

• response after randomized delay

– delay may be biased to favor certain hosts repsonding

• suppression of duplicate responses

– listen to others responses: if they say the same as you, y ou don’t need to respond

Components of the

IP Multicast Architecture

multicast routing protocols (various)

mcast addr allocation

IP Multicast Service Model

• Defined in RFC-1112 [Deering89a] • mcast groups identified by IP addr

• sending: anyone can send to mcast grp – don’t need to be a member

• receiving: hosts join and leave groups via IGMP

• network builds multicast distribution tree to send da ta

– responsibility of designated router on same LAN as host (and other rtrs in the network)

Class D IP addresses:

in “dotted decimal” notation: 224.0.0.0 — 239.255.255.255

two administrative categories:

– “well-known” multicast addresses (ex: 224.0.0.1 is “all hosts”, .2 is “all rtrs”)

IP Multicast Addresses

Address Allocation

• Outside the scope of this class, but… • Initially, random allocation

– odds of collision are low

– but are they? The Birthday Paradox says collisions will happen

• Later: more careful schemes

– ex. see “The MASC/BGMP Architecture for Multicast Routing”, SIGCOMM ’98

IP Multicast Service — Sending

• uses normal IP-Send operation, with an IP multic ast address specified as the destination

• must provide sending application a way to:

– specify outgoing network interface, if >1 available – specify IP time-to-live (TTL) on outgoing packet

– enable/disable loopback if the sending host is a memb er of the destination group on the outgoing interface

IP Multicast Service —

Receiving

• two new operations:

Join-IP-Multicast-Group ( group-address, inter face )

Leave-IP-Multicast-Group ( group-address, inter face )

• packets arrive like normal IP pkts

Why not do things at the app?

• get higher latencies/bandwidth usage

– because can’t use link-level multicast – don’t have network topology

Multicast Scope Control:

(1) TTL Expanding-Ring Search

to reach or find a nearby subset of a group

s

1

2

Multicast Scope Control:

(2) Admin TTL Boundaries

to keep multicast traffic within an administrative domain, e.g., for privacy or resource reasons

TTL threshold set on

Multicast Scope Control:

(3) Admin-Scoped Addresses

address boundary set on

the rest of the Internet

– “send with my company/country/continent/etc.” – uses address range 239.0.0.0/8

The MBone

• MBone = Multicast Backbone

• a set of routers on the Internet that can exc

hange multicast packets

– some use native multicast

– others tunnel multicast between themselves

Components of the MBone

• a method for sending multicast packets through mu lticast-ignorant routers

• IP multicast packet is encapsulated in a unicast pac ket addressed to far end of tunnel:

• like Moble IP tunnels, but manually configured

MBone Tunnels

Components of the

IP Multicast Architecture

multicast routing protocols (various)

Internet Group Management

Protocol(IGMP)

• the protocol by which hosts report their

multicast group memberships to

neighboring routers

• v1: RFC-1112; v2: RFC-2236

• operates over broadcast LANs and

point-to-point links

Relevant Protocol Layers

Within a Host

IP Service Interface

Link-Layer Service Interface

Upper-Layer Protocol Modules

IP Module

IP to link-layer address

Link-Layer

Transmission/Reception

transmission:

• IP multicast uses link-layer multicast, if pos sible

• ex: Ethernet:

reception:

• hosts link layers often filter multicast addrs • routers listen to all mcast

LAN multicast address

0000000100000000010111100

1110 28 bits 23 bits

IP multicast address group bit

IGMP Goal

• determine what IP mcast groups have receivers pr esent on the LAN

– just care about some vs. no receivers, not how many

• approach:

– designate one router as IGMP “querier” – it asks all hosts

– get at least one response per active group

How IGMP Works

• querier sends a Membership Query message to all-hosts (224.0.0.1), TTL=1

• hosts reply after random delay (0-10s) for each G

• reply goes to whole group G (and routers), suppressing ot

Q G G G routers: hosts: 1 6 4

IGMP Implications

• typically only one response (per G) is requ

ired

• routers query every 60-90 seconds

– implication: leave latency is 30-45s – IGMPv2 adds explicit leave msgs

• to reduce join latency, a host sends one or

two unsolicited responses immediately aft

er joining a new G

Components of the

IP Multicast Architecture

multicast routing protocols (various)

The Mcast Routing Problem:

How do you connect sources and sinks?

The Mcast Routing Problem:

How do you connect sources and sinks?

Mcast Routing Startup

• senders flood everyone

– “flood and prune”, DVMRP, PIM-DM

• receivers flood everyone

– MOSPF

• define a rendezvous point, senders and

receivers send towards rendezvous

How many trees per group?

• shared tree

– all senders use the same tree

– may not be optimal latency, but within 2x

• source-specific tree (or shortest path tree,

SPF)

– one tree per sender

A Shared Tree

Source-Specific Trees

Source1 _R3

Source2 can find a shorter path to R1 by following a

source-Who can send?

• Anyone (Deering’s service model)

– model used by most multicast apps

• Single-source

– only one person can send (others must make the ir own group)

Multicast Status

• MBone exists

– moderately widely used in research – but not always stable

• multi-domain routing is hard—everyone has to talk, and often p eople don’t talk about experimental services :-(

• Some commercial use (apps)

– but very little ISP support

• concerned about how to charge, and potential over-use

• Multicast widely used on LANs