IP Anycast and Multicast

IP Anycast and Multicast
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Overview

Unicast:
- One sender and one receiver
Broadcast:
- One sender and all receivers
Multicast:
- One sender and a group of receivers

Applications of Multicast

Games
Teleconferencing
Audio/Video "On Demand"
Group Chat

IP Anycast

Motivation:
- Load-balancing and failover using IP addresses rather than the DNS
Approach:
- Multiple server "instances" share an IP address
- Routing infrastructure directs packets to the appropriate server

IP Anycast - An Example

IP Anycast - An Example (cont.)

Servers Announce to Router 0:
- They both announce they are 10.0.0.1/32

Router 0's Routing Table:


                           	Destination     Mask    NextHop       Distance
        134.126.0.0     /29     127.0.0.1     0
        10.0.0.1        /32     134.126.0.1   1
        10.0.0.1        /32     134.126.0.2   2

DNS Lookup of www.example.com:


                                   www.example.com  IN   A   10.0.0.1

Client Connects to www.server.com:
- Router 0 routes through router 1
- Client/routers think there is one server

Problems with IP Anycast

Each "group" needs an entry in the global routing table
Limited by IP routing (e.g., application-layer metrics can't be used)
Connection-oriented protocols (e.g., TCP) won't failover

Approaches to Multicast

Multiple Unicast
IP Multicast
Overlay Multicast

Multiple Unicast

Process:
- Client unicasts to every recipient in the group
Assessment:
- Simple to implement
- Client must transmit \(n\) times and network must handle \(n\) times the traffic

IP Multicast

Process:
- Multicast group is defined by a special address as per RFC3171 (224.0.0.0 to 239.255.255.255)
- Sender sends packet to multicast address
- Receiver delivers packet to all appropriate subnetworks
Assessment:
- Best effort only
- Limited number of groups
- Membership is driven by receivers (as per the Internet Group Management Protocol described in RFC3376)

Group Management

Create a group
Join a group
Leave a group
Show members of a group

Multicast Routing

Link State Multicast:
- Used with link state routing
- Link state includes the groups who have members on this link
- A routing tree is calculated to build the routing table
Distance Vector Multicast:
- Used with distance vector routing
- Creates a broadcast tree for each group and then prunes it (a.k.a., broadcast-and-prune)

Adding Reliability

Issues:
- All receivers might ACK at once
- Sender does not know all receivers
Questions:
- Who to retransmit to? How?
- What kind of reliability is required? Is it application specific?

Different Approaches to Reliabile Multicast

Scalable Reliable Multicast
Pragmatic General Multicast (RFC3208)