Learning Multicast Part 1: Fundamentals

David Hedley’s
Tuesday Tech Talks
Multicast
Turning networking on it’s head
© 2018-2020 David M. Hedley All Rights Reserved.

David Hedley's Tuesday Tech Talks – Multicast Pt. 1
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David Hedley's Tuesday Tech Talks – Multicast
Pt. 1
Definition
• In computer networking, multicast is group communication[1] where data
transmission is addressed to a group of destination computers
simultaneously. Wikipedia https://en.wikipedia.org/wiki/Multicast retrieved July 5,2018
• Originally defined in RFC 966 (1985)

David Hedley's Tuesday Tech Talks – Multicast
Pt. 1
• Purpose: Understanding the fundamentals of Multicast.
• Question: What IS multicast?

• Triple Constraints
• Time
• Quality or Scope
• Money or Budget

• Traditional methods of delivering packets
• Unicast
• One to One communication
• Can be TCP, UDP, or other
• If we need to deliver to more than one host, we create a new connection for each.
• Processing burden is on the sender + network
• Broadcast
• One to all communication
• Usually link local (not crossing a layer 3 boundary)
• Directed broadcast are generally dropped or blocked in modern designs
• Processing burden is on the receivers as any that don’t want the traffic have to drop.

• Unicast vs. Multicast

• Unicast vs. Multicast
• TCP Unicast
• TCP is connection orientated
• 3 way Handshake
• Reliable
• Flow control (window size)
• UDP Unicast and Multicast
• Connectionless
• Unreliable (application layer awareness)

• Multicast disadvantages
• Best Effort Delivery
• NO Congestion Avoidance
• Duplicates
• Out of Order Delivery

• Multicast Advantages
• Eliminates Traffic Redundancy = Less Bandwidth
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Unicast vs Multicast
Mulicast Unicast

• Multicast Addressing (IPv4)
• Source (Class A, B, C) 1.0.0.0 – 223.255.255.255
• Destination (Class D) 224.0.0.0 – 239.255.255.255
• SOURCE CAN NEVER BE CLASS D GROUP ADDRESS!!!!

• Multicast Ranges (IPv4)
• Reserved Link-Local Addresses
• 224.0.0.0-224.0.0.255
• TTL=1
• Examples
• 224.0.0.5 OSPF
• Other Reserved
• 224.0.1.0 – 224.0.1.255
• TTL > 1
• Examples
• 224.0.1.1 NTP
See https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-1

• Multicast Ranges (IPv4)
• Source Specific Multicast (SSM)
• 232.0.0.0 – 232.255.255.255
• Administratively Scoped Addresses
• 239.0.0.0 – 239.255.255.255
• Like private addresses
• Example:
• 239.200.0.0/14 Enterprise range
• 239.202.0.0/17 “Regional” range
• 239.240.0.0/16 Campus range
See https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-1

• IP Multicast MAC Address Mapping
• In IPv4, all multicast address start with bits 1110, so the last 28 bits
differentiate the different multicast addresses
• Each Multicast MAC begins with 0x01005e with a zero in the 25th bit
• The last 23 bits are taken from the group address
• Example
• 239.200.128.1 = 11101111 11001000 10000000 00000001
• Mac 0000001 00000000 01011111 01001000 10000000 00000001 (01-00-5e-48-80-01)
• 224.72.128.1 = 11100000 01001000 10000000 00000001
• Mac 00000001 00000000 0101111 01001000 10000000 00000001 (01-00-5e-48-80-01)
• 32:1 Address Overlap!

David Hedley's Tuesday Tech Talks – Multicast Pt 1
• Components

• Tech Talk
• To do list!
• What are the multicast applications in your network, and what are their design
requirements?
What IGMP version do they need v2 vs v3 (MLD v1 vs v2)?
Do they need a range of group addresses or a single address?
Is the group address defined by standard or will you need to assign?
How are you documenting the addresses you use?
How will you scope your administrative range 239.0.0.0/8?

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Learning Multicast Part 1: Fundamentals

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