Domain 1 of 5 · Chapter 5 of 9

Traffic Types

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Included in this chapter:

  • The four delivery patterns and their scope
  • Addresses, IGMP, and how each pattern is identified
  • Broadcast domains, collision domains, and the IPv6 change

Four IP traffic types at a glance

PropertyUnicastMulticastAnycastBroadcast
Delivery scopeOne specific hostAll joined group membersNearest one of manyAll hosts on the segment
Address example (IPv4)Any normal host IP224.0.0.0/4 (class D)A shared unicast IP advertised from many sites255.255.255.255 or directed
Membership / selectionDestination IPIGMP group joinRouting chooses closestImplicit (whole domain)
Typical useWeb, email, SSH, file transferIPTV, conferencing, OSPF/EIGRPDNS root/TLD, CDNDHCP DISCOVER, ARP
Crosses routers?YesOnly with multicast routingYes (it is unicast on the wire)No (stops at the router)
Exists in IPv6?YesYes (replaces broadcast)YesNo

Decision tree

One specific recipient?does it name one hostYesUnicastweb, SSH, file copyNoReach a group of joiners?interested receivers onlyYesMulticast224.0.0.0/4 + IGMPIPTV, OSPF/EIGRPNoNearest of many identical?same IP from many sitesYesAnycastshared IP + routingDNS root, CDNNoEveryone on the segment?and the network is IPv4Yes, IPv4Broadcast255.255.255.255, DHCP/ARPIPv6Use multicast groupIPv6 has no broadcast (ff02::1)
Cheat sheet

Cheat sheet

  • Pick the traffic type by how many hosts the destination represents
  • Unicast is one-to-one and the default for almost all traffic
  • Multicast sends one stream the network copies only toward joiners
  • IGMP is how IPv4 hosts join and leave multicast groups
  • Multicast needs multicast routing to leave its subnet
  • Anycast advertises one address from many sites and routing picks the nearest
  • Anycast fits stateless request/response, not long stateful sessions
  • Broadcast floods every host in one broadcast domain and stops at the router
  • 255.255.255.255 is the limited broadcast and routers never forward it
  • IPv6 has no broadcast and uses multicast instead
  • The IPv6 multicast prefix is ff00::/8
  • A switch puts each port in its own collision domain but keeps one broadcast domain
  • A router or VLAN boundary breaks a broadcast domain
  • Large flat networks invite broadcast storms
  • Some low class D addresses are reserved for protocols
  • Map the scenario keywords to the traffic type
  • IPv6 anycast addresses come from the unicast space and look identical to unicast
  • In PIM Sparse Mode the Rendezvous Point is the root of the shared tree where sources register and receivers join

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References

  1. RFC 1112 — Host Extensions for IP Multicasting Whitepaper
  2. RFC 4291 — IP Version 6 Addressing Architecture Whitepaper