IP Packet Transmission Modes

Three are four basic transmission modes for IP packets:

Unicast – transmission to a single, specific destination node (most common mode, used for most Internet traffic, such as HTTP, FTP, etc). Every node that participates in network traffic must have a unique unicast address assigned to it (unique within the relevant scope, e.g. LAN, corporate network, or global Internet). Unicast transmission works over both UDP and TCP. Unicast transmission is essentially identical over IPv4 and IPv6, but there are additional “scoping” rules with IPv6. Unicast traffic easily crosses routers (subject to ACL or firewall rules, and scoping rules in IPv6).

Broadcast – transmission to all other nodes on the subnet (e.g. to find a DHCPv4 server). Broadcast transmission can be done only over UDP. Broadcast traffic requires processing by all nodes in the subnet, most of which are not intended recipients, hence they will ignore it (broadcast is “noisy”, or disruptive). Broadcast is available only over IPv4. Broadcast traffic never crosses routers – it is possible only within a subnet.

Multicast – transmission to all nodes that have “subscribed” to the destination multicast “group” (address). This works within a subnet without any need for multicast capable routers, but reaching nodes outside of the local subnet requires routers than support multicast transmission and subscription. Used for targeted server discovery (less disruptive than broadcast), streaming audio or video content to multiple subscribers (IPTV, Internet radio), etc. Multicast transmission can be done only over UDP. Multicast can be used over both IPv4 and IPv6, but there are additional “scoping” rules in IPv6. Multicast functionality is much better over IPv6, especially when crossing routers.

Anycast – transmission to the closest of multiple nodes that have had the same unicast address assigned to them. For example, there are only 13 distinct IPv4 unicast addresses for the root DNS servers, but many instances of each such address exist around the Internet (in various AS networks). A packet sent to one of those addresses will be routed to the closest one of these DNS servers, by the BGP routers, based on routing metrics. Anycast transmission can be done over UDP or TCP. To the sending node, anycast is identical to normal unicast transmission. The only difference is that the recipient node may be one of a group of potential destinations, based on routing metrics. Anycast can be done in both IPv4 and IPv6, but over IPv6 it works in smaller scopes than in IPv4.