21.10 Sending and Receiving

21.1 Introduction

As shown in Figure 20.1, a unicast address identifies a single IP interface, a broadcast address identifies all IP interfaces on the subnet, and a multicast address identifies a set of IP interfaces. Unicasting and broadcasting are the extremes of the addressing spectrum (one or all) and the intent of multicasting is to allow addressing something in between. A multicast datagram should be received only by those interfaces interested in the datagram, that is, by the interfaces on the hosts running applications wishing to participate in the multicast group . Also, broadcasting is normally limited to a LAN, whereas multicasting can be used on a LAN or across a WAN. Indeed, applications multicast across a subset of the Internet on a daily basis.

The additions to the sockets API to support multicasting are simple; they comprise nine socket options: three that affect the sending of UDP datagrams to a multicast address and six that affect the host's reception of multicast datagrams.