ld render the conversation unintelligible. In VoIP, when you lose packets, you do not request retransmissions. Instead, you hope that the user can recover from the losses by other means. Unlike TCP, UDP does not provide for error correction and sequenced packet delivery; it is up to the applica- tion itself to incorporate error correction, if required. So why use UDP? Multimedia applications typically can't tolerate the addi- tional time required for establishing TCP's virtual connection and the overhead used by TCP's delivery guarantee mechanisms. You can think of UDP as an open "pipe" between two computers that simply allows for an uninterrupted flow of data. The experience of watching a movie over the Internet under UDP is much better; you won't miss the occasional dropped datagram. Over highly controlled and error-correcting TCP, the sequencing overhead most likely would result in greater congestion and more dropped packets. SCTP and DCCP SCTP is a new Layer 4 (transport layer) protocol designed to overcome the limita- tions of TCP with respect to transport of signaling messages and VoIP networks. SCTP was originally intended for the transport of telephony signaling protocols (such as SS7) over IP, with the goal of duplicating some of the reliability character- istics of SS7 in IP. Other applications may benefit as well, such as multimedia Web browsing, video over IP, and IPTV. SCTP provides many of the features of TCP, and it also includes multistreaming, which supports independent transport and deliv- ery of multiple streams between two communicating hosts. It can apply per- stream, in-order delivery to the destination application. It also includes multihom- ing, which supports more than one path between hosts for resilience. Multihoming Chapter 8 The Internet and IP Infrastructures exchanges more than one usable IP address during association setup but can use only one at a time. DCCP is another new Layer 4 protocol that provides congestion control for unreliable data flows. Its features include unreliable 