ddress; Source Quench, used to indicate that a router is experiencing congestion and is about to begin discarding datagrams; TTL Exceeded, which indicates that a received datagram has been dis- carded because the Time-to-Live field (sounds like a soap opera for geeks, doesn't it?) reached 0; and finally, Timestamp and Timestamp Reply, which are similar to Echo messages except that they timestamp the mes- sage, giving systems the ability to a measure how long is required for remote systems to buffer and process datagrams. The Application Layer The TCP/IP application layer protocols support the actual applications and utilities that make the Internet-well, useful. They include the BGP, the DNS, the FTP, the HTTP, OSPF, the Packet Internetwork Groper (PING; how can you not love that name), the Post Office Protocol (POP), the Simple Mail Transfer Protocol (SMTP), the Simple Network Management Protocol (SNMP), the Secure Sockets Layer Protocol (SSL), and TELNET. This is a small sample of the many applications that are supported by the TCP/IP application layer. Multiprotocol Label Switching (MPLS) Before we talked about TCP/IP, we mentioned that we would discuss the evolving role of such technologies as TCP/IP and MPLS. As the network evolves, these two collections of protocols become extraordinarily impor- tant because the network is evolving at a rapid and radical pace. When establishing connections over an IP network, it is critical to manage traffic queues to ensure the proper treatment of packets that come from delay-sensitive services, such as voice and video. To do this, packets must be differentiable, that is, identifiable so that they can be classified properly. Routers, in turn, must be able to respond properly to delay-sensitive traffic by implementing queue management processes. This requires that routers establish both normal and high-priority queues, and handle the traffic found in high-priority routing queues faster than the arrival rate of the traffic. Downloaded from Digital Engineering Library @ McGra