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Figure 7-40 The UDP header
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UDP Datagram Length
UDP Checksum
because it involves minimal protocol overhead. UDP s primary responsibility is to add a port number to the IP address to create a socket for the application. The fields of a UDP message (see Figure 7-40) are described here. The Source Port field identifies the UDP port used by the sender of the datagram, while the Destination Port field identifies the port used by the datagram receiver. The Length Field field indicates the overall length of the UDP datagram. The Checksum field provides the same primitive bit error detection of the header and transported data as we saw with TCP.
The Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) is used as a diagnostic tool in IP networks to notify a sender that something unusual happened during transmission. It offers a healthy repertoire of messages, including Destination Unreachable, which indicates that delivery is not possible because the destination host cannot be reached; Echo and Echo Reply, used to check whether hosts are available; Parameter Problem, used to indicate that a router encountered a header problem; Redirect, which is used to make the sending system aware that packets should be for-
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Transport Technologies
Transport Technologies
warded to another address; 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 discarded 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 message, 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 important 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.
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Transport Technologies
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MPLS delivers QoS by establishing virtual circuits known as label switched paths (LSPs) which, in turn, are built around traffic-specific QoS requirements. An MPLS network, such as that shown in Figure 7-41, comprises label switch routers (LSRs), at the core of the network, and label edge routers (LERs), at the edge. It is the responsibility of the LERs to set QoS requirements and pass them on to the LSRs, which are responsible for ensuring that the required levels of QoS are achieved. Thus, a router can establish LSPs with explicit QoS capabilities and route packets to those LSPs as required, guaranteeing the delay that a particular flow encounters on an end-to-end basis. It s interesting to note that some industry analysts have compared MPLS LSPs to the trunks established in the voice environment. MPLS uses a two-part process for traffic differentiation and routing. First, it divides the packets into forwarding equivalence classes (FECs) based on their quality of service requirements, and then maps the FECs to their next hop point. This process is performed at the point of ingress at the edge of the network. Each FEC is given a fixed-length label that accompanies each packet from hop to hop; at each router, the FEC label is examined and used to route the packet to the next hop point, where it is assigned a new label. MPLS is a shim protocol that works closely with IP to help it deliver on QoS guarantees. Its implementation will allow for the eventual dismissal of ATM as a required layer in the multimedia network protocol stack. And while it offers a promising solution, its widespread deploy-
Figure 7-41 Schematic representation of an MPLS network showing LERs and LSRs: The LERs are at the periphery of the network; the LSRs are in the core.
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