uts and Payload Datagram TCP/UDP L2 PDU (or Packet) Frame) Figure 8.5 Datagram structure Chapter 8 The Internet and IP Infrastructures big congestion problems. Therefore, IP packets are currently segmented into 1,500-byte-maximum chunks-because this is what can fit inside a typical 1,536- byte Ethernet frame and because it is a pretty efficient size, helping to keep retransmission times down. This size is likely to increase, however, to improve performance over high-speed links. Packets between a particular source and destination might not always travel the same route. IP does its best to make the delivery to the requested destination host along the best route currently available, but if it fails for any reason, it just drops the packet. Upper-level protocols should therefore not depend on IP to deliver the packet every time. Because IP provides connectionless, best-effort ser- vice and because packets can get lost or arrive out of sequence, TCP provides a way to recover from these problems. Many network applications present data to TCP, the most common Layer 4 (trans- port layer) protocol. TCP divides the data into packets and gives each packet a sequence number that is not unique but is nonrepeating for a very long time. These packets could represent text, graphics, sound, or video-anything digital that the network can transmit. The sequence numbers help to ensure that the packets can be reassembled correctly and in the proper order at the receiving end. Thus, each packet consists of content, or data, as well as the protocol header, the information that the protocol needs to do its work. TCP uses another piece of information to ensure that the data reaches the right application when it arrives at a system-the port number, which is within the range 1 to 65,535. In an IP network, it is a number assigned to user sessions and server applications. The port number resides in the TCP header as well as in the UDP header for applications such as VoIP and videoconferencing. The source port, which can be a random number, 