tailieunhanh - ccna study guide by sybex phần 3

UDP nhận được trên các khối lớp thông tin, thay vì các dòng dữ liệu như TCP, và phá vỡ chúng thành các đoạn. Giống như TCP, mỗi phân khúc UDP là một số cho reassembly vào khối dự định tại điểm đến. Tuy nhiên, UDP không thứ tự các phân đoạn và | TCP IP and the DoD Model 111 UDP receives upper-layer blocks of information instead of data streams as TCP does and breaks them into segments. Like TCP each UDP segment is given a number for reassembly into the intended block at the destination. However UDP does not sequence the segments and does not care in which order the segments arrive at the destination. At least it numbers them though. But after that UDP sends the segments off and forgets about them. It doesn t follow through check up on them or even allow for an acknowledgment of safe arrival complete abandonment. Because of this it s referred to as an unreliable protocol. This does not mean that UDP is ineffective only that it doesn t handle issues of reliability. Further UDP doesn t create a virtual circuit nor does it contact the destination before delivering information to it. It is therefore also considered a connectionless protocol. Since UDP assumes that the application will use its own reliability method it doesn t use any. This gives an application developer a choice when running the Internet Protocol stack TCP for reliability or UDP for faster transfers. UDP Segment Format The very low overhead of UDP compared to TCP which doesn t use windowing or acknowledgments is shown in Figure . FIGURE UDP segment You need to understand what each field in the UDP segment is. The UDP segment contains the following fields Source port Port number of the host sending the data Destination port Port number of the application requested on the destination host Copyright 2000 SYBEX Inc. Alameda CA 112 Chapter 3 Internet Protocol Length of the segment Length of UDP header and UDP data CRC Checksum of both the UDP header and UDP data fields Data Upper-layer data UDP like TCP doesn t trust the lower layers and runs its own CRC. Remember that the Frame Check Sequence FCS is the field that houses the CRC which is why you can see the FCS information. The following shows a UDP segment caught on a network