tiêu đề LLC và phát hiện ra rằng gói tin đến, nói, giao thức IP tại tầng mạng. LLC cũng có thể cung cấp kiểm soát dòng chảy và sắp xếp các bit kiểm Cisco thực tế thực hành Cisco Router Bản quyền bởi Công ty Cổ phần Quế Tất cả các quyền. | Evaluate TCP IP Communication Process and Its Associated Protocols 335 FIGURE Port numbers for TCP and UDP The different port numbers that can be used are explained next Numbers below 1024 are considered well-known port numbers and are defined in RFC 3232. Numbers 1024 and above are used by the upper layers to set up sessions with other hosts and by TCP to use as source and destination addresses in the TCP segment. In the following sections we ll take a look at an analyzer output showing a TCP session. TCP Session Source Port The following listing shows a TCP session captured with Etherpeek protocol analyzer software TCP - Transport Control Protocol Source Port 5973 Destination Port 23 Sequence Number 1456389907 Ack Number 1242056456 Offset 5 Reserved 000000 Code 011000 Ack is valid Push Request Window 61320 Checksum 0x61a6 Urgent Pointer 0 No TCP Options TCP Data Area . .5. .5. .5 76 4c 19 35 11 2b 19 35 11 2b 19 35 11 2b 19 35 . 11 2b 19 Frame Check Sequence 0x0d00000f 336 Chapter 4 Technology Notice that the source host makes up the source port in this case it is 5973. The destination port is 23 which is used to tell the receiving host the purpose of the intended connection Telnet . By looking at this session you can see that the source host makes up the source port. But why does the source make up a port number To differentiate between sessions with different hosts my friend. How else would a server know where information is coming from if it didn t have a different number from a sending host TCP and the upper layers don t use hardware and logical addresses to understand the sending host s address like the Data Link and Network layer protocols do. Instead they use port numbers. And it s easy to imagine the receiving host getting thoroughly confused if all the hosts used the same port number to get to FTP TCP Session Destination Port Now you ll sometimes look at an analyzer and see that only the source port is above 1024 and the destination port is
