tailieunhanh - CompTIA Network+ Certification Study Guide part 63

CompTIA’s Network+ certification Study Guide part 63 is a globally-recognized, vendor neutral exam that has helped over 235,000 IT professionals reach further and higher in their careers. The 2009 Network+ exam (N10-004) is a major update with more focus on security and wireless aspects of networking. Our new study guide has been updated accordingly with focus on network, systems, and WAN security and complete coverage of today’s wireless networking standards. | 606 CHAPTER 12 Network Troubleshooting Methodology layer. Table illustrates some of the more common TCP applications and the ports they use Table Well-Known TCP Ports Port Number Application 20 FTP data 21 FTP control 22 SSH 23 Telnet 25 SMTP 53 DNS 80 HTTP 88 Kerberos 110 POP3 119 NNTP 139 NetBIOS 443 SSL Understanding UDP A connectionless transport protocol like UDP doesn t provide the same acknowledgment of receipt process as the connection-oriented TCP does. Because UDP doesn t sequence the packets that the data arrives in an application program that uses UDP has to be able to make sure that the entire message has arrived and is in the right order. To save processing time network applications that have very small data units to exchange and thus very little message reassembling to do may use UDP instead of TCP. For example Domain Name System DNS hostname lookup messages that will always fit in a single datagram can effectively use UDP. For these very short queries you don t need all the complexity of TCP if you don t receive an answer after a few seconds you can just ask again. UDP doesn t split data into multiple datagrams as TCP does. It also doesn t keep track of what it has sent. Data can be resent if needed and UDP doesn t guarantee delivery or protect against duplication. However it is not completely irresponsible it does provide for a checksum capability to ensure that data arrives intact and it provides port numbers to distinguish between the requests sent by different user applications. Examples of applications that use UDP for communication include Trivial File Transfer Troubleshooting the Transport Layer 607 HEAD OF THE CLASS. The Three-Way Handshake Computers using TCP to communicate have both a send window and a receive window. At the beginning of a TCP communication the protocol uses a three-way handshake to establish the session between the two computers. Because TCP unlike its transport layer sibling UDP is connection-oriented a session