Midterm Exam Review Release questions via webcourse “assignment” around 10:30am, Thur. Oct. 20 th, due via webcourse at 10:30am next day Submit format: –Word file, PDF file –Scanned answer sheets Make sure your writing is large and readable –Photos of your answer sheets if you have no scanner Make sure it is readable You can resubmit, so submit first version early! From 10:30am to 1pm, you can call me for any questions related to exam problems –Office number: (HEC 335)

Question Types –Knowledge questions –True or false statement (explain why) –Protocols –Calculations

Knowledge Question Examples Difference between TCP and UDP? Why UDP is better than TCP in some applications? How many layers in computer networking? What’s usage of TTL field in DNS record? IP packet? Why asymmetric speed in DSL and Cable Modem? What service port used by DNS, HTTP, SMTP, SSH, POP3,etc? Why use window in TCP packet transmission? What service provided by DNS? TCP or UDP used in which applications (HTTP, SMTP, DNS, VOIP, etc)?

Knowledge Question Examples What is HTTP cookie used for? TCP socket programming: –the basic socket APIs and their orders –(Page 13, Chapter2-part3.ppt) TCP/UDP header fields, header size (Page 3, Chapter3-part3.ppt) User agent uses the same protocol for sending/receiving ? Why with an attachment sent out is much bigger than the attachment? What is a torrent file? A tracker in BitTorrent? Ethernet switch checks what layer’s header? Router?

Protocol Problem Examples –Show what sender/receiver reacts when a specific incident (bit-error, lose, timeout) happens Use graph to show interactions for TCP’s SR or GBN –Example (P.16, 20 in Chapter3-part2.ppt) Show for rdt3.0 (no test on rdt1.x to rdt2.x) –Example (P. 7,8 in Chapter3-part2.ppt) –TCP syn/ack number changes in a connection Example (P. 5, Chapter3-part3.ppt) –Draw packet transmission time-line scenario Example (P. 14, 15, Chapter3-part3.ppt) –TCP three-way connection setup procedure SYN, SYN/ACK, ACK and changes of seq/ack number –Example (P.25, chapter3-part3.ppt)

Protocol Problem Examples –Why TCP delays its ACK “ Wait up to 500ms for next segment. If no next segment, send ACK ”? –DNS RR entries setup for an /Web server in a company (HW1, problem 4) –TCP AIMD (HW2, problem 6) – SMTP command and manual send spam –HTTP protocol –TCP congestion window low-pass filter (estimate RTT)

Calculation Problem Examples –Proxy impact (access time) HW1, problem 5 –UDP checksum calculation –Packet switching advantage HW1, problem 1 –TCP window usage HW2, problem 7

Try to go over 2 homework's problems Try to go over my lecture notes Covers until and including transport layer

What’s wrong? Socket programming: Char *welcomStr=“welcome”, *recvStr; …. If ( new_fd = accept(sockfd, (sockaddr *)&remote_addr, &sizeof(sockaddr)) == -1){ perror(“accept error\n”); exit(1); } …. send(new_fd, welcomeStr, strlen(welcomeStr), 0); recvNumByte=recv(new_fd, recvStr, MAXDATASIZE-1, 0); printf(“%s\n”, recvStr);