Midterm Exam Review– Time:

• Oct. 11 (Thursday, in class, close book)

– 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 email agent uses the same protocol for sending/receiving email?

• Why email 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?

Firewall?• What is SYN flooding attack? Why it is hard to defend?

Protocol Problem Examples

– Show what sender/receiver reacts when a specific incident (bit-error, lose, timeout) happens

• Use graph to show reactions 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 Email/Web server in a company (HW1, problem 6)

– TCP AIMD (HW2, problem 6)– Email SMTP command and manual send

spam– HTTP protocol (HW1, problem 4)– TCP congestion window low-pass filter (Quiz 1)

Calculation Problem Examples

– Proxy impact (access time) • HW1, problem 7

– M/M/1 queue (formula given to you)• Example, P. 9, MM1-queue.ppt

– UDP checksum calculation– Packet switching advantage

• HW1, problem 1

– TCP window usage• HW2, problem 7

• Try to go over homework problems

• Try to go over my lecture notes

What’s wrong?

Socket programming: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);

Low-pass filter programming:

void main(void){

alpha = 0.125;

for (int i=0; i<N;i++)

EstimateRTT[i] = EstimateRTT[i-1] + alpha*SampleRTT[i];

}