CS470Computer Networking

Protocols

Huiping GuoDepartment of Computer Science

California State University, Los Angeles

Final Review

Final Review

Final Time

8:00 am – 10:30am , Wed. 03/21

Open book, open note No web surfing (except for the class webpage) No emails Bring your own scratch paper

There are no spaces for your answers on the exam paper

Write down your answers (in order) on your own scratch paper

Write down your name on each paper

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Final Review

Outline Transport layer Application layer

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Final Review

Transport layer ICMP

What is ICMP for? How to transmit an ICMP message? How to avoid infinite loop?

Network Address Translation (NAT) Why NAT How it works

DHCP What is DHCP for? How it works?

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Final Review

User Datagram Protocol (UDP) Services UDP provides The applications that use UDP UDP header

Some important fields

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Final Review

Principles of reliable transfer Errors

A segment is corrupted A segment is lost

Automatic retransmission request (ARQ) When errors occur, the receiver sends a message to the

sender asking for retransmission Types of ARQ

Stop and wait Sliding window (Go back N)

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Final Review

Pros and Cons of Stop-and-wait

Pros Stop-and-wait is a complete reliable transfer protocol It’s very simple

Cons Inefficient While the sender is waiting for an ACK, it doesn’t do

anything. Why not the sender sends multiple segments at a time?

Need more than 1 bits for the sequence number.

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Final Review

Go-Back-N Based on a sliding window Use window to control number of segments to send

Number of segments that the sender can send• N-(nextseqnum – send_base)

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Final Review

Go-Back-N Cumulative ACKs

ACK (n): ACKs all segments up to, including seq # n - 1 Timer

Only one timer is used. The timer is for the oldest transmitted, yet un-acked

segment

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Final Review

TCP What services TCP provides The TCP connection TCP header format TCP reliable transfer

TCP vs. GBN How TCP detects data lost?

TCP flow control How it works Flow control vs. congestion control

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Final Review

TCP header format Some most important fields

SYN, FIN Seq, ACK Receive Window

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Final Review

TCP connection management Connection establishment

Three-way handshake In each step, what the send and the receiver do?

Connection teardown The steps

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Final Review

TCP congestion control Slow start

When the phase starts and ends

Congestion avoidance When the phase starts and ends

Timeout How to adjust the two variables

3 duplicate ACKs How to adjust the two variables

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Final Review

HTTP The service it uses Stateless HTTP connections

Non-persistent Persistent

Cookies How do cookies work?

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Final Review

FTP What is FTP for? How it works?

Two connections: control connection, data connection

Which connection is for transferring which kinds of data?

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Final Review

Email Components

user agents mail servers SMTP

Send an email SMTP is used

• Between Sender’s agent and sender’s mail server• Sender’s mail server and receiver’s mail server

Receive an email POP3 or IMAP is used

• Between receiver’s mail server and receiver’s agent Differences between POP3 and IMAP

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Final Review

DNS The service it uses Distributed hierarchical databases

Root DNS servers TLD servers Authoritative DNS servers Local DNS servers

Types of DNS queries Recursive Iterative

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Final Review

DNS DNS caching

Where and why?

DNS records RR format RR types

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