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networking chapter 2

Semantics is what a message means; syntax is how a message is organized.

Distinguish between syntax and semantics.

The simple message ordering: client sends request, and then server sends response.

Describe the simple message ordering in HTTP.

a header, a data field, and a trailer

What are the three general parts of messages?

the content being delivered by a message

What does the data field contain?

No

Is there always a data field in a message?

48 bits

How long are Ethernet MAC addresses?

switches

What devices read Ethernet destination MAC addresses?

128 bits

How long is an IPv6 address?

Three way handshake; syn, syn/ack, ack

What does TCP use to open a connection?

Protocol data units

What does PDU stand for?

Layer 2; datalink

Which layer has a header and a trailer?

Bigger

As a packet gets encapsulated does it get bigger or smaller.

Source and destination MAC address

What are the major header fields for an Ethernet frame?

Source and destination IP address

What are the major header fields for an IP packet?

Sequence number, acknowledgement number, source and destination port numbers

What are the major header fields for a TCP segment?

Source and destination port numbers

What are the major header fields for a UDP datagram?

25

Which port is used for SMTP (e-mail)?

80

Which port is used for HTTP (web server)?

20 or 21

Which port is used for FTP?

Data link

At which layer would Ethernet standard be used?

Network

At which layer would the IP standard be used?

Transport

At which layer would the transmission control protocol (TCP) standard be used?

Transport

At which layer would the User Datagram Protocol (UDP) standard be used?

Application

At which layer would the HTTP standard be used?

Data field

As a message is encapsulated from the application layer to the transport layer, in which field in the transport layer is the message from the application layer placed?

Segment

What is the TCP PDU called?

Datagram

What is the UDP PDU called?

Packet

At layer 3, what is the PDU called?

Frame

At layer 2, what is the PDU called?

None

At layer 1, what is the PDU called?

Reliable and connection oriented

What type of communication does TCP have (connection/reliability)?

Unreliable and connectionless

What type of communication does UDP have (connection/reliability)?

Network standards

Rules of operation that govern the exchange of messages between two hardware or software processes.

Semantics

What a message means

Syntax

How a message is organized

Connection-oriented

Connections with distinct openings and closings

Detects and corrects errors

Network reliability

Repeater or hubs (uncommon)

Layer 1 typical device

Switches or bridges

Layer 2 typical device

Routers; layer 3 switches

Layer 3 typical device

Layer 4 switches

Layer 4 typical devices

Client and server hosts

Layer 7 typical devices

Application programs

What are a server's port numbers associated with?

7 bits

If you need to represent 128 alternatives in a field, how many bits long must the field be?

Placing a message in the data field of another message

What is encapsulation

IETF

What is the standards agency for TCP/IP?

11 bits

You need to represent 1,026 different city names. How many bits will this take if you give each city a different binary number?

No

In HTTP, can the server transmit if it has not received a request message from the client?

None

What kind of message does the destination host send if it does not receive a segment during a TCP connection?

Host A sends SYN; Host B sends SYN/ACK; Host A sends ACK

Describe the three-way handshake in TCP connection openings.

None

What kind of message does the destination host send if it does it receive a segment that has an error during a TCP connection?

If it does not receive and ACK message from the destination host

Under what conditions will a source host TCP process retransmit a segment?

The receiver would discard the frame.

If the receiver detects an error on the basis of the value in the Frame Check Sequence field, what does it do?

Router

What device in an internet besides the destination host reads the destination IP address?

Application

Which layer has the most standards?

Converting messages into bits at the application layer

What is encoding?

2n

If a field is N bits long, how many alternatives can it represent?

It would discard one of the duplicated segments

How do you think TCP would handle the problem if an acknowledgement were lost, so that the sender retransmitted the unacknowledged TCP segment, therefore causing the receiving transport process to receive the same segment twice?

The destination port field of the TCP message

When the HTTP program sends an HTTP response message to a client PC, in what field of what message will it place the value 80?

FTP-file transfer protocol
SMTP-simple message transfer protocol
http-hypertext transfer protocol

List 5 application layer protocols by acronym, expanded name, and function.

False

The SPOT satellite tracking device shown in class assumes an underlying reliable data communications system.

64

How many characters can be represented by 6 bits?

255

What is 11111111 in base 10?

Timing (what happens when; ex. Flow control)
Semantics (what means what; ex. flags)
Syntax (what goes where; ex. Ethernet frame bits 0-16 are part of the preamble)

What three main things must communications protocols govern? Describe what each means, and give an example.

It does this so that it can apply sequence numbers to segements which are later used for error correction.

Why does TCP use a three-step handshaking protocol (also called a three-way opening) to establish connections?

The trailer is used for error detection. If there is an error, the packet is discarded.

Why does only the data link layer typically have a trailer in its protocol data unit?

This allows layer 4 to pass clean data to the application layer regardless of errors at lower levels and error correction is only done once on the two hosts which reduces how much resources are used for error correction.

Why make only layer 4 reliable?

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