HTTP Basics and Status Codes

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Questions and Answers

How many HTTP requests are involved when downloading two embedded images from the same web server using non-persistent TCP connections?

4 (correct)

What transport protocol is used for HTTP?

UDP

ICMP

TCP (correct)

SCTP

What is the size of the header in a DNS message?

16 bytes

8 bytes

10 bytes

12 bytes (correct)

Which command is used to fetch the IP address of www.school.edu?

nslookup --Type=A <a href="http://www.school.edu">www.school.edu</a> Signup and view all the answers

In a DNS response message, which UDP port is used as the source port?

53 Signup and view all the answers

Which of the following flags is set to 0 in a DNS query?

Response flag Signup and view all the answers

What is the first segment sent by the client when requesting a webpage from a web server for the first time?

DNS query in a UDP segment Signup and view all the answers

How is the length of a DNS message calculated?

Length field in UDP minus 8 bytes Signup and view all the answers

What is the size of a TCP header?

20 bytes Signup and view all the answers

During TCP connection establishment, which segment does the client send first?

SYN segment Signup and view all the answers

What happens to the client's sequence number when the client downloads objects over a TCP connection?

It stays constant. Signup and view all the answers

Which fields in the TCP segment are specifically used for flow control?

Acknowledgement number and receive window Signup and view all the answers

What is the typical size of an IPv4 header?

20 bytes Signup and view all the answers

Which flag is specifically activated for closing unused TCP ports?

RST Signup and view all the answers

How does the acknowledgement number of the client change when the client uploads objects over a TCP connection?

It stays constant. Signup and view all the answers

What is the total bit size of a TCP sequence number?

32 bits Signup and view all the answers

What is the primary protocol used for dynamic addressing?

DHCP Signup and view all the answers

How is the checksum calculated in the network layer?

By summing 16 bit words and taking the one's complement of the final sum. Signup and view all the answers

What is the size of the Identification field in bits?

16 Signup and view all the answers

What is the purpose of the Echo messages exchanged in a Ping operation?

To determine node reachability. Signup and view all the answers

What command is used to send an echo message to the IP 192.168.1.10 from 192.168.1.1?

ping 192.168.1.10 Signup and view all the answers

Which field is common in both TCP and IP headers and has the same size?

Header Length Signup and view all the answers

What statistics are expected when pinging a host?

Round-Trip Time (RTT) and packet loss. Signup and view all the answers

What is one disadvantage of static routing compared to dynamic routing?

It requires frequent updates by administrators. Signup and view all the answers

What header line is included in a conditional GET message but not in a regular HTTP GET message?

If-Modified-Since Signup and view all the answers

When comparing Date and Last Modified header lines in an HTTP response, what does the Last Modified header indicate?

Creation or modification time of the object Signup and view all the answers

Which of the following HTTP status codes corresponds to a request for a resource that has been permanently moved?

301 Moved Permanently Signup and view all the answers

In HTTP message format, what is the term used to refer to the entity that creates the client process?

User-Agent Signup and view all the answers

What characterizes persistent TCP connections as opposed to non-persistent TCP connections in HTTP?

Multiple objects are fetched over one TCP connection Signup and view all the answers

How many HTTP requests are involved when downloading a webpage with three embedded objects using a persistent TCP connection?

4 HTTP requests Signup and view all the answers

Which of the following header lines would you typically find in an HTTP request but not in an HTTP reply?

Host Signup and view all the answers

How is the total access delay calculated when multiple objects are fetched using persistent TCP?

Connection time plus 5 RTT Signup and view all the answers

What field in TCP is used to negotiate the Maximum Segment Size (MSS) between sender and receiver?

Options field Signup and view all the answers

Which of the following statements correctly distinguishes between TCP and UDP?

TCP provides reliable data transfer, while UDP does not. Signup and view all the answers

What is the method for fragmenting messages under TCP?

By dividing the message size by the MSS negotiated by both parties. Signup and view all the answers

Which routing protocol uses UDP?

RIP Signup and view all the answers

What is the purpose of error detection in both the network and transport layers?

To ensure the integrity and proper delivery of packets. Signup and view all the answers

Which command would you use to check the line statuses of all interfaces in a router?

show ip interface brief Signup and view all the answers

Which of the following Python commands correctly opens a TCP connection to a server?

socket.connect(('server_IP', server_port)) Signup and view all the answers

What is the valid range of usable IP addresses under the address space 192.168.1.128/28?

192.168.1.129 – 192.168.1.142 Signup and view all the answers

Which command is used to exclude a specific IP address range from being assigned by the DHCP server?

R1(dhcp-config)# ip dhcp excluded-address 10.0.1.10 10.0.1.20 Signup and view all the answers

What is the source IP address of the first message in the DHCP process?

0.0.0.0 Signup and view all the answers

Which of the following messages is NOT part of the DHCP exchange process?

Connection Signup and view all the answers

What does the command 'R1(dhcp-config)# network 10.0.1.0 /24' signify?

Defines the range of IPs available for DHCP assignments Signup and view all the answers

Which command allows you to view the number of messages exchanged between the DHCP server and clients?

show ip dhcp server statistics Signup and view all the answers

What is one disadvantage of using NAT (Network Address Translation)?

It destroys the end-to-end principle in TCP connections. Signup and view all the answers

When configuring a DHCP server, which command specifies the DNS server address?

R1(dhcp-config)# dns-server Signup and view all the answers

What is the purpose of the commands enabling static NAT mapping?

To create a link between public and private IP addresses Signup and view all the answers

Study Notes

HTTP Reply without Object

Last modified, Content length, and Content type headers are absent
Reply does not carry an object

GET vs. Conditional GET

Conditional GET message includes If-Modified-Since header
GET message does not include this header

Date vs. Last Modified

Date: Time of web server reply
Last Modified: Time object was created/modified

HTTP Status Codes

Examples:
- 301 Moved Permanently
- 200 OK
- 400 Bad Request
- 404 Not Found
- 302 Found
- 502 Bad Gateway
- 304 Not Modified

HTTP Request Line (Example)

GET /homepage.html HTTP/1.1

Web Server in HTTP

Web server is referred to as Host in HTTP message format
Client and server processes are referred to as User-agent and Server respectively

HTTP Header Locations

Host, Accept Language are in HTTP request
Server and Date are in HTTP reply
Connection is in both request and reply

Persistent vs. Non-Persistent TCP

Persistent TCP: Multiple objects over one TCP connection (Connection: Keep-alive)
Non-persistent TCP: Each object over a separate TCP connection (Connection: Close)

Webpage with Embedded Objects (Persistent TCP)

4 HTTP requests
Total access delay: 5 Round Trip Times (RTTs)

Webpage with Embedded Objects (Non-Persistent TCP)

4 HTTP requests
Total access delay: 8 RTTs

DNS Server and Client Ports

Server port: 53
Client port: Random 16-bit number (excluding server-defined ones)
HTTP: TCP
DNS: UDP

DNS Message Contents

DNS header (Identification, Flags, Question/Answer/Additional RR counts)
Body (Questions, Answers, Authoritative RRs, Additional RRs)

DNS Header Length

12 bytes
DNS message length determined by UDP length field minus 8 bytes

Fetching Canonical Name and IP Address

Command examples:
- nslookup –Type=CNAME www.school.edu
- nslookup –Type=A www.school.edu

Identifying Query and Response in Wireshark

Look for source/destination IPs
UDP port 53 in response, other port in query
Response flags, Recursion available flag

HTTP Request for Webpage (Client)

1. DNS query (UDP) to DNS server
1. SYN segment to web server
1. ACK segment to web server
1. HTTP request (combined with ACK)

TCP Header and IPv4 Header Sizes

TCP header: 20 bytes
Typical IPv4 header: 20 bytes

TCP Connection Establishment

Client sends SYN to server
Server replies with SYN-ACK
Client sends ACK
Connection established

TCP Connection Closing

Client sends FIN to server
Server replies with ACK
Server sends FIN to client
Client sends ACK

TCP Segment Header Fields (Congestion/Flow Control)

Sequence number
Acknowledgement number

TCP Segment Flags

SYN, ACK, FIN, PSH, URG, RST
PSH with ACK for in-line data
RST for closing unused ports

MSS Negotiation in TCP

Options field
Affects TCP header size

TCP vs. UDP

TCP: Connection-oriented, reliable, congestion control
UDP: Connectionless, unreliable (no congestion control, faster)

Routing Protocols (using TCP/UDP)

UDP: RIP
TCP: OSPF, BGP

DNS Message Fragmentation

Divided by MSS (estimated message size)
Client acknowledges received segments and client sends acknowledgment to the server when completes

Socket Library (Python)

Open/close TCP connection example
Examples for sending/receiving data using UDP/TCP sockets

Network Layer (GNS3)

Checking line statuses: show ip interface brief
Checking network reachability: show ip route

TCP Checksums

Transport Layer: Computed over entire message
Network Layer: Computed only over header portion of packets

Static vs. Dynamic Addressing

Static: Simple for a few devices, complex for many
Dynamic: DHCP, complex for many devices, works well for large networks

Network Layer Checksum Calculation

Split into 16-bit words and summed
Any carry added to the sum
Ones complement of the final sum

IP Address Ranges (e.g., 192.168.1.128/28)

Subnet mask: e.g., 255.255.255.240
Usable range: e.g., 192.168.1.129 - 192.168.1.142

DHCP Message Exchange

4 messages (Discovery, Offer, Request, Acknowledgement)
Source IP: 0.0.0.0, Destination IP: 255.255.255.255

Excluding Address Ranges (DHCP)

Prevents IP assignment in that range
Command example: R1(dhcp-config)# ip dhcp excluded-address 10.0.1.10 10.0.1.20

Configuring Gateway and DNS in DHCP Server (R1)

R1(dhcp-config)# default-router <IP address of R1's interface>
R1(dhcp-config)# dns-server <IP address of DNS server>

DHCP Server Statistics

Use show ip dhcp server statistics command

Network Address Translation (NAT)

Advantages: Increased address space, simplifies network configuration
Disadvantages: Complicates address translation for private hosts

NAT Pool (Example)

Define public IP addresses to be used

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Description

Test your knowledge of HTTP protocols, including the differences between GET and Conditional GET requests. Explore the significance of HTTP status codes, header locations, and persistent TCP connections in web communications.