Week 6 Topic 4: Layer 4 protocols

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Questions and Answers

Which layer of the OSI model is responsible for providing reliable data transfer services between end users?

  • Session Layer
  • Transport Layer (correct)
  • Data Link Layer
  • Network Layer

What is the primary function of multiplexing in the context of Layer 4 protocols?

  • Detecting and correcting errors in transmitted data.
  • Allowing multiple applications to use the same IP address simultaneously. (correct)
  • Controlling the speed at which packets are sent to prevent overwhelming the receiver.
  • Ensuring data packets arrive in the correct order.

Which of these protocols is connection-oriented and requires a three-way handshake to establish a connection?

  • UDP
  • IP
  • DNS
  • TCP (correct)

What is the purpose of sequence numbers in TCP?

<p>To identify lost packets and reassemble packets that arrive out of order. (B)</p> Signup and view all the answers

Which of the following is NOT an advantage of using TCP over UDP?

<p>Lower overhead and faster data transmission in certain scenarios. (A)</p> Signup and view all the answers

Which of these protocols relies on TCP for reliable data transfer?

<p>HTTP (B)</p> Signup and view all the answers

During the TCP handshake, what is the purpose of the SYN-ACK packet?

<p>To acknowledge the receipt of the initial SYN packet and initiate the receiver's sequence number. (A)</p> Signup and view all the answers

Which protocol is more suitable for applications that require low latency and can tolerate occasional packet loss?

<p>UDP (C)</p> Signup and view all the answers

Which of the following is a characteristic of UDP that differs from TCP?

<p>It does not provide retransmission of lost packets. (D)</p> Signup and view all the answers

Why is the sequence numbering in TCP considered a security feature?

<p>It makes it more difficult to forge TCP packets. (C)</p> Signup and view all the answers

Which of these protocols is commonly used for broadcasting messages to all devices on a network?

<p>UDP (B)</p> Signup and view all the answers

Considering its reliability features, in what scenario would TCP be preferred over UDP?

<p>File transfer where data integrity is paramount. (A)</p> Signup and view all the answers

Which of the following best describes the role of port numbers in Layer 4 protocols?

<p>To identify the specific application or service on a host. (B)</p> Signup and view all the answers

What is the main purpose of the ACK packet in the TCP three-way handshake?

<p>To acknowledge that the connection has been established. (A)</p> Signup and view all the answers

Which of the following protocols uses UDP?

<p>DNS (A)</p> Signup and view all the answers

Which aspect of TCP helps prevent a fast sender from overwhelming a slow receiver?

<p>Flow Control (C)</p> Signup and view all the answers

In what situation would using UDP be more advantageous than using TCP?

<p>Streaming a live video feed. (D)</p> Signup and view all the answers

What potential issue arises from UDP's lack of inherent reliability mechanisms?

<p>Application's increased responsibility for error handling. (B)</p> Signup and view all the answers

Which of the following is a primary advantage of TCP's connection-oriented nature?

<p>The ability to establish a reliable and ordered communication channel. (C)</p> Signup and view all the answers

If a host sends a SYN packet with sequence number 1000, what might the sequence number be in the SYN-ACK packet from the recipient?

<p>A completely different, randomly generated number to initiate its own sequence. (C)</p> Signup and view all the answers

Flashcards

What is Layer 4 responsible for?

The transport layer (Layer 4) transfers data between end-users, providing reliable data transfer services. It handles reliability, flow control, and multiplexing.

What does TCP provide?

TCP uses port numbers for multiple applications on a single IP, enhances reliability by assigning sequence numbers to packets for reordering and retransmission, and includes checksums for error detection.

What is a TCP handshake?

A process used by TCP to synchronize sequence numbers, involving SYN, SYN-ACK, and ACK packets to establish a connection before data transfer.

What happens during the SYN step?

The sender initiates the connection by sending a SYN packet which contains the port information and the senders starting sequence number.

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What happens during the SYN-ACK step?

The receiver responds to a SYN by sending a SYN-ACK packet with its own sequence number.

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What happens during the ACK step?

The sender sends an ACK packet back to the receiver to acknowledge the connection, indicating that data transfer can begin.

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What does UDP provide?

UDP uses port numbers, but it's unreliable. Packets may be lost, duplicated, or arrive out of order. Lacks error detection and flow control.

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Can you broadcast with UDP?

UDP packets can be sent to the broadcast address, reaching all devices on the network. TCP connections are point-to-point only.

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What are some popular TCP protocols?

HTTP (port 80), HTTPS (port 443), FTP (port 21), SMTP (port 25), POP3 (port 110), and SSH (port 22).

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What are some popular UDP protocols?

DNS (port 53), DHCP (ports 67 and 68), OpenVPN (port 1194), and VOIP.

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Study Notes

  • Layer 4, the transport layer, facilitates data transfer between end-users and offers reliable data transfer to upper layers.
  • Layer 4 handles reliability, ensuring packets arrive in the correct order.
  • Layer 4 manages flow control, preventing packets from being sent too quickly.
  • Layer 4 handles multiplexing, enabling multiple applications to run on the same IP address, each using a unique port number (up to 65,000 ports per IP address).
  • Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are the two most well-known Layer 4 protocols.
  • UDP and TCP packets reside within IP packets which reside within the link frame data.

Transmission Control Protocol (TCP)

  • TCP uses port numbers so multiple applications can share a single IP address.
  • TCP improves the reliability of IP packets by assigning sequence numbers.
  • Sequence numbers help TCP identify missing packets for retransmission.
  • Sequence numbers lets TCP reassemble packets that arrive out of order.
  • TCP functions with a defined beginning and end, similar to a phone call.
  • TCP provides reliability by retransmitting lost packets.
  • TCP offers reordering to correct out-of-order packets.
  • TCP includes flow control to prevent overwhelming the receiver, adjusting the transmission speed as needed.
  • TCP performs error detection via checksums on all segments.
  • TCP sequence numbering enhances security, making it difficult to forge a TCP packet.
  • TCP is connection-oriented, requiring a three-way handshake to establish each connection.
  • TCP operations are managed at the OS level, simplifying application development.
  • HTTP (port 80), HTTPS (port 443), FTP (port 21), SMTP (port 25), POP3 (port 110), and SSH (port 22) are several protocols based on TCP.

TCP Handshake

  • TCP uses sequence numbers and acknowledgment numbers.
  • When a segment is received, the receiver sends an acknowledgment back to the sender.
  • Acknowledgments confirm that the receiver received segment number xxxxx.
  • Sequence numbers are 32-bit numbers that enhance system security.
  • TCP uses a three-way handshake to synchronize sequence numbers:
    • SYN: The sender sends a SYN packet with port information and its starting sequence number to begin the connection.
    • SYN-ACK: The receiver responds with a SYN-ACK packet containing its own sequence number.
    • ACK: The sender sends an ACK packet to confirm the connection.

User Datagram Protocol (UDP)

  • Like TCP, UDP uses port numbers.
  • UDP is unreliable, packets are not automatically resent if lost.
  • UDP lacks sequence numbers, making it easier to send fake packets.
  • UDP doesn't have flow control or packet reordering mechanisms.
  • UDP is suitable for small requests needing low latency.
  • Using UDP is like sending a package without a tracking number, relying on its arrival without confirmation.
  • UDP supports broadcasting to all devices on a network, using the network's broadcast address.
  • TCP connections are limited to two hosts, unlike UDP which can broadcast.
  • DNS (port 53), DHCP (ports 67 and 68), OpenVPN (port 1194), and VOIP are some common protocols based on UDP.
  • Most multiplayer games use UDP, and may implement custom schemes for resending lost packets.

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