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Practice MCQ

Unit 6 - Notes

CSE306

Unit 6: TRANSPORT LAYER, APPLICATION LAYER & RECENT TRENDS

1. Transport Layer Services

The Transport Layer is the fourth layer of the OSI model. Its primary function is to provide process-to-process communication, ensuring that data transferred between two devices reaches the specific application process (e.g., a browser tab or an email client) running on those devices.

Core Services

  1. Process-to-Process Delivery (Port Addressing):

    • The Data Link layer handles node-to-node delivery (MAC addresses).
    • The Network layer handles host-to-host delivery (IP addresses).
    • The Transport layer handles delivery to a specific process using Port Numbers (16-bit addresses).
    • Well-known ports: 0–1023 (e.g., HTTP: 80, HTTPS: 443).

  2. Multiplexing and Demultiplexing:

    • Multiplexing (Sender side): Gathering data chunks from different sockets, encapsulating each with a header, and passing segments to the network layer.
    • Demultiplexing (Receiver side): Delivering received segments to the correct socket based on port numbers.

  3. Segmentation and Reassembly:

    • The transport layer divides the stream of data received from the application layer into manageable data units called Segments.
    • Each segment contains a sequence number to allow the receiver to reassemble the message correctly and identify lost packets.

  4. Flow Control:

    • Prevents the sender from overwhelming the receiver if the receiver is processing data slower than the sender is transmitting.
    • TCP uses a sliding window mechanism for this.

  5. Error Control:

    • Ensures the entire message arrives without error (damage, loss, or duplication).
    • Mechanisms include checksums and Automatic Repeat Request (ARQ) retransmission strategies.

  6. Connection Control:

    • Connection-Oriented: Establishes a connection (handshake) before sending data (e.g., TCP).
    • Connectionless: Sends data without setup; no guarantee of delivery (e.g., UDP).

2. TCP (Transmission Control Protocol)

TCP is a connection-oriented, reliable transport protocol. It provides a full-duplex, byte-stream service.

TCP Header Format

The TCP header is variable in length (minimum 20 bytes, maximum 60 bytes with options).

TEXT
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data |           |U|A|P|R|S|F|                               |
| Offset| Reserved  |R|C|S|S|Y|I|            Window             |
|       |           |G|K|H|T|N|N|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Checksum            |         Urgent Pointer        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Options                    |    Padding    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field Descriptions:

  • Source/Destination Port (16 bits each): Identifies the sending and receiving applications.
  • Sequence Number (32 bits): If the SYN flag is set, this is the initial sequence number. Otherwise, it is the accumulated sequence number of the first data byte in this segment.
  • Acknowledgment Number (32 bits): If the ACK flag is set, this is the value of the next sequence number the sender is expecting to receive.
  • Data Offset (HLEN) (4 bits): Indicates the size of the TCP header in 32-bit words. Minimum value is 5 (5 * 4 = 20 bytes).
  • Reserved (6 bits): For future use (set to 0).
  • Control Flags (6 bits):
    • URG: Urgent pointer field is significant.
    • ACK: Acknowledgment field is significant.
    • PSH: Push function (asks receiver to push data to application immediately).
    • RST: Reset the connection (abort).
    • SYN: Synchronize sequence numbers (initiate connection).
    • FIN: No more data from sender (terminate connection).
  • Window Size (16 bits): Used for flow control. Specifies the number of bytes the sender is willing to accept (Receive Window).
  • Checksum (16 bits): Used for error detection (covers header, data, and a pseudo-header).
  • Urgent Pointer (16 bits): Points to the end of urgent data if URG flag is set.

TCP Handshaking Operation

1. Connection Establishment (Three-Way Handshake)

Used to initialize sequence numbers and allocate buffers.

  1. SYN (Client → Server):
    • The client sets the SYN flag to 1.
    • It generates a random Initial Sequence Number (ISN), say x.
    • State: Client enters SYN-SENT.
  2. SYN + ACK (Server → Client):
    • The server receives the SYN, sets SYN and ACK flags to 1.
    • It acknowledges the client's ISN by sending Ack = x + 1.
    • It generates its own ISN, say y.
    • State: Server enters SYN-RCVD.
  3. ACK (Client → Server):
    • The client sets the ACK flag to 1.
    • It acknowledges the server's ISN by sending Ack = y + 1.
    • Sequence number is now x + 1.
    • State: Connection Established.

2. Connection Termination (Four-Way Handshake)

Either side can initiate termination.

  1. FIN (Client → Server): Client sends a segment with FIN flag set. (Client enters FIN-WAIT-1).
  2. ACK (Server → Client): Server acknowledges the FIN. (Server enters CLOSE-WAIT; Client enters FIN-WAIT-2). The connection is now "half-closed" (Server can still send data, but Client cannot).
  3. FIN (Server → Client): Once the server is done sending data, it sends its own FIN. (Server enters LAST-ACK).
  4. ACK (Client → Server): Client acknowledges the server's FIN. (Client enters TIME-WAIT to ensure ACK reaches server; Server closes).

3. UDP (User Datagram Protocol)

UDP is a connectionless, unreliable transport protocol. It adds minimal addressing and checksumming to IP but provides no guarantees of delivery, ordering, or duplicate protection.

UDP Characteristics

  • No Connection Setup: No handshaking delay (lower latency).
  • Small Header: Only 8 bytes (lower overhead compared to TCP's 20 bytes).
  • No Flow/Congestion Control: UDP pumps data as fast as desired (useful for real-time apps).

UDP Header Format (8 Bytes)

TEXT
 0      7 8     15 16    23 24    31
+--------+--------+--------+--------+
|     Source      |   Destination   |
|      Port       |      Port       |
+--------+--------+--------+--------+
|                 |                 |
|     Length      |    Checksum     |
+--------+--------+--------+--------+

  1. Source Port (16 bits): Optional. If used, indicates the port of the sending process.
  2. Destination Port (16 bits): Required. Indicates the port of the receiving process.
  3. Length (16 bits): The length of the user datagram (header + data) in bytes. Minimum value is 8.
  4. Checksum (16 bits): Used to detect errors over the header and data. Optional in IPv4, mandatory in IPv6.

4. Domain Name System (DNS)

DNS is the "phonebook" of the Internet. It translates human-friendly domain names (e.g., www.google.com) into IP addresses (e.g., 142.250.190.46).

Hierarchy

DNS uses a distributed, hierarchical database:

  1. Root DNS Servers: The top of the tree (represented by a dot .). There are 13 logical root server IP addresses worldwide.
  2. Top-Level Domain (TLD) Servers: Handle domains like .com, .org, .edu, .uk.
  3. Authoritative DNS Servers: Servers that actually hold the records for a specific domain (e.g., Google's DNS servers holding records for google.com).

How Resolution Works

  • Recursive Query: The client asks the Local DNS Server (Resolver) "What is the IP of X?" and expects the final answer. The resolver does the legwork.
  • Iterative Query: The resolver asks a Root/TLD server, which replies "I don't know, but here is the IP of the server that might know." The resolver then queries that next server.

Common DNS Records

  • A Record: Maps a hostname to an IPv4 address.
  • AAAA Record: Maps a hostname to an IPv6 address.
  • CNAME (Canonical Name): Maps an alias to a true domain name (e.g., www.example.com -> example.com).
  • MX (Mail Exchange): Specifies the mail server responsible for accepting email messages on behalf of a domain.
  • NS (Name Server): Specifies the authoritative name servers for the domain.

5. E-Mail (Electronic Mail)

Email architecture relies on three major components and specific protocols for sending and retrieving messages.

Architecture Components

  1. User Agent (UA): The software users interact with (e.g., Outlook, Gmail web interface). Used to compose and read mail.
  2. Mail Transfer Agent (MTA): Server software responsible for moving mail from the sender's server to the recipient's server.
  3. Message Access Agent (MAA): Pulls the email from the server to the client's inbox.

Protocols

  • SMTP (Simple Mail Transfer Protocol):

    • Port: 25 (or 587 for submission).
    • Function: Push protocol. Used by the sender's UA to send mail to the sender's MTA, and by the sender's MTA to transfer mail to the recipient's MTA.
    • Text-based, command-response protocol (e.g., HELO, MAIL FROM, RCPT TO, DATA).

  • POP3 (Post Office Protocol v3):

    • Port: 110.
    • Function: Pull protocol. Downloads email from the server to the local device.
    • Behavior: Usually deletes the mail from the server after downloading (poor for multi-device access).

  • IMAP (Internet Message Access Protocol):

    • Port: 143.
    • Function: Pull protocol.
    • Behavior: Syncs email. Keeps mail on the server and organizes it in folders. The client manipulates the view of the server. Good for multi-device access.

  • MIME (Multipurpose Internet Mail Extensions): Extends SMTP (which originally only supported 7-bit ASCII text) to allow non-text attachments (images, video, audio).

6. FTP (File Transfer Protocol)

FTP is a standard network protocol used for the transfer of computer files between a client and server on a computer network.

Architecture

FTP uses a Client-Server model and is unique because it establishes two separate TCP connections:

  1. Control Connection (Port 21):
    • Used for sending commands (e.g., login, change directory, list files) and receiving responses.
    • Remains open for the entire session ("stateful").
  2. Data Connection (Port 20):
    • Used strictly for transferring the actual file data.
    • Opened and closed for each file transfer.

Modes of Operation

  • Active Mode: The client opens a port and tells the server. The Server initiates the data connection back to the client. (Often blocked by client-side firewalls).
  • Passive Mode: The server opens a random port and tells the client. The Client initiates the data connection to the server. (Firewall friendly).

7. Software Defined Wide Area Network (SD-WAN)

SD-WAN is a virtual WAN architecture that allows enterprises to leverage any combination of transport services (MPLS, LTE/5G, Broadband) to securely connect users to applications.

Key Concepts

  1. Decoupling Control and Data Planes:
    • Unlike traditional routers where control logic is in every device, SD-WAN centralizes the control logic (Control Plane) in a software controller.
    • The hardware devices (Data Plane) simply forward traffic based on policies from the controller.
  2. Transport Independence:
    • Can overlay on top of any physical connection (MPLS, Internet, 4G). It creates a virtual overlay network.
  3. Application-Aware Routing:
    • Intelligent path selection.
    • Example: Critical VoIP traffic might be routed via MPLS, while bulk file backups are routed via cheap broadband. If the MPLS line fails or degrades (jitter/latency), the software automatically switches VoIP to broadband.

Benefits

  • Cost Reduction: Reduces reliance on expensive MPLS lines by utilizing cheaper broadband.
  • Performance: Optimizes traffic flow for cloud applications (Office 365, Salesforce) by allowing direct internet breakout rather than backhauling traffic to a data center.
  • Agility: Zero-touch provisioning allows new branch offices to be set up quickly.

8. Content Delivery Network (CDN)

A CDN is a geographically distributed group of servers which work together to provide fast delivery of Internet content.

The Problem

If a server is in New York and a user is in Tokyo, data packets must travel a long distance (high latency). If thousands of users access the server simultaneously, the server may crash (bottleneck).

The Solution (CDN Architecture)

  1. Origin Server: The original source where the web application lives.
  2. Edge Servers (Points of Presence - PoPs): Servers located physically closer to end-users (e.g., inside ISPs or internet exchange points).

How it Works

  1. Caching: When a user requests content (like a Netflix video or a large image), the CDN stores a copy of that content on the Edge Server closest to the user.
  2. Request Routing: Subsequent requests from other users in that region are served directly from the Edge Server, not the Origin.

Benefits

  • Reduced Latency: Content travels a shorter physical distance.
  • Higher Availability/Scalability: CDNs can absorb massive spikes in traffic (e.g., DDoS attacks or viral events) without crashing the origin server.
  • Bandwidth Savings: Offloads traffic from the origin server, reducing hosting costs.