Transport Layer and UDP Notes

Introduction to Computer Networks

TCP/IP Protocol Stack

  • Layers:

    • Ethernet
    • WIFI
    • IP
    • UDP
    • TCP
    • FTP
    • HTTP
    • TFTP
    • DNS

  • Protocols:

    • FTP: File Transfer Protocol
    • HTTP: Hypertext Transfer Protocol
    • TFTP: Trivial File Transfer Protocol
    • DNS: Domain Name System
    • TCP: Transmission Control Protocol
    • UDP: User Datagram Protocol
    • IP: Internet Protocol

Processes and Ports

  • IP Addresses: Operate at the network level, routing data between hosts.
  • Port Numbers: Operate at the transport level, directing data to specific processes within a host.
  • Analogy: IP addresses are like street addresses, while port numbers are like apartment numbers within a building.
  • Communication: Enables host-to-host (IP) and process-to-process (UDP) communication.

Sockets

  • Socket Address: Defined by the tuple .
  • Uniqueness: This tuple uniquely identifies the "pipe" or connection between processes.

Well-Known Ports

  • 7: Echo (Echoes a received datagram back to the sender)
  • 9: Discard (Discards any received datagram)
  • 11: Users (Active users)
  • 13: Daytime (Returns the date and time)
  • 17: Quote (Returns a quote of the day)
  • 19: Chargen (Returns a string of characters)
  • 20: FTP, Data (File Transfer Protocol - data connection)
  • 21: FTP, Control (File Transfer Protocol - control connection)
  • 23: TELNET (Terminal Network)
  • 25: SMTP (Simple Mail Transfer Protocol)
  • 53: DNS (Domain Name Server)
  • 67: BOOTP (Bootstrap Protocol)
  • 79: Finger (Finger)
  • 80: HTTP (Hypertext Transfer Protocol)
  • 111: Port mapper (Port mapper)

Process-to-Process Protocols

  • Responsibility: Realization of process-to-process packet delivery.
  • Expectations:
    • Guaranteed message delivery (end-to-end).
    • Ordered delivery.
    • Rejection of duplicate messages.
    • Messages of arbitrary length.
    • Congestion control to handle network overloading.

Need for Transport Layer

  • Limitations of Network Layer:
    • Can drop messages (due to buffer overflow, etc.).
    • Can reorder messages.
    • Deliver duplicate copies.
    • Limits message size.
  • Function: The transport layer addresses these limitations by providing end-to-end transport service to the application layer.

Internet Transport Protocols

  • UDP: User Datagram Protocol
    • Datagram messaging service.
    • "Best-effort" IP end-to-end service.
  • TCP: Transmission Control Protocol
    • Reliable, in-order delivery.
    • Connection set-up.
    • Discarding of corrupted packets.
    • Retransmission of lost packets.
    • Flow control.
    • Congestion control.
  • RPC: Remote Procedure Call
    • Request/Reply Service.
    • Execute a program in a remote host.

User Datagram Protocol (UDP)

  • Simplest connectionless transport protocol.
  • Functionality:
    • Multiplexing and de-multiplexing of flows.
    • UDP checksum for error detection.
    • Unreliable delivery of packets.

Multiplexing and De-multiplexing

  • Multiplexing: Multiple processes on a host generate messages for the same or different hosts, which are then multiplexed by the transport layer.
  • De-multiplexing: Reassembling messages at the receiving host and passing them to the appropriate processes.

Encapsulation Process

  • Encapsulation: Process -> Message -> UDP header -> UDP data -> IP header -> IP data -> Frame header -> Frame data
  • Decapsulation: Reverse process at the receiving end.

UDP Communication Service

  • Process Identification: Processes are identified based on ports (sockets).
  • Port Range: 16 bits per port field allows for 64K (2^16) different identifiers.
  • De-multiplexing:

UDP Message Queue

  • Ports and Queues: Each application process has a queue associated with a specific port.
  • Packet Arrival: Packets arrive and are de-multiplexed to the appropriate application process queue.

Port Discovery

  • Well-Publicized Ports: Standard ports for services (e.g., DNS uses port 53, Email uses port 25, HTTP uses port 80).
  • Port-Mapper Service: Use a well-known port (e.g., 111) as a "port-mapper" to dynamically discover ports for other services, similar to a directory assistance service.

Advantages of UDP

  • Real-Time Traffic: Control over data sending; no connection establishment delay.
  • Stateless Connection: Easier to handle many active clients due to no allocation of buffers, sequence numbers, etc.
  • Small Header: UDP header is only eight-bytes long.

Disadvantages of UDP

  • "Best Effort" Networking: No guaranteed delivery, ordered delivery, or congestion control.
  • Congestion Issues: UDP does not adapt to network congestion and can suppress TCP flows.
  • Attack Vector: Susceptible to UDP flooding attacks.

Applications Utilizing UDP

  • Simple Query Protocols: DNS benefits from low overhead and quick retransmission.
  • Multimedia Applications: Real-time applications like telephone calls, video conferencing, and gaming can tolerate some packet loss.