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Fundamentals Of TCP And UDP

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Fundamentals Of TCP And UDP

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents


Fundamentals of TCP and UDP

Feature Summary
Connection-oriented/connectionless Defines whether the protocol establishes correlation between 2 endpoints before data is sent.
Error recovery The process of noticing errors or lost segments and resending the data.
Flow control Controlling the speed of data transfer between two endpoints.
Segmenting Application layer protocols may need to send large chunks of data. Segmenting is repackaging the data so that it can fit inside multiple packets.


The Transmission Control Protocol (TCP)

TCP uses more bandwidth the UDP because it features error recovery, or more specifically, additional header fields.

  • Multiplexing using multiple port numbers
  • Error recovery (reliability)
  • Flow control using windowing
  • Connection establishment/termination (ACK and FIN)
  • End-to-end ordered data transfer
  • Segmentation


TCP Header Fields

Image:TCP-header.GIF


Multiplexing: Implies how a computer thinks when receiving data, enables the receiving computer the ability to know which app to give the data to.

- > Uses sockets --> 1. 10.1.1.1, TCP, 1030


1. IP addresses 2. Transport protocol 3. Port number

  • Connection between 2 sockets should identify unique connection
  • Well-known port numbers (HTTP, Telnet, etc). fpr Applications
  • Unused port numbers for client machines


Port Number Protocol Application
20 TCP FTP data
21 TCP FTP control
23 TCP Telnet
25 TCP SMTP
53 UDP, TCP DNS
67,68 UDP DHCP
69 UDP TFTP
80 TCP HTTP
110 TCP POP3
161 UDP SNMP

Error Recovery (Reliability): Use sequence number in 1 direction, acknowledgment in the other.

1000 Bytes of Data, SEQ = 1000 -------> 1000 Bytes of Data, SEQ = 2000 -------> Error Occurs Here 1000 Bytes of Data, SEQ = 3000 -------> No Data, ACK = 2000 <------- 1000 Bytes of Data, SEQ = 2000 ------->


Flow Control (Windowing): Uses window field; implies the max # of bytes allowed outstanding at any instant in time.

  • Starts small and grows until error occurs.
  • "Sliding Window" changes size accordingly
  • Web server must wait for clients ACK before sending more than allowed
  • If ACK received before Window exhausted, new window begins
  • Positive Acknowledgement and Retransmission (PAR) describes the error recovery and windowing


Connection Establishment and Termination: Process of initializing sequence and acknowledgement fields and agreeing to the port numbers used.

  • 3-way connection establishment
  • SYN, or "synchronize" the sequence numbers
  • ACK flag for acknowledgment
  • Sequence # and ACK # can be any number that fits in 4-byte fields.
  • 4-way connection termination
  • Uses FIN flag


Connectionless and Connection-Oriented Protocols

Connection-oriented: A protocol that needs an exchange of messages before data transfer begins, or has a required pre-established correlation between 2 endpoints (like Frame Relay).


Connectionless: A protocol that doesn't need an exchange of messages or pre-established correlation.

Connection Oriented? Reliable? Examples
Yes Yes LLC Type 2, TCP, SPX
Yes No Frame Relay, ATM, PPP
No Yes TFTP, NetWare NCP
No No UDP, Most L3 protocols


Data Segmentation and Ordered Data Transfer

Each type of data link protocol typically has a limit on maximum transmission unit (MTU) that can be sent - > the size of L3 packet that is inside Data-link frame

Segmenting = breaking data into smaller pieces

Ordered data transfer = is piecing segments on receiving end in the correct order since they may be sent out of order.


User Datagram Protocol (UDP)

  • Much less overhead,
  • No reordering of data/error recovery is possible, not reliable
  • Multiplexes like TCP, can open same ports as TCP (although not typical).
  • Not artificially slowed by ACK/Window, memory freed more quickly


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