Chapter 1. Introduction to TCP/IP Transport and Applications

This chapter covers the following exam topics:

1.0 Network Fundamentals

1.5 Compare TCP to UDP

4.0 IP Services

4.3 Explain the role of DHCP and DNS in the network

TCP/IP Layer 4 Protocols: TCP and UDP

• TCP provides retransmission (error recovery) and
• TCP helps to avoid congestion (flow control),
• UDP needs fewer bytes in its header (less overhead)
• UDP software does not slow down data transfer
• TCP can purposefully slow down data transfer
• Voice over IP (VoIP) and video over IP, do not need error recovery, so they use UDP

UDP Supports:

• Multiplexing using ports

TCP Supports:

• Multiplexing using ports

• Error recovery (reliability)

• numbering and acknowledging data with sequence and acknowledgment header fields

• Flow control using windowing

• use window sizes to protect buffer space

• Connection establishment and termination

• initialize port numbers and sequence and acknowledgment fields

• Ordered data transfer and segmentation

Transmission Control Protocol

• TCP header is 20 bytes (without options)

• source port/ destination port

• Sequence Number

• Acknowledgement Number

• Offset, Reserved, Flag bits, Window

• Checksum, Urgent

*TCP segment, Layer 4 PDU, or L4PDU

Multiplexing

example

All open on one computer:

Port 80 Web Server

Port 800 Ad Server

Port 9876 Wire Application

Socket

• Includes IP address, transport protocol, and port number

• (10.1.1.2, TCP, port 80)

Well Known (System) Ports:

• 0 to 1023

User (Registered) Ports:

• 1024 to 49151

Ephemeral (Dynamic, Private) Ports:

• 49152 to 65535,

• not assigned

• Servers use well-known ports (or user ports), whereas clients use dynamic ports

• uses the same port number for all connections. For example,

• web server with 100 clients would have only one socket (one port number)

• server looks at source port of received TCP segments.

Popular TCP/IP Applications

Simple Network Management Protocol (SNMP)

• query, compile, store, and display information about a network’s operation
• Cisco Prime software

FTP/ TFTP

• FTP allows many more features
• TFTPis very simple, good tools for embedded parts of networking devices.

SMTP/ POP3

• Simple Mail Transfer Protocol (SMTP) and Post Office Protocol version 3 (POP3)
• both used for transferring mail (TCP).

Port numbers and protocols

• FTP Data TCP 20
• FTP Control TCP 21
• SSH TCP 22
• Telnet TCP 23
• SMTP TCP 25
• DNS UDP/TCP 53
• DHCP Server UDP 67
• DHCP Client UDP 68
• TFTP UDP 69
• HTTP TCP 80
• POP2 TCP 110
• SNMP UDP 161
• SSL TCP 443
• Syslog UDP 514

Connection Establishment and Termination

TCP connection establishment (3 way handshake) occurs 1st

• SYN, DPORT=80, SPORT=49145

• SYN ACK , DPORT= 49145, SPORT=80

• ACK DPORT=80, SPORT=49145

• initializing Sequence and Acknowledgment fields

• agreeing on the port numbers

• 2 bits inside the flag fields of the TCP header. Called the SYN and ACK flags

TCP connection termination. (four-way)

- uses an additional flag, called the FIN bit

• ACK, FIN >
• < ACK
• ACK, FIN <
• < ACK

Error Recovery and Reliability

• reliability in both directions

• Sequence Number field of one direction and Acknowledgment field in the other direction

• 1000 bytes, Seq = 1000 >

• 1000 bytes, Seq = 2000 >

• 1000 bytes, Seq = 3000 >

• < no data, ACK = 4000

• received all data with sequence numbers up through one less than 4000

• ready to receive your byte 4000 next.

• ack by listing the next expected byte (forward acknowledgment)

• sequence number field identifies the data (sender)

• forward acknowledgments acknowledge the data (receiver)

• Sequence and Acknowledgment fields let the receiving host can notice lost data

• ask the sending host to resend

• acknowledge that the re-sent data arrived

• 1000 bytes, SEQ 1000 >

• 1000 bytes, SEQ 2000 X >

• 1000 bytes, SEQ 3000 >

• < no data, ACK = 2000

• (received 1000 - 1999 and 3000 - 3999, asking for 2000)

• 1000 bytes, SEQ 2000 >

• < no data, ACK 4000

• Retransmission timer

• Sender may wait a few moments to make sure no other acknowledgments arrive

Flow Control Using Windowing

Sliding window (dynamic window

• Receiver slides the window size up and down

• < ACK=1000, Window=3000

• SEQ=1000, SEQ=2000, SEQ=3000 >

• < ACK=4000, Window=4000

User Datagram Protocol

• connectionless

• no reliability,

• no windowing,

• no reordering of the received data, and

• no segmentation of large chunks of data into the right size for transmission

• provides data transfer and multiplexing using port numbers

• Less overhead and processing than TCP.

• no reordering or recovery

• DNS requests use UDP, user will retry an operation if the DNS resolution fails

• Network File System (NFS), a remote file system application, performs recovery with application layer code, so UDP features are acceptable to NFS.

• 8 byte header

• Source port, Destination Port

• Length, Checksum

TCP/IP Applications

Uniform Resource Identifiers (URI)

• clicking a link and typing a URI—refer to a URI

• referred to as web address or Universal (uniform) Resource Locator [URL]

• three key components

• before the :// identifies the protocol

• between the // and / identifies the server by name

• after the / identifies the web page.

• http:// (Scheme)

• www.certskills.com (Authority)

• /blog (path)

DNS

• < Name Resolution Request (IP Header, UDP Header, DNS Request)

• Name resolution Reply (Ip Header, UDP Header, DNS Request (IP address) >

• < TCP connection to requested web server

• DNS requests can be cached by hosts and servers

• Local DNS may need to ask for help

• The enterprise DNS acts as a recursive DNS server

• Sends repeated DNS messages to find the authoritative DNS server.

• Recursive DNS lookup

• host > Enterprise DNS >

• Root DNS

• .com TLD DNS

• Authoritative cisco.com DNS

Transferring Files with HTTP

• HTTP GET request lists file it needs

• HTTP GET response from server with a return code of 200 (meaning OK) and file’s contents.

• Server may issue a return code of 404, (file not found)

• Web pages consist of multiple files, called objects

• Objects are stored as different files on the web server

• Web browser gets the first file which can include references to other URIs that the browser also requests

• < HTTP GET /go/ccna

• HTTP OK data >

• < HTTP GET /graphics/logo1.gif

• HTTP OK data >

• < HTTP GET /graphics/ad1.gif

• HTTP OK data >

• Flow over one or more TCP connection between the client and the server.

Identifying the Correct Receiving Application

• tracks which port opened which request

Fields that identify next header

< Ethernet (Type) (0x0800)

< IPv4 (Protocol) (6)

< TCP (Destination port) (49124)