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Application Layer

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Application Tasks

• To supply services to the user– file transfer– World Wide Web browsing– mail– network management– network name services

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Application Layer Protocols

• FTP/TFTP Port 21/69

• Telnet Port 23

• HTTP Port 80

• SMTP Port 25

• SNMP Port 161

• DNS Port 53

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FTP/TFTP

• FTP (File Transfer Protocol)– uses TCP for transfer of files and data– additional parameters must be supplied– user shielded from file system properties– can be used by terminal or program

• TFTP (Trivial File Transfer Protocol)– similar to FTP but uses UDP

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FTP Commands

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FTP – Login (Two types)

• Supply server name

• Supply account (if not “anonymous”)

• Supply Password (user name if anonymous)

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FTP - Directory Operations

• Change Directory (cd)

• List directory (ls)

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FTP - File Transfer

• Set binary mode

• Put local file to server ftp directory

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Telnet• Telnet is a remote terminal protocol

• Remote logon permitted

• User gets virtual terminal (emulation)

• Uses a TCP connection protocol

• Requires two arguments– name of server– protocol port number of server (commercial

software will use default)

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Telnet - Terminal Setup

• Define terminal

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Telnet - Connect from Prompt

• Type Telnet serverName

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Telnet - Connect via Program

• Type Telnet

• Specify server and terminal type

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HTTP

• Hypertext Transfer Protocol– used for World Wide Web (WWW) to retrieve

hypertext documents– client/server mode– transaction-oriented (connection broken after

document transmission is complete)– uses TCP to connect client and server

(virtual “connection”)

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HTTP Message Format

• Request– message type– requested source

• Response– response information

• General– additional information

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HTTP Header Information

• Connection (header field names)

• Date (date and time stamp)

• Keep-alive (sender connection timeout)

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HTTP Request Methods

• Get (from specified URL)

• Put (post to URL)

• Patch (post differences to URL)

• Copy, Move, Delete (URL info)

• Link/Unlink (add or delete link in URL)

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HTTP Response Messages

• Status line– HTTP Version– Status Code– Reason Phrase

• General response

• Entity body (optional)– text, binary data, audio, images, video

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SNMP/SNMPv2

• Simple Network Management Protocol– network monitoring– network management

• Basic actions– Get (request named objects - various datatypes)– Set (request alteration of named objects)– Notify (inform request)– Uses UDP for communications

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SNMP Network Components

• Management stations– restricted access– inspect, display, and alter network data

• Managed nodes– routers, gateways, bridges, switches, hubs, etc.– computers, printers, etc.

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SNMP Management Objectives

• Fault detection

• Equipment configuration

• Gathering statistics– for accounting purposes– for diagnostic purposes

• Network security

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SNMP Model

• Objects– written in ASN.1 syntax– defined in Management Information Base

(MIB) - many types of objects pre-defined

• Agents– programs in managed nodes– supply requested information– alter information when requested

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ASN.1 Data types in SNMP

• INTEGER Type code = 2

• BIT STRING Type code = 3

• OCTET STRING Type code = 4

• NULL Type code = 5

• OBJECT IDENTIFIER Type code = 6Syntax in use:

identifier type ::= value or (range) or {idn(valn), ...}

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SNMP Data Transfer Format

• Header byte– tag (2 bits)– tag extension (1 bit)– type code (5 bits)

• Length [bytes]

• Data bytes

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Basic SNMP Objects

• System - equipment description

• Interfaces - with logged traffic

• AT - address translation

• IP - packet statistics

• ICMP - message statistics

Basic SNMP Objects (Cont’d)

• TCP - methods, parameters, statistics

• UDP - statistics

• EGP - gateway protocol statistics

• Transmission - media-specific

• SNMP - management traffic statistics

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SNMP Message Types

(0) GetRequest - get variables

(1) GetNextRequest - get next variable

(2) GetResponse- get data

(3) SetRequest - modify variable

(4) Trap - agent trap message

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SMTP

• Simple Mail Transfer Protocol

• Sent and received via Port 25 using TCP

• Envelope– list of recipients

• Body– message to be sent (7-bit ASCII)

• Outgoing messages are queued

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SMTP Header

• TO: address

• FROM: (REPLY TO:) address

• CC: address

• SUBJECT: text

• DATE: date

• ENCRYPTED: pointer

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Some SMTP Commands

• HELO (send ID - sent by client)

• MAIL FROM (identify originator)

• RCPT TO (identify recipient)

• DATA (transfer message text)

• QUIT (close TCP connection)

(See text, p660 for example)

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Some SMTP Replies

• 220 “ready” - returned by server

• 221 “destination closing”

• 250 “okay” - transfer complete

• 345 “ready for mail” - start

• 421 “mail service unavailable”

• 550 “mailbox not found”

• 554 “mail transaction failed”

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DNS• To locate addresses in network domains

• Each host has resource records of format:– DomainName– TimeToLive [seconds]– Class (IN for internet)– Type (see table)– Value (data type depends on Type field)

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DNS (continued)

• Database is distributed and hierarchical

• Names that cannot be resolved in the host are referred up the hierarchy of name servers

Name Servers (Continued)

• There is usually a specified name server for a network configuration

• A backup (second) name server is usually specified

• UDP protocol is used for name server queries and responses

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DNS Record Types

TYPE MEANING VALUE SOA Start of authority ParametersA IP address of host 4 octetsMX Mail exchange Accepts mailNS Name server Server nameCNAME Canonical name Domain namePTR Pointer IP address aliasHINFO Host description ASCII CPU and OSTXT Text ASCII text

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Example

dcs.engr.widener.edu,147.31.252.126,A,IN

End

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