internet applications chapter 14. 14.1 introduction dns electronic mail ftp tftp internet telephony...

Internet applications

Chapter 14

14.1 Introduction

DNS Electronic mail FTP TFTP Internet telephony SNMP

14.2 Domain name system

DNS(Domain name system) An efficient naming scheme Name-to-address mapping Defined in RFCs 1034 and 1035

14.2.1 Name structure and administration

Name structure Flat structure Hierarchical structure

Name administration A single host : At the lowest level of the tree Generic domains

Com:this identifies hosts that belong to a commercial organization

Edu: an educational establishment Gov:the US federal government Mil:the US armed forces Org: a non-profit organization Net: a network provider Int: an international organization

Note also that all names are case-insensitive

14.2.1 Name structure & administration (cont-1)

14.2.2 DNS resource records

DNS resource records Each domain name in the DNS name space may have information associated with it , each is indexed by the related domain name

domain name The name of the domain

type field indicates the record type

Type-A record contains an IPv4 address

Type-NS record contains the name of the name server for this domain

Type-PTR record contains an IP address stored in is its dotted decimal form

Type-HINFO record contains the type of host and its operating system

MX-record contains the name of a host - an email gateway

14.2.2 DNS resource records (cont-1)

14.2.3 DNS query messages

DNS query messageIs formed by adding a standard 12-byte header 16-bit identification field 16-bit flags field

a 1-bit field is used to indicate whether the message is a query (=0) or a response (=1)

A 4-bit field to indicate the type of search involved

14.2.4 Name servers

The total domain name space is partitioned into a number of zones

Each zone is the administered by a separate authority which is also responsible for providing one or more name servers for the zone

.

14.2.5 Service requests

Each root server holds the name and IP address of each of the second-level servers in the hierarchy

On receipt of a request from a primary server, the root server returns the name and IP address of the second-level server

The primary then proceeds to query this server and down the hierarchy until a resource record containing the required IP address

14.2.5 Service requests (cont-1)

Local name resolution


Recursive name resolution


Iterative name resolution


Pointer queries

14.3 Electronic mail

An email system : An email client runs a program Called the user agent (UA) A server has UA server to interact with the UA software in

each Client POP3 (protocol-post office protocol 3)

control messages that are exchanged Between the UA client and UA server

is specified in RFC1939 An agent (MTA) concerned with the sending and

receiving of mail messages To/from other email servers

SMTP (simple mail transfer protocol ) IS specified in RFC 821

14.3.1 Structure of email messages

Email message An envelope

Contains the email address of the sender of the message (MAIL FROM) and its intended recipient (RCPT TO)

A header A body

14.3.1 Structure of email messages (cont-1)

14.3.2 Message content

RFC 822 standard the content part of a message –the body-canonly be lines of ASCII text with the maximum length of each line set at 1000 characters

Multipurpose Internet Mail Extensions (MIME). It was first specified in RFC 1341 and later updated in RFCs 2045/8

Application:type is used when the body contents require processing by theRecipient UA before they have meaning on the user’s display

Message:type is used when the contents relate to another MIME message

Multipart:type is used to indicate that the message body contents of multiple Part/attachments

14.3.2 Message content (cont -1)

14.3.2 Message content (cont -2)

Two alternative transfer encodings are defined in RFC 1521 for use with an RFC 821-conformant message transfer system(MTA)

Quoted-printable: this is used to send messages that are composed of Characters from an alternative character set that is mostly ASCII but has a Small number of special characters which have their eighth bit set to 1.Examples are the Latin character sets

Base64: this is used to send blocks of binary date and also messages Composed of strings of characters from a character set that uses 8-bit code words such as EBCDIC

14.3.2 Message content (cont-3)

14.3.3 Message transfer

14.3.3 Message transfer (cont-1)

14.4 FTP

14.4.1 Overview File transfer protocol (FTP)

is specified in RFC 959 trivial file transfer Protocol (TFTP)

14.4.2 File content representation

File transfers involving just two different file types,ASCII and binary

A file containing 7-bit ASCII characters the file contents are first converted Into NVT ASCII by the sending side

They are then converted back again into 7-bit ASCII at the receipt side for Storage

14.4.3 FTP operation

14.4.4 Command and reply message format

Reply messages

220 FTP server read331 Password required for <username>230 User <username > logged in215 Server OS Name Type:Version200 File type acknowledged200 PORT command successful150 Opening ASCII/Binary mode date connection for <file name>226 File transfer complete221 Goodbye425 Data connection cannot be opened500 Unrecognized command501 Invalid arguments530 User access denied

14.4.5 Example

Three types of file transfer supported over the data connection:

The transfer of the contents of a named file from he client system to the Server system

A similar transfer in the server in the server-client direction The transfer of the listings of the files (or the directories in a file)

held by The server and saved in a name file on the client

A typical exchange of commands replies The client FTP control part has a resolver procedure procedure

kinked to it and,When the DNS name of the server is passed to it by the user interface,it Uses the resolver to obtain the IP address of the server

If the user had issued a put <filename .type>, then (control part) would send STOR <filename.type> command .Also, since the file transfer is in the client-server direction,if the TYPE is I,then the client would initiate the closure of the Data connection

14.4.5 Example (cont-1)

14.4.6 Anonymous FTP

Prompted for a username enters : anonymous

Prompted for password enters : email address

14.5 TFTP

TFTP is then used to download-normally referred to as bootstrapping

TFTP uses UDP as the transport protocol TFTP is given in RFC 1350

14.5.1 Protocol Opcode indicates the message type Initiates a transfer is called the client , Responds to the

server

14.5.1 Protocol (cont-1)

Filename field specify the name of the file on the server to be transferred/downloaded, is an NVT ASCII string

Mode field which is also an NVT ASCII string indicating whether the file Contents are lines of ASCII text –netascii –or a string Of 8-bit bytes –octet


The first DATA message (BN=1) is assumed to be received and acknowledged Correctly

The second DATA message (BN=2) is corrupted and hence is not received At the server side ,the absence of an ACK for BN2 means that the

retransmission timer expires and another attempt is made to send it

This time is assumed to be received correctly and both V(S) and V(R) are now incremented to 3

When the last DATA message is sent (BN=3), this is received free of errorsand hence V® is incremented to 4 and an ACK is returned with BN=3

During its transfer, the ACK is corrupted/lost ,the server retransmits another copy of BN3 which is assumed to be received error free

The client determines from the BN that the message is a duplicate-BN=3 Instead of 4-and hence

The client determines that the file has now been received by the fact that thecontents of BN3 are less than 512 bytes

The last ACK timer is used to allow for the possibility of the last ACK being lost Should the number of attempts to send a block exceed a defined limit,then an

error message is sent and the transfer aborted

14.6 Internet telephony

Session initiation protocol (SIP) the session description protocol (SDP) And the gateway location PROTOCOL(GLP)

14.6.1 SIP SIP provides services for user location, call/session establishment

, and call Participation management is defined in RFC2543 Both the request and response are made through an application

program/process called the user agent (UA) which maps therequest and its response into the standard message format used by SIP

Each UA comprises two parts, a UA client (UAC),which enables the user to send request messages

A UA server (UAS)which generates the response message

14.6.1 SIP (cont-1)

A selection of the header fields INVITE request/response message

To: The SIP address of the called participant From: The SIP address of the caller Subject: A brief title of the call Call-Id : Unique call identifier assigned by the caller Require: List of capabilities the host device can support Content-Type: Type of information in the message body Content-Length: Length of body contents

14.6.1 SIP (cont-2)

14.6.1 SIP (cont-3)

14.6.2 SDP

SDP describe the different media streams that are be involved in a call/session and also additional information relating to the call

Media streams:a multimedia call/session may involve a number of different media streams including speech,audio,video,and more general data

Stream addresses:the destination address and UDP port number for sending and /or receiving each stream is indicated

Start and stop times:broadcast sessions and enable a user to join a session during the time the broadcast is being carried out

14.6.3 GLP signaling gateway (SGW)

a gateway to convert the different signaling messages media Gateway(MGW)

a gateway to convert the different media formats

14.7 SNMP

Fault management Performance management Layer management Name management Security management Accounting management

14.7 SNMP (cont-1) SNMP allow the manager process in the manager station to exchange management-

related messages with the a management processes in the management agent Management information base (MIB)

14.7.1 Structure of management information

The current version of the MIB for the Internet is MIB-II and is defined in RFC 1213

for example: Iso(1) org(3) dod(6) internet(1) mgmt(2) mib-2(1)… 1.3.6.1.2.1 …

The meaning of the four required parameters associated with each object

SYNTAX:this defines the data type of the object

MAX-ACCESS: defines whether the variable is read-only or read-write(as viewed from the manager station)

STATUS:indicates whether the variable is current or obsolete

DESCRIPTION :an ASCII string describing what the object is used for when the macro is invoked, the final ::= sign places the variable into the object name tree of the device

14.7.1 Structure of management information (cont-1)

14.7.1 Structure of management information (cont-2)

14.7.2 Protocol

Get-request:this is used by the manager to get the current value(s) of one or more named variables from an agent. The agent then returns the value(s) using a Get-response message

Get-next-request:this is used by the manager to the next variable that is located in the MIB name tree returned in a Get-response message

Set-request:this is used by the manager to write a given set of values into the corresponding named variables

Trap:this is used by the agent in the equipment identified in the enterprise field to notify the manager of the occurrence of a previously defined event


SNMPv2A second version defined in EFC 1441

Get-bulk-request enable the retrieval process of the contents of large to be carried out more efficiently

Inform-request enable a manager process in one manager station to send information to a manager process in another manager station

An additional MIB for handling the variables associated with manager-to manager communication

The encryption of the password contained in community field

Summary

internet applications chapter 14. 14.1 introduction dns electronic mail ftp tftp internet telephony...

Documents