4-Traditional Internet Applications

Dr. John P. Abraham

Professor

UTPA

External references

• Your book provides limited amount of information. Please note that I have used Forouzan to supplement this chapter. Refer to the syllabus.

Application Layer Protocols

• Two broad types of protocols for this layer:– Private communication: programmer creates a

pair of applications that communicate over the internet with intention it is for private use.

– Standardized service. Example is a server service. All types of users will use it. The specification must be precise and unambiguous so that all can interoperate correctly.

Representation and Transfer

• Data Representation: Syntax of data items that are exchanged. Specific form used during transfer, translation of integers, characters, and files between computers.

• Data transfer. Interaction between client and server, message syntax and semantics, valid and invalid exchange error handling, termination of interaction.

Web protocols

• HTML – a representation standard of a web page.

• URL – specify the location of a page.

• HTTP – transfer protocol that browser uses to communicate with the webserver.– Request types: get, head, post, put.

• I leave detailed description because it is pretty elementary.

HTTP Requests

• GET: requests a document. Server send status information and the document.

• HEAD: Requests status information. Server sends just the status not the document.

• POST: Sends data to the server. The server appends the data to a specified item.

• PUT: Same as above, except, instead of appending it replaces previous data.

Response Header

• Version of the HTTP protocol.

• Status code (whether the server handled the request – code 200). Code 404 means item can’t be found, 400 means bad request.

• Other information. Server identification. Last Modified. Content length and Content Type. All followed by CRLF.

Caching In Browsers

• Important optimization technique to reduce download times by saving a copy of each image in a cache on the user’s disk.

• If the document changes the ‘Head request to server’ will sent last modified date and time which is compared with the cached copy. If the local copy is stale a new one is downloaded.

Browser Architecture p 59.

FTP

• Can transfer any type of data• Bidirectional transfer• Supports access restrictions• Ability to browse folders• Control messages are exchanged in ASCII• Supports heterogeneity

FTP connections

• Two connections– Control– Data– Use different port numbers: Server uses port 21 and

waits for the client (Passive open). Client uses ephemeral port and issues an active open. The server then issues an active open (client takes and active role by sending a TCP message to start the connection – SYN) using port 20 and the received port from the client and data transfer begins.

– Transmission modes: stream, block and compressed.

Figure 19.1 FTP

• The following two slides are very important. Please know the contents.

FTP Active Passive Opens

• FTP can be run in active or passive mode, which determine how the data connection is established. In active mode, the client sends the server the IP address and port number on which the client will listen, and the server initiates the TCP connection. In situations where the client is behind a firewall and unable to accept incoming TCP connections, passive mode may be used. In this mode the client sends a PASV command to the server and receives an IP address and port number in return. The client uses these to open the data connection to the server.

FTP active passive contd.

• A client makes a TCP connection to the server's port 21. This connection, called the control connection, remains open for the duration of the session, with a second connection, called the data connection, either opened by the server from its port 20 to a negotiated client port (active mode) or opened by the client from an arbitrary port to a negotiated server port (passive mode) as required to transfer file data. The control connection is used for session administration (i.e., commands, identification, passwords) exchanged between the client and server using a telnet-like protocol. Due to this two-port structure, FTP is considered an out-of-band, as opposed to an in-band protocol such as HTTP.

Table 19.1 Table 19.1 Access commands. \Access commands. \The following Slides are good references. If you are either The following Slides are good references. If you are either programming FTP, or a heavy user of FTP, you need to programming FTP, or a heavy user of FTP, you need to have this info.have this info.

Table 19.2 Table 19.2 File management commandsFile management commands

Table 19.3 Table 19.3 Data formatting commandsData formatting commands

Table 19.4 Table 19.4 Port defining commandsPort defining commands

Table 19.5 Table 19.5 File transfer commandsFile transfer commands

Table 19.5 Table 19.5 File transfer commands (continued)File transfer commands (continued)

Table 19.6 Table 19.6 Miscellaneous commandsMiscellaneous commands

Table 19.7 Table 19.7 ResponsesResponses

Table 19.7 Table 19.7 Responses (continued)Responses (continued)

Table 19.7 Table 19.7 Responses (continued)Responses (continued)

Table 19.7 Table 19.7 Responses (continued)Responses (continued)

Table 19.7 Table 19.7 Responses (continued)Responses (continued)

Table 19.7 Table 19.7 Responses (continued)Responses (continued)

Table 19.7 Table 19.7 Responses (continued)Responses (continued)

Figure 19.8 shows an example of using FTP for retrieving a list of items in a directory.

1. After the control connection to port 21 is created, the FTP server sends the 220 (service ready) response on the control connection.2. The client sends the USER command.3. The server responds with 331 (user name is OK, password is required).4. The client sends the PASS command.

5. The server responds with 230 (user login is OK)

Example 1

See Next Slide

6. The client issues a passive open on an ephemeral port for the data connection and sends the PORT command (over the control connection) to give this port number to the server.7. The server does not open the connection at this time, but it prepares itself for issuing an active open on the data connection between port 20 (server side) and the ephemeral port received from the client. It sends response 150 (data connection will open shortly).8. The client sends the LIST message.9. Now the server responds with 125 and opens the data connection.

Example 1 (cONTINUED)

See Next Slide

10. The server then sends the list of the files or directories (as a file) on the data connection. When the whole list (file) is sent, the server responds with 226 (closing data connection) over the control connection.11. The client now has two choices. It can use the QUIT command to request the closing of the control connection or it can send another command to start another activity (and eventually open another data connection). In our example, the client sends a QUIT command.12. After receiving the QUIT command, the server responds with 221 (service closing) and then closes the control connection.

Example 1 (cONTINUED)

See Next Slide

Figure 19.8 Example 1

The following shows an actual FTP session that parallels Example 1. The colored lines show the responses from the server control connection; the black lines show the commands sent by the client. The lines in white with black background shows data transfer.

Example 2

See Next Slide

$ ftp voyager.deanza.fhda.eduConnected to voyager.deanza.fhda.edu.220 (vsFTPd 1.2.1)530 Please login with USER and PASS.Name (voyager.deanza.fhda.edu:forouzan): forouzan331 Please specify the password.

Example 2

Password:230 Login successful.Remote system type is UNIX.Using binary mode to transfer files.ftp> ls reports227 Entering Passive Mode (153,18,17,11,238,169)150 Here comes the directory listing.

drwxr-xr-x 2 3027 411 4096 Sep 24 2002 businessdrwxr-xr-x 2 3027 411 4096 Sep 24 2002 personaldrwxr-xr-x 2 3027 411 4096 Sep 24 2002 school

226 Directory send OK.ftp> quit221 Goodbye.

Figure 19.9 shows an example of how an image (binary) file is stored.1. After the control connection to port 21 is created, the FTP server sends the 220 (service ready) response on the control connection.2. The client sends the USER command.3. The server responds with 331 (user name is OK, a password is required).4. The client sends the PASS command.5. The server responds with 230 (user login is OK).6. The client issues a passive open on an ephemeral port for the data connection and sends the PORT command (over the control connection) to give this port number to the server.

Example 3

See Next Slide

7. The server does not open the connection at this time, but prepares itself for issuing an active open on the data connection between port 20 (server side) and the ephemeral port received from the client. It sends the response 150 (data connection will open shortly).8. The client sends the TYPE command.9. The server responds with the response 200 (command OK).10. The client sends the STRU command.11. The server responds with 200 (command OK). 12. The client sends the STOR command.13. The server opens the data connection and sends the response 250.

Example 3 (cONTINUED)

See Next Slide

14. The client sends the file on the data connection. After the entire file is sent, the data connection is closed. Closing the data connection means end-of-file.15. The server sends the response 226 on the control connection.16. The client sends the QUIT command or uses other commands to open another data connection for transferring another file. In our example, the QUIT command is sent.17. The server responds with 221 (service closing) and it closes the control connection.

Example 3 (cONTINUED)

See Next Slide

Figure 19.9 Example 3

We show an example of anonymous FTP. We assume thatsome public data are available at internic.net.

Example 4

See Next Slide

$ ftp internic.netConnected to internic.net220 Server readyName: anonymous331 Guest login OK, send “guest” as passwordPassword: guestftp > pwd257 ’/’ is current directory

bin. . .. . .. . .

Example 4

ftp > close221 Goodbyeftp > quit

TFTP

• Simply copy a file without all the features of FTP. Mostly used now for flashing routers, diskless workstations, etc. that are directly connected.

• Uses IP and UDP. Uses port number 69 initially, once it gets the ephemeral port from the client, the server opens its own ephemeral port to communicate, freeing port 69 to be used by someone else.

Email

• MIME

• SMTP

• POP

• IMAP

Main components: User agents, message access agent, and transfer agents

Components of email

User agent: used to compose mail, read mail, store in local computer (if two users are on the same LAN, we only need two user agents). Eg. Eudora, Outlook, Netscape.

MTAs (message transfer agents) to transfer from local machine to a server, server to another server and so on. SMTP

Message Access agent – to retrieve from the local server to the local recipient computer. Pop and IMAP, MIME

Mime (multipurpose internet mail extenstion)

• Allows non-ASCII characters. Used for all languages, video, and audio.

Figure 20.9 MIME

Figure 20.10 MIME header

Table 20.1 Table 20.1 Data types and subtypes in MIMEData types and subtypes in MIME

Table 20.1 Table 20.1 Data types and subtypes in MIME (Continued)Data types and subtypes in MIME (Continued)

Table 20.2 Table 20.2 Content-transfer-encodingContent-transfer-encoding

SMTP (simple mail transfer protocol)

• Uses commands and responses to transfer messages. Each command or reply is terminated by carriage return and linefeed.

20.3 MESSAGE TRANSFER AGENT: SMTP

The actual mail transfer requires message transfer agents (MTAs). The The actual mail transfer requires message transfer agents (MTAs). The protocol that defines the MTA client and server in the Internet is called protocol that defines the MTA client and server in the Internet is called Simple Mail Transfer Protocol (SMTP).Simple Mail Transfer Protocol (SMTP).

The topics discussed in this section include:The topics discussed in this section include:

Commands and Responses Commands and Responses Mail Transfer Phases Mail Transfer Phases

Figure 20.13 SMTP range

Figure 20.14 Commands and responses

Figure 20.15 Command format

Table 20.4 Table 20.4 CommandsCommands

Table 20.5 Table 20.5 ResponsesResponses

Table 20.5 Table 20.5 Responses (Continued)Responses (Continued)

Figure 20.16 Connection establishment

Figure 20.17 Message transfer

Figure 20.18 Connection termination

Example 1

$ telnet mail.adelphia.net 25Trying 68.168.78.100...Connected to mail.adelphia.net (68.168.78.100).

Let us see how we can directly use SMTP to send an email and simulate the commands and responses we described in this section. We use TELNET to log into port 25 (the well-known port for SMTP). We then use the commands directly to send an email. In this example, forouzanb@adelphia.net is sending an email to himself. The first few lines show TELNET trying to connect to the adelphia mail server.

After connection, we can type the SMTP commands and then receive the responses as shown below. We have shown the commands in black and the responses in color. Note that we have added for clarification some comment lines, designated by the “=” sign. These lines are not part of the email procedure.

Example 1 (Continued)

================== Connection Establishment ================220 mta13.adelphia.net SMTP server ready Fri, 6 Aug 2004 . . .

HELO mail.adelphia.net250 mta13.adelphia.net

===================== Envelope ===================MAIL FROM: forouzanb@adelphia.net

250 Sender <forouzanb@adelphia.net> OkRCPT TO: forouzanb@adelphia.net

250 Recipient <forouzanb@adelphia.net> Ok=================== Header and Body ==================DATA

354 Ok Send data ending with <CRLF>.<CRLF>From: ForouzanTO: Forouzan

This is a test messageto show SMTP in action..

Example 1 (Continued)

============= Connection Termination===============

250 Message received: adelphia.net@mail.adelphia.net

QUIT

221 mta13.adelphia.net SMTP server closing connection

Connection closed by foreign host.

POP3 and IMAP4

• Message access protocols. (pull)

• Post Office protocol. Simple with limited functionality. Uses port 100. Has to modes, delete or keep.

• Internet Mail Access Protocol is used to check mail directly from the web.

20.4 MESSAGE ACCESS AGENT: POP AND IMAP

The third stage of mail delivery uses a message access agent; the client The third stage of mail delivery uses a message access agent; the client must pull messages from the server. Currently two message access must pull messages from the server. Currently two message access protocols are available: Post Office Protocol, version 3 (POP3) and protocols are available: Post Office Protocol, version 3 (POP3) and Internet Mail Access Protocol, version 4.Internet Mail Access Protocol, version 4.

The topics discussed in this section include:The topics discussed in this section include:

POP3 POP3 IMAP4 IMAP4

Figure 20.19 POP3 and IMAP4

Figure 20.20 POP3

20.5 WEB-BASED MAIL

Some websites such as Hotmail and Yahoo provide email service to Some websites such as Hotmail and Yahoo provide email service to anyone who accesses the site. Mail transfer and retrieval requires the use anyone who accesses the site. Mail transfer and retrieval requires the use of HTTP. of HTTP.

DNS (Domain Name Server)• Maps human readable symbolic names to computer

addresses• Domain names are hierarchical with most significant part

of the name on the right. Root is a dot.• Software that performs the address resolution is known

as the resolver. The resolver holds one or more DNS addresses.

• The Domain Name System distributes the responsibility of assigning domain names and mapping those names to IP addresses by designating authoritative name server for each domain.

Authoritative Name Server

• gives original, first-hand, definitive answers (authoritative answers) to DNS queries and not just cached answers that were obtained from another name server. Therefore it only returns answers to queries about domain names that are installed in its configuration system.

• An authoritative name server can either be a master server or a slave server. A master server is a server that stores the original (master) copies of all zone records. A slave server uses an automatic updating mechanism of the DNS protocol in communication with its master to maintain an identical copy of the master records.

How DNS works

• When you type a domain into your browser (or "client") the client needs to find the IP address where this site is located. The first place it will check is the operating system. The operating system routes the search (or "query") to the Resolving Name Server.

• The resolving name server serves two purposes: 1. To direct queries out to the root, TLD, and authoritative name servers, and 2. To cache DNS information once it is located.

Types of DNS entries

• Each DNS entry consists of three items: a domain name, a record type and a value.

• The record type: IP address ( A type, to be used with FTP, ping, browser etc), MX (for mail exchanger), NS (main machine incharge of the domain zone), etc.

NS Record

• NS Record A Name Server Record, or NS Record, indicates which name servers are authoritative for the zone. This will either be set to a default by the DNS provider for the domain or be customized to use Vanity NS. NS Records can also be used to assign authoritative name servers for a subdomain.

• Vanity nameservers, sometimes called custom nameservers, are a way to assign your own vanity names to our nameservers. This offers several potential benefits:

A Record

• An A record maps a domain name to an IP address. This is the most basic function of DNS. There are three values that must be specified for an A record:

• Name - The domain name. For example, dnsuniversity.com is the name of an A record. It is also the host of the domain

• TTL - The time of live, or amount of time, in seconds, the record will be stored in the cache of a resolving name server. For example, 1800 is a standard TTL.

• IP - Indicates the IP which this domain is mapped to. For example, 192.168.1.2.

• Some DNS providers allow multiple IPs per host for routing, which is called Round Robin.

• A records are used for IPv4 addresses. IPv6 addresses use AAAA Records.

MX Record

• Mail Exchange Records, or MX Records, tell mail servers where to deliver mail. Tee name field of an MX record contains the host name which appears in the email address, and the data field contains the host name of the server to which the mail should be delivered. For example:

• Name - dnsuniversity.comThis means that mail sent to user@dnsuniversity.com is directed to the mail exchangers defined by these records.

• MX Level - 10This sets the preference for the MX record. The lower the number, the higher the preference. If the mail server with the highest preference is inaccessible, the mail will be delivered to the mail server next in preference.

DNS autonomy

• Each organization is allowed to assign names to computers or change those names without informing a central authority.

• Each organization is permitted to operate DNS servers for its parts of the hierarchy.

• A given DNS can be replicated.

Figure 17.7 DNS used in the Internet

Figure 17.8 Generic domains

Table 17.1 Table 17.1 Generic domain labelsGeneric domain labels

Table 17.1 Table 17.1 Generic domain labels (Continued)Generic domain labels (Continued)

Figure 17.9 Country domains

Figure 17.10 Inverse domain

17.5 RESOLUTION

Mapping a name to an address or an address to a name is called name-Mapping a name to an address or an address to a name is called name-address resolution.address resolution.

The topics discussed in this section include:The topics discussed in this section include:

Resolver Resolver Mapping Names to Addresses Mapping Names to Addresses Mapping Addresses to Names Mapping Addresses to Names Recursive Resolution Recursive Resolution Iterative Resolution Iterative Resolution Caching Caching

Figure 17.11 Recursive resolution

Figure 17.12 Iterative resolution

DNS Caching

• To exploit temporal locality (repeated requests) DNS server caches all lookups. Cache timeouts cleans the items.

Homework

• Install a mailserver in Unix or Windows (needs to be 2003)

• Install A mail client. Learn how to backup and restore outlook

• Learn how work with add-ins.

• Learn how to change MX records