Application layer protocol data unit.

Application layer protocols are used to exchange data between programs running on the source and destination hosts.  

The device that is originating the PDU.

The presentation layer has three primary funcitons:

Coding and conversion of application  layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device

An Internet-wide system by which a hierarchical set of DNS servers collectively hold all the name-IP address mappings, with DNS servers referring users to the correct DNS server to successfully resolve a DNS name.

Domain Name System (DNS) is used to resolve Internet names to IP addresses.

A series of documents and memoranda encompassing new research, innovations, and methodologies applicable to Internet technologies. RFCs are a reference for how technologies should work.

The protocols in the TCP/IP suite are generally defined by Requests for Comments (RFC).  The Internet Engineering Task Force (IETF) maintains the RFCs as the standards for the TCP/IP suite.

The structure and order of words in a computer language.

Within the application layer, protocols specify that messages are exchanged between the source and destination hosts, the syntax of the control commands, the type and format of the data being transmitted, and the appropriate methods for error notification and recovery.

A related set of communications transaction between two or more network devices.

Both the source and destination devices use application layer protocols during a communication session.

A network device that accesses a service on another computer remotely by accessing the network.

In the client/server model, the device requesting the information is called a client and the device responding to the request is called a server.

Can refer to computer hardware that is to be used by multiple concurrent users. Alternatively, this term can refer to computer software that provides services to many users. For example, a web server consists of web server software running on some computer.

Client and server processes are considered to be in the application layer.

A computer program that runs in the background and is usually initiated as a process. Daemons often support server processes.

In a client/server network, the server runs a service, or process, sometimes called a server daemon.  Like most services, daemons typically run in the background and are not under an end user's direct control.  Daemons are described as "listening" for a request from a client, because they are programmed to respond whenever the server receives a request for the service provided by the daemon.

A host or node that participates in some form of a group. For example, peer-to-peer technology defines a group of peers that participate jointly in the same activity, each one having a server and client component.

User accounts and access rights must be set individually on each peer device.

A plan, design, or program of action to be followed. Sometimes an addressing plan is called an addressing scheme.

As you will see later in this book, the transport layer uses an addressing scheme called a port number.  Port numbers identify applications and application layer services that are the source and destination of data.

A 32 bit number, written in dotted decimal notation, used by the IP to uniquely identify an interface connected to an IP network. It is also used as a destination address in an IP header to allow routing, and as a source address to allow a computer to receive a packet and to know which IP address to send a response to.

In data networks, devices are assigned IP addresses so that they can participate in sending and receiving messages over the network.

A name, as defined by DNS, that uniquely identifies a computer in the Internet. DNS servers can then respond to DNS request by supplying the IP address that is used by the computer that has a particular domain name. This term also refers to the part of a URL that identifies a single company or organization, such as ciscopress.com.

Also, if Cisco decides to change the numeric address, it is transparent to the user, because the domain name will remain http://www.cisco.com.

A dotted decimal number defined by the IPv4 protocol to represent a network or subnet. It represents the network that hosts reside in. Also called a network number or network ID.

The DNS protocol defines an automated service that matches resource names with the required numeric network address.

DNS data records. Their precise format is defined in RFC 1035. The most important fields in a resource record are Name, Class, Type, and Data.

This message format is used for all types of client queries and server responses, error messages, and the transfer of resource record information between servers.

The client part of the DNS client-server mechanism. A DNS resolver creates queries sent across a network to a name server, interprets responses, and returns information to the requesting programs.

The DNS client, sometimes called the DNS resolver, supports name resolution for the other network applications and other services that need it.

A service or a program to look up information in the DNS (Domain Name System).

Computer operating systems also have a utility called nslookup that allows the user to manually query the name servers to resolve a given host name.  You also can use this utility to troubleshoot name resolution issues and to verify the current status of the name servers.

A request for information. Queries are answered with replies.

Computer operating systems also have a utility called nslookup that allows the user to manually query the name servers to resolve a given host name.

A temporary storage where data that has been retrieved or calculated and is accessed frequently can be stored. After the data is stored in the cache, the processes can access the cached copy instead of accessing the original data. A cache reduces the average access time and reduces the overhead of recalculating the data.

When a match is found and returned to the original requesting server, the server temporarily stores the numbered address that matches the name in the cache.  If that same name is requested again, the first server can return the address by using the value stored in its name cache.  Caching reduces both the DNS query data network traffic and the workloads of servers higher up the hierarchy.

A source of information that is highly reliable and known for its accuracy.

If a given server has resource records that correspond to its level in the domain hierarchy, it is said to be authoritative for those records.

In a web browser, an application the broswer uses, inside the browser window, to display some types of content. For example, a browser typically uses a plug-in to display video.

Other types of data, however, might required another service or program, typically referred to as a plug-in or add-on.

Defines the commands, headers, and processes by which web servers and web browsers transfer files.

HTTP, one of the protocols in the TCP/IP suite, was originally developed to publish and retrieve HTML pages and is now used for distributed, collaborative information systems.

A method of computer processing in which different parts of a program run simultaneously on two or more computers that are communicating with each other over a network.

HTTP, one of the protocols in the TCP/IP suite, was originally developed to publish and retrieve HTML pages and is now used for distributed, collaborative information systems.

Information systems that allow the creation of a document or documents that can be edited by more than one person in real time.

HTTP, one of the protocols in the TCP/IP suite, was originally developed to publish and retrieve HTML pages and is now used for distributed, collaborative information systems.

The process of obscuring information to make it unreadable without special knowledge, sometimes referred to as scrambling. The process takes the data to be encrypted and applies a mathematical formula to it along with a secret number (called an encryption key). The resulting value, which is called an encrypted packet, is sent through a network.

HTTPS can use authentication and encryption to secure data as it travels between the client and server.

A protocol that allows a computer to retrieve e-mail from a server.

Two examples of application layer protocols are Post Office Protocol (POP) and Simple Mail Transfer Protocol (SMTP).

An application protocol typically not used by end users. Instead, it is used by the network management software and networking devices to allow a network engineer to monitor and troubleshoot network problems.

Two examples of application layer protocols are Post Office Protocol (POP) and Simple Mail Transfer Protocol (SMTP).

Program used to download and send e-mail. E-mail clients us POP3 to receive e-mails and use SMTP to send e-mails. Also called an e-mail client.

When people compose e-mail messages, they typically use an application called a Mail User Agent (MUA), or e-mail client.  The MUA allows messages to be sent and places received messages into the client mailbox, both os which are distinct processes.  

Normally, a relatively general term that refers to different kinds of networking devices. historically, when routers were created, they were called gateways.

The server sends or receives e-mail through the Internet though the prudct's Internet mail gateway, which performs any necessary reformatting.

A protocol used to dynamically assign IP configuration to hosts. The services defined by the protocol are used to request and assign an IP address, default gateway, and DNS server address to a network host.

The DHCP enables clients on a network to obtain IP addresses and other information from a DHCP server.  The protocol automates the assignment of IP addresses, subnet masks, gateway, and other IP networking parameters.

DHCP-distributed addresses are not permanently assigned to hosts but are only leased for a period of time.  If the host is powered down or taken off the network, the address is returned to the pool for reuse.

DHCP can pose a security risk because any device connected to the network can receive an address.  This risk makes physical security an important factor when determining whether to use dynamic or static (manual) addressing .

DCHP is used for general-purpose hosts such as end-user devices, and static, or fixed, addresses are used for network devices such as gateways, switches, servers, and printers.

A group of IP addresses that have the same value in the first part of the IP addresses, for the purpose of allowing routing to identify the group by that initial part of the addresses. IP addresses in the same subnet tpically sit on the same network medium and are not separated from each other by any routers. IP addresses on different subnets are typically separated from one another by at least one router. Subnet is short for subnetwork.

A DHCP server replies with a DHCP OFFER, which is a lease offer message with an assigned IP address, subnet mask, DNS server, and default gateway information as well the duration of the lease.

A form of transmission where one device transmits to all devices within the network or on another network.

The client can receive multiple DHCP OFFER packets if the local network has more than one DHCP server.The client must choose between them and broadcast a DHCP REQUEST packet that identifies the explicit server and lease offer that it is accepting.

An application level network protocol mainly applied to shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network.

Server Message Block (SM) is a client/server file-sharing protocol.  It is a request response protocol.  Unlike the file sharing supported by FTP, clients establish a long-term connection to servers.  After the connection is established, the user of the client can access the resources on the server as if the resource is local to the client host.

SMB file-sharing and print services have become the mainstay of Microsoft networking.

A multiuser, multitasking operating system originally developed in the 1960s and 1970s by a group of AT7T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, and Douglas McIlroy. Today UNIX systems are split into various branches, developed over time by AT&T, as well as various commercial vendors and nonprofit organizations.

The Linux and UNIX operating systems also provide a method of sharing resources with Microsoft networks using a version of SMB called SAMBA.

The OSI reference model is a layered, abstract representation created as a guideline for network protocol. design and instruction.  The OSI model divides the networking process into seven logical layers, each of which has unique functionality and to which are assigned specific services and protocols.

In the OSI model, information is passed from one layer to the next, starting at the application layer on the transmitting host and proceeding down the hierarchy to the physical layer, then passing over the communications channel to the destination host, where the information proceeds back up the hierarchy, ending at the application layer.

The presentation layer has three primary functions:

Coding and conversion of application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device

Compression of the data in a manner that can be decompressed by the destination device

Encryption of the data for transmission and decryption of data upon receipt by the destination

The most widely known TCP/IP application layer protocols are those that provide the exchange of user information.

Among these TCP/IP protocols are the following:

Domain Name System (DNS) is used to resolve Internet names to IP addresses.

Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the web pages of the World Wide Web

Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and attachments

Telnet, a terminal emulation protocol, is used to provide remote access to servers and networking devices

File Transfer Protocol (FTP) is used for interactive file transfer between systems. 

The functions associated with the application layer protocols in both the OSI and the TCP/IP models enable the human network to interface with the underlying data network.

Each executing program loaded on a device is referred to as a process.

Other programs, such as file transfer or network print spooling, might need the assistance of application layer services to use network resources.  Although transparent to the user, these services interface with the network and prepare the data for transfer.

Each application or network service uses protocols that define the standards and data formats to be used.  A service provides the function for doing something, and a protocol provides the rules the service uses.

The application layer uses protocols that are implemented within applications and services.  Applications provide people with a way to create messages, application layer services establish an interface to the network, and protocols provide the rules and formats that govern how data is treated.

In the OSI model, applications that interact directly with people are considered to be at the top of the stack, as are the people themselves.  Like all layers within the OSI model, the application layer relies on the functions of the lower layers to complete the communication process.

Establish consistent rules for exchanging dtaa between applications and services loaded on the participating devices.

Specify how data inside the messages is tructred and the types of messages that are sent between source and destination.  Theses messages can be requests for services, acknowledgments, data messages, status messages, or error messages.

Define message dialogues, ensuring that a message being sent is met the ye expected response and that the correct services are invoked when data transfer occurs.

A single application can employ any different supporting application layer services.  Thus, what appears to the user as one request for a web page might, in fact, amount to dozens of individual requests.  For each request, multiple processes can be executed.  For example, the FTP requires a client to initiate a control process and a data stream process to a server.

Additionally, servers typically have multiple clients requesting information at the same time.  A Telnet server can have many clients requesting connections to it.  These individual client requests must be handled simultaneously and separately for the network to succeed.  The application layer processes and services rely on support from lower-layer functions to successfully manage the multiple conversations.

In a peer-to-peer network, two or more computers are connected through a network and can share resources such as printers and files without having a dedicated server.  Every connected end device, known as a peer, can function as either a server or a client.  One computer might assume the role of server for one transaction while simultaneously serve as a client  for another.  The roles of client and server are set on a per-request basis.

Unlike the client/server model, which uses dedicated servers, peer-to-peer networks decentralize the resources on a network.  Instead of locating information to be shared on dedicated servers, information can be located anywhere on any connected device.  

As you examine the different TCP/IP application layer protocols and services, you will be referring to the TCP and UDP port numbers normally associated with these services.  Some of these services are:

Domain Name System (DNS): TCP/UDP port 53

HTTP: TCP port 80

Simple Mail Transfer Protocol (SMTP): TCP port 25

Post Office Protocol (POP): UDP port 110

Telnet: TCP port 23

DHCP: UDP port 67

FTP: TCP ports 20 and 21

The DNS server stores different types of resource records used to resolve names.  These records contain the name, address, and type of record.

Some of these record types are:

A: An end device address

NS: An authoritative name server

CNAME: The canoncial name (or fully qualified domain name [FQDN] for an alias; used when multiple services have the single network address but each service has its own entry in DNS

MX: Mail exchange record; maps a domain name to a list of mail exchange servers for that domain.

DNS uses a hierarchical system to create a name database to provide name resolution.  The hierarchy looks like an inverted tree with the root at the top and branches below.

At the top of the hierarchy, the root servers maintain records about how to reach the top-level domain servers, which in turn have records that point to the secondary-level domain servers and so on.

After top-level domains are second-level domain names, and below them are other lower-level domains.

Each domain name is a path down this inverted tree starting from the root.

DNS relies on this hierarchy of decentralized servers to store and maintain these resource records.  The resource records list domain names that the server can resolve and alternative servers that can process requests.  If a given server has resource records that correspond to its level in the domain hierarchy, it is said to be authoritative for those records.

When a web address (or URL) is typed into a web browser, the web browser established a connection to the web service running on the server using HTTP.  URLs and URIs (uniform resource identifiers) are the names most people associate with web addresses.

Web browsers are the client applications computers use to connect to the World Wide Web and access resources stored on a web server.  As with most server processes, the web server runs as a background service and makes different types of files available.

To access the content, web clients make connections to the server and request the desired resources.  The server replies with the resources and, upon receipt, the browser interprets the data and presents it to the user.

http://www.cisco.com/web-server.htm

First, the broswer interprets the three parts of the URL:

http:  The protocol or scheme

www.cisco.com:  The server name

web-server.htm:  The specific filename requested

The browser then checks with a name server to convert http:www.cisco.com into a numeric address, which it uses to connect to the server.  Using the HTTP requirements, the bowser sens a GET request to the server and asks for the file web-server.htm.  The server in turn sends the HTML code for this web page to the browser.  Finally, the browser deciphers the HTML code and formats the page for the browser window.

GET:  Get is a client request for data.  A web broser sends the GET message to request pages from a web server.

POST:  Post and Put are used to send messages that upload data to the web server.

PUT:  Post and Put are used to send messages that upload data to the web server.

Although it is remarkably flexible, HTTP is not a secure protocol.  The POST messages upload information to the server in plain text that can be intercepted and read.  Similarly, the server responses, typically HTML pages, are unencrypted.

POP and POP3 (Post Office Protocol, version 3) are inbound mail delivery protocols and are typical client/server protocols.  They deliver e-mail from the e-mail server to the client (MUA).

SMTP, on the other hand, governs the transfer of outbound e-mail from the sending client to the e-mail server (MDA), as well as the transport of e-mail between e-mail servers (MTA).

To receive e-mail messages from an e-mail server, the e-mail client can use POP.  Sending e-mail from either a client or a server uses message formats and command strings defined by the SMTP protocol.  Usually an e-mail client provides the functionality of both protocols within one application.

The e-mail server operates two separate processes:

Mail Transfer Agent (MTA)

Mail Delivery Agent (MDA)

The Mail Transfer Agent (MTA) process is used to forward e-mail.

The Mail Delivery Agent (MDQA) accepts a peice of e-mail from a Mail Transfer Agent (MTA) and performs the delivery.  The MDS receives all the inbound mail from the MTA and places it into the appropriate users' mailboxes.  The MDA can also resolve final delivery issues, such as virus scanning, spam filtering, and return-recept handling.

The SMTP protocol message format uses a rigid set of commands and replies.  These commands support the procedures sused in SMTP, such as session initiation, mail transaction, forwarding mail, verifying mailbox names, expanding mailing lists, and the opening and closing exchanges.  Some of the commands specified in the SMTP protocol are:

HELO: Identifies the SMTP client process to the SMTP server process

EHLO: Is a newer version of HELO, which includes services extensions

MAIL FROM: Identifies the sender

RCPT TO: Identifies the recipient

DATA: Identifies the body of the message

When a DHCP-configured device boots up or connects to the network, the client broadcasts a DHCP DISCOVER packet to identify any available DHCP servers on the network.  A DHCP server replies with a DCHP OFFER, whcih is a lease offer message with an assigned IP address, subnet mask, DNS server, and default gateway information as well as the duration of the lease.

The client can receive multiple DHCP OFFER packets if the local network has more than one DHCP server.  The client must choose between thema nd broadcast a DCHP REQUEST pcaket that identifies the explicit server and lease offer that it is accepting.  A client can choos to request an address that it had previously been allocated by the server.

Assuming that the IP address requested by the client, or offered by the server, is still valid, the chosen server would return a DHCP ACK (acknowledgment) message.  The ACK message lets the client know that the lease is finalized.  If the offer is no longer valid for some reason, perhaps because of a timeout or another client allocating the lease, the chose server must respond to the client with a DHCP NAK (negative acknowledgment) message.   When the cleint has the lease, it must be renewed prior to the lease expiration through another DHCP REQUEST message.  The DHCP server ensures that all IP addresses are unique. (An IP address cannot be assigned to two different network devices simultaneously)

FTP was developed to allow file transfers between a client and a server.  An FTP client is an application that runs on a computer that is used to push and pull files from a server running the FTP daemon (FTPd).

To successfully transfer files, FTP requires two connections between the client and the server: one for commands and replies, and the other for the actual file transfer.

The client established the first connection to the server on TCP port 21.  This connection is used for control traffic, consisting of client commands and server replies.

The client establishes the second connection to the server over TCP port 20.  This connection is for the actual file transfer and is created every time a file is transferred.

The SMB protocol describes file system access and indicates how clients can make requests for files.  It also describes the SMB protocol interprocess communication.  ALL SMB messages share a common format.  This format uses a fixed-sized header followed by a variable-sized parameter and data component.

SMB messages can perform the following tasks:

Start, authenticate, and terminate sessions

Control file and printer access

Allow an application to send or receive messages to or from another device

Many client applications are available for accessing the Gnutella network, including BearShare, Gnucleus, LimeWire, Morpheus, WinMX, and XoloX.

Many P2P applications do not use a central database to record all the files available on the peers.  Instead, the devices on the network each tell the other what files are available when queried and use the Gnutella protocol and services to support locating resources.

When a user is connected to a Gnutella service, the client applications will search or other Gnutella nodes to connect to.  these nodes handle queries for resource locations and replies to those requests.  They also govern control messages, which help the service discover other nodes.

The Gnutella protocol defines five different packet types:

ping: For device discovery

pong: As a reply to a ping

query: For file location

query hit:  As a reply to a query

push: As a download request

Telnet is a client/server protocol that provides a standard method of emulating text-based terminal devices over the data network.  Both the protocol itself and the client software that implements the protocol are commonly referred to as Telnet.

Appropriately enough, a connection using Telnet is called a VTY (Virtual Terminal) session, or connection.  Telnet specifies how a VTY session is established and terminated.  It also provides the syntax and order of the commands used to initiate the Telnet session, and it provides control commands that can be issued during a session.  Each Telnet command consists of at least 2 bytes.  The first byte is a special character called the Interpret as Command (IAC) character.  As its name implies, the IAC character defines the next byte as a command rather than text.  Rather than using a physical device to connect to the server, Telnet uses software to create a virtual device that provides the same features of a terminal session with access to the server command-line interface (CLI).

To support Telnet client ocnnections, the server runs a service called the Telnet daemon.  A virtual terminal connection is established from an end device using a Telnet client application.

When a Telnet connection is established, users can perform any authorized function on the server, just as if they were using a command-line session on the server itself.  If authorized, they can start and stop processes, configure the device, and even shut down the system

Are You There (AYT): Enables the user to request that a response, usually a prompt icon, appear on the terminal screen to indicate that the VTY session is active

Erase Line (EL):  Deletes all text from the current line.

Interrupt Process (IP):  Suspends, interrupts, aborts, or terminates the process to which the virtual terminal is connected.  For example, if a user started a program on the Telnet server through the VTY, he or she could send an IP command to stop the program.

Challenge Questions

1.  List the six-step process for converting human communications to data.

1.  The user inputs data using a hardware interface.

2.  The application layer prepares human communication for transmission over the data network.

3.  Software and hardware convert data to a digital format.

4.  Application services initiate the data transfer.

5.  Each layer plays its role, and the OSI layers encapsulate data down the stack.  Encapsulated data travels across the media to the destination.  OSI layers at the destination decapsulate the data up the stack.

6.  Data is ready to be processed by the end device.

Challenge Questions

2.  Describe the two forms of application software and the purpose of each.

Application software has two forms: applications and services:

Applications are designed to interact with us.  Application is software for the user.  If the device is a computer, the application is typically initiated by the user.  Although there can be many layers of support underneath, application software provides an interface between humans and the hardware.  The application will initiate the data transfer process when the user clicks the Send button or performs a similar action.

Services are background programs that perform a particular function in the data network.  Services are invoked by a device connecting to the network or by an application.  For example, a network service can provide functions that transmit data or provide conversion of data in a network.  In general, services are not directly accessible or seen by the end user.  They provide the connection between an application and the network.

Challenge Questions

3.  Elaborate on the meaning of the terms server and client in the context of data networks.

Challenge Questions

4.  compare and contrast client/server with peer-to-peer data transfer over networks.

Challenge Questions

5.  List five general functions that application layer protocols specify.

Challenge Questions

6.  Give the specific purposes of the DNS, HTTP, SMB, and SMTP/POP application layer protocols.

Challenge Questions

7.  Compare and contrast the messages that application layer protocols such as DNS, HTTP, SMB, and SMTP/POP exchange between devices to enable data transfers to occur.