Application Layer

DHCPv6

-DHCP for IPv6 (DHCPv6) provides similar services for IPv6 clients. -One important difference is that DHCPv6 does not provide a default gateway address. -This can only be obtained dynamically from the Router Advertisement message of the router. -DHCPv6 has a set of messages that is similar to those for DHCPv4. The DHCPv6 messages are: SOLICIT ADVERTISE INFORMATION REQUEST REPLY

What is DHCP (Dynamic addressing) used for, and what is static addressing used for?

-DHCP is used for general purpose hosts, such as end user devices. -Static addressing is used for network devices, such as gateway routers, switches, servers, and printers.

nslookup command

-Displays information about DNS names and their corresponding IP addresses, and it can be used to diagnose DNS servers. -Nslookup that allows the user to manually query the name servers to resolve a given host name. -This utility can also be used to troubleshoot name resolution issues and to verify the current status of the name servers. -In this figure, when the nslookup command is issued, the default DNS server configured for your host is displayed. The name of a host or domain can be entered at the nslookup prompt. The Nslookup utility has many options available for extensive testing and verification of the DNS process.

File Transfer Protocol

-FTP -Is an application layer protocol -FTP was developed to allow for data transfers between a client and a server. -An FTP client is an application which runs on a computer that is being used to push and pull data from an FTP server. Figure: Based on commands sent across the control connection, data can be downloaded from the server or uploaded from the client. Process: -The client establishes the first connection to the server for control traffic using TCP port 21. (The traffic consists of client commands and server replies.) -The client establishes the second connection to the server for the actual data transfer using TCP port 20. (This connection is created every time there is data to be transferred.) -The data transfer can happen in either direction. (The client can download (pull) data from the server, or the client can upload (push) data to the server.)

HTTP and HTTPS (GET, POST, PUT)

-HTTP is a request/response protocol. -When a client, typically a web browser, sends a request to a web server, HTTP specifies the message types used for that communication. HTTP/HTTPS common essage types: 1. GET - This is a client request for data. A client (web browser) sends the GET message to the web server to request HTML pages. 2. POST - This uploads data files to the web server, such as form data. 3. PUT - This uploads resources or content to the web server, such as an image.

IMAP (Internet Message Access Protocol)

-IMAP is another protocol that describes a method to retrieve email messages. -when the user connects to an IMAP-capable server, copies of the messages are downloaded to the client application -The original messages are kept on the server until manually deleted. -Users view copies of the messages in their email client software. -Users can create a file hierarchy on the server to organize and store mail. -That file structure is duplicated on the email client as well. -When a user decides to delete a message, the server synchronizes that action and deletes the message from the server.

What are the parts of the P2P network model?

-P2P networks and P2P applications. -Both parts have similar features, but in practice work quite differently.

SMTP (Simple Mail Transfer Protocol)

-SMTP message formats require a message header and a message body. -message body can contain any amount of text, the message header must have a properly formatted recipient email address and a sender address. -When a client sends email, the client SMTP process connects with a server SMTP process on well-known port 25. After the connection is made, the client attempts to send the email to the server across the connection. When the server receives the message, it either places the message in a local account, if the recipient is local, or forwards the message to another mail server for delivery.

This layer of the OSI model is concerned with the protocols that exchange data between programs running on hosts.

Application

This is a function of the OSI session layer.

Exchange of information to initiate dialog between peers

T or F: An email client communicates directly with another email client when sending email.

FALSE Instead, both clients rely on the mail server to transport messages.

The application layer process that sends mail uses ___________.

SMTP

The ________ is a hardware/software combination that people use to directly access the resources that are stored on the server.

client

Data transfer from a client to a server is referred to as an _______ and data from a server to a client as a _________.

upload, download

P2P applications require that each end device provide a ________ _________ and run a _____________ __________.

user Interface, background service

DNS Protocol

-The DNS protocol defines an automated service that matches resource names with the required numeric network address. -It includes the format for queries, responses, and data. -The DNS protocol communications use a single format called a message. -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.

DNS Hierarchy

-The DNS protocol uses a hierarchical system to create a database to provide name resolution, as shown in the figure. DNS uses domain names to form the hierarchy. -The naming structure is broken down into small, manageable zones. -Each DNS server maintains a specific database file and is only responsible for managing name-to-IP mappings for that small portion of the entire DNS structure. -When a DNS server receives a request for a name translation that is not within its DNS zone, the DNS server forwards the request to another DNS server within the proper zone for translation. -DNS is scalable because hostname resolution is spread across multiple servers. -The different top-level domains represent either the type of organization or the country of origin. Examples of top-level domains are the following: .com - a business or industry .org - a non-profit organization .au - Australia .co - Colombia

Email clients communicate with ___________ _________ to send and receive email.

Mail Servers

__________ __________ communicate with other mail servers to transport messages from one domain to another.

Mail Servers

Common P2P Applications

eDonkey, G2, BitTorrent, Bitcoin, Gnutella, Direct Connect, Freenet -Some P2P applications are based on the Gnutella protocol, where each user shares whole files with other users. -As shown in the figure, Gnutella-compatible client software allows users to connect to Gnutella services over the internet, and to locate and access resources shared by other Gnutella peers. Many Gnutella client applications are available, including μTorrent, BitComet, DC++, Deluge, and emule.

In the peer-to-peer (P2P) networking model, the data is accessed from a _________ device without the use of a __________ _________.

peer, dedicated server

What are FQDNs?

-Fully Qualified Domain Names -They link IP address to name like www.google.com so you don't have to remember the IP address like 172.217.7.4 for google -Also when using DHCP the IP address lease times change so a FQDN allows the new IP to be associated with it so clients don't have to know the new IP address and connectivity can be maintained

POP (Post Office Protocol)

-POP is used by an application to retrieve mail from a mail server. -POP, mail is downloaded from the server to the client and then deleted on the server. (unless specified by the email client) -The server starts the POP service by passively listening on TCP port 110 for client connection requests. When a client wants to make use of the service, it sends a request to establish a TCP connection with the server, as shown in the figure. When the connection is established, the POP server sends a greeting. The client and POP server then exchange commands and responses until the connection is closed or aborted. -POP, email messages are downloaded to the client and removed from the server, so there is no centralized location where email messages are kept. Because POP does not store messages, it is not recommended for a small business that needs a centralized backup solution.

Server Message Block

-SMB -The Server Message Block (SMB) is a client/server file sharing protocol that describes the structure of shared network resources, such as directories, files, printers, and serial ports. -It is a request-response protocol. -All SMB messages share a common format. -This format uses a fixed-sized header, followed by a variable-sized parameter and data component. 3 functions of SMB messages: 1. Start, authenticate, and terminate sessions. 2. Control file and printer access. 3. Allow an application to send or receive messages to or from another device. -SMB file-sharing and print services have become the mainstay of Microsoft networking. -After the connection is established, the user of the client can access the resources on the server as though the resource is local to the client host. -With the introduction of the Windows 2000 software series, Microsoft changed the underlying structure for using SMB. In previous versions of Microsoft products, the SMB services used a non-TCP/IP protocol to implement name resolution. -Beginning with Windows 2000, all subsequent Microsoft products use DNS naming, which allows TCP/IP protocols to directly support SMB resource sharing, as shown in the figure. -The LINUX and UNIX operating systems also provide a method of sharing resources with Microsoft networks by using a version of SMB called SAMBA. The Apple Macintosh operating systems also support resource sharing by using the SMB protocol.

P2P HYBRID SYSTEM

-Some P2P applications use a hybrid system where resource sharing is decentralized, but the indexes that point to resource locations are stored in a centralized directory. -In a hybrid system, each peer accesses an index server to get the location of a resource stored on another peer. Figure: Both Clients can simultaneously send and receive messages.

Windows DNS

-The DNS cient service on Windows PCs also stores previously resolved names in memory. -The ipconfig /displaydns command displays all of the cached DNS entries.

DNS Message Format

-The DNS server stores different types of resource records that are used to resolve names. -These records contain the name, address, and type of record. Some record types: 1. A - An end device IPv4 address 2. NS - An authoritative name server 3. AAAA - An end device IPv6 address (pronounced quad-A) 4. MX - A mail exchange record -DNS uses the same message format between servers, consisting of a question, answer, authority, and additional information for all types of client queries and server responses, error messages, and transfer of resource record information. DNS Message sections and descriptions: 1. Question - The question for the name server 2. Answer - Resource Records answering the question 3. Authority - Resource Records pointing toward an authority 4. Additional - Resource Records holding additional information

Dynamic Host Configuration Protocol

-The Dynamic Host Configuration Protocol (DHCP) for IPv4 service automates the assignment of IPv4 addresses, subnet masks, gateways, and other IPv4 networking parameters. -This is referred to as dynamic addressing. -When a host connects to the network, the DHCP server is contacted, and an address is requested. -The DHCP server chooses an address from a configured range of addresses called a pool and assigns (leases) it to the host. -DHCP can allocate IP addresses for a configurable period of time, called a lease period. -When the lease period expires or the DHCP server gets a DHCPRELEASE message the address is returned to the DHCP pool for reuse. -Users can freely move from location to location and easily re-establish network connections through DHCP. -The DHCP server in most medium-to-large networks is usually a local, dedicated PC-based server. -With home networks, the DHCP server is usually located on the local router that connects the home network to the ISP.

What is a URL?

-Uniform Resource Locator: an address on the internet (www.google.com)

DNS Domain Lookup Process

-When a client makes a query, the server DNS process first looks at its own records to resolve the name. -If it is unable to resolve the name by using its stored records, it contacts other servers to resolve the name. -After a match is found and returned to the original requesting server, the server temporarily stores the numbered address in the event that the same name is requested again

DHCP Operation

-the client broadcasts a DHCP discover (DHCPDISCOVER) message to identify any available DHCP servers on the network. -A DHCP server replies with a DHCP offer (DHCPOFFER) message, which offers a lease to the client. -The offer message contains the IPv4 address and subnet mask to be assigned, the IPv4 address of the DNS server, and the IPv4 address of the default gateway. -The lease offer also includes the duration of the lease. -Assuming that the IPv4 address requested by the client, or offered by the server, is still available, the server returns a DHCP acknowledgment (DHCPACK) message that acknowledges to the client that the lease has been finalized. -If the offer is no longer valid, then the selected server responds with a DHCP negative acknowledgment (DHCPNAK) message. -If a DHCPNAK message is returned, then the selection process must begin again with a new DHCPDISCOVER message being transmitted -The client may receive multiple DHCPOFFER messages if there is more than one DHCP server on the local network. -Therefore, it must choose between them, and sends a DHCP request (DHCPREQUEST) message that identifies the explicit server and lease offer that the client is accepting. -A client may also choose to request an address that it had previously been allocated by the server. -After the client has the lease, it must be renewed prior to the lease expiration through another DHCPREQUEST message. -The DHCP server ensures that all IP addresses are unique (the same IP address cannot be assigned to two different network devices simultaneously). Most ISPs use DHCP to allocate addresses to their customers.

What are the 3 primary functions of the presentation layer?

1. Formatting, or presenting, data at the source device into a compatible format for receipt by the destination device. 2. Compressing data in a way that can be decompressed by the destination device. 3. Encrypting data for transmission and decrypting data upon receipt.

TCP/IP Application Layer Protocols

1. Name System DNS (Domain Name System): -TCP, UDP client 53 -Translates domain names, such as cisco.com, into IP addresses. 2. Host Config -BOOTP - Bootstrap Protocol UDP client 68, server 67 Enables a diskless workstation to discover its own IP address, the IP address of a BOOTP server on the network, and a file to be loaded into memory to boot the machine BOOTP is being superseded by DHCP -DHCP - Dynamic Host Configuration Protocol UDP client 68, server 67 Dynamically assigns IP addresses to be re-used when no longer needed 3. Email SMTP - Simple Mail Transfer Protocol -TCP 25 -Enables clients to send email to a mail server -Enables servers to send email to other servers POP3 - Post Office Protocol -TCP 110 -Enables clients to retrieve email from a mail server -Downloads the email to the local mail application of the client IMAP - Internet Message Access Protocol -TCP 143 -Enables clients to access email stored on a mail server -Maintains email on the server 4. File Transfer FTP - File Transfer Protocol -TCP 20 to 21 -Sets rules that enable a user on one host to access and transfer files to and from another host over a network -FTP is a reliable, connection-oriented, and acknowledged file delivery protocol TFTP - Trivial File Transfer Protocol -UDP client 69 -A simple, connectionless file transfer protocol with best-effort, unacknowledged file delivery -It uses less overhead than FTP 5. Web HTTP - Hypertext Transfer Protocol -TCP 80, 8080 -A set of rules for exchanging text, graphic images, sound, video, and other multimedia files on the World Wide Web HTTPS - HTTP Secure -TCP, UDP 443 -The browser uses encryption to secure HTTP communications -Authenticates the website to which you are connecting your browser

These three OSI layers define the same functions as the TCP/IP model application layer.

Application, Presentation, Session

Which peer-to-peer application allows users to share pieces of many files with each other at the same time? A: Hybrid B: Gnutella C: BitTorrent

C: BitTorrent Many P2P applications allow users to share pieces of many files with each other at the same time. Clients use a torrent file to locate other users who have pieces that they need so that they can then connect directly to them. This file also contains information about tracker computers that keep track of which users have specific pieces of certain files. Clients ask for pieces from multiple users at the same time. This is known as a swarm and the technology is called BitTorrent.

Which two are protocols that belong in the OSI application layer? A: PNG B: DNS C: SMTP D: QuickTime

DNS, SMTP

True or false? DHCP clients send a DHCP request message to available DHCP servers.

FALSE the client broadcasts a DHCP discover (DHCPDISCOVER) message to identify any available DHCP servers on the network.

Explain what a P2P network is.

In a P2P network, two or more computers are connected via 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 both a server and a client. One computer might assume the role of server for one transaction while simultaneously serving as a client for another. The roles of client and server are set on a per request basis. -In addition to sharing files, a network such as this one would allow users to enable networked games or share an internet connection.

A client retrieves email using one of the two application layer protocols: __________ __ ____________

POP or IMAP

MKV, GIF, and JPG standards are associated with which OSI layer?

Presentation

Email supports three separate protocols for operation, what are they?

Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), and Internet Message Access Protocol IMAP).

Steps to Browsing inet

Step 1 The browser interprets the three parts of the URL: -http (the protocol or scheme) -www.cisco.com (the server name) -index.html (the specific filename requested) Step 2 The browser then checks with a name server to convert www.cisco.com into a numeric IP address, which it uses to connect to the server. The client initiates an HTTP request to a server by sending a GET request to the server and asks for the index.html file. Step 3 n response to the request, the server sends the HTML code for this web page to the browser. Step 4 The browser deciphers the HTML code and formats the page for the browser window.

Steps of DNS from a browser

Step 1 The user types an FQDN into a browser application Address field. Step 2 A DNS query is sent to the designated DNS server for the client computer. Step 3 The DNS server matches the FQDN with its IP address. Step 4 The DNS query response is sent back to the client with the IP address for the FQDN. Step 5 The client computer uses the IP address to make requests of the server.

What are some well-known standards for video and graphic data formats?

Well-known video formats: 1. Matroska Video (MKV) 2. Motion Picture Experts Group (MPG) 3. QuickTime Video (MOV) Well-known graphic image formats: 1. Graphics Interchange Format (GIF) 2. Joint Photographic Experts Group (JPG) 3. Portable Network Graphics (PNG) format

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

client, server

A P2P application allows a device to act as both a _________ and a _________ within the same communication

client, server -Every client is a server and every server is a client.

Application layer protocols describe the _______ of the requests and responses between clients and servers. In addition to the actual data transfer, this exchange may also require user authentication and the identification of a data file to be transferred.

format