Application Layer - Modulo 15

Question 1

Application Layer
Application, Presentation, and Session
Application Layer

Answer 1

In the OSI and the TCP/IP models, the application layer is the closest layer to the end
user.
As shown in the figure, it is the layer that provides the interface between the applications
used to communicate, and the underlying network over which messages are transmitted.
Application layer protocols are used to exchange data between programs running on the
source and destination hosts.

Based on the TCP/IP model, the upper three layers of the OSI model (application,
presentation, and session) define functions of the TCP/IP application layer.
There are many application layer protocols, and new protocols are always being
developed.
Some of the most widely known application layer protocols include Hypertext Transfer
Protocol (HTTP), File Transfer Protocol (FTP), Trivial File Transfer Protocol (TFTP), Internet
Message Access Protocol (IMAP), and Domain Name System (DNS) protocol.

Question 2

Presentation Layer
The presentation layer has three primary functions:

Answer 2

• Formatting, or presenting, data at the source device into a compatible format for
  receipt by the destination device.
• Compressing data in a way that can be decompressed by the destination device.
• Encrypting data for transmission and decrypting data upon receipt.
  As shown in the figure, the presentation layer formats data for the application layer, and it
  sets standards for file formats.
  Some well-known standards for video include Matroska Video (MKV), Motion Picture
  Experts Group (MPG), and QuickTime Video (MOV).
  Some well-known graphic image formats are Graphics Interchange Format (GIF), Joint
  Photographic Experts Group (JPG), and Portable Network Graphics (PNG) format.

Question 3

Session Layer

Answer 3

As the name implies, functions at the session layer create and maintain dialogs between
source and destination applications.
The session layer handles the exchange of information to initiate dialogs, keep them active,
and to restart sessions that are disrupted or idle for a long period of time.

Question 4

TCP/IP Application Layer Protocols

Answer 4

The TCP/IP application protocols specify the format and control information necessary for
many common internet communication functions.
Application layer protocols are used by both the source and destination devices during a
communication session.
For the communications to be successful, the application layer protocols that are
implemented on the source and destination host must be compatible.

Question 5

Name System

Answer 5

DNS - Domain Name System (or Service)
* TCP, UDP 53
* Translates domain names, such as cisco.com, into IP addresses.

Question 6

Host Config

Answer 6

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

Question 7

Email

Answer 7

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

Question 8

File Transfer

Answer 8

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

Question 9

Web

Answer 9

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

Question 10

Peer-to-Peer
Client-Server Model

Answer 10

In the previous topic, you learned that TCP/IP application layer protocols implemented on
both the source and destination host must be compatible.
In this topic you will learn about the client/server model and the processes used, which are
in the application layer.
The same is true for a peer-to-peer 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.
The client is a hardware/software combination that people use to directly access the
resources that are stored on the server.
Client and server processes are considered to be in the application layer.
The client begins the exchange by requesting data from the server, which responds by
sending one or more streams of data to the client.
Application layer protocols describe the format 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.
One example of a client/server network is using the email service of an ISP to send,
receive, and store email.
The email client on a home computer issues a request to the email server of the ISP for
any unread mail.
The server responds by sending the requested email to the client.
Data transfer from a client to a server is referred to as an upload and data from a server to
a client as a download.

Question 11

Peer-to-Peer Networks

Answer 11

In the peer-to-peer (P2P) networking model, the data is accessed from a peer device
without the use of a dedicated server.
The P2P network model involves two parts: P2P networks and P2P applications.
Both parts have similar features, but in practice work quite differently.
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.
In a peer-to-peer exchange, both devices are considered equal in the communication
process.
Peer 1 has files that are shared with Peer 2 and can access the shared printer that is
directly connected to Peer 2 to print files.
Peer 2 is sharing the directly connected printer with Peer 1 while accessing the shared files
on Peer 1, as shown in the figure.

Question 12

Peer-to-Peer Applications

Answer 12

A P2P application allows a device to act as both a client and a server within the same
communication, as shown in the figure.
In this model, every client is a server and every server is a client.
P2P applications require that each end device provide a user interface and run a
background service.
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.

Question 13

Common P2P Applications

Answer 13

With P2P applications, each computer in the network that is running the application can act
as a client or a server for the other computers in the network that are also running the
application. Common P2P networks include the following:
* BitTorrent
* Direct Connect
* eDonkey
* 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.
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. BitTorrent has its own
client.
But there are many other BitTorrent clients including uTorrent, Deluge, and qBittorrent.
Note: Any type of file can be shared between users. Many of these files are copyrighted,
meaning that only the creator has the right to use and distribute them. It is against the law
to download or distribute copyrighted files without permission from the copyright holder.
Copyright violation can result in criminal charges and civil lawsuits

Question 14

Web and Email Protocols
Hypertext Transfer Protocol and
Hypertext Markup Language

Answer 14

There are application layer-specific protocols that are designed for common uses such as
web browsing and email.
The first topic gave you an overview of these protocols.
This topic goes into more detail.
When a web address or Uniform Resource Locator (URL) is typed into a web browser, the
web browser establishes a connection to the web service.
The web service is running on the server that is using the HTTP protocol.
URLs and Uniform Resource Identifiers (URIs) are the names most people associate with
web addresses.
To better understand how the web browser and web server interact, examine how a web
page is opened in a browser. For this example, use
the http://www.cisco.com/index.html URL.
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
In 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.

Question 15

HTTP and HTTPS

Answer 15

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.

Question 16

The three common message types are GET (see figure), POST, and PUT:

Answer 16

• GET - This is a client request for data. A client (web browser) sends the GET
  message to the web server to request HTML pages.
• POST - This uploads data files to the web server, such as form data.
• PUT - This uploads resources or content to the web server, such as an image.

Question 17

NOTES

Answer 17

Although HTTP is remarkably flexible, it is not a secure protocol.
The request messages send information to the server in plaintext that can be intercepted
and read.
The server responses, typically HTML pages, are also unencrypted.
For secure communication across the internet, the HTTP Secure (HTTPS) protocol is used.
HTTPS uses authentication and encryption to secure data as it travels between the client
and server.
HTTPS uses the same client request-server response process as HTTP, but the data
stream is encrypted with Transport Layer Security (TLS) or its predecessor Secure Socket
Layer (SSL) before being transported across the network.

Question 18

Email Protocols

Answer 18

One of the primary services offered by an ISP is email hosting.
To run on a computer or other end device, email requires several applications and services,
as shown in the figure.
Email is a store-and-forward method of sending, storing, and retrieving electronic
messages across a network. Email messages are stored in databases on mail servers.
Email clients communicate with mail servers to send and receive email.
Mail servers communicate with other mail servers to transport messages from one domain
to another.
An email client does not communicate directly with another email client when sending
email.
Instead, both clients rely on the mail server to transport messages.
Email supports three separate protocols for operation: Simple Mail Transfer Protocol
(SMTP), Post Office Protocol (POP), and IMAP.
The application layer process that sends mail uses SMTP.
A client retrieves email using one of the two application layer protocols: POP or IMAP.

Question 19

SMTP

Answer 19

SMTP message formats require a message header and a message body.
Although the 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.
The destination email server may not be online, or may be busy, when email messages are
sent.
Therefore, SMTP spools messages to be sent at a later time.
Periodically, the server checks the queue for messages and attempts to send them again.
If the message is still not delivered after a predetermined expiration time, it is returned to
the sender as undeliverable.

Question 20

POP

Answer 20

POP is used by an application to retrieve mail from a mail server.
With POP, mail is downloaded from the server to the client and then deleted on the server.
This is the default operation of POP.
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.
With 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.
POP3 is the most commonly used version.

Question 21

IMAP

Answer 21

IMAP is another protocol that describes a method to retrieve email messages.
Unlike POP, when the user connects to an IMAP-capable server, copies of the messages
are downloaded to the client application, as shown in the figure.
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.

Question 22

IP Addressing Services
Domain Name System

Answer 22

There are other application layer-specific protocols that were designed to make it easier to
obtain addresses for network devices.
These services are essential because it would be very time consuming to remember IP
addresses instead of URLs or manually configure all of the devices in a medium to large
network.
The first topic in this module gave you an overview of these protocols.
This topic goes into more detail about the IP addressing services, DNS and DHCP.
In data networks, devices are labeled with numeric IP addresses to send and receive data
over networks.
Domain names were created to convert the numeric address into a simple, recognizable
name.
On the internet, fully-qualified domain names (FQDNs), such as http://www.cisco.com, are
much easier for people to remember than 198.133.219.25, which is the actual numeric
address for this server.
If Cisco decides to change the numeric address of www.cisco.com, it is transparent to the
user because the domain name remains the same.
The new address is simply linked to the existing domain name and connectivity is
maintained.
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.
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.

Question 23

DNS Message Format

Answer 23

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 of these record types
are as follows:
* A - An end device IPv4 address
* NS - An authoritative name server
* AAAA - An end device IPv6 address (pronounced quad-A)
* MX - A mail exchange record
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.
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.
As shown in the table, 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 section
Description
Question
The question for the name server
Answer
Resource Records answering the question
Authority
Resource Records pointing toward an authority
Additional
Resource Records holding additional information

Question 24

DNS Hierarchy

Answer 24

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

Question 25

The nslookup Command

Answer 25

When configuring a network device, one or more DNS Server addresses are provided that the DNS client can use for name resolution. Usually the ISP provides the addresses to use for the DNS servers. When a user application requests to connect to a remote device by name, the requesting DNS client queries the name server to resolve the name to a numeric address. 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. 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.

Question 26

Dynamic Host Configuration Protocol

Answer 26

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. The alternative to dynamic addressing is static addressing. When using static addressing, the network administrator manually enters IP address information on hosts. 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. On larger networks, or where the user population changes frequently, DHCP is preferred for address assignment. New users may arrive and need connections; others may have new computers that must be connected. Rather than use static addressing for each connection, it is more efficient to have IPv4 addresses assigned automatically using DHCP. DHCP can allocate IP addresses for a configurable period of time, called a lease period. The lease period is an important DHCP setting, 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. As the figure shows, various types of devices can be DHCP servers. 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. Many networks use both DHCP and static addressing. 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. 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.

Question 27

DHCP Operation

Answer 27

As shown in the figure, when an IPv4, DHCP-configured device boots up or connects to the network, 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. 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. 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. 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. DHCPv6 has a set of messages that is similar to those for DHCPv4. The DHCPv6 messages are SOLICIT, ADVERTISE, INFORMATION REQUEST, and REPLY.

Question 28

File Sharing Services File Transfer Protocol

Answer 28

As you learned in previous topics, in the client/server model, the client can upload data to a server, and download data from a server, if both devices are using a file transfer protocol (FTP). Like HTTP, email, and addressing protocols, FTP is commonly used application layer protocol. This topic discusses FTP in more detail. 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. 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.

Question 29

Server Message Block

Answer 29

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. Here are three functions of SMB messages: * Start, authenticate, and terminate sessions. * Control file and printer access. * 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. 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 SMB file exchange process between Windows PCs is shown in the next figure. 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 though the resource is local to the client host. 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.

Question 30

Application, Presentation, and Session

Answer 30

In the OSI and the TCP/IP models, the application layer is the closest layer to the end user. Application layer protocols are used to exchange data between programs running on the source and destination hosts. The presentation layer has three primary functions: formatting, or presenting, data at the source device into a compatible form for receipt by the destination device, compressing data in a way that can be decompressed by the destination device, and encrypting data for transmission and decrypting data upon receipt. The session layer creates and maintains dialogs between source and destination applications. The session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time. TCP/IP application layer protocols specify the format and control information necessary for many common internet communication functions. These protocols are used by both the source and destination devices during a session. The protocols implemented on both the source and destination host must be compatible.

Question 31

Peer-to-Peer

Answer 31

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. The client begins the exchange by requesting data from the server, which responds by sending one or more streams of data to the client. In a P2P network, two or more computers are connected via a network and can share resources without having a dedicated server. Every 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. P2P applications require that each end device provide a user interface and run a background service. 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. Many P2P applications allow users to share pieces of files with each other at the same time. Clients use a small file called a torrent file to locate other users who have pieces that they need so that they can connect directly to them. This file also contains information about tracker computers that keep track of which users have what pieces of which files.

Question 32

Web and Email Protocols

Answer 32

When a web address or URL is typed into a web browser, the web browser establishes a connection to the web service. The web service is running on the server that is using the HTTP protocol. 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. The three common message types are GET, POST, and PUT. For secure communication across the internet, HTTPS uses the same client request-server response process as HTTP, but the data stream is encrypted with SSL before being transported across the network. Email supports three separate protocols for operation: SMTP, POP, and IMAP. The application layer process that sends mail uses SMTP. A client retrieves email using POP or IMAP. SMTP message formats require a message header and a message body. While the message body can contain any amount of text, the message header must have a properly formatted recipient email address and a sender address. POP is used by an application to retrieve mail from a mail server. With POP, mail is downloaded from the server to the client and then deleted on the server. With IMAP, unlike POP, 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.

Question 33

IP Addressing Services

Answer 33

The DNS protocol matches resource names with the required numeric network address. The DNS protocol communications use a message format for all types of client queries and server responses, error messages, and the transfer of resource record information between servers. DNS uses domain names to form a hierarchy. 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. Computer OSs use Nslookup to allow the user to manually query the name servers to resolve a given host name. DHCP for IPv4 service automates the assignment of IPv4 addresses, subnet masks, gateways, and other IPv4 networking parameters. DHCPv6 provides similar services for IPv6 clients, except that it does not provide a default gateway address. When an IPv4, DHCP-configured device boots up or connects to the network, the client broadcasts a DHCPDISCOVER message to identify any available DHCP servers on the network. A DHCP server replies with a DHCPOFFER message, which offers a lease to the client. DHCPv6 has a set of messages that is similar to those for DHCPv4. The DHCPv6 messages are SOLICIT, ADVERTISE, INFORMATION REQUEST, and REPLY.

Question 34

File Sharing Services

Answer 34

pull data from an FTP server. The client establishes the first connection to the server for control traffic using TCP port 21. The client establishes the second connection to the server for the actual data transfer using TCP port 20. The client can download (pull) data from the server, or the client can upload (push) data to the server. Here are three functions of SMB messages: start, authenticate, and terminate sessions, control file and printer access, and allow an application to send or receive messages to or from another device. 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.