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Computer Networking
12th Grade

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FTP is built on a client-server model architecture and uses separate control and data connections between the client and the server.[1] FTP users may authenticate themselves with a clear-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure transmission that protects the username and password, and encrypts the content, FTP is often secured with SSL/TLS (FTPS). SSH File Transfer Protocol (SFTP) is sometimes also used instead, but is technologically different.

The first FTP client applications were command-line programs developed before operating systems had graphical user interfaces, and are still shipped with most Windows, Unix, and Linux operating systems.[2][3]Many FTP clients and automation utilities have since been developed for desktops, servers, mobile devices, and hardware, and FTP has been incorporated into productivity applications, such as web page editors.


Simple Mail Transfer Protocol (SMTP) is an Internet standard forelectronic mail (email) transmission. First defined by RFC 821 in 1982, it was last updated in 2008 with the Extended SMTP additions by RFC 5321—which is the protocol in widespread use today.

SMTP by default uses TCP port 25. The protocol for mail submission is the same, but uses port 587. SMTP connections secured by SSL, known as SMTPS, default to port 465 (nonstandard, but sometimes used for legacy reasons).

Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For retrieving messages, client applications usually use either POP3 or IMAP.

Although proprietary systems (such as Microsoft Exchange and IBM Notes) and webmail systems (such as, Gmail and Yahoo! Mail) use their own non-standard protocols to access mail box accounts on their own mail servers, all use SMTP when sending or receiving email from outside their own systems.


In computing, the SSH File Transfer Protocol (also Secure File Transfer Protocol, or SFTP) is a network protocolthat provides file access, file transfer, and file management over any reliable data stream. It was designed by theInternet Engineering Task Force (IETF) as an extension of the Secure Shell protocol (SSH) version 2.0 to provide secure file transfer capabilities. The IETF Internet Draft states that, even though this protocol is described in the context of the SSH-2 protocol, it could be used in a number of different applications, such as secure file transfer overTransport Layer Security (TLS) and transfer of management information in VPN applications.

This protocol assumes that it is run over a secure channel, such as SSH, that the server has already authenticated the client, and that the identity of the client user is available to the protocol.

The Transmission Control Protocol (TCP) is a core protocol of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is commonly referred to as TCP/IP. TCP provides reliable, ordered, and error-checked delivery of a stream of octets between applications running on hosts communicating over an IP network. Major Internet applications such as the World Wide Webemailremote administration and file transfer rely on TCP. Applications that do not require reliable data stream service may use the User Datagram Protocol (UDP), which provides a connectionless datagram service that emphasizes reduced latency over reliability.

Telnet is an application layer protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. User data is interspersed in-band with Telnet control information in an 8-bit byte oriented data connection over the Transmission Control Protocol (TCP).

Telnet was developed in 1969 beginning with RFC 15, extended in RFC 854, and standardized as Internet Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards.

Historically, Telnet provided access to a command-line interface (usually, of an operating system) on a remote host, including most network equipment and operating systems with a configuration utility (including systems based onWindows NT).[clarification needed] However, because of serious security concerns when using Telnet over an open network such as the Internet, its use for this purpose has waned significantly in favor of SSH.

The term telnet is also used to refer to the software that implements the client part of the protocol. Telnet client applications are available for virtually all computer platforms. Telnet is also used as a verb. To telnet means to establish a connection with the Telnet protocol, either with command line client or with a programmatic interface. For example, a common directive might be: "To change your password, telnet to the server, log in and run the passwdcommand." Most often, a user will be telnetting to a Unix-like server system or a network device (such as a router) and obtaining a login prompt to a command line text interface or a character-based full-screen manager.


Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), both of which are frequently referred to as "SSL", arecryptographic protocols that provide communications security over acomputer network.[1] Several versions of the protocols are in widespread use in applications such as web browsing, email, Internet faxing, instant messaging, and voice-over-IP (VoIP). Major web sites use TLS to secure all communications between their servers and web browsers.

The primary goal of the Transport Layer Security protocol is to provide privacy and data integrity between two communicating computer applications.[1]:3 When secured by TLS, connections between a client (e.g., a web browser) and a server (e.g., have one or more of the following properties:


Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in current use. NTP was designed byDavid L. Mills of the University of Delaware.

NTP is intended to synchronize all participating computers to within a fewmilliseconds of Coordinated Universal Time (UTC).[1]:3 It uses a modified version of Marzullo's algorithm to select accurate time servers and is designed to mitigate the effects of variable network latency. NTP can usually maintain time to within tens of milliseconds over the publicInternet, and can achieve better than one millisecond accuracy in local area networks under ideal conditions. Asymmetric routes and network congestion can cause errors of 100 ms or more.[2][3]

The protocol is usually described in terms of a client-server model, but can as easily be used in peer-to-peer relationships where both peers consider the other to be a potential time source.[1]:20 Implementations send and receive timestamps using the User Datagram Protocol (UDP) on port number 123.[4][5] They can also use broadcasting or multicasting, where clients passively listen to time updates after an initial round-trip calibrating exchange.[3] NTP supplies a warning of any impending leap second adjustment, but no information about local time zones or daylight saving time is transmitted.[2][3]

The current protocol is version 4 (NTPv4), which is a proposed standard as documented in RFC 5905. It is backward compatible with version 3, specified in RFC 1305.


In computer networking, Point-to-Point Protocol (PPP) is a data link(layer 2) protocol used to establish a direct connection between twonodes. It can provide connection authentication, transmission encryption(using ECP, RFC 1968), and compression.

PPP is used over many types of physical networks including serial cable,phone line, trunk line, cellular telephone, specialized radio links, and fiber optic links such as SONET. PPP is also used over Internet accessconnections. Internet service providers (ISPs) have used PPP for customer dial-up access to the Internet, since IP packets cannot be transmitted over a modem line on their own, without some data link protocol.

Two derivatives of PPP, Point-to-Point Protocol over Ethernet (PPPoE) and Point-to-Point Protocol over ATM (PPPoA), are used most commonly by Internet Service Providers (ISPs) to establish a Digital Subscriber Line(DSL) Internet service connection with customers.

PPP is commonly used as a data link layer protocol for connection over synchronous and asynchronous circuits, where it has largely superseded the older Serial Line Internet Protocol (SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the X.25 protocol suite). The only requirement for PPP is that the circuit provided be duplex. PPP was designed to work with numerous network layer protocols, including Internet Protocol (IP), TRILL, Novell's Internetwork Packet Exchange (IPX), NBF, DECnet and AppleTalk.

Internet Protocol Suite

The Internet protocol suite is the computer networking model and set of communications protocols used on the Internet and similar computer networks. It is commonly known as TCP/IP, because its most important protocols, the Transmission Control Protocol (TCP) and the Internet Protocol (IP) were the first networking protocols defined during its development. It is occasionally known as the Department of Defense (DoD) model, because the development of the networking model was funded by DARPA, an agency of the United States Department of Defense.

TCP/IP provides end-to-end data communication specifying how data should be packetized, addressed, transmitted, routed and received. This functionality is organized into four abstraction layers which are used to sort all related protocols according to the scope of networking involved.[1][2] From lowest to highest, the layers are the link layer, containing communication methods for data that remains within a single network segment (link); the internet layer, connecting independent networks, thus providing internetworking; thetransport layer handling host-to-host communication; and the application layer, which provides process-to-process data exchange for applications.

The TCP/IP model and many of its protocols are maintained by the Internet Engineering Task Force (IETF).


The wireless data exchange standard Bluetooth uses a variety of protocols. Core protocols are defined by the trade organization Bluetooth SIG. Add protocols have been adopted from other standards bodies. This article gives an overview of the core protocols and those adopted protocols that are widely used.

The Bluetooth protocol stack is split in two parts: a "controller stack" containing the timing critical radio interface, and a "host stack" dealing with high level data. The controller stack is generally implemented in a low cost silicon device containing the Bluetooth radio and a microprocessor. The host stack is generally implemented as part of an operating system, or as an installable package on top of an operating system. For integrated devices such as Bluetooth headsets, the host stack and controller stack can be run on the same microprocessor to reduce mass production costs; this is known as a hostless system.

SSL 1.0, 2.0 and 3.0

Netscape developed the original SSL protocols.[6] Version 1.0 was never publicly released because of serious security flaws in the protocol; version 2.0, released in February 1995, "contained a number of security flaws which ultimately led to the design of SSL version 3.0".[7] Released in 1996, SSL version 3.0 represented a complete redesign of the protocol produced by Paul Kocher working with Netscape engineers Phil Karlton and Alan Freier, with a reference implementation by Christopher Allen and Tim Dierks of Consensus Development. Newer versions of SSL/TLS are based on SSL 3.0. The 1996 draft of SSL 3.0 was published by IETF as a historical document inRFC 6101.

Dr. Taher Elgamal, chief scientist at Netscape Communications from 1995 to 1998, is recognized as the "father of SSL".[8][9]

As of 2014 the 3.0 version of SSL is considered insecure as it is vulnerable to the POODLEattack that affects all block ciphers in SSL; and RC4, the only non-block cipher supported by SSL 3.0, is also feasibly broken as used in SSL 3.0.[10]

SSL 2.0 was deprecated (prohibited) in 2011 by RFC 6176.

SSL 3.0 was deprecated in June 2015 by RFC 7568.

TLS 1.0
TLS 1.0 was first defined in RFC 2246 in January 1999 as an upgrade of SSL Version 3.0, and written by Christopher Allen and Tim Dierks of Consensus Development. As stated in the RFC, "the differences between this protocol and SSL 3.0 are not dramatic, but they are significant enough to preclude interoperability between TLS 1.0 and SSL 3.0". TLS 1.0 does include a means by which a TLS implementation can downgrade the connection to SSL 3.0, thus weakening security.[11]:1–2

TLS 1.1


TLS 1.1 was defined in RFC 4346 in April 2006.[12] It is an update from TLS version 1.0. Significant differences in this version include:

  • Added protection against cipher-block chaining (CBC) attacks.
    • The implicit initialization vector (IV) was replaced with an explicit IV.
    • Change in handling of padding errors.
  • Support for IANA registration of parameters.[11]:2

TLS 1.2


TLS 1.2 was defined in RFC 5246 in August 2008. It is based on the earlier TLS 1.1 specification. Major differences include:

  • The MD5-SHA-1 combination in the pseudorandom function (PRF) was replaced with SHA-256, with an option to use cipher suite specified PRFs.
  • The MD5-SHA-1 combination in the finished message hash was replaced with SHA-256, with an option to use cipher suite specific hash algorithms. However the size of the hash in the finished message must still be at least 96 bits.[13]
  • The MD5-SHA-1 combination in the digitally signed element was replaced with a single hashnegotiated during handshake, which defaults to SHA-1.
  • Enhancement in the client's and server's ability to specify which hash and signature algorithms they accept.
  • Expansion of support for authenticated encryption ciphers, used mainly for Galois/Counter Mode (GCM) and CCM mode of Advanced Encryption Standard encryption.
  • TLS Extensions definition and Advanced Encryption Standard cipher suites were added.[11]:2

All TLS versions were further refined in RFC 6176 in March 2011 removing their backward compatibility with SSL such that TLS sessions never negotiate the use of Secure Sockets Layer (SSL) version 2.0.


TLS 1.3 (draft)


As of July 2016, TLS 1.3 is a working draft, and details are provisional and incomplete.[14][15] It is based on the earlier TLS 1.2 specification. Major differences from TLS 1.2 include:

  • Removing support for weak and lesser used named elliptic curves (see Elliptic curve cryptography)
  • Removing support for MD5 and SHA-224 cryptographic hash functions
  • Requiring digital signatures even when a previous configuration is used
  • Integrating HKDF and the semi-ephemeral DH proposal
  • Replacing resumption with PSK and tickets
  • Supporting 1-RTT handshakes and initial support for 0-RTT (see Round-trip delay time)
  • Dropping support for many insecure or obsolete features including compression, renegotiation, non-AEAD ciphers, static RSA and static DH key exchange, custom DHEgroups, point format negotiation, Change Cipher Spec protocol, Hello message UNIX time, and the length field AD input to AEAD ciphers
  • Prohibiting SSL or RC4 negotiation for backwards compatibility
  • Integrating use of session hash
  • Deprecating use of the record layer version number and freezing the number for improved backwards compatibility
  • Moving some security related algorithm details from an appendix to the specification and relegating ClientKeyShare to an appendix
  • Addition of the ChaCha20 stream cipher with the Poly1305 message authentication code
  • Addition of the Ed25519 and Ed448 digital signature algorithms
  • Addition of the x25519 and x448 key exchange protocols
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