Which Of The Following Tcp/Ip Protocols Do Email Clients Use To Download Messages UPDATED

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Which Of The Following Tcp/Ip Protocols Do Email Clients Use To Download Messages

Common Protocol

SMTP and E-mail

Walter Goralski , in The Illustrated Network (2d Edition), 2017

E-mail Access and Reading

The recipient's e-mail awarding checks in periodically with the local SMTP server to meet if any post has arrived. This checking can be either automated or when specifically run. If there is mail, the user can retrieve the mail service, open up it, and read it, and delete information technology. Usually, these are all dissever steps. This step does not utilize SMTP, simply a special post admission method and protocol such equally POP3 or IMAP4 (both are used by TCP/IP MAAs).

All v of these steps are not always necessary. Some hosts act as mail servers all on their own, and the host-local-mail service-server communication steps can be bypassed. Dial-in users oft compose, send, and receive email all at once when they send mail. But usually all five steps are needed.

4 devices are involved in the five steps. They are the sender'south client, the sender's local SMTP mail server, the recipient's local SMTP mail service server, and the recipient'southward client. The human relationship they take with one another and the protocols the email uses are shown in Effigy 25.4. Notation the symmetrical nature of the components so that ii-fashion communication is possible.

Email Protocols

At that place are three common protocols used to evangelize electronic mail over the Internet: the Simple Post Transfer Protocol (SMTP), the Mail Office Protocol (POP), and the Net Message Access Protocol (IMAP). All three employ TCP, and the terminal two are used for accessing electronic mailboxes. Special records stored in DNS servers play a role besides, using UDP. The current version of POP is version 3 (POP3) and the current version of IMAP is version 4 (IMAP4).

Although not a protocol, there is a serial of Multipurpose Net Mail Extensions (just MIME, never "MIMEs") for diverse types of email attachments (not simply simple text). Finally, a number of related specifications add hallmark to the basic email protocols. The way the protocols fit together is shown in Figure 25.5.

As we accept seen, the original SMTP was designed equally a uncomplicated host-to-host protocol. A user on 1 host created a bulletin with a program called sendmail or mail service and this text was sent directly to the destination host using SMTP as a Post Transfer Agent (MTA). Of course, if the remote user was not running an email server process to accept the SMTP session, at that place was null for the sender to do but keep trying.

Mod email systems "decouple" the sender from the receiver so that email yet goes through, even when the recipient is abroad for two weeks (only the messages keep piling upwards, merely similar regular mail). In addition, unlike almost every other TCP/IP application, email operates not from host to host only from user to user. This means that users are not required to receive e-mail on a particular host, nor is a particular host expected to accept only one user with email capabilities. (Nosotros can fifty-fifty choice up email for a recipient from the sending host, and we'll practice that later.) This user "mobility" poses special challenges for email addressing, which is why more than just a host name is required for correct e-mail delivery.

The solution, of form, is to add some other level to the hostname, this one identifying a detail user. Then, for example, [email protected] indicates a different mail destination than [email protected]. And, in fact, the bodily host on which an email user is defined is not always added to the email accost (which would yield something similar [email protected]). The email protocols all mesh together to make this work.

There are older electronic mail address formats—FIDOnet, UUCP, e-mail gateways (distinguished by the use of user% notations), and so on—but these are simply of historical involvement today. This is non to say that the development of email is uninteresting, merely that the history can be given very briefly and the discussion can turn to what is really done with email on the Internet today.

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Service Overlays

John F. Buford , ... Eng Keong Lua , in P2P Networking and Applications, 2009

Internet Routing and Isp Peering

The Internet is a collection of many different physical networks that use mutual protocols and services to create a universal routing and transport fabric for applications. Mostly, the different networks are operated as dissever autonomous systems (Ass) by their network service providers (NSPs). Inside an AS, an NSP volition utilize routing protocols that maintain detailed connectivity information for all routers in the AS. At the boundaries of the Every bit are connections to other NSPs' networks at connection points called peering points. The routers at these peering points use the Edge Gateway Protocol (BGP), which maintains information about BGP topology in the Internet and typically has limited details about routing inside a given AS. A consequence is that network traffic that is routed through an AS volition accept a restricted path. Such transit paths are determined by each NSP co-ordinate to its peering relationships with other NSPs. Each NSP manages transit traffic to balance its customer traffic with reciprocal sharing of network capacity with peer NSPs.

As discussed in Chapter 10, one consequence of BGP routing policies is that some routes crusade violations in the triangle inequality holding. That is, the directly path from A to B may be longer in terms of latency than the indirect path A-C-B. Another event is that route outages due to router failures may not be automatically rerouted since the necessary routing table information may be missing from the BGP tables.

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Data Link Management

Edward Insam PhD, BSc , in TCP/IP Embedded Internet Applications, 2003

PPP protocol field

At the framing level, the protocol and payload comprise the fields shown in Table half-dozen-ane.

Table six-1. Protocol and payload fields

Protocol ID (16 bits)

Information or payload (Variable length)

Padding (Variable length)

Each PPP parcel is preceded by a protocol identifier, a list of common protocols relevant to embedded applications is shown in Tabular array six-2. Certain rules be for protocol blazon numbering. All offset bytes must exist even, and all second bytes must be odd. That is, the to the lowest degree significant bit of the to the lowest degree pregnant byte must be one, and the to the lowest degree significant bit of the most significant byte must be a zero.

Tabular array 6-2. Common protocols relevant to embedded applications

Protocol ID	Description
0021	Cyberspace protocol (IP)
002D	VJ compressed IP
002F	VJ uncompressed IP
C021	Link control protocol (LCP)
C023	Countersign authentication protocol (PAP)
C025	Link quality report
C223	Challenge handshake authentication (CHAP)

Protocol field values in the 0000–3FFF range are used to identify the network layer protocol in use, for example, 0021 for IP. Protocols in the range 8000–BFFF identify the network control protocol, and protocols in the range C000–FFFF are link control protocols. The payload field carries the bodily data to be passed on. The minimum length is zero. The maximum length in bytes (including padding, but excluding the protocol field) is defined past the variable maximum receive unit of measurement (MRU). This variable is negotiated during link setup, and the default value is 1500. This makes PPP more or less size 'compatible' with Ethernet frames. The padding field may carry any number of bytes upward to the MRU value (normally zeros), these bytes will be ignored at the receiving end. Padding is commonly used to stitch a sequence to a give number of bytes.

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Passwords, Vulnerabilities, and Exploits

Littlejohn Shinder , Michael Cantankerous , in Scene of the Cybercrime (Second Edition), 2008

Authentication Protocols

The protocols used for authenticating identity depend on the authentication type. Some common protocols used for authentication include the following:

▪: Kerberos The default logon authentication protocol used past Windows 2000, XP, and Vista, equally well as by Windows Server 2003 and Windows Server 2008. It is also used past Mac OS X. This protocol is based on secret key (symmetric) cryptography, which we'll discuss in Chapter 12. This organisation uses tickets that a central server issues to make up one's mind whether a user can access the network and its resources. Rather than being used to log on to each server, the tickets are used by all of the servers to determine what a user is permitted to admission.
▪: Claiming Handshake Authentication Protocol (CHAP) Uses a sequence in which one party sends a challenge and the other responds with an answer. The well-nigh common course of this sequence is the server requesting a password, which the client provides to gain access to a arrangement. Microsoft developed its ain version of the protocol, called MS-CHAP.
▪: NT LAN Manager (NTLM) Another Microsoft logon authentication method that is supported by newer versions of Windows. NTLMv2 provides more security than NTLMv1, and uses a challenge-response sequence to authenticate the user. Unlike Kerberos, with NTLM, when a client wants to access a server's resources, that server must contact the domain controller to have the customer'south identity verified. The client doesn't accept credentials already issued (the session ticket in Kerberos) that the file or application server knows it can trust.
▪: Password Hallmark Protocol (PAP) A remote access authentication protocol used for Point-to-Bespeak (PPP) or punch-up connections. Its distinguishing characteristic (and the reason it should not exist used on secure networks) is the fact that it sends passwords in obviously text. This means an unauthorized person can intercept and use the passwords during manual. The just good reason to use PAP is if you face a situation in which the remote server doesn't support other, more secure authentication methods. Shiva PAP (Southward-PAP) addresses this problem past using a two-way reversible authentication method that encrypts passwords so that they will not be subject to interception and misuse.
▪: The Remote Authentication Dial-in User Service (RADIUS) Another means of authenticating remote connections that takes the hallmark responsibility off each individual remote access server by providing a centralized server to cosign clients deeply.
▪: Secure Shell (SSH) Allows users to log on to UNIX systems remotely. Both ends of the connection (client and server) are authenticated, and data—as well as passwords—can exist encrypted.

Note

In Chapter 12, we'll discuss a number of these protocols and how many of them use diverse types of encryption to ensure that data passed between a client and a server is secure.

On the Scene

Identity Confirmed; Now What?

Once a user'due south identity has been established, the next step in the security procedure is authorisation, which is concerned with what that user is permitted to do. Hallmark and potency piece of work together to provide a security system that takes into account the demand for different users to accept different capabilities on the network.

Administrators can control which files and other objects a user can access and the level of admission (read merely, change, and then on) by setting permissions. Most network operating systems provide a mechanism for associating specific permissions on an object with certain user accounts or groups. For example, Windows computers that have hard disks formatted as New Engineering science File Arrangement (NTFS) provide for ii levels of permissions: share permissions that employ only to users accessing the resources across the network, and file-level permissions (also called NTFS permissions) that utilize both across the network and to users accessing the resource from the local machine.

Administrators tin likewise control which system-wide actions a particular user (or group of users) can perform by setting user rights. User rights differ from permissions in that permissions use to access of individual files, folders, printers, and other objects.

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Using XML with SQL Server

In Designing SQL Server 2000 Databases, 2001

Additional XML Resource on the Spider web

The Internet has numerous other resource for XML. The official standards for XML are developed past the W3C. Biztalk, an industry initiative started by Microsoft, is a community of users of standards such as XML.

W3C.org

XML was adult by the W3C. The W3C was created in 1994 to develop common protocols and languages for the Web. It comprises over 400 member organizations. On the W3C Web site, you tin can find more information about the W3C and extensive information on XML, including new standards, the complete specifications, and the condition of all the XML languages. You lot tin can also come across the W3C press releases and just about whatever other information you tin can think of.

Biztalk.org

Biztalk is a customs of standards users that is supported by a wide range of organizations. Biztalk has 2 core issues: offset, that application integration is too complicated and plush; 2nd, that the side by side phase of the Net will require much greater application integration beyond disparate systems. Biztalk members are adopting an XML bulletin-passing architecture that is platform neutral to necktie systems together. Biztalk is committed to making XML interoperable by supporting standards and developing software tools to facilitate it.

XML.org

XML.org is an independent source of information and resources about using XML in industrial and commercial applications. Information technology besides provides some tools to help you make up one's mind how to use XML for your business. It is a vendor-independent organization to help with universal data exchange across all vendors. XML.org is hosted by the Organization for the Advancement of Structured Information Standards (OASIS). Oasis is an system that creates interoperability industry specifications based on public standards such every bit XML.

MSDN.Microsoft.com/XML

Y'all tin obtain Microsoft-specific information about XML from the company'south MSDN Web site, which shows highlights of new tools and features. Information technology besides contains the consummate documentation for the XML SDK, sample code, downloads, and more. This site contains all-encompassing information on using XML with Microsoft products.

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The Fundamentals in Agreement Networking Middleware

Tammy Noergaard , in Demystifying Embedded Systems Middleware, 2010

4.vi.two File Transfer Protocol (FTP) Customer Application Example

RFC959, File Transfer Protocol (FTP), is one of the simpler and more common protocols implemented within an embedded organization that is used to securely exchange files over a network. The FTP protocol is based on a communication model in which at that place is an FTP customer, also referred to as a user-protocol interpreter (user PI) that initiates a file transfer, and an FTP server or FTP site that manages and receives FTP connections. Equally shown in Effigy 4.48, the types of connections that exist between an FTP customer and server are:

•: control connections, which are connections in which commands are transmitted over
•: information connections, which are connections in which files are transmitted over.

FTP clients get-go FTP sessions by initiating a control connexion to a destination organisation with an FTP server. This FTP control connectedness is based on a TCP connectedness to port 21, because FTP requires an underlying transport layer protocol that is a reliable, ordered data stream channel. When FTP client and server communicate over a control connection, they practise and then via the interchange of commands and answer codes, such as some of the codes shown in Tabular array iv.11.

Table iv.11. Examples of FTP Commands and Respond Codes ¹

Type	Code	Definition
Command	DELE	Delete. FTP service control
	MODE	Transfer Mode. Transfer parameter control
	PASS	Countersign. Access control command
	PORT	Data Port. Transfer parameter command
	QUIT	Logout. Access command command
	Type	Representation Type. Transfer parameter command
	USER	Username. Access control control
Reply Lawmaking	110	Restart marker respond
	120	Service ready in 'x' minutes
	125	Data connection already open up
	150	File status OK
	200	Command OK
	202	Command NOT implemented
	211	System Assist

Figure 4.49 is an open source case of FTP functions, and how this source code utilizes a required underlying networking middleware layer such as TCP socket-related role calls.

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Networking

Martin Moore , Steven Hancock , in Tru64 UNIX Troubleshooting, 2003

7.2.2.iv RPC-Based Services

The Remote Procedure Call (RPC) machinery allows two networked computers to communicate with each other using a common protocol. RPC is the basis for some widely used networking protocols, including the Network File Organisation (NFS) and the Network Information Service (NIS). In improver, the daemons used in HP's TruCluster production are heavy users of the RPC mechanism.

Bug with RPC-based services may be acquired by problems with the underlying RPC machinery, which in turn normally occur if the portmap(viii) daemon (besides chosen the "portmapper") is not running or non functioning properly. The portmapper is the "traffic cop" that accepts incoming RPC requests and easily them off to the appropriate subdaemons. In this way, portmap functions very much similar the inetd daemon. However, the portmapper listens on merely one TCP/IP port and uses a program identifier to keep track of its subdaemons. This conserves the use of reserved TCP/IP ports on the system.

The programs that portmap will listen for are listed in the /etc/rpc file. When these daemons start up, they register with the portmapper. If you are trying to find out why a detail program is non communicating, check this file to verify that the program is properly listed and has a unique program number assigned. If and then, employ the rpcinfo(viii) control to verify that the subdaemon is running and has registered itself with the portmapper. To brandish the daemons registered with the portmapper on the local host, use the following command:

Alternatively, to check a host chosen "george" using rpcinfo, add the hostname to the command; for example:

This volition tell you that not only is the program properly registered, the portmapper is responding to its requests. This is a good sign that portmap is working properly.

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Applied science & Product Architectures

Rick Sherman , in Business organisation Intelligence Guidebook, 2015

Data Access APIs

Many data access Application Programming Interfaces (APIs) are used in information admission and data integration. The nigh mutual protocols include:

•: Open Database Connectivity (ODBC): standard middleware API used to access databases. ODBC was developed to exist database and operating system independent, replacing database-specific APIs.
•: Java Database Connectivity (JDBC): data admission API used past Java programming languages.
•: XML for Assay (XML/A): industry standards for accessing belittling information sources such as OLAP and data mining. It is based on manufacture-standard Extensible Markup Language (XML), Soap, and HTTP. It can execute several query languages such as Multidimensional eXpressions (MDX), SQL, Data Analysis eXpressions (DAX), and Data Mining eXtensions (DMX).
•: XQuery: query language used to extract and transform data from XML data sources and Simple Object Access Protocol (Soap) and Representational Land Transfer (REST) web services. There is a multifariousness of structured and unstructured data sources that can exist viewed equally XML.
•: Native database or data APIs: these APIs admission specific data or database sources whose providers may feel these APIs provide ameliorate functionality than industry standard APIs, or that they cannot conform to those standards.

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Os X El Capitan forensics

Shawn Hashemite kingdom of jordan , in Digital Forensics, 2016

.ssh

The {user proper noun}/.ssh file contains all the SSH keys for the user. SSH is a common protocol used for remotely accessing a system. SSH depends on private and public keys to provide encrypted advice. Fig. 7.vii shows the user's known_hosts file, which contains the addresses used with SSH and the private keys associated with the connexion. It volition also tell you the encryption method used to generate the keys. Only systems accessed volition appear in the .ssh directory. This would exist specially useful during an investigation of a network alienation. SSH is a common protocol to use for remote access, and this directory could show all systems accessed.

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Time and Frequency

Michael A. Lombardi , in Encyclopedia of Concrete Science and Technology (Third Edition), 2003

IV.C.1 Net Fourth dimension Signals

Internet time servers use standard timing protocols defined in a series of RFC (Request for Comments) documents. The three near common protocols are the Time Protocol, the Daytime Protocol, and the Network Time Protocol (NTP). An Net time server waits for timing requests sent using any of these protocols and sends a time code in the correct format when a request is received.

Client software is available for all major operating systems, and about client software is compatible with either the Daytime Protocol or the NTP. Customer software that uses the Simple Network Time Protocol (SNTP) makes the same timing request as an NTP customer but does less processing and provides less accuracy. Tabular array X summarizes the diverse protocols and their port assignments, or the port where the server "listens" for a client asking.

Table X. Internet Fourth dimension Protocols

Protocol proper noun	Document	Format	Port assignment(s)
Time protocol	RFC-868	Unformatted 32-chip binary number contains fourth dimension in UTC seconds since January 1, 1900	Port 37, tcp/ip, udp/ip
Daytime protocol	RFC-867	Exact format not specified in standard. Only requirement is that the time lawmaking is sent as ASCII characters	Port thirteen, tcp/ip, udp/ip
Network time protocol (NTP)	RFC-1305	The server provides a information bundle with a 64-scrap fourth dimension stamp containing the time in UTC seconds since Jan 1, 1900, with a resolution of 200 psec. NTP provides an accurateness of 1 to fifty msec. The client software runs continuously and gets periodic updates from the server.	Port 123, udp/ip
Unproblematic network time protocol (SNTP)	RFC-1769	The data parcel sent by the server is the aforementioned as NTP, simply the client software does less processing and provides less accuracy.	Port 123, udp/ip

NIST operates an Internet fourth dimension service using multiple servers distributed around the United States. A list of IP addresses for the NIST servers and sample customer software can exist obtained from the NIST Time and Frequency Sectionalisation spider web site: http://www.bedrock.nist.gov/timefreq. The incertitude of Internet time signals is normally <100 msec, but results vary with different computers, operating systems, and client software.

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Which Of The Following Tcp/Ip Protocols Do Email Clients Use To Download Messages UPDATED