Common Protocol

SMTP and Email

Walter Goralski , in The Illustrated Network (Second Edition), 2017

Email Access and Reading

The recipient's email application checks in periodically with the local SMTP server to see if any mail has arrived. This checking tin can be either automated or when specifically run. If in that location is mail, the user can retrieve the mail, open it, and read it, and delete it. Commonly, these are all separate steps. This step does not use SMTP, but a special mail admission method and protocol such every bit POP3 or IMAP4 (both are used past TCP/IP MAAs).

All 5 of these steps are not always necessary. Some hosts act as mail service servers all on their own, and the host-local-mail-server advice steps can be bypassed. Punch-in users often compose, ship, and receive email all at in one case when they send mail. But usually all five steps are needed.

Iv devices are involved in the five steps. They are the sender's client, the sender's local SMTP post server, the recipient's local SMTP mail service server, and the recipient'southward client. The relationship they have with one another and the protocols the email uses are shown in Effigy 25.4. Note the symmetrical nature of the components then that two-way communication is possible.

Figure 25.4. Email protocols and components, showing the components used to ship an email message. Note the symmetrical nature of the sender and recipient so that the receiver tin can answer.

Email Protocols

At that place are three common protocols used to deliver email over the Net: the Simple Mail Transfer Protocol (SMTP), the Post Office Protocol (Pop), and the Internet Message Access Protocol (IMAP). All three use TCP, and the last 2 are used for accessing electronic mailboxes. Special records stored in DNS servers play a role as well, using UDP. The electric current version of Popular is version 3 (POP3) and the electric current version of IMAP is version 4 (IMAP4).

Although not a protocol, at that place is a serial of Multipurpose Net Post Extensions (just MIME, never "MIMEs") for various types of email attachments (non only uncomplicated text). Finally, a number of related specifications add authentication to the bones email protocols. The way the protocols fit together is shown in Figure 25.5.

Figure 25.five. Email protocols, showing where they fit betwixt sender and recipient.

As nosotros have seen, the original SMTP was designed as a simple host-to-host protocol. A user on one host created a message with a program chosen sendmail or mail and this text was sent directly to the destination host using SMTP as a Mail service Transfer Agent (MTA). Of course, if the remote user was not running an email server process to take the SMTP session, there was aught for the sender to do just go along trying.

Modern electronic mail systems "decouple" the sender from the receiver so that email notwithstanding goes through, even when the recipient is away for two weeks (simply the messages keep piling up, just like regular mail). In add-on, unlike almost every other TCP/IP application, electronic mail operates non from host to host only from user to user. This means that users are not required to receive email on a item host, nor is a particular host expected to have only ane user with email capabilities. (We tin can even pick upwardly email for a recipient from the sending host, and we'll do that later.) This user "mobility" poses special challenges for email addressing, which is why more than just a host proper noun is required for correct email commitment.

The solution, of course, is to add another level to the hostname, this i identifying a particular user. Then, for case, [email protected] indicates a different mail destination than [e-mail protected]. And, in fact, the actual host on which an electronic mail user is defined is not always added to the email address (which would yield something like [email protected]). The email protocols all mesh together to make this piece of work.

There are older email address formats—FIDOnet, UUCP, e-mail gateways (distinguished by the use of user% notations), and then on—but these are only of historical interest today. This is not to say that the development of email is uninteresting, just that the history can be given very briefly and the discussion can turn to what is actually washed with email on the Cyberspace today.

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Service Overlays

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

Internet Routing and Internet access provider Peering

The Cyberspace is a collection of many different concrete networks that employ common protocols and services to create a universal routing and send cloth for applications. Generally, the different networks are operated every bit separate autonomous systems (ASs) by their network service providers (NSPs). Inside an As, an NSP will apply routing protocols that maintain detailed connectivity information for all routers in the AS. At the boundaries of the AS are connections to other NSPs' networks at connection points called peering points. The routers at these peering points employ the Border Gateway Protocol (BGP), which maintains information about BGP topology in the Internet and typically has limited details about routing within a given As. A consequence is that network traffic that is routed through an AS volition have a restricted path. Such transit paths are adamant by each NSP according 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, 1 issue of BGP routing policies is that some routes cause violations in the triangle inequality property. That is, the direct path from A to B may be longer in terms of latency than the indirect path A-C-B. Some other consequence is that road outages due to router failures may not be automatically rerouted since the necessary routing table information may exist 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 incorporate the fields shown in Tabular array half dozen-1.

Table vi-ane. Protocol and payload fields

Protocol ID (16 bits) Information or payload (Variable length) Padding (Variable length)

Each PPP packet is preceded by a protocol identifier, a list of common protocols relevant to embedded applications is shown in Table half-dozen-ii. Sure rules exist for protocol type numbering. All first bytes must be even, and all 2nd bytes must be odd. That is, the to the lowest degree significant scrap of the least pregnant byte must be 1, and the least meaning bit of the most pregnant byte must be a nil.

Table 6-2. Mutual protocols relevant to embedded applications

Protocol ID Description
0021 Internet protocol (IP)
002D VJ compressed IP
002F VJ uncompressed IP
C021 Link control protocol (LCP)
C023 Password authentication protocol (PAP)
C025 Link quality written report
C223 Claiming handshake authentication (CHAP)

Protocol field values in the 0000–3FFF range are used to identify the network layer protocol in apply, for example, 0021 for IP. Protocols in the range 8000–BFFF place the network control protocol, and protocols in the range C000–FFFF are link command protocols. The payload field carries the actual data to be passed on. The minimum length is null. The maximum length in bytes (including padding, but excluding the protocol field) is divers by the variable maximum receive unit (MRU). This variable is negotiated during link setup, and the default value is 1500. This makes PPP more than or less size 'compatible' with Ethernet frames. The padding field may carry whatever number of bytes up to the MRU value (ordinarily zeros), these bytes will be ignored at the receiving finish. Padding is usually used to sew a sequence to a give number of bytes.

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Passwords, Vulnerabilities, and Exploits

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

Authentication Protocols

The protocols used for authenticating identity depend on the hallmark blazon. Some common protocols used for hallmark include the following:

▪

Kerberos The default logon hallmark protocol used past Windows 2000, XP, and Vista, as well every bit past Windows Server 2003 and Windows Server 2008. Information technology is also used past Mac Os 10. This protocol is based on secret primal (symmetric) cryptography, which we'll discuss in Chapter 12. This system uses tickets that a central server issues to determine whether a user can access the network and its resource. 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 access.

▪

Challenge Handshake Hallmark Protocol (CHAP) Uses a sequence in which 1 party sends a challenge and the other responds with an respond. The most common course of this sequence is the server requesting a countersign, which the client provides to gain admission to a organisation. Microsoft developed its own version of the protocol, called MS-CHAP.

▪

NT LAN Manager (NTLM) Some other Microsoft logon hallmark 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'southward identity verified. The client doesn't have credentials already issued (the session ticket in Kerberos) that the file or application server knows information technology can trust.

▪

Password Authentication Protocol (PAP) A remote access authentication protocol used for Point-to-Point (PPP) or dial-up connections. Its distinguishing characteristic (and the reason it should not exist used on secure networks) is the fact that it sends passwords in plain text. This ways an unauthorized person can intercept and use the passwords during transmission. The only 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 (Due south-PAP) addresses this problem by using a two-style reversible authentication method that encrypts passwords so that they will non exist subject field to interception and misuse.

▪

The Remote Authentication Dial-in User Service (RADIUS) Another means of authenticating remote connections that takes the authentication responsibleness off each individual remote access server by providing a centralized server to authenticate clients securely.

▪

Secure Crush (SSH) Allows users to log on to UNIX systems remotely. Both ends of the connection (client and server) are authenticated, and data—likewise as passwords—tin can exist encrypted.

Note

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

On the Scene

Identity Confirmed; Now What?

In one case a user's identity has been established, the adjacent stride in the security process is authorization, which is concerned with what that user is permitted to exercise. Hallmark and authority work together to provide a security organization that takes into account the need for different users to take different capabilities on the network.

Administrators tin control which files and other objects a user can access and the level of access (read simply, change, and and then on) by setting permissions. Well-nigh 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 equally New Engineering File Organization (NTFS) provide for two levels of permissions: share permissions that apply only to users accessing the resource across the network, and file-level permissions (too chosen NTFS permissions) that apply both across the network and to users accessing the resources from the local motorcar.

Administrators tin can also command which arrangement-wide deportment a item 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 Resources on the Web

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

W3C.org

XML was developed past the W3C. The W3C was created in 1994 to develop mutual protocols and languages for the Spider web. It comprises over 400 member organizations. On the W3C Web site, you tin can discover more than information about the W3C and extensive information on XML, including new standards, the complete specifications, and the status of all the XML languages. You can also come across the W3C press releases and merely nearly any other information you tin can think of.

Biztalk.org

Biztalk is a community of standards users that is supported past a wide range of organizations. Biztalk has 2 core problems: starting time, that application integration is as well complicated and costly; second, that the next phase of the Internet volition crave much greater application integration across disparate systems. Biztalk members are adopting an XML message-passing compages that is platform neutral to tie 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 nigh using XML in industrial and commercial applications. Information technology also provides some tools to assistance you determine how to use XML for your business. Information technology is a vendor-independent organization to assist with universal data exchange across all vendors. XML.org is hosted past the Organisation for the Advancement of Structured Data Standards (OASIS). Oasis is an organization that creates interoperability manufacture specifications based on public standards such as XML.

MSDN.Microsoft.com/XML

You tin can obtain Microsoft-specific data about XML from the visitor's MSDN Web site, which shows highlights of new tools and features. It besides contains the consummate documentation for the XML SDK, sample code, downloads, and more. This site contains extensive information on using XML with Microsoft products.

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The Fundamentals in Understanding Networking Middleware

Tammy Noergaard , in Demystifying Embedded Systems Middleware, 2010

iv.6.2 File Transfer Protocol (FTP) Client Application Instance

RFC959, File Transfer Protocol (FTP), is one of the simpler and more common protocols implemented within an embedded organisation that is used to deeply substitution files over a network. The FTP protocol is based on a communication model in which there is an FTP client, also referred to every bit a user-protocol interpreter (user PI) that initiates a file transfer, and an FTP server or FTP site that manages and receives FTP connections. As shown in Figure four.48, the types of connections that be betwixt an FTP client and server are:

Figure iv.48. FTP Network
•

command connections, which are connections in which commands are transmitted over

•

data connections, which are connections in which files are transmitted over.

FTP clients start FTP sessions past initiating a control connexion to a destination organization with an FTP server. This FTP control connectedness is based on a TCP connection to port 21, considering FTP requires an underlying ship layer protocol that is a reliable, ordered information stream aqueduct. When FTP client and server communicate over a control connection, they do and then via the interchange of commands and reply codes, such every bit some of the codes shown in Table 4.11.

Table iv.xi. Examples of FTP Commands and Reply Codes ane

Type Code Definition
Command DELE Delete. FTP service command
Way Transfer Mode. Transfer parameter command
PASS Password. Access control command
PORT Data Port. Transfer parameter control
QUIT Logout. Access command command
TYPE Representation Type. Transfer parameter command
USER Username. Admission control command
Answer Code 110 Restart marker respond
120 Service ready in '10' minutes
125 Data connectedness already open up
150 File status OK
200 Control OK
202 Command NOT implemented
211 Organisation Help

Figure 4.49 is an open source instance of FTP functions, and how this source lawmaking utilizes a required underlying networking middleware layer such every bit TCP socket-related office calls.

Effigy 4.49. FTP Open Source Example thirteen

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Networking

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

vii.two.2.4 RPC-Based Services

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

Bug with RPC-based services may be caused by issues with the underlying RPC mechanism, which in turn commonly occur if the portmap(8) daemon (also called the "portmapper") is not running or not functioning properly. The portmapper is the "traffic cop" that accepts incoming RPC requests and hands them off to the appropriate subdaemons. In this manner, portmap functions very much like the inetd daemon. However, the portmapper listens on only 1 TCP/IP port and uses a programme identifier to keep rail of its subdaemons. This conserves the use of reserved TCP/IP ports on the system.

The programs that portmap volition listen for are listed in the /etc/rpc file. When these daemons first up, they register with the portmapper. If you lot are trying to observe out why a particular plan is not communicating, check this file to verify that the plan is properly listed and has a unique program number assigned. If so, employ the rpcinfo(viii) command to verify that the subdaemon is running and has registered itself with the portmapper. To display the daemons registered with the portmapper on the local host, apply the following control:

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

This will tell y'all that not only is the programme properly registered, the portmapper is responding to its requests. This is a good sign that portmap is working properly.

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Technology & Product Architectures

Rick Sherman , in Business concern Intelligence Guidebook, 2015

Data Admission APIs

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

•

Open Database Connectivity (ODBC): standard middleware API used to access databases. ODBC was developed to be database and operating system contained, replacing database-specific APIs.

•

Java Database Connectivity (JDBC): data admission API used by Java programming languages.

•

XML for Assay (XML/A): manufacture standards for accessing analytical data sources such as OLAP and information mining. Information technology is based on industry-standard Extensible Markup Language (XML), Lather, and HTTP. Information technology tin execute several query languages such equally Multidimensional eXpressions (MDX), SQL, Information Analysis eXpressions (DAX), and Data Mining eXtensions (DMX).

•

XQuery: query linguistic communication used to extract and transform data from XML data sources and Simple Object Access Protocol (SOAP) and Representational State Transfer (REST) web services. At that place is a diversity of structured and unstructured information sources that tin exist viewed every bit XML.

•

Native database or data APIs: these APIs access specific data or database sources whose providers may feel these APIs provide meliorate functionality than industry standard APIs, or that they cannot conform to those standards.

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Bone X El Capitan forensics

Shawn Jordan , in Digital Forensics, 2016

.ssh

The {user name}/.ssh file contains all the SSH keys for the user. SSH is a common protocol used for remotely accessing a organisation. SSH depends on individual and public keys to provide encrypted communication. Fig. 7.7 shows the user'south known_hosts file, which contains the addresses used with SSH and the private keys associated with the connectedness. It volition also tell you the encryption method used to generate the keys. Merely systems accessed will 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 admission, and this directory could show all systems accessed.

Figure vii.vii. SSH Information Including IP, Keys, and Encryption Method

The known_host file contains all the SSH connections for the user. This will include domains or IP addresses of connections. The file contains the method of fundamental encryption, as well every bit the individual key for this machine.

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Time and Frequency

Michael A. Lombardi , in Encyclopedia of Physical Science and Engineering science (Third Edition), 2003

Iv.C.1 Cyberspace Fourth dimension Signals

Cyberspace time servers employ standard timing protocols defined in a series of RFC (Request for Comments) documents. The iii near common protocols are the Time Protocol, the Daytime Protocol, and the Network Time Protocol (NTP). An Internet fourth dimension server waits for timing requests sent using whatsoever 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 most client software is compatible with either the Daytime Protocol or the NTP. Client software that uses the Simple Network Time Protocol (SNTP) makes the same timing asking as an NTP client but does less processing and provides less accuracy. Table X summarizes the various protocols and their port assignments, or the port where the server "listens" for a customer request.

TABLE X. Net Time Protocols

Protocol name Certificate Format Port assignment(s)
Fourth dimension protocol RFC-868 Unformatted 32-bit binary number contains time 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 code is sent as ASCII characters Port 13, tcp/ip, udp/ip
Network time protocol (NTP) RFC-1305 The server provides a data packet with a 64-scrap time stamp containing the time in UTC seconds since Jan 1, 1900, with a resolution of 200   psec. NTP provides an accurateness of 1 to 50   msec. The client software runs continuously and gets periodic updates from the server. Port 123, udp/ip
Simple network fourth dimension protocol (SNTP) RFC-1769 The data packet sent by the server is the same as NTP, just the customer software does less processing and provides less accuracy. Port 123, udp/ip

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

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