Common Protocol

SMTP and E-mail

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

Email Access and Reading

The recipient'due south email application checks in periodically with the local SMTP server to see if whatever post has arrived. This checking tin can be either automatic or when specifically run. If there is mail, the user can retrieve the mail, open up it, and read it, and delete it. Usually, these are all separate steps. This step does not use SMTP, but a special post access method and protocol such as POP3 or IMAP4 (both are used by TCP/IP MAAs).

All v of these steps are not always necessary. Some hosts deed equally mail service servers all on their own, and the host-local-mail-server communication steps can be bypassed. Punch-in users often compose, ship, and receive e-mail all at one time when they send mail service. But usually all v steps are needed.

4 devices are involved in the five steps. They are the sender'due south client, the sender's local SMTP mail server, the recipient'south local SMTP mail server, and the recipient's client. The relationship they accept with one some other and the protocols the email uses are shown in Figure 25.four. Notation the symmetrical nature of the components so that two-style communication is possible.

Effigy 25.4. Email protocols and components, showing the components used to send an e-mail message. Notation the symmetrical nature of the sender and recipient so that the receiver can respond.

Email Protocols

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

Although non a protocol, at that place is a series of Multipurpose Internet Postal service Extensions (but MIME, never "MIMEs") for various types of electronic mail attachments (not but elementary text). Finally, a number of related specifications add together authentication to the basic email protocols. The way the protocols fit together is shown in Figure 25.5.

Figure 25.5. Electronic mail protocols, showing where they fit between sender and recipient.

Every bit nosotros take seen, the original SMTP was designed as a simple host-to-host protocol. A user on one host created a message with a programme called sendmail or mail and this text was sent directly to the destination host using SMTP as a Mail Transfer Agent (MTA). Of course, if the remote user was non running an email server process to accept the SMTP session, in that location was cypher for the sender to do simply keep trying.

Modernistic email systems "decouple" the sender from the receiver so that e-mail still goes through, even when the recipient is abroad for two weeks (but the messages continue piling upwards, simply similar regular mail). In add-on, unlike about every other TCP/IP application, e-mail operates non from host to host but from user to user. This means that users are not required to receive email on a detail host, nor is a particular host expected to have just one user with email capabilities. (We can even option upward email for a recipient from the sending host, and we'll practise that after.) This user "mobility" poses special challenges for email addressing, which is why more than than just a host name is required for correct email delivery.

The solution, of course, is to add another level to the hostname, this one identifying a particular user. So, for case, [e-mail protected] indicates a dissimilar postal service destination than [email protected]. And, in fact, the actual host on which an email user is divers is non 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 so on—but these are only of historical interest today. This is not to say that the evolution of email is uninteresting, simply that the history can be given very briefly and the give-and-take 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 Internet access provider Peering

The Internet is a collection of many dissimilar concrete networks that apply common protocols and services to create a universal routing and send fabric for applications. Generally, the different networks are operated every bit divide autonomous systems (Ass) by their network service providers (NSPs). Inside an Equally, an NSP will use 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 apply the Edge Gateway Protocol (BGP), which maintains information about BGP topology in the Internet and typically has limited details well-nigh routing within a given AS. A consequence is that network traffic that is routed through an AS will 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.

Every bit discussed in Chapter 10, one consequence of BGP routing policies is that some routes crusade violations in the triangle inequality property. 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 non be automatically rerouted since the necessary routing tabular array information may be missing from the BGP tables.

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Information Link Direction

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

PPP protocol field

At the framing level, the protocol and payload contain the fields shown in Table six-1.

Table 6-1. Protocol and payload fields

Protocol ID (16 $.25) Data or payload (Variable length) Padding (Variable length)

Each PPP packet is preceded past a protocol identifier, a list of common protocols relevant to embedded applications is shown in Table 6-2. Certain rules exist for protocol type numbering. All commencement bytes must be even, and all 2nd bytes must be odd. That is, the least significant bit of the least significant byte must exist one, and the least significant flake of the most meaning byte must exist a zero.

Table 6-2. Common protocols relevant to embedded applications

Protocol ID Clarification
0021 Net protocol (IP)
002D VJ compressed IP
002F VJ uncompressed IP
C021 Link command protocol (LCP)
C023 Password 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 command protocols. The payload field carries the actual data to be passed on. The minimum length is zero. 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 or less size 'uniform' with Ethernet frames. The padding field may carry any number of bytes up to the MRU value (usually zeros), these bytes will be ignored at the receiving end. Padding is normally used to run up a sequence to a requite 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 authentication blazon. Some mutual protocols used for authentication include the following:

▪

Kerberos The default logon authentication protocol used past Windows 2000, XP, and Vista, as well equally by Windows Server 2003 and Windows Server 2008. Information technology is also used past Mac OS X. This protocol is based on secret key (symmetric) cryptography, which we'll discuss in Chapter 12. This arrangement uses tickets that a central server issues to determine whether a user tin can access the network and its resources. Rather than existence used to log on to each server, the tickets are used by all of the servers to make up one's mind what a user is permitted to access.

▪

Challenge Handshake Authentication Protocol (CHAP) Uses a sequence in which one 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 access to a system. Microsoft developed its own version of the protocol, called MS-CHAP.

▪

NT LAN Manager (NTLM) Some other 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 cosign the user. Unlike Kerberos, with NTLM, when a client wants to access a server's resource, that server must contact the domain controller to have the client'southward identity verified. The client doesn't have credentials already issued (the session ticket in Kerberos) that the file or application server knows it can trust.

▪

Countersign Authentication Protocol (PAP) A remote access hallmark protocol used for Point-to-Point (PPP) or dial-up connections. Its distinguishing characteristic (and the reason information technology should not be used on secure networks) is the fact that it sends passwords in patently text. This ways an unauthorized person can intercept and employ the passwords during transmission. The only skilful reason to use PAP is if you face up a situation in which the remote server doesn't support other, more than secure authentication methods. Shiva PAP (S-PAP) addresses this problem by using a two-mode reversible authentication method that encrypts passwords so that they will not be discipline to interception and misuse.

▪

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

▪

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—tin be encrypted.

Annotation

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

On the Scene

Identity Confirmed; Now What?

One time a user's identity has been established, the next step in the security procedure is say-so, which is concerned with what that user is permitted to do. Hallmark and authorization work together to provide a security arrangement that takes into business relationship the need for different users to accept different capabilities on the network.

Administrators can command which files and other objects a user can access and the level of admission (read simply, change, and and so on) past setting permissions. Most network operating systems provide a machinery for associating specific permissions on an object with certain user accounts or groups. For example, Windows computers that take hard disks formatted equally New Applied science File Organization (NTFS) provide for two levels of permissions: share permissions that apply only to users accessing the resource beyond the network, and file-level permissions (also called NTFS permissions) that apply both across the network and to users accessing the resource from the local machine.

Administrators tin also command which organization-broad actions a item user (or group of users) tin can perform by setting user rights. User rights differ from permissions in that permissions apply 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 resources for XML. The official standards for XML are developed by the W3C. Biztalk, an industry initiative started by Microsoft, is a customs 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 can detect more data almost the W3C and extensive information on XML, including new standards, the consummate specifications, and the status of all the XML languages. You can too see the W3C press releases and simply about whatsoever other information you tin can think of.

Biztalk.org

Biztalk is a customs of standards users that is supported by a broad range of organizations. Biztalk has two core problems: start, that application integration is besides complicated and costly; second, that the adjacent phase of the Cyberspace will require much greater application integration across disparate systems. Biztalk members are adopting an XML bulletin-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 resource virtually using XML in industrial and commercial applications. It too provides some tools to assist y'all make up one's mind how to utilize XML for your business concern. It is a vendor-independent organization to help with universal information substitution across all vendors. XML.org is hosted past the Organization for the Advancement of Structured Information Standards (Oasis). Oasis is an system that creates interoperability industry specifications based on public standards such as XML.

MSDN.Microsoft.com/XML

Y'all can obtain Microsoft-specific information about XML from the company's MSDN Web site, which shows highlights of new tools and features. It also contains the consummate documentation for the XML SDK, sample code, downloads, and more than. 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.ii File Transfer Protocol (FTP) Client Application Example

RFC959, File Transfer Protocol (FTP), is i of the simpler and more common protocols implemented within an embedded system that is used to securely exchange files over a network. The FTP protocol is based on a communication model in which there is an FTP client, 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. As shown in Figure four.48, the types of connections that exist between an FTP client and server are:

Figure 4.48. FTP Network
•

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

•

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

FTP clients starting time FTP sessions by initiating a control connection to a destination system with an FTP server. This FTP control connexion is based on a TCP connection to port 21, considering FTP requires an underlying transport layer protocol that is a reliable, ordered data stream aqueduct. When FTP client and server communicate over a control connection, they do and so via the interchange of commands and reply codes, such as some of the codes shown in Tabular array 4.11.

Table 4.xi. Examples of FTP Commands and Reply Codes one

Type Code Definition
Command DELE Delete. FTP service command
MODE 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 control
USER Username. Access command command
Reply Lawmaking 110 Restart marking answer
120 Service ready in 'x' minutes
125 Information connection already open
150 File status OK
200 Command OK
202 Command Non implemented
211 Arrangement Help

Figure iv.49 is an open source example of FTP functions, and how this source code utilizes a required underlying networking middleware layer such every bit TCP socket-related function calls.

Figure 4.49. FTP Open Source Example 13

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Networking

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

7.2.2.4 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 Arrangement (NFS) and the Network Information Service (NIS). In addition, the daemons used in HP's TruCluster product are heavy users of the RPC mechanism.

Problems with RPC-based services may exist caused by problems with the underlying RPC mechanism, which in turn usually occur if the portmap(viii) daemon (too 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 fashion, portmap functions very much like the inetd daemon. However, the portmapper listens on simply ane TCP/IP port and uses a programme identifier to go along track of its subdaemons. This conserves the use of reserved TCP/IP ports on the organisation.

The programs that portmap volition listen for are listed in the /etc/rpc file. When these daemons start up, they annals with the portmapper. If you are trying to discover out why a detail program is not communicating, check this file to verify that the plan is properly listed and has a unique programme number assigned. If and then, apply the rpcinfo(eight) command 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, apply the following control:

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

This will tell you that not just is the program properly registered, the portmapper is responding to its requests. This is a skilful sign that portmap is working properly.

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Applied science & Production Architectures

Rick Sherman , in Business Intelligence Guidebook, 2015

Data Access APIs

Many data access Application Programming Interfaces (APIs) are used in data access and data integration. The almost 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 access API used by Java programming languages.

•

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

•

XQuery: query language used to extract and transform data from XML information sources and Elementary Object Admission Protocol (Lather) and Representational Land Transfer (Remainder) web services. In that location is a variety of structured and unstructured data sources that can be viewed equally XML.

•

Native database or data APIs: these APIs admission specific information or database sources whose providers may experience these APIs provide amend functionality than industry standard APIs, or that they cannot suit to those standards.

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OS 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 system. SSH depends on private 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. Information technology will besides tell you the encryption method used to generate the keys. But systems accessed will appear in the .ssh directory. This would exist particularly useful during an investigation of a network alienation. SSH is a mutual protocol to use for remote access, and this directory could show all systems accessed.

Effigy 7.vii. SSH Data 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, also as the private key for this machine.

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Time and Frequency

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

4.C.1 Internet Time Signals

Cyberspace time servers utilise standard timing protocols defined in a series of RFC (Request for Comments) documents. The three nearly 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.

Customer software is available for all major operating systems, and most customer software is compatible with either the Daytime Protocol or the NTP. Client software that uses the Elementary Network Time Protocol (SNTP) makes the same timing request as an NTP client only 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 client request.

TABLE Ten. Cyberspace Time Protocols

Protocol name Document Format Port assignment(south)
Fourth dimension protocol RFC-868 Unformatted 32-scrap binary number contains time in UTC seconds since January one, 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 xiii, tcp/ip, udp/ip
Network time protocol (NTP) RFC-1305 The server provides a data packet with a 64-flake time stamp containing the fourth dimension in UTC seconds since Jan i, 1900, with a resolution of 200   psec. NTP provides an accuracy of 1 to fifty   msec. The client software runs continuously and gets periodic updates from the server. Port 123, udp/ip
Simple network time protocol (SNTP) RFC-1769 The data parcel sent past the server is the same equally NTP, but the client software does less processing and provides less accurateness. 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 tin can be obtained from the NIST Fourth dimension and Frequency Sectionalisation web site: http://www.boulder.nist.gov/timefreq. The uncertainty of Internet time signals is ordinarily <100   msec, only results vary with dissimilar computers, operating systems, and client software.

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