"D" Networking Definitions & Concepts...

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In digital signaling, D4 framing is a method for identifying the individual channels in a DS1 channel.

D4 framing groups twelve 193-bit frames into one D4 superframe so that DS1 channel consists of two D4 superframes.

Within each D4 superframe, the values in every one hundred ninety-third bit -- in bits 193, 386,and so on -- are used to identify the individual (DS0) channels. Also in each D4 superframe, the eighth bit in every channel of frames 6 and 12 is used for signaling between central offices. Compare with ESF (Extended Superframe Format) framing.

A program running in the background on a Unix and other systems. A daemon performs a single task the entire time that it is running. Usually, it is a program that begins executing automatically when a predefined event occurs. Daemons (pronounced "demons") are common in the OS/2 and UNIX environments and are used in artificial intelligence work. Certain terminate and stay resident (TSR) programs in a DOS environment behave like daemon programs.

Dark fibre or unlit fibre (or fiber) is the name given to fibre optic cables which have yet to be used. They are hence not yet connected to any device, and are only there for future usage.

The term was originally used when talking about the potential network capacity of telecommunication infrastructure, but now also refers to a form of telecommunication network product that is purchased by network operators from fibre providers.

Database programming begins with the database. To build effective database applications, you need to fully appreciate the work the database does for those applications. This first section addresses the best practices in data architecture, the design of relational database elements that support database applications.

Once upon a time, database programming on the Java platform was an exercise in native programming; nothing existed within the Java platform to support database programming efforts. The first tool in the database programming arsenal arrived in March 1996 in the form of Java's first proposed enterprise API, JDBC. JDBC enabled application developers to use a single API to access any database from any vendor.

JDBC, however, is the start, and not the end, of database programming. JDBC simply enables you to access a database; it does not address all elements of database programming. It does not:

• Ensure your database meets the need of your application,
• Automate the mapping of Java classes into relational entities,
• Provide a model for structuring your Java components,
• Manage application transactions.

This writeup is about database programming; it is not about JDBC. However, because JDBC plays such a critical role in database programming, it will play a critical role in this book. If you need to brush up on your JDBC skills, take a look at the tutorial in Chapter 11, in the book, Database Programming with JDBC and Java (O'Reilly). This book addresses all of the elements of database programming and their respective roles in supporting real world database applications.

Database applications require an entire network of software in order to function. Even the most basic of database applications, the command line SQL tool, is a complex system involving the database engine and a separate client utility. Architecture is the space in which all of the elements of an application operate. Before we look at each of those elements, we should first take a look at the space itself.

Architecture identifies the hardware and software necessary to support an application and specifies how those tools communicate within a network. When referring to architecture, different people tend to have different things in mind. In some cases, architecture refers to the way hardware is placed on a physical network. This kind of architecture is called network architecture. Other times, however, architecture refers to the system architecture, the way different logical and physical components work together to create a complex network application. The last kind of architecture is software architecture, when architecture refers to the design of one of the pieces of software that make up the system architecture.

The network architecture focuses on hardware issues and how they connect to one another. The quality of your network architecture affects security and bandwidth and limits the ability of your applications to talk with different parts of the system. The Figure below is a simple network architecture diagram.

It shows how the network separates the Internet from the network in which the web server runs with a firewall. Similarly, this network diagram places the database server in a separate network segment, again separated by a firewall. In spite of the fact that very little about network architecture is specific to database applications, it can make a significant impact on the performance of those applications. It is therefore helpful to understand those aspects most relevant to database systems.

There are five major requirements for a data communication network:

• Performance --

  A data communication network must deliver data in a timely manner. Performance is typically measured by the network response time. Response time is normally consider the elapsed time between the end of an inquiry to the network and the beginning of the of the response from the network or system. The response time of a communications network must match the expectations of the users. Many factors affect the response time of a network. Some of these factors are:

• Number of users on the network system
• Transmission speeds
• Type of transmission medium, and
• The type of hardware and software being employed

• Consistency -- Predictability of response time, accuracy of the data transmitted, and mean time between failures (MTBF) are important factors to consider when choosing a network. Inconsistency of response time is annoying to users, and sometimes it is worse than a slow but consistent response time (i.e., a system that response the same all the time). Accuracy of data is important if the network is to be deemed reliable. If a system loses data, then the users will not have confidence in the information generated and transmitted by the system.
• Reliability -- Network failure is any event that prohibits the user from processing transactions. Network failure can include a breakdown in hardware, the data carrying medium, and/or the network controlling software, i.e., the network operating system (NOS). The mean time between failures (MTBF) is a measure of the average time a component is expected to operate between failures. This can consist of the components within the hardware and/or the hardware itself as a component of the network. The combined components in the hardware are used to calculate the total MTBF for that piece of hardware, which is then used to calculate the total system MTBF.
• Recovery -- All networks are subject to failure. After a failure, the network must be able to recover to a prescribed level of operation. This prescribed level is a point in the network operation where the amount of lost data is nonexistent or a minimum. Recovery procedures and the extent of recovery will depend on the type of hardware and software that control the network.
• Security -- Network security is another import component in communication networks, especially when computer data is involved. A business's data must be protected from unauthorized access. Therefore, companies are placing more stringent security measures on networks in order to safeguard their data. When a communications network is being designed, security must be carefully considered and incorporated into the final design.

The layer of the OSI (Open Systems Interconnection) Reference Model that defines protocols governing data packets and transmissions, controlling the mapping of the logical to physical addresses. See the OSI Reference Model.

Are information units in connectionless protocols, which are similar to telegrams, in that they usually form self-contained messages. Thus, these information units are also called datagrams. Put simply, it is part of the Internet Protocols, which has a packet containing destination address and data.

Another way to look at a datagram is that it is a self-contained packet independent of other packets in a data stream. Since a datagram carries its own routing information (i.e., destination address), its reliable delivery does not depend on earlier exchanges between the source and destination devices. DDP (Datagram Delivery Protocol) is responsible for delivering AppleTalk transmissions as datagrams.

The AppleTalk Datagram Delivery Protocol (DDP) is responsible for ensuring delivery of datagrams between AppleTalk sockets.

A unique, four-digit value assigned to public networks and to services on those networks.

The Data Warhouse: A Place for your used data (circa 1995)

The Data Warhouse: Needs to have a Corporate Strategy:

An information management strategy in which a company's information is all accessible through a single database. The corporate information may come from many sources and departments, may come in a variety of forms, and may be stored at different levels of detail. Corporate information includes such things as product, customer, and other "departmental" databases; sales, inventory, and other transaction data; archival, or legacy, data, and so forth.

The data warehouse will also contain meta-data, which is information about the general organization of the warehouse, i.e. data about the data structure, the format and location of the various materials in the warehouse, the operations or uses allowed for various items, and possibly connections between data items. The meta-data needs to be updated whenever the actual data is changed.

The Data Warhouse: Must be easy to use:

The warehouse contents may be distributed over various hosts and locations, but should be accessible in a transparent access of the entire corporate database with simple commands that makes data warehousing so attractive. By making the entire database accessible, it becomes easier to spot trends, coordinate updates, and generally keep the data organized and consistent.

The Data Warhouse: Must be Secure:

Access to the data warehouse always assumes user authorization. That is, the integration of various databases should not make it possible for users to get access to data that were off limits before warehousing. Warehouse data should be accessible to authorized users in raw form or for analyses, and the necessary retrieval and analysis tools should be part of the data warehouse system.

The Data Warhouse: Is temporal and forever changing:

Warehouse data will vary in level of detail, or granularity. Current data, which is more likely to be active and in flux, will be more detailed (finer-grained) than older materials, which may be just summary data. Other types of data may lie between these two extremes.

The material in a data warehouse need not all be online all the time. Dormant (or, at least napping) materials may be stored on secondary media (such as tapes or companct discs), which may need to be mounted before users can make use of it. For these materials to belong to the data warehouse, it's only necessary for the meta-data (data about the data) to include information about these materials and their location.

A Data Warehousing System

A complete data warehousing system should have resources for:

• Defining and organizing the warehouse contents, and storing this as meta-data.
• Acquiring, displaying, and distrbuting data.
• Managing and overseeing both the data and the warhouse operations.
• Displaying information about the warehouse contents and organization.
• Analyzing and manipulating the data.

The advantages of data warehousing are many, as are the obstacles. One of the major issues that must be considered is how to organize and connect very heterogerneous (different) information sources. The degree to which updates and reorganizations can be automated will depend strongly on the quality of the basic organization.

The Data Warhouse: A Place for your used data (circa 1997) from Alan R. Simon; Data Warehousing for Dummies

A data warehouse is a home for "secondhand" data that originates in either other corporate applications, such as the one your company uses to fill customer orders for its products, or some data source external to your company, such as a public database that contains sales information gathered from all your competitors.

Data warehousing is the coordinated, architected, and periodic copying of data from various sources, both inside and outside the enterprise, into an environment optimized for analytical and informational processing.

Data warehousing is also, therefore, the process of creating an architected information-management solution to enable analytical and informational processing despite platform, application, organizational, and other barriers.

The Data Warhouse: A Place for your used data (circa 2001) from W.H. Inmon; Corporate Information Factory

It is a collection of integrated subject-oriented databases designed to support the DSS (Decission Support System) function, where each unit of data is relevant to some moment in time. The data warehouse contains atomic data and lightly summarized data.

It is a subject-oriented, integrated, time-variant (temporal), and nonvolatile collection of summary and detailed data used to support the strategic decision-making process for the entire company.

Here is another definition for Data Warehousing . In which the view for data warehousing is more for varification of decisions already made, i.e., Data warehousing is not necessarily for the needs of "decision makers" or used in the process of decision making. It is more operational in nature.

DECnet is Digital's routable communications protocol, which supports ethernet-style LAN's and baseband and broadband WANs over private and public lines. It interconnects PDPs, VAXs, PCs, Macs, and workstations.

In reference to networks, is the process of separating a descendant of a zone into a separate zone. The delegation is accomplished with NS records and if necessary, A records. NS records used for this purpose are called "delegation records", A records used for this purpose are called "glue records."

Records in a delegation are an exception to the rule that a record should only be defined only in the zone that owns the name of the record.

Demodulation is the opposite of modulation, which is the process of changing a carrier signal so that it carries information. For example, an AM radio broadcast modulates a carrier signal located at the frequency to which you tuned the dial. The modulation of the signal is converted to audible sound waves by the receiving device.

In computer communication, a sending modem (modulator/demodulator) modulates a digital signal into an analog wave for transmission over a voice telephone line. A modem at the receiving end demodulates the signal back into digital information.

A denial-of-service (DoS) attack is any of certain forms of malicious damage to computer systems. The aim of such an attack is to prevent legitimate users from accessing their services.

A DoS attack is generated in a number of ways. There are three basic areas of attack - the consumption of limited resources, such as bandwidth, disk space or CPU time; alterations to configuration information, such as routing information or registry entries; and the physical disruption of networking components.

The attack on resources has become increasingly popular, mainly through attempts to "flood" a network with excess or spurious packet data over the internet, thereby preventing legitimate traffic. These attacks are, or could be very costly to businesses that conduct transactions on the internet.

This is part of FLIP (Fusebox Lifecycle Process), a simple threaded messaging system that sits at the bottom of each prototype page and lets users and developers converse about the project.

Similar to nslookup. Another command line tool for querying DNS servers. Somewhat unwieldy, bundled with BIND. Like all Unix terms, dig is case sensitive and must always be used in lower case.

A digital signal uses distinct levels to represent binary ones or zeros and is the most common signaling method used on LANs and for high-speed WANs. For example, the presence of voltage, such as +5 volts, might represent a one state, and 0 volts might represent a zero state. Also, a digital signal is a representation of information which can take on only a specific set of values, and whose values exist only at specific time instants.

A data link protocol used in the DECnet architecture. DDCMP is used for point-to-point links between nodes in either asynchronous or synchronous modes. See asynchronous communication and synchronous communication.

Direct memory access is a method for transferring data from a drive or other peripheral device directly to the computer's memory, without involving the CPU (central processing unit).

The DMA process is managed by a specialized DMA controller chip, which is generally faster than the processor. When the data transfer is finished, the controller chip informs the processor, which can then proceed as if the processor had managed the transfer. Each DMA controller can handle up to four devices.

In wireless communications, an electromagnetic signal that is transmitted through the air, but low enough to reach the destination without being reflected off the earth or off the ionosphere. A direct wave requires a line of sight between sender and receiver.

Software that serves as the interface between the operating system and the hard disk; also known as a disk interface driver. The network vendor usually includes drivers for the most common types of hard disks (ESDI (Enhanced Small Device Interface), SCSI (Small Computer System Interface), and IDE (Integrated (or Intelligent) Drive Electronics)), and the hard disk manufacturer many include drivers for specific network operating systems.

A distributed denial-of-service (DDoS) attack, which is a DoS attack where many computers work in unison to attack a target system, has also gained notoriety due to the efficient tools which are available to create and launch such an attack.

One particular variant of a DoS attack on the public Internet is a smurf attack. This attack relies on mis-configured network devices that respond to so-called broadcast addresses. Abusers will send large numbers of IP packets with a faked source address (the source address is set to the address of a victim, such as an IRC server). To combat Denial of Service problems on the Internet, services like the Smurf Amplifier Registry have given Network Service Providers the ability to identify mis-configured networks and take appropriate action such as filtering.

DDoS attacks are comparable to the effect that occurs when a website gets a sudden spike in traffic due to a popular website linking to it which its server cannot handle.

A term for a network in which processing capabilities and services are distributed across the network, as opposed to being centralized in a single host or server.

The DMZ network is the intermediary network between the public Internet and an ISP's/Companies internal networks. It separates the "trusted" internal networks from the "untrusted" networksthose that are accessible to the public. The figure below shows a sample DMZ network.

Services running at the DMZ network are typically public services that require direct Internet access. For example, all of the services in the preceding figure require direct access because they are communicating with external services from the Internet.

For security reasons, servers from the Internet should not be able to directly connect to an ISP's internal servers.

POP services are often integrated on the DMZ network; acting as intermediaries between the Internet and ISP, they provide open communication channels to the Internet while maintaining secured and controlled access to ISP services.

The following paragraphs describe services that typically run at the DMZ network layer:

External DNS

The external DNS is required for name resolution of external hosts from the Internet. A traditional DNS configuration works well for most ISP environments; however, a better technique is a split DNS. This technique splits DNS into internal and external domains, and it is a fine example of separating tiers by dividing the line between internal and external access.

Separating DNS into internal and external domains has several advantages:

• A split DNS prevents internal host names and IP addresses from being revealed over the Internet. This functional characteristic allows a higher level of security, safeguarding internal hosts from some external attacks, such as denial of services (DoS).
• A split DNS enhances security and preserves public IP addresses where addresses are critically diminishing.

While having a primary external DNS server reside at the DMZ is common, we strongly recommend that you move the primary external DNS server to the content network or at least somewhere on the internal networks, because in those locations it is protected by multiple firewalls. Configure only secondary external DNS servers at the DMZ network.

All zone transfers can be one-way from a primary server to a secondary server. A list of secondary servers can be specified to ensure that only authorized servers are allowed for zone transfers.

Mail Relay

Mail relay is required for relaying incoming and outgoing mail messages between the Internet and an ISP. Its primary purpose is to accept inbound email from the Internet and send outbound email to the Internet.

For inbound mail, the mail relay plays an important role in enhancing security by functioning as an intermediary layer between the Internet and the MailStore.

Hardware required for mail relay servers is lightweight and is replicable horizontally with minimal configuration change and effort. Mail relay servers can be load balanced to provide a higher level of availability.

DHCP Relay Agent

DHCP is required for dynamic network configurations for client systems. At a minimum, configurations are hostname, IP address, netmasks, domain name, DNS server(s), and default gateway. Automatically configuring these parameters is important in maintaining a centralized administration environment.

The best location for a DHCP server is at the services network. You can place a DHCP server at the DMZ network; however, for security reasons, we recommend that you include a DHCP relay agent with this configuration.

DHCP at the DMZ network is typically configured with a DHCP relay agent. This configuration can be done two ways. One way is to have a dedicated server running DHCP relay agent software. The second and preferable way is to enable a DHCP relay agent on the router to forward DHCP messages. With this configuration, a router forwards DHCP messages to connected networks without needing a dedicated DHCP relay/server on every network.

News Feeder

News feeder is necessary to receive incoming feeds from UseNet providers or upstream news servers, as well as to propagate new articles and newsgroups to downstream news servers. For security reasons, the news feeder is typically configured at the DMZ network.

News feeders are responsible for content management and storage. Hardware requirements for news storage are extremely large. We recommend that you check the following web site for the most current storage estimate:

http://newsfeed.mesh.ad.jp/flow/size.html

Although it changes rapidly, a recent estimate for news storage is that approximately 300 Gbytes of storage is required for daily news from a full feed. Due to the sheer volume, most ISPs either outsource news service to a UseNet provider or filter news feeds and moderate newsgroups to keep content manageable and minimize cost.

RADIUS

RADIUS is required for authentication of remote users connecting to an ISP. RADIUS interfaces with a wide variety of NASs and authenticates remote users against various databases, including relational databases and LDAP. RADIUS is typically configured at the DMZ network, on the same network as NASs.

NTP

NTP is required for time synchronization with external clocks. The external clock can be a hardware clock or NTP server. A dedicated hardware clock is rarely configured for small ISPs. NTP is important in ensuring that time is accurate and synchronized between servers in an infrastructure. This synchronization is critical for firewalls to maintain proper access to an ISP, based on time of the day. Also, it is necessary for the NFS server to maintain proper file access for network file systems.

FTP

FTP (file transfer protocol) is required for uploading web content from a subscriber's system to an ISP. For security reasons, configure the FTP server at the DMZ network only.

NAS

NAS is a highly-concentrated digital modem pool with high-speed connections, such as T1 and channelized T3. Each T1 connection can provide 24 channels (23B+D), where each B channel provides 64 Kbit/sec and each D channel provides 16 Kbit/sec. The D channel is for signal provisioning and cannot be used for connection purposes.

A channelized T3, also known as CT3, is the multiplex of 28 T1s. For small ISPs, one or more T1s are sufficient, depending upon the number of concurrent users to be supported. For large ISPs, a CT3 is more economical because a single CT3 costs less than the equivalent multiplex of T1s. Because an access server provides access to an ISP for remote users, it is commonly configured at the DMZ network. However, NAS for larger sites is usually attached to a separate access network instead of the DMZ.

Cache

A cache server can be used to cache frequently accessed data such as web content, thereby enhancing performance by reducing network traffic.

Locate a cache server close to subscribers for optimal response time. You can omit a cache server for local POP, because data resides locally at the ISP. However, for remote POP, we recommend a cache server. For every remote POP, a cache server is critical to ensure an acceptable level of performance, because data resides remotely.

Gateways

A gateway is the point of interconnect between a data network and other networks that require protocol conversion. Interfacing networks can be voice or wireless networks, as well as legacy systems. For example, a wireless application protocol (WAP) gateway is used between a wireless network and a data network, where wireless markup language (WML) is converted to/from HTML format. (A WAP gateway is usually needed for serving wireless services.) Gateways typically are configured at the DMZ network. All access to an ISP's data network should be done at the point of interconnect to an ISP, that is, the DMZ network.

When a host needs a domain name resolution, it sends out a DNS question wrapped in a UDP (User Datagram Protocol) datagram (in turn, encapsulated in an IP (Internet Protocol) datagram). The question is simple: what is the IP address of the named host? The format is also fairly simple: the domain name being queried, the type of question being asked, or query type (which DNS information is being requested, like the IP address, canonical host name, or MX (Mail Exchange) information), and the type of network being queried or query class (this is pretty much exclusively IP as this point).

Each portion of the domain name is preceded by a single byte that indicates how long that part of the domain name is. The last byte of the domain name is a 0, which represents the root node. For example, the domain name:

www.starfleet.edu

would be represented within the UDP datagram as...

3 w w w 9 s t a r f l e e t 3 e d u 0

This datagram is then directed to the host's name server, which replies with the answer, again in a UDP datagram.

DNS servers respond to requests by repeating (in effect) the question and then appending the answer or answers to the question within the datagram. The answer to the question includes the type of information being included in the answer (in this case, a pointer from a domain name to an IP address), the type and class of the query, a time to live value assigned to the information being provided, the length of the reply, and the answer: the IP address associated with the domain name.

A DNS Server is any piece of software that serves as a name server, a resolver, or both.

The basic purpose of spoofing is to confuse a DNS server into giving out bad information. The way it works is that an attacker sends a recursive query to the victim's server, using the victim's server to resolve the query. The answer to the query is in a zone the attacker controls. The answer given by the attacker's name server includes an authoritative record for a domain name controlled by a third party. That authoritative record is FALSE. The victim's server caches the bogus record. (Most modern servers will not cache a fake record because it does not fall in the same parent zone as the record that was requested.)

Once spoofed the victim's resolver will continue to use the false record it has in its cache, potentially misdirecting E-Mail, or any other internet service. This is a potential major security leak for credit card information, trade secrets, and other highly sensitive information.

Recent surveys indicate that 25-30% of servers on the Internet are spoofable. Further readings on DNS spoofing.

Most often used to refer to a domain zone, domain is also used to decribe a zone, or a domain name. This ambiguity results in an unbelieveable number of technical support questions, and is a driving force in the sales rate of "DNS and Bind."

Domains define different levels of authority in a hierarchical structure. For example, in the world of government, some cities have their own management domains. Cites are within county domains, counties are within the domain of the state, and the states are within the national domain. Each domain excercises it own control, but is included in larger domains as one moves up the food chain. Likewise, the computing resources of a organization can be grouped ito domains. Workgroups or departments form domains that may belong to lager domains. Domains are set up for management and security purposes.

NOTE: A cell and a domian are similar concepts. A cell represents a distinct group of systems managed by a central authority.

A domain is a group of Windows 2000 computers that participate in the same security subtree. Active Directory consists of one or more domains. Each domain can span both local area network (LAN) and Wide Area network (WAN) links, depending on the network design and subsequent domain implementation. Multiple domains can exist on the same LAN. When there are multiple domains using different namespaces in Active Directory, it is considered to be a forest of domain trees. The forest must enclose domains that share a comman schema and configuration. They produce a GC (Global Catalog) of users, services, and resources.

A unique designator on the Internet made up of symbols separated by dots, such as:

This.is.a.sample.domain.name.com

The individual words or characters between the dots are called labels. The label furthest right represents the top level domain, com, org, uk and so on. The second most right represents the second level of domain, or "second level domain."

Other terms are child and parent.

Scrooge.com is the parent of Cratchet.Scrooge.com
Scrooge.com is the child of com.

There are certain rules required in domain name creation:

• There can be no more then 127 labels.
• No label can be longer then 63 characters.
• Labels are made up of letters, numbers and hyphens, and may not start with hyphens.
• Labels are case insensitive.
• A domain name must be defined in its closest ancestral zone (The one with the most labels).

The domain name system is a distributed database arranged hierarchically. Its purpose is to provide a layer of abstraction between other Internet services (web, email, etc.) and the numeric addresses (IP addresses) used to uniquely identify any given machine on the Internet.

DNS is the distributed naming service used on the Internet. DNS can provide a machine's IP (Internet Protocol) address, given domain names for the machine. Various products have been developed to provide DNS, such as the Berkeley Internet Name Domain (BIND). DNS is described in RFCs (Request for Comments) 1101, 1183, and 1637.

This has several advantages:

• It permits use of names instead of numbers to identify hosts (usually servers). Names are much easier to remember.
• It permits a server to change numeric addresses without requiring notification of everyone on the Internet, by simply retargeting a name to the new numeric address.
• One name can refer to multiple hosts, to share the load.

There are more benefits, of more esoteric nature.

This is all accomplished using resource records, which are organized into zones, which are in turn hosted by authoritative name servers.

Internet Domains:

The basis for the domains in the DNS may be geographical, such as an entire country, or organizational, such as a common group or activity. The top-level domains represent the most general groupings, and these domain names are standarized. There are currently 7 top-level organizational domains and 59 top-level geographical domains. See table "Internet Top-Level Organization Domains" for a list of these domains.

Domain Names in Internet Addresses:

An Internet name consists of a userid followed by an at sign (@), which is followed by one or more names separated by dots, i.e. dot notation. The most general of these names refers to domains. Domain namnes are found at the end of an Internet name.

A particular name may include references to one or more domains. The rightmost of these is a top-lovel domain. The ordering from specific to general in an Internet name is in contrast in an IP (Internet Protocol) address, in which the first (leftmost) number represents the most general division.

Any zone that isn't a reverse zone. Root is an exception; root is not a domain zone.

Sometimes called a forward domain, or forward zone. This terminology came about because of the ambiguity of the word domain, and use of the term reverse domain, which is now considered archaic.

In telecommunications, a communications link between a satellite and one or more earth stations.

To transfer data, such as a file, from a host computer to a remote machine. For example, the host may be a mainfrome or a BBS (bulletin board system) computer. Downloading requires a communications protocol that both the host and recepient can understand and use. Compare with upload.

For international standards committees, an early version of a porposed standard. The DIS is circulated to all committee members for consideration and comment.

Can be considered as noise that encroaches into the "8 bit dead time space", which can then cause communication errors for some devices on a network. One of the devices that can be affected by dribble bits is the Tektronix 4511A.

A Document Type Definition (DTD or DocType for short) is a declaration in an SGML or XML document that specifies constraints on the structure of the document. It may be included within the document file, but is normally stored in a separate ascii-text file. The syntax of SGML and XML DTDs is very similar but not identical.

Defining a DTD specifies the syntax of an application of SGML or XML, which may be a widely-used standard such as XHTML or a local application.

DTDs are usually employed to determine the structure of an XML or SGML document. A DTD will typically describe each allowable element within the document, the possible attributes and (optionally) the allowed attribute values for each element. Further, it will describe the nesting and occurrences of elements. Most of a DTD is usually composed of ELEMENT definitions and ATTLIST definitions.

You use DTDs (Document Type Definitions) to constrain the structure of XML documents and the data that they contain. A DTD contains a list of the entities that you can use in an XML document, the order and frequency in which they occur, and the data that they can contain. You can use DTDs simply as a development tool (basically a template) or more effectively as a tool for validating a given XML document. The biggest weakness in DTDs is a limited ability to define complex datatypes, and it's likely that DTDs will become obsolete as wider support becomes available for XML Scheme.

An example of a very simple XML DTD to describe a list of persons is given below:

 <!ELEMENT people_list (person*)>

 <!ELEMENT person (name, birthdate?, gender?, socialsecuritynumber?)>

 <!ELEMENT name (#PCDATA) >

 <!ELEMENT birthdate (#PCDATA) >

 <!ELEMENT gender (#PCDATA) >

 <!ELEMENT socialsecuritynumber (#PCDATA) >

 An example of an XML file which makes use of this DTD follows:

 <?xml version="1.0" encoding="UTF-8"?>

 <!DOCTYPE people_list SYSTEM "example.dtd">
 <people_list>
   <person>
     <name>Gendorff</name>
     <birthdate>27/11/1537</birthdate>
     <gender>Male</gender>
   </person>
 </people_list>

The DTD given above requires a name element within every person element; the people_list element is also mandatory, but the rest are optional.

It is possible to render this in an XML-enabled browser (such as IE5 or Mozilla) by pasting and saving the DTD component above to a text file named example.dtd and the xml file to a differently-named text file, and opening the xml file with the browser. The files should both be saved in the same directory. However, many browsers do not check that an XML document conforms to the rules in the DTD; they are only required to check that the DTD is syntactically correct.

External links:

• Different doctypes for HTML
• DocType at W3

See also: Semantic Web

A broadband wiring arrangement in which separate cables are used for transmission and receiving. Such a wiring system may be used, for example, in a 10Broad36 broadband Ethernet or a broadband (IEEE 802.4) token-bus architecture. Compare with split cable system.

In networking, a configuration in which a node can be connected to the network through more than one physical link. If one link fails, the station can still communicate via the other link.

In an electrical signal, the proportion of a time period during which the signal is on, which is when it represents a bit value of 1, vs. when the electrical signal is off, which is when the electrical signal represents a bit value of 0.

An ATP (AppleTalk Transaction Protocol) process used to implement XO (Exactly-Once) transaction service; in this process, the responder searches through a tranactions list to determine whether the request has already been received. Duplicates are not delivered to ATP's client.

A socket that is allocated to the software whenever it is required. A socket is an addressable memory location in a node on the network. Transmissions are sent directly to a socket in a node rather than to the node in general. Nodes can have more than one socket (or memory location). See DDP (Datagram Delivery Protocol)

In an AppleTalk network, dynamic addressing refers to a strategy by which nodes automatically pick unique addresses. A new node keeps trying addresses until it finds one that is not already claimed by another node on the network. Dynamic addressing is also referred to as dynamic node addressing. Dynamic addressing works as follows:

• The node selects a valid address at random and sends an enquiry control packet to that address.
• If the address belongs to a node, the node responds with an acknowledge control packet. The new node then selects another address at random and repeats the process.
• If the address does not belong to a node, the enquiring node takes it as the node's new address.

In networking, a system capability in which the file server can allocate memory as needed, subject to availability, while the network is running. Dynamic reconfiguration enables the server to allocate more resources (such as buffers, tables, etc.) as necessary in order to avoid congestion or overload on the network.

A Dynamic IP address is one that is temporarily assigned to a user by their internet service provider every time the connect. This cuts down on the number if IP addresses large consumer providers need because not all of their customers are using the service at any given time. It also cuts down on bandwidth usage by preventing consumers from hosting servers. Note: Recently a number of companies have started to offer services aimed at updating DNS for dynamically connected clients.

An addressing scheme that assigns node addresses dynamically, rather than associating a permanent address with each node. Dynamic node address assignment facilitates adding and removing nodes from the network by preventing conflicts between old nodeaddresses and new node addresses.

Each protocol stack in a given node must have a protocol address. This address is usually assigned when the stack is initialized. AARP (AppleTalk Address Resolution Protocol) provides one way of making thsi assignment. However, a protocol stack may choose to assign its protocol address using a different method and then inform AARP of this address. The only requirement is that the protocol address be unique across all nodes of a given protocol family.

When a protocol stack asks AARP to pick a unique protocol address, AARP first chooses a tentative protocol address for the node. It starts either by choosing an address value from some nonvolatile memory or by generating a random number. If a mapping for that address value already exists in the corresponding AMT (Address Mapping Table), then AARP knows that another node on the network is using this protocol address. It then picks a new random value for the protocol address until it identifies an address that is not in the AMT.

Having picked a suitable tentative protocol address, AARP must then make sure that this address is not being used by any other node on the data link. It does so by using the data link to broadcast a number of AARP Probe packets, which contain the tentative protocol address. When a node's AARP receives a Probe packet corresponding to one of its protocol stacks, it examines the protocol address of that stack. If the Probe's tetative protocol address matches the receiveing node's protocol address, AARP sends back an AARP Response packet to the probing node.

If the probing node receives an AARP Response packet, then the tentative protocol address is already in use and the node must pick a new tentative address and repeat the probing process. If the probing node does not receive a Response packet after a specified amount of time, then it retransmits the probe. If after a specified maximum number of retries the node has still not received a response, then the node's AARP accepts the tentative address as the node's protocol addrerss. AARP returns this value to its client.

Although its is unlikely, two nodes on the data link could simultaneously pick the same value for their tentative protocol addresses. To handle this situation properly, a probing node receiving a Probe packet whose tentative address matches its own tentative address concludes that this address is in use. The node then proceeds to select another tentative protocol address. While it is sending Probe packets, a node should not respond to AARP Probe or Request packets.

In various networking environments, automatic rerouting of data transmissions in order to maximize throughput or to balance traffic on transmission channels. Routing decisions are based on available and acquired data about network traffic patterns. Dynamic rounting is also known as dynamic adaptive routing.

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