INTERNET PROTOCOLS (IP)

Internet Protocols

Background

The Internet protocols are the world's most popular open-system (nonproprietary) protocol suite because they can be used to communicate across any set of interconnected networks and are equally well suited for LAN and WAN communications. The Internet protocols consist of a suite of communication protocols, of which the two best known are the Transmission Control Protocol (TCP) and the Internet Protocol (IP). The Internet protocol suite not only includes lower-layer protocols (such as TCP and IP), but it also specifies common applications such as electronic mail, terminal emulation, and file transfer. This chapter provides a broad introduction to specifications that comprise the Internet protocols. Discussions include IP addressing and key upper-layer protocols used in the Internet. Specific routing protocols are addressed individually later in this document.

Internet protocols were first developed in the mid-1970s, when the Defense Advanced Research Projects Agency (DARPA) became interested in establishing a packet-switched network that would facilitate communication between dissimilar computer systems at research institutions. With the goal of heterogeneous connectivity in mind, DARPA funded research by Stanford University and Bolt, Beranek, and Newman (BBN). The result of this development effort was the Internet protocol suite, completed in the late 1970s.

TCP/IP later was included with Berkeley Software Distribution (BSD) UNIX and has since become the foundation on which the Internet and the World Wide Web (WWW) are based.

Documentation of the Internet protocols (including new or revised protocols) and policies are specified in technical reports called Request For Comments (RFCs), which are published and then reviewed and analyzed by the Internet community. Protocol refinements are published in the new RFCs. To illustrate the scope of the Internet protocols, maps many of the protocols of the Internet protocol suite and their corresponding OSI layers. This chapter addresses the basic elements and operations of these and other key Internet protocols.

Internet Protocol (IP)

The Internet Protocol (IP) is a network-layer (Layer 3) protocol that contains addressing information and some control information that enables packets to be routed. IP is documented in RFC 791 and is the primary network-layer protocol in the Internet protocol suite. Along with the Transmission Control Protocol (TCP), IP represents the heart of the Internet protocols. IP has two primary responsibilities: providing connectionless, best-effort delivery of datagrams through an internetwork; and providing fragmentation and reassembly of datagrams to support data links with different maximum-transmission unit (MTU) sizes.

The following discussion describes the IP packet fields illustrated in :

•Version—Indicates the version of IP currently used.

•IP Header Length (IHL)—Indicates the datagram header length in 32-bit words.

•Type-of-Service—Specifies how an upper-layer protocol would like a current datagram to be handled, and assigns datagrams various levels of importance.

•Total Length—Specifies the length, in bytes, of the entire IP packet, including the data and header.

•Identification—Contains an integer that identifies the current datagram. This field is used to help piece together datagram fragments.

•Flags—Consists of a 3-bit field of which the two low-order (least-significant) bits control fragmentation. The low-order bit specifies whether the packet can be fragmented. The middle bit specifies whether the packet is the last fragment in a series of fragmented packets. The third or high-order bit is not used.

•Fragment Offset—Indicates the position of the fragment's data relative to the beginning of the data in the original datagram, which allows the destination IP process to properly reconstruct the original datagram.

•Time-to-Live—Maintains a counter that gradually decrements down to zero, at which point the datagram is discarded. This keeps packets from looping endlessly.

•Protocol—Indicates which upper-layer protocol receives incoming packets after IP processing is complete.

•Header Checksum—Helps ensure IP header integrity.

•Source Address—Specifies the sending node.

•Destination Address—Specifies the receiving node.

•Options—Allows IP to support various options, such as security.

•Data—Contains upper-layer information.

IP Addressing

As with any other network-layer protocol, the IP addressing scheme is integral to the process of routing IP datagrams through an internetwork. Each IP address has specific components and follows a basic format. These IP addresses can be subdivided and used to create addresses for subnetworks.

Each host on a TCP/IP network is assigned a unique 32-bit logical address that is divided into two main parts: the network number and the host number. The network number identifies a network and must be assigned by the Internet Network Information Center (InterNIC) if the network is to be part of the Internet. An Internet Service Provider (ISP) can obtain blocks of network addresses from the InterNIC and can itself assign address space as necessary. The host number identifies a host on a network and is assigned by the local network administrator.

IP Address Format

The 32-bit IP address is grouped eight bits at a time, separated by dots, and represented in decimal format (known as dotted decimal notation). Each bit in the octet has a binary weight (128, 64, 32, 16, 8, 4, 2, 1). The minimum value for an octet is 0, and the maximum value for an octet is 255. illustrates the basic format of an IP address.

IP Address Classes

IP addressing supports five different address classes: A, B,C, D, and E. Only classes A, B, and C are available for commercial use. The left-most (high-order) bits indicate the network class. provides reference information about the five IP address classes.

Table 30-1 Reference Information About the Five IP Address Classes

IP Address Class Format Purpose High-Order Bit(s) Address Range No. Bits Network/Host Max. Hosts
A
N.H.H.H1
Few large organizations
0
1.0.0.0 to 126.0.0.0
7/24
167772142 (224 - 2)

B
N.N.H.H
Medium-size organizations
1, 0
128.1.0.0 to 191.254.0.0
14/16
65534 (216 - 2)

C
N.N.N.H
Relatively small organizations
1, 1, 0
192.0.1.0 to 223.255.254.0
21/8
254 (28 - 2)

D
N/A
Multicast groups (RFC 1112)
1, 1, 1, 0
224.0.0.0 to 239.255.255.255
N/A (not for commercial use)
N/A

E
N/A
Experimental
1, 1, 1, 1
240.0.0.0 to 254.255.255.255
N/A
N/A

IP Subnet Addressing
IP networks can be divided into smaller networks called subnetworks (or subnets). Subnetting provides the network administrator with several benefits, including extra flexibility, more efficient use of network addresses, and the capability to contain broadcast traffic (a broadcast will not cross a router).

Subnets are under local administration. As such, the outside world sees an organization as a single network and has no detailed knowledge of the organization's internal structure.

A given network address can be broken up into many subnetworks. For example, 172.16.1.0, 172.16.2.0, 172.16.3.0, and 172.16.4.0 are all subnets within network 171.16.0.0. (All 0s in the host portion of an address specifies the entire network.)

IP Subnet Mask

A subnet address is created by "borrowing" bits from the host field and designating them as the subnet field. The number of borrowed bits varies and is specified by the subnet mask. shows how bits are borrowed from the host address field to create the subnet address field.

Subnet masks use the same format and representation technique as IP addresses. The subnet mask, however, has binary 1s in all bits specifying the network and subnetwork fields, and binary 0s in all bits specifying the host field. illustrates a sample subnet mask.


Subnet mask bits should come from the high-order (left-most) bits of the host field, as illustrates. Details of Class B and C subnet mask types follow. Class A addresses are not discussed in this chapter because they generally are subnetted on an 8-bit boundary.

Various types of subnet masks exist for Class B and C subnets.

The default subnet mask for a Class B address that has no subnetting is 255.255.0.0, while the subnet mask for a Class B address 171.16.0.0 that specifies eight bits of subnetting is 255.255.255.0. The reason for this is that eight bits of subnetting or 28 - 2 (1 for the network address and 1 for the broadcast address) = 254 subnets possible, with 28 - 2 = 254 hosts per subnet.

The subnet mask for a Class C address 192.168.2.0 that specifies five bits of subnetting is 255.255.255.248.With five bits available for subnetting, 25 - 2 = 30 subnets possible, with
23 - 2 = 6 hosts per subnet.

The reference charts shown in table 30-2 and table 30-3 can be used when planning Class B and C networks to determine the required number of subnets and hosts, and the appropriate subnet mask.

Table 30-2 Class B Subnetting Reference Chart


Number of Bits Subnet Mask Number of Subnets Number of Hosts
2
255.255.192.0
2
16382

3
255.255.224.0
6
8190

4
255.255.240.0
14
4094

5
255.255.248.0
30
2046

6
255.255.252.0
62
1022

7
255.255.254.0
126
510

8
255.255.255.0
254
254

9
255.255.255.128
510
126

10
255.255.255.192
1022
62

11
255.255.255.224
2046
30

12
255.255.255.240
4094
14

13
255.255.255.248
8190
6

14
255.255.255.252
16382
2





Table 30-3 Class C Subnetting Reference Chart

-----------------------------------------------------------------------------------
Number of Bits Subnet Mask Number of Subnets Number of Hosts
----------------------------------------------------------------------------------
2 255.255.255.192 2 62
3 255.255.255.224 6 30
4 255.255.255.240 14 14
5 255.255.255.248 30 06
6 255.255.255.252 62 02
-----------------------------------------------------------------------------------


How Subnet Masks are Used to Determine the Network Number
The router performs a set process to determine the network (or more specifically, the subnetwork) address. First, the router extracts the IP destination address from the incoming packet and retrieves the internal subnet mask. It then performs a logical AND operation to obtain the network number. This causes the host portion of the IP destination address to be removed, while the destination network number remains. The router then looks up the destination network number and matches it with an outgoing interface. Finally, it forwards the frame to the destination IP address. Specifics regarding the logical AND operation are discussed in the following section.

Logical AND Operation

Three basic rules govern logically "ANDing" two binary numbers. First, 1 "ANDed" with 1 yields 1. Second, 1 "ANDed" with 0 yields 0. Finally, 0 "ANDed" with 0 yields 0. The truth table provided in table 30-4 illustrates the rules for logical AND operations.

Table 30-4 Rules for Logical AND Operations


Input Input Output
1 1 1
1 0 0
0 1 0
0 0 0


Two simple guidelines exist for remembering logical AND operations: Logically "ANDing" a 1 with a 1 yields the original value, and logically "ANDing" a 0 with any number yields 0.

illustrates that when a logical AND of the destination IP address and the subnet mask is performed, the subnetwork number remains, which the router uses to forward the packet.

Figure 30-9 Applying a logical AND the destination IP address and the subnet mask produces the subnetwork number.







Address Resolution Protocol (ARP) Overview

For two machines on a given network to communicate, they must know the other machine's physical (or MAC) addresses. By broadcasting Address Resolution Protocols (ARPs), a host can dynamically discover the MAC-layer address corresponding to a particular IP network-layer address.

After receiving a MAC-layer address, IP devices create an ARP cache to store the recently acquired IP-to-MAC address mapping, thus avoiding having to broadcast ARPS when they want to recontact a device. If the device does not respond within a specified time frame, the cache entry is flushed.

In addition to the Reverse Address Resolution Protocol (RARP) is used to map MAC-layer addresses to IP addresses. RARP, which is the logical inverse of ARP, might be used by diskless workstations that do not know their IP addresses when they boot. RARP relies on the presence of a RARP server with table entries of MAC-layer-to-IP address mappings.

Internet Routing .

Internet routing devices traditionally have been called gateways. In today's terminology, however, the term gateway refers specifically to a device that performs application-layer protocol translation between devices. Interior gateways refer to devices that perform these protocol functions between machines or networks under the same administrative control or authority, such as a corporation's internal network. These are known as autonomous systems. Exterior gateways perform protocol functions between independent networks.

Routers within the Internet are organized hierarchically. Routers used for information exchange within autonomous systems are called interior routers, which use a variety of Interior Gateway Protocols (IGPs) to accomplish this purpose. The Routing Information Protocol (RIP) is an example of an IGP.

Routers that move information between autonomous systems are called exterior routers. These routers use an exterior gateway protocol to exchange information between autonomous systems. The Border Gateway Protocol (BGP) is an example of an exterior gateway protocol.

IP Routing

IP routing protocols are dynamic. Dynamic routing calls for routes to be calculated automatically at regular intervals by software in routing devices. This contrasts with static routing, where routers are established by the network administrator and do not change until the network administrator changes them.

An IP routing table, which consists of destination address/next hop pairs, is used to enable dynamic routing. An entry in this table, for example, would be interpreted as follows: to get to network 172.31.0.0, send the packet out Ethernet interface 0 (E0).

IP routing specifies that IP datagrams travel through internetworks one hop at a time. The entire route is not known at the onset of the journey, however. Instead, at each stop, the next destination is calculated by matching the destination address within the datagram with an entry in the current node's routing table.

Each node's involvement in the routing process is limited to forwarding packets based on internal information. The nodes do not monitor whether the packets get to their final destination, nor does IP provide for error reporting back to the source when routing anomalies occur. This task is left to another Internet protocol, the Internet Control-Message Protocol (ICMP), which is discussed in the following section.

Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) is a network-layer Internet protocol that provides message packets to report errors and other information regarding IP packet processing back to the source. ICMP is documented in RFC 792.

ICMP Messages

ICMPs generate several kinds of useful messages, including Destination Unreachable, Echo Request and Reply, Redirect, Time Exceeded, and Router Advertisement and Router Solicitation. If an ICMP message cannot be delivered, no second one is generated. This is to avoid an endless flood of ICMP messages.

When an ICMP destination-unreachable message is sent by a router, it means that the router is unable to send the package to its final destination. The router then discards the original packet. Two reasons exist for why a destination might be unreachable. Most commonly, the source host has specified a nonexistent address. Less frequently, the router does not have a route to the destination.

Destination-unreachable messages include four basic types: network unreachable, host unreachable, protocol unreachable, and port unreachable. Network-unreachable messages usually mean that a failure has occurred in the routing or addressing of a packet. Host-unreachable messages usually indicates delivery failure, such as a wrong subnet mask. Protocol-unreachable messages generally mean that the destination does not support the upper-layer protocol specified in the packet. Port-unreachable messages imply that the TCP socket or port is not available.

An ICMP echo-request message, which is generated by the ping command, is sent by any host to test node reachability across an internetwork. The ICMP echo-reply message indicates that the node can be successfully reached.

An ICMP Redirect message is sent by the router to the source host to stimulate more efficient routing. The router still forwards the original packet to the destination. ICMP redirects allow host routing tables to remain small because it is necessary to know the address of only one router, even if that router does not provide the best path. Even after receiving an ICMP Redirect message, some devices might continue using the less-efficient route.

An ICMP Time-exceeded message is sent by the router if an IP packet's Time-to-Live field (expressed in hops or seconds) reaches zero. The Time-to-Live field prevents packets from continuously circulating the internetwork if the internetwork contains a routing loop. The router then discards the original packet.

ICMP Router-Discovery Protocol (IDRP)

IDRP uses Router-Advertisement and Router-Solicitation messages to discover the addresses of routers on directly attached subnets. Each router periodically multicasts Router-Advertisement messages from each of its interfaces. Hosts then discover addresses of routers on directly attached subnets by listening for these messages. Hosts can use Router-Solicitation messages to request immediate advertisements rather than waiting for unsolicited messages.

IRDP offers several advantages over other methods of discovering addresses of neighboring routers. Primarily, it does not require hosts to recognize routing protocols, nor does it require manual configuration by an administrator.

Router-Advertisement messages enable hosts to discover the existence of neighboring routers, but not which router is best to reach a particular destination. If a host uses a poor first-hop router to reach a particular destination, it receives a Redirect message identifying a better choice.

Transmission Control Protocol (TCP)

The TCP provides reliable transmission of data in an IP environment. TCP corresponds to the transport layer (Layer 4) of the OSI reference model. Among the services TCP provides are stream data transfer, reliability, efficient flow control, full-duplex operation, and multiplexing.

With stream data transfer, TCP delivers an unstructured stream of bytes identified by sequence numbers. This service benefits applications because they do not have to chop data into blocks before handing it off to TCP. Instead, TCP groups bytes into segments and passes them to IP for delivery.

TCP offers reliability by providing connection-oriented, end-to-end reliable packet delivery through an internetwork. It does this by sequencing bytes with a forwarding acknowledgment number that indicates to the destination the next byte the source expects to receive. Bytes not acknowledged within a specified time period are retransmitted. The reliability mechanism of TCP allows devices to deal with lost, delayed, duplicate, or misread packets. A time-out mechanism allows devices to detect lost packets and request retransmission.

TCP offers efficient flow control, which means that, when sending acknowledgments back to the source, the receiving TCP process indicates the highest sequence number it can receive without overflowing its internal buffers.

Full-duplex operation means that TCP processes can both send and receive at the same time.

Finally, TCP's multiplexing means that numerous simultaneous upper-layer conversations can be multiplexed over a single connection.

TCP Connection Establishment
To use reliable transport services, TCP hosts must establish a connection-oriented session with one another. Connection establishment is performed by using a "three-way handshake" mechanism.

A three-way handshake synchronizes both ends of a connection by allowing both sides to agree upon initial sequence numbers. This mechanism also guarantees that both sides are ready to transmit data and know that the other side is ready to transmit as well. This is necessary so that packets are not transmitted or retransmitted during session establishment or after session termination.

Each host randomly chooses a sequence number used to track bytes within the stream it is sending and receiving. Then, the three-way handshake proceeds in the following manner:

The first host (Host A) initiates a connection by sending a packet with the initial sequence number (X) and SYN bit set to indicate a connection request. The second host (Host B) receives the SYN, records the sequence number X, and replies by acknowledging the SYN (with an ACK = X + 1). Host B includes its own initial sequence number (SEQ = Y). An ACK = 20 means the host has received bytes 0 through 19 and expects byte 20 next. This technique is called forward acknowledgment. Host A then acknowledges all bytes Host B sent with a forward acknowledgment indicating the next byte Host A expects to receive (ACK = Y + 1). Data transfer then can begin.

Positive Acknowledgment and Retransmission (PAR)
A simple transport protocol might implement a reliability-and-flow-control technique where the source sends one packet, starts a timer, and waits for an acknowledgment before sending a new packet. If the acknowledgment is not received before the timer expires, the source retransmits the packet. Such a technique is called positive acknowledgment and retransmission (PAR).

By assigning each packet a sequence number, PAR enables hosts to track lost or duplicate packets caused by network delays that result in premature retransmission. The sequence numbers are sent back in the acknowledgments so that the acknowledgments can be tracked.

PAR is an inefficient use of bandwidth, however, because a host must wait for an acknowledgment before sending a new packet, and only one packet can be sent at a time.

TCP Sliding Window
A TCP sliding window provides more efficient use of network bandwidth than PAR because it enables hosts to send multiple bytes or packets before waiting for an acknowledgment.

In TCP, the receiver specifies the current window size in every packet. Because TCP provides a byte-stream connection, window sizes are expressed in bytes. This means that a window is the number of data bytes that the sender is allowed to send before waiting for an acknowledgment. Initial window sizes are indicated at connection setup, but might vary throughout the data transfer to provide flow control. A window size of zero, for instance, means "Send no data."

In a TCP sliding-window operation, for example, the sender might have a sequence of bytes to send (numbered 1 to 10) to a receiver who has a window size of five. The sender then would place a window around the first five bytes and transmit them together. It would then wait for an acknowledgment.

The receiver would respond with an ACK = 6, indicating that it has received bytes 1 to 5 and is expecting byte 6 next. In the same packet, the receiver would indicate that its window size is 5. The sender then would move the sliding window five bytes to the right and transmit bytes 6 to 10. The receiver would respond with an ACK = 11, indicating that it is expecting sequenced byte 11 next. In this packet, the receiver might indicate that its window size is 0 (because, for example, its internal buffers are full). At this point, the sender cannot send any more bytes until the receiver sends another packet with a window size greater than 0.

TCP Packet Format
illustrates the fields and overall format of a TCP packet.

Figure 30-10 Twelve fields comprise a TCP packet.




TCP Packet Field Descriptions

The following descriptions summarize the TCP packet fields illustrated in :

•Source Port and Destination Port—Identifies points at which upper-layer source and destination processes receive TCP services.

•Sequence Number—Usually specifies the number assigned to the first byte of data in the current message. In the connection-establishment phase, this field also can be used to identify an initial sequence number to be used in an upcoming transmission.

•Acknowledgment Number—Contains the sequence number of the next byte of data the sender of the packet expects to receive.

•Data Offset—Indicates the number of 32-bit words in the TCP header.

•Reserved—Remains reserved for future use.

•Flags—Carries a variety of control information, including the SYN and ACK bits used for connection establishment, and the FIN bit used for connection termination.

•Window—Specifies the size of the sender's receive window (that is, the buffer space available for incoming data).

•Checksum—Indicates whether the header was damaged in transit.

•Urgent Pointer—Points to the first urgent data byte in the packet.

•Options—Specifies various TCP options.

•Data—Contains upper-layer information.

User Datagram Protocol (UDP)
The User Datagram Protocol (UDP) is a connectionless transport-layer protocol (Layer 4) that belongs to the Internet protocol family. UDP is basically an interface between IP and upper-layer processes. UDP protocol ports distinguish multiple applications running on a single device from one another.

Unlike the TCP, UDP adds no reliability, flow-control, or error-recovery functions to IP. Because of UDP's simplicity, UDP headers contain fewer bytes and consume less network overhead than TCP.

UDP is useful in situations where the reliability mechanisms of TCP are not necessary, such as in cases where a higher-layer protocol might provide error and flow control.

UDP is the transport protocol for several well-known application-layer protocols, including Network File System (NFS), Simple Network Management Protocol (SNMP), Domain Name System (DNS), and Trivial File Transfer Protocol (TFTP).

The UDP packet format contains four fields, as shown in . These include source and destination ports, length, and checksum fields.

Figure 30-11 A UDP packet consists of four fields.





Source and destination ports contain the 16-bit UDP protocol port numbers used to demultiplex datagrams for receiving application-layer processes. A length field specifies the length of the UDP header and data. Checksum provides an (optional) integrity check on the UDP header and data.

Internet Protocols Application-Layer Protocols
The Internet protocol suite includes many application-layer protocols that represent a wide variety of applications, including the following:

•File Transfer Protocol (FTP)—Moves files between devices

•Simple Network-Management Protocol (SNMP)—Primarily reports anomalous network conditions and sets network threshold values

•Telnet—Serves as a terminal emulation protocol

•X Windows—Serves as a distributed windowing and graphics system used for communication between X terminals and UNIX workstations

•Network File System (NFS), External Data Representation (XDR), and Remote Procedure Call (RPC)—Work together to enable transparent access to remote network resources

•Simple Mail Transfer Protocol (SMTP)—Provides electronic mail services

•Domain Name System (DNS)—Translates the names of network nodes into network addresses

lists these higher-layer protocols and the applications that they support.

Table 30-5 Higher-Layer Protocols and Their Applications


Application Protocols
File transfer
FTP

Terminal emulation
Telnet

Electronic mail
SMTP

Network management
SNMP

Distributed file services
NFS, XDR, RPC, X Windows

WAN TECHNOLOGIES

Introduction to WAN Technologies

What Is a WAN?

A WAN is a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer. Figure 3-1 illustrates the relationship between the common WAN technologies and the OSI model.

Figure 3-1 WAN Technologies Operate at the Lowest Levels of the OSI Model



Point-to-Point Links

A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network. Point-to-point lines are usually leased from a carrier and thus are often called leased lines. For a point-to-point line, the carrier allocates pairs of wire and facility hardware to your line only. These circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay. Figure 3-2 illustrates a typical point-to-point link through a WAN.

Figure 3-2 A Typical Point-to-Point Link Operates Through a WAN to a Remote Network



Circuit Switching

Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the
two networks are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated. Figure 3-3 illustrates an example of this type of circuit.

Figure 3-3 A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone




Packet Switching

Packet switching is a WAN technology in which users share common carrier resources. Because this allows the carrier to make more efficient use of its infrastructure, the cost to the customer is generally much better than with point-to-point lines. In a packet switching setup, networks have connections into the carrier's network, and many customers share the carrier's network. The carrier can then create virtual circuits between customers' sites by which packets of data are delivered from one to the other through the network. The section of the carrier's network that is shared is often referred to as a cloud.

Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and X.25. Figure
3-4 shows an example packet-switched circuit.

The virtual connections between customer sites are often referred to as a virtual circuit.

Figure 3-4 Packet Switching Transfers Packets Across a Carrier Network



WAN Virtual Circuits

A virtual circuit is a logical circuit created within a shared network between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit termination phase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.

PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.

WAN Dialup Services

Dialup services offer cost-effective methods for connectivity across WANs. Two popular dialup implementations are dial-on-demand routing (DDR) and dial backup.

DDR is a technique whereby a router can dynamically initiate a call on a switched circuit when it needs to send data. In a DDR setup, the router is configured to initiate the call when certain criteria are met, such as a particular type of network traffic needing to be transmitted. When the connection is made, traffic passes over the line. The router configuration specifies an idle timer that tells the router to drop the connection when the circuit has remained idle for a certain period.

Dial backup is another way of configuring DDR. However, in dial backup, the switched circuit is used to provide backup service for another type of circuit, such as point-to-point or packet switching. The router is configured so that when a failure is detected on the primary circuit, the dial backup line is initiated. The dial backup line then supports the WAN connection until the primary circuit is restored. When this occurs, the dial backup connection is terminated.

WAN Devices

WANs use numerous types of devices that are specific to WAN environments. WAN switches, access servers, modems, CSU/DSUs, and ISDN terminal adapters are discussed in the following sections. Other devices found in WAN environments that are used in WAN implementations include routers, ATM switches, and multiplexers.

WAN Switch

A WAN switch is a multiport internetworking device used in carrier networks. These devices typically switch such traffic as Frame Relay, X.25, and SMDS, and operate at the data link layer of the OSI reference model. Figure 3-5 illustrates two routers at remote ends of a WAN that are connected by WAN switches.

Figure 3-5 Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches



Access Server

An access server acts as a concentration point for dial-in and dial-out connections. Figure 3-6 illustrates an access server concentrating dial-out connections into a WAN.

Figure 3-6 An Access Server Concentrates Dial-Out Connections into a WAN



Modem

A modem is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. At the destination, these analog signals are returned to their digital form. Figure 3-7 illustrates a simple modem-to-modem connection through a WAN.

Figure 3-7 A Modem Connection Through a WAN Handles Analog and Digital Signals



CSU/DSU

A channel service unit/digital service unit (CSU/DSU) is a digital-interface device used to connect a router to a digital circuit like a T1. The CSU/DSU also provides signal timing for communication between these devices.

ISDN Terminal Adapter

An ISDN terminal adapter is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces, such as EIA/TIA-232 on a router. A terminal adapter is essentially an ISDN modem, although it is called a terminal adapter because it does not actually convert analog to digital signals. Figure 3-9 illustrates the placement of the terminal adapter in an ISDN environment.

Figure 3-9 The Terminal Adapter Connects the ISDN Terminal Adapter to Other
Interfaces



Questions

Q—What are some types of WAN circuits?

A—Point-to-point, packet-switched, and circuit-switched.

Q—What is DDR, and how is it different from dial backup?

A—DDR is dial-on-demand routing. DDR dials up the remote site when traffic needs to be transmitted. Dial backup uses the same type of services, but for backup to a primary circuit. When the primary circuit fails, the dial backup line is initiated until the primary circuit is restored.

Q—What is a CSU/DSU used for?

A—A CSU/DSU interfaces a router with a digital line such as a T1.

Q—What is the difference between a modem and an ISDN terminal adapter?

A—A modem converts digital signals into analog for transmission over a telephone line. Because ISDN circuits are digital, the conversion from digital to analog is not required.

Routing Basics

Routing Basics

What Is Routing?

Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways.

The topic of routing has been covered in computer science literature for more than two decades, but routing achieved commercial popularity as late as the mid-1980s. The primary reason for this time lag is that networks in the 1970s were simple, homogeneous environments. Only relatively recently has large-scale internetworking become popular.

Routing Components

Routing involves two basic activities: determining optimal routing paths and transporting information groups (typically called packets) through an internetwork. In the context of the routing process, the latter of these is referred to as packet switching. Although packet switching is relatively straightforward, path determination can be very complex.

Path Determination

Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a standard of measurement, such as path bandwidth, that is used by routing algorithms to determine the optimal path to a destination. To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used.

Routing algorithms fill routing tables with a variety of information. Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router representing the "next hop" on the way to the final destination. When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop. Figure 5-1 depicts a sample destination/next hop routing table.

Figure 5-1 Destination/Next Hop Associations Determine the Data's Optimal Path



Routing tables also can contain other information, such as data about the desirability of a path. Routers compare metrics to determine optimal routes, and these metrics differ depending on the design of the routing algorithm used. A variety of common metrics will be introduced and described later in this chapter.

Routers communicate with one another and maintain their routing tables through the transmission of a variety of messages. The routing update message is one such message that generally consists of all or a portion of a routing table. By analyzing routing updates from all other routers, a router can build a detailed picture of network topology. A link-state advertisement, another example of a message sent between routers, informs other routers of the state of the sender's links. Link information also can be used to build a complete picture of network topology to enable routers to determine optimal routes to network destinations.

Switching

Switching algorithms is relatively simple; it is the same for most routing protocols. In most cases, a host determines that it must send a packet to another host. Having acquired a router's address by some means, the source host sends a packet addressed specifically to
a router's physical (Media Access Control [MAC]-layer) address, this time with the protocol (network layer) address of the destination host.

As it examines the packet's destination protocol address, the router determines that it either knows or does not know how to forward the packet to the next hop. If the router does not know how to forward the packet, it typically drops the packet. If the router knows how to forward the packet, however, it changes the destination physical address to that of the next hop and transmits the packet.

The next hop may be the ultimate destination host. If not, the next hop is usually another router, which executes the same switching decision process. As the packet moves through the internetwork, its physical address changes, but its protocol address remains constant, as illustrated in Figure 5-2.

The preceding discussion describes switching between a source and a destination end system. The International Organization for Standardization (ISO) has developed a hierarchical terminology that is useful in describing this process. Using this terminology, network devices without the capability to forward packets between subnetworks are called end systems (ESs), whereas network devices with these capabilities are called intermediate systems (ISs). ISs are further divided into those that can communicate within routing domains (intradomain ISs) and those that communicate both within and between routing domains (interdomain ISs). A routing domain generally is considered a portion of an internetwork under common administrative authority that is regulated by a particular set of administrative guidelines. Routing domains are also called autonomous systems. With certain protocols, routing domains can be divided into routing areas, but intradomain routing protocols are still used for switching both within and between areas.

Routing Algorithms

Routing algorithms can be differentiated based on several key characteristics. First, the particular goals of the algorithm designer affect the operation of the resulting routing protocol. Second, various types of routing algorithms exist, and each algorithm has a different impact on network and router resources. Finally, routing algorithms use a variety of metrics that affect calculation of optimal routes. The following sections analyze these routing algorithm attributes.

Design Goals

Routing algorithms often have one or more of the following design goals:

•Optimality

•Simplicity and low overhead

•Robustness and stability

•Rapid convergence

•Flexibility

Optimality refers to the capability of the routing algorithm to select the best route, which depends on the metrics and metric weightings used to make the calculation. For example, one routing algorithm may use a number of hops and delays, but it may weigh delay more heavily in the calculation. Naturally, routing protocols must define their metric calculation algorithms strictly.

Routing algorithms also are designed to be as simple as possible. In other words, the routing algorithm must offer its functionality efficiently, with a minimum of software and utilization overhead. Efficiency is particularly important when the software implementing the routing algorithm must run on a computer with limited physical resources.

Routing algorithms must be robust, which means that they should perform correctly in
the face of unusual or unforeseen circumstances, such as hardware failures, high load conditions, and incorrect implementations. Because routers are located at network junction points, they can cause considerable problems when they fail. The best routing algorithms are often those that have withstood the test of time and that have proven stable under a variety of network conditions.

In addition, routing algorithms must converge rapidly. Convergence is the process of agreement, by all routers, on optimal routes. When a network event causes routes to either go down or become available, routers distribute routing update messages that permeate networks, stimulating recalculation of optimal routes and eventually causing all routers to agree on these routes. Routing algorithms that converge slowly can cause routing loops or network outages.

In the routing loop displayed in Figure 5-3, a packet arrives at Router 1 at time t1. Router 1 already has been updated and thus knows that the optimal route to the destination calls for Router 2 to be the next stop. Router 1 therefore forwards the packet to Router 2, but because this router has not yet been updated, it believes that the optimal next hop is Router 1. Router 2 therefore forwards the packet back to Router 1, and the packet continues to bounce back and forth between the two routers until Router 2 receives its routing update or until the packet has been switched the maximum number of times allowed.

Routing algorithms should also be flexible, which means that they should quickly and accurately adapt to a variety of network circumstances. Assume, for example, that a network segment has gone down. As many routing algorithms become aware of the problem, they will quickly select the next-best path for all routes normally using that segment. Routing algorithms can be programmed to adapt to changes in network bandwidth, router queue size, and network delay, among other variables.

Algorithm Types

Routing algorithms can be classified by type. Key differentiators include these:

•Static versus dynamic

•Single-path versus multipath

•Flat versus hierarchical

•Host-intelligent versus router-intelligent

•Intradomain versus interdomain

•Link-state versus distance vector

Static Versus Dynamic

Static routing algorithms are hardly algorithms at all, but are table mappings established by the network administrator before the beginning of routing. These mappings do not change unless the network administrator alters them. Algorithms that use static routes are simple to design and work well in environments where network traffic is relatively predictable and where network design is relatively simple.

Because static routing systems cannot react to network changes, they generally are considered unsuitable for today's large, constantly changing networks. Most of the dominant routing algorithms today are dynamic routing algorithms, which adjust to changing network circumstances by analyzing incoming routing update messages. If the message indicates that a network change has occurred, the routing software recalculates routes and sends out new routing update messages. These messages permeate the network, stimulating routers to rerun their algorithms and change their routing tables accordingly.

Dynamic routing algorithms can be supplemented with static routes where appropriate. A router of last resort (a router to which all unroutable packets are sent), for example, can be designated to act as a repository for all unroutable packets, ensuring that all messages are at least handled in some way.

Single-Path Versus Multipath

Some sophisticated routing protocols support multiple paths to the same destination. Unlike single-path algorithms, these multipath algorithms permit traffic multiplexing over multiple lines. The advantages of multipath algorithms are obvious: They can provide substantially better throughput and reliability. This is generally called load sharing.

Flat Versus Hierarchical

Some routing algorithms operate in a flat space, while others use routing hierarchies. In a flat routing system, the routers are peers of all others. In a hierarchical routing system, some routers form what amounts to a routing backbone. Packets from nonbackbone routers travel to the backbone routers, where they are sent through the backbone until they reach the general area of the destination. At this point, they travel from the last backbone router through one or more nonbackbone routers to the final destination.

Routing systems often designate logical groups of nodes, called domains, autonomous systems, or areas. In hierarchical systems, some routers in a domain can communicate with routers in other domains, while others can communicate only with routers within their domain. In very large networks, additional hierarchical levels may exist, with routers at the highest hierarchical level forming the routing backbone.

The primary advantage of hierarchical routing is that it mimics the organization of most companies and therefore supports their traffic patterns well. Most network communication occurs within small company groups (domains). Because intradomain routers need to know only about other routers within their domain, their routing algorithms can be simplified, and, depending on the routing algorithm being used, routing update traffic can be reduced accordingly.

Host-Intelligent Versus Router-Intelligent

Some routing algorithms assume that the source end node will determine the entire route. This is usually referred to as source routing. In source-routing systems, routers merely act as store-and-forward devices, mindlessly sending the packet to the next stop.

Other algorithms assume that hosts know nothing about routes. In these algorithms, routers determine the path through the internetwork based on their own calculations. In the first system, the hosts have the routing intelligence. In the latter system, routers have the routing intelligence.

Intradomain Versus Interdomain

Some routing algorithms work only within domains; others work within and between domains. The nature of these two algorithm types is different. It stands to reason, therefore, that an optimal intradomain-routing algorithm would not necessarily be an optimal interdomain-routing algorithm.

Link-State Versus Distance Vector

Link-state algorithms (also known as shortest path first algorithms) flood routing information to all nodes in the internetwork. Each router, however, sends only the portion of the routing table that describes the state of its own links. In link-state algorithms, each router builds a picture of the entire network in its routing tables. Distance vector algorithms (also known as Bellman-Ford algorithms) call for each router to send all or some portion of its routing table, but only to its neighbors. In essence, link-state algorithms send small updates everywhere, while distance vector algorithms send larger updates only to neighboring routers. Distance vector algorithms know only about their neighbors.

Because they converge more quickly, link-state algorithms are somewhat less prone to routing loops than distance vector algorithms. On the other hand, link-state algorithms require more CPU power and memory than distance vector algorithms. Link-state algorithms, therefore, can be more expensive to implement and support. Link-state protocols are generally more scalable than distance vector protocols.

Routing Metrics

Routing tables contain information used by switching software to select the best route. But how, specifically, are routing tables built? What is the specific nature of the information that they contain? How do routing algorithms determine that one route is preferable to others?

Routing algorithms have used many different metrics to determine the best route. Sophisticated routing algorithms can base route selection on multiple metrics, combining them in a single (hybrid) metric. All the following metrics have been used:

•Path length

•Reliability

•Delay

•Bandwidth

•Load

•Communication cost

Path length is the most common routing metric. Some routing protocols allow network administrators to assign arbitrary costs to each network link. In this case, path length is the sum of the costs associated with each link traversed. Other routing protocols define hop count, a metric that specifies the number of passes through internetworking products, such as routers, that a packet must take en route from a source to a destination.

Reliability, in the context of routing algorithms, refers to the dependability (usually described in terms of the bit-error rate) of each network link. Some network links might go down more often than others. After a network fails, certain network links might be repaired more easily or more quickly than other links. Any reliability factors can be taken into account in the assignment of the reliability ratings, which are arbitrary numeric values usually assigned to network links by network administrators.

Routing delay refers to the length of time required to move a packet from source to destination through the internetwork. Delay depends on many factors, including the bandwidth of intermediate network links, the port queues at each router along the way, network congestion on all intermediate network links, and the physical distance to be traveled. Because delay is a conglomeration of several important variables, it is a common and useful metric.

Bandwidth refers to the available traffic capacity of a link. All other things being equal, a 10-Mbps Ethernet link would be preferable to a 64-kbps leased line. Although bandwidth is a rating of the maximum attainable throughput on a link, routes through links with greater bandwidth do not necessarily provide better routes than routes through slower links. For example, if a faster link is busier, the actual time required to send a packet to the destination could be greater.

Load refers to the degree to which a network resource, such as a router, is busy. Load can be calculated in a variety of ways, including CPU utilization and packets processed per second. Monitoring these parameters on a continual basis can be resource-intensive itself.

Communication cost is another important metric, especially because some companies may not care about performance as much as they care about operating expenditures. Although line delay may be longer, they will send packets over their own lines rather than through the public lines that cost money for usage time.

Network Protocols

Routed protocols are transported by routing protocols across an internetwork. In general, routed protocols in this context also are referred to as network protocols. These network protocols perform a variety of functions required for communication between user applications in source and destination devices, and these functions can differ widely among protocol suites. Network protocols occur at the upper five layers of the OSI reference model: the network layer, the transport layer, the session layer, the presentation layer, and the application layer.

Confusion about the terms routed protocol and routing protocol is common. Routed protocols are protocols that are routed over an internetwork. Examples of such protocols are the Internet Protocol (IP), DECnet, AppleTalk, Novell NetWare, OSI, Banyan VINES, and Xerox Network System (XNS). Routing protocols, on the other hand, are protocols that implement routing algorithms. Put simply, routing protocols are used by intermediate systems to build tables used in determining path selection of routed protocols. Examples of these protocols include Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (Enhanced IGRP), Open Shortest Path First (OSPF), Exterior Gateway Protocol (EGP), Border Gateway Protocol (BGP), Intermediate System-to-Intermediate System (IS-IS), and Routing Information Protocol (RIP). Routed and routing protocols are discussed in detail later in this book.

Questions

Q—Describe the process of routing packets.

A— Routing is the act of moving information across an internetwork from a source to a destination.

Q—What are some routing algorithm types?

A—Static, dynamic, flat, hierarchical, host-intelligent, router-intelligent, intradomain, interdomain, link-state, and distance vector.

Q—Describe the difference between static and dynamic routing.

A—Static routing is configured by the network administrator and is not capable of adjusting to changes in the network without network administrator intervention. Dynamic routing adjusts to changing network circumstances by analyzing incoming routing update messages without administrator intervention.

Q—What are some of the metrics used by routing protocols?

A—Path length, reliability, delay, bandwidth, load, and communication cost.

SOME TRICKS IN WINDOWS XP

Internet Explorer 7 is full of many new features .
On the internet it has its reputation as Firefox killer !
Featuring : Tab Scrolling , Web Search Box , More Speed, etc.


Keyboard Shortcuts :

CTRL+click (Open links in a new tab in the background)
CTRL+SHIFT+click (Open links in a new tab in the foreground)
CTRL+T (Open a new tab in the foreground)
ALT+ENTER (Open a new tab from the Address bar)
CTRL+Q (Open Quick Tabs - thumbnail view)
CTRL+TAB/CTRL+SHIFT+TAB (Switch between tabs)
CTRL+n (n can be 1-8) (Switch to a specific tab number)
CTRL+9 (Switch to the last tab)
CTRL+W (Close current tab)
ALT+F4 (Close all tabs)
CTRL+ALT+F4 (Close other tabs)

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Hidden Programs In Windows XP !


Strange, but true that some good programs are hidden in Windows XP !!!

Programs :

1. Private Character Editor :
Used for editing fonts,etc.

** start>>Run
** Now, type eudcedit



--------------------------------------------------------------------------------


2. Dr. Watson :
This an inbuilt windows repairing software !

** start>>Run
** Now, type drwtsn32




--------------------------------------------------------------------------------


3. Media Player 5.1 :
Even if you upgrade your Media Player, you can still access your old player in case the new one fails !!!

** start>>Run
** Now, type mplay32



--------------------------------------------------------------------------------


4. iExpress :
Used to create Setups

You can create your own installers !

** start>>Run
** Now, type iexpress

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Wow Notepad Knew About 9/11/2001 !

Amazing thing but true , Notepad knew about 9/11/2001 !
The flight number which hit the WTC in New York was Q33N !

See it yourself :

1. Open Notepad
2. Type : Q33N
3. Now, go to Format menu
4. Choose Font.
5. Now, change the size to '72'
6. Now, change the font to 'Wingdings'
7. See what is displayed !

Isn't it amazing !

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Watch " Star Wars

You Don't need to Download IT !

Just :

Start>>Run , type : telnet towel.blinkenlights.nl

And Enjoy The Movie !

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Trick To Create Table In Word !


To create a table in Ms Word you can use this shortcut !

>> Just type : +======+=====+====+===+==+=+

>> And simply hit 'Enter' !

>> You will see that the text changes to a table. Here, the number '=' represent the number of characters in each cell !

>> Just it makes your work easy and fast !

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Hibernate Your XP !

Hey your Windows XP has a very good but hidden feature !

Trick Advantage :

You can leave your work in between and shutdown the PC and resume it next time as it is !
Even I switched off my PC while writing this article and when I reopened it I resumed my article from where i left !

When you want to stop your work and shutdown(Keeping your programs open)
Do The Following :

1. Click start>Turn Off Computer.
2. As the Turn Off menu comes up press 'Shift' and 'Stand By' changes to 'Hibernate'
3. Click Hibernate (Shift Kept Pressed)

Your xp will save the work and shutdown !

Now, when you switch it on again it will resume it again !

No loading of windows will take place and you will be resumed to your work as if you had just switched your monitor off and now on again !

Its Amazing !

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Crack BIOS Password !!!


Forgot BIOS Password ?

Do the following :

1. Open the CPU
2. Now, observe the motherboard.
3. You notice a coin like silver Battery(3V).

----------------------------------------- NOTE --------------------------------------------------------
This battery is 24 x 7 power supply for the BIOS, which is used to run the system clock will the main power is off. It also initiates the booting process when power is switched on.
-----------------------------------------------------------------------------------------------------------

4. Remove the battery from the motherboard.
(It is safe to remove the Battery)
5. Wait 30 seconds and place the battery back on the motherboard.
6. Now, when you start your system you won't be prompted for the BIOS password.

Enjoy !!!
------------------------------------ CAUTION -----------------------------------------------
1. Perform on your own risk !
2. You have to set the time of your computer when you start again.
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Block Or Unblock Websites without software !


Many times in schools, colleges & offices surfing Entertainment sites are banned !

To overcome this you can unblock these or block some other websites and play pranks !


Do The Following :
For eg you want to block www.abc.com !


* Open the folder C:\WINDOWS\system32\drivers\etc
* There you will find a file named HOSTS

* Click on the file and press SHIFT and now right click on it .
* From the right click menu select Open with .

* Now, select Notepad to open the file from the list !
* Now, in the file under the line 127.0.0.1 localhost add another line as 127.0.0.2 www.abc.com.

* Now, File>>Save !


Now, open your web browser and try openning www.xyz.com , it will not load !


To unblock sites just do the opposite !

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Scare Your Friend With Auto Shutdown !

Read the following :

1. Right click on desktop>select New>shortcut
2. In the shortcut window type : shutdown -s -t 60 -c "the message you want to display"
3. Select Next
4. Name it anything.

Now, double click it !

Scared ???

Nothing happened !

Remedy :

1. Go to start>Run
2. Type : shutdown -a
3. Hit Enter

Oh! You are rescued !

KNOW ABOUT BIGBANG THEORY

Big Bang Theory - The Premise

The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Discoveries in astronomy and physics have shown beyond a reasonable doubt that our universe did in fact have a beginning. Prior to that moment there was nothing; during and after that moment there was something: our universe. The big bang theory is an effort to explain what happened during and after that moment.

According to the standard theory, our universe sprang into existence as "singularity" around 13.7 billion years ago. What is a "singularity" and where does it come from? Well, to be honest, we don't know for sure. Singularities are zones which defy our current understanding of physics. They are thought to exist at the core of "black holes." Black holes are areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squished into infinite density (a mathematical concept which truly boggles the mind). These zones of infinite density are called "singularities." Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense, something - a singularity. Where did it come from? We don't know. Why did it appear? We don't know.

After its initial appearance, it apparently inflated (the "Big Bang"), expanded and cooled, going from very, very small and very, very hot, to the size and temperature of our current universe. It continues to expand and cool to this day and we are inside of it: incredible creatures living on a unique planet, circling a beautiful star clustered together with several hundred billion other stars in a galaxy soaring through the cosmos, all of which is inside of an expanding universe that began as an infinitesimal singularity which appeared out of nowhere for reasons unknown. This is the Big Bang theory.

Big Bang Theory - Common Misconceptions

There are many misconceptions surrounding the Big Bang theory. For example, we tend to imagine a giant explosion. Experts however say that there was no explosion; there was (and continues to be) an expansion. Rather than imagining a balloon popping and releasing its contents, imagine a balloon expanding: an infinitesimally small balloon expanding to the size of our current universe.

Another misconception is that we tend to image the singularity as a little fireball appearing somewhere in space. According to the many experts however, space didn't exist prior to the Big Bang. Back in the late '60s and early '70s, when men first walked upon the moon, "three British astrophysicists, Steven Hawking, George Ellis, and Roger Penrose turned their attention to the Theory of Relativity and its implications regarding our notions of time. In 1968 and 1970, they published papers in which they extended Einstein's Theory of General Relativity to include measurements of time and space.1, 2 According to their calculations, time and space had a finite beginning that corresponded to the origin of matter and energy."3 The singularity didn't appear in space; rather, space began inside of the singularity. Prior to the singularity, nothing existed, not space, time, matter, or energy - nothing. So where and in what did the singularity appear if not in space? We don't know. We don't know where it came from, why it's here, or even where it is. All we really know is that we are inside of it and at one time it didn't exist and neither did we.

Big Bang Theory -
Evidence for the Theory
What are the major evidences which support the Big Bang theory?

* First of all, we are reasonably certain that the universe had a beginning.
* Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called "Hubble's Law," named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929. This observation supports the expansion of the universe and suggests that the universe was once compacted.
* Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat. In 1965, Radioastronomers Arno Penzias and Robert Wilson discovered a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant which scientists were looking for. Penzias and Wilson shared in the 1978 Nobel Prize for Physics for their discovery.
* Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins.

Big Bang Theory - The Only Plausible Theory?

Is the standard Big Bang theory the only model consistent with these evidences? No, it's just the most popular one. Internationally renown Astrophysicist George F. R. Ellis explains: "People need to be aware that there is a range of models that could explain the observations….For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations….You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that."4

In 2003, Physicist Robert Gentry proposed an attractive alternative to the standard theory, an alternative which also accounts for the evidences listed above.5 Dr. Gentry claims that the standard Big Bang model is founded upon a faulty paradigm (the Friedmann-lemaitre expanding-spacetime paradigm) which he claims is inconsistent with the empirical data. He chooses instead to base his model on Einstein's static-spacetime paradigm which he claims is the "genuine cosmic Rosetta." Gentry has published several papers outlining what he considers to be serious flaws in the standard Big Bang model.6 Other high-profile dissenters include Nobel laureate Dr. Hannes Alfvén, Professor Geoffrey Burbidge, Dr. Halton Arp, and the renowned British astronomer Sir Fred Hoyle, who is accredited with first coining the term "the Big Bang" during a BBC radio broadcast in 1950.

Big Bang Theory - What About God?

Any discussion of the Big Bang theory would be incomplete without asking the question, what about God? This is because cosmogony (the study of the origin of the universe) is an area where science and theology meet. Creation was a supernatural event. That is, it took place outside of the natural realm. This fact begs the question: is there anything else which exists outside of the natural realm? Specifically, is there a master Architect out there? We know that this universe had a beginning.

ROUTERS

Router

A router (pronounced /'rautər/ in the USA, pronounced /'ru:tər/ in the UK, or either pronunciation in Australia) is a computer whose software and hardware are usually tailored to the tasks of routing and forwarding information. Routers generally contain a specialized operating system (e.g. Cisco's IOS or Juniper Networks JUNOS and JUNOSe or Extreme Networks XOS), RAM, NVRAM, flash memory, and one or more processors. High-end routers contain many processors and specialized Application-specific integrated circuits (ASIC) and do a great deal of parallel processing. Chassis based systems like the Nortel MERS-8600 or ERS-8600 routing switch, (pictured right) have multiple ASICs on every module and allow for a wide variety of LAN, MAN, METRO, and WAN port technologies or other connections that are customizable. Much simpler routers are used where cost is important and the demand is low, for example in providing a home internet service. With appropriate software (such as Untangle, SmoothWall, XORP or Quagga), a standard PC can act as a router.

Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router.[1] The term layer 3 switch often is used interchangeably with router, but switch is really a general term without a rigorous technical definition. In marketing usage, it is generally optimized for Ethernet LAN interfaces and may not have other physical interface types.

Routers operate in two different planes :

Control Plane, in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations,
Forwarding Plane, which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.

Control Plane

Control Plane processing leads to the construction of what is variously called a routing table or routing information base (RIB). The RIB may be used by the Forwarding Plane to look up the outbound interface for a given packet, or, depending on the router implementation, the Control Plane may populate a separate Forwarding Information Base (FIB) with destination information. RIBs are optimized for efficient updating with control mechanisms such as routing protocols, while FIBs are optimized for the fastest possible lookup of the information needed to select the outbound interface.

The Control Plane constructs the routing table from knowledge of the up/down status of its local interfaces, from hard-coded static routes, and from exchanging routing protocol information with other routers. It is not compulsory for a router to use routing protocols to function, if for example it was configured solely with static routes. The routing table stores the best routes to certain network destinations, the "routing metrics" associated with those routes, and the path to the next hop router.

Routers do maintain state on the routes in the RIB/routing table, but this is quite distinct from not maintaining state on individual packets that have been forwarded.

Forwarding Plane (a.k.a. Data Plane)

For the pure Internet Protocol (IP) forwarding function, router design tries to minimize the state information kept on individual packets. Once a packet is forwarded, the router should no longer retain statistical information about it. It is the sending and receiving endpoints that keeps information about such things as errored or missing packets.

Forwarding decisions can involve decisions at layers other than the IP internetwork layer or OSI layer 3. Again, the marketing term switch can be applied to devices that have these capabilities. A function that forwards based on data link layer, or OSI layer 2, information, is properly called a bridge. Marketing literature may call it a layer 2 switch, but a switch has no precise definition.

Among the most important forwarding decisions is deciding what to do when congestion occurs, i.e., packets arrive at the router at a rate higher than the router can process. Three policies commonly used in the Internet are Tail drop, Random early detection, and Weighted random early detection. Tail drop is the simplest and most easily implemented; the router simply drops packets once the length of the queue exceeds the size of the buffers in the router. Random early detection (RED) probabilistically drops datagrams early when the queue exceeds a configured size. Weighted random early detection requires a weighted average queue size to exceed the configured size, so that short bursts will not trigger random drops.

Types of routers

Routers may provide connectivity inside enterprises, between enterprises and the Internet, and inside Internet Service Providers (ISP). The largest routers (for example the Cisco CRS-1 or Juniper T1600) interconnect ISPs, are used inside ISPs, or may be used in very large enterprise networks. The smallest routers provide connectivity for small and home offices.

Routers for Internet connectivity and internal use

Routers intended for ISP and major enterprise connectivity will almost invariably exchange routing information with the Border Gateway Protocol. RFC 4098[3] defines several types of BGP-speaking routers:

Provider Edge Router: Placed at the edge of an ISP network, it speaks external BGP (eBGP) to a BGP speaker in another provider or large enterprise Autonomous System (AS).
Subscriber Edge Router: Located at the edge of the subscriber's network, it speaks eBGP to its provider's AS(s). It belongs to an end user (enterprise) organization.
Inter-provider Border Router: Interconnecting ISPs, this is a BGP speaking router that maintains BGP sessions with other BGP speaking routers in other providers' ASes.
Core router: A router that resides within the middle or backbone of the LAN network rather than at its periphery.
Within an ISP: Internal to the provider's AS, such a router speaks internal BGP (iBGP) to that provider's edge routers, other intra-provider core routers, or the provider's inter-provider border routers.
"Internet backbone:" The Internet does not have a clearly identifiable backbone, as did its predecessors. See default-free zone (DFZ). Nevertheless, it is the major ISPs' routers that make up what many would consider the core. These ISPs operate all four types of the BGP-speaking routers described here. In ISP usage, a "core" router is internal to an ISP, and used to interconnect its edge and border routers. Core routers may also have specialized functions in virtual private networks based on a combination of BGP and Multi-Protocol Label Switching (MPLS).

Small Office Home Office (SOHO) connectivity

Residential gateways (often called routers) are frequently used in homes to connect to a broadband service, such as IP over cable or DSL. A home router may allow connectivity to an enterprise via a secure Virtual Private Network.

While functionally similar to routers, residential gateways use port address translation in addition to routing. Instead of connecting local computers to the remote network directly, a residential gateway makes multiple local computers appear to be a single computer.

Enterprise Routers

All sizes of routers may be found inside enterprises. The most powerful routers tend to be found in ISPs but academic and research facilities, as well as large businesses, may also need large routers.

A three-layer model is in common use, not all of which need be present in smaller networks .


Access

Access routers, including SOHO, are located at customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost.


Distribution

Distribution routers aggregate traffic from multiple access routers, either at the same site, or to collect the data streams from multiple sites to a major enterprise location. Distribution routers often are responsible for enforcing quality of service across a WAN, so they may have considerable memory, multiple WAN interfaces, and substantial processing intelligence.

They may also provide connectivity to groups of servers or to external networks. In the latter application, the router's functionality must be carefully considered as part of the overall security architecture. Separate from the router may be a Firewall or VPN concentrator, or the router may include these and other security functions.

When an enterprise is primarily on one campus, there may not be a distinct distribution tier, other than perhaps off-campus access. In such cases, the access routers, connected to LANs, interconnect via core routers.


Core

In enterprises, core router may provide a "collapsed backbone" interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth.

When an enterprise is widely distributed with no central location(s), the function of core routing may be subsumed by the WAN service to which the enterprise subscribes, and the distribution routers become the highest tier.



History




A Cisco ASM/2-32EM router deployed at CERN in 1987.The very first device that had fundamentally the same functionality as a router does today, i.e a packet switch, was the Interface Message Processor (IMP); IMPs were the devices that made up the ARPANET, the first packet switching network. The idea for a router (although they were called "gateways" at the time) initially came about through an international group of computer networking researchers called the International Network Working Group (INWG). Set up in 1972 as an informal group to consider the technical issues involved in connecting different networks, later that year it became a subcommittee of the International Federation for Information Processing.

These devices were different from most previous packet switches in two ways. First, they connected dissimilar kinds of networks, such as serial lines and local area networks. Second, they were connectionless devices, which had no role in assuring that traffic was delivered reliably, leaving that entirely to the hosts (although this particular idea had been previously pioneered in the CYCLADES network).

The idea was explored in more detail, with the intention to produce real prototype system, as part of two contemporaneous programs. One was the initial DARPA-initiated program, which created the TCP/IP architecture of today. The other was a program at Xerox PARC to explore new networking technologies, which produced the PARC Universal Packet system, although due to corporate intellectual property concerns it received little attention outside Xerox until years later.

The earliest Xerox routers came into operation sometime after early 1974. The first true IP router was developed by Virginia Strazisar at BBN, as part of that DARPA-initiated effort, during 1975-1976. By the end of 1976, three PDP-11-based routers were in service in the experimental prototype Internet.

The first multiprotocol routers were independently created by staff researchers at MIT and Stanford in 1981; the Stanford router was done by William Yeager, and the MIT one by Noel Chiappa; both were also based on PDP-11s.

As virtually all networking now uses IP at the network layer, multiprotocol routers are largely obsolete, although they were important in the early stages of the growth of computer networking, when several protocols other than TCP/IP were in widespread use. Routers that handle both IPv4 and IPv6 arguably are multiprotocol, but in a far less variable sense than a router that processed AppleTalk, DECnet, IP, and Xerox protocols.

In the original era of routing (from the mid-1970s through the 1980s), general-purpose mini-computers served as routers. Although general-purpose computers can perform routing, modern high-speed routers are highly specialized computers, generally with extra hardware added to accelerate both common routing functions such as packet forwarding and specialised functions such as IPsec encryption.

Still, there is substantial use of Linux and Unix machines, running open source routing code, for routing research and selected other applications. While Cisco's operating system was independently designed, other major router operating systems, such as those from Juniper Networks and Extreme Networks, are extensively modified but still have Unix ancestry.

LINUX TUTORIAL FOR YOU

Step-by-Step Guide & Tutorial Pages

Have an old system gathering dust? Convert it into a Linux server! It's easy to do. Just follow along with our guide pages and we'll walk you through installing the Debian Linux OS and setting up a network with the most common types of Internet and LAN servers. You'll learn some things about operating systems, networking, and the Internet in the process, and you may just have some fun along the way. Even if you have never worked with Linux before, you'll be able to use our guide pages to go from zero to "sysadmin" in no time, as well as get a solid start in the knowledge needed for the Linux+ certification.



Why Not Red Hat ?

Red Hat is in a tough spot. Most of their revenue streams are based on sales, support, and training while the open nature of Linux has resulted in thousands of freely-available Linux resources on the Web. Their survival depends on having a product that is proprietary enough to make you dependent upon them for upgrades and support. And now that they are a publically-held company they are under pressure to meet the expectations of Wall Street analysts for revenue growth and cash flows every quarter. (Did you think it was just a coincidence that they churned out new versions at an average of two a year?) In time, Red Hat's dominance will likely kill off smaller commercial distributions like Mandrake and TurboLinux and dealing with Red Hat will be no different than dealing with Microsoft.


Why Debian ?

Debian is the world's leading non-commercial totally free Linux distribution. Remaining loyal to the concept upon which Linux was created, it is produced by hundreds of volunteer developers around the world. Contrary to a common misconception, Debian is not for Linux gurus only. As a matter of fact, as you will see on the guide pages, its advanced package management system makes it one of the easier distributions for new Linux users to work with. Here are just a few of its


Advantages:

Non-Proprietary: Debian is a true GNU/Linux distribution using the standard UNIX style commands. This ensures that what you learn today won't be obsolete in two years and makes it easier to also learn how to work with UNIX systems.

Easy Maintenance: A seamless, totally-integrated package management system makes it easy to keep your system up to date and free of orphan files and incompatible products. Most dependent packages are handled automatically so you don't get the "Failed dependencies" error commonly encountered when trying to add software on RPM-based systems like Red Hat and Suse.

Automated Patching: The Debian package system also allows you to use a single command to update your entire system (operating system and installed packages) over the Internet. This allows you to use a scheduler to routinely run a shell script to automatically update your system with the latest program, OS, and security patches.

Extensive: Only free software packages (applications, utilities, etc.) are allowed to be included in the official Debian distributions, and the current binary distribution comes on 21 CDs or 3 DVDs because there are over 18,000 of them. With Debian, you don't have different "server" and "workstation" or "personal" editions. It's everything all in one.

Support Options: Peer support is available through a community of listservs (mailing lists) and chat rooms. Replies to messages may even be from those who helped develop the product. And since you're likely not the first person to encounter a given issue, there are also searchable archives of listserv messages. If your company requires commercial support contracts fear not. Numerous for-profit support operations offer a variety of technical support options. With Debian, you don't have to worry about forced upgrades due to vendors dropping support for older versions.

Minimal Investment: Debian's peformance is excellent even with the modest hardware requirements Linux is famous for. While most OSs require newer, faster, bigger hardware, Debian allows you to utilize those old Pentium systems instead of throwing them into a landfill. This, along with the fact that you can load a single copy of Debian on as many systems as you want, means you can set up a full-blown enterprise at very little cost.

Reliable: Debian's focus on stability and reliability results in servers that you may have to reboot once a year, rather than once a month.
User-centric: New versions of Debian are developed when major changes warrant one, not to generate revenues from upgrades. (You need only look at the version numbers of the various distributions to verify this.) Debian disc images are available for download from www.debian.org. If you download the images, be sure to download the current "stable" release (get the "i386" set for an Intel PC system). However, downloading and burning 23 CDs or 3 DVDs takes some time and effort. You can also purchase ready-made DVD sets from Web vendors for around $20 with CD sets costing a little more.

Why Not Debian ?

If you're the type who likes to base your operations on the bleeding edge, Debian isn't for you. Debian's focus on providing a stable, reliable operating system across all platforms means it will never be "first to market" with new bells and whistles. They are incorporated into new releases once the bugs have been discovered and worked out.

SOME FLAVOURS OF LINUX

What is Ubuntu?

Ubuntu is a community developed operating system that is perfect for laptops,desktops and servers. Whether you use it at home, at school or at work Ubuntu contains all the applications you'll ever need, from word processing and email applications, to web server software and programming tools.
Ubuntu is and always will be free of charge. You do not pay any licensing fees. You can download, use and share Ubuntu with your friends, family, school or business for absolutely nothing.
We issue a new desktop and server release every six months. That means you'll always have the the latest and greatest applications that the open source world has to offer.
Ubuntu is designed with security in mind. You get free security updates for at least 18 months on the desktop and server. With the Long Term Support (LTS) version you get three years support on the desktop, and five years on the server. There is no extra fee for the LTS version, we make our very best work available to everyone on the same free terms. Upgrades to new versions of Ubuntu are and always will be free of charge.
Everything you need on one CD, which provides a complete working environment. Additional software is available online.
The graphical installer enables you to get up and running quickly and easily. A standard installation should take less than 25 minutes.
Once installed your system is immediately ready-to-use. On the desktops you have a full set of productivity, internet, drawing and graphics applications, and games.

What does Ubuntu mean?
Ubuntu is an African word meaning 'Humanity to others', or 'I am what I am because of who we all are'. The Ubuntu distribution brings the spirit of Ubuntu to the software world

Ubuntu Server Edition

The Server Edition - built on the solid foundation of Debian which is known for its robust server installations — has a strong heritage for reliable performance and predictable evolution.
Integrated and secure platform
As your business grows, so does your network. More applications need to be deployed and more servers are required. Ubuntu Server Edition offers support for several common configurations, and simplifies common Linux server deployment processes. It provides a well-integrated platform enabling you to quickly and easily deploy a new server with any of the standard internet services: mail, web, DNS, file serving or database management.
A key lesson from its Debian heritage is that of security by default. The Ubuntu Server has no open ports after the installation and contains only the essential software needed to build a secure server.
Lower Total cost of ownership with automatic LAMP installation
In around 15 minutes, the time it takes to install Ubuntu Server Edition, you can have a LAMP (Linux, Apache, MySQL and PHP) server up and ready to go. This feature, exclusive to Ubuntu Server Edition, is available at the time of installation.
The LAMP option means you don't have to install and integrate each of the four separate LAMP components, a process which can take hours and requires someone who is skilled in the installation and configuration of the individual applications. Instead, you get increased security, reduced time-to-install, and reduced risk of misconfiguration, all of which results in a lower cost of ownership.
Eliminate the cost of updating individual workstations
Ubuntu Server edition includes thin client support using LTSP (Linux Terminal Server Project). LTSP-5, the latest release, offers a simple installation and easy maintenance. All the data is stored on the server, which will substantially diminish the cost of updating individual workstations and help to ensure their security. Notable benefits of Ubuntu's thin client support are:
Simplified management: manage all clients from one system. Install new software, change their configuration, or even upgrade to a new version on the server, and all clients are instantly up to date. There is only one backup to take for all clients.
Fully automatic installation and setup: installing a thin client server is as easy as installing a single desktop system, and once it's finished, new clients can be added with no additional administration on the server
Lower TCO through shared resources: Common high-powered desktop workstations sit idle most of the day consuming power and costing your organization money. With a high-powered server and low-cost thin clients, you can get great performance and save money. Need higher performance? Just upgrade the server, and all clients instantly benefit.
Quick failure recovery: If a client system fails, simply swap in a new one and continue working. No configuration is required, and all of the user's data and settings are intact.
Locally attached devices: Users can access printers, cameras, iPods, USB sticks and other devices connected directly to the thin client.


KUBUNTU

KUBUNTU is an official derivative of Ubuntu using the KDE environment instead of GNOME. It is part of the Ubuntu project and uses the same underlying system. It is an exciting distribution that showcases the full potential of the KDE desktop. Kubuntu shares the same repositories as Ubuntu, and relies on the same underlying architecture.

The K Desktop Environment

A powerful graphical desktop environment, combines the ease of use, contemporary functionality, and outstanding graphical design with the technological superiority of the Kubuntu operating system. KDE version 3.5.5 is the current stable release and Kubuntu 6.10 is the first distribution to include it.


Photo Management

Digikam is now included by default. This advanced digital photo management application provides you with the tools necessary to view, manage, edit, enhance, organise, tag and share photographs. Organising both photos and photo albums is a snap with Digikam as it allows you to sort chronologically, by directory layout, or by custom collections.
Power Management
Kubuntu received a new power management overhaul with the latest release. Guidance, the power management system, allows users to select various functions to control the power of their portable computing system, whether it is controlling the brightness of the display during low battery, locking the system upon closing the lid or controlling access to multiple batteries.

Easy Networking and Printer Sharing
Zeroconf and print sharing let you browse the local network for available services. Both are now simple to setup and maintain requiring nothing more than ticking a box to enable the feature.
Accessibility Profiles
Kubuntu now offers users the ability to use a preconfigured accessibility profile depending on the type of disability right from the initial point of setup. This provides users the accessibility features they need in order to not only install the Kubuntu 6.10 operating system but to use the system on a daily basis for all of their computing needs. Press F5 at the CD boot screen to choose a profile.