Dynamic routing performs the same function as static routing except it is more robust. Static routing allows routing tables in specific routers to be set up in a static manner so network routes for packets are set. If a router on the route goes down the destination may become unreachable. Dynamic routing allows routing tables in routers to change as the possible routes change. There are several protocols used to support dynamic routing including RIP and OSPF.
Routing cost
Counting route cost is based on one of the following calculations:
• Hop count - How many routers the message must go through to reach the recipient.
• Tic count - The time to route in 1/18 seconds (ticks).
Dynamic routing protocols do not change how routing is done. They just allow for dynamic altering of routing tables.
There are two classifications of protocols:
1. IGP - Interior Gateway Protocol. The name used to describe the fact that each system on the internet can choose its own routing protocol. RIP and OSPF are interior gateway protocols.
2. EGP - Exterior Gateway Protocol. Used between routers of different systems. There are two of these, the first having the same name as this protocol description:
1. EGP - Exterior Gateway Protocol
2. BGP - Border Gateway Protocol.
The daemen "routed" uses RIP. The daemon "gated" supports IGP's and EGP's.
Route Discovery Methods
• Distance vector - Periodically sends route table to other routers. Works best on LANs, not WANs.
• Link-state - Routing tables are broadcast at startup and then only when they change. OSPF
uses link-state.
Routing Information Protocol (RIP)
The RIP RFC is 1058.
The routing daemon daemon adds a routing policy to the system. If there are multiple routes to a destination, it chooses the best one. The RIP message can con contain information on up to 25 routes. The RIP message contains the following components:
1. Command
2. Version - Normally 1 but set to 2 for RIP version 2.
3. family - Set to 2 for IP addresses.
4. IP address - 32 bit IP address
5. Metrics - Indicate the number of hops to a given network, the hop count.
RIP sends periodically broadcasts its routing table to neighboring routers. The RIP message format contains the following commands:
• 1 - request
• 2 - reply
• 3 & 4 - obsolete
• 5 - poll entry
• 6 - Asks for system to send all or part of routing table
When the daemon "routed" starts, it sends a request out all its interfaces for other router's routing tables. The request is broadcast if the network supports it. For TCP/IP the address family in the message is normally 2, but the initial request has address family set to 0 with the metric set to 16.
Regular routing updates are sent every 30 seconds with all or part of the route table. As each router sends routing tables (advertises routes to networks its NICs interface to) routes are determined to each network.
Drawbacks of RIP:
• RIP has no knowledge of subnet addressing
• It takes a long time to stabilize after a router or link failure.
• Uses more broadcasting than OSPF requiring more network bandwidth.
RIP Version 2
Defined by RFC 1388. It passes further information in some of the fields that are set to 0 for the RIP protocol. These additional fields include a 32 bit subnet mask and a next hop IP address, a routing domain, and route tag. The routing domain is an identifier of the daemon the packet belongs to. The route tags supports EGPs.
Open Shortest Path First (OSPF)
OSPF (RFC 1257) is a link state protocol rather than a distance vector protocol. It tests the status of its link to each of its neighbors and sends the acquired information to them. It stabilizes after a route or link failure faster than a distance vector protocol based system. OSPF uses IP directly, not relying on TCP or UDP. OSPF can:
• Have routes based on IP type of service (part of IP header message) such as FTP or Telnet.
• Support subnets.
• Assign cost to each interface based on reliability, round trip time, etc.
• Distribute traffic evenly over equal cost routes.
• Uses multicasting.
Costs for specific hops can be set by administrators. Adjacent routers swap information instead of broadcasting to all routers.
Border Gateway Protocol (BGP)
Described by RFC 1267, 1268, and 1497. It uses TCP as a transport protocol. When two systems are using BGP, they establish a TCP connection, then send each other their BGP routing tables. BGP uses distance vectoring. It detects failures by sending periodic keep alive messages to its neighbors every 30 seconds. It exchanges information about reachable networks with other BGP systems including the full path of systems that are between them.
Thursday, January 1, 2009
Saturday, December 20, 2008
Network Applications
There are three categories of applications with regard to networks:
1. Stand alone applications - Includes editors
2. Network versions of stand alone applications - May be licensed for multiple users.
3. Applications only for a network include databases, mail, group scheduling, groupware.
Models for network applications
1. Client-server - Processing is split between the client which interacts with the user and the server performing back end processing.
2. Shared file systems - The server is used for file storage and the processing of the file is done on the client computer.
3. Applications that are centralized - An example is a Telnet session. The data and the program run on the central computer and the user uses an interface such as the Telnet client or X server to send commands to the central computer and to see the results.
E-mail Systems
• Novell GroupWise - Also called Windows Messaging
• Microsoft Mail
• Microsoft Exchange - This is for the Microsoft Exchange Server. There is a Microsoft Exchange client for the Microsoft Exchange server and a client for an internet mail account only.
• Lotus Notes
cc:Mail - From Lotus and IBM
There are several types of programs used in the process of sending and receiving mail. They are:
• MUA - Mail users agent. This is the program a user will use to type e-mail. It usually incorporates an editor for support. The user types the mail and it is passed to the sending MTA. This may also be called the user agent (UA).
• MTA - Message transfer agent is used to pass mail from the sending machine to the receiving machine. There is a MTA program running on both the sending and receiving machine. Sendmail is a MTA.
• MS - Message Store is a storage area for messages that can't be delivered immediately when the recipient is off-line.
• AU - Access Unit provides access to resources like fax, telex, and teletex.
• LDA - Local delivery agent on the receiving machine receives the mail from its MTA. This program is usually procmail.
• Mail notifier - This program notifies the recipient that they have mail. Normally this requires two programs, biff and comsat. Biff allows the administrator or user to turn on comsat service.
Other components of mail service include:
• Directory services - A list of users on a system. Microsoft provides a Global Address List and a Personal Address Book.
• Post Office - This is where the messages are stored.
Mail API
Mail application programming interfaces (APIs) allow e-mail support to be integrated into application programs.
• MAPI - Microsoft's Messaging API incorporated throughout Microsoft's office products provides support for mail at the application level.
• VIM - Vendor-Independent Messaging protocol from Lotus is supported by many vendors exclusive of Microsoft.
Message Handling Service (MHS)
• MHS and Global MHS by Novell
• MHS by OSI - It is called MOTIS (message-oriented text interchange system).
X.500
This is a recommendation outlining how an organization can share objects and names on a large network. It is hierarchical similar to DNS, defining domains consisting of organizations, divisions, departments, and workgroups. The domains provide information about the users and available resources on that domain, This X.500 system is like a directory. Its recommendation comes from the International Telegraph and Telephone Consultative Committee (CCITT).
Scheduling systems
• Microsoft Schedule+
• Lotus Organizer
Groupware
Used for various electronic communication to enable a group to work together better. Functions may include group discussion, submission of reports and time sheets electronically, an on line help desk database, forms design and access, and creating a document as a group such as configuration management.
Database Management Systems (DBMS)
They are used to share data on a network. DBMS standards for distributed databases:
• SQL - Structured Query Language is a database access language. It is used by most client/server database applications.
• ODBC - Open Database Connectivity (ODBC) from Microsoft lets application developers integrate database connections in applications. It is an application programming interface (API). ODBC drivers convert an application's query int SQL and send it to the database engine program.
• DRDA - Distributed Relational Database Architecture is from IBM.
When information is processed in a distributed database, it is called a transaction. The two phases of a transaction are:
1. Write or Update - The data is temporarily updated. An abort can cancel what this phase did by removing the changed data from a temporary storage area.
2. Commit - The changed data is made permanent in the database.
Databases store multiple copies of the data which is called replication. They must be sure the various copies of the database on various servers is accurate with identical data. Data is also partitioned into smaller blocks of data.
1. Stand alone applications - Includes editors
2. Network versions of stand alone applications - May be licensed for multiple users.
3. Applications only for a network include databases, mail, group scheduling, groupware.
Models for network applications
1. Client-server - Processing is split between the client which interacts with the user and the server performing back end processing.
2. Shared file systems - The server is used for file storage and the processing of the file is done on the client computer.
3. Applications that are centralized - An example is a Telnet session. The data and the program run on the central computer and the user uses an interface such as the Telnet client or X server to send commands to the central computer and to see the results.
E-mail Systems
• Novell GroupWise - Also called Windows Messaging
• Microsoft Mail
• Microsoft Exchange - This is for the Microsoft Exchange Server. There is a Microsoft Exchange client for the Microsoft Exchange server and a client for an internet mail account only.
• Lotus Notes
cc:Mail - From Lotus and IBM
There are several types of programs used in the process of sending and receiving mail. They are:
• MUA - Mail users agent. This is the program a user will use to type e-mail. It usually incorporates an editor for support. The user types the mail and it is passed to the sending MTA. This may also be called the user agent (UA).
• MTA - Message transfer agent is used to pass mail from the sending machine to the receiving machine. There is a MTA program running on both the sending and receiving machine. Sendmail is a MTA.
• MS - Message Store is a storage area for messages that can't be delivered immediately when the recipient is off-line.
• AU - Access Unit provides access to resources like fax, telex, and teletex.
• LDA - Local delivery agent on the receiving machine receives the mail from its MTA. This program is usually procmail.
• Mail notifier - This program notifies the recipient that they have mail. Normally this requires two programs, biff and comsat. Biff allows the administrator or user to turn on comsat service.
Other components of mail service include:
• Directory services - A list of users on a system. Microsoft provides a Global Address List and a Personal Address Book.
• Post Office - This is where the messages are stored.
Mail API
Mail application programming interfaces (APIs) allow e-mail support to be integrated into application programs.
• MAPI - Microsoft's Messaging API incorporated throughout Microsoft's office products provides support for mail at the application level.
• VIM - Vendor-Independent Messaging protocol from Lotus is supported by many vendors exclusive of Microsoft.
Message Handling Service (MHS)
• MHS and Global MHS by Novell
• MHS by OSI - It is called MOTIS (message-oriented text interchange system).
X.500
This is a recommendation outlining how an organization can share objects and names on a large network. It is hierarchical similar to DNS, defining domains consisting of organizations, divisions, departments, and workgroups. The domains provide information about the users and available resources on that domain, This X.500 system is like a directory. Its recommendation comes from the International Telegraph and Telephone Consultative Committee (CCITT).
Scheduling systems
• Microsoft Schedule+
• Lotus Organizer
Groupware
Used for various electronic communication to enable a group to work together better. Functions may include group discussion, submission of reports and time sheets electronically, an on line help desk database, forms design and access, and creating a document as a group such as configuration management.
Database Management Systems (DBMS)
They are used to share data on a network. DBMS standards for distributed databases:
• SQL - Structured Query Language is a database access language. It is used by most client/server database applications.
• ODBC - Open Database Connectivity (ODBC) from Microsoft lets application developers integrate database connections in applications. It is an application programming interface (API). ODBC drivers convert an application's query int SQL and send it to the database engine program.
• DRDA - Distributed Relational Database Architecture is from IBM.
When information is processed in a distributed database, it is called a transaction. The two phases of a transaction are:
1. Write or Update - The data is temporarily updated. An abort can cancel what this phase did by removing the changed data from a temporary storage area.
2. Commit - The changed data is made permanent in the database.
Databases store multiple copies of the data which is called replication. They must be sure the various copies of the database on various servers is accurate with identical data. Data is also partitioned into smaller blocks of data.
Monday, December 15, 2008
Network Devices
Repeaters, Bridges, Routers, and Gateways
Network Repeater
A repeater connects two segments of your network cable. It retimes and regenerates the signals to proper amplitudes and sends them to the other segments. When talking about, ethernet topology, you are probably talking about using a hub as a repeater. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row. Repeaters work only at the physical layer of the OSI network model.
Bridge
A bridge reads the outermost section of data on the data packet, to tell where the message is going. It reduces the traffic on other network segments, since it does not send all packets. Bridges can be programmed to reject packets from particular networks. Bridging occurs at the data link layer of the OSI model, which means the bridge cannot read IP addresses, but only the outermost hardware address of the packet. In our case the bridge can read the ethernet data which gives the hardware address of the destination address, not the IP address. Bridges forward all broadcast messages. Only a special bridge called a translation bridge will allow two networks of different architectures to be connected. Bridges do not normally allow connection of networks with different architectures. The hardware address is also called the MAC (media access control) address. To determine the network segment a MAC address belongs to, bridges use one of:
• Transparent Bridging - They build a table of addresses (bridging table) as they receive packets. If the address is not in the bridging table, the packet is forwarded to all segments other than the one it came from. This type of bridge is used on ethernet networks.
• Source route bridging - The source computer provides path information inside the packet. This is used on Token Ring networks.
Network Router
A router is used to route data packets between two networks. It reads the information in each packet to tell where it is going. If it is destined for an immediate network it has access to, it will strip the outer packet, readdress the packet to the proper ethernet address, and transmit it on that network. If it is destined for another network and must be sent to another router, it will re-package the outer packet to be received by the next router and send it to the next router. The section on routing explains the theory behind this and how routing tables are used to help determine packet destinations. Routing occurs at the network layer of the OSI model. They can connect networks with different architectures such as Token Ring and Ethernet. Although they can transform information at the data link level, routers cannot transform information from one data format such as TCP/IP to another such as IPX/SPX. Routers do not send broadcast packets or corrupted packets. If the routing table does not indicate the proper address of a packet, the packet is discarded.
Brouter
There is a device called a brouter which will function similar to a bridge for network transport protocols that are not routable, and will function as a router for routable protocols. It functions at the network and data link layers of the OSI network model.
Gateway
A gateway can translate information between different network data formats or network architectures. It can translate TCP/IP to AppleTalk so computers supporting TCP/IP can communicate with Apple brand computers. Most gateways operate at the application layer, but can operate at the network or session layer of the OSI model. Gateways will start at the lower level and strip information until it gets to the required level and repackage the information and work its way back toward the hardware layer of the OSI model. To confuse issues, when talking about a router that is used to interface to another network, the word gateway is often used. This does not mean the routing machine is a gateway as defined here, although it could be.
Sunday, December 7, 2008
How To Become Your Own PC Repairer
If you are like me, you have probably owned quite a few personal computers. You are sure to have had the occasional problem, and rushed the machine off to the repair shop. At the time, the cost of repairs may have seemed a bit high in proportion to the original cost of the machine, and you might not have been quite sure what was actually done.
You can learn to do your own diagnosis and repair work. It is worth while knowing what is going on inside the box, even if you do not do the work yourself. With a good understanding of the likely causes of the hardware problems, you can be a lot more confident that the repairs carried out were reasonable and necessary.
To begin with, a good source of technical information is necessary. You can always ask at your local electronics shop, but if you really don’t know what you are talking about, the embarrassment can be a big turn off. There is an excellent book available called ‘Upgrading and Repairing PCs’ written by Scott Mueller. This is the most complete and most readable ‘how-to’ book I have found, and if you only buy one book on the subject, Scott’s is the one to get you hands on.
The next step is to choose a few projects to get familiar with the hardware. The best way to begin building up your skills is to pull a computer completely apart, and then put it back together. If it still works after that, then you have made a pretty good start. At this stage, you may be thinking ‘hang on a minute, I paid a lot of money for my system - No way am I going to risk messing with that baby’.
No, as a beginner, you definitely should not. You can get hold of some old computers to tear apart and rebuild. Old computers are seen as worthless, and you will find Pentium 1 and 2 machines being discarded as rubbish, even though they are still in good working order. Even a Pentium 3 can be bought very cheaply, and most of these are pretty good performers. A word of warning – be careful how widely you spread the news of you new interest in old computers. You will be surprised how many 486s and older are sitting in cartons in garages and basements; your friends and family will really be delighted to give you an amazing collection of electronic junk.
Get familiar with the various parts, and how they fit together. Learn to identify the different styles of processor chip, RAM, disk drives and other components.
As always, safety is paramount. The computer components themselves operate at low voltages, but you still have the mains strength voltage coming into the back of the computer. Always power off and unplug when you are working on the computer, and be suspicious of any signs of overheating or of burning smells. Enjoy your tinkering.
Author Johnny Mendel is the manager the PC Computer Repair and laaptop repair website that provides access to quality low priced pc computer and laptop repair products online. You can also get great free information articles on Johnny's site.
by : suegold
ARCnet Network
Topology is star and bus or a mixture. Cable type is RG-62 A/U coaxial (93 ohm), UTP or fiber-optic. A network can use any combination of this media. Connectors used include BNC, RJ-45, and others. It passes tokens passing for media access. Maximum segment length is 600 meters with RG-62 A/U, 121 meters with UTP, 3485 meters with fiber-optic, and 30 meters from a passive hub. The specification is ANSI 878.1. It can have up to 255 nodes per network. The speed is 2.5 Mbps. ARCnet Plus has operating speeds approaching 20Mbps.
Signals are broadcast across the entire network with computers processing only signals addressed to them. ARCnet tokens travel based on a station identifier (SID) which each computer has. Each network card has a DIP switch used to set the SID with an address between 1 and 255. Signals are generally sent from the lowest numbered station to the next until they wrap around back to SID of 1. To determine non-existent stations, the station with the lowest ID indicates it has the token and begins querying IDs of higher value until it gets a response. Then the next computer does the same until the original station is queried. This procedure is done when a station is added or removed from the network or when the network is originally started. How does the network know when a station has been added or removed? How is the lowest numbered SID identified? Addresses assignment is based on proximity, which helps the network operate more efficiently.
The acronym SID is used for a station identifier with regard to ARCnet, but as used in the Windows NT and Windows 95 operating systems, it refers to the security identification number of a user or group.
You can learn to do your own diagnosis and repair work. It is worth while knowing what is going on inside the box, even if you do not do the work yourself. With a good understanding of the likely causes of the hardware problems, you can be a lot more confident that the repairs carried out were reasonable and necessary.
To begin with, a good source of technical information is necessary. You can always ask at your local electronics shop, but if you really don’t know what you are talking about, the embarrassment can be a big turn off. There is an excellent book available called ‘Upgrading and Repairing PCs’ written by Scott Mueller. This is the most complete and most readable ‘how-to’ book I have found, and if you only buy one book on the subject, Scott’s is the one to get you hands on.
The next step is to choose a few projects to get familiar with the hardware. The best way to begin building up your skills is to pull a computer completely apart, and then put it back together. If it still works after that, then you have made a pretty good start. At this stage, you may be thinking ‘hang on a minute, I paid a lot of money for my system - No way am I going to risk messing with that baby’.
No, as a beginner, you definitely should not. You can get hold of some old computers to tear apart and rebuild. Old computers are seen as worthless, and you will find Pentium 1 and 2 machines being discarded as rubbish, even though they are still in good working order. Even a Pentium 3 can be bought very cheaply, and most of these are pretty good performers. A word of warning – be careful how widely you spread the news of you new interest in old computers. You will be surprised how many 486s and older are sitting in cartons in garages and basements; your friends and family will really be delighted to give you an amazing collection of electronic junk.
Get familiar with the various parts, and how they fit together. Learn to identify the different styles of processor chip, RAM, disk drives and other components.
As always, safety is paramount. The computer components themselves operate at low voltages, but you still have the mains strength voltage coming into the back of the computer. Always power off and unplug when you are working on the computer, and be suspicious of any signs of overheating or of burning smells. Enjoy your tinkering.
Author Johnny Mendel is the manager the PC Computer Repair and laaptop repair website that provides access to quality low priced pc computer and laptop repair products online. You can also get great free information articles on Johnny's site.
by : suegold
ARCnet Network
ARCnet (Attached Resource Computer Network) (CR)
Topology is star and bus or a mixture. Cable type is RG-62 A/U coaxial (93 ohm), UTP or fiber-optic. A network can use any combination of this media. Connectors used include BNC, RJ-45, and others. It passes tokens passing for media access. Maximum segment length is 600 meters with RG-62 A/U, 121 meters with UTP, 3485 meters with fiber-optic, and 30 meters from a passive hub. The specification is ANSI 878.1. It can have up to 255 nodes per network. The speed is 2.5 Mbps. ARCnet Plus has operating speeds approaching 20Mbps.
Signals are broadcast across the entire network with computers processing only signals addressed to them. ARCnet tokens travel based on a station identifier (SID) which each computer has. Each network card has a DIP switch used to set the SID with an address between 1 and 255. Signals are generally sent from the lowest numbered station to the next until they wrap around back to SID of 1. To determine non-existent stations, the station with the lowest ID indicates it has the token and begins querying IDs of higher value until it gets a response. Then the next computer does the same until the original station is queried. This procedure is done when a station is added or removed from the network or when the network is originally started. How does the network know when a station has been added or removed? How is the lowest numbered SID identified? Addresses assignment is based on proximity, which helps the network operate more efficiently.
The acronym SID is used for a station identifier with regard to ARCnet, but as used in the Windows NT and Windows 95 operating systems, it refers to the security identification number of a user or group.
Saturday, December 6, 2008
Address Resolution Protocol
ARP and RARP Address Translation
Address Resolution Protocol (ARP) provides a completely different function to the network than Reverse Address Resolution Protocol (RARP). ARP is used to resolve the ethernet address of a NIC from an IP address in order to construct an ethernet packet around an IP data packet. This must happen in order to send any data across the network. Reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network.
Address Resolution Protocol (ARP)
In an earlier section, there was an example where a chat program was written to communicate between two servers. To send data, the user (Tom) would type text into a dialog box, hit send and the following happened:
1. The program passed Tom's typed text in a buffer, to the socket.
2. The data was put inside a TCP data packet with a TCP header added to the data. This header contained a source and destination port number along with some other information and a checksum.
3. The TCP packet was be placed inside an IP data packet with a source and destination IP address along with some other data for network management.
4. The IP data packet was placed inside an ethernet data packet. This data packet includes the destination and source address of the network interface cards (NIC) on the two computers. The address here is the hardware address of the respective cards and is called the MAC address.
5. The ethernet packet was transmitted over the network line.
6. With a direct connection between the two computers, the network interface card on the intended machine, recognized its address and grabbed the data.
7. The IP data packet was extracted from the ethernet data packet.
8. The TCP data packet was extracted from the IP data packet.
The data was extracted from the TCP packet and the program displayed the retrieved data (text) in the text display window for the intended recipient to read.
In step 4 above, the IP data was going to be placed inside an ethernet data packet, but the computer constructing the packet does not have the ethernet address of the recipient's computer. The computer that is sending the data, in order to create the ethernet part of the packet, must get the ethernet hardware (MAC) address of the computer with the intended IP address. This must be accomplished before the ethernet packet can be constructed. The ethernet device driver software on the receiving computer is not programmed to look at IP addresses encased in the ethernet packet. If it did, the protocols could not be independent and changes to one would affect the other. This is where address resolution protocol (ARP) is used. Tom's computer sends a network broadcast asking the computer that has the recipient's IP address to send it's ethernet address. This is done by broadcasting. The ethernet destination is set with all bits on so all ethernet cards on the network will receive the data packet. The ARP message consists of an ethernet header and ARP packet. The ethernet header contains:
1. A 6 byte ethernet destination address.
2. A 6 byte ethernet source address.
3. A 2 byte frame type. The frame type is 0806 hexadecimal for ARP and 8035 for RARP
The encapsulated ARP data packet contains the following:
1. Type of hardware address (2 bytes). 1=ethernet.
2. Type of protocol address being mapped( 2 bytes). 0800H (hexadecimal) = IP address.
3. Byte size of the hardware address (1 byte). 6
4. Byte size of the protocol address (1 byte). 4
5. Type of operation. 1 = ARP request, 2=ARP reply, 3=RARP request, 4=RARP reply.
6. The sender's ethernet address (6 bytes)
7. The sender's IP address (4 bytes)
8. The recipient's ethernet address (6 bytes)
9. The recipient's IP address (4 bytes)
When the ARP reply is sent, the recipient's ethernet address is left blank.
In order to increase the efficiency of the network and not tie up bandwidth doing ARP broadcasting, each computer keeps a table of IP addresses and matching ethernet addresses in memory. This is called ARP cache. Before sending a broadcast, the sending computer will check to see if the information is in it's ARP cache. If it is it will complete the ethernet data packet without an ARP broadcast. Each entry normally lasts 20 minutes after it is created. RFC 1122 specifies that it should be possible to configure the ARP cache timeout value on the host. To examine the cache on a Windows, UNIX, or Linux computer type "arp -a".
If the receiving host is on another network, the sending computer will go through its route table and determine the correct router (A router should be between two or more networks) to send to, and it will substitute the ethernet address of the router in the ethernet message. The encased IP address will still have the intended IP address. When the router gets the message, it looks at the IP data to tell where to send the data next. If the recipient is on a network the router is connected to, it will do the ARP resolution either using it's ARP buffer cache or broadcasting.
Reverse Address Resolution Protocol (RARP)
As mentioned earlier, reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network. The RARP message format is very similar to the ARP format. When the booting computer sends the broadcast ARP request, it places its own hardware address in both the sending and receiving fields in the encapsulated ARP data packet. The RARP server will fill in the correct sending and receiving IP addresses in its response to the message. This way the booting computer will know its IP address when it gets the message from the RARP server.
Address Resolution Protocol (ARP) provides a completely different function to the network than Reverse Address Resolution Protocol (RARP). ARP is used to resolve the ethernet address of a NIC from an IP address in order to construct an ethernet packet around an IP data packet. This must happen in order to send any data across the network. Reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network.
Address Resolution Protocol (ARP)
In an earlier section, there was an example where a chat program was written to communicate between two servers. To send data, the user (Tom) would type text into a dialog box, hit send and the following happened:
1. The program passed Tom's typed text in a buffer, to the socket.
2. The data was put inside a TCP data packet with a TCP header added to the data. This header contained a source and destination port number along with some other information and a checksum.
3. The TCP packet was be placed inside an IP data packet with a source and destination IP address along with some other data for network management.
4. The IP data packet was placed inside an ethernet data packet. This data packet includes the destination and source address of the network interface cards (NIC) on the two computers. The address here is the hardware address of the respective cards and is called the MAC address.
5. The ethernet packet was transmitted over the network line.
6. With a direct connection between the two computers, the network interface card on the intended machine, recognized its address and grabbed the data.
7. The IP data packet was extracted from the ethernet data packet.
8. The TCP data packet was extracted from the IP data packet.
The data was extracted from the TCP packet and the program displayed the retrieved data (text) in the text display window for the intended recipient to read.
In step 4 above, the IP data was going to be placed inside an ethernet data packet, but the computer constructing the packet does not have the ethernet address of the recipient's computer. The computer that is sending the data, in order to create the ethernet part of the packet, must get the ethernet hardware (MAC) address of the computer with the intended IP address. This must be accomplished before the ethernet packet can be constructed. The ethernet device driver software on the receiving computer is not programmed to look at IP addresses encased in the ethernet packet. If it did, the protocols could not be independent and changes to one would affect the other. This is where address resolution protocol (ARP) is used. Tom's computer sends a network broadcast asking the computer that has the recipient's IP address to send it's ethernet address. This is done by broadcasting. The ethernet destination is set with all bits on so all ethernet cards on the network will receive the data packet. The ARP message consists of an ethernet header and ARP packet. The ethernet header contains:
1. A 6 byte ethernet destination address.
2. A 6 byte ethernet source address.
3. A 2 byte frame type. The frame type is 0806 hexadecimal for ARP and 8035 for RARP
The encapsulated ARP data packet contains the following:
1. Type of hardware address (2 bytes). 1=ethernet.
2. Type of protocol address being mapped( 2 bytes). 0800H (hexadecimal) = IP address.
3. Byte size of the hardware address (1 byte). 6
4. Byte size of the protocol address (1 byte). 4
5. Type of operation. 1 = ARP request, 2=ARP reply, 3=RARP request, 4=RARP reply.
6. The sender's ethernet address (6 bytes)
7. The sender's IP address (4 bytes)
8. The recipient's ethernet address (6 bytes)
9. The recipient's IP address (4 bytes)
When the ARP reply is sent, the recipient's ethernet address is left blank.
In order to increase the efficiency of the network and not tie up bandwidth doing ARP broadcasting, each computer keeps a table of IP addresses and matching ethernet addresses in memory. This is called ARP cache. Before sending a broadcast, the sending computer will check to see if the information is in it's ARP cache. If it is it will complete the ethernet data packet without an ARP broadcast. Each entry normally lasts 20 minutes after it is created. RFC 1122 specifies that it should be possible to configure the ARP cache timeout value on the host. To examine the cache on a Windows, UNIX, or Linux computer type "arp -a".
If the receiving host is on another network, the sending computer will go through its route table and determine the correct router (A router should be between two or more networks) to send to, and it will substitute the ethernet address of the router in the ethernet message. The encased IP address will still have the intended IP address. When the router gets the message, it looks at the IP data to tell where to send the data next. If the recipient is on a network the router is connected to, it will do the ARP resolution either using it's ARP buffer cache or broadcasting.
Reverse Address Resolution Protocol (RARP)
As mentioned earlier, reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network. The RARP message format is very similar to the ARP format. When the booting computer sends the broadcast ARP request, it places its own hardware address in both the sending and receiving fields in the encapsulated ARP data packet. The RARP server will fill in the correct sending and receiving IP addresses in its response to the message. This way the booting computer will know its IP address when it gets the message from the RARP server.
Saturday, November 29, 2008
Network Fault Tolerance
Redundant Array of Inexpensive disks (RAID)
RAID is a fault tolerant method of storing data, meaning that a failure can occur and the system will still function. The various RAID categories are:
• 0 - Disk striping - Data is written across multiple drives in parallel. Different parts of the data is written at the same time to more than one drive. If there are two drives, half the data is written to one drive, while the rest of the data is written to the other drive. All partitions on striped drives must be the same size. No fault tolerance is provided with RAID-0.
• 1 - Disk mirroring - All the data is written to two drives so each drive has a complete of all stored data. If one drive fails, the other can be used to get a copy of the data. To be more fault tolerant, more than one controller card may be used to control the mirrored hard drives. This is called disk duplexing and will allow the system to keep functioning if one controller card fails.
• 2 - Disk striping with error correction codes (ECC).
• 3 - Disk striping with ECC parity information stored on a separate drive.
• 4 - Disk striping with blocks with parity information stored on a separate drive.
• 5 - Disk striping with blocks with parity information stored using multiple drives. Uses five
disks with one fifth of each one to store parity information.
Sector Sparing
Sector sparing will detect when data is going to be read from or written to a bad sector on the hard drive and will move the data to a good sector. The bad sector is marked as not available so it is not used again.
Windows NT support
Supports RAID-0,1, and 5 along with sector sparing.
Terms:
• DAT - Digital Audio Tape
• Sector Sparing - A method of fault tolerance that automatically identifies and marks bad sectors as not available. It is also called hot-fixing.
• SLED - Single Large Inexpensive disk - The concept that a large disk costs less per amount of storage than several smaller ones. Somehow this concept is used as a means of fault tolerance.
RAID is a fault tolerant method of storing data, meaning that a failure can occur and the system will still function. The various RAID categories are:
• 0 - Disk striping - Data is written across multiple drives in parallel. Different parts of the data is written at the same time to more than one drive. If there are two drives, half the data is written to one drive, while the rest of the data is written to the other drive. All partitions on striped drives must be the same size. No fault tolerance is provided with RAID-0.
• 1 - Disk mirroring - All the data is written to two drives so each drive has a complete of all stored data. If one drive fails, the other can be used to get a copy of the data. To be more fault tolerant, more than one controller card may be used to control the mirrored hard drives. This is called disk duplexing and will allow the system to keep functioning if one controller card fails.
• 2 - Disk striping with error correction codes (ECC).
• 3 - Disk striping with ECC parity information stored on a separate drive.
• 4 - Disk striping with blocks with parity information stored on a separate drive.
• 5 - Disk striping with blocks with parity information stored using multiple drives. Uses five
disks with one fifth of each one to store parity information.
Sector Sparing
Sector sparing will detect when data is going to be read from or written to a bad sector on the hard drive and will move the data to a good sector. The bad sector is marked as not available so it is not used again.
Windows NT support
Supports RAID-0,1, and 5 along with sector sparing.
Terms:
• DAT - Digital Audio Tape
• Sector Sparing - A method of fault tolerance that automatically identifies and marks bad sectors as not available. It is also called hot-fixing.
• SLED - Single Large Inexpensive disk - The concept that a large disk costs less per amount of storage than several smaller ones. Somehow this concept is used as a means of fault tolerance.
Saturday, November 15, 2008
Network Drivers
Driver interfaces allow multiple protocol stacks to use one network interface card. The two in use today are listed below. They are not compatible with each other.
Open Driver Interface (ODI)
ODI is normally found on NetWare networks and was developed by Novell and Apple. It consists of:
• Multiple Protocol Interface - Provides connectivity from the data link layer to the network layer.
• Link Support Layer - It includes functions for managing protocol stack assignments and coordinating numbers assigned to MLIDs.
• Multiple-Link Interface Driver (MLID) - Passes data between the data link layer and the hardware or the network media. The drivers are protocol-independent.
Allows multiple drivers to be used on one card and lets one protocol use multiple cards.
Network Driver Interface Specification (NDIS)
NDIS, from Microsoft, is used on Microsoft networks. It allows multiple protocols to be used on a network card and supports the data link layer of the network model.
Transport Driver Interface (TDI)
This is a standard for passing messages between the drivers at the data link layer and the protocols working at the network layer such as IP or NetBEUI. It was produced by Microsoft.
Open Driver Interface (ODI)
ODI is normally found on NetWare networks and was developed by Novell and Apple. It consists of:
• Multiple Protocol Interface - Provides connectivity from the data link layer to the network layer.
• Link Support Layer - It includes functions for managing protocol stack assignments and coordinating numbers assigned to MLIDs.
• Multiple-Link Interface Driver (MLID) - Passes data between the data link layer and the hardware or the network media. The drivers are protocol-independent.
Allows multiple drivers to be used on one card and lets one protocol use multiple cards.
Network Driver Interface Specification (NDIS)
NDIS, from Microsoft, is used on Microsoft networks. It allows multiple protocols to be used on a network card and supports the data link layer of the network model.
Transport Driver Interface (TDI)
This is a standard for passing messages between the drivers at the data link layer and the protocols working at the network layer such as IP or NetBEUI. It was produced by Microsoft.
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