Routing

From Mycomputer Notes

A protocol

Is a set of rules that determines how two computers need to communicate with each other accross the network. A protocol is also a common denominator by which the different application, host and systems communicate between each others. When two computers commninicate one with another they exchange data messages (i.e data messages could be a connection to a remote machine, email, data and file transfer) To accept a message and act on this message the computer must have a definition of how the message is defined and what it means.

A protocol describes:

  • The format that a message must take.
  • The way in which computers must excange a message within the context of a particular activity, such as sending messages across networks.

Contents

[edit] Routed Protocol

IP is the routed protocol that works on the Internet. IP addressing enables packets to be routed from source to destination using the best available path.

Clarificatoin between Routed/Routable vs Routing protocol.

Routed Protocol

Any network protocol that provides enough information in its network layer address to allow a packet to be forwarded from one host to anohtohter based on the addrssing scheme.Routed protocols uses the routing table to forwarded packets. Examples of Routed Protocols:

  1. Internet Protocol (IP)
  2. Internetwork Packet Exchange (IPX)
  3. AppleTalk
Routing Protocol

Supports a routed protocol by providing mechanisms for sharing routing information. Routing protocol messages move between the routers. A routing protocol allows the routers to communicate with other routers to update and maintain tables. Routers use routing protocols to exchange routing tables and share information. Routing protocols let routers route routed protocols after a path has been determined. Examples of TCP/IP examples of routing protocols:

  1. Routing Infomation Protocol (RIP)
  2. Interior Gateway Routing Protocol (IGRP)
  3. Enhanced Interior Gateway Routing Protocol (IGRP)
  4. Open Shortest Path First (OSPF)

For a protocol to be routable it must:

  • Provide the capability to assign a network number AND a host number.
  • The IP address and network mask are required to have a routed network.
  • Network mask allows groups of sequential IP addresses to be treated as a single unit.
If this grouping were not allowd each host would ahve to be mapped individually for routing. All 254 addresses in the sequence 192.168.10.1 to 192.168.10.254 can be represented by the network address 192.168.10.0

[edit] IP As Routed Protocol

IP is the most used implementation addressing scheme. IP Is:

  • Connectionless
  • Unreliable (Does not make any effort to see if the packet was delivered, this function is haddle by higher layer protocols)
  • Best-effort delivery system protocol used on the internet.
  • Connectionless here means that no dedicated circuit connection is required. The IP protocol takes whichever route is the most efficient based on the routing protocol decision. As the information travels to the lowers layers of the OSI Model the data is encapsulated within packets called datagrams. IP determines the form of the IP packet header (addressing and other contron information) but it is not concerned with the data itself; It takes whatever is sent down by the upper layers.
Data Encapsulation Datagrams

[edit] Packert Propagation & Switching within a Router

As packet travels throught an internetwork to geto to ist final destination, the frame's headers adn trailers are stripped and replaced at every router. This reason for this it that Layer 2 units (frames) are local addressing and Layer 3 units (packets) are for end-to-end addressing. Frames are meant to work within a broadcast domain and to work with the MAC addresses that are assigned to the physical device, as data crosses a Layer 3 device the Layer 2 information changes.

TO HE CONTINUED

[edit] Connectionless & Connection Oriented Delivery.

Most of the services in the OSI model are connectionless delivery system.

Connectionless Delivery

Each packet is treated differently and send it on its way through the network. In a connnection less system , the destination is not contacted before a packet is sent. Connectionless networks processes are often referred to as being packet switched. As the packets pass from source to destination they can switch to different paths as well as arrive out of order.

Connection Oriented Delivery

Is this type of systems a connection is stablished between the sender and the recipient before any data is transferred. In the connection oriented system a connection is stablished first and then the data transfer occurs. All packets travel sequentially accross the same physical circuit.

[edit] IP Routing Protocols

Routing is an OSI Layer 3 function and allows individuals addresses to be grouped and treated as a single unit until the individual address is needed for final delivery of the data.Routing consist of finding the most efficient path from one device to another. The main device that delivery this functionality is the ROUTER. A router has 2 keys functions:

  1. Maintain routing tables and make sure other routers know of changes in the network topology. This function is done by suing the proper routing protocols and be able to communicate network information to other routers.
  2. When packets arreived at an interface, the router must used the routing table to determine where to send the packets. It switches them to the appropriate interface.

The router usees one or more routing metrics determined the optimal path along which netork traffic should be forwarded. The routing metric is a value used to determine the route's desirability. The metrics:

  • Hop count
  • Bandwith
  • Delay
  • Reliability
  • Load
  • Cost

This values are calculated in various combinations to determined the best path through internetwork. Routers interconnect network segments or entire networks. They pass the data based on LAYER 3 Information. Routers make a logical decision and then redirect packets to the appropriate output port to be encapsulated for transmission. The encapsulation/de-encapsulation process occurs each time a packet passes through a router and data is sent from one device to another. Encapsulation breaks the data stream into segments, adds the appropriate headers and trailers and transmits the data. De-encapsulation process is the opposite, removing the headers and trailers and then recombining the data into a seamless stream.

[edit] Routing Versus Switching

Routers and switches may seem to perform the same function but they are not The primary difference is that switches operate at Layer 2 of the OSI model and routers operate at Layer 3. This distinction indicates that routers and switches use different information to send data from a source to a destination.

Switching:

  • Layer 2 of the OSI Model (Data Link)
  • ARP Table of MAC Addresses to Port Number.
  • ARP Table is effective only within the same Broadcast Domain.
  • Interconnect segments beloging to the same logical network or segment.
  • Switches DO NOT Block broadcast.

Routing:

  • Layer 3 of the OSI Model (Network)
  • ARP Table of MAC to IP Addresses.(IP Addresses are organized hierarchy)
  • Routing table allows it to route data outside of the broadcast domain.
  • Routers requires organized address system so that similar addresses are group together and treat them as a single network.
  • Routers provide a higher level of security and bandwith control since they block controls.
Router and Switch Feature.

[edit] Path Determination

[edit] Routing Tables

[edit] Routing Alogarithms

An algorithm is a detailed solution to a problem. Different routing protocols use different algorithms to choose the port to which a packet should be sent. Routing algorithms depend on metrics to make these decisions.

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

  • Optimization
  • Simplicity and low overhead
  • Robustness and stability
  • Flexibility
  • Rapid convergence
Optimization

This is the capability of a routing algorithm to select the best route. The route will depend on the metrics and metric weights used in the calculation. For example, one algorithm may use both hop count and delay metrics, but may consider delay metrics as more important in the calculation.

Simplicity and low overhead

The simpler the algorithm, the more efficiently it will be processed by the CPU and memory in the router. This is important so that the network can scale to large proportions, such as the Internet.

Robustness and stability

A routing algorithm should perform correctly when confronted by unusual or unforeseen circumstances, such as hardware failures, high load conditions, and implementation errors.

Flexibility

A routing algorithm should quickly adapt to a variety of network changes. These changes include router availability, router memory, changes in bandwidth, and network delay.

Rapid convergence

Convergence is the process of agreement by all routers on available routes. When a network event causes changes in router availability, updates are needed to reestablish network connectivity. Routing algorithms that converge slowly can cause data to be undeliverable.

[edit] Routing Metrics

Routing Alogarithms used different metrics to determine the best route. The metrics can be based on a single path or can be calculated based on several characteristics. The following metrics are most commonly used by routing protocols:

  • Bandwidth
  • Delay
  • Load
  • Reliability
  • Hop Count
  • Ticks
  • Cost
Bandwidth
Bandwidth is the data capacity of a link. Normally, a 10-Mbps Ethernet link is preferable to a 64-kbps leased line.
Delay
Delay is the length of time required to move a packet along each link from a source to a destination. Delay depends on the bandwidth of intermediate links, the amount of data that can be temporarily stored at each router, network congestion, and physical distance.
Load
Load is the amount of activity on a network resource such as a router or a link.
Reliability
Reliability is usually a reference to the error rate of each network link.
Hop count
Hop count is the number of routers that a packet must travel through before reaching its destination. Each router is equal to one hop. A hop count of four indicates that data would have to pass through four routers to reach its destination. If multiple paths are available to a destination, the path with the least number of hops is preferred.
Ticks
The delay on a data link using IBM PC clock ticks. One tick is approximately 1/18 second.
Cost
Cost is an arbitrary value, usually based on bandwidth, monetary expense, or other measurement, that is assigned by a network administrator.

[edit] Interior and Exterior Gateway Protocols.

An autonomous system is a network or set of the networks under common administrative control. An autonomous systems consist of routers that present a consistent view of routing to the external world.

Two families of routing protocols are Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP) IGPs route data WITHIN an autonomous system:

  • RIP
  • IGRP
  • EIGRP
  • OSPF
  • Intermediate System-to-Intermediate System (IS-IS) protocol.

EGPs routes data BETWEEN autonomous systems. An example of the EGP is:

  • BGP

[edit] Link State & Distance Vector

Link state and Distance vector is a different way to classify routing protocols.If IGP and EGP describes the physical relationships of routers, the distance vector and link-state-categories describe how rounters interact with each other in terms of routing updates.

Distance vector routing
Determines the direction (vector) and distance (hop count) to any link in the internetwork. Distace vectors periodically (every 30 seconds) send all or some portion of their routing table to their adjacent neighbors. Routing running the Distance vector protocols send updates, periodically, even if there is no changes in the network. This process is called "routing by runor" because the information the router has is based on the neighbor's information of the network topology.

Exemples of distance vector protocols include the folllowing:

  • Routing Information Protocol(RIP) - The most commont IGP in the Internet, RIP uses hop counts as it routing metric.
  • Interior Gateway Routing Protocol(IGRP) -
  • Enhanced Interior Gateway Routing Protocol (EIGRP)
Link State Protocols
Were designed to overcome the limitations of distance vector routing protocols. Link-state routing protocols respond quickly to network changes, send trigger updates only when a network change has occurred and send periodic updates (link-state refreshes) at long time intervals, such as every 30 minutes. When a link changes state, the device that detected the change creates a link-state advertisement (LSA) concerning that link (route) and that LSA is propagated to all neighboring devices. Each routing devices takes a copy of the LSA, updates its link-state (TOPOLOGICAL) database, and forwards the LSA to all neighboring devices.The link-state database is used to calculae the best paths through the network. Link-state routers find the best paths to destination by applying the Dijkstra Shortest Path First(SPF) alogarithm againts the link state database to build the SPF tree.

[edit] Routing Protocols

The below page describes the metrics, network usability, and other significant characteristics of the most commonly used routing protocols.

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