Internet Technology Chapter 5 Suggested Exercises

Is it necessary that every autonomous system use the same intra-AS routing algorithm? Why or why not? SECTIONS 5.3-5.4

No. Each AS has administrative autonomy for routing within an AS.

What is the purpose of the SNMP trap message?

A SNMP trap message is generated as a response to an event happened on a managed device for which the device's managing server requires notification. It is used for notifying a managing server of an exceptional situation (e.g., a link interface going up or down) that has resulted in changes to MIB object values.

Compare and contrast the properties of a centralized and a distributed routing algorithm. Give an example of a routing protocol that takes a centralized and a decentralized approach.

A centralized routing algorithm computes the least-cost path between a source and destination by using complete, global knowledge about the network. The algorithm needs to have the complete knowledge of the connectivity between all nodes and all links' costs. The actual calculation can be run at one site or could be replicated in the routing component of each and every router. A distributed routing algorithm calculates the lease-cost path in an iterative, distributed manner by the routers. With a decentralized algorithm, no node has the complete information about the costs of all network links. Each node begins with only the knowledge of the costs of its own directly attached links, and then through an iterative process of calculation and information exchange with its neighboring nodes, a node gradually calculates the least-cost path to a destination or a set of destinations. OSPF (Open Shortest Path First) protocol is an example of centralized routing algorithm, and BGP (Border Gateway Protocol) is an example of a distributed routing algorithm.

Names four different types of ICMP messages

Echo reply (to ping), type 0, code 0 Destination network unreachable, type 3 code 0 Destination host unreachable, type 3, code 1. Source quench (congestion control), type 4 code 0.

True or false: When a BGP router receives an advertised path from its neighbor, it must add its own identity to the received path and then send that new path on to all of its neighbors. Explain.

False. A BGP router can choose not to add its own identity to the received path and then send that new path on to all of its neighbors, as BGP is a policy-based routing protocol. This can happen in the following scenario. The destination of the received path is some other AS, instead of the BGP router's AS, and the BGP router does not want to work as a transit router.

True or false: When an OSPF route sends its link state information, it is sent only to those nodes directly attached neighbors. Explain.

False. With OSPF, a router broadcasts its link-state information to all other routers in the autonomous system to which it belongs, not just to its neighboring routers. This is because with OSPF, each router needs to construct a complete topological map of the entire AS and then locally runs Dijkstra's shortest-path algorithm to determine its least-cost paths to all other nodes in the same AS.

What are the purposes of the SNMP GetRequest and SetRequest messages?

GetRequest is a message sent from a managing server to an agent to request the value of one or more MIB objects at the agent's managed device. SetRequest is a message used by a managing server to set the value of one or more MIB objects in a managed device.

What two types of ICMP messages are received at the sending host executing the Traceroute program?

ICMP warning message (type 11 code 0) and a destination port unreachable ICMP message (type 3 code 3).

Compare and contrast link-state and distance-vector routing algorithms.

Link state algorithms: Computes the least-cost path between source and destination using complete, global knowledge about the network. Distance-vector routing: The calculation of the least-cost path is carried out in an iterative, distributed manner. A node only knows the neighbor to which it should forward a packet in order to reach given destination along the least-cost path, and the cost of that path from itself to the destination.

What is meant by a control plane that is based on logically centralized control? In such cases, are the data plane and the control plane implemented within the same device or in separate devices? Explain. SECTION 5.2.

Logically centralized control means that a logically central routing controller computes and distributes the forwarding tables to be used by each and every router, and each router does not compute its forwarding tabel, unlike the per-router control. In the case of logically centralized control, the data plane and control plane are implemented in separate devices; the control plane is implemented in a central server or multiple servers, and the data plane is implemented in each router.

What is meant by a control plane that is based on per-router control? In such cases, when we say the network control and data planers are implemented "monolithically," what do we mean?

Per-router control means that a routing algorithm runs in each and every router; both forwarding and routing function are constrained within each router. Each router has a routing component that communicates with the routing components in other routers to compute the values for its forwarding table. In such cases, we say that the network control and data planes are implemented monolithically because each router works as an independent entity that implements its own control and data planes.

Why are different inter-AS and intra-AS protocols used in the Internet?

Policy: Among ASs, policy issues dominate. It may well be important that traffic originating in a given AS not be able to pass through another specific AS. Similarly, a given AS may want to control what transit traffic it carries between other ASs. Within an AS, everything is nominally under the same administrative control and thus policy issues a much less important role in choosing routes with in AS. Scale: The ability of a routing algorithm and its data structures to scale to handle routing to/among large numbers of networks is a critical issue in inter-AS routing. Within an AS, scalability is less of a concern. For one thing, if a single administrative domain becomes too large, it is always possible to divide it into two ASs and perform inter-AS routing between the two new ASs. Performance: Because inter-AS routing is so policy oriented, the quality (for example, performance) of the routes used is often of secondary concern (that is, a longer or more costly route that satisfies certain policy criteria may well be taken over a route that is shorter but does not meet that criteria). Indeed, we saw that among ASs, there is not even the notion of cost (other than AS hop count) associated with routes. Within a single AS, however, such policy concerns are of less importance, allowing routing to focus more on the level of performance realized on a route.

How does BGP use the NEXT-HOP attribute? How does it use the AS-PATH attribute?

Routers use the AS-PATH attribute to detect and prevent looping advertisements; they also use it in choosing among multiple paths to the same prefix. The NEXT-HOP attribute indicates the IP address of the first router along an advertised path (outside of the AS receiving the advertisement) to a given prefix. When configuring its forwarding table, a router uses the NEXT- HOP attribute.

Describe the purpose of two types of OpenFlow messages (of your choosing) that are sent from a controlled device to the controller. Describe the purpose of two types of Openflow messages (of your choosing) that are send from the controller to a controlled device.

Two types of messages from a controlled device to a controller: • Flow-removed message. Its purpose is to inform the controller that a flow table entry has been removed, for example, by a timeout or as the result of a received modify-state message. • Port-status message. Its purpose is to inform the controller of a change in port status. Two types of messages from a controller to a controlled device: 3 • Modify-state. The purpose is to add/delete or modify entries in the switch's flow table, and to set switch port properties. • Read-state. The purpose is to collect statistics and counter values rom the switch's flow table and ports.