Chapter 5 - Network Layer - Control Plane Flashcards
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 planes 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
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.
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 table, 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
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. OSPF protocol is an example of a centralized approach.
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. BGP is an example of a distributed approach.
Compare and contrast static and dynamic routing algorithms.
Static routing algorithms change the routing paths very slowly over time, often as a result of human intervention.
Dynamic routing algorithms change the routing paths as the network traffic loads or topology change. A dynamic algorithm can be run either periodically or in direct response to topology or link cost change
What is the “count to infinity” problem in distance vector routing?
The count-to-infinity problem refers to a problem of distance vector routing. The
problem means that it takes a long time for a distance vector routing algorithm to
converge when there is a link cost increase.
Increases in a link cost can cause incorrect route calculations that can take a number of iterations to resolve to a quiescent state. Solved using poison reverse.
How is a least cost path calculated in a decentralized routing algorithm?
Using the Bellman-Ford equation Dx(Y) = min ( c(x, v) + dv(y) )
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.
True or false: When an OSPF router 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 is meant by an area in an OSPF autonomous system? Why was the concept of an area introduced?
An area in an OSPF autonomous system is refers to a set of routers, in which each
router broadcasts its link state to all other routers in the same set. An OSPF AS can be configured hierarchically into multiple areas, with each area running its own OSPF link-state routing algorithm.
Within each area, one or more area border routers are responsible for routing packets outside the area. The concept of area is introduced for scalability reason, i.e., we would like to build a hierarchical routing for a large scale OSPF AS, and an area is an important building block in hierarchical routing.
Define and contrast the following terms: subnet, prefix, and BGP route.
A subnet is a portion of a larger network; a subnet does not contain a router; its boundaries are defined by the router and host interfaces.
A prefix is the network portion of a CDIRized address; it is written in the form a.b.c.d/x ; A prefix covers one or more subnets. When a router advertises a prefix across a BGP session, it includes with the prefix a number of BGP attributes.
In BGP jargon, a prefix along with its attributes is a BGP route (or simply 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 how a network administrator of an upper-tier ISP can implement policy when configuring BGP.
Typically any traffic that goes through an AS either has (1) a destination to that AS (2) a source in that AS, or (3) an agreement exists to allow non-local traffic through that AS.
A tier-1 ISP B may not to carry transit traffic between two other tier-1 ISPs, say A and C, with which B has peering agreements. To implement this policy, ISP B would not advertise to A routes that pass through C; and would not advertise to C routes that pass through A
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.
Describe the main role of the communication layer, the network-wide state management layer, and the network-control application layer in an SDN controller.
The communication layer is responsible for the communication between the SDN controller and those controlled network devices, via a protocol such as OpenFlow. Through this layer, an SDN controller controls the operation of a remote SDNenabled switch, host, or other devices, and a device communicates locally-observed events (e.g., a message indicating a link failure) to the controller.
The network-wide state-management layer provides up-to-date information about state a network’s hosts, links, switches, and other SDN-controlled devices. A controller also maintains a copy of the flow tables of the various controlled devices.
The network-control application layer represents the brain of SDN control plane. The applications at this layer use the APIs provided by a SDN controller to specify and control the data plane in the network devices. For example, a routing network-control application might determine the end-end paths between sources and destinations. Another network application might perform access control.
Suppose you wanted to implement a new routing protocol in the SDN control plane. At which layer would you implement that protocol? Explain.
. I would implement a new routing protocol at the SDN’s network-control application layer, as this is the layer where a routing protocol determines the end-to-end paths between sources and destinations.
