Networks Chapter 4/5 - Routing Algorithms

What is multicast routing?

A multicast packet is delivered to only a subset of network nodes.

What is sequence number controlled flooding?

A source node puts its address and broadcast sequence number into a broadcast packet then sends the packet to all of its neighbors. Each node maintains a list of the source address and sequence number of each broadcast packet it has already received, duplicated and forwarded

ASN

Autonomous system number

Describe AS.

Autonomous systems: a group of routers that are typically under admin control; can have ASs connected to each other Intra-AS: protocol between routers in the group and they must run the same protocols Inter-AS: protocol can be different because we are linking different autonomous systems. The outer most router will be the gateway router to an outside network

What is ABR?

Available bit rate

How much buffering is needed with a 10 Gbps link and an RTT of 250 ms?

B = RTT * C = 2.5 Gbits of buffers; however if there are multiple TCP flows (N) passing through a link, then the amount of buffering needed is B = RTT * C/sqrt(N)

What is BGP?

Border Gateway Protocol obtain subnet reachability information from neighboring ASs Propagate the reachability information to all routers internal to the AS Determine good routes to subnets based on the reachability information on AS policy BGP allows each subnet to advertise its existence to the rest of the internet

Which broadcast method completely avoids both broadcast storms and the transmission of redundant broadcast packets?

Broadcast along a spanning tree! sending packets down each unique vertice and edge

What is the decentralized routing algorithm?

Calculation of the least-cost path is carried out in an iterative distributed manner. No node has complete information about the costs of all network links. Through iteration a node gradually calculates the least cost path to a destination or set of destinations. The decentralized routing algorithm is called a distance-vector (DV) because each node maintains a vector of estimates of the costs to all other nodes in the network

Dijkstra's algorithm *need to review*

Computes the least cost path with iteration after the kth iteration of the algorithm, the least-cost paths are known to k destination nodes

What is a global routing algorithm?

Computes the least-cost path between a source and destination using complete, global knowledge about the network. Can run at one side - centralized global routing algorithm or replicated at multiple sites

What does the switching fabric of a router do?

Connects the router's input ports to its output ports

Describe how a router advertises a prefix across a BGP session with regards to AS-PATH

Contains the ASs through which the advertisement for the prefix has passed. When a prefix is passed into an AS, the AS adds its ASN to the AS-PATH attribute. If AS1 advertises the prefix to AS3, then AS-PATH would be AS2 AS1. Routers use the AS-PATH attribute to detect and prevent looping advertisements; if a router sees that its AS is contained in the path list it will reject the advertisement

What is the distance vector algorithm? *need to review*

DV is also known as the Bellman-Ford equation. It is distributed in that each node receives some information from one or more of its direct attached neighbors, performs a calculation and then distributes the results of calculation back to its neighbors. It is iterative in that the process continues until no more information is exchanges between neighbors. It is also asynchoronous in that it does not require all the nodes to operate in a lockstep manner with each other. It is decentralized and does not use global information

Describe the fatal flaw in uncontrolled flooding with regards to broadcast routing

During a flooding the source node sends a copy of the packet to all of its neighbors, and when a node receives a broadcast packet it duplicates the packet and forwards it to all of its neighbors. If however there were cycles in this network, then there would be endless flooding leading to a broadcast storm!!

What is the difference between forwarding and routing?

Forwarding involves the transfer of an incoming link to an outgoing link within a single router Routing involves all of a network's routers, whose collective interactions via routing protocols determine the paths that the packets take on their trips from source to destination node

If our IP datagram doesn't fit onto the outgoing link due to a small MTU, what shall we do?

Fragment, each of the smaller datagrams. Reassembly goes into end systems. When a datagram is created the sending host stamps the datagram with an identification number as well as a source and destination address. When a router needs to fragment a datagram, each resulting datagram is stamped with the source address, destination address and identification of the original datagram

What is the purpose of a routing algorithm?

Given a set of routers, with links connecting the routers, a routing algorithm finds a "good" path from source router to destination router-route with the least costs

How does BGP allow each AS to learn which destinations are reachable via its neighboring ASs?

In BGP, the destinations are not hosts but are CIDRized (classless interdomain routing) prefixes where each prefix represents a subnet or a collection of subnets. For example: four subnets attached to AS2: 138.16.64/24, 138.16.65/24, 138.16.66/24, 138.16.67/24. AS2 would aggregate the prefixes for these four subnets and use BGP to advertise the single prefix to 138.16.64/22 to AS1 Another example: the first three of the four subnets are in AS2 and the fourth subnet 138.16.67/24 is in AS3. Because routers use the longest-prefix matching for forwarding datagrams, AS# could advertise to AS1 the more specific prefix 138.16.67/24 and AS2 could still advertise to AS1 the aggregated prefix 138.16.64/22

What is IS-IS?

Intermediate System to Intermediate System routing protocol (ISO/IEC)

What does ICMP stand for?

Internet Control Message Protocol

What is ICMP?

Internet Control Message Protocol.

What is IGMP

Internet Group Management Protocol - operates between a host and its directly attached router

What does IP stand for?

Internet Protocol

What are link-state (LS) algorithms

LS algorithms are global state information

Describe the difference between load-sensitive algorithm and load-insensitive.

Load-sensitive algorithm: link costs vary dynamically to reflect the current level of congestion in the underlying link, if a high cost is associated with a link that is currently congested then a routing algorithm will tend to choose routes around such a congested link. Today's internet routing algorithms are load-insensitive, as the link's cost does not explicitly reflect the current level of congestion.

The maximum amount of data that a link layer frame can carry is called?

Maximum Transmission Unit (MTU)

What is NAT?

Network Address Translation

What does NAT stand for?

Network address translation

What is OSPF?

Open Shortest Path First user for intra-AS routing in the internet OSPF and IS-IS are typically deployed in the upper tier USES a flooding of link-state information and a Dijkstra least-cost path algorithm. A router constructs a complete topological map of the entire autonomous system. Locally runs Dijkstra's shortest-path algorithm to determine a shortest-path tree to all subnets with itself as the root node broadcasts routing information to all other routers in the autonomous system not just to its neighboring routers Authentication can promote security and only allow trusted routers to participate in the OSPF protocol within an AS

Discuss some functions of the input port.

Performs physical layer function of terminating an incoming physical link at a router. Performs link layer functions needed to interoperate with the link layer at the other side of the incoming link; represented by the middle boxes in the input and output ports. Look up function is also performed at the input port and occurs in the right most box of the input port; it is here that the forwarding table is consulted to determine the router output port to which an arriving packet will be forwarded by the switching fabric

What is PAT?

Port address tranlation

What is RIP?

Routing Information Protocol Distance-vector protocol that operates close to our DV protocol, uses hop count as a cost metric Response messages are also known as RIP advertisements Each router maintains an RIP table known as a routing table.

Output port

Stores packets received from the switching fabric and transmits these packets onto the outgoing link by performing the necessary link layer and physical layer functions.

Describe how a router advertises a prefix across a BGP session with regards to NEXT-HOP.

The NEXT-HOP is the router interface that begins the AS-PATH, used by routers to properly configure their forwarding tables

What is broadcast routing?

The most straightforward way to accomplish broadcast communication is for the sending node to send N packets that represent a different destination, then transmit N copies to the N destinations using unicast routing. N-way-unicast approach to broadcasting is simple. The most efficient way to forward packets for broadcast routing is to duplicate the packet when needed, otherwise there would be N packets traversing the first link.

What is the difference between a datagram service and a VC service?

The packets will take different routes with a datagram service and the packets will take the same routes between source-destination for VC

Describe an analogy similar to the details of a router's control and data plane.

The router is similar to a roundabout that a car enters in order to proceed on its journey. The car hits an attendant as it enters and she tells the car where to exit. The roundabout may have other cars in it already, and this car may encounter other cars in the roundabout.

What is the router forwarding plane?

The router's input ports, output ports and switching fabric implementing the forwarding functions and are almost always implemented in hardware

What is the solution for receiving unwanted multicast packets under RPF?

The solution to the problem is known as pruning. The multicast router that receives multicast packets and has no attached hosts joined to the group will send a prune message to its upstream router. If a router receives prune messages from each of its downstream routers, then it can forward a prune message upstream.

What is poisoned reverse?

To avoid a specific looping scenario, we will assign infinity to a particular path to avoid trying to attempt route creation from a to b Happens in our Distance-Vector Algorithm

What does TCP stand for?

Transmission Control Protocol

What does UDP stand for?

User Datagram Protocol

How do we maintain the spanning tree?

We can use the center-based approach, that is when each node joins the tree with a tree-join message it will send the message to a spcific central node

What is import policy?

When a gateway router receives a route advertisement it uses its import policy to decide whether to accept or filter the route and whether to set certain attributes such as the router preference metrics.

How can we avoid a broadcast storm?

With controlled flooding! In order to avoid a broadcast storm our node must judiciously choose when to flood a packet and when not to flood a packet.

What is a network-layer packet?

a datagram

What is a routing algorithm

algorithm with which packets flow from sender to receiver

What is a link state broadcast?

all nodes have an identical and complete view of the network. Each node can then run the LS algorithm and compute the same set of least-cost paths as every other node.

What is CBR?

constant bit rate, carrying real time constant bit rate audio and video traffic for ATM

Describe the names of the different routers and their position in the datagram packet forwarding.

default router (first-hop) - host attached directly to this one router source router- whenever a host sends a packet the default router of the host is this destination router- default router for the destination

What kind of BGPs are there?

external BGP session: a BGP session that spans two ASs internal BGP session: BGP session between routers in the same AS

What is RPF

reverse path forwarding RPF or RPB -reverse path broadcasting is known to only transmit packets on the shortest unicast path back to the source. The router simply discards the incoming packet without forwarding it on any of its outgoing links.

What is a VC?

virtual circuit, using connections at the network layer