CH4

https://gaia.cs.umass.edu/kurose_ross/videos/4/

4.1 Check all of the statements below about where (in the network) the network layer is implemented that are true.

The network layer is implemented in routers in the network core.

The network layer is implemented in hosts at the network's edge.

4.1 Consider the travel analogy discussed in the textbook - some actions we take on a trip correspond to forwarding and other actions we take on a trip correspond to routing.  Which of the following travel actions below correspond to forwarding? The other travel actions that you don't select below then correspond to routing.  

A car takes the 3rd exit from a roundabout.
A car stops at an intersection to “gas-up” and take a “bathroom break”
A car waits at light and then turns left at the intersection.

4.1 For each of the actions below, select those actions below that are primarily in the network-layer data plane. The other actions that you don't select below then correspond to control-plane actions.  

Control Plane is the decision-making layer. It determines how packets should be forwarded by building and maintaining routing tables using protocols like OSPF, BGP, or RIP.

Data Plane (or forwarding plane) is the execution layer. It moves packets from source to destination based on control plane logic

Key Differences:

• Function: Control plane decides routes; data plane forwards packets.

• Dependency: Data plane depends on control plane logic; control plane operates independently.

• Complexity: Control plane is more complex due to routing algorithms; data plane is simpler but performance-critical.

• Communication: Control plane uses routing protocols; data plane uses physical links like Ethernet/Wi-Fi.

• Examples: Control plane tasks include ARP, STP, DHCP; data plane tasks include packet switching and filtering.
  
  

Computing the contents of the forwarding table. (control-plane actions.)

Monitoring and managing the configuration and performance of an network device. (control-plane actions.)

Dropping a datagram due to a congested (full) output buffer.

Moving an arriving datagram from a router’s input port to an output port

Looking up address bits in an arriving datagram header in the forwarding table

4.1 We've seen that there are two approaches towards implementing the network control plane - a per-router control-plane approach and a software-defined networking (SDN) control-plane approach.  Which of the following actions occur in a per-router control-plane approach? The other actions that you don't select below then correspond to actions in an SDN control plane.  

A control agent in router receives a complete forwarding table, which it installs and
uses to locally control datagram forwarding.(SDN)

Routers send information about their incoming and outgoing links to other routers in the network.

A router exchanges messages with another router, indicating the cost for it (the sending router) to reach a destination host.

All routers in the network send information about their incoming and outgoing links to a logically centralized controller. (SDN)

4.1Which of the following quality-of-service guarantees are part of the Internet’s best-effort service model? Check all that apply.

None of the other services listed here are part of the best-effort service model. Evidently, best-effort service really means no guarantees at all!

4.2Where in a router is the destination IP address looked up in a forwarding table to determine the appropriate output port to which the datagram should be directed?

At the input port where a packet arrives.

4.2Where in a router does "match plus action" happen to determine the appropriate output port to which the arriving datagram should be directed?

At the input port where a packet arrives.

4.2Suppose a datagram is switched through the switching fabric and arrives to its appropriate output to find that there are no free buffers.  In this case:

The packet will either be dropped or another packet will be removed (lost) from the buffer to make room for this packet, depending on policy.  But the packet will definitely not be be sent back to the input port.

4.2What is meant by Head of the Line (HOL) blocking?

A queued datagram waiting for service at the front of a queue prevents other datagrams in queue from moving forward in the queue.