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CS 6250 Computer Networks Exam 2/68 Q/A
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CS 6250 Computer NetworksExam 2/68 Q/A
What are the main components of a router? - --Input/Output ports
-The switching fabric
-The routing processor
-What do the input ports do for a router? - --Line termination
-Data link processing unit decapsulate the packets
-Checks with the forwarding table to ensure each packet is forwarded to the
appropriate output port
-What does the switching fabric do? - -Moves packets from input to output
ports. 3 Types: memory, bus, and crossbar
-What do the output ports do? - -Receive and queue the packets that come
from the switching fabric and send them over to the outgoing link
-Routers control plane functions - -implements routing protocols, maintains
the routing tables, computers the forwarding table
-Router Lookup (Task 1 of 6) - -When a packet arrives, router looks at the
destination IP address and determines the output link by looking at the
forwarding table or FIB.
-Router Switching (Task 2 of 6) - -After lookup, the switching system takes
over to transfer the packet from the input link to the output link
-Router Queueing (Task 3 of 6) - -After switching, it will need to be queued
if the link is congested. The queue maybe simple as FIFO or some other
algorithm
-Router Header validation and checksum (4 of 6) - -The router checks the
packet's version number, decrements the time-to-live field, and recalculates
the header checksum
-Router Processing (Task 5 of 6) - -The routers build their forwarding tables
using routing protocols such as RIP, OSPF, BGP - they are implemented in the
routing processors
-Protocol Processing (Task 6 of 6) - -The routers, in order to implement their
functions, need to implement the following protocols:
-Simple Network Management Protocol which provides counters for remote
inspection
, -TCP and UDP for remote communication with the router
-Internet Control Message Protocol for sending error messages
-Switching via memory - -The ports operate as I/O devices in an operating
system. When an input port receives a packet, it sends an interrupt to the
routing processor and the packet is coped to the processors memory.
Process extracts address and looks to table to find output port. Then its
copied to the output port buffer
-Switching via bus - -When input receives a packet, it puts an internal
header that designates the output port, and sends it to a shared bus. All
output ports will receive the packet, but only the designated one will keep it.
Speed of the bus limits the speed of the router.
-Switching via Interconnection network - -Interconnect network that
connects N input ports to N output ports using 2N buses. Can carry multiple
packets at a time
-Bandwidth and Internet population scaling (problem 1 of 2 that routers
face) - -Caused by:
1. Increasing number of devices
2. increasing volumes of network traffic due to new applications
3. new technologies such as optical links that can accommodate higher
volumes of traffic
-Services at high speeds (problem 2 of 2 that routers face) - -New
applications require services such as protection against delays during
congestion, and protection during attacks or failures.
-Longest prefix matching (Router bottlenecks 1 of 4) - -Due to increasing
number of internet hosts and networks, routers need to group destinations
into prefixes, but then run into the problem of more complex algorithms for
longest prefix matching
-Service differentiation ((Router bottlenecks 2 of 4) - -Routers can provide
different quality of service to different packets. This requires routers to
classify packets based on complex criteria
-Switching Limitations (Router bottlenecks 3 of 4) - -Head of line blocking
occurs
-Bottlenecks about services (Router bottlenecks 4 of 4) - -providing
performance guarantees in high speeds is nontrivial.
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