Routing & Forwarding
7 important questions on Routing & Forwarding
- Discusswhyeachinputportinahigh-speedrouterstoresashadowcopyoftheforwarding t ab l e.
Withtheshadow copy,theforwardingdecisionismadelocally,at eachinput port, without invokingthecentralizedroutingprocessor.Suchdecentralizedforwarding avoids creating a forwarding processing bottleneck at a single point within the router.
- Three types of switching fabrics are discussed in Section 4.3. List and briefly describe each type.Which,if any,can send multiplepacketsacrossthefabricin parallel?
Switching via memory; switching via a bus; switching via an interconnection network
- Describehow packet losscan occur at input ports.Describehow packet lossat input ports can be eliminated (without using infinite buffers).
Packet loss occurs if queue size at the input port grows large because of slow switching fabric speed and thus exhausting router’s buffer space. It can be eliminated if the switching fabric speed is at least n times as fast as the input line speed, where n is the number of input ports.
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- What is HOL blocking?Does it occur in input ports or output ports?
HOLblocking:Sometimestheapacketthatisfirstinlineataninputportqueue mustwaitbecause there is no available buffer space at the output port to which it wants to be forwarded. When this occurs,all thepacketsbehind thefirst packet areblocked,even if their output queueshaveroom to accommodatethem. HOLblockingoccursattheinputport.
- What isthe32-bit binary equivalent of theIPaddress223.1.3.27?
11001010.00000011.00001110. 00011001
- Supposetherearethreeroutersbetween asourcehost and adestination host.Ignoring fragmentation, an IP datagram sent from the source host to the destination host will travel over how many interfaces?How many forwarding tables will be indexed to move the datagram from the source to the destination?
8 interfaces (2 per router and 1 of the destination host and 1 of the source host); 3 forwarding tables (1 per router).
- SupposeHost AsendsHost BaTCPsegment encapsulatedinanIPdatagram.WhenHost B receives the datagram, how does the network layer in Host B know it should pass the
s e g m e n t ( t h a t i s , t h e p a y l o a d o f t h e d a t a g r a m ) t o T CP r a t h e r t h a n t o U D P o r t o s o m e t h i n g else?
s e g m e n t ( t h a t i s , t h e p a y l o a d o f t h e d a t a g r a m ) t o T CP r a t h e r t h a n t o U D P o r t o s o m e t h i n g else?
The8-bit protocol field in theIPdatagram containsinformation about which transport layer protocol the destination host should pass the segment to.
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