CS6250 COMPUTER NETWORKS
EXAM 1
What=are=advantages=and=disadvantages=of=a=layered=architecture?=-=ans-
✔✔Each=protocol=layer=offers=different=services.=Some=advantages=are=scalability,=fle
xibility,=and=ease=of=adding=/=removing=components=making=it=easier=for=cost-
effective=implementations.=Disadvantages=include:=some=layers=functionality=depends=
on=the=information=from=the=other=layer=and=violates=the=goal=of=layer=separation;=one
=layer=may=duplicate=lower=layer=functionalities;=overhead=both=in=computation=and=in=
message=headers=caused=by=abstraction=barriers=between=layers.
What=are=the=differences=and=similarities=of=the=OSI=model=and=five-
layered=Internet=model?=-=ans-✔✔The=OSI=model=and=the=5-
layered=Internet=Model=have=many=of=the=same=layers,=with=the=difference=being=thre
e=of=the=layers=are=combined=in=the=5-layered=model.=Specifically=the=five-
layer=model=combines=the=application,=presentation,=and=session=layers=from=the=OSI=
model=into=a=single=application=layer.
What=are=sockets?=-=ans-
✔✔A=network=socket=is=a=software=structure=within=a=network=node=of=a=computer=net
work=that=serves=as=an=endpoint=for=sending=and=receiving=data=across=the=network.=
The=structure=and=properties=of=a=socket=are=defined=by=an=application=programming=i
nterface=(API)=for=the=networking=architecture.=Sockets=are=created=only=during=the=lif
etime=of=a=process=of=an=application=running=in=the=node.https://en.wikipedia.org/
wiki/
Network_socket."A=process=sends=messages=into,=and=receives=messages=from,=the=
network=through=a=software=interface=called=a=socket.=Let's=consider=an=analogy=to=he
lp=us=understand=processes=and=sockets.=A=process=is=analogous=to=a=house=and=its=
socket=is=analogous=to=its=door....a=socket=is=the=interface=between=the=application=lay
er=and=the=transport=layer=within=a=host."=-=Kurose=and=Ross,=2.1
Describe=each=layer=of=the=OSI=model.=-=ans-
✔✔Application=layer:=Service,=Interface,=Protocol.=Ex:=Turn=on=your=smartphone=and=l
ook=at=the=list=of=apps.=(HTTP,=SMTP,=FTP,=DNS)
,Presentation=layer:=Plays=intermediate=role=of=formatting=the=information=received=from
=the=layer=below=and=delivering=it=to=the=application=layer.=Ex:=converting=big=endian=t
o=little=endian.
Session=layer:=Responsible=for=the=mechanism=that=manages=the=different=transport=st
reams=that=belong=to=the=same=session=between=end-
user=and=application=process.=Ex:=teleconference=app,=it=is=responsible=for=tying=toget
her=audio=and=video=streaming.
Transport=layer:=Responsible=for=the=end-to-
end=communication=between=end=hosts.=2=transport=protocols,=TCP=and=UDP.=TCP=in
cludes=a=connection-
oriented=service=to=the=applications=that=are=running=on=the=layer=above,=guaranteed=
delivery=of=the=application-
layer=messages,=flow=control,=and=congestion=control=mechanism.=UDP=provides=a=co
nnectionless,=best-
effort=service=to=the=applications=that=are=running=in=the=layer=above=without=reliability,
=flow,=or=congestion=control.=In=this=layer=the=packet=is=called=a=segment.
Network=layer:=This=layer=is=responsible=for=moving=the=packet=of=information,=called=
a=datagram,=from=one=host=to=another.=The=network=layer=is=responsible=for=delivering
=the=datagram=to=the=Transport=layer=on=the=destination=host.=In=this=layer=there=are=t
he=IP=Protocol=and=the=routing=tables.
Data=Link=layer:=Packets=are=referred=to=as=frames.=Examples=include:=ethernet,=ppp,=
wifi.=Responsible=for=moving=the=frames=from=one=node=(host=or=router)=to=the=next=n
ode.=Services=offered=by=the=data=link=layer=protocol=include=reliable=delivery=(transmi
ssion=of=the=data=from=one=transmitting=node,=across=one=link,=to=the=receiving=node.
Physical=layer:=This=layer=is=the=actual=hardware=responsible=to=transfer=bits=within=a=f
rame=between=two=nodes=c
Provide=examples=of=popular=protocols=at=each=layer=of=the=five-
layered=Internet=model.=-=ans-
✔✔Application:=NFS,=DNS,=SNMP,=ftp,=rcp,=telnet,=HTTP
Transport:=TCP,=UDP
Internet:=IP,=ARP,=ICMP
Data=Link:=PPP,=IEEE=802.2,=Ethernet
Physical=Network:=Token=Ring,=RS-232
What=is=encapsulation,=and=how=is=it=used=in=a=layered=model?=-=ans-
✔✔Encapsulation=is=when=data=(called=a=header)=is=appended=to=the=packet=through=
each=layer=to=signify=its=on=the=correct=path=to=the=destination=host.
What=is=the=end-to-end=(e2e)=principle?=-=ans-
✔✔A=design=choice=that=shaped=the=current=internet=architecture.=It=states=the=networ
k=core=should=be=simple=and=minimal,=while=the=end=systems=should=carry=the=intellig
ence.=Network=functions=should=be=simple=and=essential=commonly=used=functions=so
=any=host=can=utilize=the=service=and=higher=form=functions=should=be=built=into=the=ap
plication=itself.=Lower=level=layers=should=be=independent=and=free=to=perform=only=th
eir=designed=function=and=the=higher-
, level=layers=deal=with=the=more=intricate=functions=that=deal=with=the=specific=applicatio
n.
What=are=the=examples=of=a=violation=of=e2e=principle?=-=ans-
✔✔Violations=include=firewalls=and=traffic=filters.=Firewalls=violate=because=they=are=int
ermediate=devices=that=are=operated=between=two=end=hosts=and=they=can=drop=the=e
nd=host=communications.=Network=Address=Translation=(NAT)=boxes=are=also=a=violati
on=because=it=uses=the=single=public=IP=address=and=distributes=a=new=IP=scheme=to=
the=hosts=connected=to=it=to=route=data=through=re-
writing=the=header=info=to=route=to=the=correct=destination=host.=NAT=boxes=are=a=viol
ation=because=they=are=not=globally=addressable=or=routable.
What=is=the=EvoArch=model?=-=ans-
✔✔An=hourglass=shaped=model=of=the=Internet=where=the=outer=bands=are=more=freq
uently=modified=or=replaced=and=the=further=in=you=go=the=harder=it=is=for=that=layer=to
=be=altered=or=modified.
Explain=a=round=in=the=EvoArch=model.=-=ans-✔✔EvoArch=is=a=discrete-
time=model=that=is=executed=over=rounds.=At=each=round,=we=perform=the=following=st
eps:=A)=We=introduce=new=nodes,=and=we=place=them=randomly=at=layers.=B)=We=exa
mine=all=layers,=from=the=top=to=the=bottom,=and=we=perform=the=following=tasks:=1)=W
e=connect=the=new=nodes=that=we=may=have=just=introduced=to=that=layer,=by=choosin
g=substrates=based=on=the=generality=probabilities=of=the=layer=below=s(l−1),=and=by=c
hoosing=products=for=them=based=on=the=generality=probability=of=the=current=layer=s(l)
.=2)=We=update=the=value=of=each=node=at=each=layer=l,=given=that=we=may=have=new
=nodes=added=to=the=same=layer=l.=3)=We=examine=all=nodes,=in=order=of=decreasing=
value=in=that=layer,=and=remove=the=nodes=that=should=die.=C)=Finally,=we=stop=the=ex
ecution=of=the=model=when=the=network=reaches=a=given=number=of=nodes.
What=are=the=ramifications=of=the=hourglass=shape=of=the=internet?=-=ans-
✔✔A.=Many=technologies=that=were=not=originally=designed=for=the=internet=have=been
=modified=so=that=they=have=versions=that=can=communicate=over=the=internet=(such=a
s=Radio=over=IP).
B.=It=has=been=a=difficult=and=slow=process=to=transition=to=IPv6,=despite=the=shortage=
of=public=IPv4=addresses.
Repeaters,=hubs,=bridges,=routers=operate=on=which=layers?=-=ans-
✔✔Repeaters=and=Hubs=work=over=L1=(Physical=Layer)
Bridges=and=Layer=2-Switches=work=over=L2=(Data=link=layer)
Routers=and=Layer=3-Switches=work=over=L3=(Network=layer)
What=is=a=bridge,=and=how=does=it="learn"?=-=ans-
✔✔A=bridge=is=a=device=with=multiple=inputs/
outputs.=A=bridge=transfers=frames=from=an=input=to=one=(or=multiple)=outputs.=Though
=it=doesn't=need=to=forward=all=the=frames=it=receives.
EXAM 1
What=are=advantages=and=disadvantages=of=a=layered=architecture?=-=ans-
✔✔Each=protocol=layer=offers=different=services.=Some=advantages=are=scalability,=fle
xibility,=and=ease=of=adding=/=removing=components=making=it=easier=for=cost-
effective=implementations.=Disadvantages=include:=some=layers=functionality=depends=
on=the=information=from=the=other=layer=and=violates=the=goal=of=layer=separation;=one
=layer=may=duplicate=lower=layer=functionalities;=overhead=both=in=computation=and=in=
message=headers=caused=by=abstraction=barriers=between=layers.
What=are=the=differences=and=similarities=of=the=OSI=model=and=five-
layered=Internet=model?=-=ans-✔✔The=OSI=model=and=the=5-
layered=Internet=Model=have=many=of=the=same=layers,=with=the=difference=being=thre
e=of=the=layers=are=combined=in=the=5-layered=model.=Specifically=the=five-
layer=model=combines=the=application,=presentation,=and=session=layers=from=the=OSI=
model=into=a=single=application=layer.
What=are=sockets?=-=ans-
✔✔A=network=socket=is=a=software=structure=within=a=network=node=of=a=computer=net
work=that=serves=as=an=endpoint=for=sending=and=receiving=data=across=the=network.=
The=structure=and=properties=of=a=socket=are=defined=by=an=application=programming=i
nterface=(API)=for=the=networking=architecture.=Sockets=are=created=only=during=the=lif
etime=of=a=process=of=an=application=running=in=the=node.https://en.wikipedia.org/
wiki/
Network_socket."A=process=sends=messages=into,=and=receives=messages=from,=the=
network=through=a=software=interface=called=a=socket.=Let's=consider=an=analogy=to=he
lp=us=understand=processes=and=sockets.=A=process=is=analogous=to=a=house=and=its=
socket=is=analogous=to=its=door....a=socket=is=the=interface=between=the=application=lay
er=and=the=transport=layer=within=a=host."=-=Kurose=and=Ross,=2.1
Describe=each=layer=of=the=OSI=model.=-=ans-
✔✔Application=layer:=Service,=Interface,=Protocol.=Ex:=Turn=on=your=smartphone=and=l
ook=at=the=list=of=apps.=(HTTP,=SMTP,=FTP,=DNS)
,Presentation=layer:=Plays=intermediate=role=of=formatting=the=information=received=from
=the=layer=below=and=delivering=it=to=the=application=layer.=Ex:=converting=big=endian=t
o=little=endian.
Session=layer:=Responsible=for=the=mechanism=that=manages=the=different=transport=st
reams=that=belong=to=the=same=session=between=end-
user=and=application=process.=Ex:=teleconference=app,=it=is=responsible=for=tying=toget
her=audio=and=video=streaming.
Transport=layer:=Responsible=for=the=end-to-
end=communication=between=end=hosts.=2=transport=protocols,=TCP=and=UDP.=TCP=in
cludes=a=connection-
oriented=service=to=the=applications=that=are=running=on=the=layer=above,=guaranteed=
delivery=of=the=application-
layer=messages,=flow=control,=and=congestion=control=mechanism.=UDP=provides=a=co
nnectionless,=best-
effort=service=to=the=applications=that=are=running=in=the=layer=above=without=reliability,
=flow,=or=congestion=control.=In=this=layer=the=packet=is=called=a=segment.
Network=layer:=This=layer=is=responsible=for=moving=the=packet=of=information,=called=
a=datagram,=from=one=host=to=another.=The=network=layer=is=responsible=for=delivering
=the=datagram=to=the=Transport=layer=on=the=destination=host.=In=this=layer=there=are=t
he=IP=Protocol=and=the=routing=tables.
Data=Link=layer:=Packets=are=referred=to=as=frames.=Examples=include:=ethernet,=ppp,=
wifi.=Responsible=for=moving=the=frames=from=one=node=(host=or=router)=to=the=next=n
ode.=Services=offered=by=the=data=link=layer=protocol=include=reliable=delivery=(transmi
ssion=of=the=data=from=one=transmitting=node,=across=one=link,=to=the=receiving=node.
Physical=layer:=This=layer=is=the=actual=hardware=responsible=to=transfer=bits=within=a=f
rame=between=two=nodes=c
Provide=examples=of=popular=protocols=at=each=layer=of=the=five-
layered=Internet=model.=-=ans-
✔✔Application:=NFS,=DNS,=SNMP,=ftp,=rcp,=telnet,=HTTP
Transport:=TCP,=UDP
Internet:=IP,=ARP,=ICMP
Data=Link:=PPP,=IEEE=802.2,=Ethernet
Physical=Network:=Token=Ring,=RS-232
What=is=encapsulation,=and=how=is=it=used=in=a=layered=model?=-=ans-
✔✔Encapsulation=is=when=data=(called=a=header)=is=appended=to=the=packet=through=
each=layer=to=signify=its=on=the=correct=path=to=the=destination=host.
What=is=the=end-to-end=(e2e)=principle?=-=ans-
✔✔A=design=choice=that=shaped=the=current=internet=architecture.=It=states=the=networ
k=core=should=be=simple=and=minimal,=while=the=end=systems=should=carry=the=intellig
ence.=Network=functions=should=be=simple=and=essential=commonly=used=functions=so
=any=host=can=utilize=the=service=and=higher=form=functions=should=be=built=into=the=ap
plication=itself.=Lower=level=layers=should=be=independent=and=free=to=perform=only=th
eir=designed=function=and=the=higher-
, level=layers=deal=with=the=more=intricate=functions=that=deal=with=the=specific=applicatio
n.
What=are=the=examples=of=a=violation=of=e2e=principle?=-=ans-
✔✔Violations=include=firewalls=and=traffic=filters.=Firewalls=violate=because=they=are=int
ermediate=devices=that=are=operated=between=two=end=hosts=and=they=can=drop=the=e
nd=host=communications.=Network=Address=Translation=(NAT)=boxes=are=also=a=violati
on=because=it=uses=the=single=public=IP=address=and=distributes=a=new=IP=scheme=to=
the=hosts=connected=to=it=to=route=data=through=re-
writing=the=header=info=to=route=to=the=correct=destination=host.=NAT=boxes=are=a=viol
ation=because=they=are=not=globally=addressable=or=routable.
What=is=the=EvoArch=model?=-=ans-
✔✔An=hourglass=shaped=model=of=the=Internet=where=the=outer=bands=are=more=freq
uently=modified=or=replaced=and=the=further=in=you=go=the=harder=it=is=for=that=layer=to
=be=altered=or=modified.
Explain=a=round=in=the=EvoArch=model.=-=ans-✔✔EvoArch=is=a=discrete-
time=model=that=is=executed=over=rounds.=At=each=round,=we=perform=the=following=st
eps:=A)=We=introduce=new=nodes,=and=we=place=them=randomly=at=layers.=B)=We=exa
mine=all=layers,=from=the=top=to=the=bottom,=and=we=perform=the=following=tasks:=1)=W
e=connect=the=new=nodes=that=we=may=have=just=introduced=to=that=layer,=by=choosin
g=substrates=based=on=the=generality=probabilities=of=the=layer=below=s(l−1),=and=by=c
hoosing=products=for=them=based=on=the=generality=probability=of=the=current=layer=s(l)
.=2)=We=update=the=value=of=each=node=at=each=layer=l,=given=that=we=may=have=new
=nodes=added=to=the=same=layer=l.=3)=We=examine=all=nodes,=in=order=of=decreasing=
value=in=that=layer,=and=remove=the=nodes=that=should=die.=C)=Finally,=we=stop=the=ex
ecution=of=the=model=when=the=network=reaches=a=given=number=of=nodes.
What=are=the=ramifications=of=the=hourglass=shape=of=the=internet?=-=ans-
✔✔A.=Many=technologies=that=were=not=originally=designed=for=the=internet=have=been
=modified=so=that=they=have=versions=that=can=communicate=over=the=internet=(such=a
s=Radio=over=IP).
B.=It=has=been=a=difficult=and=slow=process=to=transition=to=IPv6,=despite=the=shortage=
of=public=IPv4=addresses.
Repeaters,=hubs,=bridges,=routers=operate=on=which=layers?=-=ans-
✔✔Repeaters=and=Hubs=work=over=L1=(Physical=Layer)
Bridges=and=Layer=2-Switches=work=over=L2=(Data=link=layer)
Routers=and=Layer=3-Switches=work=over=L3=(Network=layer)
What=is=a=bridge,=and=how=does=it="learn"?=-=ans-
✔✔A=bridge=is=a=device=with=multiple=inputs/
outputs.=A=bridge=transfers=frames=from=an=input=to=one=(or=multiple)=outputs.=Though
=it=doesn't=need=to=forward=all=the=frames=it=receives.
