3.1 ‫ הנוגעות בתת הפרק‬3 ‫תשובות לשאלות מפרק‬
‫ תן את טבלאות המעגלים המדומים עבור כ"א מהמתגים‬,‫ עבור התרחיש הבא‬:‫ השאלה‬3.1
:‫להלן‬.3.37 ‫ברשת מאיור‬
a. Host A connects to Host B.
b. Host C connects to Host G.
c. Host E connects to Host I.
d. Host D connects to Host B.
e. Host F connects to Host J.
f. Host H connects to Host A.
II
3
3
2
3
1
1
I VCI
0
1
0
2
1
2
OI
0
1
0
0
0
3
:‫כזכור הטבלה מורכבת מ‬
II–Incoming Interface :‫ממשק נכנס‬
VCI–Virtual Circuit Identifier :‫מזהה מעגל מדומה‬
OI–Outgoing Interface :‫ממשק יוצא‬
2 ‫מתג‬
1 ‫מתג‬
O VCI
O VCI
II
I VCI
OI
0
2
0
1
0
0
3
0
1
1
1
0
0
1
2
2
1
3
2
1
3
3
4 ‫מתג‬
II
3
2
0
I VCI
0
0
0
OI
1
3
3
O VCI
0
1
2
3 ‫מתג‬
II
0
0
0
0
I VCI
0
1
2
3
OI
3
2
3
1
O VCI
0
0
1
0
(3.2) For the network given in Figure 2, give the datagram forwarding table for each
node. The links are labeled with relative costs; your tables should forward each packet
via the lowest-cost path to its destination.
3.3 Question:
Give forwarding tables for switches S1 – S4 in Figure 3. Each switch should
have a “default” routing entry chosen to forward packets with unrecognised
destination addresses towards OUT. Any specific-destination table entries duplicated
by the default entry should then be eliminated.
3.4 Question: Consider the virtual circuit switches in Fig. 3.40. table 3.5
lists, for each switch, what (port, VCI) or (VCI, Interface) pairs are
connected to what other. Connections are bidirectional. List all
endpoint-to-endpoint connections.
Figure 3.40:
Table 3.5:
Switch S1
Switch S2
Switch S3
Port
VCI
Port
VCI
Port
VCI
Port
VCI
1
1
2
2
1
1
3
2
3
1
3
2
1
1
1
2
3
3
3
2
Port VCI Port VCI
1
1
3
2
2
3
1
1
:‫כלומר עלינו לגלות בין מי למי נפתחו צינורות‬
AD
AB
BE
. Switch S1 ‫הנתונים נלקחו מהטבלה עבור‬
3.5 In the source routing example of section 3.1.3, the address received
by B is not reversible and doesn’t help B know how to reach A.
Propose a modification to the delivery mechanism that does allow for
reversibility. Your mechanism should not require giving all switches
globally unique names.
3.6 Propose a mechanism that virtual circuit switches might use so that if
one switch loses all its state regarding connections, then a sender of
packets along a path through that switch is informed of the failure.
Because in the virtual circuit mechanism the links are permanent, the
switch can send a message of failure to all its ports, the receiving
switches will check their virtual circuits table, to check whether a
connection is set through the damaged switch. If so those switches
will send to their incoming ports message of the failure of that
switch.
3.7 Propose a mechanism that might be used by datagram switches so
that if one switch loses all or part of its forwarding table, affected
senders are informed of the failure.
If a switch loses its tables, it could notify its neighbors, but we
have no means of identifying what hosts down the line might use
that switch.
So, the best we can do is to notify senders by sending them an
unable to forward message whenever a packet comes in to the
affected switch.
or
(In my opinion, the terms 'datagram switches' and 'router' are
interchangeable. You can say in that way.)
An (ICMP) unreachable responses can be used to report failures
to senders/source.
(3.9) Suppose, in Figure 4, that a new link has been added connecting switch 3 port 1
to (where G is now) and switch 1 port 0 (where D is now); neither switch is aware of
this link. Furthermore, switch 3 mistakenly thinks that host B is reached via port 1.
a. What happens if host A attempts to send to host B, using datagram
forwarding?
b. What happens if host A attempts to send to host B, using the virtual
circuit signalling mechanism?
Figure 4
(a) the packet would be sent S1 – S2 – S3, the known route towards B.
S3 would then send the packet back to S1 along the new connection,
thinking it had forwarded it to B. The packet would continue to
circulate.
(b) This time the setup message would circulate forever.
(3.12) Given the extended LAN shown in Figure 5, indicate which ports are not
selected by the spanning tree algorithm.
(3.13) Consider the arrangement of learning bridges shown in Figure 6. Assuming all
are initially empty, give the forwarding tables for each of the bridges B1 – B4 after
the
following transmissions:
i. A sends to C
j. C sends to A
k. D sends to C
Identify ports with the unique neighbour reached directly from that port;
that is, the ports for B1 are to be labelled “A” and “B2”.
(3.14) Consider hosts X, Y, Z, W and learning bridges B1, B2, B# with initially
empty forwarding tables, as shown in Figure 7.l. Suppose X sends to Z. Which
bridges learn where X is? Does Y’s network interface see this packet?
Suppose Z now sends to X. Which bridges learn where Z is? Does Y’s network
interface see this packet?
Figure 7
(3.15) Give the spanning tree generated for the external LAN shown in Figure 8. How
were the ties resolved?
Figure 8
(3.16) Suppose two learning bridges B1 and B2 form a loop as shown in Figure 9, and
do NOT implement the spanning tree algorithm. Each bridge maintains a single table
of (address, interface) pairs.
a. What will happen if M sends to L?
b. Suppose, a short while later L replies to M. Give a sequence of events
that leads to one packet from M and one packet from L circling the
loop in opposite directions.
Figure 9