CSC458 ‐ Problem Set 2 (PS2)


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1. Consider the network shown below, in which horizontal lines represent transit providers and
numbered vertical lines are inter‐provider links.
a) How many routes to P could provider Q’s BGP receive?
b) Suppose Q and P adopt the policy that outbound traffic is routed to the closest link to
the destination’s provider, thus minimizing their own cost. What paths will traffic from
host A to host B and from host B to host A take?
2. Suppose most of the Internet used some form of geographical addressing, but that a large
international organization has a single IP network address and routes its internal traffic over its
own links.
a) Explain the routing inefficiency for the organization’s inbound traffic inherent in this
b) Explain how the organization might solve this problem for outbound traffic.
c) For your method above to work for inbound traffic, what would have to happen?
d) Suppose the large organization now changes its addressing to separate geographical
addresses for each office. What will its internal routing structure have to look like if
internal traffic is still to be routed internally?
3. The sequence number field in the TCP header is 32 bits long, which is big enough to cover over 4
billion bytes of data. Even if this many bytes were never transferred over a single connection, why
might the sequence number still wrap around from 232 −1 to 0?

4. Suppose a TCP connection has a window size of eight segments and an RTT of 800 ms, the sender
sends segments at a regular rate of one every 100 ms, and the receiver sends ACKs back at the
same rate without delay. A segment is lost, and the loss is detected by the fast retransmit algorithm
on the receipt of the third duplicate ACK. At the point when the ACK of the retransmitted segment
finally arrives, how much total time has the sender lost (compared to lossless transmission) if the
sender waits for the ACK from the retransmitted lost packet before sliding the window forward
5. You are an Internet Service Provider; your client hosts connect directly to your routers. You know
some hosts are using experimental TCPs and suspect some may be using a “greedy” TCP with no
congestion control. What measurements might you make at your router to establish that a client
was not using slow start at all? If a client used slow start on startup but not after a timeout, could
you detect that?
6. Consider the arrangement of hosts H and routers R and R1 below. All links are full‐duplex, and all
routers are faster than their links. Show that R1 cannot become congested, and for any other
router R we can find a traffic pattern that congests that router alone.
7. Suppose Client A initiates a Telnet session with Server S. At about the same time, Client B also
initiates a Telnet session with Server S. Provide possible source and destination port numbers for
a) The segments send from A to S
b) The segments sent from B to S
c) The segments sent from S to A
d) The segments sent from S to B
e) If A and B are different hosts, is it possible that the source port number in the segments
from A to S is the same as that from B to S?
f) How about if they are the same host?

8. UDP and TCP use 1s complement for their checksums. Suppose you have the following three 8‐bit
bytes: 01010011, 01100110, 01110100. What is the 1s complement of the sum of these 8‐bit
bytes? (Note that although UDP and TCP use 16‐bit words in computing the checksum, for this
problem you are being asked to consider 8‐bit sums.) Show all work. Why is it that UDP takes the
1s complement of the sum; that is, why not just use the sum? With the 1s complement scheme,
how does the receiver detect errors? Is it possible that a 1‐bit error will go undetected? How about
a 2‐bit error?
9. Consider sending a large file from a host to another over a TCP connection that has no loss.
a) Suppose TCP uses AIMD for its congestion control without slow start. Assuming cwnd
increases by 1 MSS every time a batch of ACKs is received and assuming approximately
constant round‐trip times, how long does it take for cwnd to increase from 6 MSS to 12
MSS (assuming no loss events)?
b) What is the average throughout (in terms of MSS and RTT) for this connection up
through time = 6 RTT?
10. Consider the network shown below. Suppose AS3 and AS2 are running OSPF for their intra‐AS
routing protocol. Suppose AS1 and AS4 are running RIP for their intra‐AS routing protocol. Suppose
eBGP and iBGP are used for the inter‐AS routing protocol. Initially suppose there is no physical link
between AS2 and AS4.
a) Router 3c learns about prefix x from which routing protocol: OSPF, RIP, eBGP, or iBGP?
b) Router 3a learns about x from which routing protocol?
c) Router 1c learns about x from which routing protocol?
d) Router 1d learns about x from which routing protocol?

11. Consider the following network. With the indicated link costs, use Dijkstra’s shortest‐path
algorithm to compute the shortest path from x to all network nodes.
12. Consider transferring an enormous file of L bytes from Host A to Host B. Assume an MSS of 536
a) What is the maximum value of L such that TCP sequence numbers are not exhausted?
b) Recall that the TCP sequence number field has 4 bytes. For the L you obtain in (a), find how
long it takes to transmit the file. Assume that a total of 66 bytes of transport, network, and
data‐link header are added to each segment before the resulting packet is sent out over a 155
Mbps link. Ignore flow control and congestion control so A can pump out the segments back to
back and continuously.