# CS 118 Homework 6

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## Description

Problem 1
Suppose two packets arrive to two different input ports of a router at exactly the same time. Also suppose
there are no other packets anywhere in the router.
(a) Suppose the two packets are to be forwarded to two different output ports. Is it possible to forward
the two packets through the switch fabric at the same time when the fabric uses a shared bus?
(b) Suppose the two packets are to be forwarded to two different output ports. Is it possible to forward the
two packets through the switch fabric at the same time when the fabric uses switching via memory?
(c) Suppose the two packets are to be forwarded to two different output ports. Is it possible to forward
the two packets through the switch fabric at the same time when the fabric uses a crossbar?
Write your solution to Problem 1 in this box
Problem 2
Consider a router that interconnects three subnets: Subnet 1, Subnet 2, and Subnet 3. Suppose all of the
interfaces in each of these three subnets are required to have the prefix 223.1.17/24. Also suppose that Subnet
1 is required to support at least 60 interfaces, Subnet 2 is to support at least 90 interfaces, and Subnet 3
is to support at least 12 interfaces. Provide three subnet addresses (of the form a.b.c.d/x) that satisfy the
constraints. You may use the following link to help verify your result: http://jodies.de/ipcalc.
Write your solution to Problem 2 in this box
Problem 3
Consider sending a 2400 B datagram into a link that has an MTU (maximum transmission unit) of 700 B.
Suppose the original datagram is stamped with the identification number 422.
(a) How many fragments are generated?
(b) What are the values in the various fields in the IP datagram(s) generated related to fragmentation?
Write your solution to Problem 3 in this box

Problem 4
In this problem we will explore the impact of NATs on P2P applications. Suppose a peer with username
Arnold discovers through querying that a peer with username Bernard has a file it wants to download. Also
suppose that Bernard and Arnold are both behind a NAT. Try to devise a technique that will allow Arnold
to establish a TCP connection with Bernard without application-specific NAT configuration. If you have
difficulty devising such a technique, discuss why.
Write your solution to Problem 4 in this box
Problem 5
Consider the SDN OpenFlow network shown as follows. Suppose that the desired forwarding behavior for
datagrams arriving at s2 is as follows:
• Any datagrams arriving on input port 1 from hosts h5 or h6 that are destined to hosts h1 or h2 should
be forwarded over output port 2;
• Any datagrams arriving on input port 2 from hosts h1 or h2 that are destined to hosts h5 or h6 should
be forwarded over output port 1;
• Any arriving datagrams on input ports 1 or 2 and destined to hosts h3 or h4 should be delivered to
the host specified;
• Host h3 and h4 should be able to send datagram to each other.
Specify the flow table entries in s2 that implement this forwarding behavior.
Problem 5 continued on next page. . . Page 5 of 6
Write your solution to Problem 5 in this box
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