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Mohamed G. Gouda						CS 356
Spring 2019							Homework 1
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(1) Draw the  architecture of an Internet-like network that consists of
	4 hosts H1, H2, H3, and H4,
	4 routers R1, R2, R3, and R4, and
	10 subnetworks S1, S2, ..., and S10.
Each router is attached to four subnetworks and each subnetwork is
attached to two computers.
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(2) An ISP tree-like has 15 ISPs and 16 ANs where:
	V1 is a level-1 ISP,
	W1 and W2 are level-2 ISPs,
	X1, ..., and X4 are level-3 ISPs,
	Y1, ..., and Y8 are level-4 ISPs, and
	Z1, ..., and Z16 are ANs.

Assume that in this tree,
	W1 is attached to V1, X1, X2,
	W2 is attached to V1, X3, X4,
	Y1 is attached to X1, Z1, Z2,
	Y2 is attached to X1, Z3, Z4,
	Y3 is attached to X2, Z5, Z6,
	Y4 is attached to X2, Z7, Z8,
	...
	Y8 is attached to X4, Z15, Z16.

What is the set of providers of ISP Y5?
What is the set of providers of ISP V1?
What is the set of customers of AN  Z5?
What is the set of customers of ISP V1?
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(3) An application message M1 is sent from an application layer in a server
host SH to an application layer in a client host CH. Both SH and CH are
attached to a router R via two subnetworks with two link layers SL and CL,
respectively.

SH has four protocol layers: application (SA), transport (ST), network 
(SN), and link (SL). CH also has four protocol layers: application (CA),
transport (CT), network (CN), and link (CL). R also has three protocol
layers: network (RN), link (SL) which attaches SH to R, and link (CL)
which attaches CH to R.

SA generates message M1 and sends it to ST. ST receives M1, updates it
to M2, and forwards it to SN. SN receives M2, updates it to M3, and
forwards it to SL in SH. SL in SH receives M3, updates it to M4, and
forwards it to SL in R. SL in R receives M4, updates it to M3, and
forwards it to RN. RN receives M3 and forwards it to CL in R. CL in
R receives M3, updates it to M5, and forwards it to CL in CH. CL in
CH receives M5, updates it to M3, and forwards it to CN. CN receives
M3, updates it to M2, and forwards it to CT. Finally, CT receives M2,
updates it to M1, and forwards it to CA.

What are the headers in the five messages M1, M2, M3, M4, and M5?
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Solutions
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(1)
R1 is connected to H1 by S1
R1 is connected to R2 by S2
R1 is connected to R3 by S3
R1 is connected to R4 by S4

R2 is connected to H2 by S5
R2 is connected to R3 by S6
R2 is connected to R4 by S7

R3 is connected to H3 by S8
R3 is connected to R4 by S9

R4 is connected to H4 by S10
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(2)
The set of providers of Y5 is {X3}
The set of providers of V1 is { }
The set of customers of Z5 is { }
The set of customers of V1 is {W1, W2}
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(3)
M1 is (AH, text) where AH is an application header

M2 is (TH, AH, text) where TH is a transport header

M3 is (NH, TH, AH, text) where NH is a network header

M4 is (LH, NH, TH, AH, text) where LH is a header for the link layer of the 
subnetwork that connects SH with R

M5 is (LH', NH, TH, AH, text) where LH' is a header for the link layer of the 
subnetwork that connects CH with R
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