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Midterm Material
Network Layer
Link Layer
Network Security
Wireless
100
What is a web cache?
A mechanism for the temporary storage (caching) of web documents, such as HTML pages and images, to reduce bandwidth usage, server load, and perceived lag.
100
What does it mean for a routing algorithm to use global, complete knowledge?
Global - Knowledge from the entire network
Complete - Knowledge of everything in the domain
100
How is a 2 byte checksum calculated and verified?
The message (including sometimes parts of the header) are split into 2 byte chunks. These chunks are summed up and the two's compliment of this sum is the checksum value.

Upon receipt of the message the receiver calculates the sum again and XORs the result with the checksum provided. If the result is all 1s, the checksum passes.
100
What difficult mathematical problem is RSA encryption based on?
The factoring problem, which is known to be in the class of NP problems.
100
What is the difference between an access point and a router?
An access point is a link layer device, and serves as a wireless base station for nearby wireless hosts.

A router is a network layer device, and handles routing IP datagrams through a network.

It is common for an access point and a router to both be contained in the same piece of hardware.
200
Name the four types of delay that can occur in a network.
Processing delay - Time routers take to process the packet header
Queuing delay - Time the packet spends in routing queues
Transmission delay - Time it takes to push the packet's bits onto the link
Propagation delay - Time for a signal to reach its destination
200
What is a broadcast storm?
A state in which a message that has been broadcast across a network results in even more responses, and each response results in still more responses in a snowball effect.

If not stopped this can block all traffic on a network, essentially bringing the network down.
200
What is a MAC Address? And what is it used for?
MAC Address - 6 byte address, unique to the device.

Used for link layer addressing. Routing messages between physical devices.
200
Using RSA, if a message has been encoded using a person's public key pair (n, e), how is the private key pair (n, d) used to decode the message?
c = encoded message

message = c^d mod n
200
What are DIFS and SIFS?
DIFS - Distributed Inter-frame Space. The time waited after a channel is detected as idle before transmitting.

SIFS - Short Inter-frame Spacing. The time waited after receiving a data frame before sending an acknowledgement.
300
What are the different levels of DNS servers?
Root
Top-Level Domain
Authoritative
Local
300
What is datagram fragmentation? What does IPv6 do instead of datagram fragmentation?
Datagram fragmentation is used in IPv4 to split datagrams that are too large to fit into the link layer frame into smaller datagrams that can be reassembled later.

IPv6 used ICMP to send a message back to the sender saying the datagram is too large. The sender can then resend a smaller version of the datagram.
300
What is the difference between forwarding and filtering?
Filtering - Should a frame be forwarded?
Fowarding - To what interface should a frame be sent?
300
How can RSA be cracked given two moduli that have a GCD greater than one?
Each modulus can be divided by the GCD (which is far less computationally intense than computing the factors the modulus) to retrieve the two prime factors of the modulus (p and q).

Using those prime factors and the other half the public key pair, e, you can produce the private key pair associated with the public key pair.
300
What is the purpose of address 3 in the 802.11 frame when in infrastructure mode?
It is used to hold the MAC address of the first switch or router beyond the access point. It is utilized in translating from an 802.11 frame to an Ethernet frame (and vice versa).
400
Why it is unnecessary for Go-back-N to cache segments when they are received out of order?
Those segments will not be acknowledged, so eventually they will be resent.
400
What is the goal of multicast addressing?
To be able to send a single message to multiple hosts (either know or unknown) through a single address.
400
What type of MAC protocol does Ethernet use? How does this MAC protocol deal with collision?
Random Access

It utilizes collision detection. If collision is detected it sends a jamming signal and waits an exponentially growing amount of time before trying to send again.
400
How can a simple double lock cipher be broken?
Record the encrypted message after host A locks the message and sends it to host B (c1), the encrypted message after host B locks it and sends it to A (c2), and the encrypted message after host A unlocks it and sends it to B (c3).

XOR c1 and c2 to get B's key.
XOR c2 and c3 to get A's key.
400
How does a wireless host handle moving between BSSs (Basic Server Sets) within the same subnet?
1. The wireless host (H) detects the weakening signal of its current access point (AP1), and begins searching for a stronger signal to connect to.
2. H finds a stronger signal coming from the access point moving into range (AP2).
3. H dissociates itself with AP1 and associates itself with AP2.

The only remaining issue is how does the switch between the BSSs figure out that H moved. This can be handled by having AP2 transmit a frame saying H is now associated with AP2.
500
What is classeful addressing? Why it is not used in the Internet today?
Classeful Addressing - splitting the IPv4 address space into 3 classes, A, B and C, with subnet masks of 8, 16, and 24 respectively.

The available subsets of addresses were too rigid, leading to a large number of allocated, unused addresses.
500
How does the Distance Vector algorithm work?
Each router keeps a distance vector of distances to each router in the network. Messages are sent between neighbors, and values in the distance vector updated based upon the neighbors' values.

Eventually the algorithm will converge, at which point the optimal path from a given node to every other node in the network has been calculated.
500
What is the difference between Routers, Switches, and Hubs? Which of these require the use of the Ethernet MAC protocol?
Routers - Up to network layer. Deals with IP datagrams. Has traffic isolation and optimal routing.

Switch - Up to link layer. Deals with frames. Has traffic isolation and plug and play, but not optimal routing

Hub - Up to the link layer. Deals with frames. Does not have traffic isolation or optimal routing, but does have plug and play.


Only hubs require a MAC protocol with Ethernet
500
How can two host (A and B) authenticate each other using hashing if they both share a secret key?
1. Host A sends a secure random number (r) to B.
2. B XORs the r with the secret (S), and hashes the result.
3. B sends the hash back to A.
4. A also XORs r with S, and hashes the result.
5. A compares the result with what B sent back. If they match B has been authenticated to A.

This process is repeated with A and B switching roles in order for A to be authenticated to B.
500
What are the major differences between 3G and 4G? Why are LTE and WiMAX not technically 4G?
3G is circuit based, and only needs to operate at 200Kbps peak rate.

4G is packet based, and must have peak rates of 100Mbps for high mobility users (users in a car or on a bus/train) and 1Gbps for low mobility users (users walking or stationary).

LTE and WiMAX do not meet the peak rate requirements of 4G.

LTE runs at 100Mbps down and 50Mbps up.
WiMAX runs at 128Mbps down and 56Mbps up.