Computer Networks CS 6250 Actual
Exam Questions With Reviewed 100%
Correct Detailed Answers
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1. Spanning Tree (definition) - ANSWER A loop-free topology that contains
every node.
- Instead of flooding, a switch forwards packets along the spanning tree.
2. Spanning Tree (construction overview) - ANSWER 1. Elect a root
(typically switch with smallest ID)
2. At each switch, exclude a link if it's not on the shortest path to the root.
3. Spanning Tree (process per node) - ANSWER 1. Every node starts by
thinking it is the root, broadcasting this information.
2. If a node learns of another node with a smaller ID, it updates its view of the
root and computes the distance to the new root, sending this information to
other nodes as well.
3. This process repeats until you end up with a spanning tree.
4. Switches vs. Routers - ANSWER Switches operate on layer 2 (ethernet).
They are auto-configuring, and forwarding tends to be fast.
,Routers operate on layer 3 (IP). Topologies are not restricted to a spanning
tree, and they support multipath routing (a packet can be sent through
multiple possible paths).
5. Layer 2 vs. Layer 3 Switching - ANSWER Layer 2 switching is more
convenient, but its major limitation is broadcast. Spanning tree messages
and ARP queries impose a high load on the network.
6. Buffer Sizing (simple / rule of thumb) - ANSWER If RTT (round trip time)
= 2T and capacity of bottleneck = C, then the buffer size should be 2T*C.
This is the number of bits that could be outstanding along the path at any
given time.
This rule of thumb holds for a single flow, or if all flows are perfectly
synchronized. (If flows are desynchronized, we can get away with less
buffering.)
7. AIMD (definition) - ANSWER Additive Increase, Multiplicative Decrease.
Used by TCP as congestion control.
8. AIMD (process) - ANSWER The starting rate is W_max / 2.
For every W ACKs received, send W+1 packets (up to maximum of W_max).
If a packet is dropped, drop back down to W_max / 2.
, 9. Buffer Sizing (large # of flows) - ANSWER If RTT (round trip time) = 2T,
capacity of bottleneck = C, and # of unique congestion windows of flows =
n, then the buffer size should be (2T*C) / sqrt(n).
This is due to the Central Limit Theorem.
10.Autonomous System (definition) - ANSWER A network that is under a
single administrative authority.
- It may be an ISP, content provider, campus network, etc. Examples include
Comcast, Georgia Tech, and Google.
11.Two Types of Routing - ANSWER Intradomain
Interdomain
12.Intradomain Routing - ANSWER Routing within one AS
13.Interdomain Routing - ANSWER Routing between ASes
14.Two Types of Intradomain Routing - ANSWER Distance-Vector Routing
Link State Routing
15.Distance-Vector Routing (overview) - ANSWER Each node sends
distance vectors (copies of its routing table) to its neighbors.
Routers compute costs based on the shortest path available.
Exam Questions With Reviewed 100%
Correct Detailed Answers
Guaranteed Pass!!
1. Spanning Tree (definition) - ANSWER A loop-free topology that contains
every node.
- Instead of flooding, a switch forwards packets along the spanning tree.
2. Spanning Tree (construction overview) - ANSWER 1. Elect a root
(typically switch with smallest ID)
2. At each switch, exclude a link if it's not on the shortest path to the root.
3. Spanning Tree (process per node) - ANSWER 1. Every node starts by
thinking it is the root, broadcasting this information.
2. If a node learns of another node with a smaller ID, it updates its view of the
root and computes the distance to the new root, sending this information to
other nodes as well.
3. This process repeats until you end up with a spanning tree.
4. Switches vs. Routers - ANSWER Switches operate on layer 2 (ethernet).
They are auto-configuring, and forwarding tends to be fast.
,Routers operate on layer 3 (IP). Topologies are not restricted to a spanning
tree, and they support multipath routing (a packet can be sent through
multiple possible paths).
5. Layer 2 vs. Layer 3 Switching - ANSWER Layer 2 switching is more
convenient, but its major limitation is broadcast. Spanning tree messages
and ARP queries impose a high load on the network.
6. Buffer Sizing (simple / rule of thumb) - ANSWER If RTT (round trip time)
= 2T and capacity of bottleneck = C, then the buffer size should be 2T*C.
This is the number of bits that could be outstanding along the path at any
given time.
This rule of thumb holds for a single flow, or if all flows are perfectly
synchronized. (If flows are desynchronized, we can get away with less
buffering.)
7. AIMD (definition) - ANSWER Additive Increase, Multiplicative Decrease.
Used by TCP as congestion control.
8. AIMD (process) - ANSWER The starting rate is W_max / 2.
For every W ACKs received, send W+1 packets (up to maximum of W_max).
If a packet is dropped, drop back down to W_max / 2.
, 9. Buffer Sizing (large # of flows) - ANSWER If RTT (round trip time) = 2T,
capacity of bottleneck = C, and # of unique congestion windows of flows =
n, then the buffer size should be (2T*C) / sqrt(n).
This is due to the Central Limit Theorem.
10.Autonomous System (definition) - ANSWER A network that is under a
single administrative authority.
- It may be an ISP, content provider, campus network, etc. Examples include
Comcast, Georgia Tech, and Google.
11.Two Types of Routing - ANSWER Intradomain
Interdomain
12.Intradomain Routing - ANSWER Routing within one AS
13.Interdomain Routing - ANSWER Routing between ASes
14.Two Types of Intradomain Routing - ANSWER Distance-Vector Routing
Link State Routing
15.Distance-Vector Routing (overview) - ANSWER Each node sends
distance vectors (copies of its routing table) to its neighbors.
Routers compute costs based on the shortest path available.
