Computer Networks-CS 6250 Exam
Questions and Answers Latest Versions
(2025/2026) (Complete and Accurate)
Why Flow Control in TCP is Based on Byte Number (vs. packet number) - ✔✔1. Permits
the insertion of control information into the sequence space of the bytes. (later dropped)
2. Permits the TCP packet to be broken up into smaller packets if needed. (later moved to the IP
layer)
3. Permits a number of small packets to be gathered together into one larger packet
if retransmission of the data is necessary. (turned out to be critical)
DARPA Paper:
"Flow" / "Soft State" - ✔✔A building block to replace the datagram.
It would identify a sequence of packets traveling from source to destination, without
assuming any particular type of service.
The type of service would be enforced by the end points,w hich would periodically send
messages to ensure that the proper type of service was being associated with the flow.
Narrow Waist - ✔✔Every internet device "speaks" IP - allows interconnection of
existing networks while hiding underlying technology from applications.
Internet Layers
+ Examples and Guarantees - ✔✔1. Physical Layer
2. Link Layer (ethernet)
Guarantees P2P connectivity.
,3. Network Layer (IP)
Guarantees end-to-end connectivity.
4. Transport Layer (TCP, UDP)
Guarantees reliable transport and congestion control.
5. Application Layer (HTTP, SMTP)
End to End Argument (definition) - ✔✔The intelligence required to implement an
application should be placed at the endpoints.
"Dumb network, intelligent endpoints"
End to End Argument Violations - ✔✔NATs
VPN tunnels
TCP splitting - especially used when last hop is wireless
Spam filters
P2P systems
Caches & in-network aggregation
End to End Argument Paper:
Reasoning - ✔✔Functions placed at low levels of a system may be redundant or of
little value.
Many functions can be completely and correctly implemented only with the knowledge
and help of the application standing at the endpoints of the system.
, NAT (definition) - ✔✔Network Address Translation.
Maps public IP addresses / port numbers to private IP addresses / port numbers.
Why does NAT violate the end to end argument? - ✔✔Devices are not globally
addressable, so other devices can't initiate inbound connections.
ARP (definition) - ✔✔Address Reolution Protocol.
ARP table maps IP addresses to MAC addresses
ARP (process) - ✔✔1. Host queries with IP address - broadcast.
2. Host with IP address responds with MAC address - unicast.
3. When host receives reply, adds to ARP table.
4. To send packets, host puts IP packet inside Ethernet frame using MAC address destination.
Hubs vs. Switches - ✔✔Hub: Broadcast medium where all packets on the network are
seen everywhere.
Switch: Partitions LAN into separate broadcast domains / LAN segments. Frames bound for
a host in the same segment aren't forwarded to other segments.
Switch Table (definition) - ✔✔Maps destination MAC address to switch output ports
Learning Switches (process) - ✔✔1. Switch receives a packet with a destination address.
2a. If there is an entry in the table, forward accordingly and we are done. Otherwise...
2b. Flood and take note of source address + switch port in table.
Broadcast Storms + How to Prevent - ✔✔Broadcast storms can occur when there is a loop
in the network topology, and packets keep getting forwarded.
Prevent by creating a fowarding tree (spanning tree).
Questions and Answers Latest Versions
(2025/2026) (Complete and Accurate)
Why Flow Control in TCP is Based on Byte Number (vs. packet number) - ✔✔1. Permits
the insertion of control information into the sequence space of the bytes. (later dropped)
2. Permits the TCP packet to be broken up into smaller packets if needed. (later moved to the IP
layer)
3. Permits a number of small packets to be gathered together into one larger packet
if retransmission of the data is necessary. (turned out to be critical)
DARPA Paper:
"Flow" / "Soft State" - ✔✔A building block to replace the datagram.
It would identify a sequence of packets traveling from source to destination, without
assuming any particular type of service.
The type of service would be enforced by the end points,w hich would periodically send
messages to ensure that the proper type of service was being associated with the flow.
Narrow Waist - ✔✔Every internet device "speaks" IP - allows interconnection of
existing networks while hiding underlying technology from applications.
Internet Layers
+ Examples and Guarantees - ✔✔1. Physical Layer
2. Link Layer (ethernet)
Guarantees P2P connectivity.
,3. Network Layer (IP)
Guarantees end-to-end connectivity.
4. Transport Layer (TCP, UDP)
Guarantees reliable transport and congestion control.
5. Application Layer (HTTP, SMTP)
End to End Argument (definition) - ✔✔The intelligence required to implement an
application should be placed at the endpoints.
"Dumb network, intelligent endpoints"
End to End Argument Violations - ✔✔NATs
VPN tunnels
TCP splitting - especially used when last hop is wireless
Spam filters
P2P systems
Caches & in-network aggregation
End to End Argument Paper:
Reasoning - ✔✔Functions placed at low levels of a system may be redundant or of
little value.
Many functions can be completely and correctly implemented only with the knowledge
and help of the application standing at the endpoints of the system.
, NAT (definition) - ✔✔Network Address Translation.
Maps public IP addresses / port numbers to private IP addresses / port numbers.
Why does NAT violate the end to end argument? - ✔✔Devices are not globally
addressable, so other devices can't initiate inbound connections.
ARP (definition) - ✔✔Address Reolution Protocol.
ARP table maps IP addresses to MAC addresses
ARP (process) - ✔✔1. Host queries with IP address - broadcast.
2. Host with IP address responds with MAC address - unicast.
3. When host receives reply, adds to ARP table.
4. To send packets, host puts IP packet inside Ethernet frame using MAC address destination.
Hubs vs. Switches - ✔✔Hub: Broadcast medium where all packets on the network are
seen everywhere.
Switch: Partitions LAN into separate broadcast domains / LAN segments. Frames bound for
a host in the same segment aren't forwarded to other segments.
Switch Table (definition) - ✔✔Maps destination MAC address to switch output ports
Learning Switches (process) - ✔✔1. Switch receives a packet with a destination address.
2a. If there is an entry in the table, forward accordingly and we are done. Otherwise...
2b. Flood and take note of source address + switch port in table.
Broadcast Storms + How to Prevent - ✔✔Broadcast storms can occur when there is a loop
in the network topology, and packets keep getting forwarded.
Prevent by creating a fowarding tree (spanning tree).
