Computer Networks CS 6250 Q&A
DARPA Paper:
Main Goal - =Multiplexed utilization of existing interconnected networks
DARPA Paper:
Secondary Goals - =1. Survivability
2. Multiple types of service
3. Variety of networks
4. Distributed management
5. Cost effective
6. Ease of attachment
7. Accountability
Fate Sharing (definition) - =It is acceptable to lose state information for an entity if that entity is
lost.
(i.e. If a router crashes, its routing tables can be lost)
Fate Sharing (consequences) - =1. Protects against any number of intermediate failures (vs.
replication)
2. More trust placed in the host machine (vs. an architecture where the network ensures reliable
delivery)
DARPA Paper:
Survivability Mechanisms - =Fate sharing, replication
DARPA Paper:
Datagram (usefulness) - =Eliminates the need for connection state within intermediate switching
nodes.
,Provides a basic building block out of which a variety of types of service can be implemented.
Represents the minimum network service assumption, which has permitted a wide variety of
networks to be incorporated into various Internet realizations.
DARPA Paper:
Datagram (definition) - =The fundamental architectural feature of the Internet - the entity which
is transported across the underlying networks.
Important note: The role of the datagram is as a building block, and not as a service in itself.
Network Service:
Basic Assumptions - =The network can transport a packet or datagram of reasonable size, and it
will be delivered with reasonable but not perfect reliability.
The network must have some suitable form of addressing if it is more than a point-to-point link.
Network Services:
Explicitly Not Assumed - =1. Reliable or sequenced delivery
2. Network level broadcast or multicast
3. Priority ranking of transmitted packets
4. Support for multiple types of service
5. Internal knowledge of failures, speeds, or delays
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)
DARPA Paper:
Main Goal - =Multiplexed utilization of existing interconnected networks
DARPA Paper:
Secondary Goals - =1. Survivability
2. Multiple types of service
3. Variety of networks
4. Distributed management
5. Cost effective
6. Ease of attachment
7. Accountability
Fate Sharing (definition) - =It is acceptable to lose state information for an entity if that entity is
lost.
(i.e. If a router crashes, its routing tables can be lost)
Fate Sharing (consequences) - =1. Protects against any number of intermediate failures (vs.
replication)
2. More trust placed in the host machine (vs. an architecture where the network ensures reliable
delivery)
DARPA Paper:
Survivability Mechanisms - =Fate sharing, replication
DARPA Paper:
Datagram (usefulness) - =Eliminates the need for connection state within intermediate switching
nodes.
,Provides a basic building block out of which a variety of types of service can be implemented.
Represents the minimum network service assumption, which has permitted a wide variety of
networks to be incorporated into various Internet realizations.
DARPA Paper:
Datagram (definition) - =The fundamental architectural feature of the Internet - the entity which
is transported across the underlying networks.
Important note: The role of the datagram is as a building block, and not as a service in itself.
Network Service:
Basic Assumptions - =The network can transport a packet or datagram of reasonable size, and it
will be delivered with reasonable but not perfect reliability.
The network must have some suitable form of addressing if it is more than a point-to-point link.
Network Services:
Explicitly Not Assumed - =1. Reliable or sequenced delivery
2. Network level broadcast or multicast
3. Priority ranking of transmitted packets
4. Support for multiple types of service
5. Internal knowledge of failures, speeds, or delays
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)
