CS4461 - Computer Networks Exam 2

Congestion Collapse

Hosts send packets to internet as fast as advertised window allows
Congestion occurs at some routers and packets are dropped

Hosts time out and retransmit packets, resulting in more congestion

Congestion Control

Sources observe events in the network and determine capacity to control amount of packets transmitted

Congestion Window (cwnd)

Used by source to limit how much data it is allowed to have in transit at a given time

Max number of bytes of unacknowledged data is min(congestion window, advertised window)

Based on perceived congestion

Additive Increase Multiplicative Decrease (AIMD)

Saw tooth behavior

Sender increases transmission rate, probing for usable bandwidth until loss occurs

Once loss occurs cut cwnd in half and start adding again

TCP “slow” start

Initially cwnd = 1, double every RTT (increment for every ACK recieved)

On first loss set ssthresh to ½ cwnd and begin AIMD

TCP RENO

Indicated by 3 duplicate ACKs → network capably of delivering some segments

Cut cwnd in half and grow linearlly

TCP Tahoe

Indicated by 3 duplicate ACKs or timeout

Set cwnd = 1, ssthresh = ½ cwnd

Do slow start again

Estimating TCP Throughput

No congestion control: avg TCP throughput = W/RTT
With congestion control: assume ssthresh = W/2

Congestion Avoidance

Predict when congestion is about to happen and reduce the rate at which hosts send data just before packets start getting discarded

Source Base Congestion Avoidance

Check RTTs and modify cwnd appropriately

Router Based Congestion Avoidance

DEC bit - add a bit to the header of a packet that the router can set when congestion is about to occur
Random Early Detection - Implicitly notify a source of pending congestion by dropping a packet before congestion occurs

Explicit Congestion Notification (ECN)

Two bits it IP header (ToS Field) marked to indicate congestion

Congestion indication carried to receiving host by network router

Receiver sets EVE bit on receiver to sender ACK to notify of congestion

Network Layer

Provides process to process (end-to-end) communication

Runs routing algorithm so sender can efficiently locate a target receiver in network (determine path)

Helps forward message hop by hop following the path

Forwarding

Packet arrives at a router and goes out an output port based on the destination

Forwarding Table

Table of destination and next hop to go to based on the destination

Updated regularly by routing algorithms

Routing

Determining route for packets to take to get to destination

Data Plane

Local - per router function - Forwarding

determines how datagram arriving on router input port is forwarded to router output port

Control Plane

Network wide logic - Routing

Determines how datagram is routed among routers along end-end path from source host to destination host
Has two approaches (per router and SDN)

Per Router Control Plane

Individual routing algorithms components in each router interact in the control plane
Good: completely autonomous, robust
Bad: not controllable or programmable

Software Defined Networking (SDN)

Remote controller computes and installs forwarding tables in routers
Makes network controllable and programmable

Service Model of Network Layer

Best effort service (nothing guaranteed)