Last lecture -- TCP enhancement - Fast retransmit and fast recovery

Today -- Open-loop Rate-based flow control (emerging paradigm in the
Internet) 
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Classification of flow control schemes:

a) closed-loop vs. open-loop:

   - In closed-loop scheme, a source dynamically adapts its flow to
   match its current share of network resources. As this share
   increases and decreases, a source should send faster or slower.

   - In open-loop scheme, a source describes its traffic to the
   network with a few parameters. During connection establishment, the
   network reserves resources (e.g. bandwidth and buffers)
   corresponding to these parameters. During transmission, if the
   source shapes its traffic to match its traffic's description,
   network congestion is avoided. 

b) explicit feedback vs. implicit feedback in closed-loop scheme:
 
   - explicit feedback: the network explicitly informs
   the source what the network congestion status is and how much its
   service rate has changed.
   
   - implicit feedback: the source infers a change in its service rate
   by measuring it current performance.


Recall that TCP is a window-based closed-loop flow control algorithm
with implicit feedback. This is suitable because data is bursty and
does not have a good traffic characterization to enable effective
open-loop flow control.


Steps involved in Open-loop flow control

1) During connection establishment, a source describes its behavior with
a set of paramters called its traffic descriptor and negotiates
bandwidth and buffer reservation with network elements along the path
to its destination.  

2) Usually the network operator prescribes the descriptor's
parameters, and each source decides parameter values that best
describe its traffic.

3) During connection establishment, each network node examines this
description and decides whether it can support the call. If it can, it
forwards the setup request to the next element along the
path. Otherwise, it blocks the call or negotiates the parameters down
to an acceptable value. 

4) In the transmission phase, the source shapes its traffic to match
its descriptor, and each network node schedules traffic from
admitted calls to meet the bandwidth, delay and loss guarantees it
makes to them. 

Challenges in open-loop control:

a) How do we choose a right set of parameters to describe a source
   adequately? Application dependent.

b) How do we implement a scheduling mechanism at intermediate network nodes?

c) How do we admit connections so that their performance objectives
are met (call admission control)?


A traffic descriptor typically represents a behavior envelope, i.e.,
the worst possible behavior of a source, rather than its exact behavior.

Introducing the leaky-bucket/token bucket algorithm for traffic
shaping at the source.