701 EE: Communication Networks
Week 7, Part 1 Jennifer Hou
[e-mail]
Buffer Management for Congestion Control Random early drop (RED) gateway
Calculates the average queue size, using a low-pass filter with an exponential weighted moving average.
Avg is compared for two thresholds, min and max .
When avg When avg
, no packet is dropped.
!
, all arriving packets are dropped.
! "
"
$ #
When is dropped with probability
! $ # %
, every arriving packet , where
!
OHIO S ATE T
UNIVERSITY
T . H . E
Slide # 2
RED Fine Tuning of Dropping Probability
$
Let be the number of packets that arrive since the last drop. With dropping probability , we have
#
$
$
#
#
$
A more desirable alternative is for to be a uniform random . To achieve this, we assign variable from
#
!
#
%
!
#
$
#
#
OHIO S ATE T
UNIVERSITY
T . H . E
Slide # 3
Fine Tuning of Dropping Probability
where is the number of arrived packets that have not been dropped.
Then, we have
! # % # # #
for
"
#
" #
$
and
& #
for
.
OHIO S ATE T
UNIVERSITY
T . H . E
Slide # 4
Advantages of RED Advantages:
Is simple.
Achieves congestion avoidance and control.
Performs better than drop tail gateways.
Is fair the fraction of dropped packets for each connection is roughly proportional to that connection's share of the bandwidth.
OHIO S ATE T
UNIVERSITY T . H . E
Slide # 5
Disadvantage of RED
Interferes with TCP window congestion control. TCP reacts to packet loss by halving its congestion window. TCP then increases its congestion window by one packet in every RTT. It takes more time for a large-window connection to regain its original rate after a packet loss. Since a TCP connection always attempts to utilize surplus bandwidth, the connection with a small window will continue to increase its congestion window after its congestion window restores, and hence in the case that multiple TCP connections experience packet loss, the small-window connection may capture the bandwidth that originally belong to large-window connection.
T . H . E
OHIO S ATE T
UNIVERSITY
Slide # 6
Disadvantages of RED
Is in favor of small-window connections.
Buffer occupancy is sensitive to the number of active connections. Suppose we have 500 connections each with a small window of 2 packets. Dropping one packet affects only one connection, and the utilization does not degrade. If there were only one connection with a window of 1000 packets, dropping one packet would reduce the winodw to 500, and it would take 500 round trip times for the window to grow back.
OHIO S ATE T
UNIVERSITY T . H . E
Slide # 7
Fair RED (FRED)
The minimum and maximum per-flow thresholds, per flow min and per flow max , are maintained for the number of packets a flow can have in the buffer.
"
The number, , of flow 's packets in the buffer is kept as the per-active-flow information, state and compared against per flow min and per flow max .
"
"
A flow with smaller than per flow min is not subject to random early dropping even if minth avg queue maxth (i.e., the random early drop mode).
"
A connection which consistently exceeds per flow max (i.e., connections that use more packets than others) is subject to more aggressive dropping.
OHIO S ATE T
UNIVERSITY
T . H . E
Slide # 8
Balanced RED (BRED)
BRED extends FRED and imposes three thresholds, , , and , on the number of packets that a flow can have in the buffer.
"
#
The three thresholds divide the state space of into 4 regions: (0, ), ( , ), ( , ), and ( , ), each of which is labeled with a dropping probability of 0, , , and 1, respectively.
# # "
"
Each active flow keeps its queue length, , and the number of its packets accepted into the queue since last drop, .
#
"
"
The dropping probability for a packet from flow is then a is in and . function of the region
# "
The reason for figuring into the dropping probability is to prevent consecutive multiple drops from a flow.
#
OHIO S ATE T
UNIVERSITY
T . H . E
Slide # 9
Stablized RED
SRED equips each queue with a zombie list that keeps a recently seen flows. list of
The zombie list starts out empty. When a packet arrives, if the zombie is not full, the packet connection id is added to the list.
When a packet arrives, if the zombie list is full, the packet is compared with a randomly chosen zombie in the zombie list.
If the arriving packet's connection id matches the zombie, a hit occurs. Otherwise, a miss occurs, and with probability the connection id of the packet is overwritten over the zombie.
#
OHIO S ATE T
UNIVERSITY
T . H . E
Slide # 10
Stablized RED
The result ...