1. CIS, University of Delaware
Congestion control in Multicast
ElEG 667
May 7, 2003 by
Keyur Shah
CIS, University of Delaware

2. TCP Friendly
Congestion Control
Schemes
Single-rate
Multi-rate
Window-based
Rate-based
Window-based
Rate-based

3. Single Rate and Multi Rate
Data is sent to all receivers at the same rate.
The rate is typically limited by the slowest receiver
Thus a single slow receiver can drag down the data rate of the whole group

4. Multi-rate : Allows for a more flexible allocation of bandwidth along
different network paths.
Have the ability to generate the same data at different
rates over multiple streams.
Receivers try to listen to one or more streams depending
on their capacity
Receivers with different needs can be served at a rate
closer to their needs

5. Window-based and Rate-based
uses a congestion window
Each transmitted packet consumes a slot in the congestion window
Acknowledgment of a packet received, frees one slot
Sender is allowed to transmit packets only if a slot is free
Rate-based :
Dynamically adapts the transmission rate according to some network feedback mechanism that indicates
congestion

6. Issues Single rate multicast congestion control scheme between one
sender and one or more receivers
In Multicast, it is typical to use NAKs instead of ACKs to
avoid ACK implosion Also, NAK supression is done so that the Network Elements
forward only 1 NAK per group of receivers to the upstream
router.
This results in delayed feedback to the sender.

7. These delays make the system unresponsive to variations
in the network conditions. This leads to instability in the
network.
Another issue besides Stability…. TCP FRIENDLINESS
Such unresponsive flows which are slow in reacting to
congestion can drive the other competing slow,
responsive flows to a very low throughput

8. Pragmatic General Multicast
Congestion Control
(PGMCC)

9. PGMCC tries to achieve faster response
It also includes positive ACKs
But having each receiver send an ACK causes the scalability problem
Solution: Elect a group representative (called ACKER) who is
responsible for sending positive ACKs
The ACKER is dynamically selected to be the receiver which would
have the lowest throughput
Note: Due to the dynamics of the network the receiver with the lowest throughput may change from time to time…
i.e. The ACKER will also change.

10. Window based Controller used by pgmcc
Window based congestion control scheme is run between the sender and the ACKER
Uses AIMD
Uses 2 variables:
Window Size (W) – describes the current window size in packets
Token Count (T) – the no of packets that can be transmitted

11. On start up or after timeout : W = T =1
On Transmit : T = T-1
On ACK reception : W = W + 1/W
T = T /W On loss detection : W = W/2
by dupacks
Pgmcc like TCP does some exponential opening of the window (Slow Start). It is limited to a fixed size 4-6 packets
Primarily done to quickly open the window beyond the dupack threshold

12. ACKER selection
Aim – To determine the receiver which would have the lowest
throughput
Throughput for each receiver can be estimated using
Round Trip Time
Loss Rate
Initial ACKER selection
When receivers get a data packet with no ACKER selected, all
receivers generate a dummy NAK report.
The sender will select the source of the first incoming NAK as
the new ACKER

13. Whenever an ACK or NAK arrives from any receiver:
The sender computes the expected throughput (T_i) for that
receiver using the RTT and loss rate.
The sender has already stored the expected throughput for
the current ACKER (T_ACKER)
If T_i < C * T_ACKER
Node i is selected as new ACKER
Note: C is between 0-1, used to avoid too frequent ACKER changes

14. RTT measurement Explicit Timestamp :
Transmit a timestamp with every data packet
Receiver echoes the most recent timestamp in the ACK or NAK
Sender computes the RTT by subtracting the received timestamp
from the current value of the clock

15. Implicit Timestamp :
Record a timestamp for each data packet, but
timestamp isn’t transmitted
The receiver reports the most recent sequence no in the
ACK or NAK using which the sender can find the
corresponding timestamp

16. Using Sequence numbers :
Least precise technique
Doesn’t require the presence of a high resolution clock on the nodes
Sender doesn’t compute any timestamp
Receiver echoes the most recently received Sequence no
Sender computes RTT as the difference of the most recently sent
Sequence no and the one echoed in the ACK or NAK
Thus RTT is in terms of sequence numbers and not seconds

17. Timeouts
In TCP, Timeout value is calculated by accumulating statistics of
SRTT and RTTVAR
PGMCC can use a similar scheme, only that whenever the ACKER
changes the computation of SRTT and RTTVAR must be restarted
An ACKER may leave the group without notifying the sender
To avoid many successive timeouts due to absence of an ACKER,
new election of ACKER should be performed after TWO
successive timeouts

18. Fairness !!!

19. non-lossy
link
Lossy
link
Intra-protocol fairness with non-lossy and lossy links

20. non-lossy
link
Lossy
link
Inter-protocol fairness

21. Another Approach
Source based scheme is limited by the slowest receiver
The conflicting bandwidth requirements of all receivers cannot
be simultaneously met with one transmission rate
This can be achieved if we transfer the burden of rate adaption
to the receivers
The source can generate data at different rate over multiple
Streams
The receivers try to listen to one or more streams depending on their capacity.

22. Receiver driven Layered Multicast (RLM)
The source simply transmits each layer of its signal on a separate
Multicast group
Receiver has the key functionality
It adapts by joining and leaving groups
Conceptually the receiver,
- On congestion, drops a layer
- On spare capacity, adds a layer
Receiver searches for the optimal level of subscription
Similar to TCP which searches for its optimal rate using AIMD

23. S-R1 path has high capacity
- R1 subscribes to all 3 layers and gets highest quality signal
R2, R3 have to drop layer 3 because the 512 kb/s link becomes
congested

24. How many layers to subscribe ?
Receiver needs to determine whether its current level of
subscription is too high or low
Too high is easy to find- as it’ll cause congestion
Too low – there is no signal to the receiver to indicate that its
subscription is too low

25. Subscribing to layers in RLM
RLM performs a join-experinment
-spontaneously adds layers at “well chosen” times
If a join-experiment causes congestion
- the receiver quickly drops the offending layer
If a join experiment is successful
- the receiver is one step closer to the optimal operating point

26. Join experiments cause transient congestion
Need to minimize the frequency and duration of the join experiments
Solution : A learning strategy
Doing join experiments infrequently when they are likely to fail
And doing them readily when they are likely to succeed
Done by,
managing a separate Join Timer for each level of subscription

27. 4 C 3 D E B F 2 A 1
Layer #
Time

28. Scalability of RLM
If each receiver carries out the adaptation algorithm
The system scales poorly
As the session membership grows
Aggregate frequency of join experiments increases
network congestion increases

29. Also, join-experiments can interfere with each other
congestion
R1 can misinterpret the congestion and back off
layer 2 join-timer

30. Solution – Shared Learning
Receiver notifies the entire group, that it is now performing
a join-experiment on layer ‘x’
All receivers can learn from the failed join experiments of
other receivers