Congestion control in Multicast

ElEG 667May 7, 2003

byKeyur Shah

CIS, University of Delaware

TCP Friendly Congestion Control

Schemes

Single-rate Multi-rate

Window-based Rate-based Window-based Rate-based

Single Rate and Multi Rate

Single 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

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

Window-based and Rate-based

Window-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

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.

Issues

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

Pragmatic General MulticastCongestion Control

(PGMCC)

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.

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

On start up or after timeout : W = T =1

On Transmit : T = T-1

On ACK reception : W = W + 1/W T = T + 1 + 1/W

On loss detection : W = W/2by 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

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

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

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

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

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

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

Fairness !!!

Intra-protocol fairness with non-lossy and lossy links

non-lossylink

Lossylink

Inter-protocol fairness

non-lossylink

Lossylink

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.

Another Approach

Receiver driven Layered Multicast (RLM)

The source simply transmits each layer of its signal on a separateMulticast 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

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 becomescongested

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

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

Subscribing to layers in RLM

Join experiments cause transient congestion

Need to minimize the frequency and duration of the join experiments

Solution : A learning strategyDoing 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

Time

1

4

3

2

Layer #

F

ED

C

B

A

Scalability of RLM

If each receiver carries out the adaptation algorithmThe system scales poorly

As the session membership grows Aggregate frequency of join experiments increases network congestion increases

S

R1 R2

Join-2 Join-4

congestion

Also, join-experiments can interfere with each other

R1 can misinterpret the congestion and back off layer 2 join-timer

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