Coupled congestion control for RTP media
draft-welzl-rmcat-coupled-cc-03

Abstract

When multiple congestion controlled RTP sessions traverse the same network bottleneck, it can be beneficial to combine their controls such that the total on-the-wire behavior is improved. This document describes such a method for flows that have the same sender, in a way that is as flexible and simple as possible while minimizing the amount of changes needed to existing RTP applications.

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Table of Contents

• 1. Introduction
• 2. Definitions
• 3. Limitations
• 4. Architectural overview
• 5. Roles
• 5.1. SBD
• 5.2. FSE
• 5.3. Flows
• 5.3.1. Example algorithm 1 - Active FSE
• 5.3.2. Example algorithm 2 - Conservative Active FSE
• 6. Acknowledgements
• 7. IANA Considerations
• 8. Security Considerations
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Example algorithm - Passive FSE
• A.1. Example operation (passive)
• Appendix B. Change log
• B.1. Changes from -00 to -01
• B.2. Changes from -01 to -02
• B.3. Changes from -02 to -03
• Authors' Addresses

1. Introduction

When there is enough data to send, a congestion controller must increase its sending rate until the path's capacity has been reached; depending on the controller, sometimes the rate is increased further, until packets are ECN-marked or dropped. This process inevitably creates undesirable queuing delay -- an effect that is amplified when multiple congestion controlled connections traverse the same network bottleneck. When such connections originate from the same host, it would therefore be ideal to use only one single sender-side congestion controller which determines the overall allowed sending rate, and then use a local scheduler to assign a proportion of this rate to each RTP session. This way, priorities could also be implemented quite easily, as a function of the scheduler; honoring user-specified priorities is, for example, required by rtcweb [rtcweb-usecases].

The Congestion Manager (CM) [RFC3124] provides a single congestion controller with a scheduling function just as described above. It is hard to implement because it requires an additional congestion controller and removes all per-connection congestion control functionality, which is quite a significant change to existing RTP based applications. This document presents a method that is easier to implement than the CM and also requires less significant changes to existing RTP based applications. It attempts to roughly approximate the CM behavior by sharing information between existing congestion controllers, akin to "Ensemble Sharing" in [RFC2140].

2. Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Available Bandwidth:

The available bandwidth is the nominal link capacity minus the amount of traffic that traversed the link during a certain time interval, divided by that time interval.

Bottleneck:

The first link with the smallest available bandwidth along the path between a sender and receiver.

Flow:

A flow is the entity that congestion control is operating on. It could, for example, be a transport layer connection, an RTP session, or a subsession that is multiplexed onto a single RTP session together with other subsessions.

Flow Group Identifier (FGI):

A unique identifier for each subset of flows that is limited by a common bottleneck.

Flow State Exchange (FSE):

The entity that maintains information that is exchanged between flows.

Flow Group (FG):

A group of flows having the same FGI.

Shared Bottleneck Detection (SBD):

The entity that determines which flows traverse the same bottleneck in the network, or the process of doing so.

3. Limitations

Sender-side only:

Coupled congestion control as described here only operates inside a single host on the sender side. This is because, irrespective of where the major decisions for congestion control are taken, the sender of a flow needs to eventually decide the transmission rate. Additionally, the necessary information about how much data an application can currently send on a flow is typically only available at the sender side, making the sender an obvious choice for placement of the elements and mechanisms described here.

When implementing a sender-side change to a congestion control mechanism such as TFRC [RFC5348], where receiver-side calculations make assumptions about the rate of the sender, the receiver also needs to be updated accordingly. Flows that have different senders but the same receiver, or different senders and different receivers can also share a bottleneck; such scenarios have been omitted for simplicity, and could be incorporated in future versions of this document. Note that limiting the number of flows on which coupled congestion control operates merely limits the benefits derived from the mechanism.

Shared bottlenecks do not change quickly:

As per the definition above, a bottleneck depends on cross traffic, and since such traffic can heavily fluctuate, bottlenecks can change at a high frequency (e.g., there can be oscillation between two or more links). This means that, when flows are partially routed along different paths, they may quickly change between sharing and not sharing a bottleneck. For simplicity, here it is assumed that a shared bottleneck is valid for a time interval that is significantly longer than the interval at which congestion controllers operate. Note that, for the only SBD mechanism defined in this document (multiplexing on the same five-tuple), the notion of a shared bottleneck stays correct even in the presence of fast traffic fluctuations: since all flows that are assumed to share a bottleneck are routed in the same way, if the bottleneck changes, it will still be shared.

4. Architectural overview

Figure 1 shows the elements of the architecture for coupled congestion control: the Flow State Exchange (FSE), Shared Bottleneck Detection (SBD) and Flows. The FSE is a storage element that can be implemented in two ways: active and passive. In the active version, it initiates communication with flows and SBD. However, in the passive version, it does not actively initiate communication with flows and SBD; its only active role is internal state maintenance (e.g., an implementation could use soft state to remove a flow's data after long periods of inactivity). Every time a flow's congestion control mechanism would normally update its sending rate, the flow instead updates information in the FSE and performs a query on the FSE, leading to a sending rate that can be different from what the congestion controller originally determined. Using information about/from the currently active flows, SBD updates the FSE with the correct Flow State Identifiers (FSIs).

		  

-------  <---  Flow 1
| FSE |  <---  Flow 2 ..
-------  <---  .. Flow N
   ^
   |             |
-------          |
| SBD |  <-------|
-------

		

Figure 1: Coupled congestion control architecture

Since everything shown in Figure 1 is assumed to operate on a single host (the sender) only, this document only describes aspects that have an influence on the resulting on-the-wire behavior. It does, for instance, not define how many bits must be used to represent FSIs, or in which way the entities communicate. Implementations can take various forms: for instance, all the elements in the figure could be implemented within a single application, thereby operating on flows generated by that application only. Another alternative could be to implement both the FSE and SBD together in a separate process which different applications communicate with via some form of Inter-Process Communication (IPC). Such an implementation would extend the scope to flows generated by multiple applications. The FSE and SBD could also be included in the Operating System kernel.

5. Roles

This section gives an overview of the roles of the elements of coupled congestion control, and provides an example of how coupled congestion control can operate.

5.1. SBD

SBD uses knowledge about the flows to determine which flows belong in the same Flow Group (FG), and assigns FGIs accordingly. This knowledge can be derived from measurements, by considering correlations among measured delay and loss as an indication of a shared bottleneck, or it can be based on the simple assumption that packets sharing the same five-tuple (IP source and destination address, protocol, and transport layer port number pair) and having the same Differentiated Services Code Point (DSCP) in the IP header are typically treated in the same way along the path. The latter method is the only one specified in this document: SBD MAY consider all flows that use the same five-tuple and DSCP to belong to the same FG. This classification applies to certain tunnels, or RTP flows that are multiplexed over one transport (cf. [transport-multiplex]). In one way or another, such multiplexing will probably be recommended for use with rtcweb [rtcweb-rtp-usage].

5.2. FSE

The FSE contains a list of all flows that have registered with it. For each flow, it stores the following:

• a unique flow number to identify the flow
• the FGI of the FG that it belongs to (based on the definitions in this document, a flow has only one bottleneck, and can therefore be in only one FG)
• a priority P, which here is assumed to be represented as a floating point number in the range from 0.1 (unimportant) to 1 (very important). A negative value is used to indicate that a flow has terminated
• The rate used by the flow, FSE_R

In the FSE, each FG contains one static variable S_CR which is meant to be the sum of the calculated rates of all flows in the same FG (including the flow itself). This value is used to calculate the sending rate.

The information listed here is enough to implement the sample flow algorithm given below. FSE implementations could easily be extended to store, e.g., a flow's current sending rate for statistics gathering or future potential optimizations.

5.3. Flows

Flows register themselves with SBD and FSE when they start, deregister from the FSE when they stop, and carry out an UPDATE function call every time their congestion controller calculates a new sending rate. Via UPDATE, they provide the newly calculated rate and the desired rate (less than the calculated rate in case of application-limited flows, the same otherwise).

Below, two example algorithms are described. While other algorithms could be used instead, the same algorithm must be applied to all flows.

5.3.1. Example algorithm 1 - Active FSE

This algorithm was designed to be the simplest possible method to assign rates according to the priorities of flows. Simulations results in [tech-report-fse] indicate that it does however not significantly reduce queuing delay and packet loss.

              
            S_CR = S_CR + CC_R - FSE_R(f)
            

              
            S_P = 0
            for all flows i in FG do
                S_P = S_P + P(i)
            end for
            

              
            for all flows i in FG do
                FSE_R(i) = (P(i)*S_CR)/S_P
                send FSE_R(i) to the flow i
            end for
            

(1)

When a flow f starts, it registers itself with SBD and the FSE. FSE_R is initialized with the congestion controller's initial rate. SBD will assign the correct FGI. When a flow is assigned an FGI, it adds its FSE_R to S_CR.

(2)

When a flow f stops, its entry is removed from the list.

(3)

Every time the congestion controller of the flow f determines a new sending rate CC_R, the flow calls UPDATE, which carries out the tasks listed below to derive the new sending rates for all the flows in the FG. A flow's UPDATE function uses a local (i.e. per-flow) temporary variable S_P, which is the sum of all the priorities.

(a)

It updates S_CR.

(b)

It calculates the sum of all the priorities, S_P.

(c)

It calculates the sending rates for all the flows in an FG and distributes them.

5.3.2. Example algorithm 2 - Conservative Active FSE

This algorithm extends algorithm 1 to conservatively emulate the behavior of a single flow by proportionally reducing the aggregate rate on congestion. Simulations results in [tech-report-fse] indicate that it can significantly reduce queuing delay and packet loss.

              
            if Timer has expired or not set then 
              DELTA = CC_R - FSE_R(f)
              if DELTA < 0 then  // Reduce S_CR proportionally 
                S_CR = S_CR * CC_R / FSE_R(f)
                Set Timer for 2 RTTs
              else    
                S_CR = S_CR + DELTA
              end if
             end if 
            

              
            S_P = 0
            for all flows i in FG do
                S_P = S_P + P(i)
            end for
            

              
            for all flows i in FG do
                FSE_R(i) = (P(i)*S_CR)/S_P
                send FSE_R(i) to the flow i
            end for
            

(1)

When a flow f starts, it registers itself with SBD and the FSE. FSE_R is initialized with the congestion controller's initial rate. SBD will assign the correct FGI. When a flow is assigned an FGI, it adds its FSE_R to S_CR.

(2)

When a flow f stops, its entry is removed from the list.

(3)

Every time the congestion controller of the flow f determines a new sending rate CC_R, the flow calls UPDATE, which carries out the tasks listed below to derive the new sending rates for all the flows in the FG. A flow's UPDATE function uses a local (i.e. per-flow) temporary variable S_P, which is the sum of all the priorities, and a local variable DELTA, which is used to calculate the difference between CC_R and the previously stored FSE_R. To prevent flows from either ignoring congestion or overreacting, a timer keeps them from changing their rates immediately after the common rate reduction that follows a congestion event. This timer is set to 2 RTTs of the flow that experienced congestion because it is assumed that a congestion event can persist for up to one RTT of that flow, with another RTT added to compensate for fluctuations in the measured RTT value.

(a)

It updates S_CR based on DELTA.

(b)

It calculates the sum of all the priorities, S_P.

(c)

It calculates the sending rates for all the flows in an FG and distributes them.

6. Acknowledgements

This document has benefitted from discussions with and feedback from David Hayes, Andreas Petlund, and David Ros (who also gave the FSE its name).

This work was partially funded by the European Community under its Seventh Framework Programme through the Reducing Internet Transport Latency (RITE) project (ICT-317700).

7. IANA Considerations

This memo includes no request to IANA.

8. Security Considerations

In scenarios where the architecture described in this document is applied across applications, various cheating possibilities arise: e.g., supporting wrong values for the calculated rate, the desired rate, or the priority of a flow. In the worst case, such cheating could either prevent other flows from sending or make them send at a rate that is unreasonably large. The end result would be unfair behavior at the network bottleneck, akin to what could be achieved with any UDP based application. Hence, since this is no worse than UDP in general, there seems to be no significant harm in using this in the absence of UDP rate limiters.

In the case of a single-user system, it should also be in the interest of any application programmer to give the user the best possible experience by using reasonable flow priorities or even letting the user choose them. In a multi-user system, this interest may not be given, and one could imagine the worst case of an "arms race" situation, where applications end up setting their priorities to the maximum value. If all applications do this, the end result is a fair allocation in which the priority mechanism is implicitly eliminated, and no major harm is done.

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Example algorithm - Passive FSE

Active algorithms calculate the rates for all the flows in the FG and actively distribute them. In a passive algorithm, UPDATE returns a rate that should be used instead of the rate that the congestion controller has determined. This can make a passive algorithm easier to implement; however, the resulting dynamics are not fully understood. The algorithm described below is to be considered as highly experimental and did not perform as well as the active variants in simulations.

This passive version of the FSE stores the following information in addition to the variables described in Section 5.2:

• The desired rate DR. This can be smaller than the calculated rate if the application feeding into the flow has less data to send than the congestion controller would allow. In case of a bulk transfer, DR must be set to CC_R received from the flow's congestion module.

The passive version of the FSE contains one static variable per FG called TLO (Total Leftover Rate -- used to let a flow 'take' bandwidth from application-limited or terminated flows) which is initialized to 0. For the passive version, S_CR is limited to increase or decrease as conservatively as a flow's congestion controller decides in order to prohibit sudden rate jumps.

                      
            for all flows i in FG do
                new_S_CR = new_S_CR + FSE_R(i)
            end for
            DELTA =  CC_R - FSE_R(f)
                  

                      
            FSE_R(f) = CC_R
            if DELTA > 0 then  // the flow's rate has increased
                S_CR = S_CR + DELTA
            else if DELTA < 0 then
                S_CR = new_S_CR + DELTA
            end if 
            DR(f) = min(new_DR,FSE_R(f))
                  

                      
            for all flows i in FG do
               if P(i)<0 then
                  delete flow
               else
                  S_P = S_P + P(i)
               end if
            end for
            if DR(f) < FSE_R(f) then
               TLO = TLO + (P(f)/S_P) * S_CR - DR(f))
            end if
                  

                      
            Rate = min(new_DR, (P(f)*S_CR)/S_P + TLO)
                          
            if Rate != new_DR and TLO > 0 then
                TLO = 0  // f has 'taken' TLO
            end if
                  

                    
            if Rate > DR(f) then
                DR(f) = Rate
            end if
            FSE_R(f)  = Rate
                

(1)

When a flow f starts, it registers itself with SBD and the FSE. FSE_R and DR are initialized with the congestion controller's initial rate. SBD will assign the correct FGI. When a flow is assigned an FGI, it adds its FSE_R to S_CR.

(2)

When a flow f stops, it sets its DR to 0 and sets P to -1.

(3)

Every time the congestion controller of the flow f determines a new sending rate CC_R, assuming the flow's new desired rate new_DR to be "infinity" in case of a bulk data transfer with an unknown maximum rate, the flow calls UPDATE, which carries out the tasks listed below to derive the flow's new sending rate, Rate. A flow's UPDATE function uses a few local (i.e. per-flow) temporary variables, which are all initialized to 0: DELTA, new_S_CR and S_P.

(a)

For all the flows in its FG (including itself), it calculates the sum of all the calculated rates, new_S_CR. Then it calculates the difference between FSE_R(f) and CC_R, DELTA.

(b)

It updates S_CR, FSE_R(f) and DR(f).

(c)

It calculates the leftover rate TLO, removes the terminated flows from the FSE and calculates the sum of all the priorities, S_P.

(d)

It calculates the sending rate, Rate.

(e)

It updates DR(f) and FSE_R(f) with Rate.

The goals of the flow algorithm are to achieve prioritization, improve network utilization in the face of application-limited flows, and impose limits on the increase behavior such that the negative impact of multiple flows trying to increase their rate together is minimized. It does that by assigning a flow a sending rate that may not be what the flow's congestion controller expected. It therefore builds on the assumption that no significant inefficiencies arise from temporary application-limited behavior or from quickly jumping to a rate that is higher than the congestion controller intended. How problematic these issues really are depends on the controllers in use and requires careful per-controller experimentation. The coupled congestion control mechanism described here also does not require all controllers to be equal; effects of heterogeneous controllers, or homogeneous controllers being in different states, are also subject to experimentation.

This algorithm gives all the leftover rate of application-limited flows to the first flow that updates its sending rate, provided that this flow needs it all (otherwise, its own leftover rate can be taken by the next flow that updates its rate). Other policies could be applied, e.g. to divide the leftover rate of a flow equally among all other flows in the FGI.

A.1. Example operation (passive)

In order to illustrate the operation of the passive coupled congestion control algorithm, this section presents a toy example of two flows that use it. Let us assume that both flows traverse a common 10 Mbit/s bottleneck and use a simplistic congestion controller that starts out with 1 Mbit/s, increases its rate by 1 Mbit/s in the absence of congestion and decreases it by 2 Mbit/s in the presence of congestion. For simplicity, flows are assumed to always operate in a round-robin fashion. Rate numbers below without units are assumed to be in Mbit/s. For illustration purposes, the actual sending rate is also shown for every flow in FSE diagrams even though it is not really stored in the FSE.

Flow #1 begins. It is a bulk data transfer and considers itself to have top priority. This is the FSE after the flow algorithm's step 1:

      
----------------------------------------
| # | FGI |  P  | FSE_R  |  DR  | Rate |
|   |     |     |        |      |      |
| 1 |  1  |  1  |   1    |   1  |   1  |
----------------------------------------
S_CR = 1, TLO = 0
              

    

Its congestion controller gradually increases its rate. Eventually, at some point, the FSE should look like this:

      
--------------------------------------
| # | FGI |  P  |  FSE_R  |  DR  | Rate |
|   |     |     |         |      |      |
| 1 |  1  |  1  |   10    |  10  |  10  |
-----------------------------------------
S_CR = 10, TLO = 0


    

Now another flow joins. It is also a bulk data transfer, and has a lower priority (0.5):

      
----------------------------------------
| # | FGI |   P   | FSE_R  |  DR  | Rate |
|   |     |       |        |      |      |
| 1 |  1  |   1   |   10   |  10  |  10  |
| 2 |  1  |  0.5  |    1   |   1  |   1  |
------------------------------------------
S_CR = 11, TLO = 0


    

Now assume that the first flow updates its rate to 8, because the total sending rate of 11 exceeds the total capacity. Let us take a closer look at what happens in step 3 of the flow algorithm.

      
CC_R = 8. new_DR = infinity.
3 a) new_S_CR = 11; DELTA = 8 - 10 = -2.
3 b) FSE_Rf) = 8. DELTA is negative, hence S_CR = 9;
     DR(f) = 8.
3 c) S_P = 1.5.
3 d) new sending rate = min(infinity, 1/1.5 * 9 + 0) = 6.
3 e) FSE_R(f) = 6.

The resulting FSE looks as follows:
----------------------------------------
| # | FGI |   P   |  FSE_R  |  DR  | Rate |
|   |     |       |         |      |      |
| 1 |  1  |   1   |    6    |   8  |   6  |
| 2 |  1  |  0.5  |    1    |   1  |   1  |
-------------------------------------------
S_CR = 9, TLO = 0


    

The effect is that flow #1 is sending with 6 Mbit/s instead of the 8 Mbit/s that the congestion controller derived. Let us now assume that flow #2 updates its rate. Its congestion controller detects that the network is not fully saturated (the actual total sending rate is 6+1=7) and increases its rate.

      
CC_R=2. new_DR = infinity.
3 a) new_S_CR = 7; DELTA = 2 - 1 = 1.
3 b) FSE_R(f) = 2. DELTA is positive, hence S_CR = 9 + 1 = 10;
     DR(f) = 2.
3 c) S_P = 1.5.
3 d) new sending rate = min(infinity, 0.5/1.5 * 10 + 0) = 3.33.
3 e) DR(f) = FSE_R(f) = 3.33.

The resulting FSE looks as follows:
-------------------------------------------
| # | FGI |   P   |  FSE_R  |  DR  | Rate |
|   |     |       |         |      |      |
| 1 |  1  |   1   |    6    |   8  |   6  |
| 2 |  1  |  0.5  |   3.33  | 3.33 | 3.33 |
-------------------------------------------
S_CR = 10, TLO = 0


    

The effect is that flow #2 is now sending with 3.33 Mbit/s, which is close to half of the rate of flow #1 and leads to a total utilization of 6(#1) + 3.33(#2) = 9.33 Mbit/s. Flow #2's congestion controller has increased its rate faster than the controller actually expected. Now, flow #1 updates its rate. Its congestion controller detects that the network is not fully saturated and increases its rate. Additionally, the application feeding into flow #1 limits the flow's sending rate to at most 2 Mbit/s.

      
CC_R=7. new_DR=2.
3 a) new_S_CR = 9.33; DELTA = 1.
3 b) FSE_R(f) = 7, DELTA is positive, hence S_CR = 10 + 1 = 11;
     DR = min(2, 7) = 2.
3 c) S_P = 1.5; DR(f) < FSE_R(f), hence TLO = 1/1.5 * 11 - 2 = 5.33.
3 d) new sending rate = min(2, 1/1.5 * 11 + 5.33) = 2.
3 e) FSE_R(f) = 2.

The resulting FSE looks as follows:
-------------------------------------------
| # | FGI |   P   |  FSE_R  |  DR  | Rate |
|   |     |       |         |      |      |
| 1 |  1  |   1   |    2    |   2  |   2  |
| 2 |  1  |  0.5  |   3.33  | 3.33 | 3.33 |
-------------------------------------------
S_CR = 11, TLO = 5.33

              

    

Now, the total rate of the two flows is 2 + 3.33 = 5.33 Mbit/s, i.e. the network is significantly underutilized due to the limitation of flow #1. Flow #2 updates its rate. Its congestion controller detects that the network is not fully saturated and increases its rate.

      
CC_R=4.33. new_DR = infinity.
3 a) new_S_CR = 5.33; DELTA = 1.
3 b) FSE_R(f) = 4.33. DELTA is positive, hence S_CR = 12;
     DR(f) = 4.33.
3 c) S_P = 1.5.
3 d) new sending rate: min(infinity, 0.5/1.5 * 12 + 5.33 ) = 9.33.
3 e) FSE_R(f) = 9.33, DR(f) = 9.33.

The resulting FSE looks as follows:
-------------------------------------------
| # | FGI |   P   |  FSE_R  |  DR  | Rate |
|   |     |       |         |      |      |
| 1 |  1  |   1   |    2    |   2  |   2  |
| 2 |  1  |  0.5  |   9.33  | 9.33 | 9.33 |
-------------------------------------------
S_CR = 12, TLO = 0


    

Now, the total rate of the two flows is 2 + 9.33 = 11.33 Mbit/s. Finally, flow #1 terminates. It sets P to -1 and DR to 0. Let us assume that it terminated late enough for flow #2 to still experience the network in a congested state, i.e. flow #2 decreases its rate in the next iteration.

      
CC_R = 7.33. new_DR = infinity.
3 a) new_S_CR = 11.33; DELTA = -2.
3 b) FSE_R(f) = 7.33. DELTA is negative, hence S_CR = 9.33;
     DR(f) = 7.33.
3 c) Flow 1 has P = -1, hence it is deleted from the FSE.
     S_P = 0.5.
3 d) new sending rate: min(infinity, 0.5/0.5*9.33 + 0) = 9.33.
3 e) FSE_R(f) = DR(f) = 9.33.

The resulting FSE looks as follows:
-------------------------------------------
| # | FGI |   P   |  FSE_R  |  DR  | Rate |
|   |     |       |         |      |      |
| 2 |  1  |  0.5  |   9.33  | 9.33 | 9.33 |
-------------------------------------------
S_CR = 9.33, TLO = 0


        

Appendix B. Change log

B.1. Changes from -00 to -01

• Added change log.
• Updated the example algorithm and its operation.

B.2. Changes from -01 to -02

• Included an active version of the algorithm which is simpler.
• Replaced "greedy flow" with "bulk data transfer" and "non-greedy" with "application-limited".
• Updated new_CR to CC_R, and CR to FSE_R for better understanding.

B.3. Changes from -02 to -03

• Included an active conservative version of the algorithm which reduces queue growth and packet loss; added a reference to a technical report that shows these benefits with simulations.
• Moved the passive variant of the algorithm to appendix.

Authors' Addresses