--- 1/draft-ietf-rmcat-coupled-cc-05.txt 2017-03-28 06:13:13.665590757 -0700 +++ 2/draft-ietf-rmcat-coupled-cc-06.txt 2017-03-28 06:13:13.709591794 -0700 @@ -1,105 +1,106 @@ -RTP Media Congestion Avoidance S. Islam -Techniques (rmcat) M. Welzl -Internet-Draft S. Gjessing -Intended status: Experimental University of Oslo -Expires: June 10, 2017 December 7, 2016 +RTP Media Congestion Avoidance Techniques (rmcat) S. Islam +Internet-Draft M. Welzl +Intended status: Experimental S. Gjessing +Expires: September 29, 2017 University of Oslo + March 28, 2017 Coupled congestion control for RTP media - draft-ietf-rmcat-coupled-cc-05 + draft-ietf-rmcat-coupled-cc-06 Abstract When multiple congestion controlled RTP sessions traverse the same network bottleneck, combining their controls can improve the total on-the-wire behavior in terms of delay, loss and fairness. 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. It specifies how to apply the method for the NADA congestion control algorithm, and provides suggestions on how to apply it to other congestion control algorithms. -Status of this Memo +Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on June 10, 2017. + This Internet-Draft will expire on September 29, 2017. Copyright Notice - Copyright (c) 2016 IETF Trust and the persons identified as the + Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents - 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 4. Architectural overview . . . . . . . . . . . . . . . . . . . . 4 + 4. Architectural overview . . . . . . . . . . . . . . . . . . . 5 5. Roles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. SBD . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.2. FSE . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5.3. Flows . . . . . . . . . . . . . . . . . . . . . . . . . . 8 - 5.3.1. Example algorithm 1 - Active FSE . . . . . . . . . . . 8 - 5.3.2. Example algorithm 2 - Conservative Active FSE . . . . 9 - 6. Application . . . . . . . . . . . . . . . . . . . . . . . . . 10 - 6.1. NADA . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 + 5.3.1. Example algorithm 1 - Active FSE . . . . . . . . . . 9 + 5.3.2. Example algorithm 2 - Conservative Active FSE . . . . 10 + 6. Application . . . . . . . . . . . . . . . . . . . . . . . . . 11 + 6.1. NADA . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.2. General recommendations . . . . . . . . . . . . . . . . . 11 - 7. Expected feedback from experiments . . . . . . . . . . . . . . 12 - 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 - 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 - 10. Security Considerations . . . . . . . . . . . . . . . . . . . 12 - 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 - 11.1. Normative References . . . . . . . . . . . . . . . . . . . 13 - 11.2. Informative References . . . . . . . . . . . . . . . . . . 13 + 7. Expected feedback from experiments . . . . . . . . . . . . . 12 + 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 + 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 + 10. Security Considerations . . . . . . . . . . . . . . . . . . . 13 + 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 + 11.1. Normative References . . . . . . . . . . . . . . . . . . 13 + 11.2. Informative References . . . . . . . . . . . . . . . . . 14 Appendix A. Application to GCC . . . . . . . . . . . . . . . . . 15 - Appendix B. Scheduling . . . . . . . . . . . . . . . . . . . . . 15 - Appendix C. Example algorithm - Passive FSE . . . . . . . . . . . 15 - C.1. Example operation (passive) . . . . . . . . . . . . . . . 18 - Appendix D. Change log . . . . . . . . . . . . . . . . . . . . . 22 - D.1. draft-welzl-rmcat-coupled-cc . . . . . . . . . . . . . . . 22 - D.1.1. Changes from -00 to -01 . . . . . . . . . . . . . . . 22 - D.1.2. Changes from -01 to -02 . . . . . . . . . . . . . . . 22 + Appendix B. Scheduling . . . . . . . . . . . . . . . . . . . . . 16 + Appendix C. Example algorithm - Passive FSE . . . . . . . . . . 16 + C.1. Example operation (passive) . . . . . . . . . . . . . . . 19 + Appendix D. Change log . . . . . . . . . . . . . . . . . . . . . 23 + D.1. draft-welzl-rmcat-coupled-cc . . . . . . . . . . . . . . 23 + D.1.1. Changes from -00 to -01 . . . . . . . . . . . . . . . 23 + D.1.2. Changes from -01 to -02 . . . . . . . . . . . . . . . 23 D.1.3. Changes from -02 to -03 . . . . . . . . . . . . . . . 23 - D.1.4. Changes from -03 to -04 . . . . . . . . . . . . . . . 23 - D.1.5. Changes from -04 to -05 . . . . . . . . . . . . . . . 23 - D.2. draft-ietf-rmcat-coupled-cc . . . . . . . . . . . . . . . 23 - D.2.1. Changes from draft-welzl-rmcat-coupled-cc-05 . . . . . 23 - D.2.2. Changes from -00 to -01 . . . . . . . . . . . . . . . 23 - D.2.3. Changes from -01 to -02 . . . . . . . . . . . . . . . 23 + D.1.4. Changes from -03 to -04 . . . . . . . . . . . . . . . 24 + D.1.5. Changes from -04 to -05 . . . . . . . . . . . . . . . 24 + D.2. draft-ietf-rmcat-coupled-cc . . . . . . . . . . . . . . . 24 + D.2.1. Changes from draft-welzl-rmcat-coupled-cc-05 . . . . 24 + D.2.2. Changes from -00 to -01 . . . . . . . . . . . . . . . 24 + D.2.3. Changes from -01 to -02 . . . . . . . . . . . . . . . 24 D.2.4. Changes from -02 to -03 . . . . . . . . . . . . . . . 24 D.2.5. Changes from -03 to -04 . . . . . . . . . . . . . . . 24 - D.2.6. Changes from -04 to -05 . . . . . . . . . . . . . . . 24 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 + D.2.6. Changes from -04 to -05 . . . . . . . . . . . . . . . 25 + D.2.7. Changes from -05 to -06 . . . . . . . . . . . . . . . 25 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 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 when multiple congestion controlled connections traverse the same network bottleneck. @@ -109,42 +110,47 @@ connection congestion control functionality, which is quite a significant change to existing RTP based applications. This document presents a method to combine the behavior of congestion control mechanisms that is easier to implement than the Congestion Manager [RFC3124] 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. It is able to honor user-specified priorities, which is required by rtcweb [RFC7478]. + The described mechanisms are believed safe to use, but are + experimental and are presented for wider review and operational + evaluation. + 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. + stream [RFC7656], whether or not this RTP stream is multiplexed + onto an RTP session with other RTP streams. 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. @@ -214,23 +220,23 @@ ------- Figure 1: Coupled congestion control architecture Flow#1(cc) FSE Flow#2(cc) ---------- --- ---------- #1 JOIN ----register--> REGISTER REGISTER <--register-- JOIN #1 - #2 CC_R ----UPDATE----> UPDATE (in) + #2 CC_R(1) ----UPDATE----> UPDATE (in) - #3 NEW RATE <---FSE_R------ UPDATE (out) --FSE_R----> #3 NEW RATE + #3 NEW RATE <---FSE_R(1)-- UPDATE (out) --FSE_R(2)-> #3 NEW RATE Figure 2: Flow-FSE interaction 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 @@ -249,28 +255,29 @@ 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 in three basic ways: 1. From multiplexing: 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]). Such multiplexing is also a - recommended usage of RTP in rtcweb [rtcweb-rtp-usage]. + having the same values for the Differentiated Services Code Point + (DSCP) and the ECN field 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, DSCP and ECN field value to belong to + the same FG. This classification applies to certain tunnels, or + RTP flows that are multiplexed over one transport (cf. + [transport-multiplex]). Such multiplexing is also a recommended + usage of RTP in rtcweb [rtcweb-rtp-usage]. 2. Via configuration: e.g. by assuming that a common wireless uplink is also a shared bottleneck. 3. From measurements: e.g. by considering correlations among measured delay and loss as an indication of a shared bottleneck. The methods above have some essential trade-offs: e.g., multiplexing is a completely reliable measure, however it is limited in scope to two end points (i.e., it cannot be applied to couple congestion @@ -287,39 +293,40 @@ [I-D.ietf-rmcat-sbd] for details). Using system configuration to decide about shared bottlenecks can be more efficient (faster to obtain) than using measurements, but it relies on assumptions about the network environment. 5.2. FSE The FSE contains a list of all flows that have registered with it. For each flow, it stores the following: - o a unique flow number to identify the flow + o a unique flow number f to identify the flow. o 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) + be in only one FG). - o 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). + o a priority P(f), which is a positive number, greater than zero. - o The rate used by the flow in bits per second, FSE_R. + o The rate used by the flow in bits per second, FSE_R(f). - Note that the priority does not need to be a floating point value and - its value range does not matter for this algorithm: the algorithm - works with a flow's priority portion of the sum of all priority - values. Priorities can therefore be mapped to the "very-low", "low", - "medium" or "high" priority levels described in - [I-D.ietf-rtcweb-transports] using the values 1, 2, 4 and 8, - respectively. + Note that the absolute range of priorities does not matter: the + algorithm works with a flow's priority portion of the sum of all + priority values. For example, if there are two flows, flow 1 with + priority 1 and flow 2 with priority 2, the sum of the priorities is + 3. Then, flow 1 will be assigned 1/3 of the aggregate sending rate + and flow 2 will be assigned 2/3 of the aggregate sending rate. + + Priorities can be mapped to the "very-low", "low", "medium" or "high" + priority levels described in [I-D.ietf-rtcweb-transports] by simply + using the values 1, 2, 4 and 8, respectively. In the FSE, each FG contains one static variable S_CR which is the sum of the calculated rates of all flows in the same FG. 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. @@ -331,62 +338,62 @@ sending rate. Via UPDATE, they provide the newly calculated rate and optionally (if the algorithm supports it) the desired rate. The desired rate is less than the calculated rate in case of application- limited flows; otherwise, it is the same as the calculated rate. Below, two example algorithms are described. While other algorithms could be used instead, the same algorithm must be applied to all flows. Names of variables used in the algorithms are explained below. - o CC_R - The rate received from a flow's congestion controller when - it calls UPDATE. + o CC_R(f) - The rate received from the congestion controller of flow + f when it calls UPDATE. - o FSE_R - The rate calculated by the FSE for a flow. + o FSE_R(f) - The rate calculated by the FSE for flow f. o S_CR - The sum of the calculated rates of all flows in the same FG; this value is used to calculate the sending rate. o FG - A group of flows having the same FGI, and hence sharing the same bottleneck. - o P - The priority of a flow which is received from the flow's + o P(f) - The priority of flow f which is received from the flow's congestion controller; the FSE uses this variable for calculating - FSE R. + FSE_R(f). o S_P - The sum of all the priorities. 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 [fse] indicate that it does however not significantly reduce queuing delay and packet loss. (1) When a flow f starts, it registers itself with SBD and the FSE. - FSE_R is initialized with the congestion controller's initial + FSE_R(f) 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. + an FGI, it adds its FSE_R(f) to S_CR. (2) When a flow f stops or pauses, 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 + new sending rate CC_R(f), 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. - S_CR = S_CR + CC_R - FSE_R(f) + S_CR = S_CR + CC_R(f) - FSE_R(f) (b) It calculates the sum of all the priorities, S_P. S_P = 0 for all flows i in FG do S_P = S_P + P(i) end for (c) It calculates the sending rates for all the flows in an FG and distributes them. @@ -397,49 +404,49 @@ end for 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 [fse] indicate that it can significantly reduce queuing delay and packet loss. (1) When a flow f starts, it registers itself with SBD and the FSE. - FSE_R is initialized with the congestion controller's initial + FSE_R(f) 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. + an FGI, it adds its FSE_R(f) to S_CR. (2) When a flow f stops or pauses, 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 + new sending rate CC_R(f), 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. + calculate the difference between CC_R(f) and the previously + stored FSE_R(f). 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. if Timer has expired or not set then - DELTA = CC_R - FSE_R(f) + DELTA = CC_R(f) - FSE_R(f) if DELTA < 0 then // Reduce S_CR proportionally - S_CR = S_CR * CC_R / FSE_R(f) + S_CR = S_CR * CC_R(f) / FSE_R(f) Set Timer for 2 RTTs else S_CR = S_CR + DELTA end if end if (b) It calculates the sum of all the priorities, S_P. S_P = 0 for all flows i in FG do @@ -509,31 +516,32 @@ The algorithm described in this memo has so far been evaluated using simulations covering all the tests for more than one flow from [I-D.ietf-rmcat-eval-test] (see [IETF-93], [IETF-94]). Experiments should confirm these results using at least the NADA congestion control algorithm with real-life code (e.g., browsers communicating over an emulated network covering the conditions in [I-D.ietf-rmcat-eval-test]. The tests with real-life code should be repeated afterwards in real network environments and monitored. Experiments should investigate cases where the media coder's output rate is below the rate that is calculated by the coupling algorithm - (FSE_R in algorithms 1 and 2, section 5.3). Implementers and testers - are invited to document their findings in an Internet draft. + (FSE_R(i) in algorithms 1 and 2, section 5.3). Implementers and + testers are invited to document their findings in an Internet draft. 8. Acknowledgements This document has benefitted from discussions with and feedback from - Andreas Petlund, Anna Brunstrom, David Hayes, David Ros (who also - gave the FSE its name), Ingemar Johansson, Karen Nielsen, Kristian - Hiorth, Mirja Kuehlewind, Martin Stiemerling, Varun Singh, Xiaoqing - Zhu, and Zaheduzzaman Sarker. The authors would like to especially - thank Xiaoqing Zhu and Stefan Holmer for helping with NADA and GCC. + Andreas Petlund, Anna Brunstrom, Colin Perkins, David Hayes, David + Ros (who also gave the FSE its name), Ingemar Johansson, Karen + Nielsen, Kristian Hiorth, Mirja Kuehlewind, Martin Stiemerling, Varun + Singh, Xiaoqing Zhu, and Zaheduzzaman Sarker. The authors would like + to especially thank Xiaoqing Zhu and Stefan Holmer for helping with + NADA and GCC. This work was partially funded by the European Community under its Seventh Framework Programme through the Reducing Internet Transport Latency (RITE) project (ICT-317700). 9. IANA Considerations This memo includes no request to IANA. 10. Security Considerations @@ -559,106 +567,109 @@ a fair allocation in which the priority mechanism is implicitly eliminated, and no major harm is done. 11. References 11.1. Normative References [I-D.ietf-rmcat-nada] Zhu, X., Pan, R., Ramalho, M., Cruz, S., Jones, P., Fu, J., D'Aronco, S., and C. Ganzhorn, "NADA: A Unified - Congestion Control Scheme for Real-Time Media", - draft-ietf-rmcat-nada-03 (work in progress), - September 2016. + Congestion Control Scheme for Real-Time Media", draft- + ietf-rmcat-nada-03 (work in progress), September 2016. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, . [RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager", RFC 3124, DOI 10.17487/RFC3124, June 2001, . [RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP - Friendly Rate Control (TFRC): Protocol Specification", - RFC 5348, DOI 10.17487/RFC5348, September 2008, + Friendly Rate Control (TFRC): Protocol Specification", RFC + 5348, DOI 10.17487/RFC5348, September 2008, . 11.2. Informative References + [anrw2016] + Islam, S. and M. Welzl, "Start Me Up:Determining and + Sharing TCP's Initial Congestion Window", ACM, IRTF, ISOC + Applied Networking Research Workshop 2016 (ANRW 2016) , + 2016. + + [fse] Islam, S., Welzl, M., Gjessing, S., and N. Khademi, + "Coupled Congestion Control for RTP Media", ACM SIGCOMM + Capacity Sharing Workshop (CSWS 2014) and ACM SIGCOMM CCR + 44(4) 2014; extended version available as a technical + report from + http://safiquli.at.ifi.uio.no/paper/fse-tech-report.pdf , + 2014. + + [fse-noms] + Islam, S., Welzl, M., Hayes, D., and S. Gjessing, + "Managing Real-Time Media Flows through a Flow State + Exchange", IEEE NOMS 2016, Istanbul, Turkey , 2016. + [I-D.ietf-rmcat-eval-test] Sarker, Z., Singh, V., Zhu, X., and M. Ramalho, "Test - Cases for Evaluating RMCAT Proposals", - draft-ietf-rmcat-eval-test-04 (work in progress), - October 2016. + Cases for Evaluating RMCAT Proposals", draft-ietf-rmcat- + eval-test-04 (work in progress), October 2016. [I-D.ietf-rmcat-gcc] Holmer, S., Lundin, H., Carlucci, G., Cicco, L., and S. Mascolo, "A Google Congestion Control Algorithm for Real- Time Communication", draft-ietf-rmcat-gcc-02 (work in progress), July 2016. [I-D.ietf-rmcat-sbd] Hayes, D., Ferlin, S., Welzl, M., and K. Hiorth, "Shared Bottleneck Detection for Coupled Congestion Control for RTP Media.", draft-ietf-rmcat-sbd-04 (work in progress), March 2016. [I-D.ietf-rtcweb-transports] - Alvestrand, H., "Transports for WebRTC", - draft-ietf-rtcweb-transports-11.txt (work in progress), - January 2016. + Alvestrand, H., "Transports for WebRTC", Internet-draft + draft-ietf-rtcweb-transports-17.txt, October 2016. [IETF-93] Islam, S., Welzl, M., and S. Gjessing, "Updates on Coupled Congestion Control for RTP Media", July 2015, . [IETF-94] Islam, S., Welzl, M., and S. Gjessing, "Updates on Coupled Congestion Control for RTP Media", November 2015, . [RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real- Time Communication Use Cases and Requirements", RFC 7478, DOI 10.17487/RFC7478, March 2015, . - [anrw2016] - Islam, S. and M. Welzl, "Start Me Up:Determining and - Sharing TCP's Initial Congestion Window", ACM, IRTF, ISOC - Applied Networking Research Workshop 2016 (ANRW 2016) , - 2016. - - [fse] Islam, S., Welzl, M., Gjessing, S., and N. Khademi, - "Coupled Congestion Control for RTP Media", ACM SIGCOMM - Capacity Sharing Workshop (CSWS 2014) and ACM SIGCOMM CCR - 44(4) 2014; extended version available as a technical - report from - http://safiquli.at.ifi.uio.no/paper/fse-tech-report.pdf , - 2014. - - [fse-noms] - Islam, S., Welzl, M., Hayes, D., and S. Gjessing, - "Managing Real-Time Media Flows through a Flow State - Exchange", IEEE NOMS 2016, Istanbul, Turkey , 2016. + [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and + B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms + for Real-Time Transport Protocol (RTP) Sources", RFC 7656, + DOI 10.17487/RFC7656, November 2015, + . [rtcweb-rtp-usage] Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time Communication (WebRTC): Media Transport and Use of RTP", - draft-ietf-rtcweb-rtp-usage-26.txt (work in progress), - March 2016. + Internet-draft draft-ietf-rtcweb-rtp-usage-26.txt, March + 2016. [transport-multiplex] Westerlund, M. and C. Perkins, "Multiple RTP Sessions on a - Single Lower-Layer Transport", - draft-westerlund-avtcore-transport-multiplexing-07.txt - (work in progress), October 2013. + Single Lower-Layer Transport", Internet-draft draft- + westerlund-avtcore-transport-multiplexing-07.txt, October + 2013. Appendix A. Application to GCC Google Congestion Control (GCC) [I-D.ietf-rmcat-gcc] is another congestion control scheme for RTP flows that is under development. GCC is not yet finalised, but at the time of this writing, the rate control of GCC employs two parts: controlling the bandwidth estimate based on delay, and controlling the bandwidth estimate based on loss. Both are designed to estimate the available bandwidth, A_hat. @@ -686,106 +697,106 @@ 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, when round-trip times of flows are unequal, shorter-RTT flows may (depending on the congestion control algorithm) update and react to the overall FSE state more often than longer-RTT flows, which can produce unwanted side effects. This problem is more significant when the congestion control convergence depends on the RTT. While the passive algorithm works better for congestion controls with RTT-independent convergence, it can still produce oscillations on short time scales. The algorithm described below is - therefore considered as highly experimental. Results of a simplified - passive FSE algorithm with both NADA and GCC can be found in - [fse-noms]. + therefore considered as highly experimental and not safe to deploy + outside of testbed environments. Results of a simplified passive FSE + algorithm with both NADA and GCC can be found in [fse-noms]. This passive version of the FSE stores the following information in addition to the variables described in Section 5.2: - o 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. + o The desired rate DR(f) of flow f. 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(f) must be set to CC_R(f) received from the + congestion module of flow f. 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. (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. + FSE_R(f) and DR(f) 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(f) to S_CR. - (2) When a flow f stops or pauses, it sets its DR to 0 and sets P to - -1. + (2) When a flow f stops or pauses, it sets its DR(f) to 0 and sets + P(f) 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. + new sending rate CC_R(f), assuming the flow's new desired rate + new_DR(f) 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(f). 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. + Then it calculates DELTA: the difference between FSE_R(f) + and CC_R(f). for all flows i in FG do new_S_CR = new_S_CR + FSE_R(i) end for - DELTA = CC_R - FSE_R(f) + DELTA = CC_R(f) - FSE_R(f) (b) It updates S_CR, FSE_R(f) and DR(f). - FSE_R(f) = CC_R + FSE_R(f) = CC_R(f) 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)) + DR(f) = min(new_DR(f),FSE_R(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. 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 - (d) It calculates the sending rate, Rate. + (d) It calculates the sending rate, Rate(f). - Rate = min(new_DR, (P(f)*S_CR)/S_P + TLO) + Rate(f) = min(new_DR(f), (P(f)*S_CR)/S_P + TLO) - if Rate != new_DR and TLO > 0 then + if Rate(f) != new_DR(f) and TLO > 0 then TLO = 0 // f has 'taken' TLO end if - (e) It updates DR(f) and FSE_R(f) with Rate. + (e) It updates DR(f) and FSE_R(f) with Rate(f). - if Rate > DR(f) then - DR(f) = Rate + if Rate(f) > DR(f) then + DR(f) = Rate(f) end if - FSE_R(f) = Rate + FSE_R(f) = Rate(f) 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. @@ -845,122 +857,122 @@ | | | | | | | | 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. + CC_R(1) = 8. new_DR(1) = 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 b) FSE_R(1) = 8. DELTA is negative, hence S_CR = 9; + DR(1) = 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. + 3 d) new sending rate Rate(1) = min(infinity, 1/1.5 * 9 + 0) = 6. + 3 e) FSE_R(1) = 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. + CC_R(2) = 2. new_DR(2) = 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 b) FSE_R(2) = 2. DELTA is positive, hence S_CR = 9 + 1 = 10; + DR(2) = 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. + 3 d) Rate(2) = min(infinity, 0.5/1.5 * 10 + 0) = 3.33. + 3 e) DR(2) = FSE_R(2) = 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. + CC_R(1) = 7. new_DR(1) = 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. + 3 b) FSE_R(1) = 7, DELTA is positive, hence S_CR = 10 + 1 = 11; + DR(1) = min(2, 7) = 2. + 3 c) S_P = 1.5; DR(1) < FSE_R(1), hence TLO = 1/1.5 * 11 - 2 = 5.33. + 3 d) Rate(1) = min(2, 1/1.5 * 11 + 5.33) = 2. + 3 e) FSE_R(1) = 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. + CC_R(2) = 4.33. new_DR(2) = 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 b) FSE_R(2) = 4.33. DELTA is positive, hence S_CR = 12; + DR(2) = 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. + 3 d) Rate(2) = min(infinity, 0.5/1.5 * 12 + 5.33 ) = 9.33. + 3 e) FSE_R(2) = 9.33, DR(2) = 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. + Finally, flow #1 terminates. It sets P(1) to -1 and DR(1) 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. + CC_R(2) = 7.33. new_DR(2) = 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. + 3 b) FSE_R(2) = 7.33. DELTA is negative, hence S_CR = 9.33; + DR(2) = 7.33. + 3 c) Flow 1 has P(1) = -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. + 3 d) Rate(2) = min(infinity, 0.5/0.5*9.33 + 0) = 9.33. + 3 e) FSE_R(2) = DR(2) = 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 D. Change log @@ -1048,37 +1060,41 @@ o Changed several occurrences of "NADA and GCC" to "NADA", including the abstract. o Moved the application to GCC to an appendix, and made the GCC reference informative. o Provided a few more general recommendations on applying the coupling algorithm. +D.2.7. Changes from -05 to -06 + + o Incorporated comments by Colin Perkins. + Authors' Addresses Safiqul Islam University of Oslo PO Box 1080 Blindern - Oslo, N-0316 + Oslo N-0316 Norway Phone: +47 22 84 08 37 Email: safiquli@ifi.uio.no + Michael Welzl University of Oslo PO Box 1080 Blindern - Oslo, N-0316 + Oslo N-0316 Norway Phone: +47 22 85 24 20 Email: michawe@ifi.uio.no - Stein Gjessing University of Oslo PO Box 1080 Blindern - Oslo, N-0316 + Oslo N-0316 Norway Phone: +47 22 85 24 44 Email: steing@ifi.uio.no