--- 1/draft-ietf-rmcat-coupled-cc-06.txt 2017-09-15 08:13:15.100098579 -0700 +++ 2/draft-ietf-rmcat-coupled-cc-07.txt 2017-09-15 08:13:15.152099813 -0700 @@ -1,48 +1,49 @@ 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 +Expires: March 19, 2018 University of Oslo + September 15, 2017 Coupled congestion control for RTP media - draft-ietf-rmcat-coupled-cc-06 + draft-ietf-rmcat-coupled-cc-07 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 + When multiple congestion controlled Real-time Transport Protocol + (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 Network-Assisted Dynamic Adaptation (NADA) congestion control algorithm, and provides suggestions on how to apply it to other congestion control algorithms. 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 September 29, 2017. + This Internet-Draft will expire on March 19, 2018. Copyright Notice 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 @@ -86,43 +87,47 @@ 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 . . . . . . . . . . . . . . . 25 D.2.7. Changes from -05 to -06 . . . . . . . . . . . . . . . 25 + D.2.8. Changes from -06 to -07 . . . . . . . . . . . . . . . 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. + When there is enough data to send, a congestion controller attempts + to increase its sending rate until the path's capacity has been + reached. Some controllers detect path capacity by increasing the + sending rate further, until packets are ECN-marked [RFC8087] or + dropped, and then decreasing the sending rate until that stops + happening. This process inevitably creates undesirable queuing delay + when multiple congestion-controlled connections traverse the same + network bottleneck, and each connection overshoots the path capacity + as it determines its sending rate. The Congestion Manager (CM) [RFC3124] couples flows by providing a single congestion controller. 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 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]. + required by rtcweb [I-D.ietf-rtcweb-overview] [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]. @@ -130,27 +135,27 @@ 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 - stream [RFC7656], whether or not this RTP stream is multiplexed - onto an RTP session with other RTP streams. + It could, for example, be a transport layer connection, or an + RTP 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. @@ -154,29 +159,26 @@ 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 on the transmission rate. Additionally, the necessary - information about how much data an application can currently - send on a flow is often only available at the sender side, - making the sender an obvious choice for placement of the - elements and mechanisms described here. + Shared bottlenecks can exist when multiple flows originate from + the same sender, or when flows from different senders reach the + same receiver (see [I-D.ietf-rmcat-sbd], section 3). Coupled + congestion control as described here only supports the former + case, not the latter, as it operates inside a single host on + the sender side. 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 @@ -197,25 +199,30 @@ 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). This document describes both - active and passive versions, however the passive version is put into - the appendix as it is extremely experimental. Figure 2 shows the - interaction between flows and the FSE, using the variable names - defined in Section 5.2. + correct Flow Group Identifiers (FGIs). + + This document describes both active and passive versions. While the + passive algorithm works better for congestion controls with RTT- + independent convergence, it can still produce oscillations on short + time scales. The passive algorithm, described in Appendix C, is + therefore considered as highly experimental and not safe to deploy + outside of testbed environments. Figure 2 shows the interaction + between flows and the FSE, using the variable names defined in + Section 5.2. ------- <--- Flow 1 | FSE | <--- Flow 2 .. ------- <--- .. Flow N ^ | | ------- | | SBD | <-------| ------- @@ -228,54 +235,61 @@ REGISTER <--register-- JOIN #1 #2 CC_R(1) ----UPDATE----> UPDATE (in) #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 - 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. + have an influence on the resulting on-the-wire behavior. It does + not, for instance, define how many bits must be used to represent + FGIs, 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. However, only one type of coupling + algorithm should be used for all flows. Combinations of multiple + algorithms at different aggregation levels (e.g., the Operating + System coupling application aggregates with one algorithm, and + applications coupling their flows with another) have not been tested + and are therefore not recommended. 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 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 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. + in the same way along the path. This 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 @@ -524,24 +537,24 @@ Experiments should investigate cases where the media coder's output rate is below the rate that is calculated by the coupling algorithm (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, 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. + Nielsen, Kristian Hiorth, Mirja Kuehlewind, Martin Stiemerling, + Spencer Dawkins, 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 @@ -560,43 +573,46 @@ 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. + Implementers should also be aware of the Security Considerations + sections of [RFC3124], [RFC5348], and [RFC7478]. + 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. + J., and S. D'Aronco, "NADA: A Unified Congestion Control + Scheme for Real-Time Media", draft-ietf-rmcat-nada-04 + (work in progress), March 2017. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ - RFC2119, March 1997, - . + 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, - . + . 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, @@ -608,56 +624,66 @@ 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. + eval-test-05 (work in progress), April 2017. [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. + RTP Media.", draft-ietf-rmcat-sbd-08 (work in progress), + July 2017. + + [I-D.ietf-rtcweb-overview] + Alvestrand, H., "Overview: Real Time Protocols for + Browser-based Applications", draft-ietf-rtcweb-overview-18 + (work in progress), March 2017. [I-D.ietf-rtcweb-transports] 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, - . + DOI 10.17487/RFC7478, March 2015, . [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, - . + DOI 10.17487/RFC7656, November 2015, . + + [RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using + Explicit Congestion Notification (ECN)", RFC 8087, DOI + 10.17487/RFC8087, March 2017, . [rtcweb-rtp-usage] Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time Communication (WebRTC): Media Transport and Use of RTP", 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", Internet-draft draft- @@ -675,27 +701,27 @@ When applying the FSE to GCC, the UPDATE function call described in Section 5.3 gives the FSE GCC's estimate of available bandwidth A_hat. The recommended algorithm for GCC is the Active FSE in Section 5.3.1. In step 3 (c), when the FSE_R(i) is "sent" to the flow i, this means updating A_hat of flow i with the value of FSE_R(i). Appendix B. Scheduling - When connections originate from the same host, it would be possible - 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 as a function of the - scheduler. The Congestion Manager (CM) [RFC3124] also uses such a - scheduling function. + When flows originate from the same host, it would be possible 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 as a function of the scheduler. + The Congestion Manager (CM) [RFC3124] also uses such a scheduling + function. Appendix C. 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, 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 @@ -1064,20 +1090,24 @@ 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. +D.2.8. Changes from -06 to -07 + + o Addressed OPSDIR, SECDIR, GENART, AD and IESG comments. + Authors' Addresses Safiqul Islam University of Oslo PO Box 1080 Blindern Oslo N-0316 Norway Phone: +47 22 84 08 37 Email: safiquli@ifi.uio.no