One document matched: draft-welzl-rmcat-coupled-cc-00.xml
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<!-- ***** FRONT MATTER ***** -->
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
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<!-- <title abbrev="Abbreviated Title">Coupled congestion control</title> -->
<title>Coupled congestion control for RTP media</title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<!-- Another author who claims to be an editor -->
<author fullname="Michael Welzl" initials="M.W." surname="Welzl">
<organization>University of Oslo</organization>
<address>
<postal>
<street>PO Box 1080 Blindern</street>
<!-- Reorder these if your country does things differently -->
<code>N-0316</code>
<city>Oslo</city>
<region></region>
<country>Norway</country>
</postal>
<phone>+47 22 85 24 20</phone>
<email>michawe@ifi.uio.no</email>
<!-- uri and facsimile elements may also be added -->
</address>
</author>
<date month="January" year="2013" />
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<!-- Meta-data Declarations -->
<area>Transport</area>
<workgroup>RTP Media Congestion Avoidance Techniques (rmcat)</workgroup>
<!-- WG name at the upperleft corner of the doc,
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<keyword>tcp</keyword>
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<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section title="Introduction" anchor='sec-intro'>
<t><!--For interactive applications, RTP media congestion control mechanisms should minimize the amount of self-inflicted queuing delay.--> When there is enough data to send, a congestion controller must increase its sending rate until the path's available capacity has been reached; depending on the controller, sometimes the rate is increased further, until packets are ECN-marked or dropped. In the public Internet, this is currently the only way to get any feedback from the network that can be used as an indication of congestion. 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 <xref target="rtcweb-usecases"/>.</t>
<t>The Congestion Manager (CM) <xref target="RFC3124"/> provides a single congestion controller with a scheduling function just as described above. It is, however, 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. <!--, and 2) it aggregates connections by destination address, thereby implicitly assuming that all packets that share the same destination address are routed across the same bottleneck in the network. This assumption can be wrong when, e.g., NATs or Equal-Cost Multi-Path (ECMP) routing are used.--> 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 <xref target="RFC2140"/>.</t>
</section>
<section title="Definitions" anchor='sec-def'>
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
<t><list style="hanging" hangIndent="6">
<t hangText="Available Bandwidth:">
<vspace />
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.</t>
<t hangText="Bottleneck:">
<vspace />
The first link with the smallest available bandwidth along the path between a sender and receiver.</t>
<t hangText="Flow:">
<vspace />
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.</t>
<t hangText="Flow Group Identifier (FGI):">
<vspace />
A unique identifier for each subset of flows that is limited by a common bottleneck.</t>
<t hangText="Flow State Exchange (FSE):">
<vspace />
The entity which maintains information that is exchanged between flows.</t>
<t hangText="Flow Group (FG):">
<vspace />
A group of flows having the same FGI.</t>
<t hangText="Shared Bottleneck Detection (SBD):">
<vspace />
The entity that determines which flows traverse the same bottleneck in the network, or the process of doing so.</t>
</list></t>
</section>
<section title="Limitations" anchor='sec-limits'>
<t><list style="hanging" hangIndent="6">
<t hangText="Sender-side only:">
<vspace />
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. It is recognized that 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 flows on which coupled congestion control operates merely limits the benefits derived from the mechanism.
</t>
<!-- This doesn't play a role, does it?
<t hangText="Only one congested link:">As per the definition above, only one bottleneck can exist between one sender and receiver. However, in the context of this document, the point of defining a bottleneck is to define a point of congestion, i.e. a link where a queue grows and flows begin to influence each other. Due to cross traffic, there can be several such congestion points between a sender and receiver. For simplicity, this fact is ignored here.</t> -->
<t hangText="Shared bottlenecks do not change quickly:">
<vspace />
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.</t>
</list></t>
</section>
<section title="Architectural overview" anchor='sec-arch'>
<t>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. It is passive in that it does not actively initiate communication with flows and the 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 is often different from what the congestion controller originally determined. Using information about/from the currently active flows, SBD updates the FSE with the correct Flow Group Identifiers (FGIs).
</t>
<figure align="center">
<!--<preamble>Preamble</preamble>-->
<artwork align="center">
<![CDATA[
------- <--- Flow 1
| FSE | <--- Flow 2 ..
------- <--- .. Flow N
^
| |
------- |
| SBD | <-------|
-------
]]>
</artwork>
<postamble>Figure 1: Coupled congestion control architecture</postamble>
</figure>
<t>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 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.</t>
</section>
<section title="Roles" anchor='roles'>
<t>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.</t>
<section title="SBD">
<t>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) are typically routed in the same way. The latter method is the only one specified in this document: SBD MUST consider all flows that use the same five-tuple to belong to the same FG. This classification applies to certain tunnels, or RTP flows that are multiplexed over one transport (cf. <xref target="transport-multiplex"/>). In one way or another, such multiplexing will probably be recommended for use with rtcweb <xref target="rtcweb-rtp-usage"/>. Port numbers are needed as part of the classification due to mechanisms like Equal-Cost Multi-Path (ECMP) routing which use different paths for packets towards the same destination, but are typically configured to keep packets from the same transport connection on the same path.</t>
</section>
<section title="FSE">
<t>The FSE contains a list of all flows that have registered with it. For each flow, it stores:
<list style="symbols">
<t>a unique flow number to identify the flow</t>
<t>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)</t>
<t>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.</t>
<t>The calculated rate CR, i.e. the rate that was most recently calculated by the flow's congestion controller.</t>
<t>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 greedy flow, DR must be set to CR. A DR value that is larger than CR indicates that the flow has taken leftover bandwidth from a non-greedy flow.</t>
<t>S_CR, the sum of the calculated rates of all flows in the same FG (including the flow itself), as seen by the flow during its last rate update.</t>
</list>
<!-- <t>A continuously growing flow number per FGI. The first flow to register itself with the FSE is assigned number 1, the second flow is assigned number 2, etc.</t> -->
</t>
<t>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.</t>
</section>
<section title="Flows">
<t>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 non-greedy flows, the same otherwise). UPDATE returns a rate that should be used instead of the rate that the congestion controller has determined.</t>
<t>Below, an example algorithm is described. While other algorithms could be used instead, the same algorithm must be applied to all flows. The way the algorithm is described here, the operations are carried out by the flows, but they are the same for all flows. This means that the algorithm could, for example, be implemented in a library that provides registration, deregistration functions and the UPDATE function. To minimize the number of changes to existing applications, one could, however, also embed this functionality in the FSE element.</t>
<section title="Example algorithm" anchor='example-alg'>
<t><list style="format (%d)">
<t>When a flow starts, it registers itself with SBD and the FSE. CR and DR are initialized with the congestion controller's initial rate. SBD will assign the correct FGI. When a flow is assigned an FGI, its S_CR is initialized to be the sum of the calculated rates of all the flows in its FG.</t>
<t>When a flow stops, it sets its DR to 0 and negates P.</t>
<t>Every time the flow's congestion controller determines a new sending rate new_CR, assuming the flow's new desired rate new_DR to be "infinity" in case of a greedy flow with an unknown maximum rate, the flow calls UPDATE, which carries out the following tasks:
<list style="format (%c)">
<t>For all the flows in its FG (including itself), it calculates the sum of all the absolute values of all priorities, S_P, the sum of all desired rates, S_DR, and the sum of all the calculated rates, new_S_CR.</t>
<t>It updates CR if new_CR is smaller than the already stored CR value, or if new_S_CR is smaller or equal to the flow's stored S_CR value. This restriction on updating CR ensures that only one flow can make S_CR increase at a time.</t>
<t>It updates new_S_CR using its own updated CR, and updates S_CR with new_S_CR.</t>
<t>It subtracts DR from S_DR, updates DR to min(new_DR, CR), and adds the updated DR to S_DR.</t>
<t>It initializes the total leftover rate TLO to 0. Then, for every other flow i in its FG that has DR(i) < CR(i), it calculates the leftover rate as abs(P(i))/S_P * S_CR - DR(i), adds the flow's leftover rate to TLO, and sets DR(i) to CR(i). This makes flow i look like a greedy flow and ensures that the leftover rate can only once be taken from it. Finally, if P(i) is negative, it removes flow i's entry from the FSE.</t>
<t>It calculates the new sending rate as min(new_DR, P/S_P * S_CR + TLO). This gives the flow the correct share of the bandwidth based on its priority, applies an upper bound in case of an application-limited flow, and adds any potentially leftover bandwidth from non-greedy flows.</t>
<t>If the flow's new sending rate is greater than DR, then it updates DR with the flow's new sending rate.</t>
</list></t>
</list></t>
<t>The goals of the flow algorithm are to achieve prioritization, improve network utilization in the face of non-greedy 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 non-greedy 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.</t>
<t>There are more potential issues with the algorithm described here. Rule 3 b) leads to a conservative behavior: it ensures that only one flow at a time can increase the overall sending rate. This rule is probably appropriate for situations where minimizing delay is the major goal, but it may not fit for all purposes; it also does not incorporate the magnitude by which a flow can increase its rate. Notably, despite this limitation on the overall rate of all flows per FGI, immediate rate jumps of single flows could become problematic when the FSE is used in a highly asynchronous manner, e.g. when flows have very different RTTs. Rule 3 e) gives all the leftover rate of non-greedy 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.</t>
</section>
<section anchor="example-op" title="Example operation">
<t>In order to illustrate the operation of the 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.</t>
<t>Flow #1 begins. It is greedy and considers itself to have top priority. This is the FSE after the flow algorithm's step 1:</t>
<figure align="left">
<artwork align="left">
<![CDATA[
---------------------------------------------
| # | FGI | P | CR | DR | S_CR | Rate |
| | | | | | | |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 |
---------------------------------------------
]]>
</artwork>
</figure>
<t>Its congestion controller gradually increases its rate. Eventually, at some point, the FSE should look like this:</t>
<figure align="left">
<artwork align="left">
<![CDATA[
---------------------------------------------
| # | FGI | P | CR | DR | S_CR | Rate |
| | | | | | | |
| 1 | 1 | 1 | 10 | 10 | 10 | 10 |
---------------------------------------------
]]>
</artwork>
</figure>
<t>Now another flow joins. It is also greedy, and has a lower priority (0.5):</t>
<figure align="left">
<artwork align="left">
<![CDATA[
-----------------------------------------------
| # | FGI | P | CR | DR | S_CR | Rate |
| | | | | | | |
| 1 | 1 | 1 | 10 | 10 | 10 | 10 |
| 2 | 1 | 0.5 | 1 | 1 | 11 | 1 |
-----------------------------------------------
]]>
</artwork>
</figure>
<t>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.</t>
<figure align="left">
<artwork align="left">
<![CDATA[
new_CR = 8. new_DR = infinity.
3 a) S_P = 1.5; S_DR = 11; new_S_CR = 11.
3 b) new_CR < CR, hence CR = 8.
3 c) new_S_CR = 9; S_CR = 9.
3 d) DR = CR = 8; S_DR = 9.
3 e) TLO = 0; there are no other flows with DR < CR.
3 f) new sending rate: min(infinity, 1/1.5 * 9 + 0) = 6.
3 g) does not apply.
The resulting FSE looks as follows:
-----------------------------------------------
| # | FGI | P | CR | DR | S_CR | Rate |
| | | | | | | |
| 1 | 1 | 1 | 8 | 8 | 9 | 6 |
| 2 | 1 | 0.5 | 1 | 1 | 11 | 1 |
-----------------------------------------------
]]>
</artwork>
</figure>
<t>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.</t>
<figure align="left">
<artwork align="left">
<![CDATA[
new_CR=2. new_DR = infinity.
3 a) S_P = 1.5; S_DR = 9; new_S_CR = 9.
3 b) new_CR > CR but new_S_CR < S_CR, hence CR = 2.
3 c) new_S_CR = 10; S_CR = 10.
3 d) DR = CR = 2; S_DR = 10.
3 e) TLO = 0; there are no other flows with DR < CR.
3 f) new sending rate: min(infinity, 0.5/1.5 * 10 + 0) = 3.33.
3 g) new sending rate > DR, hence DR = 3.33.
The resulting FSE looks as follows:
-----------------------------------------------
| # | FGI | P | CR | DR | S_CR | Rate |
| | | | | | | |
| 1 | 1 | 1 | 8 | 8 | 9 | 6 |
| 2 | 1 | 0.5 | 2 | 3.33 | 10 | 3.33 |
-----------------------------------------------
]]>
</artwork>
</figure>
<t>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.</t>
<figure align="left">
<artwork align="left">
<![CDATA[
new_CR=9. new_DR=2.
3 a) S_P = 1.5; S_DR = 11.33; new_S_CR = 10.
3 b) new_CR > CR and new_S_CR > S_CR, hence CR is not updated
(since flow #2 has just increased S_CR, flow #1 cannot also
increase it in this iteration).
3 c) new_S_CR = 10; S_CR = 10.
3 d) DR = 2; S_DR = 5.33.
3 e) TLO = 0; there are no other flows with DR < CR.
3 f) new sending rate: min(2, 1/1.5 * 10 + 0) = 2. Note that,
without the 2 Mbit/s limitation from the application, the new
sending rate for flow #1 would now be 6.66 Mbit/s, leading to
perfect network saturation (6.66 + 3.33 = approx. 10).
3 g) does not apply.
The resulting FSE looks as follows:
-----------------------------------------------
| # | FGI | P | CR | DR | S_CR | Rate |
| | | | | | | |
| 1 | 1 | 1 | 8 | 2 | 10 | 2 |
| 2 | 1 | 0.5 | 2 | 3.33 | 10 | 3.33 |
-----------------------------------------------
]]>
</artwork>
</figure>
<t>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.</t>
<figure align="left">
<artwork align="left">
<![CDATA[
new_CR=3. new_DR = infinity.
3 a) S_P = 1.5; S_DR = 5.33; new_S_CR = 10.
3 b) new_CR > CR but new_S_CR = S_CR, hence CR = 3.
3 c) new_S_CR = 11; S_CR = 11.
3 d) DR = 3; S_DR = 5.
3 e) TLO = 0; flow #1 has DR < CR, hence TLO += 1/1.5 * 11
- 2 = 5.33.
DR of flow #1 is set to 8. Flow #1 does not have a negative
P(i) value, so its entry is not deleted.
3 f) new sending rate: min(infinity, 0.5/1.5*11 + 5.33) = 9.
3 g) new sending rate > DR, hence DR = 9.
The resulting FSE looks as follows:
-----------------------------------------------
| # | FGI | P | CR | DR | S_CR | Rate |
| | | | | | | |
| 1 | 1 | 1 | 8 | 8 | 10 | 2 |
| 2 | 1 | 0.5 | 3 | 9 | 11 | 9 |
-----------------------------------------------
]]>
</artwork>
</figure>
<t>Now, the total rate of the two flows is 2 + 9 = 11 Mbit/s, exceeding the total capacity by the 1 Mbit/s by which the congestion controller of flow #2 has increased its rate. Note that, had flow #1 been greedy, the same total rate would have resulted after this iteration. 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.</t>
<figure align="left">
<artwork align="left">
<![CDATA[
new_CR = 1. new_DR = infinity.
3 a) S_P = 1.5; S_DR = 9; new_S_CR = 11.
3 b) new_CR < CR hence CR = 1.
3 c) new_S_CR = 9; S_CR = 9.
3 d) DR = 1; S_DR = 1.
3 e) TLO = 0; flow #1 has DR < CR, hence TLO += 1/1.5 * 9 - 0 = 6.
DR of flow #1 is set to 8. Flow #1 has a negative P(i) value, so
its entry is deleted.
3 f) new sending rate: min(infinity, 0.5/1.5 * 9 + 6) = 9.
3 g) new sending rate > DR, hence DR = 9.
The resulting FSE looks as follows:
-----------------------------------------------
| # | FGI | P | CR | DR | S_CR | Rate |
| | | | | | | |
| 1 | 1 | -1 | 8 | 0 | 10 | 2 | (before deletion)
| 2 | 1 | 0.5 | 1 | 9 | 9 | 9 |
-----------------------------------------------
]]>
</artwork>
</figure>
<t>Now, the total rate, used only by flow #2, is 9 Mbit/s, which is the rate that it would have had alone upon reacting to congestion after a sending rate of 11 Mbit/s.</t>
</section>
</section>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>This document has benefitted from discussions with and feedback from Stein Gjessing, David Hayes, Safiqul Islam, Naeem Khademi, Andreas Petlund, and David Ros (who also gave the FSE its name).</t>
</section>
<!-- Possibly a 'Contributors' section ... -->
<section anchor="IANA" title="IANA Considerations">
<t>This memo includes no request to IANA.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>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.</t>
<t>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.</t>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
<!-- References split into informative and normative -->
<!-- There are 2 ways to insert reference entries from the citation libraries:
1. define an ENTITY at the top, and use "ampersand character"RFC2629; here (as shown)
2. simply use a PI "less than character"?rfc include="reference.RFC.2119.xml"?> here
(for I-Ds: include="reference.I-D.narten-iana-considerations-rfc2434bis.xml")
Both are cited textually in the same manner: by using xref elements.
If you use the PI option, xml2rfc will, by default, try to find included files in the same
directory as the including file. You can also define the XML_LIBRARY environment variable
with a value containing a set of directories to search. These can be either in the local
filing system or remote ones accessed by http (http://domain/dir/... ).-->
<references title="Normative References">
<!--?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?-->
&RFC2119;
&RFC2140;
&RFC3124;
</references>
<references title="Informative References">
<reference anchor="rtcweb-usecases" target="">
<front>
<title>Web Real-Time Communication Use-cases and Requirements</title>
<author initials="C." surname="Holmberg" fullname="C. Holmberg"></author>
<author initials="S." surname="Hakansson" fullname="S. Hakansson"></author>
<author initials="G." surname="Eriksson" fullname="G. Eriksson"></author>
<date month="December" year="2012"/>
</front>
<seriesInfo name="Internet-draft" value="draft-ietf-rtcweb-use-cases-and-requirements-10.txt"/>
</reference>
<reference anchor="transport-multiplex" target="">
<front>
<title>Multiple RTP Sessions on a Single Lower-Layer Transport</title>
<author initials="M." surname="Westerlund" fullname="M. Westerlund"></author>
<author initials="C." surname="Perkins" fullname="C. Perkins"></author>
<date month="October" year="2012"/>
</front>
<seriesInfo name="Internet-draft" value="draft-westerlund-avtcore-transport-multiplexing-04.txt"/>
</reference>
<reference anchor="rtcweb-rtp-usage" target="">
<front>
<title>Web Real-Time Communication (WebRTC): Media Transport and Use of RTP</title>
<author initials="C." surname="Perkins" fullname="C. Perkins"></author>
<author initials="M." surname="Westerlund" fullname="M. Westerlund"></author>
<author initials="J." surname="Ott" fullname="J. Ott"></author>
<date month="October" year="2012"/>
</front>
<seriesInfo name="Internet-draft" value="draft-ietf-rtcweb-rtp-usage-05.txt"/>
</reference>
</references>
<!--<section anchor="app-additional" title="Additional Stuff">
<t>This becomes an Appendix.</t>
</section>-->
<!-- Change Log
v00 2006-03-15 EBD Initial version
v01 2006-04-03 EBD Moved PI location back to position 1 -
v3.1 of XMLmind is better with them at this location.
v02 2007-03-07 AH removed extraneous nested_list attribute,
other minor corrections
v03 2007-03-09 EBD Added comments on null IANA sections and fixed heading capitalization.
Modified comments around figure to reflect non-implementation of
figure indent control. Put in reference using anchor="DOMINATION".
Fixed up the date specification comments to reflect current truth.
v04 2007-03-09 AH Major changes: shortened discussion of PIs,
added discussion of rfc include.
v05 2007-03-10 EBD Added preamble to C program example to tell about ABNF and alternative
images. Removed meta-characters from comments (causes problems). -->
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 20:58:02 |