[rmcat] Review on draft-ietf-rmcat-eval-test-02
Stefan Holmer <holmer@google.com> Sun, 07 February 2016 12:05 UTC
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From: Stefan Holmer <holmer@google.com>
Date: Sun, 07 Feb 2016 12:05:02 +0000
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To: rmcat WG <rmcat@ietf.org>, "Michael Ramalho (mramalho)" <mramalho@cisco.com>, "Xiaoqing Zhu (xiaoqzhu)" <xiaoqzhu@cisco.com>, Varun Singh <varun@comnet.tkk.fi>, Zaheduzzaman Sarker <zaheduzzaman.sarker@ericsson.com>
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Subject: [rmcat] Review on draft-ietf-rmcat-eval-test-02
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Hi, Sorry for being late with this review, here are my comments: *General comments:* - We consider audio and video content, but should we also consider slide shows and/or screen content? In my experience those often sources are widely different in their rate characteristics, and may be very bursty. I think it could be interesting to see how different candidates handle the case where they are source limited most of the time (static image, encoder produces < 50 kbps), and every now and then there's a slide change. - Each video stream is often accompanied by a constant bitrate audio stream in these test cases, but it's not clear to me why? I think we need to clarify how the audio stream is going to be evaluated. Are we going to look at end-to-end latency and jitter? - It's suggested that video resolution should be specified in the test cases, but we don't seem to suggest that video quality should be evaluated (PSNR/SSIM). How are we then expecting that video resolution will matter in these tests? Do we allow the encoder to internally scale down the resolution depending on the available bandwidth? In that case, will be really be evaluating CC candidates or are we starting to lean towards comparing full RTC systems? *Raw comments:* 4.1. Evaluation metircs . . . . . . . . . . . . . . . . . . . 7 Comments: - metrics *Section 3* It should be noted that depending on the test cases it is possible to have different path characteristics in of the either directions. Comments: - "in either of the directions" - There is a bullet which looks misplaced after this. In a testbed environment laboratory there may exist a significant amount of traffic on portions of the network path between the endpoints that is not desired for the purposes of these RMCAT tests. Comments: - I think we should define what a testbed environment laboratory is somewhere. Is it a LAN with some network emulator, or is it a fully simulated network? + Bottleneck queue size: defines size of queue in terms of queuing time when the queue is full (in milliseconds). Comments: - "the size of the queue" * Application-related: defines the traffic source behaviour for implementing the test case Comments: - In this section we mention Video/Voice as media types. Should we also consider screen content/slide shows? See general comments. - Under adaptability we mention resolution as being an option of adaptation. It's not clear to me how we should evaluate something like that without comparing the visual quality in some way. See general comments. *Section 4.3* Also it is possible that the media traffic generator used in a particular simulator or testbed if not capable of generating higher bitrate. Hence we have selected a suitable bitrate range Comments: - Spelling errors: - "testbed is not" - "bit rate" *Section 5.1* maximum Media Bit Rate is Greater than Link Capacity. In this case, the application will attempt to ramp up to its maximum bit rate, since the link capacity is limited to a value lower, the congestion control scheme is expected to stabilize the sending bit rate close to the available bottleneck capacity. This situation can occur when the endpoints are connected via thin long networks even though the advertised capacity of the access network may be higher. Comments: - I don't understand the last sentence here. Isn't it simpler to refer to e.g. cable/adsl links where the uplink may be in the order of 256 - 1024 kbps. It should be noted that the exact variation in available capacity due to any of the above depends on the under-lying technologies. Hence, we describe a set of known factors, which may be extended to devise a more specific test case targeting certain behaviour in a certain network environment. Expected behavior: the candidate algorithm is expected to detect the path capacity constraint, converges to bottleneck link's capacity and Comments: - "underlying" - "targeting* a* certain" - "converges to *the* bottleneck link's capacity" o Path characteristics: as described in Section 4.2 Comments: - I don't think there is a point in mentioning that we are using the defaults. * This test uses the following one way propagation delays of 50 ms and 100 ms. Comments: - "This test uses one way propagation delays of 50 ms and 100 ms" - Is this referring to two instances of the test, or to forward and backward delays? *Section 5.2* Expected behavior: the candidate algorithms is expected to detect the variation in available capacity and adapt the media stream(s) accordingly. The flows stabilize around their maximum bitrate as the as the maximum link capacity is large enough to accommodate the flows. When the available capacity drops, the flow(s) adapts by decreasing its sending bit rate, and when congestion disappears, the flow(s) are again expected to ramp up. Comments: - spelling error "maximum bitrate as the maximum link capacity" - Rewrite last sentence as plural. *Section 5.3* It is expected that the candidate algorithms is able to cope with the lack of feedback information and adapt to minimize the performance degradation of media flows in the forward channel. It should be noted that for this test case: logs are compared with the reference case, i.e, when the backward channel has no impairments Comments: - "algorithms *are* able" - End last sentence with ". - I think the test duration should be longer since this is a fairly complicated scenario. What about 300 s? *Section 5.4* In this test case, more than one RMCAT media flow shares the bottleneck link and each of them uses the same congestion control algorithm. This is a typical scenario where a real-time interactive application sends more than one media flows to the same destination and these flows are multiplexed over the same port. Comments: - - I don't think multiplexing is a good example here. There is no reason why they would actually fight for bandwidth between each other if they are multiplexed over the same port, as they may as well run a single CC and allocate the available bandwidth between them. Better to take an example of two or more different endpoints in the same office calling two or more endpoints in another office, or similar. - misspelled: "one media flows" should be "one media flow" Testbed topology: Three media sources S1, S2, S3 are connected to respective R1, R2, R3. Comments: - Should probably be "are connected to R1, R2 and R3 respectively". +---------+------------+------------+----------+ | Flow IF | Media type | Start time | End time | +---------+------------+------------+----------+ Comments: - Flow ID *Section 5.1* * Path capacity ratio: 1.0 Comments: - Not necessary to mention as I think it is the default? Also mentioned in other places. + Congestion control: Default TCP congestion control. Comments: - Should we suggest what the default TCP CC could be? New reno, Cubic? *Section 5.8* In this test case, more than one real-time interactive media flows share the link bandwidth and all flows reach to a steady state by utilizing the link capacity in an optimum way. At these stage one of the media flow is paused for a moment. This event will result in more available bandwidth for the rest of the flows and as they are on a shared link. When the paused media flow will resume it would no longer have the same bandwidth share on the link. It has to make it's way through the other existing flows in the link to achieve a fair share of the link capacity. This test case is important specially for real-time interactive media which consists of more than one media flows and can pause/resume media flow at any point of time during the session. This test case directly addresses the requirement number 5 in [I-D.ietf-rmcat-cc-requirements]. One can think it as a variation of test case defined in Section 5.4. However, it is different as the candidate algorithms can use different strategies to increase its efficiency, for example the fairness, convergence time, reduce oscillation etc, by capitalizing the fact that they have previous information of the link. Comments: - Several spelling errors: - "At *this* stage one of the media *flows* is paused for a moment." - - "This event will result in more available bandwidth for the rest of the flows *(and)* as they are on a shared link. When the paused media flow *resumes* it *will* no longer have the same bandwidth share on the link." - - " ... one media flows and can pause/resume media *flows* at any point of time during the session." - - "However, it is different as the candidate algorithms can use different strategies to increase *their* efficiency, for example *(the)* fairness" *Section 6* It has been noticed that there are other interesting test cases besides the basis test cases listed above. Comments: - "base" *Section 6.2* Testbed attributes: o Test duration: 120s o Path characteristics: * Reference bottleneck capacity between A and B = 2Mbps. ... * One-Way propagation delay: 1. Between S1 and R1: 100ms 2. Between S2 and R2: 40ms 3. Between S3 and R3: 40ms Comments: - Duration should probably be at least 300s given the complexity of this setup. - Does the reference bottleneck capacity apply to all links or only A and B? If only A and B, what's the reference for the other links? - In my opinion this test case is complex enough without different one-way propagation delays on the links. *Section 7* Additional wireless network (both cellular network and WiFi network) specific test cases are define in [I-D.ietf-rmcat-wireless-tests] Comments: - "defined in"
- [rmcat] Review on draft-ietf-rmcat-eval-test-02 Stefan Holmer
- Re: [rmcat] Review on draft-ietf-rmcat-eval-test-… Zaheduzzaman Sarker