November 9, 2011

What's up in networks (3/3): dash

The last post in this mini-series. After openFlow and hetnets, here is dash.

DASH or Dynamic Adaptive Streaming over HTTP
Although it is not exactly what the MPEG scientists have promoted for a decade, most of today's video traffic is based on HTTP and TCP (Netflix player, Microsoft Smooth Streaming and Adobe OSMF). And it works. The video traffic is exploding: adaptive streaming already represents more than one third of the Internet traffic at peak time, and it is expected to prevail, even on mobiles. Facing this plebiscite, the MPEG consortium has launched the process of standardizing DASH into MPEG.

In short, for a given movie, the video server publishes a manifest file in which it declares several video formats. Each format corresponds to a certain encoding, so a certain quality and a certain bit-rates. All these different videos of the same movie are cut into chunks. A client requesting a movie selects a given video format and then starts downloading the chunks. On a periodic manner, the client tries to estimate whether this video encoding fits the capacity of the network link between her and the server. If she is not satisfied, she considers switching to another encoding for the next chunks. What is the best chunk size, how to estimate the link capacity, what is the best delay between consecutive estimation, how to react to short-term bandwidth changes, how to switch to another encoding… are among the questions that have not received the attention of the scientific community, so every DASH client implements some magic parameters without any concern for potential impacts on the network.

Despite the multimedia scientific community and the video standardization group are large lively communities, many research issues related to DASH have not been anticipated and sufficiently addressed. Among them, I highlight:
  • When several concurrent DASH connections share the same bottleneck, the congestion control mechanism of TCP may be compromise. In fact, a DASH connection is based on TCP, which implements an adaptive congestion control with proven convergence toward a fair sharing of the bottleneck among concurrent connections. By incessantly adapting the flow bit-rate DASH may prevent the convergence of TCP. If network bottlenecks locate on links that are shared by hundreds of concurrent DASH flows, the lack of convergence of the congestion control mechanism is a risk. I may overestimate the impact, but at least understanding the impact of DASH adaptive policy (which seems to use a lot of random parameter settings) on the eventual convergence of a congestion control policy is an exciting scientific topic.
  • When multiple servers store different video encodings of the same movie, the client may incessantly switch from a video encoding to another. A DASH connection works especially well when the bottleneck is always the same, whatever the chosen video encoding. In this case, the adaptive mechanism converges toward the video encoding that fits the bottleneck capacity. But in today's Internet, the content can be located in various distinct locations: CDN servers, Internet proxies, and content routers with caching capabilities. If the links toward the different encodings have different congestion level, the DASH adaptive algorithm may become crazy. 
  • A DASH connection does not support swarming. Swarm downloading (one client fetching a large video content from multiple servers) was expected to be enabled by both the multiple copies of the same content and the chunk-based video format. If every chunk comes from a different server, the congestion cannot be accurately measured. In fact, DASH cannot implement a consistent behavior when multiple paths are used to retrieve the video chunks. 
By the way, DASH is yet another point in favor of HTTP, which is becoming the de facto narrow waist of the Internet. The motivations for using HTTP include its capacity to traverse firewalls and NATs, its nice human-readable names and its capacity to leverage on Internet proxies and CDNs. Somehow, DASH adds congestion control and adaptive content, making the HTTP protocol even more powerful. But the gap between its huge utilization over the Internet and the lack of understanding of its behavior at large scale has the potential to scare network operators. I guess it is the way Internet has always evolved.

2 comments:

  1. DASH does not define behaviour, DASH defines the format for the MPD and segments based on ISOBMFF and M2TS. Keep that in mind when writing "behavior of DASH is still|also mysterious" because some of them might by simply out of scope for good reasons.

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  2. Thank you for the remark. I tried to fix it. In fact, I wanted to say that the adaptive algorithm may not converge.

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