October 2, 2015

Can Multipath Boost the Network Performances of Real-time Media?

I would like to emphasize now a paper that deals with multipath networking for video streaming. This paper is:
Varun Singh, Saba Ahsan, and Jörg Ott, “MPRTP: Multipath Considerations for Real-time Media”, in Proc. 4th ACM Multimedia Systems Conference (MMSys '13), Oslo, Norway, Feb. 2013
and it has led to multiple actions in IETF standardization group.

There are multiple routes between two hosts in the current Internet. This statement tends to be even truer when considering the flattening Internet topology, where Internet Service Providers (ISPs) have multiple options to reach a distant host. It is also truer with the multiple network interfaces available in the modern mobile devices and the multiple wireless network accesses that co-exist in the urban environment. The question now is about the exploitation of these multiple routes. The network protocols that are in used today stick to the traditional monopath paradigm. Yet, scientists have shown that leveraging multipath can bring many advantages, including better traffic load balancing, higher throughput and more robustness. 

This paper, which is already two years old, studies multipath opportunities for the specific case of conversational and interactive communication systems between mobile devices (e.g. Skype). These applications are especially challenging because the traffic between communicating hosts should meet tight real-time bounds. The idea of this paper is to study whether the most widely used network protocol for the applications, namely Real Time Transport Protocol (RTP), can be turned into a multipath protocol. They thus propose a backwards-compatible extension to RTP called Multipath RTP (MPRTP).

In short, this paper presents the MPRTP extension and evaluates its performance in several scenarios. First, the authors comment the main challenges that an extension of RTP protocol must face in order to split a single RTP stream into multiples subflows. Second, the authors present the protocol details as well as the algorithms that are considered to solve these challenges. Third, simulations are conducted to evaluate the performance of the proposal.

Authors point out that a MPRTP protocol should be able to adapt to bandwidth changes on the paths by redistributing the traffic load among them in a smooth way to avoid oscillations. This is especially important in the case of mobile communications where quick capacity changes are common. To guarantee fast adaptation, the authors propose packet-scheduling mechanisms that do not abruptly reallocate traffic among congested and non-congested paths if a path becomes suddenly congested. 

Other important issue is the variation on packet inter-arrival time (packet-skew) among the different paths.  The fact of having multiple diverse paths make harder to estimate the right buffer size to prevent this issue. To overcome this problem the authors propose an adaptive playout buffer, which individually considers the path skew in each path. They also privilege the selection of paths with similar latencies.

The choice of suitable transmission paths should consider the path characteristics in terms of QoS metrics as losses, latency or capacity. The authors propose several extensions to the RTP protocol, including a new RTP reporting message (where the receiver provides QoS data per sub-flow) and a scheduling algorithm (where the sender uses these reports to decide a traffic distribution among the available paths). 

All the aforementioned extensions are always designed to be backwards compatibility, i.e. traditional RTP hosts can interoperate with hosts equipped with MPRTP extensions in single-path scenarios. 

An exhaustive battery of simulations is conducted to evaluate the MPRTP performance in a broad range of scenarios: (i) path properties (losses, delays, and capacities) vary along time; (ii) paths share a common bottleneck, and (iii) MPRTP is deployed over mobile terminals using WLAN and/or 3G paths. These evaluations show that (1) the dynamic MPRTP performance is not far from the static performance for single and multipath cases, (2) MPRTP successfully offloads traffic from congested paths to the other ones keeping some proportional fairness among them, and (3) on lossy links multipath is more robust and produces fewer losses with respect to single path.

Overall, this paper addresses a significant problem (how to make a real-time UDP-based protocol multipath) with a comprehensive study. It is one of the first attempts to exploit multipath functionalities in the framework of multimedia communications, and especially with tight real time limitations. This paper thus perfectly completes the works that have been done by the network community on multipath TCP protocols. That being said, many problems related to multipath multimedia protocols are still open. Among others, let us cite rate-adaptive streaming and multiview video in the context of multipath.

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