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<pre>Internet Engineering Task Force (IETF) Y. Zhang
Request for Comments: 6972 Coolpad
Category: Informational N. Zong
ISSN: 2070-1721 Huawei Technologies
July 2013
<span class="h1">Problem Statement and Requirements of</span>
<span class="h1">the Peer-to-Peer Streaming Protocol (PPSP)</span>
Abstract
Peer-to-Peer (P2P) streaming systems becoming more and more popular
on the Internet, and most of them are using proprietary protocols.
This document identifies problems associated with proprietary
protocols; proposes the development of the Peer-to-Peer Streaming
Protocol (PPSP), which includes the tracker and peer protocols; and
discusses the scope, requirements, and use cases of PPSP.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc6972">http://www.rfc-editor.org/info/rfc6972</a>.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
<span class="grey">Zhang & Zong Informational [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-1.1">1.1</a>. Backgrounds . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-1.2">1.2</a>. Requirements Language . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-2">2</a>. Terminology and Concepts . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3">3</a>. Problem Statement . . . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-3.1">3.1</a>. Heterogeneous P2P Traffic and P2P Cache Deployment . . . . <a href="#page-5">5</a>
<a href="#section-3.2">3.2</a>. QoS Issue and CDN Deployment . . . . . . . . . . . . . . . <a href="#page-5">5</a>
3.3. Extended Applicability in Mobile and Wireless
Environments . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-4">4</a>. Tasks of PPSP: Standard Peer-to-Peer Streaming Protocols . . . <a href="#page-7">7</a>
<a href="#section-4.1">4.1</a>. Tasks and Design Issues of the Tracker Protocol . . . . . <a href="#page-8">8</a>
<a href="#section-4.2">4.2</a>. Tasks and Design Issues of the Peer Protocol . . . . . . . <a href="#page-9">9</a>
<a href="#section-5">5</a>. Use Cases of PPSP . . . . . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5.1">5.1</a>. Worldwide Provision of Live/VoD Streaming . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5.2">5.2</a>. Enabling CDN for P2P VoD Streaming . . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-5.3">5.3</a>. Cross-Screen Streaming . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-5.4">5.4</a>. Cache Service Supporting P2P Streaming . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-5.5">5.5</a>. Proxy Service Supporting P2P Streaming . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-5.5.1">5.5.1</a>. Home Networking Scenario . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-5.5.2">5.5.2</a>. Browser-Based HTTP Streaming . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-6">6</a>. Requirements of PPSP . . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-6.1">6.1</a>. Basic Requirements . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-6.2">6.2</a>. Operational and Management Requirements . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-6.2.1">6.2.1</a>. Operational Considerations . . . . . . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-6.2.2">6.2.2</a>. Management Considerations . . . . . . . . . . . . . . <a href="#page-17">17</a>
<a href="#section-6.3">6.3</a>. PPSP Tracker Protocol Requirements . . . . . . . . . . . . <a href="#page-17">17</a>
<a href="#section-6.4">6.4</a>. PPSP Peer Protocol Requirements . . . . . . . . . . . . . <a href="#page-18">18</a>
<a href="#section-7">7</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
<a href="#section-8">8</a>. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-21">21</a>
<a href="#section-9">9</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-21">21</a>
<a href="#section-9.1">9.1</a>. Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-21">21</a>
<a href="#section-9.2">9.2</a>. Informative References . . . . . . . . . . . . . . . . . . <a href="#page-21">21</a>
<span class="grey">Zhang & Zong Informational [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Backgrounds</span>
Streaming traffic is among the largest and fastest growing traffic on
the Internet [<a href="#ref-Cisco" title=""Cisco Visual Networking Index: Forecast and Methodology, 2012 - 2017"">Cisco</a>]. Peer-to-Peer (P2P) streaming contributes
substantially to this growth. With the advantage of high scalability
and fault tolerance against a single point of failure, P2P streaming
applications are able to distribute large-scale, live, and video-on-
demand (VoD) streaming programs to a large audience with only a
handful of servers. More and more providers are joining the P2P
streaming ecosystem, e.g., Content Distribution Networks (CDN)
providers started using P2P technologies to distribute their
streaming content.
Given the increasing integration of P2P streaming in the global
content delivery infrastructure, there is a need for an open and
standard streaming signaling protocol suite. Almost all existing
systems use proprietary protocols. Having multiple proprietary
protocols that perform similar functions results in repetitious
development efforts for new systems, and the lock-in effects lead to
substantial integration difficulties with other players (e.g., CDN).
For example, in the enhancement of existing caches and CDN systems to
support P2P streaming, proprietary protocols may increase the
complexity of interactions with different P2P streaming applications.
In this document, we propose the development of an open, P2P
Streaming Protocol, which is abbreviated as PPSP, to standardize
signaling operations in the P2P streaming system to solve the above-
mentioned problems.
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>. Requirements Language</span>
The key words "MUST" and "MUST NOT" in this document are to be
interpreted as described in <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>] and indicate
requirement levels for compliant implementations.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Terminology and Concepts</span>
CHUNK: A CHUNK is a basic unit of data organized in P2P streaming for
storage, scheduling, advertisement, and exchange among peers [<a href="#ref-VoD" title=""Challenges, Design and Analysis of a Large-Scale P2P-VoD System"">VoD</a>].
A CHUNK size varies from several KBs to several MBs in different
systems. In the case of the MB size CHUNK scenario, a sub-CHUNK
structure named piece is often defined to fit in a single transmitted
packet. A streaming system may use different granularities for
different usage, e.g., using CHUNKs during data exchange and using a
larger unit such as a set of CHUNKs during advertisement.
<span class="grey">Zhang & Zong Informational [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
CHUNK ID: The identifier of a CHUNK in a content stream.
CLIENT: A CLIENT refers to a participant in a P2P streaming system
that only receives streaming content. In some cases, a node not
having enough computing and storage capabilities will act as a
CLIENT. Such a node can be viewed as a specific type of PEER.
CONTENT DISTRIBUTION NETWORK (CDN): A CDN is a collection of nodes
that are deployed, in general, at the network edge, like Points of
Presence (POP) or Data Centers (DC), and store content provided by
the original content servers. Typically, CDN nodes serve content to
the users located nearby topologically.
LIVE STREAMING: LIVE STREAMING refers to a scenario where all the
audiences receive streaming content for the same ongoing event. It
is desired that the lags between the play points of the audiences and
streaming source be small.
P2P CACHE: A P2P CACHE refers to a network entity that caches P2P
traffic in the network and, either transparently or explicitly,
streams content to other PEERs.
PEER: A PEER refers to a participant in a P2P streaming system that
not only receives streaming content, but also caches and streams
streaming content to other participants.
PEER LIST: A list of PEERs that are in the same SWARM maintained by
the TRACKER. A PEER can fetch the PEER LIST of a SWARM from the
TRACKER or from other PEERs in order to know which PEERs have the
required streaming content.
PEER ID: The identifier of a PEER such that other PEERs, or the
TRACKER, can refer to the PEER by using its ID.
PEER-TO-PEER STREAMING PROTOCOL (PPSP): PPSPs refer to the primary
signaling protocols among various P2P streaming system components,
including the TRACKER and the PEER.
TRACKER: A TRACKER refers to a directory service that maintains a
list of PEERs participating in a specific audio/video channel or in
the distribution of a streaming file. Also, the TRACKER answers PEER
LIST queries received from PEERs. The TRACKER is a logical component
that can be centralized or distributed.
VIDEO ON DEMAND (VoD): VIDEO ON DEMAND refers to a scenario in which
different audiences may watch different parts of the same recorded
streaming with downloaded content.
<span class="grey">Zhang & Zong Informational [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
SWARM: A SWARM refers to a group of PEERs that exchange data to
distribute CHUNKs of the same content (e.g., video/audio program,
digital file, etc.) at a given time.
SWARM ID: The identifier of a SWARM containing a group of PEERs
sharing a common streaming content.
SUPER-NODE: A SUPER-NODE is a special kind of PEER deployed by ISPs.
This kind of PEER is more stable with higher computing, storage, and
bandwidth capabilities than normal PEERs.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Problem Statement</span>
The problems caused by proprietary protocols for P2P streaming
applications are described in this section.
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Heterogeneous P2P Traffic and P2P Cache Deployment</span>
ISPs are faced with different P2P streaming applications introducing
substantial traffic into their infrastructure, including their
backbone and their exchange/interconnection points. P2P caches are
used by ISPs to locally store content and hence reduce the P2P
traffic. P2P caches usually operate at the chunk or file
granularity.
However, unlike web traffic that is represented by HTTP requests and
responses and therefore allows any caching device to be served (as
long as it supports HTTP), P2P traffic is originated by multiple P2P
applications that require the ISPs to deploy different type of caches
for the different types of P2P streams.
This increases both engineering and operational costs dramatically.
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. QoS Issue and CDN Deployment</span>
When compared to client/server streaming, P2P streaming is often
criticized due to its poorer QoS performance (e.g., longer startup
delay, longer seek delay, and channel switch delay). Hybrid CDN/P2P
is a good approach to address this problem [<a href="#ref-CDN-P2P" title=""Analysis of a CDN-P2P Hybrid Architecture for Cost-Effective Streaming Media Distribution"">CDN-P2P</a>].
In order to form the hybrid P2P+CDN architecture, the CDN must be
aware of the specific P2P streaming protocol in the collaboration.
Similar to what is described in <a href="#section-3.1">Section 3.1</a>, proprietary P2P
protocols introduce complexity and the deployment cost of CDN.
<span class="grey">Zhang & Zong Informational [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. Extended Applicability in Mobile and Wireless Environments</span>
Mobile and wireless networks, which make considerable use of
streaming service, are becoming increasingly important in today's
Internet. It's reported that the average volume of video traffic on
mobile networks had risen up to 50% in the early part of 2012
[<a href="#ref-ByteMobile">ByteMobile</a>]. There are multiple prior studies exploring P2P
streaming in mobile and wireless networks [<a href="#ref-Mobile-Streaming1">Mobile-Streaming1</a>]
[<a href="#ref-Mobile-Streaming2">Mobile-Streaming2</a>].
However, it's difficult to directly apply current P2P streaming
protocols (even assuming we can reuse some of the proprietary ones)
in mobile and wireless networks.
Following are some illustrative problems:
First, P2P streaming assumes a stable Internet connection in
downlink and uplink directions, with decent capacity and peers
that can run for hours. This isn't the typical setting for mobile
terminals. Usually, the connections are unstable and expensive in
terms of energy consumption and transmission (especially in uplink
direction). To make mobile/wireless P2P streaming feasible,
trackers may need more information on peers like packet loss rate,
peer battery status, and processing capability during peer
selection as compared to fixed peers. Unfortunately, current
protocols don't convey this kind of information.
Second, current practices often use a "bitmap" message in order to
exchange chunk availability. The message size is in kilobytes and
is exchanged frequently, e.g., an interval of several seconds or
less. In a mobile environment with scarce bandwidth, the message
size may need to be shortened, or it may require more efficient
methods for expressing and distributing chunk-availability
information, which is different from wireline P2P streaming.
Third, for resource-constrained peers, like mobile handsets or
set-top boxes (STB), there are multiple systems competing for
severely limited resources when using proprietary protocols. The
terminal has to install different streaming client software for
different usages, e.g., some for movies and others for sports.
Each of these applications will compete for the same set of
resources, even when one of the applications is running in
background mode. PPSP can alleviate this problem with the basic
idea that the "one common client software with PPSP and different
scheduling plug-ins" is better than "different client software
running at the same time" in memory and disk consumption.
<span class="grey">Zhang & Zong Informational [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Tasks of PPSP: Standard Peer-to-Peer Streaming Protocols</span>
PPSP aims to solve the problems mentioned above by standardizing
signaling protocols that support either live or VoD streaming. PPSP
supports both centralized and distributed trackers. In distributed
trackers, the tracker functionality is distributed in decentralized
peers. In this section, the tracker is a logic conception that can
be implemented in a dedicated tracker server or in peers.
The PPSP design includes a signaling protocol between trackers and
peers (the PPSP "tracker protocol") and a signaling protocol among
the peers (the PPSP "peer protocol") as shown in Figure 1. The two
protocols enable peers to receive streaming content within the time
constraints.
+------------------------------------------------+
| |
| +--------------------------------+ |
| | Tracker | |
| +--------------------------------+ |
| | ^ ^ |
|Tracker | | Tracker |Tracker |
|Protocol| | Protocol |Protocol |
| | | | |
| V | | |
| +---------+ Peer +---------+ |
| | Peer |<----------->| Peer | |
| +---------+ Protocol +---------+ |
| | ^ |
| | |Peer |
| | |Protocol |
| V | |
| +---------------+ |
| | Peer | |
| +---------------+ |
| |
| |
+------------------------------------------------+
Figure 1: PPSP System Architecture
The PPSP design, in general, needs to solve the following challenges:
1) When joining a swarm, how does a peer know which peers it
should contact for content?
2) After determining a set of peers, how does a peer make contact
with these peers? In which manner?
<span class="grey">Zhang & Zong Informational [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
3) How to choose peers with better service capabilities and how to
collect such information from peers?
4) How to improve the efficiency of the communication, e.g., which
compact on-the-wire message format and suitable underlying
transport mechanism (UDP or TCP)?
5) How to improve the robustness of the system using PPSP, e.g.,
when the tracker fails? How to make the tracker protocol and the
peer protocol loosely coupled?
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Tasks and Design Issues of the Tracker Protocol</span>
The tracker protocol handles the initial and periodic exchange of
meta-information between trackers and peers, such as a peer list and
content information.
Therefore, the tracker protocol is best modeled as a request/response
protocol between peers and trackers, and will carry information
needed for the selection of peers suitable for real-time/VoD
streaming.
Special tasks for the design of the tracker protocol are listed
below. This is a high-level task list. The detailed requirements on
the design of the tracker protocol are explicated in <a href="#section-6">Section 6</a>.
1) How should a peer be globally identified? This is related to
the peer ID definition but irrelevant to how the peer ID is
generated.
2) How to identify different peers, e.g., peers with public or
private IP addresses, peers behind or not behind NAT, peers with
IPV4 or IPV6 addresses, peers with different properties?
3) The tracker protocol must be light weight, since a tracker may
need to serve a large number of peers. This is related to the
encoding issue (e.g., Binary based or Text based) and keep-alive
mechanism.
4) How can the tracker report an optimized peer list to serve
particular content? This is related to the status statistic, with
which the tracker can be aware of the peer status and content
status.
The PPSP tracker protocol will consider all these issues in the
design according to the requirements from both the peer and tracker
perspectives and will also take into consideration deployment and
operation perspectives.
<span class="grey">Zhang & Zong Informational [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Tasks and Design Issues of the Peer Protocol</span>
The peer protocol controls the advertising and exchange of content
between the peers.
Therefore, the peer protocol is modeled as a gossip-like protocol
with periodic exchanges of neighbor and chunk-availability
information.
Special tasks for the design of the peer protocol are listed below.
This is a high-level task-list. The detailed requirements on the
design of the peer protocol are explicated in <a href="#section-6">Section 6</a>.
1) How is certain content globally identified and verified? Since
the content can be retrieved from everywhere, how to ensure the
exchanged content between the peers is authentic?
2) How to identify the chunk availability in certain content?
This is related to the chunk-addressing and chunk-state
maintenance. Considering the large amount of chunks in certain
content, light-weight expression is necessary.
3) How to ensure the peer protocol efficiency? As we mentioned in
<a href="#section-3">Section 3</a>, the chunk availability information exchange is quite
frequent. How to balance the information exchange size and amount
is a big challenge.
The PPSP peer protocol will consider all the above issues in the
design according to the requirements from the peer perspective.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Use Cases of PPSP</span>
This section is not a to-do list for the WG; it provides details on
how PPSP could be used in practice.
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Worldwide Provision of Live/VoD Streaming</span>
The content provider can increase live streaming coverage by
introducing PPSP between different providers. This is quite similar
to the case described in CDNI [<a href="./rfc6707" title=""Content Distribution Network Interconnection (CDNI) Problem Statement"">RFC6707</a>] [<a href="./rfc6770" title=""Use Cases for Content Delivery Network Interconnection"">RFC6770</a>].
Let us assume a scenario in which there is only provider A (e.g., in
China) providing live streaming service in provider B's (e.g., in the
USA) and C's (e.g., in Europe) coverage. Without PPSP, when a user
(e.g., a Chinese American) in the USA requests the program to the
tracker (which is located in A's coverage), the tracker may generally
return a peer list to the user including most of the peers in China,
because generally most users are in China and there are only few
<span class="grey">Zhang & Zong Informational [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
users in the USA. This may affect the user experience. But, if we
can use the PPSP tracker protocol to involve B and C in the
cooperative provision, as shown in Figure 2, even when the streaming
does no attract many users in the USA and Europe, the tracker in A
can optimally return a peer list to the user including B's and C's
Super-Nodes (SN for short) to provide a better user performance.
Furthermore, B's User2 and C's User3 can exchange data (availability)
with these local SNs using the peer protocol.
+-------------------------------------------------------------------+
| |
| +------------------+ |
| +------------>| A's Tracker |<----------+ |
| | +------------------+ | |
| Tracker| ^ ^ | |
| Protocol| Tracker| |Tracker |Tracker |
| | Protocol| |Protocol |Protocol |
| | | | | |
| | | | | |
| v v v v |
| +------+ Peer +------+ +------+ +------+ |
| | B's |<------->| B's | | C's | | C's | |
| | SN1 |Protocol | SN2 | | SN1 | | SN2 | |
| +------+ +------+ +------+ +------+ |
| ^ ^ ^ ^ |
| | | | | |
| | | Peer Protocol Peer Protocol| | |
| Peer | +-------------+ +--------------+ |Peer |
| Protocol| | | |Protocol|
| | | | | |
| | | | | |
| | | | | |
| v v v v |
| +------+ Peer +------+ +---------+ Peer +---------+ |
| | A's |<------> | B's | |A's |<------> |C's | |
| | User1|Protocol | User2| | User1 |Protocol | User3 | |
| +------+ +------+ +---------+ +---------+ |
| |
+-------------------------------------------------------------------+
Figure 2: Cooperative Vendors Interaction
<span class="grey">Zhang & Zong Informational [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Enabling CDN for P2P VoD Streaming</span>
Figure 3 shows an example of enabling CDN to support P2P VoD
streaming from different content providers by introducing PPSP inside
CDN overlays. It is similar to Figure 2, except that the
intermediate SNs are replaced by 3rd party CDN surrogates. The CDN
nodes talk with the different streaming systems (including trackers
and peers) using the same PPSP protocols.
+-------------------------------------------------------------------+
| |
| +-------------+ +--------------+ |
| +----->| A's Tracker | | B's Tracker |<---+ |
| | +-------------+ +--------------+ | |
| Tracker| ^ ^ ^ ^ | |
| Protocol| Tracker| |Tracker | |Tracker |Tracker |
| | Protocol| |Protocol| |Protocol |Protocol|
| | | | | | | |
| | | | | | | |
| v v | | v v |
| +------+ Peer +------+| | +------+Internal+------+ |
| | CDN |<------>| CDN || | | CDN |<-----> | CDN | |
| | Node1|Protocol| Node2|| | | Node3|Protocol| Node4| |
| +------+ +------+| | +------+ +------+ |
| ^ ^ | | ^ ^ |
| | | | | | | |
| | | Peer Protocol | | HTTP | | |
| Peer | +-------------+ | | +------+ |Peer |
| Protocol| | | | | Protocol |Protocol|
| | | +-+ | | | |
| | | | | | | |
| | | | | | | |
| v v v v v v |
| +------+ Peer +------+ +---------+ Peer +---------+ |
| | A's |<------> | A's | |B's |<------> |B's | |
| | User1|Protocol | User2| | User3 |Protocol | User4 | |
| +------+ +------+ +---------+ +---------+ |
| |
+-------------------------------------------------------------------+
Figure 3: CDN Supporting P2P Streaming
Furthermore, the interaction between the CDN nodes can be executed by
either existing (maybe proprietary) protocols or the PPSP peer
protocol. The peer protocol is useful for building new CDN systems
(e.g., operator CDN) that support streaming at a low cost.
<span class="grey">Zhang & Zong Informational [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
Note that for compatibility reasons, both HTTP and P2P streaming can
be supported by CDN from the users' perspective.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Cross-Screen Streaming</span>
In this scenario, PC, STB/TV, and mobile terminals from both fixed
and mobile/wireless networks share the streaming content. With PPSP,
peers can identify the types of access networks, average load, and
peer abilities and get to know what content other peers have even in
different networks (potentially with the conversion of the content
availability expression in different networks) as shown in Figure 4.
+------------------------------------------------------------------+
| |
| Tracker Protocol +---------+ Tracker Protocol |
| +-------------> | Tracker |<------------------+ |
| | +---------+ | |
| | ^ | |
| | | | |
| | | | |
| V | V |
| +------+ | +------------+ |
| | STB | Tracker Protocol |Mobile Phone| |
| +------+ | +------------+ |
| ^ | ^ |
| | | | |
| | | | |
| | V | |
| |Peer Protocol +---------+ Peer Protocol | |
| +-------------> | PC |<------------------+ |
| +---------+ |
| |
+------------------------------------------------------------------+
Figure 4: Heterogeneous P2P Streaming with PPSP
Such information will play an important role in selecting suitable
peers, e.g., a PC or STB is more likely to provide stable content,
and a mobile peer within a high-load cell is unlikely to be selected,
which may lead to a higher load on the base station.
<span class="grey">Zhang & Zong Informational [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>. Cache Service Supporting P2P Streaming</span>
In Figure 5, when peers request the P2P streaming data, the cache
nodes intercept the requests and ask for the frequently visited
content (or part of) on behalf of the peers. To do this, it asks the
tracker for the peer list and the tracker replies with external peers
in the peer list. After the cache nodes exchange data with these
peers, it can also act as a peer and report what it caches to the
tracker and serve inside requesting peers afterward. This operation
greatly decreases the inter-network traffic in many conditions and
enhances the user experience.
+----------------------------------------------------------------+
| |
| Tracker Protocol +---------+ |
| +----------------> | Tracker | |
| | +---------+ |
| | ^ |
| | | |
| | | Tracker Protocol |
| | | |
| | | |
| | +---------|-------------------------------------|
| | | V |
| | | +---------+ |
| | +----------|---> | Cache |<-------------------+ |
| | | | +---------+ Tracker/Peer| |
| | | Peer | Protocol | |
| | | Protocol | | |
| | | | | |
| | | | | |
| V V | V |
| +-----------+ | ISP Domain +------------+ |
| | External | | | Inside | |
| | Peer | | | Peer | |
| +-----------+ | +------------+ |
+----------------------------------------------------------------+
Figure 5: Cache Service Supporting Streaming with PPSP
The cache nodes do not need to update their library when new
applications supporting PPSP are introduced, which reduces the cost.
<span class="grey">Zhang & Zong Informational [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a>. Proxy Service Supporting P2P Streaming</span>
<span class="h4"><a class="selflink" id="section-5.5.1" href="#section-5.5.1">5.5.1</a>. Home Networking Scenario</span>
For applications where the peer is not colocated with the Media
Player in the same device (e.g., the peer is located in a Home Media
Gateway), we can use a PPSP Proxy, as shown in Figure 6.
+---------------------------------------------------------------+
| |
| Tracker Protocol +--------+ |
| +----------------> | Tracker| |
| | +--------+ |
| | ^ |
| | | |
| | | Tracker Protocol |
| | | |
| | +---------|------------------------------------|
| | | V |
| | | +--------+ |
| | +----------|---> | PPSP |<------------------+ |
| | | | | Proxy | DLNA | |
| | | Peer | +--------+ Protocol | |
| | | Protocol| | |
| | | | | |
| V V | V |
| +-----------+ | Home Domain +-----------+ |
| | External | | |DLNA Pres.| |
| | Peer | | |Devices | |
| +-----------+ | +-----------+ |
+---------------------------------------------------------------+
Figure 6: Proxy Service Supporting P2P Streaming
As shown in Figure 6, the PPSP Proxy terminates both the tracker and
peer protocol, allowing the legacy presentation devices to access P2P
streaming content. In Figure 6, the Digital Living Network Alliance
(DLNA) protocol [<a href="#ref-DLNA" title=""DLNA"">DLNA</a>] is used in order to communicate with the
presentation devices, thanks to its wide deployment. Obviously,
other protocols can also be used.
<span class="h4"><a class="selflink" id="section-5.5.2" href="#section-5.5.2">5.5.2</a>. Browser-Based HTTP Streaming</span>
P2P Plug-ins are often used in browser-based environments to stream
content. With P2P plug-ins, HTTP streaming can be turned into P2P
streaming. From the browser (and hence the user) perspective, it's
just HTTP-based streaming, but the PPSP-capable plug-in can actually
accelerate the process by leveraging streams from multiple sources/
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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
peers [<a href="#ref-P2PYoutube">P2PYoutube</a>]. In this case, the plug-ins behave just like the
proxy.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Requirements of PPSP</span>
This section enumerates the requirements that should be considered
when designing PPSP.
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. Basic Requirements</span>
PPSP.REQ-1: Each peer MUST have a unique ID (i.e., peer ID).
It's a basic requirement for a peer to be uniquely identified in a
P2P streaming system so that other peers or trackers can refer to
the peer by ID.
Note that a peer can join multiple swarms with a unique ID or
change swarm without changing its ID.
PPSP.REQ-2: The streaming content MUST be uniquely identified by a
swarm ID.
A swarm refers to a group of peers sharing the same streaming
content. A swarm ID uniquely identifies a swarm. The swarm ID
can be used in two cases: 1) a peer requests the tracker for the
peer list indexed by a swarm ID; 2) a peer tells the tracker about
the swarms it belongs to.
PPSP.REQ-3: The streaming content MUST be partitioned into chunks.
PPSP.REQ-4: Each chunk MUST have a unique ID (i.e., chunk ID) in the
swarm.
Each chunk must have a unique ID in the swarm so that the peer can
understand which chunks are stored in which peers and which chunks
are requested by other peers.
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. Operational and Management Requirements</span>
This section lists some operational and management requirements based
on the checklist presented in <a href="./rfc5706#appendix-A">Appendix A of [RFC5706]</a>.
<span class="grey">Zhang & Zong Informational [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h4"><a class="selflink" id="section-6.2.1" href="#section-6.2.1">6.2.1</a>. Operational Considerations</span>
PPSP.OAM.REQ-1: PPSP MUST be sufficiently configurable.
According to basic requirements, when setting up PPSP, a content
provider should generate chunk IDs and a swarm ID for each stream
of content. An original content server and tracker are configured
and set up. The content provider should then publish this
information, typically by creating web links.
The configuration should allow the proxy-based and end-client
scenarios.
PPSP.OAM.REQ-2: PPSP MUST implement a set of configuration parameters
with default values.
PPSP.OAM.REQ-3: PPSP MUST support diagnostic operations.
Mechanisms must be supported by PPSP to verify correct operation.
The PPSP tracker should collect the status of the peers including
the peer's activity, whether it obtained chunks in time, etc.
Such information can be used to monitor the streaming behavior of
PPSP.
PPSP.OAM.REQ-4: PPSP MUST facilitate achieving quality acceptable to
the streaming application.
There are basic quality requirements for streaming systems. The
setup time to receive a new streaming channel or to switch between
channels should be reasonably small. End-to-end delay, which
consists of the time between content generation (e.g., a camera)
and content consumption (e.g., a monitor), will become critical in
case of live streaming, especially in provisioning of sporting
events where an end-to-end delay of 1 minute or more are not
acceptable.
For instance, the tracker and peer protocol can carry quality
related parameters (e.g., video quality and delay requirements)
together with the priorities of these parameters, in addition to
the measured QoS situation (e.g., performance, available uplink
bandwidth) of content providing peers.
PPSP implementations may use techniques such as scalable streaming
to handle bandwidth shortages without disrupting playback.
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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h4"><a class="selflink" id="section-6.2.2" href="#section-6.2.2">6.2.2</a>. Management Considerations</span>
PPSP.OAM.REQ-5: When management objectives need to be supported in
implementations, PPSP MUST support remote management using a standard
interface, as well as a basic set of management information.
Due to large-scale peer networks, PPSP tracker service or seeders
should remotely collect information from peers and expose the
information via a standard interface for management purposes.
Peer information can be collected via a PPSP tracker protocol or
peer protocol.
The minimum set of management objects should include swarm
information such as content characteristics and rate limits;
tracking information such as swarm list and log events; and peer
information such as peer activity, chunk statistics, and log
event.
PPSP.OAM.REQ-6: PPSP MUST support fault monitoring including peer and
server health, as well as the streaming behavior of peers.
Peer and server health will at least include node activity and
connectivity, especially for peers behind NAT. As mentioned in
PPSP.OAM.REQ-4, streaming behavior of the peer can be learned from
chunk distribution information.
PPSP.OAM.REQ-7: PPSP MUST support configuration management to define
the configuration parameters.
A set of configurable parameters related to chunk generation in
the PPSP setup stage can be defined by content providers via a
management interface to content servers.
PPSP.OAM.REQ-8: PPSP MUST support performance management with respect
to streaming performance based on chunk distribution statistics,
network load, and tracker and peer monitoring.
PPSP.OAM.REQ-9: PPSP MUST support security management. See <a href="#section-7">Section 7</a>
of this document.
<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>. PPSP Tracker Protocol Requirements</span>
PPSP.TP.REQ-1: The tracker protocol MUST allow the peer to solicit a
peer list in a swarm generated and possibly tailored by the tracker
in a query and response manner.
The tracker request message may include the requesting peer's
preference parameter (e.g., preferred number of peers in the peer
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list) or preferred downloading bandwidth. The tracker will then
be able to select an appropriate set of peers for the requesting
peer according to the preference.
The tracker may also generate the peer list with the help of
traffic optimization services, e.g., Application-Layer Traffic
Optimization [<a href="#ref-ALTO" title=""ALTO Protocol"">ALTO</a>].
PPSP.TP.REQ-2: The tracker protocol MUST report the peer's activity
in the swarm to the tracker.
PPSP.TP.REQ-3: The tracker protocol MUST take the frequency of
message exchange and efficient bandwidth use into consideration when
communicating chunk availability information.
For example, the chunk availability information between peer and
tracker can be presented in a compact method, e.g., to express a
sequence of continuous "1" more efficiently.
PPSP.TP.REQ-4: The tracker protocol MUST have a provision for the
tracker to authenticate the peer.
This ensures that only the authenticated users can access the
original content in the P2P streaming system.
<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>. PPSP Peer Protocol Requirements</span>
PPSP.PP.REQ-1: The peer protocol MUST allow the peer to solicit the
chunk information from other peers in a query and response manner.
PPSP.PP.REQ-2: The chunk information exchanged between a pair of
peers MUST NOT be passed to other peers, unless the chunk information
is validated (e.g., preventing hearsay and DoS attacks).
PPSP.PP.REQ-3: The peer protocol MUST allow the peer to solicit an
additional list of peers to that received from the tracker.
It is possible that a peer may need additional peers for certain
streaming content. Therefore, the peer is allowed to communicate
with other peers in the current peer list to obtain an additional
list of peers in the same swarm.
PPSP.PP.REQ-4: When used for soliciting an additional list of peers,
the peer protocol MUST contain swarm-membership information of the
peers that have explicitly indicated they are part of the swarm,
which is verifiable by the receiver.
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PPSP.PP.REQ-5: The additional list of peers MUST only contain peers
that have been checked to be valid and online recently (e.g.,
preventing hearsay and DoS attacks).
PPSP.PP.REQ-6: The peer protocol MUST report the peer's chunk
availability update.
Due to the dynamic change of the buffered streaming content in
each peer and the frequent join/leave of peers in the swarm, the
streaming content availability among a peer's neighbors (i.e., the
peers known to a peer by getting the peer list from either the
tracker or peers) always changes, and thus requires being updated
on time. This update should be done at least on demand. For
example, when a peer requires finding more peers with certain
chunks, it sends a message to some other peers in the swarm for a
streaming content availability update. Alternatively, each peer
in the swarm can advertise its streaming content availability to
some other peers periodically. However, the detailed mechanisms
for this update, such as how far to spread the update message, how
often to send this update message, etc., should be left to the
algorithms, rather than protocol concerns.
PPSP.PP.REQ-7: The peer protocol MUST take the frequency of message
exchange and efficient bandwidth use into consideration when
communicating chunk information.
For example, the chunk availability information between peers can
be presented in a compact method.
PPSP.PP.REQ-8: The peer protocol MUST exchange additional
information, e.g., status about the peers.
This information can be, for instance, information about the
access link or information about whether a peer is running on
battery or is connected to a power supply. With such information,
a peer can select more appropriate peers for streaming.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Security Considerations</span>
This document discusses the problem statement and requirements around
P2P streaming protocols without specifying the protocols. However,
we believe it is important for the reader to understand areas of
security introduced by the P2P nature of the proposed solution. The
main issue is the usage of untrusted entities (peers) for service
provisioning. For example, malicious peers/trackers may:
o Originate DoS attacks to the trackers by sending a large number of
requests with the tracker protocol;
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o Originate fake information on behalf of other peers;
o Originate fake information about chunk availability;
o Originate fake reply messages on behalf of the tracker;
o Leak private information about other peers or trackers.
We list some important security requirements for PPSP protocols
below:
PPSP.SEC.REQ-1: PPSP MUST support closed swarms, where the peers are
authenticated or in a private network.
This ensures that only the trusted peers can access the original
content in the P2P streaming system. This can be achieved by
security mechanisms such as peer authentication and/or key
management schemes.
Another aspect is that confidentiality of the streaming content in
PPSP needs to be supported. In order to achieve this, PPSP should
provide mechanisms to encrypt the data exchange among the peers.
PPSP.SEC.REQ-2: Integrity of the streaming content in PPSP MUST be
supported to provide a peer with the possibility of identifying
unauthentic content (undesirable modifications by other entities
rather than its genuine source).
In a P2P live streaming system, a polluter can introduce corrupted
chunks. Each receiver integrates into its playback stream the
polluted chunks it receives from its neighbors. Since the peers
forward chunks to other peers, the polluted content can
potentially spread through the P2P streaming network.
The PPSP protocol specifications will document the expected
threats (and how they will be mitigated by each protocol) and also
considerations on threats and mitigations when combining both
protocols in an application. This will include privacy of the
users and protection of the content distribution.
PPSP.SEC.REQ-3: The security mechanisms in PPSP, such as key
management and checksum distribution, MUST scale well in P2P
streaming systems.
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<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Acknowledgements</span>
Thanks to J. Seng, G. Camarillo, R. Yang, C. Schmidt, R. Cruz, Y. Gu,
A. Bakker, and S. Previdi for contributing to many sections of this
document. Thank you to C. Williams, V. Pascual, and L. Xiao for
contributing to the PPSP requirements section.
We would like to acknowledge the following people who provided
review, feedback, and suggestions to this document: M. Stiemerling,
D. Bryan, E. Marocco, V. Gurbani, R. Even, H. Zhang, D. Zhang,
J. Lei, H. Song, X. Jiang, J. Seedorf, D. Saumitra, A. Rahman,
J. Pouwelse, W. Eddy, B. Claise, D. Harrington, J. Arkko, and all the
AD reviewers.
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. References</span>
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC5706">RFC5706</a>] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
<a href="./rfc5706">RFC 5706</a>, November 2009.
[<a id="ref-RFC6707">RFC6707</a>] Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content
Distribution Network Interconnection (CDNI) Problem
Statement", <a href="./rfc6707">RFC 6707</a>, September 2012.
[<a id="ref-RFC6770">RFC6770</a>] Bertrand, G., Stephan, E., Burbridge, T., Eardley, P., Ma,
K., and G. Watson, "Use Cases for Content Delivery Network
Interconnection", <a href="./rfc6770">RFC 6770</a>, November 2012.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. Informative References</span>
[<a id="ref-ALTO">ALTO</a>] Alimi, R., Penno, R., and Y. Yang, <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&q=inurl:draft-+%22ALTO+Protocol%22'>"ALTO Protocol"</a>, Work
in Progress, December 2009.
[<a id="ref-ByteMobile">ByteMobile</a>]
Bytemobile, "Mobile Video Traffic Hits Nearly 70% on
Certain Networks", February 2012,
<<a href="http://www.bytemobile.com/news-events/2012/archive_230212.html">http://www.bytemobile.com/news-events/2012/</a>
<a href="http://www.bytemobile.com/news-events/2012/archive_230212.html">archive_230212.html</a>>.
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<span class="grey"><a href="./rfc6972">RFC 6972</a> PPSP Problem Statement July 2013</span>
[<a id="ref-CDN-P2P">CDN-P2P</a>] Xu, D., Kulkarni, S., Rosenberg, C., and H-K. Chai,
"Analysis of a CDN-P2P Hybrid Architecture for
Cost-Effective Streaming Media Distribution", Multimedia
Systems, vol. 11, no. 4, pp. 383-399, 2006.
[<a id="ref-Cisco">Cisco</a>] Cisco, "Cisco Visual Networking Index: Forecast and
Methodology, 2012 - 2017", Visual Networking Index (VNI),
<<a href="http://www.cisco.com/en/US/solutions/collateral/ns341/">http://www.cisco.com/en/US/solutions/collateral/ns341/</a>
ns525/ns537/ns705/ns827/ white_paper_c11-481360_
ns827_Networking_Solutions_White_Paper.html>.
[<a id="ref-DLNA">DLNA</a>] "DLNA", <<a href="http://www.dlna.org">http://www.dlna.org</a>>.
[<a id="ref-Mobile-Streaming1">Mobile-Streaming1</a>]
Noh, J., Makar, M., and B. Girod, "Streaming To Mobile
Users In A Peer-to-Peer Network", MOBIMEDIA , 2009.
[<a id="ref-Mobile-Streaming2">Mobile-Streaming2</a>]
Peltotalo, J., Harju, J., Saukkoh, M., Vaatamoinen, L.,
Bouazizi, I., Curcio, I., and J. van Gassel, "A Real-Time
Peer-to-Peer Streaming System for Mobile Networking
Environment", Proceedings of the INFOCOM and Workshop on
Mobile Video Delivery (MoVID '09), 2009.
[<a id="ref-P2PYoutube">P2PYoutube</a>]
"Youtube Extension-Opera Add-Ons", Opera Software,
<<a href="https://addons.opera.com/en/extensions/details/p2p-youtube/">https://addons.opera.com/en/extensions/details/</a>
<a href="https://addons.opera.com/en/extensions/details/p2p-youtube/">p2p-youtube/</a>>.
[<a id="ref-VoD">VoD</a>] Huang, Y., Fu, T., Chiu, D-M., Lui, J., and C. Huang,
"Challenges, Design and Analysis of a Large-Scale P2P-VoD
System", SIGCOMM , 2008.
Authors' Addresses
Yunfei Zhang
Coolpad
EMail: hishigh@gmail.com
Ning Zong
Huawei Technologies
EMail: zongning@huawei.com
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