In the field of content distribution, CDN products have always dominated before. The quality of CDN is good, but the price is high, which is exactly the old saying "you get what you get for every penny". Quality and price seem to be contradictory, and you can't have both. With the rapid development of cloud computing, various black technologies continue to emerge, so that "cost performance" is no longer a slogan, and good products can be bought at low prices. The black technology product of Alibaba Cloud introduced today is PCDN. The quality is not lower than that of CDN, and the price can be as low as half of CDN.
Based on P2P technology, PCDN is a low-cost and high-quality content distribution network service constructed by mining and utilizing the massive fragmented idle resources of the edge network. After accessing the service by integrating the PCDN SDK, the distribution quality equal to or higher than that of CDN can be obtained, and the distribution cost can be significantly reduced. It is suitable for business scenarios such as video on demand, live broadcast, and large file download. From the birth of P2P technology to today, from the beginning to C-end users, extensive development, to the official commercialization of B-end large customers, it can provide comprehensive service quality assurance, and it has been updated and iterated in many versions.
1. Development and evolution of P2P technology
The early P2P technology, called version 1.0, mainly relies on P2P to do its best to provide services, and there is no service quality guarantee. 2C applications such as Thunder, BT, and eDonkey that appeared around 2000. This type of application is a tool for content dissemination and sharing on the Internet, and there is no strict quality of service guarantee. From a technical point of view, such applications are based on terminal traffic uploading and need to read and write the local disk of the computer at the same time.
After several years of development, around 2005, it developed to the 2.0 version of P2P. The main feature of this version is P2P+CDN, which has a certain quality of service guarantee. Video websites such as Fengxing, PPlive, PPS, Youku, etc. have combined P2P and CDN to make their own technical solutions. However, in terms of technical implementation, it is still based on terminal-based traffic transmission, mainly in the form of PC client and Flash.
After 2014, P2P technology has developed to version 3.0. The feature of this version is CDN + P2P technology + P2P node resources, which can provide comprehensive service quality assurance, and its performance is not lower than CDN. During this period, smart hardware products such as Youku Routing Bao and Xunlei Moneymaking Treasure were launched, which absorbed idle bandwidth resources in users' homes and commercial places and packaged them into P2P distribution services. P2P nodes have since developed into a model of the sharing economy. Technically, using intelligent hardware products, the client APP no longer needs the terminal to upload and read and write to the disk. It also supports comprehensively in business scenarios, including long video/short video on demand, live broadcast of large-scale events, live broadcast of shows, and large file downloads. Good quality, low price, and comprehensive support for business scenarios have contributed to the continuous development of P2P CDN products, and large-scale commercialization cases have been accumulated at this stage.
2. Features of Alibaba Cloud PCDN
Based on the P2P 3.0 version, Alibaba Cloud PCDN has made more optimizations and innovations. Regardless of the coverage of business scenarios, performance indicators, or price, it has the advantage of being far ahead of other P2P products. In terms of P2P node reserves, Alibaba Cloud PCDN provides a better business cooperation model by developing node alliances, and has reached good cooperative relations with major operators, IDCs, and terminal equipment vendors across the country. The high-quality P2P node resources of operators in various regions of the country are distributed, and the P2P bandwidth reserve exceeds 10Tbps. Alibaba Cloud PCDN has independently developed an industry-leading node scheduling system, with more than 100 core technology patents, enabling global scheduling of millions of nodes, completing data transmission through the optimal path, accessing nearby, and giving full play to the advantages of P2P technology. It has obvious advantages in transmission, weak network optimization, download acceleration, etc., ensuring excellent distribution service quality and lower prices.
We know that on mobile terminals, uploading traffic usually brings many negative effects, such as consumption of user traffic, more power consumption, frequent reading and writing of T cards/ROMs, affecting the life of hardware devices, etc. Alibaba Cloud PCDN utilizes a large number of P2P node resources, enabling terminals to only download without uploading, which greatly expands the scope of use of P2P CDN. This advantage is significantly ahead of those P2P products that rely solely on client upload traffic to obtain P2P efficiency .
另外，阿里云PCDN针对各类业务场景做了进一步优化，使之对各场景的支持更加完善，给客户提供了更多差异化价值。在下载业务方面，支持下载速率控制（限速）。在带宽峰期对下载速度进行限制，可以有效控制带宽峰值。对于一些不追求下载速度的后台下载场景，同样可以对下载速度进行控制，使后台下载不至于影响前台的游戏、视频等网络体验。在安全方面，支持https，防止内容被篡改。并且集成了httpdns，保证了调度精准性，同时有效避免了域名劫持的发生。在防盗链的处理上，PCDN除了继承CDN的referer、鉴权策略、IP黑名单等防盗链机制，还从云+端的角度，使SDK跟本地业务结合，实现鉴权逻辑、DRM等，弥补了单纯靠云端鉴权的不足，使防盗链机制更加完善。
三、PCDN保证高质量的基本原理
CDN对外服务是单节点、单链路的方式，这要求CDN节点有很高的稳定性。一旦CDN节点出现故障或者链路出现抖动、拥塞等，将势必影响服务质量。而PCDN对外服务是多点对单点、多链路的方式，这种特性使PCDN能够有效避免节点故障、链路网络问题带来的影响，使整个传输更加稳定。
PCDN对外服务时，CDN将被当做超级种子，同时也有很多P2P节点提供服务。虽然单个P2P节点的能力弱于CDN，但依赖数量众多的P2P节点和优良的调度算法，可以很好的保证整网的可靠性。CDN在节点水位问题、稳定性、网络波动等问题将要发生之前，P2P调度会尽可能从P2P节点把用户所要的内容返回给SDK，来抑制缓冲和卡顿的情况。所以PCDN相对于CDN，服务质量有所优化。
有数据显示，以视频点播业务为例，PCDN的首播时间等同于CDN，流畅性同比CDN平均提升1~3%。在下载业务方面，PCDN的下载速度、下载完成率等指标全面领先CDN。
四、PCDN保证低价格的基本原理
PCDN的带宽分为一、二、三级节点带宽，一级节点带宽为CDN带宽；二级节点带宽为分布全国各地P2P节点提供的带宽，包含接入层节点、路由器等提供的运营商/家庭出口带宽；三级带宽为客户端之间互相分享的带宽。如下图所示，各级节点能力不同，从一级CDN节点到三级客户端节点，根据其自身能力，按网络质量、存储容量、稳定性、计算能力、节点数量、可控性六个基本维度进行划分。其中，CDN各方面能力是最强的，但是节点数量是最少的，成本是最高的。逐一往右推移，网络质量和存储能力、计算能力在不同程度的下降，但是节点数量在提升，成本在降低。
所以，阿里云PCDN在对外提供服务时，调度系统根据质量优先、兼顾成本的原则，对各级节点进行合理配比。以视频点播的业务为例，当播放器开始播放的时候，无论从用户还是从业务角度，都希望尽快拿到数据，不愿意缓冲和等待，这个时候PCDN默认CDN的响应是比较及时，调度系统首先从CDN上拿数据，即首帧时间与CDN等效。在正常播放过程中，播放器预缓冲时，播放器对数据的请求不是很着急，这时优先使用低成本的P2P，如果P2P没有命中或者调度系统评估要很慢才能回传，再回源CDN。通过这样的方法，能有效减少高成本的CDN用量。
通过这种方式，根据业务场景和使用量，PCDN的价格一般比CDN降低30%~50%。目前支持按日峰值带宽、月95峰值带宽和日流量三种计费方式。从客户的时间使用情况看，对于一个峰值带宽30G左右的业务，使用PCDN一年相比CDN可以节省至少100万。对于带宽峰值超过100G的业务，成本节省效果将更加可观。
阿里云PCDN目前已支持视频点播、视频直播、大文件下载各类典型的业务场景，包括但不限于长视频点播、短视频点播、互动娱乐直播、晚会赛事直播、应用市场分发等各类产品，全面支持Android、iOS、OTT、Flash、PC-Client等各种主流平台和主流协议。通过在娱乐、教育、体育、广电等多个行业积累的众多成熟服务客户案例，阿里云PCDN对各业务场景的理解与服务也不断升级优化，目前已能支持更多的下载场景、提供更好的防劫持防盗链方案和更好的视频播放体验，让质量和价格不再是矛盾体，达到高质量低成本这一超高性价比的效果，鱼和熊掌兼得。阿里云PCDN，期待为您提供更好的服务。