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P2P技术科普(三)——- BT工作原理简介

2009年5月27日 没有评论

上海之行结束,CCGRID2009收获良多,云计算之复杂以及前景超出想象,以后再慢慢表吧。

2002年左右,程小胖告诉我有一种软件可以下片,人越多越快。很神奇。请注意,并不是所有的P2P技术都能做到这点,其他的P2P基本上只是提高网络资源的利用率但是,网络中的每一单买卖都仍然采用,11的客户端服务器模式。回顾一下之前讲过的P2P,去掉技术细节,做一单买卖可以这样描述:

1, 有人向网络发一个资源请求(请求者)。有很多种搜索方法可供选择。

2, 当一个有该的资源的人(提供者)收到这个的请求,他会设法通知请求者:他那有其想要的东西。

3, 于是请求者去找提供者下载。

 

这样做的一个缺点就是,比如,我选择了一个比较矬的提供者,而那人的带宽只有30k,然而网络中还有很多潜在提供者,可能有的人能达到30m。但是由于我在选择时只无法对对方的带宽做出判断而无福消受。

这样做还有一个致命的漏洞比如,我收到一个关于“海贼王”的请求,而且我有全套。按理说我应该通知请求者,让他来我这下。但是,让他来我这里下东西,对我来说,除了占我带宽,增加我中毒的风险,没有任何的好处。那我凭什么主动邀请他来??我何不装ytd,对该请求视而不见?非常不幸的是,跟我有相同想法的孙子在一般p2p网络里面占了大多数,甚至绝大多数。这就是搭便车问题(free riding)。所幸还是有一些大侠的,无偿的提供着大量资源,他们的存在使得那些一般的P2P得以侥幸存活。

 

其实,如果仅仅依赖活雷锋们,P2P就失去了其人人平等的意义。大侠们回复了网络中绝大多数的请求,于是人人都愿意与这些人建立连接,因为这样能更快的找到资源。于是这些大侠们就成了变得越来越红,网络的拓扑结构(我装b了,这个词没法解释,但是我实在找不出别的词了,别在意,看下图。)将会发生变化。

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这样的变化很不好,因为P2P又逐渐变成了客户端/服务器模式了。

 

 

于是,不论是学术界还是产业界都意识到,如果没有一种鼓励机制来刺激提供者,P2P将失去其天生的魅力。在众多解决方案中BT脱颖而出,迅速发展了起来。BT是简称,在国外可能会产生歧义,如British Telecommunications等等。所以大家尽量还是用全称,尤其是和外国人说的时候Bit-Torrenttɔrhttp://cache.lexico.com/dictionary/graphics/luna/thinsp.pngənt)。有人说BT是第三代的P2P,我认为这是因为他把P2P的理念更加深入的实现了。

 

BitTorrent当中,所有的资源都被切成很小的等份(碎片)。这里,你不需要知道怎么切,只需要知道有一种技术可以把一个文件切成很多小等份,还能把这些等份再重新的组装。在这个技术的支持下,bittorrent 中所有有相同请求的用户可以相互传资源的碎片。而且谁传的多,谁就将获得更多。具体技术如下:

1, 首先,有同样资源请求的人怎么能相互认识呢?

bittorrent中每一个资源都对应有一个叫tracker的服务器。只要你对一个资源有意思,你就必须先联系这个资源相应的tracker。或者你想要共享一个资源你也必须联系tracker。这样的话,tracker就掌握着整个网络中谁想要这个资源以及谁有这个资源。当然,为了节约,一个tracker可以负责多种资源。

2, 那么,怎么找到tracker呢?

大家可能听说过做种子,每个有完整资源的人都可以做种子,其实就是生成一个后缀是.torrent的文件。每一款bittorrent应用软件都会自动帮你生成,所以不用担心。你只要知道这个文件中包括两部分内容:1tracker的地址。2,相关资源的一些属性,比如大小,名字等等。这个torrent文件一般都可以发布在网上,比如某某人的博客,或者论坛上。

3, 于是,大家可以从网上找到torrent文件,并从中知道tracker在哪。再通过联系tracker

得到一个名单,其中包括一部分正在下载或有完整资源的用户。

这时候,你就可以与这些人建立链接,并分别从他们身上要不同的碎片。当然要相同资源的人越多,你可以建立连接的人就越多,也就更可能早日得到所有碎片。

4, 还有一点很重要就是,bittorrent有自己的鼓励机制,就是说,你做的贡献大就会被鼓励,你不做贡献就会被惩罚。具体的操作是,每个人在下载的同时也上传。上传给谁呢?谁给我给的多,我就传给谁。而且我只传个前4名的(视具体软件具体分析,也可能是前8名或其他)。

 

举个例子,比如kaka下载‘春晚’,先从某春晚发烧友论坛下载了一个叫“春晚全集.torrent”的文件。kaka可以联系文件中的tracker,并得知‘程胖,日天,威,候泡,鸣’有想关资源。于是kaka分别与这五个人建立连接。通过检测链接的流量,kaka可知,比如从程胖那下载速度可达200k/s, 日天100k/s, 150k/s, 候泡70k/s,2k/s。于是kaka在上传的时候就不给鸣上传,而只给前4名传。由于鸣的上传只有2k/s,所以不太可能有人给他传,所以时间一长他可能会意识到,可能自己给的太少了,于是把上传带宽增加到了500k/skaka突然意识到,从鸣那里可以得到很好的速度,于是停止给候泡(70k/s)传转而给鸣传。(总给那些速度最好的人上传是为了能留住他们,当然,只有你传的足够多才能留得住人家)。

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bittorrent大概的工作原理就是这样了,还有些细节这里不多说了。我个人认为Bittorrent的设计还是很不错的,但是其只适用于文件共享,或视频共享。然而网络中的资源又何止电影音乐??绝大多数的资源是不可分的,bittorrent的局限性还是相当大! 另一点可悲的是,大多数宽带用户是adsl,也就是说他们的下载上传的带宽上限差别很大,比如,下载最大2m/s,上传却只有100k/s。这也限制了bittorrent的发挥。

 

下集预告:P2P的搭便车问题

这次提高如果搭便车问题不能得以解决,P2P将变回客户端服务器模式。那么Bittorrent是否就是完美的?还有什么别的方法来解决呢?

分类: p2p科普 标签:

P2P技术科普(二)—- 非结构化P2P及其搜索方法

2009年5月13日 没有评论

上集回顾,传统的‘客户端/服务器’模式有一些问题,比如,单一故障点,低资源利用率,高带宽要求。我说P2P客服了这些问题,因为没有中央服务器了。这里,我要强调一下,并不是说只要有中央的东西就是失败,只要这个中央的东西不是那个实际掌握资源的人就可以接受。下面,简要介绍一下P2P技术的发展。

 

第一代,Napster

这一代的p2p只是把资源从服务器上拿掉了。中央服务器上只有一个目录。这个目录记录着哪个用户有哪些资源。要建立这样的一个目录,要求网络中的每个人都要告诉这个服务器:他有什么资源。

1,  Kaka发一个资源请求向中央服务器。

2,  这个服务器然后检索目录,并告诉Kaka,日天有他要的东西。

3,  Kaka直接去找日天下东西。

这样做,解决了高带宽需求(服务器只需要转发一些消息,而不用真格的提供资源),低资源利用率的问题(网络里谁都可以发挥作用了)。但是,他仍然面临严重的‘单一故障点’风险。同时,napster的出现严重挑战了知识产权法,并最终导致其在2001年被勒令关闭。

 

第二代,非结构化P2P

Napster帝国的坍塌,导致了江湖上群雄并起,老的规矩也就法不责众了,其中Gnutella等非结构的P2P脱颖而出。非结构化说白了就是,用户之间瞎连,没有规定谁必须和谁有连接,基本随机。其基本搜索方法是地毯式的,如果这样的搜索能够遍及整个网络。那么,只要资源存在就一定能一网打尽。

但是上天要求每一个用户都要给自己资源请求加一个限制,于是每个资源请求都只能走几步就必须停了,即使什么都还没找到。因为地毯式的搜索要消耗大量的网络资源,比如,平均一个人有4个邻居,如果资源请求的限制为5. 那么一共需要4+42+43+44+45=1364个信息,每个信息多大呢?大概k到几十k,也就是说一个信息就至少几m。一个人一个信息就这么大了,想想一个大网络,每人十几个邻居,限制提高到78. 这就是为什么中国很多地方,尤其是网吧限制p2p. 因为他们可怜的带宽都被p2p的请求信息占用了。

 

当然,p2p的设计者们都看到了问题,于是提出了很多新的搜索方法。这些方法可以分成两种:一种根据以前的搜索信息对以后的搜索进行预测。一种就是瞎找,碰运气

1,根据经验的搜索准确率可能会高一点,但是这种方法要求每个用户都要建一些表来记录以前的数据,而且每次发送或接到请求都要经过一系列的计算才能决定发给哪个邻居。而且程序上难以实现,更重要的是平均准确率并不比瞎找好很多。这里就不多做介绍了。

2,瞎B找的,还基本上以地毯式为基础。我介绍两种最常见的:

l  K-walkers: 第一个人先把消息发给k邻居,具体多少都自己定。 然后从第一轮邻居开始都只把消息转发给他们的一个邻居。

l  水纹式(涟漪式):先定一个限制。第一个人先发给第一层的邻居。如果第一层不灵,他们再发给他们的邻居。一旦找到资源就不再继续了。这样的话最坏的情况就是原始的地毯式。好点的情况就是,消息没到限制就找到了相应的资源,就停了。

 

这种非结构的p2p的优点在于,操作简单,完全不需要中央控制,所以又称为‘存(纯)p2p’。其缺点同样明显,要么牺牲搜索成功率,要么牺牲带宽占有量。

 

于是又有人提出了结构化的P2P。数学的东西我就不说了,基本的思想就是:

1,  每个用户,资源都得有个名字。

2,  有一种数学加密方法,对这些名字加密。且保证如果名字不同,那么得到的结果就不同。

3,  如一资源和一用户加密后结果接近,就把该资源的地址放到相应的用户身上。

 

举个例子:

有三个人,日天,kaka,威哥。三个资源:“一剪梅.mp3,“八荣八耻歌歌词.txt,xxx.avi

对这留个东西加密后分别得到: 1002003000 3002 23398

‘xxx.avi’的下载地址就会被分配到‘日天’的机子上,于是当你想搜索“xxx.avi”,你的消息就会通过某种方法被传到到日天那。

 

这样,结构化的P2P就可以保证,只要网络里有相应的资源,就一定能找到。但是由于其程序实现的难度,加之维护网络所需的投入太大,并不被经常使用。尽管他有很漂亮的数据。

 

当然还有第三代的,以后再做专题。

 

下集预告:BT基本原理

(为什么只有BitTorrent才是人越多就越快? 有何缺陷?)

 

分类: p2p科普 标签:

P2P技术科普(一)——-P2P基本原理

2009年5月9日 2 条评论

P2P技术之前,网络中的所有人大多都是通过一种叫做“客户端/服务器”的模式被组织在一起。一个服务器向很多客户提供服务。举个例子吧:  kaka有一台机子,里面存有全套的央视春晚,他想把他的挚爱分享给所有人。于是不论是通过广告还是别的什么方式,很多人知道了kaka有这个资源的消息。通过网络,他们就可以向kaka建立连接并从他的机子上下载。这样就形成了一个以kaka为中心的小型网络。这当中,kaka是服务器,其他人都是客户端。如图一

图一 

图一

 

这样的模式中,对服务器的性能及相应带宽要求比较高,如果有成千上万的人都想下春晚,kaka的海淀在线肯定就操蛋了。当然一般的公司不会用一个家用电脑做服务器。假设有存在一个特别强大的服务器,以及足够的带宽。我们来看看这种模式天生的一些弱点:

1.      服务器扮演的角色太重了,网络的一切都指着他。假如kaka不小心把电脑从6楼扔下去,整个网络就都瘫痪了。请注意,想要把一个服务器弄失灵的方法太多了,所以使用这中模式的风险是比较大的。

2.      网络的资源利用率低下。所有客户都只能从服务器那获得资源,因为他们之间不认识,甚至不知道相互的存在。然而,现实中,可能每个人都有自己独门的资源,比如日天有无数游戏,威哥有无数动漫电影…….总之,整个网络的资源可能是服务器的无数倍。但是苦无客户端之间没有交流的渠道,所以资源无法互通

3.      关于完美服务器的假设,基本不成立。

 

P2P技术就是要解决上述的问题已达到如下的效果: 所有人的资源都可以被别人找到并使用。没有哪个人特别重要,人人平等(peer-to-peer就是这个意思),任何一个人失灵都不会导致网络的死亡。

 

这里已Gnutella为例,对P2P基本原理进行介绍。 Gnutella是一种典型的P2P网络,很具代表性,以其为基础的软件有Limewire。这种网络里,没有服务器。用户之间任意连接,如图二左所示

kkk2 

图二

 

Kaka认识 ‘日天’‘curt’‘威’, 日天认识‘小胖’,威认识‘候泡’,Curt 认识 ‘路’。当然真正的网络有至少有成百上千个用户,这里只是展示一个小的局部。

 

这个网络如何工作呢?比如kaka想要找一首叫‘走进新时代’的歌。一个基本交易要经过如下几步:

1, kaka给他的邻居们每人发一个请求消息,‘你们丫谁有走进新时代?’

2, 日天,威,curt吐血,‘谁tm听这歌啊,我没有’。于是把kaka的消息转发给他们的邻居:日天发给小胖,威发给候泡,curt发给路总。

3, kaka邻居的邻居们收到这个消息后,同样,先看看自己有没有。比如,候泡有这个歌,他就可以给kaka发个消息:‘爷这有,来下吧’

4, 卡卡于是可以直接去候泡那下载。

 

很多人说在p2p里面,所有人即是服务器又是客户端。应该这样理解,比如,kaka在上例中就是个客户端。 但当有人要想要春晚的时候,只要那人的请求消息能被转到kaka这,kaka就可以成为一个服务器。

 

最后要说的是P2P的现状以及应用。目前P2P与传统的‘客户端/服务器’模式可以说分庭抗礼,互为补充。那么为什么没能取而代之呢?因为,P2P也有自身的弱点,其中包括,1,占带宽(50%以上的网络带宽都是P2P占用的,搜索方法不灵)2,安全(不具体解释了,太专业了,总之和加密那些东西有关)3 诚信(没有中央控制,你传个毛片也可以告诉你是新闻联播)4,自私(我什么都不付出,有也说没有)5,知识产权保护(在中国相当于盗版的集散地)等等。 至于这些问题的对策以及其不足,以后会做专题介绍。

P2P的应用不少但大都停留在比较低端的服务,比如文件共享,msn,在线视频这一类。实际上作为一种新的模式,其生命力应该不止如此。

 

下期预告: 非结构P2P及其搜索方法

(由于没有中央的服务器的控制。定位资源就成了P2P的一个核心技术。上例中用的是‘flooding’说白了就是地毯式的。还有很多更高效的)

分类: p2p科普 标签:

P2P技术科普(前言)

2009年5月9日 没有评论

迅雷,msnskypepplive,  ppstream, BT……铺天盖地的P2P应用已然一定程度上改变了我们上网的习惯,方式以及质量。如果你想了解P2P技术,又懒得找,懒得看那些繁琐,模棱两可又及其抽象的中文资料,这里应该是一个你想来的地方。 我的目的是用比较俗的语言来对P2P技术做一个系统的介绍。这里,我会尽量避开不必要的术语,只告诉并解释你需要知道的。当然我不是什么专家,不过请你相信我回尽量保证我所提供信息的准确性和严谨性。我们没有任何商业目的,您的每一次点击都将对我国的慈善事业做出一份贡献。在此谨代表站长kakarott先生向诸位表示感谢。

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