读《RTP.Audio.And.Video.For.The.Internet》笔记

2004-11-02

Internet包传输的特点 Internet是一个复杂的异构网络，对Internet中IP包的传输性能很多人通过统计的方法来分析，分析主要集中在一下几个方面: 1.average packet loss 2.patterns of loss 3.packet corruption and duplication 4.transit time(Delay and Jitter) 5.and the effects of multicast 有很多尺度可以衡量网络的丢包,包括平均丢包率,丢包的模式. 平均丢包率 gives a general measure of network congestion, while loss patterns and correlation give insights into the dynamics of the network. 1.平均丢包率 统计表明丢包率是和包的传输路径非常相关的,并且和一天中的时间有关,并不是一个常数,而且在各个时间段内的丢包率并不是平滑的变化. 如: The Internet Weather Report,120 a monthly survey of the loss rate measured in a range of routes across the Internet, showed average packet loss rates within the United States ranging from 0% to 16%, depending on the ISP, as of May 2001. The monthly average loss rate is about 2% in the United States, but for the Internet as a whole, the average is slightly higher, at about 3%. 在的Measuring IP Network Performance 一节中引用了几篇文章和数据说明了以上几个问题. 针对我们的网络传输模拟软件,我们可以使用以下的结论: 平均丢包率是随时间和地点变化的,因此我们必须提供这个功能. 2.丢包的模式 统计表明各个包丢失的概率在时间上并不是平均分布的,如Measurements by Vern Paxson have shown, in some cases, a five- to tenfold increase in loss probability for a particular packet if the previous packet was lost, clearly implying that packet loss is not uniformly distributed in time.即各个包丢失的概率并不是完全不相关的. in 1995, by Handley and Yajnik et al. in 1996的研究表明大约有90%包丢失是单个包丢失,