6.3.4 Analyzing sender and receiver reports

6.3.4 Analyzing sender and receiver reports

It is expected that reception quality feedback will be useful not only for the sender but also for other receivers and third-party monitors.The sender may modify its transmissions based on the feedback; receivers can determine whether problems are local, regional or global;network managers may use profile-independent monitors that receive only the RTCP packets and not the corresponding RTP data packets to evaluate the performance of their networks for multicast distribution.

Cumulative counts are used in both the sender information and receiver report blocks so that differences may be calculated between any two reports to make measurements over both short and long time periods, and to provide resilience against the loss of a report. The difference between the last two reports received can be used to estimate the recent quality of the distribution. The NTP timestamp is included so that rates may be calculated from these differences over the interval between two reports. Since that timestamp is independent of the clock rate for the data encoding, it is possible to implement encoding- and profile-independent quality monitors.

An example calculation is the packet loss rate over the interval between two reception reports. The difference in the cumulative number of packets lost gives the number lost during that interval. The difference in the extended last sequence numbers received gives the number of packets expected during the interval. The ratio of these two is the packet loss fraction over the interval. This ratio should equal the fraction lost field if the two reports are consecutive, but otherwise not. The loss rate per second can be obtained by dividing the loss fraction by the difference in NTP timestamps, expressed in seconds. The number of packets received is the number of packets expected minus the number lost. The number of packets expected may also be used to judge the statistical validity of any loss estimates. For example, 1 out of 5 packets lost has a lower significance than 200 out of 1000.

From the sender information, a third-party monitor can calculate the average payload data rate and the average packet rate over an interval without receiving the data. Taking the ratio of the two gives the average payload size. If it can be assumed that packet loss is independent of packet size, then the number of packets received by a particular receiver times the average payload size (or the corresponding packet size) gives the apparent throughput available to that receiver.

In addition to the cumulative counts which allow long-term packet loss measurements using differences between reports, the fraction lost field provides a short-term measurement from a single report. This becomes more important as the size of a session scales up enough that reception state information might not be kept for all receivers or the interval between reports becomes long enough that only one report might have been received from a particular receiver.

The interarrival jitter field provides a second short-term measure of network congestion. Packet loss tracks persistent congestion while the jitter measure tracks transient congestion. The jitter measure may indicate congestion before it leads to packet loss. Since the interarrival jitter field is only a snapshot of the jitter at the time of a report, it may be necessary to analyze a number of reports from one receiver over time or from multiple receivers, e.g., within a single network.

 

    接收质量反馈不仅对发送者有用，而且对于其它接收者和第三方监视器也有作用。发送者可以基于反馈修正发送信息量；接收者可以判断问题是本地的，区域内的还是全局的；网络管理者可以利用与协议无关的监视器(只接收RTCP包而不接收相应的RTP包)去评估多点传送网络的性能。
    在发送者信息和接收者报告块中都连续统计丢包数，因此可以计算任何两个报告块中的差别。在短时间和长时间内都可以进行测算。最近收到的两个包之间差值可以评估当前传输质量。包中有NTP时间戳，可以用两个报告间隔的差值计算传输速率。由于此时间间隔与数据编码速率独立，因此可以实现与编码及协议独立的质量监视。
    一个例子是计算两个报告间隔时间内的丢包率。丢包率＝此间隔内丢失的包／此间隔内期望收到的包。如果此值与“丢失比例”字段中的值相同，说明包是连续的；若否，说明包不是连续的。间隔时间内的丢包率／间隔时间＝每秒的丢包率。
    从发送者信息中，第三方监视器可以在一个时间间隔内计算平均负载数据发送速率和平均发包速率，而无需考虑数据接收。两个值的比就是平均负载大小（平均每个包的负载大小）。（即：平均负载大小＝平均负载数据发送速率／平均发包率。）若能假定丢包与包的大小无关，那么某个特定接收者收到的包数乘以平均负载大小(或相应的包大小)就得出接收者可得到的外在吞吐量。
除了累计计数允许利用报告间差值进行长期包损测量外，单个报告的“丢包比例”字段提供一个短时测量数据。当会话规模增加到无法为所有接收者保存接收状态信息，或者报告间隔变得足够长以至于从一个特定接收者只能收到一个报告时，短时测量数据变得更重要。
到达间隔抖动字段提供另一个有关网络阻塞的短时测量量。丢包反映了长期阻塞，抖动测量反映出短时间的阻塞。抖动测量可以在导致丢包前预示阻塞。由于到达间隔抖动字段仅仅是发送报告时刻抖动的一个快照，因此需要在一个网络内在一段时间内分析来自某个接收者的报告，或者分析来自多个接收者的报告。