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本帖最后由 爱卫生 于 2011-5-26 10:07 编辑
回复 chenhaonan 的帖子
引入一个官方的解释。参考规范TS24.008的10.5.6.12章节关于TFT的描述,“The TFT may contain packet filters for the downlink direction, the uplink direction or packet filters that apply for both directions. The packet filters determine the traffic mapping to PDP contexts. ”。也就是说TFT是用来做路由寻址的,用于和PDP上下文做一个映射的。具体是怎么做的呢?可以参考下面的英文解释(暂不翻译了):
“ A TFT is created for a new PDP context using the same PDP address and the same APN as an existing PDP context but with a different QoS profile. This new PDP context is called a secondary PDP context and is activated during a secondary PDP context activation procedure. After a TFT has been created for a new secondary PDP context, it is sent by the MS to the network during the secondary PDP context activation procedure. A TFT may be modified during a PDP context modification procedure initiated by the MS. A TFT is deleted when the associated PDP context is deactivated.
During packet transmission between the MS and the external packet network, the GGSN will compare the parameters of the IP PDU header with packet filters of the TFT. If a match is found between the IP PDU header and a packet filter, the GGSN is able to direct the IP PDU from the interconnected external PDN to the suitable activated PDP context identified by the NSAPI parameter. This is illustrated in Figure below. ”
图例:TFT在下行方向映射到PDP上下文的示意图 上面的这段英文,直白一点就是说。TFT肯定是和二次激活有关。并且包含了一个packet filter可以来区分MS的不同上层应用,例如在“Secondary PDP Context激活流程及实例”这篇帖子当中,就给出了抓到的包,看到了二次激活请求(#5号包)中用的TFT,包括应用服务器的IP和应用端口号554,代表是一个流媒体业务。 为什么不能用TEID和NSAPI来区分,一定要用TFT呢?因为前者只能在Gn接口来区分出MS的Primary PDP Context和Secondary PDP Context,但在Gi接口中不行。我们的下行数据肯定是从Gi口先收到。这个Gi口收到的下行数据是一个纯IP包,没有GTP头部的。所以GGSN怎么能和PDP上下文映射呢?就需要用到TFT了。因为这个下行数据中有端口号和IP地址等信息。和PDP Context映射以后,到了Gn接口,再由TEID和NSAPI来区分。 |
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