TFT（Traffic Flow Template)

爱卫生

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.

我的理解：
用在同一个APN的同一个PDP地址的不同QOS请求。例如一个多媒体的业务，同一个APN，同一个IP，但不同的视频和语音业务有不同的QOS profile。
实际上TFT就是靠五元组来区分的（IP地址，端口号，协议号）。在图例中很清楚，一个下行的数据包在SGSN中，要查找TFT的映射表格，再映射到不同的bearer上，每个bearer的NSAPI，QOS都是不一样的。所以要TFT来进行区分。

imwoohan

在图例中很清楚，一个下行的数据包在SGSN中，要查找TFT的映射表格，再映射到不同的bearer上，每个bearer的NSAPI，QOS都是不一样的。所以要TFT来进行区分。

查找TFT映射表的是GGSN吧？要GGSN查找映射表，找到对应的PDP上下文，然后加上GTP发送给SGSN。

imwoohan

我的理解，就是从PDN来了数据以后，GGSN首先会根据数据包的目的IP地址找到相对应的PDP上下文的集合，(当然这个目的IP地址可能是经过NAT映射了的，GGSN会把这个IP地址还原成分配给MS的IP地址)，然后就是依次查找PDP上下文集合，找到拥有packet filter为该数据包的源ip的TFT的PDP上下文后，在把该IP包加上GTP头，最后发送给SGSN。