我的个人理解是这样的:
其实这些概念和2G/3G的GPRS网络类似。
SDF:就是一种用户传递的上层应用。例如HTTP、FTP。每一种应用的Qos需求不同,就需要不同的EPS承载,所以EPC网络需要能区分出不同的SDF,怎么区分,不光是看端口号,还要看源IP,目的IP等。实际上就是通过TFT里的packet filter来区分的。TS23.203的4.2.3举例了怎么来基于SDF计费:
“A network server provides an FTP service. The FTP server supports both the active (separate ports for control and data) and passive modes of operation. A PCC rule is configured for the service data flows associated with the FTP server for the user. The PCC rule uses a filter specification for the uplink that identifies packets sent to port 20 or 21 of the IP address of the server, and the origination information is wildcarded. In the downlink direction, the filter specification identifies packets sent from port 20 or 21 of the IP address of the server.“
TFT:通过定义packet filter来区分SDF。如果两个包TFT完全一样,网络侧就认定是同一个SDF,享受同一种Qos待遇,映射到相同的EPS承载中。TFT和Packet Filter的定义都在TS24.008有。其中,Packet Filter就包括:
0 0 0 1 0 0 0 0 IPv4 source address type
0 0 1 0 0 0 0 0 IPv6 source address type
0 0 1 1 0 0 0 0 Protocol identifier/Next header type
0 1 0 0 0 0 0 0 Single destination port type
0 1 0 0 0 0 0 1 Destination port range type
0 1 0 1 0 0 0 0 Single source port type
0 1 0 1 0 0 0 1 Source port range type
0 1 1 0 0 0 0 0 Security parameter index type
0 1 1 1 0 0 0 0 Type of service/Traffic class type
1 0 0 0 0 0 0 0 Flow label type
通过以上信息,完全能唯一的识别一个flow出来。
TFA:是一组SDF的集合。这是因为,可能有多个SDF,但他们的Qos需求其实是一样的,应该映射到同一个EPS承载,所以需要做聚合。一个典型的例子,UE可能会浏览雅虎的主页,也会浏览新浪的主页,虽然网站不同,但其实都是同一种应用HTTP,都应该放到背景类,享用Non-GBR的服务。但实际上由于雅虎、新浪的IP地址不同,会对应到不同的SDF,因此要做聚合。所以,规范里的图就很好解释了。
我的理解是TFT和SDF template实际上是相同的作用,只不过穿了不同的马甲。
能够确定的是,UE到PCEF侧的上行数据,肯定是需要通过TFT来进行SDF的分类并映射到上行EPS承载的。而SDF template也可以做SDF分类和映射,但这个应该是在PCEF和PCRF之间的术语,和UE应该没有关系的。证据是TS23.203里还有一段话:
“For the PDP address of an UE, the PCEF routes downlink packets to the different PDP contexts based on the downlink parts of the service data flow templates, in the active PCC rules and their routeing associations to the PDP contexts. The association between an active PCC rule and a PDP context shall correspond to the downlink TFT received from the UE. Each active PCC rule shall have a single routeing association to a PDP context. Upon reception of a packet, the PCEF evaluates the downlink part of the service data flow templates of the PCC rules activated for the PDP address in order of precedence to find a match. When the first match is found, the packet is tunnelled to the SGSN via the PDP context, for which the PCC rule has the routeing association. If no match is found, the PCEF shall silently discard the packet.
The UE shall define TFTs that enable successful binding at the PCRF for service data flows requiring a binding to occur. For each uplink packet, the UE should choose the PDP context that is used for the downlink direction of the same service data flow, as declared in the TFT information. The PCEF shall only apply the uplink parts of the service data flow templates of the PCC rules, which are associated with the same PDP context as the uplink packet arrived on.“
蓝色文字翻译过来就是,PCEF根据PCEF下发的PCC rule里的SDF template来路由下行数据,而这个active 的PCC rule要和从UE收到的下行TFT对应。
紫色部分是UE应定义好TFT,使得PCRF能为SDF完成一个绑定。对每个上行数据报文,UE应该选择一个用于相同SDF下行方向使用的PDP上下文,而这个相同的SDF是在TFT的信息中声明的。
关于TFT的实例,论坛有一个抓包,可以参考:http://www.gprshome.com/forum.php?mod=viewthread&tid=240&extra=page%3D1。Secondary PDP上下文激活流程实例。
这个例子应该是说的TFA,即SDF的聚合,目的就是映射到同一个EPS承载。但后面这句“TFT设计成仅限HTTP业务端口号的SDF通过”我可能理解不大一样,应该还是有两个TFT,分别对应yahoo和google,但通过TFA聚合后映射到同一个EPS承载。
TS23.203里介绍EPS承载绑定的时候,还有一段,在A1.3.1.1。
“For the GPRS case bearer binding is performed by:
- PCRF, when the selected operation mode is UE-only, see [12], either due to PCRF decision or network/UE capability;
- PCRF and PCEF (i.e. the PCRF performs the binding of the PCC rules for user controlled services while the PCEF performs the binding of the PCC rules for the network controlled services), when the selected operation mode is UE/NW.
In order to identify the candidate PDP context the bearer binding shall compare:
- the PCC rule service data flow template with the TFT filters; and
- the PCC rule QoS parameters with the PDP context QoS parameters.”
翻译是:GPRS的承载绑定在PCEF及PCRF上执行,而蓝色部分提到,在绑定的时候,PCEF及PCRF都需要比较PCC Rule里的SDF template和TFT filter(后者是由UE提供的)。
欢迎光临 51学通信技术论坛 (http://www.51xuetongxin.com/bbs/) | Powered by Discuz! X2 |