mtpppoe中为什么要有 ++a->pppoeHostUniq; 语句呀？

 我参考mtpppoe开发自己的pppoe客户端，但是++a->pppoeHostUniq;不明白， rfc2516上关于HostUniq的描述是：0x0103 Host-Uniq
 This TAG is used by a Host to uniquely associate an Access
Concentrator response (PADO or PADS) to a particular Host request (PADI or PADR).  The TAG_VALUE is binary data of any value and length that the Host chooses.  It is not interpreted by the Access Concentrator.  The Host MAY include a Host-Uniq TAG in a PADI of PADR.  If the Access Concentrator receives this TAG, it MUST include the TAG unmodified in the associated PADO or PADS response.
这表明在一次协商中这个值是不应该变化的，可是为什么在PADI 和PADR函数中都有 ++语句呀？ 请教高手呀^_^