主要实现报文重组。单个IP报文由帧缓冲区数据结构struct sk_buff描述，其中成员lst用来组织链表结构，iph指向报文中IP首部的起始位置，成员head，end分别用来标识整个数据缓冲区的范围，而data、tail用来标识有效数据缓冲的范围，注意end与tail都是指向范围外的第一个字节。-A single IP packet data structure by the fr a me buffer struct sk_buff descr iption, including members to organization developed iph list structure, pointing in the first message, the starting position of IP members used to identify a head, end the whole data buffer respectively, and the scope of data, tail is used to identify effective data buffer area, attention with tail are pointing to end the first byte beyond.