mbuf header


/* header at beginning of each mbuf: */
struct m_hdr {
        struct  mbuf *mh_next;     /* next mbuf in chain */
        struct  mbuf *mh_nextpkt;  /* next chain in queue/record */
        caddr_t mh_data;           /* location of data */
        u_int   mh_len;            /* amount of data in this mbuf */
        short   mh_type;           /* type of data in this mbuf */
        u_short mh_flags;          /* flags; see below */
};

mbufs can get chained
so data is split over them

3 different kinds of mbufs
holding data itself
external data storage (M_EXT set in flags)
packet header (M_PKTHDR set in flags)