Tentacles Passing data to from input to output path The struct pkthdr_pf appears directly in struct mbuf_hdr: struct pkthdr_pf { void *hdr; /* saved hdr pos in mbuf for ECN */ u_int rtableid; /* alternate routing table id */ u_int32_t qid; /* queue id */ u_int16_t tag; /* tag id */ u_int8_t flags; u_int8_t routed; }; Small amount of data, huge performance improvement vs using mbuf tags. In the reald world, packets come on mbuf clusters, so this space in the header is usually unused anyways.