Function ip_reass

freebsd/netinet/ip_reass.c:182–557

 */
#define	M_IP_FRAG	M_PROTO9
struct mbuf *
ip_reass(struct mbuf *m)
{
	struct ip *ip;
	struct mbuf *p, *q, *nq, *t;
	struct ipq *fp;
	struct ifnet *srcifp;
	struct ipqhead *head;
	int i, hlen, next, tmpmax;
	u_int8_t ecn, ecn0;
	uint32_t hash, hashkey[3];
#ifdef	RSS
	uint32_t rss_hash, rss_type;
#endif

	/*
	 * If no reassembling or maxfragsperpacket are 0,
	 * never accept fragments.
	 * Also, drop packet if it would exceed the maximum
	 * number of fragments.
	 */
	tmpmax = maxfrags;
	if (V_noreass == 1 || V_maxfragsperpacket == 0 ||
	    (tmpmax >= 0 && atomic_load_int(&nfrags) >= (u_int)tmpmax)) {
		IPSTAT_INC(ips_fragments);
		IPSTAT_INC(ips_fragdropped);
		m_freem(m);
		return (NULL);
	}

	ip = mtod(m, struct ip *);
	hlen = ip->ip_hl << 2;

	/*
	 * Adjust ip_len to not reflect header,
	 * convert offset of this to bytes.
	 */
	ip->ip_len = htons(ntohs(ip->ip_len) - hlen);
	/*
	 * Make sure that fragments have a data length
	 * that's a non-zero multiple of 8 bytes, unless
	 * this is the last fragment.
	 */
	if (ip->ip_len == htons(0) ||
	    ((ip->ip_off & htons(IP_MF)) && (ntohs(ip->ip_len) & 0x7) != 0)) {
		IPSTAT_INC(ips_toosmall); /* XXX */
		IPSTAT_INC(ips_fragdropped);
		m_freem(m);
		return (NULL);
	}
	if (ip->ip_off & htons(IP_MF))
		m->m_flags |= M_IP_FRAG;
	else
		m->m_flags &= ~M_IP_FRAG;
	ip->ip_off = htons(ntohs(ip->ip_off) << 3);

	/*
	 * Make sure the fragment lies within a packet of valid size.