diff -r 1fc9d0e88dac sys/kern/uipc_mbuf.c
--- a/sys/kern/uipc_mbuf.c	Sat Mar 29 19:05:05 2008 +0100
+++ b/sys/kern/uipc_mbuf.c	Sat Mar 29 21:09:39 2008 +0100
@@ -1539,6 +1539,92 @@ nospace:
 	return (NULL);
 }
 
+/*
+ * Defragment an mbuf chain, returning at most maxfrags separate
+ * mbufs+clusters.  If this is not possible NULL is returned and
+ * the original mbuf chain is left in it's present (potentially
+ * modified) state.  We use two techniques: collapsing consecutive
+ * mbufs and replacing consecutive mbufs by a cluster.
+ *
+ * NB: this should really be named m_defrag but that name is taken
+ */
+struct mbuf *
+m_collapse(struct mbuf *m0, int how, int maxfrags)
+{
+	struct mbuf *m, *n, *n2, **prev;
+	u_int curfrags;
+
+	/*
+	 * Calculate the current number of frags.
+	 */
+	curfrags = 0;
+	for (m = m0; m != NULL; m = m->m_next)
+		curfrags++;
+	/*
+	 * First, try to collapse mbufs.  Note that we always collapse
+	 * towards the front so we don't need to deal with moving the
+	 * pkthdr.  This may be suboptimal if the first mbuf has much
+	 * less data than the following.
+	 */
+	m = m0;
+again:
+	for (;;) {
+		n = m->m_next;
+		if (n == NULL)
+			break;
+		if ((m->m_flags & M_RDONLY) == 0 &&
+		    n->m_len < M_TRAILINGSPACE(m)) {
+			bcopy(mtod(n, void *), mtod(m, char *) + m->m_len,
+				n->m_len);
+			m->m_len += n->m_len;
+			m->m_next = n->m_next;
+			m_free(n);
+			if (--curfrags <= maxfrags)
+				return m0;
+		} else
+			m = n;
+	}
+	KASSERT(maxfrags > 1,
+		("maxfrags %u, but normal collapse failed", maxfrags));
+	/*
+	 * Collapse consecutive mbufs to a cluster.
+	 */
+	prev = &m0->m_next;		/* NB: not the first mbuf */
+	while ((n = *prev) != NULL) {
+		if ((n2 = n->m_next) != NULL &&
+		    n->m_len + n2->m_len < MCLBYTES) {
+			m = m_getcl(how, MT_DATA, 0);
+			if (m == NULL)
+				goto bad;
+			bcopy(mtod(n, void *), mtod(m, void *), n->m_len);
+			bcopy(mtod(n2, void *), mtod(m, char *) + n->m_len,
+				n2->m_len);
+			m->m_len = n->m_len + n2->m_len;
+			m->m_next = n2->m_next;
+			*prev = m;
+			m_free(n);
+			m_free(n2);
+			if (--curfrags <= maxfrags)	/* +1 cl -2 mbufs */
+				return m0;
+			/*
+			 * Still not there, try the normal collapse
+			 * again before we allocate another cluster.
+			 */
+			goto again;
+		}
+		prev = &n->m_next;
+	}
+	/*
+	 * No place where we can collapse to a cluster; punt.
+	 * This can occur if, for example, you request 2 frags
+	 * but the packet requires that both be clusters (we
+	 * never reallocate the first mbuf to avoid moving the
+	 * packet header).
+	 */
+bad:
+	return NULL;
+}
+
 #ifdef MBUF_STRESS_TEST
 
 /*