diff --git a/net/rxrpc/ar-ack.c b/net/rxrpc/ar-ack.c
index fc07a92..657ee69 100644
--- a/net/rxrpc/ar-ack.c
+++ b/net/rxrpc/ar-ack.c
@@ -543,6 +543,38 @@
 }
 
 /*
+ * process the extra information that may be appended to an ACK packet
+ */
+static void rxrpc_extract_ackinfo(struct rxrpc_call *call, struct sk_buff *skb,
+				  unsigned latest, int nAcks)
+{
+	struct rxrpc_ackinfo ackinfo;
+	struct rxrpc_peer *peer;
+	unsigned mtu;
+
+	if (skb_copy_bits(skb, nAcks + 3, &ackinfo, sizeof(ackinfo)) < 0) {
+		_leave(" [no ackinfo]");
+		return;
+	}
+
+	_proto("Rx ACK %%%u Info { rx=%u max=%u rwin=%u jm=%u }",
+	       latest,
+	       ntohl(ackinfo.rxMTU), ntohl(ackinfo.maxMTU),
+	       ntohl(ackinfo.rwind), ntohl(ackinfo.jumbo_max));
+
+	mtu = min(ntohl(ackinfo.rxMTU), ntohl(ackinfo.maxMTU));
+
+	peer = call->conn->trans->peer;
+	if (mtu < peer->maxdata) {
+		spin_lock_bh(&peer->lock);
+		peer->maxdata = mtu;
+		peer->mtu = mtu + peer->hdrsize;
+		spin_unlock_bh(&peer->lock);
+		_net("Net MTU %u (maxdata %u)", peer->mtu, peer->maxdata);
+	}
+}
+
+/*
  * process packets in the reception queue
  */
 static int rxrpc_process_rx_queue(struct rxrpc_call *call,
@@ -606,6 +638,8 @@
 		       rxrpc_acks[ack.reason],
 		       ack.nAcks);
 
+		rxrpc_extract_ackinfo(call, skb, latest, ack.nAcks);
+
 		if (ack.reason == RXRPC_ACK_PING) {
 			_proto("Rx ACK %%%u PING Request", latest);
 			rxrpc_propose_ACK(call, RXRPC_ACK_PING_RESPONSE,
@@ -801,9 +835,9 @@
 	struct msghdr msg;
 	struct kvec iov[5];
 	unsigned long bits;
-	__be32 data;
+	__be32 data, pad;
 	size_t len;
-	int genbit, loop, nbit, ioc, ret;
+	int genbit, loop, nbit, ioc, ret, mtu;
 	u32 abort_code = RX_PROTOCOL_ERROR;
 	u8 *acks = NULL;
 
@@ -899,9 +933,30 @@
 	}
 
 	if (test_bit(RXRPC_CALL_ACK_FINAL, &call->events)) {
-		hdr.type = RXRPC_PACKET_TYPE_ACKALL;
 		genbit = RXRPC_CALL_ACK_FINAL;
-		goto send_message;
+
+		ack.bufferSpace	= htons(8);
+		ack.maxSkew	= 0;
+		ack.serial	= 0;
+		ack.reason	= RXRPC_ACK_IDLE;
+		ack.nAcks	= 0;
+		call->ackr_reason = 0;
+
+		spin_lock_bh(&call->lock);
+		ack.serial = call->ackr_serial;
+		ack.previousPacket = call->ackr_prev_seq;
+		ack.firstPacket = htonl(call->rx_data_eaten + 1);
+		spin_unlock_bh(&call->lock);
+
+		pad = 0;
+
+		iov[1].iov_base = &ack;
+		iov[1].iov_len	= sizeof(ack);
+		iov[2].iov_base = &pad;
+		iov[2].iov_len	= 3;
+		iov[3].iov_base = &ackinfo;
+		iov[3].iov_len	= sizeof(ackinfo);
+		goto send_ACK;
 	}
 
 	if (call->events & ((1 << RXRPC_CALL_RCVD_BUSY) |
@@ -971,8 +1026,6 @@
 
 	/* consider sending an ordinary ACK */
 	if (test_bit(RXRPC_CALL_ACK, &call->events)) {
-		__be32 pad;
-
 		_debug("send ACK: window: %d - %d { %lx }",
 		       call->rx_data_eaten, call->ackr_win_top,
 		       call->ackr_window[0]);
@@ -997,12 +1050,6 @@
 		ack.serial	= 0;
 		ack.reason	= 0;
 
-		ackinfo.rxMTU	= htonl(5692);
-//		ackinfo.rxMTU	= htonl(call->conn->trans->peer->maxdata);
-		ackinfo.maxMTU	= htonl(call->conn->trans->peer->maxdata);
-		ackinfo.rwind	= htonl(32);
-		ackinfo.jumbo_max = htonl(4);
-
 		spin_lock_bh(&call->lock);
 		ack.reason = call->ackr_reason;
 		ack.serial = call->ackr_serial;
@@ -1116,6 +1163,15 @@
 	ack.maxSkew = htons(atomic_read(&call->conn->hi_serial) -
 			    ntohl(ack.serial));
 send_ACK:
+	mtu = call->conn->trans->peer->if_mtu;
+	mtu -= call->conn->trans->peer->hdrsize;
+	ackinfo.maxMTU	= htonl(mtu);
+	ackinfo.rwind	= htonl(32);
+
+	/* permit the peer to send us jumbo packets if it wants to */
+	ackinfo.rxMTU	= htonl(5692);
+	ackinfo.jumbo_max = htonl(4);
+
 	hdr.serial = htonl(atomic_inc_return(&call->conn->serial));
 	_proto("Tx ACK %%%u { m=%hu f=#%u p=#%u s=%%%u r=%s n=%u }",
 	       ntohl(hdr.serial),
diff --git a/net/rxrpc/ar-error.c b/net/rxrpc/ar-error.c
index 2c27df1..6cb3e88 100644
--- a/net/rxrpc/ar-error.c
+++ b/net/rxrpc/ar-error.c
@@ -100,8 +100,10 @@
 		}
 
 		if (mtu < peer->mtu) {
+			spin_lock_bh(&peer->lock);
 			peer->mtu = mtu;
 			peer->maxdata = peer->mtu - peer->hdrsize;
+			spin_unlock_bh(&peer->lock);
 			_net("Net MTU %u (maxdata %u)",
 			     peer->mtu, peer->maxdata);
 		}
diff --git a/net/rxrpc/ar-output.c b/net/rxrpc/ar-output.c
index 5cdde4a..591c442 100644
--- a/net/rxrpc/ar-output.c
+++ b/net/rxrpc/ar-output.c
@@ -582,7 +582,7 @@
 			max &= ~(call->conn->size_align - 1UL);
 
 			chunk = max;
-			if (chunk > len)
+			if (chunk > len && !more)
 				chunk = len;
 
 			space = chunk + call->conn->size_align;
diff --git a/net/rxrpc/ar-peer.c b/net/rxrpc/ar-peer.c
index d399de4..ce08b78 100644
--- a/net/rxrpc/ar-peer.c
+++ b/net/rxrpc/ar-peer.c
@@ -19,6 +19,7 @@
 #include <net/sock.h>
 #include <net/af_rxrpc.h>
 #include <net/ip.h>
+#include <net/route.h>
 #include "ar-internal.h"
 
 static LIST_HEAD(rxrpc_peers);
@@ -28,6 +29,47 @@
 static void rxrpc_destroy_peer(struct work_struct *work);
 
 /*
+ * assess the MTU size for the network interface through which this peer is
+ * reached
+ */
+static void rxrpc_assess_MTU_size(struct rxrpc_peer *peer)
+{
+	struct rtable *rt;
+	struct flowi fl;
+	int ret;
+
+	peer->if_mtu = 1500;
+
+	memset(&fl, 0, sizeof(fl));
+
+	switch (peer->srx.transport.family) {
+	case AF_INET:
+		fl.oif = 0;
+		fl.proto = IPPROTO_UDP,
+		fl.nl_u.ip4_u.saddr = 0;
+		fl.nl_u.ip4_u.daddr = peer->srx.transport.sin.sin_addr.s_addr;
+		fl.nl_u.ip4_u.tos = 0;
+		/* assume AFS.CM talking to AFS.FS */
+		fl.uli_u.ports.sport = htons(7001);
+		fl.uli_u.ports.dport = htons(7000);
+		break;
+	default:
+		BUG();
+	}
+
+	ret = ip_route_output_key(&rt, &fl);
+	if (ret < 0) {
+		kleave(" [route err %d]", ret);
+		return;
+	}
+
+	peer->if_mtu = dst_mtu(&rt->u.dst);
+	dst_release(&rt->u.dst);
+
+	kleave(" [if_mtu %u]", peer->if_mtu);
+}
+
+/*
  * allocate a new peer
  */
 static struct rxrpc_peer *rxrpc_alloc_peer(struct sockaddr_rxrpc *srx,
@@ -47,7 +89,8 @@
 		peer->debug_id = atomic_inc_return(&rxrpc_debug_id);
 		memcpy(&peer->srx, srx, sizeof(*srx));
 
-		peer->mtu = peer->if_mtu = 65535;
+		rxrpc_assess_MTU_size(peer);
+		peer->mtu = peer->if_mtu;
 
 		if (srx->transport.family == AF_INET) {
 			peer->hdrsize = sizeof(struct iphdr);