[TCP]: MTU probing

Implementation of packetization layer path mtu discovery for TCP, based on
the internet-draft currently found at
<http://www.ietf.org/internet-drafts/draft-ietf-pmtud-method-05.txt>.

Signed-off-by: John Heffner <jheffner@psc.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 301 insertions, 37 deletions

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 16984d4a8a06..ebf2e0b363c4 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -664,6 +664,22 @@ ctl_table ipv4_table[] = {
                 .mode           = 0644,
                 .proc_handler   = &proc_dointvec,
         },
+        {
+                .ctl_name       = NET_TCP_MTU_PROBING,
+                .procname       = "tcp_mtu_probing",
+                .data           = &sysctl_tcp_mtu_probing,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec,
+        },
+        {
+                .ctl_name       = NET_TCP_BASE_MSS,
+                .procname       = "tcp_base_mss",
+                .data           = &sysctl_tcp_base_mss,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec,
+        },
 
         { .ctl_name = 0 }
 };
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e9a54ae7d690..0ac388e3d01d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1891,6 +1891,34 @@ static void tcp_try_to_open(struct sock *sk, struct tcp_sock *tp, int flag)
         }
 }
 
+static void tcp_mtup_probe_failed(struct sock *sk)
+{
+        struct inet_connection_sock *icsk = inet_csk(sk);
+
+        icsk->icsk_mtup.search_high = icsk->icsk_mtup.probe_size - 1;
+        icsk->icsk_mtup.probe_size = 0;
+}
+
+static void tcp_mtup_probe_success(struct sock *sk, struct sk_buff *skb)
+{
+        struct tcp_sock *tp = tcp_sk(sk);
+        struct inet_connection_sock *icsk = inet_csk(sk);
+
+        /* FIXME: breaks with very large cwnd */
+        tp->prior_ssthresh = tcp_current_ssthresh(sk);
+        tp->snd_cwnd = tp->snd_cwnd *
+                       tcp_mss_to_mtu(sk, tp->mss_cache) /
+                       icsk->icsk_mtup.probe_size;
+        tp->snd_cwnd_cnt = 0;
+        tp->snd_cwnd_stamp = tcp_time_stamp;
+        tp->rcv_ssthresh = tcp_current_ssthresh(sk);
+
+        icsk->icsk_mtup.search_low = icsk->icsk_mtup.probe_size;
+        icsk->icsk_mtup.probe_size = 0;
+        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
+}
+
+
 /* Process an event, which can update packets-in-flight not trivially.
  * Main goal of this function is to calculate new estimate for left_out,
  * taking into account both packets sitting in receiver's buffer and
@@ -2023,6 +2051,17 @@ tcp_fastretrans_alert(struct sock *sk, u32 prior_snd_una,
                         return;
                 }
 
+                /* MTU probe failure: don't reduce cwnd */
+                if (icsk->icsk_ca_state < TCP_CA_CWR &&
+                    icsk->icsk_mtup.probe_size &&
+                    tp->snd_una == icsk->icsk_mtup.probe_seq_start) {
+                        tcp_mtup_probe_failed(sk);
+                        /* Restores the reduction we did in tcp_mtup_probe() */
+                        tp->snd_cwnd++;
+                        tcp_simple_retransmit(sk);
+                        return;
+                }
+
                 /* Otherwise enter Recovery state */
 
                 if (IsReno(tp))
@@ -2243,6 +2282,13 @@ static int tcp_clean_rtx_queue(struct sock *sk, __s32 *seq_rtt_p)
                         tp->retrans_stamp = 0;
                 }
 
+                /* MTU probing checks */
+                if (icsk->icsk_mtup.probe_size) {
+                        if (!after(icsk->icsk_mtup.probe_seq_end, TCP_SKB_CB(skb)->end_seq)) {
+                                tcp_mtup_probe_success(sk, skb);
+                        }
+                }
+
                 if (sacked) {
                         if (sacked & TCPCB_RETRANS) {
                                 if(sacked & TCPCB_SACKED_RETRANS)
@@ -4101,6 +4147,7 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                 if (tp->rx_opt.sack_ok && sysctl_tcp_fack)
                         tp->rx_opt.sack_ok |= 2;
 
+                tcp_mtup_init(sk);
                 tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
                 tcp_initialize_rcv_mss(sk);
 
@@ -4211,6 +4258,7 @@ discard:
                 if (tp->ecn_flags&TCP_ECN_OK)
                         sock_set_flag(sk, SOCK_NO_LARGESEND);
 
+                tcp_mtup_init(sk);
                 tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
                 tcp_initialize_rcv_mss(sk);
 
@@ -4399,6 +4447,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                                  */
                                 tp->lsndtime = tcp_time_stamp;
 
+                                tcp_mtup_init(sk);
                                 tcp_initialize_rcv_mss(sk);
                                 tcp_init_buffer_space(sk);
                                 tcp_fast_path_on(tp);
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 233bdf259965..57e7a26e8213 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -900,6 +900,7 @@ struct sock *tcp_v4_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
                 inet_csk(newsk)->icsk_ext_hdr_len = newinet->opt->optlen;
         newinet->id = newtp->write_seq ^ jiffies;
 
+        tcp_mtup_init(newsk);
         tcp_sync_mss(newsk, dst_mtu(dst));
         newtp->advmss = dst_metric(dst, RTAX_ADVMSS);
         tcp_initialize_rcv_mss(newsk);
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9f498a6c8895..8197b5e12f1f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -51,6 +51,12 @@ int sysctl_tcp_retrans_collapse = 1;
  */
 int sysctl_tcp_tso_win_divisor = 3;
 
+int sysctl_tcp_mtu_probing = 0;
+int sysctl_tcp_base_mss = 512;
+
+EXPORT_SYMBOL(sysctl_tcp_mtu_probing);
+EXPORT_SYMBOL(sysctl_tcp_base_mss);
+
 static void update_send_head(struct sock *sk, struct tcp_sock *tp,
                              struct sk_buff *skb)
 {
@@ -681,6 +687,62 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
         return 0;
 }
 
+/* Not accounting for SACKs here. */
+int tcp_mtu_to_mss(struct sock *sk, int pmtu)
+{
+        struct tcp_sock *tp = tcp_sk(sk);
+        struct inet_connection_sock *icsk = inet_csk(sk);
+        int mss_now;
+
+        /* Calculate base mss without TCP options:
+           It is MMS_S - sizeof(tcphdr) of rfc1122
+         */
+        mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);
+
+        /* Clamp it (mss_clamp does not include tcp options) */
+        if (mss_now > tp->rx_opt.mss_clamp)
+                mss_now = tp->rx_opt.mss_clamp;
+
+        /* Now subtract optional transport overhead */
+        mss_now -= icsk->icsk_ext_hdr_len;
+
+        /* Then reserve room for full set of TCP options and 8 bytes of data */
+        if (mss_now < 48)
+                mss_now = 48;
+
+        /* Now subtract TCP options size, not including SACKs */
+        mss_now -= tp->tcp_header_len - sizeof(struct tcphdr);
+
+        return mss_now;
+}
+
+/* Inverse of above */
+int tcp_mss_to_mtu(struct sock *sk, int mss)
+{
+        struct tcp_sock *tp = tcp_sk(sk);
+        struct inet_connection_sock *icsk = inet_csk(sk);
+        int mtu;
+
+        mtu = mss +
+              tp->tcp_header_len +
+              icsk->icsk_ext_hdr_len +
+              icsk->icsk_af_ops->net_header_len;
+
+        return mtu;
+}
+
+void tcp_mtup_init(struct sock *sk)
+{
+        struct tcp_sock *tp = tcp_sk(sk);
+        struct inet_connection_sock *icsk = inet_csk(sk);
+
+        icsk->icsk_mtup.enabled = sysctl_tcp_mtu_probing > 1;
+        icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
+                               icsk->icsk_af_ops->net_header_len;
+        icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, sysctl_tcp_base_mss);
+        icsk->icsk_mtup.probe_size = 0;
+}
+
 /* This function synchronize snd mss to current pmtu/exthdr set.
 
    tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
@@ -708,25 +770,12 @@ unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct inet_connection_sock *icsk = inet_csk(sk);
-        /* Calculate base mss without TCP options:
-           It is MMS_S - sizeof(tcphdr) of rfc1122
-         */
-        int mss_now = (pmtu - icsk->icsk_af_ops->net_header_len -
-                       sizeof(struct tcphdr));
+        int mss_now;
 
-        /* Clamp it (mss_clamp does not include tcp options) */
-        if (mss_now > tp->rx_opt.mss_clamp)
-                mss_now = tp->rx_opt.mss_clamp;
+        if (icsk->icsk_mtup.search_high > pmtu)
+                icsk->icsk_mtup.search_high = pmtu;
 
-        /* Now subtract optional transport overhead */
-        mss_now -= icsk->icsk_ext_hdr_len;
-
-        /* Then reserve room for full set of TCP options and 8 bytes of data */
-        if (mss_now < 48)
-                mss_now = 48;
-
-        /* Now subtract TCP options size, not including SACKs */
-        mss_now -= tp->tcp_header_len - sizeof(struct tcphdr);
+        mss_now = tcp_mtu_to_mss(sk, pmtu);
 
         /* Bound mss with half of window */
         if (tp->max_window && mss_now > (tp->max_window>>1))
@@ -734,6 +783,8 @@ unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
 
         /* And store cached results */
         icsk->icsk_pmtu_cookie = pmtu;
+        if (icsk->icsk_mtup.enabled)
+                mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
         tp->mss_cache = mss_now;
 
         return mss_now;
@@ -1063,6 +1114,140 @@ static int tcp_tso_should_defer(struct sock *sk, struct tcp_sock *tp, struct sk_
         return 1;
 }
 
+/* Create a new MTU probe if we are ready.
+ * Returns 0 if we should wait to probe (no cwnd available),
+ *         1 if a probe was sent,
+ *         -1 otherwise */
+static int tcp_mtu_probe(struct sock *sk)
+{
+        struct tcp_sock *tp = tcp_sk(sk);
+        struct inet_connection_sock *icsk = inet_csk(sk);
+        struct sk_buff *skb, *nskb, *next;
+        int len;
+        int probe_size;
+        unsigned int pif;
+        int copy;
+        int mss_now;
+
+        /* Not currently probing/verifying,
+         * not in recovery,
+         * have enough cwnd, and
+         * not SACKing (the variable headers throw things off) */
+        if (!icsk->icsk_mtup.enabled ||
+            icsk->icsk_mtup.probe_size ||
+            inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
+            tp->snd_cwnd < 11 ||
+            tp->rx_opt.eff_sacks)
+                return -1;
+
+        /* Very simple search strategy: just double the MSS. */
+        mss_now = tcp_current_mss(sk, 0);
+        probe_size = 2*tp->mss_cache;
+        if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high)) {
+                /* TODO: set timer for probe_converge_event */
+                return -1;
+        }
+
+        /* Have enough data in the send queue to probe? */
+        len = 0;
+        if ((skb = sk->sk_send_head) == NULL)
+                return -1;
+        while ((len += skb->len) < probe_size && !tcp_skb_is_last(sk, skb))
+                skb = skb->next;
+        if (len < probe_size)
+                return -1;
+
+        /* Receive window check. */
+        if (after(TCP_SKB_CB(skb)->seq + probe_size, tp->snd_una + tp->snd_wnd)) {
+                if (tp->snd_wnd < probe_size)
+                        return -1;
+                else
+                        return 0;
+        }
+
+        /* Do we need to wait to drain cwnd? */
+        pif = tcp_packets_in_flight(tp);
+        if (pif + 2 > tp->snd_cwnd) {
+                /* With no packets in flight, don't stall. */
+                if (pif == 0)
+                        return -1;
+                else
+                        return 0;
+        }
+
+        /* We're allowed to probe.  Build it now. */
+        if ((nskb = sk_stream_alloc_skb(sk, probe_size, GFP_ATOMIC)) == NULL)
+                return -1;
+        sk_charge_skb(sk, nskb);
+
+        skb = sk->sk_send_head;
+        __skb_insert(nskb, skb->prev, skb, &sk->sk_write_queue);
+        sk->sk_send_head = nskb;
+
+        TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
+        TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
+        TCP_SKB_CB(nskb)->flags = TCPCB_FLAG_ACK;
+        TCP_SKB_CB(nskb)->sacked = 0;
+        nskb->csum = 0;
+        if (skb->ip_summed == CHECKSUM_HW)
+                nskb->ip_summed = CHECKSUM_HW;
+
+        len = 0;
+        while (len < probe_size) {
+                next = skb->next;
+
+                copy = min_t(int, skb->len, probe_size - len);
+                if (nskb->ip_summed)
+                        skb_copy_bits(skb, 0, skb_put(nskb, copy), copy);
+                else
+                        nskb->csum = skb_copy_and_csum_bits(skb, 0,
+                                         skb_put(nskb, copy), copy, nskb->csum);
+
+                if (skb->len <= copy) {
+                        /* We've eaten all the data from this skb.
+                         * Throw it away. */
+                        TCP_SKB_CB(nskb)->flags |= TCP_SKB_CB(skb)->flags;
+                        __skb_unlink(skb, &sk->sk_write_queue);
+                        sk_stream_free_skb(sk, skb);
+                } else {
+                        TCP_SKB_CB(nskb)->flags |= TCP_SKB_CB(skb)->flags &
+                                                   ~(TCPCB_FLAG_FIN|TCPCB_FLAG_PSH);
+                        if (!skb_shinfo(skb)->nr_frags) {
+                                skb_pull(skb, copy);
+                                if (skb->ip_summed != CHECKSUM_HW)
+                                        skb->csum = csum_partial(skb->data, skb->len, 0);
+                        } else {
+                                __pskb_trim_head(skb, copy);
+                                tcp_set_skb_tso_segs(sk, skb, mss_now);
+                        }
+                        TCP_SKB_CB(skb)->seq += copy;
+                }
+
+                len += copy;
+                skb = next;
+        }
+        tcp_init_tso_segs(sk, nskb, nskb->len);
+
+        /* We're ready to send.  If this fails, the probe will
+         * be resegmented into mss-sized pieces by tcp_write_xmit(). */
+        TCP_SKB_CB(nskb)->when = tcp_time_stamp;
+        if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
+                /* Decrement cwnd here because we are sending
+                * effectively two packets. */
+                tp->snd_cwnd--;
+                update_send_head(sk, tp, nskb);
+
+                icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
+                icsk->icsk_mtup.probe_seq_start = TCP_SKB_CB(nskb)->seq;
+                icsk->icsk_mtup.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;
+
+                return 1;
+        }
+
+        return -1;
+}
+
+
 /* This routine writes packets to the network.  It advances the
  * send_head.  This happens as incoming acks open up the remote
  * window for us.
@@ -1076,6 +1261,7 @@ static int tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle)
         struct sk_buff *skb;
         unsigned int tso_segs, sent_pkts;
         int cwnd_quota;
+        int result;
 
         /* If we are closed, the bytes will have to remain here.
          * In time closedown will finish, we empty the write queue and all
@@ -1085,6 +1271,14 @@ static int tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle)
                 return 0;
 
         sent_pkts = 0;
+
+        /* Do MTU probing. */
+        if ((result = tcp_mtu_probe(sk)) == 0) {
+                return 0;
+        } else if (result > 0) {
+                sent_pkts = 1;
+        }
+
         while ((skb = sk->sk_send_head)) {
                 unsigned int limit;
 
@@ -1455,9 +1649,15 @@ void tcp_simple_retransmit(struct sock *sk)
 int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
 {
         struct tcp_sock *tp = tcp_sk(sk);
+        struct inet_connection_sock *icsk = inet_csk(sk);
         unsigned int cur_mss = tcp_current_mss(sk, 0);
         int err;
 
+        /* Inconslusive MTU probe */
+        if (icsk->icsk_mtup.probe_size) {
+                icsk->icsk_mtup.probe_size = 0;
+        }
+
         /* Do not sent more than we queued. 1/4 is reserved for possible
          * copying overhead: fragmentation, tunneling, mangling etc.
          */
@@ -1883,6 +2083,7 @@ static void tcp_connect_init(struct sock *sk)
         if (tp->rx_opt.user_mss)
                 tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
         tp->max_window = 0;
+        tcp_mtup_init(sk);
         tcp_sync_mss(sk, dst_mtu(dst));
 
         if (!tp->window_clamp)
@@ -2180,3 +2381,4 @@ EXPORT_SYMBOL(tcp_make_synack);
 EXPORT_SYMBOL(tcp_simple_retransmit);
 EXPORT_SYMBOL(tcp_sync_mss);
 EXPORT_SYMBOL(sysctl_tcp_tso_win_divisor);
+EXPORT_SYMBOL(tcp_mtup_init);
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index e1880959614a..7c1bde3cd6cb 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -119,8 +119,10 @@ static int tcp_orphan_retries(struct sock *sk, int alive)
 /* A write timeout has occurred. Process the after effects. */
 static int tcp_write_timeout(struct sock *sk)
 {
-        const struct inet_connection_sock *icsk = inet_csk(sk);
+        struct inet_connection_sock *icsk = inet_csk(sk);
+        struct tcp_sock *tp = tcp_sk(sk);
         int retry_until;
+        int mss;
 
         if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
                 if (icsk->icsk_retransmits)
@@ -128,25 +130,19 @@ static int tcp_write_timeout(struct sock *sk)
                 retry_until = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries;
         } else {
                 if (icsk->icsk_retransmits >= sysctl_tcp_retries1) {
-                        /* NOTE. draft-ietf-tcpimpl-pmtud-01.txt requires pmtu black
-                           hole detection. :-(
-
-                           It is place to make it. It is not made. I do not want
-                           to make it. It is disgusting. It does not work in any
-                           case. Let me to cite the same draft, which requires for
-                           us to implement this:
-
-   "The one security concern raised by this memo is that ICMP black holes
-   are often caused by over-zealous security administrators who block
-   all ICMP messages.  It is vitally important that those who design and
-   deploy security systems understand the impact of strict filtering on
-   upper-layer protocols.  The safest web site in the world is worthless
-   if most TCP implementations cannot transfer data from it.  It would
-   be far nicer to have all of the black holes fixed rather than fixing
-   all of the TCP implementations."
-
-                           Golden words :-).
-                   */
+                        /* Black hole detection */
+                        if (sysctl_tcp_mtu_probing) {
+                                if (!icsk->icsk_mtup.enabled) {
+                                        icsk->icsk_mtup.enabled = 1;
+                                        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
+                                } else {
+                                        mss = min(sysctl_tcp_base_mss,
+                                                  tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low)/2);
+                                        mss = max(mss, 68 - tp->tcp_header_len);
+                                        icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
+                                        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
+                                }
+                        }
 
                         dst_negative_advice(&sk->sk_dst_cache);
                 }