dccp ccid-2: Schedule Sync as out-of-band mechanism

The problem with Ack Vectors is that

  i) their length is variable and can in principle grow quite large,
 ii) it is hard to predict exactly how large they will be.

Due to the second point it seems not a good idea to reduce the MPS; in
particular when on average there is enough room for the Ack Vector and an
increase in length is momentarily due to some burst loss, after which the
Ack Vector returns to its normal/average length.

The solution taken by this patch is to subtract a minimum-expected Ack Vector
length from the MPS (previous patch), and to defer any larger Ack Vectors onto
a separate Sync - but only if indeed there is no space left on the skb.

This patch provides the infrastructure to schedule Sync-packets for transporting
(urgent) out-of-band data. Its signalling is quicker than scheduling an Ack, since
it does not need to wait for new application data.

It can thus serve other parts of the DCCP code as well.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

diff --git a/include/linux/dccp.h b/include/linux/dccp.h
index 7187bd8a75f62c23b13ac73f4416488a0e42a0c7..83197b601a4f95a00a02a40e8b524490f9739725 100644 (file)
--- a/include/linux/dccp.h
+++ b/include/linux/dccp.h
@@ -462,6 +462,7 @@ struct dccp_ackvec;
  * @dccps_hc_rx_insert_options - receiver wants to add options when acking
  * @dccps_hc_tx_insert_options - sender wants to add options when sending
  * @dccps_server_timewait - server holds timewait state on close (RFC 4340, 8.3)
+ * @dccps_sync_scheduled - flag which signals "send out-of-band message soon"
  * @dccps_xmit_timer - timer for when CCID is not ready to send
  * @dccps_syn_rtt - RTT sample from Request/Response exchange (in usecs)
  */
@@ -502,6 +503,7 @@ struct dccp_sock {
        __u8                            dccps_hc_rx_insert_options:1;
        __u8                            dccps_hc_tx_insert_options:1;
        __u8                            dccps_server_timewait:1;
+       __u8                            dccps_sync_scheduled:1;
        struct timer_list               dccps_xmit_timer;
 };
 

diff --git a/net/dccp/options.c b/net/dccp/options.c
index b11d7b7167f0a00f79cda29f518dd3921dee0316..791e07853a7951d8775cab6045c2485dfea609c6 100644 (file)
--- a/net/dccp/options.c
+++ b/net/dccp/options.c
@@ -430,6 +430,7 @@ static int dccp_insert_option_ackvec(struct sock *sk, struct sk_buff *skb)
 {
        struct dccp_sock *dp = dccp_sk(sk);
        struct dccp_ackvec *av = dp->dccps_hc_rx_ackvec;
+       struct dccp_skb_cb *dcb = DCCP_SKB_CB(skb);
        const u16 buflen = dccp_ackvec_buflen(av);
        /* Figure out how many options do we need to represent the ackvec */
        const u8 nr_opts = DIV_ROUND_UP(buflen, DCCP_SINGLE_OPT_MAXLEN);
@@ -438,10 +439,25 @@ static int dccp_insert_option_ackvec(struct sock *sk, struct sk_buff *skb)
        const unsigned char *tail, *from;
        unsigned char *to;
 
-       if (DCCP_SKB_CB(skb)->dccpd_opt_len + len > DCCP_MAX_OPT_LEN)
+       if (dcb->dccpd_opt_len + len > DCCP_MAX_OPT_LEN) {
+               DCCP_WARN("Lacking space for %u bytes on %s packet\n", len,
+                         dccp_packet_name(dcb->dccpd_type));
                return -1;
-
-       DCCP_SKB_CB(skb)->dccpd_opt_len += len;
+       }
+       /*
+        * Since Ack Vectors are variable-length, we can not always predict
+        * their size. To catch exception cases where the space is running out
+        * on the skb, a separate Sync is scheduled to carry the Ack Vector.
+        */
+       if (len > DCCPAV_MIN_OPTLEN &&
+           len + dcb->dccpd_opt_len + skb->len > dp->dccps_mss_cache) {
+               DCCP_WARN("No space left for Ack Vector (%u) on skb (%u+%u), "
+                         "MPS=%u ==> reduce payload size?\n", len, skb->len,
+                         dcb->dccpd_opt_len, dp->dccps_mss_cache);
+               dp->dccps_sync_scheduled = 1;
+               return 0;
+       }
+       dcb->dccpd_opt_len += len;
 
        to   = skb_push(skb, len);
        len  = buflen;
@@ -482,7 +498,7 @@ static int dccp_insert_option_ackvec(struct sock *sk, struct sk_buff *skb)
        /*
         * Each sent Ack Vector is recorded in the list, as per A.2 of RFC 4340.
         */
-       if (dccp_ackvec_update_records(av, DCCP_SKB_CB(skb)->dccpd_seq, nonce))
+       if (dccp_ackvec_update_records(av, dcb->dccpd_seq, nonce))
                return -ENOBUFS;
        return 0;
 }

diff --git a/net/dccp/output.c b/net/dccp/output.c
index 1b3168307586fba46db5610888ab4638820c8eec..bfda071559f4d2979c980dc6193d5536a91a9db5 100644 (file)
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -305,6 +305,8 @@ void dccp_write_xmit(struct sock *sk, int block)
                        if (err)
                                DCCP_BUG("err=%d after ccid_hc_tx_packet_sent",
                                         err);
+                       if (dp->dccps_sync_scheduled)
+                               dccp_send_sync(sk, dp->dccps_gsr, DCCP_PKT_SYNC);
                } else {
                        dccp_pr_debug("packet discarded due to err=%d\n", err);
                        kfree_skb(skb);
@@ -591,6 +593,12 @@ void dccp_send_sync(struct sock *sk, const u64 ackno,
        DCCP_SKB_CB(skb)->dccpd_type = pkt_type;
        DCCP_SKB_CB(skb)->dccpd_ack_seq = ackno;
 
+       /*
+        * Clear the flag in case the Sync was scheduled for out-of-band data,
+        * such as carrying a long Ack Vector.
+        */
+       dccp_sk(sk)->dccps_sync_scheduled = 0;
+
        dccp_transmit_skb(sk, skb);
 }