2 files changed, 25 insertions, 7 deletions

diff --git a/net/dccp/ccid.h b/net/dccp/ccid.h
index e3cdd8afc801..fdeae7b57319 100644
--- a/net/dccp/ccid.h
+++ b/net/dccp/ccid.h
@@ -27,6 +27,7 @@ struct tcp_info;
  *  struct ccid_operations  -  Interface to Congestion-Control Infrastructure
  *
  *  @ccid_id: numerical CCID ID (up to %CCID_MAX, cf. table 5 in RFC 4340, 10.)
+ *  @ccid_ccmps: the CCMPS including network/transport headers (0 when disabled)
  *  @ccid_name: alphabetical identifier string for @ccid_id
  *  @ccid_owner: module which implements/owns this CCID
  *  @ccid_hc_{r,t}x_slab: memory pool for the receiver/sender half-connection
@@ -45,6 +46,7 @@ struct tcp_info;
  */
 struct ccid_operations {
         unsigned char           ccid_id;
+        __u32                   ccid_ccmps;
         const char              *ccid_name;
         struct module           *ccid_owner;
         struct kmem_cache       *ccid_hc_rx_slab,
diff --git a/net/dccp/output.c b/net/dccp/output.c
index 5589a5e581f4..3b763db3d863 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -133,15 +133,31 @@ static int dccp_transmit_skb(struct sock *sk, struct sk_buff *skb)
         return -ENOBUFS;
 }
 
+/**
+ * dccp_determine_ccmps  -  Find out about CCID-specfic packet-size limits
+ * We only consider the HC-sender CCID for setting the CCMPS (RFC 4340, 14.),
+ * since the RX CCID is restricted to feedback packets (Acks), which are small
+ * in comparison with the data traffic. A value of 0 means "no current CCMPS".
+ */
+static u32 dccp_determine_ccmps(const struct dccp_sock *dp)
+{
+        const struct ccid *tx_ccid = dp->dccps_hc_tx_ccid;
+
+        if (tx_ccid == NULL || tx_ccid->ccid_ops == NULL)
+                return 0;
+        return tx_ccid->ccid_ops->ccid_ccmps;
+}
+
 unsigned int dccp_sync_mss(struct sock *sk, u32 pmtu)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct dccp_sock *dp = dccp_sk(sk);
-        int mss_now = (pmtu - icsk->icsk_af_ops->net_header_len -
-                       sizeof(struct dccp_hdr) - sizeof(struct dccp_hdr_ext));
+        u32 ccmps = dccp_determine_ccmps(dp);
+        int cur_mps = ccmps ? min(pmtu, ccmps) : pmtu;
 
-        /* Now subtract optional transport overhead */
-        mss_now -= icsk->icsk_ext_hdr_len;
+        /* Account for header lengths and IPv4/v6 option overhead */
+        cur_mps -= (icsk->icsk_af_ops->net_header_len + icsk->icsk_ext_hdr_len +
+                    sizeof(struct dccp_hdr) + sizeof(struct dccp_hdr_ext));
 
         /*
          * FIXME: this should come from the CCID infrastructure, where, say,
@@ -151,13 +167,13 @@ unsigned int dccp_sync_mss(struct sock *sk, u32 pmtu)
          * make it a multiple of 4
          */
 
-        mss_now -= ((5 + 6 + 10 + 6 + 6 + 6 + 3) / 4) * 4;
+        cur_mps -= ((5 + 6 + 10 + 6 + 6 + 6 + 3) / 4) * 4;
 
         /* And store cached results */
         icsk->icsk_pmtu_cookie = pmtu;
-        dp->dccps_mss_cache = mss_now;
+        dp->dccps_mss_cache = cur_mps;
 
-        return mss_now;
+        return cur_mps;
 }
 
 EXPORT_SYMBOL_GPL(dccp_sync_mss);