tcp/dccp: Consolidate common code for RFC 3390 conversion

This patch consolidates the code common to TCP and CCID-2:
 * TCP uses RFC 3390 in a packet-oriented manner (tcp_input.c) and
 * CCID-2 uses RFC 3390 in packet-oriented manner (RFC 4341).

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

1 files changed, 15 insertions, 0 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 8983386356a..6bc4b8148ca 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -782,6 +782,21 @@ static inline __u32 tcp_current_ssthresh(const struct sock *sk)
 /* Use define here intentionally to get WARN_ON location shown at the caller */
 #define tcp_verify_left_out(tp) WARN_ON(tcp_left_out(tp) > tp->packets_out)
 
+/*
+ * Convert RFC3390 larger initial windows into an equivalent number of packets.
+ *
+ * John Heffner states:
+ *
+ *      The RFC specifies a window of no more than 4380 bytes
+ *      unless 2*MSS > 4380.  Reading the pseudocode in the RFC
+ *      is a bit misleading because they use a clamp at 4380 bytes
+ *      rather than a multiplier in the relevant range.
+ */
+static inline u32 rfc3390_bytes_to_packets(const u32 bytes)
+{
+        return bytes <= 1095 ? 4 : (bytes > 1460 ? 2 : 3);
+}
+
 extern void tcp_enter_cwr(struct sock *sk, const int set_ssthresh);
 extern __u32 tcp_init_cwnd(struct tcp_sock *tp, struct dst_entry *dst);