syncookies: update mss tables

- ipv6 msstab: account for ipv6 header size
- ipv4 msstab: add mss for Jumbograms.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 34 insertions, 43 deletions

diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index c9dac8615958..a7cbcc4b726b 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -138,23 +138,23 @@ static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
 }
 
 /*
- * This table has to be sorted and terminated with (__u16)-1.
- * XXX generate a better table.
- * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
+ * MSS Values are taken from the 2009 paper
+ * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
+ *  - values 1440 to 1460 accounted for 80% of observed mss values
+ *  - values outside the 536-1460 range are rare (<0.2%).
+ *
+ * Table must be sorted.
  */
 static __u16 const msstab[] = {
-        64 - 1,
-        256 - 1,
-        512 - 1,
-        536 - 1,
-        1024 - 1,
-        1440 - 1,
-        1460 - 1,
-        4312 - 1,
-        (__u16)-1
+        64,
+        512,
+        536,
+        1024,
+        1440,
+        1460,
+        4312,
+        8960,
 };
-/* The number doesn't include the -1 terminator */
-#define NUM_MSS (ARRAY_SIZE(msstab) - 1)
 
 /*
  * Generate a syncookie.  mssp points to the mss, which is returned
@@ -169,10 +169,10 @@ __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 
         tcp_synq_overflow(sk);
 
-        /* XXX sort msstab[] by probability?  Binary search? */
-        for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
-                ;
-        *mssp = msstab[mssind] + 1;
+        for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
+                if (mss >= msstab[mssind])
+                        break;
+        *mssp = msstab[mssind];
 
         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 
@@ -202,7 +202,7 @@ static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
                                             jiffies / (HZ * 60),
                                             COUNTER_TRIES);
 
-        return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
+        return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 }
 
 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
index 9fcb3ec7c838..70d330f8c990 100644
--- a/net/ipv6/syncookies.c
+++ b/net/ipv6/syncookies.c
@@ -27,28 +27,17 @@ extern __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
 #define COOKIEBITS 24   /* Upper bits store count */
 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
 
-/*
- * This table has to be sorted and terminated with (__u16)-1.
- * XXX generate a better table.
- * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
- *
- * Taken directly from ipv4 implementation.
- * Should this list be modified for ipv6 use or is it close enough?
- * rfc 2460 8.3 suggests mss values 20 bytes less than ipv4 counterpart
- */
+/* Table must be sorted. */
 static __u16 const msstab[] = {
-        64 - 1,
-        256 - 1,
-        512 - 1,
-        536 - 1,
-        1024 - 1,
-        1440 - 1,
-        1460 - 1,
-        4312 - 1,
-        (__u16)-1
+        64,
+        512,
+        536,
+        1280 - 60,
+        1480 - 60,
+        1500 - 60,
+        4460 - 60,
+        9000 - 60,
 };
-/* The number doesn't include the -1 terminator */
-#define NUM_MSS (ARRAY_SIZE(msstab) - 1)
 
 /*
  * This (misnamed) value is the age of syncookie which is permitted.
@@ -134,9 +123,11 @@ __u32 cookie_v6_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 
         tcp_synq_overflow(sk);
 
-        for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
-                ;
-        *mssp = msstab[mssind] + 1;
+        for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
+                if (mss >= msstab[mssind])
+                        break;
+
+        *mssp = msstab[mssind];
 
         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 
@@ -154,7 +145,7 @@ static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
                                             th->source, th->dest, seq,
                                             jiffies / (HZ * 60), COUNTER_TRIES);
 
-        return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
+        return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 }
 
 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)