path: root/Documentation
diff options
authorEric Dumazet <>2019-06-06 09:15:31 -0700
committerDavid S. Miller <>2019-06-15 18:47:31 -0700
commit5f3e2bf008c2221478101ee72f5cb4654b9fc363 (patch)
tree7be18a95aa24dc68564ab7d84bb158468efb8a09 /Documentation
parentf070ef2ac66716357066b683fb0baf55f8191a2e (diff)
tcp: add tcp_min_snd_mss sysctl
Some TCP peers announce a very small MSS option in their SYN and/or SYN/ACK messages. This forces the stack to send packets with a very high network/cpu overhead. Linux has enforced a minimal value of 48. Since this value includes the size of TCP options, and that the options can consume up to 40 bytes, this means that each segment can include only 8 bytes of payload. In some cases, it can be useful to increase the minimal value to a saner value. We still let the default to 48 (TCP_MIN_SND_MSS), for compatibility reasons. Note that TCP_MAXSEG socket option enforces a minimal value of (TCP_MIN_MSS). David Miller increased this minimal value in commit c39508d6f118 ("tcp: Make TCP_MAXSEG minimum more correct.") from 64 to 88. We might in the future merge TCP_MIN_SND_MSS and TCP_MIN_MSS. CVE-2019-11479 -- tcp mss hardcoded to 48 Signed-off-by: Eric Dumazet <> Suggested-by: Jonathan Looney <> Acked-by: Neal Cardwell <> Cc: Yuchung Cheng <> Cc: Tyler Hicks <> Cc: Bruce Curtis <> Cc: Jonathan Lemon <> Signed-off-by: David S. Miller <>
Diffstat (limited to 'Documentation')
1 files changed, 8 insertions, 0 deletions
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 288aa264ac26..22f6b8b1110a 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -255,6 +255,14 @@ tcp_base_mss - INTEGER
Path MTU discovery (MTU probing). If MTU probing is enabled,
this is the initial MSS used by the connection.
+tcp_min_snd_mss - INTEGER
+ TCP SYN and SYNACK messages usually advertise an ADVMSS option,
+ as described in RFC 1122 and RFC 6691.
+ If this ADVMSS option is smaller than tcp_min_snd_mss,
+ it is silently capped to tcp_min_snd_mss.
+ Default : 48 (at least 8 bytes of payload per segment)
tcp_congestion_control - STRING
Set the congestion control algorithm to be used for new
connections. The algorithm "reno" is always available, but