diff --git a/shadowsocks/obfsplugin/auth.py b/shadowsocks/obfsplugin/auth.py
index 74959ad..3b9b739 100755
--- a/shadowsocks/obfsplugin/auth.py
+++ b/shadowsocks/obfsplugin/auth.py
@@ -1212,7 +1212,7 @@ class auth_aes128_sha1(auth_base):
     def rnd_data_len(self, buf_size, full_buf_size):
         if full_buf_size >= self.server_info.buffer_size:
             return 0
-        tcp_mss = self.server_info.tcp_mss if self.server_info.tcp_mss < 4096 else 4096
+        tcp_mss = self.server_info.tcp_mss if self.server_info.tcp_mss < 1500 else 1500
         rev_len = tcp_mss - buf_size - 9
         if rev_len == 0:
             return 0