From a817ed0bdd278a446faf8df06a200f4097eca68c Mon Sep 17 00:00:00 2001
From: "Mikkel K. Andersen" <mka@copenhagen-energy.com>
Date: Thu, 16 Jul 2026 15:39:10 +0200
Subject: [PATCH] Allow disabling client_max_window_bits/server_max_window_bits
 in the deflate offer

permessage-deflate always appended "; server_max_window_bits=15; client_max_window_bits=15"
to the handshake header. 15 is the RFC 7692 default window size, so those parameters
convey nothing, and some servers and gateways reject a client offer that carries them.

Add a sendClientServerMaxWindowBits flag on WebSocketPerMessageDeflateOptions (default
true, so existing behavior is unchanged) that gates emitting the two parameters. Set it
to false to send a bare "permessage-deflate" offer. Also initialize the flag in the
header-parsing constructor, which previously left it unset.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 .../IXWebSocketPerMessageDeflateOptions.cpp     | 17 ++++++++++++++---
 .../IXWebSocketPerMessageDeflateOptions.h       |  5 ++++-
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/ixwebsocket/IXWebSocketPerMessageDeflateOptions.cpp b/ixwebsocket/IXWebSocketPerMessageDeflateOptions.cpp
index b583bac3..285588d6 100644
--- a/ixwebsocket/IXWebSocketPerMessageDeflateOptions.cpp
+++ b/ixwebsocket/IXWebSocketPerMessageDeflateOptions.cpp
@@ -27,13 +27,15 @@ namespace ix
         bool clientNoContextTakeover,
         bool serverNoContextTakeover,
         uint8_t clientMaxWindowBits,
-        uint8_t serverMaxWindowBits)
+        uint8_t serverMaxWindowBits,
+        bool sendClientServerMaxWindowBits)
     {
         _enabled = enabled;
         _clientNoContextTakeover = clientNoContextTakeover;
         _serverNoContextTakeover = serverNoContextTakeover;
         _clientMaxWindowBits = clientMaxWindowBits;
         _serverMaxWindowBits = serverMaxWindowBits;
+        _sendClientServerMaxWindowBits = sendClientServerMaxWindowBits;
 
         sanitizeClientMaxWindowBits();
     }
@@ -59,6 +61,7 @@ namespace ix
         _enabled = false;
         _clientNoContextTakeover = false;
         _serverNoContextTakeover = false;
+        _sendClientServerMaxWindowBits = true;
         _clientMaxWindowBits = kDefaultClientMaxWindowBits;
         _serverMaxWindowBits = kDefaultServerMaxWindowBits;
 
@@ -128,8 +131,11 @@ namespace ix
         if (_clientNoContextTakeover) ss << "; client_no_context_takeover";
         if (_serverNoContextTakeover) ss << "; server_no_context_takeover";
 
-        ss << "; server_max_window_bits=" << static_cast<int>(_serverMaxWindowBits);
-        ss << "; client_max_window_bits=" << static_cast<int>(_clientMaxWindowBits);
+        if (_sendClientServerMaxWindowBits)
+        {
+            ss << "; server_max_window_bits=" << static_cast<int>(_serverMaxWindowBits);
+            ss << "; client_max_window_bits=" << static_cast<int>(_clientMaxWindowBits);
+        }
 
         ss << "\r\n";
 
@@ -168,6 +174,11 @@ namespace ix
         return _serverMaxWindowBits;
     }
 
+    bool WebSocketPerMessageDeflateOptions::getSendClientServerMaxWindowBits() const
+    {
+        return _sendClientServerMaxWindowBits;
+    }
+
     bool WebSocketPerMessageDeflateOptions::startsWith(const std::string& str,
                                                        const std::string& start)
     {
diff --git a/ixwebsocket/IXWebSocketPerMessageDeflateOptions.h b/ixwebsocket/IXWebSocketPerMessageDeflateOptions.h
index 95751b61..0650bbc7 100644
--- a/ixwebsocket/IXWebSocketPerMessageDeflateOptions.h
+++ b/ixwebsocket/IXWebSocketPerMessageDeflateOptions.h
@@ -19,7 +19,8 @@ namespace ix
             bool clientNoContextTakeover = false,
             bool serverNoContextTakeover = false,
             uint8_t clientMaxWindowBits = kDefaultClientMaxWindowBits,
-            uint8_t serverMaxWindowBits = kDefaultServerMaxWindowBits);
+            uint8_t serverMaxWindowBits = kDefaultServerMaxWindowBits,
+            bool sendClientServerMaxWindowBits = true);
 
         WebSocketPerMessageDeflateOptions(std::string extension);
 
@@ -29,6 +30,7 @@ namespace ix
         bool getServerNoContextTakeover() const;
         uint8_t getServerMaxWindowBits() const;
         uint8_t getClientMaxWindowBits() const;
+        bool getSendClientServerMaxWindowBits() const;
 
         static bool startsWith(const std::string& str, const std::string& start);
         static std::string removeSpaces(const std::string& str);
@@ -40,6 +42,7 @@ namespace ix
         bool _enabled;
         bool _clientNoContextTakeover;
         bool _serverNoContextTakeover;
+        bool _sendClientServerMaxWindowBits;
         uint8_t _clientMaxWindowBits;
         uint8_t _serverMaxWindowBits;