feat(rtsp): add option to limit packetsize for clients that cannot configure it (#5153)

Signed-off-by: Georgi Valkov <gvalkov@gmail.com>

docs/configuration.md

@@ -1753,6 +1753,67 @@ editing the `conf` file in a text editor. Use the examples as reference.
     </tr>
 </table>
 
+### packetsize
+
+<table>
+    <tr>
+        <td>Description</td>
+        <td colspan="2">
+            Limit the packetsize to avoid fragmentation on a low MTU link.
+            @note{This helps avoid packet loss and micro-stutter on a layer 2 VPN with
+            clients that cannot configure this value, e.g. Moonlight for Android/iOS.
+            }
+            @tip{To discover the optimal value:
+            <ul>
+                <li>Send ping to the server with don't fragment flag (DF)</li>
+                <li>Find the size of the largest replay, and subtract 16</li>
+                <li>Monitor the traffic to ensure no fragmentation</li>
+            </ul>
+            If using a VPN tunnel:
+            <ul>
+                <li>Set MTU on the TUN/TAP interface, and</li>
+                <li>Ensure no fragmentation both inside and outside the tunnel</li>
+                <li>Max UDP size = MTU size - 28</li>
+                <li>`packetsize` = max UDP size - 16</li>
+                <li>Monitor the traffic to ensure no fragmentation</li>
+            </ul>
+            Sample calculation for OpenVPN layer 2, using IPv4:
+            <ul>
+                <li>1428 bytes for max ICMP/UDP size outside the tunnel</li>
+                <li>Subtract the OpenVPN overhead: 24 bytes (may vary)</li>
+                <li>1404 bytes for Ethernet inside the tunnel</li>
+                <li>Subtract the Ethernet header: 14 bytes</li>
+                <li>1390 bytes for MTU inside the tunnel</li>
+                <li>Subtract the IPv4 header: 20 bytes</li>
+                <li>Subtract the UDP header: 8 bytes</li>
+                <li>1362 bytes for UDP payload</li>
+                <li>Subtract: 16 bytes</li>
+                <li>1346 bytes for `packetsize`</li>
+            </ul>
+            }
+            @warning{Reduce the bitrate when using low values.
+            Values larger than 1456 require jumbo frames.
+            }
+        </td>
+    </tr>
+    <tr>
+        <td>Default</td>
+        <td colspan="2">@code{}
+            0
+            @endcode</td>
+    </tr>
+    <tr>
+        <td>Range</td>
+        <td colspan="2">0, 200-65535</td>
+    </tr>
+    <tr>
+        <td>Example</td>
+        <td colspan="2">@code{}
+            packetsize = 1346
+            @endcode</td>
+    </tr>
+</table>
+
 ## Config Files
 
 ### file_apps

src/config.cpp

@@ -544,6 +544,7 @@ namespace config {
 
     ENCRYPTION_MODE_NEVER,  // lan_encryption_mode
     ENCRYPTION_MODE_OPPORTUNISTIC,  // wan_encryption_mode
+    0,  // packetsize
   };
 
   nvhttp_t nvhttp {
@@ -1252,6 +1253,7 @@ namespace config {
 
     int_between_f(vars, "lan_encryption_mode", stream.lan_encryption_mode, {0, 2});
     int_between_f(vars, "wan_encryption_mode", stream.wan_encryption_mode, {0, 2});
+    int_between_f(vars, "packetsize", stream.packetsize, {0, PACKETSIZE_MAX});
 
     path_f(vars, "file_apps", stream.file_apps);
 #ifndef __ANDROID__

src/config.h

@@ -16,6 +16,12 @@
 #include "nvenc/nvenc_config.h"
 
 namespace config {
+  // Valid range for the packetsize limit
+  constexpr int PACKETSIZE_MIN = 200;
+  constexpr int PACKETSIZE_MAX = 65535;
+  constexpr int PACKETSIZE_SMALL = 500;
+  constexpr int PACKETSIZE_LARGE = 1456;
+
   // track modified config options
   inline std::unordered_map<std::string, std::string> modified_config_settings;
 
@@ -177,6 +183,9 @@ namespace config {
     // Video encryption settings for LAN and WAN streams
     int lan_encryption_mode;
     int wan_encryption_mode;
+
+    // Limit the packetsize to avoid fragmentation on a low MTU link
+    int packetsize;
   };
 
   struct nvhttp_t {

src/rtsp.cpp

@@ -1004,6 +1004,23 @@ namespace rtsp_stream {
         config.encryptionFlagsEnabled |= SS_ENC_AUDIO;
       }
 
+      // Limit the packetsize to avoid fragmentation with clients that cannot configure this value
+      if (config::stream.packetsize && config::stream.packetsize < config.packetsize) {
+        if (config::stream.packetsize < config::PACKETSIZE_MIN || config::stream.packetsize > config::PACKETSIZE_MAX) {
+          BOOST_LOG(warning) << "packetsize range: ["sv << config::PACKETSIZE_MIN << "-"sv << config::PACKETSIZE_MAX
+                             << "] invalid value: "sv << config::stream.packetsize;
+        } else {
+          if (config::stream.packetsize < config::PACKETSIZE_SMALL) {
+            BOOST_LOG(info) << "packetsize is small < "sv << config::PACKETSIZE_SMALL << " bytes, reduce bitrate if the stream breaks"sv;
+          } else if (config::stream.packetsize > config::PACKETSIZE_LARGE) {
+            BOOST_LOG(info) << "packetsize is large > "sv << config::PACKETSIZE_LARGE << " bytes, jumbo frames may be used"sv;
+          }
+
+          BOOST_LOG(info) << "packetsize limit: "sv << config.packetsize << " -> "sv << config::stream.packetsize << " bytes"sv;
+          config.packetsize = config::stream.packetsize;
+        }
+      }
+
       config.monitor.height = (int) util::from_view(args.at("x-nv-video[0].clientViewportHt"sv));
       config.monitor.width = (int) util::from_view(args.at("x-nv-video[0].clientViewportWd"sv));
       config.monitor.framerate = (int) util::from_view(args.at("x-nv-video[0].maxFPS"sv));

src_assets/common/assets/web/config.html

@@ -253,6 +253,7 @@
               "lan_encryption_mode": 0,
               "wan_encryption_mode": 1,
               "ping_timeout": 10000,
+              "packetsize": 0,
             },
           },
           {

src_assets/common/assets/web/configs/tabs/Network.vue

@@ -173,6 +173,13 @@ const effectivePort = computed(() => +config.value?.port ?? defaultMoonlightPort
       <div class="form-text">{{ $t('config.ping_timeout_desc') }}</div>
     </div>
 
+    <!-- Packet Size Limit -->
+    <div class="mb-3">
+      <label for="packetsize" class="form-label">{{ $t('config.packetsize') }}</label>
+      <input type="number" min="0" max="65535" class="form-control" id="packetsize" placeholder="0" v-model="config.packetsize" />
+      <div class="form-text">{{ $t('config.packetsize_desc') }}</div>
+    </div>
+
   </div>
 </template>
 

src_assets/common/assets/web/public/assets/locale/en.json

@@ -319,6 +319,8 @@
     "output_name": "Display Id",
     "output_name_desc_unix": "During Sunshine startup, you should see the list of detected displays. Note: You need to use the id value inside the parenthesis. Below is an example; the actual output can be found in the Troubleshooting tab.",
     "output_name_desc_windows": "Manually specify a display device id to use for capture. If unset, the primary display is captured. Note: If you specified a GPU above, this display must be connected to that GPU. During Sunshine startup, you should see the list of detected displays. Below is an example; the actual output can be found in the Troubleshooting tab.",
+    "packetsize": "Packet Size Limit",
+    "packetsize_desc": "Limit the packet size to avoid fragmentation on a low MTU link. This helps reduce packet loss and micro-stuttering, while streaming over a layer 2 VPN to clients that cannot configure this value, e.g. Moonlight for Android/iOS. Reduce the bitrate when using low values. Values larger than 1456 require jumbo frames. Range: 0, 200-65536. A value of 0 will disable the limit.",
     "ping_timeout": "Ping Timeout",
     "ping_timeout_desc": "How long to wait in milliseconds for data from moonlight before shutting down the stream",
     "pkey": "Private Key",