diff --git a/app/src/main/java/io/timelimit/android/sync/network/api/HttpClient.kt b/app/src/main/java/io/timelimit/android/sync/network/api/HttpClient.kt
index 72a7ba5..a99ec49 100644
--- a/app/src/main/java/io/timelimit/android/sync/network/api/HttpClient.kt
+++ b/app/src/main/java/io/timelimit/android/sync/network/api/HttpClient.kt
@@ -1,5 +1,5 @@
 /*
- * TimeLimit Copyright <C> 2019 - 2020 Jonas Lochmann
+ * TimeLimit Copyright <C> 2019 - 2023 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -31,7 +31,7 @@ val httpClient: OkHttpClient by lazy {
     if (BuildConfig.DEBUG) {
         builder.addInterceptor (HttpLoggingInterceptor {
             Log.d("HttpClient", it)
-        })
+        }.apply { level = HttpLoggingInterceptor.Level.HEADERS })
     }
 
     builder.build()
diff --git a/app/src/main/java/io/timelimit/android/sync/network/api/HttpServerApi.kt b/app/src/main/java/io/timelimit/android/sync/network/api/HttpServerApi.kt
index c234c9b..8b34ccb 100644
--- a/app/src/main/java/io/timelimit/android/sync/network/api/HttpServerApi.kt
+++ b/app/src/main/java/io/timelimit/android/sync/network/api/HttpServerApi.kt
@@ -23,12 +23,15 @@ import io.timelimit.android.async.Threads
 import io.timelimit.android.coroutines.executeAndWait
 import io.timelimit.android.coroutines.waitForResponse
 import io.timelimit.android.sync.network.*
+import io.timelimit.android.util.okio.LengthSink
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.RequestBody
 import okhttp3.Response
+import okhttp3.internal.closeQuietly
 import okio.BufferedSink
 import okio.GzipSink
+import okio.Sink
 import okio.buffer
 import java.io.OutputStreamWriter
 
@@ -63,24 +66,37 @@ class HttpServerApi(private val endpointWithoutSlashAtEnd: String): ServerApi {
 
         private val JSON = "application/json; charset=utf-8".toMediaTypeOrNull()
 
-        private fun createJsonRequestBody(serialize: (writer: JsonWriter) -> Unit) = object: RequestBody() {
-            override fun contentType() = JSON
-            override fun writeTo(sink: BufferedSink) {
+        private fun createJsonRequestBody(
+            serialize: (writer: JsonWriter) -> Unit,
+            measureContentLength: Boolean
+        ): RequestBody {
+            fun write(sink: Sink) {
                 val writer = JsonWriter(
-                        OutputStreamWriter(
-                                GzipSink(sink)
-                                        .buffer().outputStream()
-                        )
+                    OutputStreamWriter(
+                        GzipSink(sink)
+                            .buffer().outputStream()
+                    )
                 )
 
                 serialize(writer)
 
-                writer.flush()
                 writer.close()
             }
+
+            val length =
+                if (measureContentLength) LengthSink().also { write(it) }.length
+                else null
+
+            return object: RequestBody() {
+                override fun contentType() = JSON
+                override fun writeTo(sink: BufferedSink) = write(sink)
+                override fun contentLength(): Long = length ?: -1
+            }
         }
     }
 
+    private var sendContentLength = false
+
     override suspend fun getTimeInMillis(): Long {
         httpClient.newCall(
                 Request.Builder()
@@ -595,10 +611,30 @@ class HttpServerApi(private val endpointWithoutSlashAtEnd: String): ServerApi {
         path: String,
         requestBody: (writer: JsonWriter) -> Unit
     ): Response {
+        if (!sendContentLength) {
+            val response = postJsonRequest(path, requestBody, transmitContentLength = false)
+
+            if (response.code != 411) return response
+
+            Threads.network.executeAndWait { response.closeQuietly() }
+
+            sendContentLength = true
+        }
+
+        return postJsonRequest(path, requestBody, transmitContentLength = true)
+    }
+
+    private suspend fun postJsonRequest(
+        path: String,
+        requestBody: (writer: JsonWriter) -> Unit,
+        transmitContentLength: Boolean
+    ): Response {
+        val body = createJsonRequestBody(requestBody, transmitContentLength)
+
         return httpClient.newCall(
             Request.Builder()
                 .url("$endpointWithoutSlashAtEnd/$path")
-                .post(createJsonRequestBody(requestBody))
+                .post(body)
                 .header("Content-Encoding", "gzip")
                 .build()
         ).waitForResponse()
diff --git a/app/src/main/java/io/timelimit/android/util/okio/LengthSink.kt b/app/src/main/java/io/timelimit/android/util/okio/LengthSink.kt
new file mode 100644
index 0000000..869c10e
--- /dev/null
+++ b/app/src/main/java/io/timelimit/android/util/okio/LengthSink.kt
@@ -0,0 +1,31 @@
+/*
+ * TimeLimit Copyright <C> 2019 - 2023 Jonas Lochmann
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+package io.timelimit.android.util.okio
+
+import okio.Buffer
+import okio.Sink
+import okio.Timeout
+
+class LengthSink: Sink {
+    private var lengthInternal = 0L
+
+    val length get() = lengthInternal
+
+    override fun write(source: Buffer, byteCount: Long) { lengthInternal += byteCount }
+    override fun timeout(): Timeout = Timeout.NONE
+    override fun flush() = Unit
+    override fun close() = Unit
+}
\ No newline at end of file