diff --git a/.env.example b/.env.example
index 8d40b28..5d7961e 100644
--- a/.env.example
+++ b/.env.example
@@ -27,22 +27,22 @@ LAYLINK_KCP_INTERVAL_MS=10
 # KCP 内部 update 间隔；单位毫秒。常用 10、20、30，越小越低延迟但 CPU/发包更高。
 LAYLINK_KCP_FAST_RESEND=2
 # KCP 快速重传阈值；0 关闭，2 是常见低延迟设置，丢包网络可尝试 2-4。
-LAYLINK_KCP_NO_CONGESTION_CONTROL=0
-# 是否关闭 KCP 拥塞控制；0 开启拥塞控制更稳，1 更激进但容易挤爆 UDP 发送缓冲。
-LAYLINK_KCP_SEND_WINDOW=256
-# KCP 发送窗口；越大吞吐潜力越高但更容易拥堵，常用 128、256、512、1024。
-LAYLINK_KCP_RECV_WINDOW=512
-# KCP 接收窗口；应不小于发送窗口，常用 256、512、1024。
-LAYLINK_KCP_MTU_BYTES=1200
-# KCP MTU；建议 1200 避免公网路径分片，内网可尝试 1350。
+LAYLINK_KCP_NO_CONGESTION_CONTROL=1
+# 是否关闭 KCP 拥塞控制；1 为高吞吐模式，接近早期 100Mbps 默认值；丢包/拥堵明显时改 0。
+LAYLINK_KCP_SEND_WINDOW=1024
+# KCP 发送窗口；高吞吐建议 1024，保守可用 256 或 512。
+LAYLINK_KCP_RECV_WINDOW=1024
+# KCP 接收窗口；应不小于发送窗口，高吞吐建议 1024。
+LAYLINK_KCP_MTU_BYTES=1350
+# KCP MTU；高吞吐建议 1350，若公网链路分片/丢包明显则改 1200。
 LAYLINK_KCP_TICK_MS=10
 # PHP transport tick 间隔；单位毫秒。通常与 LAYLINK_KCP_INTERVAL_MS 一致。
-LAYLINK_KCP_UDP_SEND_QUEUE_BYTES=16777216
-# UDP EAGAIN 发送队列上限；单位字节。发送缓冲暂满时会排队重试，超过后关闭该 KCP 会话。
-LAYLINK_KCP_UDP_FLUSH_PACKETS=256
-# 每次 tick 最多刷出的 UDP packet 数；拥堵时可调低到 64 或 128，追求吞吐可调高。
-LAYLINK_KCP_OUTPUT_DRAIN_PACKETS=256
-# 每次从 native KCP 输出队列搬到 UDP 发送队列的最大 packet 数；调低可减少单个大下载占用事件循环时间。
+LAYLINK_KCP_UDP_SEND_QUEUE_BYTES=67108864
+# UDP EAGAIN 发送队列上限；单位字节。高吞吐建议 67108864，内存紧张或拥堵明显时调低。
+LAYLINK_KCP_UDP_FLUSH_PACKETS=1024
+# 每次 tick 最多刷出的 UDP packet 数；高吞吐建议 1024，拥堵时可调低到 256 或 128。
+LAYLINK_KCP_OUTPUT_DRAIN_PACKETS=1024
+# 每次从 native KCP 输出队列搬到 UDP 发送队列的最大 packet 数；高吞吐建议 1024，单连接影响事件循环时调低。
 
 [client-agent]
 NODE_ID=client-01
diff --git a/contract.md b/contract.md
index 6caddf0..9281dd3 100644
--- a/contract.md
+++ b/contract.md
@@ -2,7 +2,7 @@
 
 ## Implementation Status
 
-Last updated: 2026-05-29 Asia/Shanghai.
+Last updated: 2026-05-30 Asia/Shanghai.
 
 Current phase: MVP bootstrap in progress.
 
@@ -240,6 +240,21 @@ Completed in this checkpoint:
   * `POP_AGENT_KCP_WORKERS` is exposed but currently clamped to `1` in `bin/pop-server.php`.
   * KCP/UDP must remain single-worker in the current architecture because KCP session state is process-local and UDP packets for one conv can otherwise be handled by different workers.
   * Native KCP output draining is capped per tick by `LAYLINK_KCP_OUTPUT_DRAIN_PACKETS` to reduce single-flow event-loop monopolization during large downloads.
+* Fixed KCP POP-side session lookup for real-world UDP/NAT behavior:
+  * POP no longer depends only on `remote ip:port + conv` for KCP session lookup.
+  * POP keeps a `conv -> session` index and migrates the current UDP remote address when a known `conv` arrives from a changed source port.
+  * KCP callbacks now resolve the active connection by `conv`, so migrated sessions continue delivering frames instead of silently dropping `DATA`.
+* Fixed native KCP send-buffer accounting after heavy speedtest-style traffic:
+  * The previous PHP-side native KCP `queuedBytes` counter could grow during large transfers and never fall back to zero unless user payload was received in the opposite direction.
+  * This could make a long-lived KCP Agent connection permanently appear full after upload/download tests, causing later `OPEN_OK` / `DATA` sends to fail and audit rows to show zero transferred bytes.
+  * Added native wrapper `laylink_kcp_waitsnd()` around upstream `ikcp_waitsnd()`.
+  * `NativeKcpSession::getSendBufferQueueSize()` now derives watermarks from real KCP pending segment count plus pending UDP output bytes.
+  * POP now treats failure to send `OPEN_OK` as `agent_buffer_overflow` instead of leaving a target connection open with no client-visible success.
+* Restored KCP high-throughput defaults:
+  * Early native KCP testing used hardcoded `nodelay=1, interval=10, resend=2, nc=1, sndwnd=1024, rcvwnd=1024, mtu=1350`.
+  * The first exposed `.env` defaults were more conservative (`nc=0`, smaller windows, `mtu=1200`) and could reduce throughput dramatically on speedtest-style high-BDP paths.
+  * `NativeKcpSession`, KCP UDP queue/flush fallback defaults, current `.env`, and `.env.example` now use the high-throughput profile by default.
+  * For lossy or congested paths, tune down to `LAYLINK_KCP_NO_CONGESTION_CONTROL=0`, `LAYLINK_KCP_MTU_BYTES=1200`, smaller windows, or lower flush/drain packet counts.
 
 Known MVP limitations:
 
diff --git a/native/kcp/laylink_kcp.c b/native/kcp/laylink_kcp.c
index 1c1e133..2c98e9b 100644
--- a/native/kcp/laylink_kcp.c
+++ b/native/kcp/laylink_kcp.c
@@ -164,6 +164,13 @@ void laylink_kcp_flush(laylink_kcp* session) {
     ikcp_flush(session->kcp);
 }
 
+int laylink_kcp_waitsnd(laylink_kcp* session) {
+    if (session == NULL || session->kcp == NULL) {
+        return 0;
+    }
+    return ikcp_waitsnd(session->kcp);
+}
+
 int laylink_kcp_pending_output_size(laylink_kcp* session) {
     if (session == NULL || session->output_head == NULL) {
         return 0;
diff --git a/native/kcp/laylink_kcp.h b/native/kcp/laylink_kcp.h
index 6e0a7ba..f9180c9 100644
--- a/native/kcp/laylink_kcp.h
+++ b/native/kcp/laylink_kcp.h
@@ -19,6 +19,7 @@ unsigned int laylink_kcp_check(laylink_kcp* session, unsigned int current);
 int laylink_kcp_peeksize(laylink_kcp* session);
 int laylink_kcp_recv(laylink_kcp* session, char* buffer, int len);
 void laylink_kcp_flush(laylink_kcp* session);
+int laylink_kcp_waitsnd(laylink_kcp* session);
 int laylink_kcp_pending_output_size(laylink_kcp* session);
 int laylink_kcp_pop_output(laylink_kcp* session, char* buffer, int len);
 
diff --git a/src/Server/AgentListener.php b/src/Server/AgentListener.php
index 56481ae..68f15c3 100644
--- a/src/Server/AgentListener.php
+++ b/src/Server/AgentListener.php
@@ -237,10 +237,13 @@ final class AgentListener
 
         $target->onConnect = function () use ($session, $agentConnection): void {
             $session->state = TunnelSession::OPEN;
-            $this->send($agentConnection, new Frame(FrameType::OPEN_OK, $session->sessionId, [
+            $sent = $this->send($agentConnection, new Frame(FrameType::OPEN_OK, $session->sessionId, [
                 'target_host' => $session->targetHost,
                 'target_port' => $session->targetPort,
             ]));
+            if ($sent === false) {
+                $this->closeSession($session, 'failed', 'agent_buffer_overflow');
+            }
         };
         $target->onMessage = function (AsyncTcpConnection $target, string $data) use ($session, $agentConnection): void {
             $session->bytesTargetToClient += strlen($data);
diff --git a/src/Transport/KcpFrameClientTransport.php b/src/Transport/KcpFrameClientTransport.php
index 920ce58..aeb2aa2 100644
--- a/src/Transport/KcpFrameClientTransport.php
+++ b/src/Transport/KcpFrameClientTransport.php
@@ -37,7 +37,7 @@ final class KcpFrameClientTransport implements FrameClientTransport
         $this->connection = $connection;
         $this->sender = new KcpUdpPacketSender(
             $connection,
-            KcpConfig::int('LAYLINK_KCP_UDP_SEND_QUEUE_BYTES', 16 * 1024 * 1024, 1024 * 1024, 512 * 1024 * 1024),
+            KcpConfig::int('LAYLINK_KCP_UDP_SEND_QUEUE_BYTES', 64 * 1024 * 1024, 1024 * 1024, 512 * 1024 * 1024),
         );
 
         $connection->onConnect = function () use ($connection): void {
@@ -130,14 +130,14 @@ final class KcpFrameClientTransport implements FrameClientTransport
                 'type' => KcpPacketCodec::SYN,
                 'conv' => $this->conv,
             ]));
-            $this->sender?->flush(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 256, 1, 8192));
+            $this->sender?->flush(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 1024, 1, 8192));
             return;
         }
 
         $before = $this->getSendBufferQueueSize();
-        $this->sender?->flush(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 256, 1, 8192));
+        $this->sender?->flush(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 1024, 1, 8192));
         $this->session?->tick();
-        $this->sender?->flush(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 256, 1, 8192));
+        $this->sender?->flush(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 1024, 1, 8192));
         if ($before >= $this->maxSendBuffer && $this->getSendBufferQueueSize() < $this->maxSendBuffer) {
             ($this->onBufferDrain)($this);
         }
diff --git a/src/Transport/KcpFrameServerListener.php b/src/Transport/KcpFrameServerListener.php
index 5cadefb..9bb3212 100644
--- a/src/Transport/KcpFrameServerListener.php
+++ b/src/Transport/KcpFrameServerListener.php
@@ -15,6 +15,8 @@ final class KcpFrameServerListener
     private array $connections = [];
     /** @var array<string, KcpReliableSession|NativeKcpSession> */
     private array $sessions = [];
+    /** @var array<int, string> */
+    private array $keysByConv = [];
     private ?int $timerId = null;
     private int $nextConnectionId = 1_000_000;
 
@@ -48,6 +50,7 @@ final class KcpFrameServerListener
                 return;
             }
             $key = $this->key($connection, $conv);
+            $key = $this->knownKey($connection, $conv, $key);
             $wrapped = $this->connections[$key] ?? null;
             $session = $this->sessions[$key] ?? null;
             if ($wrapped === null || $session === null) {
@@ -72,6 +75,7 @@ final class KcpFrameServerListener
             $this->handleSyn($connection, $key, $packet['conv']);
             return;
         }
+        $key = $this->knownKey($connection, $packet['conv'], $key);
 
         $wrapped = $this->connections[$key] ?? null;
         $session = $this->sessions[$key] ?? null;
@@ -98,31 +102,35 @@ final class KcpFrameServerListener
 
     private function handleSyn(UdpConnection $connection, string $key, int $conv): void
     {
+        if (isset($this->keysByConv[$conv]) && $this->keysByConv[$conv] !== $key) {
+            $key = $this->migrateConnection($this->keysByConv[$conv], $key, $connection, $conv);
+        }
+
         if (isset($this->connections[$key])) {
             $this->connections[$key]->updateConnection($connection);
             $this->connections[$key]->sendUdpPacket(KcpPacketCodec::encode([
                 'type' => KcpPacketCodec::SYN_ACK,
                 'conv' => $conv,
             ]));
-            $this->connections[$key]->flushUdp(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 256, 1, 8192));
+            $this->connections[$key]->flushUdp(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 1024, 1, 8192));
             return;
         }
 
         $sender = new KcpUdpPacketSender(
             $connection,
-            KcpConfig::int('LAYLINK_KCP_UDP_SEND_QUEUE_BYTES', 16 * 1024 * 1024, 1024 * 1024, 512 * 1024 * 1024),
+            KcpConfig::int('LAYLINK_KCP_UDP_SEND_QUEUE_BYTES', 64 * 1024 * 1024, 1024 * 1024, 512 * 1024 * 1024),
         );
         $session = $this->createSession(
             $conv,
             fn (string $packet): bool|null => $sender->send($packet),
-            function (Frame $frame) use ($key): void {
-                $wrapped = $this->connections[$key] ?? null;
+            function (Frame $frame) use ($conv): void {
+                $wrapped = $this->connectionForConv($conv);
                 if ($wrapped !== null) {
                     ($this->onFrame)($wrapped, $frame);
                 }
             },
-            function (\Throwable $e) use ($key): void {
-                $wrapped = $this->connections[$key] ?? null;
+            function (\Throwable $e) use ($conv): void {
+                $wrapped = $this->connectionForConv($conv);
                 if ($wrapped !== null) {
                     ($this->onInvalidFrame)($wrapped, $e);
                 }
@@ -131,6 +139,7 @@ final class KcpFrameServerListener
         $wrapped = new KcpFrameServerConnection($this->nextConnectionId++, $connection, $session, $sender, $this->maxSendBuffer);
         $this->sessions[$key] = $session;
         $this->connections[$key] = $wrapped;
+        $this->keysByConv[$conv] = $key;
 
         $sender->send(KcpPacketCodec::encode([
             'type' => KcpPacketCodec::SYN_ACK,
@@ -149,9 +158,9 @@ final class KcpFrameServerListener
             }
 
             $before = $session->getSendBufferQueueSize();
-            $wrapped->flushUdp(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 256, 1, 8192));
+            $wrapped->flushUdp(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 1024, 1, 8192));
             $session->tick();
-            $wrapped->flushUdp(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 256, 1, 8192));
+            $wrapped->flushUdp(KcpConfig::int('LAYLINK_KCP_UDP_FLUSH_PACKETS', 1024, 1, 8192));
             if ($before >= $this->maxSendBuffer && $session->getSendBufferQueueSize() < $this->maxSendBuffer) {
                 ($this->onBufferDrain)($wrapped);
             }
@@ -161,7 +170,11 @@ final class KcpFrameServerListener
     private function closeConnection(string $key): void
     {
         $wrapped = $this->connections[$key] ?? null;
+        $session = $this->sessions[$key] ?? null;
         unset($this->connections[$key], $this->sessions[$key]);
+        if ($session !== null) {
+            unset($this->keysByConv[$session->conv()]);
+        }
         if ($wrapped !== null) {
             ($this->onClose)($wrapped);
         }
@@ -182,4 +195,45 @@ final class KcpFrameServerListener
     {
         return $connection->getRemoteAddress() . '#' . $conv;
     }
+
+    private function knownKey(UdpConnection $connection, int $conv, string $candidate): string
+    {
+        if (isset($this->connections[$candidate])) {
+            return $candidate;
+        }
+
+        $known = $this->keysByConv[$conv] ?? null;
+        if ($known === null || !isset($this->connections[$known], $this->sessions[$known])) {
+            return $candidate;
+        }
+
+        return $this->migrateConnection($known, $candidate, $connection, $conv);
+    }
+
+    private function migrateConnection(string $oldKey, string $newKey, UdpConnection $connection, int $conv): string
+    {
+        if ($oldKey === $newKey) {
+            return $newKey;
+        }
+
+        $wrapped = $this->connections[$oldKey] ?? null;
+        $session = $this->sessions[$oldKey] ?? null;
+        if ($wrapped === null || $session === null) {
+            return $newKey;
+        }
+
+        unset($this->connections[$oldKey], $this->sessions[$oldKey]);
+        $wrapped->updateConnection($connection);
+        $this->connections[$newKey] = $wrapped;
+        $this->sessions[$newKey] = $session;
+        $this->keysByConv[$conv] = $newKey;
+
+        return $newKey;
+    }
+
+    private function connectionForConv(int $conv): ?KcpFrameServerConnection
+    {
+        $key = $this->keysByConv[$conv] ?? null;
+        return $key === null ? null : ($this->connections[$key] ?? null);
+    }
 }
diff --git a/src/Transport/NativeKcpLibrary.php b/src/Transport/NativeKcpLibrary.php
index 9b9d048..ede92a3 100644
--- a/src/Transport/NativeKcpLibrary.php
+++ b/src/Transport/NativeKcpLibrary.php
@@ -22,6 +22,7 @@ final class NativeKcpLibrary
         int laylink_kcp_peeksize(laylink_kcp* session);
         int laylink_kcp_recv(laylink_kcp* session, char* buffer, int len);
         void laylink_kcp_flush(laylink_kcp* session);
+        int laylink_kcp_waitsnd(laylink_kcp* session);
         int laylink_kcp_pending_output_size(laylink_kcp* session);
         int laylink_kcp_pop_output(laylink_kcp* session, char* buffer, int len);
     CDEF;
diff --git a/src/Transport/NativeKcpSession.php b/src/Transport/NativeKcpSession.php
index 4a4a680..361987d 100644
--- a/src/Transport/NativeKcpSession.php
+++ b/src/Transport/NativeKcpSession.php
@@ -18,9 +18,9 @@ final class NativeKcpSession
     private FrameParser $parser;
     private bool $paused = false;
     private bool $closed = false;
+    private int $mtuBytes;
     /** @var Frame[] */
     private array $pausedFrames = [];
-    private int $queuedBytes = 0;
 
     public function __construct(
         private readonly int $conv,
@@ -40,17 +40,15 @@ final class NativeKcpSession
             KcpConfig::int('LAYLINK_KCP_NODELAY', 1, 0, 1),
             KcpConfig::int('LAYLINK_KCP_INTERVAL_MS', 10, 1, 1000),
             KcpConfig::int('LAYLINK_KCP_FAST_RESEND', 2, 0, 10),
-            KcpConfig::int('LAYLINK_KCP_NO_CONGESTION_CONTROL', 0, 0, 1),
+            KcpConfig::int('LAYLINK_KCP_NO_CONGESTION_CONTROL', 1, 0, 1),
         );
         $this->ffi->laylink_kcp_wndsize(
             $this->session,
-            KcpConfig::int('LAYLINK_KCP_SEND_WINDOW', 256, 16, 8192),
-            KcpConfig::int('LAYLINK_KCP_RECV_WINDOW', 512, 16, 8192),
-        );
-        $this->ffi->laylink_kcp_setmtu(
-            $this->session,
-            KcpConfig::int('LAYLINK_KCP_MTU_BYTES', 1200, 576, 1400),
+            KcpConfig::int('LAYLINK_KCP_SEND_WINDOW', 1024, 16, 8192),
+            KcpConfig::int('LAYLINK_KCP_RECV_WINDOW', 1024, 16, 8192),
         );
+        $this->mtuBytes = KcpConfig::int('LAYLINK_KCP_MTU_BYTES', 1350, 576, 1400);
+        $this->ffi->laylink_kcp_setmtu($this->session, $this->mtuBytes);
 
         $this->parser = new FrameParser();
     }
@@ -70,7 +68,7 @@ final class NativeKcpSession
 
     public function sendFrame(Frame $frame, int $maxSendBuffer): bool|null
     {
-        if ($this->closed || $this->queuedBytes >= $maxSendBuffer) {
+        if ($this->closed || $this->getSendBufferQueueSize() >= $maxSendBuffer) {
             return false;
         }
 
@@ -80,7 +78,6 @@ final class NativeKcpSession
             return false;
         }
 
-        $this->queuedBytes += strlen($bytes);
         $this->ffi->laylink_kcp_flush($this->session);
         $this->drainOutput();
         return true;
@@ -146,7 +143,10 @@ final class NativeKcpSession
 
     public function getSendBufferQueueSize(): int
     {
-        return max($this->queuedBytes, $this->pendingOutputBytes());
+        $waitSegments = $this->ffi->laylink_kcp_waitsnd($this->session);
+        $waitBytes = max(0, $waitSegments) * $this->mtuBytes;
+
+        return $waitBytes + $this->pendingOutputBytes();
     }
 
     private function drainFrames(): void
@@ -164,7 +164,6 @@ final class NativeKcpSession
                 return;
             }
 
-            $this->queuedBytes = max(0, $this->queuedBytes - $read);
             $bytes = FFI::string($buffer, $read);
             try {
                 foreach ($this->parser->push($bytes) as $frame) {
@@ -183,7 +182,7 @@ final class NativeKcpSession
     private function drainOutput(): void
     {
         $drained = 0;
-        $maxPackets = KcpConfig::int('LAYLINK_KCP_OUTPUT_DRAIN_PACKETS', 256, 1, 8192);
+        $maxPackets = KcpConfig::int('LAYLINK_KCP_OUTPUT_DRAIN_PACKETS', 1024, 1, 8192);
         while ($drained < $maxPackets && ($size = $this->ffi->laylink_kcp_pending_output_size($this->session)) > 0) {
             $buffer = $this->ffi->new("char[$size]");
             $read = $this->ffi->laylink_kcp_pop_output($this->session, $buffer, $size);