diff --git a/src/shrpx_error.h b/src/shrpx_error.h
index eacbcf7c..c89611f1 100644
--- a/src/shrpx_error.h
+++ b/src/shrpx_error.h
@@ -39,6 +39,7 @@ enum ErrorCode {
   SHRPX_ERR_DCONN_CANCELED = -103,
   SHRPX_ERR_RETRY = -104,
   SHRPX_ERR_TLS_REQUIRED = -105,
+  SHRPX_ERR_SEND_BLOCKED = -106,
 };
 
 } // namespace shrpx
diff --git a/src/shrpx_http3_upstream.cc b/src/shrpx_http3_upstream.cc
index a2906861..4551dfed 100644
--- a/src/shrpx_http3_upstream.cc
+++ b/src/shrpx_http3_upstream.cc
@@ -127,7 +127,10 @@ Http3Upstream::Http3Upstream(ClientHandler *handler)
       downstream_queue_{downstream_queue_size(handler->get_worker()),
                         !get_config()->http2_proxy},
       idle_close_{false},
-      retry_close_{false} {
+      retry_close_{false},
+      tx_{
+          .data = std::unique_ptr<uint8_t[]>(new uint8_t[64_k]),
+      } {
   ev_timer_init(&timer_, timeoutcb, 0., 0.);
   timer_.data = this;
 
@@ -700,6 +703,19 @@ int Http3Upstream::init(const UpstreamAddr *faddr, const Address &remote_addr,
 int Http3Upstream::on_read() { return 0; }
 
 int Http3Upstream::on_write() {
+  int rv;
+
+  if (tx_.send_blocked) {
+    rv = send_blocked_packet();
+    if (rv != 0) {
+      return -1;
+    }
+
+    if (tx_.send_blocked) {
+      return 0;
+    }
+  }
+
   if (write_streams() != 0) {
     return -1;
   }
@@ -711,14 +727,13 @@ int Http3Upstream::on_write() {
 
 int Http3Upstream::write_streams() {
   std::array<nghttp3_vec, 16> vec;
-  std::array<uint8_t, 64_k> buf;
   auto max_udp_payload_size = std::min(
       max_udp_payload_size_, ngtcp2_conn_get_path_max_udp_payload_size(conn_));
   size_t max_pktcnt =
       std::min(static_cast<size_t>(64_k), ngtcp2_conn_get_send_quantum(conn_)) /
       max_udp_payload_size;
   ngtcp2_pkt_info pi, prev_pi;
-  uint8_t *bufpos = buf.data();
+  uint8_t *bufpos = tx_.data.get();
   ngtcp2_path_storage ps, prev_ps;
   size_t pktcnt = 0;
   int rv;
@@ -803,8 +818,6 @@ int Http3Upstream::write_streams() {
 
       last_error_ = quic::err_transport(nwrite);
 
-      handler_->get_connection()->wlimit.stopw();
-
       return handle_error();
     } else if (ndatalen >= 0) {
       rv = nghttp3_conn_add_write_offset(httpconn_, stream_id, ndatalen);
@@ -817,12 +830,26 @@ int Http3Upstream::write_streams() {
     }
 
     if (nwrite == 0) {
-      if (bufpos - buf.data()) {
-        send_packet(static_cast<UpstreamAddr *>(prev_ps.path.user_data),
-                    prev_ps.path.remote.addr, prev_ps.path.remote.addrlen,
-                    prev_ps.path.local.addr, prev_ps.path.local.addrlen,
-                    prev_pi, buf.data(), bufpos - buf.data(),
-                    max_udp_payload_size);
+      if (bufpos - tx_.data.get()) {
+        auto faddr = static_cast<UpstreamAddr *>(prev_ps.path.user_data);
+        auto data = tx_.data.get();
+        auto datalen = bufpos - data;
+
+        rv = send_packet(faddr, prev_ps.path.remote.addr,
+                         prev_ps.path.remote.addrlen, prev_ps.path.local.addr,
+                         prev_ps.path.local.addrlen, prev_pi, data, datalen,
+                         max_udp_payload_size);
+        if (rv == SHRPX_ERR_SEND_BLOCKED) {
+          on_send_blocked(faddr, prev_ps.path.remote, prev_ps.path.local,
+                          prev_pi, data, datalen, max_udp_payload_size);
+
+          ngtcp2_conn_update_pkt_tx_time(conn_, ts);
+          reset_idle_timer();
+
+          signal_write_upstream_addr(faddr);
+
+          return 0;
+        }
 
         reset_idle_timer();
       }
@@ -842,54 +869,99 @@ int Http3Upstream::write_streams() {
       prev_pi = pi;
     } else if (!ngtcp2_path_eq(&prev_ps.path, &ps.path) ||
                prev_pi.ecn != pi.ecn) {
-      send_packet(static_cast<UpstreamAddr *>(prev_ps.path.user_data),
-                  prev_ps.path.remote.addr, prev_ps.path.remote.addrlen,
-                  prev_ps.path.local.addr, prev_ps.path.local.addrlen, prev_pi,
-                  buf.data(), bufpos - buf.data() - nwrite,
-                  max_udp_payload_size);
+      auto faddr = static_cast<UpstreamAddr *>(prev_ps.path.user_data);
+      auto data = tx_.data.get();
+      auto datalen = bufpos - data - nwrite;
 
-      send_packet(static_cast<UpstreamAddr *>(ps.path.user_data),
-                  ps.path.remote.addr, ps.path.remote.addrlen,
-                  ps.path.local.addr, ps.path.local.addrlen, pi,
-                  bufpos - nwrite, nwrite, max_udp_payload_size);
+      rv = send_packet(faddr, prev_ps.path.remote.addr,
+                       prev_ps.path.remote.addrlen, prev_ps.path.local.addr,
+                       prev_ps.path.local.addrlen, prev_pi, data, datalen,
+                       max_udp_payload_size);
+      switch (rv) {
+      case SHRPX_ERR_SEND_BLOCKED:
+        on_send_blocked(faddr, prev_ps.path.remote, prev_ps.path.local, prev_pi,
+                        data, datalen, max_udp_payload_size);
+
+        on_send_blocked(static_cast<UpstreamAddr *>(ps.path.user_data),
+                        ps.path.remote, ps.path.local, pi, bufpos - nwrite,
+                        nwrite, max_udp_payload_size);
+
+        signal_write_upstream_addr(faddr);
+
+        break;
+      default: {
+        auto faddr = static_cast<UpstreamAddr *>(ps.path.user_data);
+        auto data = bufpos - nwrite;
+
+        rv = send_packet(faddr, ps.path.remote.addr, ps.path.remote.addrlen,
+                         ps.path.local.addr, ps.path.local.addrlen, pi, data,
+                         nwrite, max_udp_payload_size);
+        if (rv == SHRPX_ERR_SEND_BLOCKED) {
+          on_send_blocked(faddr, ps.path.remote, ps.path.local, pi, data,
+                          nwrite, max_udp_payload_size);
+        }
+
+        signal_write_upstream_addr(faddr);
+      }
+      }
 
       ngtcp2_conn_update_pkt_tx_time(conn_, ts);
       reset_idle_timer();
 
-      handler_->signal_write();
-
       return 0;
     }
 
     if (++pktcnt == max_pktcnt ||
         static_cast<size_t>(nwrite) < max_udp_payload_size) {
-      send_packet(static_cast<UpstreamAddr *>(ps.path.user_data),
-                  ps.path.remote.addr, ps.path.remote.addrlen,
-                  ps.path.local.addr, ps.path.local.addrlen, pi, buf.data(),
-                  bufpos - buf.data(), max_udp_payload_size);
+      auto faddr = static_cast<UpstreamAddr *>(ps.path.user_data);
+      auto data = tx_.data.get();
+      auto datalen = bufpos - data;
+
+      rv = send_packet(faddr, ps.path.remote.addr, ps.path.remote.addrlen,
+                       ps.path.local.addr, ps.path.local.addrlen, pi, data,
+                       datalen, max_udp_payload_size);
+      if (rv == SHRPX_ERR_SEND_BLOCKED) {
+        on_send_blocked(faddr, ps.path.remote, ps.path.local, pi, data, datalen,
+                        max_udp_payload_size);
+      }
 
       ngtcp2_conn_update_pkt_tx_time(conn_, ts);
       reset_idle_timer();
 
-      handler_->signal_write();
+      signal_write_upstream_addr(faddr);
 
       return 0;
     }
 #else  // !UDP_SEGMENT
-    send_packet(static_cast<UpstreamAddr *>(ps.path.user_data),
-                ps.path.remote.addr, ps.path.remote.addrlen, ps.path.local.addr,
-                ps.path.local.addrlen, pi, buf.data(), bufpos - buf.data(), 0);
+    auto faddr = static_cast<UpstreamAddr *>(ps.path.user_data);
+    auto data = tx_.data.get();
+    auto datalen = bufpos - data;
+
+    rv = send_packet(faddr, ps.path.remote.addr, ps.path.remote.addrlen,
+                     ps.path.local.addr, ps.path.local.addrlen, pi, data,
+                     datalen, 0);
+    if (rv == SHRPX_ERR_SEND_BLOCKED) {
+      on_send_blocked(faddr, ps.path.remote, ps.path.local, pi, data, datalen,
+                      0);
+
+      ngtcp2_conn_update_pkt_tx_time(conn_, ts);
+      reset_idle_timer();
+
+      signal_write_upstream_addr(faddr);
+
+      return 0;
+    }
 
     if (++pktcnt == max_pktcnt) {
       ngtcp2_conn_update_pkt_tx_time(conn_, ts);
       reset_idle_timer();
 
-      handler_->signal_write();
+      signal_write_upstream_addr(faddr);
 
       return 0;
     }
 
-    bufpos = buf.data();
+    bufpos = tx_.data.get();
 #endif // !UDP_SEGMENT
   }
 
@@ -1760,6 +1832,11 @@ int Http3Upstream::send_packet(const UpstreamAddr *faddr,
   case -EMSGSIZE:
     max_udp_payload_size_ = NGTCP2_MAX_UDP_PAYLOAD_SIZE;
     break;
+  case -EAGAIN:
+#if EAGAIN != EWOULDBLOCK
+  case -EWOULDBLOCK:
+#endif // EAGAIN != EWOULDBLOCK
+    return SHRPX_ERR_SEND_BLOCKED;
   default:
     break;
   }
@@ -1767,6 +1844,70 @@ int Http3Upstream::send_packet(const UpstreamAddr *faddr,
   return -1;
 }
 
+void Http3Upstream::on_send_blocked(const UpstreamAddr *faddr,
+                                    const ngtcp2_addr &remote_addr,
+                                    const ngtcp2_addr &local_addr,
+                                    const ngtcp2_pkt_info &pi,
+                                    const uint8_t *data, size_t datalen,
+                                    size_t max_udp_payload_size) {
+  assert(tx_.num_blocked || !tx_.send_blocked);
+  assert(tx_.num_blocked < 2);
+
+  tx_.send_blocked = true;
+
+  auto &p = tx_.blocked[tx_.num_blocked++];
+
+  memcpy(&p.local_addr.su, local_addr.addr, local_addr.addrlen);
+  memcpy(&p.remote_addr.su, remote_addr.addr, remote_addr.addrlen);
+
+  p.local_addr.len = local_addr.addrlen;
+  p.remote_addr.len = remote_addr.addrlen;
+  p.faddr = faddr;
+  p.pi = pi;
+  p.data = data;
+  p.datalen = datalen;
+  p.max_udp_payload_size = max_udp_payload_size;
+}
+
+int Http3Upstream::send_blocked_packet() {
+  int rv;
+
+  assert(tx_.send_blocked);
+
+  for (; tx_.num_blocked_sent < tx_.num_blocked; ++tx_.num_blocked_sent) {
+    auto &p = tx_.blocked[tx_.num_blocked_sent];
+
+    rv = send_packet(p.faddr, &p.remote_addr.su.sa, p.remote_addr.len,
+                     &p.local_addr.su.sa, p.local_addr.len, p.pi, p.data,
+                     p.datalen, p.max_udp_payload_size);
+    if (rv == SHRPX_ERR_SEND_BLOCKED) {
+      signal_write_upstream_addr(p.faddr);
+
+      return 0;
+    }
+  }
+
+  tx_.send_blocked = false;
+  tx_.num_blocked = 0;
+  tx_.num_blocked_sent = 0;
+
+  return 0;
+}
+
+void Http3Upstream::signal_write_upstream_addr(const UpstreamAddr *faddr) {
+  auto conn = handler_->get_connection();
+
+  if (faddr->fd != conn->wev.fd) {
+    if (ev_is_active(&conn->wev)) {
+      ev_io_stop(handler_->get_loop(), &conn->wev);
+    }
+
+    ev_io_set(&conn->wev, faddr->fd, EV_WRITE);
+  }
+
+  conn->wlimit.startw();
+}
+
 int Http3Upstream::handle_error() {
   if (ngtcp2_conn_is_in_closing_period(conn_)) {
     return -1;
diff --git a/src/shrpx_http3_upstream.h b/src/shrpx_http3_upstream.h
index e8c8dee4..3759ede2 100644
--- a/src/shrpx_http3_upstream.h
+++ b/src/shrpx_http3_upstream.h
@@ -157,6 +157,14 @@ public:
   void qlog_write(const void *data, size_t datalen, bool fin);
   int open_qlog_file(const StringRef &dir, const ngtcp2_cid &scid) const;
 
+  void on_send_blocked(const UpstreamAddr *faddr,
+                       const ngtcp2_addr &remote_addr,
+                       const ngtcp2_addr &local_addr, const ngtcp2_pkt_info &pi,
+                       const uint8_t *data, size_t datalen,
+                       size_t max_udp_payload_size);
+  int send_blocked_packet();
+  void signal_write_upstream_addr(const UpstreamAddr *faddr);
+
 private:
   ClientHandler *handler_;
   ev_timer timer_;
@@ -174,6 +182,23 @@ private:
   bool idle_close_;
   bool retry_close_;
   std::vector<uint8_t> conn_close_;
+
+  struct {
+    bool send_blocked;
+    size_t num_blocked;
+    size_t num_blocked_sent;
+    // blocked field is effective only when send_blocked is true.
+    struct {
+      const UpstreamAddr *faddr;
+      Address local_addr;
+      Address remote_addr;
+      ngtcp2_pkt_info pi;
+      const uint8_t *data;
+      size_t datalen;
+      size_t max_udp_payload_size;
+    } blocked[2];
+    std::unique_ptr<uint8_t[]> data;
+  } tx_;
 };
 
 } // namespace shrpx