3 files changed, 44 insertions, 6 deletions

diff --git a/src/event/quic/ngx_event_quic_connid.c b/src/event/quic/ngx_event_quic_connid.c
index a30e7ef35..4dbb2bbd6 100644
--- a/src/event/quic/ngx_event_quic_connid.c
+++ b/src/event/quic/ngx_event_quic_connid.c
@@ -325,6 +325,35 @@ ngx_quic_next_client_id(ngx_connection_t *c)
 }
 
 
+ngx_quic_client_id_t *
+ngx_quic_used_client_id(ngx_connection_t *c, ngx_quic_path_t *path)
+{
+    ngx_queue_t            *q;
+    ngx_quic_socket_t      *qsock;
+    ngx_quic_connection_t  *qc;
+
+    qc = ngx_quic_get_connection(c);
+
+    /* best guess: cid used by active path is good for us */
+    if (qc->socket->path == path) {
+        return qc->socket->cid;
+    }
+
+    for (q = ngx_queue_head(&qc->sockets);
+         q != ngx_queue_sentinel(&qc->sockets);
+         q = ngx_queue_next(q))
+    {
+        qsock = ngx_queue_data(q, ngx_quic_socket_t, queue);
+
+        if (qsock->path && qsock->path == path) {
+            return qsock->cid;
+        }
+    }
+
+    return NULL;
+}
+
+
 ngx_int_t
 ngx_quic_handle_retire_connection_id_frame(ngx_connection_t *c,
     ngx_quic_header_t *pkt, ngx_quic_retire_cid_frame_t *f)
diff --git a/src/event/quic/ngx_event_quic_connid.h b/src/event/quic/ngx_event_quic_connid.h
index fc7850a9c..a0552e9dd 100644
--- a/src/event/quic/ngx_event_quic_connid.h
+++ b/src/event/quic/ngx_event_quic_connid.h
@@ -23,6 +23,8 @@ ngx_int_t ngx_quic_create_server_id(ngx_connection_t *c, u_char *id);
 ngx_quic_client_id_t *ngx_quic_create_client_id(ngx_connection_t *c,
     ngx_str_t *id, uint64_t seqnum, u_char *token);
 ngx_quic_client_id_t *ngx_quic_next_client_id(ngx_connection_t *c);
+ngx_quic_client_id_t *ngx_quic_used_client_id(ngx_connection_t *c,
+    ngx_quic_path_t *path);
 void ngx_quic_unref_client_id(ngx_connection_t *c, ngx_quic_client_id_t *cid);
 
 #endif /* _NGX_EVENT_QUIC_CONNID_H_INCLUDED_ */
diff --git a/src/event/quic/ngx_event_quic_migration.c b/src/event/quic/ngx_event_quic_migration.c
index f1c923f83..74dc0113b 100644
--- a/src/event/quic/ngx_event_quic_migration.c
+++ b/src/event/quic/ngx_event_quic_migration.c
@@ -348,16 +348,23 @@ ngx_quic_update_paths(ngx_connection_t *c, ngx_quic_header_t *pkt)
         }
     }
 
+    /* prefer unused client IDs if available */
     cid = ngx_quic_next_client_id(c);
     if (cid == NULL) {
-        qc = ngx_quic_get_connection(c);
-        qc->error = NGX_QUIC_ERR_CONNECTION_ID_LIMIT_ERROR;
-        qc->error_reason = "no available client ids for new path";
 
-        ngx_log_error(NGX_LOG_ERR, c->log, 0,
-                      "no available client ids for new path");
+        /* try to reuse connection ID used on the same path */
+        cid = ngx_quic_used_client_id(c, path);
+        if (cid == NULL) {
 
-        return NGX_ERROR;
+            qc = ngx_quic_get_connection(c);
+            qc->error = NGX_QUIC_ERR_CONNECTION_ID_LIMIT_ERROR;
+            qc->error_reason = "no available client ids for new path";
+
+            ngx_log_error(NGX_LOG_ERR, c->log, 0,
+                          "no available client ids for new path");
+
+            return NGX_ERROR;
+        }
     }
 
     ngx_quic_connect(c, qsock, path, cid);