http: return more than data from server app

Server app could return headers in front of body/data buffer.
Offers apis for building and serialization of headers section.
HTTP layer now only add Date, Server and Content-Lengths headers,
rest is up to app. Well known header names are predefined.

Type: improvement

Change-Id: If778bdfc9acf6b0d11a48f0a745a3a56c96c2436
Signed-off-by: Matus Fabian <matfabia@cisco.com>

5 files changed, 351 insertions, 97 deletions

diff --git a/src/plugins/http/http.c b/src/plugins/http/http.c
index c5e2cc1df7d..db101539ac4 100644
--- a/src/plugins/http/http.c
+++ b/src/plugins/http/http.c
@@ -36,12 +36,6 @@ const char *http_status_code_str[] = {
 #undef _
 };
 
-const char *http_content_type_str[] = {
-#define _(s, ext, str) str,
-  foreach_http_content_type
-#undef _
-};
-
 const http_buffer_type_t msg_to_buf_type[] = {
   [HTTP_MSG_DATA_INLINE] = HTTP_BUFFER_FIFO,
   [HTTP_MSG_DATA_PTR] = HTTP_BUFFER_PTR,
@@ -374,22 +368,17 @@ http_ts_reset_callback (session_t *ts)
  */
 static const char *http_error_template = "HTTP/1.1 %s\r\n"
 					 "Date: %U GMT\r\n"
-					 "Content-Type: text/html\r\n"
 					 "Connection: close\r\n"
-					 "Pragma: no-cache\r\n"
 					 "Content-Length: 0\r\n\r\n";
 
-static const char *http_redirect_template = "HTTP/1.1 %s\r\n";
-
 /**
  * http response boilerplate
  */
 static const char *http_response_template = "HTTP/1.1 %s\r\n"
 					    "Date: %U GMT\r\n"
-					    "Expires: %U GMT\r\n"
 					    "Server: %v\r\n"
-					    "Content-Type: %s\r\n"
-					    "Content-Length: %lu\r\n\r\n";
+					    "Content-Length: %u\r\n"
+					    "%s";
 
 static const char *http_request_template = "GET %s HTTP/1.1\r\n"
 					   "User-Agent: %v\r\n"
@@ -1004,49 +993,54 @@ http_state_wait_app_reply (http_conn_t *hc, transport_send_params_t *sp)
       goto error;
     }
 
-  http_buffer_init (&hc->tx_buf, msg_to_buf_type[msg.data.type], as->tx_fifo,
-		    msg.data.len);
+  if (msg.code >= HTTP_N_STATUS)
+    {
+      clib_warning ("unsupported status code: %d", msg.code);
+      return HTTP_SM_ERROR;
+    }
 
   /*
-   * Add headers. For now:
+   * Add "protocol layer" headers:
    * - current time
-   * - expiration time
    * - server name
-   * - content type
    * - data length
    */
   now = clib_timebase_now (&hm->timebase);
-
-  switch (msg.code)
-    {
-    case HTTP_STATUS_NOT_FOUND:
-    case HTTP_STATUS_METHOD_NOT_ALLOWED:
-    case HTTP_STATUS_BAD_REQUEST:
-    case HTTP_STATUS_INTERNAL_ERROR:
-    case HTTP_STATUS_FORBIDDEN:
-    case HTTP_STATUS_OK:
-      header =
-	format (0, http_response_template, http_status_code_str[msg.code],
-		/* Date */
-		format_clib_timebase_time, now,
-		/* Expires */
-		format_clib_timebase_time, now + 600.0,
-		/* Server */
-		hc->app_name,
-		/* Content type */
-		http_content_type_str[msg.content_type],
-		/* Length */
-		msg.data.len);
-      break;
-    case HTTP_STATUS_MOVED:
-      header =
-	format (0, http_redirect_template, http_status_code_str[msg.code]);
-      /* Location: http(s)://new-place already queued up as data */
-      break;
-    default:
-      clib_warning ("unsupported status code: %d", msg.code);
-      return HTTP_SM_ERROR;
+  header = format (0, http_response_template, http_status_code_str[msg.code],
+		   /* Date */
+		   format_clib_timebase_time, now,
+		   /* Server */
+		   hc->app_name,
+		   /* Length */
+		   msg.data.body_len,
+		   /* Any headers from app? */
+		   msg.data.headers_len ? "" : "\r\n");
+
+  /* Add headers from app (if any) */
+  if (msg.data.headers_len)
+    {
+      HTTP_DBG (0, "got headers from app, len %d", msg.data.headers_len);
+      if (msg.data.type == HTTP_MSG_DATA_PTR)
+	{
+	  uword app_headers_ptr;
+	  rv = svm_fifo_dequeue (as->tx_fifo, sizeof (app_headers_ptr),
+				 (u8 *) &app_headers_ptr);
+	  ASSERT (rv == sizeof (app_headers_ptr));
+	  vec_append (header, uword_to_pointer (app_headers_ptr, u8 *));
+	}
+      else
+	{
+	  u32 orig_len = vec_len (header);
+	  vec_resize (header, msg.data.headers_len);
+	  u8 *p = header + orig_len;
+	  rv = svm_fifo_dequeue (as->tx_fifo, msg.data.headers_len, p);
+	  ASSERT (rv == msg.data.headers_len);
+	}
     }
+  HTTP_DBG (0, "%v", header);
+
+  http_buffer_init (&hc->tx_buf, msg_to_buf_type[msg.data.type], as->tx_fifo,
+		    msg.data.body_len);
 
   offset = http_send_data (hc, header, vec_len (header), 0);
   if (offset != vec_len (header))
diff --git a/src/plugins/http/http.h b/src/plugins/http/http.h
index debdebcf050..07d5472346e 100644
--- a/src/plugins/http/http.h
+++ b/src/plugins/http/http.h
@@ -51,6 +51,14 @@ typedef struct http_conn_id_
 STATIC_ASSERT (sizeof (http_conn_id_t) <= TRANSPORT_CONN_ID_LEN,
 	       "ctx id must be less than TRANSPORT_CONN_ID_LEN");
 
+typedef struct
+{
+  char *base;
+  uword len;
+} http_token_t;
+
+#define http_token_lit(s) (s), sizeof (s) - 1
+
 typedef enum http_conn_state_
 {
   HTTP_CONN_STATE_LISTEN,
@@ -235,50 +243,100 @@ typedef enum http_status_code_
     HTTP_N_STATUS
 } http_status_code_t;
 
-#define HTTP_HEADER_ACCEPT		      "Accept"
-#define HTTP_HEADER_ACCEPT_CHARSET	      "Accept-Charset"
-#define HTTP_HEADER_ACCEPT_ENCODING	      "Accept-Encoding"
-#define HTTP_HEADER_ACCEPT_LANGUAGE	      "Accept-Language"
-#define HTTP_HEADER_ACCEPT_RANGES	      "Accept-Ranges"
-#define HTTP_HEADER_ALLOW		      "Allow"
-#define HTTP_HEADER_AUTHENTICATION_INFO	      "Authentication-Info"
-#define HTTP_HEADER_AUTHORIZATION	      "Authorization"
-#define HTTP_HEADER_CLOSE		      "Close"
-#define HTTP_HEADER_CONNECTION		      "Connection"
-#define HTTP_HEADER_CONTENT_ENCODING	      "Content-Encoding"
-#define HTTP_HEADER_CONTENT_LANGUAGE	      "Content-Language"
-#define HTTP_HEADER_CONTENT_LENGTH	      "Content-Length"
-#define HTTP_HEADER_CONTENT_LOCATION	      "Content-Location"
-#define HTTP_HEADER_CONTENT_RANGE	      "Content-Range"
-#define HTTP_HEADER_CONTENT_TYPE	      "Content-Type"
-#define HTTP_HEADER_DATE		      "Date"
-#define HTTP_HEADER_ETAG		      "ETag"
-#define HTTP_HEADER_EXPECT		      "Expect"
-#define HTTP_HEADER_FROM		      "From"
-#define HTTP_HEADER_HOST		      "Host"
-#define HTTP_HEADER_IF_MATCH		      "If-Match"
-#define HTTP_HEADER_IF_MODIFIED_SINCE	      "If-Modified-Since"
-#define HTTP_HEADER_IF_NONE_MATCH	      "If-None-Match"
-#define HTTP_HEADER_IF_RANGE		      "If-Range"
-#define HTTP_HEADER_IF_UNMODIFIED_SINCE	      "If-Unmodified-Since"
-#define HTTP_HEADER_LAST_MODIFIED	      "Last-Modified"
-#define HTTP_HEADER_LOCATION		      "Location"
-#define HTTP_HEADER_MAX_FORWARDS	      "Max-Forwards"
-#define HTTP_HEADER_PROXY_AUTHENTICATE	      "Proxy-Authenticate"
-#define HTTP_HEADER_PROXY_AUTHENTICATION_INFO "Proxy-Authentication-Info"
-#define HTTP_HEADER_PROXY_AUTHORIZATION	      "Proxy-Authorization"
-#define HTTP_HEADER_RANGE		      "Range"
-#define HTTP_HEADER_REFERER		      "Referer"
-#define HTTP_HEADER_RETRY_AFTER		      "Retry-After"
-#define HTTP_HEADER_SERVER		      "Server"
-#define HTTP_HEADER_TE			      "TE"
-#define HTTP_HEADER_TRAILER		      "Trailer"
-#define HTTP_HEADER_TRANSFER_ENCODING	      "Transfer-Encoding"
-#define HTTP_HEADER_UPGRADE		      "Upgrade"
-#define HTTP_HEADER_USER_AGENT		      "User-Agent"
-#define HTTP_HEADER_VARY		      "Vary"
-#define HTTP_HEADER_VIA			      "Via"
-#define HTTP_HEADER_WWW_AUTHENTICATE	      "WWW-Authenticate"
+#define foreach_http_header_name                                              \
+  _ (ACCEPT, "Accept")                                                        \
+  _ (ACCEPT_CHARSET, "Accept-Charset")                                        \
+  _ (ACCEPT_ENCODING, "Accept-Encoding")                                      \
+  _ (ACCEPT_LANGUAGE, "Accept-Language")                                      \
+  _ (ACCEPT_RANGES, "Accept-Ranges")                                          \
+  _ (ACCESS_CONTROL_ALLOW_CREDENTIALS, "Access-Control-Allow-Credentials")    \
+  _ (ACCESS_CONTROL_ALLOW_HEADERS, "Access-Control-Allow-Headers")            \
+  _ (ACCESS_CONTROL_ALLOW_METHODS, "Access-Control-Allow-Methods")            \
+  _ (ACCESS_CONTROL_ALLOW_ORIGIN, "Access-Control-Allow-Origin")              \
+  _ (ACCESS_CONTROL_EXPOSE_HEADERS, "Access-Control-Expose-Headers")          \
+  _ (ACCESS_CONTROL_MAX_AGE, "Access-Control-Max-Age")                        \
+  _ (ACCESS_CONTROL_REQUEST_HEADERS, "Access-Control-Request-Headers")        \
+  _ (ACCESS_CONTROL_REQUEST_METHOD, "Access-Control-Request-Method")          \
+  _ (AGE, "Age")                                                              \
+  _ (ALLOW, "Allow")                                                          \
+  _ (ALPN, "ALPN")                                                            \
+  _ (ALT_SVC, "Alt-Svc")                                                      \
+  _ (ALT_USED, "Alt-Used")                                                    \
+  _ (ALTERNATES, "Alternates")                                                \
+  _ (AUTHENTICATION_CONTROL, "Authentication-Control")                        \
+  _ (AUTHENTICATION_INFO, "Authentication-Info")                              \
+  _ (AUTHORIZATION, "Authorization")                                          \
+  _ (CACHE_CONTROL, "Cache-Control")                                          \
+  _ (CACHE_STATUS, "Cache-Status")                                            \
+  _ (CAPSULE_PROTOCOL, "Capsule-Protocol")                                    \
+  _ (CDN_CACHE_CONTROL, "CDN-Cache-Control")                                  \
+  _ (CDN_LOOP, "CDN-Loop")                                                    \
+  _ (CLIENT_CERT, "Client-Cert")                                              \
+  _ (CLIENT_CERT_CHAIN, "Client-Cert-Chain")                                  \
+  _ (CLOSE, "Close")                                                          \
+  _ (CONNECTION, "Connection")                                                \
+  _ (CONTENT_DIGEST, "Content-Digest")                                        \
+  _ (CONTENT_DISPOSITION, "Content-Disposition")                              \
+  _ (CONTENT_ENCODING, "Content-Encoding")                                    \
+  _ (CONTENT_LANGUAGE, "Content-Language")                                    \
+  _ (CONTENT_LENGTH, "Content-Length")                                        \
+  _ (CONTENT_LOCATION, "Content-Location")                                    \
+  _ (CONTENT_RANGE, "Content-Range")                                          \
+  _ (CONTENT_TYPE, "Content-Type")                                            \
+  _ (COOKIE, "Cookie")                                                        \
+  _ (DATE, "Date")                                                            \
+  _ (DIGEST, "Digest")                                                        \
+  _ (DPOP, "DPoP")                                                            \
+  _ (DPOP_NONCE, "DPoP-Nonce")                                                \
+  _ (EARLY_DATA, "Early-Data")                                                \
+  _ (ETAG, "ETag")                                                            \
+  _ (EXPECT, "Expect")                                                        \
+  _ (EXPIRES, "Expires")                                                      \
+  _ (FORWARDED, "Forwarded")                                                  \
+  _ (FROM, "From")                                                            \
+  _ (HOST, "Host")                                                            \
+  _ (IF_MATCH, "If-Match")                                                    \
+  _ (IF_MODIFIED_SINCE, "If-Modified-Since")                                  \
+  _ (IF_NONE_MATCH, "If-None-Match")                                          \
+  _ (IF_RANGE, "If-Range")                                                    \
+  _ (IF_UNMODIFIED_SINCE, "If-Unmodified-Since")                              \
+  _ (KEEP_ALIVE, "Keep-Alive")                                                \
+  _ (LAST_MODIFIED, "Last-Modified")                                          \
+  _ (LINK, "Link")                                                            \
+  _ (LOCATION, "Location")                                                    \
+  _ (MAX_FORWARDS, "Max-Forwards")                                            \
+  _ (ORIGIN, "Origin")                                                        \
+  _ (PRIORITY, "Priority")                                                    \
+  _ (PROXY_AUTHENTICATE, "Proxy-Authenticate")                                \
+  _ (PROXY_AUTHENTICATION_INFO, "Proxy-Authentication-Info")                  \
+  _ (PROXY_AUTHORIZATION, "Proxy-Authorization")                              \
+  _ (PROXY_STATUS, "Proxy-Status")                                            \
+  _ (RANGE, "Range")                                                          \
+  _ (REFERER, "Referer")                                                      \
+  _ (REPR_DIGEST, "Repr-Digest")                                              \
+  _ (SET_COOKIE, "Set-Cookie")                                                \
+  _ (SIGNATURE, "Signature")                                                  \
+  _ (SIGNATURE_INPUT, "Signature-Input")                                      \
+  _ (STRICT_TRANSPORT_SECURITY, "Strict-Transport-Security")                  \
+  _ (RETRY_AFTER, "Retry-After")                                              \
+  _ (SERVER, "Server")                                                        \
+  _ (TE, "TE")                                                                \
+  _ (TRAILER, "Trailer")                                                      \
+  _ (TRANSFER_ENCODING, "Transfer-Encoding")                                  \
+  _ (UPGRADE, "Upgrade")                                                      \
+  _ (USER_AGENT, "User-Agent")                                                \
+  _ (VARY, "Vary")                                                            \
+  _ (VIA, "Via")                                                              \
+  _ (WANT_CONTENT_DIGEST, "Want-Content-Digest")                              \
+  _ (WANT_REPR_DIGEST, "Want-Repr-Digest")                                    \
+  _ (WWW_AUTHENTICATE, "WWW-Authenticate")
+
+typedef enum http_header_name_
+{
+#define _(sym, str) HTTP_HEADER_##sym,
+  foreach_http_header_name
+#undef _
+} http_header_name_t;
 
 typedef enum http_msg_data_type_
 {
@@ -669,11 +727,17 @@ typedef struct
 {
   u8 *name;
   u8 *value;
+} http_header_ht_t;
+
+typedef struct
+{
+  http_token_t name;
+  http_token_t value;
 } http_header_t;
 
 typedef struct
 {
-  http_header_t *headers;
+  http_header_ht_t *headers;
   uword *value_by_name;
 } http_header_table_t;
 
@@ -685,7 +749,7 @@ typedef struct
 always_inline void
 http_free_header_table (http_header_table_t *ht)
 {
-  http_header_t *header;
+  http_header_ht_t *header;
   vec_foreach (header, ht->headers)
     {
       vec_free (header->name);
@@ -713,7 +777,7 @@ http_parse_headers (u8 *headers, http_header_table_t **header_table)
   u8 *pos, *end, *name_start, *value_start, *name;
   u32 name_len, value_len;
   int rv;
-  http_header_t *header;
+  http_header_ht_t *header;
   http_header_table_t *ht;
   uword *p;
 
@@ -779,7 +843,7 @@ always_inline const char *
 http_get_header (http_header_table_t *header_table, const char *name)
 {
   uword *p;
-  http_header_t *header;
+  http_header_ht_t *header;
 
   p = hash_get_mem (header_table->value_by_name, name);
   if (p)
@@ -791,6 +855,67 @@ http_get_header (http_header_table_t *header_table, const char *name)
   return 0;
 }
 
+/**
+ * Add header to the list.
+ *
+ * @param headers Header list.
+ * @param name Pointer to header's name buffer.
+ * @param name_len Length of the name.
+ * @param value Pointer to header's value buffer.
+ * @param value_len Length of the value.
+ *
+ * @note Headers added at protocol layer: Date, Server, Content-Length
+ */
+always_inline void
+http_add_header (http_header_t **headers, const char *name, uword name_len,
+		 const char *value, uword value_len)
+{
+  http_header_t *header;
+  vec_add2 (*headers, header, 1);
+  header->name.base = (char *) name;
+  header->name.len = name_len;
+  header->value.base = (char *) value;
+  header->value.len = value_len;
+}
+
+/**
+ * Serialize the header list.
+ *
+ * @param headers Header list to serialize.
+ *
+ * @return New vector with serialized headers.
+ *
+ * The caller is always responsible to free the returned vector.
+ */
+always_inline u8 *
+http_serialize_headers (http_header_t *headers)
+{
+  u8 *headers_buf = 0, *dst;
+  u32 headers_buf_len = 2;
+  http_header_t *header;
+
+  vec_foreach (header, headers)
+    headers_buf_len += header->name.len + header->value.len + 4;
+
+  vec_validate (headers_buf, headers_buf_len - 1);
+  dst = headers_buf;
+
+  vec_foreach (header, headers)
+    {
+      clib_memcpy (dst, header->name.base, header->name.len);
+      dst += header->name.len;
+      *dst++ = ':';
+      *dst++ = ' ';
+      clib_memcpy (dst, header->value.base, header->value.len);
+      dst += header->value.len;
+      *dst++ = '\r';
+      *dst++ = '\n';
+    }
+  *dst++ = '\r';
+  *dst = '\n';
+  return headers_buf;
+}
+
 #endif /* SRC_PLUGINS_HTTP_HTTP_H_ */
 
 /*
diff --git a/src/plugins/http/http_content_types.h b/src/plugins/http/http_content_types.h
new file mode 100644
index 00000000000..ddc02566db7
--- /dev/null
+++ b/src/plugins/http/http_content_types.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: Apache-2.0
+ * Copyright(c) 2024 Cisco Systems, Inc.
+ */
+
+#ifndef SRC_PLUGINS_HTTP_HTTP_CONTENT_TYPES_H_
+#define SRC_PLUGINS_HTTP_HTTP_CONTENT_TYPES_H_
+
+#include <http/http.h>
+
+static http_token_t http_content_types[] = {
+#define _(s, ext, str) { http_token_lit (str) },
+  foreach_http_content_type
+#undef _
+};
+
+#define http_content_type_token(e)                                            \
+  http_content_types[e].base, http_content_types[e].len
+
+#endif /* SRC_PLUGINS_HTTP_HTTP_CONTENT_TYPES_H_ */
diff --git a/src/plugins/http/http_header_names.h b/src/plugins/http/http_header_names.h
new file mode 100644
index 00000000000..99acac786db
--- /dev/null
+++ b/src/plugins/http/http_header_names.h
@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: Apache-2.0
+ * Copyright(c) 2024 Cisco Systems, Inc.
+ */
+
+#ifndef SRC_PLUGINS_HTTP_HTTP_HEADER_NAMES_H_
+#define SRC_PLUGINS_HTTP_HTTP_HEADER_NAMES_H_
+
+#include <http/http.h>
+
+static http_token_t http_header_names[] = {
+#define _(sym, str) { http_token_lit (str) },
+  foreach_http_header_name
+#undef _
+};
+
+#define http_header_name_token(e)                                             \
+  http_header_names[e].base, http_header_names[e].len
+
+#define http_header_name_str(e) http_header_names[e].base
+
+#endif /* SRC_PLUGINS_HTTP_HTTP_HEADER_NAMES_H_ */
diff --git a/src/plugins/http/http_plugin.rst b/src/plugins/http/http_plugin.rst
index c4c4d2c8234..2f7a58ef9bf 100644
--- a/src/plugins/http/http_plugin.rst
+++ b/src/plugins/http/http_plugin.rst
@@ -121,6 +121,7 @@ Following example shows how to parse headers:
 
 .. code-block:: C
 
+  #include <http/http_header_names.h>
   if (msg.data.headers_len)
     {
       u8 *headers = 0;
@@ -134,7 +135,7 @@ Following example shows how to parse headers:
           /* your error handling */
         }
       /* get Accept header */
-      const char *accept_value = http_get_header (ht, HTTP_HEADER_ACCEPT);
+      const char *accept_value = http_get_header (ht, http_header_name_str (HTTP_HEADER_ACCEPT));
       if (accept_value)
         {
           /* do something interesting */
@@ -154,3 +155,97 @@ Finally application reads body:
       rv = svm_fifo_peek (ts->rx_fifo, msg.data.body_offset, msg.data.body_len, body);
       ASSERT (rv == msg.data.body_len);
     }
+
+Sending data
+""""""""""""""
+
+When server application sends response back to HTTP layer it starts with message metadata, followed by optional serialized headers and finally body (if any).
+
+Application should set following items:
+
+* Status code
+* target form
+* header section offset and length
+* body offset and length
+
+Application could pass headers back to HTTP layer. Header list is created dynamically as vector of ``http_header_t``,
+where we store only pointers to buffers (zero copy).
+Well known header names are predefined.
+The list is serialized just before you send buffer to HTTP layer.
+
+.. note::
+    Following headers are added at protocol layer and **MUST NOT** be set by application: Date, Server, Content-Length
+
+Following example shows how to create headers section:
+
+.. code-block:: C
+
+  #include <http/http.h>
+  #include <http/http_header_names.h>
+  #include <http/http_content_types.h>
+  http_header_t *resp_headers = 0;
+  u8 *headers_buf = 0;
+  http_add_header (resp_headers,
+		   http_header_name_token (HTTP_HEADER_CONTENT_TYPE),
+		   http_content_type_token (HTTP_CONTENT_TEXT_HTML));
+  http_add_header (resp_headers,
+		   http_header_name_token (HTTP_HEADER_CACHE_CONTROL),
+		   http_token_lit ("max-age=600"));
+  http_add_header (&hs->resp_headers,
+		   http_header_name_token (HTTP_HEADER_LOCATION),
+		   (const char *) redirect, vec_len (redirect));
+  headers_buf = http_serialize_headers (hs->resp_headers);
+
+The example below show how to create and send response HTTP message metadata:
+
+.. code-block:: C
+
+  http_msg_t msg;
+  msg.type = HTTP_MSG_REPLY;
+  msg.code = HTTP_STATUS_MOVED
+  msg.data.headers_offset = 0;
+  msg.data.headers_len = vec_len (headers_buf);
+  msg.data.type = HTTP_MSG_DATA_INLINE;
+  msg.data.body_len = vec_len (tx_buf);
+  msg.data.body_offset = msg.data.headers_len;
+  msg.data.len = msg.data.body_len + msg.data.headers_len;
+  ts = session_get (hs->vpp_session_index, hs->thread_index);
+  rv = svm_fifo_enqueue (ts->tx_fifo, sizeof (msg), (u8 *) &msg);
+  ASSERT (rv == sizeof (msg));
+
+Next you will send your serialized headers:
+
+.. code-block:: C
+
+  rv = svm_fifo_enqueue (ts->tx_fifo, vec_len (headers_buf), headers_buf);
+  ASSERT (rv == msg.data.headers_len);
+  vec_free (headers_buf);
+
+Finally application sends response body:
+
+.. code-block:: C
+
+  rv = svm_fifo_enqueue (ts->tx_fifo, vec_len (tx_buf), tx_buf);
+  if (rv != vec_len (hs->tx_buf))
+    {
+      hs->tx_offset = rv;
+      svm_fifo_add_want_deq_ntf (ts->tx_fifo, SVM_FIFO_WANT_DEQ_NOTIF);
+    }
+  else
+    {
+      vec_free (tx_buf);
+    }
+  if (svm_fifo_set_event (ts->tx_fifo))
+    session_send_io_evt_to_thread (ts->tx_fifo, SESSION_IO_EVT_TX);
+
+Example above shows how to send body data by copy, alternatively you could pass it as pointer:
+
+.. code-block:: C
+
+  msg.data.type = HTTP_MSG_DATA_PTR;
+  /* code omitted for brevity */
+  uword data = pointer_to_uword (tx_buf);
+  rv = svm_fifo_enqueue (ts->tx_fifo, sizeof (data), (u8 *) &data);
+  ASSERT (rv == sizeof (data));
+
+In this case you need to free data when you receive next request or when session is closed.