The most notable point in nghttp2 library architecture is it does not perform any I/O. nghttp2 only performs HTTP/2 protocol stuff based on input byte strings. It will calls callback functions set by applications while processing input. The output of nghttp2 is just byte string. An application is responsible to send these output to the remote peer. The callback functions may be called while producing output.
Not doing I/O makes embedding nghttp2 library in the existing code base very easy. Usually, the existing applications have its own I/O event loops. It is very hard to use nghttp2 in that situation if nghttp2 does its own I/O. It also makes light weight language wrapper for nghttp2 easy with the same reason. The down side is that an application author has to write more code to write complete application using nghttp2. This is especially true for simple "toy" application. For the real applications, however, this is not the case. This is because you probably want to support HTTP/1 which nghttp2 does not provide, and to do that, you will need to write your own HTTP/1 stack or use existing third-party library, and bind them together with nghttp2 and I/O event loop. In this point, not performing I/O in nghttp2 has more point than doing it.
The primary object that an application uses is :type:`nghttp2_session` object, which is opaque struct and its details are hidden in order to ensure the upgrading its internal architecture without breaking the backward compatibility. An application can set callbacks to :type:`nghttp2_session` object through the dedicated object and functions, and it also interacts with it via many API function calls.
An application can create as many :type:`nghttp2_session` object as it wants. But single :type:`nghttp2_session` object must be used by a single thread at the same time. This is not so hard to enforce since most event-based architecture applicatons use is single thread per core, and handling one connection I/O is done by single thread.
To feed input to :type:`nghttp2_session` object, one can use nghttp2_session_recv() or nghttp2_session_mem_recv() functions. They behave similarly, and the difference is that nghttp2_session_recv() will use :type:`nghttp2_read_callback` to get input. On the other hand, nghttp2_session_mem_recv() will take input as its parameter. If in doubt, use nghttp2_session_mem_recv() since it is simpler, and could be faster since it avoids calling callback function.
To get output from :type:`nghttp2_session` object, one can use nghttp2_session_send() or nghttp2_session_mem_send(). The difference between them is that the former uses :type:`nghttp2_send_callback` to pass output to an application. On the other hand, the latter returns the output to the caller. If in doubt, use nghttp2_session_mem_send() since it is simpler. But nghttp2_session_send() might be easier to use if the output buffer an application has is fixed sized.
In general, an application should call nghttp2_session_mem_send() when it gets input from underlying connection. Since there is great chance to get something pushed into transmission queue while the call of nghttp2_session_mem_send(), it is recommended to call nghttp2_session_mem_recv() after nghttp2_session_mem_send().
There is a question when we are safe to close HTTP/2 session without waiting for the closure of underlying connection. We offer 2 API calls for this: nghttp2_session_want_read() and nghttp2_session_want_write(). If they both return 0, application can destroy :type:`nghttp2_session`, and then close the underlying connection. But make sure that the buffered output has been transmitted to the peer before closing the connection when nghttp2_session_mem_send() is used, since nghttp2_session_want_write() does not take into account the transmission of the buffered data outside of :type:`nghttp2_session`.
To use the public APIs, include nghttp2/nghttp2.h:
#include <nghttp2/nghttp2.h>
The header files are also available online: :doc:`nghttp2.h` and :doc:`nghttp2ver.h`.
Do not call nghttp2_session_send(), nghttp2_session_mem_send(), nghttp2_session_recv() or nghttp2_session_mem_recv() from the nghttp2 callback functions directly or indirectly. It will lead to the crash. You can submit requests or frames in the callbacks then call these functions outside the callbacks.
nghttp2_session_send() and nghttp2_session_mem_send() send first 24 bytes of client magic string (MAGIC) (:macro:`NGHTTP2_CLIENT_MAGIC`) on client configuration. The applications are responsible to send SETTINGS frame as part of connection preface using nghttp2_submit_settings(). Similarly, nghttp2_session_recv() and nghttp2_session_mem_recv() consume MAGIC on server configuration unless nghttp2_option_set_no_recv_client_magic() is used with nonzero option value.
By default, nghttp2 library checks HTTP messaging rules described in HTTP/2 specification, section 8. Everything described in that section is not validated however. We briefly describe what the library does in this area. In the following description, without loss of generality we omit CONTINUATION frame since they must follow HEADERS frame and are processed atomically. In other words, they are just one big HEADERS frame. To disable these validations, use nghttp2_option_set_no_http_messaging().
For HTTP request, including those carried by PUSH_PROMISE, HTTP message starts with one HEADERS frame containing request headers. It is followed by zero or more DATA frames containing request body, which is followed by zero or one HEADERS containing trailer headers. The request headers must include ":scheme", ":method" and ":path" pseudo header fields unless ":method" is not "CONNECT". ":authority" is optional, but nghttp2 requires either ":authority" or "Host" header field must be present. If ":method" is "CONNECT", the request headers must include ":method" and ":authority" and must omit ":scheme" and ":path".
For HTTP response, HTTP message starts with zero or more HEADERS frames containing non-final response (status code 1xx). They are followed by one HEADERS frame containing final response headers (non-1xx). It is followed by zero or more DATA frames containing response body, which is followed by zero or one HEADERS containing trailer headers. The non-final and final response headers must contain ":status" pseudo header field containing 3 digits only.
All request and response headers must include exactly one valid value for each pseudo header field. Additionally nghttp2 requires all request headers must not include more than one "Host" header field.
HTTP/2 prohibits connection-specific header fields. The following header fields must not appear: "Connection", "Keep-Alive", "Proxy-Connection", "Transfer-Encoding" and "Upgrade". Additionally, "TE" header field must not include any value other than "trailers".
Each header field name and value must obey the field-name and field-value production rules described in RFC 7230, section 3.2.. Additionally, all field name must be lower cased. While the pseudo header fields must satisfy these rules, we just ignore illegal regular headers (this means that these header fields are not passed to application callback). This is because these illegal header fields are floating around in existing internet and resetting stream just because of this may break many web sites. This is especially true if we forward to or translate from HTTP/1 traffic.
For "http" or "https" URIs, ":path" pseudo header fields must start with "/". The only exception is OPTIONS request, in that case, "*" is allowed in ":path" pseudo header field to represent system-wide OPTIONS request.
With the above validations, nghttp2 library guarantees that header field name passed to nghttp2_on_header_callback() is not empty. Also required pseudo headers are all present and not empty.
nghttp2 enforces "Content-Length" validation as well. All request or response headers must not contain more than one "Content-Length" header field. If "Content-Length" header field is present, it must be parsed as 64 bit signed integer. The sum of data length in the following DATA frames must match with the number in "Content-Length" header field if it is present (this does not include padding bytes).
Any deviation results in stream error of type PROTOCOL_ERROR. If error is found in PUSH_PROMISE frame, stream error is raised against promised stream.
As of nghttp2 v1.8.0, we have added HTTP/2 non-critical extension framework, which lets application send and receive user defined custom HTTP/2 non-critical extension frames. nghttp2 also offers built-in functionality to send and receive official HTTP/2 extension frames (e.g., ALTSVC frame). For these built-in handler, refer to the next section.
To send extension frame, use nghttp2_submit_extension(), and implement :type:`nghttp2_pack_extension_callback`. The callback implements how to encode data into wire format. The callback must be set to :type:`nghttp2_session_callbacks` using nghttp2_session_callbacks_set_pack_extension_callback().
For example, we will illustrate how to send ALTSVC frame.
typedef struct {
const char *origin;
const char *field;
} alt_svc;
ssize_t pack_extension_callback(nghttp2_session *session, uint8_t *buf,
size_t len, const nghttp2_frame *frame,
void *user_data) {
const alt_svc *altsvc = (const alt_svc *)frame->ext.payload;
size_t originlen = strlen(altsvc->origin);
size_t fieldlen = strlen(altsvc->field);
uint8_t *p;
if (len < 2 + originlen + fieldlen || originlen > 0xffff) {
return NGHTTP2_ERR_CANCEL;
}
p = buf;
*p++ = originlen >> 8;
*p++ = originlen & 0xff;
memcpy(p, altsvc->origin, originlen);
p += originlen;
memcpy(p, altsvc->field, fieldlen);
p += fieldlen;
return p - buf;
}This implements :type:`nghttp2_pack_extension_callback`. We have to set this callback to :type:`nghttp2_session_callbacks`:
nghttp2_session_callbacks_set_pack_extension_callback(
callbacks, pack_extension_callback);To send ALTSVC frame, call nghttp2_submit_extension():
static const alt_svc altsvc = {"example.com", "h2=\":8000\""};
nghttp2_submit_extension(session, 0xa, NGHTTP2_FLAG_NONE, 0,
(void *)&altsvc);Notice that ALTSVC is use frame type 0xa.
To receive extension frames, implement 2 callbacks: :type:`nghttp2_unpack_extension_callback` and :type:`nghttp2_on_extension_chunk_recv_callback`. :type:`nghttp2_unpack_extension_callback` implements the way how to decode wire format. :type:`nghttp2_on_extension_chunk_recv_callback` implements how to buffer the incoming extension payload. These callbacks must be set using nghttp2_session_callbacks_set_unpack_extension_callback() and nghttp2_session_callbacks_set_on_extension_chunk_recv_callback() respectively. The application also must tell the library which extension frame type it is willing to receive using nghttp2_option_set_user_recv_extension_type(). Note that the application has to create :type:`nghttp2_option` object for that purpose, and initialize session with it.
We use ALTSVC again to illustrate how to receive extension frames. We
use different alt_svc struct than the previous one.
First implement 2 callbacks. We store incoming ALTSVC payload to
global variable altsvc_buffer. Don't do this in production code
since this is not thread safe:
typedef struct {
const uint8_t *origin;
size_t originlen;
const uint8_t *field;
size_t fieldlen;
} alt_svc;
/* buffers incoming ALTSVC payload */
uint8_t altsvc_buffer[4096];
/* The length of byte written to altsvc_buffer */
size_t altsvc_bufferlen = 0;
int on_extension_chunk_recv_callback(nghttp2_session *session,
const nghttp2_frame_hd *hd,
const uint8_t *data, size_t len,
void *user_data) {
if (sizeof(altsvc_buffer) < altsvc_bufferlen + len) {
altsvc_bufferlen = 0;
return NGHTTP2_ERR_CANCEL;
}
memcpy(altsvc_buffer + altsvc_bufferlen, data, len);
altsvc_bufferlen += len;
return 0;
}
int unpack_extension_callback(nghttp2_session *session, void **payload,
const nghttp2_frame_hd *hd, void *user_data) {
uint8_t *origin, *field;
size_t originlen, fieldlen;
uint8_t *p, *end;
alt_svc *altsvc;
if (altsvc_bufferlen < 2) {
altsvc_bufferlen = 0;
return NGHTTP2_ERR_CANCEL;
}
p = altsvc_buffer;
end = altsvc_buffer + altsvc_bufferlen;
originlen = ((*p) << 8) + *(p + 1);
p += 2;
if (p + originlen > end) {
altsvc_bufferlen = 0;
return NGHTTP2_ERR_CANCEL;
}
origin = p;
field = p + originlen;
fieldlen = end - field;
altsvc = (alt_svc *)malloc(sizeof(alt_svc));
altsvc->origin = origin;
altsvc->originlen = originlen;
altsvc->field = field;
altsvc->fieldlen = fieldlen;
*payload = altsvc;
altsvc_bufferlen = 0;
return 0;
}Set these callbacks to :type:`nghttp2_session_callbacks`:
nghttp2_session_callbacks_set_on_extension_chunk_recv_callback(
callbacks, on_extension_chunk_recv_callback);
nghttp2_session_callbacks_set_unpack_extension_callback(
callbacks, unpack_extension_callback);In unpack_extension_callback above, we set unpacked alt_svc
object to *payload. nghttp2 library then, calls
:type:`nghttp2_on_frame_recv_callback`, and *payload will be
available as frame->ext.payload:
int on_frame_recv_callback(nghttp2_session *session,
const nghttp2_frame *frame, void *user_data) {
switch (frame->hd.type) {
...
case 0xa: {
alt_svc *altsvc = (alt_svc *)frame->ext.payload;
fprintf(stderr, "ALTSVC frame received\n");
fprintf(stderr, " origin: %.*s\n", (int)altsvc->originlen, altsvc->origin);
fprintf(stderr, " field : %.*s\n", (int)altsvc->fieldlen, altsvc->field);
free(altsvc);
break;
}
}
return 0;
}Finally, application should set the extension frame types it is willing to receive:
nghttp2_option_set_user_recv_extension_type(option, 0xa);The :type:`nghttp2_option` must be set to :type:`nghttp2_session` on its creation:
nghttp2_session_client_new2(&session, callbacks, user_data, option);In the previous section, we talked about the user defined HTTP/2 extension frames. In this section, we talk about HTTP/2 extension frame support built into nghttp2 library.
As of this writing, nghttp2 supports ALTSVC extension frame. To send ALTSVC frame, use nghttp2_submit_altsvc() function.
To receive ALTSVC frame through built-in functionality, application has to use nghttp2_option_set_builtin_recv_extension_type() to indicate the willingness of receiving ALTSVC frame:
nghttp2_option_set_builtin_recv_extension_type(option, NGHTTP2_ALTSVC);This is very similar to the case when we used to receive user defined frames.
If the same frame type is set using nghttp2_option_set_builtin_recv_extension_type() and nghttp2_option_set_user_recv_extension_type(), the latter takes precedence. Application can implement its own frame handler rather than using built-in handler.
The :type:`nghttp2_option` must be set to :type:`nghttp2_session` on its creation, like so:
nghttp2_session_client_new2(&session, callbacks, user_data, option);When ALTSVC is received, :type:`nghttp2_on_frame_recv_callback` will be called as usual.