# HTTP2 window update and priority **URL:** \ **Category:** NGINX\ **Created:** [May 9, 2016, 3:27pm UTC](https://www.ruby-forum.com/t/http2-window-update-and-priority/244591 "2016-05-09T15:27:53Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\ **Post date:** [May 9, 2016, 3:27pm UTC](https://www.ruby-forum.com/t/http2-window-update-and-priority/244591/1 "2016-05-09T15:27:53Z")
Hi According to my view on the source code of Nginx 1.9.15, I noticed some observation below: 1. when there is no enough window, nginx won’t send back the response frame, though according to the log, nginx said it has sent the headers frame out, but client won’t receive it. This is not right according to RFC 7540 because window update will only limit the data frames 2.when there is no enough window, nginx will first store the request in the queue, when it received a window update, nginx will start fetch the streams stored in the queue use ngx\_queue\_head within a while loop(line 2161 in ngx\_http\_v2.c). I noticed the stream order I got is the same as the sending order with no relationship of the dependency tree, then nginx will create the corresponding data frames according to the stream order. But I noticed whether nginx will put the frame into the output queue depends on the value of wev-\>ready, here a problem is that I noticed when the request files is big, the wev-\>ready is set 0, only when this happens, the priority mechanism can work otherwise the frame will be sent out directly with no dependency observation. I am still confused on how does nginx implement the priority. Could you please give me some suggestions. A failed priority mechanism example debug log: > Best Regards Muhui Jiang ---
**Author:** ![Muhui\_Jiang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/eada6e/32.png) [@Muhui\_Jiang](https://www.ruby-forum.com/u/Muhui_Jiang)\ **Post date:** [May 10, 2016, 3:15pm UTC](https://www.ruby-forum.com/t/http2-window-update-and-priority/244591/2 "2016-05-10T15:15:46Z")
On Monday 09 May 2016 21:27:31 Muhui Jiang wrote: > Hi > > According to my view on the source code of Nginx 1.9.15, I noticed some > observation below: > > 1. when there is no enough window, nginx won’t send back the response > frame, though according to the log, nginx said it has sent the headers > frame out, but client won’t receive it. This is not right according to RFC > 7540 because window update will only limit the data frames You’ve misread the source code. HTTP/2 windows are handled in ngx\_http\_v2\_send\_chain(), which is only called for response body data. > dependency observation. I am still confused on how does nginx implement the > priority. Could you please give me some suggestions. > > A failed priority mechanism example debug log: > [LOG/example.log at master · valour01/LOG · GitHub](https://github.com/valour01/LOG/blob/master/example.log) The prioritization has effect when there is concurrency for connection. In your case from nginx point of view there was only one stream in processing at any moment. The previous stream was completed before the new one has been received. wbr, Valentin V. Bartenev ---
**Author:** ![Muhui\_Jiang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/eada6e/32.png) [@Muhui\_Jiang](https://www.ruby-forum.com/u/Muhui_Jiang)\ **Post date:** [May 10, 2016, 4:13pm UTC](https://www.ruby-forum.com/t/http2-window-update-and-priority/244591/3 "2016-05-10T16:13:37Z")
Hi > You’ve misread the source code. HTTP/2 windows are handled in > ngx\_http\_v2\_send\_chain(), which is only called for response body > data. when the window is blocked, Nginx logs that the headers frame has been sent out, however I cannot receive any response headers. The same client tried on h2o and I can receive the response headers. > The prioritization has effect when there is concurrency for > connection. In your case from nginx point of view there was > only one stream in processing at any moment. The previous > stream was completed before the new one has been received. Let me explain my strategy in detail. I used some requests(stream id 1-15) to consume the window (65535) And then block the window without sending any window update frame(I have sent a very large stream update before). Then I send the next 6 streams with different priorities. At this moment, the 6 streams should be blocked. Nginx should have enough time to handle the priority. Then I send a very large window update frame, but nginx will process the stream one by one with no priority observation. What I want to say is, only when the file is really large then the priority can be observed. But I have give enough time to Nginx by block the request. Nginx should handle the priority though the file is really small. If you need more information, please tell me. Many Thanks Best Regards Muhui Jiang 2016-05-10 21:15 GMT+08:00 Valentin V. Bartenev [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![Muhui\_Jiang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/eada6e/32.png) [@Muhui\_Jiang](https://www.ruby-forum.com/u/Muhui_Jiang)\ **Post date:** [May 10, 2016, 5:12pm UTC](https://www.ruby-forum.com/t/http2-window-update-and-priority/244591/4 "2016-05-10T17:12:02Z")
Hi To say more, I may need your comment on my understanding of Nginx 1. Is the reason that I cannot receive the response headers frame because the response headers are stored in the SSL buffer? 2.Nginx will fetch the blocked request in the queue when receiving a window update frame (is the request order in the queue the same as my sending order?) 3.Then Nginx will handle the request one by one in a non-block mode. I noticed that when the requested file is small, nginx will create all the data frames, push the data frames into out frames queue and sent it out. ( the handling order is the same as my sending request order) When the requested file is large(say 10Mb), nginx will create only a part of data frames and then create some other data frames (the handling order is related to the dependency tree). So: - when a file is large, how many data frames to be generated once before generating the other frames, is it related to the SSL buffer size and the maximum frame size? and when Nginx are going to send out the frames and start a new SSL buffer. - I dig the debug log and noticed the real process to reflect the priority handling is the order to generate the data frames and connect those data frames before sending then out. I cannot find the handling process related to priority when generating the data frames and connect them together. It would be great if you can tell me the strategy or locate the source code. - Like the case I mentioned above. when the requested file is small, clients can not get the expected receiving response order. How large a file should be then nginx can handle them concurrently. I think in the real environment, many js , css file are not so big. So Nginx can not give priority mechanism in those cases. I suggest that Nginx should change the request queue order(Here I noticed the request queue is the same as the receiving order) rather than handling priority when generating data frames, it would be too late. So that even the files is small, the response order will also follows the dependency tree. Best Regards Muhui Jiang 2016-05-10 22:33 GMT+08:00 Valentin V. Bartenev [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![Muhui\_Jiang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/eada6e/32.png) [@Muhui\_Jiang](https://www.ruby-forum.com/u/Muhui_Jiang)\ **Post date:** [May 10, 2016, 4:34pm UTC](https://www.ruby-forum.com/t/http2-window-update-and-priority/244591/5 "2016-05-10T16:34:03Z")
On Tuesday 10 May 2016 22:12:59 Muhui Jiang wrote: > Hi > > > You’ve misread the source code. HTTP/2 windows are handled in > > ngx\_http\_v2\_send\_chain(), which is only called for response body > > data. > > when the window is blocked, Nginx logs that the headers frame has been sent > out, however I cannot receive any response headers. The same client tried > on h2o and I can receive the response headers. In addition, nginx uses SSL buffer to optimize record sizes. You can tune it using the “ssl\_buffer\_size” directive: > **[Module ngx\_http\_ssl\_module](http://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_buffer_size)** > > The ngx\_http\_ssl\_module module provides the necessary support for HTTPS. > priority. Then I send a very large window update frame, but nginx will > process the stream one by one with no priority observation. What I want to > say is, only when the file is really large then the priority can be > observed. But I have give enough time to Nginx by block the request. Nginx > should handle the priority though the file is really small. If you need > more information, please tell me. Many Thanks [..] Ok, now I understand your case. You are right, currently in this particular case priorities aren’t handled. wbr, Valentin V. Bartenev