|1HTTP/2|

|^ HTTP/2 is the most recent standard (RFC 9113, 2022) for implementing how
HTTP is represented by, and transported between, client and server. It is not a
ground-up rewrite of the established standard, HTTP/1.1 (RFC 2616, 1999). 
Those elements and semantics remain substantially the same.  Instead HTTP/2
modifies how the data is encapsulated (framed) and transferred between agents,
abstracting the complexity of this within the new protocol layer, leaving the
application level largely insulated from change.  As a result all existing
HTTP/1.1 web-based environments should be able to continue without
modification.

|^ The focus of the protocol is on performance, in particular end-user
perceived page rendering and web application responsiveness.  With the original
web use case being a relatively simple, single resource request-response, and
early markup involving text with a few illustrative images, the single network
connection, back-to-back request-response paradigm was simple to implement and
worked well enough.  In short time this moved to multiple network connections,
each loading elements in parallel as the complexity and density of the
individual elements on the pages increased, and to the introduction of HTTP/1.1 
|/pipelining| (back-to-back requests over a single connection) in an
attempt to avoid request-response-request latency.  Modern web documents and
applications tend to have dozens of fine-grained elements that dynamically load
resources based on the content of the page and/or user interaction.  The
single, then multiple network connections, each with its round-trip TCP
connection establishment overhead and request-response blocking of resources,
did not scale effectively.  HTTP/2 replaces it with a single TCP connection on
which multiple resources concurrently can be requested, pushed, and
transferred.  A more rigorous and effective implementation of the pipeline
concept.

|^ While multiplexing communication over a single network connection is a core
performance technology there are other contributing elements. The framing layer
uses binary tokens and parameters.  The plain-text request and response headers
of HTTP/1.|/n| are replaced with tokenised, encoded and dynamically cached
equivalents, commonly providing compression in excess of eighty percent.  The
relationship and priority of resources can be established allowing inferior
resources to be delivered after or dependent on superior ones.  The HTTP/2
server can send multiple responses to a single request.  Known as |/server
push| it can be used to pre-load the browser (cache) with resources it has not
encountered yet.

|^ HTTP/2 has the potential to place additional load on the client and server
in comparison to HTTP/1.|/n||.  One particular consideration for WASD sites is
the |/stream concurrency| setting of the HTTP/2 connection.  The server
specifies to the client the maximum number of concurent request-response (and
server push) |/streams| it will accept. RFC 9113 contains, "This limit is
directional: it applies to the number of streams that the sender permits the
receiver to create. Initially, there is no limit to this value.  It is
recommended that this value be no smaller than 100, so as to not unnecessarily
limit parallelism."  This translates to a hypothetical ten browsers connected
to the site each with up to one hundred concurrent streams, or potentially one
thousand active requests!  Time to check those server configuration and SYSGEN
parameters|...|

|^ Note that HTTP/1.1 has recently been revisited with RFC 7230 family of
specifications (2014) providing some clarifications and refinements on the
original.

|2WASD HTTP/2|

|^ WASD HTTP/2 implements all of the essential requirements of RFC 9113
(naturally enough).  This includes the framing protocol, datagram (message) and
stream management, header compression (RFC 7541), connection settings and flow
control, along with HTTP/2 connection establishment and termination (TLS ALPN
and HTTP upgrade).  It does not ((perhaps) currently) provide server-push or
stream prioritisation and dependency.

|^ Prior to the introduction of HTTP/2, WASD's fundamental abstraction was the
request, with each request interfacing directly with the network stack.  With
an HTTP/2 protocol connection somewhat supplanting the role of a Transmission
Control Protocol (TCP) connection in HTTP/1.|/n||, a new level of communication
abstraction was required between the request processing and the network
processing.  It should be noted that HTTP/2 itself is transported on TCP.

|^ Another new layer of abstraction required interfacing each protocol's
request/response header formats with the underlying server processing (avoiding
excessive duplication of code).  HTTP/1.|/n| has a plain-text,
carriage-control separated format, while HTTP/2 has a binary, compressed,
lookup-table oriented format (RFC 7541).  The layer was implemented using a
|/key||-|/value| dictionary.

|^ The accomodations for handling both HTTP/2 and HTTP/1.1, along with related
and ancilliary design and code changes, have not measurably impacted overall
WASD performance, although as noted below there is a server process CPU impost
associated with HTTP/2.

|note|
|0It's fair to say|...||
Reimplementing the complexities and subtleties of TCP |--| and adding a few of
its own |--| up in the application layer has made HTTP/2 a significantly more
complicated and less transparent protocol of HTTP/1.1 and while solving
some minor annoyances with that has sacrificed the usefulness and elegance of a
once readable byte-stream.  Certainly added layers and associated processing to
WASD, breaking the original I/O event driven design for possibly minor
performance improvements.
|!note|

|0HTTP/2 and WATCH|

|^ WATCH reports have the network item: [x]HTTP/2.  This provides a detailed
overview of the underlying framing and connection management exchanges between
client and server.  WATCH reports are available to HTTP/2 connected clients
with one consideration.  Due to multiplexed requests over the single network
connection, WATCHing the [x]HTTP/2 item of another request in the same browser
(using the same HTTP/2 connection - and there |/can| be multiple from a single
browser) is not possible (or at least more code than it's worth).  The HTTP/2
activity of the WATCHing generates more report items which generate |...| a
descent into reporting oblivion.

|^  WASD detects when a request is initiated on the same HTTP/2 connection as
an [x]HTTP/2 WATCHing client and if this sort of reporting cascade is possible
(any |/networking| group item) advises 

|code|
\|Time_______\|Module__\|Line\|Item\|Category__\|Event...\|
\|22:00:55.22 WATCH    1823 0004 CONNECT    HTTP/2 with 192.168.1.2,62446 on https://klaatu.private,443 (0.0.0.0)\|
\|22:00:55.22 WATCH    1454 0004 CONNECT    HTTP/2 rabbit hole\|
|!code|
 Such a request is not reported on further.

|^ Workarounds?

|bullet#|
|& WATCH from an independent browser instance.  Often requires a separate host
or different browser (e.g. Chrome and Firefox on the same host).
|& Have an HTTP/1.1 (only) service on the same server and use WATCH from that.
|!bullet|

|2HTTP/2 and Performance|

|^ With HTTP/2 not modifying the fundamentals of HTTP/1.1 semantics the
commonly touted payoff for all the additional complexity (in implementation) is
performance.  While this is often stated in terms of page rendering speeds or
web application responsiveness there is another significant measure of
performance - efficiency.  HTTP/2 much more efficiently utilises each network
(TCP) connection, as well as reducing the (time and processing) overhead of
setting-up and tearing-down of each of these required for parallelism under
HTTP/1.1.

|0Is it all worth it?\ \ As might be expected |-| that depends.|

|^ There are a number of sufficiently good analyses of both the factors that
affect HTTP/2 performance and the actual performance relative to HTTP/1.1. See
the references section and search the Web.  This section contains some
observations made during WASD HTTP/2 development. All of these seem to
correspond with others' observations, as well as what might reasonably be
expected considering the strategies employed by the protocol.

|bullet|

|item| For simple request-response use cases (e.g. download a file) HTTP/2
makes no observable performance difference.

|item| Where multiple resources need to be loaded by a page the measurable
performance improvement is proportional to the number of resources and the
latency of the network.

|item| In a low-latency environment such as the average LAN (e.g. 5mS RTT)
HTTP/2 makes minimal  difference irrespective of the number of resources loaded
(until it reaches rediculous quantities).

|item| In a high-latency environment such as a VPN spanning half the globe
(e.g. 350mS RTT) HTTP/2 makes an obvious and of course measurable improvement
for anything other than a trivial number of resources. 

|item| On a CPU constrained system HTTP/1.|/n| is significantly more
responsive than HTTP/2.  This unsurprising considering the explicit
multiplexing and header marshalling employed by HTTP/2.

|item| On the developer's bench there is ~10% more CPU consumed for the same load
profile** via HTTP/2 compared to HTTP/1.1 for similar durations.  This is
(probably) due to header compression and multiplexed stream processing.  It is
(probably) offset (to some degree) by fewer resources consumed in the network
stack managing the multiple TCP connections of HTTP/1.1.

|^ As also related in |link|Server Performance||, using the same load profile
as above** and using HTTP/1.1, WASD v11.0 compared to v10.4 showed ~5%
additional CPU and duration.  This is (probably) largely due to dictionary
processing.

|^+ ** |/100 individual files, size 2kB to 250kB, 50 concurrent, ~30%
CPU utilisation (~5% USER mode, mostly INTERRUPT servicing), batched 10,000 at
a time over a LAN.|

|!bullet|

|note|
|0YMMV!|
After some months (and now years) accessing WASD HTTP/2 over various LANs and
WANs the developer, FWIW, can't shake the perception that it |/seems||
generally more responsive in the real world.  Yet interestingly |...|
|!note|

|0Performance Assessment|

|^ As described in |link%|../config/##Server and Site Testing++in++WASD Configuration|
the OWASP ZAP application is integral to WASD test and exercise.  It can
generate an intense stream of traffic via cleartext (port 80) or TLS (port
443).

|draw|
+-----------+                    +------------+
|           |<--HTTP/1\.1 clear-->|            |
| OWASP ZAP |                    |    WASD    |
|           |<---HTTP/1\.1 TLS--->|            |
+-----------+                    +------------+
|!draw|

|^ Using the |/nghttpx| proxy utility (see reference below) it is also used to
exercise WASD's HTTP/2. 

|draw|
+-----------+                    +------------+                +------------+ 
|           |<--HTTP/1\.1 clear-->|            |                |            |
| OWASP ZAP |                    |  nghttpx   |<--HTTP/2 TLS-->|    WASD    |
|           |<---HTTP/1\.1 TLS--->|            |                |            |
+-----------+                    +------------+                +------------+
|!draw|

|^ On the development bench (modest) X*6 system formal performance assessment
is an encouraging ~50% of the average duration of HTTP/1.1 (see image
immediately below).

|0HTTP Report|

|^ WASD keeps track of HTTP family statistics.

|^ After 3.8 million requests via OWASP ZAP using the above configuration over
a number of spider-generated scans, one third of which were HTTP/2, one third
over TLS HTTP/1.1, and another third cleartext HTTP/1.1, the following image
suggests requests using HTTP/2 take approximately 50% of HTTP/1.1.

|image%%|./http_report.png|

|0Other Assessment|

|^ The simplest tool for getting a |/feel| for, and elementary measurement of
HTTP/2 may be found in the |link%|/wasd_root/exercise/*.*|WASD_ROOT:[EXERCISE]|
directory.  The document DOTTY.HTML and its companion files provide a page that
loads a selectable number of resources (images) in a consistent and
reproducible manner.  This DOTTY.HTML can be accessed via unencrypted HTTP
(http://), encrypted HTTP (https://) and services configured to provide HTTP/2
or HTTP/1.1.   Using these combinations with the selectable volume of
resources, elementary comparisons may be made in target environments.

|^ The Server Admin, HTTP Report (|link|Server Administration||) contains
comparative duration and bytes-per-second minimum/maximum/average for total
server HTTP/2 and HTTP/1.|/n| requests.  These cannot simply be taken
at face value without some consideration of the respective load profile but
under controlled conditions can provide useful metrics.

|^ Other development and load/performance tools were employed from a Linux
platform.  For someone educated in computing during the (19)70s, the
availability of VM technology for such purposes is just brilliant!\ \ \ \
|/But you know, we were happy in those days, though we were poor.| 

|^ Indispensible were 

|simple#|
|& |%https://nghttp2.org/documentation/nghttp.1.html|
|& |%https://nghttp2.org/documentation/h2load.1.html|
|& |%https://nghttp2.org/documentation/nghttpx.1.html|
|& |%https://www.zaproxy.org|
|!simple|

|^ Many thanks to the developer(s) of this package.

|2HTTP/2 Configuration|

|^ While effectively transparent to the end-user, HTTP/2 has some aspects that
need to be carefully considered by the server administrator.

|bullet|

|item| The level of (request) concurrency suggested by RFC 9113 section 6.5.2
would likely  require redimensioning a web server and possibly the supporting
system.  Environments historically expecting per-client resource demand to be
limited by the number of concurrent (HTTP/1.|/n||) network connections
an agent will deploy per origin server, often limited to less than a dozen,
might behave entirely differently when presented with many dozens, or
potentially hundreds of requests.  WASD's default of 100 is the RFC
recommendation in part because browsers tend to open multiple connections
to maintain the parallelism sought, so a reduction in HTTP/2 stream
concurrency often just increases HTTP/2 connection concurrency.

|item| Secure HTTP requires a minimum of TLS 1.2 with SNI and ALPN (RFC 9113
section 9.2).

|item| The ciphers available for use with HTTP/2 secure HTTP are quite specific
(at least in what the RFC prohibits - RFC 9113 Appendix A).  This and the
overall encryption requirements for HTTP/2 can cause issues with established
(older) agents and with mainstream browsers strictly enforcing the RFC
definitions making support for combined /2-/1.1 services sometimes problematic.

|^ Use of elliptic curve ciphers (ECDHE), as an element of Perfect Forward
Security (PFS), is mandated for HTTP/2 (RFC 9113 section 9.2.2).  The keys for
the elliptic curve ciphers are stored in PEM-encoded files  ocated in
WASD_ROOT:[LOCAL].  These can be copied from the WASD OpenSSL package using

|code|
$ copy WASD_ROOT:[SRC.OPENSSL-|/n_n_n||.WASD.CERT]DH_PARAM_*.PEM WASD_ROOT:[LOCAL]
|!code|

or locally generated as described in |link|Forward Secrecy||. 

|^ This SSL configuration and minimum cipher list seems to work for all major
browsers at the time of writing:

|code|
# WASD_CONFIG_GLOBAL
[SecureSocket]  enabled
[SSLversion]  TLSvall
[SSLoptions]  +OP_CIPHER_SERVER_PREFERENCE
[SSLcipherList] EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH:-DSS:
|!code|

|*YMMV!||

|item| TLS renegotiation (e.g. for a client certificate) must not be performed
on an HTTP/2 secure connection.  This precludes having selected paths perform
authorisation based on X509 and means that the service itself must request a
client certificate at connection establishment (RFC 9113 section 9.2.1).

|item| While the protocol provides for HTTP/2 using non-TLS (non-SSL)
connections the major browsers (Chrome, Edge (MSIE), FireFox, Safari) only
support it when using TLS.  To |/encourage| naive users to a TLS service the
following mapping rule approach may be used to redirect non-TLS home page
connections.

|code|
# WASD_CONFIG_MAP
[[*:80]]
if (!ssl:) redirect / https:///
|!code|

|!bullet|

|3Global Configuration|

|^ HTTP/2 and its features are globally enabled and configured using
directives contained in the WASD_CONFIG_GLOBAL configuration file.

|0HTTP/2 Global Configuration||

|table|
|~_ |: Directive |: Description |:> Default
|~
|~#* |. [Http2Protocol] |. enabled or disabled on a whole-of-server basis
|.> disabled
|~ |. [Http2FrameSizeMax] |. maximum frame size in octets (bytes) the server
is prepared to receive |.> 16384
|~ |. [Http2HeaderListMax] |. maximum number of octets (bytes) permitted in
a received header once uncompressed |.> 65535
|~ |. [Http2HeaderTableMax] |. maximum number of bytes permitted in the
server-end header cache |.> 4096
|~ |. [Http2PingSeconds] |. number of seconds between connection RTT
pings |.> 300
|~ |. [Http2StreamsMax] |. maximum number of concurrent streams (requests)
the server permits on the connection |.> 32
|~ |. [Http2InitWindowSize] |. initial window size (number of octets in
transit) for flow-control purposes |.> 6291456
|!table|

|^ These largely reflect settings and defaults from RFC 9113 6.5.1

|bullet|
|item| The minimum frame size is defined by the RFC at 16384.
|item| WASD automatically pings a connection every configured seconds.  The
latest value is available as real-number milliseconds in dictionary entry
"http2_ping" and CGI variable HTTP2_PING.
|!bullet|

|3Service Configuration|

|^ Using the WASD_CONFIG_SERVICE directive [ServiceHttp2Protocol] HTTP/2
may be disabled on a per-service basis.  The default is enabled if HTTP/2 is
enabled globally.

|3HTTP/2 Set Rules|

|^ WASD request processing rules may be used on a per-path basis to modify
(some) global configuration settings and provide other WevDAV configuation. 
See |link%|../config/##Request Processing Configuration++of++WASD Configuration||).

|table|
|~_ |: Rule|: Description
|~
|~#* |. HTTP2=PROTOCOL=1.1 |. send a "HTTP_1_1_REQUIRED" error
causing the client to use HTTP/1.1 (RFC 9113 section 7)
|~ |. HTTP2=SEND=GOAWAY |. send a "GOAWAY" frame to the client
resulting in it dropping the HTTP/2 connection
|~ |. HTTP2=SEND=PING |. send a "PING" frame to the client
calculating the Round Trip Time (RTT) of the connection
|~ |. HTTP2=SEND=RESET |. send a "RST_STREAM" frame to the client 
causing it to drop the HTTP/2 stream (request in progress)
|~ |. HTTP2=STREAMS=MAX=|/integer| |. set the maximum concurrent
streams on a per-path basis
|~ |. HTTP2=WRITE=|/low\|normal\|high| |. When request
data  is written it is queued at the specified priority, where high priority
are written before normal (default) and low priority, and normal priority
before low.  This is only for associated stream (request) and is not a
connection or whole-of-server prioritisation.
|!table|

|^ Use path SETings to prioritise some resources (e.g. CSS and JavaScript)
over others (e.g. images) and potentially improve page rendering speed.  Where
multiple concurrent requests are being serviced on the one HTTP/2 connection
this will deliver the |/high||er priority content before others.

|code|
# WASD_CONFIG_MAP
SET **.css http2=write=high
SET **.js  http2=write=high
|!code|

|2HTTP/2 Detection|

|^ A request using HTTP/2 may be detected during processing with the
|/http2:| conditional.

|code|
if (http2:)
   |/do this||
endif
|!code|

|^ See |link%|../config/##Conditional Configuration++of++WASD Configuration||).

|^ A script may detect HTTP/2 using the REQUEST_PROTOCOL CGI variable with the
value "HTTP/2".  Other protocol versions are similarly represented.

|^ A Server-Side Includes (SSI) document can use variations on the following
construct (and similar to the script suggestion immediately above) to detect
and process the request protocol.

|code|
<!--#if var={request_protocol} eqs="HTTP/2" -->
HTTP/2
<!--#else-->
HTTP/1.n
<!--#endif-->
|!code|

This is demonstrated in the example SSI document:
|^+ |link%|/wasd_root/exercise/shtml.shtml|WASD_ROOT:[EXERCISE]SHTML.SHTML|

|^ At the time of writing there is no browser-supported mechanism for a dynamic
document (i.e. JavaScript) determining the underlying HTTP protocol used to
access a resource.  To access this information the server must be used.  The
suggested method, and the one employed by the DOTTY.HTML tool described above,
is to provide one JavaScript source for HTTP/2 and another for everything else.

|^ The document would contain

|code|
<script type="text/javascript" src="/example-path/http.js"></script>
|!code|

and the server configuration

|code|
# WASD_CONFIG_MAP
if (http2:)
   map /example-path/http.js /example-path/http2.js
else
   map /example-path/http.js /example-path/http1.js
endif
|!code|

where each contains a minimum variable setting or similar flag detectable by
the document.

|2HTTP/2 References|

|^ The following provide a starting-point for investigating HTTP/2 (verified
available at time of publication).

|bullet|

|item| |%https://http2.github.io/||
|^- Home page for HTTP/2 maintained by the IETF HTTP Working Group.

|item| |%https://en.wikipedia.org/wiki/HTTP/2||

|item|  |%https://httpwg.github.io/specs/rfc9113.html||
|^- |%https://tools.ietf.org/html/rfc9113||
|^- HTTP/2 specification

|item|  |%https://httpwg.github.io/specs/rfc7541.html||
|^- |%https://tools.ietf.org/html/rfc7541||
|^- HPACK (header compression) specification

|item|  |%https://httpwg.github.io/specs/rfc7230.html||
|^- |%https://tools.ietf.org/html/rfc7230||
|^- Most recent HTTP/1.1 specifications (30, 31, 32, 33, 34 and 35)

|item| |%http://http2-explained.haxx.se/||
|^- Useful overview of HTTP/2 by the developer of cURL.

|item| |%https://hpbn.co/http2/||
|^- Another useful and more detailed overview of the protocol.
|^- From the excellent |%https://hpbn.co/| 

|item| |%http://undertow.io/blog/2015/04/27/An-in-depth-overview-of-HTTP2.html||
|^- A concise and useful summary.

|item| |%https://blog.cloudflare.com/tools-for-debugging-testing-and-using-http-2/||
|^- Not much here for VMS but a useful survey nonetheless.

|!bullet|