On 12/02/2025 02:50, Chuck Caldarale wrote:
On 2025 Feb 11, at 19:21, Amit Pande <amit.pa...@veritas.com.invalid> wrote:
Am planning to update the Tomcat configuration to support HTTP/2.
Wanted to understand the difference between
<UpgradeProtocol className="org.apache.coyote.http2.Http2Protocol"/> nested
within the HTTP/1.1 connector
Vs
Supporting protocol=org.apache.coyote.http2.Http2Protocol in the connector
configuration.
I don’t see anywhere in the documentation for supported Tomcat levels where the
“protocol=…Http2Protocol” setting is allowed.
Correct. It isn't.
Where did you come up with that?
As I understand using the upgrade protocol mechanism allows to fallback on HTTP
1.1 in case the client and server don't agree on HTTP2. Is this right
understanding?
> >
Yes - hence the <UpgradeProtocol> element within the <Connector> element rather
than an additional protocol attribute.
It is a little more complicated than that.
Because either HTTP/1.1 or HTTP/2 can be used via the same host+port
combination (Connector in Tomcat configuration) we needed a way to link
multiple protocols to the Connector which is why the <UpgradeProtocol>
element is used. It does more than just support HTTP upgrade though.
The available mechanisms to select the protocol depend on whether TLS is
being used or not.
The possible ways to start an HTTP/2 connection are:
For non-TLS connections:
- HTTP Upgrade from HTTP/1.1 (deprecated)
- HTTP/2 connection preface
For TLS connections
- ALPN
HTTP upgrade requires an existing HTTP/1.1 connection. The client can
request an upgrade. If the server doesn't agree, they continue using
HTTP/1.1.
If the server receives an HTTP/2 connection preface and doesn't
understand it, the likely result is a 4xx response which will almost
certainly result in the connection being closed. There is no fallback
within the connection preface process.
ALPN allows the client to specify a list of protocols and the server
picks the one it wants to use (the server as a list in preference
order). If the client doesn't support any of the protocols the server
wants to use, the connection fails. Servers that advertise h2 over ALPN
do not have to advertise any other HTTP version.
Note that all of the above is HTTP/2 specific. If a third protocol was
to be supported, it would define how connections to that protocol could
be started.
Were there any other design considerations to support HTTP/2 via upgrade
protocol mechanisms? My apologies but I didn't find details around this.
The Tomcat does doesn't get a choice on how to do this. The available
processes to start an HTTP/2 connection are defined by RFC 9113.
Mark
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