Jered Floyd commented on TS-4468:

I believe I considered this approach and ruled it out -- it's been a while and 
I'm just waking up so let me see if I can reconstruct why...

I think the disadvantage with matching session reuse on the "Host" header 
(rather than the origin server FQDN) is that is reduces the value of server 
session reuse for non-SSL connections.  That is, the semantics for your 
approach are fine for HTTPS but don't allow valid session reuse for HTTP when 
multiple targets map to the same origin server.

So, to retain current HTTP session reuse behavior, you would need to index HTTP 
sessions based on origin server name while changing HTTPS sessions to be 
indexed on the Host header.  Now, I don't know how much HTTP session reuse goes 
on in the scenario I describe (multiple targets on the same origin server) so 
maybe changing the index to be on "host" rather than "server" is just fine, but 
it's a more extreme change to the semantics of 
proxy.config.http.server_session_sharing.match = "host" or "both".

> http.server_session_sharing.match = both unsafe with HTTPS
> ----------------------------------------------------------
>                 Key: TS-4468
>                 URL: https://issues.apache.org/jira/browse/TS-4468
>             Project: Traffic Server
>          Issue Type: Bug
>          Components: HTTP, SSL
>    Affects Versions: 6.1.1
>            Reporter: Jered Floyd
>            Assignee: Susan Hinrichs
>             Fix For: 7.0.0
>         Attachments: TS-4468.patch
>          Time Spent: 3h 10m
>  Remaining Estimate: 0h
> proxy.config.http.server_session_sharing.match has a default value of "both", 
> which compares IP address, port, and FQDN when determining whether a 
> connection can be reused for further user agent requests.
> The "host" (FQDN) matching does not behave safely when ATS is operating as a 
> reverse proxy.  The compared value is the origin server FQDN after mapping, 
> rather than the initial "Host" target.
> If multiple Hosts map to the same origin server and the scheme is HTTPS, ATS 
> will attempt to reuse a connection that may have an SNI Host that does not 
> match the HTTP Host.  With Apache 2.4 origin servers this results in 400 Bad 
> Request to the user agent.
> You can observe this behavior with two mapping rules such as:
> map https://example.com/ https://origin.example.com/
> map https://www.example.com/ https://origin.example.com/
> Non-caching clients alternately fetching URIs from the two targets will see 
> 400 Bad Request responses intermittently.
> proxy.config.http.server_session_sharing.match should have a default value of 
> "none" when proxy.config.reverse_proxy.enabled is "1"
> In order of completeness:
> 1) Do not share server sessions on reverse_proxy requests.
> 2) Do not share server sessions on reverse_proxy requests where scheme is 
> 3) Compare target host (SNI host) rather than replacement host when 
> determining if reuse of server session is allowed (when 
> server_session_sharing.match is set to "host" or "both").

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