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Update ALPN example
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bashi committed Oct 18, 2023
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22 changes: 13 additions & 9 deletions draft-happy-eyeballs-v3.md
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Expand Up @@ -315,18 +315,22 @@ seconds.
The `alpn` and `no-default-alpn` SvcParamKeys in SVCB RRs indicate the
"SVCB ALPN set," which specifies the underlying transport protocols supported
by the associated service endpoint. When the client requests SVCB RRs, it
SHOULD perform the procedure specified in
{{Section 7.1.2 of SVCB}} to determine the underlying
transport protocols that both the client and the service endpoint support. The
client SHOULD NOT attempt to make a connection to a service endpoint whose SVCB
ALPN set does not contain any protocols that the client supports. For example,
suppose the client is an HTTP client that only supports TCP-based versions such
as HTTP/1.1 and HTTP/2, and it receives the following HTTPS RR:
SHOULD perform the procedure specified in {{Section 7.1.2 of SVCB}} to determine
the underlying transport protocols that both the client and the service endpoint
support. The client SHOULD NOT attempt to make a connection to a service
endpoint whose SVCB ALPN set does not contain any protocols that the client
supports. For example, suppose the client is an HTTP client that only supports
TCP-based versions such as HTTP/1.1 and HTTP/2, and it receives the following
HTTPS RR:

~~~
example.com. 60 IN HTTPS 1 . alpn="h3" no-default-alpn ipv6hints=2001:db8::2
example.com. 60 IN HTTPS 1 svc1.example.com. (
alpn="h3" no-default-alpn ipv6hint=2001:db8::2 )
~~~

In this case, attempting a connection to 2001:db8::2 would not make sense
because example.com only supports the UDP-based version of HTTP (HTTP/3).
because the RR indicates that svc1.example.com would only support the
UDP-based version of HTTP (HTTP/3).

If the client is an HTTP client that supports both Alt-Svc {{?AltSvc=RFC7838}}
and SVCB (HTTPS) RRs, the client SHOULD ensure that connection attempts are
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