Re: [core] RFC 7252 - Forward Proxies

[Achim Kraus <[e-mail]>]

Hi Klaus,

thanks for your answers and reference explaining the term "IPv4address"!

 > For example, if a client wants to GET a representation of
<coap://example.com:12345/.well-known/core>, then the CoAP options
always encode <coap://198.51.100.1:12345/.well-known/core>, regardless
of whether the destination of the UDP datagram is (198.51.100.1, 12345)
or the IP address and UDP port number of a forward proxy.

In my understanding the, CoAP URI-Host option will be "example.com" and
not "198.51.100.1", otherwise hosts with multiple virtual server will
not be able to differentiate the logical request destination.

Also in my understanding, the port, IP-literal and IPv4address (the none
regname variants), are only included, if they differ from the
destination (which will be the porxy's address in case of a proxy).
Otherwise the note to point 5 of
https://tools.ietf.org/html/rfc7252#section-6.4

"NOTE: In the usual case where the request's destination IP
address is derived from the host part, this ensures that a Uri-
Host Option is only used for a <host> component of the form regname."

will not make sense.

 >> "6 ... otherwise": if a Proxy-Scheme is used, should that default port
 >> be use? Or still the URI-Scheme's port? If the Proxy-Scheme is to be
 >> considered, that default port maybe unclear/unknown by the client
 >> implementation.

 > It's the port number of the request URI (or, if the port number in
the request URI is omitted, the default port number for the scheme of
the request URI). It's not the port number of the forward proxy.

My question was rather, if it's the default port of the proxy scheme
(maybe http), or the requests coap-uri-scheme. My intention of the
question is:

"logical URI"  => "http://example.com/test"

Will be either:
Proxy-Scheme : "http"
CoAP-URI: "coap://example.com/test"
or
Proxy-Scheme : "http"
CoAP-URI: "coap://example.com:80/test"

Best regards

Achim Kraus


Am 10.01.20 um 10:47 schrieb Klaus Hartke:
> Hej Achim!
>
> Achim Kraus wrote:
>> A informal description would be in my opinion, add the uri-host option,
>> if it's not the IP-literal of the destination
>
> Yes. A bit more precisely: The Uri-Host option is omitted in a request
> if and only if its value equals the destination IP address of the
> containing UDP datagram.
>
>> (that assumes, that the
>> term "destination" refers to the actual proxy to send the request to).
>
> Yes. When sending a CoAP request to a forward proxy, the destination
> of the UDP datagram is set to the IP address and UDP port number of
> the forward proxy.
>
>> But the text seems not to differentiate between the request's (which
>> maybe the proxy) and the URI destination (which will be the "final
>> destination", or something like that).
>
> The CoAP options always encode the request URI, i.e., the URI of the
> target resource. For example, if a client wants to GET a
> representation of <coap://example.com:12345/.well-known/core>, then
> the CoAP options always encode
> <coap://198.51.100.1:12345/.well-known/core>, regardless of whether
> the destination of the UDP datagram is (198.51.100.1, 12345) or the IP
> address and UDP port number of a forward proxy.
>
>> I'm also not sure, what the "or
>> IPv4address" refers to.
>
> This refers to the syntax rules for hosts in RFC 3986:
>
>      host = IP-literal / IPv4address / reg-name
>
> Example IP-literal: "[2001:db8::1]"
> Example IPv4address: "198.51.100.1"
> Example reg-name: "example.com"
>
>> For me it seems, that stick strict to 6.4 would cause different option
>> values, if a proxy is used and the "final destination" URI contains also
>> a (different) literal IP address. For me "strict" will then result in a
>> wrong request to the proxy.
>
> This may need a bit clarification. When a request is intended for a
> server, the request URI is encoded using the Uri-Host, Uri-Port,
> Uri-Path, and Uri-Query options. (So, all the request URI components
> are present, except for the scheme.) When a request is intended for a
> forward proxy, the request URI is either encoded using the Proxy-Uri
> option or the Proxy-Scheme, Uri-Host, Uri-Port, Uri-Path, and
> Uri-Query options. (So, all the request URI components are present,
> including the scheme.) Even though some of these options have the word
> "Proxy" in their name, they still contain the components of the
> request URI, not the IP address or UDP port number of the forward
> proxy.
>
>> "6 ... otherwise": if a Proxy-Scheme is used, should that default port
>> be use? Or still the URI-Scheme's port? If the Proxy-Scheme is to be
>> considered, that default port maybe unclear/unknown by the client
>> implementation.
>
> It's the port number of the request URI (or, if the port number in the
> request URI is omitted, the default port number for the scheme of the
> request URI). It's not the port number of the forward proxy.
>
> Does that answer your questions?
>
> Best regards,
> Klaus
>