Subject: [alto] Endpoint Cost Service with IP prefixes

From: Hans Seidel <[email-masked]>
Date: Thu, 10 Sep 2015 17:30:11 +0200

Hi All,

Contrary to my knowledge I just found out that the endpoint cost service 
is not limited to endpoint addresses but also permits prefixes as stated 
in section 11.5 of the RFC7285.
However, I took a more detailed look at this section and recognized that 
prefixes are no accepted input parameter.

Here are some more details:

11.5.   Endpoint Cost Service
[...]
In particular, this service allows lists of endpoint prefixes (and
addresses, as a special case) to be ranked (ordered) by an ALTO
server.

The accept input parameter for endpoints in ECS are of type 
TypedEndpointAddr

11.5.1.3. Accept Input Parameters
[...]
object {
     [TypedEndpointAddr srcs<0..*>;]
     [TypedEndpointAddr dsts<0..*>;]
} EndpointFilter;

which is defined here:

10.4.1.  Typed Endpoint Addresses

     When an endpoint address is used, an ALTO implementation must be able
     to determine its type. For this purpose, the ALTO Protocol allows
     endpoint addresses to also explicitly indicate their types. This
     document refers to such addresses as "Typed Endpoint Addresses".

     Typed endpoint addresses are encoded as strings of the format
     AddressType:EndpointAddr, with the ’:’ character as a separator. The
     type TypedEndpointAddr is used to indicate a string of this format.

and EndpointAddr is defined here:

10.4.3. Endpoint Address

     The EndpointAddr component of TypedEndPointAddr is also encoded as a
     string. The exact characters and format depend on AddressType. This
     document defines EndpointAddr when AddressType is "ipv4" or "ipv6".

10.4.3.1. IPv4

     IPv4 Endpoint Addresses are encoded as specified by the IPv4address
     rule in Section 3.2.2 of [RFC3986].
10.4.3.2. IPv6

IPv6 endpoint addresses are encoded as specified in Section 4 of
     [RFC5952]

 From my point of view it is not clear whether prefixes are allowed as 
ECS endpoints or not.
Can someone clarify? Maybe I overlooked something.

Thanks
Hans

---

From: Wendy Roome <[email-masked]>
Date: Thu, 10 Sep 2015 12:11:23 -0400

Hans,

Thanks for pointing that out. I never noticed that paragraph before, and I
do not see any reason for ECS to give costs between prefixes.

I believe that paragraph is what is politely called a "blooper". (There
are other less polite descriptions.)

My guess is that someone added that in an early draft, thinking it was a
good idea, but later abandoned that approach. Subsequent reviews
concentrated on the detailed protocol descriptions and examples, and we
glossed over the section headers. So no one noticed that dangling thread.

I suggest creating an errata sheet to delete that paragraph, and to fix
any other, ah, *bloopers* we discover.

	- Wendy Roome

On 09/10/2015, 11:30, "Hans Seidel" <[email-masked]> wrote:

>    ...

---

From: "Y. Richard Yang" <[email-masked]>
Date: Fri, 11 Sep 2015 16:52:25 -0400

Wendy, Hans,

Thanks for clarifying this. Please see below.

> >addresses, as a special case) to be ranked (ordered) by an ALTO

If we want to limit to no prefixes, the change will be a simple revision:
Sec. 11.5:
  "... this service allows lists of endpoint addresses to be ranked
(ordered) ...

On the other hand, if we want to include prefixes in the grammar, we need
to use the definition in Sec. 10.4.4. (Endpoint Prefixes).

I am fine with an errata to do the first change. Although I do see an value
in using prefixes, although the client and the server will need the context
for the semantics: is it the average of between two sets?

Richard

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