Subject: Re: [alto] Draft 16

From: Wendy Roome <[email-masked]>
Date: Thu, 23 May 2013 10:06:56 -0400

{6.3} says:

  "Two Network Maps are the same if (1) they are retrieved from the same
ALTO Server (i.e., same Hostname + Port) and (2) ...."

Isn't (1) overly restrictive?  That means an organization cannot run
multiple ALTO servers, on different hosts/ports, which share the same
network map and cost maps.

Yes, the last sentence of that paragraph says "If a logical ALTO Server is
implemented by multiple load balance servers with the same Hostname +
Port, the servers need to ensure the consistency." But isn't that
referring to a transparent load balancer, where the client always uses the
same host/port and has the illusion that it's a single server?

BTW, in {8.5.4}, the ALTO server at custom.alto.example.com presumably
uses the same network map as the server at alto.example.com in {8.5.3}.

I'm not sure how to resolve that without introducing some notion of ALTO
servers being "compatible". Maybe change the sentence to

	"Two Network Maps are the same if (1) they are retrieved from compatible
ALTO servers and (2) .... Two ALTO servers are compatible if one server's
Information Resource Directory {8.5} references services, including the
Information Resource Directory, provided by the other server."

	- Wendy Roome

---

From: "Y. Richard Yang" <[email-masked]>
Date: Thu, 23 May 2013 11:08:05 -0400

> network map and cost maps.

The current spec gives only one explicit condition to verify that two
network maps are the same: same Hostname+Port, and same Version tag. Your
example of different hosts/ports could be resolved by matching on relaxed
conditions (e.g., some higher level of domain name such as example.com).
But we have not introduced any protocol mechanism to explicitly convey
such. You can think about the current spec as providing one sufficient
condition. One possibility to indicate more clearly is to change the
wording to:

======
  "One sufficient test to verify that two Network Maps are the same
consists of the following
  two conditions: (1) they are retrieved from the same ALTO Server (i.e.,
same Hostname
   + Port) and (2) they have the same Version Tag, where two Version Tags
match only
   if their strings are the same.  Whenever the content of the Network Map
maintained by an
   ALTO Server changes, the Version Tag MUST also be changed.
   Possibilities for generating a Version Tag include the last-modified
   timestamp for the Network Map, or a hash of its contents, where the
   collision probability is considered zero in practical deployment
   scenarios.  If a logical ALTO Server is implemented by multiple load
   balance servers with the same Hostname + Port, the servers need to
   ensure the consistency.

    A deployment may relax the preceding sufficient test. One example
relaxation is to
   relax condition (1) to match on a suffix of the domain name instead of
the exact Hostname.
   This is  outside the scope of this document. "
 ====

What do you think?

Note that even matching Hostname is already somehow "relaxed". One
potential issue is that the same server (Hostname) may provide two network
maps, one coarse-grained, and one fine-grained. Since the matching does not
consider the complete URI, this can cause some ambiguity, in particular
when a Cost Map refers to a Network Map.

Richard

> Yes. The sentence is a reminder about the complexity when load balance
servers are involved.

>         - Wendy Roome
>
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