Subject: Re: Evaluation of Gateway Location proposals

From: Jose Costa Requena <[email-masked]>
Date: Wed May 05 17:19:03 1999

Hi Hussein,

Here is another comment that I couldn't answer in the last mail.
Regarding the Cache Alignment state you said:

>And I don't see what advantage call routing can gain from this phase?

This phase could seems that will be inefficient but the utility appears when 
one of the connections between a pair of servers is down.The rest of servers 
in the server Group are continuing their transactions and updating their 
databases. The FSM which takes care of that pair will check the connection 
after the Time out has expired(Hello Interval) and it has to restart all the 
process till it reaches again the bidirectional state. In this moment, the 
server that was isolated from the SG has to update its database. When the 
server is again reachable can be long time ago from the last update or it 
can be after a short interval, anyway in the Cache Alignment State that 
Server can ask only for the CSA records that have changed since the last 
time that it was connected. It means that in that state there will be 
required only the records that are out of date and not the rest. But if the 
failure was only during a short interval the server does not have to update 
any record and it transitions immediately to the Cache Update State and it 
means that the protocol will perform nearly only two phases from the Hello 
directly to the Cache State Update.

Another question is about your comment:

>From reading the SCSP spec, I thought that in the Cache State Update
>phase no CSA Solicit messages are required? Just CSA Request/CSA Reply 
>messages. If a CSA Solicit message is required, this makes SCSP's overhead 
>three messages per update as compared to BGP's one message per update.

In this case, the CSUS are required for specific situations, one example 
could be the moment that the entry that the server is asking for is no 
longer in the database.As I comment in the last mail in that situation the 
Server will receive the same CSAS that it sent in the CSUS but with the N 
bit on. In this case we get the benefit of not sending and receiving all the 
record, only the notification saying that it was a null entry. I guess that 
in this and other cases is useful to send only the summary and compare if it 
is  newest or not, instead of compare all the record. I guess that when the 
data is quite big is better to send and receive only the summaries.

Sorry for so long explanation but I have tried to describe my point of view, 
maybe I'm wrong and is better only use two phases in the updating and only 
two kind of messages for that, but I hope some comments about my idea.
Thanks very much.
Best regards.
Jose

Helsinki University of Technology
Laboratory of Telecommunications Technology
Jose Costa Requena
Research Scientist
Room: SG 227B
Tel: + 358 9 451 2475
email(internet): [email-masked]
http://keskus.tct.hut.fi/tutkimus/ipana/pers/jose.shtml

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From: "Hussein F. Salama" <[email-masked]>
Date: Wed, 05 May 1999 16:18:48 -0700

> directly to the Cache State Update.

I think in call routing, when the connection between LS A and LS B is
disconnected (due to any reason), LS A should immediately delete any
call routes it has previously learned from LS B. As a result, when the
connection between LS A and LS B is restored, both LSs will have to
exchange their entire call routing databases. 

For call routing, and layer 3 routing, purposes, I don't think it's wise
for an LS A to keep a call route X after LS B,  which advertised X to A,
disappears. The reason is: how will we A learn about changes to the
status of X while B is down? 

> data is quite big is better to send and receive only the summaries.

Please see my previous message.

Hussein

-- 
Hussein F. Salama
Cisco Systems
Mail Stop SJC6/3, 170 W. Tasman Drive, San Jose, CA 95134
Voice: +1 (408) 527-7147, Fax: +1 (408) 527-1714

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