Compiling on Windows, and a question of policy

Vernon Schryver
Mon Dec 2 17:21:12 UTC 2002

> From: "John R Levine" <>

> ...
> > The 15-bit DCC server-ID makes having a zillion small servers impossible
> > without using uglinesses such as the server-ID translation kludge.
> If they were all my users, I'd want a translation kludge anyway that hid
> my set of internal servers behind a single public server ID to avoid
> broadcasting my customer list to the world.

Dccd can translate IDs.  See discussion of the flod file in the end
of the dccd man page.

> > However, a low-end appliance could pay for the use of external servers.
> The capacity would probably be fine, but I was wondering about network
> latency.

For the clients, probably, but what about the servers?  10,000 DCC
clients each handling 1000 mail messages/day would involve only about
1 GByte of bandwidth, but 10,000 DCC servers each with only a single
flooding peer would need 100 GByte.  You wouldn't want the boxes to
have only a single flooding peer, because that would not be reliable.

>           I use DNS caches for DNSBL queries, including full zone mirrors
> for the zones I use a lot, to minimize network delays.  With a lot of DCC
> traffic, the issues would be similar.

The latencies would be similar, depending on server load, network
paths, and so forth.  A small difference is that a single DCC operation
requires only 1 exchange of packets, but a DNS operation can need
several.  A possibly larger difference is that DCC retransmission
delays are based on measured RTTs instead of a constant 30 seconds.

How large must the latencies be to matter?  Each SMTP mail message
typically involves at least 2 DNS operations to check the reverse DNS
name of the SMTP client. and to do a forward look-up of that name.
If the Mail_From domain differs, a third DNS operation is required.
Unless it is receiving spam or the the SMTP server is extremely busy,
those first two DNS operations will involve at least two sub-operations
over the Internet to find a server and then get an answer, for a total
of 4 round trips.  Would a fifth round trip over the Internet matter?

Vernon Schryver

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