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From: tong@aussie.enet.dec.com (Benny Tong)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: choosing ethernet packet type automatically
Message-ID: <1990Sep26.091724@aussie.enet.dec.com>
Date: 25 Sep 90 23:27:23 GMT
References: <9009241126.AA00500@asylum.sf.ca.us> <9009241320.AA23877@decwrl.dec.com>
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: tong@aussie.enet.dec.com (Benny Tong)
Organization: Digital Equipment Corporation
Lines: 25

RFC1122 says:

    Furthermore, it is not useful or even possible for a
    dual-format host to discover automatically which format to
    send, because of the problem of link-layer broadcasts.

I suggest:
    When an IP datagram needs to be sent to an unknown Internet host, ARP
    request packets of both ethernet and 802.3 types are broadcasted to the
    ethernet. The IP datagram is queued for later transmission. When an ARP
    reply comes back, the same packet type and ethernet address as the ARP
    reply is used for sending the IP datagram. If replies with two packet
    types come back, a default packet type can be used.

In article <9009241126.AA00500@asylum.sf.ca.us>, romkey@ASYLUM.SF.CA.US (John
Romkey) writes:
|>The ARP scheme you suggested is what I and other people suggested.
|>When we talked about it in the Host Requirements group, problem after
|>problem surfaced until someone finally said, "Look, why don't we do it
|>the right way? They're different encapsulations; put a router between
|>them." And we were enlightened.
|>

What are the problems with link-layer broadcasts? Can you list some of them
please?