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From: tong@aussie.enet.dec.com (Benny Tong)
Newsgroups: comp.protocols.tcp-ip
Subject: choosing ethernet packet type automatically
Message-ID: <1990Sep24.150326@aussie.enet.dec.com>
Date: 24 Sep 90 05:30:51 GMT
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: tong@aussie.enet.dec.com (Benny Tong)
Organization: Digital Equipment Corporation
Lines: 49


    RFC 1122 2.3.3 (p.24) says:
    
    The IP encapsulation for Ethernets is described in RFC-894
    [LINK:3], while RFC-1042 [LINK:4] describes the IP
    encapsulation for IEEE 802 networks.  RFC-1042 elaborates and
    replaces the discussion in Section 3.4 of [INTRO:2].
    
    Every Internet host connected to a 10Mbps Ethernet cable:
    
    o    MUST be able to send and receive packets using RFC-894
    	 encapsulation;
    
    o    SHOULD be able to receive RFC-1042 packets, intermixed
    	 with RFC-894 packets; and
    
    o    MAY be able to send packets using RFC-1042 encapsulation.
    
    
    An Internet host that implements sending both the RFC-894 and
    the RFC-1042 encapsulations MUST provide a configuration switch
    to select which is sent, and this switch MUST default to RFC-894.
    
    [...]
    
    Note that 894-only systems cannot interoperate directly
    with 1042-only systems.  If the two system types are set
    up as two different logical networks on the same cable,
    they can communicate only through an IP gateway.
    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.
    
    <end of quote>
    
    Question :
    
    Why is it not useful, or even possible for a dual-format host
    to determine automatically which format to send? What is the
    problem with link-layer broadcasts?
    
    Will this scheme work?
    
    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, the default packet type can be used.