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From: guy@auspex.UUCP (Guy Harris)
Newsgroups: comp.sys.ibm.pc.rt
Subject: Re: AIX TCP/IP inet_len
Message-ID: <1299@auspex.UUCP>
Date: 26 Mar 89 23:50:08 GMT
References: <932@applix.UUCP>
Reply-To: guy@auspex.UUCP (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 34

>I can't find any documentation on this "maximum packet length -
>r_inetlen" paramter and am wondering if 1518 is "standard"
>in the rest of the TCP/IP world or what...why does IBM ship 1518 as
>the default inetlen if a popular platform like Suns can't handle packets
>over 1500??

More to the point, why does IBM ship 1518 as the default "inetlen" -
assuming "inetlen" is the maximum size of an IP datagram - if the
DEC/Intel/Xerox Ethernet 2.0 spec says, quite clearly, that an Ethernet
packet must have between 46 and 1500 octets?  A limit of 1500 wasn't Sun's
idea....

The part that's somewhat suspicious here is that the largest valid
*Ethernet* packet is 1518 octets long - 6 octets of destination address,
6 octests of source address, 2 octets of type, 1500 octets of data, and
4 octets of frame check sequence.  If "inetlen" is really the maximum
size of an IP datagram, perhaps somebody at IBM was confused and thought
1518 was the maximum number of *data* bytes in an Ethernet frame, rather
than the maximum *total* number of bytes in an Ethernet frame?

>(At 1518, the Sun console would print "ie0: huge packet")

Yeah, by the Ethernet 2.0 spec, it *is* huge - or, at least, larger than
is considered valid - if it has 1518 bytes of data, and thus 1536 bytes
in the frame as a whole....

>From Sun:
>/usr/include/net/if_ieee802.h:#define	MAX_8023_DLEN		1500

Well, that's IEEE 802.3, not Ethernet; however, as I parse stuff in the
May 7, 1982 IEEE P802 draft, the recommended MaxFrameSize for a
10Mbit/second CSMA/CD network (for which read "the IEEE 802 version of
Ethernet", basically) is 1518, which means the maximum data size in the
packet is 1500.