Packet length limit under NBT.

Christopher R. Hertel chertel at
Thu Dec 7 02:43:58 GMT 2000

At the CIFS conference, there was a discussion regarding support for
multiple IP address entries under a group name in the NBNS.  I mentioned
that the RFCs originally allowed for negotiation of TCP over port 137 so
that long lists of IPs could be returned from a group name query.

I mentioned that there was a limit to the size of the UDP packets and others
felt that the limit was too low.  That is, that the maximum UDP packet size
was larger than I was suggesting.

...and so it is, but the RFCs state:

    - MAX_DATAGRAM_LENGTH: the maximum length of an IP datagram.  The
      minimal maximum length defined in for IP is 576 bytes.  This
      value is used when determining whether to fragment a NetBIOS
      datagram.  Implementations are expected to be capable of
      receiving unfragmented NetBIOS datagrams up to their maximum size.

Basically, 576 is used as the maximum size for an IP datagram under NetBIOS
because it is the smallest maximum size that can be expected to be
supported. The RFCs provide for fragmentation but, at least with respect to
group name queries, no one has implemented that support.

RFC1001 also says that the largest NetBIOS datagram is 1064 bytes.

I'm digging through to figure out where these numbers come from.  Clues

Chris -)-----

Samba Team --         -)-----   Christopher R. Hertel
jCIFS Team --   -)-----   ubiqx development, uninq.
ubiqx Team --     -)-----   crh at

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