6.2 mtu now limits size of incomming packet

[Chuck Swiger]

On Jul 13, 2007, at 12:27 PM, Bill Moran wrote:
>> I agree with others that MTU means "limit what I transmit".  It  
>> does not
>> mean "limit what someone else can transmit to me."
>
> Interesting viewpoint.  I disagree with it, but I can't quote any  
> standard
> or otherwise to support my view.  You didn't either.
>
> Does anyone know of a publicised, authoritative standard that would  
> clear this up?

Sure.  RFC-791:

"Fragmentation

     Fragmentation of an internet datagram is necessary when it
     originates in a local net that allows a large packet size and must
     traverse a local net that limits packets to a smaller size to reach
     its destination.

     An internet datagram can be marked "don't fragment."  Any internet
     datagram so marked is not to be internet fragmented under any
     circumstances.  If internet datagram marked don't fragment  
cannot be
     delivered to its destination without fragmenting it, it is to be
     discarded instead.

     Fragmentation, transmission and reassembly across a local network
     which is invisible to the internet protocol module is called
     intranet fragmentation and may be used [6]."

RFC-879:

"     HOSTS MUST NOT SEND DATAGRAMS LARGER THAN 576 OCTETS UNLESS THEY
       HAVE SPECIFIC KNOWLEDGE THAT THE DESTINATION HOST IS PREPARED TO
       ACCEPT LARGER DATAGRAMS."

"8.  Maximum Packet Size

    Each network has some maximum packet size, or maximum transmission
    unit (MTU).  Ultimately there is some limit imposed by the
    technology, but often the limit is an engineering choice or even an
    administrative choice.  Different installations of the same network
    product do not have to use the same maximum packet size.  Even  
within
    one installation not all host must use the same packet size (this  
way
    lies madness, though).

    Some IP implementers have assumed that all hosts on the directly
    attached network will be the same or at least run the same
    implementation.  This is a dangerous assumption.  It has often
    developed that after a small homogeneous set of host have become
    operational additional hosts of different types are introduced into
    the environment.  And it has often developed that it is desired to
    use a copy of the implementation in a different inhomogeneous
    environment.

    Designers of gateways should be prepared for the fact that  
successful
    gateways will be copied and used in other situation and
    installations.  Gateways must be prepared to accept datagrams as
    large as can be sent in the maximum packets of the directly attached
    networks.  Gateway implementations should be easily configured for
    installation in different circumstances.

    A footnote:  The MTUs of some popular networks (note that the actual
    limit in some installations may be set lower by administrative
    policy):

       ARPANET, MILNET = 1007
       Ethernet (10Mb) = 1500
       Proteon PRONET  = 2046"

RFC-894:

"  The minimum length of the data field of a packet sent over an
    Ethernet is 1500 octets, thus the maximum length of an IP datagram
    sent over an Ethernet is 1500 octets.  Implementations are  
encouraged
    to support full-length packets.  Gateway implementations MUST be
    prepared to accept full-length packets and fragment them if
    necessary.  If a system cannot receive full-length packets, it  
should
    take steps to discourage others from sending them, such as using the
    TCP Maximum Segment Size option [4].

    Note:  Datagrams on the Ethernet may be longer than the general
    Internet default maximum packet size of 576 octets.  Hosts connected
    to an Ethernet should keep this in mind when sending datagrams to
    hosts not on the same Ethernet.  It may be appropriate to send
    smaller datagrams to avoid unnecessary fragmentation at intermediate
    gateways.  Please see [4] for further information on this point."

And RFCs 1122 and 1191 are also somewhat relevant.  My reading of the  
above is that ethernet-capable gateways must be willing to accept  
packets as large as 1500 octets and fragment such traffic to meet the  
MTU settings as needed, except if DF is set.  If DF is set, but the  
packet is addressed to the gateway itself, then it should be  
delivered unfragmented even if that packet exceeded the MTU set on  
the receiving interface.

For hosts which are not network gateways, one should not assume them  
to be capable of receiving packets larger than 576 octets, but the  
TCP MSS option is almost universally available to indicate the  
appropriate maximum size that host is willing to receive during the  
3WHS setup...

-- 
-Chuck