RE: [Manet-dt] RE: SMF: Usage of IPv4 Identification field

> I leave implementation details up to you. The only hint I can 
> provide is the
> number of fragments for a full IP datagram is typically low, 
> as the maximum
> IPv4 packet is 64KB and the minimum supported MTU is often 
> mentioned as 576.
> 576 is the minimum size of IP datagram to be supported, so is 
> not exactly
> MTU (see RFC791).

(RFC791, Section 3.2) says:

    "Every internet module must be able to forward a datagram of 68
    octets without further fragmentation.  This is because an internet
    header may be up to 60 octets, and the minimum fragment is 8
octets."

This means that the minimum MTU all links must support is only
68 bytes; not 576. (RFC791, Section 3.2) also says:

    "Every internet destination must be able to receive a datagram of
576
    octets either in one piece or in fragments to be reassembled."

This means that all end systems must have a Maximum Receive
Unit (MRU) of at least 576 bytes. This is sustained by
(RFC1122, Section 3.2.2), which calls it the "Effective
MTU to Receive" (EMTU_R).

Main point - it is possible for both source hosts and
middleboxes to fragment datagrams into "tinygrams" as
small as 68 bytes - so this has to be accounted for by
any in-the-network function that might draw conclusions
about fragment offset. 

Fred
fred.l.templin@boeing.com

_______________________________________________
Manet-dt mailing list
Manet-dt@ietf.org
https://www1.ietf.org/mailman/listinfo/manet-dt