Re: [RRG] Re: [RAM] Tunneling overheads and fragmentation

>> Granted, this is a fairly old box in a small network, but I don't  
>> see anyone seriously claiming that ALL ISP networks support  
>> packets larger than 1500 bytes on ALL their internal links (and  
>> also on inter-ISP links).
>
> I did a survey about a month ago and it is true. I will yield to  
> those folks who responded to me to protect their privacy. ;-)
>
> But the main gist was:
>
> o We are going to 9K MTUs on all our internal links.
> o Where we don't have 9K MTUs, we use 4470.
> o Virtually no one runs ISP links at 1500.

And hence why I put this paragraph (the second one below) in section  
5 of draft-farinacci-lisp-03.txt to reflect Brian Carpenter's comment  
on the subject:

-----

    Since additional tunnel headers are prepended, the packet becomes
    larger and in theory can exceed the MTU of any link traversed from
    the ITR to the ETR.  It is recommended, in IPv4 that packets do not
    get fragmented as they are encapsulated by the ITR.  Instead, the
    packet is dropped and an ICMP Too Big message is returned to the
    source.

    In practice, this is not really a problem.  Hosts typically do not
    originate IP packets larger than 1500 bytes.  And second, a survey
    has been taken (from a list of ISPs, see Acknowledgement section)
    where nearly all ISP link MTUs are either 4470 bytes or support
    Ethernet jumbo frames of 9000 bytes.  Therefore, we don't anticipate
    any problems with prepending additional headers.

-----

Dino

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