Subject: (IPng 6550) Re: Packet Loss (was Re: Host Fragmentation

From: [email-masked]
Date: Mon, 28 Sep 1998 15:27:45 EDT

In a message dated 9/25/98 4:29:26 AM Eastern Daylight Time,
[email-masked] writes:

> I'm afraid it remains a fact of the Internet that if an IP fragment
>  gets dropped:

>      a)	The packet cannot be reassembled by the recipient host
>      and
>      b) the rest of the fragments move on and keep filling the links.
  
>  And, since IP packets/fragments may move along different paths, we
>  cannot even do the "horrible ATM-like PPD/EPD trick".
  
>  Then, whether or not this is a PROBLEM may and can be argued and
>  discussed in lengthy pieces of email.
  
>  My personal take is that b) is a disaster - it can give you 100%
>  utilized links and 0% goodput AND it is likely to appear when your
>  network is pushed near its limit. Whether this happens because up-
>  stream links are overloaded or its the Cessna 172 router who drops
>  packets from the Proteon interface is less important - when it
>  happens goodput rapidly drops.

Actually, the Cessna 172 router example described a situation
where network fragmentation decreased packet lossage while
TCP segmentation or host fragmentation would produce
consistent packet losses.
  
>  It can also be argued whether the cure is better than the disease,
>  whether the added complexity in Path MTU Discovery really pays off
>  by avoiding b). Maybe some other problem takes over. Since the SW
>  industry (e.g. Sun/Solaris) has put MTU Discovery into IPv4, one
>  would assume that someone has actually looked into live Internet
>  traffic and/or made realistic simulations. References from those
>  people would be really useful and probably terminate this thread.

While it is possible that the Cessna 172 style packet loss is taking
place in the network today, outside of topological instabilities
due to architectural problems associated with Cisco routers,
unless traffic is crossing an area of the internet that is severely
traffic misengineered, loss of packets in the Internet is probably
much lower than seems to be believed in this forum.  However,
if packet loss is as high as the specifiers of IPv6 seem to believe,
using ICMP in the PMTU procedures is probably inappropriate.

And if we really take 
<A HREF="http://members.aol.com/Volsinians/fragcomments.pdf">that ridiculous
Mogul paper</A> seriously,
what if the ICMP messages themselves were subject to
deterministic packet loss (not to mention issues of
ICMP filtering, NAT, etc.)?

Joachim Martillo
<A HREF="http://members.aol.com/Telford001/">Telford Tools, Inc.</A>
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