Re: What's your favorite MTU?

[[e-mail] (Art Berggreen)]

>A little research in the RFC archives, however, suggests that
>there are actually very few reasonable values for Path MTUs.
>I count about 9, although I could be missing some.  I went
>through all the RFCs listed in RFC1130 as describing IP encapsulations,
>made a chart of the MTUs specified, and arranged them in declining
>order.  Then I grouped similar values together, and picked a set
>of plateaus corresponding to the least element in each group.
>In most cases, the choice of groups is quite clear; I'm a little
>concerned about the plateaus I picked at smaller MTU sizes, but
>none of my values is more than 17% from optimal for its group.
>
>The new algorithm is to step down through the plateaus until
>you hit one which is lower than your previous attempted IP Length.
>In the FDDI-SLIP-FDDI scenario, this converges in 5 RTTs, not 10.
>FDDI-Ethernet-FDDI converges in two steps, to within 3% of optimal.

I'm a bit concerned about building into a protocol the MTUs that happen
to be used on today's network technologies.

>The table at the end of this message shows my results (the figures
>in parentheses show how far from optimal the worst-case member of
>each group lies).
>
>I would appreciate it if people could correct any mistakes in this
>table, and tell me about data links (such as Amateur Packet Radio)
>that I've left out.  Also, if I've included any data links that
>are completely obsolete, please let me know!
>
>-Jeff
>
>PMTU-plateau	MTU	Protocol   Comments			Specification
>		
>		65535	IP	   Official Maximum MTU		RFC 791
>		65535	IP-HC      Hyperchannnel		RFC 1044
>65535
>		 8166	IP-IEEE    IEEE 802.4			RFC 1042
>8166
>		 4478	IP-FDDI    FDDI				RFC 1103
>4478
>		 2048?	IP-WB      Wideband Network		RFC 907
>		 2002	IP-IEEE    IEEE 802.5			RFC 1042

Don't 16Mbit TRs support much larger MTUs?

>2002 (2%)
>		 1536	IP-EE      Exp. Ethernet Nets		RFC 895
>		 1500	IP-E       Ethernet Networks		RFC 894
>		 1492	IP-IEEE    IEEE 802.3			RFC 1042
>1492 (3%)
>		 1006	IP-SLIP    Serial Lines			RFC 1055
>		 1006	IP-ARPA    ARPANET			BBN 1822
>1006
>		  576	IP-X25     X.25 Networks		RFC 877

There is no inherent MTU limit in X25 (using M-bit sequences).  In fact
if large X.25 packet sizes can be negotiated, there are real advantages
in using a larger MTU.

>		  544	Portal	   DEC IP Portal		--------
>		  512	IP-NETBIOS NETBIOS			RFC 1088
>		  508	IP-IEEE    IEEE 802/Source-Rt Bridge	RFC 1042
>		  508	IP-ARC     ARCNET/extended		RFC 1051
>508 (13%)
>		  296	IP-SLIP    Serial Lines (Van Jacobson)	RFC 1055
>		  253	IP-ARC     ARCNET			RFC 1051
>252 (17%)
>
>68			IP	   Official minimum MTU		RFC 791
>
>	    Don't Know	IP-DC      DC Networks			RFC 891
>
>

Art