[e2e] MTU variability

Cannara cannara at attglobal.net
Sun Aug 10 22:20:11 PDT 2003

Just one correction...CSMA/CD operation is not affected by frame sizes greater
than the 512-bit Ethernet minimum.  The minimum Ethernet frame size and the
worst-case round-trip LAN-segment delay must satisfy RTD <= 512 bit times, to
allow CD to function.  The effect of Jumbos, or anything larger than the
minimum, is simply to extend the sender's slot time and prevent (via CS)
others from sharing the segment for that time.  1518B (12144b) was simply a
judgement call on buffer sizes and courteousness in sharing when Ethernet II
was defined by Xerox.  Ethernet for years, has deserved a larger max frame
size, as has IP.  In fact, it's been possible to mis-install station/server
NIC drivers as Token Ring (to the bottom of some OS stacks) and yet install an
Ethernet card.  If the card's buffering is adequate, you suddenly have 4kB
Ethernet frames on your LAN and it all works just fine.

Also, 1Gb Enet can be CSMA/CD (bus rather than FDX), but the segment's
physical size is puny.


RJ Atkinson wrote:
> On Sunday, Aug 10, 2003, at 06:28 America/Montreal, Lloyd Wood wrote:
> > I think multihoming and handover the real cause of MTU variability.
> Other sources of MTU variability that are increasingly common:
>         - encrypted/authenticated tunnels using ESP/AH
>         - frame-relay-like deployments of MPLS-based tunnels [1]
>         - Gigabit Ethernet (and 10 GigE) with manually configured
>                 jumbo frame (9180 byte IP MTU) support [2]
> Ran Atkinson
> rja at extremenetworks.com
> [1] Sometimes erroneously referred to as a Virtual Private Network
> (VPN).  However, there is no security in this, hence no privacy,
> hence it isn't really a "private" network.
> [2] Officially, IEEE 802 doesn't admit that jumbo frames exist,
> because an Ethernet frame much larger than 1518 bytes is incompatible
> with Ethernet subnets that use CSMA/CD transmission.  However,
> CSMA/CD is virtually non-existent with 1 GigE products.  CSMA/CD
> is not permitted with 10 GigE.  So virtually all GigE (and 10 GigE)
> products support a 9K byte MTU -- and have done for years now.

More information about the end2end-interest mailing list