[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Reopening jumbo frames in IS-IS

Thus spake "Curtis Villamizar" <curtis at faster-light.net> 
In message <015a01c57a75$4e1e7b70$6801a8c0 at stephen>
"Stephen Sprunk" writes:

That seems excessively restrictive.  There are many environments
today using jumbo frames (8k-10k MTU) on end hosts without
problems.

This also begs for a definition of what exactly a "jumbo" frame is;
does an MPLS label (or several) on the front of a 1500-byte Ethernet
frame count?  Does a packet on media with a >1500 byte native
MTU count?  Should a host on FDDI or a direct HDLC/PPP link be
limited to 1500 byte packets -- even though its native MTU is 4470
-- because we fear it might need to talk to an Ethernet-connected
host?


Jumbo frames specificly refers to ethernet.  FDDI, HIPPI, HDLC, PPP,
etc are not affected.  It was these interfaces at end systems with
100baseT, GbE, and 10GbE in provider POPs that created the
requirement for ethernet jumbo frames.

Only as it relates to IS-IS; jumbos existed long before in other contexts despite the IEEE's efforts to the contrary.

No one in their right mind (note that I this is a qualified "no one")
would bridge a PPP link to an ethernet link.  Standards should
discourage doing incredibly stupid things rather than try to
accomodate people who do them with no good reason for doing so.

Then a large number of ISPs are out of their minds... The two main ways to deploy ADSL service are (a) Ethernet bridged to PPPoATM, and (b) PPPoE directly to the host. I think the latter is also commonly used for DOCSIS.

If bridging POS and Ethernet were safer (i.e. jumbos could be assumed), we'd probably see that become common in ISPs as well.

TCP and most other modern protocols handle the end system MTU quite
nicely and TCP path MTU discovery is widely implemented and easy
enough to enable though I think it is still disabled by default in
most implementations.

Win2k and later have PMTUD on by default, but unfortunately not PMTU black hole detection. I'm not sure about earlier Windows versions or MacOS/Linux/etc.

However, there's no current means for MTU detection for hosts on the same subnet. I can envision a number of ways to fix this for IP traffic, but I don't know enough about IS-IS to know if any of them are applicable.

S

Stephen Sprunk "Those people who think they know everything
CCIE #3723 are a great annoyance to those of us who do."
K5SSS --Isaac Asimov