Meeting report

mogul (Jeffrey Mogul) Wed, 13 December 1989 02:35 UTC

Received: by (5.54.5/4.7.34) id AA17191; Tue, 12 Dec 89 18:35:51 PST
From: mogul (Jeffrey Mogul)
Message-Id: <>
Date: 12 Dec 1989 1835-PST (Tuesday)
To: mtudwg
Subject: Meeting report

This is the result of about 45 minutes of frenzied typing; I wanted
to get this down before I forgot anything.  Doubtless I have both
put too much in, and left a lot out.  Meeting attendees, please correct
me if I have made any serious mistakes.  I will then send a corrected
report to the IETF poobahs.

Please do NOT try to re-argue things over this report; I am trying
to record what happened, not continue our arguments.  After the final
draft of the meeting report is agreed upon, we can start arguing protocols

Thank you.


	MTU Discovery Working Group
	Chairperson: Jeffrey Mogul/DECWRL

	Reported by Jeffrey Mogul


		a) Discuss proposed MTU Discovery protocols
		b) Attempt a consensus protocol
		c) Find a victim to write up a draft
		d) Set next meeting date


	Art Berggreen
	Noel Chiappa		jnc@PTT.LCS.MIT.EDU
	Farokh Deboo		sun!iruucp!ntrlink!fjd
	Steve Deering
	Rich Fox		sytek!
	Ivan Liu
	Keith Mc Cloghrie	sytek!kzm@hplabs.HP.COM
	Jeff Mogul
	Nuggehalli Pradeep
	Stephanie Price		cmcvax!

	[Noel Chiappa participated via telephone]


	This was the first meeting of the MTU Discovery Working Group.
	We had already made and discussed a number of proposals for
	a protocol design, and had also discussed (over the mailing
	list) a number of technical and non-technical constraints
	on such a protocol.
	The most important non-technical constraint was that we wanted
	to devise a protocol that would work fairly well, or at least
	no worse than "no protocol at all", in the presence of large
	numbers of unconverted hosts and routers.
	Technical constraints include issues such as the cost of
	processing options; the possible use of the reserved bit
	in the IP header [probably not available to us]; the layers
	in which the MTU discovery protocol is implemented; the need
	to support TOS, security, and asymmetric paths; deciding when
	to send IP options; the problem of LANs with more than one
	MTU [a possible consequence of the use of bridges between
	Ethernets and FDDI]; the delays in propagation of information
	through the network; the realization that the path MTU is
	first known at the wrong end of the path;

	The proposals made before the meeting basically fell into two
	categories: those that probed the network to find out the path
	MTU (the minimum MTU over a path), essentially by asking the
	routers along the path to report the MTU, and those that asked
	the receiving host to report fragmentation when it occurs, with
	the understanding that the size of first fragment of a packet
	received probably reflects the path MTU.  In the latter case,
	the sender must send large packets occasionally in order to
	discover if they will be fragmented.
	The "Report Fragmentation" approach would be cleanest if we
	could have used the reserved bit in the IP header as a flag
	to tell the receiver that the sender is willing to receive
	and utilize a (new) ICMP Fragmentation Occured message.  However,
	we were told that this bit is unlikely to be released for this
	purpose.  "Report Fragmentation" (R-F) can also be done using an
	IP header option, again "allowing" the receiver to send the
	ICMP report.
	R-F has the advantage that it requires no changes in the routers,
	but the disadvantage that it requires changes in most end-hosts
	before it does much good.  (If the receiving host does not implement
	it, the sending host could obliviously continue to send packets
	that are being fragmented and perhaps lost.)  During the meeting,
	we discussed how to decide when and how often to send the ICMP
	Fragmentation Occurred message, and which layer should send it.

	Since option-processing is expensive for routers, we believe
	that the RF option cannot be sent on every packet.  Thus, the
	receiving host should remember that a sender has specified
	Report Fragmentation, and if fragmentation does occur later
	on, the ICMP should be sent even if the fragmented packet did
	not carry the option.

	Since some protocols on the sending host (e.g., NFS) might not
	be able to use the MTU information even thought others (e.g., TCP)
	might have requested it, we felt that it was necessary to send
	fragmentation reports in response only to those host+protocol+port
	tuples that had sent the RF option; this means that the receiving
	host must keep a table keyed in this way, and probably that it
	has to be maintained by the transport protocols (TCP, UDP, etc.)
	At the same time, it was felt to be unfortunate that the transport
	protocols would be burdened with this chore; we consider it an
	"implementation suggestion" rather than a protocol specification.
	Talk then turned to the "MTU Probe Option" approach.  In this
	approach (as it evolved from RFC1063 before the meeting), the
	sender would (occasionally) send this option on packets flowing
	on the connection.  The option would have three fields: a "next
	hop IP address field", an "OK" field, and an "minimum MTU" field.
	The initial values would be the first hop router address, "true",
	and the MTU of the first-hop link.  Each router along the path
	would set the MTU to the min of the current value and the MTUs
	of the incoming and outgoing paths.  If its own address was NOT
	the same as the "next hop" field, it would set the OK field to
	"false"; otherwise, it would change the next-hop field.  In any
	case, the option is forwarded (but not copied on fragmentation).
	The last-hop router (it should be able tell that it is such;
	we'll address the FDDI-Ethernet bridge issue somehow) does the
	same thing to the option as the previous routers, but if the OK
	field is still "true", it now knows the path MTU.  It therefore
	sends a (new) ICMP Path MTU message back to the sender, including
	the usual IP header info for matching at the sender.  In any
	case, the option continues to the receiving host.
	Note that the sender will not get a Path MTU message unless
	every router along the way understands this protocol.  This
	is because we could otherwise be misled by a low-MTU link
	bordered by two unconverted routers.  However, we believe that
	routers will be converted much sooner than most end hosts.
	Since the option is also received by the ultimate receiving host,
	that host can also interpret this option as "Report Fragmentation"
	flag (as above).  This is a backup mechanism; if the Path MTU
	message is not generated, or if the MTU then decreases enough
	to cause fragmentation, the sender will still find out.  Of course,
	the sender cannot know that the receiver is doing this.  One
	partial solution is that if the receiver gets a MTU Probe option
	with the OK field = false, then it should send an Path MTU message
	to the sender with a code indicating "unknown"; this tells the
	sender that it is OK to use the "Report Fragmentation" approach.
	If the sender receives neither kind of Path MTU message, then it
	must assume that it will not receive Fragmentation Occurred
	reports, and it should stick to the current "no more than 576
	bytes if non-local" rule.
	This hybrid approach seems (to the attendees, at least) to
	combine the best of both methods: it gives accurate, early
	results (i.e., before a connection has to start sending
	big datagrams) without incurring fragmentation if the routers
	cooperate, it detects fragmentation if the receiver cooperates,
	and it causes conservative behavior otherwise.
	There are still several issues to nail down, including
	how often to send the MTU Probe option, how many times to
	send the Fragmentation Occurred ICMP message, etc.
	Keith McCloghrie and Rich Fox were volunteered to write up
	a draft RFC, which we hope to circulate several weeks before
	the next IETF meeting.
	We expect to meet again at the February IETF meeting.