[pmtud] Minutes from the pmtud meeting at IETF57 - VIE

[Matthew J Zekauskas <[e-mail]>]

Path Maximum Transmission Unit Discovery Pre-WG (pmtud)
Thursday, 17-July-2003 from 13:00 to 15:00
=======================================================

The meeting was chaired by Matt Mathis and Matt Zekauskas.
Al Morton, Itojun Hagino and Matt Z. took notes that were
assembled into these minutes by the chairs.

Agenda
------
 * Preliminaries: Blue sheets, Note takers, etc
 * WG Status
 * Short history and work to date
 * Robustness Issues
 * Other Stakeholders
 * Plans

Matt Mathis led off the meeting presenting the new co-chair, the
agenda, the changes to the proposed charter, and the aggressive
milestones.

The group status is that some parts of the administrative preparation
did not get done, but IESG has approved, hence "Pre WG".  This will be
a fast development, silence will be acceptance (at the start as sections
are integrated).  PMTUD is a re-activation of path MTU WG, which was
a very similar effort.  To participate, you must subscribe to IETF list
([e-mail]).  Matt Zekauskas volunteered to Co-chair the group.
The charter was broadened so as not to restrict to a single method.
Milestones are aggressive, and need for implementation to test is clear.

No one disagreed, nor were there suggestions for other methods to
study.  Matt Mathis will make another editorial pass at the document
(which was a rewrite, starting from RFC 1981, instead of an update to
the previous BOF input).  Sections will be added based on mailing list
comments and any input from stakeholder communities.

Matt Mathis sketched the previous algorithm and noted some of the
problems.  He then sketched the new algorithm, noting that there
is just a small amount of MUST/SHOULD language: under what circumstances
can losses be ignored as a congestion signal.  The rest is heuristics;
it doesn't need to be the same for every application, and permits
vendor diversity.

Question: when can the algorithm be used by TCP?  Just after 3-way
handshake, or before real communication?  Matt responded that it
uses live payload data, and the draft has a recommendation not
to attempt the algorithm unless the congestion window is at least
twenty packets, so the connection is well established before the
algorithm starts.  Thus, this could slow down tiny files -- the
exact algorithm is a heuristic, so you could choose to perform
it differently.  There are tradeoffs.

The request for collaborators in the IPsec & security area led to a
big discussion on tunnel issues.  People were positive about the
method, but there are corner cases to consider.  Input has been
promised to the mailing list.  (In the IPv4 world, lack of PMTUD is
noted as a major problem with IPsec VPNs and providing services.)

This started around slide 11, "Plans for the Next Draft".  One
of the collaborators was folks from the multicast area; one possibility
is a generalization of the algorithm for reliable delivery.
This would solve an ICMP implosion problem if the current MTU
discovery technique was used.  Dave Thaler noted that the behavior
as specified in IPv4 and IPv6 was different, in 6 you respond,
in v4 you don't.

Another collaborating group would be IPsec; currently the security
architecture document has major sections dealing with the interaction
of MTU discovery and IPsec (because tunnels are created); the new
technique might obsolete many of those sections.

Itojun Hagino noted that the interaction between IPsec and and TCP
depends on if the TCP stack is aware of IPsec.  If the TCP stack does
not take care of the IPsec header size, the algorithm would need to be
revised.

Matt M. responded that the detail in the draft needs to be resolved
in a consistent way.  You can count the IPsec header as part of the
IP header or TCP header.  The really nasty cases involve additional
layers, for example IPsec on a VPN, ICMP messages could go back to
the wrong place.

Michael Richardson expanded on this as an IPsec implementer.  The
worst case is common at meetings such as this -- you have a corporate
address on your laptop, and a VPN back to the corporate space, so
all traffic goes back to HQ.  Try to visit a bank, and they have
ICMP filters.  Your gateway is sending out ICMP messages to the
bank, and they drop them.  This proposed algorithm should work
really well.  Many times VPNs are blamed (since they are the newest
element in the path), when the problem is really a bad ICMP filter.

However, there is a problem, if you raise the MTU and the tunnels do
not toss large messages but fragment them anyway, you will end up
always fragmenting.  Michael noted that his Linux implementation (he's
the FreeS/WAN technical lead) did not honor the DF bit by default.
Having a poor(er) performing implementation was better than one that
didn't work at all.  ("Poor performance is better than no security.")
Perhaps there could be a heuristic that worked for a short term
solution so these mechanisms don't interact badly... the endpoint
would need to be updated for this algorithm, so IPsec tunneling could
be updated at the same time.  This behavior is often a kernel option,
too.  [In reviewing the minutes, Michael related that:
 "The key point is that I, the IPsec developer can't control:
      1) the ICMP filter.
      2) the TCP on the machine behind it.
 I can have *some* influence on the TCP at the receiving end of the flow,
 but not a lot. *If* the IPsec tunnel terminates on the same machine as the
 TCP, then in theory, the TCP can learn about the reduced MTU, and set the
 MSS appropriately. In practice, probably only KAME, Microsoft and Sun are
 well enough integrated to do this right now. Probably Linux 2.6 will be able
 to do so as well.
  We do have an option to hack the MSS on the encapsulator's side already,
 alas."]

Perhaps you could fragment into tunnel, but retain the DF bit, and if
set don't do anything weird.  Itojun related KAME experience; there
they ended up not setting the DF bit on output header when IPsec
tunnels are created.

Another point was that IPv6 on IPv4 tunnels have the same issue.
IPv6 tunnels should have a MTU of 1280 by default so a minimum MTU
can be maintained.

Matt M. mentioned that he's aware that a large number of tunneling
implementations don't copy the DF bit from inner packet to outer
header.  He's not yet sure if the document needs a specific section
covering tunnels and tunnel migration; an intermediate ground that
works is to let tunnels behave this way in the interim, and discourage
a mode where end systems ignore can't fragment messages.

Dave Thaler noted that mobility might add additional headers; so a
tunnel MTU of 1280 might not be enough; 1380 would be better.

Itojun stated that he was thinking of configured tunnels and not mobile
IPv6.  If you send a packet with mobile headers, the TCP stack needs
to be aware of the size of the mobile IP headers and reduce MSS
appropriately -- maintain the total MTU size.

Matt Mathis noted that there was definitely a subgroup interested in
considering tunneling and MTU discovery; he encouraged folks to
join the mailing list and contribute the various circumstances where
there are potential problems.

Lars Eggert mentioned that RFC2003 specifies some things related
to MTU discovery, and RFC2401 specifically prohibits some of the
mechanisms in RFC2003 for security reasons.  Joe Touch also has
a relevant draft: draft-touch-ipsec-vpn.

As to other transport protocols, Matt Z. reported that he had
quickly skimmed SCTP and DCCP documents, and that SCTP looked
possible, but DCCP says specifically that the MTU can't be raised.
No one that claimed to be an SCTP expert was in the room (or at
least didn't comment negatively on the applicability to SCTP).
Eddie Kohler noted that this behavior was revised in the
DCCP WG meeting this week.   Matt Z. prompted him to send some DCCP text.

Matt M. emphasized that the point with getting a draft done early
is to encourage implementation as soon as possible.  The algorithm
will use specific details of other protocols, and we're dependent
on the uniformity of implementation of certain features.  We need
to learn what implementations really do; ideally get a custom
implementation run on servers and real field data to feed back
into the document.

Matt M also noted two cases that he's worried about (although
these are just examples; others are encouraged to consider other
cases, or report back implementation experience).  First, what
happens if a path is striped across multiple links, and the MTU
is not the same across the stripes?  You can require that the
MTU is not raised until a certain number of segments are received
successfully.  You need to understand the interaction between random
losses and whether the MTU is or is not raised.  Second, what happens
if there is a parametric failure -- when raising the MTU causes the
error rate to increase?  An actual case is one particular 10G gbic;
it was error free with 1500 byte packets, but not with 9000 byte
packets.  There is an opportiunity for different heuristics here,
for example use a smaller MSS if you cannot fill a window.

Michael thought a "brokenness test page" was needed -- a good testbench.

For hard, repeated, timeouts the first thing you want to do is
reduce congestion variables, then reduce MTU.  At some point want
to restart the checks to increase MTU.

There are other possible protocol interactions, too: for example,
SCTP can use multiple endpoints.  What if it changes addresses,
and the new path has a smaller MTU?

Michael felt that it was important to focus where the production
environments hurt most with the current MTU scheme.  Matt M noted
that different things hurt in different environments.  Michael
expanded that the most frequent case will likely be large port 80
responses to a client.  And it's the client that would decide that
the path is stupid or broken, and other A record should be tried.
The web server is getting the timeouts, not the client.  This won't
deploy if we can't solve the web server case.

Matt M noted another case he had thought about, but not seen: what
happens if raising the MTU causes link stability problems (as opposed
to hard failures) -- say the link "goes away" for 10 seconds and then
returns.  He's thought about using a state machine to catch this case...
the link is broken, and we don't necessarily want to fix it with
an MTU discovery algorithm.

On ignoring DF bits, so that a tunnel fragments large packets: Matt M
contended it was worse for a 1500 tunnel fragmenting a 9000 packet
than a tunnel fragmenting a 1500 packet by the tunnel overhead.
Michael didn't understand this at first; Matt explained that
the problem is that with many fragments the odds are greater that
you lose a fragment, and hence the whole packet than if there
are only two fragments.

In thinking about other stakeholders, Magnus Westerlund felt that the
algorithm would work for RTP over UDP with the use of RTCP extensions
for packet loss vectors.

Itojun said that we should contact [e-mail] for SCTP.

In the multicast case, Dave Thaler argued that this algorithm might
cause an ACK implosion that is worse than a ICMP-message-too-big
implosion, since there are typically far more receivers than there are
bottleneck-MTU links.

Magnus commented that the document as written is very TCP specific.
The algorithm should be better separated from actual deployment.
Matt M said that's the intention.

Another question was what, exactly, is the definition of MTU?  End-to-end
or link-specific.  Matt M said that we were talking about IP MTU
when using a particular link layer; how the IETF uses it, not what
hardware specifications say.

Dave Thaler argued that IPv6 might not need this at all; the algorithm
could arguably make performance worse (since the MSS size would ramp
up instead of being decided once by ICMP-message-too-big).  Matt M.
said the new algorithm would prevent against implementation or
configuration bugs and also work in the cases where L2 MTUs were
different on a switch.

One audience member said that this should be documented in detail.

Dave Thaler mentioned that filtering ICMPv6 has larger problems.  IPv6
neighbor discovery uses ICMPv6, so if ICMP is filtered you won't get
connectivity.  In addition, since v6 has no DF bit (but implied DF on
all packets) blocking ICMP definitely leads to black hole problems in
the network.  Thus, there is already a natural incentive to allow PMTU
using ICMP.

Matt M noted that some stacks have IPv4 mimic IPv6 -- they always set
DF, even on fragments,  and attempt to fragment only at the endpoints.

However, there's no requirement that routers send the too big messages
in v4, but there is a requirement in v6.

Another comment was tha the MTU in Router Advertisement messages should
solve this problem.  If operational experience says this isn't happening,
it should be reflected to the v6 working group.

Matt M said that in all cases the problems with path MTU discovery
are bugs.  There are a large set of problems.



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