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Independent submission                                     M. Richardson
Internet-Draft                                                       SSW
Expires: January 15, 2005                                  July 17, 2004


    An interim solution to the Path MTU discovery problem for IPsec
                                gateways
                 draft-richardson-ipsec-fragment-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   Path MTU discovery depends upon proper respect for the Don't Fragment
   (DF) bit. IPsec gateways often present an Maximum Transmission Unit
   (MTU) constraint, and therefore must send ICMP Fragment Needed
   messages when the DF bit is set. This document proposes to ignore it
   in certain cases.








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Heuristic  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1 Step 0 - selection . . . . . . . . . . . . . . . . . . . . . .  4
   2.2 Step 1 - tracking  . . . . . . . . . . . . . . . . . . . . . .  4
   2.3 Step 2 - size check  . . . . . . . . . . . . . . . . . . . . .  4
   2.4 Step 3 - error throttling  . . . . . . . . . . . . . . . . . .  4
   2.5 Step 4 - send  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       Normative references . . . . . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  6
       Intellectual Property and Copyright Statements . . . . . . . .  7






































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1. Introduction

   Path MTU discovery [1] depends upon proper respect for the Don't
   Fragment (DF) bit. IPsec gateways often present an Maximum
   Transmission Unit (MTU) constraint, and therefore must send ICMP
   Fragment Needed messages when the DF bit is set.

   At the same time, increasing numbers of firewalls and networks are
   misconfigured, and drop all ICMP messages. For road-warriors that
   operate on an extruded IP address (i.e. one from behind their
   security gateway, and route all traffic through the corporate
   firewall), they find that they can not reach certain sites. When full
   size datagrams arrive at the VPN gateway with the DF bit set (such as
   from a web server's response), they are too big to enter the tunnel.
   An ICMP is sent (rate limited) and the oversize datagram is
   discarded. The ICMP is filtered out, and the sender never reduces its
   segment size. The result is that the web site stalls. Ironically,
   this occurs more often for more efficient web servers, as they tend
   to fill the datagram more regularly.

   Although the site in question is misconfigured, the IPsec system is
   blamed, since the site "works fine" when the IPsec tunnel is removed.
   The result is that many IPsec security gateway vendors are resorting
   to ignoring the DF bit, and fragmenting the datagram anyway (either
   before encapsulation, or fragmenting the resulting ESP packet).

   To complicate the situation, the PMTUD WG, has recognized the
   proliferation of missconfigured systems wrt ICMP, and is proposing a
   new method of determining the MTU.  The new method does not rely on
   ICMPs, but rather on the non-delivery of occasional larger probes.
   The only requirement that the new method places on lower layers is
   that they reliably discard probes that are larger than the path MTU.
   With this new method, devices that do not honor the DF bit are at
   risk of invoking needless fragmentation.

   Further, for high speed networks, that employ larger TCP windows, the
   result of the fragmentation can cause TCP segments to become
   corrupted.  {ref please}

   This document proposes a heuristic for IPsec security gateways to
   observe such that they will interop with the both the current PMTU
   methods (given that ICMP is broken) and new mechanisms.









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2. Heuristic

   Summary: If the system is keeping per flow state, preferentially
   error packets that suddenly reach a new high-water mark for each
   particular flow, because they arelikely to be probes, or classic
   PMTUD.

   For systems that have per-flow [Host to Host] (Ed. per-microflow -
   5-tuple?) tracking, step 1 is included. Otherwise, it is skipped.

2.1 Step 0 - selection

   Is the datagram is too big for the tunnel, and has the DF bit set? If
   not, encapsulate as normal.

2.2 Step 1 - tracking

   Keep track of the largest datagram size received. When there is a new
   high water mark, do standard ICMP Need Fragment processing. If this
   is the first time the datagram was too big, then goto step 4. If not,
   then drop datagram.

2.3 Step 2 - size check

   Is the amount that the packet is too big exactly due to the tunnel
   overhead? (In particular, this would never apply when the media on
   both sides is dissimilar). If not, do standard ICMP processing, and
   drop the datagram.

2.4 Step 3 - error throttling

   Does error rate limiting permit an ICMP error message be sent at this
   time? (rate limited to about 1 packet per second) If so, then do
   standard ICMP Need Fragment processing, and drop the datagram.

2.5 Step 4 - send

   Fragment the datagram prior to encapsulation. Divide the datagram
   into two equal pieces and encapsulate each one seperately. No attempt
   to send an ICMP is made.











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3. Example

   A 1500 packet to which a 20 byte IP and 28 byte ESP header is added,
   trying to fit on a 1500 byte network is fragmented anyway.

   A 9000 byte packet with a 20 byte IP and 28 byte ESP header trying to
   fit on a 1500 byte network is dropped.












































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Normative references

   [1]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
        November 1990.


Author's Address

   Michael C. Richardson
   Sandelman Software Works
   470 Dawson Avenue
   Ottawa, ON  K1Z 5V7
   CA

   EMail: mcr@sandelman.ottawa.on.ca
   URI:   http://www.sandelman.ottawa.on.ca/



































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