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
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3667.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 15, 2005.
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.
Richardson Expires January 15, 2005 [Page 1]
Internet-Draft opportunistic July 2004
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
Richardson Expires January 15, 2005 [Page 2]
Internet-Draft opportunistic July 2004
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.
Richardson Expires January 15, 2005 [Page 3]
Internet-Draft opportunistic July 2004
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.
Richardson Expires January 15, 2005 [Page 4]
Internet-Draft opportunistic July 2004
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.
Richardson Expires January 15, 2005 [Page 5]
Internet-Draft opportunistic July 2004
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/
Richardson Expires January 15, 2005 [Page 6]
Internet-Draft opportunistic July 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the IETF's procedures with respect to rights in IETF Documents can
be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Richardson Expires January 15, 2005 [Page 7]