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Network Working Group      Michael Richardson mcr@sandelman.ottawa.on.ca
INTERNET-DRAFT                             Sandelman Software Works Corp
draft-richardson-ipsec-pmtu-discov-02.txt              v1.1, 17 Aug 1998
Expires in six months


        Path MTU discovery in the presence of security gateways

Status of This memo

This document is an Internet-Draft. 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
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Abstract

This document describes the problem of getting accurate Path MTU infor-
mation in the presence of untrusted routers. Typical Path MTU discovery
is done by sending packets with the don't fragment bit set, and listen-
ing for ICMP messages from routers that want to fragment the packets.
Unfortunately, these messages could be forged, and IPsec based security
system(s) can not pass make direct use of these messages. An alternate,
backwards compatible algorithm is suggested.






















Michael Richardson mcr@sandelman.ottawa.on.ca                   [page 1]


INTERNET-DRAFT                                         v1.1, 17 Aug 1998

Table of Contents

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
  1.1.  Definition of terminology   . . . . . . . . . . . . . . . . .  2
2.  Introduction to the problem   . . . . . . . . . . . . . . . . . .  2
  2.1.  Requirement for PMTU information  . . . . . . . . . . . . . .  3
3.  Authenticated PMTU information  . . . . . . . . . . . . . . . . .  4
  3.1.  Proposal one  . . . . . . . . . . . . . . . . . . . . . . . .  4
  3.2.  Proposal two  . . . . . . . . . . . . . . . . . . . . . . . .  4
  3.3.  Differences   . . . . . . . . . . . . . . . . . . . . . . . .  5
4.  Limits to this solution: IPv6   . . . . . . . . . . . . . . . . .  5
5.  Security Considerations:  . . . . . . . . . . . . . . . . . . . .  6
6.  References:   . . . . . . . . . . . . . . . . . . . . . . . . . .  6
  6.1.  Author's Address  . . . . . . . . . . . . . . . . . . . . . .  6
  6.2.  Expiration and File Name  . . . . . . . . . . . . . . . . . .  6



1.  Introduction

1.1.  Definition of terminology

Here is a network of two security gateways, a client node and a server
node.

                  C---{G1}--{R1}--{R2}...{R3}--{R4}...{Rn}--{G2}---S

            C is the TCP initiator.
            G1/G1 are security gateways.
            Rx are routers.
            .. is a link with a restricted MTU.
            S is the TCP listener.

There are both TCP endpoints and security association end points, they
will be distinguished with the following terms:

   C  is the transport layer originator. TLO

   S  is the transport layer target.     TLT

   C/G1
      is a network layer originator/target pair. NLO/NLT/

   G1/G2
      is a network layer originator/target pair.

   G2/S
      is a network layer originator/target pair.

2.  Introduction to the problem

RFC1191 describes a mechanism for finding the maximum transmission unit
of an arbitrary internet path. It says:


Michael Richardson mcr@sandelman.ottawa.on.ca                   [page 2]


INTERNET-DRAFT                                         v1.1, 17 Aug 1998

          The basic idea is that a source host initially assumes that
          the PMTU of a path is the (known) MTU of its first hop, and
          sends all datagrams on that path with the DF bit set.  If
          any of the datagrams are too large to be forwarded without
          fragmentation by some router along the path, that router
          will discard them and return ICMP Destination Unreachable
          messages with a code meaning "fragmentation needed and DF
          set" 7.  Upon receipt of such a message (henceforth called a
          "Datagram Too Big" message), the source host reduces its
          assumed PMTU for the path.

The are several problems:

1. the ICMP "Datagram Too Big" messages are sent from a intermiate
   router (Rx in the diagram) to the gateway machine. They are not
   authenticated in anyway, nor does it appear that there is any
   reasonable way for the routers to prove they are legitimate members
   of the routing path.

   An attacker could influence the MTU used, possibly reducing the MTU
   of the route to an unacceptably low value. This may consistute
   unacceptably bad service. This is an issue to the Internet Metrics
   WG.

   A too high an MTU would result in excessive fragmentation, which on a
   loosy link, may result in very high retransmission rates. IPsec
   tunnels do not retransmit encrypted packets, rather they depend on
   the TLO node to do a retransmit, so retransmitted packets result in
   higher encryption loads as well. A gateway with limited CPU may start
   discarding more datagram fragments as it spends more time encrypting.

2. the PMTU information in the ICMP messages is difficult to relay back
   to the TCP/UDP (or other) stacks of the sending node. So, nodes C and
   S continue to send using whatever MTU they started with. This defeats
   the point of doing PMTU in conjunction with IPsec.

3. it would be preferable to IPsec gateways for TLO nodes to have PMTU
   available. This allows the IPsec gateway to ask the TLO node to
   reduce its PMTU by the amount of overhead the ESP takes. Otherwise,
   the resulting ESP datagram has to be fragmented.

There are two path MTUs:

1. the TLO/TLT PMTU

2. the NLO/NLT PMTU

The ideal transport layer PMTU is the NLx PMTU minus the overhead of the
ESP header and transform. For rfc1829 ESP this number is 36 bytes, for
the KSM draft ESP rfcXXXX this is 52 bytes (for DES, DES/HMAC-MD5-96).

2.1.  Requirement for PMTU information



Michael Richardson mcr@sandelman.ottawa.on.ca                   [page 3]


INTERNET-DRAFT                                         v1.1, 17 Aug 1998

The information must be authenticated. This implies that none of the
routers Rx may provide this information. It must come from either
nodes/routers on the trusted side, or from the gateways themselves.

Only the two gateway nodes know the effective number of bytes of
overhead.

Only the decrypting node can observe the fragmentation resulting from
the sequence of routers, R1..Rn.

IPv6 does not allow for intermediate routers to fragment packets. Only
the originating node may do so. Intermediate routers MUST send ICMP
Datagram Too Big messages, and drop the packet. It should be noted,
again, that there are two originators: C and G1.

3.  Authenticated PMTU information

Both proposal one and two must be adapted slightly for IPv6. This is
discussed later.

3.1.  Proposal one

Gateway G1 MUST drop all non-local ICMP Host Unreachable datagrams
(including "Datagram too bid") which arrive on its unprotected
interface. The gateway MAY accept ICMP packets that are addressed to
itself.

ICMP datagrams arriving via an authenticated (whether encrypted or not,
depending only policy) at G1 SHOULD be passed to their destination node
as normal.

Gateway G2 upon receiving an ESP or AH packet that needs to be
reassembled, MUST take note of the size largest fragment received. This
value is compared to the previous largest fragment size. If this size
has changed by more than 10%, or more than 2*MSL time (i.e. 2 minutes)
has passed since the previous ICMP message, then an ICMP Datagram Too
Big message is generated. The largest fragment size is initialized to
576 bytes.

The ICMP datagram is addressed from gateway G2 to the originating node
C, and gives a size that is based on the maximum fragment size (above),
minus the IPsec overhead. The ICMP datagram is sent via the tunnel on
which the IPsec packet was a member. I.e. the ICMP is encapsulated.

A packet arriving at G1 with the DF bit set, does not cause the DF bit
to be set on the encapsulating datagram.

3.2.  Proposal two

Gateway G1 MUST drop all non-local ICMP Host Unreachable datagrams
(including "Datagram too bid") which arrive on its unprotected
interface. The gateway MAY accept ICMP packets that are addressed to
itself.


Michael Richardson mcr@sandelman.ottawa.on.ca                   [page 4]


INTERNET-DRAFT                                         v1.1, 17 Aug 1998

ICMP datagrams arriving via an authenticated (whether encrypted or not,
depending only policy) at G1 SHOULD be passed to their destination node
as normal.

Gateway G1 MUST maintain a PMTU value with its SPI/Security Association
state. Packets arriving from node C with the DF bit set, and that are
bigger than the PMTU value, MUST be discarded, and an ICMP Datagram Too
Big message sent. In other words, the security gateway acts as a router
would if the IPsec tunnel were in fact a physical interface. The PMTU
value is initialized to either to the MTU of the interface on which
outgoing ESP packets would travel, minus the ESP overhead.

Gateway G2 upon receiving an ESP or AH packet that needs to be
reassembled, MUST take note of the size largest fragment received. This
value is compared to the previous largest fragment size. If this size
has changed by more than 10%, or more than 2*MSL time (i.e. 2 minutes)
has passed since the previous ICMP message, then an ICMP Datagram Too
Big message is generated. The largest fragment size is initialize to
576.

The ICMP datagram is addressed from gateway G2 to gateway G1, and gives
a size that is based on the maximum fragment size (above), minus the
IPsec overhead. The ICMP datagram is sent via the tunnel on which the
IPsec packet was a member. I.e. the ICMP is encapsulated and encrypted.

A packet arriving at G1 with the DF bit set (but fitting in the MTU of
the SA), does not cause the DF bit to be set on the encapsulating
datagram. If the DF bit was copied, and a routing change reduced the
PTMU, the datagram to be dropped, and never reach G2, so news of the
PMTU change would not be relayed.
3.3.  Differences

This section is still under construction. Input is requested:

1. the ICMP is generated by the near router in proposal two.

2. the ICMP in the tunnel potentially carries addresses which would not
   satisfy filtering rules.

4.  Limits to this solution: IPv6

The major problem in the IPv6 case is that the far end gateway G2 will
not see no packets if the PMTU estimate is too big. An ICMP will only be
received by G1 if the PMTU estimate is small enough to transit all
routers.

In order to grow the PMTU, either initially, or to take advantage of a
routing change, the gateway G1 must therefore send probe packets of a
larger size, knowing that the packet will be lost if the probe is too
big. There are other reasons why the packet, or the response may be
lost, so the probe must be done again anyway.

Further, the path may suddendly experience a drop in PMTU due to a


Michael Richardson mcr@sandelman.ottawa.on.ca                   [page 5]


INTERNET-DRAFT                                         v1.1, 17 Aug 1998

routing change. In that case, no packets will be received at G2, so G1
must also occasionally send probes of a smaller size if it hasn't
received an ICMP message in 2*MSL time. (note, this number is probably
too big)

Making smaller packets is easy: the gateway can use the fragmentation
facilities of IPv6 to split up an encrypted packet. A larger packet can
be produced by adding more padding before encryption.

5.  Security Considerations:

This entire document discusses a security protocol.

6.  References:

   RFC-1825
      R. Atkinson, "Security Architecture for the Internet Protocol",
      RFC-1825, August 1995.

   RFC-1191
      J. Mogul, S. Deering, "Path MTU Discovery", RFC-1191, November
      1990.

   KSM-AH
      New AH draft.

   metrics
      I. M. ISP, "How fast can it go?", draft-ietf-metrics-00.txt, work
      in progress: Jan. 20, 1997

   Gupta97-1
      V. Gupta, S. Glass, "Firewall Traversal for Mobile IP: Goals and
      Requirements", draft-ietf-mobileip-ft-req-00.txt, work in
      progress: Jan. 20, 1997

   Gupta97-2
      V. Gupta, S. Glass, "Firewall Traversal for Mobile IP: Guidelines
      for Firewalls and Mobile IP entities", draft-ietf-mobileip-
      firewall-trav-00.txt, work in progress: March 17, 1997

6.1.  Author's Address

             Michael C. Richardson
             Sandelman Software Works Corp.
             152 Rochester Street
             Ottawa, ON K1R 7M4
             Canada

             Telephone:   +1 613 233-6809
             EMail:       mcr@sandelman.ottawa.on.ca

6.2.  Expiration and File Name



Michael Richardson mcr@sandelman.ottawa.on.ca                   [page 6]


INTERNET-DRAFT                                         v1.1, 17 Aug 1998

This draft expires January 9, 1997

Its file name is draft-richardson-ipsec-pmtu-discov-02.txt



















































Michael Richardson mcr@sandelman.ottawa.on.ca                   [page 7]


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