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Network Working Group P. Savola
Request for Comments: 4459 CSC/FUNET
Category: Informational April 2006
MTU and Fragmentation Issues with In-the-Network Tunneling
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Tunneling techniques such as IP-in-IP when deployed in the middle of
the network, typically between routers, have certain issues regarding
how large packets can be handled: whether such packets would be
fragmented and reassembled (and how), whether Path MTU Discovery
would be used, or how this scenario could be operationally avoided.
This memo justifies why this is a common, non-trivial problem, and
goes on to describe the different solutions and their characteristics
at some length.
Table of Contents
1. Introduction ....................................................2
2. Problem Statement ...............................................3
3. Description of Solutions ........................................4
3.1. Fragmentation and Reassembly by the Tunnel Endpoints .......4
3.2. Signalling the Lower MTU to the Sources ....................5
3.3. Encapsulate Only When There is Free MTU ....................6
3.4. Fragmentation of the Inner Packet ..........................8
4. Conclusions .....................................................9
5. Security Considerations ........................................10
6. Acknowledgements ...............................................11
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................12
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1. Introduction
A large number of ways to encapsulate datagrams in other packets,
i.e., tunneling mechanisms, have been specified over the years: for
example, IP-in-IP (e.g., [1] [2], [3]), Generic Routing Encapsulation
(GRE) [4], Layer 2 Tunneling Protocol (L2TP) [5], or IP Security
(IPsec) [6] in tunnel mode -- any of which might run on top of IPv4,
IPv6, or some other protocol and carrying the same or a different
protocol.
All of these can be run so that the endpoints of the inner protocol
are co-located with the endpoints of the outer protocol; in a typical
scenario, this would correspond to "host-to-host" tunneling. It is
also possible to have one set of endpoints co-located, i.e.,
host-to-router or router-to-host tunneling. Finally, many of these
mechanisms are also employed between the routers for all or a part of
the traffic that passes between them, resulting in router-to-router
tunneling.
All these protocols and scenarios have one issue in common: how does
the source select the maximum packet size so that the packets will
fit, even encapsulated, in the smallest Maximum Transmission Unit
(MTU) of the traversed path in the network; and if you cannot affect
the packet sizes, what do you do to be able to encapsulate them in
any case? The four main solutions are as follows (these will be
elaborated in Section 3):
1. Fragmenting all too big encapsulated packets to fit in the paths,
and reassembling them at the tunnel endpoints.
2. Signal to all the sources whose traffic must be encapsulated, and
is larger than fits, to send smaller packets, e.g., using Path
MTU Discovery (PMTUD)[7][8].
3. Ensure that in the specific environment, the encapsulated packets
will fit in all the paths in the network, e.g., by using MTU
bigger than 1500 in the backbone used for encapsulation.
4. Fragmenting the original too big packets so that their fragments
will fit, even encapsulated, in the paths, and reassembling them
at the destination nodes. Note that this approach is only
available for IPv4 under certain assumptions (see Section 3.4).
It is also common to run multiple layers of encapsulation, for
example, GRE or L2TP over IPsec; with nested tunnels in the network,
the tunnel endpoints can be the same or different, and both the inner
and outer tunnels may have different MTU handling strategies. In
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particular, signalling may be a scalable option for the outer tunnel
or tunnels if the number of innermost tunnel endpoints is limited.
The tunneling packet size issues are relatively straightforward in
host-to-host tunneling or host-to-router tunneling where Path MTU
Discovery only needs to signal to one source node. The issues are
significantly more difficult in router-to-router and certain
router-to-host scenarios, which are the focus of this memo.
It is worth noting that most of this discussion applies to a more
generic case, where there exists a link with a lower MTU in the path.
A concrete and widely deployed example of this is the usage of PPP
over Ethernet (PPPoE) [11] at the customers' access link. These
lower-MTU links, and particularly PPPoE links, are typically not
deployed in topologies where fragmentation and reassembly might be
unfeasible (e.g., a backbone), so this may be a slightly easier
problem. However, this more generic case is considered out of scope
of this memo.
There are also known challenges in specifying and implementing a
mechanism that would be used at the tunnel endpoint to obtain the
best suitable packet size to use for encapsulation: if a static value
is chosen, a lot of fragmentation might end up being performed. On
the other hand, if PMTUD is used, the implementation would need to
update the discovered interface MTU based on the ICMP Packet Too Big
messages and originate ICMP Packet Too Big message(s) back to the
source(s) of the encapsulated packets; this also assumes that
sufficient data has been piggybacked on the ICMP messages (beyond the
required 64 bits after the IPv4 header). We'll discuss using PMTUD
to signal the sources briefly in Section 3.2, but in-depth
specification and analysis are described elsewhere (e.g., in [4] and
[2]) and are out of scope of this memo.
Section 2 includes a problem statement, section 3 describes the
different solutions with their drawbacks and advantages, and section
4 presents conclusions.
2. Problem Statement
It is worth considering why exactly this is considered a problem.
It is possible to fix all the packet size issues using solution 1,
fragmenting the resulting encapsulated packet, and reassembling it by
the tunnel endpoint. However, this is considered problematic for at
least three reasons, as described in Section 3.1.
Therefore, it is desirable to avoid fragmentation and reassembly if
possible. On the other hand, the other solutions may not be
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practical either: especially in router-to-router or router-to-host
tunneling, Path MTU Discovery might be very disadvantageous --
consider the case where a backbone router would send ICMP Packet Too
Big messages to every source that would try to send packets through
it. Fragmenting before encapsulation is also not available in IPv6,
and not available when the Don't Fragment (DF) bit has been set (see
Section 3.4 for more). Ensuring a high enough MTU so encapsulation
is always possible is of course a valid approach, but requires
careful operational planning, and may not be a feasible assumption
for implementors.
This yields that there is no trivial solution to this problem, and it
needs to be further explored to consider the trade offs, as is done
in this memo.
3. Description of Solutions
This section describes the potential solutions in a bit more detail.
3.1. Fragmentation and Reassembly by the Tunnel Endpoints
The seemingly simplest solution to tunneling packet size issues is
fragmentation of the outer packet by the encapsulator and reassembly
by the decapsulator. However, this is highly problematic for at
least three reasons:
o Fragmentation causes overhead: every fragment requires the IP
header (20 or 40 bytes), and with IPv6, an additional 8 bytes for
the Fragment Header.
o Fragmentation and reassembly require computation: splitting
datagrams to fragments is a non-trivial procedure, and so is their
reassembly. For example, software router forwarding
implementations may not be able to perform these operations at
line rate.
o At the time of reassembly, all the information (i.e., all the
fragments) is normally not available; when the first fragment
arrives to be reassembled, a buffer of the maximum possible size
may have to be allocated because the total length of the
reassembled datagram is not known at that time. Furthermore, as
fragments might get lost, or be reordered or delayed, the
reassembly engine has to wait with the partial packet for some
time (e.g., 60 seconds [9]). When this would have to be done at
the line rate, with, for example 10 Gbit/s speed, the length of
the buffers that reassembly might require would be prohibitive.
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When examining router-to-router tunneling, the third problem is
likely the worst; certainly, a hardware computation and
implementation requirement would also be significant, but not all
that difficult in the end -- and the link capacity wasted in the
backbones by additional overhead might not be a huge problem either.
However, IPv4 identification header length is only 16 bits (compared
to 32 bits in IPv6), and if a larger number of packets are being
tunneled between two IP addresses, the ID is very likely to wrap and
cause data misassociation. This reassembly wrongly combining data
from two unrelated packets causes data integrity and potentially a
confidentiality violation. This problem is further described in
[12].
IPv6, and IPv4 with the DF bit set in the encapsulating header,
allows the tunnel endpoints to optimize the tunnel MTU and minimize
network-based reassembly. This also prevents fragmentation of the
encapsulated packets on the tunnel path. If the IPv4 encapsulating
header does not have the DF bit set, the tunnel endpoints will have
to perform a significant amount of fragmentation and reassembly,
while the use of PMTUD is minimized.
As Appendix A describes, the MTU of the tunnel is also a factor on
which packets require fragmentation and reassembly; the worst case
occurs if the tunnel MTU is "infinite" or equal to the physical
interface MTUs.
So, if reassembly could be made to work sufficiently reliably, this
would be one acceptable fallback solution but only for IPv6.
3.2. Signalling the Lower MTU to the Sources
Another approach is to use techniques like Path MTU Discovery (or
potentially a future derivative [13]) to signal to the sources whose
packets will be encapsulated in the network to send smaller packets
so that they can be encapsulated; in particular, when done on
routers, this includes two separable functions:
a. Forwarding behaviour: when forwarding packets, if the IPv4-only
DF bit is set, the router sends an ICMP Packet Too Big message to
the source if the MTU of the egress link is too small.
b. Router's "host" behaviour: when the router receives an ICMP
Packet Too Big message related to a tunnel, it (1) adjusts the
tunnel MTU, and (2) originates an ICMP Packet Too Big message to
the source address of the encapsulated packet. (2) can be done
either immediately or by waiting for the next packet to trigger
an ICMP; the former minimizes the packet loss due to MTU changes.
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Note that this only works if the MTU of the tunnel is of reasonable
size, and not, for example, 64 kilobytes: see Appendix A for more.
This approach would presuppose that PMTUD works. While it is
currently working for IPv6, and critical for its operation, there is
ample evidence that in IPv4, PMTUD is far from reliable due to, for
example firewalls and other boxes being configured to inappropriately
drop all the ICMP packets [14], or software bugs rendering PMTUD
inoperational.
Furthermore, there are two scenarios where signalling from the
network would be highly undesirable. The first is when the
encapsulation would be done in such a prominent place in the network
that a very large number of sources would need to be signalled with
this information (possibly even multiple times, depending on how long
they keep their PMTUD state). The second is when the encapsulation
is done for passive monitoring purposes (network management, lawful
interception, etc.) -- when it's critical that the sources whose
traffic is being encapsulated are not aware of this happening.
When desiring to avoid fragmentation, IPv4 requires one of two
alternatives [1]: copy the DF bit from the inner packets to the
encapsulating header, or always set the DF bit of the outer header.
The latter is better, especially in controlled environments, because
it forces PMTUD to converge immediately.
A related technique, which works with TCP under specific scenarios
only, is so-called "MSS clamping". With that technique or rather a
"hack", the TCP packets' Maximum Segment Size (MSS) is reduced by
tunnel endpoints so that the TCP connection automatically restricts
itself to the maximum available packet size. Obviously, this does
not work for UDP or other protocols that have no MSS. This approach
is most applicable and used with PPPoE, but could be applied
otherwise as well; the approach also assumes that all the traffic
goes through tunnel endpoints that do MSS clamping -- this is trivial
for the single-homed access links, but could be a challenge
otherwise.
A new approach to PMTUD is in the works [13], but it is uncertain
whether that would fix the problems -- at least not the passive
monitoring requirements.
3.3. Encapsulate Only When There is Free MTU
The third approach is an operational one, depending on the
environment where encapsulation and decapsulation are being
performed. That is, if an ISP would deploy tunneling in its
backbone, which would consist only of links supporting high MTUs
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(e.g., Gigabit Ethernet or SDH/SONET), but all its customers and
peers would have a lower MTU (e.g., 1500, or the backbone MTU minus
the encapsulation overhead), this would imply that no packets (with
the encapsulation overhead added) would have a larger MTU than the
"backbone MTU", and all the encapsulated packets would always fit
MTU-wise in the backbone links.
This approach is highly assumptive of the deployment scenario. It
may be desirable to build a tunnel to/from another ISP, for example,
where this might no longer hold; or there might be links in the
network that cannot support the higher MTUs to satisfy the tunneling
requirements; or the tunnel might be set up directly between the
customer and the ISP, in which case fragmentation would occur, with
tunneled fragments terminating on the ISP and thus requiring
reassembly capability from the ISP's equipment.
To restate, this approach can only be considered when tunneling is
done inside a part of specific kind of ISP's own network, not, for
example, transiting an ISP.
Another, related approach might be having the sources use only a low
enough MTU that would fit in all the physical MTUs; for example, IPv6
specifies the minimum MTU of 1280 bytes. For example, if all the
sources whose traffic would be encapsulated would use this as the
maximum packet size, there would probably always be enough free MTU
for encapsulation in the network. However, this is not the case
today, and it would be completely unrealistic to assume that this
kind of approach could be made to work in general.
It is worth remembering that while the IPv6 minimum MTU is 1280 bytes
[10], there are scenarios where the tunnel implementation must
implement fragmentation and reassembly [3]: for example, when having
an IPv6-in-IPv6 tunnel on top of a physical interface with an MTU of
1280 bytes, or when having two layers of IPv6 tunneling. This can
only be avoided by ensuring that links on top of which IPv6 is being
tunneled have a somewhat larger MTU (e.g., 40 bytes) than 1280 bytes.
This conclusion can be generalized: because IP can be tunneled on top
of IP, no single minimum or maximum MTU can be found such that
fragmentation or signalling to the sources would never be needed.
All in all, while in certain operational environments it might be
possible to avoid any problems by deployment choices, or limiting the
MTU that the sources use, this is probably not a sufficiently good
general solution for the equipment vendors. Other solutions must
also be provided.
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3.4. Fragmentation of the Inner Packet
A final possibility is fragmenting the inner packet, before
encapsulation, in such a manner that the encapsulated packet fits in
the tunnel's path MTU (discovered using PMTUD). However, one should
note that only IPv4 supports this "in-flight" fragmentation;
furthermore, it isn't allowed for packets where the Don't Fragment
bit has been set. Even if one could ignore IPv6 completely, so many
IPv4 host stacks send packets with the DF bit set that this would
seem unfeasible.
However, there are existing implementations that violate the standard
that:
o discard too big packets with the DF bit not set instead of
fragmenting them (this is rare);
o ignore the DF bit completely, for all or specified interfaces; or
o clear the DF bit before encapsulation, in the egress of configured
interfaces. This is typically done for all the traffic, not just
too big packets (allowing configuring this is common).
This is non-compliant behaviour, but there are certainly uses for it,
especially in certain tightly controlled passive monitoring
scenarios, and it has potential for more generic applicability as
well, to work around PMTUD issues.
Clearing the DF bit effectively disables the sender's PMTUD for the
path beyond the tunnel. This may result in fragmentation later in
the network, but as the packets have already been fragmented prior to
encapsulation, this fragmentation later on does not make matters
significantly worse.
As this is an implemented and desired (by some) behaviour, the full
impacts e.g., for the functioning of PMTUD (for example) should be
analyzed, and the use of fragmentation-related IPv4 bits should be
re-evaluated.
In summary, this approach provides a relatively easy fix for IPv4
problems, with potential for causing problems for PMTUD; as this
would not work with IPv6, it could not be considered a generic
solution.
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4. Conclusions
Fragmentation and reassembly by the tunnel endpoints are a clear and
simple solution to the problem, but the hardware reassembly when the
packets get lost may face significant implementation challenges that
may be insurmountable. This approach does not seem feasible,
especially for IPv4 with high data rates due to problems with
wrapping the fragment identification field [12]. Constant wrapping
may occur when the data rate is in the order of MB/s for IPv4 and in
the order of dozens of GB/s for IPv6. However, this reassembly
approach is probably not a problem for passive monitoring
applications.
PMTUD techniques, at least at the moment and especially for IPv4,
appear to be too unreliable or unscalable to be used in the
backbones. It is an open question whether a future solution might
work better in this aspect.
It is clear that in some environments the operational approach to the
problem, ensuring that fragmentation is never necessary by keeping
higher MTUs in the networks where encapsulated packets traverse, is
sufficient. But this is unlikely to be enough in general, and for
vendors that may not be able to make assumptions about the operators'
deployments.
Fragmentation of the inner packet is only possible with IPv4, and is
sufficient only if standards-incompliant behaviour, with potential
for bad side-effects (e.g., for PMTUD), is adopted. It should not be
used if there are alternatives; fragmentation of the outer packet
seems a better option for passive monitoring.
However, if reassembly in the network must be avoided, there are
basically two possibilities:
1. For IPv6, use ICMP signalling or operational methods.
2. For IPv4, packets for which the DF bit is not set can be
fragmented before encapsulation (and the encapsulating header
would have the DF bit set); packets whose DF bit is set would
need to get the DF bit cleared (though this is non-compliant).
This also minimizes the need for (unreliable) Internet-wide
PMTUD.
An interesting thing to explicitly note is that when tunneling is
done in a high-speed backbone, typically one may be able to make
assumptions on the environment; however, when reassembly is not
performed in such a network, it might be done in software or with
lower requirements, and there exists either a reassembly
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implementation using PMTUD or using a separate approach for passive
monitoring -- so this might not be a real problem.
In consequence, the critical questions at this point appear to be 1)
whether a higher MTU can be assumed in the high-speed networks that
deploy tunneling, and 2) whether "slower-speed" networks could cope
with a software-based reassembly, a less capable hardware-based
reassembly, or the other workarounds. An important future task would
be analyzing the observed incompliant behaviour about the DF bit to
note whether it has any unanticipated drawbacks.
5. Security Considerations
This document describes different issues with packet sizes and in-
the-network tunneling; this does not have security considerations on
its own.
However, different solutions might have characteristics that may make
them more susceptible to attacks -- for example, a router-based
fragment reassembly could easily lead to (reassembly) buffer memory
exhaustion if the attacker sends a sufficient number of fragments
without sending all of them, so that the reassembly would be stalled
until a timeout; these and other fragment attacks (e.g., [15]) have
already been used against, for example, firewalls and host stacks,
and need to be taken into consideration in the implementations.
It is worth considering the cryptographic expense (which is typically
more significant than the reassembly, if done in software) with
fragmentation of the inner or outer packet. If an outer fragment
goes missing, no cryptographic operations have been yet performed; if
an inner fragment goes missing, cryptographic operations have already
been performed. Therefore, which of these approaches is preferable
also depends on whether cryptography or reassembly is already
provided in hardware; for high-speed routers, at least, one should be
able to assume that if it is performing relatively heavy
cryptography, hardware support is already required.
The solutions using PMTUD (and consequently ICMP) will also need to
take into account the attacks using ICMP. In particular, an attacker
could send ICMP Packet Too Big messages indicating a very low MTU to
reduce the throughput and/or as a fragmentation/reassembly
denial-of-service attack. This attack has been described in the
context of TCP in [16].
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6. Acknowledgements
While the topic is far from new, recent discussions with W. Mark
Townsley on L2TP fragmentation issues caused the author to sit down
and write up the issues in general. Michael Richardson and Mika
Joutsenvirta provided useful feedback on the first version. When
soliciting comments from the NANOG list, Carsten Bormann, Kevin
Miller, Warren Kumari, Iljitsch van Beijnum, Alok Dube, and Stephen
J. Wilcox provided useful feedback. Later, Carlos Pignataro provided
excellent input, helping to improve the document. Joe Touch also
provided input on the memo.
7. References
7.1. Normative References
[1] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
1996.
[2] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", RFC 4213, October 2005.
[3] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
Specification", RFC 2473, December 1998.
[4] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[5] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[6] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[7] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[8] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for
IP version 6", RFC 1981, August 1996.
[9] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
[10] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
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7.2. Informative References
[11] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., and
R. Wheeler, "A Method for Transmitting PPP Over Ethernet
(PPPoE)", RFC 2516, February 1999.
[12] Mathis, M., "Fragmentation Considered Very Harmful", Work in
Progress, July 2004.
[13] Mathis, M. and J. Heffner, "Path MTU Discovery", Work in
Progress, March 2006.
[14] Medina, A., Allman, M., and S. Floyd, "Measuring the Evolution
of Transport Protocols in the Internet", Computer
Communications Review, Apr 2005, <http://www.icir.org/tbit/>.
[15] Miller, I., "Protection Against a Variant of the Tiny Fragment
Attack (RFC 1858)", RFC 3128, June 2001.
[16] Gont, F., "ICMP attacks against TCP", Work in Progress,
February 2006.
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Appendix A. MTU of the Tunnel
Different tunneling mechanisms may treat the tunnel links as having
different kinds of MTU values. Some might use the same default MTU
as for other interfaces; some others might use the default MTU minus
the expected IP overhead (e.g., 20, 28, or 40 bytes); some others
might even treat the tunnel as having an "infinite MTU", e.g., 64
kilobytes.
As [2] describes, having an infinite MTU, i.e., always fragmenting
the outer packet (and never the inner packet) and never performing
PMTUD for the tunnel path, is a very bad idea, especially in
host-to-router scenarios. (It could be argued that if the nodes are
sure that this is a host-to-host tunnel, a larger MTU might make
sense if fragmentation and reassembly are more efficient than just
sending properly sized packets -- but this seems like a stretch.)
Author's Address
Pekka Savola
CSC/FUNET
Espoo
Finland
EMail: psavola@funet.fi
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