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Network Working Group                                         J. HeffnerRequest for Comments: 4963                                     M. MathisCategory: Informational                                      B. Chandler                                                                     PSC                                                               July 2007IPv4 Reassembly Errors at High Data RatesStatus 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 IETF Trust (2007).Abstract   IPv4 fragmentation is not sufficiently robust for use under some   conditions in today's Internet.  At high data rates, the 16-bit IP   identification field is not large enough to prevent frequent   incorrectly assembled IP fragments, and the TCP and UDP checksums are   insufficient to prevent the resulting corrupted datagrams from being   delivered to higher protocol layers.  This note describes some easily   reproduced experiments demonstrating the problem, and discusses some   of the operational implications of these observations.Heffner, et al.              Informational                      [Page 1]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 20071.  Introduction   The IPv4 header was designed at a time when data rates were several   orders of magnitude lower than those achievable today.  This document   describes a consequent scale-related failure in the IP identification   (ID) field, where fragments may be incorrectly assembled at a rate   high enough that it is likely to invalidate assumptions about data   integrity failure rates.   That IP fragmentation results in inefficient use of the network has   been well documented [Kent87].  This note presents a different kind   of problem, which can result not only in significant performance   degradation, but also frequent data corruption.  This is especially   pertinent due to the recent proliferation of UDP bulk transport tools   that sometimes fragment every datagram.   Additionally, there is some network equipment that ignores the Don't   Fragment (DF) bit in the IP header to work around MTU discovery   problems [RFC2923].  This equipment indirectly exposes properly   implemented protocols and applications to corrupt data.2.  Wrapping the IP ID Field   The Internet Protocol standard [RFC0791] specifies:      "The choice of the Identifier for a datagram is based on the need      to provide a way to uniquely identify the fragments of a      particular datagram.  The protocol module assembling fragments      judges fragments to belong to the same datagram if they have the      same source, destination, protocol, and Identifier.  Thus, the      sender must choose the Identifier to be unique for this source,      destination pair and protocol for the time the datagram (or any      fragment of it) could be alive in the Internet."   Strict conformance to this standard limits transmissions in one   direction between any address pair to no more than 65536 packets per   protocol (e.g., TCP, UDP, or ICMP) per maximum packet lifetime.   Clearly, not all hosts follow this standard because it implies an   unreasonably low maximum data rate.  For example, a host sending   1500-byte packets with a 30-second maximum packet lifetime could send   at only about 26 Mbps before exceeding 65535 packets per packet   lifetime.  Or, filling a 1 Gbps interface with 1500-byte packets   requires sending 65536 packets in less than 1 second, an unreasonably   short maximum packet lifetime, being less than the round-trip time on   some paths.  This requirement is widely ignored.Heffner, et al.              Informational                      [Page 2]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 2007   Additionally, it is worth noting that reusing values in the IP ID   field once per 65536 datagrams is the best case.  Some   implementations randomize the IP ID to prevent leaking information   out of the kernel [Bellovin02], which causes reuse of the IP ID field   to occur probabilistically at all sending rates.   IP receivers store fragments in a reassembly buffer until all   fragments in a datagram arrive, or until the reassembly timeout   expires (15 seconds is suggested in [RFC0791]).  Fragments in a   datagram are associated with each other by their protocol number, the   value in their ID field, and by the source/destination address pair.   If a sender wraps the ID field in less than the reassembly timeout,   it becomes possible for fragments from different datagrams to be   incorrectly spliced together ("mis-associated"), and delivered to the   upper layer protocol.   A case of particular concern is when mis-association is self-   propagating.  This occurs, for example, when there is reliable   ordering of packets and the first fragment of a datagram is lost in   the network.  The rest of the fragments are stored in the fragment   reassembly buffer, and when the sender wraps the ID field, the first   fragment of the new datagram will be mis-associated with the rest of   the old datagram.  The new datagram will be now be incomplete (since   it is missing its first fragment), so the rest of it will be saved in   the fragment reassembly buffer, forming a cycle that repeats every   65536 datagrams.  It is possible to have a number of simultaneous   cycles, bounded by the size of the fragment reassembly buffer.   IPv6 is considerably less vulnerable to this type of problem, since   its fragment header contains a 32-bit identification field [RFC2460].   Mis-association will only be a problem at packet rates 65536 times   higher than for IPv4.3.  Effects of Mis-Associated Fragments   When the mis-associated fragments are delivered, transport-layer   checksumming should detect these datagrams as incorrect and discard   them.  When the datagrams are discarded, it could create a   performance problem for loss-feedback congestion control algorithms,   particularly when a large congestion window is required, since it   will introduce a certain amount of non-congestive loss.   Transport checksums, however, may not be designed to handle such high   error rates.  The TCP/UDP checksum is only 16 bits in length.  If   these checksums follow a uniform random distribution, we expect mis-   associated datagrams to be accepted by the checksum at a rate of one   per 65536.  With only one mis-association cycle, we expect corrupt   data delivered to the application layer once per 2^32 datagrams.Heffner, et al.              Informational                      [Page 3]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 2007   This number can be significantly higher with multiple concurrent   cycles.   With non-random data, the TCP/UDP checksum may be even weaker still.   It is possible to construct datasets where mis-associated fragments   will always have the same checksum.  Such a case may be considered   unlikely, but is worth considering.  "Real" data may be more likely   than random data to cause checksum hot spots and increase the   probability of false checksum match [Stone98].  Also, some   applications or higher-level protocols may turn off checksumming to   increase speed, though this practice has been found to be dangerous   for other reasons when data reliability is important [Stone00].4.  Experimental Observations   To test the practical impact of fragmentation on UDP, we ran a series   of experiments using a UDP bulk data transport protocol that was   designed to be used as an alternative to TCP for transporting large   data sets over specialized networks.  The tool, Reliable Blast UDP   (RBUDP), part of the QUANTA networking toolkit [QUANTA], was selected   because it has a clean interface which facilitated automated   experiments.  The decision to use RBUDP had little to do with the   details of the transport protocol itself.  Any UDP transport protocol   that does not have additional means to detect corruption, and that   could be configured to use IP fragmentation, would have the same   results.   In order to diagnose corruption on files transferred with the UDP   bulk transfer tool, we used a file format that included embedded   sequence numbers and MD5 checksums in each fragment of each datagram.   Thus, it was possible to distinguish random corruption from that   caused by mis-associated fragments.  We used two different types of   files.  One was constructed so that all the UDP checksums were   constant -- we will call this the "constant" dataset.  The other was   constructed so that UDP checksums were uniformly random -- the   "random" dataset.  All tests were done using 400 MB files, sent in   1524-byte datagrams so that they were fragmented on standard Fast   Ethernet with a 1500-byte MTU.   The UDP bulk file transport tool was used to send the datasets   between a pair of hosts at slightly less than the available data rate   (100 Mbps).  Near the beginning of each flow, a brief secondary flow   was started to induce packet loss in the primary flow.  Throughout   the life of the primary flow, we typically observed mis-association   rates on the order of a few hundredths of a percent.Heffner, et al.              Informational                      [Page 4]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 2007   Tests run with the "constant" dataset resulted in corruption on all   mis-associated fragments, that is, corruption on the order of a few   hundredths of a percent.  In sending approximately 10 TB of "random"   datasets, we observed 8847668 UDP checksum errors and 121 corruptions   of the data due to mis-associated fragments.5.  Preventing Mis-Association   The most straightforward way to avoid mis-association is to avoid   fragmentation altogether by implementing Path MTU Discovery [RFC1191]   [RFC4821].  However, this is not always feasible for all   applications.  Further, as a work-around for MTU discovery problems   [RFC2923], some TCP implementations and communications gear provide   mechanisms to disable path MTU discovery by clearing or ignoring the   DF bit.  Doing so will expose all protocols using IPv4, even those   that participate in MTU discovery, to mis-association errors.   If IP fragmentation is in use, it may be possible to reduce the   timeout sufficiently so that mis-association will not occur.   However, there are a number of difficulties with such an approach.   Since the sender controls the rate of packets sent and the selection   of IP ID, while the receiver controls the reassembly timeout, there   would need to be some mutual assurance between each party as to   participation in the scheme.  Further, it is not generally possible   to set the timeout low enough so that a fast sender's fragments will   not be mis-associated, yet high enough so that a slow sender's   fragments will not be unconditionally discarded before it is possible   to reassemble them.  Therefore, the timeout and IP ID selection would   need to be done on a per-peer basis.  Also, it is likely NAT will   break any per-peer tables keyed by IP address.  It is not within the   scope of this document to recommend solutions to these problems,   though we believe a per-peer adaptive timeout is likely to prevent   mis-association under circumstances where it would most commonly   occur.   A case particularly worth noting is that of tunnels encapsulating   payload in IPv4.  To deal with difficulties in MTU Discovery   [RFC4459], tunnels may rely on fragmentation between the two   endpoints, even if the payload is marked with a DF bit [RFC4301].  In   such a mode, the two tunnel endpoints behave as IP end hosts, with   all tunneled traffic having the same protocol type.  Thus, the   aggregate rate of tunneled packets may not exceed 65536 per maximum   packet lifetime, or tunneled data becomes exposed to possible mis-   association.  Even protocols doing MTU discovery such as TCP will be   affected.  Operators of tunnels should ensure that the receiving   end's reassembly timeout is short enough that mis-association cannot   occur given the tunnel's maximum rate.Heffner, et al.              Informational                      [Page 5]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 20076.  Mitigating Mis-Association   It is difficult to concisely describe all possible situations under   which fragments might be mis-associated.  Even if an end host   carefully follows the specification, ensuring unique IP IDs, the   presence of NATs or tunnels may expose applications to IP ID space   conflicts.  Further, devices in the network that the end hosts cannot   see or control, such as tunnels, may cause mis-association.  Even a   fragmenting application that sends at a low rate might possibly be   exposed when running simultaneously with a non-fragmenting   application that sends at a high rate.  As described above, the   receiver might implement to reduce or eliminate the possibility of   conflict, but there is no mechanism in place for a sender to know   what the receiver is doing in this respect.  As a consequence, there   is no general mechanism for an application that is using IPv4   fragmentation to know if it is deterministically or statistically   protected from mis-associated fragments.   Under circumstances when it is impossible or impractical to prevent   mis-association, its effects may be mitigated by use of stronger   integrity checking at any layer above IP.  This is a natural side   effect of using cryptographic authentication.  For example, IPsec AH   [RFC4302] will discard any corrupted datagrams, preventing their   deliver to upper layers.  A stronger transport layer checksum such as   SCTP's, which is 32 bits in length [RFC2960], may help significantly.   At the application layer, SSH message authentication codes [RFC4251]   will prevent delivery of corrupted data, though since the TCP   connection underneath is not protected, it is considered invalid and   the session is immediately terminated.  While stronger integrity   checking may prevent data corruption, it will not prevent the   potential performance impact described above of non-congestive loss   on congestion control at high congestion windows.   It should also be noted that mis-association is not the only possible   source of data corruption above the network layer [Stone00].  Most   applications for which data integrity is critically important should   implement strong integrity checking regardless of exposure to mis-   association.   In general, applications that rely on IPv4 fragmentation should be   written with these issues in mind, as well as those issues documented   in [Kent87].  Applications that rely on IPv4 fragmentation while   sending at high speeds (the order of 100 Mbps or higher) and devices   that deliberately introduce fragmentation to otherwise unfragmented   traffic (e.g., tunnels) should be particularly cautious, and   introduce strong mechanisms to ensure data integrity.Heffner, et al.              Informational                      [Page 6]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 20077.  Security Considerations   If a malicious entity knows that a pair of hosts are communicating   using a fragmented stream, it may be presented with an opportunity to   corrupt the flow.  By sending "high" fragments (those with offset   greater than zero) with a forged source address, the attacker can   deliberately cause corruption as described above.  Exploiting this   vulnerability requires only knowledge of the source and destination   addresses of the flow, its protocol number, and fragment boundaries.   It does not require knowledge of port or sequence numbers.   If the attacker has visibility of packets on the path, the attack   profile is similar to injecting full segments.  Using this attack   makes blind disruptions easier and might possibly be used to cause   degradation of service.  We believe only streams using IPv4   fragmentation are likely vulnerable.  Because of the nature of the   problems outlined in this document, the use of IPv4 fragmentation for   critical applications may not be advisable, regardless of security   concerns.8.  Informative References   [Kent87]     Kent, C. and J. Mogul, "Fragmentation considered                harmful", Proc. SIGCOMM '87 vol. 17, No. 5, October                1987.   [RFC2923]    Lahey, K., "TCP Problems with Path MTU Discovery",RFC2923, September 2000.   [RFC0791]    Postel, J., "Internet Protocol", STD 5,RFC 791,                September 1981.   [RFC1191]    Mogul, J. and S. Deering, "Path MTU discovery",RFC1191, November 1990.   [Stone98]    Stone, J., Greenwald, M., Partridge, C., and J. Hughes,                "Performance of Checksums and CRC's over Real Data",                IEEE/ ACM Transactions on Networking vol. 6, No. 5,                October 1998.   [Stone00]    Stone, J. and C. Partridge, "When The CRC and TCP                Checksum Disagree", Proc. SIGCOMM 2000 vol. 30, No. 4,                October 2000.Heffner, et al.              Informational                      [Page 7]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 2007   [QUANTA]     He, E., Alimohideen, J., Eliason, J., Krishnaprasad, N.,                Leigh, J., Yu, O., and T. DeFanti, "Quanta: a toolkit                for high performance data delivery over photonic                networks", Future Generation Computer Systems Vol. 19,                No. 6, August 2003.   [Bellovin02] Bellovin, S., "A Technique for Counting NATted Hosts",                Internet Measurement Conference, Proceedings of the 2nd                ACM SIGCOMM Workshop on Internet Measurement, November                2002.   [RFC2460]    Deering, S. and R. Hinden, "Internet Protocol, Version 6                (IPv6) Specification",RFC 2460, December 1998.   [RFC2960]    Stewart, R., Xie, Q., Morneault, K., Sharp, C.,                Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,                Zhang, L., and V. Paxson, "Stream Control Transmission                Protocol",RFC 2960, October 2000.   [RFC4251]    Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)                Protocol Architecture",RFC 4251, January 2006.   [RFC4301]    Kent, S. and K. Seo, "Security Architecture for the                Internet Protocol",RFC 4301, December 2005.   [RFC4302]    Kent, S., "IP Authentication Header",RFC 4302, December                2005.   [RFC4459]    Savola, P., "MTU and Fragmentation Issues with In-the-                Network Tunneling",RFC 4459, April 2006.   [RFC4821]    Mathis, M. and J. Heffner, "Packetization Layer Path MTU                Discovery",RFC 4821, March 2007.Heffner, et al.              Informational                      [Page 8]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 2007Appendix A.  Acknowledgements   This work was supported by the National Science Foundation under   Grant No. 0083285.Authors' Addresses   John W. Heffner   Pittsburgh Supercomputing Center   4400 Fifth Avenue   Pittsburgh, PA  15213   US   Phone: 412-268-2329   EMail: jheffner@psc.edu   Matt Mathis   Pittsburgh Supercomputing Center   4400 Fifth Avenue   Pittsburgh, PA  15213   US   Phone: 412-268-3319   EMail: mathis@psc.edu   Ben Chandler   Pittsburgh Supercomputing Center   4400 Fifth Avenue   Pittsburgh, PA  15213   US   Phone: 412-268-9783   EMail: bchandle@gmail.comHeffner, et al.              Informational                      [Page 9]

RFC 4963       IPv4 Reassembly Errors at High Data Rates       July 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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, THE IETF TRUST 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.Intellectual Property   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 procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Heffner, et al.              Informational                     [Page 10]