Draft RFC for the "DF scheme"
mogul (Jeffrey Mogul) Thu, 19 April 1990 00:16 UTC
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From: mogul (Jeffrey Mogul)
Date: 18 Apr 1990 1616-PST (Wednesday)
Subject: Draft RFC for the "DF scheme"
Please read this BEFORE the IETF meeting next month. A PostScript version is available if you really must have one. -Jeff (with lots of help from Steve Deering) Internet Engineering Task Force J. Mogul Internet Draft DECWRL Obsoletes: RFC 1063 S. Deering Stanford University April 1990 WORKING DRAFT -- Do Not Redistribute Path MTU Discovery Abstract This memo describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. It specifies a small change to the way routers generate one type of ICMP message. For a path that passes through a router that has not been so changed, this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by current practice. Status of this memo ---to be determined--- Distribution of this memo is unlimited. Acknowledgements This proposal is a product of the IETF MTU Discovery Working Group. The mechanism proposed here was first suggested by Geof Cooper , who in two short paragraphs set out all the basic ideas that took the Working Group months to reinvent. 1. Introduction When one IP host has a large amount of data to send to another host, the data is transmitted as a series of IP datagrams. It is usually preferable that these datagrams be of the largest size that does not require fragmentation anywhere along the path from the source to the destination. (For the case against fragmentation, see .) This datagram size is referred to as the Path MTU (PMTU), and it is equal to the minimum of the MTUs of each hop in the path. A shortcoming of the current Internet protocol suite is the lack of a standard mechanism for a host to discover the PMTU of an arbitrary path. The current practice  is to use the lesser of 576 and the first-hop MTU as the PMTU for any destination that is not connected Mogul/Deering [page 1] Internet Draft Path MTU Discovery April 1990 to the same network or subnet as the source. In many cases, this results in the use of smaller datagrams than necessary, because many paths have a PMTU greater than 576. A host sending datagrams much smaller than the Path MTU allows is wasting Internet resources and probably getting suboptimal throughput. Furthermore, current practice does not prevent fragmentation in all cases, since there are some paths whose PMTU is less than 576. It is expected that future routing protocols will be able to provide accurate PMTU information within a routing area, although perhaps not across multi-level routing hierarchies. It is not clear how soon that will be ubiquitously available, so for the next several years the Internet needs a simple mechanism that discovers PMTUs without wasting resources and that works before all hosts and routers are modified. 2. Protocol overview In this memo, we describe a technique for using the Don't Fragment (DF) bit in the IP header to dynamically discover the PMTU of a path. 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''. Upon receipt of such a message (henceforth called a ``Datagram Too Big'' message), the source host reduces its assumed PMTU for the path. The PMTU discovery process ends when the host's estimate of the PMTU is low enough that its datagrams can be delivered without fragmentation. Or, the host may elect to end the discovery process by ceasing to set the DF bit in the datagram headers; it may do so, for example, because it is willing to have datagrams fragmented in some circumstances. Normally, the host continues to set DF in all datagrams, so that if the route changes and the new PMTU is lower, it will be discovered. Unfortunately, the Datagram Too Big message, as currently specified, does not report the MTU of the hop for which the rejected datagram was too big, so the source host cannot tell exactly how much to reduce its assumed PMTU. To remedy this, we propose that a currently unused header field in the Datagram Too Big message be used to report the MTU of the constricting hop. This is the only change specified for routers in support of PMTU Discovery. Mogul/Deering [page 2] Internet Draft Path MTU Discovery April 1990 The PMTU of a path may change over time, due to changes in the routing topology. Reductions of the PMTU are detected by Datagram Too Big messages, except on paths for which the host has stopped setting the DF bit. To detect increases in a path's PMTU, a host periodically reinitializes its assumed PMTU to be the first-hop MTU (and if it had stopped, resumes setting the DF bit). This will almost always result in datagrams being discarded and Datagram Too Big messages being generated, because in most cases the PMTU of the path will not have changed, so it should be done infrequently. Since this mechanism essentially guarantees that host will not receive any fragments from a peer doing PMTU Discovery, it may aid in interoperating with certain hosts that (improperly) are unable to reassemble fragmented datagrams. 3. Host specification When a host receives a Datagram Too Big message, it must reduce its estimate of the PMTU to the relevant destination, based on the value of the Next-Hop MTU field in the message (see section 4). We do not specify the precise behavior of a host in this circumstance, since different applications may have different requirements, and since different implementation architectures may favor different strategies. We do require that after receiving a Datagram Too Big message, a host MUST attempt to avoid eliciting more such messages in the near future. The host may either reduce the size of the datagrams it is sending along the path, or cease setting the Don't Fragment bit in the headers of those datagrams. Clearly, the former strategy may continue to elicit Datagram Too Big messages for a while, but since each of these messages (and the dropped datagrams they respond to) consume Internet resources, the host MUST force the PMTU discovery process to converge. Hosts using MTU Discovery MUST detect decreases in Path MTU as fast as possible. Hosts SHOULD detect increases in Path MTU, but because doing so requires sending datagrams larger than the current estimated PMTU, and because the likelihood is that the PMTU will not have increased, this SHOULD be done at infrequent intervals (10 minutes or so). Hosts must be able to deal with Datagram Too Big messages that do not include the next-hop MTU, since it is not feasible to upgrade all the routers in the Internet in any finite time. A Datagram Too Big message from an unmodified router can be recognized by the presence Mogul/Deering [page 3] Internet Draft Path MTU Discovery April 1990 of a zero in the (newly-defined) Next-Hop MTU field. (This is required by the ICMP specification , which says that ``unused'' fields must be zero.) In section 5, we discuss possible strategies for a host to follow in response to an old-style Datagram Too Big message (one sent by an unmodified router). A host MUST never reduce its estimate of the Path MTU below 68 octets. A host MUST not increase its estimate of the Path MTU in response to the contents of a Datagram Too Big message. A message purporting to announce an increase in the Path MTU might be a stale datagram that has been floating around in the Internet, a false packet injected as part of a denial-of-service attack, or the result of having multiple paths to the destination. 3.1. TCP MSS Option A host doing PMTU Discovery must obey the rule that it not send IP datagrams larger than 576 octets unless it has permission from the receiver. For TCP connections, this means that a host must not send datagrams larger than the Maximum Segment Size (MSS) sent by its peer. Section 126.96.36.199 of ``Requirements for Internet Hosts -- Communication Layers''  says: Some TCP implementations send an MSS option only if the destination host is on a non-connected network. However, in general the TCP layer may not have the appropriate information to make this decision, so it is preferable to leave to the IP layer the task of determining a suitable MTU for the Internet path. Actually, many TCP implementations always send an MSS option, but set the value to 536 if the destination is non-local. Note: The TCP MSS is defined to be the relevant IP datagram size minus 40 . The default of 576 octets for the maximum IP datagram size yields a default of 536 octets for the TCP MSS. This behavior was correct when the Internet was full of hosts that did not follow the rule that datagrams larger than 576 octets should not be sent to non-local destinations. Now that most hosts do follow this rule, it is unnecessary to limit the value in the TCP MSS option Mogul/Deering [page 4] Internet Draft Path MTU Discovery April 1990 to 536 for non-local peers. Moreover, doing this prevents PMTU Discovery from discovering PMTUs larger than 576, so hosts SHOULD no longer lower the value they send in the MSS option. The MSS option should now reflect the size of the largest datagram the host is able to reassemble (MMS_R, as defined in ); in many cases, this will be the architectural limit of 65535 octets. A host MAY send an MSS value derived from the MTU of its connected network (the maximum MTU over its connected networks, for a multi-homed host); this should not cause problems for PMTU Discovery, and may dissuade a broken peer from sending enormous datagrams. 4. Router specification When a router is unable to forward a datagram because it exceeds the MTU of the next-hop network and its Don't Fragment bit is set, the router is required to return an ICMP Destination Unreachable message to the source of the datagram, with the Code indicating ``fragmentation needed and DF set''. To support the Path MTU discovery technique specified in this memo, the router MUST include the MTU of that next-hop network in the low-order 16 bits of the ICMP header field that is labelled ``unused'' in the ICMP specification . The high-order 16 bits remain unused, and MUST be set to zero. Thus, the message has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 3 | Code = 4 | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | unused = 0 | Next-Hop MTU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Internet Header + 64 bits of Original Datagram Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The value carried in the Next-Hop MTU field is: The size in octets of the largest datagram that could be forwarded, along the path of the original datagram, without being fragmented at this router. The size includes the IP header and IP data, and does not include any lower-level headers. This field will never contain a value less than 68, since every router ``must be able to forward a datagram of 68 octets without Mogul/Deering [page 5] Internet Draft Path MTU Discovery April 1990 fragmentation'' . 5. Host processing of old-style messages In this section we outline several possible strategies for a host to follow upon receiving a Datagram Too Big message from an unmodified router (i.e., one where the Next-Hop MTU field is zero). This section is not part of the protocol specification. The simplest thing for a host to do in response to such a message is to assume that the PMTU is the minimum of its currently-assumed PMTU and 576, and to stop setting the DF bit in datagrams sent on that path. Thus, the host falls back to the same PMTU as it would choose under current practice (see section 3.3.3 of ``Requirements for Internet Hosts -- Communication Layers'' ). This strategy has the advantage that it terminates quickly, and does no worse than existing practice. It fails, however, to avoid fragmentation in some cases, and to make the most efficient utilization of the internetwork in other cases. More sophisticated strategies involve ``searching'' for an accurate PMTU estimate, by continuing to send datagrams with the DF bit while varying their sizes. A good search strategy is one that obtains an accurate estimate of the Path MTU without causing many packets to be lost in the process. Several possible strategies apply algorithmic functions to the previous PMTU estimate to generate a new estimate. For example, one could multiply the old estimate by a constant (say, 0.75). We do NOT recommend this; it either converges far too slowly, or it substantially underestimates the true PMTU. A more sophisticated approach is to do a binary search on the packet size. This converges somewhat faster, although it still takes 4 or 5 steps to converge from an FDDI MTU to an Ethernet MTU. A serious disadvantage is that it requires a complex implementation in order to recognize when a datagram has made it to the other end (indicating that the current estimate is too low). We also do not recommind this strategy. One strategy that appears to work quite well starts from the observation that there are, in practice, relatively few MTU values in use in the Internet. Thus, rather than blindly searching through arbitrarily chosen values, we can search only the ones that are likely to appear. Moreover, since designers tend to chose MTUs in similar ways, it is possible to collect groups of similar MTU values Mogul/Deering [page 6] Internet Draft Path MTU Discovery April 1990 and use the lowest value in the group as our search ``plateau.'' (It is clearly better to underestimate an MTU by a few per cent than to overestimate it by one octet.) In section 7, we describe how we arrived at a table of representative MTU plateaus for use in PMTU estimation. With this table, convergence is as good as binary search in the worst case, and is far better in common cases (for example, it takes only two round-trip times to go from an FDDI MTU to an Ethernet MTU). Since the plateaus lie near powers of two, if an MTU is not represented in this table, the algorithm will not underestimate it by more than a factor of 2. Any search strategy must have some ``memory'' of previous estimates in order to chose the next one. One approach is to use the currently-cached estimate of the Path MTU, but in fact there is better information available in the Datagram Too Big message itself. All ICMP Destination Unreachable messages, including this one, contain the IP header of the original datagram, which contains the Total Length of the datagram that was too big to be forwarded without fragmentation. Since this Total Length may be less than the current PMTU estimate, but is nonetheless larger than the actual PMTU, it is a good input to the method for chosing the next PMTU estimate. Note: routers based on implementations derived from 4.2BSD Unix send an incorrect value for the Total Length of the original IP datagram. The value sent by these routers is the sum of the original Total Length and the original Header Length (expressed in octets). Since it is impossible for the host receiving such a Datagram Too Big message to know if it sent by one of these routers, the host must be conservative and assume that it is. If the Total Length field returned is not less than the current PMTU estimate, it must be reduced by 4 times the value of the returned Header Length field. The strategy we recommend, then, is to use as the next PMTU estimate the greatest plateau value that is less than the returned Total Length field [corrected, if necessary, according to the Note above]. 6. Host implementation In this section we discuss how PMTU Discovery is implemented in host software. This is not a specification, but rather a set of suggestions. The issues include: Mogul/Deering [page 7] Internet Draft Path MTU Discovery April 1990 - What layer or layers implement PMTU Discovery? - Where is the PMTU information cached? - How is stale PMTU information removed? - What must transport and higher layers do? 6.1. Layering In the IP architecture, the choice of what size datagram to send is made by a protocol at a layer above IP. We refer to such a protocol as a ``packetization protocol.'' Packetization protocols are usually transport protocols (for example, TCP) but can also be higher-layer protocols (for example, protocols built on top of UDP). Implementing PMTU Discovery in the packetization layers simplifies some of the inter-layer issues, but has several drawbacks: the implementation may have to be redone for each packetization protocol, it becomes hard to share PMTU information between different packetization layers, and the connection-oriented state maintained by some packetization layers may not easily extend to save PMTU information for long periods. We therefore believe that the IP layer should store PMTU information and that the ICMP layer should process received Datagram Too Big messages. The packetization layers must still be able to respond to changes in the Path MTU, by changing the size of the datagrams they send, and must also be able to specify that datagrams are sent with the DF bit set. We do not want the IP layer to simply set the DF bit in every packet, since it is possible that a packetization layer, perhaps a UDP application outside the kernel, is unable to change its datagram size. Protocols involving intentional fragmentation, while inelegant, are sometimes successful (NFS the primary example), and we do not want to break such protocols. To support this layering, packetization layers requires an extension of the IP service interface defined in : A way to learn of changes in the value of MMS_S, the ``maximum send transport-message size,'' which is derived from the Path MTU by subtracting the minimum IP header size. Mogul/Deering [page 8] Internet Draft Path MTU Discovery April 1990 6.2. Storing PMTU information In general, the IP layer should associate each PMTU value that it has learned with a specific path. A path is identified by a source address, a destination address and an IP type-of-service. (Some implementations do not record the source address of paths; this is acceptable for single-homed hosts, which have only one possible source address.) Note: Some paths may be further distinguished by different security classifications. The details of such classifications are beyond the scope of this memo. The obvious place to store this association is as a field in the routing table entries. A host will not have a route for every possible destination, but it should be able to cache a per-host route for every active destination. (This requirement is already imposed by the need to process ICMP Redirect messages.) When the first packet is sent to a host for which no per-host route exists, a route is chosen either from the set of per-network routes, or from the set of default routes. The PMTU fields in these route entries should be initialized to be the MTU of the associated first-hop data link, and MUST never be changed by the PMTU Discovery process. Until a Datagram Too Big message is received, the PMTU associated with the initially-chosen route is presumed to be accurate. When a Datagram Too Big message is received, the ICMP layer determines a new estimate for the Path MTU (either from a non-zero Next-Hop MTU value in the packet, or using the method described in section 5). If a per-host route for this path does not exist, then one is created (almost as if a per-host ICMP Redirect is being processed; the new route uses the same first-hop router as the current route). If the PMTU estimate associated with the per-host route is higher than the new estimate, then the value in the routing entry is changed. The packetization layers must be notified about decreases in the PMTU. Any packetization layer instance (for example, a TCP connection) that is actively using the path must be notified if the PMTU estimate is decreased. Note: even if the Datagram Too Big message contains an Original Datagram Header that refers to a UDP packet, the TCP layer must be notified if any of its connections use the given path. Mogul/Deering [page 9] Internet Draft Path MTU Discovery April 1990 Also, the instance that sent the datagram that elicited the Datagram Too Big message should be notified that its datagram has been dropped, even if the PMTU estimate has not changed, so that may retransmit the dropped datagram. Note: The notification mechanism can be analogous to the mechanism used to provide notification of an ICMP Source Quench message. In some implementations (such as 4.2BSD-derived systems), the existing notification mechanism is not able to identify the specific connection involved, and so an additional mechanism is necessary. Alternatively, an implementation can avoid the use of an asynchronous notification mechanism for PMTU decreases by postponing notification until the next attempt to send a datagram larger than the PMTU estimate. In this approach, when an attempt is made to SEND a datagram with the DF bit set, and the datagram is larger than the PMTU estimate, the SEND function should fail and return a suitable error indication. This approach may be more suitable to a connectionless packetization layer (such as one using UDP), which may be hard to ``notify'' from the ICMP layer. In this case, the normal timeout-based retransmission mechanisms would be used to recover from the dropped datagrams. 6.3. Purging stale PMTU information Internetwork topology is dynamic; routes change over time. The PMTU discovered for a given destination may be wrong if a new route comes into use. Thus, PMTU information cached by a host can become stale. Because a host using PMTU discovery always sets the DF bit, if the stale PMTU value is too large, this will be discovered almost immediately once a datagram is sent to the given destination. No such mechanism exists for realizing that a stale PMTU value is too small, so an implementation should ``age'' cached values. When a PMTU value has not been decreased for a while (on the order of 10 minutes), the PMTU estimate should be set to the first-hop data-link MTU, and the packetization layers should be notified of the change. This will cause the complete PMTU discovery process to take place again. Note: an implementation should provide a means for changing the timeout duration, including setting it to ``infinity.'' For example, hosts attached to an FDDI network which is then attached to the rest of the Internet via a slow serial line Mogul/Deering [page 10] Internet Draft Path MTU Discovery April 1990 are never going to discover a new non-local PMTU, so they should not have to put up with dropped datagrams every 10 minutes. An upper layer MUST not retransmit datagrams in response to an increase in the PMTU estimate, since this increase never comes in response to an indication of a dropped datagram. One approach to implementing PMTU ageing is to add a timestamp field to the routing table entry. This field is initialized to a ``reserved'' value, indicating that the PMTU has never been changed. Whenever the PMTU is decreased in response to a Datagram Too Big message, the timestamp is set to the current time. Once a minute, a timer-driven procedure runs through the routing table, and for each entry whose timestamp is not ``reserved'' and is older than the timeout interval: - The PMTU estimate is set to the MTU of the associated first hop. - Packetization layers using this route are notified of the increase. PMTU estimates may disappear from the routing table if the per-host routes are removed; this can happen in response to an ICMP Redirect message, or because certain routing-table daemons delete old routes after several minutes. Also, on a multi-homed host a topology change may result in the use of a different source interface. When this happens, if the packetization layer is not notified then it may continue to use a cached PMTU value that is now too small. One solution is to notify the packetization layer of a possible PMTU change whenever a Redirect message causes a route change, and whenever a route is simply deleted from the routing table. 6.4. TCP layer actions The TCP layer must track the PMTU for the destination of a connection; it should not send datagrams that would be larger than this. A simple implementation could ask the IP layer for this value (using the GET_MAXSIZES interface described in ) each time it created a new segment, but this could be inefficient. Moreover, TCP implementations that follow the ``slow-start'' congestion-avoidance algorithm  typically calculate and cache several other values derived from the PMTU. It may be simpler to receive asynchronous notification when the PMTU changes, so that these variables may be Mogul/Deering [page 11] Internet Draft Path MTU Discovery April 1990 updated. A TCP implementation must also store the MSS value received from its peer (which defaults to 536), and not send any datagram larger than this MSS, regardless of the PMTU. In 4.xBSD-derived implementations, this requires adding an additional field to the TCP state record. Finally, when a Datagram Too Big message is received, it implies that a datagram was dropped by the router that sent the ICMP message. It is sufficient to treat this as any other dropped segment, and wait until the retransmission timer expires to cause retransmission of the segment. If the PMTU Discovery process requires several steps to estimate the right PMTU, this could delay the connection by many round-trip times. Alternatively, the retransmission could be done in immediate response to a notification that the Path MTU has changed, but only for the specific connection specified by the Datagram Too Big message. Note: One MUST not retransmit in response to every Datagram Too Big message, since a burst of several oversized segments will give rise to several such messages and hence several retransmissions of the same data. If the new estimated PMTU is still wrong, the process repeats, and there is an exponential growth in the number of superfluous segments sent! This means that the TCP layer must be able to recognize when a notification actually decreases the PMTU that it has already used to send a datagram on the given connection, and should ignore any other notifications. NOTE: SOMEONE SHOULD LOOK AT THE INTERACTION BETWEEN THESE RETRANSMISSIONS AND SLOW-START. SHOULD THE CONGESTION WINDOW BE SET BACK TO ONE PMTU? 6.5. Issues for other transport protocols Some transport protocols (such as ISO TP4 ) are not allowed to repacketize when doing a retransmission. That is, once an attempt is made to transmit a datagram of a certain size, its contents cannot be split into smaller datagrams for retransmission. In such a case, the original datagram should be retransmitted without the DF bit set, allowing it to be fragmented as necessary to reach its destination. Subsequent datagrams, when transmitted for the first time, should be no larger than allowed by the Path MTU, and should have the DF bit set. Mogul/Deering [page 12] Internet Draft Path MTU Discovery April 1990 NOTE: COULD SAY SOMETHING ABOUT NFS HERE. 6.6. Management interface We suggest that an implementation provide a way for a system utility program to: - Specify that PMTU Discovery not be done on a given route. - Change the PMTU value associated with a given route. The latter can be accomplished by associating a flag with the routing entry; when a packet is sent via a route with this flag set, the IP layer leaves the DF bit clear no matter what the upper layer requests. These features might be used to work around an anomalous situation, or by a routing protocol implementation that is able to obtain Path MTU values. The implementation should also provide a way to change the timeout period for ageing stale PMTU information. 7. Likely values for Path MTUs The algorithm recommended in section 5 for ``searching'' the space of Path MTUs is based on a table of values that severely restricts the search space. We describe here a table of MTU values that, as of this writing, represents all major data-link technologies in use in the Internet. In table 7-1, data links are listed in order of decreasing MTU, and grouped so that each set of similar MTUs is associated with a ``plateau'' equal to the lowest MTU in the group. (The table also includes some entries not currently associated with a data link, and gives references where available). Where a plateau represents more than one MTU, the table shows the maximum inaccuracy associated with the plateau, as a percentage. We do not expect that the values in the table, especially for higher MTU levels, are going to be valid forever. The values given here are an implementation suggestion, NOT a specification or requirement. Implementors should use up-to-date references to pick a set of plateaus; it is important that the table not contain too many entries or the process of searching for a PMTU might waste Internet Mogul/Deering [page 13] Internet Draft Path MTU Discovery April 1990 resources. Implementors should also make it convenient for customers without source code to update the table values in their systems (for example, the table in a BSD-derived Unix kernel could be changed using a new ``ioctl'' command). Plateau MTU Comments Reference 65535 Official maximum MTU RFC 791 65535 Hyperchannel RFC 1044 65535 32000 Just in case 17914 16Mb IBM Token Ring ref.  17914 8166 IEEE 802.4 RFC 1042 8166 4408 4Mb IBM Token Ring ref.  4352 FDDI (Revised) RFC XXX 4352 (1%) 2048? Wideband Network RFC 907 2002 IEEE 802.5 RFC 1042 2002 (2%) 1536 Exp. Ethernet Nets RFC 895 1500 Ethernet Networks RFC 894 1500 Point-to-Point (max MTU) RFC 1134 1492 IEEE 802.3 RFC 1042 1492 (3%) 1006 SLIP RFC 1055 1006 ARPANET BBN 1822 1006 576 X.25 Networks RFC 877 544 DEC IP Portal ref.  512 NETBIOS RFC 1088 508 IEEE 802/Source-Rt Bridge RFC 1042 508 ARCNET RFC 1051 508 (13%) 296 Serial Lines ??? 296 68 Official minimum MTU RFC 791 Table 7-1: Common MTUs in the Internet Mogul/Deering [page 14] Internet Draft Path MTU Discovery April 1990 8. Security considerations This Path MTU Discovery mechanism makes possible two denial-of- service attacks, both based on a malicious party sending false Datagram Too Big messages to an Internet host. In the first attack, the false message indicates a PMTU much smaller than reality. This should not entirely stop data flow, since the victim host should never set its PMTU estimate below the absolute minimum, but at 8 octets of IP data per datagram, progress could be slow. In the other attack, the false message indicates a PMTU greater than reality. If believed, this could cause temporary blockage as the victim sends datagrams that will be dropped by some router. Within one round-trip time, the host would discover its mistake (receiving Datagram Too Big messages from that router), but frequent repetition of this attack could cause lots of datagrams to be dropped. A host, however, should never raise its estimate of the PMTU based on a Datagram Too Big message, so should not be vulnerable to this attack. A malicious party could also cause problems if it could stop a victim from receiving legitimate Datagram Too Big messages, but in this case there are simpler denial-of-service attacks available. References  R. Braden, ed. Requirements for Internet Hosts -- Communication Layers. RFC 1122, SRI Network Information Center, October, 1989.  Geof Cooper. IP Datagram Sizes. Electronic distribution of the TCP-IP Discussion Group, Message-ID <8705240517.AA01407@apolling.imagen.uucp>cp>.  ISO. ISO Transport Protocol Specification: ISO DP 8073. RFC 905, SRI Network Information Center, April, 1984.  Van Jacobson. Congestion Avoidance and Control. In Proc. SIGCOMM '88 Symposium on Communications Architectures and Protocols, pages 314-329. Stanford, CA, August, 1988. Mogul/Deering [page 15] Internet Draft Path MTU Discovery April 1990  C. Kent and J. Mogul. Fragmentation Considered Harmful. In Proc. SIGCOMM '87 Workshop on Frontiers in Computer Communications Technology. August, 1987.  Drew Daniel Perkins. Private Communication.  J. Postel. Internet Control Message Protocol. RFC 792, SRI Network Information Center, September, 1981.  J. Postel. Internet Protocol. RFC 791, SRI Network Information Center, September, 1981.  J. Postel. The TCP Maximum Segment Size and Related Topics. RFC 879, SRI Network Information Center, November, 1983.  Michael Reilly. Private Communication. Authors' Addresses Steve Deering Computer Science Department Stanford University Stanford, CA 94305 Phone: (415) 723-9427 EMail: firstname.lastname@example.org Jeffrey Mogul Digital Equipment Corporation Western Research Laboratory 100 Hamilton Avenue Palo Alto, CA 94301 Phone: (415) 853-6643 EMail: email@example.com Mogul/Deering [page 16]