RFC 1063

IP MTU discovery options

📅 published July 1988, from ietf

IP MTU Discovery Options 🦋

okay bestie, this is RFC 1063 🎀 it's from the Network Working Group and it's a Request For Comments: 1063, which is the standards-y way of saying "here's an idea, tell us what you think" 💅

Here's the title block tea ☕

Authors: J. Mogul (DEC), C. Kent (DEC), C. Partridge (BBN), and K. McCloghrie (TWG) 🫶
Date: July 1988 🌸
Status: an Experimental protocol proposal 🧪 (it's a proposal, not a final thing, so no cap there)
Obsoletes and updates: the title block doesn't list anything it obsoletes or updates ✨

STATUS OF THIS MEMO 🌸

This memo is all about a pair of IP options that can be used to learn the minimum MTU of a path through an internet 🧚, along with what you can do with them. It's a proposal for an Experimental protocol, and distribution of this memo is unlimited 💖.

INTRODUCTION 💅

Okay so the Internet Protocol lets gateways fragment packets that are too big to forward, but fragmentation is not always desirable 🙈. It can lead to poor performance or even total communication failure in situations that are surprisingly common 💀 (for a thorough discussion, see [1]).

A datagram gets fragmented if it's bigger than the Maximum Transmission Unit (MTU) of some network along the path it follows. So to avoid fragmentation, a host sending an IP datagram MUST ensure that the datagram is no larger than the Minimum MTU (MINMTU) over the entire path 🎀.

It has long been recognized that the methods for discovering the MINMTU of an IP internetwork path are inadequate 😭. The methods currently available fall into two categories: (1) choosing small MTUs to avoid fragmentation or (2) using additional probe packets to discover when fragmentation will occur. Both have problems, bestie.

Choosing MTUs is a whole balancing act ⚖️. You want network utilization, which means using the largest possible datagram, but you also want fragmentation avoidance. Without knowing the network path, fragmentation avoidance pushes you toward small, and thus too many, datagrams. So any choice for the MTU size made without information from the network is likely to either fail to properly utilize the network or fail to avoid fragmentation 💀.

Probe packets burden the network with unnecessary packets 🙃. And because network paths often change during the lifetime of a TCP connection, probe packets will have to be sent on a regular basis to detect any changes in the effective MINMTU.

Implementors sometimes mistake the TCP MSS option as a mechanism for learning the network MINMTU, but that's not it bestie 🙅‍♀️. In fact, the MSS option is only a mechanism for learning about buffering capabilities at the two TCP peers. Separate provisions MUST be made to learn the IP MINMTU ✨.

In this memo, we propose two new IP options that, when used in conjunction, will permit two peers to determine the MINMTU of the paths between them 🦋. In this scheme, one option is used to determine the lowest MTU in a path, and the second option is used to convey this MTU back to the sender. That can happen in the IP datagram carrying the transport acknowledgement to the datagram that held the MTU discovery option 👑.

OPTION FORMATS 🧚

Probe MTU Option (Number 11) 🔥

Format

              +--------+--------+--------+--------+
              |00001011|00000100|   2 octet value |
              +--------+--------+--------+--------+

Definition

This option always contains the lowest MTU of all the networks that have been traversed so far by the datagram 🫧.

A host that sends this option MUST initialize the value field to be the MTU of the directly-connected network. If the host is multi-homed, this SHOULD be for the first-hop network.

Each gateway that receives a datagram containing this option MUST compare the MTU field with the MTUs of the inbound and outbound links for the datagram. If either MTU is lower than the value in the MTU field of the option, the option value SHOULD be set to the lower MTU. (Note that gateways conforming to RFC-1009 MAY not know either the inbound interface or the outbound interface at the time that IP options are processed. Accordingly, support for this option MAY require major gateway software changes.)

Any host receiving a datagram containing this option SHOULD confirm that the value of the MTU field of the option is less than or equal to that of the inbound link. If it isn't, it SHOULD reduce the MTU field value, before processing the option.

If the receiving host is not able to accept datagrams as large as specified by the value of the MTU field of the option, then it SHOULD reduce the MTU field to the size of the largest datagram it can accept.

Reply MTU Option (Number 12) 🌈

Format

              +--------+--------+--------+--------+
              |00001100|00000100|   2 octet value |
              +--------+--------+--------+--------+

Definition

This option is used to return the value learned from a Probe MTU option to the sender of the Probe MTU option 💌.

RELATION TO TCP MSS 🎀

Note that there are two superficially similar problems in choosing the size of a datagram 👯. First, there is the restriction [2] that a host not send a datagram larger than 576 octets unless it has assurance that the destination is prepared to accept a larger datagram. Second, the sending host SHOULD NOT send a datagram larger than MINMTU, in order to avoid fragmentation. The datagram size SHOULD normally be the minimum of these two lower bounds.

In the past, the TCP MSS option [3] has been used to avoid sending packets larger than the destination can accept. Unfortunately, this is not the most general mechanism. It is not available to other transport layers, and it cannot determine the MINMTU, because gateways do not parse TCP options 😤.

Because the MINMTU returned by a probe cannot be larger than the maximum datagram size that the destination can accept, this IP option could, in theory, supplant the use of the TCP MSS option, providing an economy of mechanism 🧠. (Note however, that some researchers believe that the value of the TCP MSS is distinct from the path's MINMTU. The MSS is the upper limit of the data size that the peer will accept, while the MINMTU represents a statement about the data size supported by the path 🙈.)

Note that a failure to observe the MINMTU restriction is not normally fatal. Fragmentation will occur, but this is supposed to work. A failure to observe the TCP MSS option, however, could be fatal, because it might lead to datagrams that can never be accepted by the destination 💀. Therefore, unless and until the Probe MTU option is universally implemented, at least by hosts, the TCP MSS option MUST be used as well 🫶.

IMPLEMENTATION APPROACHES ✨

Who Sends the Option 🎀

Okay so there are at least two ways to build MTU discovery 🦋. Method one makes the transport layer in charge. Method two makes the IP layer in charge. A host system SHOULD support one of the two schemes 💅.

Transport Discovery 🚀

In the transport case, the transport layer can pop a Probe MTU option onto an outbound datagram ✨. When a datagram with a Probe MTU option comes in, that option MUST be passed up to the receiving transport layer, and then that layer SHOULD acknowledge the Probe with a Reply MTU option in the next return datagram 💖.

Bestie, heads up 😭: because these options ride on unreliable datagrams, the original sender will have to resend Probes (possibly once per window of data) until it gets a Reply option. Also, the Reply MTU option MAY come back on an IP datagram for a different transport protocol than the one it was sent on (e.g., TCP generated the probe but the Reply was received on a UDP datagram) 🌈.

IP Discovery 🌸

Ngl, the better scheme is putting MTU discovery into the IP layer, using control mechanisms in the routing cache 👑. Whenever an IP datagram is sent, the IP layer checks the routing cache to see if a Probe MTU or Reply MTU option needs to go in the datagram. Whenever a datagram with either option arrives, the info in those options gets placed in the routing cache 🧚.

The basic working is kinda complex 💀, so we trace it through one round-trip. Implementors SHOULD realize that there may be cases where both options are contained in one datagram. For this explanation, the sender of the probe is called the Probe-Sender, and the receiver is the Probe-Receiver 🫶.

When the IP layer is asked to send a Probe MTU option (see the section below on when to probe), it makes some record in the routing cache that indicates the next IP datagram to Probe-Receiver SHOULD contain the Probe MTU option.

When the next IP datagram to Probe-Receiver is sent, the Probe MTU option is inserted. The IP layer in Probe-Sender SHOULD continue to send an occasional Probe MTU in subsequent datagrams until a Reply MTU option is received. It is strongly RECOMMENDED that the Probe MTU not be sent in all datagrams, but only at such a rate that, on average, one Probe MTU will be sent per round-trip interval. (Another way of saying this: we would hope that only one datagram in a transport protocol window worth of data has the Probe MTU option set.) This mechanism might be implemented by sending every Nth packet, or, in those implementations where the round-trip time estimate to the destination is cached with the route, once every estimated RTT. 🍓

When a Probe MTU option is received by Probe-Receiver, the receiving IP SHOULD place the value of this option in the next datagram it sends back to Probe-Sender. The value is then discarded. In other words, each Probe MTU option causes the Reply MTU option to be placed in one return datagram. 💜

When Probe-Sender receives the Reply MTU option, it SHOULD check the value of the option against the current MINMTU estimate in the routing cache. If the option value is lower, it becomes the new MINMTU estimate. If the option value is higher, Probe-Sender SHOULD be more conservative about changing the MINMTU estimate. If a route is flapping, the MINMTU may change frequently. In such situations, keeping the smallest MINMTU of various routes in use is preferred. As a result, a higher MINMTU estimate SHOULD only be accepted after a lower estimate has been permitted to "age" a bit. In other words, if the probe value is higher than the estimated MINMTU, only update the estimate if the estimate is several seconds old or more. Finally, whenever the Probe-Sender receives a Reply MTU option, it SHOULD stop retransmitting probes to Probe-Receiver. ⭐

A few additional issues complicate this discussion 🙈.

One problem is setting the default MINMTU when no Reply MTU options have been received. We recommend the use of the minimum of the supported IP datagram size (576 octets) and the connected network MTU for destinations not on the local connected network, and the connected network MTU for hosts on the connected network.

The MINMTU information, while kept by the Internet layer, is in fact only of interest to the transport and higher layers. Accordingly, the Internet layer MUST keep the transport layer informed of the current value of the estimated MINMTU. Furthermore, minimal transport protocols, such as UDP, MUST be prepared to pass this information up to the transport protocol user. 🧚

It is expected that there will be a transition period during which some hosts support this option and some do not. As a result, hosts SHOULD stop sending Probe MTU options and refuse to send any further options if they do not receive either a Probe MTU option or a Reply MTU option from the remote system after a certain number of Probe MTU options have been sent. In short, if Probe-Sender has sent several probes but has gotten no indication that Probe-Receiver supports MTU probing, then Probe-Sender SHOULD assume that Probe-Receiver does not support probes. (Obviously, if Probe-Sender later receives a probe option from Probe-Receiver, it SHOULD revise its opinion.) 🔥

Implementations SHOULD NOT assume that routes to the same destination that have a different TOS have the same estimated MINMTU. We recommend that the MTU be probed separately for each TOS. 💅

Respecting the TCP MSS 💖

One issue concerning TCP MSS is that it is usually negotiated assuming an IP header that contains no options. If the transport layer is sending maximum size segments, it may not leave space for IP to fit the options into the datagram. Thus, insertion of the Probe MTU or Reply MTU option may violate the MSS restriction. Because, unlike other IP options, the MTU options can be inserted without the knowledge of the transport layer, the implementor MUST carefully consider the implications of adding options to an IP datagram. 😳

One approach is to reserve 4 bytes from the MINMTU reported to the transport layer. This will allow the IP layer to insert at least one MTU option in every datagram (it can compare the size of the outgoing datagram with the MINMTU stored in the route cache to see how much room there actually is). This is simple to implement, but does waste a little bandwidth in the normal case. 🌸

Another approach is to provide a means for the IP layer to notify the transport layer that space MUST be reserved for sending an option. The transport layer would then make a forthcoming segment somewhat smaller than usual. 🦋

When a Probe Can Be Sent 🔥

A system that receives a Probe MTU option SHOULD always respond with a Reply MTU option, unless the probe was sent to an IP or LAN broadcast address. 👑

A Probe MTU option SHOULD be sent in any of the following situations:

The MINMTU for the path is not yet known;
A received datagram suffers a fragmentation re-assembly timeout. (This is a strong hint the path has changed; send a probe to the datagram's source);
An ICMP Time Exceeded/Fragmentation Reassembly Timeout is received (this is the only message we will get that indicates fragmentation occurred along the network path);
The transport layer requests it.

Implementations MAY also wish to periodically probe a path, even if there is no indication that fragmentation is occurring. This practice is perfectly reasonable! If fragmentation and reassembly is working perfectly, the sender may never get any indication that the path MINMTU has changed unless a probe is sent. We recommend, however, that implementations send such periodic probes sparingly. Once every few minutes, or once every few hundred datagrams, is probably sufficient. 🧚

There are also some scenarios in which the Probe MTU SHOULD NOT be sent, even though there may be some indication of a MINMTU change:

Probes SHOULD NOT be sent in response to the receipt of a probe option. Although the fact that the remote peer is probing indicates that the MINMTU may have changed, sending a probe in response to a probe causes a continuous exchange of probe options. 🙈
Probes MUST NOT be sent in response to fragmented datagrams except when the fragmentation reassembly of the datagram fails. The problem in this case is that the receiver has no mechanism for informing the remote peer that fragmentation has occurred, unless fragmentation reassembly fails (in which case an ICMP message is sent). Thus, a peer may use the wrong MTU for some time before discovering a problem. If we probe on fragmented datagrams, we may probe, unnecessarily, for some time until the remote peer corrects its MTU. 💀
For compatibility with hosts that do not implement the option, no Probe MTU Option SHOULD be sent more than ten times without receiving a Reply MTU Option or a Probe MTU Option from the remote peer. Peers which ignore probes and do not send probes MUST be treated as not supporting probes. 🍓
Probes SHOULD NOT be sent to an IP or LAN broadcast address. 🌈
We recommend that Probe MTUs not be sent to other hosts on the directly-connected network, but that this feature be configurable. There are situations (for example, when Proxy ARP is in use) where it may be difficult to determine which systems are on the directly-connected network. In this case, probing may make sense. 💜

SAMPLE IMPLEMENTATION SKETCH ✨

okay bestie, this part is where the spec gets concrete 🧚 Here's how an IP-layer implementation of MTU probing might be designed, step by step.

First, the routing cache entries get a glow-up 💅 They store seven extra values:

MINMTU: The current MINMTU of the path.
ProbeRetry: A timestamp indicating when the next probe should be sent.
LastDecreased: A timestamp showing when the MTU was last decreased.
ProbeReply: A bit indicating a Reply MTU option should be sent.
ReplyMTU: The value to go in the Reply MTU option.
SupportsProbes: A bit indicating that the remote peer can deal with probes (always defaults to 1=true).
ConsecutiveProbes: The number of probes sent without the receipt of a Probe MTU or Reply MTU option.

There are also several configuration parameters 🎀 These should be configurable by appropriate network management software, and the values we suggest are "reasonable" 🫶

Default_MINMTU: The default value for the MINMTU field of the routing cache entry, to be used when the real MINMTU is unknown. Recommended value: 576.
Max_ConsecutiveProbs: The maximum number of probes to send before assuming that the destination does not support the probe option. Recommended value: 10.
ProbeRetryTime: The time (in seconds) to wait before retrying an unanswered probe. Recommended value: 60 seconds, or 2*RTT if the the RTT is available to the IP layer.
ReprobeInterval: The time to wait before sending a probe after receiving a successful Reply MTU, in order to detect increases in the route's MINMTU. Recommended value: 5 times the ProbeRetryTime.
IncreaseInterval: The time to wait before increasing the MINMTU after the value has been decreased, to prevent flapping. Recommended value: same as ProbeRetryTime.

When a brand-new route gets entered into the routing cache 🌸 the initial values should be set like this:

MINMTU = Default_MINMTU
ProbeRetry = Current Time
LastDecreased = Current Time - IncreaseInterval
ProbeReply = false
SupportsProbes = true
ConsecutiveProbes = 0

This initialization happens before the first packet goes out on this route, so that the first packet will contain a Probe MTU option 🦋

Whenever the IP layer sends a datagram on this route, it checks the SupportsProbes bit to see if the remote system supports probing. If the SupportsProbes bit is set, and the timestamp in ProbeRetry is less than or equal to the current time, a Probe option should be sent in the datagram, and the ProbeRetry field gets incremented by ProbeRetryTime ⭐

Whether or not the Probe MTU option is sent in a datagram, if the ProbeReply bit is set, then a Reply MTU option with the value of the ReplyMTU field is placed in the outbound datagram. After that, the ProbeReply bit is cleared 💜

Every time a Probe option is sent, the ConsecutiveProbes value should be incremented. If this value reaches Max_ConsecutiveProbes, the SupportsProbe bit should be cleared 💀

When an IP datagram containing the Probe MTU option is received, the receiving IP sets the ReplyMTU to the Probe MTU option value and sets the ProbeReply bit in its outbound route to the source of the datagram. The SupportsProbe bit is set, and the ConsecutiveProbes value is reset to 0. Main character energy 👑

If an IP datagram containing the Reply MTU option is received, the IP layer must locate the routing cache entry corresponding to the source of the Reply MTU option. If no such entry exists, a new one (with default values) should be created. The SupportsProbe bit is set, and the ConsecutiveProbes value is reset to 0. The ProbeRetry field is set to the current time plus ReprobeInterval 🔥

Four cases are possible when a Reply MTU option is received, and this part is kinda the heart of the whole thing 🫶

The Reply MTU option value is less than the current MINMTU: the MINMTU field is set to the new value, and the LastDecreased field is set to the current time.
The Reply MTU option value is greater than the current MINMTU and the LastDecreased field plus IncreaseInterval is less than the current time: set the ProbeRetry field to LastDecreased plus IncreaseInterval, but do not change MINMTU.
The Reply MTU option value is greater than the current MINMTU and the LastDecreased field plus IncreaseInterval is greater than the current time: set the MINMTU field to the new value.
The Reply MTU option value is equal to the current MINMTU: do nothing more. Iconic restraint 😌

Whenever the MTU field changes, the transport layer should be notified 📣 either by an upcall, or by a change in a shared variable (which may be accessed from the transport layer by a downcall).

If a fragmentation reassembly timeout occurs, if an ICMP Time Exceeded/Fragmentation Reassembly Timeout is received, or if the IP layer is asked to send a probe by a higher layer, the ProbeRetry field for the appropriate routing cache entry is set to the current time. This will cause a Probe option to be sent with the next datagram (unless the SupportsProbe bit is turned off) 🌈

MANAGEMENT PARAMETERS 🎀

We suggest that the following parameters be made available to local applications and remote network management systems:

The number of probe retries to be made before determining a system is down. The value of 10 is certain to be wrong in some situations 🙈
The frequency with which probes are sent. Systems may find that more or less frequent probing is more cost effective.
The default MINMTU used to initialize routes.
Applications should have the ability to force a probe on a particular route. There are cases where a probe needs to be sent but the sender doesn't know it. An operator must be able to cause a probe in such situations. Furthermore, it may be useful for applications to "ping" for the MTU 🍓

REFERENCES 📚

[1] Kent, C. and J. Mogul, "Fragmentation Considered Harmful", Proc. ACM SIGCOMM '87, Stowe, VT, August 1987.

[2] Postel, J., Ed., "Internet Protocol", RFC-791, USC/Information Sciences Institute, Marina del Rey, CA, September 1981.

[3] Postel, J., Ed., "Transmission Control Protocol", RFC-793, USC/Information Sciences Institute, Marina del Rey, CA, September 1981.

[4] Postel, J., "The TCP Maximum Segment Size and Related Topics", RFC-879, USC/Information Sciences Institute, Marina del Rey, CA, November 1983.

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ietf: rfc1063 1988-07
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