📄 rfc1063.txt
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Network Working Group J. MogulRequest For Comments: 1063 C. Kent DEC C. Partridge BBN K. McCloghrie TWG July 1988 IP MTU Discovery OptionsSTATUS OF THIS MEMO A pair of IP options that can be used to learn the minimum MTU of a path through an internet is described, along with its possible uses. This is a proposal for an Experimental protocol. Distribution of this memo is unlimited.INTRODUCTION Although the Internet Protocol allows gateways to fragment packets that are too large to forward, fragmentation is not always desirable. It can lead to poor performance or even total communication failure in circumstances that are surprisingly common. (For a thorough discussion of this issue, see [1]). A datagram will be fragmented if it is larger than the Maximum Transmission Unit (MTU) of some network along the path it follows. In order 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 methods have problems. Choosing MTUs requires a balance between network utilization (which requires the use of the largest possible datagram) and fragmentation avoidance (which in the absence of knowledge about the network path encourages the use of small, and thus too many, datagrams). Any choice for the MTU size, without information from the network, is likely to either fail to properly utilize the network or fail to avoid fragmentation. Probe packets have the problem of burdening the network withMogul, Kent, Partridge, & McCloghrie [Page 1] RFC 1063 IP MTU Discovery Options July 1988 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. 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; the second option is used to convey this MTU back to the sender (possibly in the IP datagram containing the transport acknowledgement to the datagram which contained 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 value of the MTU field of the option is less than or equal to that of the inbound link, and if necessary, reduce theMogul, Kent, Partridge, & McCloghrie [Page 2] RFC 1063 IP MTU Discovery Options July 1988 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 fatalMogul, Kent, Partridge, & McCloghrie [Page 3] RFC 1063 IP MTU Discovery Options July 1988 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 There are at least two ways to implement the MTU discovery scheme. One method makes the transport layer responsible for MTU discovery; the other method makes the IP layer responsible for MTU discovery. A host system should support one of the two schemes. Transport Discovery In the transport case, the transport layer can include the Probe MTU option in an outbound datagram. When a datagram containing the Probe MTU option is received, the option must be passed up to the receiving transport layer, which should then acknowledge the Probe with a Reply MTU option in the next return datagram. Note that because the options are placed on unreliable datagrams, the original sender will have to resend Probes (possibly once per window of data) until it receives a Reply option. Also note that the Reply MTU option may be returned on an IP datagram for a different transport protocol from which it was sent (e.g., TCP generated the probe but the Reply was received on a UDP datagram). IP Discovery A better scheme is to put MTU discovery into the IP layer, using control mechanisms in the routing cache. Whenever an IP datagram is sent, the IP layer checks in the routing cache to see if a Probe or Reply MTU option needs to be inserted in the datagram. Whenever a datagram containing either option is received, the information in those options is placed in the routing cache. The basic working of the protocol is somewhat complex. We trace it here through one round-trip. Implementors should realize that there may be cases where both options are contained in one datagram. For the purposes of this exposition, the sender of the probe is called the Probe-Sender and the receiver, 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.Mogul, Kent, Partridge, & McCloghrie [Page 4] RFC 1063 IP MTU Discovery Options July 1988 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 is that 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 protocolMogul, Kent, Partridge, & McCloghrie [Page 5] RFC 1063 IP MTU Discovery Options July 1988 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 it does not receive either a Probe MTU option or 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⌨️ 快捷键说明
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