7. Likely Values for Path MTUs
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 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).
Note: It might be a good idea to add a few table entries for values equal to small powers of 2 plus 40 (for the IP and TCP headers), where no similar values exist, since this seems to be a reasonably non-arbitrary way of choosing arbitrary values.
The table might also contain entries for values slightly less than large powers of 2, in case MTUs are defined near those values (it is better in this case for the table entries to be low than to be high, or else the next lowest plateau may be chosen instead).
| 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. [6] | |
| 17914 | |||
| 8166 | IEEE 802.4 | RFC 1042 | |
| 8166 | |||
| 4464 | IEEE 802.5 (4Mb max) | RFC 1042 | |
| 4352 | FDDI (Revised) | RFC 1188 | |
| 4352 (1%) | |||
| 2048 | Wideband Network | RFC 907 | |
| 2002 | IEEE 802.5 (4Mb recommended) | RFC 1042 | |
| 2002 (2%) | |||
| 1536 | Exp. Ethernet Nets | RFC 895 | |
| 1500 | Ethernet Networks | RFC 894 | |
| 1500 | Point-to-Point (default) | 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. [10] | |
| 512 | NETBIOS | RFC 1088 | |
| 508 | IEEE 802/Source-Rt Bridge | RFC 1042 | |
| 508 | ARCNET | RFC 1051 | |
| 508 (13%) | |||
| 296 | Point-to-Point (low delay) | RFC 1144 | |
| 296 | |||
| 68 | Official minimum MTU | RFC 791 |
Table 7-1: Common MTUs in the Internet