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RFC 25 UNKNOWN ARPANET · CONSTRAINTS

No High Link Numbers

Limit link numbers to 63 instead of allowing all 256 values.

On rfc-editor.org ↗
PUBLISHED
October 1969
AUTHORS
S.D. Crocker
STREAM
Legacy
PROPOSED MAXIMUM LINK NUMBER
63
MORE REALISTIC ESTIMATE
32
DOI
10.17487/RFC0025

RFC text

Original text · RFC Editor View on rfc-editor.org ↗ 
Network Working Group                                      Steve Crocker
Request for Comments: 25                                            UCLA
                                                         30 October 1969




                           NO HIGH LINK NUMBERS




Because it may be desirable to reserve one or more link numbers for
instrumentation purposes, and because 256 link numbers are many more
than are needed, we suggest that no link number over 63 be used.  At
UCLA, we will implement our tables to take advantage of this limitation.
We also note that 32 may be even more realistic, but 64 is certainly
sufficient.

AI ELI5

On this page
PROBLEMThe host-host protocol allowed link numbers from 0 to 255, but hosts needed to maintain tables and manage many simultaneous links.
MECHANISMPropose a practical limit of 63 link numbers, reserving higher numbers for instrumentation purposes. Even 32 might be sufficient for practical use.
DESIGN GUIDANCEThe proposal set a practical expectation for early network implementations.

§1Why Limit Link Numbers?

RFC 25 proposes that no link number over 63 be used in the host-host protocol. The rationale is twofold: 256 link numbers (0-255) are far more than needed for practical host-to-host communication, and reserving the higher numbers (64-255) for instrumentation and future purposes is prudent design.

The protocol allowed 256 link numbers, but that is too many for normal use.

§2Practical Limits

Most hosts will not need to maintain more than 63 simultaneous connections for normal operation. UCLA implemented its link tables to take advantage of this limitation, which reduces table size and implementation complexity. The RFC notes that 32 link numbers may be even more realistic for many purposes, though 64 is certainly sufficient to cover peak usage.

UCLA built their system to handle at most 63 links, and that works fine for everything they need to do.

§3Instrumentation and Future Use

By suggesting an upper limit of 63, RFC 25 reserves link numbers 64-255 for special purposes such as instrumentation, network monitoring, or future extensions. This gives protocol designers flexibility to add new features without conflicting with the standard connection numbering scheme.

Keeping higher link numbers in reserve lets network managers and designers add special features later.