[aprssig] Max packet size

[John Langner WB2OSZ]

Brek wrote:

> I believe the maximum packet size for ax.25 is 255 bytes.

AX.25 v2.0 has a maximum *information* size of 256 (not 255) bytes.  The
total packet size will be larger after adding up to 10 addresses, of 7 bytes
each, and control/protocol bytes.

AX.25 v2.2 starts with the same maximum, as the default, but stations can
negotiate a larger size if both support it. (Not applicable to APRS.)

APRS has a maximum *information* field length of 256 bytes.  (See Chapter 3
of the APRS protocol spec.)   Worst case you would need another 72 for
addresses, control, and protocol.  That does not include the "flag" fields
and FCS.

> Is there a smaller maximum size for an APRS packet, ...?

Smaller sizes are mentioned for various data type formats.  For example, the
Position Report mentions a maximum comment length of 43.  In reality, this
limit is rarely, if ever, enforced.  Observing recent local activity, I see
examples, from several different implementations, with longer comments.