Re: controlling IP "type of service"

Phil R. Karn (thumper!
12 May 88 04:21:14 GMT

> Do any of the popular implementations actually try to set this
> field, [TOS] and does it affect the delivered performance of the network?

Mine (the KA9Q package) does. My IP interprets the "delay" and
"reliability" bits in the AX.25 (amateur packet radio) subnet driver as

1. If the "low delay" bit is set, send the datagram in an unnumbered
information (UI) frame. There is no link level flow control or
acknowledgment on these frames.

2. Else if the reliability flag is set, send the datagram as one (or more)
regular information (I) frames, opening a new link connection if
necessary. I frames are acknowledged and flow controlled at the link
layer according to LAPB (the protocol on which AX.25 is based). The "or
more" part refers to the possible use of subnet fragmentation and
reassembly to keep the physical frames small without imposing an
unreasonably small MTU on the IP layer.

3. Else use the interface's default encapsulation mode.

Although the driver does look at these bits, this is just a hook for
future use. At present my applications do not give the user the ability
to control these bits in generated datagrams. The default mode is
presently used for all datagrams.


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