Re: Ether loopback...


A Hamilton (mcvax!ukc!etive!adam@uunet.uu.net)
16 Aug 88 14:21:07 GMT


In article <12420888971.28.BILLW@MATHOM.CISCO.COM> BILLW@MATHOM.CISCO.COM (William Westfield) writes:

: Some ethernet controllers are capable of hearing packets that they are
: transmitting as they are being transmitted. If this is true, then
: sending a packet to yourself is a good way to check whether the
: ethernet is in good shape - it checks the transceiver and the ethernet
: cable as well as the controller.
:
Actually not so. Many ethernet controllers have a smart processor which will
check for a loopback packet and return it to the host as if it had been
received from the Ethernet. But the transceivers CANNOT listen during
their own transmission.

In article <88Aug7.130257hst.4295@dorsai.ics.hawaii.edu> torben@DORSAI.ICS.HAWAII.EDU ("Torben N. Nielsen") writes:
>
>Anyone happen to know if there're legitimate reasons for sending to yourself
>onan Ethernet? That is, Ethernet packets where the source and the destination
>address are identical? Seems pointless since as far as I know, you cannot read
>packets you yourself transmitted off of the wire..... Am I wrong?
>
> Torben

No, you are right (in terms of hardware but it has to be fixed by some
form of s/w [see above]) but it is not pointless.
There are 2 reasons I am aware of.
1) The IEEE 802.3 spec. says that all stations should be capable of
receiving a self-addressed Ethernet packet as though it were received from the
net (section 2.3.3.5). The IEEE 802.2 spec. requires a station to transmit
such a packet at startup so as to perform a "duplicate address check".
(The packet is an IEEE 802.2 TEST packet). It is required that exactly
one reply is received in an appropriate period of time (this one packet is
from the originating station itself which replies to this TEST packet as
to any other). Section 6.9.2 is the relevant bit in my (out-of-date) copy.

2) Loopback tests are often required if no other station supports the
relevant s/w. They can also be useful as they give very different timing
effects. Under these circumstances it can be very useful if packets go out
onto the real network where they can be picked up by a monitor, rather than
being soft looped entirely.

                Adam Hamilton



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