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From: mitton@dave.enet.dec.com (Dave Mitton)
Newsgroups: comp.dcom.lans
Subject: Re: More questions regarding the LAN
Keywords: Token Ring, 802.5
Message-ID: <20513@shlump.nac.dec.com>
Date: 25 Feb 91 23:58:35 GMT
References: <2334@enuxha.eas.asu.edu> <31710002@hpctdlb.HP.COM>
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: mitton@dave.enet.dec.com (Dave Mitton)
Organization: Digital Equipment Corporation, Littleton MA
Lines: 46

>From: franka@hpctdlb.HP.COM (Frank Actis)
>Newsgroups: comp.dcom.lans
>Subject: Re: More questions regarding the LAN
>Date: 21 Feb 91 23:50:56 GMT

>
>>  From: gdrao@enuxha.eas.asu.edu (Giridhar Rao)
>>  Subject: More questions regarding the LAN
...
>>  	What consequences arise if the destination station removes the
>>  	frame instead  of the sender ? The IEEE 802.5 standard says that
>>  	sender removes the frame ? Any advantages in that ? Isn't it a
>>  	lot of overhead ?
....
>The sending station knows that the frame has been received by looking at
>a couple of bits on the frame that has been set by the destination system.
>These bits are the "address recognized" (I saw the frame sent to me) and 
>the "frame copied" (I was able to pull the frame off the LAN and into my 
>buffers) which tell the sender if the remote station is up and that it
>received the frame.
...
>Frank Actis

Frank, part your answer is some what misleading.
An 802.5 Token Ring transmitting station strips his own packet based on
recognizing his own source address and knowing his outstanding transmitted
packet count. The ARI and FCI bits in the Frame Status field are not used
for anything other than status information and retransmission at the MAC
level.  Note that they are outside of CRC and Code Violation protection
and can easily be undetected garbage.

An 802.5 station can send multiple packets on the wire until the Token Holding
Timer (THT) expires (however the TI chip set does not allow this).  After every
transmit, a counter is incremented, which causes the receiver to watch for
the incoming packet with its source address and strip it.  With Early Token
Release (ETR) that may not be the next packet.  If the packet does not arrive
within Timer, Return to Repeat (TRR) then the frame is declared lost, an error.

To the original question, the questioner should consider how broadcast,
multicast, and bridging were to work in such a network where the dest node
stripped the frame.  Also, what if the node was not present?  All these cases
are handled with source node stripping.


	Dave Mitton, Consultant Engineer, Token Ring Program
	Digital Equipment Corp