Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!amdcad!phil
From: phil@amdcad.AMD.COM (Phil Ngai)
Newsgroups: comp.dcom.lans
Subject: Re: Can ethernet TCP/IP lock up?
Keywords: I don't know; someone says it can
Message-ID: <21680@amdcad.AMD.COM>
Date: 19 May 88 18:22:06 GMT
References: <299@fedeva.UUCP> <21674@amdcad.AMD.COM>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Organization: Advanced Micro Devices
Lines: 38

In article <21674@amdcad.AMD.COM> rpw3@amdcad.UUCP (Rob Warnock) writes:
>In article <299@fedeva.UUCP> wrd3156@fedeva.UUCP (Bill Daniels) writes:
>| Some folks in my organization have been led to believe that a "screaming" 
>| modem/transceiver can lock up an ethernet by asserting carrier forever.
>| Supposedly token stuff like GM/MAP does not permit this...
>| What do you think?
>
>Any piece of hardware can fail. Your token-ring transmitters can go into
>"screaming" mode, too. But well-designed hardware tries to avoid failing in
>ways that will take down the whole net. In particular, Ethernet transceivers
>(see the DEC/Intel/Xerox Ethernet spec) have what is called "jabber control"
>to prevent exactly this kind of "screaming" (which is usually a controller
>board fault, b.t.w., not the transceiver itself). The odds that a controller
>will go beserk *AND* that the jabber control will have failed at the same time
>are much less than either fault alone. And each fault is itself very rare.

In addition to Rob's comments, it should be noted that the jabber
control is supposed to be implemented so as to eliminate the chance of
the network being overloaded if any one component fails. In one design
I saw, the jabber control was replicated three times. Any one of them
could shut down the transceiver if carrier were asserted too long. 

The jabber control functions independently of the controller or the
transmit or receive circuitry. It listens to the trunk cable; there is
no possible failure of the transmitter or receiver that could disable it. 

Of course, this increases the chance that one node will be cut off if
the jabber control activates when it shouldn't. But this is consistent
with the philosophy of protecting the network even at the cost of
slightly decreased availability for a particular node. 

All this not withstanding, we have hundreds of transceivers in active
use at my company and I haven't ever seen one fail. 

-- 
Make Japan the 51st state!

I speak for myself, not the company.
Phil Ngai, {ucbvax,decwrl,allegra}!amdcad!phil or phil@amd.com