Andy Fleming wrote:
On Fri, Feb 6, 2009 at 1:30 PM, Jerry Van Baren gerald.vanbaren@ge.com wrote:
Andy Fleming wrote:
On Fri, Feb 6, 2009 at 7:32 AM, Jerry Van Baren gerald.vanbaren@ge.com wrote:
Peter Tyser wrote:
[snip]
My experience is that "The second case assumes that the far end link partner fully supports Auto-Negotiation:" / "Min spec" time is what occurs. I've never seen a PHY take anything other than bang-on 2 seconds (OK, 2.054 seconds).
IIRC, the "min spec" time is the minimum time a PHY is allowed to take for autonegotiation. The "max spec" time is the maximum time the PHY is allowed to take for autonegotiation. The PHY *has* everything it needs to know by the "min spec" time, which is why all PHYs (that I've ever used) take exactly 2 seconds for autonegotiation.
If you wanted to be safer than my semi-empirical 2 second number, 2.8 (round up to 3?) seconds is probably a good choice.
Based on my reading of the below, "safe" is 3.5 seconds.
Note that the first case mentioned below is oddball in that the far end doesn't support autonegotiation. In real life, you are probably screwed in this case regardless of the time it takes for autonegotiation to fail.
If the link partner doesn't support autonegotiation, many PHYs
s/many/all/
support "Parallel detection" which allows the PHY to essentially notice by the link response what speeds seem to be supported. And,
(speed, but not duplex)
really, my issue is not with autonegotiation failing, but failing due to no link actually being present. If the link isn't there, it's annoying to wait and wait and wait.
I understand "parallel detection". The problem is that the percentage of times that it results in a properly configured link is (in my experience) very very low. Not because "parallel detection" doesn't work, but because the humans configuring the fixed side don't understand parallel detection and set it up for full duplex.
Ignoring the configuration where both ends are (presumably correctly) manually configured, you end up with five cases, two of them misconfigured and WRONG: 1) Autonegotiation <-> autonegotiation - reliable. 2) 10bT half duplex <-> autonegotiation - reliable. 3) 100bT half duplex <-> autonegotiation - reliable. 4) 10bT *FULL* duplex <-> autonegotiation - *UNreliable*. 5) 100bT *FULL* duplex <-> autonegotiation - *UNreliable*.
The problem that I've observed is that the *humans* (the weak links) that do the manual configuration don't understand that "parallel detection" *must be* half duplex by definition in the spec (it is hard to define a reliable algorithm to detect full duplex capability so the spec writers punted). As a result, the human invariably picks "full duplex" because everybody knows full duplex is better... and end up as case (4) or (5). They inadvertently end up with a slower unreliable link (lots of "collisions" resulting in runt packets) rather than the faster better link they thought they were picking (d'oh!). The really bad thing is that the network works fine in testing on an isolated LAN with no traffic and absolutely craps its pants when it hits the real world.
That is my reasoning behind my statement that we can generally ignore the autonegotiation <-> fixed configuration case because the odds of it working properly are poor anyway.
Having said all that, I don't have any problem with using 3.5 seconds as the safe timeout value. it isn't worth timing out too soon just to shave 0.5 or even 1.0 seconds off the negotiation timeout time.
I recall verifying the DP83840 calculations against chapter 28 of the IEEE 802.3u and I *clearly* recall not wanting to go back there ever again. If you doubt the numbers, *you* can go there. ;-)
I'll trust your math. My curiosity has to do with whether the link status will be reported as up once the break link time has elapsed. But 3.5 is still better than what at some point was a much longer timeout.
Andy
Best regards, gvb