Re: Question about Link Fault Signalling
Bob,
I'll go one step further, in line with a recent note from Rich.
The presence or absence of LF or RF ordered sets should decide
what state the RS is in. Requiring some number of IDLEs to
exit a state is undesirable (in my opinion). The case where
a MAC is sending data when an RS leaves the LF state results
in the link partner seeing RF followed directly by packets.
Though this first packet will likely be trashed (and I have
no problem with that), it should not require 8 words of IDLEs
to exit the RF state, resulting in the loss of perhaps many
packets if they all have minimum IPGs.
Ben
David Gross wrote:
>
> Hi Bob,
>
> I think I might have been a bit unclear in my question judging from the
> range of responses, but I think Ben saw what I was getting at. The
> question arose because of the wording of a paragraph, namely:
>
> "The reception of four status messages of the same type shall indicate
> that the corresponding fault condition has occurred. The reception of
> four Idle control characters on successive RX_CLK edges (eight
> consecutive Idle control characters) shall clear all fault conditions."
>
> The point is you make no mention that one particular fault condition can
> be cleared if another one is being detected (or likewise if it stops
> being detected, but the RS is NOT recieving IDLEs).
>
> This is the case we seem to be talking about when we come out of an LF
> condition and immediately start recieving RF at the RS. There is no
> continuous IDLE stream to remove fault conditions as seems to be
> signified. There should be a sentence adding that since faults are
> exclusive when recieved at the RS (it can get LF or RF or neither, but
> not both), if a different fault condition is detected while a previous
> condition has not yet been cleared by IDLEs, then that fault shall be
> asserted and the previous one cleared.
>
> I hope this is a bit clearer than my last attempt at this :)
>
> cheers,
> Dave Gross
>
> Grow, Bob wrote:
> >
> > See responses in line <RMG>
> >
> > --Bob Grow
> >
> > -----Original Message-----
> > From: Ben Brown [mailto:bbrown@amcc.com]
> > Sent: Wednesday, January 03, 2001 5:23 PM
> > To: 802.3ae
> > Subject: Re: Question about Link Fault Signalling
> >
> > Bob,
> >
> > Please explain how LF and RF can be simultaneously detected
> > or rather simultaneously exist on a link? When an RS is
> > sending RF, if the link is up and working fine, the RF will
> > make it all the way to the link partner's RS. If the link
> > is broken somewhere, the RF is lost due to the broken link
> > and a device somewhere in the link will generate LF to
> > report the broken link.
> >
> > <RMG> I think we agree on the important things, we are differing on
> > "detect". The remote DTE is transmitting RF, and an LF is generated because
> > XAUI can't deskew. The RF could be detected below the fault (it exists
> > within the DTE), but the RS can't see it because of the LF. Thus a LF
> > naturally has precedence over an RF for signaling. (In this case your
> > saying the RF is lost.)
> >
> > Also, I think David's point was this:
> >
> > Device A Device B
> > Time X PCS not in sync PCS not in sync
> > PCS sending LF PCS sending LF
> > RS detecting LF RS detecting LF
> > RS sending RS sending RF
> >
> > Time Y PCS in sync PCS not in sync
> > PCS forwarding RF PCS sending LF
> > RS detecting RF RS detecting LF
> > RS sending IDLE RS sending RF
> >
> > In the transition from Time X to Time Y, the RS doesn't
> > see enough IDLEs to transition from LF to IDLE so it must
> > have a means of transitioning directly from LF to RF.
> >
> > <RMG>I don't see anything in Shimon's rules that requires Idle to change
> > from LF to RF. The Idle clears the LinkStatus variable, not which type of
> > fault is received. The fault type is determined by receiving the specified
> > number of messages of the same type.
> >
> > It must be clear that detection of RF clears the detection
> > of LF and vice versa. You can't always expect IDLEs between
> > LF and RF detection.
> >
> > <RMG>I agree, I think it is clear in Shimon's rules.
> >
> > Regards,
> > Ben
> >
> > "Grow, Bob" wrote:
> > >
> > > Your proposed change is too subtitle for me while I am trying to finish my
> > > ballot comments, but I think you erred in thinking RF is repeated, it
> > isn't.
> > >
> > > I don't think the existing text has a fatal embrace nor a race condition
> > iso
> > > can't see any reason to change it. RF is not repeated, so once LF goes
> > > away, only Idle is transmitted until RF is no longer received.
> > >
> > > BTW, RF and LF can be simultaneously detected though the RS will only see
> > > the higher priority LF.
> > >
> > > FYI, someone else has pointed out that I may have misread Shimon's
> > > SuggestedRemedy that the reset to LinkStatus = OK occurs on 8 * 4 bytes of
> > > Idle, not 2 * 4 bytes as my edited version records.)
> > >
> > > --Bob
> > >
> > > -----Original Message-----
> > > From: David Gross [mailto:dgross@nortelnetworks.com]
> > > Sent: Wednesday, January 03, 2001 2:35 PM
> > > To: Grow, Bob
> > > Cc: rtaborek@earthlink.net; HSSG
> > > Subject: Re: Question about Link Fault Signalling
> > >
> > > Thanks for the response Bob,
> > >
> > > I'd just like to make one clarification which I think might be
> > > necessary. I'd like to see "In the case of a Local Fault condition..."
> > > rather than "Upon detection of a Local Fault condition..." (and likewise
> > > for Remote Fault). The reason for this is that since upon start-up, one
> > > can assume that both devices will be in LF and transmitting RF. This
> > > implies that once a device can start recieving data (i.e.: no longer
> > > have a LF) it will be recieving RF. As a result, as the definition seems
> > > to imply, the Fault conditions won't be cleared (The IDLE control words
> > > won't be detected for 2 clock edges), but instead Remote Fault will be
> > > detected. Since RF and LF cannot be detected at the same time (LF
> > > prevents the transmission of recieved RF), it is logical that LF will be
> > > cleared while RF will be achieved. There should be something in there
> > > which allows for the clearing of LF in such a case, and jumping from the
> > > LF condition immediately to the RF condition. Let me know what you
> > > think.
> > >
> > > -Dave Gross
> > >
> > > Grow, Bob wrote:
> > > >
> > > > Shimon submitted a comment proposing changing the entry to link down
> > > (eitner
> > > > RF or LF) from 3 to 4 status messages, with exit on 8 consecutive idle
> > > > bytes. While I am open to discussion on the numbers, I think his
> > proposed
> > > > text with improved description of the protocol is a great starting point
> > > for
> > > > discussion. Since this has come up again, here is a slightly edited
> > > version
> > > > of his proposed text.
> > > >
> > > > "46.2.6 Link fault signaling
> > > >
> > > > "Two link fault conditions are specified for 10Gb/s operation: Local
> > Fault
> > > > and Remote Fault. The Local Fault condition at the Reconciliation
> > Sublayer
> > > > indicates that a link failure has been detected on the receive path by a
> > > > local DTE sublayer. The source of the failure could be at the remote
> > > > transmitter, the interconnect between the two DTEs, at one of the local
> > > > DTE's devices or the interconnect between the local DTE's devices. The
> > > > Remote Fault condition is generated by the Reconciliation Sublayer, and
> > > when
> > > > received by at a Reconciliation sublayer indicates that a link failure
> > has
> > > > been detected by the remote DTE. The source of the failure could be at
> > > the
> > > > local transmitter, the interconnect between the two DTEs, at one of the
> > > > remote DTE's devices or the interconnect between the remote DTE's
> > devices.
> > > >
> > > > " Fault conditions are conveyed over the XGMII using status messages.
> > All
> > > > status messages are four bytes in length, and are sent on a single XGMII
> > > > clock edge. A status message is indicated by a Pulse control character
> > > > aligned to lane 0, with the status condition encoded in the three data
> > > bytes
> > > > of lanes 1, 2 and 3. The status encodings are shown in Table 46-4."
> > > >
> > > > <Table 46-4>
> > > > <For the sake of completeness, also show Lane 0 encoding>
> > > >
> > > > "A PHY indicates Local Fault conditions to the Reconciliation sublayer
> > by
> > > > alternating the corresponding status message with Idle control
> > characters
> > > on
> > > > RXC<3:0> and RXD<31:0>. The Reconciliation sublayer sends the Remote
> > > Fault
> > > > indication to the remote DTE by alternating the Remote Fault message
> > with
> > > > Idle control characters on TXC<3:0> and TXD<31:0>.
> > > >
> > > > "The PHY repeats a Remote Fault indication received from the remote DTE
> > > > unless a Local Fault condition is detected resulting in the PHY over
> > > writing
> > > > the received data with the Local Fault indication.
> > > >
> > > > "The Reconciliation sublayer continuously monitors RXC<3:0> and
> > RXD<31:0>
> > > > for status messages. The reception of four status messages of the same
> > > type
> > > > shall indicate that the corresponding fault condition has occurred. The
> > > > reception of four Idle control characters on successive RX_CLK edges
> > > (eight
> > > > consecutive Idle control characters) shall clear all fault conditions.
> > > >
> > > > " Upon detection of a Local Fault condition, the Reconciliation sublayer
> > > > shall:
> > > > 1) Set the link_fail status indication.
> > > > 2) Inhibit the transmission of MAC frames.
> > > > 3) Continuously send alternating Remote Fault messages and Idle control
> > > > characters.
> > > >
> > > > "Upon detection of a Remote Fault condition, the Reconciliation
> > sublayer
> > > > shall:
> > > > 1) Set the link_fail status indication.
> > > > 2) Inhibit the transmission of MAC frames.
> > > > 3) Continuously send Idle characters.
> > > >
> > > > "After detecting that the Fault condition has cleared (both Local and
> > > > Remote), the Reconciliation sublayer shall:
> > > > 1) Clear the link_fail status indication.
> > > > 2) Enable the transmission of MAC frames."
> > > >
> > > > --Bob Grow
> > > >
> > > > -----Original Message-----
> > > > From: David Gross [mailto:dgross@nortelnetworks.com]
> > > > Sent: Wednesday, January 03, 2001 8:48 AM
> > > > To: rtaborek@earthlink.net
> > > > Cc: HSSG
> > > > Subject: Re: Question about Link Fault Signalling
> > > >
> > > > Hi Rich,
> > > >
> > > > I have a quick question about Remote Fault I was hoping you could
> > > > answer. In 46.2.6, it says:"Reception of multiple local fault messages
> > > > causes the Reconcilliation Sublayer to inhibit the transmission of
> > > > frames by MAC, and to encode remote fault status messages on TXC<3:0>
> > > > and TXD<31:0>" It goes on to specify that reception of three LF messages
> > > > sets link_fail to 1, and none n 6 clock periods clears link_fail.
> > > >
> > > > My question is this: I believe we said that upon recieving RF, the RS
> > > > will output an IDLE stream until it no longer recieves RF. If this is
> > > > so, how many RF messages set this condition to be true, and in how many
> > > > clocks do we say that this condition is cleared if no RFs are detected.
> > > > Is it similar to LF, or do we only require that one RF be detected (and
> > > > then for how long before we reset this IDLE output condition of the RS
> > > > Tx?)
> > > >
> > > > Thanks in advance.
> > > >
> > > > -Dave Gross
> >
> > --
> > -----------------------------------------
> > Benjamin Brown
> > AMCC
> > 2 Commerce Park West
> > Suite 104
> > Bedford NH 03110
> > 603-641-9837 - Work
> > 603-491-0296 - Cell
> > 603-626-7455 - Fax
> > 603-798-4115 - Home Office
> > bbrown@amcc.com
> > -----------------------------------------
--
-----------------------------------------
Benjamin Brown
AMCC
2 Commerce Park West
Suite 104
Bedford NH 03110
603-641-9837 - Work
603-491-0296 - Cell
603-626-7455 - Fax
603-798-4115 - Home Office
bbrown@amcc.com
-----------------------------------------