[EFM] Re: Pause frame usage in transport networks
Ben,
You are right about the OLT buffer depth (BD), minus head room, as setting the maximum
burst size (MBS). Here are the three main scenarios that impact the discussion:
1) Subscriber responds to Pause frames: In this case there is no configuration or shaper
complexity at CPE. Bursts of up to MBS go through, and anything beyond is shaped by flow
control. No packet loss at ingress due to flow control.
2) Subscriber has built in shaper (no Pause support): More complexity because of CPE shaper
and need for configuration. Would work just fine under normal conditions. But if for any
reason there is a discrepancy between the configured parameters and the network, packets
would be lost. Such reasons may be misbehaving subscriber, miss configuration, faults in
network that would shrink the bandwidth on the backbone pipe, etc.
3) Subscriber does NOT respond to Pause frames: Performance would suffer as any bursts
beyond buffer depth (BD) result in lost packets. Lost packets mean time outs and
re-transmissions hence good put goes down. You may say that I like to live dangerously.
But for EoS the wide area pipe is fixed in bandwidth. There is no stat-muxing and its not
like that one may get away with it if the network is idle. So could we size BD to capture
the bursts. Well traces show that traffic coming out of the enterprise has a heavy tail.
Traffic coming out of homes, today, maybe. Can one get creative with buffer management,
sure you can. But it seems to me that we end up with a lossy scenario anyway.
I have to disagree about the 3rd scenario being more reliable (argument that there is no
Pause frame to lose). If measure of reliability is probability of packet loss, then the 3rd
scenario is by far the least reliable. The probability of losing packets in scenario-3 far
exceeds the probability of losing a Pause frame.
So it seems to me that in the balance, Pause frame is a useful mechanism. Whether it is
activated is left to configuration. However I expect it from my Service Provider or I will
vote with my feet.
Best regards, Siamack
Ben Brown wrote:
> Siamack,
>
> Pause frames defeat the whole notion of peak bursts, don't they?
> Isn't the true peak burst limit the depth of the ingress buffering
> at the OLT? Once the OLT is beyond this peak limit, Pause frames
> simply limit the ONU to its agreed throughput. In fact, due to
> worst case response times, the ONU will likely be paused before
> the OLT's ingress buffer is full. If the ONU chooses to ignore
> (or loses) a Pause frame, the OLT must be capable of discarding
> frames that exceed its ingress buffer depth (buffer overflow).
>
> Pause frames can be used to smooth out an ONU's burst. So could
> a rate limiter at the output of the ONU. Either would work to
> enforce the SLA. Ultimately, the only guaranteed SLA enforcer
> is the ability to discard frames at the OLT. If you can trust
> the ONU to respond to Pause frames, why not trust it to rate
> limit its output? This is actually more reliable because you
> don't have to worry about a Pause frame getting lost due to
> a network error.
>
> Regards,
> Ben
>
> Siamack Ayandeh wrote:
> >
> > Ben,
> >
> > Please see my comments interleaved.
> >
> > Regards, Siamack
> >
> > Ben Brown wrote:
> >
> > > Siamack,
> > >
> > > This comment is way off track of the original question but I
> > > feel a need to ask this question. That's why I changed the
> > > subject. I'll even redraw the network so that we're all using
> > > the same context.
> > >
> > > ONU1 ------ OLT1/GFP ------------------- GFP/OLT2 ------ ONU2
> > > Ethernet SONET Ethernet
> > >
> > > Why are Pause frames used on the Ethernet links? The ONU should
> > > never be allowed to Pause the OLT as that would back-pressure
> > > the entire WAN. Since the WAN doesn't support back-pressure,
> > > packets over the SONET link that exceed the OLT's egress buffers
> > > would wind up being dropped at the OLT.
> >
> > That's basically what I said "OLT can simply buffer and subsequently drop packets."
> >
> > >
> > >
> > > The OLT could Pause the ONU but for what reason? The only reason
> > > that I can think of would be to enforce the ONU's SLA. The point
> > > of putting an SLA in place is to enforce some set of rules,
> > > usually having to do with the minimum and maximum throughput
> > > guaranteed at the OLT for the ONU. The reason an SLA needs to
> > > be in place is because each side doesn't really trust the
> > > other's "handshake" and a legal document of sorts is needed.
> > > So, if neither side "trusts" the other, why do you rely on
> > > Pause frames to enforce the SLA? If the ONU will attempt to
> > > use as much bandwidth as possible, it will likely do so by
> > > ignoring the Pause frames from the OLT. This means that the
> > > only way the OLT can truly enforce the SLA is to be able to
> > > discard the frames that exceed the bandwidth agreed to in
> > > the SLA. If the OLT is capable of this, why even bother with
> > > Pause frames?
> >
> > You forget two things:
> > 1. The bursty nature of traffic and the fact that peak is greater than the committed
> > rate of service
> > 2. That networks by definition protect themselves i.e. can not rely on subscriber
> > alone
> >
> > If the subscriber exceeds its SLA then yes, frames would be dropped. But if it's just
> > a burst, i.e. on average the subscriber is in compliance then buffering and Pause
> > should do the job.
> >
> > >
> > >
> > > Sorry for being long winded but I'm trying to make a logical
> > > argument. What assumptions did I make that aren't valid?
> > >
> > > Thanks,
> > > Ben
> > >
> > > Siamack Ayandeh wrote:
> > > >
> > > > Shahram,
> > > >
> > > > It would be helpful to this discussion if you indicate what you had in mind
> > > > with regard to OAM frames so one can think of a pragmatic answer. For example
> > > > if you are worried about Pause frames: In this case the SONET/SDH (OLT1-OLT2)
> > > > is often the bottleneck link. So Pause is used on the local link to protect the
> > > > OLT-1/2 buffers. If the egress ONU back pressures the network, then OLT can
> > > > simply buffer and subsequently drop packets. Otherwise fairly large buffers
> > > > would be required to absorb the round trip time of the wide area. If you think
> > > > about it as a two port bridge, again Pause is not propagated over the wide
> > > > area. If you look at various IETF Pseudowire flavors, again following the lead
> > > > of IEEE, Pause is not propagated over the wide area.
> > > >
> > > > Siamack
> > > >
> > > > Geoff Thompson wrote:
> > > >
> > > > > Matt-
> > > > >
> > > > > To further enrich the information that you provided below....
> > > > >
> > > > > 802.3 has a long, well established tradition of not supporting media
> > > > > converters.
> > > > >
> > > > > It would be my opinion that any "features" designed to specifically support
> > > > > media converters were out of scope unless they were specifically mentioned
> > > > > in the PAR.
> > > > >
> > > > > Geoff
> > > > >
> > > > > At 01:05 AM 2/12/2003 -0500, Matt Squire wrote:
> > > > >
> > > > > >My 2 cents.
> > > > > >
> > > > > >If there is a MAC layer in the ONU, then OAM terminates there. If a
> > > > > >vendor builds something without a MAC layer, then it doesn't terminate
> > > > > >there. If you put MACs in your ONU, you're really building something
> > > > > >akin to a 2-port bridge (likely with STP disabled permanently). If you
> > > > > >don't, then its more along the lines of a media converter. Both models
> > > > > >can work. Both models can interoperate. So do it anyway you want, and
> > > > > >maybe the market will agree with you.
> > > > > >
> > > > > >- Matt
> > > > > >
> > > > > >Shahram Davari wrote:
> > > > > > >
> > > > > > > Roy,
> > > > > > >
> > > > > > > Since EOS is a private line service, It seems that you agree with me
> > > > > > and Chak that for the EOS it seems from architectural point of view the
> > > > > > OAM MAC frames should not be terminated at OLTs. Is that correct?
> > > > > > >
> > > > > > > Yours,
> > > > > > > -Shahram
> > > > > > >
> > > > > > > > -----Original Message-----
> > > > > > > > From: Roy Bynum [mailto:rabynum@xxxxxxxxxxxxxx]
> > > > > > > > Sent: Tuesday, February 11, 2003 12:26 PM
> > > > > > > > To: Chau, Chak; stds-802-3-efm@ieee.org
> > > > > > > > Subject: RE: [EFM] Question regarding OAM in 802.3ah D1.3
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > Chak,
> > > > > > > >
> > > > > > > > Depending on the service definition and who owns the service
> > > > > > > > facilities,
> > > > > > > > end to end OAM functionality would not necessarily available. In all
> > > > > > > > packet services, the service provider would own at least a
> > > > > > > > portion, if not
> > > > > > > > all of the communications facilities. The OAM functionality
> > > > > > > > would then be
> > > > > > > > for the use of the service provider, not the customer. Only
> > > > > > > > with a Leased
> > > > > > > > Circuit type of service (also referred to as "Private Line")
> > > > > > > > would end to
> > > > > > > > end, OLT1 to OLT2 OAM functionality exist. For all other types of
> > > > > > > > services, the OAM for Link 1 would not be the OAM for Link 2.
> > > > > > > > Other than
> > > > > > > > with a Leased Circuit service, the services are defined to
> > > > > > > > alter, filter,
> > > > > > > > or drop customer originated frames/packets in one way or
> > > > > > > > another, including
> > > > > > > > the OAM frames. This is the gist of the work that is being done by
> > > > > > > > SG15/Q.10 under E.Ethna. Other than with a Leased Circuit
> > > > > > > > service, true
> > > > > > > > "transparency" does not exist.
> > > > > > > >
> > > > > > > > Thank you,
> > > > > > > > Roy Bynum
> > > > > > > >
> > > > > > > > At 10:15 AM 2/11/2003 -0600, Chau, Chak wrote:
> > > > > > > > >Hi David and All,
> > > > > > > > >
> > > > > > > > >I though for a PtP application the OAMPDUs message should be
> > > > > > > > transparent
> > > > > > > > >form OLT1 to OLT2.
> > > > > > > > >Or else, this will defeat the purpose of PtP, i.e., no L2 frame
> > > > > > > > >processing. Which means that OAM for Link1 can be OAM for
> > > > > > > > Link2, is that
> > > > > > > > >correct? This topic may be reviewed outside of EFM if prefered.
> > > > > > > > >
> > > > > > > > >Kind Regards,
> > > > > > > > >Chak
> > > > > > > > >
> > > > > > > > >Chak Chau
> > > > > > > > >FUJITSU, Transmission Development
> > > > > > > > >Phone: (972) 479-2795
> > > > > > > > >chak.chau@xxxxxxxxxxxxxxx
> > > > > > > > >chakavuth.chau@xxxxxxxxxxxx
> > > > > > > > >-----Original Message-----
> > > > > > > > >From: David Martin [mailto:dwmartin@xxxxxxxxxxxxxxxxxx]
> > > > > > > > >Sent: Monday, February 10, 2003 2:59 PM
> > > > > > > > >To: stds-802-3-efm@ieee.org
> > > > > > > > >Subject: RE: [EFM] Question regarding OAM in 802.3ah D1.3
> > > > > > > > >
> > > > > > > > >Shahram,
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Agreed, the EoS (e.g. GFP-F) mapping is a simple
> > > > > > > > port-to-port mapping and
> > > > > > > > >doesn't include the full MAC sublayer processing (i.e. only
> > > > > > > > terminates
> > > > > > > > >IPG, preamble, SFD). Inspecting the MAC DA and filtering off
> > > > > > > > EFM OAMPDUs
> > > > > > > > >and processing them is required by the network application,
> > > > > > > > since the
> > > > > > > > >Ethernet link / PHY to which they apply is terminated. OAM
> > > > > > > > for link 1
> > > > > > > > >cannot be mixed with OAM for link 2 on the other side of the
> > > > > > > > provider's
> > > > > > > > >network.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >ONU1 -------------- OLT1/GFP------------------ GFP/OLT2
> > > > > > > > ------------ ONU2
> > > > > > > > > Ethernet SONET
> > > > > > > > Ethernet
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >It might be more appropriate to continue this privately, or
> > > > > > > > on the Q.12/15
> > > > > > > > >reflector.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >...Dave
> > > > > > > > >David W. Martin
> > > > > > > > >Nortel Networks
> > > > > > > > ><mailto:dwmartin@xxxxxxxx>dwmartin@xxxxxxxx
> > > > > > > > >+1 613 765-2901 (esn 395)
> > > > > > > > >~~~~~~~~~~~~~~~~~~~~
> > > > > > > > >-----Original Message-----
> > > > > > > > >From: Shahram Davari [mailto:Shahram_Davari@xxxxxxxxxxxxxx]
> > > > > > > > >Sent: Monday, February 10, 2003 3:16 PM
> > > > > > > > >To: Martin, David [SKY:QW10:EXCH]; stds-802-3-efm@ieee.org
> > > > > > > > >Subject: RE: [EFM] Question regarding OAM in 802.3ah D1.3
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >David,
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >I like to agree with you, but from layering architectural
> > > > > > > > point of view,
> > > > > > > > >the EOS box does not have to implement MAC
> > > > > > > > >layer (i.e., do any MAC lookup), rather a P-2-P EOS is a
> > > > > > > > kind of port
> > > > > > > > >transport in which all traffic coming form an Ethernet port
> > > > > > > > are send over
> > > > > > > > >a specific SONET channel.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Please see further comments in-line:
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Thanks,
> > > > > > > > >-Shahram
> > > > > > > > >-----Original Message-----
> > > > > > > > >From: David Martin [mailto:dwmartin@xxxxxxxxxxxxxxxxxx]
> > > > > > > > >Sent: Monday, February 10, 2003 3:00 PM
> > > > > > > > >To: stds-802-3-efm@ieee.org
> > > > > > > > >Subject: RE: [EFM] Question regarding OAM in 802.3ah D1.3
> > > > > > > > >Shahram,
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >This question is somewhat out of scope wrt EFM, but the
> > > > > > > > answer is yes, the
> > > > > > > > >EFM OAMPDU flow must be terminated at the Provider Edge.
> > > > > > > > Otherwise it
> > > > > > > > >would flow through the provider's SONET network and get
> > > > > > > > mixed in with a
> > > > > > > > >separate EFM OAMPDU flow at the far end.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >SD=> Which separate OAMPDU flow? do you mean from ONU2 to OLT2?
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > Note that you have the terms ONU / OLT reversed.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >So the ONU is the customer side?
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >This "filter function" is being defined in the ITU-T SG15 /
> > > > > > > > Q.12 work in
> > > > > > > > >draft G.ethna (was G.etna) and in the OIF UNI v2.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Thanks I will have a look.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >The PE-PE, or SONET portion is not an EFM link, but rather a
> > > > > > > > SONET path,
> > > > > > > > >which has its own OAM (i.e. POH).
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Agree.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >...Dave
> > > > > > > > >David W. Martin
> > > > > > > > >Nortel Networks
> > > > > > > > ><mailto:dwmartin@xxxxxxxx>dwmartin@xxxxxxxx
> > > > > > > > >+1 613 765-2901 (esn 395)
> > > > > > > > >~~~~~~~~~~~~~~~~~~~~
> > > > > > > > >-----Original Message-----
> > > > > > > > >From: Shahram Davari [mailto:Shahram_Davari@xxxxxxxxxxxxxx]
> > > > > > > > >Sent: Monday, February 10, 2003 2:13 PM
> > > > > > > > >To: stds-802-3-efm@ieee.org
> > > > > > > > >Subject: [EFM] Question regarding OAM in 802.3ah D1.3
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Hi,
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >802.3ah section 57 says that the OAM defined is for single link (or
> > > > > > > > >emulated link), and should not be forwarded by bridges/switches.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >My question is, in case of Ethernet over SONET transport
> > > > > > > > (GFP + Virtual
> > > > > > > > >concatenation), should the OAMPDU be terminated at the EOS
> > > > > > > > device or it
> > > > > > > > >should be transparently transported?
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Consider this example:
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >OLT1 -------------- ONU 1------------------ ONU 2 ------------ OLT 2
> > > > > > > > > Ethernet SONET Ethernet
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Assume OLT1 and OLT2 are the customer equipments and ONU1
> > > > > > > > and ONU2 are
> > > > > > > > >provider
> > > > > > > > >transport equipments that transport Ethernet over SONET (but
> > > > > > > > don't do any
> > > > > > > > >switching/bridging).
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >In this case should ONU1 terminate OAMPDUs from OLT1 or it
> > > > > > > > should sent
> > > > > > > > >them transparently to OLT2?
> > > > > > > > >In other words is OLT1--ONU1 considered a single link? what
> > > > > > > > about ONU1 to
> > > > > > > > >ONU2, is this also a link?
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >Thanks in advance,
> > > > > > > > >Shahram Davari
> > > > > > > > >Senior Product Research Engineer
> > > > > > > > >R&D Research Ottawa
> > > > > > > > >PMC-Sierra, Inc.
> > > > > > > > >Phone: (613) 271-4018
> > > > > > > > >Fax: (613) 271-6468
> > > > > > > > >
> > > > > > > >
> > >
> > > --
> > > -----------------------------------------
> > > Benjamin Brown
> > > AMCC
> > > 200 Minuteman Rd
> > > 3rd Floor
> > > Andover, MA 01810
> > > 978-247-8022 - Work
> > > 603-491-0296 - Cell
> > > 978-247-0024 - Fax
> > > 603-798-4115 - Home Office/Fax
> > > bbrown@xxxxxxxx
> > > -----------------------------------------
>
> --
> -----------------------------------------
> Benjamin Brown
> AMCC
> 200 Minuteman Rd
> 3rd Floor
> Andover, MA 01810
> 978-247-8022 - Work
> 603-491-0296 - Cell
> 978-247-0024 - Fax
> 603-798-4115 - Home Office/Fax
> bbrown@xxxxxxxx
> -----------------------------------------
begin:vcard
n:Ayandeh;Siamack
tel;fax:781 271 9988
tel;work:781 276 4192
x-mozilla-html:FALSE
url:www.txc.com
org:Transwitch Corporation (Onex)
adr:;;34 Crosby Drive;Bedford;MA;01730;USA
version:2.1
email;internet:sayandeh@xxxxxxx
title:Senior Consulting Engineer
end:vcard