RE: Does Ten-Gigabit Ethernet need fault tolerance?
The traffic restoration time of P802.3ad will be as fast as:
1) Failure of an individual link (in a set of aggregated links) can be
detected.
2) Each of the connected systems (routers, switches, or end stations) can
move 'flows' from that link to another link.
The second of these, moving flows, is entirely under the control of the
system that is transmitting the frames (distribution of flows is not
symmetric). That is to say each transmitter at each end of the link can
independently start use another link other than the failed link.
The constraints on moving the flows are therefore:
(a) how fast the transmitting system can react to link failure
(b) any deliberate allowance that that system may make to preserve ordering
because the remote system takes time to 'collect' the frames from each
individual link - see the P802.3ad draft for a detailed discussion of this,
which makes it easier for links to be terminated on separate physical pieces
of hardware (cards etc.).
Disregarding (b) for the present - see more below - recovery from a failed
link takes place without any protocol being exchanged between the
communicating systems so that (2) can be as short a time as the implementor
cares to make it. Given the simplicity of the decision to be made - it can
be precomputed, if this link fails I do X - it is not unreasonable to do
this at interrupt time in a software system. So it happens that failover has
been implemented in very low cost (as cheap as managed Ethernet gets) volume
products within 20 milliseconds. More attention to the subsystem design
could naturally achieve failover rather more quickly though I don't know
anyone is bothered to strive for sub 10 millisecond yet.
How much allowance is made for (b) is an implementation issue, but can be
minimized by receive side switch design. If this became a hot area I am sure
that vendors would start quoting it.In the IP world the right answer is to
make no allowance at all.
Going on from P802.3ad, a similar approach can be used to dramatically
improve 802.1D spanning tree reconfiguration. Again a choice of the link (or
set of aggregated links) to failover to can be preselected for dual
redundant tree topologies - failover can be initiated without the
transmission of additional protocol messages - and again 20 milliseconds
failover is achieved. Achieving these failover times for failure of any
single link in the network requires careful consideration of the topolgy to
be used, however work underway in 802.1 is designed to free spanning tree
reconfiguration times from all worse case end to end timer estimates, so
where messages need to be exchanged the 'old' very slow spanning tree
reconfiguration times no longer apply. This work is underway as P802.1w
Rapid Reconfiguration. It will result in protocol enhancements that will
plug and play with existing switches - though those switches won't achieve
the rapid reconfiguration times (new switches attached to them may well do,
depends on topology). Rapid reconfiguration at this level protects a level 2
network of switches.
It should be noted that P802.3ad may well reconfigure from link failure on
an aggregated link basis. This is the best approach for aggregated links
since it does not disrupt topology at all (to first order, capacity changes
may well require traffic redistribution at layer 3 or more probably at layer
2.5 (MPLS)). Above this spanning tree can be used for rapid local
reconfiguration to avoid disrupting routing. Above that routing may have to
kick in to rediscover best paths etc. Again it is possible for routing to
precompute alternates on a local level and failover to them rapidly as a
first response to failure, good network topology is however required for
this to be effective and is may be impossible on a long haul basis. MPLS
failover may well be more challenging and require setting up the MPLS label
switching path again - I think this is the only recovery strategy where
RSVP-like MPLS signalling is used. However these problems, which are very
real, will not be helped at all by local link recovery except as in as much
as that completely masks failure as in the P802.3ad Link Aggregation case.
Note that in the case where protocol messages are sent to aid rapid
reconfiguration the design challenge is to minimize the number of messages
sent to achieve a given timing. Almost any protocol (well any correct
protocol of which there are rather fewer) can achieve rapid reconfiguration
if the designer is allowed to send many messages, just depends on how much
processing is tolerated.
All of the above, IMHO, indicates to me that Ethernet should stick to
providing timely indication of link failure, and making sure that both
communicating systems see the failure. Timely indication of link recovery is
also desirable but since this will undoubtedly be confirmed by a protocol
excahnge before the link is brought into service, it is less critical.
Mick
-----Original Message-----
From: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
[mailto:owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx]On Behalf Of Roy Bynum
Sent: Sunday, July 18, 1999 5:33 PM
To: mick@xxxxxxxxxxx
Cc: stds-802-3-hssg@xxxxxxxx
Subject: Re: Does Ten-Gigabit Ethernet need fault tolerance?
Mick,
When implemented how fast is the FT traffic restoration of P802.3ad supposed
to
work? From that restoration time, calculate how much data got lost. One of
the
major features of tightly coupled error detection is that the traffic
restoration times are greatly reduced. At present, the fastest that L3
Matrix
(I can not use any vendor implementation names.), MPLS, or L1/REI/L3 is
proposed
is 10 times slower than 802.3 that is directly mapped into SONET. (There
are
several transport vendors that are doing this.) There are other issues with
fiber maintenance that have not been issues before because the amount of
aggregated data and the "round the clock" that 10GbE will likely see. This
makes the FT as much a function of the PHY as anywhere else.
Thank you,
Roy Bynum
MCI WorldCom
"Mick Sea,man" wrote:
> What needs to be built in is the detection of failure. What we don't need
to
> do is to build everything into the MAC. I suggest you look at the fault
> tolerant capabilities provided by P802.3ad and the work on Rapid
> Reconfiguration starting in 802.1.
>
> Both these (will) provide a degree of fault tolerance based on using
> protocols that are independent of MAC details to allow network nodes to
> precalculate their response to a low level indication of failure. There is
> really no need to build these protocols into the MAC.
>
> Mick
>
> -----Original Message-----
> From: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> [mailto:owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx]On Behalf Of Joe Gwinn
> Sent: Friday, July 16, 1999 3:15 PM
> To: stds-802-3-hssg@xxxxxxxx
> Subject: Does Ten-Gigabit Ethernet need fault tolerance?
>
> The purpose of this note is to present a case for inclusion of fault
> tolerance in 10GbE, and to offer a suitable proven technology for
> consideration. However, no salesman will call.
>
> Why Fault Tolerance? Ten-Gigabit Ethernet is going to be a relatively
> expensive, high-performance technology intended for major backbones,
> perhaps even nibbling at the bottom end of the wide-area network (WAN)
> market. In such applications, high availability is very much desired;
loss
> of such a backbone or WAN is much too disruptive (and therefore expensive)
> to be much tolerated, and this kind of a market will gladly pay a
> reasonable premium to achieve the needed fault tolerance.
>
> Why add Fault Tolerance now? Because it's easiest (and thus cheapest) if
> done from the start, and because having FT built in and therefore becoming
> ubiquitous will be a competitive discriminator, neutralizing one of the
> remaining claimed advantages of ATM.
>
> Isn't Fault Tolerance difficult? In hub-and-spoke (logical star, physical
> loop) topologies such as GbE and10GbE, it's not hard to achieve both fault
> tolerance (FT) and military-level damage tolerance (DT). In networks of
> unrestricted topology, it's a lot harder. The presence of bridges does
not
> affect this conclusion.
>
> How do I know that FT is so easily achieved? Because it's already been
> done, may be bought commercially, and is in use on one military system and
> is proposed for others. The FT/DT technology mentioned here was developed
> on a US Navy project, and is publically available without intellectual
> property restrictions. Why was the technology made public? To encourage
> its adopotion and use in COTS products, so that defense contractors can
buy
> FT/DT lans from catalogs, rather than having to develop them again and
> again, at great risk and expense.
>
> What is the difference between Fault Tolerance and Damage Tolerance? In
> fault tolerance, faults are rare and do not correlate in either time or
> place. The classic example is the random failure of hardware components.
> (Small acts of damage, such as somebody tripping over a wire or breaking a
> connector somewhere, are treated as faults here because they are also rare
> and uncorrelated.) In damage tolerance, the individual faults are sharply
> correlated in time and place, and are often massive in number. The classic
> military example is a weapon strike. In the commercial world, a major
power
> failure is a good example. Damage tolerance is considered much harder to
> accomplish than fault tolerance. If you have damage tolerance, you also
> have fault tolerance, but fault tolerance does not by itself confer damage
> tolerance.
>
> How is this Damage Tolerance achieved? All changes in LAN segment
topology
> (the loss or gain of nodes (NICs), hubs, or fibers) are detected in MAC
> hardware by the many link receivers, which report both loss and
acquisition
> of modulated light. This surveillance occurs all the time on all links,
and
> is independent of data traffic. Any change in topology provokes the
> hardware into "rostering mode", the automatic exploration of the segment
> using a flood of special "roster" packets to find the best path, where
> "best" is defined as that path which includes the maximum number of nodes
> (NICs).
>
> Just how fault tolerant and damage tolerant is this scheme? A segment
will
> work properly with any number of nodes and hubs, if sufficient fibers
> survive to connect them together, and will automatically configure itself
> into a working segment within a millisecond of the last fault. If the
> number of broken fibers is less than the number of hubs, all surviving
> nodes will remain accessible, regardless of the fault pattern. If the
> number of fiber breaks is equal to or greater than the number of hubs,
> there is a simple equation to predict the probability of loss of access to
> a typical node due to loss of hubs and/or fibers, given only the number of
> hubs and the probability of any fiber breaking: Pnd[p,r]= ((2p)(1-p))^r,
> where p is the probability of fiber breakage and r is the number of
> surviving hubs (which ranges from zero to four in a quad system). This
> equation is exact (to within 1%) for fiber breakage probabilities of 33%
or
> less, and applies for any number of hubs.
>
> The simplicity of this equation is a consequence of the simplicity of this
> protocol, which is currently implemented in standard-issue FPGAs (not
> ASICs), and works without software intervention. It can also be
> implemented in firmware.
>
> To give a numerical example, in a 33-node 4-hub segment, loss of 42 fibers
> (16% of the segment's 264 fibers) would lead to only 0.5% of the nodes
> becoming inaccessible, on average. Said another way, after 42 fiber
breaks,
> there are only five chances out of a thousand that a node will become
> inaccessible. This is very heavy damage, with one fiber in six broken. To
> take a more likely example, with three broken fibers, all nodes will be
> accessible, and with four broken fibers, there is less than one chance in
a
> million that a node will become inaccessible. Recovery takes two ring tour
> times plus settling time (electrical plus mechanical), typically less than
> one millisecond in ship-size networks, measured from the last fault.
> Chattering and/or intermittent faults can be handled by a number of
> mechanisms, including delaying node entry by up to one second. Few current
> LAN technologies approach this degree of resilience, or speed of recovery.
>
> In commercial systems and some military systems, a dual-ring solution is
> sufficient. Up to quad-ring solutions are comercially available, needed
> for some military systems. However, the ability to support up to quad
> redundant systems should be provided in 10GbE, for two reasons. First,
> quad is needed for the military market, which may be economically
> significant in the early years of 10GbE. Second, quad provides a clear
> growth path and a way to reassure non-military customers that their most
> stringent problems can be solved: One can ask them if their needs really
> exceed those of warships duelling with supersonic missiles.
>
> The basic technical document, the RTFC Principles of Operation, is on the
> GbE website as "http://grouper.ieee.org/ groups/802/3/ 10G_study/public/
> email_attach/ gwinn_1_0699.pdf" and "http://grouper.ieee.org/
> groups/802/3/10G_study/ public/ email_attach/ gwinn_2_0699.pdf". I was a
> member of the team that developed the technology, and am the author of
> these documents.
>
> Although these documents assume RTFC, a form of distributed shared memory,
> the basic rostering technology can easily be adapted for Gigabit and
> Ten-Gigabit Ethernet as well. For nontechnical reasons, RTFC originally
> favored smart nodes connected via dumb hubs. However, the overall design
> can be somewhat simplified if one goes the other way, to dumb nodes and
> smart hubs. This also allows the same dumb nodes to be used in both
non-FT
> and FT networks, increasing node production volume, and does not force
> users to throw nodes away to upgrade to FT.
>
> I therefore would submit that 10GbE would greatly benefit from fault
> tolerance, and also that it's very easily achieved if included in the
> original design of 10GbE.
>
> Joe Gwinn