Re: [mpls] Last Call: <draft-ietf-mpls-in-udp-04.txt> (Encapsulating MPLS in UDP) to Proposed Standard

[<[e-mail]>]

Curtis

http://lmgtfy.com/?q=Jonathan+Stone+CRC+checksum

HDLC is just whatever is over the last hop. You said HDLC, I reused that as
an example.

Any link technology could be substituted - 10Mbps Ethernet, say, though
you'd criticise that as not being 10Gbps Ethernet and therefore out of date.

Again, the point is that the link check is not end-to-end, and that errors
can creep in from the most unexpected places. By analogy with security,
if I have security across each hop, why would I need security end-to-end?
I already have it across each hop! Each link is highly and absolutely
unrbreakably secure! What is this end-to-end of which you speak?

If you don't get that point because it's a bit abstract and timeless, that's
fine. (and if you have to explain the joke, the joke wasn't funny.)
The link CRC doesn't apply across the entire path; do the maths for
the path and a series of concatenated links.

[As it happens, I'm familiar with UDP/IP/HDLC internet infrastructure installed
within the decade and  in daily operational use to deliver imagery from orbit.
But the paper I wrote on that dates from 2007, so is old and
won't be of interest to you.]

Wait, this thread is all about putting 90s MPLS technology over UDP
technology specified in 1980. Clearly, if MPLS has to rely on an older
technology in this way, the MPLS crowd should give up and go home. 

We've learned repeatedly that zero checksums are a bad idea. IPv6
RFC2460:

        Unlike IPv4, when UDP packets are originated by an IPv6 node,
         the UDP checksum is not optional.  That is, whenever
         originating a UDP packet, an IPv6 node must compute a UDP
         checksum over the packet and the pseudo-header, and, if that
         computation yields a result of zero, it must be changed to hex
         FFFF for placement in the UDP header.  IPv6 receivers must
         discard UDP packets containing a zero checksum, and should log
         the error.

which RFC6935 simply rewote as inconvenient to tunnelers without
considering how it affected everything else in the network.

Those that do not learn from history are doomed to repeat it.
(Unless it's recent history, in which case they're doomed to reject it
as irrelevant.)

Lloyd Wood
http://about.me/lloydwood
________________________________________
From: Curtis Villamizar [[e-mail]]
Sent: 15 January 2014 01:42
To: Wood L  Dr (Electronic Eng)
Cc: [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]
Subject: Re: [mpls] Last Call: <draft-ietf-mpls-in-udp-04.txt> (Encapsulating MPLS in UDP) to Proposed Standard

In message <[e-mail]>
[e-mail] writes:

> The HDLC part here is last link, not the scope of the whole path.  Any
> 'low' bit error rate given actually becomes quite high once you
> consider no of bits per packet and line rate...
>
> Do read Jonathan Stone's papers on where errors creep in - not just in
> the link, by at any point along the path, including regeneration
>
> Lloyd Wood


Lloyd,

There is no HDLC hop.  No one has used HDLC for internet
infrastructure in ages.  It was a joke, like Scott's comment on
wanting to use X.25.  HDLC was disappearing when the Stone/Partridge
Sigcomm 2000 paper was written.

Links please.  And how old is that paper?  Not another 15 year old
work is it?

If you have one bit error per day, how many packets do you lose that
day?  (hint: one).

If you have one bit error per day, how many undetactable packet errors
do you have?  (hint: crc32 gets all one bit errors, therefore zero).

10^-12 bit errors is one per 10 second on 100 Gb/s, one per 100 second
on 10 Gb/s and is generally considered high enough to take a link down
immediately.  A 1500 byte packet is 12,000 bits, about ~10^4.  That
would yield a packet rate as high as 10^-8 if bit errors were mostly
one bit error per packet.  In that case all errors would be
detectable.  It is only when there are a lot of bit errors or more per
packet that the CRC can be defeated and then its about 10^-9 chance.

So at an error rate much less than 10^-8 packets (tightly bunched
errors with multiple bit errors per packet) some 10^-9 might be
undetectable with a CRC32.  One packet every 10^6 seconds at 100 Gb/s
could have an undetectable error.  About one undetectable error a day
or one a week for continuous full out 100 Gb/s link.

Note that the same low error rate does not apply to a GbE or 10GbE
over colored optics over ROADM in the metro since there is no FEC
there.  It also may not apply to the enterprise or campus Ethernets.
In those hops the error rate is likely to be higher.  Needless to say,
wireless hops can have very high error rates.

This is why it could make sense to have the UDP checksum optional in
MPLS over UDP.  It wouldn't hurt to provide the checksums but in some
cases it might be OK to disable them.  That is what SHOULD is for in
an IETF document.

Curtis


> From: Curtis Villamizar [[e-mail]]
> Sent: 14 January 2014 20:54
> To: Wood L  Dr (Electronic Eng)
> Cc: [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]
> Subject: Re: [mpls] Last Call: <draft-ietf-mpls-in-udp-04.txt> (Encapsulating MPLS in UDP) to Proposed Standard
>
> In message <[e-mail]>
> [e-mail] writes:
>
> > It stands to reason that if tunnelers can turn off udp checksums
> > because their performance is degraded, they can turn off
> > congestion control because it will degrade their performance.
> >
> > Rest of the internet getting congested and getting
> > misdelivered corrupted packets? Really not their problem.
> >
> > There are important vendors trying to sell products here,
> > and they need performance to do so.
> > Get with the program!
> >
> > Lloyd Wood
> > http://about.me/lloydwood
>
>
> OK, perhaps if you are running MPLS/UDP/IP over HDLC and the HDLC
> configuration is set to count FCS errors but not drop you will still
> *really* need the UDP checksum.  Otherwise its isn't going to do much
> for you.  Any checksum is really bad for some types of errors such as
> chunk reordering and multiple bit errors.
>
> Maybe on HDLC or PPP with 16 bit CRC you may see a low error rate, but
> in theory that would be much less than 10^-5 since few multiple bit
> errors will be coincidence match the CRC, even for a 16 bit CRC.
>
> I suspect most routers would be able to do the checksum anyway and for
> modern links if they come up with a zero error count that's fine.
>
> <ot>
>
> Modern OTN based transport networks use forward error correction FEC
> which accounts for a fair amount of overhead and a lot of processing
> gates on the receiving end.  The measure of effectiveness of given FEC
> is in dB with 10 dB being a reduction of a factor of 10 in bit errors
> and typical FEC in the high tens of dB.  The target corrected error
> rate is often 10^-15 or one bit error in 24 hours for 10 Gb/s, one bit
> error in 2.5 hours for 100 Gb/s.  Any link with corrected bit error
> rates approaching 10^-12 is taken out of service.  This is roughly
> equivalent to the old ES (errored seconds) and SES (severely errored
> seconds) metric where a ES is one second with any bit errors and an
> SES is one second with 10 or more errors (I think its 10).  More than
> some number of ES or SES and a link is taken down.  The uncorrected
> errors are passed through.
>
> A packet may traverse an entire continent with 2-3 such links
> separated by regeneration or could stop at a number of routers along
> the way.  Typically today the router uses 10GbE or 100GbE (growing
> use) which are then passed as a bit stream in the transport network.
> At the other end the uncorrected errors from transport are picked up
> by Ethernet 32 bit FCS.  Since a 32 bit FCS picks up 100% of single
> bit errors and most instances where a small number of bits are in
> error, and all but 1 in 2^32 where many bits are in error, few errors
> are going to get through.  If GFP is used, the per packet FCS is
> checked at each hop and for GFP-T also checked end to end.
>
> A bad local ethernet is more likely to contribute an error (again
> better than 1 in 2^32 detection is expected) due to something like a
> bad CAT-{5,5e,6} connection or too many sharp turns.  A DSL or DOCSIS
> link is also more likely to contribute an error.  With CRC32 on all
> links and no bad hardware in between (ie: circa 1990s equipment with
> no parity RAM and no correction on DMA, buses, etc) you would expect
> on the order of 10^-8 errors (10^-9 per hop, a few errored hops).
>
> For example, two hosts on my home LAN had non-zero tcp checksums.
> Each had < 10^-6 packet error rate.  It is hard to tell if this is
> host errors at the other end.  The only hosts I have with non-zero are
> on the service provider DMZ LAN so that would include any bot attacks,
> etc, where sending hosts could be old junk.  Host behind those have
> zero UDP and TCP checksum errors.  This seems similar to Stewart's
> quick check.
>
> In the T1/T3 days the transport layer just had parity and just counted
> parity errors.  Providers in those days were notorious for ignoring ES
> and SES counters until the customer complained.  HDLC then had its 16
> bit CRC, optional 32 bit.  If an ISP wasn't paying attention to their
> HDLC error counters then it was up to the IP end customer to complain
> and hope the problem got escallated rather than dropped.
>
> </ot>
>
> As to whether congestion control is in practice needed see
> http://www.ietf.org/mail-archive/web/mpls/current/msg11222.html
>
> Its fine to make them both optional and to make congestion control
> mechanisms out of scope and the topic of a later document if needed.
>
> Curtis
>
>
>
> > ________________________________________
> > From: ietf [[e-mail]] On Behalf Of Joel M. Halpern [[e-mail]]
> > Sent: 10 January 2014 15:36
> > To: Eggert, Lars; Xuxiaohu
> > Cc: [e-mail]; IETF
> > Subject: Re: Last Call: <draft-ietf-mpls-in-udp-04.txt> (Encapsulating MPLS in UDP) to Proposed Standard
> >
> > Maybe I am completely missing things, but this looks wrong.
> > If the MPLS LSP is carrying fixed rate pseudo-wires, adding congestion
> > control will make it more likely that the service won't work.  Is that
> > really the goal?
> >
> > We do not perform congestion control on MPLS LSPs.
> > Assuming that a UDP tunnel is carrying just MPLS and was established
> > just for MPLS, why would we expect it to behave differently than an MPLS
> > LSP running over the exact same path, carrying the exact same traffic?
> >
> > Yours,
> > Joel
> >
> > On 1/10/14 3:47 AM, Eggert, Lars wrote:
> > > Hi,
> > >
> > > that sounds good. What congestion control are you going to be specifying for your tunnel?
> > >
> > > Lars
> > >
> > > On 2014-1-10, at 4:46, Xuxiaohu <[e-mail]> wrote:
> > >
> > >> Hi Lars,
> > >>
> > >> Thanks a lot for your comments.
> > >>
> > >> I wonder whether the following modified text for Congestion Consideration section is OK from your point of view:
> > >>
> > >> Since the MPLS-in-UDP encapsulation causes MPLS packets to be forwarded through "UDP tunnels", the congestion control guidelines for UDP tunnels as defined in Section 3.1.3 of [RFC5405] SHOULD be followed. Specifically, MPLS can carry a number of different protocols as payloads. When the payload traffic is IP-based and congestion-controlled, the UDP tunnel SHOULD NOT employ its own congestion control mechanism, because congestion losses of tunneled traffic will already trigger an appropriate congestion response at the original senders of the tunneled traffic. When the payload traffic is not known to be IP-based, or is known to be IP-based but not congestion-controlled, the UDP tunnel SHOULD employ an appropriate congestion control mechanism. Furthermore, because UDP tunnels are usually bulk-transfer applications as far as the intermediate routers are concerned, the guidelines as defined in Section 3.1.1 of [RFC5405] SHOULD apply.
> > >>
> > >> Best regards,
> > >> Xiaohu
> > >>
> > >>> -----ÓʼþÔ­¼þ-----
> > >>> ·¢¼þÈË: mpls [mailto:[e-mail]] ´ú±í Eggert, Lars
> > >>> ·¢ËÍʱ¼ä: 2014Äê1ÔÂ8ÈÕ 18:22
> > >>> ÊÕ¼þÈË: IETF
> > >>> ³­ËÍ: [e-mail]
> > >>> Ö÷Ìâ: Re: [mpls] Last Call: <draft-ietf-mpls-in-udp-04.txt> (Encapsulating MPLS
> > >>> in UDP) to Proposed Standard
> > >>>
> > >>> Hi,
> > >>>
> > >>> On 2014-1-2, at 16:14, The IESG <[e-mail]> wrote:
> > >>>> - 'Encapsulating MPLS in UDP'
> > >>>> <draft-ietf-mpls-in-udp-04.txt> as Proposed Standard
> > >>>
> > >>>
> > >>> this document needs to describe how it addresses the issues raised in BCP145
> > >>> (RFC5405). It already contains some text about messages sizes and congestion
> > >>> considerations, which is great. Unfortunately, the text about congestion
> > >>> considerations is not fully in line with RFC5405.
> > >>>
> > >>> Lars
> > >
> > _______________________________________________
> > mpls mailing list
> > [e-mail]
> > https://www.ietf.org/mailman/listinfo/mpls
>
>