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RE: [RRG] Sprite & IPTM while PMTU probing is in progress
Robin,
See below:
> -----Original Message-----
> From: Robin Whittle [mailto:rw@firstpr.com.au]
> Sent: Thursday, November 29, 2007 4:36 AM
> To: Routing Research Group list
> Cc: Templin, Fred L
> Subject: Re: [RRG] Sprite & IPTM while PMTU probing is in progress
>
> Hi Fred,
>
> Thanks for your response. You wrote, in part:
>
> > On- and off-list discussions have explored the idea of requiring
> > a 2KB EMTU_R on all ETRs and accommodating all 1500-byte and
> > smaller packets; even if fragmentation is needed and the inner
> > DF=1. This would uphold the "principle of least surprise" to the
> > SH, but the issue comes in knowing the EMTU_R of the ETR and in
> > avoiding excessive fragmentation.
>
> I don't clearly understand this. Iljitsch has not been able to
> convince me - or anyone else AFAIK - that it it will be practical to
> insist on massive hardware upgrades broad enough for the wide range
> of location I think ITRs and ETRs need to be located. (Host to host,
> Man - Coast to coast. http://www.firstpr.com.au/ip/ivip/tv-ad/)
Sorry for asking, but do you have a PR department that
writes this stuff up? :^}
> Maybe some people envisage a more restricted range, or are more
> upbeat about the prospects of a global jumboframes and gigabit
> Ethernet upgrade.
>
> For the purposes of discussing sprite-mtu and IPTM, I assume that
> this upgrade is not practical.
IMHO, the H/W upgrades will occur independently of any
tunnel MTU handling. But, the tunnel MTU handling should
be capable of taking advantage of the larger MTUs when
they become available.
> > Since the ITR cannot know the EMTU_R of the ETR a priori unless
> > there is some spec that says: "all ETRs MUST configure an EMTU_R
> > of at least X bytes", the ITR should not simply fragment the
> > outer packets (or, allow the network to fragment them) since they
> > could black-hole.
>
> You and most other people on this list have much more experience in
> these matters than I do, but I don't see why fragmenting packets for
> a few seconds will lead to a "black hole".
If the ETR does not have a large-enough EMTU_R, I think
it would have to drop the packet silently.
> I recognise that fragmentation involves several costs:
>
> 1 - More work for the routers at each end.
That brings up another question; were you expecting to
fragment the packet in the ITR's stack before sending,
or just send with DF=0 and let the network fragment?
The literature and recent discussion consensus seems
to greatly favor the former.
> 2 - Two packets rather than one being handled by all
> routers en-route.
Or more than 2, if the network also fragments.
> 3 - More data being sent, due to an extra packet's worth of
> overhead.
>
> 4 - Most seriously, I think, a greater chance of loss of the total
> packet due to its delivery depending on two packets, rather than
> one.
>
> Each packet has some risk of being lost, and the risk is
> a little higher than would otherwise be the case, since the
> two packets take up more time on the wire and more router
> resources than the single original packet.
>
> 5 - Fragmentation failures due to 16 bit ID wraparound. You
> mentioned this being a problem with IPv4 and high packet rates
> with longer delays, causing the wrong packets to be reassembled,
> due to the 16 bit counter wrapping and them having the same
> sequence number.
>
> Tacking point 5 first, in message 608, quoting Iljitsch, I asked
> whether this could be largely resolved by a shorter time window for
> reassembly in the ETR:
>
> - - - -
>
> > I don't have any references, but in short, the issue is that you
> > have a 16 bit ID space with a reassembly timeout of something
> > like a few minutes. This means you can only send 65536 packets
> > during that "few minute" window or you'll incorrectly reassemble
> > fragments from different packets if you lose a fragment. This is
> > especially problematic if the fragmented packets belong to a
> > tunnel because in that case the IP source/dest addresses are
> > always the same.
>
> In a fresh system such as an ITR-ETR scheme, perhaps a workaround
> for this would be to set the maximum reassembly time at the ETR to
> something very much shorter, such as 1, 2 or 3 seconds?
>
> That could be a pain where the ETR is an additional function in a
> server or router with a pre-existing TCP/IP stack. Then it would
> probably not be possible to shorten the time just for the ITR to ETR
> packets.
>
> - - - -
>
> Can anyone comment on the prospects of this being an acceptable
> solution?
During some of the discussions (can't recall whether it
was on- or off-list) some Linux code was shown that uses an
out-of-order upper bound in terms of number of reassemblies
ourstanding instead of a shortened timer when deciding whether
to purge an incomplete reassembly. That seemed like the right
way to go to me.
Setting a shorter timer might not be such a good idea for
reasons outlined in RFC1122, Section 3.3.2. In some earlier
efforts, I delved into the RFC1122 suggestion of managing
round-trip times:
+ It has been suggested that a cache might be kept of
+ round-trip times measured by transport protocols for
+ various destinations, and that these values might be used
+ to dynamically determine a reasonable reassembly timeout
+ value. Further investigation of this approach is
+ required.
My conclusion was that a protocol between the ITR and ETR
could be devised to do this, but it was far too complicated
for practical purposes and had too many things that could
go wrong.
> In an ITR-ETR scheme, we probably don't want to sit around waiting
> for packets which somehow get lost and arrive more than a second
> late via some circuitous route through Manangatang.
There is also the consideration of whether we want to make
this delay-tolerant, e.g., for multiple satellite hops,
interplanetary communications, etc.
> I will read:
>
> IPv4 Reassembly Errors at High Data Rates
> http://tools.ietf.org/html/rfc4963
OK.
> The other points don't, to me, constitute a "black hole" or a
> serious problem. They seem to involve marginally greater efforts,
> marginally less efficiency and marginally greater chances of the
> final packet not being delivered.
>
> To me, this is a far better thing to do, for a few seconds or so
> into a potentially long flow, than dropping packets.
>
>
> > There are also other factors to consider, including that the ITR
> > may not have ultimate control over the setting of the ip_id. And,
> > the ETR may not be able to receive non-initial fragments in the
> > first place. (These factors can be mitigated by the placement of
> > the ITR and ETR in some use cases, however.)
>
> I don't clearly understand these points. Can you explain
> them further?
If the ITR is located behind a NAT or translating firewall,
the ip_id could be re-written. I believe a common approach
is to rewrite with a randomly-chosen 16-bit value. When
there are many SH's behind the same NAT/firewall and using
the same ITR, this could present a problem.
About non-initial fragments, I am told that some NATs/firewalls
simply pass the first fragment and drop all others. Problem
being that non-initial fragments do not include the transport
layer header.
> >> A "long" inner packet is one which, once encapsulated, would
> >> exceed the ITR's current best estimate of the PMTU. This would
> >> initially be a default such as 1280 bytes.
> >>
> >> If this default value is above the minimum for the protocol,
> >> eg. 576 for IPv4, then this value of PMTU to the "core" of the
> >> net must be available to every ITR and ETR and would be part of
> >> the specification of the ITR-ETR scheme.
> >
> > The pathMTU cannot be known a priori; all that can be known a
> > priori is the EMTU_R of the ETR if there is a specification for
> > the minimum size.
>
> Yes. I envisage some value around 1280 - at least something so that
> a 1500, 1520 etc. byte packet would only be fragmented into
> two packets.
Unless some new spec comes along, the only values for
EMTU_R that can be assumed are 576 bytes for IPv4 and
1500 bytes for IPv6.
> >> The default value would be replaced by a higher value once the
> >> probe process was complete. The pattern would be something
> >> like this, assuming the SH's initial idea of PMTU to the
> >> Destination Host was 1500. I will assume an ENCAPS overhead of
> >> 20 bytes (as with IPv4 Ivip, though other ITR-ETR schemes have
> >> higher overheads) and that all ITRs and ETRs are located so
> >> they have an MTU of at least 1280 from the DFZ.
> >
> > Do you mean to say "pathMTU" or "linkMTU"? In terms of "pathMTU",
> > do we need to consider links with configurable linkMTUs that
> > might have either mis-configured or overly- conservative values?
>
> I don't understand this clearly enough to respond at present.
May not have been worded very well. A while back, I think
it was Iljitsch who indirectly pointed out that any link
for which an MTU can be manually configured can also be
misconfigured. For example, what if an operator means to
set a linkMTU of 4500, but his finger slips and he ends
up setting only 450? Flows that use that link would
experience the inefficiency - but, they should still work.
> (My example is not in quotes - just your response.)
>
> Inner ITR action on SH's idea DH gets
> packet outer packet of PMTU
> length following encapsul- to DH
> ation of inner packet
>
> 1500
>
>
> 200 Send outer packet - 1500 The packet
> the length is less than
> 1280.
>
> 1500 Fragment packet and 1500 The packet
> commence probing (Less efficient
> and more error-
> prone tunnel with
> 2 packets instead
> of 1, but this is
> only for a few
> seconds, I hope.)
>
> 1500 Fragment packet and 1500 The packet
> continue probing
>
> ... etc.
>
> > This could black hole and appear as congestion-related loss to
> > the SH.
>
> As noted above, I foresee only marginal efficiency and reliability
> impact, where you see unacceptable packet losses.
It's not so much the loss I am concerned with; IMHO,
sustained and unmitigated fragmentation is dangerous
even for short periods of time.
> Probing complete:
> PMTU to ETR decided
> to be 1460.
>
> 1400 Send outer packet - 1500 The packet
> the length is <= 1460.
> (This length would not
> necessarily be sent - it
> is just to show that the
> ITR will now send longer
> packets without frag-
> mentation than before.)
>
> 1500 Drop the packet and send
> the SH a PTB message
> with value 1440. 1440 Nothing, but the
> ITR is usually
> close to the SH,
> and it doesn't
> take long for...
>
>
> > Depending on the placement of the ITR, this PTB might not make it
> > back to the SH.
>
> If there is a filter blocking PTB messages from the ITR to the SH,
> then I think ordinary, non-RFC4821, PMTUD would be clobbered anyway
> - with or without an ITR-ETR scheme This situation would not
> persist, I think when an ITR-ETR scheme was widely deployed, as I
> discussed in:
>
> http://psg.com/lists/rrg/2007/msg00636.html
See RFC2923 for a discussion of PTB filtering implications
for classical PMTUD.
> 1440 Send the outer packet - 1440 The packet
> the length is <= 1460 (Now the tunnel
> is handling
> optimal length
> packets.)
>
> >> This pattern would continue unless the ITR, with periodic
> >> probing, decides that the PMTU is less than 1460 (it might do
> >> this quickly if it got a PTB message from a router in a new,
> >> more MTU-challenged, path to the ETR), and if the SH sends a
> >> packet which would be too big for the new lower value of PMTU.
> >> Then the ITR would send another another PTB message to the SH,
> >> with a lower value than 1440.
>
> > There is not strictly any periodic probing needed to detect
> > pathMTU reductions, since the data packets serve as virtual
> > probes. The data packets will be lost and might be considered by
> > the SH as congestion-related loss if the PTB can't be translated
> > by the ITR and sent back to the SH.
>
> In my IPTM proposal, the ITR would only get PTB messages from
> routers in the tunnel if the outer source address was that of the
> ITR. This would be the case if IPTM was applied to LISP, eFIT-APT
> or TRRP. Ivip uses the sending host's address in the outer header,
> so the ITR would never get a PTB message if the PMTU to the ETR
> suddenly became lower than what it had assumed, based on previous
> explicit probes acknowledge by the ETR.
I believe this represents a departure from some of the other
proposals. Are you suggesting that the SH will always be
globally addressable from within the core? So, there is no
locator/id split? No public addressing for the ITR and
private addressing for the SH? No IP version mismatches
between the SH and ITR?
In any event, assuming that the outer source is that of
the SH (and not the ITR), then the PTBs coming from within
the tunnel and delivered directly to the SH will forever
report a too-large MTU because the SH has no way of knowing
that an ITR on the path will be inserting ENCAPS overhead.
Even worse, the data included in the PTB will include the
ENCAPS inserted by the ITR and will be unrecognizable
to the SH.
> This would cause a black hole for all packets which, once
> encapsulated, were longer than the new, lower, PMTU between the ITR
> and ETR. To guard against this, the ITR needs to periodically probe
> an ETR to which it is continually sending long packets. There could
> be other techniques, such as an ITR-ETR protocol which enables the
> ITR to receive acknowledgement of packets it sends, but this gets
> pretty complex, and I am hoping to avoid such things.
Unless the period for probing is very short, there is
opportunity for a self-sustaining denial-of-service to
the SH.
> > But, the ITR will be able to return the correct PTB when the SH
> > retransmits. This is the same as for sprite-mtu.
>
> Once the ITR has a correct idea of the new, lower, PMTU to the ETR,
> it can drop (IPTM) or send (sprite-mtu) the packet, and generate a
> PTB to the SH when the SH next sends a packet which, once
> encapsulated, would be too long for that new PMTU.
This loses out on the opportunity to use the packets
sent into the tunnel as probes to detect MTU increases.
And, if you are expecting your ITR to periodically
re-probe to detect MTU increases in time to satisfy
MTU-probing SH, it is going to require excessive probing
overhead and may not get the job done in time to avoid
black-holing.
> >> Alternatively, occasional probing by the ITR might discover a
> >> higher value of PMTU to this ETR, and the SH could discover
> >> this increase by trying its luck with a larger packet - and
> >> either having it accepted, or rejected with a PTB containing
> >> the new higher value, minus 20.
> >
> > SHs that don't implement RFC4821 will have to wait for a long
> > time before trying a larger packet (RFCs 1191 and 1981 say 10min,
> > I believe).
>
> Indeed. This is why my critique of sprite-mtu seems to be
> important. As I understand it, the SH first gets a rather low value
> in the PTB message from the ITR, at a part of the exchange at which
> an IPTM ITR would have simply fragmented the packet without any PTB,
> and commenced or continued probing the ETR.
That depends on what you consider to be "low". Depending
on the placement of the ITR, "low" will only go as low
as 1280 but may go as large as, e.g., 1480.
> If it takes at least 10 minutes for a non-RFC4821 compliant host to
> try sending larger packets, then this is a long time for the
> communication to be restricted to the shorter packets.
SHs are therefore advised to begin implementing RFC4821.
Deployment is incremental and involves only the SH.
> I am not assuming widespread adoption of RFC4821 at any time. It
> looks really complex to implement, involving applications and the
> TCP layer communicating with a new function in the OS in ways which
> were not originally part of the protocol stack. Writing all this
> code, for marginal immediate benefit, and then trying to debug it in
> all its possible combinations of applications, live network settings
> etc. sounds really, really, complex.
I disagree; active end-to-end involvement in MTU determination
is important for the long term.
> > SHs that implement RFC4821 can retry end-to-end probing more
> > frequently than that, since loss of a probe does not
> > expose data to silent loss.
>
> Yes.
>
>
> >> I think Sprite will fragment outer packets, as does IPTM,
> >> irrespective of the DF flag of the inner packet.
> >
> > That is true.
>
> OK.
>
>
> >> The criteria for which IPv4 outer packets are fragmentable is
> >> complex (5.6.4).
> >
> > That text borrows from the tunnel MTU and fragmentation
> > discipline set forth in RFC4213, Section 3.2. But, I don't know
> > what you mean by complex?
>
> What I meant to write was that when meant I tried to read this and
> the rest of the ID in order to answer the questions I tried to pose
> in my second example, I couldn't understand it clearly enough to
> envisage how your proposal would operate.
>
> My questions were something like:
>
> Does the ITR first send a rather low value in the PTB message (for
> the first "long" packet)?
>
> While probing is taking place, does the ITR generate PTBs for
> packets which are too long for its currently too low PMTU
> estimate (and sends the packet too, without fragmentation), or
> does it fragment them, like IPTM?
>
> I think the answers are:
>
> Yes.
>
> The ITR generates PTBs and sends the packets holus bolus.
>
>
> >> I am not sure how Sprite handles "large" outer packets while it
> >> is probing. Does it fragment them as IPTM does? Or does it do
> >> the following, which is the same as what it does for a "long"
> >> packet once the PMTU has been reliably ascertained:
> >>
> >> (5.6.5) "... admits the packet but also sends a PTB message
> >> ..."
> >
> > It does the latter; this borrows from RFC2003, Section 5.
>
> OK, without carefully reading RFC2003, which I would if I had more
> time right now, I understand that the ITR both sends the complete
> outer packet, without fragmentation, and sends a PTB message
> to the SH.
>
> >> It seems strange to me to send the packet (unfragmented, I
> >> assume) while also sending back a PTB message to the sending
> >> host. Wouldn't this cause needless traffic and/or confusing
> >> signals to the SH if the outer packet does in fact arrive at
> >> the ETR and therefore the inner packet is delivered to the
> >> destination host?
> >
> > To the SH, it would appear that there is a router on the path
> > returning inaccurate information. This can happen already today,
> > since routers can be misconfigured, and spoofed PTBs can be sent
> > from any node in the network.
>
> It still seems strange, confusing and inefficient to me.
I disagree; there is value in sending the packet into the
tunnel on at least three levels: 1) it serves as a virtual
probe so that the ITR can detect MTU restrictions further
down the tunnel, 2) the packet may be an MTU probe of the
SH, 3) Packet delivery ratio may benefit in some use cases.
> > SHs that implement RFC4821 should not have a problem
> > deconflicting the (suspect) PTB information from (authentic)
> > end-to-end feedback from the DH, but should benefit from the PTB
> > info when the actual data is not delivered to the DH.
>
> Yes, but I am assuming that none, or few, hosts will implement
> RF4821 any time soon.
Incrementally deployable; touches SH only; realizing
larger MTUs gives incentive for deployment.
> > ITRs can help the situation by sending sprites of, e.g., 1500
> > bytes into the tunnel early in the process so that most if not
> > all SHs that use the tunnel will see a 1500 byte or larger MTU.
>
> Does "early in the process" mean when only shorter packets have so
> far needed to be tunneled to the ETR?
>
> If so, then the ITRs could be generating large volumes (in bytes) of
> probe packets in response to only small traffic flows, and to some
> flows which never in fact require PMTU knowledge, since the flows
> never actually use long packets.
There is nothing mandated here, and implementations will
be evaluated on the merits of their probing strategies.
> >> Here I will assume IPv4 only, with 1280 bytes for the default
> >> PMTU for every ETR the ITR has not yet probed. I will also
> >> assume an encapsulation overhead of 20, although this would
> >> typically be higher for Sprite and non-Ivip ITR-ETR schemes.
> >
> > I don't understand "higher for sprite-mtu"?
>
> This was a low-key aside. In my second example, trying to explain
> how I thought sprite-mtu might work, I kept the same 20 byte
> encapsulation overhead I used in my first example, which was for
> IPTM, assuming Ivip's 20 byte overhead.
>
> Other ITR-ETR schemes, and I guess most other tunneling schemes
> sprite-mtu would be applied to, have higher encapsulation overheads,
> I think.
The ENCAPS overhead is orthogonal to the use- or non-use
of sprite-mtu.
> >> If the ITR sends a PTB message to the SH when the first packet
> >> (or multiple packets) length exceeds the default PMTU value and
> >> then, after probing, decides the PMTU is 1480, then I am
> >> concerned that the SH would get contradictory values in these
> >> PTB messages.
> >>
> >> At first the SH would be told to send packets no longer than
> >> (1280 - 20 = 1260) and later, it would be told to send packets
> >> no longer than (1480 - 20 = 1460).
> >
> > Note: in the next draft version I would like to rewrite the
> > second bullet of Section 5.6.4 as:
> >
> > o for IPv4/*/IPv4 tunnels, 'pathMTU' is less than MIN(EMTU_R,
> > 1280+ENCAPS) bytes and the inner IPv4 packet is no larger than
> > MIN(EMTU_R-ENCAPS, 1280).
>
> The current version is:
>
> o for IPv4/*/IPv4 tunnels, 'pathMTU' is less than MIN(EMTU_R,
> 1280) bytes and the inner IPv4 packet is no larger than
> MIN(EMTU_R, 1280) minus ENCAPS. (When EMTU_R for the TFE is
> not known, 576 bytes must be assumed.)
>
> OK. My eyes are glazing over right now.
Reaching that point also myself...
> I will probably be able
> to understand this in its full context in the future.
>
> >> But if the SH took notice of the first PTB message, it probably
> >> wouldn't send any longer packets which would trigger the
> >> second. So, if my understanding of Sprite is correct, the SH
> >> would experience something like this:
>
> Inner ITR action on SH's idea DH gets
> packet outer packet of PMTU
> length following encapsul- to DH
> ation of inner packet
>
> 1500
>
>
> 200 Send outer packet - 1500 The packet
> the length is less than
> 1280.
>
> > Probing can start here also.
>
> As I noted above, at this point the ITR can't reasonably assume that
> it will need PMTU information to the ETR. It is just a short packet.
Again, nothing is mandated and some implementations may
make have better probing strategies than others.
> Perhaps smart ITRs could have heuristics to help them decide when to
> start probing earlier, based on prior statistics (of all sorts of
> aspects of packets that the ITR might analyse) of shorter packets
> often being followed by longer packets. But that is not something I
> would require of an IPTM ITR.
>
>
> 1500 Send packet and Probably nothing
> commence probing - however, if the
> Send PTB with value packet was a little
> 1260 1260 shorter, such as
> 1440, then it may
> arrive at the ETR
> in one piece,
> despite the SH
> being told it was
> too big.
>
> > With the note above, the returned size would be 1280; not 1260.
> > Also, I'm not sure as to the "probably nothing" as linkMTUs
> > increase above 1500 (perhaps someone could send the IEEE
> > reference that proposes the increase for 802 linkMTUs).
>
> OK - I guess in the future it might be worth trying longer than 1500
> byte packets to the ETR.
Right.
> My current IPTM proposal is not aimed at optimising the
> opportunities for doing so. I will think about this whenever I
> revise it.
>
> SH breaks the message
> into smaller packets
> and retries:
>
> 1260 Send packet and 1260 The packet
> continue probing
>
> ... etc.
>
> Probing complete:
> PMTU to ETR decided
> to be 1460.
>
> > By probing, do you mean by the ITR or by the SH?
>
> I meant the ITR sends sprite probes to the ETR.
Do you mean sprite-mtu probes and not traceroutes? One
thing I do not understand about your proposal is how
you expect the traceroutes to be efficient and converge
within a reasonable amount of time? Also, there is no
guarantee of getting the ICMPs back from the network
middleboxes. Am I missing something?
> > I am assuming that SHs will begin using RFC4821 and will
> > probe the path for themselves independent of any probing
> > done by the ITR.
>
> I am not assuming hosts will be any different than they are today.
> As far as I know, few, if any, implement RFC4821.
If hosts see the value of larger MTUs, they will begin
to deploy RFC4821. (Or, if vendors see the value, they
will begin to push out RFC4821 in automated S/W updates.)
IMHO, PMTUD cannot be efficiently and correctly handled
within network middleboxes alone; end-to-end involvement
is needed as well.
> If you are assuming this, I think it would be good to make it an
> explicit condition you are designing sprite-mtu to function within.
sprite-mtu works independently of RFC4821, but end systems
benefit from using RFC4821.
> 1260 Send outer packet - 1260 The packet
> the length is <= 1460.
>
> 1260 SH would probably keep The packet -
> sending packets of but more and
> length <= 1260. Unless shorter
> the SH was pushy, it packets than
> would never discover the the ITR-ETR
> PMTU it could use was tunnel can
> in fact 1440. handle.
>
> > IMHO, SHs that use RFC4821 can be "pushy" within reason.
>
> Yes, but I think it will be a long time before there are many such
> hosts.
Why not?
> > I would like to add one other note about the 1280. That number
> > comes from the SHOULD in RFC4213, Section 3.2.1. The reason I am
> > taking the SHOULD is that there can be additional encapsulations
> > on the path between the ITR and ETR (e.g., tunnel-mode IPsec) and
> > we don't want to cause fragmentation for those, either. If the
> > ITR and ETR are arranged such that there will be no additional
> > encapsulations on the path (and the ITR has a way of knowing
> > this) then the spec could use the RFC4213, Section 3.2.1
> > "configuration knob" to push the 1280 up to as much as 1480 or
> > perhaps even 1500. I would not want to go any higher than this,
> > since it could involve excessive fragmentation resulting in
> > undetected data corruption.
>
> OK - I won't try to chase this up right now, but thanks for the
> pointers.
>
> > Maybe I should add something about this to the spec?
>
> Yes, I think the more explanation of where numbers like 1280 come
> from, the better.
OK.
> Thanks again for this detailed discussion.
Sure.
Fred
fred.l.templin@boeing.com
> - Robin http://www.firstpr.com.au/ip/ivip/pmtud-frag/
>
>
>
>
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