Re: [Int-area] Call for WG adoption of draft-templin-intarea-parcels-10

[Joel Halpern <[e-mail]>]

Fred, I understood full well that you only envision a small number of 
reassembly devices.  After all, on any given path only one device will 
likely reassemble.  Still, that device will be spending a lot of 
resources in a very expensive part of the path (fast path forwarding) to 
provide a small benefit to some hosts.

Fundamentally you are asking the archtiecture to spend those resources 
for use case that you have not explained.  "I have proof" i snot 
relevant.  Without knowing the scenarios and the assumptions, it does 
not help us to judge.  It is worse than the case in the early days of 
the MANET working group where the competing proposal repeatedly said "my 
simulation shows ..."

Fundamentally, it is not the network's job to reassemble packets for a 
host.  If you want NICs to do that, as Tom has said, that's fine.  It is 
a private matter between the host and the NIC.  But you are asking for 
functionality in the network.

I note also that you are assuming that hosts have links that support 
actual MTUs larger than 64K.  I know of no link that has those 
properties in current use.  (I am vaguely familiar with HIPPI and 
FiberChannel.  Neither appears to be relevant.)

Yours,

Joel

On 7/12/2022 10:02 AM, Templin (US), Fred L wrote:
>
> Joel, you are misunderstanding what nodes would be involved in 
> reassembly; this would
>
> not be at every single IP layer router in the path. It would only be 
> at possibly 0, 1 or 2
>
> adaptation layer middleboxes in the path from source to destination. 
> And, then most
>
> likely only at a near-end middlebox very near the destination that 
> happens to know the
>
> destination would prefer to receive larger parcels.
>
> About segment size, I have proof that using segment sizes 
> significantly larger than the
>
> path MTU can often produce dramatic performance increases even when 
> fragmentation
>
> is intentionally invoked. I also have proof that packaging multiple 
> segments in the same
>
> system call can drive performance even higher an without reducing the 
> segment size.
>
> IP parcels takes it the logical next step of allowing multiple 
> segments to travel together
>
> in the same packet, which may or may not be subject to fragmentation 
> and reassembly.
>
> But, let’s not get so hung up on the middlebox question that we forget 
> the benefits
>
> for end-to-end.
>
> Fred
>
> *From:*Joel Halpern [mailto:[e-mail]]
> *Sent:* Monday, July 11, 2022 4:02 PM
> *To:* Templin (US), Fred L <[e-mail]>
> *Cc:* [e-mail]
> *Subject:* Re: [Int-area] Re: Call for WG adoption of 
> draft-templin-intarea-parcels-10
>
>
> No, intermediate reassembly is not an optimization.
>
> First, it is a bad idea.  It is very painful for routers to perform 
> reassembly.  They have to burn expensive resources managing such 
> attttempted reassesmbly.  It has major cost even if the router decides 
> to give up and forward the pieces.
>
> And second, unless one makes some unstated assumptions in the absence 
> of such reassembly the sending host will be throttled to the receiving 
> host rate.  So the benefit of the entire system is markedly reduced.
>
> Net: we should not adopt this draft.
>
> Yours,
>
> Joel
>
> On 7/11/2022 6:41 PM, Templin (US), Fred L wrote:
>
>     Tom,
>
>     > Why would someone put six segments in a parcel if they already
>     have a 9K link MTU?
>
>     > Why not just send one segment in 9K?
>
>     This is the mindset that we need to overcome. We have had it
>     drilled into our heads
>
>     that MSS must be the same as the path MTU, but it does not need to
>     be that way.
>
>     If the MSS is smaller than the path MTU, but we can send multiple
>     segments in a
>
>     single parcel that more closely approaches the size of the path
>     MTU then
>
>     amortization savings are possible.
>
>     >The algorithm isn't the problem, it's supporting new protocols and
>     multiple
>
>     >checksums in a packet in hardware.
>
>     But Tom, how hard can this be? Instead of running the Internet
>     checksum 1 time
>
>     over N octets of data simply run it M times over N/M octet chunks
>     of the data in
>
>     succession but still in a single pass. You spoke before of NICs
>     adapting to support
>
>     TCP jumbograms – if they can do that, why not a very
>     straightforward application
>
>     of Internet checksum? I haven’t looked at this in a long while,
>     but isn’t this also
>
>     similar to what UDP-lite did?
>
>     > Either you're trivializing reassembly or maybe you're thinking of
>     some new method that
>
>     > somehow avoids all the pitfalls and problems we've had with
>     reassembly over the years!
>
>     Intermediate node parcel reassembly is really just an optimization
>     to try to pass the
>
>     largest possible parcels on to the next hop instead of passing
>     many smaller ones. It is
>
>     really just a concatenation of segments of sub-parcels belonging
>     to the same original
>
>     parcel. Reordering is unimportant – it is OK to concatenate
>     sub-parcels 3,8,5,2 in that
>
>     order and without even waiting for any other sub-parcels to show
>     up. The application
>
>     will simply perceive it as a case of network reordering and the
>     upper layer protocol
>
>     will do the correct thing with the sequence numbers. AFAICT, the
>     only hard requirement
>
>     is that the final sub-parcel must not be concatenated as an
>     intermediate sub-parcel.
>
>     This stuff will all work, and it will work for the betterment of
>     the Internet.
>
>     Fred
>
>     *From:*Tom Herbert [mailto:[e-mail]
>     <mailto:[e-mail]>]
>     *Sent:* Monday, July 11, 2022 2:57 PM
>     *To:* Templin (US), Fred L <[e-mail]>
>     <mailto:[e-mail]>
>     *Cc:* Richard Li <[e-mail]>
>     <mailto:[e-mail]>; Juan Carlos Zuniga (juzuniga)
>     <juzuniga=[e-mail]>
>     <mailto:juzuniga=[e-mail]>; [e-mail]
>     *Subject:* Re: [EXTERNAL] Re: [Int-area] Call for WG adoption of
>     draft-templin-intarea-parcels-10
>
>
>     	
>
>     EXT email: be mindful of links/attachments.
>
>     On Mon, Jul 11, 2022 at 2:20 PM Templin (US), Fred L
>     <[e-mail]> wrote:
>
>         Tom, some rejoinders:
>
>         >Yes, I agree if the packet is fragmented by the network then this
>         is a nice feature.
>
>         >However, today we already have this from a host perspective property
>         by just
>
>         >sending "small" packets.
>
>         It can be readily shown that some applications get much
>         greater performance by
>
>         sending larger packets that trigger fragmentation/reassembly
>         than by sending
>
>         smaller packets that do not. Multiple order of magnitude
>         performance increases
>
>         are indeed possible.
>
>         >I'm not sure the savings qualify as significant. 9K MTUs are
>         becoming common in data centers
>
>         >and the standard TCP/IPv6 header is 80 bytes so that's already
>         less than 1% overhead.
>
>         I think 9K is only a starting point, and IP parcels pave the
>         way to much larger link MTUs,
>
>         possibly even in excess of 64KB. And, doing the math, even for
>         just a 9K link sending a
>
>         single parcel that contains 6x 1440 octet segments would save
>         5 * 60 == 300 octets in
>
>     Why would someone put six segments in a parcel if they already
>     have a 9K link MTU? Why not just send one segment in 9K?
>
>         comparison with sending 6x  1500 octet packets with 60 octets
>         of IP/TCP headers per
>
>         packet. For links with larger MTUs, the savings for sending
>         parcels with lots of segments
>
>         (up to 64) becomes even greater.
>
>         >As I already mentioned, this is addressed by the BiGTCP work
>         (https://lwn.net/Articles/884104).
>
>         >Sending or receiving multi-megabytes TCP segments in one system call
>         is now feasible. Also, it's
>
>         >inevitable that NIC vendors will apply this also to be able to offload
>         TCP jumbo grams. Given this
>
>         >is just software that doesn't require hardware change or
>         on-the-wire protocols to change, it's
>
>         >immediately deployable with just a softwar change which is a huge benefit
>         to datacenter operators.
>
>         As I have said, IP parcels has the same advantage within the
>         host system-call (user-space
>
>         to kernel-space) context. But, IP parcels goes a step further
>         to provide efficient packaging
>
>         over-the-wire, whereas the approach you are referring to opens
>         the box inside the
>
>         kernel and sends individual packets instead of aggregates.
>
>         >All modern NIC HW can deal with offloading a single checksum per
>         packet, it's going to be
>
>         >a major effort for them to offload multiple checksum like IP
>         parcels needs. Without checksum
>
>         >offload, this would be a non-starter for a lot of deployments.
>
>         Check the latest spec (now at -12 and likely to stay that way
>         until IETF114. Any H/W checksum
>
>         that can run over the first segment of a packet should be
>         possible to make run over the N-1
>
>         additional segments of the same packet (parcel) by applying
>         the very familiar Internet
>
>         checksum algorithm.
>
>     The algorithm isn't the problem, it's supporting new protocols and
>     multiple checksums in a packet in hardware.
>
>         >I'm not convinced of that. For instance, I'm skeptical that
>         intermediate devices trying to reassemble
>
>         >packets that aren't addressed to themselves could ever be robust or
>         efficient (i.e. complexity, non-work
>
>         >conserving resource requirements, security issues with reassembly,
>         multi-path that causes latency
>
>         >increase, potential DoS vector, etc.). Can you comment on this?
>
>         Perhaps what is confusing this matter is that the intermediate
>         devices referred to
>
>         here most certainly do not refer to all routers in the path.
>         Instead, what is intended
>
>         here is an OMNI intermediate device, of which there may be
>         something on the order
>
>         of 0, 1, or 2 of them on the path between the OMNI source and
>         destination even
>
>         though there may be many 10’s or even 100’s of ordinary IP
>         routers on the path.
>
>         And, again, this is not a strict reassembly case – instead, it
>         is an opportunistic
>
>         “combine if convenient; else forward” swift decision.
>
>     Either you're trivializing reassembly or maybe you're thinking of
>     some new method that somehow avoids all the pitfalls and problems
>     we've had with reassembly over the years! Consider that many NIC
>     vendors have tried, and largely failed, to get any sort of device
>     reassembly widely deployed (e.g. IP reassembly, TCP segmentation
>     reassembly, etc.). The reason they failed is because they can't
>     give the host stack transparency and control over the reassembly
>     process.
>
>     In its nature reassembly can only be done with at least packets.
>     That means a device performing reassembly has to receive one
>     packet, hold it, and wait for the following packet to perform
>     reassembly. That makes reassembly, unlike fragmentation, a
>     non-work conserving process. Many issues and policies arise from
>     this. For instance, what happens if a packet is held and the
>     following packet is never seen? (usually implies a reassembly
>     timer). What happens if a packet is received OOO and is already
>     forwarded, but the preceding packet is then received, do we try to
>     reassemble that one? (the solution here seems to be to maintain
>     some sort of flow state)? What about overlapping fragments and the
>     security issues around that?
>
>     IMO, if the WG does pursue this, I believe a lot of the effort
>     will be in specifying how reassembly in intermediate nodes works.
>
>     Tom
>
>         Thanks - Fred
>
>         *From:*Tom Herbert [mailto:[e-mail]]
>         *Sent:* Monday, July 11, 2022 1:34 PM
>         *To:* Templin (US), Fred L <[e-mail]>
>         *Cc:* Richard Li <[e-mail]>; Juan Carlos
>         Zuniga (juzuniga) <juzuniga=[e-mail]>;
>         [e-mail]
>         *Subject:* [EXTERNAL] Re: [Int-area] Call for WG adoption of
>         draft-templin-intarea-parcels-10
>
>
>         	
>
>         EXT email: be mindful of links/attachments.
>
>         On Mon, Jul 11, 2022 at 12:22 PM Templin (US), Fred L
>         <[e-mail]> wrote:
>
>             Richard and others, thank you for these comments and for
>             the ensuing discussion that
>
>             took place over the time I was away on vacation. Strange
>             how the timing hit when I
>
>             was away from the office and off the grid - I was on a
>             camping trip in Canada not far
>
>             from where Steve Deering lives although I did not visit him.
>
>             In any event, I was able to push out a new draft version
>             ahead of the deadline that
>
>             may address some (but likely not all) of your concerns:
>
>             https://datatracker.ietf.org/doc/draft-templin-intarea-parcels/
>
>             The major change is that the draft now talks about
>             interactions with upper layer
>
>             protocols including TCP and UDP, whereas the previous
>             draft versions were silent
>
>             regarding upper layer protocol framing.
>
>             To others who have commented, I beg to differ and maintain
>             that IP parcels do
>
>             represent a significant improvement over the current state
>             of affairs and over
>
>             just regular IP jumbograms. In particular:
>
>         Hi Fred, some comments in line.
>
>             1) IP parcels make it so that the loss unit is a single
>             segment instead of the entire
>
>             packet/parcel, and loss of a segment often results in
>             retransmission of just that
>
>             segment instead of the entire packet/parcel.
>
>         Yes, I agree if the packet is fragmented by the network then
>         this is a nice feature. However, today we already have this
>         from a host perspective property by just sending "small" packets.
>
>             2) IP parcels are more efficient than sending a single
>             segment per IP packet, since
>
>             the parcel includes a single IP header plus single full
>             {TCP,UDP} header for possibly
>
>             many segments. This can result in significant savings in
>             terms of bits over the wire
>
>             for omitting unnecessary header bytes.
>
>         I'm not sure the savings qualify as significant. 9K MTUs are
>         becoming common in data centers and the standard TCP/IPv6
>         header is 80 bytes so that's already less than 1% overhead.
>
>             Consider the postal service analogy; when
>
>             many items can be sent together in a single package/parcel
>             there is a large savings
>
>             in shippeing and handling costs than when each individual
>             item is shipped separately.
>
>         As I already mentioned, this is addressed by the BiGTCP work
>         (https://lwn.net/Articles/884104). Sending or receiving
>         multi-megabytes TCP segments in one system call is now
>         feasible. Also, it's inevitable that NIC vendors will apply
>         this also to be able to offload TCP jumbo grams. Given this is
>         just software that doesn't require hardware change or
>         on-the-wire protocols to change, it's immediately deployable
>         with just a softwar change which is a huge benefit to
>         datacenter operators.
>
>             3) IP parcels improve large packet integrity by including
>             a separate checksum for
>
>             each segment instead of a single checksum for the entire
>             packet.
>
>         All modern NIC HW can deal with offloading a single checksum
>         per packet, it's going to be a major effort for them to
>         offload multiple checksum like IP parcels needs. Without
>         checksum offload, this would be a non-starter for a lot of
>         deployments.
>
>             This means that
>
>             large parcels (up to a few MB) can be sent in one piece
>             over links with sufficiently
>
>             large MTU without requiring the link itself to provide
>             strong integrity checks over
>
>             the entire length of the parcel. This means that link MTUs
>             significantly larger than
>
>             9KB are now safely possible.
>
>             4) IP parcels offer all of the efficiency advantages to
>             upper layers as are offered
>
>             by GSO/GRO, etc. but also provide benefits 1) through 3)
>             above that are not
>
>             offered by GSO/GRO.
>
>         Most of this is doable in GSO/GRO.
>
>             5) Plus, the idea is just plain neat. Better packaging is
>             good. More efficient
>
>             handling is good. Reduced header overhead is good. SAFE
>             larger MTUs are
>
>             good. The idea itself is good.
>
>         I'm not convinced of that. For instance, I'm skeptical that
>         intermediate devices trying to reassemble packets that aren't
>         addressed to themselves could ever be robust or efficient
>         (i.e. complexity, non-work conserving resource requirements,
>         security issues with reassembly, multi-path that causes
>         latency increase, potential DoS vector, etc.). Can you comment
>         on this?
>
>         Tom
>
>             Fred
>
>             *From:*Int-area [mailto:[e-mail]] *On
>             Behalf Of *Richard Li
>             *Sent:* Friday, July 01, 2022 3:11 PM
>             *To:* Juan Carlos Zuniga (juzuniga)
>             <juzuniga=[e-mail]>
>             *Cc:* [e-mail]
>             *Subject:* Re: [Int-area] Call for WG adoption of
>             draft-templin-intarea-parcels-10
>
>             Chairs and Authors,
>
>             I always like every new idea and effort to improve the
>             Internet performance, and thus I have read this draft with
>             a great interest. The following are my
>             observations/comments/questions. If they don’t make any
>             sense to you, please accept my apology, and disregard them.
>
>             1.The text “multiple upper layer protocol segments” is
>             ambiguous. It seems that you really mean “multiple
>             segments from ‘the same’ upper layer protocol”, doesn’t
>             it? It seems that multiple segments from different upper
>             layer protocols are not allowed in your parcel.
>
>             2.Is the following a fair statement? All segments in the
>             same packet come from the same application identified by
>             the 5-tupe (source address, destination address, source
>             port, destination port, protocol number).
>
>             3.Segment size
>
>             You require that their sizes be the same except for the
>             last one. Is this required for easy implementation or
>             what? Do you require it for any other reasons?
>
>             4.TTL issue
>
>             You described how parcels are forwarded over the
>             Internetwork, and in particular you described what the
>             ingress/egress middlebox does about parcels. I understand
>             that the ingress middlebox may break the parcel into
>             smaller ones, which may rejoin at the egress middlebox. My
>             question is about TTL. As different smaller parcels may
>             traverse along different paths, as a result their TTLs may
>             be different when they reach the egress middlebox . How
>             does the egress middlebox set up the TTL value? Please
>             provide more descriptions.
>
>             5.Reordering at the egress middlebox
>
>             The parcels would arrive one after another, and therefore
>             the egress middlebox would “wait” for a little bit to
>             identify and pick up enough parcels/packets for their
>             rejoining and repackaging. A description of the egress
>             middlebox behavior would be useful and helpful, in
>             particular I would like to know more about the waiting
>             time if any, and how you deal with the reordering and loss.
>
>             6.IPv4 option
>
>             Does IETF still allow to change/add IPv4 option fields? I
>             might be wrong, but aren’t they frozen? Also, do
>             commercial routers still care about IPv4 options?
>
>             7.IPv6 option
>
>             This draft has defined a hop-by-hop option, it will
>             require every intermediate IPv6 router to inspect this
>             option. There have been some discussions on the pros/cons
>             about Hop-by-Hop IPv6 Option. Is there any feedback from
>             WG 6man?
>
>             8.Parcel Path Qualification
>
>             This draft has described a method for parcel path
>             qualification probe from end to end. It is nice to have
>             it, but it is unreliable simply for the following reason:
>             a probe parcel goes along one specific path, and your real
>             application parcels may take different paths.
>
>             9.Integrity
>
>             First paragraph of Section 7. More explanation/elaboration
>             should be useful. I might have missed it in previous
>             paragraphs, but if I do, please provide a reference to it
>             such as “as described in …”.
>
>             10.Implementation Status
>
>             In section 10. TSO’s performance gain and Parcel’s gain
>             should be regarded as two different things. Since this
>             draft is adding a hop-by-hop option, every intermediate
>             router is required to process the hop-by-hop option, which
>             will, theoretically speaking, lead to performance
>             downgrade. Of course, the whole performance would depend
>             on many other factors, such as the total numbers of
>             routing table lookups and number of segments.
>
>             11.General observation
>
>             This proposal essentially tries to solve a problem caused
>             by MTU. If MTU be very big, one would simply put the whole
>             data in a single packet. Since MTU is limited, a packet
>             has to be cut into many smaller pieces (segments). In the
>             existing specification, when an intermediate router sees a
>             packet with its size larger than MTU, the router would be
>             expected to fragment it so that the fragments could be
>             forwarded. Here let me call it “fragmentation as needed”.
>             In reality, however, some (if not all) commercial routers
>             don’t do “fragmentation as needed”, instead of fragmenting
>             the packet they simply discard it in order to achieve the
>             wire-speed. This draft defines a new way to address the
>             MTU issue: when a router sees a packet with its size
>             larger than MTU, the router is asked to fragment it in a
>             prescribed way (fragment it into pre-packaged segments).
>             If I may, let me call it “fragmentation as prescribed”.
>             Both “fragmentation as needed” and “fragmentation as
>             prescribed” would require the support from intermediate
>             routers. As the same as fragmentation as needed,
>             fragmentation as prescribed may downgrade the performance
>             of intermediate routers. What is more, intermediate
>             routers/boxes may perform “rejoining and repackaging”,
>             which will adversely impact the performance of the
>             intermediate routers/boxes.
>
>             Best regards,
>
>             Richard
>
>             *From:*Int-area <[e-mail]> *On Behalf Of
>             *Juan Carlos Zuniga (juzuniga)
>             *Sent:* Wednesday, June 22, 2022 12:25 PM
>             *To:* [e-mail]
>             *Subject:* [Int-area] Call for WG adoption of
>             draft-templin-intarea-parcels-10
>
>             Dear IntArea WG,
>
>             We are starting a 2-week call for adoption of the
>             IP-Parcels draft:
>
>             https://www.ietf.org/archive/id/draft-templin-intarea-parcels-10.html
>             <https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.ietf.org%2Farchive%2Fid%2Fdraft-templin-intarea-parcels-10.html&data=05%7C01%7Crichard.li%40futurewei.com%7C715b5db213134932c70208da5484f702%7C0fee8ff2a3b240189c753a1d5591fedc%7C1%7C1%7C637915227299598680%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000%7C%7C%7C&sdata=w4G5ypaSRv%2FR31%2F%2B857XT2xUqHdEXv90ubD5GGjqBEQ%3D&reserved=0>
>
>
>             The document has been discussed for some time and it has
>             received multiple comments.
>
>             If you have an opinion on whether this document should be
>             adopted by the IntArea WG please indicate it on the list
>             by the end of Wednesday July 6^th .
>
>             Thanks,
>
>             Juan-Carlos & Wassim
>
>             (IntArea WG chairs)
>
>             _______________________________________________
>             Int-area mailing list
>             [e-mail]
>             https://www.ietf.org/mailman/listinfo/int-area
>
>
>
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