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

[Joel Halpern <[e-mail]>]

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]]
> *Sent:* Monday, July 11, 2022 2:57 PM
> *To:* Templin (US), Fred L <[e-mail]>
> *Cc:* Richard Li <[e-mail]>; Juan Carlos Zuniga 
> (juzuniga) <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)
>
>         _______________________________________________
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