Re: [DMM] Questions and comments on draft-ietf-dmm-5g-uplane-analysis-00

[Shunsuke Homma <[e-mail]>]

Hi Sridhar,

Thank you for your review and comments. (And I'm sorry for the late 
replay...)

Please find my replies in-line.(Tagged with [SH].)

Best regards,

Shunsuke



On 2019/01/09 12:00, Sridhar Bhaskaran wrote:
> Dear authors of draft-ietf-dmm-5g-uplane-analysis-00,
> 
> Thank you for the draft.
> 
> I have the following questions for clarification and comments on draft-ietf-dmm-5g-uplane-analysis-00
> 
> Questions
> ========
> 1. Section 3.6 - could you elaborate on what you mean by
> 
>>> [GTP-U-6]:  Does not support to response ICMP PTB for Path MTU
>                 Discovery.
> 
[SH] What we want to say in this observations is that 3GPP does not 
define PMTUD in user plane while TS23.060 requires well managed MTU 
size.Thereby there's no specification on how to handle ICMP Packet Too 
Big message at U-Plane functions like UPF. But if ICMP PTB message is 
generated at an intermediate router, the UPF which receives PTB message 
may not take any action due to lack of 3GPP specification. It may cause 
black hole for mobile user.


> 2. Section 4.1
> 
>>> These tunnels are available to be handled by other
>     authorized functions through the control plane.
> 
> Could you elaborate on what you mean by "other" authorized functions? Right now only SMF is allows to setup / teardown tunnels via N4 at UPF.
> 
[SH] "other authorized functions" means functions which are external to 
the 5GS owned by the NOP. The 5GS has NEF and it provides some 
controllabilties to external parties. For example, an MVNO may handle UP 
tunnels/traffic flows on UPFs via NEF, SMF, and N4 interface. I can 
modify this text to describe the above more clearly.


> 3. Section 4.2 Arch-Req-3: Could you please clarify the following sentence? First part of sentence talks about multiple PDU sessions but end of the sentence talks about one PDU session. So its not clear to me which case this is talking about.
> 
> However
>     it should be the multiple PDU sessions multihoming case where the
>     destination gNB or UPF needs to maintain multiple tunnel states under
>     the one PDU session to one UP tunnel architectural principle.
> 
[SH] What we want to express here is that multihoming with P2P needs to 
maintain a tunnel state for each source UPF which leads increase of 
loads on management of tunnel states.


> 
> 4. Section 4.2 - Arch-Req-5. I am not able to understand the following sentences. Could you clarify what you mean by "connecting them without extra anchor points"? Also what does "them" refer to here? Does it refer to UE or UPF?
> 
> In addition, deployment of multiple UPFs as anchors closed to UEs'
>     site and connecting them without extra anchor points enable to make
>     data path more efficient.
> 
[SH] In the current LTE, all of UP traffic is forwarded to a P-GW which 
is centralized and it may cause trombone routing. On the other hand, the 
5GS allows flexible deployment of UPFs mainly for MEC use cases. For 
example, in case these UPFs are distributed geographically, UP flows can 
be applied LBO or forwarded between UPFs nearby src and dst directly. 
Anyway, I think it should be described in ARCH-Req-4: Flexible UPF 
selection, and I'll move this text to there.


> 
> 5. Section Arch-Req-5: Are the following statements an architectural requirement derived from 23.501 or an architectural requirement this draft is putting on 3GPP? Atleast the words " UP protocol shall support to aggregate several PDU sessions into a tunnel or shall be a session-less tunnel." Seems like this draft is putting a requirement on 3GPP.
> 
> It is expected that multiple UPFs with per session tunnel handling
>     for a PDU session becomes complicated task more and more for a SMF by
>     increasing number of UPFs, and UP protocol shall support to aggregate
>     several PDU sessions into a tunnel or shall be a session-less tunnel.
> 
[SH] It's not requirement to 3GPP from IETF. We are assuming that the 
current 5GS potencially have this requirement on UP protocol. If this 
text seems not to be appropriate, we can change it.


> Comments:
> ==========
> 1. Section 4.1.1 - traffic detection based on UE IP address and SDF filters is missing in the below list
> 
> o  For IPv4 or IPv6 PDU Session type
> 
>        *  PDU Session
> 
>        *  QFI
> 
>        *  Application Identifier: The Application ID is an index to a set
>           of application detection rules configured in UPF
> 
[SH] Thanks. I'll add UE IP address and SDF filters into the list. BTW, 
can you tell me which sections should be referred? I'm referring section 
5.7.6 and 5.8.2 in TS23.501. Are there any others?


> 2. Section 4.2 Arch-Req-2:
> 
>>> The 5G system requires IP connectivity for N3, N6, and N9 interfaces.
> 
> There is a specific case where IP connectivity on N6 is not mandatory. For Ethernet PDU sessions, the anchor UPF could use L2 switching on N6 side. You refer clause 5.6.10.2 of TS 23.501 especially the statements below
> 
> -	Configurations, where more than one PDU Session to the same DNN (e.g. for more than one UE) corresponds to the same N6 interface. In this case the UPF acting as PSA needs to be aware of MAC addresses used by the UE in the PDU Session in order to map down-link Ethernet frames received over N6 to the appropriate PDU Session. Forwarding behaviour of the UPF acting as PSA is managed by SMF as specified in clause 5.8.2.5.
> 
[SH] Agreed, thank you.


> 
> 3. Section 4.2 Arch-Req-3:
> 
> Multihoming is provided with Branching Point (BP) or Uplink
>     Classifier (UL CL) which are functionalities of UPF.
> 
> ULCL is not used for multihoming. ULCL is used for traffic splitting towards a local DN. Only BP is used for multihoming case.
> 
[SH] I reviewed the section 5.6.4 in TS23.501, and I understood that 
multiple anchor UPFs is realized by either ULCL or IPv6 multi-homing and 
a way with ULCL is not called multihoming. I'll modify the description 
about multihoming and add suplementaly expanation on difference between 
ULCL and IPv6 multi-homing.



> 4. Section 5 is missing one evaluation aspect. GTP-U supports "End markers" to help RAN sequence the packets when there is a change of UPF during mobility procedures. So any user plane protocol that is to be evaluated need to support some mechanism to help the last downlink node on path (e.g gNB) to sequence the packets coming from multiple UPFs during mobility cases.
> 
[SH] Thanks. I'll add it as one evaluation aspect.


> 5. Section 5.7 - Need justification for the following statement:
> 
> However some means need to indicate a slice on the shared
>     underlying networks of the UP over the wire.
> 
> What is broken or what is the issue if slice for transport is not indicated on the UP over the wire? What are the issues with providing a "network instance" (which could be mapped to a transport path) in the forwarding action rule of a PDU session?
> 
> What are the advantages of carrying slice information in every packet?
> 
[SH] Is to provide a greater affinity between the UP and the underlying 
network infrastructure. For example, if a UP session
requires certain level of latency with dedicated BW requirements, 
traffic engineering (TE) embodies appropriate forwarding policy through 
the underlay transport network to that specific UP session.​



> Regards
> Sridhar Bhaskaran
> 
>   
> 
> 
> 
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> 
> 


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Shunsuke Homma
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