Dear Sridhar,

It’s nice to see you in DMM list as well. :-)

> [...snip...]

> 1. Section 5.2.2 of the draft says,
> UPF2_in : (Z,A)                              -> UPF2 maps the flow w/
>                                            SID list <C1,S1, gNB> 
> When the packet arrives to the UPF2, the UPF2 will map that
>   particular flow into a UE session.
> Does this mean the UPF2 is aware of the gNB the UE is latched on and hence 
> after each mobility, the information regarding current gNB is signaled / 
> programmed till the UPF2 (which could be the PDU session anchor UPF)?

Right. It’s equivalent with GTP-U case where there’s no N9 along the path.
One significant difference between GTP-U and enhanced mode is that the packets 
of PDU sessions which belong to a same SR policy will use same set of SIDs in 
the header.

> 2. For traditional mode (basic mode), could you please elaborate on the MTU 
> overhead being less than GTPU? GTPU encap MTU overhead = 20 octets outer IPv4 
> header + 8 octets UDP header + 12 octets GTPU header + Extension header for 
> QFI. SRv6 encap MTU overhead = 40 octets IPv6 header + EH for carrying QFI 
> (this part is not clear - see my next question). In your blog 
>  I noticed in Fig.1 the MPLS / TE headers included in calculation. So does 
> the MTU overhead saving talked about for SRv6 assume that underlay TE 
> technologies are replaced? It is not clear from the draft. If there are any 
> other drafts that provide a clear calculation on the MTU savings, could you 
> please point to them?

I hope that the following spreadsheet helps for that purpose. I’ve updated it 
align to the latest draft after I shared it in last month.

As the draft says that traditional mode is as same as existing user plane 
protocol, it is also equivalent in terms of overhead size.
You can find that both GTP-U and traditional mode of SRv6 cost 58 bytes as the 

> 3. How to encode QFI, RQI in SRv6 (both traditional mode and enhanced mode)? 
> In GTPU the QFI/RQI markings are carried in a GTPU extension header, defined 
> as a container. Please refer the following documents for details

Thank you for those latest references! Actually I was seeking them from last 
CT4 meeting folder. ;-)

And yes, we still leave it at this time since defining 3GPP specific extension 
headers and messages should be up to 3GPP decision. 
We don’t want to violate that responsibility. Nevertheless, in case that those 
GTP-U extension header and messages are carried as its encoding, TLV in SRH 
would work to do that with just one or few code points to 3GPP which looks not 
big deal. If you are interested, it would be nice if we can write a draft with 
that idea with you and John.


> Incoming LS from RAN3 to CT4: 
> Corresponding agreed CR in CT4: 
> LS out from CT4 to RAN3: 
> Thanks
> Sridhar
> -----Original Message-----
> From: dmm [] On Behalf Of Satoru Matsushima
> Sent: Tuesday, March 13, 2018 6:55 AM
> To: John Kaippallimalil <>
> Cc: dmm <>
> Subject: Re: [DMM] Fwd: I-D Action: draft-ietf-dmm-srv6-mobile-uplane-01.txt
> John,
>> 2018/03/13 7:37、John Kaippallimalil <>のメール:
>> A few questions for clarification:
>> 1. Section 5.1.1, "..Since there is only one SID, there is no need to push 
>> an SRH..."
>> In this case the outer IPv6 address representing a SID would contain a TEID.
>> So for 1000 PDU sessions - would there be as many IPv6 addresses 
>> (representing SIDs/TEID in the outer header)
> Right. It is not limited but in a typical case given that a /64 per node for 
> the SID space which allows each node allocate a SID per session basis.
>> 2.  Section 5.2 (& Figure 3), suppose there were more than 1 UPF on 
>> path (gNB --> UPF1 --> UPF2 as in the example in 5.1 but now with other fns 
>> - S1, C1) Would the SR path at gNB (uplink) be (a) the list of SIDs to chain 
>> functions along the path to UPF1,
>>      or, would it be (b) the list of SIDs to chain functions, plus UPF1, .. 
>> upto the anchor UPF2. 
> Please see the section 5.2.1 of packet flow on uplink.
> We assume that there’s no UPF1 along the path in the diagram.
>> 3.  Section 5.2, "The SR policy MAY include SIDs for traffic engineering and 
>> service chaining ..."
>> 3GPP service chaining is at N6 interface, but in this case, is the policy 
>> for traffic steering implemented at the N3  interface.
>> Would PCF/SMF provision this at gNB.
> When it comes to enhanced mode for control-plane side, it may literally be 
> enhanced.
> But at this revision of the draft, it is assumed that gNB is capable to 
> resolve SR policy from remote endpoint address of tunnel to SIDs list while 
> N2 is unchanged and kept as it is.
> Cheers,
> --satoru
>> Thanks,
>> John
>> -----Original Message-----
>> From: Satoru Matsushima []
>> Sent: Mittwoch, 7. März 2018 12:23
>> To: Marco Liebsch
>> Cc: dmm
>> Subject: Re: [DMM] Fwd: I-D Action: 
>> draft-ietf-dmm-srv6-mobile-uplane-01.txt
>> Marco,
>>> 2018/03/07 18:41、Marco Liebsch <>のメール:
>>> Satoru,
>>> since I read this at different places, let me ask one clarifying question 
>>> about the stateless motivation: 
>>> I see that for SRv6 you may not need a state at the egress (at least 
>>> not for traffic forwarding) but for Uplink/Downlink (UL/DL) you need 
>>> a state at both edges of the communication since the DL egress serves as 
>>> uplink ingress, correct?
>> 2x unidirectional tunnels to form bidirectional paths require 4 states in 
>> total at both the ingress and egress.
>> In SR case it requires just 2 states at the ingresses for both directions.
>> Cheers,
>> --satoru
>>> marco
>>> -----Original Message-----
>>> From: dmm [] On Behalf Of Satoru 
>>> Matsushima
>>> Sent: Dienstag, 6. März 2018 17:23
>>> To: Tom Herbert
>>> Cc: dmm
>>> Subject: Re: [DMM] Fwd: I-D Action: 
>>> draft-ietf-dmm-srv6-mobile-uplane-01.txt
>>> Hello Tom,
>>>>> A Big progress is that the draft supports interworking with GTP 
>>>>> over
>>>>> IPv6 in addition to GTP over IPv4.
>>>>> And we have made change SRv6 function to IPv6 encapsulation with 
>>>>> SRH instead of SRH insertion by default.
>>>> Hi Satoru,
>>>> If there are no intermediate hops od SIDs being set when 
>>>> encapsulating would a SR header still be needed or could this just 
>>>> be simple IP in IP encpasulation?  If is no SR header then it's 
>>>> possible that ILA might then be used to completely eliminate the 
>>>> encapsulation overhead.
>>> I think you’re right. You would find that case in the draft as ‘Traditional 
>>> Mode’ which is equivalent with traditional GTP-U case. You seem you say ILA 
>>> is also equivalent with that mode. In addition, this draft introduces 
>>> ‘Enhance Mode’ to cover more advanced cases.
>>> IMO SR is designed not to maintain path states except at an ingress node. 
>>> So the packet need to preserve original DA in the header that keep the 
>>> egress node in stateless. It would be great if ILA is designed in the 
>>> similar concept as well.
>>> If it’s not, it looks a kind of tradeoff, between reducing the overhead and 
>>> keeping the statelessness. It’s not apple-to-apple comparison. To decide to 
>>> choose which one need to be prioritized would depend on each deployment 
>>> case in operators IMO.
>>> Cheers,
>>> --satoru
>>> _______________________________________________
>>> dmm mailing list
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