Dear Satoru,

I am Sridhar Bhaskaran - 3GPP CT4 delegate. In addition to John's questions, I 
have few more questions for clarification.

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)?

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 

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

Incoming LS from RAN3 to CT4:

Corresponding agreed CR in CT4:

LS out from CT4 to RAN3:

-----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


> 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 
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.


> 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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