Re: [sipcore] FW: New Version Notification for draft-dmudric-sipcore-sipv6-addr-selection-00.txt

["Mudric, Dusan (Dusan)" <[e-mail]>]

Hi Dale,

Please see inlined.

Thanks,
Dusan.

> -----Original Message-----
> From: Dale R. Worley <[e-mail]>
> Sent: Saturday, January 25, 2020 11:36 AM
> To: Mudric, Dusan (Dusan) <[e-mail]>
> Cc: [e-mail]; [e-mail]; Grebovich, Dragan (Dragan)
> <[e-mail]>
> Subject: Re: [sipcore] FW: New Version Notification for draft-dmudric-
> sipcore-sipv6-addr-selection-00.txt
> 
> "Mudric, Dusan (Dusan)" <[e-mail]> writes:
> > This draft describes real connectivity issues when SIP signaling and
> > media are run on multihomed hosts. [...]
> 
> Does it handle cases where initial messages have to be sent to a target that
> has multiple addresses?  I worked on that in
> draft-worley-sipcore-happy-earballs-00 and there are a lot of considerations
> involved.
> 
> Dale

[Dusan] 
[Dusan] Good question. 

This draft does cover some of the use cases in that category. Sections 3.4 and 4.1 talk about the host registering a list of prioritized IPv6 addresses, and how host addresses should be selected by SIP Proxy, when sending SIP messages to the host. Section 5 talks about offering multiple IPv6 addresses in SDP media line and using DASA to select the best destination address. However, section 5 does not elaborate on the SIP Proxy "best" address selection. 

The proposed algorithm in section 4 is about selecting one, the "best", address for SIP Via header. If uplink R3-ISP1 fails (see Figure 2), and SIP Proxy sends 200 OK to the "best" Via address over R3, 200 OK will be lost. The proposed default route change in section 3.4 would fix the problem. But, if HostA had means to advertise all its reachable addresses in Via, SIP Proxy would be able to try another destination address in Via, select a different source address, and route 200 OK over ISP2. However, this would require SIP to support multiple IPv6 addresses per host (or even SIP Proxy) in Via.

The main reason the algorithms are focused on the "best" address selection is to advertise one IPv6 address and avoid SIP protocol changes. Underlined recovery mechanisms make sure the "new best" address is used, or a SIP message is routed over a different first hop router and ISP. The proposed solution avoids probing (hence delays) by integrating IPv6 ND protocol (see the end of section 4), address selection SASA/DASA, and SIP address selection. It also uses SIP proactive mechanisms, Re-REGISTER and Re-INVITE, to transition to a new "best" address (this eliminates probing for new requests). Should I summarize these intents in a separate section (like SIP keeps using one address per host and avoids probing)?

Thanks,
Dusan.