Subject: [sipcore] sip/tcp with primary & secondary outbound proxies (behavior upon receipt of tcp rst)

From: "Penar, Russell" <[email-masked]>
Date: Tue, 7 Jul 2015 17:22:30 +0000

I'm seeing non-ideal behavior from cpe and hoping this group can provide guidelines specific to the scenario below, and ultimately standardize ideal cpe behavior for the given context.  I haven't found any specifications defining the ideal behavior, and it was suggested during sipnoc to contact sipcore as an appropriate next step.

Scenario context:
Example platform has two geographically redundant access sbcs (each sbc is a highly available mated pair). Both sbc ip addresses are provided to cpe in a primary/secondary priority configuration via srv record lookups.  Most, if not all, vendor sbc mated pairs maintain signaling/media state, but not tcp state (reportedly doing so is significantly hard/expensive).  Sbc mated pair failovers are fairly common (e.g. often required for software upgrades) relative to primary geographic site failure.

Problem/Variance:
Where a standing call is setup against the primary sbc; after a primary sbc mated pair failover, calls are at risk of being dropped if a tcp connection is not established back to the primary sbc.  This can be due to session audits from the platform, or more specific to my ask is a cpe re-Invite scenario.  For example, when cpe puts standing call on hold (after sbc failover), the sbc target for re-establishing the signaling tcp session is critical.

I've seen cpe exhibit ideal behavior, where upon receipt of tcp rst the cpe connects back to the primary sbc.  In that ideal scenario, the call stays up, mid-call control doesn't fail, etc.  And I've seen non-ideal behavior where once the cpe attempts to put the call on hold, it reacts to the tcp rst by failing over to the secondary sbc and dropping the standing call.  Fwiw, I'm told the vendor exhibiting the ideal behavior only recently began to do so (reportedly from customer pressure to properly support geo-redundant setups as described above).

Regards,
Russ

Work - +1.303.626.2812
Mobile - +1.425.753.6191

This communication is the property of CenturyLink and may contain confidential or privileged information. Unauthorized use of this communication is strictly prohibited and may be unlawful. If you have received this communication in error, please immediately notify the sender by reply e-mail and destroy all copies of the communication and any attachments.