SSC mode — What happens to your IP address when you move
Sessions, flows and slices
Where it sits
What it is
What it is. Session and Service Continuity mode — the rule deciding what happens to your IP address when you move far enough that the UPF anchoring your session is no longer the right one. TS 23.501 §5.6.9. Chosen per PDU session, and the SMF "determines the SSC mode of a session" (§6.2.2).
| Mode | The anchor | Your address | The cost |
|---|---|---|---|
| 1 | never changes | kept, for the life of the session | the path can grow arbitrarily long |
| 2 | may change — break before make | lost, then a new one | a visible gap; connections drop |
| 3 | may change — make before break | old and new coexist, briefly | two anchors at once |
This is one trade, stated three ways: address stability against path length. A UPF anchor owns the device's address, so keeping the address means keeping the anchor. Keep the anchor while the device travels a hundred kilometres and every packet is hauled back to where the session started — the trombone problem. Move the anchor and the address changes, which the application above will notice.
Mode 1 is what a phone uses, and it is why your IP address does not change as you cross a city. The path inefficiency is real and acceptable, because the anchor is in a central data centre anyway.
Mode 3 is the one that makes edge computing usable, and its trick is worth stating: the new session is established before the old is released, so the device holds both for a window. An application can migrate its connections across at its own pace. It is a soft handover for IP addresses — and it is only possible because the CP/UP split lets the SMF stand up a second anchor without disturbing the first.
Mode 2 is the blunt one: release, then re-establish. Simple, and honest about the fact that something will break.
Where it meets the rest of the folder. Mobility in the RAN keeps the radio connection alive as a device moves between cells. SSC mode is the question of what happens when the device outruns not the cell but the anchor — a core-network form of mobility with no radio counterpart, operating on a scale of kilometres rather than metres.
Read on
This concept is read as part of one argument in ref-session, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: