Subject: [DMM] More info on the different OnDemand Session Continuity services

From: "Moses, Danny" <[email-masked]>
Date: Sun, 30 Jul 2017 12:09:27 +0000

In IETF99 (Prague) we introduced the "Graceful-Replacement" session continuity service in addition to the services specified in previous versions as a result of 3GPP specifying SSC (Service and Session Continuity) mode 3.

This led to some questions at IETF 99 regarding this new service and the need for the other services.

I am sending this note (as promised in the DMM session), to refresh everyone's memory and to provide use-cases for each service type.

Session Continuity Services are associated with source IP prefixes assigned to mobile nodes by the network. The TCP/IP stack in the mobile node creates IP addresses using those prefixes and assigns them upon request to specific IP sockets. Applications on the mobile nodes request and receive source IP addresses with a specific Session Continuity service.

The draft specifies the following session continuity services (or "type of source IP addresses" as they are referred to) in section 3.1:

- Fixed IP Address
A Fixed IP address is an address with a guarantee to be valid for a very long time, regardless of whether it is being used in any packet to/from the mobile host, or whether or not the mobile host is
connected to the network, or whether it moves from one point-of-attachment to another (with a different IP prefix) while it is connected.

- Session-lasting IP Address
A session-lasting IP address is an address with a guarantee to be valid throughout the IP session(s) for which it was requested. It is guaranteed to be valid even after the mobile host had moved from one
point-of-attachment to another (with a different IP prefix).

- Non-persistent IP Address
This type of IP address does not provide IP session continuity nor IP address reachability. The IP address is created from an IP prefix that is obtained from the serving IP gateway and is not maintained across gateway changes. In other words, the IP prefix may be released and replaced by a new one when the IP gateway changes due to the movement of the mobile host forcing the creation of a new source
IP address with the updated allocated IP prefix.

- Graceful Replacement IP Address
In some cases, the network cannot guarantee the validity of the provided IP prefix throughout the duration of the IP session, but can provide a limited graceful period of time in which both the original
IP prefix and a new one are valid. This enables the application some flexibility in the transition from the existing source IP address to the new one.

This gracefulness is still better than the non-persistence type of address for applications that can handle a change in their source IP address but require that extra flexibility.

Some use-cases:
Servers require Fixed IP addresses for reachability:
A server that is connected to a mobile network requires that it be reachable by its clients. One of the common ways of achieving reachability is by advertising its source IP address via DNS. For this purpose it is very useful to have an IP address that never changes (even when the server is disconnected from the network for maintenance). A change of source IP address will require a DNS update to re-enable clients to reach it. This is a time-consuming process that is not desirable in many cases.

Real-time application require session-lasting addresses for contiguous connection:
A real-time application that requires constant connection to peers through the network requires the connection to be session-lasting. A change of the source IP address will cause the destination address of packets sent back to become obsolete and, as a result, will not reach their destination. Although this error could be detected by higher-layer protocols, the time to detect the loss and overcome it might be too large for some applications. A session-lasting source IP address guarantees that this event never occurs.

Common viewers use non-persistent addresses:
There are various non-real time applications that people use like: web browsing (for non-streaming activities) and email clients. These applications do not require any special service from the network and can recover from a socket error caused by a change of the source IP address with minimal effect on their user's experience. Non-persistent source IP addresses will do. Using such addresses is beneficial for these applications and networks. Applications do not suffer from their traffic having to be encapsulated, decapsulated and routed through non-optimal routes and through congested mobility anchors, and networks can save their precious resources and still provide good connectivity.

Video viewers use Graceful-replacement addresses:
The Graceful-replacement service is a compromise between session-lasting service and non-persistent service. They are the best fit for applications like video applications that can handle a short disconnect from their server if they accumulate enough content in local buffers. Although, in many cases, the session-lasting service is the most appropriate service for such applications, they can still work fairly well with graceful-replacement service, since they can still control the exact time of switching to the new IP address.

In some cases, Graceful-replacement service is even better than session-lasting service for video viewers. When the content streamed to the viewer comes from a cached (or replicate server) source in the local access network and the host of the viewer handoffs to a point-of-attachment in a different access network, it is preferable for the viewer to disconnect from the old (and remote by now) cache and reconnect to a new local cache. This process may take some time as the new cache needs to be located. While finding the new cache, the user can still enjoy the movie that is continued to be streamed from the old cache. The graceful time, enables the viewer to be prepared to switch to the new source without affecting the user's experience.

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