224 lines
6.8 KiB
Plaintext
224 lines
6.8 KiB
Plaintext
== SGSN Pooling
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SGSN pooling is described in 3GPP TS 23.236 <<3gpp-ts-23-236>>, and is supported
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by OsmoGbProxy since early 2021.
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The aim of SGSN pooling is to distribute load from a BSS across multiple SGSNs,
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which are equivalent and redundant infrastructure for the same core network.
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The main mechanism for SGSN pooling is the TLLI/P-TMSI, which an SGSN hands out
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to its attached subscribers. Typically 10 bits of the P-TMSI are designated as a
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Network Resource Identifier (NRI) that identifies the originating SGSN, and
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allows OsmoGbProxy to direct a subscriber back to the same SGSN instance that
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previously negotiated the Attach procedure. Typically, the full NRI value
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range available is divided into N even ranges, where each SGSN is assigned one
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NRI range.
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Subscribers attaching without a TLLI, or those with unknown NRI value,
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are evenly distributed across SGSN instances. OsmoGbProxy uses a hash-based
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approach to distribute load across all connected SGSNs.
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A Paging Response from a subscriber is always returned back to whichever SGSN
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initiated the Paging, regardless of the Mobile Identity used.
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Finally, a NULL-NRI is a special NRI value that indicates that the SGSN wishes
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to offload this subscriber to a different SGSN. A NULL-NRI is an arbitrary NRI
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value that is chosen distinctly for each PLMN served by a BSS, so that a
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subscriber can be reassigned within that PLMN. Upon (periodic) Location
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Updating, an offloading SGSN hands out a NULL-NRI value in the assigned TLLI,
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along with a non-broadcast LAI. The subscriber will notice the LAI mismatch,
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and immediately re-attempt the attach using the TLLI containing the NULL-NRI.
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OsmoGbProxy recognises the NULL-NRI and redirects the subscriber to one of the
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other SGSNs. A prerequisite for this to work well is that the particular SGSN is
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previously marked as not accepting new subscribers, in OsmoGbProxy's configuration.
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The mechanisms described above make up the NAS node selection function
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implemented in OsmoGbProxy.
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3GPP TS 23.236 also defines that an offloading SGSN hands subscriber information
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to the newly assigned SGSN, which takes place outside the scope of OsmoGbProxy.
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=== Configuring SGSN Pooling
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The NRI ranges assigned to each SGSN must match in the OsmoGbProxy and the SGSN
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configuration. If inconsistent NRI value ranges are configured,
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attached subscribers would be redirected to SGSN instances that did not perform the
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attach, possibly rendering the core network unusable.
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==== Connecting Multiple SGSNs
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----
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! Configure the Network Service
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ns
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bind udp sgsn
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listen 10.0.0.1 23000
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nse 1
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ip-sns-bind sgsn
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ip-sns-remote 10.0.1.1 23000
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ip-sns-remote 10.0.1.1 23001
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nse 2
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ip-sns-bind sgsn
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ip-sns-remote 10.0.1.2 23000
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nse 3
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ip-sns-bind sgsn
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ip-sns-remote 10.0.1.3 23000
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! configure NRI value ranges
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gbproxy
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nri bitlen 10
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nri null add 0
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sgsn nsei 1
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name SGSN01
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nri add 1 341
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sgsn nsei 2
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name SGSN02
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nri add 342 682
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sgsn nsei 3
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name SGSN03
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nri add 683 1023
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----
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==== NRI Value Bit Length
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In OsmGbProxy, the NRI value's bit length is freely configurable from 0 to 15
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bits. 3GPP TS 23.236 suggests a typical bit length of 10. Setting the length
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to 0 disables SGSN pooling, this is also the default.
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The NRI bit length must be identical across the entire SGSN pool.
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Change the NRI value bit length in OsmoGbProxy's VTY configuration like this:
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----
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gbproxy
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nri bitlen 10
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----
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In the TMSI bits, regardless of the NRI bit length, the NRI value always starts
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just after the most significant octet of a TMSI (most significant bit at TMSI's
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bit 23).
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==== NULL-NRI
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Since OsmoGbProxy supports serving only one PLMN, NULL-NRI are configured globally.
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Even though 3GPP TS 23.236 indicates that there is a single NULL-NRI per PLMN,
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OsmoGbProxy allows configuring multiple NULL-NRI values.
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----
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network
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nri null add 0
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nri null add 423
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----
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==== Assigning NRI Ranges to SGSNs
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Each SGSN configured in OsmoGbProxy must be assigned a distinct NRI value range.
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Overlapping NRI value ranges will cause failure to serve subscribers.
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NRI values are typically configured in ranges, here dividing a 10bit range
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(0..1023) into three equal ranges, while leaving 0 available to be configured
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as NULL-NRI:
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----
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sgsn nsei 1
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nri add 1 341
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sgsn nsei 2
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nri add 342 684
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sgsn nsei 3
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nri add 685 1023
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----
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NRI can also be assigned in single values:
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----
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sgsn nsei 1
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nri add 23
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----
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Ranges can be constructed arbitrarily by a sequence of `add` and `del`
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configurations, here a contrived example:
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sgsn nsei 1
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nri add 0 342
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nri del 23
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nri del 42 235
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nri add 1000 1023
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----
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On the VIEW and ENABLE VTY nodes, `show nri all` shows all SGSNs:
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----
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OsmoGbProxy> show nri all
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sgsn nsei 1
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nri add 1 341
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sgsn nsei 2
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nri add 342 684
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sgsn nsei 3
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nri add 685 1023
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----
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When configuring overlapping NRI value ranges across SGSNs, the telnet VTY warns
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about it, and starting OsmoGbProxy with such a configuration will fail:
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----
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sgsn nsei 1
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nri add 1 511
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sgsn nsei 2
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nri add 512 1023
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sgsn nsei 3
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nri add 500 555
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----
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This results in:
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----
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$ osmo-gbproxy
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% Warning: NSE(00003/SGSN): NRI range [500..555] overlaps between NSE 00003 and NSE 00001. For overlaps, NSE 00001 has higher priority than NSE 00003
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% Warning: NSE(00003/SGSN): NRI range [500..555] overlaps between NSE 00003 and NSE 00002. For overlaps, NSE 00002 has higher priority than NSE 00003
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----
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==== SGSN Offloading
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To effectively offload a particular SGSN, it must be marked as no longer taking
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new subscribers in OsmoGbProxy. This can be achieved in the telnet VTY by:
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----
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sgsn nsei 1
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no allow-attach
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----
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This SGSN will, as long as it is connected, continue to serve subscribers
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already attached to it: those that yield an NRI matching this SGSN, and those
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that are being paged by this SGSN. But OsmoGbProxy will no longer direct new
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subscribers to this SGSN.
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To re-enable an SGSN for attaching new subscribers:
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----
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sgsn nsei 1
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allow-attach
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----
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==== Traffic allocation
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In a SGSN pool, osmo-gbproxy is facing the problem of dividing the downlink
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capacity of a cell towards the SGSN. The BSS advertises the per-BVC capacity
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by means of the BSSGP FLOW-CONTROL-BVC messages, but as there are multiple
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SGSN in a pool, they all have to share / divide that total capacity.
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By default, osmo-gbproxy advertises the full capacity to _each_ of the SGSN
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pool members, which results in significant over-provisioning and can lead to
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overload situations.
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The administrator can configure the _percentage_ of the overall BSS-advertised
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capacity that shall be reported to each pool member SGSN using the
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`pool bvc-flow-control-ratio <1-100>` configuration command.
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A setting of 100 means that each pool member is informed of 100% of the
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BSS side capacity.
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A setting of 25 means that each pool member is informed of 25% of the
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BSS side capacity. This would make most sense in a set-up with four
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SGSN of equal share.
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More complex capacity division schemes are so far not supported by
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osmo-gbproxy.
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