Outer Header Removal
Outer Header Removal feature is used to remove the GPRS Tunneling Protocol User Plane (GTP-U) header from the uplink GTP-U packets.
The following format is used for encoding Outer Header Removal Information Element (IE):
Bits | ||||||||
Octets | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
1–2 | Type = 95 (decimal) | |||||||
3–4 | Length = n | |||||||
5 | Outer Header Removal Description | |||||||
6 |
GTP-U Extension Header Deletion |
|||||||
7 to (n+4) | These octets are present only if explicitly specified |
Per 3GPP TS 29.244, the Outer Header Removal Description field, when present, is encoded as specified in the following table.
Outer Header to be Removed from the Incoming Packet | Value (Decimal) |
---|---|
GTP-U/UDP/IPv4 (See Notes 1, 2), | 0 |
GTP-U/UDP/IPv6 (See Notes 1, 2) | 1 |
UDP/IPv4 (See Notes 3, 6) | 2 |
UDP/IPv6 (See Notes 3, 6) | 3 |
IPv4 (See Note 6) | 4 |
IPv6 (See Note 6) | 5 |
GTP-U/UDP/IP (See Note 4) | 6 |
VLAN S-TAG (See Note 5) | 7 |
S-TAG and C-TAG (See Note 5) | 8 |
For future use. Not sent. If received, it’s interpreted as the value "1". | 9–255 |
NOTES:
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The SGW-U/I-UPF stores GTP-U extension headers. These headers are forwarded for the packets that aren’t requested to be deleted by the GTP-U Extension Header Deletion field.
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The SGW-U/I-UPF stores the GTP-U message type for a GTP-U signaling message, which must be forwarded. For example, an End Marker message
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This value applies to DL packets received by a PGW-U for non-IP PDN connections. These connections use SGi tunneling based on UDP/IP encapsulation.
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The CP function uses this value for instructing the UP function to remove the GTP-U/UDP/IP header regardless of the IP version (IPv4 or IPv6).
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This value applies to DL packets received by a UPF over N6 for Ethernet PDU sessions.
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This value applies to DL packets received by a UPF (PDU Session Anchor) over N6, when explicit N6 traffic routing information is provided to the SMF.
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The Local F-TEID in the Created PDR or Traffic Endpoint IE is populated with the local GTP-U endpoint. This endpoint is allocated based on the configuration irrespective of the Outer Header Removal received in the Created PDR IE. For example, if the GTP-U service configuration supports both the IPv4 and IPv6 addresses, then both these addresses are allocated and advertised in the Created PDR and Traffic Endpoint IEs.
In addition to the Local F-TEID allocation process, the User Plane advertises the allocated F-TEID to the Control Plane through the Created PDR or Traffic Endpoint IE with the following scenarios:
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The Outer Header Removal that is received in the Create PDR IE is also considered while populating the Created PDR or Traffic Endpoint IE.
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If Outer Header Removal is received as GTP-U, UDP, or IP, then the local GTP-U endpoint is advertised as allocated, which implies that if both the IPv4 and IPv6 addresses are configured in the GTP-U service, then both these addresses are allocated and advertised. If an IPv4 or IPv6 address is configured, then only the required address is allocated and advertised through the Created PDR or Traffic Endpoint IE.
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If Outer Header Removal is received as GTP-U, UDP, or IPv4, then only the IPv4 address is advertised even if both these addresses are configured.
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If Outer Header Removal is received as GTP-U, UDP, or IPv6, then only the IPv6 address is advertised even if both these addresses are configured.
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Software Requirements
The software requirements are as follows:
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The feature requires UPF support to identify, encode, and decode the wildcard tunnel type "GTP-U/UDP/IP-6" on the N4 interface.
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If IPv4 and IPv6 addresses are received as part of Outer Header Creation (OHC), priority is given to the IPv6 endpoint, and hence the IPv6 Outer Header Removal (OHR) endpoint is retained by the UPF.
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GTP-U/UDP/IP-6 on the N4 interface, can be received over Sx Establishment or Sx Modification request messages. UPF must support type-6 on both cases.
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In Handoff scenarios, for all the PDRs with OHR value- 6, uplink packets are buffered until an appropriate OHC IE is received for PDRs corresponding to the downlink FAR.
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The uplink packets are forwarded only after the appropriate OHR type is set at UPF.
Limitations
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When the Outer Header Removal value as 6 is received for uplink PDR, the UPF maintains only IPv6 Outer Header Removal IE for uplink PDR. The UPF maintains it until an appropriate Outer Header Creation IE is received for downlink FAR.
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This feature is applicable to the N4 interface only.