Export of GTP-U Information in IP Flow Information Export (IPFIX)

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Table of Contents

1. Introduction

A dedicated header, called GPRS Tunneling Protocol Header (GTP), is defined by 3GPP for use of GTP-C Control plane and GTP-U User Plane [TS.29281] traffic of mobile subscribers.¶

This document specifies six IPFIX Information Elements (IEs) [RFC7012] for fields within a GTP-U header.¶

These IEs are used to export the GTP-U Tunnel Endpoint Identifier (TEID), QoS Flow Identifier (QFI) and PDU Type from the PDU Session Container extension header.¶

Some examples are provided in Appendix A.¶

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This document makes use of the terms defined in [RFC7011]¶

• IPFIX¶

• IPFIX Information Elements¶

• Template¶

• Template Record¶

• Options Template¶

• Options Template Record¶

• Data Record¶

• Data Set¶

3. IPFIX GTP-U Information Elements

This section defines IPFIX IEs corresponding to various fields in the GTP-U.¶

gtpuFlags

8-bit flags field defined in the GTP-U.¶

gtpuMsgType

8-bit message type field defined in the GTP-U.¶

gtpuTEid

32-bit tunnel endpoint identifier field defined in GTP-U which unambiguously identifies a tunnel endpoint in the receiving GTP-U protocol entity for a given UDP/IP endpoint.¶

gtpuSequenceNum

16-bit sequence number field defined in the GTP-U. This field is interpreted based on the corresponding flag value from gtpuFlags.¶

gtpuQFI

6-bit QoS flow identifier field defined in PDU Session Container extension header of GTP-U.¶

gtpuPduType

4-bit PDU Type field defined in PDU Session Container extension header of GTP-U.¶

4. Sample Use Cases

In order to identify the transport performance of PDU Sessions with specific QoS within a slice or within a group of slices hosted on the same UPF the GTP User Plane related IEs would be much helpful.¶

For example when in case of one or a couple of dedicated UPFs are deployed per 5G slice, the slice is identified first using list of gNodeB IPs composing the slice and list of IPs of User Plane Function dedicated for the slice. The gNodeB and the User Plane Function form the tunnel endpoints. Also the traffic for individual PDU session per direction is identified using the GTP-U TEID, GTP-U PDU Type together with above mentioned tunnel endpoints. Further the traffic for specific QoS within a PDU session per direction is identified using the combination of GTP-U TEID, GTP-U PDU Type and GTP-U QFI attributes. It is possible that there may be multiple IP flows having the same attributes.¶

In another scenario when multiple 5G slices are served by the same User Plane Function, the slice is identified using a separated list of gNodeB IPv6 addresses per slice. If Intermediate User Plane Function or Uplink Classifier is deployed there is an addition of a GTP-U tunnel between the Intermediate/Uplink-Classifier UPF and the final UPF. These brings a challenge for identifying the end to end path for a certain PDU session - where the GTP-U PDU Type and GTP-U QFI attributes from the gNodeB and Intermediate/Uplink-Classifier UPF tunnel will be the same on the Intermediate/Uplink-Classifier and final UPF tunnel, however the GTP-U TEIDs will be different since this is a different tunnel.¶

5. IANA Considerations

IANA has added the following new IEs to the "IPFIX Information Elements" registry [RFC7012] available at [IANA-IPFIX].¶

Table 1 lists the GTP-U IEs:¶

     +-------+-----------------+
     |Element| Name            |
     |       |                 |
     |   ID  |                 |
     |       |                 |
     +-------+-----------------+
     | 505   | gtpuFlags       |
     |       |                 |
     +-------+-----------------+
     | 506   | gtpuMsgType     |
     |       |                 |
     +-------+-----------------+
     | 507   | gtpuTEid        |
     |       |                 |
     |       |                 |
     +-------+-----------------+
     | 508   | gtpuSequenceNum |
     |       |                 |
     +-------+-----------------+
     | 509   | gtpuQFI         |
     |       |                 |
     |       |                 |
     |       |                 |
     |       |                 |
     +-------+-----------------+
     | 510   | gtpuPduType     |
     |       |                 |
     |       |                 |
     |       |                 |
     +-------+-----------------+

  Table 1: GTP-U IEs in the "IPFIX Information Elements" Registry

6. Acknowledgements

The authors would like to thank Ketan Talaulikar and Dhananjay Patki for their reviews and valuable comments.¶

7. Contributors

8. Implementation Status

Note to the RFC-Editor: Please remove this section before publishing.¶

9. Security Considerations

There exists no extra security considerations regarding allocation of these IPFIX IEs compared to [RFC7012].¶

The IEs described in this document export GTP user plane data metrics on how packets are being forwarded in 5G network. Applications and operators using the IEs described in this document must evaluate the sensitivity of this information in their implementation context, and apply the data-at-rest storage guidance in Section 11.8 of [RFC7011] as appropriate.¶

Appendix A. IPFIX Encoding Examples

In this section, an example is provided to show IPFIX encoding format for the GTP-U introduced IEs. Template definition and data set corresponding to an observed GTP-U header is illustrated below.¶

          Observed GTP-U Header:
          Flags = 0x36, Message Type = 0xff, TEID = 0x1,
          Sequence number = 0x0000,
          Next extension header type = 0x85 (PDU Session container),
          PDU Type = 0, QFI = 8

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          SET ID = 2           |       Length = 32             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Template ID = 256        |      Field Count = 6          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|  gtpuFlags = 505            |      Field Length = 1         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|  gtpuMsgType = 506          |      Field Length = 1         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|  gtpuTEid = 507             |      Field Length = 4         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|  gtpuSequenceNum = 508      |      Field Length = 2         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|  gtpuQFI = 509              |      Field Length = 1         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|  gtpuPduType = 510          |      Field Length = 1         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 1: Sample Template Record

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         SET ID = 256          |           Length = 14         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | gtpuFlags     |  gtpuMsgType  |      gtpuTEid = 0x1           |
   | = 0x36        |  = 0xff       |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |   gtpuSequenceNum = 0x0000    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | gtpuQFI = 8   | gtpuPduType   |
   |               | = 0           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



           Figure 2: Data Set Encoding Format

Authors' Addresses