Abstract:
A technique for controlling and handling set up of a probe tunnel stretching from an access network node through a core network towards a core network node is described. In a method implementation, probe tunnel set up control comprises determining a First Fully Qualified Tunnel Endpoint Identifier (F-TEID) associated with the core network node and sending a probe tunnel set up instruction to the access network node. The instruction commands the access network node to locally set up the probe tunnel towards the core network node based on the F-TEID.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure generally relates to the field of communication network probing. In particular, a technique for controlling the set up of a probe tunnel for network probing purposes is described. 
       BACKGROUND  
       [0002]    Communication network probing is a technique increasingly used to monitor and analyze network traffic and transmission paths. The resulting insights are typically exploited by network operators for network management and optimization. As an example, network probing permits to identify possible bottlenecks within a communication network. 
         [0003]    Network probing can be based on both live user traffic and dedicated probe (or test) traffic. For probe traffic generation specific test equipment, such as probe traffic generators, are installed at one or more network locations. The probe traffic generators are configured to initiate probe traffic communication with conventional network nodes, other probe traffic generators or special servers (such as reflectors). On the basis of the probe traffic, path properties such as packet loss, delay, jitter and throughput can be determined. 
         [0004]    Network tunnels are transmission paths that require particular considerations when it comes to network probing. Generally, communication networks use tunneling protocols when a first network protocol (e.g., a delivery protocol) encapsulates a second network protocol (e.g., a payload protocol). As an example, the General Packet Radio Service (GPRS) Tunneling Protocol, also referred to as GTP, is a tunneling protocol used to transport in a GPRS core network user traffic between an access network and an external Packet Data Network (PDN) or another access network. The access network may be configured according to the Global System for Mobile Communication (GSM), Universal Mobile Telecommunications System (UMTS) or Long Term Evolution (LTE) specifications. 
         [0005]    GTP is in fact a protocol suite that comprises multiple individual protocols, such as GTP-U and GTP-C. GTP-U is the user plane protocol applied to transport user data within the GPRS core network and between an access network and the core network. GTP-C, on the other hand, is the associated control plane protocol. 
         [0006]    On the user plane, multiple tunnels may be set up for an individual user. Each tunnel is identified locally at a network endpoint by a Tunnel Endpoint Identifier (TEID). The TEIDs are randomly allocated by the tunnel endpoints. A Fully Qualified TEID (F-TEID) additionally contains address information (typically the Internet Protocol, or IP, address) of a given tunnel endpoint. Accordingly, a tunnel stretching between two endpoints can uniquely be identified by the F-TEID pair associated with the two end-points. 
         [0007]    For network probing purposes, the GTP ECHO protocol permits to probe the connectivity between two devices supporting GTP. A network probing system on the basis of GTP ECHO messages is exemplarily described in WO 2008/138509 A. 
         [0008]    It has been found that the end-to-end path from an access network via a core network to, for example, a PDN cannot yet be probed satisfactorily. This drawback can be attributed to the fact that tunnels specifically set up for probing purposes (“probe tunnels” hereinafter) do no route test traffic through certain network nodes that would need to be probed, such as Serving Gateways (S-Gws), Serving GPRS Support Nodes (SGSNs), PDN Gateways (P-Gws) and Gateway GPRS Support Nodes (GGSN). The same applies to network nodes above the P-Gw or GGSN (e.g., PDN servers on the Gi interface). 
         [0009]    A further problem that has been observed is the fact that mobile communication systems often have two Transport Control Protocol (TCP)/IP layers. Most existing network probing solutions are only capable of probing the lower TCP/IP layer between two network nodes. This means that the upper TCP/IP layer that actually represents the end-to-end connectivity across the mobile communication system will not be probed. 
         [0010]    To overcome this problem, mobile terminal-based probing solutions have been proposed to monitor the end-to-end connectivity. Such systems have the drawbacks that they consume precious radio resources and that they require additional hardware. As an alternative, an emulation of mobile terminals on the network side has been considered for end-to-end path probing. The emulation approach, however, necessitates the implementation of a complete mobile terminal stack and of user authentication mechanisms on the network side. 
         [0011]    In sum, the presently available network probing approaches for tunnel-based transmission paths do not yet permit a satisfactory monitoring of an end-to-end connection and of individual network nodes in the core network. This drawback is based on the nature of the probe tunnels defining the transmission paths to be probed. 
       SUMMARY  
       [0012]    There is a need for a technique to set up probe tunnels for an efficient network probing that avoids one or more of the problems identified above. 
         [0013]    According to a first aspect, a method of controlling set up of a probe tunnel stretching from an access network node through a core network towards a core network node is proposed. The method comprises determining a first F-TEID associated with a core network node and sending a first probe tunnel set up instruction to the access network node for instructing the access network node to locally set up the probe tunnel towards the core network node based on the first F-TEID. 
         [0014]    The core network may be or include a part of a packet-oriented network, such as a GPRS network. The core network node may be or comprised by any node of a core network, such as a serving node or a gateway node interfacing another network domain different from the core network. The probe tunnel may stretch through the complete core network (e.g., from an end facing an access network to an end facing a PDN or another access network) or through a portion thereof (e.g., from an end facing an access network to a serving node of the core network). Moreover, the probe tunnel may stretch from a first access network node, via the core network, to a second access network node. 
         [0015]    The access network may be a Radio Access Network (RAN) conforming to the GSM, UMTS and/or LTE specifications. In such a case, the access network node may be configured as a Base Station (BS), for example as a NodeB or eNodeB, or a Radio Network Controller (RNC). 
         [0016]    As understood herein, an F-TEID comprises a tunnel identifier for a given tunnel as well as address information pertaining to a particular network node acting as tunnel endpoint. The tunnel identifier permits to at least locally identify the given tunnel by the particular network node. According to the present disclosure, the term F-TEID is meant to include, but not to be limited to the particular definition in the applicable specifications of the 3 rd  Generation Partnership Project (3GPP). Accordingly, the term encompasses every identifier suitable to convey tunnel identity information on the one hand and related network node address information on the other. 
         [0017]    In one implementation, the first probe tunnel set up instruction includes the first F-TEID. In another implementation, the first F-TEID is sent to the access network node in a separate messaging step (e.g., before, after or together with the first set up instruction). 
         [0018]    A second probe tunnel set up instruction may sent to the core network node for instructing the core network node to locally set up the probe tunnel. The second probe tunnel set up instruction may be sent before, after or concurrently with the first probe tunnel set up instruction. 
         [0019]    The probe tunnel may be set up by the core network node based on a second F-TEID associated with the access network node. The second F-TEID may be included in the second probe tunnel set up instruction or may be sent in a separate messaging step. 
         [0020]    The individual F-TEIDs may be determined in various ways. In one variant, at least one of the first F-TEID and the second F-TEID is determined from a predetermined pool comprising one or more F-TEIDs specifically allocated for probing purposes. The individual network nodes may have a priori knowledge of the one or more F-TEIDs allocated for probing purposes. Such a priori knowledge may result from a configuration step preceding the probe tunnel set up signaling. 
         [0021]    In another variant, the individual F-TEIDs are obtained from the individual network nodes. As an example, the step of determining the first F-TEID may comprise receiving the first F-TEID from the core network node (using, e.g., standardized procedures). The first F-TEID may be received from the core network node in response to receipt of the second probe tunnel set up instruction by the core network node. In a similar manner, determining the second F-TEID may comprise receiving the second F-TEID from the access network node. 
         [0022]    Further probe tunnel set up instructions may be sent to other network nodes of the access network or the core network. As an example, a third probe tunnel set up instruction may be sent to a gateway node of the core network for instructing the gateway node to locally set up the probe tunnel (e.g., based on the first F-TEID). The gateway node may interface a network domain different from the core network, such a PND, and the core network node may be arranged between the access network node and the gateway node. As such, the resulting probe tunnel may stretch from the access network node, via the core network node, to the gateway node. 
         [0023]    In one implementation, the gateway node is one of a PDN-Gw and a GGSN. Additionally, or in the alternative, the core network node may be a serving node, such as one of a S-Gw and a SGSM. 
         [0024]    The steps of the method may be performed at least partially by an Operation Support Subsystem (OSS) or a Mobility Management Entity (MME). Additionally, one or more steps of the method may be performed by the OSS and one or more further steps may be performed by the MME. As an example, the OSS may instruct the MME to perform a standard tunnel set up procedure for the probe tunnel towards any core network node (e.g., the serving node and/or the gateway node). In such an implementation the transmission of an explicit probe tunnel set up instruction to the core network node may be omitted. 
         [0025]    Once the probe tunnel has been set up, measurement data pertaining to probe traffic transmitted via the probe tunnel may be received. The measurement data may be received from one or more of the access network node, the core network node (e.g., the gateway node or the serving node) and a network node located in a network domain (e.g., a PDN) interfacing the core network via the gateway node. The measurement data may pertain to one or more of packet loss, delay, jitter, throughput and any other parameter of interest. 
         [0026]    The technique presented herein may be performed in relation to a plurality of access network nodes. In such a case, a coordinated network probing via the resulting plurality of probe tunnels can be performed. 
         [0027]    According to another aspect, a method of setting up a probe tunnel stretching from an access network node through a core network towards a core network node is presented, wherein the method is performed by the access network node and comprises receiving a probe tunnel set up instruction to locally set up the probe tunnel towards a core network node, wherein the instruction is accompanied by an F-TEID associated with the core network node, and setting up the probe tunnel towards the core network node based on the F-TEID associated with the core network node. As mentioned above, the F-TEID may be received together with the probe tunnel set up instruction or in a separate messaging step. 
         [0028]    The method performed by the access network node may further comprise assigning the probe tunnel to a probe traffic generator. The probe traffic generator may be configured to generate probe traffic once the probe tunnel has been set up. Still further, the access network node may locally perform measurements pertaining to the probe traffic transmitted via the probe tunnel and may report the corresponding measurement data (e.g., statistics) to the OSS or any other network node. 
         [0029]    Also provided is a computer program product comprising program code portions for performing the steps of any of the methods or method aspects disclosed herein when the computer program product is run on one or more computing devices. The computer program product may be stored on a computer readable recording medium such as a hard disk, CD-ROM or DVD. The computer program product may also be provided for download via a wireless or wired communication network. 
         [0030]    According to a further aspect, a tunnel management control function apparatus for controlling set up of a probe tunnel stretching from an access network node through a core network towards a core network node is provided. The apparatus comprises a determination unit adapted to determine an F-TEID associated with a core network node and an instructing unit adapted to sent a probe tunnel set up instruction to the access network node to instruct the access network node to locally set up the probe tunnel towards the core network node based on the F-TEID. 
         [0031]    The tunnel management control function apparatus may further comprise an interface adapted to receive measurement data pertaining to probe traffic transmitted via the probe tunnel. Additionally, an evaluation unit adapted to evaluate the measurement data thus received may be provided. 
         [0032]    According to a still further aspect, a tunnel management handling function apparatus for an access network node is provided, wherein the apparatus is adapted to set up a probe tunnel stretching from the access network node through a core network towards a core network node and comprises an interface adapted to receive a probe tunnel set up instruction to locally set up the probe tunnel towards the core network node, wherein the instruction is accompanied by an F-TEID of the core network node. The apparatus further comprises a set up unit adapted to set up the probe tunnel towards the core network node based on the F-TEID associated with the core network node. The F-TEID may be received together with the probe tunnel set up instruction or during a separate messaging step. 
         [0033]    The tunnel management handling function apparatus may further comprise a probe traffic generator adapted to be assigned to the probe tunnel. The probe traffic generator may be adapted to generate probe traffic that is transmitted, via the probe tunnel, towards the core network node. Moreover, a reporting unit may be present that is configured to transmit measurement data pertaining to the probe traffic to the OSS or any other network node. In one implementation, the measurement data pertains to reflected probe traffic that has been received from the tunnel management handling function apparatus via the probe tunnel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    Further aspects, details and advantages of the technique presented herein will become apparent from the following description of preferred embodiments in conjunction with the drawings, wherein: 
           [0035]      FIG. 1  illustrates a network system embodiment in which a probe tunnel is set up; 
           [0036]      FIG. 2  an embodiment of a tunnel management control function apparatus and an embodiment of a tunnel management handling function apparatus; 
           [0037]      FIG. 3  a flow diagram illustrating a method embodiment of controlling set up of a probe tunnel; 
           [0038]      FIG. 4  a flow diagram illustrating a method embodiment of handling set up a probe tunnel; 
           [0039]      FIG. 5  a first signaling embodiment illustrating the set up of a probe tunnel; 
           [0040]      FIG. 6  a second signaling embodiment illustrating the set up of a probe tunnel; and 
           [0041]      FIG. 7  a third signaling embodiment illustrating the set up of a probe tunnel. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular network configurations and signaling procedures, in order to provide a thorough understanding of the technique presented herein. It will be apparent to one skilled in the art that the technique may be practiced in other embodiments that depart from these specific details. For example, while the following description will mainly focus on GPRS (including Evolved GPRS) core networks and LTE access networks, it will be appreciated that the technique presented herein could also be implemented in other kinds of core and access networks. 
         [0043]    Those skilled in the art will further appreciate that the methods, functions and steps disclosed herein may be implemented in the form of software, hardware or a combination of software and hardware. As an example, the methods, functions and steps may be embodied in a processor (e.g., a microcontroller) and a memory coupled to the processor, wherein the memory is encoded with one or more programs that control the processor to perform the methods, steps and functions disclosed herein upon execution. 
         [0044]      FIG. 1  schematically illustrates probe tunnel set up in a network system embodiment comprising a core network  10 , an access network  12  as well as an optional packet data network  14 . The core network  10  comprises one or multiple core network nodes  16 ,  16 ′, such as serving nodes, gateway nodes, and so on. The core network  10  further comprises a Tunnel Management Control Function apparatus (TMCF)  18  in charge of probe tunnel set up control. Generally, the TMCF  18  is responsible for locally initiating probe tunnel set up in each network node that forms a probe tunnel endpoint. The TMCF  18  may be incorporated in any existing or dedicated control node of the core network  10  or any other control node, or may be distributed among two or more control nodes. 
         [0045]    The access network  12  comprises an access network node  20  (e.g., a BS) and a Traffic Management Handling Function apparatus (TMHF)  22 . The TMHF  22  is in charge of handling probe tunnel set up in the access network node  20 . The TMHF  22  may be incorporated in the access network node  20  or in any other component of the access network  12  capable of communicating with the access network node  20 . 
         [0046]    It should be noted that the TMHF  22  could additionally be provided for core network nodes acting as tunnel endpoints (such as the core network nodes  16 ,  16 ′) for a similar purpose and in a similar manner as will be described for the access network node  20  below. Alternatively, standard tunnel set up procedures as defined, for example, in the 3GPP specifications may be used for this purpose for network nodes different from the access network node  20 . 
         [0047]    As illustrated in  FIG. 1 , a communication link stretches between the TMCF  18  and the TMHF  22 . A further communication link exists between the TMCF  18  and each core network node  16 ,  16 ′ involved in the probe tunnel set up procedure. The communication link between the TMCF  18  and the TMHF  22  is used for communication purposes in the context of setting up a probe tunnel  24  from the access network node  20  through the core network  10  towards one or more of the core network nodes  16 ,  16 ′. In the present embodiment the probe tunnel  24  comprises multiple tunnel sections (i.e., a first section between the access network node  20  and the core network node  16  and a second section between the core network node  16  and a further node  16 ′ of the core network  10 ). Each tunnel section is defined by two tunnel endpoints and associated F-TEIDs. 
         [0048]    The structure and purpose of each of the TMCF  18  and the TMHF  22  will now be explained in more detail with reference to  FIG. 2 . 
         [0049]    As illustrated in  FIG. 2 , the TMCF  18  comprises a determination unit  30  adapted to determine an F-TEID associated with the core network node  16  and an instructing unit  32  adapted to send a probe tunnel set up instruction, via an interface  34 , to the access network node  20 . The probe tunnel set up instruction instructs the access network node  20  to locally set up the probe tunnel  24  towards the core network node  16  based on the F-TEID, and to initiate test traffic generation. Local probe tunnel set up may include one or several actions, including, for example, F-TEID handling (e.g., F-TEID storage or F-TEID-related messaging) and probe traffic generator assignment. In general, the actions to be performed for local probe tunnel set up will include one or more actions as defined in the applicable standard for tunnel set up in general. 
         [0050]    The interface  34  is further adapted to receive measurement data pertaining to probe traffic transmitted via the probe tunnel  24  from, for example, the access network node  20  and the core network nodes  16 ,  16 ′. Measurement data may also be received from one or more network nodes located in the PDN  14 . 
         [0051]    As further shown in  FIG. 2 , the TMHF  22  comprises an interface  40  adapted to receive a probe tunnel set up instruction from the TMCF  18 . The probe tunnel set up instruction is directed at a local set up of the probe tunnel  24  at the access network node  20  towards the core network node  16  and is accompanied by an F-TEID of the core network node  16 . The TMHF  22  further comprises a set up unit  42  adapted to set up the probe tunnel  24  towards the core network node  16  based on the F-TEID associated with a core network node  16 . Additionally, a probe traffic generator  44  is present that may be assigned to the probe tunnel  24  in response to receipt of the probe tunnel set up instruction. 
         [0052]    In the following, the operation of the TMCF  18  and the TMHF  22  will be described in more detail with reference to the flow diagrams  300  and  400  illustrated in  FIGS. 3 and 4  and with further reference to the three signaling diagrams of  FIGS. 5 ,  6  and  7 . The following embodiments will specifically be described for a GPRS-compliant core network  10 , an LTE-compliant access network  12  as well as probe tunnel set up, or establishment, based on GTP-U. It should be noted that the access network  12  could also be implemented on the basis of, for example, the GSM or UMTS specifications. In a similar manner, the core network  10  could also be different from a GPRS (including Evolved GPRS in LTE/SAE) network. 
         [0053]    As shown in  FIGS. 4 ,  5  and  6 , the main components of an LTE/GPRS implementation include an access network node in the form of an eNodeB  20 , a first core network component in the form an S-Gw  16 , a second core network component in the form of a PDN-Gw (or simply P-Gw)  16 ′, as well as a first control node in the form of an OSS  50 , and an optional second control node in the form on an MME  52 . It should be noted that the first and second core network components  16 ,  16 ′ could also be realized as SGSN and GGSN, respectively. 
         [0054]    In the PDN, a further node  54  involved in network probing exists. This further node  54  may take the form of a server capable of acting as at least one of a probe traffic receiver, probe traffic receiver, probe traffic generator and probe traffic reflector. The node  54  may communicate with the P-Gw  16 ′ via the Gi interface. 
         [0055]    Both the PDN server  54  and the eNodeB  20  comprise a probe traffic handling function, namely a Probe Traffic Control Server (PTC-S)  56  and a Probe Traffic Control eNodeB (PTC-E)  58 , respectively. Both the PTC-S  56  and the PTC-E  58  may potentially act as probe traffic generator, probe traffic reflector and Measurement Reporting Function (MRF). As for the eNodeB  20 , the PTC-E  58  may thus realize the probe traffic generation function of the TMHF  22 . Generally, probe traffic generation follows the conventional approaches. For example, voice-like probe traffic, probe traffic in the form of WWW-like TCP downloads, etc. may be generated by the PTC-S  56  and PTC-E  58 . 
         [0056]    Although not specifically shown in  FIGS. 5 ,  6  and  7 , any network node along the probe tunnel  24  may incorporate an MRF. The MRF may be configured to report measurement data, such as probe traffic statistics (e.g., probe packet statistics) to the OSS  50 . 
         [0057]    In the LTE/GPRS implementation, the TMCF  18  could either be implemented in the OSS  50  (see  FIG. 4 ), in the MME  52  (see  FIG. 6 ) or could be distributed between the OSS  50  and the MME  52  (see  FIG. 5 ). The TMHF  22 , on the other hand, will be implemented in the eNodeB  20 . Additionally, the TMHF  22  may be implemented in the S-Gw  16  and P-Gw  16 ′ (see  FIG. 5 ). Alternatively, standard tunnel handling functions as defined in the 3GPP specifications may be utilized by the core network nodes  16 ,  16 ′ (see  FIGS. 6 and 7 ). 
         [0058]    The signaling diagram of  FIG. 5  illustrates probe tunnel set up under control of the OSS  50 . In the embodiment illustrated in  FIG. 5 , an MME is thus not involved. 
         [0059]    Initially, the TMCF  18  in the OSS  50  uses conventional or dedicated messaging steps (not illustrated in  FIG. 5 ) to determine the F-TEIDs of the eNodeB  20 , the S-Gw  16  and P-Gw  16 ′ for probe tunnel set up purposes. Thus, according to step  302  in flow diagram  300  of  FIG. 3 , the F-TEID of at least the S-Gw  16  is determined. In this regard, 3GPP signaling procedures for F-TEID determination may be performed by the TMCF  18 . The corresponding signaling procedures can be omitted if there exists a set of F-TEIDs of all network nodes participating in the probe tunnel set up procedure (i.e., tunnel endpoints) that are reserved for such probe tunnels. In such a case, the TMCF  18  of the OSS  50  may keep track of all F-TEIDs allocated for probing purposes in the different network nodes and request the network nodes, via probe tunnel set up instructions, to assign the pre-allocated F-TEIDs to the probe tunnels. 
         [0060]    With reference to  FIG. 5 , in a first signaling phase  1 ), the TMCF  18  in the OSS  50  requests the TMHF  22 ′ in the S-Gw  16  to set up the probe tunnel  24  locally. To this end, a probe tunnel set up instruction including the previously acquired (e.g., pre-allocated) F-TEIDs of the eNodeB  20  and the P-Gw  16 ′ is sent to the S-Gw  16 . Receipt of the probe tunnel set up instruction triggers the S-Gw  16  to return its F-TEID for the probe tunnel  24  that needs to be set up. It should be noted that the F-TEIDs of the NodeB  20 , the S-Gw  16  and the P-Gw  16 ′ need not be transmitted in case there exists a set of pre-allocated F-TEIDs for probing purposes (see signaling phase  0 , in  FIG. 5 ). 
         [0061]    In the next signaling phase  2 ), the TMCF  18  in the OSS  50  requests the TMHF  22 ″ in the P-Gw  16 ′ to set up the probe tunnel  24  locally. The corresponding probe tunnel set up instruction may be accompanied by the F-TEIDs of the S-Gw  16  as received from the S-Gw  16  in the signaling phase  1 ) or as pre-allocated for probing purposes. 
         [0062]    Then, in signaling phase  3 ), the TMCF  18  in the OSS  50  requests the TMHF  22  in the eNodeB  20  to set up the probe tunnel  24  locally. A probe tunnel set up instruction is sent to the eNodeB  20  in this regard as indicated by step  304  in flow diagram  300 . The probe tunnel set up instruction may include the F-TEID of the S-Gw  16  as received from the S-Gw  16  in signaling phase  1 ) or as pre-allocated by the S-Gw  16  for probing purposes. Receipt of the probe tunnel set up instruction by the eNodeB  20  corresponds to step  402  in flow diagram  400  of  FIG. 4 . In response to receipt of the probe tunnel set up instruction from the TMCF  18  in the OSS  50 , the TMHF  22  in the eNodeB  20  sets up the probe tunnel  24  towards the S-Gw  16  based on the F-TEID associated with the S-Gw  16  as illustrated by step  404  in flow diagram  400 . 
         [0063]    Then, in signaling phase  4 ), the TMHF  22  in the eNodeB  20  assigns the probe tunnel  24  that has been set up towards the S-Gw  16  to the probe traffic generator of the PTC-E  58 . In signaling phase  5 ) the PTC-E  58  starts generating probe traffic for IP packet probing with respect to the probe tunnel  24 . The corresponding probe traffic is routed through the S-Gw  16  and through the P-Gw  16 ′. From the P-Gw  16 ′ the probe traffic may be forwarded in the PDN to the server  54 . The PTC-S  56  in the server  54  may operate in a reflector mode and reflect the probe traffic back towards the eNodeB  20 . 
         [0064]    In signaling phase  6 ), the PTC-E  58 , PTC-S  56  and optional MRF functions along the probe tunnel  24  (e.g., within the S-Gw  16  and/or the P-Gw  16 ′) provide flow reports including measurement data (e.g., IP packet statistics) to the OSS  50  for network traffic and transmission path analysis. 
         [0065]    As illustrated in the signaling diagram of  FIG. 6 , the TMCF functionalities may be distributed between the OSS  50  and the MME  52 . Thus, a first TMCF portion  18 A may be located in the OSS  50 , and a second TMCF portion  18 B may be located in the MME  52 . The TMCF portion  18 A in the OSS  50  uses proprietary protocols in accordance with the present disclosure to communicate with the TMCF portion  18 B in the MME  52  and the TMHF  22  in the eNodeB  20 . On the other hand, the MME  52  uses 3GPP standard procedures for probe tunnel set up. In other words, the S-Gw  16  and the P-Gw  16 ′ may not be aware that a particular tunnel  24  that is locally set up by them will actually be used for probing purposes. Rather, the S-Gw  16  and the P-Gw  16 ′ will implement standard tunnel set up procedures (and can thus be set up like legacy nodes). Thus, the TMHF is not to be implemented in the core network nodes such as the S-Gw  16  and P-Gw  16 ′. 
         [0066]    With reference to  FIG. 6 , in a first signaling phase  1 ), the TMCF portion  18 A in the OSS  50  initiates a probe session set up in the core network towards an eNodeB  20 . To this end a corresponding session initiation message (probe session request) is sent to the TMCF portion  18 B in the MME  52 . As has already been explained with reference to the signaling diagram of  FIG. 5 , the TMCF portion  18 A in the OSS  50  may in a previous messaging step have obtained the F-TEIDs of all network nodes participating in the probe tunnel set up (or may have determined such F-TEIDs from an F-TEID pool pre-allocated for probing purposes) in accordance with step  302  in flow diagram  300 . 
         [0067]    In response to receipt of the probe session request from the OSS  50  in the first signaling phase  1 ), the MME  52  will initiate a standard 3GPP session establishment procedure towards the S-Gw  16  for tunnel set up in signaling phase  2 ). The S-Gw  16  will then set up the tunnel session in the core network with the P-Gw  16 ′. Alternatively, or in addition, the F-TEID of the P-Gw  16 ′ may be communicated to its peer node (the S-Gw  16 ) via the standard 3GPP protocols. 
         [0068]    As for the eNodeB  20 , the F-TEID for probing purposes may be pre-allocated (which is feasible since the eNodeB  20  comprises the proprietary TMHF  22 , whereas the S-Gw  16  and P-Gw  16 ′ may be set up as legacy nodes). Alternatively, the OSS  50  could in an initial signaling phase  0 ) preceding the first signaling phase  1 ) request the eNodeB  20  to allocate a local F-TEID for probe tunnel set up. The locally allocated F-TEID of the eNodeB  20  may then be distributed towards the S-Gw  16  and P-Gw  16 ′ subsequently (e.g., in signaling steps  1 ) and  2 )). 
         [0069]    In a next signaling phase  3 ), the TMCF portion  18 A in the OSS  50  sends a probe tunnel set up instruction to the TMHF  22  in the eNodeB  20  (see step  304  in flow diagram  300 ). The tunnel set up instruction may be accompanied by the F-TEID assigned by the S-Gw  16  for the (probe) tunnel  24  to be set up and instructs the eNodeB  20  to locally set up the probe tunnel  24  towards the S-Gw  16 . The probe tunnel set up instruction received by the TMHF  22  of the eNodeB  20  (see step  402  of the flow diagram  400  of  FIG. 4 ) triggers the eNode  20  to set up the probe tunnel  24  towards the S-Gw based on the F-TEID associated with the S-Gw  16  as illustrated by step  404  of flow diagram  400  of  FIG. 4 . 
         [0070]    Signaling phases  4 ),  5 ) and  6 ) are the same as the corresponding signaling phases illustrated in  FIG. 5 . For this reason a more detailed description thereof is omitted here. 
         [0071]    According to the signaling diagram illustrated in  FIG. 7 , the TMCF  18  is realized in the MME  52 . In this implementation, the signaling with respect to probe tunnel set up is handled in the MME  52 , which has full control over the probe tunnel set up procedure. In a similar manner as described above with reference to  FIG. 6 , the TMHF  22  is only implemented in the eNodeB  20 , but not in the S-Gw  16  or the P-Gw  16 ′. 
         [0072]    The probe tunnel set up procedure may be triggered by receipt of an optional probe session set up request from the OSS  50  by the TMCF  18  in the MME  52 . In a signaling phase not illustrated in  FIG. 7 , the TMCF  18  in the MME  52  may then determine the F-TEIDs of all network nodes involved in the (probe) tunnel set up procedure. The subsequent signaling phases ( 1 ) to ( 5 ) corresponds to the signaling phase ( 2 ) to ( 6 ) discussed above with reference to  FIG. 6 , respectively, but now involving the TMCF  18  in the MME  52  (the F-TEID of the eNodeB  20  is allocated as part of signaling phase  1 ) as discussed above and communicated to the S-Gw  16  in signaling phase  2 ). For this reason, a more detailed description thereof is omitted here. 
         [0073]    As has become apparent from the above description of exemplary embodiments, the technique presented herein permits a comparatively simple set up of probe tunnels for network probing purposes. In certain scenarios (see  FIGS. 6 and 7 ), probe tunnel set up remains transparent for the core network nodes that form tunnel endpoints (such as the S-Gw and the P-Gw). Nonetheless, the probe traffic is actually routed through the S-Gw (or a functionally equivalent SGSN) and P-Gw (or a functionally equivalent GGSN), so that also path performance through such network nodes becomes possible to analyze. Additionally, network nodes (such as PDN servers) above the P-Gw (or GGSN) on the Gi interface can be monitored and anlysed with respect to, for example, path performance. Still further, since probe tunnel set up and start of probe traffic generation can be centralized (by involving one or more central control nodes such as the OSS and the MME), it is possible to perform coordinated network probing involving a plurality of radio access nodes at a time. 
         [0074]    The technique presented herein may be embodied in many different forms, not all of which are described above, and all such forms are contemplated to be within the scope of the present invention. The particular embodiments described above are merely illustrative and should not be considered restrictive in any way. The scope of the present invention is determined by the claims that follow, and all variations and equivalents that fall within the range of the claims are intended to be embraced therein.