Protocol Anomaly prevention options

Use protocol anomaly detection options to detect or deny protocol anomalies according to GTP standards and tunnel state. Protocol anomaly attacks involve malformed or corrupt packets that typically fall outside of the protocol specifications. Packets cannot pass through if they fail the sanity check.

Protocol Anomaly
Invalid Reserved Field	GTP version 0 (GSM 09.60) headers specify a number of fields that are marked as ”Spare” and contain all ones (1). GTP packets that have different values in these fields are flagged as anomalies. GTP version 1 (GSM 29.060) makes better use of the header space and only has one, 1bit, reserved field. In the first octet of the GTP version1 header, bit 4 is set to zero.
Reserved IE	Both versions of GTP allow up to 255 different Information Elements (IE). However, a number of Information Elements values are undefined or reserved. Packets with reserved or undefined values will be filtered.
Miss Mandatory IE	GTP packets with missing mandatory Information Elements (IE) will not be passed to the GGSN.
Out of State Message	The GTP protocol requires a certain level of state to be kept by both the GGSN and SGSN. Some message types can only be sent when in a specific GTP state. Packets that do not make sense in the current state are filtered or rejected Both versions of GTP allow up to 255 different message types. However, a number of message type values are undefined or reserved. Best practices dictate that packets with reserved or undefined values will be filtered.
Out of State IE	GTP Packets with out of order Information Elements are discarded.
Spoofed Source Address	The End User Address Information Element in the PDP Context Create & Response messages contain the address that the mobile station (MS) will use on the remote network. If the MS does not have an address, the SGSN will set the End User Address field to zero when sending the initial PDP Context Create message. The PDP Context Response packet from the GGSN will then contain an address to be assigned to the MS. In environments where static addresses are allowed, the MS will relay its address to the SGSN, which will include the address in the PDP Context Create Message. As the MS address is negotiated within the PDP Context creation handshake, any packets originating from the MS that contain a different source address are detected and dropped.

Encapsulated non-IP end user traffic filtering options

Depending on the installed environment, it may be beneficial to detect GTP packets that encapsulate non-IP based protocols. You can configure the FortiOS Carrier firewall to permit a list of acceptable protocols, with all other protocols denied.

The encoded protocol is determined in the PDP Type Organization and PDP Type Number fields within the End User Address Information Element. The PDP Type Organization is a 4-bit field that determines if the protocol is part of the ETSI or IETF organizations. Values are zero and one, respectively. The PDP Type field is one byte long. Both GTP specifications list only PPP, with a PDP Type value of one, as a valid ETSI protocol. PDP Types for the IETF values are determined in the “Assigned PPP DLL Protocol Numbers” sections of RFC1700. The PDP types are compressed, meaning that the most significant byte is skipped, limiting the protocols listed from 0x00 to 0xFF.

Encapsulated Non-IP End User Address Filtering

Enable Non-IP Filter                Select to enable encapsulated non-IP traffic filtering.

Default Non-IP Action Select the default action for encapsulated non-IP traffic filtering. If you select Allow, all sessions are allowed except those blocked by individual encapsulated non-IP traffic filters. If you select Deny, all sessions are blocked except those allowed by individual encapsulated non-IP traffic filters.

Type                                      The type chosen, AESTI or IETF.

Start Protocol                         The beginning protocol port number range.

End Protocol                          The end of the protocol port number range.

Action                                    The type of action that will be taken.

Modify a non-IP filter’s settings in the list. When you select Edit, the Edit Edit window appears, which allows you to modify the Non-IP policy settings.

Delete                                    Remove a non-IP policy from the list.

Add a new encapsulated non-IP traffic filter. When you select Add Non-IP Add Non-IP Policy Policy, you are automatically redirected to the New page.

New (window)

Type                                       Select AESTI or IETF.

Start Protocol                        Select a start and end protocol from the list of protocols in RFC 1700. Allowed range includes 0 to 255 (0x00 to 0xff). Some common protocols End Protocol                          include: •  33 (0x0021)   Internet Protocol •  35 (0x0023)   OSI Network Layer •  63 (0x003f)    NETBIOS Framing •  65 (0x0041)   Cisco Systems •  79 (0x004f)    IP6 Header Compression •  83 (0x0053)   Encryption Action                                    Select Allow or Deny.

Encapsulated IP traffic filtering options

You can use encapsulated IP traffic filtering to filter GTP sessions based on information contained in the data stream. to control data flows within your infrastructure. You can configure IP filtering rules to filter encapsulated IP traffic from mobile stations by identifying the source and destination policies. For more information, see When to use encapsulated IP traffic filtering.

Expand Encapsulated IP Traffic Filtering in the GTP profile to reveal the options.

Encapsulated IP Traffic Filtering

Enable IP Filter                       Select to enable encapsulated IP traffic filtering options.

Default IP Action Select the default action for encapsulated IP traffic filtering. If you select Allow, all sessions are allowed except those blocked by individual encapsulated IP traffic filters. If you select Deny, all sessions are blocked except those allowed by individual encapsulated IP traffic filters.

Select a source IP address from the configured firewall IP address or Source                                   address group lists. Any encapsulated traffic originating from this IP address will be a match if the destination also matches.

Destination                             Select a destination IP address from the configured firewall IP address or address group lists. Any encapsulated traffic being sent to this IP address will be a match if the destination also matches.

The type of action that will be taken. Action Select to Allow or Deny encapsulated traffic between this source and Destination.

Edit                                        Modifies the source, destination or action settings.

Adds a new encapsulated IP traffic filter. When you select Add IP Policy, Add IP Policy the New window appears which allows you to configure IP policy settings.

New (window)

Source                                  Select the source firewall address or address group.

Destination                            Select the destination firewall address or address group.

Action                                    Select Allow or Deny.

Information Element (IE) removal policy options

In some roaming scenarios, the unit is installed on the border of the PLMN and the GRX. In this configuration, the unit supports information element (IE) removal policies to remove any combination of R6 IEs (RAT, RAI, ULI, IMEI-SV and APN restrictions) from the types of messages described in “Advanced filtering options”, prior to forwarding the messages to the HGGSN (proxy mode).

IE removal policy
Enable	Select to enable this option.
SGSN address of message IE	The firewall address or address group that contains the SGSN addresses.
IEs to be removed	The IE types that will be removed. These include APN Restriction, RAT, RAI, ULI, and IMEI.
Add	Adds an IE removal policy. When you select Add, the New window appears, which allows you to configure the IE policy.
Edit	Modifies settings from within the IE removal policy. When you select Edit, the Edit window appears, which allows you to modify the settings within the policy.
Delete	Removes the IE removal policy from the list.
New IE policy page
SGSN address	Select a firewall address or address group that contains SGSN addresses.
IEs to be removed	Select one or more IE types to be removed. These include APN Restriction, RAT, RAI, ULI, and IMEI.

GTP Profile

You can configure multiple GTP profiles within the GTP menu. GTP profiles concern GTP activity flowing through the unit. These GTP profiles are then applied to a security policy.

GTP profile configuration settings

The following are GTP profile configuration settings in Security Profiles > GTP Profiles.

GTP Profile Lists each GTP profile that you have created. On this page, you can edit, delete or create a new GTP profile.

Creates a new GTP profile. When you select Create New, you are Create New automatically redirected to the New page.

Edit      Modifies settings within a GTP profile in the list. When you select Edit, you are automatically redirected to Edit page.

 

Removes a GTP profile from the list. To remove multiple GTP profiles from within the list, on the GTP Profile page, in each of the rows of the profiles you want removed, select the Delete check box and then select Delete. To remove all GTP profiles from within the list, on the GTP Profile page, select the check box in the check box column and then select Delete.
Name                                     The name of the GTP profile.
Displays the number of times the object is referenced to other objects. For example, av_1 profile is applied to a security policy; on the Profile page (Security Profiles > Antivirus > Profiles), 1 appears in Ref. . To view the location of the referenced object, select the number in Ref., and the Object Usage window appears displaying the various locations of the referenced object. To view more information about how the object is being used, use one of the following icons that is available within the Object Usage window: • View the list page for these objects – automatically redirects you to Ref. the list page where the object is referenced at. • Edit this object – modifies settings within that particular setting that the object is referenced with. For example, av_1 profile is referenced with a security policy and so, when this icon is selected, the user is redirected to the Edit Policy page. • View the details for this object – table, similar to the log viewer table, contains information about what settings are configured within that particular setting that the object is referenced with. For example, av_1 profile is referenced with a security policy, and that security policy’s settings appear within the table.
New GTP Profile Provides settings for configuring a GTP profile.
Name                                     Enter a name for the GTP profile.
General Settings                    Configure general options for the GTP profile.
Message Type Filtering          Configure filtering for messages.
APN Filtering                          Configure filtering options for APN.
Basic Filtering                        Configure filtering options for IMSI.
Advanced Filtering	Configure advanced filtering options.
IE removal policy	Configure IE removal policy options.
Encapsulated IP Traffic Filtering	Configure filtering options for encapsulated IP traffic.
Encapsulated Non-IP End User Address Filtering	Configure filtering options for encapsulated non-IP end user addresses.
Protocol Anomaly	Configure protocol anomaly options.
Anti-Overbilling	Configure anti-overbilling options.
Log	Configure log options.

General settings options

The following are mostly house keeping options that appear in the General Settings area of the GTP configuration page.

General Settings section of the New GTP Profile

GTP-in-GTP                            Select Allow to enable GTP packets to be allowed to contain GTP packets, or a GTP tunnel inside another GTP tunnel. To block all GTP-in-GTP packets, select Deny.

Enter the shortest possible message length in bytes. Normally this is controlled by the protocol, and will vary for different message types. If a packet is smaller than this limit, it is discarded as it is likely malformed and Minimum Message Length a potential security risk. The default minimum message length is 0 bytes.

Maximum Message Length      Enter the maximum allowed length of a GTP packet in bytes. A GTP packet contains three headers and corresponding parts GTP, UDP, and IP. If a packet is larger than the maximum transmission unit (MTU) size, it is fragmented to be delivered in multiple packets. This is inefficient, resource intensive, and may cause problems with some applications. By default the maximum message length is 1452 bytes.

 

General Settings section of the New GTP Profile

Enter the maximum number of tunnels allowed open at one time. For additional GTP tunnels to be opened, existing tunnels must first be closed. This feature can help prevent a form of denial of service attack on your network. This attack involves opening more tunnels than the network can Tunnel Limit handle and consuming all the network resources doing so. By limiting the number of tunnels at any one time, this form of attack will be avoided. The tunnel limiting applies to the Handover Group, and Authorized SGSNs and GGSNs.

Tunnel Timeout                      Enter the maximum number of seconds that a GTP tunnel is allowed to remain active. After the timeout the unit deletes GTP tunnels that have stopped processing data. A GTP tunnel may hang for various reasons. For example, during the GTP tunnel tear-down stage, the “delete pdap context response” message may get lost. By setting a timeout value, you can configure the FortiOS Carrier firewall to remove the hanging tunnels. The default is 86400 seconds, or 24 hours.

Enter the number of packets per second to limit the traffic rate to protect the GSNs from possible Denial of Service (DoS) attacks. The default limit of 0 does not limit the message rate. GTP DoS attacks can include: Control plane message rate limit • Border gateway bandwidth saturation: A malicious operator can connect to your GRX and generate high traffic towards your Border Gateway to consume all the bandwidth. • GTP flood: A GSN can be flooded by illegitimate traffic

Handover Group           Select the allowed list of IP addresses allowed to take over a GTP session when the mobile device moves locations. Handover is a fundamental feature of GPRS/UMTS, which enables subscribers to seamlessly move from one area of coverage to another with no interruption of active sessions. Session hijacking can come from the SGSN or the GGSN, where a fraudulent GSN can intercept another GSN and redirect traffic to it. This can be exploited to hijack GTP tunnels or cause a denial of service. When the handover group is defined it acts like a white list with an implicit default deny at the end — the GTP address must be in the group or the GTP message will be blocked. This stops handover requests from untrusted GSNs.

General Settings section of the New GTP Profile

Use Authorized SGSNs to only allow authorized SGSNs to send packets through the unit and to block unauthorized SGSNs. Go to Firewall Objects > Address > Addresses and add the IP addresses of the authorized SGSNs to a firewall address or address group. Then set Authorized SGSNs Authorized SGSNs to this firewall address or address group. You can use Authorized SGSNs to allow packets from SGSNs that have a roaming agreement with your organization.

Authorized GGSNs                 Use Authorized GGSNs to only allow authorized GGSNs to send packets through the unit and to block unauthorized GGSNs. Go to Firewall Objects > Address > Addresses and add the IP addresses of the authorized GGSNs to a firewall address or address group. Then set Authorized GGSNs to this firewall address or address group. You can use Authorized GGSNs to allow packets from SGSNs that have a roaming agreement with your organization.

Message type filtering options

On the New GTP Profile page, you can select to allow or deny the different types of GTP messages, which is referred to as message type filtering. You must expand the Message Type Filtering section to access the settings.

The messages types include Path Management, Tunnel Management, Location Management, Mobility Management, MBMS, and GTP-U and Charging Management messages.

For enhanced security, Fortinet best practices dictate that you set Unknown Message Action to deny. This will block all unknown GTP message types, some of which may be malicious.

To configure message type filter options, expand Message Type Filtering in the GTP profile.

APN filtering options

An Access Point Name (APN) is an Information Element (IE) included in the header of a GTP packet. It provides information on how to reach a network.

An APN has the following format:

<network_id>[.mnc<mnc_int>.mcc<mcc_int>.gprs] Where:

• <network_id> is a network identifier or name that identifies the name of a network, for example, com

or internet.

• [.mnc<mnc_int>.mcc<mcc_int>.gprs] is the optional operator identifier that uniquely identifies the operator’s PLMN, for example mcc456.gprs.

Combining these two examples results in a complete APN of internet.mnc123.mcc456.gprs.

By default, the unit permits all APNs. However, you can configure APN filtering to restrict roaming subscribers’ access to external networks.

APN filtering applies only to the GTP create pdp request messages. The unit inspects GTP packets for both APN and selected modes. If both parameters match and APN filter entry, the unit applies the filter to the traffic.

Additionally, the unit can filter GTP packets based on the combination of an IMSI prefix and an APN.

APN Filtering
Enable APN Filter	Select to enable APN filtering.
Default APN Action	Select the default action for APN filtering. If you select Allow, all sessions are allowed except those blocked by individual APN filters. If you select Deny, all sessions are blocked except those allowed by individual APN filters.
Value	The APN to be filtered.
Mode	The type of mode chosen that indicates where the APN originated and whether the Home Location Register (HLR) has verified the user subscription:
Action	The type of action that will be taken.
Edit	Modifies the settings within the filter. When you select Edit, the Edit window appears, which allows you to modify the settings of the APN.
Delete	Removes the APN from the list within the table, in the APN Filtering section.
Add APN	Adds a new APN filter to the list. When you select Add APN, the New window appears, which allows you to configure the APN settings.
New APN page
Value	Enter an APN to be filtered. You can include wild cards to match multiple APNs. For example, the value internet* would match all APNs that being with internet.
Mode	Select one or more of the available modes to indicate where the APN originated and whether the Home Location Register (HLR) has verified the user subscription.
Mobile Station provided		MS-provided APN, subscription not verified, indicates that the mobile station (MS) provided the APN and that the HLR did not verify the user’s subscription to the network.
Network provided		Network-provided APN, subscription not verified, indicates that the network provided a default APN because the MS did not specify one, and that the HLR did not verify the user’s subscription to the network.
Subscription Verified		MS or Network-provided APN, subscription verified, indicates that the MS or the network provided the APN and that the HLR verified the user’s subscription to the network
Action		Select Allow or Deny.

Basic filtering options

The International Mobile Station Identity (IMSI) is used by a GPRS Support Node (GSN) to identify a mobile station. Three elements make up every IMSI:

l the mobile country code (MCC) l the mobile network code (MNC) l the mobile subscriber identification number (MSIN).

The subscriber’s home network—the public land mobile network (PLMN)—is identified by the IMSI prefix, formed by combining the MCC and MNC.

By default, the unit allows all IMSIs. You can add IMSI prefixes to deny GTP traffic coming from non-roaming partners. Any GTP packets with IMSI prefixes not matching the prefixes you set will be dropped. GTP Create pdp request messages are filtered and only IMSI prefixes matching the ones you set are permitted. Each GTP profile can have up to 1000 IMSI prefixes set.

An IMSI prefix and an APN can be used together to filter GTP packets if you set an IMSI filter entry with a nonempty APN.

IMSI Filtering section of the New GTP Profile
Enable IMSI Filter                      Select to enable IMSI filtering.
Default IMSI Action Select the default action for IMSI filtering. If you select Allow, all sessions are allowed except those blocked by individual IMSI filters. If you select Deny, all sessions are blocked except those allowed by individual IMSI filters.
APN                                          The APN that is part of the IMSI that will be filtered.
MCC-MNC	The MCC-MNC part of the IMSI that will be filtered.
Mode	The type of mode that indicates where the APN originated and whether the Home Location Register (HLR) has verified the user subscription.
Action	The type of action that will be taken.
Edit	Modifies settings to an IMSI filter. When you select Edit, the Edit window appears, which allows you to modify the IMSI filter’s settings.
Delete	Removes an IMSI filter from within the table, in the IMSI Filtering section.
Add IMSI	Adds a new IMSI filter to the list. When you select Add IMSI, the New window appears, which allows you to configure IMSI filter settings.
New IMSI page
APN	Enter the APN part of the IMSI to be filtered.
MCC-MNC	Enter the MCC-MCC part of the IMSI to be filtered.
Mode	Select one or more of the available modes to indicate where the APN originated and whether the Home Location Register (HLR) has verified the user subscription.
Mobile Station provided	MS-provided APN, subscription not verified, indicates that the mobile station (MS) provided the APN and that the HLR did not verify the user’s subscription to the network.
Network provided	Network-provided APN, subscription not verified, indicates that the network provided a default APN because the MS did not specify one, and that the HLR did not verify the user’s subscription to the network.
Subscription Verified	MS or Network-provided APN, subscription verified, indicates that the MS or the network provided the APN and that the HLR verified the user’s subscription to the network
Action	Select Allow or Deny.

Advanced filtering options

The FortiOS Carrier firewall supports advanced filtering against the attributes RAT, RAI, ULI, APN restriction, and IMEI-SV in GTP to block specific harmful GPRS traffic and GPRS roaming traffic. The following table shows some of the GTP context requests and responses that the firewall supports.

Attributes supported by FortiCarrier firewalls

	GTP Create PDP Context Request	GTP Create PDP Context Response	GTP Update PDP GTP Update PDP Con- Context text Request Response
APN	yes	yes	–
APN Restriction	yes	–	–	yes
IMEI-SV	yes	–	–	–
IMSI	yes	–	yes	–
RAI	yes	–	yes	–
RAT	yes	–	yes	–
ULI	yes	–	yes	–

When editing a GTP profile, select Advanced Filtering > Create New to create and add a rule. When the rule matches traffic it will either allow or deny that traffic as selected in the rule.

Advanced Filtering
Enable	Select to enable advanced filtering.
Default Action	Select the default action for advanced filtering. If you select Allow, all sessions are allowed except those blocked by individual advanced filters. If you select Deny, all sessions are blocked except those allowed by individual advanced filters.
Messages	The messages, for example, Create PDP Context Request.
APN Restriction	The APN restriction.
RAT Type	The RAT types associated with that filter.
ULI	The ULI pattern.
RAI	The RAI pattern.
IMEI	The IMEI pattern.
Action	The action that will be taken.
Edit	Modifies the filter’s settings. When you select Edit, the Edit window appears, which allows you to modify the filter’s settings.

 

Delete	Removes a filter from the list.
Add	Adds a filter to the list. When you select Add, the New window appears, which allows you to configure settings for messages, APN, IMSI, MSISDN, RAT type, ULI, RAI, IMEI patterns as well as the type of action.
New Filtering page
Messages	The PDP content messages this profile will match.
Create PDP Context Request	Select to allow create PDP context requests.
Create PDP Context Response	Select to allow create PDP context responses.
Update PDP Context Request	Select to allow update PDP context requests.
Update PDP Context Response	Select to allow update PDP context responses.
APN	Enter the APN.
APN Mode	Select an APN mode as one or more of •  Mobile Station provided •  Network provided •  Subscription provided This field is only available when an APN has been entered.
Mobile Station provided	MS-provided PAN, subscription not verified, indicates that the mobile station (MS) provided the APN and that the HLR did not verify the user’s subscription to the network.
Network provided	Network-provided APN, subscription not verified, indicates that the network provided a default APN because the MS did not specify one, and that the HLR did no verify the user’s subscription to the network.
Subscription verified	MS or Network-provided APN, subscription verified, indicates that the MS or the network provided the APN and that the HLR verified the user’s subscription to the network.
APN Restriction	Select the type of restriction that you want. You can choose all of the types, or one of the types. You cannot choose multiple types. Types include: •  all •  Public-1 •  Public-2 •  Private-1 •  Private-2
IMSI	Enter the IMSI.
MSISDN	Enter the MSISDN.
RAT Type	Optionally select the RAT type as any combination of the following: •  Any •  UTRAN •  GERAN •  Wifi •  GAN •  HSPA Some RAT types are GTPv1 specific.
ULI pattern	Enter the ULI pattern.
RAI pattern	Enter the RAI pattern.
IMEI pattern	Enter the IMEI pattern.
Action	Select either Allow or Deny.

Adding an advanced filtering rule

When adding a rule, use the following formats:

• Prefix, for example, range 31* for MCC matches MCC from 310 to 319. l Range, for example, range 310-319 for MCC matches MCC from 310 to 319.
• Mobile Country Code (MCC) consists of three digits. The MCC identifies the country of domicile of the mobile subscriber.
• Mobile Network Code (MNC) consists of two or three digits for GSM/UMTS applications. The MNC identifies the home PLMN of the mobile subscriber. The length of the MNC (two or three digits) depends on the value of the MCC. Best practices dictate not to mix two and three digit MNC codes within a single MCC area. l Location Area Code (LAC) is a fixed length code (of 2 octets) identifying a location area within a PLMN. This part of the location area identification can be coded using a full hexadecimal representation except for the following reserved hexadecimal values: 0000 and FFFE. These reserved values are used in some special cases when no valid LAI exists in the MS (see 3GPP TS 24.008, 3GPP TS 31.102 and 3GPP TS 51.011).
• Routing Area Code (RAC) of a fixed length code (of 1 octet) identifies a routing area within a location. l CI or SAC of a fixed length of 2 octets can be coded using a full hexadecimal expression.
• Type Allocation Code (TAC) has a length of 8 digits.
• Serial Number (SNR) is an individual serial number identifying each equipment within each TAC. SNR has a length of 6 digits. l Software Version Number (SVN) identifies the software version number of the mobile equipment. SVN has a length of 2 digits.

GTP Configuration

The GTP (GPRS Tunneling Protocol) is one of the major mobile core protocols used since to transfer data in the core mobile network. Mobility and data are exploding and this trend will continue with VoLTE, 5G, and the Internet of Things (IoT). The role of GTP in mobile networks will continue to remain critical.

With the mobile network ever growing importance as the communication channel for data rich application on mobile devices, connected intelligent devices and the IoT, comes the growing potential for attacks on the mobile infrastructure.

Introduction to GTP

GTP as a Potential Attack Vector

GTP’s role in transferring data in the core mobile infrastructure makes it a potential ideal attack vector. To understand the security features for GTP we need to understand the risks that might compromise this protocol. The business impact might varies in-between the different attacks from Denial of Service (DoS) attacks that hinders the capability of performing a legitimate operation due to resource starvation (for example – not being able to charge the customer for GPRS traffic use due to denial of service attack on the Charging GW) to remote compromise attacks that allows the hacker to have remote control of a critical device (for example – take control over a GGSN).

GTP-based attacks may have a wide range of business impact, based on the attacked devices’ vulnerability, ranging from service unavailability, compromise customer information, and gaining control over infrastructure elements, just to give a few examples.

Listed below are the main categories of GTP-based attacks:

• Protocol anomaly attacks are packets and packets formats that should not be expected on the GTP protocol. These can include malformed packets, reserved packets’ fields and types, etc.
• Infrastructure attacks are attempts to connect to restricted core elements, such as the GGSN, SGSN, PGW, etc. l Overbilling attacks results in customers charged for traffic they did not use or the opposite of not paying for the used traffic.

Protecting Against GTP-Based Attacks: The Carrier Grade GTP Firewall

With the evolution of the mobile network so has GTP evolved. The awareness to the potential of GTP-based attacks has led mobile core vendors to harden their software to better deal with a potential attack. Alongside this evolution, network security vendors, such as Fortinet, has led the way in providing specific GTP aware firewalls to secure and protect the different versions of the GTP protocol from potential attacks.

A GTP firewall should be placed where GTP traffic and session originate and terminate, as shown in the below diagram, and has to inspect both the GTP-C (Control Plane) and GTP-U (Data Plane) packets that, together, constitute the GPRS Tunneling Protocol.

The GTP firewall in both cases is placed in line between the SGSN / SGW and the GGSN / PGW which are the initiator and terminator of the GTP traffic. One of the main roles of GTP firewall is also to be able to support the roaming between different versions of GTP without interrupting the service.

The GTP firewall must be carrier grade in its ability to scale and provide high availability without impact its ability to provide effective protection.

FortiGate with FortiCarrier – The Leading GTP Firewall

FortiGate is Fortinet’s physical security platform, built specifically for high performance and scalability with the utilization of specialized FortiASIC technology. Fortinet Content Processors (CP) and Network Processors (NP) enable, offloading CPU intensive tasks and allowing the FortiGate to provide carrier grade performance and scalability. Utilizing the power of the FortiGate platform, FortiOS, Fortinet’s security Operating System, provides threat intelligence and advanced functionalities to provide effective security, ranging from Carrier Grade NAT (CGNAT), firewalling, IPSec, etc.

FortiCarrier is the part of FortiOS which was specifically designed to provide security for specific carriers and mobile operators’ protocols and requirements, such as awareness and security for GTP. The wide range of FortiGate platforms with FortiOS and FortiCarrier enables mobile operators to cost effectively secure their mobile network against GTP-based attacks, while ensuring unparalleled performance, availability and security effectiveness.

GPRS network common interfaces

There are interfaces for each connection on the GPRS network. An interface is an established standard form of communication between two devices. Consider a TCP/IP network. In addition to the transport protocol (TCP) there are other protocols on that network that describe how devices can expect communications to be organized, just like GPRS interfaces.

GPRS network common interfaces

Interfaces between devices on the network

There are a series of interfaces that define how different devices on the carrier network communicate with each other. There interfaces are called Ga to Gz, and each one defines how a specific pair of devices will communicate. For example Gb is the interface between the base station and the SGSN, and Gn is one possible interface between the SGSN and GGSN.

The SGSN and GGSN keep track of the CDR information and forward it to the Charging Data Function (CDF) using the Gr interface between the SGSN and home location register (HLR), Gs interface between the SGSN and MSC (VLR), Gx interface between the GGSN and the Charging Rules Function (CRF), Gy between the GGSN and online charging system (OCS), and finally Gz which is the off-line (CDR-based) charging interface between the GSN and the CG that uses GTP’.

Each of these interfaces on the GPRS network is has a name in the format of Gx where x is a letter of the alphabet that determines what part of the network the interface is used in. It is common for network diagrams of GPRS networks to include the interface name on connections between devices.

GPRS network interfaces, their roles, and billing

Name	Device connections that use Traffic Protocol this interface                          used		Its role or how it affects billing
Ga	CDR and GSN (SGSNs and GGSNs)	GTP‘ – GTP modified to include CDR role	CDR have the accounting records, that are compiled in the GSN and then sent to the Charging Gateway (CG)
Gb	MS and SGSN	Frame Relay or IP	When an IP address moves to a new MS, the old MS may continue to use and bill that IP address.
Gi	GGSN and public data networks (PDNs)	IP based	This is the connection to the Internet. If the GTP tunnel is deleted without notifying the Gi interface, the connection may remain open incurring additional charges. FortiOS Carrier adds this interface to a firewall. See Anti-overbilling with FortiOS Carrier.
Gn	SGSN and external SGSNs and internal GGSNs	GTP	When the GTP tunnel is deleted, need to inform other interfaces immediately to prevent misuse of connections remaining
Gp	Internal SGSN and external GGSNs	GTP	open. FortiOS Carrier adds this interface to a firewall.
Gz	GSN (SGSN and GGSN) and the charging gateway (CG)	GTP‘	Used for the offline charging interface. Ga is used for online charging.

GPRS network common interfaces

Corporate customers may have a direct connection to the Gi interface for higher security. The Gi interface is normally an IP network, though a tunnelling protocol such as GRE or IPsec may be used instead.

 

Introduction to GTP

GTP basic concepts

GPRS currently supports data rates from 9.6 kbps to more than 100 kbps, and is best suited for burst forms of traffic. GPRS involves both radio and wired components. The mobile phone sends the message to a base station unit (radio based), and the base station unit sends the message to the carrier network and eventually the Internet (wired carrier network).

The network system then either sends the message back to a base station and to the destination mobile unit, or forwards the message to the proper carrier’s network where it gets routed to the mobile unit.

PDP Context

The packet data protocol (PDP) context is a connection between a mobile station and the end address that goes through the SGSN and GGSN. It includes identifying information about the mobile customer used by each server or device to properly forward the call data to the next hop in the carrier network, typically using a GTP tunnel between the SGSN and GGSN.

When a mobile customer has an active voice or data connection open, both the SGSN and GGSN have the PDP context information for that customer and session.

When a mobile phone attempts to communicate with an address on an external packet network, either an IP or X.25 address, the mobile station that phone is connected to opens a PDP context through the SGSN and GGSN to the end address. Before any traffic is sent, the PDP context must first be activated.

The information included in the PDP context includes the customer’s IP address, the IMSI number of the mobile handset, and the tunnel endpoint ID for both the SGSN and GGSN. The ID is a unique number, much like a session ID on a TCP/IP firewall. All this information ensures a uniquely identifiable connection is made.

Since one mobile device may have multiple connections open at one time, such as data connections to different Internet services and voice connections to different locations, there may be more than one PDP context with the same IP address making the extra identifying information required.

The endpoint that the mobile phone is connecting to only knows about the GGSN — the rest of the GPRS connection is masked by the GGSN.

Along the PDP context path, communication is accomplished in using three different protocols.

l The connection between the Mobile Station and SGSN uses the SM protocol. l Between SGSN and GGSN GTP is used. l Between GGSN and the endpoint either IP or X.25 is used.

FortiOS Carrier is concerned with the SGSN to GGSN part of the PDP context — the part that uses GTP.

For more about PDP context, see Tunnel Management Messages.

Creating a PDP context

While FortiOS Carrier is concerned mostly with the SGSN to GGSN part of the PDP Context, knowing the steps involved in creating a PDP context helps understand the role each device, protocol, and message type plays.

Both mobile stations and GGSNs can create PDP contexts.

A Mobile Station creates a PDP context

1. The Mobile Station (MS) sends a PDP activation request message to the SGSN including the MS PDP address, and APN.
2. Optionally, security functions may be performed to authenticate the MS.
3. The SGSN determines the GGSN address by using the APN identifier.
4. The SGSN creates a down link GTP tunnel to send IP packets between the GGSN and SGSN.
5. The GGSN creates an entry in its PDP context table to deliver IP packets between the SGSN and the external packet switching network.
6. The GGSN creates an uplink GTP tunnel to route IP-PDU from SGSN to GGSN.
7. The GGSN then sends back to the SGSN the result of the PDP context creation and if necessary the MS PDP address.
8. The SGSN sends an Activate PDP context accept message to the MS by returning negotiated the PDP context information and if necessary the MS PDP address.
9. Now traffic can pass from the MS to the external network endpoint.

A GGSN creates a PDP context

1. The network receives an IP packet from an external network.
2. The GGSN checks if the PDP Context has already been created.
3. If not, the GGSN sends a PDU notification request to the SGSN in order to initiate a PDP context activation.
4. The GGSN retrieves the IP address of the appropriate SGSN address by interrogating the HLR from the IMSI identifier of the MS.
5. The SGSN sends to the MS a request to activate the indicated PDP context.
6. The PDP context activation procedure follows the one initiated by the MS. See “A Mobile Station creates a PDP context”.
7. When the PDP context is activated, the IP packet can be sent from the GGSN to the MS.

Terminating a PDP context

A PDP context remains open until it is terminated. To terminate the PDP context an MS sends a Deactivate PDP context message to the SGSN, which then sends a Delete PDP Context message to the GGSN.

When the SGSN receives a PDP context deletion acknowledgment from the GGSN, the SGSN confirms to the MS the PDP context deactivation. The PDP can be terminated by the SGSN or GGSN as well with a slight variation of the order of the messages passed.

When the PDP Context is terminated, the tunnel it was using is deleted as well. If this is not completed in a timely manner, it is possible for someone else to start using the tunnel before it is deleted. This hijacking will result in the original customer being over billed for the extra usage. Anti-overbilling helps prevent this. See Configuring Anti-overbilling in FortiOS Carrier.

GPRS security

The GPRS network has some built-in security in the form of GPRS authentication. However this is minimal, and is not sufficient for carrier network security needs. A GTP firewall, such as FortiOS Carrier, is required to secure the Gi, Gn, and Gp interfaces.

GPRS authentication

GPRS authentication is handled by the SGSN to prevent unauthorized GPRS calls from reaching the GSM network beyond the SGSN (the base station system, and mobile station). Authentication is accomplished using some of the customer’s information with a random number and uses two algorithms to create ciphers that then allow authentication for that customer.

User identity confidentiality ensures that customer information stays between the mobile station and the SGSN — no identifying information goes past the SGSN. Past that point other numbers are used to identify the customer and their connection on the network.

Periodically the SGSN may request identity information from the mobile station to compare to what is on record, using the IMEI number.

Call confidentiality is achieved through the use of a cipher, similar to the GPRS authentication described earlier. The cipher is applied between the mobile station and the SGSN. Essentially a cipher mask is XORd with each outgoing frame, and the receiving side XORs with its own cipher to result in the original frame and data.

Parts of a GTPv1 network

A sample GTP network consists of the end handset sender, the sender’s mobile station, the carrier’s network including the SGSN and GGSN, the receiver’s mobile station, and the receiver handset.

When a handset moves from one mobile station and SGSN to another, the handset’s connection to the Internet is preserved because the tunnel the handset has to the Internet using GTP tracks the user’s location and information. For example, the handset could move from one cell to another, or between countries.

The parts of a GPRS network can be separated into the following groups according to the roles of the devices:

• Radio access to the GPRS network is accomplished by mobile phones and mobile stations (MS).
• Transport the GPRS packets across the GPRS network is accomplished by SGSNs and GGSNs, both local and remote, by delivering packets to the external services. l Billing and records are handled by CDF, CFR, HLR, and VLR devices.

GPRS networks also rely on access points and PDP contexts as central parts of the communication structure. These are not actual devices, but they are still critical .

These devices, their roles, neighboring devices, the interfaces and protocols they use are outlined in the following table.

Carrier network showing the interfaces used (GTPv1)

Devices on a GTPv1 network

Device role	Neighboring Devices	Interfaces used	Protocols used
Mobile Users	Mobile Stations (MS)	Radio Access Technology (RAT)
Mobile Stations (MS)	Mobile Users, SGSN	Gb	IP, Frame Relay
SGSN (local)	MS, SGSN (local or remote), GGSN (local and remote), CDR, CFR, HLR, VLR	Ga, Gb, Gn, Gp, Gz	IP, Frame Relay, GTP, GTP’
SGSN (remote)	SGSN (local)	Gn	GTP
GGSN (local)	SGSN (local or remote), GGSN (local and remote), CDR, CFR, HLR, VLR	Ga, Gi, Gn, Gp, Gz	IP, GTP, GTP’
GGSN (remote)	SGSN (local), WAP gateway, Internet, other external services	Gi, Gp	IP, GTPv1
CDR, CFR	SGSN (local), GGSN (local)	Ga, Gz	GTP’
HLR, VLR	SGSN (local), GGSN (local)	Ga, Gz	GTP’

Radio access

For a mobile phone to access the GPRS core network, it must first connect to a mobile station. This is a cellular tower that is connected to the carrier network.

How the mobile phone connects to the mobile station (MS) is determined by what Radio Access Technologies (RATs) are supported by the MS.

Transport

Transport protocols move data along the carrier network between radio access and the Internet or other carrier networks.

FortiOS Carrier should be present where information enters the Carrier network, to ensure the information entering is correct and not malicious. This means a Carrier-enabled FortiGate unit intercepts the data coming from the SGSN or foreign networks destined for the SSGN or GGSN onto the network, and after the GGSN as the data is leaving the network.

GTP

GPRS Tunnelling Protocol (GTP) is a group of IP-based communications protocols used to carry General Packet

Radio Service (GPRS) within Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) networks. It allows carriers to transport actual cellular packets over a network via tunneling. This tunneling allows users to move between SGSNs and still maintain connection to the Internet through the GGSN.

GTP has three versions version 0, 1, and 2. GTP1 and GTP2 are supported by FortiOS Carrier. The only GTP commands that are common to all forms of GTP are the echo request/response commands that allow GSNs to verify up to once every 60 seconds that neighboring GSNs are alive.

GTPv0

There have been three versions of GTP to date. The original version of GTP (version 0) has the following differences from version GTPv1.

l the tunnel identification is not random l there are options for transporting X.25 l the fixed port number 3386 is used for all functions, not just charging l optionally TCP is allowed as a transport instead of UDP l not all message types are supported in version 0

GTPv1

On a GPRS network, Packet Data Protocol (PDP) context is a data structure used by both the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). The PDP context contains the subscribers information including their access point, IP address, IMSI number, and their tunnel endpoint ID for each of the

SGSN and GGSN.

The Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its geographical service area. Its tasks include packet routing and transfer, mobility management

(attach/detach and location management), logical link management, and authentication and charging functions.

The location register of the SGSN stores location information (e.g., current cell, current VLR) and user profiles (e.g., IMSI, address(es) used in the packet data network) of all GPRS users registered with this SGSN.

GTPv1-C

GTPv1-C refers to the control layer of the GPRS Transmission network. This part of the protocol deals with network related traffic.

FortiOS Carrier handles GTPv1-C in GTPv1 by using the Tunnel Endpoint IDentifier (TEID), IP address and a Network layer Service Access Point Identifier (NSAPI), sometimes called the application identifier, as an integer value that is part of the PDP context header information used to identify a unique PDP context in a mobile station, and SGSN.

For more information on GTPv1-C, see GTP-C messages.

GTPv1-U

GTPv1-U is defined in 3GPP TS 29.281 and refers to the user layer of the GPRS Tunneling network. This part of the protocol deals with user related traffic, user tunnels, and user administration issues.

A GTPv1-U tunnel is identified by a TEID, an IP address, and a UDP port number. This information uniquely identifies the limb of a GTPv1 PDP context. The IP address and the UDP port number define a UDP/IP path, a connectionless path between two endpoints (i.e. SGSN or GGSN). The TEID identifies the tunnel endpoint in the receiving GTPv1-U protocol entity; it allows for the multiplexing and demultiplexing of GTP tunnels on a UDP/IP path between a given GSN-GSN pair. For more information on GTPv1-U, see GTP-U messages.

The GTP core network consists of one or more SGSNs and GGSNs.

GGSN

The Gateway GPRS Support Node (GGSN) connects the GPRS network on one side via the SGSN to outside networks such as the Internet. These outside networks are called packet data networks (PDNs). The GGSN acts as an edge router between the two different networks — the GGSN forwards incoming packets from the external PDN to the addressed SGSN and the GGSN also forwards outgoing packets to the external PDN. the GGSN also converts the packets from the GPRS packets with SGSN to the external packets, such as IP or X.25.

SGSN

The Serving GPRS Support Node (SGSN) connects the GPRS network to GTPv1 compatible mobile stations, and mobile units (such as UTRAN and ETRAN) on one side and to the gateway node (GGSN), which leads to external networks, on the other side. Each SGSN has a geographical area, and mobile phones in that area connect to the GPRS network through this SGSN. The SGSN also maintains a location register that contains customer’s location and user profiles until they connect through a different SGSN at which time the customer information is moved to the new SGSN. This information is used for packet routing and transfer, mobility management also known as location management, logical link management, and authentication and billing functions.

GTPv2

GTPv2, defined in 3GPP TS 29.274, is dramatically different from GTPv1, defined in 3GPP TS 29.060. Where in

GTPv1 the tunnel is between the SGSN and the GGSN, in GTPv2 The SGSN is between the MME and the LTE Serving Gateway (S-GW), beyond which is the PDN gateway (P-GW). Even tunnel management messages have changed significantly.

Network diagram for GTPv2

Device roles on a GTPv2 network

Device role	Neighboring Devices	Interfaces used	Protocols used
Mobile Users	Mobile Stations (MS)	Radio Access Technology (RAT)	—
GTPv1 Mobile Stations (MS)	Mobile Users, SGSN	Gb	IP, Frame Relay
GTPv2 Mobile Stations (MS)	Mobile Users, MME	???	IP, Frame Relay
SGSN (local)	GTPv1 MS, SGSN, S-GW	???	IP, Frame Relay, GTPv1, GTP’
S-GW	SGSN, MME, P-GW	???	IP, GTPv2, GTP’
P-GW	S-GW, Internet, other external services	???	IP, GTPv2

GTPv2-C

GTPv2-C is the control layer messaging for GTPv2. It is used by LTE mobile stations, SGSN units for backwards compatibility, and SGWs that are the gateway to other networks. The messaging is very different from GTPv1. GTPv2-C is required to communicate with the Mobility Management Entity (MME) to create, change and delete EPS bearers when handover events happen, and to create Forwarding tunnels. The protocol is also used to communicate with the Serving Gateway (SGW) which has the S-GW and PDN-GW interfaces, and the Serving GPRS Support Node (SGSN).

MME

MME essentially fills the role of the SGSN in a GTPv1 network — it is how the mobile stations gain access to the

Carrier network. GTPv2 supports different mobile stations than GTPv1, so MME handles the GTPv2 MSes and SGSN handles the GTPv1 MSes

 

GPRS network common interfaces

Billing and records

A major part of the GPRS network is devoted to billing. Customer billing requires enough information to identify the customer, and then billing specific information such as connection locations and times, as well as amount of data transferred. A modified form of GTP called GTP’ is used for billing. The home location records and visitor location records store information about customers that is critical to billing.

GTP’ (GTP prime)

GTP is used to handle tunnels of user traffic between SGSNs and GGSNs. However for billing purposes, other devices that are not supported by GTP are required. GTP’ (GTP prime) is a modified form of GTP and is used to communicate with these devices such as the Charging Data Function (CDF) that communicates billing information to the Charging Gateway Function (CGF). In most cases, GTP‘ transports user records from many individual network elements, such as the GGSNs, to a centralism computer which then delivers the charging data more conveniently to the network operator’s billing center, often through the CGF. The core network sends charging information to the CGF, typically including PDP context activation times and the quantity of data which the end user has transferred.

GTP’ is used by the Ga and Gz interfaces to transfer billing information. GTP’ uses registered UDP/TCP port 3386. GTP’ defines a different header, additional messages, field values, as well as a synchronization protocol to avoid losing or duplicating CDRs on CGF or SGSN/GGSN failure. Transferred CDRs are encoded in ASN.1.

HLR

The Home Location Register (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. There can be several logical, and physical, HLRs per public land mobile network (PLMN), though one international mobile subscriber identity (IMSI)/MSISDN pair can be associated with only one logical HLR (which can span several physical nodes) at a time. The HLRs store details of every SIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is the primary key to each HLR record.

VLR

The Visitor Location Register (VLR) is a database which stores information about all the mobile devices that are currently under the jurisdiction of the Mobile Switching Center which it serves. Of all the information the VLR stores about each Mobile Station, the most important is the current Location Area Identity (LAI). This information is vital in the call setup process.

Whenever an MSC detects a new MS in its network, in addition to creating a new record in the VLR, it also updates the HLR of the mobile subscriber, informing it of the new location of that MS.

For more information on GTP‘, see GTP-U and Charging Management Messages.