MMS Concepts

MMS background

MMS is a common method for mobile users to send and receive multimedia content. A Carrier network supports MMS across its network. This makes up the MMS Service Provider Network (MSPN).

Messages can be sent or received between the MMSC and a number of other services including the Internet, content providers, or other carriers. Each of these different service connections uses different MMS formats including MM1 and MM7 messages (essentially HTTP format), and MM3 and MM4 messages (SMTP formatted). These different formats reflect the different purposes and content for each type of MMS message.

MMS content interfaces

MMS content interfaces

MMS messages are sent from devices and servers to other devices and servers using MMS content interfaces

There are eight interfaces defined for the MMS standard, referred to as MM1 through MM8. The most important of these interfaces for the transfer of data is the MM1 interface, as this defines how mobile users communicate from the mobile network to the Multimedia Message Service Center (MMSC). MMS content to be monitored and controlled comes from these mobile users and is going to the provider network.

Other MMS content interfaces that connect a service provider network to other external sources can pose threats as well. MM3 handles communication between the Internet and the MMSC and is a possible source of viruses and other content problems from the Internet. MM4 handles communication between different content provider MMSCs. Filtering MM4 content protects the service provider network from content sent from foreign service providers and their subscribers. Finally MM7 is used for communication between content providers and the MMSC. Filtering MM3 content can also keep harmful content off of the service provider network.

MMS content interfaces

How MMS content interfaces are applied

As shown below, the sender’s mobile device encodes the MMS content in a form similar to MIME email message (MMS MIME content formats are defined by the MMS Message Encapsulation specification). The encoded message is then forwarded to the service provider’s MMSC. Communication between the sending device and the MMSC uses the MM1 content interface. The MM1 content interface establishes a connection and sends an MM1 send request (m-send.req) message that contains the MMS message. The MMSC processes this request and sends back an MM1 send confirmation (m-send.conf) HTTP response indicating the status of the message — accepted or an error occurred, for example.

MM1 transactions between senders and receivers and the MMSC

If the recipient is on another carrier, the MMSC forwards the message to the recipient’s carrier. This forwarding uses the MM4 content interface for forwarding content between operator MMSCs (see the figure below).

Before the MMSC can forward the message to the final recipient, it must first determine if the receiver’s handset can receive MMS messages using the MM1 content interface. If the recipient can use the MM1 content interface, the content is extracted and sent to a temporary storage server with an HTTP front-end.

To retrieve the message, the receiver’s handset establishes a connection with the MMSC. An HTTP get request is then sent from the recipient to the MMSC. This message contains the URL where the content of the message is stored. The MMSC responds with a retrieve confirmation (m-retrieve.conf) HTTP response that contains the message.

MM4 messages sent between operator MMSCs

                                                                               Receiving Operator

MMSC                                                                                                        MMSC

This causes the receiver’s handset to retrieve the content from the embedded URL. Several messages are exchanged to indicate status of the delivery attempt. Before delivering content, some MMSCs also include a content adaptation service that attempts to modify the multimedia content into a format suitable for the recipient’s handset.

If the receiver’s handset is not MM1 capable, the message can be delivered to a web based service and the receiver can view the content from a normal Internet browser. The URL for the content can be sent to the receiver in an SMS text message. Using this method, non-MM1 capable recipients can still receive MMS content.

The method for determining whether a handset is MMS capable is not specified by the standards. A database is usually maintained by the operator, and in it each mobile phone number is marked as being associated with a legacy handset or not. It can be a bit hit and miss since customers can change their handset at will and this database is not usually updated dynamically.

Email and web-based gateways from MMSC to the Internet use the MM3 content interface. On the receiving side, the content servers can typically receive service requests both from WAP and normal HTTP browsers, so delivery via the web is simple. For sending from external sources to handsets, most carriers allow MIME encoded message to be sent to the receiver’s phone number with a special domain.

How FortiOS Carrier processes MMS messages

MMS messages can be vectors for propagating undesirable content such as spam and viruses. FortiOS Carrier can scan MMS messages sent using the MM1, MM3, MM4, and MM7 content interfaces. You can configure FortiOS Carrier to scan MMS messages for spam and viruses by configuring and adding MMS protection profiles and adding the MMS protection profiles to security policies. You can also use MMS protection profiles to apply content blocking, carrier endpoint filtering, MMS address translation, sending MMS notifications, DLP archiving of MMS messages, and logging of MMS message activity.

FortiOS Carrier MMS processing

FortiOS Carrier can send MMS messages to senders informing those senders that their devices are infected. FortiOS Carrier can also send MMS notifications to administrators to inform them of suspicious activity on their networks.

For message floods and duplicate messages, FortiOS Carrier does not send notifications to message senders but does send notifications to administrators and sends messages to sender handsets to complete MM1 and MM4 sessions.

Where MMS messaging uses the TCP/IP set of protocols, SMS text messaging uses the Signaling System Number 7 (SS7) set of protocols, which is not supported by FortiOS.

FortiOS Carrier and MMS content scanning

The following section applies to MMS content scanning, including virus scanning, file filtering, content spam filtering, carrier endpoint filtering, and MMS content checksum filtering.

MM1 Content Scanning

During MM1 content scanning a message is first transmitted from the sender, establishing a connection with the MMSC. FortiOS Carrier intercepts this connection and acts as the endpoint. FortiOS Carrier then establishes its own connection to the MMSC. Once connected, the client transmits its m-send.req HTTP post request to FortiOS Carrier which scans it according to the MMS protection profile settings. If the content is clean, the message is forwarded to the MMSC. The MMSC returns m-send.conf HTTP response through FortiOS Carrier to the sender.

If FortiOS Carrier blocks the message (for example because a virus was found, see the figure below), FortiOS Carrier resets the connection to the MMSC and sends m-send.conf HTTP response back to the sender. The response message can be customized using replacement messages. FortiOS Carrier then terminates the connection. Sending back an m-send.conf message prevents the sender from trying to send the message again.

MM1                            message sent by sender (blocking m.send.req messages)

FortiOS Carrier also sends m-send.rec notifications messages to the MMSC that are then forwarded to the sender to notify them of blocked messages.

Filtering message retrieval

FortiOS Carrier intercepts the connection to the MMSC, and the m-retrieve.conf HTTP response from the MMSC is scanned according to the MMS content scanning settings. If the content is clean, the response is forwarded back to the client. If the content is blocked, FortiOS Carrier drops the connection to the MMSC. It then builds an m-retrieve.conf message from the associated replacement message and transmits this back to the client.

FortiOS Carrier also sends m-send.rec notifications messages to the MMSC that are then forwarded to the receiver to notify them of blocked messages.

MM1                                             received by receiver (blocking m.retrieve.conf messages)

Filtering MM3 and MM4 messages works in an similar way to MM1 (see the figures below). FortiOS Carrier intercepts connections to the MMSC, and scans messages as configured. When messages are blocked, FortiOS Carrier closes sessions as required, sends confirmation messages to the sender, notifies administrators, and notifies senders and receivers of messages.

MM3                                                    from a sender on the Internet to an MMSC

1. Open TCP session
2. Send full email message
3. Content blocked
4. Send 550 Error and replacement message
5. MM3 notification message

Sent once per notification period, regardless of how many messages are blocked

         MM4                                                     between operator MMSCs

1. Open TCP session
2. Send full MM4-forward.req message
3. m-retrieve.conf mesage
4. Content blocked
5. Send 250 response

         MM7                                                     between a VASP and an MMSC

FortiOS Carrier and MMS duplicate messages and message floods

FortiOS Carrier detects duplicate messages and message floods for the MM1 and MM4 interfaces. How FortiOS Carrier detects and responds to duplicate messages and message floods is different from how FortiOS Carrier detects and responds to viruses and other MMS scanning protection measures.

For message floods and duplicate messages, the sender does not receive notifications about floods or duplicate messages, as if the sender is an attacker they can gain useful information about flood and duplicate thresholds. Plus, duplicate messages and message floods are usually a result of a large amount of messaging activity and filtering of these messages is designed to reduce the amount of unwanted messaging traffic. Adding to the traffic by sending notifications to senders and receivers could result in an increase in message traffic.

You can create up to three thresholds for detecting duplicate messages and message floods. For each threshold you can configure the FortiOS Carrier unit to respond by logging the activity, archiving or quarantining the messages, notifying administrators of the activity, and by blocking the messages. In many cases you may only want to configure blocking for higher activity thresholds, and to just monitor and send administrator notifications at lower activity thresholds.

When a block threshold is reached for MM1 messages, FortiOS Carrier sends m-send.conf or m-retrieve.conf messages to the originator of the activity. These messages are sent to end the MM1 sessions, otherwise the originator would continue to re-send the blocked message. When a block threshold is reached for MM4, FortiOS Carrier sends a MM4-forward.res message to close the MM4 session. An MM4 message is sent only if initiated by the originating MM4-forward.req message.

MM1 message flood and duplicate message blocking of sent messages

MM1 message flood and duplicate message blocking of received messages

MMS protection

MM4 message flood and duplicate message blocking

2. Open TCP session
3. Send full MM4-forward.req message Without ‘.’ on single line

7. Reset TCP session

MMS protection profiles

An MMS protection profile is a group of settings that you can apply to an MMS session matched by a security policy.

MMS protection profiles are easy to configure and can be used by more than one security policy. You can configure a single MMS protection profile for the different traffic types handled by a set of security policies that require identical protection levels and types. This eliminates the need to repeatedly configure those same MMS protection profile settings for each individual security policy.

Bypassing MMS protection profile filtering based on carrier endpoints

For example, while traffic between trusted and untrusted networks might need strict protection, traffic between trusted internal addresses might need only moderate protection. You would configure two separate MMS protection profiles to provide the different levels of protection: one for traffic between trusted networks, and one for traffic between trusted and untrusted networks.

Once you have configured the MMS Protection Profile, you need to add it to a security policy to apply the profile to MMS traffic.

Bypassing MMS protection profile filtering based on carrier endpoints

You can use carrier endpoint filtering to exempt MMS sessions from MMS protection profile filtering. Carrier endpoint filtering matches carrier endpoints in MMS sessions with carrier endpoint patterns. If you add a carrier endpoint pattern to a filter list and set the action to exempt from all scanning, all messages from matching carrier endpoints bypass MMS protection profile filtering. See Bypassing message flood protection based on user’s carrier endpoints.

Applying MMS protection profiles to MMS traffic

To apply an MMS protection profile you must first create the MMS protection profile and then add the MMS protection profile to a security policy by enabling the Carrier security profile. The MMS protection profile then applies itself to the traffic accepted by that security policy.

MMS protection profiles can contain settings relevant to many different services. Each security policy uses the subset of the MMS protection profile settings that apply to the sessions accepted by the security policy. In this way, you might define just one MMS protection profile that can be used by many security policies, each policy using a different or overlapping subset of the MMS protection profile.

To add an MMS protection profile to a security policy

1. Go to Security Profiles > MMS Profile.
2. Select Create New to add an MMS protection profile.
3. Configure as needed, and save.
4. Go to Policy & Objects > IPv4 Policy.
5. Select Create New to add a security policy, or select an existing policy and Edit to add the MMS profile.
6. Configure the security policy as required.
7. Enable MMS Profile, and select the MMS profile to add to the security policy.
8. Select OK.