Tag Archives: Multimedia Message

MMS Deep Dive – MM1 – Mobile Terminated MMS

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In our last post we talked about sending an Multimedia Message, and in this post, we’re going to cover the process of receiving a Multimedia Message.

Carl Sagan once famously said “If you wish to make an apple pie from scratch, you must first invent the universe”, we don’t need to go that far back, but if you want to deliver an MMS to a subscriber, first you must deliver an SMS.

Wait, but we’re talking about MMS right? So why are we talking SMS?

Modern MMS transport relies on HTTP, which is client-server based, the phone / UE is the client, and the MMSc is the Server.

The problem with this client-server relationship, is the client requests things from the server, but the server can’t request things from the client.

This presents a problem when it comes to delivering the MMS – The phone / UE will need to request the MMSc provide it the message to be received, but needs to know there is a message to request in the first place.

So this is where SMS comes in. When the MMSc has a message destined for a Subscriber, it sends the phone/UE an SMS, informing that there is an MMS waiting, and providing the URL the MMS can be retrieved from.

This is typically done by MAP or SMPP, to link the MMSc to the SMSc to allow it to send these messages.

This SMS contains the URL to retrieve the MMS at, once the UE receives this SMS, it knows where to retrieve the MMS.

It can then send an HTTP GET to the URL to retrieve the MMS, and lastly sends an HTTP POST to confirm to the MMSc it retrieved it all OK.

MMS Mobile Terminated message flow

So that’s the basics, let’s look at each part of the dialog in some more detail, starting with this magic SMS to tell the UE where to retrieve the MMS from.

WAP PUSH from MMSc sent via SMS

So some things to notice, the user data, which would usually carry the body of our SMS instead contains another protocol, “Wireless Session Protocol” (WSP), and this is the method “Push”.

That in turn is followed by MMS Message Encapsulation, again inside the SMS message body, this time with the MMS specific data.

The From: header contains the sender of the MMS, this is how you can see who the MMS is from, while it’s still downloading.

The expiry indicates to the handset, it it doesn’t download the MMS within the specified time period, it shouldn’t bother, as the message will have expired.

And lastly, and perhaps most importantly, we have the X-MMS-Content-Location header, which tells our subscriber where to download the MMS from.

After this, the UE sends an HTTP GET to the URL in the X-MMS-Content-Location header (typically on the “mms” APN), to retrieve the MMS from the MMSc.

HTTP GET from the UE to the MMSc

The HTTP GET is pretty normal, there’s the usual MMS headers we talked about in the last post, and we just GET the path provided by the MMSc in the WAP PUSH.

The response from the MMSc contains the actual MMS itself, which is almost a mirror of the sending process (the Data component is unchanged from when the sender sent it).

Response to HTTP GET for message retrieval

At this stage our subscriber has retrieve the MMS, but may not have retrieved it fully, or may have had an issue retrieving it.

Instead the UE sends an HTTP POST with the MMS-Message-Type m-notifyresp-ind with the transaction ID, to indicate that it has successfully retrieved the MMS, and at this point the MMS can notify the sender if delivery receipts are enabled, and delete the message from the cache.

And finally the MMSc sends back a 200 OK with no body to confirm it got that too.

Some notes on MMS Security

Reading about unauthenticated GET requests, you may be left wondering what security does MMS have, and what stops you from just going through and sending HTTP GET requests to all the possible URL paths to vacuum up all the MMS?

In the standard, nothing!

Typically the MMSc has some layer of security added by the implementer, to ensure the user retrieving the MMS, is the user the MMS is destined for.
Because MMS has no security in the standard, this is typically achieved through Header Enrichment, whereby the P-GW adds a HTTP header with the MSISDN or IMSI of the subscriber, and then the MMSc can evaluate if this subscriber should be able to retrieve that URL.

Another attack vector I played with was sending a SMS based MMS-Notify with a different URL, which if retrieved, would leak the subscriber’s IP, as it would cause the UE to try and get data from that URL.

Looking inside the MMS Exchange (With call flow and PCAP)

GSM, LTE, Mobile Networks, RFCs & Standards, , ,

So you want to send an MMS?

We’ve covered SMS in the past, but MMS is a different kettle of fish.

Let’s look at how the call flow goes, when Bob wants to send a picture to Alice.

Before Bob sends the MMS, his phone will have to be setup with the correct settings to send MMS.
Sometimes this is done manually, for others it’s done through the Carrier provisioning SMS that preloads the settings, and for others it’s baked in based on the Android Carrier settings XML,

APN settings for Telstra in Australia for MMS

It’s made up of the APN to send MMS traffic over, the MMSC address (Multimedia Message Switching Center) and often an MMS proxy and port combination for where the traffic will actually go.

Message Flow – Bob to MMSC (Mobile Originated MMS)

Bob opens his phone, creates a new message to Alice, selects the picture (or other multimedia filetype) to send to her and hits the send button.

For starters, MMS has a file size limit, like MTU it’s not advertised, so you don’t know if you’ve hit it, so rather like MTU is a “lowest has the highest success of getting through” rule. So Bob’s phone will most likely scale the image down to fit inside 300K.

Next Bob’s phone knows it has an MMS to send, for this is opens up a new bearer on the MMS APN, typically called MMS, but configured in the phone by Bob.

Why use a separate APN for sending 300K of MMS traffic?
Once upon a time mobile data was expensive.
By having a separate APN just for MMS traffic (An APN that could do nothing except send / receive MMS) allowed easier billing / tariffing of data, as MMS traffic was sent over a APN which was unmetered.

After the bearer is setup on the MMS APN, Bob’s phone begins crafting a HTTP 1.1 Post to be sent to the MMSC.
The content type of this request will be application/vnd.wap.mms-message and the body of the HTTP post will be made up of MMS Message Encapsulation, with the body containing the picture he wants to send to Alice.

Note: Historically Wireless Session Protocol (WSP) was used in lieu of HTTP. These clients would now need a WAP gateway to translate into HTTP.

This HTTP Post is then sent to the MMSC Address, or, if present, the MMSC Proxy address.
This traffic is sent over the MMS APN that we just brought up.

HTTP POST Headers for the MO MMS Message

MMS Message Encapsulation from MO MMS Message

The MMSC receives this information, and then, if all was successful, responds with a 200 OK,

200 OK response to MO MMS Message

So now the MMSC has the information from Bob, let’s flip over to Alice.

Message Flow – MMSC to Alice (Mobile Terminated MMS)

For the purposes of simplicity, we’re going to rule out the MMSC from doing clever things like converting the media, accepting email (SMPP) as MMS, etc, etc. Instead we’re going to assume Alice and Bob are on the same Network, and our MMSC is just doing store-and-forward.

The MMSC will look at the To address in the MMS Message Encapsulation of the request Bob sent, to determine that this message is destined for Alice.

The MMSC will load the media content (photo) sent by Bob destined for Alice and serve it via HTTP. The MMSC generates a random URL to serve it this particular file on, with each MMS the MMSC handles being assigned a random URL containing the media content.

Next the MMSC will need to tell Alice’s phone, that she has an MMS waiting for her. This is done by generating an SMS to send to Alice’s phone,

The user-data of this SMS is the Wireless Session Protocol with the method PUSH – Aka WAP Push.

SMS alerting the user of an MMS waiting for delivery

This specially encoded SMS is parsed by the Alice’s phone, which tells the her there is an MMS message waiting for her.

On some operating systems this is pulled automatically, on others, users need to select “Download” to actually get the file.

The UE then just runs an HTTP get to the address in the X-Mms-Content-Location: Header to pull the multimedia content that Bob sent.

HTTP GET from Alice’s Phone / UE to retrieve MMS sent by Bob (MT-MMS)

All going well the URL is valid and Alice’s phone retrieves the message, getting a 200 OK back from the server with the message content.

HTTP Response (200 OK) for MT-MMS, sent by the MMSC to Alice’s phone with the MMS Body

So now Alice’s phone has the MMS content and renders it on the screen, Alice can see the Photo Bob sent her.

Lastly Alice’s phone sends a HTTP POST again to the MMSC, this time indicating the message status is “Retrieved”,

And to close everything off the MMSC confirms receipt of the Retrieved status with a 200 OK, and we are done.

What didn’t we cover?

So that’s a basic MMS message flow, but there’s a few parts we didn’t cover.

The overall architecture beyond just the store-and forward behaviour, charging and authentication we didn’t cover. So let’s look at each of these points.

Overall Architecture

What we just covered what what’s defined as the MM1 interface.

There’s obviously a stack of other interfaces, such as for charging, messaging between MMSC/Carriers, subscriber locating / user database, etc.

Charging

MMSCs would typically have a connection to trigger charging events / credit-control events prior to processing the message.

For online charging the Ro interface can be used, as you would for IMS charging events.

3GPP 3GPP TS 32.270 covers the charging architecture for online/offline charging for MMS.

Authentication

Unfortunately authentication was a bit of an afterthought for the MMS standard, and can be done several different ways.

The most common is to correlate the IP Address on the MMS APN against a subscriber.