IP networking

The fundamental data networking function that the AT&T wireless network provides is transport of packets between a mobile device and an external network via Internet Protocol (IP). IP address management is therefore an important aspect of routing data between external networks and AT&T GSM/GPRS, EDGE, and UMTS/HSDPA networks.

Overview

This discussion assumes an understanding of protocol stacks in the Internet Protocol suite (TCP/IP).

The AT&T wireless network does not restrict which protocols your applications use in the higher layers (i.e., transport and application) of the IP protocol stack. For example, at the transport layer, wireless applications can use either User Datagram Protocol (UDP) or Transmission Control Protocol (TCP). In the application layer, common network application protocols such as Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP) operate readily over the AT&T wireless network.

IP packets in a Web-based application travel between the mobile device and a server located on an external network. The mobile device accessing the data packets has a Web browser which uses:

• HTML for rendering content.
• HTTP for requesting Web pages.
• TCP for reliable transfer of the pages.
• IP for actually sending/receiving data across the network.

The Device could be a mobile telephone or PC card modem; or both the Device and Mobile Computer could be integrated parts of a single mobile device, such as a Smartphone. Layer one (the physical connection between the computer and device) can be a cable, infrared, Bluetooth, or PC card interface. At layer two, the computer and device interface communicate either by using Point-to-Point Protocol (PPP) or via the Network Driver Interface Specification (NDIS). From a networking perspective, the device and the network infrastructure (which includes base stations, the core network, and gateways to external networks) all operate in layer three to route IP packets between the mobile device and the external network.

IP networking and wireless considerations

There are a number of aspects of IP networking that are unique to wireless networks in general, and to AT&T wireless network specifically.

Computers on wireless networks must be mobile across large geographic areas and be able to move in and out of coverage areas without losing IP communications. EDGE and UMTS technologies address these challenges by allowing a mobile device to maintain the same IP address as it moves from the coverage of one base station to another in a continuous session, even if the mobile device temporarily moves out of coverage. In addition, AT&T has designed capabilities into its network to give customers control over the types of addresses their mobile devices use and the networks with which their mobile devices can communicate.

To actually engage in a data session, the mobile device must activate a packet data protocol (PDP) context, which results in the network assigning an IP address.

AT&T provides multiple options for IP addressing, including public, private, customer-supplied, and network-assigned static IP addresses. Part of activating the PDP context includes specifying an Access Point Name (APN). The selected APN defines:

• What type of IP address to use (e.g., public/private, dynamic/static).
• How mobile devices communicate with external networks (e.g., customer networks).
• Which fixed-end connections to use.
• Which value-added services the mobile device can access (e.g., MEdia Net).

Most customers use the two general-purpose APNs wap.att and isp.att. Special purpose and custom APNs are also available to address unique requirements. Content providers can determine the appropriate IP management solution for their applications by defining the following criteria.

Application type

Depending on the application type, decide whether you want IP addresses to be:

• Private or public.
• Dynamic or static.
• From a specified range. If so, who supplies the range, AT&T or the content provider?

Service level

Decide what level of service is required between the customer network and the AT&T wireless network in order to determine your approach for fixed-end connectivity.

GSM/GPRS, EDGE, and UMTS technologies allow communications via the Internet. In addition, AT&T offers Commercial Connectivity Services that provide either a Frame Relay or network VPN connection between the customer network and the AT&T wireless network for enterprise-focused applications.

Security requirements

Decide what your security requirements are:

• Should the network allow mobile terminated communications?
• Should the network allow communication with the Internet?
• What is the allowed range of IP addresses of hosts on external networks?

General IP networking considerations

Most IP-based applications will function normally over the AT&T wireless network. However, some applications may be affected or may require customers to choose appropriate addressing and APN options. Here are some general rules of thumb.

Public vs. private IP addresses

Some applications may not work properly with a private IP address because the application might require the same source IP address to be used end-to-end (as in some VPN systems in specific operating modes, for example). You can resolve this problem by using a public IP address. This is generally not an issue for laptop applications, which use the isp.AT&T APN and public IP addresses. Smartphones, however, generally use the wap.AT&T APN, which uses private IP addresses.

Device authentication or identification based on IP address

Since the network assigns addresses dynamically, the IP address cannot always be used to identify or authenticate a device. For firewall purposes, content providers can use a pre-designated block of IP addresses in conjunction with a custom APN. With this arrangement, the content provider can specify a contiguous block of IP addresses, or AT&T can provide the addresses. Alternatively, an application could use network-assigned static IP addresses (which require a custom APN). With static IP addresses, the mobile station always has the same IP address, simplifying authentication and identification. Yet another option is to use some other form of persistent identifier, such as the phone number of the device (referred to as the MSISDN), a manually assigned mobile device name, or the user login name, and not the IP address at all.

Change of IP address with new PDP context

Except in the case of static IP addresses, a new PDP context can result in a new IP address, which may affect applications that are already open. However, the AT&T network protects against temporary loss of connection by maintaining a Packet Data Protocol (PDP) context for up to approximately an hour even if the mobile station is unreachable during that period.

Out-of-coverage situations

Mobile applications should be tolerant of temporary out-of-coverage situations. If the application has timers to detect no response, these should be set long enough to account for predicable gaps in communication due to the mobile station being temporarily out of wireless coverage.

Mobile terminated applications

These applications require a custom APN to allow unsolicited incoming IP traffic from the content provider's network servers, or from the Internet. The traffic can only travel through a dedicated fixed-end connection. As an alternative, if the phone number (MSISDN) of the device is known, the application can also push messages using Short Message Service (SMS). All devices on the AT&T wireless network have a phone number and support SMS. An option for WAP-based applications is to use WAP Push.

Available destinations

The APN determines which destinations may be available. For example, a custom APN can be configured to block Internet access.

Security based on filtering the IP address

If a content provider wishes to filter incoming traffic from mobile stations based on the source address, the content provider may wish to use a custom APN and to use a predefined block of IP addresses. Network-assigned static IP addresses may also be useful in this case.

Mobility middleware

Developers should be aware that there are mobile middleware solutions available from third-party software vendors that enhance IP address management. These enhancements could include enabling the mobile station to continue using the same IP address even if the network assigns a different IP address or if a new network is used. They also could include enabling applications to maintain sessions even if connections are temporarily unavailable.

IP address management

An Internet Protocol (IP) address works like a phone number or street address to identify a network device on the Internet (be it a PC, Web server, mobile phone, or other device). IP addresses are used in the transmission of data across networks, and each node on a network needs a unique IP address. Technically, an IP address is a 32-bit number written in four 8-bit bytes in dotted-decimal format. An IP address looks like this: 166.121.121.121.

The range of 32-bit IP addresses is not infinite. Many are reserved for special purposes, such as private networks or multicast addresses, which reduces the number of addresses that can be allocated worldwide for public Internet addresses.

There are two types of IP addresses: public and private.

Public IP addresses

A public IP address is a unique and registered Internet address assigned to a user. IP packets sent to and from the user maintain the same address as they traverse from the AT&T wireless network to other networks. A public address is necessary for some applications, such as certain virtual private networking programs. When connecting via the APN isp.ATT, the network assigns public IP addresses.

Private IP addresses

Private IP addresses are non-routable across the Internet. They are used primarily within a single domain and typically start with "10." Private IP addresses prevent access into and out of a proprietary network. An example of a private IP address is 10.196.210.141.

Private IPs are valuable because several different domains can use the same private IPs at the same time and not conflict with each other. When connecting via the APN wap.ATT, the network assigns private IP addresses.

Understanding IP address management

For a mobile device to communicate across the AT&T wireless network, it needs an IP address. The IP address is used both by the wireless modem, which uses the IP address for forwarding packets to and from the mobile computer, and the TCP/IP protocol stack in the mobile computer.

When a modem requests data service with the network, the network assigns an IP address as part of the Packet Data Protocol Context. For the APNs isp.ATT and wap.ATT, the addresses are assigned dynamically. Static IP addresses are an option with custom APNs.

When a mobile device sends packets with a private address to an external host, the network translates the address to a public IP address. The reverse translation occurs for packets sent to the user from external networks. This process is called Network Address Translation (NAT).

Since the network can allow multiple users to share the same public IP address, the operator can conserve IP addresses. This is beneficial given the current shortage of IP addresses and the growing number of mobile devices needing them.

NOTE: Due to the dynamic assignment of IP addresses, applications should not depend on an IP address for identifying or communicating with a mobile device. The exception to this rule of thumb is if you are using a custom APN.

Determining the mobile device IP address

So how can an application operating outside the AT&T wireless network obtain the current IP address of the mobile station? One way is for the mobile station to initiate communication with your server to identify itself.

Another way is to use Dynamic Domain Name Service (Dynamic DNS). External hosts can direct a DNS query to obtain the IP address of a mobile station based on its name. The format of the name is 1X.internet.myatt.com, where X is the Mobile Station Integrated Services Digital Network number (MSISDN), which is the phone number of the device. An example of such a name is 14045551111.internet.myatt.com.

AT&T supports Dynamic DNS for public Internet name resolution only. Customer intranet entries are not supported.

Dynamic DNS also is available as an option for custom APNs, but most customers elect to use static IP addresses instead.

Learn more about AT&T IP addressing options, including pricing.