There are a number of mechanisms, existing or proposed, capable of transporting IP effectively over ATM. The key functions of these mechanisms are to manage the set of virtual circuits used to transport IP over the ATM infrastructure; aggregate the appropriate sets of IP packets onto the correct VCs; and provide the mechanisms to interoperate with existing routers.

To interoperate with existing routers, the IP/ATM infrastructure must implement standard routing protocols such as routing information protocol (RIP), open shortest path first (OSPF), border gateway protocol (BGP), distance vector multicast routing protocol (DVMRP) and protocol independent multicast (PIM), as well as the standard non–ATM interfaces. This permits the infrastructure to accept and deliver both user data and control information to and from existing routers.

Internally, the IP/ATM infrastructure must construct the set of VCs (either permanent or switched) required to provide connectivity across the infrastructure. The VCs may be established in response to demand (and torn down when the demand no longer exists) or by administrative control or some combination of both options. The IP/ATM infrastructure must also ensure that each virtual channel connection (VCC) has the appropriate traffic descriptors to achieve the required CoS.

Given that a set of VCs is, or can be, established, the IP/ATM infrastructure must ensure the appropriate packets are aggregated onto the correct VCs. The most fundamental piece of information in this regard is given by the IP–forwarding information. However, implementation of VPN services and policy-based treatment of traffic (such as CoS) can also influence the selection.

Figure 3. The Relationship between ATM and SONET

There are a number of ATM–based technologies required to support such an infrastructure. One of the key technologies is next hop resolution protocol (NHRP), which allows the IP/ATM infrastructure to use a set of shortcut virtual circuits to carry IP traffic over ATM without causing problems for the routing protocols.

Transporting IP over ATM has the following advantages:

• secure, easy to provision VPNs

• CoS options for public Internet and VPN connectivity

• superior bandwidth-management capabilities

• shared network and management infrastructure with other transport services, including frame relay, cell relay, circuit emulation, voice services, and SMDS

• IP traffic–path monitoring for and by specific customers to assure that SLAs are being met

• variable bandwidth capability via over-subscription

• advanced network management

Figure 4. An ATM–Based IP Service