ched at wire speed as ATM but on real IP packets. These are notorious about their vary- ing lengths; hence, switching has to occur at layer 3 without the nuisance (in this case) of the fixed- cell length that ATM is imposing. In fact, some researchers2 openly admit that MPLS has borrowed the good design attributes of ATM without the need to set up calls and without the need for a fixed- length cell. These factors were once perceived as the two major drawbacks of ATM. The classifica- tion issue we just discussed becomes absolutely critical for the performance of the MPLS networks, as switching must occur in extremely high speeds at the backbone of the Internet (where wire speed attains at least several tens of gigabits per second) based on the content of a small prefix (MPLS label tag) attached to the IP packet. Similar concerns are found when implementing other applications such as load balancers for server networks. We also discussed relevant issues in the section "Packet Classification and Fast Forwarding" earlier in this chapter. Modification Modification is a generic term that can be applied to several contexts. In a typical modification, a packet must be encapsulated. This means that new headers must be calculated. In some instances, new trailers and CRC checksums must also be calculated. This is done for example when IP over ATM running, where an extra overhead of 8 bytes is created and added to an IP packet. ATM Adaptation Layer level 5 (AAL5) is subsequently used to carry the encapsulated packet, which now carries the original content along with the appropriate ATM headers, the required AAL5 padding, and the nec- essary trailers. Another example that we will see in the security coprocessor Chapter 17 is the encap- sulation of encrypted and authenticated packets in an IP Security (IPsec) environment. This involves the creation or removal of the Authentication Header (AH) and Encapsulating Security Payload (ESP) headers, and especially the dressing (or undressing for that matter) of packets depending on whe