will not be required to find out by extensive real-time analysis on a per-packet basis what treatment should be applied to each packet. Instead, routers use a series of per-hop behaviors (PHBs) to determine how to handle a specific flow. The PHB is the externally observable behavior of a packet in a DS-compliant router. The IETF has defined three major PHBs, which are in effect specific packet forwarding treatments: Assured Forwarding (AF) (RFC 2597) This PHB has four classes of traffic, which each have three possible levels of drop-precedence. Expedited Forwarding (EF) (RFC 2598) Also known by some as virtual leased line service, provides capabilities of low latency, low jitter, no packet loss, and an assured level of guaranteed bandwidth. Best Efforts Forwarding. Note that DiffServ only defines the DS field and a series of PHBs. These do not represent multi- ple levels of network services per se. The individual carrier or service provider is responsible for cre- ating different levels of service and for deciding which combination of DS bits and PHBs reflect which service and how they are mapped onto each other through tools that network equipment manufactur- ers develop and put at the carriers' disposal. It should be emphasized that DiffServ, unlike IntServ, does not offer performance guarantees. Even if packets are marked for preferential or priority treatment, it does not mean they will get it. For exam- ple, a router downstream that may be experiencing local traffic congestion will not hesitate to drop newly arriving packets no matter how they are marked. In addition, no QoS-related signaling provi- sions are provided in the protocol and consequently applications, which might need to adapt their behavior, depending on network status, do not have any means as to how go about implementing such a solution. However, it remains a simpler protocol computationally and structurally, and it is becom- ing a preferred method of working. Neither DiffServ nor IntServ can assign paths, though. The net- work equipment that