Network Sales and Services Handbook (Cisco Press Networking Technology)
Internet Protocol (IP) is based on the notion that datagrams with source and destination IP addresses can traverse a network of (IP) routers without the help of the sender or receiver. The Internet was built on this concept of a dumb network, with the intelligence provided by the sender and receiver.
IP provides a best-effort service; making no guarantees about when and how data arrives at the destination. IP provides the following features:
Network addressing Enables independent handling of each IP datagram across the network.
Datagram fragmentation and reassembly (by the sending and receiving routers) Enables carrying of traffic across different network media, such as Ethernet and Token Ring, each supporting different datagram sizes.
Being a best-effort protocol, IP does not provide reliable data delivery. Routers can discard IP datagrams in transit without notifying sender or receiver. IP relies on upper-level protocols, such as Transmission Control Protocol (TCP), to track datagrams and retransmit as required. These "reliability" mechanisms can assure only the delivery not the timeliness of data delivery and do not provide any throughput guarantees.
This issue of "data delivery without guarantee" is not a problem for traditional Internet applications such as Web browsing, e-mail, or file transfer. New applications, such as streaming video and audio, demand high throughput capacity in the form of bandwidth requirements and have low-latency demands when used in two-way communications, such as conferencing and telephony.
Three solutions to the "data delivery without guarantee" issue have yielded the following proposed methodologies:
Differentiated QoS IETF standards (RFC 2474, 2475) define a differentiated services (DiffServ) field for IPv4 and IPv6 headers. This DiffServ field provides classes of service that can be assigned to specific types of voice, video, or data applications.
Resource Reservation Protocol (RSVP) IETF standard (RFC 2205) provides for guaranteed, on-time delivery of data over an IP network. RSVP works in cooperation with the DiffServ standard by reserving bandwidth in response to client requests.
IEEE 802.1p Defines frame format extensions for Ethernet, Token Ring, FDDI, and other data-link layer protocols. This frame format extension carries extra bits used to specify the priority of the packet; priority ranges from 0 (lowest) to 7 (highest).