Network Service Models

Network Service Models

Before looking into the network layer, let's take the wider view and examine the different types of service that might be offered by the network layer. When the transport layer at a sending host transmits a packet into the network (that is, passes it down to the network layer at the sending host), can the transport layer count on the network layer to deliver the packet to the destination? When several packets are sent, will they be delivered to the transport layer in the receiving host in the order in which they were sent? Will the amount of time between the sending of two sequential packet transmissions be the same as the amount of time between their reception? Will the network provide any feedback about congestion in the network? What is the abstract view (properties) of the channel connecting the transport layer in the sending and receiving hosts? The answers to these questions and others are determined by the service model provided by the network layer. The network service model defines the characteristics of end-to-end transport of packets between sending and receiving end systems.

Let's now examine some possible services that the network layer could provide. In the sending host, when the transport layer passes a packet to the network layer, specific services that could be provided by the network layer include:

●  Guaranteed delivery. This service guarantees that the packet will finally arrive at its destination.

●  Guaranteed delivery with bounded delay. This service not only guarantees delivery of the packet, but delivery within a particular host-to-host delay bound (for instance, within 100 msec).

Moreover, the following services could be provided to a flow of packets between a given source and destination:

●  In-order packet delivery. This service guarantees that packets arrive at the destination in the order that they were sent.

●  Guaranteed minimal bandwidth. This network-layer service emulates the behavior of a transmission link of a particular bit rate (for  instance, 1 Mbps) between sending and receiving hosts (even though the actual end-to-end path may traverse numerous physical links). As long as the sending host transmits bits (as part of packets) at a rate below the specified bit rate, then no packet is lost and each packet arrives within a specified host-to-host delay (for instance, within 40 msec).

●  Guaranteed maximum jitter. This service guarantees that the amount of time between the transmission of two successive packets at the sender is equal to the amount of time between their receipt at the destination (or that this spacing changes by no more than some particular value).

●  Security services. Using a secret session key known only by a source and destination host, the network layer in the source host could encrypt the payloads of all datagrams being sent to the destination host. The network layer in the destination host would then be responsible for decrypting the payloads. With such a service, confidentiality would be provided to all transport-layer segments (TCP and UDP) between the source and destination hosts. In addition to confidentiality, the network layer could provide data integrity and source authentication services.

This is only a partial list of services that a network layer could provide - there are countless variations possible.

The Internet's network layer provides a single service, known as best-effort service. From Table 1, it might appear that best-effort service is a euphemism for no service at all. With best-effort service, timing between packets is not guaranteed

Internet ATM CBR and ATM ABR service models

to be preserved, packets are not guaranteed to be received in the order in which they were sent, nor is the eventual delivery of transmitted packets guaranteed. Given this definition, a network that delivered no packets to the destination would satisfy the definition of best-effort delivery service. As we'll discuss shortly, however, there are sound reasons for such a minimalist network-layer service model. We'll cover additional, still-evolving, Internet service models in "Multimedia Networking".

Other network architectures have defined and implemented service models that go beyond the Internet's best-effort service. For instance, the ATM network architecture [MFA Forum 2009, Black 1995] provides for multiple service models, meaning that different connections can be provided with different classes of service within the same network. A discussion of how an ATM network provides such services is well beyond the scope of this blog; our aim here is only to note that alternatives do exist to the Internet's best-effort model. Two of the more important ATM service models are constant bit rate and available bit rate service:

●  Constant bit rate (CBR) ATM network service. This was the first ATM service model to be standardized, reflecting early interest by the telephone companies in ATM and the suitability of CBR service for carrying real-time, constant bit rate audio and video traffic. The goal of CBR service is conceptually simple - to provide a flow of packets (known as cells in ATM terminology) with a virtual pipe whose properties are the same as if a dedicated fixed-bandwidth transmission link existed between sending and receiving hosts. With CBR service, a flow of ATM cells is carried across the network in such a way that a cell's end-to-end delay, the variability in a cell's end-end delay (that is, the jitter), and the fraction of cells that are lost or delivered late are all guaranteed to be less than specified values. These values are agreed upon by the sending host and the ATM network when the CBR connection is first established.

●  Available bit rate (ABR) ATM network service. With the Internet offering so-called best-effort service. ATM's ABR might best be characterized as being a slightly-better-than-best-effort service. As with the Internet service model, cells may be lost under ABR service.  Unlike in the Internet, on the other hand, cells cannot be reordered (although they may be lost), and a minimum cell transmission rate (MCR) is guaranteed to a connection using ABR service. If the network has enough free resources at a given time, a sender may also be able to send cells successfully at a higher rate than the MCR. Additionally, as we saw in "Principles of Congestion Control", ATM ABR service can provide feedback to the sender (in terms of a congestion notification bit, or an explicit rate at which to send) that controls how the sender adjusts its rate between the MCR and an allowable peak cell rate.


network layer, transport layer, end systems

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