Answered Essay: (a). Give the forwarding tables for S1 through S4 assuming packets to adjacent nodes are sent along the con

Four switches are arranged as below. The destinations are S1 through S4 themselves.

S1──────S2
│       │
│       │
S4──────S3

(a). Give the forwarding tables for S1 through S4 assuming packets to adjacent nodes are sent along the connecting link, and packets to diagonally opposite nodes are sent clockwise.

(b). Give the forwarding tables for S1 through S4 assuming the S1–S4 link is not used at all, not even for S1⟷S4 traffic.

Expert Answer

 

There are many interconnections in the field of networking, as in most technical fields, and it is difficult to find an order of presentation that does not involve endless “forward references” to future chapters; this is true even if – as is done here – a largely bottom-up ordering is followed. I have therefore taken here a different approach: this first chapter is a summary of the essentials – LANs, IP and TCP – across the board, and later chapters expand on the material here.

Local Area Networks, or LANs, are the “physical” networks that provide the connection between machines within, say, a home, school or corporation. LANs are, as the name says, “local”; it is the IP, or Internet Protocol, layer that provides an abstraction for connecting multiple LANs into, well, the Internet. Finally, TCP deals with transport and connections and actually sending user data.

This chapter also contains some important other material. The section on datagram forwarding, central to packet-based switching and routing, is essential. This chapter also discusses packets generally, congestion, and sliding windows, but those topics are revisited in later chapters. Firewalls and network address translation are also covered here and not elsewhere.

1.1   Layers

These three topics – LANs, IP and TCP – are often called layers; they constitute the Link layer, the Internetwork layer, and the Transport layer respectively. Together with the Application layer (the software you use), these form the “four-layer model” for networks. A layer, in this context, corresponds strongly to the idea of a programming interface or library (though some of the layers are not accessible to ordinary users): an application hands off a chunk of data to the TCP library, which in turn makes calls to the IP library, which in turn calls the LAN layer for actual delivery.

The LAN layer is often conceptually subdivided into the “Physical layer” dealing with, eg, the electrical signaling mechanisms involved, and above that an abstracted “Logical link layer” that describes how packets can be addressed from one LAN node to another. The physical layer is generally of direct concern only to those designing LAN hardware; the kernel software interface to the LAN corresponds to the logical link layer. This LAN physical/logical division gives us the Internet five-layer model. This is less a formal hierarchy as an ad hoc classification method. We will return to this below in 1.15   IETF and OSI.

1.2   Bandwidth and Throughput

Any one network connection – eg at the LAN layer – has a data rate: the rate at which bits are transmitted. In some LANs (egWi-Fi) the data rate can vary with time. Throughput refers to the overall effective transmission rate, taking into account things like transmission overhead, protocol inefficiencies and perhaps even competing traffic. It is generally measured at a higher network layer than the data rate.

The term bandwidth can be used to refer to either of these, though we here try to use it mostly as a synonym for data rate. The term comes from radio transmission, where the width of the frequency band available is proportional, all else being equal, to the data rate that can be achieved.

In discussions about TCP, the term goodput is sometimes used to refer to what might also be called “application-layer throughput”: the amount of usable data delivered to the receiving application. Specifically, retransmitted data is counted only once when calculating goodput but might be counted twice under some interpretations of “throughput”.

Data rates are generally measured in kilobits per second (Kbps) or megabits per second (Mbps); in the context of data rates, a kilobit is 103 bits (not 210) and a megabit is 106 bits. The use of the lowercase “b” means bits; data rates expressed in terms of bytes often use an upper-case “B”.

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