ISO Layer Model

Lecture 9

October 16, 2000

The Need for Protocols

Multiple hardware platforms need to have the ability to communicate.

Writing communications code in this instance is non-portable (platform-dependent).

Protocols provide a high-level interface method to the communications medium.

The protocol itself is a set of rules by which the software and hardware must follow in order to communicate properly.

Need for Protocols (cont.)

Multiple types of protocols exist. RS-232 is form of a protocol in the sense that strict

rules must be followed for communication to occur between devices.

Network protocols cover almost all communications between two or more computers.

– Transferring an ASCII file– Point-to-point encryption

Protocol Suites

A single, all-encompassing protocol is not only inefficient, but also wasteful on CPU, memory resources.

Programmers have created smaller suites which provide the rule sets for communication within that protocol.

The suites are also divided into smaller pieces, called layers.

The ISO Layer Model

Why the Layer Model?

It is simple to visualize how software interacts with hardware.

Topmost layer is closest to the user. Bottom layer is the furthest away from the user.

Layer Breakdown

Layer 1: Physical– The physical network hardware, medium.

Layer 2: Data Link– How data is organized into frames, and how to

transmit those frames. Byte/bit stuffing, checksums. Layer 3: Network

– How addresses are assigned, and how packets are transmitted from one end of the network to the other.

Layer Breakdown (cont.)

Layer 4: Transport– How to reliably transfer data.

Layer 5: Session– How to start sessions (connections) with remote devices,

machines. (Sockets) Layer 6: Presentation

– How to represent data. Does int, char replacements. Layer 7: Application

– How one user-level program requests a connection to another machine, and how the machine responds.

Protocol Stacks

The protocol is divided into modules. Each module corresponds to a different level in

the layer model. All of the modules put together is the “stack.” Theoretically, a given module can only

communicate with the module above and below it.

Stack Examples

Communication With Stacks

Stack Interaction

Because the stack dictates how computers communicate, machines utilizing one type of stack cannot communicate with a machine running another type of stack.

Netware v. TCP/IP But machines can concurrently run more than

one stack!

Layering

At each level where the packet is processes, additional information beyond the actual data is added or removed.

At the Layer 2, a checksum is added before being sent to Layer 1.

When the Layer 2 of the receiving machine receives the packet, it strips off the header and the checksum footer.

A Layered Frame

Protocol Dependent Information

The previous picture is a general idea. Some protocols specify that all additional

information must be appended instead of prepended, etc.

Layering Overhead

Because the sending computer adds information to the frame, the receiving machine must undo the changes before the frame is passed to the next layer.

Because layers must undo changes that the counterpart creates, code for the other layers can be independently created and updated without affecting the other layers!

Data At Each Layer

Protocol Optimization Techniques

Protocols utilize different methods depending on the focus and goal of the protocol.

Sequencing– To avoid out-of-order delivery– To avoid duplicate packets

Packet Retransmission– When a packet is received, an ACK is sent back. If

the ACK isn’t received by the transmitter, packet is resent. (Bound by time.)

Optimization Techniques (cont.)

Avoiding Excessive Delay– Each transmission is assigned an ID. Packets must

contain a valid ID to be accepted for a given “conversation.” p. 213

Flow Control to Avoid Overrun– Machines do no communicate at the same rate.– Sliding Window method.

The Sliding Window

The sliding window concept greatly increases protocol throughput.

All packets in a given “window” are sent sequentially, without waiting for an immediate ACK.

Once all packets in a window have been sent, transmitter waits for all ACKs to be received before continuing on to next window.

Network Congestion

If a single computer sends, no congestion on network link.

If more than one computer sends, packet queuing must occur, so some congestion exists.

Network Congestion (cont.)

If congestion persists over time, packets fill the switch’s memory, and will begin to be discarded.

Retransmission can fix this problem, but if the congestion still exists when the retransmission occurs, those packets will also be discarded.

This scenario is called congestion collapse.

Avoiding Congestion

If a switch is having congestion, send a message back to the sender informing them of the congestion.

Packet loss as an estimate of congestion. Rate control

– Temporary rate reduction– Sliding window size reduction

Protocol Design

Flow control vs. congestion control– Flow control sends more packets to improve

throughput– Congestion control reduces the number of packets

Sequence number– Too small, miscommunication, confusion– Too big, wasted bandwidth, overhead