COMMUNICATION NETWORK PROTOCOLS

Simply the rules and standards set up to govern communications over a network architecture

More specifically, communication protocols are sets of rules that regulate the exchange of

messages to provide a reliable and orderly flow of information among communicating processes

CATEGORIES OF COMMUNICATION SERVICES

Connection-Oriented – require explicit set up of connection before communication commences

Messages are delivered reliably and in sequence

Connectionless – no initial connection set up required

Messages are delivered on a best-effort basis in timing and route, and may arrive in arbitrary order – useful analogy is the postal service

OSI PROTOCOL SUITE A protocol suite consists of the layers of a standardized

network architecture

The first of the two most popular of these is the the Open Systems Interconnection (OSI) from the International Standards Organization

Consists of 7 layers

Serves to abstract away the details of the network communication from the communicating processes

The above is achieved through the process of encapsulation

Specifically, each layer in the protocol receives a Protocol Data Unit (PDU) from its upper layer and encapsulates it with a header of information for its peer layer in the communicating process, which peels these encapsulations off in reverse order

OSI MODEL

THE PHYSICAL LAYER

The physical layer specifies the electrical and mechanical characteristics of the physical communication link between two nodes

This is where electrical or optical signals are converted to bits, or vice versa

Bit synchronization is the process of detecting the beginning of a bit, or sequence of bits

BIT SYNCHRONOUS VERSUS CHARACTER ASYNCHRONOUS

Bit sequences may be bit synchronous or character asynchronous

Bit synchronous sequences are large blocks of bits transmitted at a regular rate

Results in higher data transfer rate and more efficient utilization of the link

Character asynchronous data are small fixed-size bit sequences transmitted asynchronously

Low-speed character-oriented terminals generally use this method

STANDARDIZATION OF PHYSICAL LINKS

Standardization required to allow peers to communicate extends all the way down to the physical level, for things like coding method, modulation technique, and wire and connector specifications

RS232C and X.21 are two typical such standardizations

THE DATALINK LAYER

Ensures reliable data transfer for groups of bits, called frames

Protocols here handle configuration setup, error controls, sequencing and flow control of frames

Configuration concerns the establishment and termination of a connection, and whether it should be full or half duplex, and synchronous or asynchronous

Errors include transmission errors and loss or replication of data frames

Sequencing uses sequence numbers to maintain orderly delivery of frames

Flow control is necessary when receiver cannot keep pace with sender, perhaps because of limited buffer space

THE DATALINK LAYER

All of these control functions contribute overhead bits which are added to frames as headers or trailers

Consequentially, each control protocol at this layer has a particular format so that bits can be interpreted correctly

THE NETWORK LAYER

The Network Layer is concerned with sending packets over multiple links, as opposed to the previous two layers which are each concerned with only one physical link

Packets, the fundamental data unit of the network layer, are not analogous to frames, as they are not bounded by the size of the physical link

The routing function of the network layer consists of determining, given the destination address of a packet, what link to forward that packet to

Congestion control is the alleviation of node bottlenecks in the network through routing changes

THE NETWORK LAYER

When routing decisions are made when a connection is established it is called a virtual circuit

These deliver packets, called datagrams, in order Datagrams require reassembly in proper sequence Not all packets are data packets Control packets are also used for network address

resolution and status broadcasting Such protocols generally use datagram services

since packets are small and short-lived

THE TRANSPORT LAYER Serves as the interface between the communication

subnetwork (physical, data link and network layers), and network independent layers (session, presentation and application)

Primary responsibility is to provide reliable end-to-end communication between processes

All network-dependent issues are abstracted away before we reach this level

At the Transport layer is where messages are broken into packets, or, at the receiving process, packets are assembled into messages

Several sessions may be multiplexed into a single transport connection, or alternatively a single session could occupy multiple transport connections

SESSIONS

Sessions are categorized according to their requirements for error handling and multiplexing

OSI defines 5 classes of transport services to support sessions, TP0-TP4

TP4 is the most common transport service, and allows multiplexing, sessions, error detection, and retransmission

It is a reliable connection-oriented service for unreliable networks

SESSION, PRESENTATION AND APPLICATION LAYERS

Layers above the Transport Layer are considered to be 'on top' of the implementation of the system, and thus can be considered together

The Session Layer provides additional dialog and synchronization services to the Transport Layer

The Presentation Layer provides data encryption, compression and code conversion for messages that use different coding schemes

The standard for the Application Layer is left entirely up to the developer of the application in question

TCP/IP

There are two major types of system interactions – interprocess and internode

Stated another way, as system designers we are primarily concerned with 1) how is communication maintained between a pair of processes, and 2) how are messages routed through network nodes?

Thus the Transport and Network Layers are most important to us

ENTER TCP/IP

TCP/IP is a protocol suite designed to address these needs

TCP is a Transport Layer protocol equivalent to TP4 in the OSI

IP is an internet protocol which encompasses a bit more than the network layer of the OSI

The primary focus of TCP/IP is connecting networks, rather than connecting computers as in the OSI

TCP/IP

TCP/IP

At the Transport Layer we have the option of connection-oriented or connectionless service

Either can be implemented through virtual circuit or datagram at the network level

Connection-oriented communication has a more stringent requirement for correct and orderly delivery of of messages

On the other hand, a datagram is a simpler and more efficient implementation at the network level

PHILOSOPHY OF TCP/IP

By combining the connection-oriented transport layer with a datagram network layer we accommodate a large class of network applications

In other words shift the burden of providing reliable from the network to the operating system level

Here we have more control of the host than we do in the network

PROCESS COMMUNICATION

Generally uses ports, identified by a port id A process may have multiple ports, and

different processes may share a common destination port

Only unique within a local host A nonambiguous network-wide endpoint can

thus be established by concatenating the port id with the host and network addresses

INTERNET ADDRESSES

Full internet address consists of a network address and a host address within that network

May contain subnetwork address Internet IP address is 32 bits long

SOCKETS A socket specifies the interface to a port It is an abstraction of network I/O that allows

standard read and write operations The socket is configured to conform to a

particular protocol family and communication service

For TCP the socket must be bound to a destination transport port before read and write operations can take place

The destination port number is chosen by the operating system

Efficiency Study of TCP Protocols in Infrastructured Wireless Networks

Goal of research to isolate most efficient solution to non-congestion packet loss in TCP protocol over wireless network

TCP is inherently inefficient over wireless networks

TCP Tahoe and Reno, the two most common TCP distributions, both assume packet loss to be the result of congestion, since random loss occurs in wired networks less than 1% of the time

Efficiency Study of TCP Protocols in Infrastructured Wireless Networks

However, in a wireless network schema two other kinds of packet loss become common, these being random packet loss, which manifests through bit corruption, and disconnection packet loss, which occurs when a mobile host disconnects from the network

Both Tahoe and Reno will interpret this as congestion and trigger defensive action, which slows throughput rates

FOUR PROPOSED SOLUTIONS

TCP Westwood – Relies on a more optimistic estimation of the available bandwidth after a loss event has occurred

TCP Jersey – Actually distinguishes between congestion and noncongestion loss

TCP Veno focuses on solving the random non-congestion loss problem by using two parameters, the expected rate, and the actual rate, to calculate the backlog in the router queue that is used as an indication of congestion

Expected rate is calculated as the ratio of the congestion window over the best RTT

Actual rate is calculated as the ratio of the congestion window over the latest RTT

JTCP – assumes network congestion can be inferred from the difference in interarrival times of successive packet ACKs

Interarrival jitter – time difference between two packets on the sender side versus the time difference between the same two packets on the receiver side

If greater than 0, then the second packet traveled longer through the network than the first

Jitter ratio – variance of the queue rate – remember that queues form because rate of arrival of packets is greater than service rate at the router

JTCP uses jitter ratio, or variance in queue length, to determine types of loss in the network

BENCHMARK METRICS

Throughput – the ratio of the total data transferred to the time it took to transfer

Average Congestion Window – the sum of all congestion window sizes divided by the number of transmissions – provides an idea of the protocols resilience to loss and ability to recover

Time to Complete – time to transfer a continuous block of memory (file)

RESULTS

RESUTS

RESULTS

REFERENCES

Milan Todorovi´c, No´e L´opez-Benitez. Efficiency Study of TCP Protocols in Infrastructured Wireless NetworksNetworking and Services, 2006.

ICNS '06. International conference on

103,16-18 July 2006.