Communication Protocols

• Communication protocols are complex, and involve computers, terminals, and data processing devices exchanging data
• There must be a data path between two computers, including the following considerations:
• Activate communications path to inform the network destination
• Source must check the destination is prepared to receive data
• File transfer application on source must confirm the destination file management system accepts/stores the file
• May need file format translation

Protocol Architecture

• Complex implementation is achieved by breaking down tasks into subtasks or modules, implemented separately
• Modules are arranged into a vertical stack
• Each layer performs a related subset of functions
• Each layer relies on the layer below it for more primitive functions
• Each layer provides services to the layer above it
• Peer layers communicate via rules or conventions known as a "protocol"

Key Elements of a Protocol

• Communication is achieved when peer layers in two different systems communicate
• Key features include:
• Syntax, concerning the data block format
• Semantics, which dictates control info for coordination and error handling
• Timing, which considers speed matching and sequencing

TCP/IP Protocol Architecture

• Result of protocol research and development conducted by ARPANET
• ARPANET's research was funded by the Defense Advanced Research Projects Agency (DARPA)
• Used by the global Internet
• Referred to as the TCP/IP protocol suite, which is a large grouping of protocols
• Protocols have been issued as Internet standards by the Internet Activities Board (IAB)

Simplified Network Architecture

• Communications involve three agents:
• Applications such as file transfer
• Computers such as PCs and servers
• Networks
• Applications execute on computers which can support simultaneous applications
• Computers are connected to networks where data is exchanged between computers
• Tasks are partitioned into 3 layers

TCP/IP Layers

• Does not have an official layer model and predates the OSI Reference Model
• Operates on a 5-layer model:
• Physical layer
• Network access layer
• Internet layer
• Host-to-host(or transport) layer
• Application Layer

TCP/IP - Physical Layer

• This layer covers the physical interface between a data transmission device and a transmission medium/network
• Specifies characteristics for:
• The transmission medium
• The nature of the signals
• The data rate and related considerations

TCP/IP - Network Access Layer

• Concerned with data exchange between an end system and the network it is attached to
• The sending computer considers issues like:
• Providing the destination address
• Invoking certain services (priority)
• Accessing and routing data across a network link between systems
• Different standards available for circuit switching, packet switching, and LANs

TCP/IP - Internet Layer

• Uses Internet Protocol (IP) to traverse multiple interconnected networks
• IP provides communication between devices attached to different networks
• Internet Protocol (IP) at this layer:
• Provides the routing function across multiple networks
• Implemented not only in end systems but also in routers
• A router is a processor that connects two networks to relay data from one network to the other on its best route from the source to destination

TCP/IP - Transport Layer

• The host-to-host, or transport layer:
• Collects mechanisms in a common layer shared by all applications
• Provides reliable data delivery, regardless of the underlying application
• Guarantees data arrives at the destination in the same order it was sent
• Commonly uses TCP

TCP/IP - Application Layer

• This layer contains the logic needed to support various user apps like file transfer
• A separate module handles each different application type

Operation of TCP/IP

• Protocols are configured for communications
• The total communication facility may contain multiple networks or subnetworks
• A network access protocol is used, like Ethernet logic
• Ethernet connects a computer to a subnetwork
• Enables the host to send data across the subnetwork to another host
• Or, send data to a router that will forward the data outside the subnetwork
• IP is implemented in end-systems and routers & acts as a relay to move data blocks
• TCP is implemented only in the end systems to ensure all data is reliably delivered to the appropriate application

Data Addressing

• Two addressing levels are needed:
• Each host on a subnetwork must have a unique global address called an IP address
• This allows data to be delivered to the correct host
• Each process within a host must have an address called a port
• This address allows for data delivery to the correct process

PDU Transmission

• Process:
• The sending process generates a block of data and passes it to TCP
• TCP may break the block into smaller pieces, appending control information called a TCP header, to form a TCP segment
• TCP hands each segment to IP to transmit it to the receiving process at host B
• IP appends a header with control information to each TCP segment, to form an IP datagram
• IP hands each datagram over to the network access layer, to transmit across the first subnetwork
• The network access layer appends its own header, creating a packet or frame
• The packet is transmitted across the subnetwork to router J

Transmission Control Protocol (TCP)

• The transport layer protocol
• Provides a reliable connection for data transfer between applications
• A logical connection is defined by a given pair of port values
• Throughout the connection, each entity keeps track of TCP segment flow in order to regulate that flow and recover lost/damaged segments

TCP Header Format

• A TCP segment includes a header of at least 20 octets or 160 bits
• Source Port and Destination Port fields identify the source and destination system applications using the connection
• Sequence Number, Acknowledgment Number, and Window fields enable flow control and error control
• The checksum is a 16-bit frame check sequence for error detection in the TCP segment

User Datagram Protocol (UDP)

• An alternative to TCP
• UDP is connectionless, has no guaranteed delivery, no sequence preservation, no protection against duplication, but offers minimum overhead
• Adds port addressing to IP

IP Header Format

• Minimum of 20 octets, or 160 bits
• Version indicates the version of IP being used
• IP Header Length (IHL) indicates the datagram header length in 32-bit words
• Type-of-Service specifies how an upper-layer protocol wants a datagram handled, and assigns importance levels
• Total Length specifies the length, in bytes, of the IP packet -- including data and header
• The ID, Flags, and Fragment Offset fields are used in the fragmentation and reassembly process
• Time-to-Live maintains a counter that decrements to zero, at which point the datagram is discarded
• Protocol indicates which upper-layer protocol receives incoming packets after IP processing completes
• Header Checksum helps ensure IP header integrity
• Source Address specifies the sending node
• Destination Address specifies the receiving node
• Options allows IP to support various options like security
• Data contains the upper-layer data sent in the packet

IPv6 Header

• Provides a number of functional enhancements, over existing IPv4, to handle increased network speeds and varied data streams

TCP/IP - Application Layer Protocols

• Standardized protocols operate on top of TCP:
• Simple Mail Transfer Protocol (SMTP)
• Provides a basic electronic mail transport facility for transferring messages between separate hosts
• Stores messages to a user mailbox
• File Transfer Protocol (FTP)
• Sends files from one system to another under user command
• Accommodates both text and binary files
• TELNET
• Provides a remote logon capability for users at a terminal to log onto a remote computer and work as if directly connected

OSI (Open Systems Interconnection)

• Developed by the International Organization for Standardization(ISO)
• It has seven layers
• TCP/IP is considered the de facto standard

OSI v TCP/IP

• The TCP/IP architecture has come to dominate
• TCP/IP protocols were mature and well tested when similar OSI protocols were in development
• The OSI model is unnecessarily complex with seven layers

Standardized Protocol Architectures - OSI Model

• Motivation for developing the OSI model was to provide a standardization framework
• Defines what functions can be performed at that layer and facilitate the standards
• Changes in standards at one layer shouldn't affect software at another layer
• The communications function is broken down into seven distinct layers
• Each layer provides services to the next higher layer and implements a protocol to the peer layer in other systems.

Layer specific Standards

• Nature of needed standardization at each layer
• Three elements are necessary for operating systems:
• Protocol specification
• Includes the format ofprotocol data units exchanged, semantics of all fields, and the allowable sequence of PDUs
• Service definition
• Describes what services are provided but not how they are provided
• Addressing.
• Referenced by means of a service access point (SAP)
• Network service access points (NSAP) indicate a transport entity that is a user of the network service

Service Primitives & Parameters

• Used to define services between adjacent layers
• Primitives specify functions performed
• Parameters pass data and control information
• Process:
• Data is transferred from an N entity to a peer N entity, and the following occurs:
• The source N entity invokes its N-1 entity with a request primitive
• This includes needed parameters
• The source N-1 entity prepares an N-1 PDU to send to the peer N-1 entity.
• The destination N-1 entity gives the appropriate destination N entity via an indication primitive.
• The destination entity issues a response primitive to its N-1 entity (if an acknowledgment is needed)
• N-1 entity conveys the acknowledgment in an N-1 PDU
• Finally, the acknowledgment is delivered to the N entity as a confirm primitive
• This is a confirmed service because the initiator receives confirmation that the requested service worked at the other end

Primitive Types

• Request
• A primitive issued by a service user to invoke some service, and to pass the parameters needed to specify the requested service
• Indication
• A primitive issued by a service provider to indicate that a procedure has been invoked by the peer service user or notify them of action
• Response
• A primitive issued by a service user to acknowledge or complete a procedure previously invoked by an indication to that user
• Confirm
• A primitive issued by a service provider to acknowledge a request by the service user or to complete so