Transport Layer

Transport Layer Overview

Transport layer is layer 4 of the OSI model. Refer to the previous graphic used

Transport Layer

It responds to service requests from the session layer and issues service requests to the network layer.

The transport layer provides transparent transfer of data between hosts.

It is responsible for end-to-end error recovery and flow control.

It ensures complete data transfer.

Transport Layer

In the IP protocol Stack this function is achieved by the connection oriented Transmission Control Protocol (TCP) or the datagram type User Datagram Protocol (UDP).

The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer.

Transport Layer

Optional services that can be provided at layer 4 are: Connection Oriented. This is normally easier to

deal with than Connectionless models, so where the Network layer only provides a connectionless service, often a connection oriented service is built on top of that in the Transport layer.

Connection vs Connectionless Connectionless describes communication between

two network end points in which a message can be sent from one end point to another without prior arrangement. The device at one end of the communication transmits data to the other, without first ensuring that the recipient is available and ready to receive the data. The device sending a message simply sends it addressed to the intended recipient. If there are problems with the transmission, it may be necessary to resend the data several times. The Internet Protocol (IP) and User Datagram Protocol (UDP) are connectionless protocols.

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The other method of transmitting data is the connection-oriented approach, in which the devices use a preliminary protocol to set up an end-to-end connection before any data can be sent. Connection-oriented protocol service is sometimes called a "reliable" network service, because it guarantees that data will arrive in the proper sequence. For connection-oriented communications, each end point must be able to transmit so that it can communicate. Transmission Control Protocol (TCP) is a connection-oriented protocol.

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Connection vs Connectionless

Transport Layer

Optional services that can be provided at layer 4 are: Same Order Delivery. The Network layer doesn't

generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature, so the Transport layer provides it. The simplest way of doing this is to give each packet a number, and allow the receiver to reorder the packets.

Transport Layer

Optional services that can be provided at layer 4 are: Error 'Free' Data. The underlying network may well be

noisy, and the data received may not always be the same as the data sent. The Transport layer can fix this: typically by providing a checksum of the data which detects if there has been a glitch of some kind. Of course, error free is impossible, but it is possible to substantially reduce the numbers of undetected errors. This layer may also retransmit packets which have gone missing en route.

Transport Layer

Optional services that can be provided at layer 4 are: Flow Control. The amount of memory on a

computer is limited, and without flow control a larger computer might flood a computer with so much information that it can't hold it all before dealing with it. Nowadays, this is not a big issue, as memory is cheap while bandwidth is comparatively expensive, but in earlier times it was more important.

Transport Layer

Optional services that can be provided at layer 4 are: Byte Orientation. Rather than dealing with things

on a packet-by-packet basis, the Transport layer may add the ability to view communication just as a stream of bytes. This is nicer to deal with.

Transport Layer

Optional services that can be provided at layer 4 are: Ports. Ports are essentially ways to address

multiple entities in the same location. For example, the first line of a postal address is a kind of port, and distinguishes between different occupants of the same house. Computer applications will each listen for information on their own ports, which is why you can use more than one network-based application at the same time.

Transport Layer & The Internet

Two most common transport services are UDP and TCP.

TCP is the more complicated, providing a connection and byte oriented stream which is almost error free, with flow control, multiple ports, and same order delivery.

UDP is a very simple 'datagram' service, which provides limited error reduction and multiple ports. TCP stands for Transport Control Protocol, while UDP stands for User Datagram Protocol.

In-Depth TCP

Definition from Wikipedia: Transmission Control Protocol (TCP) is a

connection-oriented, reliable delivery byte-stream transport layer protocol currently documented in IETF RFC 793.

In the TCP/IP model, TCP provides an interface between a network layer below and an application layer above. Applications send streams of 8-bit bytes to TCP for delivery onto the network.

In-Depth TCP

TCP connections contain three phases: connection establishment data transfer connection termination.

A 3-way handshake is used to establish a connection. A four-way handshake is used to tear-down a connection. During connection establishment, parameters such as sequence numbers are initialized to help ensure ordered delivery and robustness.

In-Depth TCP – Connection Establishment

While it is possible for a pair of end hosts to initiate a connection between themselves simultaneously, typically one end opens a socket and listens passively for a connection from the other.

In-Depth TCP – Data Transfer

During the data transfer phase, a number of key mechanisms determine TCP's reliability and robustness. These include using sequence numbers for ordering received TCP segments and detecting duplicate data, checksums for segment error detection, and acknowledgements and timers for detecting and adjusting to loss or delay.

In-Depth TCP – Connection Termination

The connection termination phase uses a four-way handshake, with each side of the connection terminating independently. Therefore, a typical teardown requires a pair of FIN and ACK segments from each TCP endpoint.

TCP Ports

TCP uses the notion of port numbers to identify sending and receiving applications.

Each side of a TCP connection has an associated 16-bit unsigned port number assigned to the sending or receiving application.

Ports are categorized into three basic categories: well known, registered and dynamic/private. The well known ports are assigned by the Internet Assigned Numbers Authority (IANA) and are typically used by system-level or root processes.

TCP Ports

Well known applications running as servers and passively listening for connections typically use these ports. Some examples include: FTP (21), TELNET (23), SMTP (25) and HTTP (80).

Registered ports are typically used by end user applications as source ports when contacting servers, but they can also identify named services that have been registered by a third party.

TCP/IP Introduction

TCP and IP were developed by a Department of Defense (DOD) research project to connect a number different networks designed by different vendors into a network of networks (the "Internet"). It was initially successful because it delivered a few basic services that everyone needs (file transfer, electronic mail, remote logon) across a very large number of client and server systems.

TCP/IP Introduction

Several computers in a small department can use TCP/IP on a single LAN. The IP component provides routing from the department to the enterprise network, then to regional networks, and finally to the global Internet. On the battlefield a communications network will sustain damage, so the DOD designed TCP/IP to be robust and automatically recover from any node or phone line failure.

TCP/IP In-Depth Like other protocols, composed of layers: IP - is responsible for moving packet of data from node

to node. IP forwards each packet based on a four byte destination IP address. The Internet authorities assign ranges of numbers to different organizations. The organizations assign groups of their numbers to departments.

TCP - is responsible for verifying the correct delivery of data from client to server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.

Sockets - is a name given to the package of subroutines that provide access to TCP/IP on most systems.

TCP/IP In-Depth

To insure that all types of systems from all vendors can communicate, TCP/IP is absolutely standardized on the LAN. However, larger networks based on long distances and phone lines are more volatile. In the US, many large corporations would wish to reuse large internal networks based on IBM's SNA. In Europe, the national phone companies traditionally standardize on X.25.

Addressing

Each technology has its own convention for transmitting messages between two machines within the same network. On a LAN, messages are sent between machines by supplying the six byte unique identifier (the "MAC" address).

IP Addressing

On top of these local or vendor specific network addresses, TCP/IP assigns a unique number to every workstation in the world. This "IP number" is a four byte value that, by convention, is expressed by converting each byte into a decimal number (0 to 255) and separating the bytes with a period. For example, Microsoft’s web server IP address is 207.46.156.220.

IP Addressing An organization begins by registering a name or

address with a company that provides services, requesting assignment of a network number. It is still possible for almost anyone to get assignment of a number for a small "Class C" network in which the first three bytes identify the network and the last byte identifies the individual computer.

Larger organizations can get a "Class B" network where the first two bytes identify the network and the last two bytes identify each of up to 64 thousand individual workstations. Yale's Class B network is 130.132, so all computers with IP address 130.132.*.* are connected through Yale.

IP Addressing & DNS The only way for your system to know what

www.microsoft.com is in an actual address is to use DNS. This is Domain Name System, and serves as a look-up table for name resolution.

Definition from wikipedia.com: The Domain Name System stores information about host and domain names on the Internet. Most importantly, it provides an IP address for each host name, and lists the mail exchange servers accepting e-mail for each domain.

The DNS is a vital part of the Internet, because IP addresses are needed for routing but host names and domain names are used by humans, for example in URLs and email addresses.

How DNS Works

A domain name consists of two or more parts separated by periods. The rightmost label is the top-level (for example, the top-level domain for www.wikipedia.org is org). Each label to the left specifies a subdivision or subdomain (for example, wikipedia.org is a subdomain of org and www.wikipedia.org is a subdomain of wikipedia.org).

In theory, this subdivision can be up to 127 levels deep, and each label can be up to 63 characters long, as long as the whole domain name is no longer than 254 characters.

DNS Records A Record (address record) maps a host name to its

IP address CNAME Record (canonical name record) makes

one domain name an alias of another MX Record (mail exchange record) maps a domain

name to a list of mail exchange servers for that domain

PTR Record (pointer record) maps a host name to the canonical name of that host; it is used for reverse DNS lookup

NS Record (name server record) maps a domain name to a list of DNS Servers for that domain

Addressing & Network Paths

Every time a message arrives at an IP router, it makes an individual decision about where to send it next. There is concept of a session with a pre-selected path for all traffic. Consider a company with facilities in New York, Los Angeles, Chicago and Atlanta. It could build a network from four phone lines forming a loop (NY to Chicago to LA to Atlanta to NY). A message arriving at the NY router could go to LA via either Chicago or Atlanta. The reply could come back the other way.

Addressing & Network Paths/Routing

How does the router make a decision between routes? There is no correct answer. Traffic could be routed by the "clockwise" algorithm (go NY to Atlanta, LA to Chicago). The routers could alternate, sending one message to Atlanta and the next to Chicago. More sophisticated routing measures traffic patterns and sends data through the least busy link.

Addressing & Network Paths/Routing If one phone line in this network breaks down, traffic can still

reach its destination through a roundabout path. After losing the NY to Chicago line, data can be sent NY to Atlanta to LA to Chicago. This provides continued service though with degraded performance. This kind of recovery is the primary design feature of IP.

The loss of the line is immediately detected by the routers in NY and Chicago, but somehow this information must be sent to the other nodes. Otherwise, LA could continue to send NY messages through Chicago, where they arrive at a "dead end." Each network adopts some Router Protocol which periodically updates the routing tables throughout the network with information about changes in route status