Practical Approach - Application Layer with Transport Layer

Agenda

• Review of models

• Application layer interacts with transport / Scenarios

• Port Addressing

• Responsibilities of Transport Layer

• Tools to help understand and monitor

The entire picture – top down approach

• OSI – theoretical, reference model

• Internet Protocol Suite – practical & widely used, protocol stack on which the internet works.

Cisco Systems(2007), Application Layer Functionality and Protocols, Retrieved from http://ciscoserver.eastms.edu/scoleman/ccna1.html

Application layer protocols

• DNS – Matches/ resolves domain names with IP addresses

• HTTP – Used to transfer data between clients/servers using a web browser

• SMTP & POP3 – used to send email messages from clients to servers over the internet

• FTP – allows the download/upload of files between a client/server

• Telnet – allows users to login to a host from a remote location and take control as if they were sitting at the machine (virtual connection)

• DHCP – assigns IP addresses, subnet masks, default gateways, DNS servers, to users as they login the network

Cisco Systems(2007), Application Layer Functionality and Protocols, Retrieved from http://ciscoserver.eastms.edu/scoleman/ccna1.html

Applications – Processes and Services

DNS Server

• Telnet Server DHCP

• Email Server

• Web Server

• FTP Server

DNS Telnet SMTP, POP, IMAP HTTP DHCP FTP

In and IT Environment – Companies, Enterprises etc.

Examples DHCP

Retrieved from http://wintips4u.blogspot.com/2002/11/dhcp-gujaratikanudo.html

TELNET DNS

Retrieved from http://www.cisco.com/en/US/products/hw/optical/ps2006/products_tech_note09186a008052e85f.shtml

Retrieved from http://ciscoserver.eastms.edu/scoleman/ccna1.html

Applications need transport

Transport layer = LIAISON Protocols at this layer oversee the delivery of data from an application program on one computer to an application program on another computer. More importantly, they act as a liaison between the application-layer protocols and the services provided by the lower layers (network, data link, and physical). Retrieved from http://highered.mcgraw-hill.com/sites/0072397020/student_view0/chapter3/ Stallings (2010), Each application within a host must have an address that is unique within the host; this allows the host-to-host protocol (TCP) to deliver data to the proper process. These latter addresses are known as Ports. How is the application identified? – Transport layer uses application identifier – PORTS Internal address reserved for a specific application. It determines which application layer protocols is used.

Application connects to transport layer

Link layer Network

IP Internet

TCP port UDP port

20 21 80 69 53

FTP HTTP TFTP DNS Application

TCP UDP Transport

Socket interface Socket

IP address & TCP/UDP port

PORT ADDRESSING TRANSPORT LAYER

• Many common application have default port assignments.

• Port numbers are assigned in different ways depending on whether it is a request or response.

• The source port in a segment of a client request is generated randomly once it does not conflict with existing

used ports.

• The destination port is either assigned by default or manually IANA assignment ports – International Assigned Numbers Authority http://www.iana.org/

• 0 to 1023 - Well Known (Contact) Ports • 1024 to 49151 – Registered Ports • 49152 to 65535 – Private and/or Dynamic Ports

Responsibility of Transport Layer – Port Addressing

Retrieved from http://ciscoserver.eastms.edu/scoleman/ccna1.html

Other responsibilities - TRANSPORT Layer

• Segmentation • Session Multiplexing • Identification of different applications • Sequencing • Flow Control • Connection Oriented • Reliability

RELIABLE BEST EFFORT

Connection Type Connection-oriented Connectionless

Protocol TCP UDP

Sequencing Yes No

Uses Email / File Sharing /Downloading

Voice Streaming / Video Streaming/ Online Gaming

TCP / UDP

TCP

TRANSPORT LAYER

UDP

TCP FTP – 20-21 Telnet – 23 SMTP – 25 DNS – 53 (Both TCP & UDP) HTTP – 80

UDP DHCP – 67 & 68 DNS - 53 TFTP – 69 www.iana.org RFC 1700 - http://www.cse.ohio-state.edu/cgi-bin/rfc/rfc1700.html

NETWORK UTILITIES

TCP – 3 WAY HANDSHAKE IN TCP / Connection Oriented CONNECTION ESTABLISHMENT

Retrieved from – http://blogs.ixiacom.com/ixia-blog/tcp-portals-the-handshakes-a-lie/

Protocol Analyzer

Connection Request

Retrieved from - http://www.highteck.net/IT/Transport/Livello_OSI_Transport.html

Protocol Analyzer

• Connection confirmation

Retrieved from - http://www.highteck.net/IT/Transport/Livello_OSI_Transport.html

Protocol Analyzer

• Acknowledgement of confirmation

Retrieved from - http://www.highteck.net/IT/Transport/Livello_OSI_Transport.html

NetStat

• Examine open TCP connections over the network host

• Unexplained TCP connections which can pose a threat by consuming resources

Retrieved from - http://www.highteck.net/IT/Transport/Livello_OSI_Transport.html

Troubleshooting Network Utility tools

• FIDDLER

• Microsoft Network monitor

SUMMARY • When we use apps over the internet the application layer protocols takes the request

down to lower layers.

• The transport layer is the liaison, responsible for delivering the segments to lower layer.

• A port is what connects the application layer to the transport layer. Tells the transport layer which protocol is speaking and whether to use tcp/udp to deliver.

• TCP – Reliable connection oriented

• UDP – Connectionless

• Application protocols + ports tells us whether TCP/UDP to use.

• Tools to use for monitoring and troubleshooting TCP connections.

REFERENCES

Coleman S.. (2007), Application Layer Functionality and Protocols, In Network Fundamentals CCNA Cisco Systems– (Chapter 3)

Retrieved from http://ciscoserver.eastms.edu/scoleman/ccna1.html

Stallings ,W. (2010), Cryptography and Network Security, Fifth Edition William Stallings Prentice Hall.

http://www.highteck.net/IT/Transport/Livello_OSI_Transport.html

http://wintips4u.blogspot.com/2002/11/dhcp-gujaratikanudo.html

http://www.cisco.com/en/US/products/hw/optical/ps2006/products_tech_note09186a008052e85f.shtml

http://blogs.ixiacom.com/ixia-blog/tcp-portals-the-handshakes-a-lie/

http://www.iana.org/numbers