Transport and Network Layer
1 – Introduction
Responsible for moving messages from end-to-end in a network
Closely tied together
TCP/IP: most commonly used protocol o Used in Internet
o Compatible with a variety of Application Layer protocols as well as with many Data Link Layer protocols
2 - Transport & Network Layer Protocols
Transport
end-to-end delivery of messages
o Sets up virtual circuits (when needed)
segmentation and reassembly
o Breaking the message into several smaller pieces at the sending end
o Reconstructing the original message into a single whole at the receiving end
Interacts with Application Layer Network
addressing and routing of messages
o Selects the best path from computer to computer until the message reaches destination
Performs encapsulation on sending end
o Adds network layer header to message segments
Performs decapsulation on receiving end
o Removes the network layer header at receiving end and passes them up to the transport layer
Transmission Control Protocol - TCP
Links the application layer to the network layer
Performs packetization and reassembly
o Breaks up a large message into smaller packets o Numbers the packets
o Reassembles the packets at the destination end
Ensures reliable delivery of packets
Internet Protocol
addressing and routing of packets o Two versions in current in use
IPv4: a 192 bit (24 byte) header, uses 32 bit addresses.
IPv6: Mainly developed to increase IP address space due to the huge growth in Internet usage (128 bit addresses)
o Both versions have a variable length data field
Max size depends on the data link layer protocol.
e.g., Ethernet’s max message size is 1,492 bytes, so max size of TCP message field:
1492 – 24 – 24 = 1444 bytes
– TCP/IP
3 Transport Layer Functions
Linking to the Application Layer
TCP may serve several Application Layer protocols at the same time o Problem: Which application layer program to send a message to?
o Solution: Port numbers located in TCP header fields; 2-byte each (source, destination)
Standard port numbers
o Usual practice numbers: 80 for HTTP, 25 for SMTP, 21 for FTP …
Nonstandard port numbers
o Possible, but requires configuration of TCP
o Can be used to enhance security from commonly known ports
Segmenting
Session management
Connection Oriented is provided by TCP
o Setting up a virtual circuit, or a TCP connection
o TCP asks IP to route all packets in a message by using the same path (from source to destination)
o Packet deliveries are acknowledged o Used by HTTP, SMTP, FTP
Connectionless Routing is provided by UDP
Sending packets individually without a virtual circuit
o Each packet is sent independently of one another, and will be routed separately, following different routes and arriving at different times
QoS Routing (provided by RTP)
– A special kind connection oriented routing with priorities More about User Diagram Protocol
Protocol used for connectionless routing in TCP/IP suite that uses no acks, no flow control
Uses only a small packet header
o Only 8 bytes containing only 4 fields:
Source port
Destination port
Message length
Header checksum
Commonly used for control messages that are usually small, such as DNS, DHCP, RIP and SNMP.
Can also be used for applications where a packet can be lost, such as information rich video More about Q of S
QoS defines and assigns priorities to “classes of service”
Timeliness - timely delivery of packets
o Packets be delivered within a certain period of time (to produce a smooth, continuous output
o Required by some applications, especially real time applications (e.g., voice and video frames)
o (e-mail doesn’t require this)
QoS routing
o Defines classes of service, each with a different priority:
Real-time applications such as VoIP- highest
A graphical file for a Web page - a lower priority
E-mail - lowest (can wait a long time before delivery)
Asynchronous Transfer Mode (ATM)
o A high-speed data link layer protocol
TCP/IP protocol suite
o Resource Reservation Protocol (RSVP)
Sets up virtual circuits for general purpose real-time applications o Real-Time Streaming Protocol (RTSP)
Sets up virtual circuits for audio-video applications o Real-Time Transport Protocol (RTP)
Used after a virtual connection setup by RSVP or RTSP
Adds a sequence number and a timestamp for helping applications to synchronize delivery
Uses UDP (because of its small header) as transport
4 – Addressing
Addressing
o Each device on the path between source and destination must have an address o Internet Addresses
o Assignment of addresses
o Translation between network layer addresses and other addresses (address resolution)
About assignment of address
Application Layer address (URL)
o For servers only (clients don’t need it)
o Assigned by network managers and placed in configuration files.
o Some servers may have several application layer addresses
Network Layer Address (IP address)
o Assigned by network managers, or by programs such as DHCP, and placed in configuration files
o Every network on the Internet is assigned a range of possible IP addresses for use on its network
Data Link Layer Address (MAC address)
o Unique hardware addresses placed on network interface cards by their manufacturers ( based on a standardized scheme)
Servers have permanent IP addresses, clients usually do not
Internet addresses and domain names
IPv 4
32 bits
Dotted Decimal Notation
134.139.1.2
IPv6
128 bits
3.2 x 1038 address Subnets
Subnet mask
To separate the subnet part of the address from the host part Dynamic Addressing
Give out IP address for client computers on network
Bootstrap Protocol (bootp)
Dynamic Host Control Protocol (DHCP)
Different approaches, but same basic operations:
o A program residing in a client establishes connection to bootp or DHCP server o A client broadcasts a message requesting an IP address (when it is turned on and
connected)
o Server (maintaining IP address pool) responds with a message containing IP address (and its subnet mask)
o IP addresses can also be assigned with a time limit (leased IP addresses) o When expires, client must send a new request
Domain Name Service
Used to determine IP address for a given URL
Provided through a group of name servers
o Databases containing directories of domain names and their corresponding IP addresses
Large organizations maintain their own name servers
o smaller organizations rely on name servers provided by their ISPs
When a domain name is registered, IP address of the DNS server must be provided to registrar for all URLs in this domain
o Example: Domain name: indiana.edu
o URLs: www.indiana.edu, www.kelly.indiana.edu, abc.indiana.edu How DNS workd
Each client has an address table
Use DNS server to resolve the address
Mac Address Resolution
Find MAC address of a host of a specific IP address
ARP
Broadcast an ARP message to all nodes on a LAN asking which node has a certain IP address
Host with that IP address then responds by sending back its MAC address
Store this MAC address in its address table
Send the message to the destination node
Example of a MAC address: 00-0C-00-F5-03-5A
5 – Routing
Process of identifying what path to have a packet take through a network from sender to receiver
Routing Tables
o Used to make routing decisions
o Shows which path to send packets on to reach a given destination o Kept by computers making routing decisions
Routers
o Special purpose devices used to handle routing decisions on the Internet o Maintain their own routing tables
Type of Routing
Centralized versus Decentralized
Centralized routing
o Decisions made by one central computer o Used on small, mainframe-based networks
Decentralized routing
o Decisions made by each node independently of one another o Information needs to be exchanged to prepare routing tables o Used by the Internet
More about decentralized routing
Static routing versus dynamic routing
Static routing
o Fixed routing table created by network manager o Table changes when computers added or removed
Dynamic Routing
o Routing tables are updated dynamically
o Routers exchange information for routers to update routing tables More about dynamic routing algorithms
Distance vector versus link state
Distance vector
o Uses the least number of hops to decide the best route o RIP and RIP-2
Link state
o Use a combination of information types to decide the best route: number of hops, congestion, speed of link
o Nodes exchanged link state info periodically to keep the network up to date o More reliable
Routing Protocols
Rules to exchange info among routers, so that each can build and maintain routing tables
Interior routing protocols versus exterior protocols
Exterior routing protocols operate between autonomous systems (networks of different companies)
Interior routing protocols operate within an autonomous system; RIP, OSPF, EIGRP, ICMP RIP
OSPF
Other interior routing protocols
Exterior routing protocol
For a computer to work in TCP/IP network
Required Network layer addressing information o Computer’s own IP address
o Its subnet mask
To determine what addresses are part of its subnet o Local DNS server’s IP address
To translate URLs into IP addresses o IP address of the router (gateway) on its subnet
To route messages going outside of its subnet
Address information is obtained from a configuration file or provided by a DHCP server
Servers also need to know their own application layer addresses (domain names)
6 - TCP/IP Example
About Router
Case 1a:
7 – Implications for Management
Most organizations moving toward a single standard based on TCP/IP o Decreased cost of buying and maintaining network equipment o Decreased cost of training networking staff
Telephone companies with non-TCP/IP networks are also moving toward TCP/IP o Significant financial implications for telcos
o Significant financial implications for networking equipment manufacturers
