Ch 5 Network Layer Essays

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Business Data Communications & Networking
Chapter 5
Network and Transport Layers
Outline
Transport Layer Protocols
Network Layer Protocols
Transport Layer Functions
Linking to the application layer
Segmenting
Session Management
Network Layer Functions
Addressing
Routing
TCP/IP Examples
Implications for Management

Network and Transport Layers
Transport Layer
Layer 4 in the Internet model
Links application and network layers
Responsible for segmentation and reassembly
Session management
Responsible for end-to-end delivery of messages
Network Layer
Layer 3 in the Internet model
Responsible for addressing and routing of messages
Protocols
TCP/IP
Originally developed as a single internetworking protocol by Vint Cerf and Bob Kahn in 1974
Later divided into the TCP and IP protocols
Most common protocols of the Internet and in LANs, WANs, and backbone networks

Transport Layer Protocols
Transmission Control Protocol (TCP)
Most common transport layer protocol
PDU called a segment
Used for reliable transmission of data
160 - 192 bits (20 -24 bytes) of overhead
Options field is not required
User Datagram Protocol (UDP)
Operates at the transport layer
PDU called a segment
Used in time-sensitive situations, for control messages, or when reliability is handled by the application layer
32-64 bits (4-8 bytes) of overhead
Source port is optional in IPv4 and IPv6, Checksum is optional in IPv4
Network Layer Protocols
Internet Protocol (IP)
IP version 4 (IPv4)
Most common version of IP used
32-bit addresses (232 or ~4.29 billion possible)
Exhaustion of address space
IP version 6 (IPv6)
128-bit addresses (2128 or ~3.4 × 1038 possible)
Slowly being adopted due to IPv4 exhaustion
Network Protocols
IPv4 Packet
160-192 bits (20-24 bytes) of overhead
Options field rarely used
IPv6 Packet
Fixed Header
320 bits (40 bytes) of overhead
Transport Layer Functions
1. Linking to the application layer
TCP/UDP may serve multiple application layer protocols
Ports used to identify application (2-byte numbers)
Many source/destination ports follow standards
Common port standards
HTTP: TCP port 80
HTTPS: TCP port 443
FTP: TCP ports 20 and 21
SMTP: TCP port 25
IMAP: TCP port 143
POP3: TCP port 110 (more commonly TCP port 995 secure version)
DNS: TCP or UDP port 53 (most commonly UDP)
Transport Layer Functions
Transport Layer Functions

2. Segmenting
Breaking up large files into smaller segments (and putting them back together)
Segments may be passed individually to application layer or after reassembly
How large are the segments?
Size depends on the network and data link layer protocols
Maximum Segment Size (MSS) is negotiated during TCP handshake
e.g., if the maximum size of the data in an Ethernet frame is 1,500 bytes and TCP and IP use 20 byte headers, the maximum segment size is 1460 bytes
Transport Layer Functions 3. Session management
A session can be thought of as a conversation between two computers or creating a virtual circuit
Using a session to send data is also called connection-oriented messaging (TCP)
Sending messages without establishing a session is connectionless messaging (UDP)
TCP connections are opened using a three-way handshake
SYN
SYN-ACK
ACK
Sessions provide reliable end-to-end connections
Network Layer Functions
Addressing
Used to direct messages from source to destination
Addresses are assigned in various ways (e.g., by system administrators, ICANN, hardware vendors, etc.)
Addresses exist at different layers
Addresses may be translated (resolved) from one layer to another (e.g., DNS, ARP)
Address Type
Example
Example Address
Application layer
Uniform Resource Locator (URL) www.indiana.edu Network layer
IP address
129.79.78.193 (4 bytes)
Data link layer
MAC address
1C-6F-65-F8-33-8A (6 bytes)

Addressing
IPv4 addresses are 32 bits
Most common way to write is using dot-decimal notation
Easier for people to read and remember
Breaks the address into four bytes and writes each byte in decimal notation instead of binary
Example:
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