Transport and Network Layers

Transport and Network Layers

• The transport layer is layer 4 in the Internet model and links the application and network layers
• The transport layer is responsible for segmentation and reassembly and session management
• The transport layer handles the end-to-end delivery of messages
• The network layer is layer 3 in the Internet model
• The network layer is responsible for addressing and routing messages

TCP/IP Protocol

• Originally developed as a single internetworking protocol in 1974 by Vint Cerf and Bob Kahn
• TCP/IP was later divided into TCP and IP protocols
• TCP/IP are the most common protocols on the Internet and in LANs, WANs, and backbone networks

Transport Layer Protocols

• Transmission Control Protocol (TCP) is the most common transport layer protocol
• TCP's PDU is called a segment
• TCP is used for reliable transmission of data, with a 160-192 bit (20-24 byte) overhead
• The options field in TCP is not always required
• User Datagram Protocol (UDP) operates at the transport layer
• UDP's PDU is also called a segment
• UDP is used in time-sensitive situations and for control messages
• UDP overhead is 32-64 bits (4-8 bytes)
• In UDP, the source port is optional in IPv4 and IPv6; Checksum is optional in IPv4

Network Layer Protocols

• The IPv4 is the most common version of IP used
• IPv4 uses 32-bit addresses (~4.29 billion possible), but has led to an exhaustion of address space
• IPv6 uses 128-bit addresses (~3.4 x 1038 possible) and is slowly being adopted due to IPv4 exhaustion

IPv4 Packet

• IPv4 packet overhead is 160-192 bits (20-24 bytes)
• An options field is rarely used

IPv6 Packet

• IPv6 has a fixed header with 320 bits (40 bytes) of overhead
• IPv6 includes optional headers like hop-by-hop options and destination options

Transport Layer Functions

• The transport layer links to the application layer
• TCP/UDP may serve multiple application layer protocols
• Ports are used to identify applications, using 2-byte numbers, with many source/destination ports following standards
• Common port standards include:
  • 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 (or TCP port 995 for secure version)
  • DNS: TCP or UDP port 53 (most commonly UDP)

Segmentation

• Segmentation involves breaking up large files into smaller segments and putting them back together
• Segments may be passed individually to the application layer or after reassembly
• Segment size depends on the network and data link layer protocols
• Maximum Segment Size (MSS) is negotiated during TCP handshake
• 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

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 called connection-oriented messaging (TCP)
• Sending messages without establishing a session is called connectionless messaging (UDP)
• TCP connections are opened using a three-way handshake using SYN, SYN-ACK, and ACK
• Sessions provide reliable end-to-end connections

Addressing

• Addressing is used to direct messages from source to destination
• Addresses are assigned in various ways, like by system administrators, ICANN, hardware vendors, etc
• Addresses exist at different layers and may be translated from one layer to another, like DNS or ARP
• Examples of addresses:
  • Application layer: Uniform Resource Locator (URL), like www.indiana.edu
  • Network layer: IP address, like 129.79.78.193 (4 bytes)
  • Data link layer: MAC address, like 1C-6F-65-F8-33-8A (6 bytes)
• IPv4 addresses are 32 bits and are commonly written using dot-decimal notation
• A portion of an IP address represents the network, and the rest identifies the host
• Classful addressing uses the first bits to determine the number of hosts, though it is discontinued, the nomenclature is still used

Addressing Types

• Classless Inter-Domain Routing (CIDR) uses subnet masks to more flexibly divide address space into subnets
• Dynamic addressing involves a server supplying IP addresses automatically
• Dynamic Host Configuration Protocol (DHCP) is the most common protocol for dynamic addressing
• DHCP involves a device sending out a broadcast message and addresses being "leased" for a length of time
• Address resolution involves host (server) name resolution and MAC address resolution
• Domain Name Service (DNS) translates a host name to an IP address
• Address Resolution Protocol (ARP) identifies the MAC address of the next device in the circuit

Routing

• Routing is the process of identifying the path a packet takes through a network from sender to receiver
• Routing tables are used to make routing decisions, showing which path to send packets on to reach a given destination
• Routers are special purpose devices used to handle routing decisions on the Internet, and maintain their own routing tables
• Centralized routing involves one computer making routing decisions, which is not common anymore
• Decentralized routing involves each node independently making decisions and exchanging information
• The Internet uses decentralized routing

Routing Types

• Static routing utilizes fixed routing tables and are manually configured by network managers
• Dynamic routing has routing tables that are updated periodically; routers exchange information using protocols to update tables
• Distance vector algorithms are based on the number of "hops" between two devices
• Link state algorithms are based on the number of hops, circuit speed, and traffic congestion, which can provide more reliable, up-to-date paths to destinations
• Routing Information Protocol (RIP) is a dynamic distance vector protocol used for interior routing, wherein the network manager builds the routing table and routers counts "hops" and selects the shortest route when new computers are added
• Open Shortest Path First (OSPF) is a dynamic link state protocol used for interior routing in large enterprise networks
• Enhanced Interior Gateway Routing Protocol (EIGRP) is a dynamic link state protocol
• EIGRP records transmission capacity, delay time, reliability and load for all paths and keeps the routing tables for its neighbors and uses this information in its routing decisions as well
• Border Gateway Protocol (BGP) is a dynamic distance vector protocol used for exterior routing that provides routing information only on selected routes (e.g., preferred or best route)

Multicasting

• Unicast is one computer to another computer
• Broadcast is one computer to all computers in the network
• Multicast is one computer to a group of computers (e.g., videoconference)
• The same data needs to reach multiple receivers, avoiding transmitting it once for each receiver
• Multicasting is particularly useful if the access link has bandwidth limitations
• In IP multicast, hosts dynamically join and leave multicast groups using Internet Group Management Protocol (IGMP)

TCP/IP Example

• Required network addressing information:
  • Device’s own IP address
  • Subnet mask
  • IP address of default gateway (most commonly the router)
  • IP address of at least one DNS server

TCP/IP Examples

• A Client (128.192.98.130) requests a Web page from a server (www1.anyorg.com); the client knows the server's IP
• A Client (128.192.98.130) requests a Web page from a server (www2.anyorg.com) on a different subnet; the client knows the server’s IP
• A Client (128.192.98.130) requests a Web page from a server (www1.anyorg.com); the client does not know the server’s IP

TCP/IP and Layers

• Packets move through all layers of host computers
• At gateways and routers, packets move from the Physical layer to the Data Link Layer through the network Layer
• Ethernet packets are removed and a new one is created for the next node in each stop along the way
• IP and above packets never change in transit (created by the original sender and destroyed by the final receiver)

Implications for Management

• Organizations are standardizing on TCP/IP, which decreases equipment and training costs
• Network providers are also moving toward standardization
• Transition to IPv6 is slow