COE332: Computer Networks - Network Layer

COE332: Computer Networks - Network Layer

• Course Material: Computer Networking: A Top-Down Approach, 6th edition, by Kurose and Ross.
• Chapter 4 Focus: Network Layer
• Chapter Goals: Understanding network layer services, including network layer service models, forwarding versus routing, how a router works (path selection), broadcast and multicast, and instantiation/implementation in the Internet.
• Network Layer Function: Transport segments from the sending host to the receiving host. On the sending side, it encapsulates segments into datagrams. On the receiving side, it delivers segments to the transport layer. Network layer protocols operate in every host and router. Routers examine header fields in all IP datagrams.
• Two Key Network Layer Functions:
  • Forwarding: Moving packets from a router's input to appropriate output. Analogy: traveling from one place to another.
  • Routing: Determining the route packets take from source to destination. Analogy: planning a trip from one place to another. This includes routing algorithms.
• Interplay of Routing and Forwarding: The routing algorithm determines the end-to-end path through a network. The forwarding table determines the local forwarding at the router taking into account the network header information.
• Connection Setup: An important function in some architectures (ATM, frame relay, X.25). Before datagrams flow, end hosts and routers establish virtual connections. Routers handle the network layer connection, while transport handles connections between transport layer processes.
• Network Service Model: Models for transporting datagrams from sender to receiver include guaranteed delivery with minimal delay or guaranteed minimum bandwidth, in-order delivery, etc.
• Network Layer Service Models:
  • Internet: Best effort, no guarantees, no congestion feedback.
  • ATM: Various models like CBR (constant bit rate), VBR (variable bit rate with guaranteed rate), ABR (variable bit rate with guaranteed minimum rate), and UBR (unspecified bit rate) with varying guarantees and congestion control.
• Connection-less vs. Connection Service: - Datagram networks provide connectionless service. - Virtual circuit networks provide connection service, analogous to TCP/UDP. Host-to-host service (one or other available). Implementation in network core.
• Datagram Networks: No call setup at the network layer, no state about end-to-end connections, no network-level concept of connection, packets forwarded using destination host address.
• Datagram Forwarding Table: 4 billion IP addresses so ranges (aggregate table) listed rather than individual addresses. Forwarding table has destination address, output link.
• Longest Prefix Matching: When searching the forwarding table, the longest matching address prefix is used.
• IP Datagram Format: Includes protocol version, header length, type of data, max number of remaining hops, upper layer protocol, source and destination IP addresses, options (variable length), and total datagram length, used for segmentation/reassembling. Includes how to calculate overhead to ensure enough space for information.
• IP Fragmentation and Reassembly: Network links have MTUs (maximum transmission units), and large IP datagrams are divided (fragmented) within a network. A single datagram becomes multiple smaller datagrams. Reassembly only at final destination.
• IP Addressing: Introduction 32-bit identifiers for hosts and router interfaces, one per interface, e.g., 223.1.1.1.
  • How interfaces are connected: Ethernet, WiFi.
  • DHCP Protocol: Dynamic Host Configuration Protocol for dynamically obtaining IP addresses from the server.
• Subnets: Device interfaces with the same subnet part of the IP address can physically reach each other without an intervening router. Determine subnets by detaching interfaces from host/router, creating isolated networks
• IP Addressing: CIDR (Classless Inter-Domain Routing): subnet portion length of address is variable. IP address format: a.b.c.d/x (x is #bits in subnet).
• IP Addresses: How to Get One hard-coding is an option, but Dynamic Host Configuration Protocol (DHCP) allows dynamically obtaining an address from a server.
• DHCP: Dynamic Host Configuration Protocol Allows the host to dynamically obtain an IP address from a server. Server gives an IP address, first-hop router, and DNS server info.
• NAT: Network Address Translation Local networks may use a single IP address to the outside world. Outbound datagrams have source IP/port changed (NAT IP/new port #). Incoming datagrams have dest IP/port changed. 16-bit port number field for many simultaneous connections.
• NAT Traversal Problems A client may want to connect to a server inside a NAT'd network. Static configuration or protocols like UPnP can address this.
• IPv6: Motivation 32-bit address space soon allocated. Header format helps with speed, and processing/forwarding, and header changes facilitate QoS.
• IPv6 Datagram Format Fixed-length 40-byte header, no fragmentation allowed
• Other Changes From IPv4 Checksum removed for processing time reduction. Options field outside the header, indicated by Next Header field. ICMPv6 new version, extra message types. Multicast group management functions.
• Tunneling: IPv6 Datagram carried as payload in IPv4 for routers in a mixed IPv4/IPv6 environment.
• Interplay Between Routing and FW Routing algorithm determines end-to-end path, FW table determines local forwarding on a router.
• Graph Abstraction Useful in network contexts. Nodes are routers, links cost or connection.
• Routing Algorithm Classification Global (link state) / Decentralized (distance vector), Static/Dynamic (routes change).
• Hierarchical Routing Aggregate routers into regions (Autonomous Systems). Intra-AS routing protocols for internal destinations, and inter-AS for external destinations.
• Inter-AS Tasks: AS1 must learn destinations, propagate reachability, and determine appropriate gateway routers.

This quiz focuses on Chapter 4 of 'Computer Networking: A Top-Down Approach,' covering the critical functions of the network layer. You'll explore topics such as network layer services, routing, and how routers operate. Test your understanding of datagrams and packet forwarding.