Network Layer Functions and Concepts

Network Layer

• The network layer is responsible for transporting segments from the sending to the receiving host.
• It encapsulates segments into datagrams.
• On the receiving side, it delivers segments to the transport layer.

Network Layer Protocols

• Network layer protocols are present in every host and router.
• Routers examine the header fields in all IP datagrams passing through them.

Two Key Network Layer Functions

• Forwarding: Moving packets from a router's input to the appropriate output.
• Routing: Determining the path taken by packets from source to destination. This involves routing algorithms.

Interplay Between Routing and Forwarding

• The routing algorithm determines the end-to-end path through the network.
• The forwarding table determines the local forwarding actions at each router.
• The value in the arriving packet's header determines the output link.

Connection Setup

• In some network architectures (e.g., ATM, frame relay, X.25), before datagrams flow, two end hosts and intervening routers establish a virtual connection.
• This involves routers getting involved.
• Network layer connection service is between two hosts.
• Transport layer connection service is between two processes.

Network Service Models

• The network service model describes how datagrams are transported from sender to receiver.
• Service models for individual datagrams include guaranteed delivery and guaranteed delivery with less than 40msec delay.
• Models for datagram flows include in-order delivery and guaranteed bandwidth.

Chapter 4 Outline

• The chapter covers introduction, virtual circuits and datagram networks, what's inside a router, IP protocol, datagram format, IPv4 addressing, ICMP, IPv6, routing algorithms, routing in the Internet, broadcast, and multicast routing.

Virtual Circuits

• A virtual circuit behaves much like a telephone circuit in terms of its performance.
• Each packet carries a VC identifier (not destination host address).
• Every router on the path maintains state information for each connection.
• Link resources (bandwidth, buffers) can be allocated to a VC.

VC Implementation

• A VC consists of a path from source to destination, VC numbers for each link, and entries in the forwarding tables in routers along the path.
• Packets belonging to a VC carry a VC number(rather than destination address).
• The VC number can change on each link.
• The new VC number comes from the forwarding table.

Virtual Circuits: Signaling Protocols

• Used for setting up, maintaining, and tearing down virtual circuits in ATM, frame relay, and X.25.
• Not used in today's internet.

Datagram Networks

• No call setup at the network layer for end-to-end connections.
• No network-level concept of connection.
• Packets are forwarded based on the destination host address.

Datagram Forwarding Table

• The forwarding table lists ranges of destination addresses (aggregate table entries) and the corresponding output link.
• It uses longest prefix matching for finding the forwarding entry.

Routing Algorithms

• Link state: Routing algorithms that use all the link state information from all routers to compute the shortest path.
• Distance vector: Routing algorithms that use information from directly connected neighbors to calculate the shortest path.
• Hierarchical routing: A routing strategy that divides the network into smaller regions or hierarchical levels.

Routing in the Internet

• Protocols like RIP, OSPF, and BGP are used for routing in the internet.

Broadcast and Multicast Routing

• Methods for routing broadcast and multicast traffic within a network.

Router Architecture Overview

• The two functions are protocols(RIP, OSPF, BGP), and forwarding.
• A routing processor runs routing algorithms and the high-speed switching fabric handles packet forwarding.

Input Port Functions

• Input ports perform bit-level reception, data link layer receive function, and decentralized switching operations in routers.
• Given a datagram's destination, it looks up the output port using the forwarding table.
• Queuing is done if datagrams arrive faster than the forwarding rate.

Switching Fabrics

• Switching fabrics transfer packets from input to output buffers.
• Switching rate is the rate at which packets are transferred from inputs to outputs.
• Common fabrics include memory, bus, and crossbar.

Switching via Memory

• In the first-generation routers, packet copying to system memory limits speed by memory bandwidth.

Switching via a Bus

• Datagrams transfer between input port memory and output port memory via a shared bus.
• Bus contention limits switching speed by bus bandwidth

Switching via Interconnection Network

• It overcomes bus speed limitations by using a banyan network, crossbar, or other interconnection networks to connect processors in a multiprocessor.
• Datagrams are fragmented into fixed-length cells and switch cells through the fabrics.

Output Ports

• Output ports perform buffering and scheduling datagrams if the arrival rate from the fabric exceeds the output line speed.
• Datagram loss can occur due to congestion.

How Much Buffering

• RFC 3439 suggests average buffering should equal the RTT multiplied by link capacity.
• Recent recommendations suggest buffering equal to (RTT * C) / N, where N is the number of flows.

Input Port Queueing

• If the fabric is slower than the combined input ports, queueing occurs at input queues.
• Head-of-the-Line blocking occurs when a queued datagram prevents others in the queue from progressing.
• Output port contention can cause queueing (delay) and loss due to overflow
• Only one red datagram can be transferred at a time, the lower red packet is blocked.

ICMP: Internet Control Message Protocol

• Used by hosts and routers to communicate network-level information.
• Includes error reporting, unreachable host, network, port, protocol, echo request/reply (ping), and network-layer messages (e.g. source quench messages).

IPv6: Motivation

• The initial motivation was the impending exhaustion of 32-bit address space.
• Additional improvements include faster processing/forwarding and header modifications for QoS support.
• IPv6 uses a fixed-length header that allows no fragmentation.

IPv6 Datagram Format

• Includes IPv6 header fields (version, priority, payload length, flow label, next header, hop limit, source address, destination address), and data.

Other Changes From IPv4

• The checksum is removed to reduce processing time.
• Options are allowed but are outside the header, indicated by "Next Header."
• ICMPv6 includes new message types like "Packet Too Big," and multicast group management functions.

Transition From IPv4 to IPv6

• Tunneling is a method for carrying IPv6 datagrams as payload in IPv4 datagrams among IPv4 routers.
• Routers can carry different types of headers concurrently.

Tunneling

• IPv6 datagrams are carried as payload in IPv4 datagrams for transition between IPv4 routers.
• This allows IPv6 traffic to traverse networks running IPv4 infrastructure.

IPv6 Adoption

• The US National Institutes of Standards (NIST) have estimated the adoption rate of IPv6 in industry and government.
• Deployment has been gradual, with application-level changes taking place in 20 years, starting with WWW and Facebook.

IP Addressing: Introduction

• IP address is a 32-bit identifier for hosts and routers along with interfaces.
• An interface connects a host or router to a physical link.
• Routers and typically have multiple interfaces.

IP Addressing: CIDR

• Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses that uses variable length subnet masks. It defines an address format (a.b.c.d/x).
• X identifies how many 'bits' are used for the subnet portion of the address.

IP Addresses: How to Get One?

• Hosts can get IP addresses through several methods, such as hardcoded by system admins or using DHCP.
• DHCP is used for dynamic assignment of IP addresses via broadcast. It allows devices to obtain their IP addresses, and additional data, from a DHCP server on the network.

DHCP: More Than IP Addresses

• DHCP can return more than just an IP address, such as the address of the first-hop router and the DNS server's address.
• It also provides network mask information.

DHCP: Client-Server Scenario

• The DHCP consists of client-server interactions where a client requests and obtains an IP address from a DHCP server.
• A series of messages (DHCP discover, DHCP offer, DHCP request, DHCP ack) are exchanged.

DHCP: Example

• The DHCP server encapsulates its responses in UDP, which further gets encapsulated in IP, and that encapsulated in an Ethernet frame and broadcast onto the LAN.

IP Addresses: How to Get One?

• ISPs receive blocks of IP addresses allocated by ICANN (Internet Corporation for Assigned Names and Numbers).
• Organizations receive portions of address space for their networks.

NAT: Network Address Translation

• NAT allows a local network to use a single IP address to the outside world.
• NAT translates local addresses to a global address.

NAT: Network Address Translation (Implementation)

• NAT routers translate source IP addresses and port numbers in outgoing datagrams.
• They also translate destination fields in incoming datagrams.
• Using a NAT translation table, the NAT router maintains source and destination pairings.

NAT Traversal Problem

• The NAT traversal problem happens when a client wants to connect to a server, but the server's address is local to the LAN.
• A solution is to statically configure NAT to forward incoming requests to the server at a given port.

NAT: Motivation

• Local networks use one IP address for the outside world.
• This saves the need for address space from an ISP.
• It allows changes of devices and ISP without notifying the outside world.
• Devices inside a local network do not need to be explicitly addressable to the outside world.