Network Layer Functions and Protocols

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Questions and Answers

Which of the following is a function of the network layer?

Ensuring data integrity

Data encryption and decryption

Session management

Path selection (correct)

What is the role of ICMP in the network layer?

Data segmentation

Traffic encryption

Address assignment

Error reporting (correct)

What does a VC number carry in a packet?

Destination address

Connection type

Source address

VC number (correct)

How is a VC number changed along a path?

From the forwarding table of the router Signup and view all the answers

What characteristic distinguishes datagram networks from virtual circuit networks?

They do not have a network-level concept of connection Signup and view all the answers

In the VC forwarding table, what does the 'Incoming interface' represent?

The interface of the next hop router Signup and view all the answers

What type of information do VC routers maintain?

Connection state information Signup and view all the answers

Which layer in the network model initiates the call for a virtual circuit?

Application layer Signup and view all the answers

What is the role of routers in datagram networks regarding connections?

They have no state about end-to-end connections Signup and view all the answers

What does the NAT translation table maintain?

Source IP address to NAT IP address translations Signup and view all the answers

How does a NAT router modify outgoing datagrams?

It replaces the source address with the NAT IP and new port number Signup and view all the answers

What happens when incoming datagrams arrive at a NAT router?

Their destination fields are replaced with stored source address and port Signup and view all the answers

Which of the following is NOT a function of a NAT router?

Sending unmodified datagrams to the Internet Signup and view all the answers

What is stored in the NAT translation table for each pair?

Source IP address and port number with corresponding NAT IP and port Signup and view all the answers

What is the primary function of a router in a network?

Forwarding datagrams from incoming to outgoing links Signup and view all the answers

Which of the following describes 'smart' end systems?

They can perform control and error recovery. Signup and view all the answers

What is a primary characteristic of traditional routers that use memory for switching?

Direct control of CPU for packet processing. Signup and view all the answers

Which type of switching fabric is specifically designed to eliminate bandwidth limitations of a bus?

Switching through an interconnection network Signup and view all the answers

What is the purpose of routing algorithms like RIP and OSPF in a router?

To determine the optimal path for datagram forwarding Signup and view all the answers

What is meant by 'queueing' in the input port functions of a router?

Managing packets arriving faster than the forwarding rate Signup and view all the answers

What does 'line speed' refer to in the context of input port processing?

Maximum data transfer rate of the transmission line Signup and view all the answers

Which routing method is characterized by always sending data in a predestined manner without reevaluation?

Distance vector routing Signup and view all the answers

Which of the following statements is true regarding 'dumb' end systems such as telephones?

They rely on the network for most of the processing. Signup and view all the answers

What is the goal of a switching fabric within a router?

To transfer packets from input buffer to output buffer efficiently Signup and view all the answers

What is an essential characteristic of devices within the same subnet?

They can physically reach each other without an intervening router. Signup and view all the answers

What is the subnet mask indicated in the document?

/24 Signup and view all the answers

What action is necessary to determine the subnets from a host or router?

Detach each interface to create isolated networks. Signup and view all the answers

Which part of the IP address represents the subnet?

High order bits Signup and view all the answers

What is another type of interface besides wired Ethernet mentioned?

Wireless WiFi interfaces Signup and view all the answers

What does isolating networks create?

A subnet Signup and view all the answers

How is the subnet part of the IP address defined?

By the first n bits specified in the subnet mask Signup and view all the answers

What defines a 'subnet' in networking?

An isolated network segment with a common subnet part Signup and view all the answers

What does tunneling involve in the context of IPv6 and IPv4 networks?

Carrying IPv6 datagrams as payloads in IPv4 datagrams Signup and view all the answers

What potential issue arises from the transition from IPv4 to IPv6?

Mixed environments with both IPv4 and IPv6 routers Signup and view all the answers

Which statement regarding the adoption of IPv6 is true?

Approximately 11% of US government routers are using IPv6 Signup and view all the answers

What is a characteristic difference between the logical and physical views of tunneling?

The physical view represents the actual path taken by packets Signup and view all the answers

What is the significance of not having 'flag days' in the transition to IPv6?

It implies that both IPv4 and IPv6 can operate concurrently Signup and view all the answers

In a mixed IPv4 and IPv6 environment, how are data packets transmitted between routers?

IPv6 packets can be wrapped in IPv4 packets during transit Signup and view all the answers

What percentage of industry IP routers reportedly support IPv6 according to estimates?

3% Signup and view all the answers

What does the logical view of a network typically represent?

The flow of data between logical channels Signup and view all the answers

Study Notes

Chapter 4: Network Layer

Computer Networking: A Top-Down Approach (6th edition) by Jim Kurose and Keith Ross, Addison-Wesley, March 2012
Chapter Goals: Understand principles behind network layer services, including network layer service models, forwarding versus routing, how a router works, routing (path selection), broadcast, multicast, instantiation, and implementation in the Internet.

Chapter 4: Outline

4.1 Introduction
4.2 Virtual Circuit and Datagram Networks
4.3 What's Inside a Router
4.4 IP: Internet Protocol
- Datagram format
- IPv4 addressing
- ICMP
- IPv6
4.5 Routing Algorithms
- Link state
- Distance vector
- Hierarchical routing
4.6 Routing in the Internet:
- RIP
- OSPF
- BGP
4.7 Broadcast and Multicast Routing

Network Layer

Transport Segment: Transports segments from sending to receiving host.
Encapsulation: Encapsulates segments into datagrams on sending side.
Delivery: Delivers segments to transport layer on receiving side.
Protocols: Network layer protocols operate in every host and router.
Header Fields: Routers examine header fields in all IP datagrams passing through.

Two Key Network Layer Functions

Forwarding: Moves packets from a router's input to the appropriate output port.
Routing: Determines the route taken by packets from source to destination.
Routing Algorithms: Used to determine routes.

Interplay Between Routing and Forwarding

Routing Algorithm: Determines the end-to-end path through the network.
Local Forwarding/Table: Determines the local forwarding at the router.

Connection Setup

Network layer connection services: Between two hosts (or potentially intervening routers in Virtual Circuits).
Transport Layer connection services: Between two processes.

Network Service Models

Example Services for Individual Datagrams:
- Guaranteed delivery
- Guaranteed delivery with less than 40 msec delay
Example Services for a Flow of Datagrams:
- In-order datagram delivery
- Guaranteed minimum bandwidth to flow
- Restrictions on changes in inter-packet spacing

Virtual Circuits

Analogy: Source-to-destination path behaves like a telephone circuit for performance and actions along that path.
Call Setup: Call setup/teardown for each call before data flows.
VC Identifier: Each packet carries a VC identifier (not destination host address).
State Maintenance: Every router maintains state for each passing connection on the source-destination path.
Resource Allocation: Link and router resources (bandwidth, buffers) may be allocated to VC (predictable service).

VC Implementation

VC path: Constitutes the path from source to destination.
VC numbers: Assigned one number for each link along the path (entries in forwarding tables).
Packet carrying VC number: Packet belonging to VC carries VC number (rather than dest address).
VC number changes: VC number can be changed on each link.
New VC number from forwarding tables: New VC number comes from forwarding tables.

VC Forwarding Table

Incoming interface: Interface the packet arrives on.
Incoming VC#: The VC number of the incoming packet.
Outgoing interface: Interface the packet leaves on.
Outgoing VC#: The VC number of the outgoing packet.

Virtual Circuits: Signaling Protocols

Used in: ATM, frame-relay, X.25.
Not Used in: Today's Internet.

Datagram Networks

No call setup: No call setup at network layer.
No state: No network-level concept of "connection."
Destination address: Packets are forwarded using destination host address.

Datagram Forwarding Table

Dest Address: Destination address or range of addresses.
Output link: Corresponding output link for the destination.

Longest Prefix Matching

Matching: When looking for a forwarding table entry, use the longest address prefix that matches destination address.

Datagram or VC Network: Why?

Internet (datagram): Data exchange among computers; "elastic" service, no strict timing; many link types; different characteristics; "smart" end systems (can adapt, perform control, error recovery).
ATM (VC): Evolved from telephony (human conversation); strict timing, reliability requirements; need for guaranteed service; "dumb" end systems (telephones); complexity inside network.

Router Architecture Overview

Routing algorithms/protocol : Run routing protocol (RIP, OSPF, BGP).
Forwarding datagrams: Forward datagrams from incoming to outgoing links.

Input Port Functions

Line termination: Bit-level reception.
Link Layer (receive): Link layer protocol receive.
Lookup/forwarding: Lookup forwarding, and queueing.
Switch fabric: Switch fabric acts as a connection point.
Decentralized switching: Given a datagram, look up output port using forwarding table.

Switching Fabrics

Packet transfer: Transfer packets from input buffer to appropriate output buffer.
Switching rate: Rate at which packets can transfer from inputs to outputs.
Types: Memory switching, bus switching, crossbar switching.

Switching via Memory

First-generation routers: Traditional computers with switching under direct control of CPU.
Packet Copying: Packet copied to system memory.
Speed limitation: Limited by memory bandwidth.

Switching via Bus

Datagram Transfer: Datagram is transferred from input port memory to output port memory via a shared bus.
Bus contention: Switching speed limited by the bus bandwidth.
Cisco 5600 example: 32 Gbps bus in Cisco 5600 is sufficient for access/enterprise routers.

Switching via Interconnection Network

Bus/Bandwidth Limitation issues: Overcome bus bandwidth limitations with banyan networks, crossbar, other interconnection networks.
Advanced design: Fragmenting datagram into fixed-length cells, switching cells through fabrics (e.g., Cisco 12000).

Output Ports

Buffering: Buffering is required from fabric faster rate.
Scheduling: Priority scheduling.

Output Port Queuing

Arrival rate: Buffering when arrival rate via switch exceeds output line speed.
Loss and delay: Queueing (delay) and loss due to the output port buffer overflow.

How Much Buffering?

RFC 3439: Average buffering is equal to typical RTT(say 250msec).
Capacity (C): Times link capacity C (e.g., 10 Gbps link: ~2.5 Gbit buffer).
Recommendation for many flows: With N flows, buffering equal to RTT.C / N

Input Port Queuing

Fabric slower: Fabric slower than input ports combined which can lead to queueing at input queues.
Delay/Loss: Queueing delay and loss due to input buffer overflow.
HOL blocking: Head-of-the-line blocking.

ICMP

Internet Control Message Protocol: Used for error reporting from hosts and routers.

IPv6: Motivation

Address space: 32-bit address space is shrinking.
Header format: Header format improvement for faster processing and QoS.

IPv6 Datagram Format

Priority and flow labels: Identifies priority and flow, respectively.
Fixed-length header: Fixed-length 40 byte header.
No fragmentation: No fragmentation (to avoid issues at lower levels/routers).

Other Changes from IPv4

Checksum removal: Processing time reduced through checksum removal.
Options: Options outside of the header, indicated by "Next Header" field in order to save space.
ICMPv6: Extensions to ICMP.
Message Types: New/Additional message types are added.

Transition from IPv4 to IPv6

Tunneling: IPv6 datagram carried as payload in an IPv4 datagram for transit between IPv4 and IPv6 routers.

IP Addressing

Introduction: IP addresses are identifiers for hosts and routers on an internet.
Interface: An interface is a connection between a host/router and a physical link.
Host/routers: Routers typically have multiple interfaces; Hosts typically has one or two interfaces (Wired/Wireless).

Subnets

IP address: High-order bits in subnet and low-order bits in host part.
What's a Subnet: A subnet is a set of interfaces with the same subnet part of the IP address.
Recipe: To determine subnets, detach each interface from its host and create islands of isolated networks.

IP Addressing: CIDR

Classless Inter-Domain Routing: Allows assigning arbitrary-length subnet portions of an IP address.
Format: Format is a.b.c.d/x where x is the number of bits in subnet portion of address.

IP Addresses: How to Get One?

Hard-coded: Hard-coded by system administrator.
DHCP: Dynamic Host Configuration Protocol: Dynamically obtain IP address from the server.

DHCP: Dynamic Host Configuration Protocol

Goal: Allows hosts to dynamically obtain IP addresses from a network server.
Lease renewal: Renew IP leases automatically.
Address reuse: Allows reuse of addresses when a device is no longer connected on the network.

DHCP Client-Server Scenario

Client's Request: Arriving DHCP client needing an address from the network.

DHCP: More than IP addresses

More Data: Provides more data beyond the IP address: first-hop router, DNS server, subnet mask.

NAT: Network Address Translation

Motivation: Local networks use one IP address to the outside world; range of addresses not needed; can change addresses without notifying outside world; can change ISPs.
Implementation: NAT router must translate addresses when packets are sent out or received by the router.

NAT Traversal Problem

External visibility: NATted devices are only visible externally through a single address.
Solution 1: Statically configure NAT to forward incoming connection requests at a given port to the server.

IPv6: Adoption

Estimates: US National Institutes of Standards estimate (2013) about ~3% of industry IP routers and ~11% of US government routers use IPv6.
Deployment and usage: Long periods for deployment and widespread usage.

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Description

Test your knowledge on the various functions and protocols associated with the network layer in computer networks. This quiz covers topics like ICMP, routing protocols such as OSPF and BGP, and the distinction between virtual circuit networks and datagram networks.