Computer Networks - Topic 6B: Network Layer: IP Fundamentals, IP Addressing, IPv4, IPv6

Why IP Fundamentals, IP Addressing, IPv4, IPv6?

1. The Internet Protocol (IP) is the basis of the Internet and central to how it functions, enabling seamless data transfer across global distances and disparate networks. Understanding IP is essential.

2. Key functional aspects of contemporary networks depend on the operation of IP and the routing functions it enables.

3. Designing and coding any networked application requires an understanding of the function and especially addressing schemes used by IP

4. Managing any network requires an understanding of the function and especially addressing schemes used by IP

OSI Context

• The OSI model is a conceptual model that describes the functions of a networking system. It's characterized by layers:

  • Application

  • Presentation

  • Session

  • Transport

  • Network

  • Data Link

  • Physical

• The TCP/IP model is a practical implementation of the OSI model, collapsed into fewer layers:

  • Application

  • Transport

  • Internet

  • Link

  • Physical

• The IEEE 802 reference model focuses on the Data Link layer, further dividing it into:

  • Logical Link Control (LLC)

  • Medium Access Control (MAC)

Internet Protocol Fundamentals

• The Internet Protocol (IP) was designed to:

  • Permit traffic to be routed across a multiplicity of different networks each using their own and possibly dissimilar or disparate datalink protocols, each with their own addressing schemes, limits on packet sizes and unique flow and error control schemes

  • Operate across a global network regardless of the underlying datalink protocols and their specific MAC layer addressing schemes

  • Operate with a network of routing nodes that receive, store and forward packets to other routing nodes over datalink channels between these nodes – the earliest “routers” were general purpose computers with routing software and datalink interface hardware installed, later dedicated router devices were developed

  • Permit fragmentation and defragmentation of incoming packets produced by higher layer protocols. The ability to operate across datalink channels, hop by hop, with disparate frame/packet sizes.

• Top 10 principles

  1. Make sure it works

  2. Keep it simple

  3. Make clear choices

  4. Exploit modularity

  5. Expect heterogeneity

  6. Avoid static options and parameters

  7. Look for a good design; it need not be perfect

  8. Be strict when sending and tolerant when receiving

  9. Think about scalability

  10. Consider performance and cost

• Topics:

  • IP Version 4 Protocol

  • IP Addresses

  • IP Version 6 Protocol

  • Internet control protocols

  • Label switching and MPLS

• The Internet is an interconnected collection of many networks.

Internet Protocol Addressing

• The Internet Protocol (IP) was designed around an addressing model that permits operation across a global network regardless of:

  • underlying datalink protocol MAC layer addressing

  • underlying datalink protocol packet sizes

  • underlying datalink protocol flow control

  • underlying datalink protocol error control

• The addressing model has evolved from IPv1 through the widely used IPv4 to the current IPv6 model

• Key concepts:

  • Prefixes - a contiguous block of IP address space

  • Subnets

  • CIDR — Classless InterDomain Routing

  • Classful and special addressing

  • NAT — Network Address Translation

A Prefix and a Subnet Mask

A prefix and a subnet mask
$Prefix length = L bits$
$Network$
$Host$
$Subnet mask 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 000$

Subnets

Splitting an IP prefix into separate networks with subnetting

CIDR-Classless InterDomain Routing

• A set of IP address assignments example:

  • Cambridge: first address 194.24.0.0, last address 194.24.7.255, 2048, Prefix 194.24.0.0/21

  • Edinburgh: first address 194.24.8.0, last address 194.24.11.255, 1024, Prefix 194.24.8.0/22

  • Available: first address 194.24.12.0, last address 194.24.15.255, 1024, Prefix 194.24.12.0/22

  • Oxford: first address 194.24.16.0, last address 194.24.31.255, 4096, Prefix 194.24.16.0/20

• Aggregation of IP prefixes

Classful and Special Addressing

• IP address formats

• special IP addresses

  • This host $0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0$

  • Network 127

  • A host on this network $(Anything)$

  • Broadcast on the local network $1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1$

  • Broadcast on a distant network

  • Loopback

• Private internets:

  • 10.0.0.0 - 10.255.255.255 (10/8 prefix)

  • 172.16.0.0 - 172.31.255.255 (172.16/12 prefix)

  • 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

• An enterprise that decides to use IP addresses out of the address space defined in this document can do so without any coordination with IANA or an Internet registry. The address space can thus be used by many enterprises.

NAT―Network Address Translation

Placement and operation of a NAT box

NAT-Network Address Translation Example

NAT operation

IPv4 and IPv6

• Packets flow down and up the protocol stack as they flow through the network

• Routing nodes only care about forwarding a packet to its intended destination via a specified route

• Therefore, packets are only buffered and retransmitted on another datalink interface

• Fragmentation is the process of breaking a packet payload into multiple “fragments” usually of equal size, and sending them between two routers

• Fragmentation is performed when the frame (packet) size of the datalink channel between two routers is too small for the received packet.

• The router receiving fragments will reassemble them into the original packet – fragments must be numbered so they can be reassembled (offset)

• Fragmentation allows use of arbitrary datalink channels

The IP Version 4 (IPv4) Protocol

$Version$
$IHL$
$Differentiated services$
$Total length$
$Identification$
$Flags$
$Fragment offset$
$Time to live$
$Protocol$
$Header checksum$
$Source address$
$Destination address$
$Options (0 or more words)$

• Version (IPv4)

• Internet header length - in 32 bit words - including options

• Type of service

• Total length of datagram, in octets

• Identification - sequence number, used with addresses and user protocol to identify datagram uniquely

• Flags - More bit, Don’t fragment

• Fragmentation offset

• Time To Live – decremented with each hop between routers

• Protocol - Next higher layer to receive data field at destination

• Header checksum - reverified and recomputed at each router, 16 bit ones complement sum of all 16 bit words in header, set to zero during calculation

• Source Address, Destination Address, Options, Padding to fill to multiple of 32 bits

The IP Version 4 (IPv4) Protocol

Option's Description:

IP Version 6

• IPv6 major goals

  • Support billions of hosts

  • Reduce routing table size

  • Simplify the protocol

  • Provide better security

  • Attention to type of service

  • Aid multicasting

  • Roaming host without changing address

  • Allow future protocol evolution

  • Permit coexistence of old and new protocols for years

• IP version 6 improvements

  • Longer addresses than IPv4

  • Simplification of the header

  • Better support for options

  • Big advance is in security

  • Quality of service

The Main IPv6 Header

$Version$
$Diff. services$
$Flow label$
$Payload length$
$Next header$
$Hop limit$
$Source address (16 bytes)$
$Destination address (16 bytes)$

IPv6 Extension Headers Description

• The hop-by-hop extension header for large datagrams (jumbograms)

• The extension header for routing

Point-to-Point Protocol (PPP)

• PPP provides an encapsulation protocol over both bit-oriented synchronous links and asynchronous links with 8 bits of data and no parity. These links MUST be full-duplex, but MAY be either dedicated or circuit-switched.

• PPP uses HDLC as a basis for the encapsulation.

• PPP has been carefully designed to retain compatibility with most commonly used supporting hardware.

• The PPP encapsulation also provides for multiplexing of different network-layer protocols simultaneously over the same link. It is intended that PPP provide a common solution for easy connection of a wide variety of hosts, bridges and routers.

• Some protocols expect error free transmission, and either provide error detection only on a conditional basis, or do not provide it at all. PPP uses the HDLC Frame Check Sequence for error detection. This is commonly available in hardware implementations, and a software implementation is provided.

• PPP is comprised of three main components:

  1. A method for encapsulating datagrams over serial links.

  2. A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection.

  3. A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols.

• The Point-to-Point Protocol uses the principles, terminology, and frame structure of the International Organization For Standardization's (ISO) High-level Data Link Control (HDLC) procedures (ISO 3309-1979 [2]), as modified by ISO 3309:1984/PDAD1 "Addendum 1: Start/stop transmission" [5]. ISO 3309-1979 specifies the HDLC frame structure for use in synchronous environments. ISO 3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to allow its use in asynchronous environments.

• PPP has very wide range of configuration options for different payload protocol types with Unique LCP and NCP options.

Internet Control Protocols

• ICMP — RFC 792/4443 Internet Control Message Protocol

• ARP — RFC 826 Address Resolution Protocol

• DHCP — RFC 2131 Dynamic Host Configuration Protocol

• These protocols are used to manage nodes in a local network, but some also collect in some instances status information from network nodes

• Utilities are usually available to permit administrators to use these protocols from the command line

• These protocols and utilities are widely used

ICMP-The Internet Control Message Protocol Types

• Internet Control Message Protocol

• RFC 792 (get it and read it)

• Transfer of (control) messages from routers and hosts to hosts

• Feedback about problems

• e.g. time to live expired

• Encapsulated in IP datagram

• Not reliable

ping - the ICMP echo Utility

• ICMP Echo —The Internet Control Message Protocol Echo type

• The ping utility sends an echo command and measures the round trip delay

• Used for status monitoring, testing and debugging of network installations

RFC 826 ARP-The Address Resolution Protocol

Two switched Ethernet LANs joined by a router

• The arp utility displays and modifies the Internet-to-Ethernet address translation tables used by the address resolution protocol (arp(4)).

• With no flags, the program displays the current ARP entry for hostname.

• The host may be specified by name or by number, using Internet dot notation

• -a The program displays or deletes all of the current ARP entries.

• -d A super-user may delete an entry for the host called hostname with the -d flag.

• -n Show network addresses as numbers

• -s hostname ether_addr Create an ARP entry for the host called hostname with the Ethernet address ether_addr.

RFC 2131 DHCP—The Dynamic Host Configuration Protocol

• “The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCPIP network.

• DHCP is based on the Bootstrap Protocol (BOOTP), adding the capability of automatic allocation of reusable network addresses and additional configuration options.”

• “There are two primary differences between DHCP and BOOTP. First, DHCP defines mechanisms through which clients can be assigned a network address for a finite lease, allowing for serial reassignment of network addresses to different clients.

• Second, DHCP provides the mechanism for a client to acquire all of the IP configuration parameters that it needs in order to operate.”

• Often network routers or modem-router devices have embedded DHCP servers

DHCP-The Dynamic Host Configuration Protocol

• DHCP servers can be run as daemons on host systems, or be embedded in other devices such as routers, or modem-routers

• ADSL and other home user modem-routers are usually supplied with embedded DHCP server software designed to autonomously build and maintain a DHCP table across the network

• Typically a DHCP server installation includes, usually proprietary, management utilities to assign static IP address mappings so users can control which IP addresses are assigned to which devices

• A typical DHCP management utility allows the specification of IP addresses and lease durations (in the case of static assignments for as long as the user intends)

Reading Materials

1. Tanenbaum, Feamster, Wetherall, Computer Networking, 6th Edition, Pearson Education, Chapter 5

2. Stallings, Data and Computer Communications, International Edition, 10th edition, Chapter 15

3. Kreyszig, Advanced Engineering Mathematics, 10th Ed, Chapter 23