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Notes of Internet and Intranet [CT 754]

Internet Protocol Overview


TCP/IP and IP Layer Overview

TCP/IP Architecture

- TCP/IP stands for Transmission Control Protocol and Internet Protocol.
- It is a four layer conceptual model.
- The four layers are application layer, transport layer, Internet layer and Network layer.
- It provides a flexible architecture in the sense that adding new machines to the network is easy.
- The network is robust and connectionless.

IP Layer:

- It is responsible for addressing, routing and packaging functions.
- It uses the protocols like IP, ARP, ICMP and IGMP.
- IP is a routable protocol responsible for IP addressing, routing and fragmentation plus reassembly of the packets.
- This layer helps the packets to travel independently to the destination.
- The order in which the packets are received in the destination is different from the order they are sent.

For details of Transmission Control Protocol and User Datagram Protocol, view chapter 5 of Computer Networks.

IPv4: Address Format, Header Structure

Limitations of IPv4

- Due to address class allocation practices, public IPv4 addresses are becoming scarce. Because of this, it forces deployment of network address translator to share a public IPv4 address among several private addresses. But, NAT adds complexity and also becomes barrier for applications.
- IPv4 works with flat routing infrastructure in which individual address prefixes were assigned and each prefix became a new route in the routing table.
- IPv4 must be configured either manually or through DHCP.
- It do not have built-in security and rely upon Ipsec for security.
- Due to lack of infrastructure, communication with IPv4 mobile node are inefficient.

IPv6 as replacement of IPv4

- IPv6 addresses are 128 bits long, creating a huge amount of address space.
- It uses hierarchical routing infrastructure. It results in relatively few routing entries in the routing table.
- It is automatically configure with the host’s IPv6 address.
- It supports for Ipsec protocol headers is required. IPv6 packets are not required to be protected with Authentication header (AH) or Encapsulating Security Payload (ESP).
- It is capable of supporting mobility more efficiently.

IPv4 Header:

- The IPv4 header consists of following:
1. Version : Version number of Internet Protocol used.
2. Internet Header Length (IHL) : Length of entire IP header.
3. Differentiated Services Code Point (DSCP) : It is a type of service.
4. Explicit Congestion Notification (ECN) : Carries information about congestion seen in the route.
5. Total Length : Length of entire IP packet.
6. Identification : It helps fragments to identify original IP packet they belong to.
7. Flags : It tells whether to fragment or not.
8. Fragment Offset : It tells exact position of the fragment in the original IP packet.
9. Time to live : It tells how many hops a packet can cross.
10. Protocol : Tells network layer in destination about to which protocol the packet belongs to.
11. Header Checksum : Keeps checksum of the entire header.
12. Source Address : 32 bit address of the sender.
13. Destination Address : 32 bit address of the receiver.
14. Options : It is optional field which is used if the value of IHL is greater than 5.

- The header structure is shown in given figure:


IPv4 Address:

- IPv4 address is a 32 bit address composed of 4 octets, each of 8 bit; separated by ‘.’
- IPv4 uses hierarchical addressing scheme.
- A single IP address can contain information about the network, its sub-networks and also the hosts.
- It is hierarchical because a network can have many sub-networks, which in turns have many hosts.
- The general addressing scheme is shown below:

| Network(8) | Network(8) | Sub-Network(8) | Host(8) |

- Example :

IPv4 Fragmentation:

- The process of fragmentation in IPv4 is managed by a 32 bit field of the IPv4 header present at 4th octet.
- Identification, flag and fragment offset are responsible for fragmentation.
- A 16 bit identifier allows fragments to share a common value so that they can be identified as fragments from the same original packet in the destination.
- A 3 bit flags provides the status. The first bit is unused. The second bit if set, the packet can not be fragmented and must be discarded if it can not be forwarded. The third bit is More-fragments-bit, which is set for all the fragments except the last one.

Example: Suppose a router is attempting to pass 1320 octets of IP packet into a network whose maximum packet size is 532 octet. So fragmentation is needed. Here, the original packet is divided into three fragments. The first fragment with 532 octet (IP payload of 512 octet), second with 532 octet (IP payload of 512 octet) and third one with 296 octets (IP payload of 376 octet).
The demonstration is shown in given figure:


IPv6: Address Format, Header Structure

IPv6 Header:

- IPv6 has one fixed header and zero or more optional extension headers.
- The extension headers consists of information that helps routers to handle packet flow.
- IPv6 fixed header is of 40 bytes long with following information:
1. Version : Version of Internet Protocol used.
2. Traffic Class : The 6 MSB are used to indicate type of service and the 2 LSB are used for Explicit Congestion Notification (ECN).
3. Flow Label : It maintains the sequential flow of packets belonging to a communication. It avoids reordering of data packets in the destination.
4. Payload Length : It tells routers how much information a particular packet contains in its payload.
5. Next Header : It indicates types of extension header or if extension header is not present, it indicates upper layer PDU.
6. Hop Limit : It tells how many hops a packet can cross.
7. Source Address : 128 bit address of the sender.
8. Destination Address : 128 bit address of the receiver.

The extension headers are as follows:
1. Hop-by-hop Options Header
2. Destination Options Header
3. Routing Header
4. Fragment Header
5. Authentication Header
6. Encapsulating Security Payload Header


IPv6 Address:

- IPv6 address is made of 128 bits divided into eight 18 bits blocks.
- Each block is separated by colon ‘:’
- Example: 2001:0000:3238:DFE3:0063:0000:0000:AB4F
- Some rules are specified to shorten this length. They are as follows:
1. Discard leading zeros. In block 5, 0063 can be written as 63.
2. If two or more blocks contain consecutive zero, omit them all and replace with double colon ‘::’. Block 6 and 7 can be replace with ::
3. Consecutive zero blocks can be replaced by :: only one. So if zeros still prevails, they should be shrunk down to a single zero. In second block, 0000 can be written as 0
- After shortening, the IPv6 address is : 2001:0:3238:DFE3:63::AB4F

IPv6 Fragmentation:

- Fragmentation is handled by fragment header.
- It consists of only one flag bit (More-fragment bit) and other two bits are reserved.
- The packet identifier field is of 32 bits.
- IPv6 router is not able to fragment IPv6 packets. So IPv6 sender is responsible for fragmenting the IPv6 packet at the source.

Internet RFCs

Internet RFC

- RFC stands for Request for Comments.
- RFC documents is the documents that is used by the Internet community as a way to define new standards and share technical information.
- It is published by the researchers from universities and corporations to offer best practices and solicit feedback on Internet technologies.
- RFC is managed by an organization known as Internet Engineering Task Force (IETF).
- RFC should be in plain text format.
- RFC can also be used as the reference to study the glimpse of the early days of computer networking.
- Some of the early stages of computer networking technologies are documented in RFC including:
1. Internet domain name concepts (RFC 1034)
2. Address allocation for private intranets (RFC 1918)
3. HTTP (RFC 1945)
4. IPv6 (RFC 2460)
- Comments on RFC are given through the RFC Editor site (
- Each RFC gets a serial number.
- The RFC is static. If it is changed, then it gets a new serial number.

RFC Streams:

- There are four streams of RFC. They are as follows:
3. IAB
4. Independent Submission

- Only IETF can create Best Current Practice (BCP) and RFC on standard track.
- An independent submission is checked by IESG for conflicts with IETF work. The quality is assessed by the independent submission editorial board.
- IRTF and independent submission are supposed to be experiments for the Internet without any conflicts with IETF.

RFC Status:

- Each RFC is assigned a designation with regard to status within the Internet standardization process.
- The status may be:
1. Informational
2. Experimental
3. Best Current Practice
4. Standard Track
5. Historic

- Only IETF approves the standard tracks RFC.
- Standard track is again divided into proposed standard, draft standard and Internet standard.
- Once RFC is approved as Internet standard, it is provided with a STD number.
- Informational RFC can be anything that provides information about the Internet.
- Experimental RFC can be IETF document or individual submission. A draft is assigned experimental if it is unclear whether the proposal will work or widely accepted.
- BCP covers technical documents for how to practice Internet standards.
- Historic RFC are the RFC with the technologies that are no longer recommended for use.

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