Lec-55: Options & Padding in IPv4 Header | Computer Networks

Summary

In this video, the instructor explains IPv4 options and the padding field in the IPv4 header. Key topics include the 'Record Route' option, which records the IP addresses of routers a packet passes through, and 'Source Routing,' where the source defines the packet's path through specific routers. The instructor also covers padding, where extra bytes are added to the header to ensure its size is a multiple of four. The video provides a detailed understanding of these concepts, their use, and how they impact network communication, with examples for better clarity.

Takeaways

• 😀 IPv4 options and padding are additional fields that can be included in the IPv4 header, but their use depends on the specific packet or fragment.
• 😀 The IPv4 header size ranges from 20 bytes to 60 bytes, with 20 bytes being the minimum and 60 bytes being the maximum. If the header is 20 bytes, no options are used.
• 😀 The options field in IPv4 can be from 0 to 40 bytes, and if the header size is 60 bytes, this means 40 bytes are dedicated to options.
• 😀 The 'Record Route Option' allows for the recording of the IP addresses of routers that the packet passes through. However, due to security concerns, this option has been removed in IPv6.
• 😀 In the 'Record Route Option,' a maximum of 9 router addresses can be recorded (since the options field is 40 bytes and each address is 4 bytes).
• 😀 The 'Source Routing' option allows the sender to define the exact path the packet will take through the network, either strictly or loosely.
• 😀 There are two types of source routing: strict (defining the exact path through all routers) and loose (allowing flexibility in some parts of the path).
• 😀 Source routing is typically restricted to network administrators, and regular users do not have the option to use it.
• 😀 Padding is used to ensure the IPv4 header size is a multiple of 4 bytes. If the header size is not a multiple of 4, padding is added to make it one.
• 😀 If the IPv4 header is 23 bytes, for example, 1 byte of padding (8 bits of zeros) will be added to make it 24 bytes, which is a multiple of 4.

Q & A

What is the size range of IPv4 options in the header?

The size of IPv4 options can range from 0 to 40 bytes. This means the IPv4 header size can vary between a minimum of 20 bytes (with no options) and a maximum of 60 bytes (with 40 bytes of options).

What does the term 'Record Route' option in IPv4 refer to?

The 'Record Route' option in IPv4 allows the recording of the IP addresses of the routers the packet passes through from the source to the destination. A maximum of 10 routers can be recorded, as each address takes up 4 bytes and the options field can hold up to 40 bytes.

Why is the 'Record Route' option not used in IPv6?

The 'Record Route' option is not used in IPv6 due to security reasons. Storing the IP addresses of all routers along the path could pose a security risk, and thus, this option was removed in IPv6.

What is the concept of 'Source Routing' in IPv4?

'Source Routing' allows the source to specify the exact route a packet should take through the network. The source can define the path, listing the routers (e.g., R1, R2, R3) the packet must go through to reach the destination.

What is the difference between 'Strict' and 'Loose' source routing?

'Strict Source Routing' requires the packet to follow a defined path exactly, with all routers specified. In 'Loose Source Routing', the packet can follow part of the path as defined, but other routers can be chosen freely in between.

Can users implement 'Source Routing' in IPv4?

No, 'Source Routing' is generally used by network administrators, not end-users. Users do not have the capability to specify the routing path themselves.

What is padding in the context of IPv4 headers?

Padding in IPv4 headers is the practice of adding extra bytes to ensure the header size is a multiple of 4. If the header size isn't a multiple of 4, padding is added (usually 0s) to make it a valid size.

Why is the header size required to be a multiple of 4 in IPv4?

IPv4 requires the header size to be a multiple of 4 to maintain proper alignment and efficiency when processing the header. If the header size is not a multiple of 4, padding is added to make it so.

What happens if the IPv4 header size is not a multiple of 4?

If the IPv4 header size is not a multiple of 4, padding bytes (usually 0s) are added to ensure the total size becomes a multiple of 4, ensuring proper alignment.

What is the maximum number of routers that can be recorded in the 'Record Route' option of IPv4?

The maximum number of routers that can be recorded in the 'Record Route' option is 9. This is because the maximum options size is 40 bytes, and each IP address takes 4 bytes, so 40 bytes can store the addresses of up to 9 routers.