IP Packet Forwarding by Routers

IP (Internet Protocol) packet forwarding is the process of transmitting packets from one network to another network. Routers play a vital role in forwarding packets from the source network to the destination network. During the process of IP packet forwarding by routers, some of the packet fields are updated.

Updated Packet Fields

Among the various packet fields, the following fields are updated during normal IP packet forwarding by routers:

1. IP Header TTL

TTL (Time to Live) is a field in the IP header that indicates the maximum number of hops that a packet can traverse before being discarded. Each router that forwards the packet decrements the TTL value by one. If the TTL value reaches zero, the packet is discarded, and an ICMP message is sent back to the source. Therefore, routers update the TTL value in the IP header to ensure that packets do not loop indefinitely.

2. IP Header Checksum

The IP header checksum is a value calculated over the entire IP header to ensure that the header is error-free. Each router that forwards the packet recalculates the checksum value based on the updated header information. Therefore, routers update the IP header checksum to ensure that the header is valid and error-free.

Unchanged Packet Fields

The following packet fields remain unchanged during normal IP packet forwarding by routers:

1. IP Header Source Address

The source IP address in the IP header indicates the IP address of the sender of the packet. This field remains unchanged during packet forwarding by routers.

2. IP Header Destination Address

The destination IP address in the IP header indicates the IP address of the intended recipient of the packet. This field also remains unchanged during packet forwarding by routers.

Conclusion

In summary, routers update the IP header TTL and IP header checksum fields during normal IP packet forwarding. The source and destination IP addresses remain unchanged during the forwarding process.