Question

10. Suppose that host HI sends a frame F1 to host 112. The network topology is illustrated in the following figure. Fl is fragmented into two fragments router R1. F22 at R1, R2, and R3: Routers F1, F21, and F22: Frames H2 H1 Network Network 2 Network 3 Frame F21 and F22 Frame Fl R1 R2 R3 Internetwork N

Answer 1

When a router receives a packet, it examines the destination address and determines the outgoing interface to use and that interface's MTU(maximum transmission unit ). If the packet size is bigger than the MTU, and the Do not Fragment (DF) bit in the packet's header is set to 0, then the router may fragment the packet.

The router divides the packet into fragments. The max size of each fragment is the MTU minus the IP header size (20 bytes minimum; 60 bytes maximum). The router puts each fragment into its own packet, each fragment packet having following changes:

• The total length field is the fragment size.
• The more fragments (MF) flag is set for all fragments except the last one, which is set to 0.
• The fragment offset field is set, based on the offset of the fragment in the original data payload. This is measured in units of eight-byte blocks.
• The header checksum field is recomputed.

Reassembly:

A receiver knows that a packet is a fragment if at least one of the following conditions is true:

• The "more fragments" flag is set. (This is true for all fragments except the last.)
• The "fragment offset" field is nonzero. (This is true for all fragments except the first.)

The receiver identifies matching fragments using the foreign and local internet address, the protocol ID, and the identification field. The receiver will reassemble the data from fragments with the same ID using both the fragment offset and the more fragments flag. When the receiver receives the last fragment (which has the "more fragments" flag set to 0), it can calculate the length of the original data payload, by multiplying the last fragment's offset by eight, and adding the last fragment's data size.

When the receiver has all the fragments, it can put them in the correct order, by using their offsets. It can then pass their data up the stack for further processing.