Consider three IP networks and . Host in network sends messages each containing bytes of application data to a host in network . The layer prefixes byte header to the message.
This passes through an intermediate network .The maximum packet size, including byte IP header, in each network is:
- A:
- B:
- C:
The network and are connected through a Mbps link, while and
are connected by a Kbps link (bps = bits per second).
Assuming that the packets are correctly delivered, how many bytes, including headers, are delivered to the layer at the destination for one application message, in the best case? Consider only data packets.
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Answer: D
To find the total number of bytes delivered to the IP layer at the destination, we trace the packet size and fragmentation through each network:
- At Host (Network A):
- Application data = bytes.
- TCP layer adds a -byte header, making the TCP segment size = bytes.
- Network A has an MTU of bytes. The IP layer adds a -byte IP header, resulting in a single packet of bytes. Since , no fragmentation occurs in Network A.
- At Network B:
- Network B has an MTU of bytes, which includes a -byte IP header. Thus, the maximum IP payload size per fragment is bytes.
- Since is a multiple of , it is a valid fragment payload size.
- The -byte TCP segment payload is fragmented into three packets:
- Fragment 1: bytes payload + bytes IP header = bytes
- Fragment 2: bytes payload + bytes IP header = bytes
- Fragment 3: bytes payload + bytes IP header = bytes
- At Network C:
- Network C has an MTU of bytes. Since the incoming fragments (, , and bytes) are well within this limit, no further fragmentation or reassembly occurs on the path.
- These three fragments are delivered directly to the IP layer of the destination host .
Therefore, the total bytes delivered to the IP layer at the destination is:
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