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What is the fastest line speed at which a host can blast out 1500-byte TCP payloads
with a 120-sec maximum packet lifetime without having the sequence numbers wrap
around? Take TCP, IP, and Ethernet overhead into consideration. Assume that Ethernet
frames may be sent continuously.
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Case 1: Assume that there is no segmentation needed.

TCP payload can be 1500 bytes.

Ethernet header = 8 + 6 + 6 + 2 bytes

Ethernet checksum = 4 bytes

Therefore, the size of first frame is 1566 bytes.

The time to live = 120 sec.

The length of sequence number = 32 bits.

Therefore, the maximum number of bytes that a host can send out without wrapping around the sequence number is 2 ^32/ 1500.

Since Ethernet frames may be sent continuously, the maximum line speed, V can be:

V <= 1566 * 8 * (2^32 / 1500) (bits)/120 (sec)

= 298929723.8016 bits/sec

= 298.9 Mbps

The maximum line speed is 298.9 Mbps.

Case 2:  Since the maximum size of Ethernet payload is 1500 bytes, the TCP payload needs to be segmented as followings.

Frame 1:

Ethernet header = 8 + 6 + 6 + 2 bytes

Ethernet checksum = 4 bytes

Therefore, the size of first frame is 1526 bytes

Frame 2:

TCP payload remained after segmentation = 40 bytes (= 1500 – 1460 bytes)

Ethernet header = 8 + 6 + 6 + 2 bytes

Ethernet checksum = 4 bytes

Therefore, the size of second frame is 106 bytes

The time to live = 120 sec.

The length of sequence number = 32 bits.

Therefore, the maximum number of bytes that a host can send out without wrapping around the sequence number is 2 ^32/ 1500.

Since Ethernet frames may be sent continuously, the maximum line speed, V can be:

V <= (1526 + 106) * 8 * (2^32 / 1500) (bits)/120 (sec)

= 1632 * 8 * (2^32 /1500) (bits)/ 120 (sec)

= 311528294.5365 bits/sec

= 311.5 Mbps

The maximum line speed is 311.5 Mbps.

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