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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.
in Computer Networks by Boss (11k points) | 49 views

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Case 1: Assume that there is no segmentation needed.

 

IP header = 20 bytes

TCP header = 20 bytes

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:

IP header = 20 bytes

TCP header = 20 bytes

Since the maximum Ethernet Payload is 1500 bytes including IP header and TCP header, TCP payload can be 1460 bytes.

Ethernet header = 8 + 6 + 6 + 2 bytes

Ethernet checksum = 4 bytes

Therefore, the size of first frame is 1526 bytes

 

Frame 2:

IP header = 20 bytes

TCP header = 20 bytes

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.

 

 

 

by (23 points)

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