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SNOOP+α PROTOCOL
13

Tcp snoop protocols

Dec 04, 2014

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Technology

Amr Nasr

snoop + alpha protocol
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Page 1: Tcp snoop protocols

SNOOP+α PROTOCOL

Page 2: Tcp snoop protocols

The snoop agent

Page 3: Tcp snoop protocols

THE SNOOP PROTOCOL

1-a snoop agent, is introduced at the AP (Access Point) that acts as the last-hop router to the MT(Mobile Terminal).

2-The snoop agent monitors every packet that passes through the AP in either direction and maintains a soft state for each TCP connection.

3-he Snoop agent operates differently on TCP data and ACK channels, as well as on the uplink (from the MT) and downlink (to the MT) TCP data segments.

4-For data transfer from an MT to the FH, the Snoop agent detects packet losses on the wireless link by monitoring the data segments it forwards. It then informs the MT that the packet loss was not related to congestion, thus allowing the sender to retransmit without triggering normal congestioncontrol procedures.

Page 4: Tcp snoop protocols

P1P2P3

P4

P1

P2

Ack1

Ack2Ack1

Ack2 Ack2P3

P4Ack3

Ack4

Ack3

Ack4

FH BS snoop agentMH

X

X

1-If BS sends duplicate A2 FH will think that there is congestion and lowers speed.2-Disadvantage will send P4 and after P3 he will send P3 and P4.

Page 5: Tcp snoop protocols

Advantages of Snoop protocol

1-offered End-to-End connections for TCP2-prevents Fixed Host from generating congestion algorithms3-Improved performance4-decreased End-to-End Average delay.5-High throughput

Page 6: Tcp snoop protocols

Protocols combined with snoop protocol

TCP TahoeRenoNew RenoSACKVegas

Page 7: Tcp snoop protocols

Simulation Results

Page 8: Tcp snoop protocols

Improvement of TCP protocols using snoop

1-As expected, most TCP versions improve compared to theplain wireless environment without Snoop. For instance,TCP Tahoe, Reno and New Reno improve.

2- However, two important aspects need further investigation. TCP Vegas, which was the worst performing version without Snoop, now is the best performing version, and TCP SACK that was the best performing version without Snoop is now the worst version.

Page 9: Tcp snoop protocols

Snoop+ α Protocol

1-snoop has a problem; when there are no duplicate ACKs, the Snoop protocol cannot notice the packet toss until the local retransmission timer is expired. This results in the degradation of TCP throughput.

2- the Snoop+ α protocol to solve the Snoop protocol’s problem. Because the Snoop+ α agent adaptively checks packet loss whenever it receives new ACKs, it performs packet retransmission fast compared to the Snoop protocol.

Page 10: Tcp snoop protocols

These ACKs fall into one of three categories:

1-Duplicate ACK : this is an ACK identical to the previously received ACK number. DUPACK are generated when packets, the number of which is larger than the lost packet number , are received by the MH.2- A spurious ACK : this is an ACK whose number is less than that of the last ACK seen by the Snoop+ α agent. This rarely happens and the packet is discarded.3-A new ACK : this is an ACK not received previously by the snoop+ α agent .this ACK initiates the cleaning of the snoop agent buffer and all acknowledged packets are freed.

Page 11: Tcp snoop protocols

Procedure for packets loss

For data transfer from an FH (Fixed Host) to an MT, theSnoop agent caches unacknowledged TCP data segments,which it forwards to the TCP receiver, and monitors thecorresponding ACKs. In particular, it provides for:

1. Retransmission of any lost data segments locally by1.using local timers or TCP duplicate acknowledgments to

identify packet loss, instead of waiting for the FH to do so.2. Suppressing the duplicate ACKs on their way fromthe MT back to the FH, thus avoiding fast retransmit andcongestion control at the latter.

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