Which three TCP enhancements can be used with TCP selective acknowledgments? (Choose
three.)
A.
header compression
B.
explicit congestion notification
C.
keepalive
D.
time stamps
E.
TCP path discovery
F.
MTU window
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TCP Selective Acknowledgement (SACK) prevents unnecessary retransmissions by specifying successfully received subsequent data. Let’s see an example of the advantages of TCP SACK.
TCP_ACK.jpgTCP (Normal) Acknowledgement TCP_SACK.jpg
TCP Selective Acknowledgement
For TCP (normal) acknowledgement, when a client requests data, server sends the first three segments (named of packets at Layer 4): Segment#1,#2,#3. But suppose Segment#2 was lost somewhere on the network while Segment#3 stills reached the client. Client checks Segment#3 and realizes Segment#2 was missing so it can only acknowledge that it received Segment#1 successfully. Client received Segment#1 and #3 so it creates two ACKs#1 to alert the server that it has not received any data beyond Segment#1. After receiving these ACKs, the server must resend Segment#2,#3 and wait for the ACKs of these segments.
For TCP Selective Acknowledgement, the process is the same until the Client realizes Segment#2 was missing. It also sends ACK#1 but adding SACK to indicate it has received Segment#3 successfully (so no need to retransmit this segment. Therefore the server only needs to resend Segment#2 only. But notice that after receiving Segment#2, the Client sends ACK#3 (not ACK#2) to say that it had all first three segments. Now the server will continue sending Segment #4,#5, …
The SACK option is not mandatory and it is used only if both parties support it.
The TCP Explicit Congestion Notification (ECN) feature allows an intermediate router to notify end hosts of impending network congestion. It also provides enhanced support for TCP sessions associated with applications, such as Telnet, web browsing, and transfer of audio and video data that are sensitive to delay or packet loss. The benefit of this feature is the reduction of delay and packet loss in data transmissions. Use the “ip tcp ecn” command in global configuration mode to enable TCP ECN.
The TCP time-stamp option provides improved TCP round-trip time measurements. Because the time stamps are always sent and echoed in both directions and the time-stamp value in the header is always changing, TCP header compression will not compress the outgoing packet. Use the “ip tcp timestamp” command to enable the TCP time-stamp option.
The TCP Keepalive Timer feature provides a mechanism to identify dead connections. When a TCP connection on a routing device is idle for too long, the device sends a TCP keepalive packet to the peer with only the Acknowledgment (ACK) flag turned on. If a response packet (a TCP ACK packet) is not received after the device sends a specific number of probes, the connection is considered dead and the device initiating the probes frees resources used by the TCP connection.
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