TCP Flashcards
What is binary exponential backoff?
Doubling of time between TCP retransmissions.
What is Retransmission Timeout?
TCP sets a timer when it sends data, and if the data is not acknowledged when the timer expires, a timeout or timer-based retransmission of data occurs. The timeout occurs after an interval called the retransmission timeout (RTO).
What is the term for TCPs clock “tick”?
Granularity
What is the initial TCP RTO?
According to [RFC6298], the initial setting for the RTO should be 1s, although 3s is used in the event of a timeout on the initial SYN segment
What is the first part of Karns algorithm?
If a retransmitted packet is acknowledged, ignore any round trip measurement based on the packet. This solves the retransmission ambiguity problem.
What is the second part of Karns algorithm?
When a retransmitted packet is acknowledged, don’t reset the backed off RTO. Wait until the NEXT TCP packet is acknowledged before doing so.
What is the retransmission ambiguity problem?
If a retransmitted packet is acknowledged, it is not possible to figure out what the RTT was because it isn’t clear if the original or retransmitted packet is being ACKed.
What triggers a fast retransmit?
Three duplicate ACK in a row.
What’s up with delayed ACK?
It’s desirable for TCP to piggyback an ACK on top of additional data. For this reason, ACK can be delayed up to 500ms (typically 200ms)
What’s the Nagle algorithm?
If a link is saturated, small packets suck because of all the overhead. Nigel algorithm is a self governing tool that says that a packet smaller than SMSS can’t be sent until all unacknowledged data is acknowledged. If ACKs are slow, then good, it bunched up traffic. If ACKs are fast, then it didn’t matter.
ICMP dest unreachable, packet too large? What message
ICMP type 3 code 4. Response also includes MTU
What does the sending TCP do when window size is 0?
Send 1 byte window probe (regular TCP packet) at RTO interval. This prompts a response which will have the window size. No response = keep sending, but double the time between each message.
