Term
| what mechanisms does TCP have for acclomplishing retransmission? |
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Definition
a. timer-based - when the retransmission timeout (RTO) expires.
B. fast retransmission (ACK-based) - when cumulative ACKs fail to increment or SACKs indicate out of order packets. |
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Term
| Which retransmission mechanism is preferred and why? |
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Definition
| Fast retransmission is preferred as it is more efficient. Timer-based retransmissions lead to underutilization of the network. |
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Term
| Why must TCP constantly evaluate the RTO? |
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Definition
| RTT may change over time due to changing network conditions. |
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Term
| What must tcp monitor the RTT for? |
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Definition
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Term
| Describe the retransmission ambiguity problem and how it can be overcome. |
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Definition
| This is when an ACK is received for a packet that has been retransmitted, the sender will not know which packet is being ACK'd. |
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Term
| When tcp is retransmitting segments, what will cause it to stop doubling the RTO backoff factor? |
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Definition
| When it receives an ACK for a segment that was only sent once. |
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Term
| When the tcp timestamps option is being used, what TSER value is returned when a receiver is acknowledging a number of reordered packet? |
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Definition
| The value is taken from the TSV of the last packet to arrive. This results in the RTT being overestimated, which is desirable as it will make the sender less aggressive to retransmit. This can be overcome by using the tcp timestamps option or just ignoring RTT measurements for packets that have been sent more than once. |
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Term
| what must tcp do when it receives an out of order segment? |
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Definition
| It must generate an immediate ACK (duplicate ACK). |
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Term
| when a duplicate ACK arrives at a sender, what does is indicate? |
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Definition
| Either packet loss or packet reordering in the network. |
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Term
| what is assumed when the duplicate ACK threshold (3) is exceeded and what action does the sender take? |
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Definition
| Packet loss is assumed, fast transmit and congestion control procedures are invoked. |
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Term
| What benefit does SACK add to fast retransmit? |
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Definition
| SACK allows a sender to fill more than one hole in the receiver per RTT. |
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Term
| why can the sack option only specify 3 blocks? |
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Definition
| a sack option that specifies N blocks requires 8N+2 bytes and it usually accompanies the 10-byte TSOPT (plus 2 bytes padding). A TCP header only has 40 bytes available for options. |
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Term
| what may prevent a sender from filling 3 holes, that have just been reported in a sack, with a single retransmission? |
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Definition
| congestion control - i.e. reduced window size. |
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Term
| under what conditions would a lost ack/sack message be retransmitted? |
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Definition
| If it contained data or the SYN or FIN flags were set. |
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Term
| why are the first sack blocks from previous sacks repeated in later positions in subsequent sacks? |
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Definition
| To guard against ACK loss. If the ACK was lost, the original sender would know if a subsequent sack message didn't indicate that the block was still missing. |
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Term
| what is a spurious retransmission and what are the possible causes? |
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Definition
| TCP may initiate a retransmission even when no data has been lost. Possible causes are spurious timeouts, packet reordering, packet duplication or lost ACKS. |
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Term
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Definition
Duplicate SACK enables a SACK receiver to indicate when a duplicate segment has been received.
It's purpose is to highlight when spurious transmissions have occurred.
Interoperable with conventional sack senders. |
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Term
| describe tcp repacketization? |
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Definition
When tcp times out and retransmits, it doesn't have to send an identical segment.
This allows tcp to send a bigger segment, which can help to combat retransmission ambiguity.
The larger segment can't exceed the MSS of the receiver or the path MTU. |
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