Transport Layer Concepts and Protocols

These flashcards cover key concepts from the Transport Layer of networking protocols, focusing on TCP, UDP, connection management, and congestion control.

What does the transport layer provide if the network layer provides logical communication between hosts?

Logical communication between apps on different hosts.

How does a server handle demultiplexing when it receives three TCP segments with the same destination IP and port 80?

It directs them to different sockets based on their 4-tuple.

Why might an application developer choose UDP over TCP?

To avoid the latency overhead of establishing a connection.

In the RDT 2.0 protocol, how does the sender respond to a corrupted ACK or NAK?

It simply retransmits the current packet.

How does RDT 2.2 become a 'NAK-free' protocol?

Receiver sends an ACK for the last correctly received packet.

What is the primary reason the stop-and-wait operation of RDT 3.0 is inefficient?

Sender utilization is low due to waiting a full RTT per packet.

What happens if the sender's timer expires in the Go-Back-N protocol?

Sender retransmits unACKed packet and all higher sequence packets.

Why must Selective Repeat's window size be less than or equal to half the size of the sequence number space?

Prevents confusing a retransmitted old packet with a new one.

What will the acknowledgment number be for a TCP segment with sequence number 100 and containing 20 bytes of data?

120.

How does TCP calculate its timeout interval?

Calculates EWMA of recent RTTs and adds a safety margin.

What specifically triggers 'Fast Retransmit' in TCP?

The receipt of three duplicate ACKs for the same sequence number.

What is the purpose of TCP Flow Control?

To prevent the sender from overwhelming the receiver's application.

What is the 'half-open connection' problem in TCP 3-way handshake?

Accepting duplicate data from an old, delayed connection request.

What represents an un-needed duplicate retransmission in TCP?

Premature timeout causing sender to re-transmit a delayed packet.

What does the AIMD algorithm accomplish in TCP Congestion Control?

Probes bandwidth by increasing rate, strictly backs off on loss.

How does the congestion window (cwnd) grow during TCP Slow Start?

Grows exponentially, doubling every RTT until threshold or loss.

What dictates the transition from TCP Slow Start to Congestion Avoidance?

Window (cwnd) reaches or exceeds slow start threshold (ssthresh).

What is the core principle behind the TCP CUBIC congestion control algorithm?

Window size grows as a cubic function of time since last loss.

How does the delay-based approach to TCP congestion control differ from loss-based TCP?

Keeps bottleneck fully utilized without filling router buffers.

Why is classic TCP considered 'fair' under idealized conditions?

AIMD mechanics naturally converge flows to an equal bandwidth share.

What is the expected outcome when one application opens 9 TCP connections and another opens 1 on the same congested link?

First app gets roughly 90% of bandwidth, second app gets 10%.

What do the Ann and Bill characters in the Transport Layer analogy represent?

The Transport Protocols (like TCP or UDP).

Which of the following is NOT a service provided by TCP?

Delay and bandwidth guarantees.

What happens if the UDP Checksum calculation results in an overflow?

Overflow bit wraps around and adds to the least significant bit.

What is a significant weakness of the Internet Checksum used in UDP?

Fooled if multiple bits flip in a way that offsets the sum.

How does Network-assisted congestion control differ from End-to-end congestion control?

Routers explicitly mark packets to indicate congestion.

What happens to the congestion window (cwnd) after a TCP sender experiences a timeout?

It drops all the way down to 1 MSS.

What is the purpose of the 'rwnd' field in the TCP segment structure?

Indicates bytes receiver can accept without overflowing buffer.

Why is the calculation of EstimatedRTT weighted?

Recent network conditions better predict immediate future.

What is the consequence of 'wasted capacity' during heavy congestion?

Upstream link capacity is wasted when a packet drops late.