English
Menu

What is Round-trip time (RTT)?

The time from sending data until the response comes back. It includes the time signals take to travel the distance plus processing and queuing in devices along the way. It is used to calculate TCP's retransmission timeout and to estimate how long it takes to establish a connection (number of round trips × RTT), and it can be measured with ping. Light travels at about 200,000 km/s in optical fiber, so 1,000 km of distance alone adds about 10 ms of round-trip time.

Japanese name: RTT(往復遅延時間)

Round-trip time (RTT) appears in 9 practice questions across 3 topics, including Computer Networking Basics, Web Technology Basics, Applied Cryptography.

Field: Computer Networking Basics

Try it: questions about Round-trip time (RTT)

A client makes a new HTTPS connection (TLS 1.3 over TCP) to a web server with a round-trip time (RTT) of 100 ms. Roughly how long does it take until the response to the first request starts to arrive? Assume DNS resolution is already done, and ignore server processing time and data transfer time.

  1. AAbout 100 ms
  2. BAbout 300 ms
  3. CAbout 200 ms
Show answer and explanation

Answer: B. About 300 ms

First, the TCP three-way handshake takes one round trip (the client can send its first TLS message together with the final ACK). Next, the TLS 1.3 handshake takes one round trip, and then the HTTP request and response take one more, for three round trips in total: 3 × 100 = 300 ms. With TLS 1.2 the key exchange takes two round trips, giving four round trips, or 400 ms. This waiting time depends on the number of RTTs, which is set by distance, so adding link bandwidth does not shorten it. QUIC, used by HTTP/3, combines the transport and cryptographic handshakes into one round trip and also offers a way to send data in the first flight when reconnecting (0-RTT).

A TCP connection has a fixed receive window of 65536 bytes and an RTT of 10 ms. The link bandwidth is ample and there is no loss. Which is closest to the upper limit on this connection's throughput?

  1. AAbout 52 Mbps
  2. BAbout 6.6 Mbps
  3. CAbout 524 Mbps
Show answer and explanation

Answer: A. About 52 Mbps

Under window control, at most one window of data can be sent per round trip (RTT), the time until ACKs return. The throughput limit is therefore window ÷ RTT = 65536 bytes × 8 ÷ 0.01 s ≈ 52.4 Mbps (about 52 Mbps), and it goes no higher even on a 1 Gbps link. To fill a fast, distant link, the window must be at least the product of bandwidth and RTT (the bandwidth-delay product): for 1 Gbps and an RTT of 10 ms, 10^9 × 0.01 = 10^7 bits = 1.25 MB. The window field in the TCP header is 16 bits, for a maximum of 65535 bytes, but the window scale option can enlarge it.

Round-trip time (RTT) in the explanations

Questions that mention Round-trip time (RTT) (9)

  1. A client makes a new HTTPS connection (TLS 1.3 over TCP) to a web server with a round-trip time (RTT) of 100 ms. Roughly how long does it take until the response to the first request starts to arrive? Assume DNS resolution is already done, and ignore server processing time and data transfer time.
  2. A TCP connection has a fixed receive window of 65536 bytes and an RTT of 10 ms. The link bandwidth is ample and there is no loss. Which is closest to the upper limit on this connection's throughput?
  3. Over a link with an RTT of 50 ms and a bandwidth of 100 Mbps, a request that returns a 10 KB response is repeated 100 times, each sent only after the previous response has been fully received. Which is closest to the total time? Take the time per request as the RTT plus the response transfer time (data size ÷ bandwidth), 1 KB = 1000 bytes, and ignore server processing time.
  4. From a web server with an RTT of 40 ms, one HTML file and the four images it references are fetched one after another. Assume that establishing a TCP connection takes one round trip and each request and response take one round trip, and ignore data transfer time, server processing time, TLS and connection teardown. Which of the following is the difference in total time between opening a new TCP connection for each object and reusing one TCP connection (a persistent connection)?
  5. 1.5 MB of text data is compressed with gzip to one third of its size and sent. The bandwidth is 10 Mbps and the RTT is 100 ms, and before the data starts to be sent, the TCP connection, the TLS handshake and the HTTP request take three round trips in total. Assume 1 MB = 10^6 bytes, and ignore compression and decompression time, headers and waiting caused by TCP congestion control. Which of the following is closest to the time until all the data has been received?
  6. Which of the following is a correct caution about 0-RTT in TLS 1.3 (sending "early data" together with the first message, using a pre-shared key obtained in an earlier connection)?
  7. In the three-way handshake that establishes a TCP connection, what is the correct order of the packets exchanged?
  8. What triggers a TCP sender to decide that a segment was lost and retransmit it?
  9. Which of the following correctly describes a change from TLS 1.2 in TLS 1.3 (RFC 8446)?

Books on Computer Networking Basics Ad

As an Amazon Associate I earn from qualifying purchases.

Related terms

TCP (Transmission Control Protocol) TLS HTTP (Hypertext Transfer Protocol) Three-way handshake Forward secrecy Packet HTTPS DNS (Domain Name System) QUIC Throughput

Back to the glossary