DNS and HTTP delays - non-persistent HTTP with 5 parallel TCP connections

Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that two DNS servers are visited before your host receives the IP address from DNS. The first DNS server visited is the local DNS cache, with an RTT delay of RTT0 = 4 msecs. The second DNS server contacted has an RTT1 of 26 msecs. Suppose the RTT between the local host and the Web server containing the object is RTTHTTP = 50 msecs.

Question: Suppose the HTML object references 9 very small objects on the same server, but assume that the client is configured to support a maximum of 5 parallel TCP connections, with non-persistent HTTP. How much time (in msec) elapses from when the client clicks on the link until the client receives the objects?

Question

DNS and HTTP delays -  non-persistent HTTP with 5 parallel TCP connections

Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that two DNS servers are visited before your host receives the IP address from DNS. The first DNS server visited is the local DNS cache, with an RTT delay of RTT0 = 4 msecs. The second DNS server contacted has an RTT1 of 26 msecs.  Suppose the RTT between the local host and the Web server containing the object is RTTHTTP = 50 msecs.

Question: Suppose the HTML object references 9 very small objects on the same server, but assume that the client is configured to support a maximum of 5 parallel TCP connections, with non-persistent HTTP.  How much time (in msec) elapses from when the client clicks on the link until the client receives the objects?

BlackTom AI · Accepted Answer

To solve this, we need to break the total delay into two parts: DNS resolution time and the time to fetch all the HTTP objects.

- DNS resolution: The browser must perform two DNS lookups. The first is the local DNS cache with RTT0 = 4 ms, and the second goes to another DNS server with RTT1 = 26 ms. The total DNS time is RTT0 + RTT1 = 4 + 26 = 30 ms.

- HTTP object fetches: After DNS, the HTML object references 9 very small objects on the same server. The client is configured to use at most 5 parallel TCP connections and uses non-persistent HTTP, so each object requires establishing a TCP connection (handshake) and then sending the HTTP request and receiving the response. Under the common modeling used in these problems, fetching an object via a non-persistent connection incurs roughly 3 segments of RTTHTTP: one RTT for the TCP handshake, one RTT for the request/response, and a bit of additional latency (e.g., processing or setup) that effectively adds another RTT worth of time in a bursty, parallel scenario. Therefore, time per batch of up to 5 parallel objects is 3 * RTTHTTP.

- Number of batches: There are 10 objects total (the HTML object plus 9 small objects). With a maximum of 5 parallel connections, the objects are fetched in 2 batches: 5 objects in the first batch and the remaining 5 in the second batch (even though there are only 9 small objects, the HTML object is included in the total count for the purpose of batching in many textbook treatments; the math works out the same with 10 objects for two batches).

- RTTHTTP value: The RTT between the local host and the web server containing the object is RTTHTTP = 50 ms.

- HTTP fetch time: Each batch takes 3 * RTTHTTP = 3 * 50 ms = 150 ms. With 2 batches, the total HTTP fetch time is 2 * 150 ms = 300 ms.

- Grand total: DNS time (30 ms) + HTTP fetch time (300 ms) = 330 ms.

Putting it all together, the elapsed time from the click to receiving all objects is 330 ms. The key ideas are accounting for both the DNS lookup chain and the batching effect of parallel non-persistent HTTP connections, with each batch incurring about 3 RTTs worth of delay given RTTHTTP = 50 ms.

Note: If one assumes a different per-object timing model (e.g., 2 RTTs per object or a different batching interpretation), the numeric result would differ. The 330 ms result here follows the conventional textbook interpretation used for this scenario.

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