Homework 2: HTTP, DNS, Error Recovery

Due on Tuesday, February 4th at 11:59 pm

Submit your homework solution PDF to Gradescope in any format you please; however, make sure that each question part is on its own page.

#Question 1: HTTP (25 points)

1. (9 points) From class, we know that the If-Modified-Since header is used in HTTP Web Caching requests. Explain how this compares to the If-None-Match header. What header must the server send in responses to support If-None-Match? What benefits do we gain from this approach? (The Mozilla docs are a great resource).

2. (8 points) HTTP Cookies (among other features) add state to what otherwise would be a stateless protocol. When visiting Piazza via Bruin Learn (the left-hand side tab), the browser sends the stored cookies to Piazza-proving your identity. Let’s say that Piazza set its cookies with the attribute SameSite=Strict. Would we still be able to access Piazza inside an iframe within Bruin Learn (meaning Piazza is loaded cross-site)? What about SameSite=Lax? (Optional: by default, LTI integrations in Bruin Learn don’t work in macOS Safari. Why is that?)

3. (8 points) What improvements does HTTP/2 have over HTTP/1.1 to mitigate Head-of-Line blocking? Even with these mitigations, explain how HTTP/2 still suffers from HoL blocking.

#Question 2: DNS (25 points)

Professor Paul Eggert is fed up with UCLA IT. His high performance websites at web.cs.ucla.edu require the lowest latency access from any global location. Relying on UCLA servers alone won’t do. He’s experimenting with switching to Flarecloud—a global CDN network. To test this out, he asks his students to visit beta.cs.ucla.edu. This domain resolves to Flarecloud’s CDN servers (which caches his websites) instead of UCLA’s.

One of his students decided to perform dig queries on both those domains.

$ dig web.cs.ucla.edu
[...]
;; QUESTION SECTION:
;web.cs.ucla.edu.	        IN      A

;; ANSWER SECTION:
web.cs.ucla.edu.	11702	IN	A	131.179.128.29

$ dig beta.cs.ucla.edu
[...]
;; QUESTION SECTION:
;beta.cs.ucla.edu.              IN      A

;; ANSWER SECTION:
beta.cs.ucla.edu.	11702	IN	CNAME   cdn.flarecloud.com
cdn.flarecloud.com.	172800	IN	A       162.159.38.2

Assume that:

• the student’s caching resolver is completely empty;
• only iterative resolution is used;
• ns.ucla.edu is the authoritative NS for all ucla.edu subdomains (there is no separate NS for cs.ucla.edu); and
• it takes
  • 5 ms to contact the caching resolver,
  • 60 ms for the caching resolver to contact the root, com, and edu nameservers, and
  • 25 ms for the caching resolver to contact the Flarecloud and UCLA nameservers (all round trip).

1. (10 points) How long does it take to resolve web.cs.ucla.edu? Show all the steps needed to resolve this query.

2. (15 points) How about beta.cs.ucla.edu? (Caches are present from the previous request-this is only asked a second later). Show all the steps needed to resolve this query.

#Question 3: Error Recovery (25 points)

This question was written by Professor George Varghese.

The figure below shows two examples of a Go-Back-N protocol working with a window size of 4 and a modulus of 4. Recall that if the modulus is 4, all sequence numbers and \(\rm ACK\)s are incremented modulo 4. We want to show that the modulus is too small. In both cases, the sender starts by sending the first 4 packets it is allowed to in its current window (\(P_0\) through \(P_3\)) without waiting for an \(\rm ACK\).

1. (5 points) In case 1 shown on the left, the \(\rm ACK\) to the first data packet \(P_0\) is received. On receipt of \(\mathrm{ACK}_1\), the window slides, and the sender sends a different \(P_0\) from the one it sent earlier.

   What should the receiver ideally do in this case? Specify how the receiver number changes if it does, whether the data packet should be accepted, and what \(\rm ACK\) number should be sent back.

2. (5 points) In Case 2 shown on the right, the \(\rm ACK\) to the first data packet \(P_0\) is lost. After a timeout occurs, the sender retransmits the old \(P_0\).

   What should the receiver do in this case? Specify how the receiver number changes if it does, whether the data packet should be accepted, and what \(\rm ACK\) number should be sent back.

3. (5 points) Argue from these 2 cases, that the modulus is too small to work correctly.

4. (5 points) Using the ideas in this example, argue briefly why the modulus should be at least \(w + 1\), for a general window size of \(w\). (Try to make your argument general that works for any \(w\) not just 4.)

5. (5 points) Explain how this relates to the Alternating Bit protocol.

#Question 4: KEEPALIVE (25 points)

This question was written by Professor George Varghese.

Songwu Lu has been consulting with an Internet Service Provider and finds that they use an unusual error recovery protocol shown in the figure below.

The protocol is very similar to Go-Back-N with numbered data packets; the key difference is that the sender does not normally send ACKs. The receiver sends a message called a NACK only if it detects an error in the received sequence or if it receives a so-called KEEPALIVE packet.

In the figure below, the sender sends off the first three data packets. The second one is lost; thus when the receiver gets the third packet, it detects an error and sends a NACK which contains the highest number the receiver has received in sequence. When NACK 1 gets to the sender, the sender retransmits data packets 2 and 3. Periodically, based on a timer, the sender transmits a KEEPALIVE packet. The receiver always replies to a KEEPALIVE packet using a NACK.

1. (5 points) Why is the KEEPALIVE packet needed? What can go wrong if the sender does not send KEEPALIVE packets?

2. (5 points) A KEEPALIVE packet is sent when a KEEPALIVE timer expires. The sender maintains the following property:

   While there remains unacknowledged data, the KEEPALIVE timer is running.

   Why does this property guarantee that any data packet given to the sender will eventually reach the receiver (as long as the link delivers most packets without errors)?

3. (5 points) Under what conditions must the timer be stopped and started to maintain the property?

4. (10 points) Consider sending a single data packet D that is lost. After that, no packets get lost. What is the worst-case latency before the receiver receives D and the sender knows the receiver has received D?

   Write your answer in terms of:

   • packet roundtrips (\(\rm RTT\); assume no transmission delays), and
   • KEEPALIVE timer durations (\(T\)).

   Assume that \(\mathrm{RTT} << T\).