Networking Exam One Review

The HTTP GET: What is the purpose of the HTTP GET message?

The HTTP GET request message is used by a web client to request a web server to send the requested object from the server to the client.

What is the purpose of a manifest file in a streaming multimedia setting?

To let a client know where it can retrieve different video segments, encoded at different rates

What is the Internet?: Which of the following descriptions below correspond to a "nuts-and-bolts" view of the Internet? Select one or more of the answers below that are correct. [Hint: more than one of the answers below are correct].

- A "network of networks". - A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. - A collection of billions of computing devices, and packet switches interconnected by links.

"HTTP is stateless.": What do we mean when we say "HTTP is stateless"? In answering this question, assume that cookies are not used. Check all answers that apply.

- An HTTP server does not remember anything about what happened during earlier steps in interacting with this HTTP client. - An HTTP client does not remember anything about what happened during earlier steps in interacting with any HTTP server.

UDP service. When an application uses a UDP socket, what transport services are provided to the application by UDP? Check all that apply.

- Best effort service. The service will make a best effort to deliver data to the destination but makes no guarantees that any particular segment of data will actually get there. - Congestion control. The service will control senders so that the senders do not collectively send more data than links in the network can handle.

Packet switching versus circuit switching (1): Which of the characteristics below are associated with the technique of packet switching? Select all correct answers. [Hint: more than one of the answers is correct].

- Congestion loss and variable end-end delays are possible with this technique. - This technique is used in the Internet. - Data may be queued before being transmitted due to other user's data that's also queueing for transmission. - Resources are used on demand, not reserved in advance. - Reserves resources needed for a call from source to destination.

The client-server paradigm: Which of the characteristics below are associated with a client-server approach to structuring network applications (as opposed to a P2P approach)?

- HTTP uses this application structure. - There is a server that is always on. - There is a server with a well-known server IP address.

Comparing and contrasting HTTP and SMTP (2): Which of the following characteristics apply to SMTP only (and do not apply to HTTP)?

- Operates mostly as a "client push" protocol. - Uses server port 25. - Uses CRLF.CRLF to indicate end of message.

Performance: Utilization: Consider the scenario shown below, with a single source client sending to a server over two links of capacities R1=10 Mbps and R3=100 Mbps. What is the utilization of the link of capacity R1?

1.0

Performance: Maximum end-end throughput.: Consider the scenario shown below, with a single source client sending to a server over two links of capacities R1=10 Mbps and R3=100 Mbps. What is the maximum achievable end-end throughput (in Mbps, give an integer value) for the client-to-server pair, assuming that the client is trying to send at its maximum rate?

10 Mbps

Performance: Maximum Throughput (2). Now consider the network shown below (just an extended version of the network above), with two senders on the left sending packets to a common receiver on the right. The links have transmission rates of R1 = R2 = 100 Mbps, and R3 = 50 Mbps. What is the maximum end-to-end throughput achieved by each session, assuming both sessions are sending at the maximum rate possible?

25 Mbps

Simple HTTP GET request response time: Suppose an HTTP client makes a request to the gaia.cs.umass.edu web server. The client has never before requested a given base object, nor has it communicated recently with the gaia.cs.umass.edu server. You can assume, however, that the client host knows the IP address of gaia.cs.umass.edu. How many round trip times (RTTs) are needed from when the client first makes the request to when the base page is completely downloaded, assuming the time needed by the server to transmit the base file into the server's link is equal to 1/2 RTT and that the time needed to transmit the HTTP GET into the client's link is zero? (You should take into account any TCP setup time required before the HTTP GET is actually sent by the client, the time needed for the server to transmit the requested object, and any propagation delays not accounted for in these amounts of time.)

2.5 RTT

Performance: Maximum end-end throughput: Consider the scenario shown below, with 10 different servers (four shown) connected to 10 different clients over ten three-hop paths. The pairs share a common middle hop with a transmission capacity of R = 200 Mbps. Each link from a server has to the shared link has a transmission capacity of RS = 25 Mbps. Each link from the shared middle link to a client has a transmission capacity of RC = 50 Mbps. What is the maximum achievable end-end throughput (in Mbps, give an integer value) for each of ten client-to-server pairs, assuming that the middle link is fairly shared and all servers are trying to send at their maximum rate?

20 Mbps

E-mail delays: How many RTTs are there from when a client first contacts an email server (by initiating a TCP session) to when the client can begin sending the email message itself - that is following all initial TCP or SMTP handshaking required? Recall the figure below from our class notes:

3

Download delays for 100 objects (HTTP 1.1 with browser caching): Consider an HTTP 1.1 client and server. The RTT delay between the client and server is 2 seconds. Suppose the time a server needs to transmit an object into its outgoing link is 3 seconds, as shown below for the first of these 100 requests. You can assume that any other HTTP message not containing an object sent by the client and server has a negligible (zero) transmission time. Suppose the client makes 100 requests, one after the other, waiting for a reply to a request before sending the next request. Using HTTP 1.1, how much time elapses between the client transmitting the first request, and the receipt of the last requested object, assuming the client uses the IF-MODIFIED-SINCE header line, and 50% of the objects requested have not changed since the client downloaded them (before these 100 downloads are performed)?

352 secs

Playout delay: smallest initial playout delay. Consider the following scenario, in which a server is streaming chunks of video to a client. The first chunk is transmitted by the server at t=2, is received at the client at t=10, and is played out at the client at t=16. The retransmission, reception, and playout of 11 chunks is shown. In this example, the playout delay is 6, and no chunks miss their playout deadline. What is the smallest initial playout delay such that the (periodic) playout of chunks is timed such that no chunks miss their playout deadline? You can assume that if a chunk arrives (black staircase) at the same time as it is scheduled for playout, then the chunk is played out successfully without being lost.

4

Performance: Maximum Throughput. Consider again the network shown above. The links again have transmission rates of R1 = R2 = 100 Mbps Assume that the link R2 is fairly shared (as we've seen is done via TCP) between the two sessions. What is the maximum end-to-end throughput achieved by each session, assuming both sessions are sending at the maximum rate possible?

50 Mbps

Consider the network shown in the figure below, with three links, each with a transmission rate of 1 Mbps, and a propagation delay of 1 msec per link. Assume the length of a packet is 1000 bits. What is the end-end delay of a packet from when it first begins transmission on link 1, until is it received in full by the server at the end of link 3. You can assume that queueing delays and packet processing delays are zero, but make sure you include packet transmission time delay on all links. Assume store-and forward packet transmission.

6 msec

How many calls can be carried?: Consider the circuit-switched network shown in the figure below, with four circuit switches A, B, C, and D. Suppose there are 20 circuits between A and B, 19 circuits between B and C, 15 circuits between C and D, and 16 circuits between D and A. What is the maximum number of connections that can be ongoing in the network at any one time?

70

Download delays for 100 objects (HTTP 1.0): Consider an HTTP 1.0 client and server. The RTT delay between the client and server is 2 seconds. Suppose the time a server needs to transmit an object into its outgoing link is 3 seconds, as shown below for the first of these 100 requests. You can assume that any other HTTP message not containing an object sent by the client and server has a negligible (zero) transmission time. Suppose the client makes 100 requests, one after the other, waiting for a reply to a request before sending the next request. Using HTTP 1.0, how much time elapses between the client transmitting the first request, and the receipt of the last requested object?

700 secs

What's in the DNS type A resource record?: What information does the type "A" resource record hold in the DNS database?

A hostname and an IP address.

Streaming video definitions: Match the definition/function of an element or approach in a networked streaming video system, with its name.

A unit of video, each of which may be encoded at multiple different rates, stored in different files. Chunk A file containing the location and encoding rate of files corresponding to video segments in a video. Manifest An approach that allows a client to adapt the encoding rate of retrieved video to network congestion conditions. DASH A CDN approach that stores content in access networks, close to clients. Enter deep

What's a "packet" really called? Match the name of an Internet layer with unit of data that is exchanged among protocol entities at that layer, using the pulldown menu.

Application layer Message Transport layer Segment Network layer Datagram Link layer Frame Physical layer Bit

Access network per-subscriber speeds: Match the access network with the approximate speeds that a subscriber might experience. (Note: if you look these up, do so in the 8E textbook, slides, or video -- not in the 7E or earlier versions, since link access speeds are always increasing over the years).

Ethernet Wired. Up to 100's Gbps per link. 4G cellular LTE Wireless. Up to 10's Mbps per device. 802.11 WiFi Wireless. 10's to 100's of Mbps per device. Cable access network Wired. Up to 10's to 100's of Mbps downstream per user. Digital Subscriber Line Wired. Up to 10's of Mbps downstream per user.

Routing versus forwarding: Choose one of the following two definitions that makes the correct distinction between routing versus forwarding.

Forwarding is the local action of moving arriving packets from router's input link to appropriate router output link, while routing is the global action of determining the source-destination paths taken by packets.

The DNS authoritative name server. What is the role of an authoritative name server in the DNS?

It provides the definitive answer to the query with respect to a name in the authoritative name server's domain.

DNS functions: Match the function of a server to a given type of DNS server in the DNS server hierarchy.

Provides authoritative hostname to IP mappings for organization's named hosts. Authoritative DNS server Replies to DNS query by local host, by contacting other DNS servers to answer the query. Local DNS server Responsible for a domain (e.g., *.com, *.edu); knows how to contact authoritative name servers. Top Level Domain (TLD) servers Highest level of the DNS hierarchy, knows how to reach servers responsible for a given domain (e.g., *.com, *.edu). DNS root servers

Which e-mail protocol?: Match the functionality of a protocol with the name of the email protocol (if any) that implements that functionality.

Pushes email from a mail client to a mail server. SMTP Pulls mail from one mail server to another mail server. Neither SMTP nor IMAP does this. Pulls email to a mail client from a mail server. IMAP

Security defenses. Match the description of a security defense with its name.

Specialized "middleboxes" filtering or blocking traffic, inspecting packet contents inspections. Firewall Provides confidentiality by encoding contents. Encryption Used to detect tampering/changing of message contents, and to identify the originator of a message. Digital signatures Limiting use of resources or capabilities to given users. Access control Proving you are who you say you are. Authentication

What is "encapsulation"? Which of the definitions below describe what is meant by the term "encapsulation"?

Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the "packet" for that layer.

Consider the following DNS requests, made by the local host at the given times: t=0, the local host requests that the name gaia.cs.umass.edu be resolved to an IP address. t=1, the local host requests that the name icann.org be resolved to an IP address. t=5, the local host requests that the name cs.umd.edu be resolved to an IP address. (Hint: be careful!) t=10, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. t=12, the local host requests that the name cs.mit.edu be resolved to an IP address. t=30, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. (Hint: be careful!) Which of the requests require 2 time units to be resolved?

The request at t=10.

Conditional HTTP GET: What is the purpose of the conditional HTTP GET request message?

To allow a server to only send the requested object to the client if this object has changed since the server last sent this object to the client.

What is DASH? In DASH (Dynamic, Adaptive Streaming over HTTP), a server divides a video file into chunks that ... (pick best completion from below)

... are stored, each encoded at multiple rates (video quality). The client plays the video chunk-by-chunk, with each chunk requested at encoding rate that fits the available bandwidth at the time.

Computing Packet Transmission Delay(1): Suppose a packet is L = 1500 bytes long (one byte = 8 bits), and link transmits at R = 1 Gbps (i.e., a link can transmit bits 1,000,000,000 bits per second). What is the transmission delay for this packet?

.000012 secs

Suppose that the browser makes an HTTP request to the server for an HTTP object, that the browser has a copy of that object in its cache, but that copy may or may not be up-to-date in its cache (and so the browser requests the object using the HTTP If-Modified-Since header field). Assume that the time taken to transmit an HTTP reply (by the sender into the TCP connection) that contains a requested object in the body of an HTTP reply is 50 msec, but that the time taken to transmit a HTTP reply with a "304 Not Modified" reply code without an included object is negligible (e.g., zero). You can assume that a TCP connection has already been set up, so do not include that delay in your answer below. What is the time from when the browser issues the initial HTTP GET request until it is able to display the requested object in the case that the browser does have the requested object in its browser cache?

200 msec