ist 220 final

Use the pulldown menu to match a congestion control approach to how the sender detects congestion. 1 The sender infers segment loss from the absence of an ACK from the receiver. 2Bits are set at a congested router in a sender-to-receiver datagram, and bits are in the returned to the sender in a receiver-to sender ACK, to indicate congestion to the sender. 3The sender measures RTTs and uses the current RTT measurement to infer the level of congestion.

1 end-end 2 network-assisted 3 delay-based

Over HTTP, a response that the request has succeeded, or OK, uses which code number?

200

How big is the MAC address space? The IPv4 address space? The IPv6 address space?

2^48 MAC addresses; 2^32 IPv4 addresses; 2^128 IPv6 addresses.

What advantage does a circuit-switched network have over a packet-switched network? What advantages does TDM have over FDM in a circuit-switched network?

A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for the duration of a call. Most packet-switched networks today (including the Internet) cannot make any end-to-end guarantees for bandwidth. FDM requires sophisticated analog hardware to shift signal into appropriate frequency bands.

Let's review some of the terminology used in this textbook. Recall that the name of a transport-layer packet is segment and that the name of a link-layer packet is frame. What is the name of a network-layer packet? Recall that both routers and link-layer switches are called packet switches. What is the fundamental difference between a router and link-layer switch?

A network-layer packet is a datagram. A router forwards a packet based on the packet's IP (layer 3) address. A link-layer switch forwards a packet based on the packet's MAC (layer 2) address.

describe what a proxy server is.

A proxy server, also known as a Web cache, is a network entity that satisfies HTTP requests on the behalf of an origin Web server. A user's browser can be configured so that all of the user's HTTP requests are first directed to the Web cache. This can substantially reduce the response time for a client request, and can substantially reduce traffic on an institution's access link to the Internet.

Suppose that an arriving packet matches two or more entries in a router's forwarding table.

A router uses longest prefix matching to determine which link interface a packet will be forwarded to if the packet's destination address matches two or more entries in the forwarding table.

Why is an ARP query sent within a broadcast frame? Why is an ARP response sent within a frame with a specific destination MAC address?

An ARP query is sent in a broadcast frame because the querying host does not which adapter address corresponds to the IP address in question. For the response, the sending node knows the adapter address to which the response should be sent, so there is no need to send a broadcast frame

What is an application-layer message? A transport-layer segment? A network-layer datagram? A link-layer frame?

Application-layer message: data which an application wants to send and passed onto the transport layer; transport-layer segment: generated by the transport layer and encapsulates application-layer message with transport layer header; network-layer datagram: encapsulates transport-layer segment with a network-layer header; link-layer frame: encapsulates network-layer datagram with a link-layer header.

What is the name of the variable that the TCP sender uses to constrain its sending rate?

Congestion window

Which of the following network applications runs over UDP?

DNS

encapsulation as used in the context of protocol layers and their service models.

Encapsulation is the process of creating a packet which includes two fields: header fields and a payload field; the payload field is typically a packet from the layer above.

Suppose end system A wants to send a large file to end system B. At a very high level, describe how end system A creates packets from the file. When one of these packets arrives to a router, what information in the packet does the router use to determine the link onto which the packet is forwarded? Why is packet switching in the Internet analogous to driving from one city to another and asking directions along the way?

End system A breaks the large file into chunks. It adds header to each chunk, thereby generating multiple packets from the file. The header in each packet includes the IP address of the destination (end system B). The packet switch uses the destination IP address in the packet to determine the outgoing link.

If a file consists of F bits and the transfer takes T seconds for the receiver to receive all of the bits, what is the average throughput?

F/T

A TCP segment contains the source and destination IP addresses among its header fields.

False

Packet switching pre-allocates use of the transmission link regardless of demand, with allocated but unneeded link time going unused.

False

SMTP is a protocol that you can use to access your e-mail.

False

SMTP is primarily a pull protocol—it is used to load information at the client's convenience.

False

The Internet's network core contains packet switches.

False

The UDP segment structure includes header fields for source and destination IP address.

False

Host A is sending Host B a large file over a TCP connection. Assume Host B has no data to send Host A. Host B will not send acknowledgments to Host A because Host B cannot piggyback the acknowledgments on data. The size of the TCP rwnd never changes throughout the duration of the connection. Suppose Host A is sending Host B a large file over a TCP connection. The number of unacknowledged bytes that A sends cannot exceed the size of the receive buffer. Suppose Host A is sending a large file to Host B over a TCP connection. If the sequence number for a segment of this connection is m, then the sequence number for the subsequent segment will necessarily be m + 1. The TCP segment has a field in its header for rwnd . Suppose that the last SampleRTT in a TCP connection is equal to 1 sec. The current value of TimeoutInterval for the connection will necessarily be >= 1 sec. Suppose Host A sends one segment with sequence number 38 and 4 bytes of data over a TCP connection to Host B. In this same segment the acknowledgment number is necessarily 42.

False False True False True False False

Assume a TCP connection exists between Host A and Host B. True or false: The acknowledgement number that Host A puts in its segment is the sequence number of the last byte Host A received from Host B.

False It's not the sequence number of the last byte Host A received; it's the sequence number of the next byte Host A is expecting from Host B.

SMTP uses UDP for ultra-reliable delivery from the sender's mail server to the recipient's mail server.

False. It uses TCP

List five tasks that a layer can perform. Is it possible that one (or more) of these tasks could be performed by two (or more) layers?

Five generic tasks are error control, flow control, segmentation and reassembly, multiplexing, and connection setup.

In a few sentences, describe the difference between forwarding and routing.

Forwarding refers to the router-local action of transferring a packet from an input link interface to the appropriate output link interface. Routing refers to the network-wide process that determines the end-to-end paths that packets take from source to destination.

What are some of the possible services that a link-layer protocol can offer to the network layer? Which of these link-layer services have corresponding services in IP? In TCP?

Framing: there is also framing in IP and TCP; link access; reliable delivery: there is also reliable delivery in TCP; flow control: there is also flow control in TCP; error detection: there is also error detection in IP and TCP; error correction; full duplex: TCP is also full duplex.

Is HFC transmission rate dedicated or shared among users? Are collisions possible in a downstream HFC channel? Why or why not?

HFC bandwidth is shared among the users. On the downstream channel, all packets emanate from a single source, namely, the head end. Thus, there are no collisions in the downstream channel.

Which of the following terms is not descriptive of TCP?

Half duplex

Which of the following fields is used to ensure data integrity?

Header checksum

Who is responsible for developing many of the most important Internet standards?

IETF (Internet Engineering Task Force)

Which action should the TCP receiver take in response to the following event: "Arrival of out-of-order segment with higher-than-expected sequence number. Gap detected."?

Immediately send duplicate ACK, indicating sequence number of next expected byte.

Because POP3 is an extremely simple mail access protocol, it doesn't require authorization (i.e., a username and password).

It's simple, but it does require authorization.

Compare and contrast link-state and distance-vector routing algorithms.

Link state algorithms: Computes the least-cost path between source and destination using complete, global knowledge about the network. Distance-vector routing: The calculation of the least-cost path is carried out in an iterative, distributed manner

What does the term "logical communication" mean, in the context of a transport-layer protocol? This question can be answered fully in one or two short sentences.

Logical communication means that, from an application's perspective, it is as if the hosts running the processes were directly connected.

What is meant by a control plane that is based on logically centralized control? In such cases, are the data plane and the control plane implemented within the same device or in separate devices?

Logically centralized control means that a logically central routing controller computes and distributes the forwarding tables to be used by each and every router, and each router does not compute its forwarding table, unlike the per-router control. In the case of logically centralized control, the data plane and control plane are implemented in separate devices; the control plane is implemented in a central server or multiple servers, and the data plane is implemented in each router.

Which is not one of the types of delay which contributes to total nodal delay?

Loss delay

What do we call packets of information that are exchanged at the application layer?

Messages

Alternatively, what are the two best reasons for selecting a proprietary protocol for Wazzup?

More control/security over the product The ability to offer a similar experience across platforms Reduced confusion/complexity for applications The ability to monetize the protocol by being the only provider of applications which support it

What is meant by a control plane that is based on per-router control? In such cases, when we say the network control and data planes are implemented "monolithically," what do we mean?

Per-router control means that a routing algorithm runs in each and every router; both forwarding and routing function are constrained within each router. Each router has a routing component that communicates with the routing components in other routers to compute the values for its forwarding table. monolithically because each router works as an independent entity that implements its own control and data planes.

Suppose two TCP connections are present over some bottleneck link of rate R bps. Both connections have a huge file to send (in the same direction over the bottleneck link). The transmissions of the files start at the same time. What transmission rate would TCP like to give to each of the connections?

R/2

Which layers in the Internet protocol stack does a router process? Which layers does a link-layer switch process? Which layers does a host process?

Routers process network, link and physical layers (layers 1 through 3).(This is a little bit of a white lie, as modern routers sometimes act as firewalls or caching components, and process Transport layer as well.) Link layer switches process link and physical layers (layers 1 through2). Hosts process all five layers.

By which name are transport-layer packets known?

Segments

What are the two best reasons for selecting an open protocol for Wazzup?

Speed of development - you get feedback and more people working on it Allows for more competition Flexibility - users can tailor the clients to their preferences Free for everybody

Which of the following is defined as the area or component which connects a router's input ports to its output ports?

Switching fabric

In a few sentences, describe the purpose and operation of TCP's flow control and congestion control mechanisms. A strong answer will include a discussion of how these mechanisms differ.

TCP's flow control mechanism is designed to eliminate the possibility of the sender overflowing the receiver's buffer. It matches the rate at which the sender is sending against the rate at which the receiving application is reading. The mechanism works by having the sender maintain a variable called the receive window. The receive window is included in the header of every segment the receiver sends to the sender. TCP's congestion control mechanism is designed to have each sender limit the rate at which it sends traffic into its connection as a function of perceived network congestion. The mechanism operating at the sender keeps track of a variable called the congestion window, which is used to limit the amount of unacknowledged data at the sender and therefore indirectly limits the sender's send rate. TCP flow control is implemented on the sender side, while congestion control is implemented on the sender side the flow control variable (receive window) will vary only with respect to the TCP receiver's free buffer space, while the congestion control variable (congestion window) will vary with respect to network-wide conditions.

What information is used by a process running on one host to identify a process running on another host?

The IP address of the destination host and the port number of the socket in the destination process.

List five nonproprietary Internet applications and the application-layer protocols that they use.

The Web: HTTP; file transfer: FTP; remote login: Telnet; e-mail: SMTP; BitTorrent file sharing: BitTorrent protocol

Why do HTTP, SMTP, and POP3 run on top of TCP rather than on UDP?

The applications associated with those protocols require that all application data be received in the correct order and without gaps. TCP provides this service whereas UDP does not.

In the reading for topic 1.6, "Networks Under Attack," the authors describe four categories of attacks that "the bad guys" can launch across the Internet, and gave several examples of each.

The bad guys can put malware in your host via the internet . Malware: self-replicating or not . Viruses Worms The bad guys can attack servers and network infrastructure. Denial-of-service (DoS) attacks Distributed DoS (DDoS) The bad guys can sniff packets. Packet sniffer - passive receiver that records a copy of every packet that flies by The bad guys can masquerade as someone you trust. IP spoofing

Consider sending a packet from a source host to a destination host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable?

The delay components are processing delays, transmission delays, propagation delays, and queuing delays. All of these delays are fixed, except for the queuing delays, which are variable.

What are the key differences between routing and forwarding?

The key differences between routing and forwarding is that forwarding is a router's local action of transferring packets from its input interfaces to its output interfaces, and forwarding takes place at very short timescales (typically a few nanoseconds), and thus is typically implemented in hardware. Routing refers to the network-wide process that determines the end-to-end paths that packets take from sources to destinations. Routing takes place on much longer timescales (typically seconds), and is often implemented in software.

We noted that network layer functionality can be broadly divided into data plane functionality and control plane functionality. What are the main functions of the data plane? Of the control plane?

The main function of the data plane is packet forwarding, which is to forward datagrams from their input links to their output links.

For a communication session between a pair of processes, which process is the client and which is the server?

The process which initiates the communication is the client; the process that waits to be contacted is the server.

Current versions of HTTP use UDP connections.

They use TCP connections.

TCP's connection-establishment procedure is commonly known as:

Three-way handshake

What field in the IP header can be used to ensure that a packet is forwarded through no more than N routers?

Time-to-live.

All activity in the Internet that involves two or more communicating remote entities is governed by a protocol.

True

An HTTP server is stateless.

True

Once a connection is established using SMTP, the server and client perform some application-layer handshaking before transferring information from one to another.

True

The first line of an HTTP request message is called the request line

True

Web servers implement the server side of HTTP.

True

When a packet is dropped along a path, the transmission capacity that was used at each of the upstream links to forward that packet to the point at which it is dropped ends up having been wasted.

True

When using Go-Back-N, the TCP receiver sends the TCP sender a cumulative ACK.

True

Describe how Web caching can reduce the delay in receiving a requested object. Will Web caching reduce the delay for all objects requested by a user or for only some of the objects? Why?

Web caching can bring the desired content "closer" to the user, possibly to the same LAN to which the user's host is connected. Web caching can reduce the delay for all objects, even objects that are not cached, since caching reduces the traffic on links.

Is it possible for an organization's Web server and mail server to have exactly the same alias for a hostname (for example, foo.com)? What would be the type for the RR that contains the hostname of the mail server?

Yes, an organization's mail server and Web server can have the same alias for a host name. The MX record is used to map the mail server's host name to its IP address.

Is it possible for an application to enjoy reliable data transfer even when the application runs over UDP? If so, how?

Yes. The application developer can put reliable data transfer into the application layer protocol. This would require a significant amount of work and debugging, however.

a. A user requests a Web page that consists of some text and three images. For this page, the client will send one request message and receive four response messages. b. Two distinct Web pages (for example, www.mit.edu/research.html and www.mit.edu/students.html) can be sent over the same persistent connection. c. With nonpersistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages. d. The Date: header in the HTTP response message indicates when the object in the response was last modified. e. HTTP response messages never have an empty message body.

a) F b) T c) F d) F e) F

List the four broad classes of services that a transport protocol can provide. For each of the service classes, indicate if either UDP or TCP (or both) provides such a service.

a) Reliable data transfer: TCP provides a reliable byte-stream between client and server but UDP does not. b) A guarantee that a certain value for throughput will be maintained: Neither c) A guarantee that data will be delivered within a specified amount of time: Neither d) Confidentiality (via encryption): Neither

The HTTP mechanism which allows a cache to verify that its objects are up to date is called the

conditional GET

An HTTP server is considered stateless. While this can simplify server design and make it easier for servers to handle multiple requests simultaneously, it is often useful to identify users. This is done using:

cookies

routing algorithm is the same as?

forwarding table

The three major components of e-mail are user agents, mail servers, and the Simple Mail Transfer Protocol (SMTP).

true