CS 6250 Quiz 3 (All Quizzes) Latest Update | Verified
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Question:
What is the drawback to using the traditional approach of having a single, publicly accessible web server?
Answer:
Geographical distance to all users
Wasteful for a single data center to send the same data over and over
Single point of failure
Question:
What is a CDN?
Answer:
Multiple geographically distributed data centers with copies of content that direct users to server to best serve their
request.
Question:
What are the six major challenges that Internet applications face?
Answer:
Peering point congestion
Inefficient routing protocols
Unreliable networks
Inefficient communication protocols
Scalability
Application limitations
Question:
What are the major shifts that have impacted the evolution of the Internet ecosystem?
Answer:
Increased demand for online content, especially videos
"Topological Flattening"
Question:
Compare the "enter deep" and "bring home" approach of CDN server placement.
Answer:
Enter Deep: Deploy many small networks around the world. Goal is to make distance between user and closest server
as small as possible
Bring Home: Deploy fewer, larger servers in critical areas. Fewer clusters to maintain, but users will experience higher
delay and slower throughput
Question:
What is the role of DNS in the way CDN operates?
Answer:
DNS intercepts requests so that CDNs can choose where to direct users based on location/other conditions
Question:
What are the two main steps in CDN server selection?
Answer:
Map a client to a cluster
Select a server from the cluster
Question:
What is the simplest approach to select a cluster? What are the limitations of this approach?
Answer:
"As the crow flies": Pick the closest geographic cluster
Limitations: actually picking the location closest to the Local Domain Name Server, not the user. The closest
geographical server may also not be the best in end-to-end performance
Question:
What metrics could be considered when using measurements to select a cluster?
Answer:
Network-layer metrics: delay, bandwidth
Application-layer metrics: re-buffering ratio, average bitrate
Question:
How are the metrics for cluster selection obtained?
Answer:
Active measurements: LDNS can probe multiple clusters. Most cannot do this though and it would create a lot of
traffic
Passive measurements: server in CDN can track performance metrics based on current traffic conditions. This would
require a real-time controller with real-time view at all cluster-client pairs
Question:
Explain the distributed system that uses a 2-layered system. What are the challenges of this system?
Answer:
Coarse-grained global layer: Operates at large time scales. Has global view of client quality metrics. Builds prediction
model of video quality
Fine-grained per-client layer: Operates at millisecond timescale. Makes decisions upon client request
Challenges: Hard to design a centralized system with scale of today's networks. Needs data for different subnet pairs,
so some clients need to be routed to sub-optimal clusters
Question:
What are the strategies for server selection? What are the limitations of these strategies?
Answer:
Assign server randomly: might end up assigning server with high workload while lower workload was available
Load-Balancing to route to least-loaded server: not all servers have same content so they'd need to fetch all content for
every server
Question:
What is consistent hashing? How does it work?
Answer:
Tends to balance load by assigning roughly the same number of content IDs, and little movement is needed as nodes
join and leave the system. (uniform circle example)
Question:
Why would a centralized design with a single DNS server not work?
Answer:
Single point of failure
Difficult to handle all volume of querying traffic
Cannot be close to all clients
Updating a huge db for every host on Internet
Question:
What are the main steps that a host takes to use DNS?
Answer:
User host runs client side of DNS application
Browser extracts hostname and passes it to client side of DNS application
DNS client sends a query containing the hostname of DNS
DNS client receives a reply including IP address for the hostname
Host can initiate TCP connection to HTTP server at that port IP
Question:
What are the services offered by DNS, apart from hostname resolution?
Answer:
Mail server/Host aliasing
Load distribution
Question:
What is the structure of DNS hierarchy?
Answer:
Root DNS servers
Top level domain (TLD) servers
Authoritative servers
Local DNS servers
Question:
Why does DNS use a hierarchical scheme?
Answer:
Hierarchy scheme solves the scaling issue
Question:
What is the difference between iterative and recursive DNS queries?
Answer:
Iterative: querying host is referred to a different DNS server in the chain until it can fully resolve the request
Recursive: querying host and each DNS server in chain queries next server and delegates query to it
Question:
What is DNS caching?
Answer:
Once a server receives the DNS reply of mapping any host to IP address, it stores that info in the Cache memory before
sending it to the client
Question:
What is a DNS resource record?
Answer:
Mapping between hostnames and IP addresses
Question:
What are the most common types of resource records?
Answer:
TYPE=A: domain name - IP address
TYPE=NS: domain name - DNS server that can get IP addresses for hosts in that domain
TYPE=CNAME: alias hostname - canonical name
TYPE=MX: alias hostname of mail server - canonical name of email server
Question:
Describe the DNS message format.
Answer:
ID: So client can match queries with responses
Flags: Specifications about DNS message, query, etc
Question: Info about the query like the hostname
Answer: Resource record for hostname that was queried
Authority: Resource records for more authoritative servers
Additional Info
Question:
What is IP Anycast?
Answer:
Route a client to the "closest" server as determined by BGP.
Question:
What is HTTP Redirection?
Answer:
If a client sends a GET request to a server A, it can redirect the client to another server B by sending a HTTP response
with code 3xx and name of new server. Can slow things down but useful for load balancing. Does not require any
central coordination
What is the difference between forwarding and routing?
Answer:
Forwarding refers to transferring a packet from an incoming link to an outgoing link within a single router.
Routing refers to how routers work together using routing protocols to determine the good paths (or good routes as we
call them) over which the packets travel from the source to the destination node
What is the main idea behind a link-state routing algorithm?
Answer:
Link state routing protocols are underpinned by Dijkstra's algorithm. The link costs and network topology are known
to all nodes.
Key equation
D(v) = min(D(v), D(w) + c(w, v))
What is an example of a link-state routing algorithm?
Answer:
OSPF - Open Shortest Path First protocol
Walk through an example of the link-state routing algorithm.
Answer:
Lecture slides.
What is the computational complexity of the link-state routing algorithm?
Answer:
O(N^2)
This is derived from the fact that each iteration, we need to search through the nodes not in N' for the node with
smallest D. This group starts off as size N and decreases by 1 each iteration
N + (N - 1) + .. + 1 = O(N^2)
What is the main idea behind the distance vector routing algorithm?
Answer:
The distance vector routing algorithm is underpinned by the Bellman Ford algorithm. Unlike link state, DV is a
distributed algorithm. Nodes exchange information to update their view of the network.
Key equation:
For node x,
Dx(y) = minv{c(x,v) + Dv(y)} for each destination node y in the network
where minv in the equation is taken over all of x's neighbors v
Walk through an example of the distance vector algorithm.
Lecture slides.
When does the count-to-infinity problem occur in the distance vector algorithm?
A count-to-infinity problem occurs when we have a routing loop e.g. in order to get to x, y routes through z, and z
routes through y.
We saw in the lecture where this happened with a large increase in link cost, that took a long time to propagate among
nodes in the network.
How does poison reverse solve the count-to-infinity problem?
The idea of poison reverse is to lie about a node z's ability to reach a particular other node x, so that other nodes y don't
try to reach x via z. In this manner, we prevent a routing loop.
This technique will solve the problem with 2 nodes, however poison reverse will not solve a general count to infinity
problem involving 3 or more nodes that are not directly connected.
What is the Routing Information Protocol (RIP)?
RIP is a distance vector routing protocol. It uses hop count as a metric (link cost = 1), and messages exchanged are
known as RIP advertisements.
RIP uses UDP to send request and response messages, and is actually implemented as an application-level process.
Some of the challenges with RIP include updating routes, reducing convergence time, and avoiding loops/count-to
infinity problems.
What is the Open Shortest Path First (OSPF) protocol?
OPSF is a link-state routing protocol. OSPF was introduced as an advancement of the RIP Protocol, operating in
upper-tier ISPs. It is a link-state protocol that uses flooding of link-state information and a Dijkstra least-cost path
algorithm.
OPSF involves configuring an AS into areas. One area in an AS is the backbone area.
Routing topologies are communicated between routers in an area with link state advertisements (LSAs).
How does a router process advertisements?
In the context of OSPF: Updating the link-state database, calculating the shortest path using the shortest path first
SPF algorithm., feeding information to the Forwarding Information Base...
What is hot potato routing?
A technique/practice of choosing a path within the network, by choosing the closest egress point based on intradomain
path cost (Interior Gateway Protocol/IGP cost).
This is useful when packets are intended for destinations outside the current network, and we want to get the traffic
out ASAP.
Hot potato routing simplifies computations for the routers as they are already aware of the IGP path costs. It makes
sure that the path remains consistent, since the next router in the path will also choose to send the packet to the same
egress point.
"Routing" and "forwarding" are interchangeable terms.
True
False
False
Consider a source and destination host. Before packets leave the source host, the host needs to define the path over
which the packets will travel to reach the destination host.
True
False
False
Intradomain routing refers to routing that takes place among routers that belong to the same administrative domain.
In contrast when routers belong to different administrative domains, we refer to routing as interdomain routing.
True
False
True
Consider the link-state routing protocol. The link costs are known to all nodes.
True
False
True
As is the network topology
Consider the link-state routing protocol. The network topology is known to all nodes.
True
False
True
As are all link costs.
Consider the link-state routing protocol and a node u as our source node. The goal of the algorithm is to compute the
least-cost paths from the source node u to every other node in the network.
True
False
True
Consider the link-state routing protocol with a node u as our source node. Consider the initialization of the algorithm.
We initialize the least-cost path from node u to directly attached neighbors to be the cost of the direct links, and for
non-directly attached neighbors we initialize the least-cost path to be infinity.
True
False
True
The distance vector (DV) routing algorithm continues iterating as long as neighbors send new updates to each other.
True
False
True
The distance vector (DV) routing algorithm is an example of a ______________ algorithm.
A. decentralized
B. centralized
A.
The distance vector (DV) routing algorithm requires synchronization between routers.
True
False
False
In the distance vector (DV) routing algorithm, each node maintains and updates its own view of the network.
True
False
True
Consider the distance (DV) routing algorithm. Which is used by each node to update the node's distance vector?
A. Dijkstra's algorithm
B. None of the above
C. The spanning tree protocol
D. The Bellman Ford equation
D.
The count-to-infinity problem states that good news (e.g., a decrease in a link cost) propagates slowly among nodes in
the network.
True
False
False
Bad news (increased link cost) propagates slowly.
The poison reverse technique solves the count-to-infinity problem for all network topologies.
True
False
False
Not a general solution.
The Routing Information Protocol (RIP) is based on the Distance Vector protocol.
True
False
True
Open Shortest Path First (OSPF) is based on the link state routing algorithm.
True
False
True
The number of egress points that a network has is upper bounded, e.g., two or three egress points per network.
True
False
False
Consider a network with multiple egress points. Further consider that these egress points offer different paths to the
same external destinations. Then these paths must have different costs.
True
False
False
According to the hot potato routing technique, it is in a network's best interest to route the traffic so that it exits the
network at the router geographically closest to the one from which it entered the network.
True
False
False
The one with the best IGP cost
Assume a source and a destination host. As packets travel over a path from the source host to the destination host the
packets are handled by multiple routers over that path. If these routers belong to different administrative domains,
they need to run the same intradomain routing algorithm, since they are on the same path for that pair of hosts.
True
False
False
What is the difference between forwarding and routing.
We refer to forwarding as the action of transferring a packet from an incoming link to an outgoing link within a single
router.
By routing we refer to how routers work together using routing protocols to determine the good paths over which the
packets travel from the source to the destination node.
Dijkstra's algorithm is an example of what?
A Global, Link State Algorithm
What is the main idea behind link state algorithm
The link state routing algorithm looks to determine the shortest paths (determined by the link costs) between a source
node and all other nodes in the network. The link costs and the network topology are known to all nodes
In the initialization step, we initialize the least cost path to all directly attached neighbors and set all others to infinity.
What is the computational complexity of the link state routing algorithm?
The complexity of the algorithm is in the order of n squared O(n^2). The algorithm searches through n (n+1) / 2
nodes.
What is the main idea behind distance vector routing algorithm?
Each node maintains its own distance vector, with the costs to reach every other node in the network. The neighboring
nodes exchange their distance vectors to update their own view of the network.
Algorithm is Iterative, Asynchronous, distributed, and based on BFA.
When does the count-to-infinity problem occur in the distance vector algorithm?
___ occurs in the distance vector algorithm when the cost of a link increases and two nodes think they can get to a
third node through each other based on their previous outdated costs. This link cost change took a long time to
propagate among the nodes of the network
How does poison reverse solve the count-to-infinity problem?
The way it works is that a node a, that uses node b to get a node c, will tell node b that the cost for its path to node c is
infinity (Da(c)=infinity). Node b assumes that node a has no path to node c except through node b, so it will never send
packets to node c via node a.
Poison reverse helps prevent the count-to-infinity problem only for 2 nodes, it does not solve a general count to
infinity involving 3 or more nodes that are not directly connected.
What is the Routing Information Protocol (RIP)?
is based on the Distance Vector protocol.
The metric for choosing a path could be shortest distance, lowest cost or a load-balanced path.
Routing updates between neighbors are done periodically, using ___ advertisements which contain information about
sender's distances to destination subnets.
Each router maintains a routing table, which contains its own distance vector as well as the router's forwarding table.
Routers send request and response messages over UDP, using port number 520, which is layered on top of network
layer IP protocol.
Some of the challenges with RIP include updating routes, reducing convergence time, and avoiding loops/count-to
infinity problems.
What is Open Shortest Path First (OSPF)?
is a routing protocol which uses a link state routing algorithm to find the best path between the source and the
destination router. intradomain.
How does a router process advertisements?
Process begins when an LS update is received.
a. Every Link State Advertisements (LSA) is unpacked, the OSPF protocol checks whether it is a new or a duplicate
LSA, compared to the link-state (LS) DB.
i.If it is a duplicate, it sends an LS ACK packet back immediately.
ii.If it is new, it updates the LS DB, schedules a Shortest Path First (SPF) algorithm calculation and it determines
which interface the LSA needs to be flooded out of.
Once the LS update packet has been processed, it prepares new LSAs updates into a new LS update packet and sends it
to the next router.
After this, the SPF calculations are computed.
And finally the Forwarding Information Base (FIB) is updated. The information in the FIB is used to decide which
outgoing interface card is the incoming packet forwarded to.
What is hot potato routing?
is a technique/practice of choosing a path within the network, by choosing the closest egress point based on intra
domain path cost (Interior Gateway Protocol/IGP cost).
effectively reduces the network's resource consumption by getting the traffic out as soon as possible.
simplifies computations for the routers as they are already aware of the IGP path costs. It makes sure that the path
remains consistent, since the next router in the path will also choose to send the packet to the same egress point.
What could weights on the graph edges be represented by when we are seeking the least cost path between two nodes?
Cable Length, Time Delay, Monetary Cost, Link Capacity, Current load on the link.
Forwarded
A packet is ___ when it is moved from the routers input link to the appropriate output link.
What is true about the link state algorith?
The number of immediate neighbors a node has will have no impact on the number of iterations required to complete
the algorithm.
Routing loops
Can cause the count to infinity problem.
The Routing Information Protocol (RIP) is an example of
a DV algorithm and intradomain algorithm.
what is Interior gateway protocol (IGP)
a type of protocol used for exchanging routing information between gateways (commonly routers) within an
autonomous system. ___ can be divided into two categories: distance-vector routing protocols and link-state routing
protocols.
Additional details of the Distance Vector Algorithm
Each node maintains and updates its own view of the network (doesn't have the complete picture).
Keeps iterating as long as neighbors send new updates
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