Lesson 12:
Having a single server for providing Internet content has the following disadvantages:
Single point of failure.
Bandwidth waste in high demand for the same content.
Scalability issues.
Potentially big geographic distance between Internet hosts/users and the server.
Single point of failure.
Bandwidth waste in high demand for the same content.
Scalability issues
Potentially big geographic distance between Internet hosts/users and the server.
Lesson 12 (T/F):
One of the advantages of using CDNs is that the routing protocols they use take important aspects into consideration, such as congestion, latency, etc., in order to best deliver the content to the Internet users.
False
Lesson 12 (T/F):
There are several factors that can make a CDN network unreliable, such as misconfigured routers, power outages, malicious attacks or natural disasters.
True
Lesson 12 (T/F):
As the Internet evolves, the topology of the ISPs has become flatter, and the number of IXPs increases as the time progresses due to the services they offer and the lower costs for the ISPS.
True
Lesson 12 (T/F):
The major drawback of the “Enter Deep” approach is that, if one server is lost, that geographic area will experience a higher delay and lower throughput.
False
Enter deep downside:
However, the downside to thishighly distributed approach is that it is much more difficult to manage and maintain so many clusters.
Bring home downside:
The downside is that the users will experience higher delay and lower throughput.
Lesson 12 (T/F):
When using CDN servers for content delivery, there is more overhead than when using the traditional approach.
True
When a user requested some content, with the old traditional approach, it was pretty straightforward to figure out which server cluster should service the request – because with the traditional approach, there was only one server cluster that could service the request.
Lesson 12 (T/F):
For a CDN to deliver content to an Internet user, a cluster is mapped to a client first and then a server within that cluster is selected.
True
- The first thing that needs to be done in order to serve the content to an end-user is to select the server to serve the content from. This process is quite important as it significantly impacts the end-user performance.
- There are two main steps in this process. The first step consists of mapping the client to a cluster. Recall that a CDN constitutes of geographically distributed clusters with each cluster containing a set of servers. In the next step, a server is selected from the cluster. Let us now look at the policies and mechanisms for this server selection process in detail.
Lesson 12 (T/F):
Picking the geographically closest cluster location for a user is always the best choice in terms of performance for content delivery.
False
- ExampleCDN name server system (which is where the server selection happens) didn’t directly interact with the user, but rather with the user’s Local Domain Name Server (LDNS).
- So, picking the geographically closest cluster is really “picking the cluster closest to the LDNS” and not “closest to the user.” Most of the time, that works ok, but some customers use a remote LDNS. To overcome this, there have been suggestions made to include the client IP in the interaction between the ExampleCDN name server system and the LDNS. This allows the ExampleCDN name server system to know where the client is located.
- Another very important limitation is that the cluster which is geographically closest may not be the best choice in terms of actual end-to-end network performance. This can happen because of multiple reasons.
- One, due to routing inefficiencies in routing protocols such as BGP, the actual network path to the geographically closest server might actually be longer than the path to an alternative cluster. This can lead to a higher RTT, even though the server is closest geographically.
- Another important reason could be that the path to the geographically closest cluster or the cluster itself could be congested.
Lesson 12 (T/F):
By using consistent hashing for server selection, in the case of a server failure, the objects that the server was responsible for can be taken care of by a random server within the same ID space.
False
Let us see, what happens when a server (#41 in this case) leaves. The objects that the server was responsible for can now be served by the next server.
Lesson 12:
When using DNS caching, what would happen if a host A makes a request for a domain that was just previously queried by another host?
The local DNS server will send the request to the appropriate DNS server to fetch the IP address.
The local DNS server will immediately answer the host with the IP address.
The request will be forward to the root servers first.
None of the above.
The local DNS server will immediately answer the host with the IP address.
Lesson 12:
What is the type of the following resource record: (amazon.com, dns.amazon.com, ?, TTL)?
A
NS
CNAME
MX
NS
- TYPE=A:the name is a domain name and value is the IP address of the hostname. (abc.com, 190.191.192.193, A)
- TYPE=NS:the name is the domain name, and the value is the appropriate authoritative DNS server that can obtain the IP addresses for hosts in that domain. (abc.com, dns.abc.com, NS)
- TYPE=CNAME:the name is the alias hostname, and the value is the canonical name, (abc.com, relay1.dnsserver.abc.com, CNAME)
- TYPE=MX:the name is the alias hostname of a mail server, and the Value is the canonical name of the email server. (abc.com, mail.dnsserver.abc.com, MX)
Lesson 12 (T/F):
IP Anycast assigns the same IP address to multiple servers in order to deliver content from CDNs by using the closest server to a client based on BGP path length.
True
- The main goal of IP anycast is to route a client to the “closest” server, as determined by BGP (Border Gateway Protocol), a routing protocol used for inter-AS routing. This is achieved by assigning the same IP address to multiple servers belonging to different clusters.
- Now, each of these servers will use the standard BGP to advertise this IP address. Thus multiple BGP routes for the same IP address corresponding to different cluster locations will propagate in the public Internet.
- Now when a BGP router receives multiple route advertisements for this IP address, it would treat them as multiple paths to the same locations, although, in reality these routes correspond to different physical locations. In the end, the shortest path will be stored and used for routing packets.
Lesson 12 (T/F):
HTTP redirection can only be used in order to share the load of content requests among servers.
False
One particular case is for load-balancing
