计网homework

计网一些课后作业的答案，总结如下：

1.1 List the available residential access technologies in your city. For each type of access, provide the advertised downstream rate, upstream rate, and monthly price.
答：dial-up (up to 56kbps); DSL (up to 1 Mbps upstream , up to 8 Mbps downstream); cable modern (up to 30Mbps downstream, 2 Mbps upstream.)

1.3 Why will two ISPs at the same level of the hierarchy often peer with each other? How does an IXP earn money?

1.4 Some content providers have created their own networks. Describe Google’s network. What motivates content providers to create these networks?

hubs, switches, bridges and routers.
答：hubs operate at Layer 1 of the OSI model, while bridges and switches work with MAC addresses at Layer 2. Hubs broadcast incoming traffic on all ports, whereas bridges and switches only route traffic towards their addressed destinations.

2.1 The benefits of IPv6 when compared with IPv4
More Efficient Routing、More Efficient Packet Processing、Directed Data Flows
Simplified Network Configuration、Support For New Services、Security.

2.2 Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in common?
IPv6 has a fixed length header, which does not include most of the options an IPv4 header can include. Even though the IPv6 header contains two 128 bit addresses (source and destination IP address) the whole header has a fixed length of 40 bytes only. Several of the fields are similar in spirit. Traffic class, payload length, next header and hop limit in IPv6 are respectively similar to type of service, datagram length, upper-layer protocol and time to live in IPv4.

2.3 Give more detailed information about the data plane and control plane of network layer.
Data plane refers to all the functions and processes that forward packets/frames from one interface to another. Control plane refers to all the functions and processes that determine which path to use.

2.4 What is connection flooding?
Flooding is a Denial of Service (DoS) attack that is designed to bring a network or service down by flooding it with large amounts of traffic. By flooding a server or host with connections that cannot be completed, the flood attack eventually fills the hosts memory buffer.

2.5 Wireshark
Wireshark is the world’s leading network traffic analyzer, and an essential tool for any security professional or systems administrator. This free software lets you analyze network traffic in real time, and is often the best tool for troubleshooting issues on your network.

3.1 Where does queuing occur in a router?
Queueing can occur at both the input ports and the output ports of a router. Queueing occurs at the output port when the arriving rate of packets to the outgoing link exceeds the link capacity.

3.2 SDN (software defined networking)
Software-defined networking (SDN) is designed to make a network flexible. SDN lets you design, build, and manage networks, separating the control and forwarding planes. The control plane is directly programmable, and it abstracts the underlying infrastructure for applications and network services.

3.3 Working process of a load balancer
A load balancer is a virtual hardware that acts like a reverse proxy to distribute network or application traffic across different servers. A load balancer is used to improve the concurrent user capacity and overall reliability of applications.

3.4 Present MPLS applications
MPLS stands for Multi-Protocol Label Switching. It is a mechanism for routing traffic within a telecommunications network, as data travels from one network node to the next. MPLS can provide applications including VPNs (Virtual Private Networks), traffic engineering (TE) and Quality of Service (QoS).

4.1 Is TCP a GBN or an SR protocol?
TCP does something in between GBN and SR, closer to SR. TCP sends only one packet that it thinks is lost, not entire window, so not like GBN. TCP receiver buffers out of order segments. But ACK indicates the sequence number that is missing (unlike SR).

4.2 Compare flow control and congestion control.
The flow control is a mechanism that controls the traffic between sender and receiver. The congestion control mechanism controls the traffic that is placed by the transport layer into the network.

4.3 the phenomenon of “global synchronization” caused by TCP
TCP global synchronization occurs when a sudden burst of traffic causes simultaneous packet loss across many TCP sessions using a single (congested) link. Each affected TCP session backs off its send rate at the same time, causing link utilization to go way down.

4.4 TCP congestion control
TCP Congestion Control. If an acknowledgement for a segment is not received within the timeout, it is re-transmitted. TCP uses a congestion window in the sender side to do congestion avoidance. The congestion window indicates the maximum amount of data that can be sent out on a connection without being acknowledged.

4.5 the fairness problem of TCP
TCP fairness requires that a new protocol receive no larger share of the network than a comparable TCP flow. This is important as TCP is the dominant transport protocol on the Internet, and if new protocols acquire unfair capacity they tend to cause problems such as congestion collapse.

5.1 Compare three service models, i.e., best-effort service, DiffServ, IntServ
IntServ uses the per-flow approach to provide guarantees to individual streams, while DiffServ provides aggregate assurances for a group of applications.
Best effort refers to a network service that attempts to deliver messages to their intended destinations but which does not provide any special features that retransmit corrupted or lost packets. Thus, there are no guarantees regarding delivery.

5.2 How VoIP works?
VoIP stands for Voice over Internet Protocol. A VoIP system works by taking your analogue voice signals, converting them into digital signals, then sending them as data over your broadband line.

5.3 CDN’s work principles
A content delivery network (CDN) is a system of distributed servers (network) that deliver webpages and other web content to a user, based on the geographic location of the user. IT works on the principle that the closer the content of the web hosting provider is to a user, the faster it will be accessible to that person. The response time of the nearby server is shortened, and the content can be downloaded quickly. CDNs are primarily used for streaming audio and video content, but also for websites with particularly wide coverage to optimize the load distribution of servers and improve the user experience.

5.4 Three categories of Streaming video systems . List three disadvantages of UDP streaming.
HTTP streaming、UDP streaming、Adaptive HTTP streaming
The sender and the receiver bandwidth is volatile.
Distribution of the video system is also complex due to require tracking of each client to server interactions and media controls.
Most of the firewalls do not agree the UDP traffic.

5.5 Principles Usage of token bucket / leaky bucket
The token bucket is an algorithm used in packet switched computer networks and telecommunications networks. It can be used to check that data transmissions, in the form of packets, conform to defined limits on bandwidth and burstiness
The leaky bucket algorithm is a method of temporarily storing a variable number of requests and organizing them into a set-rate output of packets in an asynchronous transfer mode (ATM) network. The leaky bucket is used to implement traffic policing and traffic shaping in Ethernet and cellular data networks.

6.1 Compare and contrast distance-vector routing and link-state algorithms
Algorithm： Bellman ford ——Dijsktra
Updates： Full routing table ——Link state updates
Updates frequency： Periodic updates——Triggered updates
Convergence time： Moderate——Fast
Updates： On broadcast——On multicast

6.2 Comparison among RIP, OSPF and IS-IS
Compare to RIP, OSPF has no limitation due to hops (RIP has a limit of 15 hops so any network with more than 15 hops cannot be achieved by RIP. OSPF can handle Variable Length Subnet Masks (VLSM) but RIP cannot. The most important is that OSPF converges much faster than RIP due to its calculation algorithm.

6.3 a pathology of link-state algorithm
It is a dynamic routing algorithm in which each router shares knowledge of its neighbors with every other router in the network. A router sends its information about its neighbors only to all the routers through flooding. Information sharing takes place only whenever there is a change.

6.4 the “count-to-infinity” problem of the DV algorithm
Counting to infinity is just another name for a routing loop. In distance vector routing, routing loops usually occur when an interface goes down. It can also occur when two routers send updates to each other at the same time.

6.5 the concept of “poisoned reverse”
poison reverse is a way in which a gateway node tells its neighbor gateways that one of the gateways is no longer connected.

7.1 Give an example about BGP incidents and explain the reasons.
BGP hijacking (sometimes referred to as prefix hijacking, route hijacking or IP hijacking) is the illegitimate takeover of groups of IP addresses by corrupting Internet routing tables maintained using the Border Gateway Protocol (BGP).

7.2 Why are there different Intra- and Inter-AS routing ?
Border Gateway Protocol(BGP) is used for inter-AS( Autonomous System (AS) ) protocols.But, Router Information Protocol(RIP) and Open Shortest Path First(OSPF) protocol are used for intra-AS protocols.
Inter-AS protocol provides for the controlled distribution of routing information, but Intra-As protocol are the policy issues play a much less important role in choosing routes
Inter-AS protocol provides is dominates the quality and the performance, but Intra-As protocol focuses on the performance.

7.3 peering and transit
IP peering and IP transit are ISP definitions used to describe how connections are made between entities that make up the internet. IP peering is a mutual exchange of data between two ISPs, and the amount of data exchanged is typically close to equal.

7.4 Hot potato routing、Cold potato routing
In hot-potato routing, an ISP hands off traffic to a downstream ISP as quickly as it can. Cold-potato routing is the opposite of hot-potato routing where an ISP carries traffic as far as possible on its own network before handing it off to a downstream ISP. These two policies reflect different priorities for the ISP.

8.1 SSL / TLS Give at least one example.
Transport Layer Security (TLS), and its now-deprecated predecessor, Secure Sockets Layer (SSL), are cryptographic protocols designed to provide communications security over a computer network. The connection is private (or secure) because symmetric cryptography is used to encrypt the data transmitted.

8.2 Advantages and disadvantages of VPN
Enhanced Security、Bypass Geo-restrictions on Websites and Content、Anonymous Downloads.
Easy File Sharing Between Remote Clients——Speed Issues、Increased Network Complexity

8.3 symmetric encryption techniques.
Symmetric encryption is a type of encryption where only one key (a secret key) is used to both encrypt and decrypt electronic information. The entities communicating via symmetric encryption must exchange the key so that it can be used in the decryption process.

8.4 public key systems.
Public-key encryption is a cryptographic system that uses two keys – a p加粗样式ublic key known to everyone and a private or secret key known only to the recipient of the message. Example: When John wants to send a secure message to Jane, he uses Jane’s public key to encrypt the message

9.1 How can we provide reliable multicast?
A reliable multicast protocol is a computer networking protocol that provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications such as multi-receiver file transfer.

10.1 Mobility comparison between GSM and Mobile IP
The main difference between both is: GSM is the radio network you use to access data. Mobile IP is used to transport your IP data from your device to a centralized exit point where it enters the public internet.

10.2 IP subnet
A portion of a network that shares a common address component. On TCP/IP networks, subnets are defined as all devices whose IP addresses have the same prefix. Dividing a network into subnets is useful for both security and performance reasons. IP networks are divided using a subnet mask.

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