CS代考 GY 6843 Topics for final exam Spring 2022

CS-GY 6843 Topics for final exam Spring 2022
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CS-GY 6843 Topics for midterm exam Spring 2022
Sliding Window Protocol
CS-GY 6843 Topics for final exam Spring 2022 1

Flag in TCP header signal the beginning of a connection
Contains first sequence number to be used in the connection ACK
Attached to data packet traveling in the reverse direction
In TCP, the ack field contains the next packet expected by the receiver
The SYN flag is used to indicate that a sender is ready to transmit data. The ACK is used to indicate that the receiver is ready to accept data. Since the TCP connection is bi- directional. So two SYNs and two ACKs must be sent. However, the server SYN + ACK shared the same packet. SO only three packets needed. That is why we called Three-way handshake
A FIN packet is sent by the side who wants to tear down the connection.
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An ACK is sent back by the other side.
Since each direction of the TCP connection can be closed at different times, the two FINs and two ACKs required to tear down the TCP connection may all travel in different packets. Therefore, up to four packets can be sent during this final phase of the TCP connection.
Nagle’s Algorithm
Nagle’s algorithm woks by combining a number of small outgoing messages and sending them all at once. Specifically, as long as there is a sent packet for which the sender has received no acknowledgment, the sender should keep its output until it has a full packet’s worth of output, thus allowing output to be sent all at once.
This Algorithm solve problem about an application repeatedly send data in small chunks
TCP Delayed Ack Protocol
In essence, several ACK responses may be combined together into a single response, reducing protocol overhead.
The additional wait time introduced by the delayed ACK can cause further delays when interacting with certain applications and configurations.
If the application is transmitting data in smaller chunks and expecting periodic acknowledgment replies, this negative interaction can occur.
for example, consider a situation where Bob is sending data to Carol. Bob’s socket layer has less than a complete packet’s worth of data remaining to send. Per Nagle’s algorithm, it will not be sent until he receives an ACK for the data that has already been sent. At the same time, Carol’s application layer will not send a response until it gets all of the data. If Carol is using delayed ACKs, her socket layer will not send an ACK until the timeout is reached
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Advantages
Delayed ACKs can give the application the opportunity to update the TCP receive window and also possibly to send an immediate response along with the ACK.
For certain protocols such as Telnet, delayed ACKs can reduce the number of responses sent by the server
UDP vs. TCP
TCP: connection-oriented, makes sure data is delivered, sends packets again and again until everything is received, sends acknowledgments that data is received
UDP: connectionless, does not guarantee delivery (faster!!!)
Definition: Domain name System Name space
the Name space is the structure of the DNS database An inverted tree with the root node at the top
Each node has a label
The root has a null label
Domain Names
A domain name is the sequence of labels from a node to the root, separated by dots, read left to right
Domain names identifies its position in the name space Domains &Subdomains
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One domain is a subdomain of another if its domain name ends in the other’s domain name
for example, sales.nominum.com is a subdomain of nominum.com& com, nominum.com is a subdomain of com
Each time an administrator delegates a subdomain, a new unit of administration is created. These units called zones
Scalability Authoritative servers
The name servers that load a complete zone are the authoritative server of that zone. More than one name servers are authoritative for the same zone. A single name server might be authoritative for many zones.
Authoritative: maintain the data
Master: Where the data is edited Slave: Where the data is replicated to
There are two main types of name severs: Authoritative and Cashing. Cashing: stores data obtained from an authoritative sever
Reverse lookup
A reverse DNS lookup is a DNS query for the domain name associated with a given IP address.
Reverse DNS lookups query DNS servers for a PTR (pointer) record
for example, PTR records store IP addresses with their segments reversed, and they append “.in-addr.arpa” to that. For example if a domain has an IP address of 192.0.2.1, the PTR record will store the domain’s information under 1.2.0.192.in-addr.arpa.
DNS record types
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A(Host address) NS(Name Sever) PTR(Pointer)
reverse lookup MX(Mail eXchange) SOA(Start of Authority)
A SOA-record is automatically created when you create a new zone Recursive DNS & Iterative DNS
In the recursive DNS, a DNS server continues querying other DNS servers recursively until it gets an IP address to send to the client. Recursive methods can reduce communication cost. Recursive DNS puts higher performance demand on each name server. Recursive works better with caching.
In the Iterative DNS, each DNS query responds directly to the client with an address for another DNS server to ask, and the client continues querying DNS servers until one of them responds with the correct IP address for the given domain, or it time out, or an error occurs.
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Recursive DNS
In the situation where you want faster performance. Recursive DNS usually solves the situation faster than iterative queries when recursive DNS server caches the results for each query and saves it for a set amount of time determined by time to live.
Iterative DNS
However, if you want a more secure DNS, iterative is more favorable than recursive DNS because recursive DNS queries have security issues. It is vulnerable for DNS amplification attacks and DNS cache poisoning.
802.11(wireless frames) vs. 802.3(Ethernet)
802.11 uses CSMA/CA. CA stands for collision avoidance which cannot detect the collision but only to avoid it. The data is sent only the channel is idle and if no acknowledgement received, it resends the data. In this way, there is no collision occurs, so 802.11 is not able to perform frame collision. RTS
802.3 uses CSMA/CD. CD stands for Collision Detection, which means the collision is only detected not avoided. When two station send the frame simultaneously via the same medium, the collision occurs and detected. The participant will send a Jam signal so all other can detect the collision. Then counters for retransmission is reset and successfully complete frame transmission if maximum number of transmission attempts is not reached. In conclusion the 802.3 performs frame collision
RTS request-to-send
CTS clear-to-send
Collision Avoidance
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Access Point
An access point is a type of hardware, such as a wireless router, that acts as a connection portal for other devices in the network
No assigned IP: access point needs to connect to a router to connect to the outside world.
MPLS(Multi-Protocol Label Switching)
Virtual Circuits vs. Packet Routing Virtual Circuit
Combination of packet and circuit switching
Logical circuit is identified by a VC ID
The packet will then have a fixed length label in the header MLPS routing
path to destination can be based on source and destination address
CS-GY 6843 Topics for final exam Spring 2022 8

fast reroute
precompute backup routes in case of link failure IP routing
path to destination determined by destination address alone Label Switching vs. IP routing
In label switching, once a path is established between two networks, it follows the same path for the entire communication irrespective of traffic. This makes it less flexible than ip routing.
In dynamic routing, routing table are changing continuously to find the best path.
RSVP vs. RSVP-TE RSVP
In an MPLS network ,RSVP is to reserve bandwidth of the LSP. it form source LER to destination LER to reserve bandwidth and then back to the source to set the maximum bandwidth of the LSP
Resource Reservation Protocol with traffic engineering
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Using constrained routing, RSVP-TE looks for the shortest path with enough available bandwidth to carry a particular LSP.
1+1 & 1+ m backups
Fast reroute
An MPLS fast reroute is a backup
LSP that is used is and when the primary LSP fails for some reason. Since it is already setup, traffic quickly switches to the backup LSP
One to one protection/detour: an individual backup path is fully signaled through RSVP for every LSP, at every point where protection is provided (i.e. every node).
Many-to-one protection: a single bypass LSP is created between two nodes to be protected. During a failure, multiple LSPs are rerouted over the bypass LSP.
switching vs. routing
Switching is to switch data packets between devices on the same network(or same LAN)
Routing is to route packets between different networks( between different LAN ) LSP
Label Switch Path
LSP is a unidirectional tunnel between a pair of routers, routed across an MPLS network
LSP is required for any MPLS forwarding to occur LER
Label Edge Router
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An LER is a router on the edge of the MPLS network. On ingress, it checks on the final destination and decides which of the LSPs to use. It then assigns a label as it sends packets into the MPLS network. On egress it removes the label and the packet is delivered by standard routing protocols
Label Switching Router
This router does the MPLS switching in the middle of LSP
SVC vs. PVC
“PVC” is a virtual circuit which is available permanently. It is a type of virtual circuit where the end points do not signal the circuit. The virtual circuit values are manual. However, “SVC” is a circuit which is an on-demand circuit established by the user signals.
“PVC” is a permanent circuit while “SVC” must be re-established every time when there is a need for a data transfer.
The SVC disappears as soon as the data is transferred Circuit setup
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IP Multicast Group Membership
28 Bits of 32 Bit IP Address Reserved for Multicast Class D IP Address Range Reserved for Multicast
224.0.0.0 to 239.255.255.25
Layer 2 Multicast Address Derived From Layer 3 IP Address No net mask for IP multicast group
Multicast addresses allow a source device to send a packet to a group of devices. Devices that belong to a multicast group are assigned a multicast group IP address.
Ethernet Multicast
23 Bits of 48 Bit MAC Address Reserved for Multicast
By Default: A Layer 2 Switch Will Forward Multicast Packets Out All Ports (except origin port)
To Eliminate “Flooding” – IGMP Snooping is Utilized IGMP snooping
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The IGMP is a network layer protocol that allows several devices to share one IP address so they can all receive the same data. Networked devices use IGMP to join and leave multicasting groups, and each multicasting group shares an ip address
IP Multicast Limited to “UDP” Packet Deliver
It uses various algorithms to transmit the data stream as quickly and efficiently as possible to all members of the particular multicast group. https://www.ionos.com/digitalguide/server/know-how/multicast/
Announced networks LSA(Link State Ack) Flooding
Router R1 that detects a topology change adjusts its LSA and floods the LSA
Router R1 notifies all OSPF neighbors using 224.0.0.5, or, on LAN links, all OSPF DRs and BDRs using 224.0.0.6
All(ospf) router IP Multicast
In OSPF router sends a “hello” packet to other OSPF routers on the network. The OSPF router must send this “hello” to an assigned multicast address which is 224.0.0.5. And the other routers will respond.
Default route (0.0.0.0) DHCP
In OSPF it mean default route as it will match all the ip address with subnet /0 (the /0 means the lowest priority, so when it check the forwarding table, it will check all the other routing info, and check the /0 last to go to the default route)
Longest prefix match
CS-GY 6843 Topics for final exam Spring 2022 13

refers to an algorithm used by router in IP networking to select an entry from a routing table
The most specific of the matching table entries — the one with the longest subnet mask — is called the longest prefix match.
Forwarding tables often contain a default which has the shortest possible prefix match, to fall back on in case matches with all other entries fail.
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