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83 Cards in this Set
- Front
- Back
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What is the internet? |
Network of routers and user devices, server devices connected together via links (ex: ethernet, wifi) Topology view: physical connection of computers to a network of networks Services view: application programmers view socket API Protocol view: software implementing the networking stack on computers and routers |
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What is a website? |
An application that runs over the internet (ex: email, video chat, video streaming, social media, ftp) |
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What are network protocols? Give some examples |
Defines the syntax and semantics of messages Ex: HTTP - web; SMTP, IMAP, POP - email |
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What does IETF stand for? |
Internet engineering task force |
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What does RFC stand for? |
Request for comments |
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What is a host? |
Source (or origin) of a message or destination of a message; original sender or ultimate receiver |
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What is a router? |
Forwards messages from source hosts to destination hosts along a least cost path |
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When would a router act like a host? |
When trying to configure it |
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What is a link? |
Physical communications medium that connects two nodes in a network; propagate packets within the link |
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What is a node? |
Either a host or router |
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What is a NIC? |
Network interface card also referred to as a network adapter; transmits data onto link and receives data from link (ex: wifi interface, Ethernet interface) |
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Can hosts have multiple interfaces? |
Yes, but we will assume only one interface is active at a time |
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Can a router also act as a host? |
Yes, when you configure a router or with routing protocols *based on IP address of the destination host |
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What is an IP packet? |
Chunk of data that is sent at a time |
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What 2 problems was the internet solving? |
1) user on 1 computer couldn't share data with another computer on the same domain 2) user on 1 network could share data with other users on the same network but not with users on different networks |
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What is a forwarding table? |
Indicates which is the best "next hop" to get to the destination IP address |
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What is the order of the protocol stack from top to bottom? Give example of each |
L7: Application - web, echo L4: Transport - UDP/TCP L3: Network - IP L2: Link Layer - Ethernet, wifi L1: Physical - copper, fiber optics |
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What is the link layer? |
Responsible for communication between 2 nodes on a single link |
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What is a MAC address? |
Uniquely identifies a node interface on the link |
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What is the network layer? |
Supports communication between 2 hosts on the global internet using the services of the link layer; forwards IP packets from source to destination host |
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What is an IP address? |
Uniquely identifies host or router interfaces |
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What is the transport layer? |
Supports communication between 2 application processes using services of the network layer for host-to-host communication |
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What are ports? |
Uniquely identify source and destination application processes |
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What is the application layer? |
Consists of distributed apps making use of underlying services to transmit and receive messages via the socket API |
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What are the 3 layers of the stack used in routers? |
Physical, Link, and Network |
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What is encapsulation? |
Add protocol header when sending a packet |
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What is decapsulation? |
Delete protocol header; Ethernet/Wi-Fi header is stripped off and rest of packet is kept; at router, will check IP address to see where to forward packet to next |
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What are the parts a packet from left to right? |
1)Ethernet/wifi header 2) IP header 3) UDP/TCP header 4) App data |
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What is a bottleneck link? |
Slowest link along a path |
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What is throughput? |
Usually used in an application context and refers to the effective date that data is transferred |
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How is max throughput constrained? |
By the bottleneck link transmission rate |
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Which ports are reserved for well known services, assigned by IANA? |
0-1023 |
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How are port numbers picked? |
The OS can allocate unique port number to an application dynamically or you can specify what port number to use |
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What is a socket? |
Door between application process and end to end transport protocol |
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Describe the client/server socket interaction |
Client: send request to server Server: receive request from client, process request, and send response to client Client: receive response from server and process response |
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What are the 4 sources of packet delay? |
Transmission Propagation Processing Queuing |
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What is delay/latency? |
How long does it take for a packet to get to a destination (in seconds); one way trip time (OTT) or roundtrip time (RTT) |
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What is loss? |
Percentage of packets that get dropped or never arrive |
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Why does loss occur? |
Loss due to queue overflow in router under congestion or packets may be corrupted by links or bad memory |
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What is transmission delay? |
Time to transmit a packet of size (L) at a transmission rate (R) onto the link. d = L / R |
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What is propagation delay? |
Time to propagate a bit across a link of length (D) with a speed (S); d = D / S |
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What is processing delay? |
Time to process packet in hosts and routers (ex: in router, processing delay due to forwarding table lookup, encapsulation and decapsulation or errors, error detection) |
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What is queuing delay? |
Time waiting at output link for transmission due to network congestion; occurs when arrival rate exceeds the transmission rate |
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What is the domain name system (DNS)? |
Maps a host name to IP address(es) of host; "phonebook" for the internet |
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Does DNS map URLs to IP addresses? |
No, url format: http://www.njit.edu/cs/courses.html www.njit.edu: host name looked up in DNS cs/courses.html: path to object not looked up in DNS |
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What is a name server? |
Responsible for storing host name to IP address mappings for all hosts with a host name suffix of, for example, njit.edu |
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What are top-level domain name servers? |
Ex: .edu Store host name and IP address of name servers below them in the hierarchy (typically authoritative name servers, ex: njit.edu) |
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What is the root server? |
Stores hostname and IP address of TLD name servers |
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What are the two approaches for resolving a host name? |
Iterative: host (DNS client) bears the burden of hostname lookup processing; scales better with # hosts and networks joining the net Recursive: name servers (especially root and TLD servers) bear the burden of hostname lookup processing; simple hosts, less processing |
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What are the advantages of caching? |
1) Minimizes RTT 2) shorter distance = lower propagation delays 3) fewer routers on path = lower processing delays |
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What are the disadvantages of caching? |
1) cached results can become out of date ("stale") 2) in DNS, records have a time to live (TTL) value, cache only for the period of TTL 3) subsequent request goes to the origin server |
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What is a resource record and what is its format? |
Stores the hostname to IP mappings Tuple: (name, type, class, TTL, data corresponding to domain name) |
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What does a Type A resource record look like? |
Hostname A IN TTL (ip address) Ex: Gaia.cs.umass.edu A IN 3600 128.119.245.12 |
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What does a Type NS resource record look like? |
Domainname NS IN TTL (hostname of the NS for the domain)Ex: njit.edu NS IN 3600 dns1.njit.edu |
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What does a type CNAME (canonical name) resource record do and what's its format? |
Maps "alias" host name to "real" hostname Ex: njit.edu CNAME IN 3600 dprh0.stratcomm.njit.edu dprh0.stratcomm.njit.edu A IN 3600 128.235.21.11 |
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Which layer is responsible for managing ports? |
Transport layer |
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Which layer is responsible for forwarding IP packets from a source host to a destination host on the global internet via routers? |
Network layer (IP) |
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Describe an IP address |
Uniquely identifies a host (interface) Routers and hosts have IP addresses IPv4: 32 bits, 4 bytes, ~4 billion IP addresses IPv6: 128 bits, 16 bytes Expressed in dotted decimal notation Each byte is a decimal number separated by a dot, can be represented in binary |
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Describe hierarchical structure of the IP address structure |
Network part: uniquely identifies the network the host is attached to Host part: uniquely identifies the host on that network |
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Describe network part hierarchy of the IP address structure |
Network id: organization Subnetwork id: link in organization's network Host id: a specific host on the link |
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One prefix used by ISP to route to all networks and machines at an organization |
Route aggregation |
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What is the purpose of the hierarchy? |
Indicates the location of a host in the network topology and helps to scale the forwarding process |
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What are the 2 types of IP addresses? |
Unicast: identify a single host Broadcast: identifies multiple hosts on a network |
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By convention, a broadcast IP address is the __ value of an IP address within a given range |
Maximum |
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What is the general broadcast IP address and an example of a targeted broadcast? |
255.255.255.255 128.1.2.255 |
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Describe these unicast IP addresses: 127.0.0.1: 192.168.X.X: 10.X.X.X: 172.16.0.0: |
127.0.0.1: reserved for local use, typically used to identify the loopback interface192.168.X.X: private home interface, first 16 bits reserved (192.168.0.0 - 192.168.255.255)10.X.X.X: private (like NJIT network), first 8 bits reserved (10.0.0.0 - 10.255.255.255)172.16.0.0: reserved for private use, not globally unique, only locally unique within some scope, first 12 bits reserved |
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What is a data plane? |
Used with forwarding: 1) router consults the forwarding table when packet arrives 2) examines network prefix of destination IP address in IP header 3) Uses forwarding table to determine outgoing link |
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Discuss the process of IP Forwarding |
When forwarding, routers need to distinguish the network port from the host port; routers need to determine the network boundary |
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What are the 2 ways for a router to determine the network boundary? |
1) Network mask: 32 bit numbers with 1s corresponding to the network part and 0s corresponding to the host part Ex: Network part is in the first 24 bits of IP address Ex IP: 128.1.2.3 network mask: 255.255.255.0, network IP address: 128.1.2.0 2) Network prefix length: # bits corresponding to the network part Ex: 128.1.2.3/24 |
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What make use of a forwarding table at the network layer to make the forwarding decisions? |
Routers |
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What is the Maximum Transmission Unit (MTU)? |
Defined by link layer; max length of IP packet (in bytes); includes IP header and TCP/UDP header + app data length; excludes link layer header; unit is bytes typically |
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What is a subnet? |
Numbers a link in a network uniquely; each has a unique subnet id; hosts on the same subnet share a subnet id; hosts on same link can communicate directly; hosts on same subnet can communicate directly without going through a router; hosts on different subnets must send their packets via a router |
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Where can you find MAC address? |
Hardcoded into NIC, 48 bits and unique |
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Describe where each part comes from in the message to get from Host A to B on the same subnet directly with no router |
Ethernet Source MAC: hardcoded on NIC Dest MAC: ARP IP Source: DHCP Dest: DNS UDP header Source port: OS assigns Dest port: Reserved, well known, standard |
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What is Address Resolution Protocol? (ARP) |
Maps IP address to a MAC address; ARP broadcasts a request message asking for the MAC address corresponding to the specified IP address; host with the specified IP address responds with its MAC address to source host; ARP keeps IP to MAC bindings in an ARP cache |
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What are the 4 items received from DHCP? |
1) Host's own IP address 2) IP address of first hop router (default gateway) 3) IP address(es) of DNS server (typically local server) 4) Subnet mask |
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1 byte = _ bits |
8 |
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1 Mbps = _ bits |
1000000 |
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1Gbps = _ bits |
1000000000 |
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How does a host typically acquire the information found in network settings? |
DHCP during the link up event -> DHCP request allocates an IP address for a lease time DHCP client will renew the lease |
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Why can the root server only query iteratively? |
The root DNS server doesn’t accept a recursive query, but rathers gives you the reference to the next name server that is to be queried in |
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What is MTU? |
the maximum transmission unit is the size of the largest protocol data unit that can be communicated in a single network layer transaction |
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What is TTL? |
tells a network router whether or not the packet has been in the network too long and should be discarded |
