Midterm 1 Flashcards
What is ARP and how is it used?
Address Resolution Protocol - Broadcast query to discover an IP to MAC mapping with a unicast response of the MAC from the target IP.
What is a NAT?
Network Address Translation - Converts to/from public IP:Port to private IP:Port on the local network. Replaces the source IP upon leaving the NAT to the public and replaces the destination IP upon entering the NAT to the local network.
What is the STP? Explain it’s purpose.
Spanning Tree Protocol - Physically can have redundant links (topology loops), but ports switched off at the logical layer so that there are no loops. Ports can then be switched back on if necessary in case something else goes down.
What is the narrow waist?
Internet Protocol (IP) (AKA Network Layer) is the center of the waist. Every internet device must implement the IP stack. Provides guarantees to above layers: End-to-end connectivity with best effort delivery. This is about the only requirement to ‘get on the internet’.
What is the end to end principle?
As much functionality as possible should be implemented at the endpoints, not in the middle. Keeps core IP very simple. Allows innovation at the endpoints. It’s not doing more than you need. “Dumb Network, Intelligent Endpoints”
What is the longest matching prefix length?
The most specific subnet, eg: /20 is more descriptive than /16. You should forward according to the longest prefix.
What is an autonomous system?
An independently operated network.
List advantages/disadvantages of packet switching.
Advantages: No busy signal! The links are shared so multiple people can use the same paths. Potentially better resilience properties.
Disadvantages: Variable delay, potential for lost or dropped packets.
List advantages/disadvantages of circuit switching.
Advantages: resource control, better accounting, reservation of resources, ability to pin paths b/t sender/receiver. Better throughput as the only congestion is your own.
Disadvantages: Only one person can use a given path at any one time. More difficult to design and implement.
What is a broadcast storm?
Loop connections where one broadcast is sent to a switch which emits the same broadcast which goes back to the source which re-emits it and it continues in a loop this way.
What is fate sharing?
Acceptable to lose state information for that entity if the entity is lost itself. State in device shares the fate of the device itself.
What were the original goals of the internet?
Main Goal: “Multiplexed utilization of existing interconnected networks”
Smaller Goals:
Interconnection - Support multiple types of communication services (protocols)
Work on a variety of networks - physical layers
Survivability - Network works even if some devices fail
Distributed Management - No single owner
Cost Effectiveness - ‘fairly’ cost effective as it is
Ease of Attachment - Narrow Waist makes it easy to use optical, cable, or wireless (physical layer). Anything that wants to be on the internet just needs to speak IP - that’s it.
Accountability - ability to bill. Harder in the current internet of packet switching than if it was circuit switched.
What are the major layers of the OSI network model?
Application Layer: HTTP/SMTP/etc.
Transport Layer: provides ‘reliably transport’, ‘congestion control’ (TCP/UDP)
Network Layer: “IP over anything”
Link Layer: often Ethernet (p2p connectivity)
Physical Layer: optical, electrical, etc
What are some violations of the end-to-end argument?
NAT, VPN Tunnels, TCP splitting, Spam (network should deliver all main to user, but with spam, sometimes it gets filtered), P2P, Caches
How does NAT violate the end-to-end argument?
It interferes by rewriting the source and destination ports as they go in and out of the router’s NAT. Machines behind the NAT aren’t publicly addressable.
What is the difference between switches, hubs, and routers?
Hubs: broadcast medium - all packets are seen everywhere.
Switch: Isolates Traffic - only forwards packets to the ports where the destination is. Typically operate at layer 2. Major limitation: broadcast - spanning tree/arp queries can eat up a lot of bandwidth.
Routers: Typically operate at layer 3 (IP layer). Topologies are not restricted to a spanning tree. Can have multiple paths to the same destination.
What is a switch table (forwarding table)?
A mapping of host IP’s to output ports.
- If entry is not found in table, it floods (broadcasts)
- It will forward ARP requests properly (broadcasts)
- If entry exists, it will just send to that port
How does STP work?
- Elect a root (swtich w/ smallest ID) - initially every node assumes it is the root.
- At each switch, exclude link if not on the shortest path to root
- Tiebreakers go to the node with the smallest ID
- Messages are passed with the (origin, claimed root, and distance to claimed root)
- Each switch adds 1 to the distance received in the message and checks to see if msg claimed root is < it’s claimed root, if so it updates claimed root and its distance to claimed root.
How are buffering needs determined in router/switch design?
used to be 2TC, but now it’s 2TC/sqrt(n).
2T*C only applies if flows are synchronized. If they are not synchronized, they gaussian becomes narrower as a function of 1/sqrt(n).
Define the major two types of routing.
Interdomain: Routing outside an AS
Intradomain: Routing inside an AS