CSE 4344 Final Exam Review
How many bits are in the following?
IPv4
IPv6
MAC Address - ANS-- IPv4 - 32-bit
- IPv6 - 128-bit (in hex)
- MAC Address - 48-bit (in hex)
What is the difference between Pure Aloha and Slotted Aloha? - ANS-Slotted Aloha:
- slots must be synchronized, frames can only be transmitted at the beginning of the slot.
Pure Aloha:
- slots are not synchronized, frames can be transmitted anytime.
What are the two planes in the Network Layer? - ANS-- Control Plane
- Data Plane
T/F - All devices (excluding switches) connected to the internet have Layer 3 in them. - ANS--
True, Anything connected to the internet must have Layer 3 (Network Layer) except for switches
What are the two main functions of a Router? - ANS-- Forwarding (within the router)
- Routing (route from source to destination)
Data Plane - ANS-- Forwarding
- Local, per-router function
- Determines how datagrams go through the router (input to output of the router)
- Forwards the datagrams based on the forwarding table created in the control plane
Control Plane - ANS-- Routing
- Global, network wide logic
- Determines datagrams route from source to destination (end to end)
,What are the two control plane approaches - ANS-- Traditional routing algorithms (independent
router decisions)
- Software Define Network (SDN: global decision by a remote server)
Per router control plane - ANS-- Individual routing algorithm components in each and every
router interact in the control plane. Each router has their own forwarding table which it uses to
independently decide where to send the datagrams next. (Meaning each router makes its own
forwarding table with info from all other routers in the network)
Forwarding based on the forwarding table - ANS-- Data plane
Making the decisions (makes the forwarding tables) - ANS-- Control plane
SDN (Software Defined Networking) - ANS-- Remote controller computes and installs the
forwarding tables in each router. (Control Plane)
Decentralized Switching - ANS-- Using header field values, look up the output port using
forwarding table in the input port memory (match plus action)
When does input port queuing occur? - ANS-- When datagrams arrive faster than they can be
forwarded into the switching fabric (datagrams arrival rate > forwarding rate).
Destination-based forwarding - ANS-- Forwarding based only on the destination IP address
(traditional).
Generalized forwarding - ANS-- Forwarding based on any set of header field values.
Forwarding Table Entry - ANS-Destination Address range:
- If the destination IP address falls into the Address range, go to that link interface (output port)
Link Interface:
- Each link interface has its Address range.
Longest Prefix Matching - ANS-- When looking for a forwarding table entry for the given
destination address, use the longest address prefix that matches the destination address.
Switching Fabric - ANS-- Moving packets within the router.
Switching Rate - ANS-- Rate at which packets can be transferred from inputs to outputs.
- Often measured as multiple of input/output line rate.
3 Major types of Switching Fabrics - ANS-- Memory
, - Bus
- Interconnection network
Switching via Memory - ANS-- Uses memory to transfer datagrams. Packets copied through
system memory.
- Switching speed limited by memory bandwidth. Still used for some software routers.
Switching via Bus - ANS-- Datagram from input port memory transferred to output port memory
via a shared bus.
- Switching speed limited by bus bandwidth (single bus).
- Only 1 packet can be transferred on the bus at a time.
Used on low-end routers.
Switching via Interconnection Network - ANS-- Overcame bus bandwidth limitations by using
crossbar switch.
- Crossbar switch is an interconnection net that has 2*N buses connect to N input ports to N
output ports.
- Can control each bus intersection to open/close at will.
- Could potentially enable parallelism in switching!
What causes queuing delay and loss in Input Ports? - ANS-- Input buffer overflow! (i.e., the
datagrams are arriving to the input port faster than the switching fabric can transfer them to the
output, which fills up the buffer and causes overflows).
Head-of-the-Line (HOL) blocking - ANS-- Queued datagram at front of queue prevents others in
queue from moving forward.
What happens if you have a very fast input port with slow switching fabric? - ANS-- You will get
a lot of dropped packets.
Input Port Buffer - ANS-- Usually very small, 1 or 2 packets.
Output Port Queuing - ANS-- Buffering required when datagrams arrive from fabric faster than
link transmission rate. Datagrams can be lost due to congestion, lack of buffers.
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