Chapter 4 - Network Layer (Data Plane) Flashcards
What does a sender do in the Network Layer
Encapsulates segments into datagrams and passes this to the link layer
What do routers do?
- Examines header fields for all IP datagrams
- moves datagrams from input ports to output ports
What are the two network-layer functions?
Forwarding: move packets from routers input link to the appropriate output link
Routing: Determines the route taken by packets from source to destination (routing algorithms)
Data Plane vs Control Plane
Data plane: (per router) determines how datagram on router input port is forwarded to router output port
Control Plane: (Network wide logic) determines how datagram is routed from souce router to destination router
Per router control plane?
Individual routing algorithm components in each and every router interact in the control plane
Software-Defined Networking (SDN) control plane
Remote controller computes, installs tables in routers
Example network service models for INDIVIDUAL datagrams
- guaranteed delivery
- guaranteed delivery with less than 40 msec delay
Example services for a FLOW of datagrams
- in order datagram delivery
- guaranteed minimum bandwidth to flow
- restrictions on changes in inter-packet spacing
Best effort service model
Bandwidth - none
Loss - no
Order - no
Timing - no
Why Best effort model?
- Simplicity
- sufficient provisioning of bandwidth allows performance to be “good enough”
- replicated, application-layer distributed services
- congestion control of “elastic” services
Router Architecture
- Routing processor
- High speed switching fabric
- Input ports
- Output ports
Input Port Functions
Physical Layer: bit-level reception
Link layer protocols
Decentralized switching:
What is decentralized switching
- using the header field values, lookup the output port from the forwarding table in the input port memory
- the goal is to complete the input port processing at “line speed”
- if the datagrams arrive faster than the forwarding rate into switch fabric, then they will be queued.
What is longest prefix matching
When looking for a forwarding table entry for given destination address, you use the longest address prefix that matches the destination address.
What do switching fabrics do?
- transfer the packet from the input port to the appropriate output port
What is the switching rate?
rate at which packets can be transferred from inputs to outputs
Switching via memory
- packets are copied to the system’s memory
- the speed is limited by the memory bandwidth (2 bus crossings per datagram)
Switching via bus
- datagram from the input port memory is transferred to the output port memory via a SHARED BUS
- Bus contention: the switching speed is limited by bus bandwidth
Switching via interconnection network
-Crossbar: interconnected busses
- multistage switch: nxn switch from multiple stages of smaller switches
- exploiting parallelism: fragment datagram into fized length cells on entry. Switch cells through the fabric, reassemble datagram at exit
What is input port queuing?
If the switch fabric is slower than the input ports combined, the the queuing may occur at input queues
What is head-of-the-line (HOL) blocking?
a queued datagram at the front of the queue prevents others in the queue from moving foward.
What are the two types of output port queuing?
- buffering
- scheduling dscipline
What is buffering?
- required when datagrams arrive from fabric faster than link transmission rate.
- datagrams can be lost due to congestion, lack of buffers
What is scheduling discipline
chooses among the queued datagrams for transmission
- through PRIORITY SCHEDULING, you can determine who gets the best performance.
What are the two types of buffer management?
- dropping packets when buffers are full
- marking packets to signal congestion
what is tail dropping?
dropping the arriving packet
what is the priority dropping
drop or remove packets based on a priority basis
What are the 4 types of packet scheduling
- first come first serve aka First in first out
- priority
- round robin
- weighted fair queuing
What is First in first out packet scheduling
- which ever packets arrive first, those will be processed first followed by subsequent packets
what is priority packet scheduling
- packets are defined to be high priority or low priority
- packets that are high priority are processed first, followed by low priority packets
what is round robin packet scheduling?
-packets are divided into classes
- the server will cycle through the classes, processing a packet from each class each time.
- will also go to high priority classes first
What is weighted fair queuing packet scheduling?
- generalized round robin
- each class has a weight and gets a weighted amount of service in each cycle: w_i/sum(w_j)
- minimum bandwidth guarantee (per traffic class)
What are path -selection algorithms?
-routing protocols
- SDN controller
IP protocols?
-datagram format
- addressing
- packet handling conventions
ICMP protocol
- error reporting
- router “signaling”
What is an IP address?
32 bit identifier associated with each host or router interface
What is an interface?
- connection between host/router and physical link
-routers typically have multiple interfaces - host typically has one or two interfaces
What is a subnet?
- device interfaces that can physically reach each other WITHOUT PASSING THOUGH AN INTERVENING ROUTER
What are the parts of an IP address?
subnet part: devices in same subnet have common high order bits
host part: the remaining low order bits
What is CIDR?
- subnet portion of address or arbitrary length
- a.b.c.d/x where c is the # bits in subnet portion of address.
