[컴퓨터네트워크] 1. Introduction (3)

Performance: Loss, Delay, Throughput

How do loss and delay occur?

• Packets queue in router buffers --> Packets queue, wait for their turn.

 

Four sources of packet delay

Four sources of packet delay

▶ d-pro: nodal processing

• check bit errors at L2, L3
• determine output link

▶ d-queue: queueing delay

: time waiting at output link for transmission.

• depends on congestion level of router.

▶ d-trans: transmission delay = L/R

• L: packet length (bits)
• R: link bandwidth (bps)

▶ d-prop: propagation delay = d/s

• d: length of physical link
• s: propagation speed

 

Packet queueing delay (revisited)

▶ Traffic intensity = La/R

• L: packet length (bits)
• a: average packet arrival rate
• R: link bandwidth (bps) - 컴퓨터나 네트워크가 일정 시간 내에 보낼 수 있는 정보량

Traffic Intensity (TI)

--> queuing delay는 traffic intensity에 따라 달라진다.

• La/R = 0: queuing delay small
• La/R = 1: queuing delay large
• La/R > 1: queuing delay infinite --> packet loss

 

Traffic intensity(TI) at Output Queue

• La/R = Average rate at which bits arrive at the queue (bps)/the rate at which bits are pushed out of the queue (bps)

 

"Real" Internet delays and routes

• traceroute program
  : Provides delay measurements from source to router along end-end Internet path towards destination.
  

traceroute program

 

 

Packet loss

Packet loss

• Queue (aka buffer) preceding link in buffer has finate capacity.
  --> Packet arriving to full queue dropped (aka lost).
• Lost packet may be retransmitted by previous node(loss 복구), by source end system, or not at all.

 

 

Throughput

: Rate at which bits are being sent from sender to receiver.

Server --> Router --> Host

▶ Bottleneck link

: Link on end-end path that constrains end-end throughput.

Bottleneck link

 

Throughput: Internet scenario

Throughput: Internet scenario

• Mainly, affected by transmission rate.
• End-end throughput: min(Rc, Rs, R/10)
  --> In practice: Rc or Rs is often bottleneck.
  --> A link with high transmission rate may be the bottleneck link due to the intervening traffic.

 

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Protocol layers, service models

5-layer Internet protocol stack

5-layer Internet protocol stack

▶ Application: supporting network applications.

• FTP, SMTP, HTTP

▶ Transport: process-process data transfer.

• TCP, UDP

▶ Network: routing of datagrams from source to destination.

• IP protocol, Routing protocol

▶ Data link: data transfer between neighboring network elements.

• Ethernet, 802.11 WiFi, PPP

▶ Physical: bits "on the wire".

 

 

Encapsulation

Encapsulation

▷ Protocol data unit의 이름이 계층별로 다르다.

• Application: message
• Transport: segment
• Network: datagram
• Data link: frame
• Physical

▷ 어떤 계층 앞의 헤더는 앞선 source/switch/router/destionation의 동일 계층에서 보낸 것이다.