230914 ch1 p30-65

질문) 1 hop의 개념? A  source host로부터 switch까지는 1 hop이라고 하는가?

1. Pckt sw vs Circuit sw

bursty data를 전달할 때 simpler, no call setup : 웬만하면 멈추지 않고 그냥 보냄

그 결과 excessive congestion, 즉 output buffer에 몰릴 수 있음.

해결방법 : congestion contrl by TCP

 

 

2. Internet structure: multi-tier hierarchy

1. hosts : access ISP가 관리, global ISP(관념적, 불가능)
2. Two ways of connecting different networks (usually different ISPs): IXP and Peering(돈 안내는 관계)
3. What is Point of Presence (PoP)? A group of one or more routers (at the same location) in the provider network where customer ISPs can connect into the provider ISP (이화 customer ISP에서 인터넷 더 멀리 보내주는 KT ISP로 갈 수 있는데 이 때 KT ISP에 links to peering networks) 
4. What is the advantage of a multi-homed network? it can continue to send and receive packets into the Internet even if one of its providers has a failure (ex. 스타벅스 불나도 인터넷 계속 연결됨) content provider에 연결되어 있으면 그건 multi-homed 로 안쳐줌

 

• Content Provider Network (CPN) or Content Delivery/Distribution Network (CDN)

1. CPN and CDN are compatible, but CDN is more widely used. ex.넷플릭스
2. The goal of CDN is to reduce payment to upper-tier ISPs & reduce latency which content users feel by wisely locating caches (usually closer to users) edge computing (tier-1 commercial ISPs 는 돈안내도 됨, 최상위급)

 

3. Loss, Delay, Throughput

1. 1-hop delay (or nodal delay) : processing delay --> queueing delay -> transmission delay --> propagation delay
   • processing delay : 라우터 안에서 switching
   • d.proc : nodal processing @ L2, L3
   • d.trans : 라우터 안에서 bit->signal, L/R 이고 L=packet length ,R = link bandwidth
   • d.prop : time taken for 1 bit to travel from e2e of 1 link , d/s 이고 d=length of physical link, s=빛의 속도
2. e2e Delay = Number of hop X 4 kinds of delays per hop
3. Queueing delay in an output queue can be estimated by an index called "Traffic Intensity (TI)", 얼마나 찼는지 나타내주는 index
4.  TI = (arrival rate into output queue) / (departing rate from output queue) = 들어오는거/나가는거
5. arrival rate into output queue = L-bits x a packets/sec. (초당 a개의 L-bit 패킷이 큐에 진입)
6.  departing rate from output queue = outgoing link의 transmission rate R bps
7.  As a value of IT gets closer to 1, queueing delay rapidly increases.

 

• How to measure a real delay to a host (A).

1. Type "tracert (A's IP address)" on your Window system.
2. Type "traceroute (A's IP address)" on your Linux or Unix systems. router를 10번 거친 후 못 가게 막기 for 목적지 헤매는 pkt 죽이기
3. This command uses TTL(Time-To-Live) field in an IP header and ICMP (Internet Control Management Protocol) messages.

 

• Can you define “Throughput”? How is “Throughput” different from “transmission rate or link bandwidth” ?

1. Throughput: amount of bits delivered between two end hosts over a specific period of time; it is a real-time measurement. (수업시간에 external link에 부과되는 데이터량이 증가하면 external link가 bottleneck 링크가 될 수 있다고 한 것이 real-time measurement를 의미함. 일시적인 부하는 괜찮으나 지속적으로 external link가 bottleneck 링크가 되면 외부 링크 증설이 필요할 수 있음.)
2.  transmission rate: usually about link’s capacity (amount of bits transmitted, i.e. bit ---> signal per second; it is not a real-time measurement. (링크의 성능, 고정된 값. 링크사면 끝)
3.  Unit is bps (bits per second) for both.
4.  Throughput is determined by a bottleneck link (real-time measurement, 속도 제일 낮은 link; packet이 몰리거나 그래픽 카드를 꽂으면 달라질 수 있음) on an e2e path between two hosts. 
5. packet loss (p54): buffer가 유한해서 발생함. reserve restoration을 하지 않는 circuit sw과는 달리, packet sw은 기다리려고 buffer을 마련했는데 그 buffer에도 못 들어가면 loss가 발생
   • 그렇다면 책임자는? previous node(router, L2) or source end system (보낸애의 TCP L4) 

 

4. Layers, service models

networks는 각 계층별로 구성되어 따로따로 발전이 가능하다.

 

layers : each layer implements a service relying on services provided by layer below.

• Application Layer: (network application, message)
• Transport Layer: (process to Process communication, (TCP,UDP,SCTP,QUIC) segment, port number)
• Network Layer: (e2e data delivery(routing), datagram or packet, IP address)
• Link Layer: (1-hop data delivery, frame, MAC address) local한 개념
• Physical Layer: (bit transfer)       

※ 4계층 TCP는 5계층의 message를 break-down하므로 TCP segment라고 명명하나, UDP는 break-down을 하지 않고 그대로 IP 에 내려 보내므로 UDP segment 라는 말은 하지 않습니다. 오히려 3계층 PDU의 body에 있는 내용과 동일하다고 하여 UDP datagram 이라고 부릅니다.

P.S. 
Try questions in slides #47, #58. The answers will be posted later next week. ;)