Computer Network CH4 network layer data plane

Network layer: our goals

understand principles behind network layer services, focusing on data plane:

• network layer service models
• forwarding versus routing
• how a router works
• addressing
• generalized forwarding
• Internet architecture

instantiation, implementation in the Internet

• IP protocol
• NAT, middleboxes

question for CH3:TCP receiver

會延遲回復ACK

• 500ms，delay ack
  • 之後一次回覆連續的ACK，避免網路中都是ACK的訊息

如果有封包沒收到，後續收到的封包都會回傳那個沒收到封包的series number

• 對於傳送端而言就是duplicate
• 會進行重傳

network layer: overview

Network-layer services and protocols

sender:

• 封裝segments變成datagram

receiver:

• 丟給transport layer
• Ip datagram解封裝

internet的host每個都有network layer protocol

• 

routers:

• check header fields of each datagram
  • from input to output

functions

• forwarding
  • 知道甚麼來源的IP要丟到哪裡?
• routing
  • 決定從source 到 destination要走哪裡
• 

Network layer: data plane, control plane

based source IP and destination IP from datagram to decide which port to go

Data plane:

• local, per-router function
• determines how datagram arriving on router input port is forwarded to router output port
• 

Control plane

• network-wide logic
• determines how datagram is routed among routers along end-end path from source host to destination host

two control-plane approaches:

• 規劃整個datagram的路徑
• traditional routing algorithms: implemented in routers
  • 
• software-defined networking (SDN): implemented in (remote) servers
  • 

network service model

Ex.

• individual of datagram
  • guaranteed delivery
• for a flow of datagram
  • in-order datagram delivery
  • gurantee minimum bandwith to flow
  • …

Best effort網際網路在使用的

• No guarantees on:
  • successful datagram delivery to destination
  • timing or order of delivery
  • bandwidth available to end-end flow

其他不會考(看過就好)

reflection on best-effort service

for most of the time

• provisioning bandwith

What’s inside a router?

router architecture overview

each router has a routing processor

• 

input port function p4.16

input port queueing

• datagram進入速度大於forwarding rate
• 

forwarding method

Destination-based

最長前綴匹配(這個方法不重要)

• 盡量匹配最長的情況
• 

switching fabrics

via memory

• 1st generation
• traditional
• speed depends on memory bandwith
  • 2 bus crossing per datagram
• 

via bus

• speed limited by bus bandwith
  • 32Gps bus, Cisco 5600
• 

via interconnection network

• multistage switch
  • n*n switch from multiple stages of smaller switches
• speed up , via parallel
• 

input port queueing

If switch fabric slower than input ports combined -> queueing may occur at input queues

• queueing delay and loss due to input buffer overflow!

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

• 

output port queueing

How much buffering?

Buffer Management

packet scheduling FCFS

決定哪個封包接下來會被送向link

FCFS

• first come, first served
  

priority sheduling

• 封包有不同的優先權
  • 根據優先權做分類
    

如果已經處理，就繼續執行
但如果有封包同時進來

• 根據優先權排序

Round Robin scheduling

• 會輪流的傳遞不同優先度的封包
• weighted fair queueing
  • 每個cycle分配給不同優先權封包的時間，基於各個優先權的權重
  • minimum bandwith guarantee
    

sidebar: network neutrality(看過就好 偏無聊)

網路中立性

• 只要合法，ISP業者就應該中立
• 不能歧視
  

不考這個小節的名詞解釋

ISP?(不重要)

ISP業者究竟是提供甚麼東西?

IP:the internet protocol

network layer

Internet

• host, router network layer functions:
• 
• path-selection algo.
  • decide how packet go
  • put to forwarding table

IP datagram format

IPV4

• IP address
  • 32 bits
    

packet length最大長度是1500 bytes

• 因為使用的是etherNet
  • 這種網路中一個最大的傳送單位就是1500 bytes

overhead

• IP有五列總共5*4 = 20 bytes

IP addressing: introduction

子網路的高bit會一樣

• 223.1.1.1
• 223.1.1.2
  

每一個host可以有多個interface

• connection between host/router and physical link
• 前提是有多張網卡

subnet

中間沒有其他中介的路由器

• 有相同的high level class

IP address structure

• subnet part
  • same high order bit
• host part
  • low order bit
    

Recipe for defining subnets:

• detach each interface from its host or router, creating “islands” of isolated networks
• each isolated network is called a subnet

Ex.

• 223.1.3.0/24
  • 24代表前24個bits 是subnet part
    

CIDR

how to get one ip

host如何取得

DHCP

• 會服務這個subnet底下連接的裝置
• 分配IP給這些裝置
  
• 可以知道發IP的router在哪裡

DHCP EX.(沒甚麼好講的)

電信業者跟學校就是跟更上層的人要IP

• ICANN

network address translation IPv6

NAT: network address translation

• 
• 
• 可以不用告知外界便調整內網中的裝置IP

source ip 要轉換成 NAT ip

• 路由器要透過轉譯表(NAT ip + port)
  • 轉換內網中正確的ip
    

NAT has been controversial:

• routers “should” only process up to layer 3
• address “shortage” should be solved by IPv6
• violates end-to-end argument (port # manipulation by network-layer device)

transparent的問題?

• NAT traversal
  • 外網的host沒辦法找到內網的host

IPv6: motivation

40 bytes fixed length header

• address become 128 bits
• flow label
  

transition

• v4 and v6 可以同時存在
• router可能不支援v6
  • 可以把v6塞進v4裡面 = tunneling
    
    

IPv6

• 發展太久了

Generalized Forwarding, SDN

match plus action

• traditional
  • basic forwarding table(aka flow table)
  • destination based
• generalized forwarding
  • any header field can determine action
  • 可以讀header中很多的資料
  • 有多種action
• 

Flow table abstraction

總而言之，可以看很多欄位的資料

• 執行很多不同的動作
  • 
  • 
• link, network and transport layer header could be used

可以藉此實現firewall(會考喔)

• 
• 

要使用上述的功能，要滿足下面的條件

• openflow
• generalized forwarding
• 硬體要支援
• 可以有以下的功能
  

Ex.(看過就好)

summary

• 
• 

middleboxes(沒什麼重點)(不重要)