1.6 Delay( 时延 ) P29-31 （ in packet-switched networks ） packets experience delay on end-to-end path A B propagation transmission nodal processing queueing P29 nodal processing delay ( 节点处理时延 ) queueing delay ( 排队时延 ) Transmission delay ( 传输时延 ) Propagation delay ( 传播时延 )

nodal processing( 节点处理时延 ): –check bit errors –determine route A B propagation transmission nodal processing queueing queueing( 排队时延 ) –time waiting at output link for transmission –depends on congestion level of router Delay in packet-switched networks

Transmission delay( 传输时延 ): R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R Propagation delay ( 传播时延 ) : d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s A B propagation transmission nodal processing queueing Note: s and R are very different quantities!

end-to-end delay nodal processing delay queueing delay Transmission delay Propagation delay packet lost( 分组丢失 )

Internet delays and routes 1 cs-gw ( ) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms 4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms 5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms ( ) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms ( ) 126 ms 128 ms 126 ms 17 *fantasia.eurecom.fr ( ) 132 ms 128 ms traceroute: routers, rt delays on source-dest path also: pingplotter, various windows programs 请阅读 P33 tracert 命令的使用及含义

1.7 Protocol “ Layers ” ( 协议层次 ) P34-39 Networks are complex! many “ pieces ” : –hosts –routers –links of various media –applications –protocols –hardware, software Question: Is there any hope of organizing structure of network?

Organization of air travel a series of steps ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing P35

Organization of air travel : a different view Layers: each layer implements a service –via its own internal-layer actions –relying on services provided by layer below ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing

Distributed implementation of layer functionality ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing Departing airport arriving airport intermediate air traffic sites airplane routing

发信人 收信人 通信者活动 邮局转送业务 邮局服务业务 邮局转送业务 邮局服务业务 通信者活动 邮件运输业务 邮政通信系统

分层可以带来如下好处： （ 1 ）各层之间是独立的。 （ 2 ）灵活性好。 （ 3 ）结构上可分割开。 （ 4 ）易于实现和维护。 （ 5 ）能促进标准化工作。 P22

OSI( 开放系统互联 ) 体系结构的分层 原则：  网中各节点具有相同的层次  不同节点的对等层具有相同的功能，按协议通信  同一节点的相邻层间通过接口通信  每层使用下层服务，并为上层提供服务

网络体系结构 (network architecture) ： 计算机网络的各层及其协议的集合。也就是说， 计算机网络的体系结构就是这个计算机网络及其部 件所应完成的功能的精确定义。

Internet protocol stack application( 应用层 ): supporting network applications –ftp, smtp, http transport ( 传输层 ) : transfer application data between client and server. –tcp, udp network ( 网络层 ) : routing of datagrams from source to destination –ip, routing protocols link ( 链路层 ) : data transfer between neighboring network elements –ppp, ethernet physical ( 物理层 ) : transfer bits “ on the wire ” application transport network link physical P38

Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical Each layer: distributed “ entities( 实体 ) ” implement layer functions at each node entities perform actions, exchange messages with peers ( 对等层 )

Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data E.g.: transport take data from application add addressing, reliability check information to form “ datagram ” send datagram to peer( 对等层 ) wait for peer to ack receipt data transport ack

Layering: physical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data router receiver sender

Protocol layering and data Each layer takes data from above adds header( 头部 ) information to create new data unit passes new data unit to layer below application transport network link physical application transport network link physical source destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame

function of layer Each layer may have functions as folllows: error control( 差错控制 ) flow control( 流量控制 ) segmentation and reassembly( 分片和重组 ) multiplexing( 复用 ) connection setup( 建立连接 ) P37

HTTP 、 FTP 、 SMTP 、 TELNET 、 DNS 、 SNMP TCP UDP ICMP 、 IP 、 ARP 、 RARP various data link protocols TCP/IP Protocol Stack （协议栈） various physical protocols

1.8 Internet History P40( 自学 ) 1961: Kleinrock - queueing theory shows effectiveness of packet- switching 1964: Baran - packet- switching in military nets 1967: ARPAnet conceived by Advanced Research Projects Agency 1969: first ARPAnet node operational 1972: –ARPAnet demonstrated publicly –NCP (Network Control Protocol) first host- host protocol –first program –ARPAnet has 15 nodes : Early packet-switching principles

Internet History 1970: ALOHAnet satellite network in Hawaii 1973: Metcalfe ’ s PhD thesis proposes Ethernet 1974: Cerf and Kahn - architecture for interconnecting networks late70 ’ s: proprietary architectures: DECnet, SNA, XNA late 70 ’ s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes Cerf and Kahn ’ s internetworking principles: –minimalism, autonomy - no internal changes required to interconnect networks –best effort service model –stateless routers –decentralized control define today ’ s Internet architecture : Internetworking, new and proprietary nets

Internet History 1983: deployment of TCP/IP 1982: smtp protocol defined 1983: DNS defined for name-to-IP-address translation 1985: ftp protocol defined 1988: TCP congestion control new national networks: Csnet, BITnet, NSFnet, Minitel 100,000 hosts connected to confederation of networks : new protocols, a proliferation of networks

Internet History Early 1990 ’ s: ARPAnet decommissioned 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995) early 1990s: WWW –hypertext [Bush 1945, Nelson 1960 ’ s] –HTML, http: Berners-Lee –1994: Mosaic, later Netscape –late 1990 ’ s: commercialization of the WWW Late 1990 ’ s: est. 50 million computers on Internet est. 100 million+ users backbone links running at 1 Gbps 1990 ’ s: commercialization, the WWW

Introduction: Summary Covered a “ ton ” of material! Internet overview what ’ s a protocol? network edge, core, access network –packet-switching versus circuit-switching performance: loss, delay layering and service models backbones, NAPs, ISPs history You now have: context, overview, “ feel ” of networking more depth, detail later in course