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Chapter 4 Network Layer A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007. Thanks and enjoy! JFK/KWR All material copyright 1996-2007 J.F Kurose and K.W. Ross, All Rights Reserved Network Layer 4-1 Chapter 4: Network Layer Chapter goals: understand principles behind network layer services: IP addressing (寻址) forwarding versus routing (转发 vs 路由) routing (path selection) advanced topics: IPv6, mobility instantiation, implementation in the Internet Network Layer 4-2 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-3 Vocabulary and Term Virtual Circuit (VC) /Datagram 虚电路/数据报 Router/Switch/Hub/Repeater 路由器/交换机/集线器/中继器 Input/Output Port 输入/输出 端口 Switch Fabric 、Crossbar 交换结构、交叉开关 Forwarding/Routing 转发/路由 FIB (Forwarding Information Base)/Route Table 转发表/路由表 Network Layer 4-4 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-5 Protocol:comm rules Host 2 Host 1 Segment App. Layer 4 L4 34 L3 Assemble App. Layer L4 Protocol L4 4 L3 34 L2 2 34 Data Data App. Protocol Header 2 34 1 2 34 L2 L1 L3 Protocol L2 Protocol Interface L1 Protocol Media L1 1 2 34 Network Layer 4-6 Network layer transport segment from sending to receiving host on sending side encapsulates segments into datagrams on receiving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP datagrams passing through it application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network network data link data link physical physical network data link physical network data link physical network data link physical network data link physical Network Layer application transport network data link physical 4-7 Journey of a Packet Host Host 5 Router 4 4 3 L3 3 3 Switch 2 3 Hub 2 1 5 L2 L1 2 L2 1 2 3 L1 2 3 1 2 3 1 L1 Network Layer 4-8 Two Key Network-Layer Functions forwarding: move packets from router’s input to appropriate router output routing: determine route taken by packets from source to destination analogy: routing: process of planning trip from source to destination forwarding: process of getting through single interchange routing algorithms Network Layer 4-9 Router Functions Dest Packet Next-hop Cal. Dest Port/NH ------- ------- ---- ---- Select Best Route Forwarding (Interface ij) Control(Congest, Security) Next-Hop Network Layer 4-10 Router Startup Power-On Self Test (POST) Boot ROM Loading OS IOS in Flash Loading Configure Information Config file in NVRAM Run applications Network Layer 4-11 Interplay between routing and forwarding routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 0111 1 3 2 Network Layer 4-12 Connection setup 3rd important function in some network architectures: ATM, frame relay (帧中继), X.25 before datagrams flow, two end hosts and intervening routers establish virtual connection routers get involved network vs. transport layer connection service: network: between two hosts (may also involve inervening routers in case of Virtual Circuits) transport: between two processes Network Layer 4-13 Network service model Q: What network service model for “channel” transporting datagrams from sender to receiver? Example services 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 Network Layer 4-14 Network layer service models: Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed rate guaranteed minimum none no no no yes yes yes yes yes yes no yes no no (inferred via loss) no congestion no congestion yes no yes no no Network Layer 4-15 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-16 Network layer connection and connection-less service datagram network provides network-layer connectionless service VC network provides network-layer connection service analogous to the transport-layer services, but: service: host-to-host no choice: network provides one or the other implementation: in network core Network Layer 4-17 Virtual circuits “source-to-destination path behaves much like telephone circuit” performance-wise network actions along source-to-dest path call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host address) every router on source-destination path maintains “state” for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) Network Layer 4-18 Recall Circuit Switching dedicated resources: no sharing guaranteed performance Network Layer 4-19 VC implementation a VC consists of: 1. 2. 3. path from source to destination VC numbers, one number for each link along path entries in forwarding tables in routers along path packet belonging to VC carries VC number (rather than destination address) VC number can be changed on each link. New VC number comes from forwarding table Network Layer 4-20 Forwarding table VC number 22 12 1 Forwarding table in northwest router: Incoming interface 1 2 3 1 … 2 32 3 interface number Incoming VC # 12 63 7 97 … Outgoing interface 3 1 2 3 … Outgoing VC # 22 18 17 87 … Routers maintain connection state information! Network Layer 4-21 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet application transport 5. Data flow begins network 4. Call connected data link 1. Initiate call physical 6. Receive data application 3. Accept call 2. incoming call transport network data link physical Network Layer 4-22 Datagram networks no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets forwarded using destination host address packets between same source-dest pair may take different paths application transport network data link 1. Send data physical application transport network 2. Receive data data link physical Network Layer 4-23 Summary Circuit Exchange Exchange Technology Message Switching Store & Forwarding Virtual Circuit Packet Switching Datagram Network Layer 4-24 Forwarding table Destination Address Range 4 billion possible entries ? Link Interface 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 0 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 1 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 2 otherwise 3 Network Layer 4-25 Longest prefix matching Prefix Match 11001000 00010111 00010 11001000 00010111 00011000 11001000 00010111 00011 otherwise Link Interface 0 1 2 3 Examples DA: 11001000 00010111 00010110 10100001 Which interface? DA: 11001000 00010111 00011000 10101010 Which interface? Network Layer 4-26 Datagram or VC network: why? Internet (datagram) data exchange among ATM (VC) evolved from telephony computers human conversation: “elastic” service, no strict strict timing, reliability timing req. requirements “smart” end systems need for guaranteed (computers) service can adapt, perform “dumb” end systems control, error recovery telephones simple inside network, complexity inside complexity at “edge” network many link types different characteristics uniform service difficult Network Layer 4-27 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-28 CISCO Routers Cisco12000 Cisco7500 Cisco7200 As5800 As5200/5300 Cisco4000 Cisco3600 Cisco2600 Cisco2500 Cisco1600 Network Layer 4-29 Routers: Low/Middle Performance Network Layer 4-30 Router: High-Performance CISCO 12000 Lucent NX64000 Juniper T640 Network Layer 4-31 Router Inside Cisco 2600 Front end Back end Network Layer 4-32 COM Port Network Layer 4-33 Router Organization Network Layer 4-34 Network Layer 4-35 Router Architecture Overview Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link Network Layer 4-36 Input Port Functions Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5 Decentralized switching: given datagram destination, lookup output port using forwarding table in input port memory goal: complete input port processing at ‘line speed’ queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Layer 4-37 Three types of switching fabrics Network Layer 4-38 Switching Via Memory First generation routers: traditional computers with switching under direct control of CPU packet copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram) Input Port Memory Output Port System Bus Network Layer 4-39 Switching Via a Bus datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers Network Layer 4-40 Switching Via An Interconnection Network overcome bus bandwidth limitations Banyan networks, other interconnection nets initially developed to connect processors in multiprocessor advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 12000: switches 60 Gbps through the interconnection network Network Layer 4-41 Output Ports Buffering required when datagrams arrive from fabric faster than the transmission rate Scheduling discipline chooses among queued datagrams for transmission Network Layer 4-42 Output port queueing buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow! Network Layer 4-43 How much buffering? RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C e.g., C = 10 Gbps link: 2.5 Gb buffer Recent recommendation: with N flows, buffering equal to RTT. C N Network Layer 4-44 Input Port Queuing Fabric slower than input ports combined -> queueing may occur at input queues Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward queueing delay and loss due to input buffer overflow! Network Layer 4-45 Fourth Generation Routers Optical links 100s of metres Switch Core Linecards 160Gb/s - 20Tb/s routers in development Network Layer 4-46 Homework Reviews:2,3,7,10 Network Layer 4-47 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-48 Network Layer 4-49 Network Layer 4-50 Network Layer 4-51 Packet Ethernet Header IP TCP Header Header HTTP Header e.g., HTTP Body Network Layer 4-52 The Internet Network Layer Host, router network layer functions: Transport layer: TCP, UDP Network layer IP protocol •addressing conventions •datagram format •packet handling conventions Routing protocols •path selection •RIP, OSPF, BGP forwarding table ICMP protocol •error reporting •router “signaling” Link layer physical layer Network Layer 4-53 IP datagram format IP protocol version number header length (bytes) “type” of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead 32 bits head. type of length ver len service fragment 16-bit identifier flgs offset upper time to header layer live checksum total datagram length (bytes) for fragmentation/ reassembly 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) E.g. timestamp, record route taken, specify list of routers to visit. Network Layer 4-54 IP Format (Chinese Version) Network Layer 4-55 TTL head. type of length ver len service fragment flgs 16-bit identifier offset time to upper Header layer live checksum 32 bit source IP address 32 bit destination IP address Options (if any) Time to live max number remaining hops decremented at each router TTL=0: discard (only in LAN when initial TTL=1) Too small to reach the destination can be used to count hops to destination Network Layer 4-56 Internet Protocol Numbers :2008-02-27 www.iana.org/assignments/protocol-numbers 0-133:SAME 134 RSVP-E2E-IGNORE [RFC3175] 135 Mobility Header [RFC3775] 136 UDPLite [RFC3828] 137 MPLS-in-IP [RFC4023] 138 manet MANET Protocols [RFC-ietf-manet-iana-07.txt] 139 HIP Host Identity Protocol [RFC-ietf-hip-base-10.txt] 140-252 Unassigned [IANA] 253 Use for experimentation and testing [RFC3692] 254 Use for experimentation and testing [RFC3692] 255 Reserved [IANA] Network Layer 4-57 IP Fragmentation & Re-assemble fragmentation: in: one large datagram out: 3 smaller datagrams network links have MTU (max.transfer size) - largest possible link-level frame. different link types, different MTUs large IP datagram divided (“fragmented”) within net one datagram becomes several datagrams reassembly “re-assembled” only at final destination IP header bits used to identify, order related fragments Network Layer 4-58 IP Fragmentation & Re-assemble length ID fragflag offset =4000 =x =0 =0 Example: Ethernet 4000 byte datagram MTU = 1500 bytes One large datagram becomes several smaller datagrams Flag = 1 there is more fragment Flag = 0 this is the last fragment Offset: byte number of the 1st byte of the fragment (specified in units of 8-byte chunks) length ID fragflag offset =1500 =x =1 =0 length ID fragflag offset =1500 =x =1 =185 length ID fragflag offset =1040 =x =0 =370 1480 bytes in data field offset = 1480/8 Network Layer 4-59 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-60 Address Allocation IANA:Internet Assigned Numbers Authority(互联网号码分配机构) Domain name, IP address, protocol parameters, root server management ICCAN:Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes ASO (Address Supporting Organization) Advisory on policy and structure Network Layer 4-61 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes Network Layer 4-62 IP Addressing IP Address: Classful Addressing (有类寻址) A,B,C,D,E --5 Classes Network Mask (网络掩码) Classless Addressing (无类寻址) Subnet (子网) & Supernet (超网) NAT (Network Address Translation) Network Layer 4-63 IP Address Classes 8 bits 8 bits 8 bits 8 bits Host Host Host Host Host • Class A: Network • Class B: Network Network • Class C: Network Network Network • Class D: Multicast • Class E: Research purpose Host Network Layer 4-64 Classful Addressing Network Layer 4-65 IP in China 万个 16,000 14,000 13,527 中国IPv4地址 11,825 12,000 9,802 10,000 8,000 6,000 4,146 4,942 5,995 6,830 7,439 8,479 4,000 2,000 0 2003.12 2004.06 2004.12 2005.06 2005.12 2006.06 2006.12 2007.06 2007.12 Network Layer 4-66 IP in China (2005) Telcom:34.70% Netcom:21.40% CERNET:15.08% CNNIC:14.47% Unicom:3.07% Mobile:2.84% 其他:8.43% Network Layer 4-67 China IPv4 Allocation List (2007.1) Network Layer 4-68 IP Address: 点分十进制表示法 主机的 IP 地址 是 32 位二进制代码 每隔 8 位插入一个空格 以提高可读性 将每 8位的二进制数 转换为十进制数表示 readable! 10000000000010110000001100011111 10000000 00001011 00000011 00011111 128 11 3 31 128.11.3.31 Network Layer 4-69 Number of Networks and Hosts Class Network Number of Networks Hosts for each network A 1.0.0.0 126.0.0.0 27 –2 (125) 224 -2 (16 777 216) B 128.1.0.0 191.254.0.0 214 -2 (16384) 216 ( 64K )-2 (65 534) C 192.0.1.0 223.255.254.0 221 –2 (>2million) 28-2 (254) Network Layer 4-70 How to get Network ID:by Network Mask Class Bits for Network ID Bits for host ID Network Mask A 8 24 255.0.0.0 B 16 16 255.255.0.0 C 24 8 255.255.255.0 IP Address “AND” mask=> Network ID Network Layer 4-71 Example: Example1: First 8 bits are network ID Address 8.1.4.5 0000 1000 0000 0001 0000 0100 0000 0101 Mask 255.0.0.0 1111 1111 0000 0000 0000 0000 0000 0000 Result 8.0.0.0 0000 1000 0000 0000 0000 0000 0000 0000 Example2: First 16 bits are network ID Address 130.4.100.1 1000 0010 0000 0100 0110 0100 0000 0101 Mask 255.255.0.0 1111 1111 1111 1111 0000 0000 0000 0000 Result 130.4.0.0 1000 0010 0000 0100 0000 0000 0000 0000 Network Layer 4-72 TCP/IP Configuration Network Layer 4-73 IP Address Classes Exercise Address Class Network Host 10.2.1.1 128.63.2.100 201.222.5.64 192.6.141.2 130.113.64.16 256.241.201.10 Network Layer 4-74 IP Address Classes Exercise Answers Address Class 10.2.1.1 A 10.0.0.0 0.2.1.1 128.63.2.100 B 128.63.0.0 0.0.2.100 201.222.5.64 C 201.222.5.0 0.0.0.64 192.6.141.2 C 192.6.141.0 0.0.0.2 130.113.64.16 B 130.113.0.0 0.0.64.16 256.241.201.10 Network Host Nonexistent Network Layer 4-75 Special IP Address Network Layer 4-76 Host Addresses 172.16.2.2 10.1.1.1 10.6.24.2 E1 172.16.3.10 E0 172.16.2.1 10.250.8.11 172.16.12.12 172.16 Network . 12 . 12 Host 10.180.30.118 Routing Table Network Interface 172.16.0.0 E0 10.0.0.0 E1 Network Layer 4-77 IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router’s typically have multiple interfaces host typically has one interface IP addresses associated with each interface 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 223.1.3.2 223.1.3.1 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Network Layer 4-78 Why Subnetting? Computer School 172.26.0.0 255.255.0.0: 172.26.0.0/16 Labs 601:172.26.1.0-172.26.1.255 602:172.26.2.0-172.26.2.255 601: 172.26.1.0 255.255.255.0: 172.26.1.0/24 602: 172.26.2.0 255.255.255.0: 172.26.2.0/24 Network Layer 4-79 Subnets 223.1.1.1 IP address: subnet part (high order bits) host part (low order bits) What’s a subnet ? device interfaces with same subnet part of IP address 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 subnet 223.1.3.1 223.1.3.2 can physically reach each other without intervening router network consisting of 3 subnets Network Layer 4-80 Subnets 223.1.1.2 How many? 223.1.1.1 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.2.1 223.1.3.27 223.1.2.2 223.1.3.1 223.1.3.2 Network Layer 4-81 Smaller Subnets 172.26.1.0 : Lab 601 IP Address How to divide into 4 Sub-Groups? (11000000 00011010 00000001 00000000) 172.26.1.0-172.26.1.63 172.26.1.64-172.26.1.127 172.26.1.128-172.26.1.191 172.26.1.192-172.26.1.255 (11000000 00011010 00000001 11111111) Network Layer 4-82 What is the network number and mask for each group? each subnet has 64 (256/4) hosts number of hosts is 64 (26) network mask has 32-6=26 bits, originally network mask has 24 bits, i.e., 172.26.1.0/24 Now, we have the sub network mask 172.26.1.0/26 172.26.1.64/26 172.26.1.128/26 172.26.1.192/26 1111 1111 1111 1111 1111 1111 1100 0000 255. 255. 255. ? Network Layer 4-83 IP addressing: CIDR CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part host part 11001000 00010111 00010000 00000000 200.23.16.0/23 Network Layer 4-84 How to get IP Address for a Host? Question? hard-coded by system admin in a file Wintel: control-panel->network->configuration->tcp/ip>properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play” Network Layer 4-85 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an “on” Support for mobile users who want to join network (more shortly) DHCP overview: host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg Network Layer 4-86 DHCP client-server scenario A B 223.1.2.1 DHCP server 223.1.1.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.2.2 223.1.1.3 223.1.3.1 223.1.3.27 223.1.3.2 E arriving DHCP client needs address in this network Network Layer 4-87 DHCP client-server scenario DHCP server: 223.1.2.5 DHCP discover arriving client src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 Lifetime: 3600 secs DHCP request time src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs Network Layer 4-88 How to get IP Address for a Host? Q: How does network get subnet part of IP address? A: gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 Organization 1 Organization 2 ... 11001000 00010111 00010000 00000000 11001000 00010111 00010010 00000000 11001000 00010111 00010100 00000000 ….. …. 200.23.16.0/23 200.23.18.0/23 200.23.20.0/23 …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 Network Layer 4-89 Network Configuration 172.26.1.1 172.26.1.17 172.26.1.0/24 172.26.2.1 172.26.1.99 172.26.2.36 172.26.2.99 172.26.2.0/24 Network Layer 4-90 Number of Networks and Hosts Class Network Number of Networks Hosts for each network A 1.0.0.0 126.0.0.0 27 –2 (125) 224 -2 (16 777 216) B 128.1.0.0 191.254.0.0 214 -2 (16384) 216 ( 64K )-2 (65 534) C 192.0.1.0 223.255.254.0 221 –2 (>2million) 28-2 (254) Network Layer 4-91 Special IP Address Network Layer 4-92 用网络掩码划分子网号 从IP地址的主机号中“借用”若干位作为子网编号,主 机号相应缩短,并通过网络掩码来识别。 子网-id 因特网部分 两级 IP 地址 网络号 net-id 因特网部分 三级 IP 地址 本地部分 主机号 host-id 本地部分 host-id net-id 主机号 网络号 子网掩码 1111111111111111 11000000 00000000 (IP 地址) AND (子网掩码) = 网络号(Net-id) Network Layer 4-93 用网络掩码划分子网号 示例: 对于B类IP地址 168.16.0.0,把B类IP子网掩码从 255.255.0.0/16 改为: 255.255.192.0/18 由于将2位主机号作为子网ID, 可以组成 4 个子网号:00 ~ 11,再加上原来的 Net ID: 10101000.00010000.00000000.00000000,即168.16.0.0 各子网的实际 Net ID 最终就成了: 10101000.00010000.00000000.00000000 (168.16.0.0) 10101000.00010000.01000000.00000000 (168.16.64.0) 10101000.00010000.10000000.00000000 (168.16.128.0) 10101000.00010000.11000000.00000000 (168.16.192.0) Network Layer 4-94 用网络掩码划分子网号 示例: 对于B类IP地址 168.16.0.0,若子网数要求210个 需利用8位主机号作为子网ID,可提供254个子网 8位子网号 子网掩码: 255.255.255.0 10101000.00010000.XXXXXXXX.00000000 若子网数要求900个, 需将10位主机号作为子网ID,可提供1022 个子网。 子网掩码: 255.255.255.192 11111111 11111111 1 1 1 1 1 1 1 1 11000000 10位子网号 10101000.00010000.XXXXXXXX.XX000000 Network Layer 4-95 NAT: Network Address Translation rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.4 10.0.0.1 10.0.0.2 138.76.29.7 10.0.0.3 All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) Network Layer 4-96 NAT: Network Address Translation Motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just one IP address for all devices can change addresses of devices in local network without notifying outside world can change ISP without changing addresses of devices in local network devices inside local net not explicitly addressable, visible by outside world (a security plus). Network Layer 4-97 NAT: Network Address Translation Implementation: NAT router must: outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #) . . . remote clients/servers will respond using (NAT IP address, new port #) as destination address remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair incoming datagrams: replace (NAT IP address, new port #) in destination fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table Network Layer 4-98 NAT: Network Address Translation 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table 2 NAT translation table WAN side addr. LAN side addr. 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 138.76.29.7, 5001 10.0.0.1, 3345 …… …… S: 10.0.0.1, 3345 D: 128.119.40.186, 80 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 138.76.29.7 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3: Reply arrives dest. address: 138.76.29.7, 5001 3 1 10.0.0.4 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 10.0.0.1 10.0.0.2 4 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 Network Layer 4-99 NAT: Network Address Translation 16-bit port-number field: Consider 60,000 simultaneous connections with a single LAN-side address! NAT is controversial: routers should only process up to layer 3 violates end-to-end argument • NAT possibility must be taken into account by app designers, e.g., P2P applications address IPv6 shortage should instead be solved by Network Layer 4-100 NAT problem and its Solution client want to connect to server with address 10.0.0.1 server address 10.0.0.1 local Client to LAN (client can’t use it as destination address) only one externally visible NATted address: 138.76.29.7 solution 1: statically configure NAT to forward incoming connection requests at given port to server 10.0.0.1 ? 138.76.29.7 10.0.0.4 NAT router e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0.1 port 25000 Network Layer 4-101 NAT problem and its Solution solution 2: NAT Traversal Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to: learn public IP address (138.76.29.7) enumerate existing port mappings add/remove port mappings (with lease times) 10.0.0.1 IGD 10.0.0.4 138.76.29.7 NAT router i.e., automate static NAT port map configuration Network Layer 4-102 NAT problem and its Solution solution 3: relaying (used in Skype) NATed server establishes connection to relay External client connects to relay relay bridges packets between to connections 2. connection to relay initiated by client Client 3. relaying established 1. connection to relay initiated by NATted host 138.76.29.7 10.0.0.1 NAT router Network Layer 4-103 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-104 ICMP: Internet Control Message Protocol used by hosts & routers to communicate network- level information error reporting: unreachable host, network, port, protocol echo request/reply (ping) network-layer “above” IP: ICMP messages carried in IP datagrams ICMP message: type, code + first 8 bytes of IP datagram causing error Network Layer 4-105 ICMP Internet Control Message Protocol(RFC792) ICMP Header 8-bit type 8-bit code 16-bit checksum 4Byte(variable content ) ICMP data( variable ) ICMP Message Network Layer 4-106 ICMP Format 8 bits ICMP Datagram IP Frame 帧头 8bits Type ICMP 头部 Code 16bits 16bits 16bits 32bits Checksum ICMP field1数据field2 IP 头部 field3 IP 数据区 帧数据区 Network Layer 4-107 ICMP Function Error Report (差错报告) Destination Unreachable: Timeout (TTL =0 ) : Parameter Problem : 3 11 12 Data Control (数据控制) Data Quench(信源抑制) : Redirect (重定向) : 5 4 (D-R1-R2-S,S-R2S-R1) Query(request/response) Echo (回送) request/response : Timestamp (时戳) request/response: Router (路由器) request/announcement:15/16 Netmask (掩码) request/reply: 8/0 13/14 17/18 Network Layer 4-108 ICMP: Internet Control Message Protocol Type Code description 0 3 3 3 3 3 3 4 8 9 10 11 12 0 0 1 2 3 6 7 0 0 0 0 0 0 echo reply (ping) destination network unreachable destination host unreachable destination protocol unreachable destination port unreachable destination network unknown destination host unknown source quench (congestion control - not used) echo request (ping) route advertisement router discovery TTL expired bad IP header Network Layer 4-109 Traceroute and ICMP Source sends series of UDP segments to destination First has TTL =1 Second has TTL=2, etc. Unlikely port number When nth datagram arrives to nth router: Router discards datagram And sends to source an ICMP message (type 11, code 0) Message includes name of router& IP address Network Layer 4-110 Traceroute and ICMP When ICMP message arrives, source calculates RTT Traceroute does this 3 times Stopping criterion: UDP segment eventually arrives at destination host Destination returns ICMP “host unreachable” packet (type 3, code 3) When source gets this ICMP, stops. Network Layer 4-111 ICMP:request/reply Yes. The path is nice! Host B is OK? B A type=8 Echo Request Echo Reply type=0 Ping & Traceroute Network Layer 4-112 Tracert Network Layer 4-113 Network Layer 4-114 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-115 IPv6 Initial motivation: IPv4 32-bit address space soon to be completely allocated. Additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 BYTE header no fragmentation allowed Network Layer 4-116 IPv6 Header (Cont.) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). Next header: identify upper layer protocol for data Network Layer 4-117 IPv6 Address Examples 2007:1022:0000::0001:0000:0000:0000:1234 2007:1022: 1::1234 ::00EC:A0BF:FFFF How to map IPv4 address to IPv6 address? IPV4: 202.197.10.1 IPV6: ? Network Layer 4-118 Header: IPv4 vs. IPv6 Network Layer 4-119 Other Changes from IPv4 Checksum: removed entirely to reduce processing time at each hop Options: allowed, but outside of header, indicated by “Next Header” field ICMPv6: new version of ICMP additional message types, e.g. “Packet Too Big” multicast group management functions Network Layer 4-120 Transition From IPv4 to IPv6 Not all routers can be upgraded simultaneous no “flag days” How will the network operate with mixed IPv4 and IPv6 routers? Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers Network Layer 4-121 Tunneling Logical view: Physical view: E F IPv6 IPv6 IPv6 A B E F IPv6 IPv6 IPv6 IPv6 A B IPv6 tunnel IPv4 IPv4 Network Layer 4-122 Tunneling Logical view: Physical view: tunnel A B IPv6 IPv6 A B C IPv6 IPv6 IPv4 Flow: X Src: A Dest: F data A-to-B: IPv6 E F IPv6 IPv6 D E F IPv4 IPv6 IPv6 Src:B Dest: E Src:B Dest: E Flow: X Src: A Dest: F Flow: X Src: A Dest: F data data B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 Flow: X Src: A Dest: F data E-to-F: IPv6 Network Layer 4-123 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-124 Interplay between routing, forwarding routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 0111 1 3 2 Network Layer 4-125 Graph abstraction 5 2 U 2 1 Graph: G = (N,E) v x 3 w 3 1 5 Z 1 y 2 N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Remark: Graph abstraction is useful in other network contexts Example: P2P, where N is set of peers and E is set of TCP connections Network Layer 4-126 Graph abstraction: costs 5 2 U v 2 1 x • c(x, y) = cost of link (x, y) 3 w 3 1 5 1 y 2 - e.g., c(w,z) = 5 z • cost could always be 1, or inversely related to bandwidth, or inversely related to congestion Cost of path (x1, x2, x3,…, xp) = c(x1, x2) + c(x2, x3) + … + c(xp-1, xp) Question: What’s the least-cost path between u and z ? Routing algorithm: algorithm that finds the least-cost path Network Layer 4-127 Routing Algorithm classification Global: all routers have complete topology, link cost info “link state” algorithms Decentralized: router knows physically-connected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors “distance vector” algorithms Network Layer 4-128 Routing Algorithm classification Static: routes change slowly over time Dynamic: routes change more quickly periodic update in response to link cost changes Network Layer 4-129 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-130 A Link-State Routing Algorithm Dijkstra’s algorithm net topology, link costs known to all nodes accomplished via “link state broadcast” all nodes have same info. computes least cost paths from one node (‘source”) to all other nodes gives forwarding table for that node iterative: after k iterations, know least cost path to k destination’s Notation: c(x,y): link cost from node x to y; = ∞ if not direct neighbors D(v): current value of cost of path from source to dest. v p(v): predecessor node along path from source to v N': set of nodes whose least cost path definitively known Network Layer 4-131 Dijsktra’s Algorithm 1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N' Network Layer 4-132 Dijkstra’s algorithm: example Step 0 1 2 3 4 5 N' u ux uxy uxyv uxyvw uxyvwz D(v),p(v) D(w),p(w) 2,u 5,u 2,u 4,x 2,u 3,y 3,y D(x),p(x) 1,u D(y),p(y) ∞ 2,x D(z),p(z) ∞ ∞ 4,y 4,y 4,y 5 2 U 3 v 2 1 x w 3 1 5 Z 1 y 2 Network Layer 4-133 Step 0 1 2 3 4 5 N' u ux uxy uxyv uxyvw uxyvwz D(v),p(v) D(w),p(w) 2,u 5,u 2,u 4,x 2,u 3,y 3,y D(x),p(x) 1,u D(y),p(y) ∞ 2,x D(z),p(z) ∞ ∞ 4,y 4,y 4,y Step 0 D(z) = ∞ Step 1 D(z) = ∞ Step 2 D(z) = min( D(z), D(y) + c(y, z) )= min(∞, 2 + c(y, z) )= 4 Step 3 D(z) = min( D(z), D(v) + c(v, z) )= min(4, 2 + c(v, z) )= 4 Step 4 D(z) = min( D(z), D(w) + c(w, z) )= min(4, 4 + 5 )= 4 5 2 U v 2 1 x 3 w 3 1 5 1 y 2 Z Network Layer 4-134 Dijkstra’s algorithm: example (2) Resulting shortest-path tree from U: v w u z x y Resulting forwarding table in u: destination link v x (u,v) (u,x) y (u,x) w (u,x) z (u,x) Network Layer 4-135 Dijkstra’s algorithm, discussion Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amount of carried traffic D 1 1 0 A 0 0 C e 1+e B e initially 2+e D 0 1 A 1+e 1 C 0 B 0 … recompute routing 0 D 1 A 0 0 2+e B C 1+e … recompute 2+e D 0 A 1+e 1 C 0 B e … recompute Network Layer 4-136 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-137 Distance Vector Algorithm Bellman-Ford Equation (dynamic programming) Define dx(y) := cost of least-cost path from x to y Then, we have dx(y) = min {c(x,v) + dv(y) } v where min is taken over all neighbors v of x Network Layer 4-138 Bellman-Ford example Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3 5 2 U B-F equation says: 3 v 2 1 x w Z 1 3 1 5 y 2 du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 Node that achieves minimum is next hop in shortest path ➜ forwarding table Network Layer 4-139 From V W X u 2 5 1 c(u,x) + dx(z), v 0 3 2 c(u,w) + dw(z) } w 3 0 3 x 2 3 0 y 3 1 1 z 5 5 3 U’s route information from his neighbors (dynamically): v, w, x du(z) = min { c(u,v) + dv(z), U的转发表 距离 下一跳 u 0 - v 2 v w x y 5 1 2 w,v x x z 4 x Network Layer 4-140 Distance vector algorithm Basic idea: Each node periodically sends its own distance vector estimate to neighbors When a node x receives new DV estimate from neighbor, it updates its own DV using B-F equation: Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N Under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y) Network Layer 4-141 Distance Vector Algorithm (5) Iterative, asynchronous: each local iteration caused by: local link cost change DV update message from neighbor Distributed: each node notifies neighbors only when its DV changes neighbors then notify their neighbors if necessary Each node: wait for (change in local link cost or msg from neighbor) recompute estimates if DV to any destination has changed, notify neighbors Network Layer 4-142 Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 node x table cost to x y z cost to x y z from from x 0 2 7 y ∞∞ ∞ z ∞∞ ∞ node y table cost to x y z Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 x 0 2 3 y 2 0 1 z 7 1 0 x ∞ ∞ ∞ y 2 0 1 z ∞∞ ∞ node z table cost to x y z from from x x ∞∞ ∞ y ∞∞ ∞ z 71 0 time 2 y 7 1 z Network Layer 4-143 Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 node x table cost to x y z x ∞∞ ∞ y ∞∞ ∞ z 7 1 0 from from from from x 0 2 7 y 2 0 1 z 7 1 0 cost to x y z x 0 2 7 y 2 0 1 z 3 1 0 x 0 2 3 y 2 0 1 z 3 1 0 cost to x y z x 0 2 3 y 2 0 1 z 3 1 0 x 2 y 7 1 z cost to x y z from from from x ∞ ∞ ∞ y 2 0 1 z ∞∞ ∞ node z table cost to x y z x 0 2 3 y 2 0 1 z 7 1 0 cost to x y z cost to x y z from from x 0 2 7 y ∞∞ ∞ z ∞∞ ∞ node y table cost to x y z cost to x y z Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 x 0 2 3 y 2 0 1 z 3 1 0 time Network Layer 4-144 V-D路由算法 算法原理: 让每个路由器动态计算并维护一张距离向量表,表中为 从某节点能到达其他节点的最佳路径代价和输出接口; 具体地: (1)每个路由器维持到各个已知目的节点的距离(distance) (2)向每个邻居路由器周期宣告自己知道的网络信息 (3)收到邻居送来的信息进行路由计算 (4)然后把学来的路由再告诉其他邻居 如果从本路由器不能到某目的地,则其代价为无穷大; 路径代价的度量单位可以是时间延迟、物理距离、经过 的路由器个数(跳数)或其它参数; Network Layer 4-145 V-D算法原理图示: 每隔一段时间(如30 秒)计算到相邻路由器代价; 每隔一段时间(如30 秒)相邻路由器之间交换一次 向量表,并据此计算并更新自己的路由表; Network Layer 4-146 Distance Vector: link cost changes Link cost changes: node detects local link cost change updates routing info, recalculates distance vector if DV changes, notify neighbors 1 x y 4 50 1 z At time t0, y detects the link-cost change, updates its DV, and informs its neighbors. At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV. At time t2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z. “good news travels fast” Network Layer 4-147 Convergence “Bad news travels slow” Network Layer 4-148 Distance Vector: link cost changes Link cost changes: good news travels fast bad news travels slow - “count to infinity” problem! 44 iterations before algorithm stabilizes: 60 x 4 y 50 1 z Poisoned reverse: If Z routes through Y to get to X : Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) will this completely solve count to infinity problem? Network Layer 4-149 V-D路由算法-Still have problem? 采用水平分割法,若B路由器出故障,A-B-C或者A- B-D可确保不再发生问题,但不幸的是C听说D可以2跳 到达A,D亦听说可通过C到A,因此,下一次交换均将 到A的距离增加到3。。。直至无穷大。 D C B A 1979年以后,ARPANet就以链路-状态算法取代它。 Network Layer 4-150 V-D路由算法特点: 该算法思想非常简单,基于该算法的路由协议(如RIP )易配置、维护和使用。 最大问题是收敛慢,且在收敛过程中,有可能产生路由 循环。 不适合于大型、复杂的网络。 只考虑了简单的距离(代价)因素,没有考虑物理链路 带宽; 交换的是相邻路由器之间的部分路由信息,信息不充分 ,路由器并未获知全网的结构以及各节点状态。 Network Layer 4-151 Comparison of LS and DV algorithms Message complexity LS: with n nodes, E links, O(nE) messages sent DV: exchange between neighbors only convergence time varies Speed of Convergence LS: O(n2) algorithm requires O(nE) messages may have oscillations DV: convergence time varies may be routing loops count-to-infinity problem Network Layer 4-152 Comparison of LS and DV algorithms Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes only its own table DV: DV node can advertise incorrect path cost each node’s table used by others error propagate through network Network Layer 4-153 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-154 Hierarchical Routing Our routing study thus far - idealization all routers identical network “flat” … not true in practice scale: with 200 million destinations: can’t store all destination’s in routing tables! routing table exchange would swamp links! administrative autonomy internet = network of networks each network administrator may want to control routing in its own network Network Layer 4-155 Hierarchical Routing aggregate routers into regions, “autonomous systems” (AS) routers in the same AS run same routing protocol Gateway router Direct link to router in another AS “intra-AS” routing protocol routers in different AS can run different intraAS routing protocol Network Layer 4-156 Autonomous System AS (自治系统) Network Layer 4-157 Interconnected ASes 3c 3a 3b AS3 1a 2a 1c 1d 1b Intra-AS Routing algorithm 2c AS2 AS1 Inter-AS Routing algorithm Forwarding table 2b forwarding table configured by both intra- and inter-AS routing algorithm intra-AS sets entries for internal destinations inter-AS & Intra-As sets entries for external destinations Network Layer 4-158 Inter-AS tasks AS1 must: 1. learn which destinations reachable through AS2, which through AS3 2. propagate this reachability info to all routers in AS1 Job of inter-AS routing! suppose router in AS1 receives datagram destination outside of AS1 router should forward packet to gateway router, but which one? 3c 3b 3a AS3 1a 2a 1c 1d 1b 2c AS2 2b AS1 Network Layer 4-159 Example: Setting forwarding table in router 1d suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2. inter-AS protocol propagates reachability info to all internal routers. router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c. installs forwarding table entry (x,I) x 3c 3a 3b AS3 1a 2a 1c 1d 1b AS1 2c 2b AS2 Network Layer 4-160 Example: Choosing among multiple ASes now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2. to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. this is also job of inter-AS routing protocol! x 3c 3a 3b AS3 1a 2a 1c 1d 1b 2c AS2 2b AS1 Network Layer 4-161 Example: Choosing among multiple ASes now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2. to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. this is also job of inter-AS routing protocol! hot potato routing: send packet towards closest of two routers. Learn from inter-AS protocol that subnet x is reachable via multiple gateways Use routing info from intra-AS protocol to determine costs of least-cost paths to each of the gateways Hot potato routing: Choose the gateway that has the smallest least cost Determine from forwarding table the interface I that leads to least-cost gateway. Enter (x,I) in forwarding table Network Layer 4-162 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-163 Intra-AS Routing also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Network Layer 4-164 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-165 RIP ( Routing Information Protocol) distance vector algorithm included in BSD-UNIX Distribution in 1982 distance metric: # of hops (max = 15 hops) From router A to subsets: u v A z C B D w x y destination hops u 1 v 2 w 2 x 3 y 3 z 2 Network Layer 4-166 RIP advertisements distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) ech advertisement: list of up to 25 destination nets within AS Network Layer 4-167 RIP: Example z w A x D B y C Destination Network w y z x …. Next Router Num. of hops to dest. …. .... A B B -- 2 2 7 1 Routing table in D Network Layer 4-168 RIP: Example Dest w x z …. Next C … w hops 1 1 4 ... A Advertisement from A to D z x Destination Network w y z x …. D B C y Next Router Num. of hops to dest. …. .... A B B A -- Routing table in D 2 2 7 5 1 Network Layer 4-169 RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if tables changed) link failure info quickly (?) propagates to entire net poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) Network Layer 4-170 RIP Table processing RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated routed routed Transprt (UDP) network (IP) link physical Transprt (UDP) forwarding table forwarding table network (IP) link physical Network Layer 4-171 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-172 OSPF (Open Shortest Path First) “open”: publicly available uses Link State algorithm LS packet dissemination topology map at each node route computation using Dijkstra’s algorithm OSPF advertisement carries one entry per neighbor router advertisements disseminated to entire AS (via flooding) carried in OSPF messages directly over IP (rather than TCP or UDP Network Layer 4-173 OSPF “advanced” features (not in RIP) security: all OSPF messages authenticated (to prevent malicious intrusion) multiple same-cost paths allowed (only one path in RIP) For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time) integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology data base as OSPF hierarchical OSPF in large domains. Network Layer 4-174 Hierarchical OSPF Network Layer 4-175 Hierarchical OSPF two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. backbone routers: run OSPF routing limited to backbone. boundary routers: connect to other AS’s. Network Layer 4-176 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-177 Internet inter-AS routing: BGP BGP (Border Gateway Protocol): the de facto standard BGP provides each AS a means to: 1. 2. 3. Obtain subnet reachability information from neighboring ASs. Propagate reachability information to all ASinternal routers. Determine “good” routes to subnets based on reachability information and policy. allows subnet to advertise its existence to rest of Internet: “I am here” Network Layer 4-178 BGP basics pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions BGP sessions need not correspond to physical links. when AS2 advertises prefix to AS1: AS2 promises it will forward any addresses datagrams towards that prefix. AS2 can aggregate prefixes in its advertisement eBGP session 3c 3a 3b AS3 1a AS1 iBGP session 2a 1c 1d 1b 2c AS2 2b Network Layer 4-179 Distributing reachability info using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use iBGP do distribute new prefix info to all routers in AS1 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session when router learns of new prefix, creates entry for prefix in its forwarding table. eBGP session 3c 3a 3b AS3 1a AS1 iBGP session 2a 1c 1d 1b 2c AS2 2b Network Layer 4-180 Path attributes & BGP routes advertised prefix includes BGP attributes. prefix + attributes = “route” two important attributes: AS-PATH: contains ASs through which prefix advertisement has passed: e.g, AS 67, AS 17 NEXT-HOP: indicates specific internal-AS router to next-hop AS. (may be multiple links from current AS to next-hop-AS) when gateway router receives route advertisement, uses import policy to accept/decline. Network Layer 4-181 BGP route selection router may learn about more than 1 route to some prefix. Router must select route. elimination rules: 1. 2. 3. 4. local preference value attribute: policy decision shortest AS-PATH closest NEXT-HOP router: hot potato routing additional criteria Network Layer 4-182 BGP messages BGP messages exchanged using TCP. BGP messages: OPEN: opens TCP connection to peer and authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also used to close connection Network Layer 4-183 BGP routing policy legend: B W X A provider network customer network: C Y A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks X does not want to route from B via X to C .. so X will not advertise to B a route to C Network Layer 4-184 BGP routing policy (2) legend: B W X A provider network customer network: C Y A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C? No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers B wants to force C to route to w via A B wants to route only to/from its customers! Network Layer 4-185 Why different Intra- and Inter-AS routing ? Policy: Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no policy decisions needed Scale: hierarchical routing saves table size, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance Network Layer 4-186 Chapter 4: Network Layer 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Network Layer 4-187 Broadcast Routing deliver packets from source to all other nodes source duplication is inefficient: duplicate duplicate creation/transmission R1 R1 duplicate R2 R2 R3 R4 source duplication R3 R4 in-network duplication source duplication: how does source determine recipient addresses? Network Layer 4-188 In-network duplication flooding: when node receives brdcst pckt, sends copy to all neighbors Problems: cycles & broadcast storm controlled flooding: node only brdcsts pkt if it hasn’t brdcst same packet before Node keeps track of pckt ids already brdcsted Or reverse path forwarding (RPF): only forward pckt if it arrived on shortest path between node and source spanning tree No redundant packets received by any node Network Layer 4-189 Spanning Tree First construct a spanning tree Nodes forward copies only along spanning tree A B c F A E B c D F G (a) Broadcast initiated at A E D G (b) Broadcast initiated at D Network Layer 4-190 Spanning Tree: Creation Center node Each node sends unicast join message to center node Message forwarded until it arrives at a node already belonging to spanning tree A A 3 B c 4 E F 1 2 B c D F 5 E D G G (a) Stepwise construction of spanning tree (b) Constructed spanning tree Network Layer 4-191 Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting routers having local mcast group members tree: not all paths between routers used source-based: different tree from each sender to rcvrs shared-tree: same tree used by all group members Shared tree Source-based trees Approaches for building mcast trees Approaches: source-based tree: one tree per source shortest path trees reverse path forwarding group-shared tree: group uses one tree minimal spanning (Steiner) center-based trees …we first look at basic approaches, then specific protocols adopting these approaches Shortest Path Tree mcast forwarding tree: tree of shortest path routes from source to all receivers Dijkstra’s algorithm S: source LEGEND R1 1 2 R4 R2 3 R3 router with attached group member 5 4 R6 router with no attached group member R5 6 R7 i link used for forwarding, i indicates order link added by algorithm Reverse Path Forwarding rely on router’s knowledge of unicast shortest path from it to sender each router has simple forwarding behavior: if (mcast datagram received on incoming link on shortest path back to center) then flood datagram onto all outgoing links else ignore datagram Reverse Path Forwarding: example S: source LEGEND R1 R4 router with attached group member R2 R5 R3 R6 R7 router with no attached group member datagram will be forwarded datagram will not be forwarded • result is a source-specific reverse SPT – may be a bad choice with asymmetric links Reverse Path Forwarding: pruning forwarding tree contains subtrees with no mcast group members no need to forward datagrams down subtree “prune” msgs sent upstream by router with no downstream group members LEGEND S: source R1 router with attached group member R4 R2 P R5 R3 R6 P R7 P router with no attached group member prune message links with multicast forwarding Shared-Tree: Steiner Tree Steiner Tree: minimum cost tree connecting all routers with attached group members problem is NP-complete excellent heuristics exists not used in practice: computational complexity information about entire network needed monolithic: rerun whenever a router needs to join/leave Center-based trees single delivery tree shared by all one router identified as “center” of tree to join: edge router sends unicast join-msg addressed to center router join-msg “processed” by intermediate routers and forwarded towards center join-msg either hits existing tree branch for this center, or arrives at center path taken by join-msg becomes new branch of tree for this router Center-based trees: an example Suppose R6 chosen as center: LEGEND R1 3 R2 router with attached group member R4 2 R5 R3 1 R6 R7 1 router with no attached group member path order in which join messages generated Internet Multicasting Routing: DVMRP DVMRP: distance vector multicast routing protocol, RFC1075 flood and prune: reverse path forwarding, source-based tree RPF tree based on DVMRP’s own routing tables constructed by communicating DVMRP routers no assumptions about underlying unicast initial datagram to mcast group flooded everywhere via RPF routers not wanting group: send upstream prune msgs DVMRP: continued… soft state: DVMRP router periodically (1 min.) “forgets” branches are pruned: mcast data again flows down unpruned branch downstream router: reprune or else continue to receive data routers can quickly regraft to tree following IGMP join at leaf odds and ends commonly implemented in commercial routers Mbone routing done using DVMRP Tunneling Q: How to connect “islands” of multicast routers in a “sea” of unicast routers? physical topology logical topology mcast datagram encapsulated inside “normal” (non-multicast- addressed) datagram normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router receiving mcast router unencapsulates to get mcast datagram PIM: Protocol Independent Multicast not dependent on any specific underlying unicast routing algorithm (works with all) two different multicast distribution scenarios : Dense: Sparse: group members # networks with group densely packed, in “close” proximity. bandwidth more plentiful members small wrt # interconnected networks group members “widely dispersed” bandwidth not plentiful Consequences of Sparse-Dense Dichotomy: Dense group membership by Sparse: no membership until routers assumed until routers explicitly join routers explicitly prune receiver- driven data-driven construction construction of mcast on mcast tree (e.g., RPF) tree (e.g., center-based) bandwidth and non bandwidth and non-groupgroup-router processing router processing profligate conservative PIM- Dense Mode flood-and-prune RPF, similar to DVMRP but underlying unicast protocol provides RPF info for incoming datagram less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm has protocol mechanism for router to detect it is a leaf-node router PIM - Sparse Mode center-based approach router sends join msg to rendezvous point (RP) router can switch to source-specific tree increased performance: less concentration, shorter paths R4 join intermediate routers update state and forward join after joining via RP, R1 R2 R3 join R5 join R6 all data multicast from rendezvous point R7 rendezvous point PIM - Sparse Mode sender(s): unicast data to RP, which distributes down RP-rooted tree RP can extend mcast tree upstream to source RP can send stop msg if no attached receivers “no one is listening!” R1 R4 join R2 R3 join R5 join R6 all data multicast from rendezvous point R7 rendezvous point Chapter 4: summary 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP Network Layer 4209 Virtual Circuit VC number 22 12 1 Forwarding table in northwest router: Incoming interface 1 2 3 1 … 2 32 3 interface number Incoming VC # 12 63 7 97 … Outgoing interface 3 1 2 3 … Outgoing VC # 22 18 17 87 … Routers maintain connection state information! Network Layer 4-210 Virtual circuits: signaling protocols used to setup, maintain, and teardown Virtual Circuit used in ATM, frame-relay, X.25 not used in today’s Internet application transport 5. Data flow begins network 4. Call connected data link 1. Initiate call physical 6. Receive data application 3. Accept call 2. incoming call transport network data link physical Network Layer 4-211 Datagram networks no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets forwarded using destination host address packets between same source-destination pair may take different paths application transport network data link 1. Send data physical application transport network 2. Receive data data link physical Network Layer 4-212 Forwarding table Destination Address Range 4 billion possible entries ? Link Interface 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 0 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 1 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 2 otherwise 3 Network Layer 4-213 Three types of switching fabrics Network Layer 4-214 IP Address Format IP Address ::= { <Network ID>, <Host ID>} All network Ids are assigned by INIC, and the host Ids are assigned by AS Network Layer 4-215 IP Address: 点分十进制表示法 主机的 IP 地址 是 32 位二进制代码 每隔 8 位插入一个空格 以提高可读性 将每 8位的二进制数 转换为十进制数表示 readable! 10000000000010110000001100011111 10000000 00001011 00000011 00011111 128 11 3 31 128.11.3.31 Network Layer 4-216 IP 地址中的网络号字段和主机号字段 A 类地址 0 net-id 8 bit B 类地址 host-id 24 bit 10 net-id 16 bit C 类地址 host-id 16 bit 110 net-id 24 bit D 类地址 1110 E 类地址 11110 host-id 8 bit 多播地址 保留为今后使用 Network Layer 4-217 Address “AND” mask= network ID First 8 bits are network ID Address 8.1.4.5 0000 1000 0000 0001 0000 0100 0000 0101 Mask 255.0.0.0 1111 1111 0000 0000 0000 0000 0000 0000 Result 8.0.0.0 0000 1000 0000 0000 0000 0000 0000 0000 Address 130.4.100.1 1000 0010 0000 0100 0110 0100 0000 0101 Mask 255.255.0.0 1111 1111 1111 1111 0000 0000 0000 0000 Result 130.4.0.0 1000 0010 0000 0100 0000 0000 0000 0000 First 16 bits are network ID Network Layer 4-218 IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router’s typically have multiple interfaces host typically has one interface IP addresses associated with each interface Subnet 2 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 Subnet 1 223.1.2.2 Subnet 3 223.1.3.2 223.1.3.1 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Network Layer 4-219 IP Packet format:header and Data 分组表示符:用于判断 IP版本号: 通常5个字长 IPv4分段属于哪个分组 ( 20个字节 ) 用来说明分段在当前分组中的 3位优先级: 头部加数据最 位置(偏移量以8字节为单位) D,T,R;两位未用 大:65536字节 校验:仅对头 部进行校验 DF=1:不可分段 MF=1:还有分段 生存期:用于限制分组生命 周期的计时器(缺省64)。 用于说明IP分组应被传送到的 选项:用于扩充协议功能。例如安 数据高层协议(TCP: 6, UDP: 17) 全性,记录路由,时间戳等 Network Layer 4220 ICMP (Internet Control Message Protocol) 由于IP层采用数据报方式,当通信介质或路由器出现故障、 目的主机不可达、IP报文生存超时、路由器或通信拥塞等问 题出现时,IP报文无法正常到达目的主机,而源端却无法判 断发送失败的原因。 How to do? 让路由器向源主机报告差错情况-引入ICMP协议。 ICMP协议是IP层协议的一部分; ICMP报文通过IP报文来传输。当路由器要发送ICMP报文 时,它会创建一个IP报文并将ICMP报文封装到IP报文的数 据区中,然后象普通IP报文一样通过因特网。 Network Layer 4-221 The 用于路由器或主机向出错数据报的源端 types of ICMP packet 报告出错情况 目的端不可达(type = 3) 超时报文(type = 11) 差错报文 用于拥塞控制的源抑制报文和 参数出错(type = 12) 用于路径控制的重定向报文 ICMP报 文类型 源抑制报文(type = 4) 用于获得某些有关网络状态的信 重定向报文(type = 5) 控制报文 息,以便进行网络故障诊断和控 制 请求/应答 报文 回应请求/应答报文(type=8/0) 时戳请求/应答报文(type=13/14) 地址请求/应答报文(type=17/18) Network Layer 4222 链路-状态(L-S)路由算法 1、工作原理: -为每个路由器构造一张全网的拓扑结构图,图中包含 相邻节点之间的代价,使用最短路径算法为路由器计算 出到每一个节点的距离(代价)。 各自路由器之间发现邻居关系(确定位置) 计算链路传输时延 构造L-S报文 广播L-S报文 每个路由器计算到其他路由器的最短路径 Network Layer 4223 距离向量(V-D)路由算法 算法原理: 让每个路由器动态计算并维护一张距离向量表,表中为 从某节点能到达其他节点的最佳路径代价和输出接口; 具体地: (1)每个路由器维持到各个已知目的节点的距离(distance) (2)向每个邻居路由器周期宣告自己知道的网络信息 (3)收到邻居送来的信息进行路由计算 (4)然后把学来的路由再告诉其他邻居 如果从本路由器不能到某目的地,则其代价为无穷大; 路径代价的度量单位可以是时间延迟、物理距离、经过 的路由器个数(跳数)或其它参数; Network Layer 4224 Hierarchical routing Network Layer 4225