Networking and Internetworking

Transcription

1 Networking and Internetworking Dr. Xiaobo Zhou Adopted from Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 4, Addison-Wesley /4/ Protocol Layers in the ISO (OSI) Model Layers Application Presentation Session Transport Network Data link Physical Sender Message sent Communication medium Message received Recipient 2 1

2 Encapsulation Applied in Layered Protocols Application-layer message Presentation header Session header Transport header Network header 3 OSI Protocol Summary Layer Description Examples Application Protocols that are designed to meet the communication requirements of specific applications, often defining the interface to a service. HTTP, FTP, SMTP, CORBA IIOP Presentation Protocols at this level transmit data in a network representation that is independent of the representations used in individual computers, which may differ. Encryption is also performed in this layer, if required. Secure Sockets (SSL),CORBA Data Rep. Session At this level reliability and adaptation are performed, such as detection of failures and automatic recovery. Transport This is the lowest level at which messages (rather than packets) are handled. TCP, UDP Messages are addressed to communication ports attached to processes, Protocols in this layer may be connection-oriented or connectionless. Network Data link Physical Transfers data packets between computers in a specific network. In a WAN or an internetwork this involves the generation of a route passing through routers. In a single LAN no routing is required. Responsible for transmission of packets between nodes that are directly connected by a physical link. In a WAN transmission is between pairs of IP, ATM virtual circuits Ethernet MAC, ATM cell transfer, routers or between routers and hosts. In a LAN it is between any pair of hosts. PPP The circuits and hardware that drive the network. It transmits sequences of Ethernet base- band binary data by analogue signalling, using amplitude or frequency modulation signalling, ISDN of electrical signals (on cable circuits), light signals (on fibre optic circuits) or other electromagnetic signals (on radio and microwave circuits). 4 2

3 TCP/IP Suite 5 Internet Protocol (IP) Routers (gateways) interconnect different networks Host computers prepare IP packets and transmit them over their attached network Routers forward IP packets across networks Best-effort IP transfer service, no retransmission Net 1 Net 2 Router 2/4/2008 Addison-Wesley Publishers

4 Addressing & Routing Hierarchical address: Net ID + Host ID IP packets routed according to Net ID Routers compute routing tables using distributed algorithm H H H Net 1 G Net 2 G G Net 3 G G Net 4 G Net 5 H 7 Transport Protocols Host computers run two transport protocols on top of IP to enable process-to-process communications User Datagram Protocol (UDP) enables best-effort transfer of individual block of information Transmission Control Protocol (TCP) enables reliable transfer of a stream of bytes Transport Protocol Internet 8 4

5 Routing in Packet Networks Node (switch or router) Three possible (loopfree) routes from 1 to 6: 1-3-6, , Which is best? Min delay? Min hop? Max bandwidth? Min cost? Max reliability? 9 An Example of Routing Information Protocol (RIP) (a) A subnet. (b) Input from A, I, H, K, and the new routing table for J. 2/4/ Addison-Wesley Publishers

6 Count-to-Infinity Problem It converges to the correct answer quickly to good news but slowly to bad news. B knows A is 1 hop away while all other routers still think A is down, why? What is the spreading rate of good news? How many exchanges needed in a N-hop subnet? Does B know that C s path runs through B? Why spreading rate of bad news so slow? What is the core problem? Link-State Algorithm Basic idea: two stage procedure Each source node gets a map of all nodes and link metrics (link state) t of the entire network Learning who the neighbors are and what are delay to them Construct a link state packet, and deliver it to others Find the shortest path on the map from the source node to all destination nodes; Dijkstra s algorithm Broadcast of link-state information Every node i in the network broadcasts to every other node in the network: ID s of its neighbors: N i =set of neighbors of i Distances to its neighbors: {C ij j N i } Flooding is a popular method of broadcasting packets 6

7 Building Link State Packets A state packet starts with the ID of the sender, a seq#, age, and a list of neighbors with delay information. (a) A subnet. (b) The link state packets for this subnet. When to build the link state packets? Periodically, or when significant event occurs. Distributing the Link State Packets Flooding is used to distribute the link state packets. What is the major problem with flooding? How to handle the problem? (source router, sequence number) How to make the sequence number unique? 32-bit sequence number What happens if a router crashes, losing its track, and starts again? What happens if sequence number is corrupted, say 65,540, not 4. Age field (in sec; usually a packet comes in 10 sec.) All packets should be ACKed. 7

8 Names and IP Addresses Routing is done based on 32-bit IP addresses Dotted-decimal notation Hosts are also identified by name Easier to remember Hierarchical name structure tesla.comm.utoronto.edu Domain Name System (DNS) provided conversion between names and addresses 15 Internet address structure, showing field sizes in bits 7 24 Class A: 0 Network ID Host ID Class B: 1 0 Network ID Host ID 21 8 Class C: Network ID Host ID 28 Class D (multicast): Multicast address 27 Class E (reserved): unused 16 8

9 Subnets A campus network consisting of LANs for various departments. Subnetting: how to allow a network to be split into several parts for internal use but still act like a single network to the outside - When a packet comes into the main router, how does it know which subnet to give the packet to? 17 Subnet Addressing Does a LAN need a unique network address? Subnet addressing introduces another hierarchical level Transparent to remote networks Simplifies management of multiplicity of LANs Masking used to find subnet number Original address 1 0 Net ID Host ID Subnetted address 1 0 Net ID Subnet ID Host ID 9

10 Subnetting Example Organization has Class B address (16 host ID bits) with network ID: Create subnets with up to 100 hosts each 7 bits sufficient for each subnet 16-7=9 bits for subnet ID Apply subnet mask to IP addresses to find corresponding subnet Example: Find subnet for IP add = Mask = AND = Subnet = Subnet address used by routers within organization IP Address Problems In the 1990, two problems became apparent IP addresses were being exhausted IP routing tables were growing very large IP Address Exhaustion how to be scalable? Class A, B, and C address structure inefficient Class B too large for most organizations, but future proof Class C too small Rate of class B allocation implied exhaustion by 1994 IP routing table size Growth in number of networks in Internet reflected in # of table entries From 1991 to 1995, routing tables doubled in size every 10 months Stress on router processing power and memory allocation Short-term solution: Classless Interdomain Routing (CIDR), RFC 1518 New allocation policy (RFC 2050) Private IP Addresses set aside for intranets (NAT) Long-term solution: IPv6 with much bigger address space 2/4/ Addison-Wesley Publishers

11 Classless Inter-Domain Routing (CIDR) CIDR deals with Routing Table Explosion Problem Networks represented by prefix and mask Summarize a contiguous group of class C addresses using variablelength mask, if all of them use the same outgoing line Solution: Route according to prefix of address, not class Routing table entry has <IP address, network mask> Example: / min address mask IP prefix max address 21 A Typical NAT-based Home Network DSL or Cable connection to ISP xx subnet Modem / firewall / router (NAT enabled) Ethernet switch WiFi base station/ access point printer Laptop Game box Media hub TV monitor PC 1 PC 2 Bluetooth adapter Bluetooth printer Camera 22 11

12 IPv4 Packet Header Version IHL Type of Service Total Length Identification Flags Fragment Offset Time to Live Protocol Header Checksum Source IP Address Destination IP Address Options Padding Minimum 20 bytes Up to 40 bytes in options fields 23 IPv6 Header Layout Version (4 bits) Traffic class (8 bits) Flow label (20 bits) Payload length (16 bits) Next header (8 bits) Hop limit (8 bits) Source address (128 bits) Destination address (128 bits) 2/4/ Addison-Wesley Publishers

13 The Mobile IP Routing Mechanism Address of FA returned to sender First IP packet addressed to MH Sender Home agent Subsequent IP packets tunnelled to FA Internet First IP packet tunnelled to FA Mobile host MH Foreign agent FA 25 Network Performance Wired: Example Range Bandwidth Latency (Mbps) (ms) LAN Ethernet 1-2 kms WAN IP routing worldwide MAN ATM 250 kms Internetwork Internet worldwide Wireless: WPAN Bluetooth ( ) 1) 10-30m WLAN WiFi (IEEE ) km WMAN WiMAX (802.16) 550 km WWAN GSM, 3G phone nets worldwide