Bài giảng Mạng máy tính - Chương 4: The network layer and Internetworking - Nguyễn Trung Dũng

28-May-09 1 Chapter 4 The Network Layer & Internetworking Content • Internetworking • The Network Layer in the Internet • Network Layer Design Issues • Routing Algorithms • Congestion Control Algorithms 28-May-09 2 Internetworking • Overview • How Networks Differ ? • How Networks Can Be Connected ? • Concatenated Virtual Circuits • Connectionless Internetworking • Tunneling • Internetwork Routing • Fragmentation Overview 28-May-09 3 How Networks Differ ? How Networks Can Be Connected ? 28-May-09 4 Concatenated Virtual Circuits Connectionless Internetworking 28-May-09 5 Tunneling Internetwork Routing 28-May-09 6 Routed versus Routing • Routed protocol: used at the network layer that transfer data from one host to another across a router • Routing protocols: allow routers to choose the best path for data from source to destination • Examples: Internet Protocol (IP); Novell's Internetwork Packet Exchange (IPX); DECnet, AppleTalk, Banyan VINES, and Xerox Network Systems (XNS). Routing protocol • Provides processes for sharing route information • Allows routers to communicate with other routers to update and maintain the routing tables • Examples: Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and Enhanced IGRP (EIGRP) 28-May-09 7 Path Determination • Path determination enables a router to compare the destination address to the available routes in its routing table, and to select the best path • Static or Dynamic routing Transportation Analogy 28-May-09 8 The Routing Process Routing Table 28-May-09 9 Information in Routing Table • Protocol type – The type of routing protocol that created the routing table entry • Destination/next-hop associations – These associations tell a router that a particular destination is either directly connected to the router, or that it can be reached using another router called the “next-hop” on the way to the final destination • Routing metric – Different routing protocols use different routing metrics. • Outbound interfaces – The interface that the data must be sent out on Routing Algorithms & Metrics • Design goals of Routing Protocols – Optimization – Simplicity & Low Overhead – Robustness & stability – Flexibility – Rapid Convegence • Some metrics used by Routing Protocols: – Bandwidth – Delay – Load – Reliability – Hop count – Ticks, cost 28-May-09 10 IGP and EGP • Autonomous system is a network or set of networks under common administrative control. An autonomous system consists of routers that present a consistent view of routing to the external world. • Interior Gateway Protocols (IGP): route data within an autonomous system. Eg: RIP and RIPv2; IGRP; EIGRP; OSPF; IS-IS; • Exterior Gateway Protocols (EGP): route data between autonomous systems. Eg: BGP Link state and Distance Vector • The distance-vector routing approach determines the distance and direction, vector, to any link in the internetwork. Routers using distance-vector algorithms send all or part of their routing table entries to adjacent routers on a periodic basis. This happens even if there are no changes in the network. Eg: RIP, IGRP, EIRP • Link state routing protocols send periodic update at longer time interval (30’), Flood update only when there is a change in topology. Link state use their database to creat routing table. Eg: OSPF, IS-IS 28-May-09 11 Routing Protocols • RIP:distance vector; uses hop count as its metric; RIP cannot route a packet beyond 15 hops. RIPv1 requires all devices in the network use the same subnet mask. RIPv2 supports VLSM. • IGRP:distance-vector; routing protocol developed by Cisco. IGRP can select the fastest path based on delay, bandwidth, load, and reliability. It also has a much higher maximum hop count limit than RIP. • OSPF • IS-IS • EIGRP • BGP Fragmentation Transparent and Non-Transparent Fragmentation 28-May-09 12 The Network Layer in the Internet • TCP/IP model • Internet Protocol (IP) • Addressing • IP Address • Internet Control Protocols o Internet Control Message Protocol o ARP - Address Resolution Protocol o RARP, BOOTP, and DHCP • OSPF - Interior Gateway Routing Protocol • BGP - Exterior Gateway Routing Protocol • Internet Multicasting • Mobile IP • IPv6 TCP/IP IP TCP UDP Ethernet PPP HDLC ... HTTP SMTP SNMP ... 28-May-09 13 Internet Protocol (IP) o Connectionless: Different packets may take different paths to get through the network; reassembled at the destination, the destination is not contacted before a packet is sent. o Connection-oriented: A connection is established between the sender and the recipient before any data is transferred. The IPv4 header 28-May-09 14 The IPv4 header • 4 bits • Indicates version of IP used • IPv4: 0100; IPv6: 0110 The IPv4 header • 4 bits • Indicates datagram header length in 32 bit words 28-May-09 15 The IPv4 header • 8 bits • Specifies the level of importance that has been assigned by upper-layer protocol The IPv4 header • 16 bits • Specifies the length of the entire packet in bytes, including data and header 28-May-09 16 The IPv4 header • 16 bits • Identifies the current datagram The IPv4 header • 3 bits • The second bit specifies if the packet can be fragmented; the last bit specifying whether the packet is the last fragment in a series of fragmented packets. 28-May-09 17 The IPv4 header • 13 bits • Used to help piece together datagram fragments The IPv4 header • 8 bits • Specifies the number of hops a packet may travel. This number is decreased by one as the packet travels through a router 28-May-09 18 The IPv4 header • 8 bits • Indicates which upper-layer protocol, such as TCP(6) or UDP(17), receives incoming packets after IP processing has been completed The IPv4 header • 16 bits • Helps ensure IP header integrity • Not caculated for the encapsulation data 28-May-09 19 The IPv4 header • 32 bits • Specifies the sending node IP address The IPv4 header • 32 bits • Specifies the receiving node IP address 28-May-09 20 The IPv4 header • Variable length • Allows IP to support various options, such as security The IPv4 header • Variable length • Extra zeros are added to this field to ensure that the IP header is always a multiple of 32 bits. 28-May-09 21 The IPv4 header • Variable length up to 64 Kb • Contains upper-layer information • For any two systems to communicate, they must be able to identify and locate each other. We call it “addressing”. • The hosts are “grouped” into networks. In the illustration, we use the A or B to identify the network and the number sequence to identify the individual host. • The combination of letter (network address) and the number (host address) create a unique address for each device on the network. Addressing 28-May-09 22 • An address generally represents the connection to the network. A device that have two connection points may need two addresses beloging to two networks. • Each connection points (espcially in LAN technologies) also has its ID (example: MAC address) which is called physical address. There is also the need to map between physical adresses (layer 2) and logical addresses (layer 3). Addressing • Every IP address has two parts. One part identifies the network where the system is connected, and a second part identifies that particular system on the network. • Two different networks must have different network address (net-id), and two different hosts in the same network must have different host address (host-id). Of cause, hosts in the same network have the same network address. Addressing Rule 28-May-09 23 IP Address (IPv4) 28-May-09 24 When all host-bits are zeros, we have a number that represents network address. This address is reserved, namely it cannot be assigned to any host. Network Address • When host-bits are all one, we have a number that represents broadcast address. This address is also reserved, namely it cannot be assigned to any host. • Example where Broadcast addresses are used: a host need to locate a specific service. Broadcast Address 28-May-09 25 Unicast and Broadcast Transmission Unicast transmission Broadcast transmission The concept of unicast and broadcast transmission exist in both layer 2 and layer 3 protocols. There are refelections in the addressing scheme Certain host addresses are reserved and cannot be assigned to devices on a network. These reserved host addresses include the following: – Host-bits = all zeros (network address); – Host-bits = all ones (broadcast address); – Network-bits = all ones; – Network-bits = all zeros; – 127.x.x.x (loopback address = 127.0.0.1). Reserved IP Address 28-May-09 26 • The stability of the Internet depends directly on the uniqueness of publicly used network addresses. • In the figure, there is an “IP conflict” issue. • A procedure was needed to make sure that addresses were in fact unique. Originally, an organization known as the Internet Network Information Center (InterNIC) handled this procedure. InterNIC no longer exists and has been succeeded by the Internet Assigned Numbers Authority (IANA). Required Unique Address • Public IP addresses are unique. No two machines that connect to a public network can have the same IP address. • Public IP addresses must be obtained from an Internet service provider (ISP) or a registry at some expense. • With the rapid growth of the Internet, public IP addresses were beginning to run out (IP address depletion). • New addressing schemes, such as classless interdomain routing (CIDR) and IPv6 were developed to help solve the problem. Private IP addresses are another solution. Public IP Addresses 28-May-09 27 • RFC 1918 sets aside three blocks of IP addresses for private, internal use. These three blocks consist of one Class A, a range of Class B addresses, and a range of Class C addresses. • Addresses that fall within these ranges are not routed on the Internet backbone. Internet routers immediately discard private addresses. Private IP Addresses • When addressing a nonpublic intranet, a test lab, or a home network, we normally use private addresses instead of globally unique addresses. • Private addresses can be used to address point-to-point serial links without wasting real IP addresses. • Connecting a network using private addresses to the Internet requires translation of the private addresses to public addresses. This translation process is referred to as Network Address Translation (NAT). Using Private Addresses 28-May-09 28 Introduction to Subnetting • Subnetting is another method of managing IP addresses. This method of dividing full network address classes into smaller pieces has prevented complete IP address exhaustion. • The network is no longer limited to the default Class A, B, or C network masks and there is more flexibility in the network design. • Analogy: telephone. • Subnet addresses include the network portion, plus a subnet field and a host field. • To create a subnet address, a network administrator borrows bits from the host field and designates them as the subnet field. Reason for Subnetting 28-May-09 29 Establishing SM address The number of bits in the subnet will depend on the maximum number of hosts required per subnet. The subnet mask: using binary ones in the host octet(s) (2 power of borrowed bits) – 2 = usable subnets (2 power of remaining host bits) –2= usable hosts Applying the Subnet Mask 28-May-09 30 The Logical ANDing process • ANDing is a binary process by which the router calculates the subnetwork ID for an incoming packet • ANDding process is handled at the binary level • (IP address) AND (subnetmask address) = subnetwork ID (router uses that information to forward the packet across the correct interface) Practice Router1 LAN4 LAN5 LAN7LAN2 LAN1 LAN8 Switch1 WAN S0/1 S0/0 F0/0 F0/0 Loopback2 Loopback7 DNS Server DHCP Server1 PC2 Router2 Switch2 TFTP Server DHCP Server2 WEB Server1 (www.cisco.com) Loopback3LAN3 LAN6 Loopback6F0/1 F0/1 PC1 PC3 PC4 Switch3 Switch4 28-May-09 31 IPv4 and IPv6 Addresses IPv4 32-bit IPv6 128-bit IPv4 and IPv6 4.3 e 9 IP addresses 3.4 e 38 IP addresses 28-May-09 32 Internet Control Protocols • ICMP - Internet Control Message Protocol • ARP - Address Resolution Protocol • RARP, BOOTP, and DHCP ICMP - Internet Control Message Protocol 28-May-09 33 ARP The issue of address mapping between level-2 and level-3 addresses are quite relevent. In TCP/IP communication, a host needs to know both IP address and MAC address of the destination host in order to send packet to it. So there comes Address Resolution Protocol (ARP) which helps hosts in the same LAN segments to find each other MAC addresses. Proxy ARP Communications among LAN segments have an additional task. TCP/IP has a variation on ARP called Proxy ARP that will provide the MAC address of an intermediate device (example router) for transmission outside the LAN to another network segment. 28-May-09 34 • Some devices keep the IP-MAC mapping in a so-called ARP table which is stored in RAM. • Example: arp -a, arp -d *. • When a devices needs to send data to a host --whose IP is known but MAC is unknown-- it send an ARP request as a broadcast frame. Then the destination reply with ARP reply. • Another way to build ARP table is to monitor the traffic. • Router generally do not forward such the broadcast. If this feature is turned on, a router performs a Proxy ARP. • However, in reality, we apply the default gateway feature. When the destination host is of the different network, then the IP packet is sent to the default gateway (MAC) while IP address is set to the final destination. • If there is neither default gateway nor Proxy ARP, no traffic can leave the local network. ARP Please remember that both ARP and RARP use the same message structure. Packet Propagation and Switching 28-May-09 35 Router Protocol Stripping Router Protocol Stripping 28-May-09 36 Router Protocol Stripping Router Protocol Stripping 28-May-09 37 Router Protocol Stripping Router Protocol Stripping 28-May-09 38 Router Protocol Stripping Router Protocol Stripping 28-May-09 39 Router Protocol Stripping Encapsulation changes in a Router 28-May-09 40 Routing vs. Switching Switching occurs at Layer 2, routing occurs at Layer 3. Routing and switching use different information in the process of moving data from source to destination Switching and Layer 2 Routing 28-May-09 41 ARP table and Routing table Router and Switch • Each computer and router interface maintains an ARP table for Layer 2 communication. The ARP table is only effective for the broadcast domain (or LAN) that it is connected to • MAC addresses are not logically organized, but IP addresses are organized in a hierarchical manner 28-May-09 42 Obtaining IP Addresses Devices come with MAC addresses (layer-2). However, IP addresses (layer-3) require proper configuration. There are basically two ways to obtain IP addresses: static and dynamic. Static assignment works best on small, infrequently changing networks. The system administrator manually assigns and tracks IP addresses for each computer, printer, or server on the intranet. Good recordkeeping is critical to prevent problems which occur with duplicate IP addresses. 28-May-09 43 RARP Reverse Address Resolution Protocol (RARP) associates a known MAC addresses with an IP addresses. This association allows network devices to encapsulate data before sending the data out on the network. A network device, such as a diskless workstation, might know its MAC address but not its IP address. RARP allows the device to make a request to learn its IP address. RARP Operation: 1: ARP request 2: ARP response 3: RARP request 4: RARP response 5: Dynamic RARP request 6: Dynamic RARP response 7: Dynamic RARP error 8: InARP request 9: InARP response ARP and RARP share the same packet format, which is encapsulated on layer-2 frames. They differentiate themselves by the “operation” field. 28-May-09 44 RARP • Hardware Type specifies a hardware interface type for which the sender requires a response (ie. ~layer 2). • Protocol Type specifies the type of high level protocol address the sender has supplied (ie. ~layer 3). RARP • HLen: Hardware address length. • PLen: Protocol address length. 28-May-09 45 RARP • Sender Hardware Address: Hardware address of the sender. • Sender Protocol Address: Protocol address of the sender. • Target Hardware Address: Hardware address of the targer. • Target Protocol Address: Protocol address of the target. RARP • The workstation boots, and then generates an RARP request. • It broadcasts the request to all hosts (using layer-2 broadcast address). • All other host discard the request, except the RARP server, who accepts it. I needs an IP address! 28-May-09 46 RARP • The RARP server generates the RARP response which contain its’ answer. • It broadcasts the response to all the hosts. • The workstation receives the answer and set its IP address. Thanks. From now on, I am 192.168.10.36 Here it is: 192.168.10.36! BOOTP The bootstrap protocol (BOOTP) operates in a client-server environment and only requires a single packet exchange to obtain IP information. However, unlike RARP, BOOTP packets can include the IP address, as well as the address of a router, the address of a server, and vendor-specific information, etc. BOOTP is encapsulated on UDP datagram. 28-May-09 47 BOOTP • Op: Message operation code; can be BOOTREQUEST or BOOTREPLY. • Htype: Hardware address type. • HLen: Hardware address length. • Hops: Clients place zero, this field is used by BOOTP server to send request to another network. BOOTP • Xid: Transaction ID • Seconds: Seconds elapsed since the client began the address acquisition or renewal process. 28-May-09 48 BOOTP • Ciadder: Client IP address. • Yiadder: “Your” (client) IP address. • Siadder: IP address of the next server to use in bootstrap. • Giadder: Relay agent IP address used in booting via a relay agent. • Chadder: Client hardware address. BOOTP • Server Host Name: Specifies particular server to get BOOTP information from. • Boot File Name:Alow multiple boot files (example: for different OSes). • Vendor Specific Area: Optional vendor information that can be passed to the host. 28-May-09 49 BOOTP • The workstation boots, and then generates an BOOTP request. • It broadcasts the request to all hosts (IP source address = unknown). • All other host discard the request, except the BOOTP server, who accepts it. I needs an IP address! BOOTP • The BOOTP server generates the BOOTP reply which contain its’ answer. • It broadcasts the response to all the hosts. • The workstation receives the answer and set its IP address. Thanks. From now on, I am 192.168.10.36 Here it is: 192.168.10.36! I keep a profile for each clients: IP=..., MAC=... IP=..., MAC=... IP=..., MAC=... 28-May-09 50 DHCP Dynamic host configuration protocol (DHCP) is the successor to BOOTP. Unlike BOOTP, DHCP allows a host to obtain an IP address dynamically without individual profile that the network administrator having to set up for each device. All that is required when using DHCP is a defined range of IP addresses on a DHCP server. The major advantage that DHCP has over BOOTP is that it allows users to be mobile. DHCP offers a one to many ratio of IP addresses and that an address is available to anyone who connects to the network. client server request client server request client server request request RARP BOOTP DHCP DHCP DHCP uses the same message structure of BOOTP, with some extentions (subnet masks, etc.) The idea is that the entire network configuration of a computer can be obtained in one message. 28-May-09 51 DHCP • The laptop boots, and then generates an DHCP request. • It broadcasts the request to all hosts (to discover who is the server). • All other host discard the request, except the DHCP servers, who accept it. I needs an IP address! Both of us are supposed to receive DHCP messages. DHCP • The DHCP servers generates the DHCP offers which contain its’ answer. • It broadcasts the response to all the hosts. • The laptop receives the answers, select one of them (the first one). OK, I accept one offer of yours. My offer is: 192.168.10.35! My offer is: blah blah! 28-May-09 52 DHCP • The laptop sends DCHPREQUEST addressed to the specific DHCP server that has sent the accepted offer. Here’s my DHCPREQUEST, specifically for you. Good, got it! DHCP • The DHCP server sends the DHCPACK • And the laptop sets the IP address accordingly. Thanks! From now on I am 192.168.10.35! Here is the ACK