CS 520: Network Architecture I Winter Lecture 3: Internetworking Architecture and Addressing

Transcription

1 CS 520: Architecture I Winter 2006 Lecture 3: Internetworking Architecture and Addressing This lecture provides an overview of the internetworking concept (Chapter 3), then looks at Internet addressing (Chapter 4). I. Internetworking Concept Chapter 3 - Internetworking Concept and Architectural Model Goal: Coordinate diverse underlying network technologies into a coordinated whole. Create "The Internet" out of "networks". Result is a high-level abstraction that can be used to make design decisions for the entire Internet. Two Types of Interconnection 1. Application-Level Interconnection Have applications running on different machines work together to send and route data. Examples: Mail servers, web caches. Each application makes routing decisions on where to send to the next machine running that application. Send mail messages, web pages, files, etc. Cumbersome approach. Applications must do more than originally intended. They must understand how the network is configured and connected. They must send messages from machine to machine to relay messages to their final destination. Lecture 3, Page 1 of 13

2 Some of this is done today under the name of. Which has also been called Layer 7 Switching". Applications send packets between each other in ways to avoid the routing of the standard Internet. - To avoid poor performance given by the default Internet path. Organizations place machines strategically and route packets as they wish between those machines. - With default Internet routing between those machines. - But not default Internet routing on the end-to-end path. Organizations then have some control over their own performance. - But this may hurt overall Internet traffic engineering and design. 2. -Level Interconnection Sends small packets of information instead of files or large messages. Does not use intermediate application programs. Benefits: 1. Maps directly to underlying network hardware. 2. Separates data communication from applications. 3. Keeps entire system flexible. 4. Allows network managers to add new network technologies while keeping application programs unchanged, or vice versa (can change applications without requiring changes to the network). Based on the concept known as internetworking. Independent of the network hardware used at lower layers. Also independent of the nature of a network - LAN or WAN. Independent of applications at higher layers. Lecture 3, Page 2 of 13

3 Motivations No single network hardware technology can satisfy all constraints. Users desire universal interconnection (to be able to communicate to anyone anywhere). II. Internet Architecture Question: How are networks interconnected to form an internetwork? Two networks can only be connected if there is a computer physically attached to both of them. Called a router or gateway. This computer must forward packets between the networks. Lecture 3, Page 3 of 13

4 Functions of Routers R 1 forwards packets between Net 1 and Net 2 R 1 also forwards packets to Net 3 From Net 1 Destined for Net 3 Through Net 2 to R 2 Assumes R 2 will send the packet to Net 3 The main requirement of a router is to find the for every packet. Capabilities of Routers Routers need not know about every possible destination computer. Routers use information about the destination when forwarding a packet, not the destination. Must have information about how to route to every possible destination network. Obviously, there are much fewer networks in the Internet than there are hosts. Routers usually are small computers, usually dedicated only to routing. Lecture 3, Page 4 of 13

5 User's View From the viewpoint of a user, all computers are connected to one big network. User View All s are Equal Actual Structure From the Internet Architecture point of view, all networks are equal. All are capable of transferring packets. Will not choose one network as being better to transport packets than another. Does not consider - Throughput characteristics - Maximum packet size - Geographic scale - etc. All types of networks are considered one network each. TCP/IP hides the details of particular physical networks. Lecture 3, Page 5 of 13

6 Recall SMILES Scalability Manageability Intricacies Layering Efficiency Service Differentiation How is this internetworking concept good from a SMILES perspective? Easy to connect end-to-end Little complexity (low intricacy) Scalable Manageable Strictly follows layering principle. What are the limitations of this concept? No service differentiation May sacrifice efficiency Unanswered Questions Form of addresses that are globally recognized and unique (Chapters 4-5). Address of a network Address of an individual host How internet addresses relate to physical network addresses (Ethernet, FDDI, ATM, etc.). Formats of packets (Chapter 6). Handling of multiple packets at once. Error handling. How internet routers learn about routes. Lecture 3, Page 6 of 13

7 Summary Interconnected systems agree to a convention that allows each computer to communicate with every other computer. Allows use of intermediate networks to forward packets. The key piece of hardware is the network router which forwards packets between networks. Chapter 4 - Classful Internet Addresses III. Original IP Addressing To have universal communication (throughout the world), a computer must use a universally accepted naming/addressing mechanism. Naming can be done through alphanumeric names, numerical addresses, or route signifiers. People prefer Easier to remember But can be very bulky - 10's of octets, 100's of bits Compact numerical addresses are better More efficient to process Less is used. The Domain Name System (DNS - Chapter 23) provides a mechanism for using alphanumeric names and translating them into physical addresses. Alphanumeric names are used to start a communication session. But only the numeric address is used in every packet. IP Addresses Original Addressing Scheme A 32-bit integer address is used Called the Internet Address or IP address. Each host is assigned a unique IP address. All IP addresses in a given network share a common prefix (first set of bits). Lecture 3, Page 7 of 13

8 Each address is a pair (netid, hostid) netid identifies the network hostid identifies a host on that network. Three main types of netid's were used Class A, B, and C networks. Figure 4.1 Example: Class A - Begins with a "0" - Next 7 bits are for the netid (bits 1 through 7). - Last 24 bits are for the hostid (bits 8 through 31). - How many possible Class A networks are there? 2 7 = How many Class A hosts are possible? 2 24 = 16,77,216 => 16 million - Not all bit sequences for hostid are valid, however, to identify a specific host. Major benefit of this approach: Each address is self-identifying. - Just by looking at the first 3 bits, one can tell the class of a network. - By knowing the class, then it is easy to parse the netid and hostid. Different sizes of networks can be obtained as organizations need them. Lecture 3, Page 8 of 13

9 Scalability Manageability Intricacies Layering Efficiency Service Differentiation How does this approach affect how well addressing SMILES? Scalable Intricacy easy to parse address Poor efficiency wastes addresses because of fixed blocks. Manageable network admin can decide hostid s on its own Note: To be precise, an IP address does NOT identify a host. If a host is connected to two networks, it will have two addresses. - One address for the physical connection to each network. - Each address will have a different netid. - Called a multi-homed host. - Example? IP addresses do not specify an individual computer, but rather a connection (network interface) to a network. Lecture 3, Page 9 of 13

10 D G B A E F C Specialized IP Addresses address hostid 0 is never assigned to an individual host - This address is used to refer to the network itself. - All bits of hostid are 0. - Therefore, an internet address can be used to refer to either networks or hosts. - Example: The address of the Class A network for IBM is Directed broadcast address Use this address to send a packet to all hosts on a network. Signified by a hostid with bits that are all 1's. One packet is sent to the network, then the devices in the network broadcast the message throughout the network. - Some types of networks inherently support broadcast, like Ethernet. - Some network hardware does not support broadcast at all so the broadcast address would not work there. Powerful but somewhat dangerous. - RFC 2644 modified the original definition of directed broadcast A router MAY have a configuration option to allow it to receive directed broadcast packets, however this option MUST be disabled by default, and thus the router MUST NOT receive Directed Broadcast packets unless specifically configured by the end user. - A damaging way to disable a non-broadcast network (i.e., ATM, Frame Relay, etc.) with a flood of traffic could be to use broadcast addresses. - One could explicitly allow directed broadcast, but should carefully filter the packets that are allowed to use it. Lecture 3, Page 10 of 13

11 Limited broadcast address Also called local broadcast address. Is sent from a network to all others in a network. Full address is all 1's (all 32 bits) - No specific netid is specified. Can be used at startup before a host knows its own netid. Use of zeroes and ones (see RFC 1700 for clarification, if needed) 1's mean "all" - Never used as a source address. - A packet cannot come from all hosts. 0's mean "this" - Used as a source address. - hostid=0 means packet came from this host, even when hostid is not known. - netid=0 means packet came from this network. - Only allowed to be used at system startup (bootstrapping) as a source address (i.e., from this network or host). Subnet addressing Allows multiple small physical networks to be identified within an overall netid will see this more later. Supernetting Allows division of netid and hostid at arbitrary points - Not just Class A, B, or C specifications. - Also called classless addressing under the overall name of Classless Inter-Domain Routing (CIDR). Will look at this in Chapter 9. Multicast Addresses Shown as Class D on Figure 4.1. Packet is delivered to a specific subset of hosts. This is discussed in Chapter 16 (we will likely not cover this topic). Lecture 3, Page 11 of 13

12 Key weaknesses of original Internet addressing: Mobility If a computer is moved to another network, it needs a new IP address. Partial solution: Mobile IP allows a computer to use a temporary address. Outgrowing Class C addresses Multi-homed hosts A host with multiple IP addresses can be reached multiple ways. If a path to the host is lost, an alternate path cannot be used unless its other IP addresses are known. Dotted Decimal Notation Four decimal integers are used, separated by decimal points is the IP address for Loopback Addresses prefix is to be used for loopback. A message intended never to leave a host. Used for testing TCP/IP communication between processes in a host. A packet with a network 127 address should never appear in a network. 127 is confusing anyway. It would be all 1 s for the netid, which is allowed but ambiguous. This reduced the number of Class A addresses that were available. Lecture 3, Page 12 of 13

13 IP address assignments must be unique As long as the network is not completely private. Private network operators can assign numbers as they wish. But must be sure they will never connect to the public network. Originally controlled by IANA (Internet Assigned Number Authority) until Now controlled by ICANN (Internet Corporation for Assigned Names and Numbers) A list of addresses assigned to particular organizations can be seen at Assigned based on the size of network requested. ISP's may have a Class A address and assign subnetwork addresses as they wish to their customers. Using some of the hostid bits. ICANN only deals with. can be assigned within a network as the owner wishes - Owner does not need to notify ICANN Next lecture: Mapping Internet addresses to physical addresses (Chapter 5). Then we will look specifically at the Internet Protocol. Lecture 3, Page 13 of 13