EE228a - Lecture 2 - Spring 2006 Internet

Transcription

1 EE228a - Lecture 2 - Spring 2006 Internet Jean Walrand, Scribed by Bonnie Zhu 1 Abstract In today s class, we cover the topic of the Internet with emphasis on review of TCP/IP. ( A good reference is [1] and a slightly more readable but less comprehensive is [2] I. NETWORKS Communication networks are usually defined by their size and complexity. From top-down, we have Wide area networks(wan) These networks connect computers and other terminals over large distances. They often require multiple communication connections, including microwave radio links and satellite. WAN consists of the back-bone networks that are connected via high capacity fiber optics cables. Metropolitan area networks(man) These networks are interconnected within a WAN and are spread around, say, a town or city. This kind of network is a high speed network using optical fiber connections. Local area networks(lan) These networks are components of a MAN to connect computer and other terminals distributed in a localized area. The connection is usually cable (twisted pair copper) or fiber. Figure 1 shows the possible components of a LAN Fig. 1. The possible components of a LAN II. HISTORY Table I lists the milestones in the development of the Internet. TABLE I MILESTONES IN DEVELOPMENT OF INTERNET 1962 L. Kleinrock proposes Packet Switching 1966 L. Roberts proposes architecture to Darpa 1969 First demonstration of packet switching: 4-node Arpanet 1969 S. Croker introduces RFCs (managed by J. Postel) 1972 R. Kahn proposes an open architecture: Inter-networking with stateless routers, best effort, no control plane 1973 Kahn and V. Cerf propose early ideas of IP (32-bit address) and TCP (end-to-end ACKs with a window scheme) 1973 R. Metcalfe invents Ethernet 1982 IGP and EGP Late 1970s, early 1980s Berkeley develops BSD, a modified implementation of UNIX that includes TCP/IP 1983 Arpanet switches to TCP/IP 1983 P. Mockapetris invents DNS 1988 Van Jacobson fixes TCP Internet supported by NSF and other agencies 1993? T. Berners-Lee invents WWW 1995 Internet is privatized 2000 WiFi,VoIP, P2P

2 2 III. KEY IDEAS We will address some of the key ideas to the development of Internet. Packet Switching End-to-End error and flow control internetworking Multiple Access DNS P2P VoIP A. Packet Switching Figure 2 illustrates the idea of packet switching. Packets (units of information carriage) are individually routed between nodes over data links which might be shared by many other nodes. Once all the packets forming a message arrive at the destination, they are recompiled into the original message. This contrasts with circuit switching, which sets up a dedicated connection between the two nodes for their exclusive use for the duration of the communication. Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it s sent. This is the case with most real-time data, such as live audio and video. Because packets follow different routes, packet switching introduces different delays and jitter. It is more efficient and robust for data that can withstand some delays in transmission, such as messages and Web pages. In Packet Switching, the intermediate routers are required to know the networking structures, i.e., how the connections are made between huge numbers of servers and clients. This turns out to be an issue for graph encoding, as well as for the applications in DNS. In contrast to traditional methods relying on the canonical ordering of a connected plannar graph, orderly spanning trees are much more adaptable to create efficient and easy encodings [3], [4]. Fig. 2. Packet Switching B. End-to-End Error and Flow Control The purpose of end-to-end error and flow control is to ensure complete data transfer. As shown in Figure 3, node A can have up to N packets for node B that have not been acknowledged. By selecting N, A adjusts the rate of transmissions to enable B handle all the incoming data without buffer-overflow thus limit congestion. This is particularly important where the sending device is capable of sending data much faster than the receiving device can receive it. Error control is implemented as two separate functions: error detection and retransmission. As depicted in Figure 3, if ACK is late, i.e., an error is detected, node A retransmits packets. Fig. 3. End-to-End Error and Flow Control C. Internetworking The goal of internetworking is to connect different networks. As illustrated in Figure 4, for a packet travels through two networks, only local addresses are involved when it s within one network, then router that connects two networks looks at IP address to transfer it to the other network. Once the packet is delivered into the other network, only local address are used again.

3 3 Fig. 4. Internetworking D. Multiple Access The scheme allows temporary access to the network by individual users, on a demand basis, for the purpose of transmitting information while sharing one medium. (e.g.,wifi,ethernet,..). As illustrated in Figure 5, if there is a collision detected, then random waiting time will be allocated after collision. Or to avoid collision, when the network is detected to be busy, the scheme ensures that it will wait till idle before more transmission. Note that WiFi uses collision avoidance whereas wired schemes usually use collision detection. Fig. 5. Multiple Access E. DNS:Domain Name System The Domain Name System(DNS) translates domain names into IP addresses, such as google.com into The directory of DNS is a distributed database. The mapping between domain name and IP address can depend on source address and also can be done to multiple addresses. F. P2P: Peer-to-peer The purpose of P2P scheme is to allow all peers become servers thus to utilize the computing power and bandwidth of the participants in the network. The examples can be a case that each peer knows the addresses of a set of peers: his friends or a case that to find a file, peer A asks his friends who in turn ask their friends, and so on, until someone, B, says he has the file (can limit search); A then asks B for the file. Many variations are possible. The key idea is that there is no specialized server; every client becomes a server G. VOIP:Voice over IP The idea of VoIP is to place phone calls over Internet. It should be cheaper because the Internet infrastructure is much lighter than that of the phone network. It has the following steps: Gateway converts phone signal into IP packets and vice-versa Protocol to convert phone number into gateway IP address Gateway converts usual control signals (dial tone, ringing, busy, ) into IP packets and vice-versa IV. PROTOCOLS Protocol is a convention or standard that controls or enables the connection, communication and data transfer between two endpoints. There are an array of protocols, among them we list several most important and widely used ones, such as IP: Internet Protocol

4 4 TCP: Transmission Control Protocol UDP: User Datagram Protocol HTTP: Hypertext Transfer Protocol FTP: File Transfer Protocol ARP: Address Resolution Protocol Figure 6 shows the protocol stack. Fig. 6. Protocol Stack Figure 7 illustrates a more comprehensive view in the OSI network layers Fig. 7. Network Protocols IP:Internet Protocol The Internet Protocol delivers packets between any two hosts, organizes address and manages routing tables. The 32 bits IP addresses are arranged so that prefix determines next router toward destination. The routing algorithm employed by IP is essentially shortest path. and is done in two levels: nodes are grouped; shortest path inside each group and across groups. The routing tables updated periodically to adjust to changes IPv6 follows IPv4 as the second version of the Internet Protocol to be formally adopted for general use. IPv6 supports addresses while IPv TCP:Transmission Control Protocol TCP Implements reliable delivery of byte stream between hosts. It multiplexes multiple connections to and from a host by adding a port number. It involves end-to-end retransmissions to guarantee reliable and in-order delivery of sender to receiver data. It regulates flow to avoid congestion: at destination by flow control; at routers by congestion control. By using ACK, the acknowledgement receipt of a packet, TCP controls the number of unacknowledged packets to guarantee its reliability. Also, TCP implements flow control mechanism to use bandwidth efficiently. Figure 8 illustrates the idea of TCP service. TCP is the intermediate layer between the Internet Protocol below it, and an application above it. Applications send streams of bytes to TCP for delivery through the network, and TCP divides the byte stream into appropriately sized segments. TCP then passes the resulting packets to IP. TCP makes sure the data delivery is ordered, reliable and well-paced. Congestion Control: As shown in Figure 9, in the reality of networks, flows share links. Then it s natural to ask the question of how to share the links bandwidth. Congestion control is a resource allocation problem involving many flows, many links, and complicated global dynamics. Broadly speaking, the idea of TCP congestion control is for each source to determine how much capacity is available in the network, so it knows how many packets it can safely have in transit. TCP connection has window to control number of unacknowledged packets. The sending rate depends on the ratio of window size to RTT(round trip time). So TCP can vary window size to control sending rate. The rate adjustment algorithm depends on congestion or not and tries to solve three subproblems: finding fixed bandwidth, adjusting to bandwidth variations and sharing

5 5 Fig. 8. TCP Service Fig. 9. Flows at links bandwidth. The basic idea is to increase rate upon receipt of ACK of new data and to decrease rate upon detection of loss. Depending on what problem we are solving, we choose different increase/decrease functions. For the case of multiple flows, we want steady state to be fair, i.e. two identical flows end up with same bandwidth, then Additive Increase Multiplicative Decrease (AIMD) is a solution as shown in Figure 10 The important thing to understand about AIMD is that the source is willing to reduce its congestion window at a much faster rate than it is willing to increase its congestion window. While AIMD was originally proposed as an ad hoc solution to the congestion problem, it has recently been shown to be nearly theoretically optimal. In the phase of slow-start, which is called slow because it s from a cold starting point, rapidly increase rate until packet drop occurs to have an estimate of the bandwidth, the congestion window initially sets to 1 then increases exponentially upon receipt of ACK. When there is no congestion, the sending rate increases by one packet/rtt every RTT while it decreases by factor 2 when there s a congestion detected. When a new ACK is received, if congestion window (cwnd) is less than the threshold value, cwnd increases by one until it hits the threshold value (ssthresh), otherwise, i.e. in the phase of congestion avoidance, cwnd is incremented by 1/cwnd. The recovery mechanism used by TCP is a combination of fast transmit/recovery and time out. TCP retransmits after 3 duplicated ACK s to prevent expensive timeouts. At steady state, cwnd oscillates around the optimal window size. During timeout, the thresh hold value for slow start is decreased to half and cwnd is reset to be 1. Figure 11 illustrates the idea. One more refinement scheme utilized by TCP to achieve congestion control is Flow Control. Its purpose is to avoid saturating destination. Given a receiver advertised window RAW, the actual window = min {RAW, W } actual window open = actual window - OUT where OUT = Outstanding = Last sent - last ACKed W = Cong.Window from AIMD + refinements The scheme is shown in Figure 12 Figure 13 illustrates TCP phases. UDP User Datagram Protocol (UDP) unreliably delivers packets with error detection. It multiplexes multiple connections to and from a host by adding a port number. UDP adds error detection code for the packet. It doesn t involve any retransmission or flow control thus why it s unreliable.

6 6 Fig. 10. TCP congestion control - TCP Algorithm: AIMD Fig. 11. TCP Congestion Control Summary FTP File Transfer Protocol (FTP) enables reliable delivery of files between hosts. It converts file into byte stream for TCP. HTTP HyperText Transfer Protocol (HTTP) is the primary method used to transfer or convey information on the World Wide Web. The original purpose was to provide a way to publish and receive HTML pages. It sets up TCP connections and FTP to transfer files. Connections are closed after transfer. ARP Address Resoluton Protocol (ARP) discovers local address from IP address. One example of its usage as we seen in Figure 4 for internetworking, host 1.1 uses ARP to discover local address b of 1.2. The protocol carries out in two steps, Host 1.1 broadcasts request on local network: Who is 1.2? Host 1.2 replies to 1.1; local address of source (b) is in the packet. V. TECHNOLOGY In this section, we discuss the technical devices of networks including communication links and switches. A. Communication Links Figure 14 shows different types of links used in communication networks, such as optical fiber, coaxial cable and twisted pairs etc. Communication links convert bits string into signals that propagates as electromagnetic waves. Depends on the medium, the links can be categorized as the following, Optical: They typically utilize On/off pulses of light and equip with encoding to make sure there are enough transitions for the receiver to remain in sync. The capacity of optic fibers is up to 10Gbps over 100 km per wavelength and up to 128 wavelengths per fiber. Wired and wireless: They typically utilize pulses of sine waves at different frequencies or phases to encode groups of bits. Their capacities can be

7 7 Fig. 12. TCP flow control Fig. 13. TCP Phases Wires: 100 Mbps over 100 m Wireless: 100 Mbps over 30 m with multiple antennas (802.11n) B. Switches Switches include telephone switch and router as show in Figure 15 They are installed at various links such as optical, cable or wires. (While optical may work better in communication links, there is a problem with optical switching since it is not easy to buffer light pulses.) Switches loop up destination address and send packet to corresponding output port. They may perform some traffic policing such as limiting rate and some differentiated services such as assigning priority to VoIP packets. Depends on their task, Fast router : total throughput of 500 Gbps From a few to 128 ports. REFERENCES [1] W. Richard Stevens TCP/IP Illustrated: Volume 1 - the Protocols [2] Douglas Comer Internetworking with TCP/IP Vol. 1: Principles, Protocols, and Architecture [3] Y.-T. Chiang, C.-C. Lin, and H.-I. Lu, Orderly Spanning Trees with Applications, SIAM Journal on Computing, 34(4): , 2005 [4] X. He, M.-Y. Kao, and H.-I. Lu, A Fast General Methodology for Information-Theoretically Optimal Encodings of Graphs SIAM Journal on Computing, 30(3): , 2000.

8 8 Fig. 14. Communication Links Fig. 15. Telephone Switch and Router