Distributed Systems: Networking

Transcription

1 Distributed Systems: Networking 3 o IEIC Pedro F. Souto (pfs@fe.up.pt) February 22, 2010 Distributed Application Definition An application with two or more processes that execute in different processors that to not share memory. Corollary Processes of a distributed application communicate with each other by exchanging messages. Process Process essage Is an indivisible sequence of bits. The format and meaning of a message is specified by the communications protocol.

2 essages and Networks The transport of a message from its source to its destination is done via a computer network. Host Host Application Host Application Channel Host Host Often, we can abstract the network as a communications channel. What are the properties of that channel? Building blocks Nodes Computers and special purpose computers, such as: hubs switches routers Links Coaxial cable, optical fiber, air/space point-to-point multiple access

3 Computer networks A network can be defined as: two or more nodes connected by a link, or recursively, as two or more networks connected by two or more nodes Computer network: Example

4 Addressing and Routing In packet-switched networks, communication between end-hosts is by means of messages/packets that are stored and forwarded by the network nodes. To help locate the source and the destination of a packet, nodes have addresses Usually, they are unique. They can be of different types: unicast: a single node broadcast: all nodes on a network multicast: some subset of nodes on a network Forwarding is the process of sending a message received on a link to another link. Routing is the process of determining on which link a message should be forwarded. Statistical ultiplexing Problem How can a network support multiple distributed applications, each using its own logical channel? Solution Use statistical multiplexing. I.e.: Links are shared among packets. Packets from different sources are interleaved on link. Packets contending for a link are buffered.

5 What can go wrong in a network? essage loss this can have different causes: Bit-level errors, as a result of electromagnetic interference, e.g. Packet loss, as a result of congestion, e.g. Link and node failures. Other problems these may be almost as bad as message loss: essages are delayed essages are delivered out of order essages can be eavesdropped Layering Problem How to design a network to support the communication between computers at different corners of a network (e.g. Internet)? Solution Decompose the problem into smaller problems, using layering: Each layer adds some functionality by building on the functionality of the layers below. The functionality of each layer is provided by processes that execute in different computers: A computer network is itself a distributed application Host 1 Layer 3 protocol Host 2 Layer 3 Layer 3 Layer 2 Layer 2 protocol Layer 2 Layer 1 Layer 1 protocol Physical medium Layer 1

6 Protocols and encapsulation In order to provide the functionality of a layer, processes in different computers have to execute a protocol, which specifies: 1. The format and the meaning of the messages exchanged. 2. The rules used to exchange these messages. In addition, a protocol offers a service interface that is used by the layer above it: Host 1 Host 2 Layer j Service interface Layer j Layer i Protocol Layer i Protocols and encapsulation Usually, protocols use a mechanism called encapsulation: Layer j Layer j H i T i Layer i Layer i H i T i H i T i When a layer receives a message to be sent on the network from the layer above it, it: 1. adds some protocol-dependent data usually at the head, but possibly also at the tail of the message; 2. passes the resulting message to the layer below it, if any. Otherwise, it sends that message to the (same layer at) the next hop Upon reception of a message from a layer below, it does the reverse.

7 Layer odels Internet odel This is the 5-layer model upon which the Internet is built. OSI Reference odel This is a 7-layer model. Roughly, it breaks: The 4th layer of the Internet model into 3 layers. Application Transport Network Link Physical Internet odel Application Presentation Session Transport Network Link Physical OSI Reference odel Physical Layer Concerns the way bits are sent in the physical medium, e.g. encoding/modulation schemes, shape of signals, voltage levels,, etc. This is the domain of the electrical/telecommunications engineer.

8 Link Layer Concerns the way messages (usually called frames) are exchanged between two devices (computers or network devices) that are directly connected to the same physical medium. When the links are multiaccess, it is common to divide the link layer in two sublayers: edium Access Control Which specifies a protocol to prevent the interference between devices that transmit on the same physical medium When two devices transmit simultaneously on the physical medium their signals will be composed, and the resulting signal may be interpreted as completely different from either of the composing signals. Logical Link Which concerns other functionality, such as the structure of the frame, including mechanisms to determine the beginning and the end of a frame, as well as to determine whether a frame has errors. Network Layer What it does? Its main functionality is to allow the communication between devices that are not directly connected, i.e. routing. Why do we need it? For reasons related to: Physics laws Scalability Ease of deployment Ease of management Cost directly connecting each computer to all other computers is not an option. Note Sometimes, the data link layer also includes routing functionality. The main differences have to do with: The distance between devices, e.g. whether devices are in the same building or in the same town. Which addresses are used for routing: layer 3 vs. layer 2 routing.

9 Transport Layer What it does? Essentially it provides end-to-end communication, i.e. communication between application processes. Whereas the network layer supports only communication between end hosts. In addition, it may provide additonal functionality/properties, e.g.: Reliability i.e. that messages are not lost nor duplicated Order i.e. that messages are delivered in the order they are sent Fragmentation/reconstruction i.e. to allow the exchange of messages whose size is larger than the maximum size of the messages ( packets) used at the network layer Application Layer This layer provides communication services (and protocols) supporting specific applications. E.g.: to access documents in the Web; to transport messages from the sender to the destination s mailbox; to transfer files. In the OSI-R this layer is divided in three: Session layer which supports (application) sessions on top of the channel provided by the transport layer. Presentation layer which deals with issues related to the format of the data exchanged (e.g. number of bits used for integers, the order of multi-byte integers, the code used for characters,...) Application layer which supports the different applications. The idea was to factor out some common functionality, so that different application could use common session and presentation services.

10 Path of a message in a network Further Reading Section 4.1 (Fundamentals) of Tanenbaum and van Steen, Distributed Systems, 2nd Ed. Chapter 1 of J. Kurose and Keith Ross, Computer Networking: A top-down approach featuring the Internet, 3rd Ed., International Edition, Addison-Wesley, 2005 Chapter 1 of L. Peterson and B. Davie, Computer Networks: A systems approach, 2nd Ed., organ Kaufman Publishers, 2000.