Distributed Systems Interconnec=ng Them Fundamentals of Distributed Systems Alvaro A A Fernandes School of Computer Science University of Manchester

Transcription

1 Distributed Systems Interconnec=ng Them Fundamentals of Distributed Systems lvaro Fernandes School of Computer Science University of Manchester

2 Goals 1. To highlight the role of the interconnect in characterizing the no=on of a distributed system 2. To consider how different classes of distributed systems arise from the scale of the interconnect 3. To explain how the most familiar, large- scale interconnect in existence, i.e., the Internet, is a sokware- driven interconnect over a worldwide physical fabric 4. To introduce the no=on of a protocol stack over a packet switched network and instan=ate it with the best known Internet protocols.

3 Interac=on: Goals and Modes Interac=on is for coordina(on and coopera(on, implies that the rela(onships between events in the (meline of each interac=ng process maoers, and may lead to compe((on, which in turn implies the need for management if global goals are to be achieved. Interac=on can be by sharing an address space, or by synchronous communica(on mechanisms (e.g., rendezvous, remote procedure calls), or by message passing (in which case protocols at different abstrac=on levels are relied upon to deal with transport, rou=ng, integrity security, reliability, etc.)

4 Fundamental Difference If components are spa=ally apart, interac=on implies an interconnect, which centralized systems do not need to take into account explicitly. The crucial new element in distributed computa=on compared to the centralized case is the need to consider an interconnect between different elements.

5 No Interconnec=on In the basic model, without an interconnect, a single algorithm has (cost- neutral) access to a single data space. In computer architecture, this is oken referred to as SISD (single instruc=on from an instruc=on pool onto a single item from a data pool). LGORITHM DT L G O D LGO T L G O Older PCs and mainframes were of this kind, newer ones are not, as they have mul=ple cores. D T DT

6 Interconnec=ng Storage If we introduce an interconnect, we can, e.g., apply an algorithm to dis=nct par==ons of a data space we have distributed for that purpose. Now, access to the data is not cost- neutral anymore: there is a cost in going through the interconnect. In computer architecture, this is oken referred to as SIMD (single instruc=on from an instruc=on pool onto a mul=ple items from a data pool). Modern GPUs are of this kind. D LGORITHM DT L G O T D LGO T L G DT O

7 Interconnec=ng Processors We could also use the interconnect, not on par==ons of the data but rather, to apply several algorithms to the same data. ccess to the data is again not cost- neutral: there is a cost in going through the interconnect. In computer architecture, this is oken referred to as MISD (mul=ple instruc=ons from an instruc=on pool onto a single item from a data pool). This is not as widely useful, but can be used in fault tolerance. D LGORITHM DT L G O T D LGO T L G DT O

8 Interconnec=ng Machines The inherently- distributed system case is one in which an interconnect binds together different centralized systems taken as autonomous components. The interconnect is s=ll not cost- neutral, but the grain of func=onality is much larger now: the cost is paid for complete computa=ons, not just data accesses. In computer architecture, this is oken referred to as MIMD (mul=ple instruc=ons from an instruc=on pool onto mul=ple items from a data pool). Modern PCs are of this kind. D LGORITHM DT L G O T D LGO T L G DT O

9 Interconnects at Various Scales Chip: a bus connects, e.g., mul=ple cores. Parallel Machines/ ppliances: a very fast interconnect (e.g., InfiniBand) binds together mul=ple, specially- assembled compu=ng and storage resources. Clusters of worksta=ons (COW): the interconnect is now across racks that pile up commodity machines. Network of worksta=ons (NOW): the interconnect is now a local area network (LN), such as an Ethernet. Web/Grid: the interconnect is now a wide- area network (WN) such as the Internet.

10 Networks as Interconnects The distributed systems that are most familiar to everyone can be understood as MIMD, also called shared- nothing, architectures. nother feature is that the interconnect in their case is a full- blown network, not just a piece of electronics or advanced cabling technology. bove all, in their case, the network exists independently as a complex, physical fabric, the use of which must then be mediated by complex, sophis=cated sokware.

11 The Physical Fabric vbns OC3 phone/ cable ISP point-ofpresence T 3 trunk network access point T 1 trunk home ethernet

12 The Physical Components From the smaller to the larger (and ignoring wireless elements), individual machines in homes connect to a server in the service provider (ISP), using dial- up lines (DS0), or cable (ISDN = 2*DS0), or digital subscriber lines (DSL). In organiza=ons, LNs are formed using Ethernet as a fabric and connected to a point- of- presence (POP) through (op=cal) T1 = 24*DS0 trunks. One step above we find T3 = 28*T1 trunks, giving access to network access points (NP) that are linked by very- high- speed backbone network services (vbns) with links like OC3 = 84*T1, and above.

13 Machines are Nodes that Host Components host 1 host 2 host 3 host 4

14 Forming Networks The Internet is a packet switched network. network (of networks) is formed by the routers that link them. Informa=on from an applica=on is broken down into packets (also called segments, or frames, or blocks) to which control data is added (typically a header) to help the packets reach their des=na=on (across various autonomous, physical networks), there to be reassembled before delivery to the applica=on.

15 Rou=ng Packets On their way, packets pass through routers. router examines a packet s des=na=on (in the header) and, taking into account of, e.g., traffic volumes, forwards the packet. The packet is forwarded to a router that is closer to the des=na=on and on the way to which there seems to be a lightly- loaded route. Routers, therefore, are crucial in avoiding conges=on due to compe==on between very, very many sources for a route to one target (or just a few).

16 The SoKware Stack over the Fabric application transport internet link physical

17 Protocol Layers The Internet can be understood as having four main protocol layers. The applica(on layer is the one that only deals with messages in the form that the applica=on sends and receives. The transport layer breaks a message down into packets and wraps it with control informa=on. The internet layer may break data further in order to fulfill its main task, i.e., to route packets forward to their des=na=on. The link layer oversees the working of the physical fabric and may request and send confirma=on of error- free transmission.

18 Example Protocols at Each Layer application transport internet link physical http tcp ip mac b

19 Packet Encapsula=on at each Layer udp header data udp data http udp ip header ip data ip frame header frame data frame footer mac

20 Protocol Instances (1) Each protocol layer can be instan=ated by many dis=nct, specific protocols. The best known applica=on layer protocol is the hypertext transfer protocol (HTTP), used to retrieving text documents by link traversal. SMTP + POP3/IMP are used for ; VoIP for audio. The best know transport protocol is the transmission control protocol (TCP), though the user datagram protocol (UDP) is widely used where it is most suited for. The internet protocol (IP) is the almost universal internet layer protocol for rou=ng in the Internet.

21 Protocol Instances (2) Some basic differences between TCP and UDP include: TCP is connec(on- oriented, UDP is not. TCP is reliable, it ensures that the message payload will be: complete (by reques=ng acknowledgement of receipt of a packet and resending if it fails to receive that acknowledgement), intact (by adding a checksum to each packet that reveals en- route corrup=on if it happens), in the right order and without duplica(on (by adding a sequence number), even if it resends unnecessarily. UDP only ensures that there will be no corrup=on. UDP is oken used when the payload can be made small, the set- up of communica=on must be quick, the loss (or out- of- order arrival) of the odd packet is not problema=c (e.g., in audio and video transfer).

22 Mul=- hop Rou=ng host 2 gateway point-ofpresence (POP) network access point (NP) NP POP host 3 ethernet T1 T3 OC3 T3 T1

23 Summary 1. In distributed systems, interconnects allow interac=on. 2. Explicit considera=on of the purpose and impact of the interconnects, at various scales, is a fundamental difference between distributed and centralized systems. 3. network of networks (like the Internet) is a large- scale, oken worldwide, interconnect. 4. The Internet is a sokware- driven en=ty over a public physical fabric. 5. It uses packet switching based on a 4- layer protocol stack, of which HTTP, TCP/UDP, and IP are among the best known instances.