Transcript ppt - NOISE
Intro (continued) and Design Principles Nick Feamster CS 3251: Computer Networking I Spring 2013 Step 1: How to Draw a Network Node Link Node • Electrical questions – Voltage, frequency, … – Wired or wireless? • Link-layer issues: How to send data? – When to talk – can everyone talk at once? – What to say – low-level format? – Stay tuned for lecture 5 • Okay… what about more nodes? Some Important Concepts • Protocols • Layering and the Internet Protocol Stack – The Internet’s “Narrow Waist” • Resource Sharing – Circuit switching vs. packet switching – “Best effort” networks What is a Protocol? • The syntax and semantics by which hosts and nodes agree on how to talk – Must be standardized and agreed upon by all parties – Standardization process • IETF Requests for Comments (RFC) • De-facto standards • Format of messages • Expectations for message delivery 4 From Signals to Packets Analog Signal “Digital” Signal Bit Stream Packets 0 0 1 0 1 1 1 0 0 0 1 0100010101011100101010101011101110000001111010101110101010101101011010111001 Header/Body Packet Transmission Sender Header/Body Header/Body Receiver Analog versus Digital Encoding • Digital transmissions. – Interpret the signal as a series of 1’s and 0’s – E.g. data transmission over the Internet • Analog transmission – Do not interpret the contents – E.g broadcast radio • Why digital transmission? Layering • Helps manage complexity • Each layer: – Relies on services from layer below – Provides services to layer above • For example: IP (network) layer – IP relies on connectivity to next hop, access to medium – IP provides a datagram service • Best effort delivery • Packets may be lost, corrupted, reordered, etc. – Layers on top of IP (e.g., TCP) may guarantee reliable, in-order delivery 7 Layering Mechanism: Encapsulation User A User B Application (message) Get index.html Transport (segment) Connection ID Network (datagram) Source/Destination Link (frame) Link Address • This can be more complex • Example: Network layers can be encapsulated within another network layer 8 The Internet Protocol Stack • Need to interconnect many existing networks • Hide underlying technology from applications • Decisions – Network provides minimal functionality – IP as the “Narrow waist” email WWW phone... SMTP HTTP RTP... Applications TCP UDP… IP ethernet PPP… CSMA async sonet... Technology copper fiber radio... 9 The “Narrow Waist” • Facilitates interconnection and interoperability • IP over anything, anything over IP – Has allowed for much innovation both above and below the IP layer of the stack – Any device with an IP stack can “get on the Internet” • Drawback: very difficult to make changes to IP 10 Resource Sharing • How? Multiplexing – Switched network – Party “A” gets resources sometimes – Party “B” gets them sometimes • Interior nodes (“Routers” or “Switches”) arbitrate access to resources 11 An Age-Old Debate! Circuit Switching • Resource control, accounting, ability to “pin” paths, etc. Packet Switching • Sharing of resources, soft state (good resilience properties), etc. It is held that packet switching was one of the Internet’s greatest design choices. Of course, there are constant attempts to shoehorn the best aspects of circuits into packet switching. Examples: MPLS, ATM, IntServ QoS, etc. Circuit Switching • Resources are reserved • Source first establishes a connection (circuit) to the destination • Source sends the data over the circuit – Constant transmission rate • Example: telephone network – Early early versions: Human-mediated switches. – Early versions: End-to-end electrical connection – Today: Virtual circuits or lambda switching 13 Resource Sharing in Circuit-Switched Networks • Frequency-Division Multiplexing (FDM) – Link dedicates a frequency to each connection – Width of this frequency band is called “bandwidth” – We will discuss the capacity in Lecture 10 • Time-Division Multiplexing – Each circuit gets all of the bandwidth on a link for brief periods of time 14 Circuit Switching • Advantages – Fast and simple data transfer, once the circuit has been established – Predictable performance since the circuit provides isolation from other users • Guaranteed bandwidth • Disadvantages – What about bursty traffic? – Users with differing needs for bandwidth – What if all resources are allocated? 15 Packet Switching (Our Focus) • Resources are not reserved • Packets are self-contained – Each has a destination address – Source may have to break up single message • Each packet travels independently to the destination host – Routers and switches use the address in the packet to determine how to forward the packets 16 Resource Sharing: Packet Switching • Statistical multiplexing • Switches arbitrate between inputs • Can send from any input that’s ready – – – – Links are never idle when traffic to send Efficiency! Requires buffering/queues Implies a service model/discipline (Lecture 21) 17 Type of Packet Switching: Datagrams • Information for forwarding traffic is contained in destination address of packet • No state established ahead of time Alternatives • Source routing: Complete route is contained in each data packet • Circuit Switching: Signaling protocol sets up entire path out-of-band. (cf. the phone network) • Virtual Circuits: Hybrid approach. Packets carry “tags” to indicate path, forwarding over IP Forwarding: Packet-Switched Networks • Each packet contains a destination in the header – Much like a postal address on an envelope • Each hop (“router” or “switch”) inspects the destination address to determine the next hop • Will a packet always take the same path? • How do the hops know how to forward packets? 19 Delay in Packet Switched Networks • Four contributors to hop-by-hop delay – Processing: Lookup, etc. – Queueing: Time the packet must wait before being transmitted – Transmission: time to push the packet onto the link – Propagation: time for the packet to propagate from A to B • End-to-end performance metrics – Latency – Throughput 20