Intro (continued) and Design Principles Nick Feamster CS 3251: Computer Networking I Spring 2013

Step 1: How to Draw a Network NodeLinkNode 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

4 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

From Signals to Packets Analog Signal “ Digital ” Signal Bit Stream Packets Header/Body ReceiverSender Packet Transmission

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?

7 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

8 Layering Mechanism: Encapsulation This can be more complex Example: Network layers can be encapsulated within another network layer Get index.html Connection ID Source/Destination Link Address User AUser B Application (message) Transport (segment) Network (datagram) Link (frame)

9 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” Technology Applications WWW phone... SMTP HTTP RTP... TCP UDP… IP ethernet PPP… CSMA async sonet... copper fiber radio...

10 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

11 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

An Age-Old Debate! Resource control, accounting, ability to “pin” paths, 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 Packet Switching Sharing of resources, soft state (good resilience properties), etc.

13 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

14 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

15 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?

16 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

17 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)

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

19 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?

20 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