Network and Telecomm Notes

POTS: Plain Old Telephone Service

Network and Transport Layers

Work together similarly to how Physical and Datalink work together.

Layer 3: Network

Home of our beloved protocol: IP.

Also:

• IPv6
• IPX/SPX
• NetBEUI
• AppleTalk 

Concerned with getting data from one destination to another.  Specifically it routes data through multiple hops.

Two ways to move data:

• Connection-oriented. (TCP)  More reliable.
• Connectionless (UDP)  Less overhead.

Virtual Circuit: Both ends are aware of the path that their data will take through the network.

In connection-oriented all packets take the same path and are guaranteed to be received correctly.  In connectionless, packets can take any path and there is no guarantee of reception or non-corruption.  However there is less overhead so it is significantly faster.

SNA: System Network Architecture.  Connection-oriented, developed by IBM.  Assumes all devices are aware of other networked devices.  Packets are routed through a specific path.  Considered clumsy, but reliable.

Majority of protocols are connectionless.

Routing

Routers are layer 3 devices (operate on IPs instead of MACs)

Subnet: Which bits in an IP address (192.168.1.1) designate a node. (255.255.255.0)  Also can be expressed as a number from 0-32 to indicate the number of bits (24).  OR mask gives you network address (192.168.1).  NOR gives you node address (1).

Router is aware of it's own IP address and the IP and MAC addresses of the nodes on the network (Routing Table).

If an address is part of the network address, the router will search it's routing table for the machine and deliver the packet.

Routing Protocols

• RIP
• BGP: Border Gateway Protocol

If not, there is another interface that is connected to another router, which has it's own network connected to it.

Default Gateway: The address where packets will be forwarded if the destination does not reside on the local network (as per the subnet).  Both PCs and routers use this, routers use this if they do not know where to send the packet.

Each router queries it's interfaces and discovers the networks around it.  Then it is able to route packets accordingly.  It also knows which routers can route a packet to another router that it belongs to.

Metrics: Tie-breakers on deciding which path to take.  Determined by a pre-chosen "cost" of each path.  Lowest metric wins.  Multiple factors can determine cost, such as bandwidth, cost of transmission, number of hops, etc.

ARP: Address Resolution Protocol.  Used to resolve IPs to MACs.

Layer 4: Transport

The more sophisticated Network is, the less sophisiticated Transport needs to be.

TCP is one example. 

Error Checking

Making sure every packet reaches the destination intact.

Sequencing

Resequences packets so they are in the correct order on the receiving end.

Addressing

Port: Not physical, a virtual 16-bit integer.  An id that identifies a protocol.

Well Known Ports: Ports that are commonly associated with a specific protocol (HTTP 80, FTP 21, HTTPS 443, etc)

Three-way Handshake

The process by which the Transport layer establishes it's virtual circuit.

• (A->B) Connection request
• (B->A) Acknowledgement
• (A->B) Acknowledgement of the acknowledgement
• Circuit established, meaningful traffic begins to flow.
• Meaningful traffic ended.
• Both PCs verify all data received.

Data Prioritization

Data is placed in a buffer when it arrives, either normal or expidited.  The latter is processed first.

Flow Control

• I got the packet, send more
• I got the packet, buffer full, don't send
• I didn't get the packet, resend
• I didn't get the packet, don't send, buffer full

Retransmission

Selective retransmission used as opposed to whatever that other method was.

Failiure Recovery

• Failure of network.  One hop on the virtual circuit becomes unavailable.  Another circuit can be established.  Prof thinks file resuming is built into TCP (actually it's a feature of HTTP and FTP).
• System failure.  One of the computers shuts down.
  

Multiplexing

Allows the transmission of data to multiple ports on a remote machine.  Or from one machine to multiple machines.

TCP and UDP

TCP's header is 20 octets, UDP's is 8.  UDP becomes more ubiquitous as errors become less.  It is used where retransmission is detrimental (real-time data).  Voice-over IP, games.