Networking:

• So far, we've only worked on configuring specific hosts. This is easy in isolation from other hosts.
• The real action comes in network administration when we must coordinate actions of multiple hosts.
• To do this, we must understand the network that connects them.
• Reference: Burgess Chapter 3

How a network is born:

• Standards: encourage multiple vendors/bodies to agree on how the net works.
• Three main standards organizations:
  • IEEE: Institute of Electrical and Electronic Engineers.
  • ISO: International Standards Organization.
  • Internet RFC's: Requests for comments
• Standards formation: like passing a law in congress:
  • Propose idea.
  • Committee studies idea.
  • If good, propose as a standard.
  • Discussion.
  • Vote.
  • We have a new standard!

One small editorial comment on standards

• Standards committees are supposed to be made of technical experts.
• In practice, they're made of politicians, just the Internet kind.
• Funny story: the standards committee on C and C++ was working on `standardizing' floating point hexadecimal notation.
  • 0x34.2 = 00111000.0010 = 001110000010 / 010000 (base 2) !
  • this is silly enough, except that there is a conflicting standard:
    • a trailing 'f' means single precision (1.2f).
  • So what does 0x1f.ff mean?
    • 0x1f.f in single precision?
    • 0x1f.ff in double precision?
  • Still no resolution to this issue as far as I know!

Networking concepts

• Protocols: how network devices communicate.
• Layers: how one builds complex protocols from simple ones.
• Client/server computing: how network devices provide services.

Protocols

• A protocol is a method by which two devices achieve communication and mutual understanding.
• analogy is diplomacy: the process by which two countries communicate about their needs.
• simple protocol:
  • I say "hello".
  • You say "How are you?"
  • I say "Fine, thanks."
• meaning of protocol: everything's OK.

Layers

• Complex protocols are built incrementally in layers
• Each layer uses the features of the previous one.
• Rough example: speaking English:
  • Layer 1: meaning of words: dictionary
  • Layer 2: context: determine which definition to use based upon context of conversation.
  • Layer 3: task: determines which words to use to accomplish a specific mission.
• Rough example: party conversation.
  • Layer 1: smalltalk, establishing communication.
  • Layer 2: image, telling the stories you want to tell, projecting an image.
  • Layer 3: mission, accomplishing goals, etc.

Client/server computing

• Client: that which requires information/services.
• Server: that which provides required information/services.
• "It is illogical to withhold required information!" (Star Trek, the movie).
• Examples:
  • network file system: coordinate file users (clients) with file sources (servers).
  • name services: coordinate stations needing name translations (clients) with hosts providing name service (servers).

Network layers:

• start with physical wire.
• end at application level, e.g., "electronic mail".
• By layering more complex protocols on top of simpler ones.
• Key to understanding networking: OSI/ISO reference model.

OSI/ISO Reference Model:

 +---------------------+
 | Application Layer   |  how devices agree on the meaning of data
 +---------------------+     mayor
 | Presentation Layer  |  how devices agree on the format of data
 +---------------------+     town meetings
 | Session Layer       |  how neighboring devices establish channels of info
 +---------------------+     community cable
 | Transport Layer     |  how to avoid communication errors between devices
 +---------------------+     libel suits/lawyers
 | Network Layer       |  how non-neighboring devices communicate 
 +---------------------+     gossip
 | Data Link Layer     |  how neighboring devices communicate
 +---------------------+     talking over the fence
 | Physical Layer      |  how devices are physically connected 
 +---------------------+     back fences

Running joke among netadmins: the 9 layers of the OSI reference model:

 +---------------------+
 | Political Layer     |  how devices agree on what to spend. (you are here) 
 +---------------------+
 | Financial Layer     |  how devices agree on what to buy. 
 +---------------------+
 | Application Layer   |  how devices agree on the meaning of data.
 +---------------------+     
 | Presentation Layer  |  how devices agree on the format of data
 +---------------------+     
 | Session Layer       |  how neighboring devices establish channels of info
 +---------------------+    
 | Transport Layer     |  how to avoid communication errors between devices
 +---------------------+   
 | Network Layer       |  how non-neighboring devices communicate 
 +---------------------+  
 | Data Link Layer     |  how neighboring devices communicate
 +---------------------+
 | Physical Layer      |  how devices are physically connected 
 +---------------------+

General comments:

• network devices understand layers from the bottom up.
• the more levels a device understands, the more sophisticated it is and the more involved it is to configure.
• we roughly name devices according to the level of information they can interpret.

   +---------------------+
   | Application Layer   | \
   +---------------------+  +- `gateways' (application semantics) 
   | Presentation Layer  | /
   +---------------------+ 
   | Session Layer       | \
   +---------------------+  +- routers (global addressing)
   | Transport Layer     | /
   +---------------------+  
   | Network Layer       |  switches (local address) 
   +---------------------+  
   | Data Link Layer     |  hubs, repeaters (signal multiplexing) 
   +---------------------+         
   | Physical Layer      |  transceivers (signal conversion) 
   +---------------------+  
  

Internet concepts:

• Local area network(LAN): a small cluster of machines networked together within a site or building.
• local address: the address of a machine on a local-area network.
• Wide-area network (WAN): a network spanning multiple organizations or buildings.
• Internet: a global network connecting all machines in the world.
• global address: the address of a machine on the whole internet.

A brief map of internet technologies to ISO model:

 +---------------------+
 | Application Layer   |  IMAP, POP, TELNET, SSH, .... 
 +---------------------+  
 | Presentation Layer  |  RPC
 +---------------------+ 
 | Session Layer       |  Transmission Control Protocol (TCP)  \  Internet
 +---------------------+                                        + Protocol (IP)
 | Transport Layer     |  User Datagram Protocol (UDP)         /  (Internet
 +---------------------+                                           address) 
 | Network Layer       |  Media Access Control (MAC address: ethernet) 
 +---------------------+  
 | Data Link Layer     |  point-to-point communication 
 +---------------------+         
 | Physical Layer      |  point-to-point connectivity 
 +---------------------+  

Protocol stack

• This set of layers acts like a stack (``the protocol stack'').
• Each layer uses the one beneath it to communicate.
• Information to be sent starts at a particular level and gets sent downward in the stack until it is sent on the physical level.
• The receiver interprets the info by allowing it to propogate upward to the appropriate level.

   +---------------------+                         +---------------------+
   | Application Layer   | v                     ^ | Application Layer   |  
   +---------------------+ |                     | +---------------------+ 
   | Presentation Layer  | |                     | | Presentation Layer  |  
   +---------------------+ |                     | +---------------------+ 
   | Session Layer       | |                     | | Session Layer       |  
   +---------------------+ |                     | +---------------------+
   | Transport Layer     | |                     | | Transport Layer     |  
   +---------------------+ |                     | +---------------------+
   | Network Layer       | |                     | | Network Layer       | 
   +---------------------+ |                     | +---------------------+
   | Data Link Layer     | |                     | | Data Link Layer     | 
   +---------------------+ |                     | +---------------------+ 
   | Physical Layer      | +---------------------+ | Physical Layer      |
   +---------------------+                         +---------------------+  
  

Typical example of layering in action:

 host        repeater    router      host
 [ App ]>    [ App ]     [ App ]    >[ App ] 
 [ Pre ]|    [ Pre ]     [ Pre ]    |[ Pre ]
 [ Ses ]|    [ Ses ]     [ Ses ]    |[ Ses ]
 [ Trn ]|    [ Trn ]    +[ Trn ]+   |[ Trn ]
 [ Net ]|    [ Net ]    |[ Net ]|   |[ Net ]
 [ Lnk ]|   +[ Lnk ]+   |[ Lnk ]|   |[ Lnk ]
 [ Phy ]+-->+[ Phy ]+-->|[ Phy ]+-->+[ Phy ]

Encapsulation:

• A device can initiate transfer of information at almost any level.
• A level introduces information in the form of packets that it forwards to lower levels.
• Each level adds its own header and trailer and forwards the packet to the next level down.

   [ App ]                           DATA
   [ Pre ]                      PreH DATA PreT
   [ Ses ]                 SesH PreH DATA PreT SesT
   [ Trn ]            TrnH SesH PreH DATA PreT SesT TrnT
   [ Net ]       NetH TrnH SesH PreH DATA PreT SesT TrnT NetT
   [ Lnk ]  LnkH NetH TrnH SesH PreH DATA PreT SesT TrnT NetT LnkT
   [ Phy ]  
  

• The receiving device UNWRAPS the packet in the same way it was wrapped, removing info upper layers SHOULDN'T KNOW!
• A layer can have an empty header and/or trailer.

Case study: telnet/ssh

• Telnet and ssh are application protocols.
• When you type a character, it's sent in a packet.
• The packet gets wrapped with a TCP header, then an IP header, then a MAC header, then sent.
• When received, it's unwrapped and the server receives the character.
• But on the transmission line, we didn't just send the characters, but also three headers (TCP, IP, and MAC).
• Here's what a single character message looks like using Sun's `snoop' program: (in this case, from server to client)

   ETHER:  ----- Ether Header -----
   ETHER:  
   ETHER:  Packet 2 arrived at 15:07:10.24
   ETHER:  Packet size = 94 bytes
   ETHER:  Destination = 8:0:20:7a:67:e9, Sun LAN address 
   ETHER:  Source      = 8:0:20:b0:17:e, Sun LAN address 
   ETHER:  Ethertype = 0800 (IP)
   ETHER:  
   IP:   ----- IP Header -----
   IP:   
   IP:   Version = 4
   IP:   Header length = 20 bytes
   IP:   Type of service = 0x00
   IP:         xxx. .... = 0 (precedence)
   IP:         ...0 .... = normal delay
   IP:         .... 0... = normal throughput
   IP:         .... .0.. = normal reliability
   IP:   Total length = 80 bytes
   IP:   Identification = 6587
   IP:   Flags = 0x4
   IP:         .1.. .... = do not fragment
   IP:         ..0. .... = last fragment
   IP:   Fragment offset = 0 bytes
   IP:   Time to live = 255 seconds/hops
   IP:   Protocol = 6 (TCP)
   IP:   Header checksum = 2ea1
   IP:   Source address = 130.64.23.39, Conbrio.EECS.Tufts.EDU
         global 
   IP:   Destination address = 130.64.23.164, Blackhole.EECS.Tufts.EDU
         global 
   IP:   No options
   IP:   
   TCP:  ----- TCP Header -----
   TCP:  
   TCP:  Source port = 22  ssh daemon 
   TCP:  Destination port = 33158  ssh client
   TCP:  Sequence number = 278287127
   TCP:  Acknowledgement number = 1357088868
   TCP:  Data offset = 20 bytes
   TCP:  Flags = 0x18
   TCP:        ..0. .... = No urgent pointer
   TCP:        ...1 .... = Acknowledgement
   TCP:        .... 1... = Push
   TCP:        .... .0.. = No reset
   TCP:        .... ..0. = No Syn
   TCP:        .... ...0 = No Fin
   TCP:  Window = 8760
   TCP:  Checksum = 0xda12
   TCP:  Urgent pointer = 0
   TCP:  No options
  

• That's right, a single character takes 94 characters to transmit!

TCP/IP: Transmission Control Protocol/Internet Protocol

• IP: provides global naming scheme:

   [  ?  ]                                Data raw data 
   [ Net ]                 [IP Addresses] Data global address
   [ Lnk ] [MAC Addresses] [IP Addresses] Data local address
  

• TCP: allows sessions on top of that

   [  ?  ]                              Data
   [ Ses ]                    [TCP head]Data packet/sequence number 
   [ Net ]           [IP head][TCP head]Data global address 
   [ Lnk ] [MAC head][IP head][TCP head]Data local address
  

   OSI                     TCP/IP          TCP             UDP
   Application             Application     stream          message
   Session/Transport       Transport       segment         packet
   Network                 Internet        datagram        datagram
  

TCP: Transmission Control Protocol

• stream based; handles stream dissassembly and reassembly.
• connection: stateful:
  • negotiation: two machines start talking
  • session: one conversation.
  • closure: stop talking.
  • EX: telnet, ssh.
• reliable: you're assured that your data is sent.
• roughly corresponds to ISO session layer.
• metaphor: a two-way pipe of data between processes on differing machines.

UDP: User Datagram Protocol

• packet based: one `message' at a time. `Message passing'.
• unreliable: you have no guarantee that the packet you sent actually got there. It could be dropped.
• roughly corresponds to ISO transport layer
• metaphor: a single message between two processes on differing machines.

Reliability and `drops'

• Networks aren't perfect.
• Packets can be dropped.
• A dropped packet simply never arrives at its destination.

Let's build ourselves a network:

• Example: our Lin* network:

   Internet
     | 2xT3
   Tufts backbone
     | 1000/FX
   EECS NAT firewall 
     | 2x1000/FX  1000/FX
   EECS router----------- [switch]
     |1000/FX            /    |   \  100/T : 100 Mbits/second
     |                 lin00 lin01 lin02 ...
   [switch]
     | 100/T
   conmoto
  

  • 100/T: '100-base-T': 100 megabits/second over copper cables.
  • 1000/T: '1000-base-T': 1 gigabit/second over copper cables.
  • 1000/FX: '100-base-FX': 1 gigabit/second over optical fiber.

First step: the physical layer:

• Many kinds of Ethernet
  • Thinnet
  • Thicknet
  • Twisted-Pair (10/T)
• Other kinds of physical layers:
  • Token-ring
  • Frame-relay
• For this class, we concentrate on Ethernet.

thicknet(AUI):

• original 'ethernet'
• bulky (multi-conductor)
• fragile
• limited runlength
• segments must be multiples of fixed length
• This is a pain.

thinnet:

• coax (two conductor cable)
• thin
• long run-length,
• vampire-taps,
• segments must be on multiples of .5 meters.
• BNC connectors (like video)
• easy to wire: snake the cable everywhere and tap into where needed.
• failure of cable (or station) takes down whole segment (and all stations connected to it).
• This is cheap but even more of a pain.

twisted pair:

• telco-like cable (not telco),
• RJ-45 connectors (similar to, but larger than RJ11 two line phone connectors).
• six conductors in cable.
• robust
• point-to-point
• failure points isolated.
• wiring cost much higher because one must wire one's network in a star!
• must buy lots of hubs/switches!

The link layer: ethernet.

• thicknet, thinnet: 'bussed' communication (IEEE 802.3).
• IEEE 802.3: standard for
  • wiring
  • protocol for machine-machine communication.

Basics of ethernet: simple model (thinnet)

• all machines connected to one wire (with another wire as a ground reference).

    ---------+------------+----------+-----------+---------- signal
             |            |
            machine 1   machine 2
             |            |
    ---------+------------+----------+-----------+---------- ground
  

• if any station shorts the signal wire to ground, the signal becomes the same as ground from the point of view of all stations.
• Every other station sees this short when one station does it.
• one bit wire-or `bus'. anyone can transmit or receive on it.

Representing a message

• messages on the wire are expressed as binary voltage differences between ground and a reference voltage.
• each message is called an `ethernet packet'.
• messages are binary streams of 1's and 0's.
• each 1 or 0 is expressed by a transition in voltage (manchester encoding).

Sending a message

• to send a message, machines listen on the wire for a lull in activity.
• A machine starts sending a message when it thinks the wire is free of messages from other machines.
• There can be collisions due to the delay between a machine's inference that the line is free and the machine's transmission of a packet.
• Messages from all machines are wire-or'd into one data stream.

      [[[[[[[[[ is a packet. 
      -----------[[[[[[[[[------------------[[[[[[[[-------[[[[[[[[--- M1
    + -----------------------[[[[[[[[[[[------------------------------ M2
    + ---------------------------------------------------[[[[[[[[----- M3
    = -----------[[[[[[[[[---[[[[[[[[[[[----[[[[[[[[-----[[[[[[[[[[--- result
                 clean!      clean!         clean!       collision!
  

• logical 'or' of packets can result in clean packet or collision.

Packet structure

• Each packet starts with a preamble that determines source and destination machine addresses.
• These are local addresses (also called MAC addresses, `Media Access Control')
• Each machine reads and writes two kinds of packets:
  • broadcasts intended for all machines (special MAC address)
  • directed packets intended for one and only one machine.
• Each machine reads all broadcasts and only those directed packets that are intended for that one machine. (unless you configure it differently).
• except for a special promiscuous mode': in this mode the interface reads every packet whether or not it's for you. nice for debugging networks and stealing passwords.

Modern ethernet: 10/T, 100/T, 1000/T

• 10/T: 10 MegaBits/second transmit rate
• 100/T: 100 MegaBit/second transmit rate
• 1000/T: 1 GigaBit/second transmit rate
• point to point.
• hubs replace busses, contain complex electronics ($).
• `twisted pair'

Wiring 10,100,1000/T:

• 4 conductor phone wires, RJ-11 8 conductor jack, using only pins:

     DTE mapping: on the back of a workstation
     1  TX+             [  Transmit pair wire 1
     2  TX-             [  Transmit pair wire 2
     3  RX+             [  Receive pair wire 1
     4  UNUSED          (ground plane) 
     5  UNUSED          (ground plane) 
     6  RX-             [  Receive pair wire 2
     7  UNUSED          (ground plane) 
     8  UNUSED          (ground plane) 
   
     DCE mapping: on the back of a hub or concentrator
     1  RX+             [  Receive pair wire 1
     2  RX-             [  Receive pair wire 2
     3  TX+             [  Transmit pair wire 1
     4  UNUSED          (ground plane) 
     5  UNUSED          (ground plane) 
     6  TX-             [  Transmit pair wire 2
     7  UNUSED          (ground plane) 
     8  UNUSED          (ground plane) 
  

• DTE: Data Terminal Equipment: End-user node, server node.
• DCE: Data Communications Equipment: hub, router, switch, modem, etc.
• Basic game: connect transmit to receive, receive to transmit!

        SWITCH          COMPUTER
     1  RX+             TX+
     2  RX-             TX-
     3  TX+             RX+
     4  GND             GND
     5  GND             GND
     6  TX-             RX-
     7  GND             GND
     8  GND             GND
  

How 10,100,1000/T works:

• wire-or bus replaced by two busses between switch and machine.
• Each device talks on its TX and listens on its RX.
• Switches enable talking to computers in full duplex mode (TX and RX independent, both devices can talk at same time).
• Hubs only allow talking in half-duplex mode (RX echoes TX, wire-or occurs inside hub).
• This is why switches are much more expensive than hubs!
• In the dorms: have hubs, want switches.

Crossover:

• you can always connect DTE DIRECTLY to DCE by means of a CROSSOVER cable (NULL cable) :

     1 ----- 3   (transmit to receive, receive to transmit) 
     2 ----- 6
     3 ----- 1
     6 ----- 2
  

• If you have two machines, you don't need a hub! $100 saved.

Comments on all ethernet link layers:

• collisions: packets get lost
• protocol: complex requirements for avoiding collisions and for packet recovery when lost.
  • half hardware, half software,
  • usually encoded into special hardware contained on the 'ethernet board'.
  • older boards do this in software.
• lies, damn lies, and bandwidth estimates.

Collisions:

• Machines try to avoid sending messages at the same time.
• look at wire
• if someone talking, desist.
• if no one talking, you can send a packet.
• if this process fails, collisions occur.
• happens when two machines find a free wire at roughly the same time.

      -----------[[[[[[[[[------------------[[[[[[[[------------------ M1
    + ------------[[[[[[[[[------------------------------------------- M2
    = -----------[XXXXXXXX[-----------------[[[[[[[[------------------ result
                  ^^^^^^^^ collision between two packets. both packets lost
  

Detecting collisions:

• collisions are detected via use of checksums:
• sum up words in message, and append sum to header of message.
• if the words you get don't sum up to the sum you get, oops!

Dealing with collisions:

• The result of a collision is a garbled message.
• 802.3: After a collision, both machines try again with increasing delays between tries (exponential backoff).
• This guarantees that eventually, one machine will send its message before the other can mistake the channel for blank.
• distance between packets in time is shorter than time it takes to set up communication after seeing a clear line.
• This and the speed of light determine line length limits!

      -----[[[[[[[[---------[[[[[[[[----------------------------------
      ------[[[[[[[[------------........------------------------------
           ^^^^^^^^ sends too close for collision avoidance: oops!
                            ^^^^^^^^^ here machine 1 gets the wire 
                                ^^^^^^^^ machine 2 checks the wire, loses race
  

lies, damn lies, and bandwidth calculations

• ethernet's advertised bandwidth is 10, 100, or 1000 megabits/second.
• But collision resolution gets completely out of hand when a half-duplex wire exceeds 3,30, or 300 megabits/second respectively. (30% overhead).
• At 50% overhead, almost no information gets through.
• Only exception: two machines connected together by an otherwise unfettered wire in full duplex.
• Point-to-point independent connections can run at advertised bandwidth.
  • half duplex: one machine can talk at a time.
  • full duplex: both machines on a p-to-p link can talk simultaneously.
• 10 mbit half to 100 mbit full = x30.

Other link layers: connecting over the phone

• PPP point-to-point protocol
• SLIP serial line internet protocol
• both of these based upon a serial line connection (IEEE RS232, RS422).

RS232 (modem-to-computer) connections: also called SERIAL

• DB25 connector (25 pins) or DB9 connector (PC, 9 pins)
• Meant to connect a local computer and a remote one through a modem:

   computer <------> modem <-------> modem <-------> computer
          DTE      DCE       phone       DCE       DTE
                             line 
  

• Pin map looks similar to the map for T/P ethernet:
  • DCE: Data Communications Equipment: on the back of your modem.

     DCE pin map: (most common signals)               direction of signal
     1       chassis ground (shield for cable).                 ..
     2       TXD     transmit data                              -> 
     3       RXD     receive data                               <-
     4       RTS     request to send                            ->
                     (1 when terminal ready to send data)
     5       CTS      clear to send                             <-
                     (1 when ok to send it)
                     'hardware flow control' 
     7       signal ground
     8       DCD     data carrier detect                        ->
                     (1 when modem connected to another) 
     20      DTR     data terminal ready                        <-
                     (1 when terminal ready)
     
    

  • DTE: Data Terminal Equipment: on the back of your computer.

     DTE pin map: (most common signals)               direction of signal
     1       chassis ground (shield for cable).                 ..
     2       RXD     receive data                               <-
     3       TXD     transmit data                              -> 
     5       RTS     request to send                            ->
     4       CTS     clear to send                              <-
     7       signal ground
     8       DTR     data terminal ready                        <-
     20      DCD     data carrier detect                        ->
    

RS232 function:

• by nature, point to point communication (no network layer).
• normally full duplex: talk on TXD, listen on RXD.
• To talk, must connect TXD to RXD and RXD to TXD.

Null modems:

• connect two serial devices without the modem
• utilize _CROSSOVER_ cables between devices.

         1  --------  1
   TXD   2  --------  3  RXD
   RXD   3  --------  2  TXD
   RTS   4  --------  5  CTS
   CTS   5  --------  4  RTS
         7  --------  7
   DCD   8  -------- 20  DTR
   DTR  20  --------  8  DCD
  

RS232 speeds:

• Standard bank of speeds.
• 300,600,1200,2400,4800,9600,19200,38400,57600,112000 bits per second (baud).
• baud/10 =~ characters per second (raw rate)
• compression technologies allow one to send more.

RS232 data format:

• Two logic levels corresponding to voltage level: 1 and 0.
• line starts out at logic 0.
• After a start bit there are 6, 7, or 8 data bits followed by an optional parity bit and one or two stop bits.
• Confusing: bits are transmitted in binary right to left!
• Example: one start bit, 8 data, no parity, one stop bit:
  • ASCII 'C' = 67 (base 10) = 1000011 (base 2) so to send it one would send

     1 1 1 0 0 0 0 1 0 1
     ^                    start bit
       ^^^^^^^^^^^^^^^    8 data bits backwards
                       ^ one stop bit
    

• Parity bit is binary parity of data bits
  • 1 represents odd number of 1's,
  • 0 represents even number of 1's

Point-to-point protocol:

• IP over RS232.
• one host connected to another through serial link and optional modem.
• software unwraps IP packets, sends through interface, re-wraps as IP packets on other end.
• two phases in a ppp connection:
  1. negotiation: two computers talk to one another, figure out what to agree upon for ip addresses, compression methods, etc. Ppp uses a symmetric negotiation: either host can specify the appropriate parameters.
  2. link: use these agreed upon parameters and establish a link.
• typical use: home internet access (to be obsoleted by Cable Modems).

Illusions:

• If your modem and your computer both compress data, they work against one another.
• compression reduces the length of data, not content.
  • compress once -> get to minimal length.
  • compress again -> no gain.
• This means a slowdown of compression, and decompression!