Networking

Q and A for this lecture

Notes:

For the purposes of these lectures, we make some simplifications.
An RFC is a Request For Comment. These documents become the standard specifications for protocols, hardware, software that make the WWW run (and other things). You can find all RFCS at faqs.org. For example: RFC 2151
Required reading: See the TCP/IP Tutorial for more details. Or see the local copy
Optional reading: See this lan howto, or check out some ones notes
octet: Eight bits. This term is used in networking, in preference to byte, because some systems use the term "byte" for things that are not 8 bits long.
The diagrams below assume that we are using Ethernet to transmit IP datagrams which contain TCP datagrams.

Ethernet

RFCS: 894 and 826 for sending IP over Ethernet.
Purpose: LAN protocol, packet based, broadcast
Addressing: MAC Addresses (1/card) uniquely assigned by card manufacturer. 48-bits broken into 6 octets. (ie. 00:10:A4:0E:5D:94)
Hardware: Ethernet card, RJ45 cable, HUBS
Method of operation: To send a packet to host (on same broadcast network) simply broadcast packet. All hosts on this network will hear the transmitted packet. Only the one with the target MAC address will listen. If more than one broadcast occurs at the same time, retransmit after a random delay.

Packet Structure:

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Ethernet destination address (first 32 bits)            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Ethernet dest (last 16 bits)  |Ethernet source (first 16 bits)|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Ethernet source address (last 32 bits)                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        Type code              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  IP header, then TCP header, then your data                   |
    |                                                               |
        ...
    |                                                               |
    |   end of your data                                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Ethernet Checksum                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Ethernet destination/Source: MAC addresses of sender/intended reciever
Type code: Protocol type of payload. In our case, IP. Other protocols are possible.
Ethernet Checksum: function calculated on entire ethernet packet to ensure integrity at recieving end.

Examples: ifconfig

IP (Internet Protocol)

RFC 791
Location: On top of Ethernet (this is a simplification)
Purpose: internet protocol, datagram based, point to point, route between networks, responsible for routing individual datagrams.
Hardware: Interconnected Ethernet networks (hooked together with gateways (computers with Ethernet cards on more than one ethernet network)

Packet Structure:

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Version|  IHL  |Type of Service|          Total Length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Identification        |Flags|      Fragment Offset    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Time to Live |    Protocol   |         Header Checksum       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Source Address                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Destination Address                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  TCP header, then your data ......                            |
    |                                                               |

Source/Destination Address: Source and destination IP address for this packet. Destination is necessary for routing.
Protocol: Determines the payload type. In our case, the payload will be a TCP packet. There are other possibilities.
Header Checksum: function calculated on IP header to ensure integrity at recieving end.
Time to Live: Maximum number of hops packet can make until it is discarded. This prevents packets living forever (and looping).

Addressing: IP Address (1/card (this is a simplification)), assigned by the network administrator
- Format: 32-bits broken into 4 octets (ie. 128.6.4.194)
- Networks: Local IP addresses are all similar (are all on the same network).
- Class A (1-126), B (128.1 through 191.254.), C(192.1.1 to 223.254.254)
- Special machine addresses: 0 is reserved for machines that don't know their address. 255 is used for "broadcast". 127.0.0.1 is reserved for "me".
- Example:
  - 128.6.4.19 is on network 128.6 (along with possibly 254*254=64516 other systems).
  - If this system wants to broadcast to it's neighbours, it would address packets to 128.6.255.255 or to 255.255.255.255.
- Netmask: Mask used to (locally) subdivide a Class A/B/C address. Outside world does not see these subdivisions.
- Example:
- Example:
- Example: 142.150.10.197 is on a network with subnetmask 255.255.252.0. Determine if another machine is on the same subnet by ANDing both with the subnet mask. The result should be the same. machines on this subnet of this Class B network are...
- Question:
Routing
- Gateway: a system that connects networks together. Usually containing 2 ethernet cards (one for each of two networks it connects).
- Routing tables: lists network this system is on as well as gateways and the networks they are on.
- Default gateway: the machine packets are routed to in case no other gateway is appropriate for routing a datagram.
- netstat, traceroute examples
- A good note on Routing
Method of operation: Consult routing table, if destination IP address is on this subnetwork, send to that host (possibly using ARP to discover its MAC address). If destination IP is not on this network, send packet on to appropriate gateway (determined by routing table). They will pass it on. Again, ARP may be used to discover intermediate recipient.
ARP request:(Address Resolution Protocol) RFC= 826
Examples: ifconfig

TCP (Transmission Control Protocol)

RFCS: 793
Location: On top of IP (this is a simplification)
Purpose: point to point, conversation based protocol,
Hardware: (nothing additional required)
Method of operation: Break long messages into datagrams, give each a sequence number and pair of ports. Send this down to the IP level. Recipient reassembles incomming datagrams into long message. Lost packets are resent.
Addressing: Port (associated with a running application)
- A pair of ports (one at each end of the TCP link) determines a communication pipe (conversation).
- At each end, the port connects to a single running application.
- Format: 16 bits (2 octets), that is, an nteger from 1 to 65535
- Special ports: /etc/services

Packet Structure:

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Source Port          |       Destination Port        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sequence Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Acknowledgment Number                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Data |           |U|A|P|R|S|F|                               |
    | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
    |       |           |G|K|H|T|N|N|                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Checksum            |         Urgent Pointer        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   your data ... next 500 octets                               |
    |   ......                                                      |

Source/Destination port: identifier for the conversation
Sequence number: octet number of first octet in data
Checksum: function calculated on data to ensure integrity at recieving end.

Putting it all together

Application protocols (built on top of TCP/IP)

Server: Application waiting for connections. Will provide a service.
Client: Application connecting to a server
ftp RFC=542
telnet
smtp RFC=821 example
http RFC=2616, 1945, 2068,2660
Client/Server connection example

Exercises:

Use netstat, traceroute
telnet (to an smtp server) and send mail to your friend (after reading the appropriate RFC)
Read about the Domain Name System (DNS), use nslookup
Read the TCP/IP Tutorial
If I establish a TCP connection to an application and follow a packet from source to destination, which of the following addresses change during transit? Which remain the same? TCP (Port), IP, Ethernet (MAC)
True or false: The body of a TCP packet is read at each hop along its path from source to destination.
Consider I am sending mail from dvp.cs.toronto.edu to mail.cs.toronto.edu (aka yonge.cs.toronto.edu).
- What does the following tell you? nslookup.txt What do you think means?
- I executed netstat -tn on both dvp.cs.toronto.edu and mail.cs.toronto.edu. Look through this and find the TCP connection (on both ends) that I established in sending mail from dvp.cs.toronto.edu to mail.toronto.edu. Hint: I am using the smtp protocol. You will need to find the port number for this (either in the RFC or in /etc/services).
- Is it possible for mail.cs.toronto.edu to accept more emails (even while I am sending an email)? What line in which file above convinces you that this is the case?

References

Zubair says...about 309...