Notes on TCP/IP

References:

• Andrew S. Tanenbaum, Computer Networks, Prentice Hall, 1981. Old, but good, fundamental discussion of OSI.
• Andrew S. Tanenbaum, Modern Operating Systems, Chapter 10, Prentice Hall, 1992.
• Craig Hunt, TCP/IP Network Administration, O'Reilly, 1992.
• OSI seven layer model
• Connected: An Internet Encyclopedia by Brent Baccala.
• Request For Comment index for Internet standards.
• Topical core: Pages focusing on major Internet concepts and features.

Open Systems Interconnect (OSI)

Seven layer

1. The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. E.g.: this layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method.
2. The Data Link Layer describes the logical organization of data bits transmitted on a particular medium. E.g.: this layer defines the framing, addressing and checksuming of Ethernet packets.
3. The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. E.g.: this layer defines the addressing and routing structure of the Internet.
4. The Transport Layer describes the quality and nature of the data delivery. E.g.: this layer defines if and how retransmissions will be used to ensure data delivery.
5. The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. E.g.: this layer describes how request and reply packets are paired in a remote procedure call.
6. The Presentation Layer describes the syntax of data being transferred. E.g.: this layer describes how floating point numbers can be exchanged between hosts with different math formats.
7. The Application Layer describes how real work actually gets done. E.g.: this layer would implement file system operations.

Mnemonic: "Please Do Not Throw Salami Pizza Away"

TCP/IP Protocol Architecture

1. The Network Access Layer is responsible for delivering data over the particular hardware media in use. Different protocols are selected from this layer, depending on the type of physical network.
2. The Internet Layer is responsible for delivering data across a series of different physical networks that interconnect a source and destination machine. Routing protocols are most closely associated with this layer, as is the IP Protocol, the Internet's fundamental protocol.
3. The Host-to-Host Layer handles connection rendezvous, flow control, retransmission of lost data, and other generic data flow management. The mutually exclusive TCP and UDP protocols are this layer's most important members.
4. The Process Layer contains protocols that implement user-level functions, such as mail delivery, file transfer and remote login.

Network Access Layer

Map IP addresses to physical addresses used by the network.

Example RFCs:

RFC 826, Address Resolution Protocol (ARP), maps IP --> Ethernet addresses.

RFC 894, A Standard for the Transmission of IP Datagrams over Ethernet Networks.

Internet Layer

RFC 791, Internet Protocol (IP). Notice the dates :-)

Is this message for me?

Functions include:

• Defining the
datagram;
• Defining the Internet addressing scheme;
• Moving data between Network Access Layer and the Host-to-Host Transport Layer;
• Routing datagrams to remote hosts;
• Performing fragmentation and re-assembly of datagrams.

IP is a connectionless protocol

IP is an unreliable protocol because it contains no error detection and recovery

IP Datagram format (after Hunt, Fig. 1.5.; see also zytracx

                       ---    b i t s ---

       |                        1       1       2       2       2     3|
       |0      |4      |8      |2      |6      |0      |4      |8     1|
       +-------+-------+---------------+-------------------------------+
   W  1|version|  IHL  | TypeOfService |           Total Length        |
       +-------+-------+---------------+-------+-----------------------+
   o  2|       Identification          | Flags | Fragmentation offset  |
       +---------------+---------------+-------+-----------------------+
   r  3|  TimeToLive   |    Protocol   |        Header Checksum        |
       +---------------+---------------+-------------------------------+
   d  4|                       Source Address                          |
       +---------------------------------------------------------------+
   s  5|                    Destination Address                        |
       +---------------------------------------------------------------+
      6|               Options                         |  Padding      |
       +---------------------------------------------------------------+
       |                    data begins here ...                       |

Routing a datagram:

Host A1                                            Host C1
  Application                                        Application
  Transport       Gateway G1       Gateway G2        Transport
  Internet          Internet         Internet        Internet  
  Network Access    Network Access   Network Access  Network Access
      v              ^   v            ^   v           ^
       `--->--------'     `--->------'     `--->------'
         Network A          Network B        Network C

RFC 792, Internet Control Message Protocol:

• Flow control
• Detecting unreachable destinations
• Redirecting routes
• Checking remote hosts

Host-to-Host Transport Layer

Which app gets this message? Ports

Supports

User Datagram Protocol (UDP). Unreliable, connectionless, fast, especially for "query-response" applications.
    UDP header

Transmission Control Protocol (TCP). Reliable, connection-oriented, byte-stream.
    TCP header

Process (Application) Layer

Continued ...