TCP/IP & OSI:

TCP/IP & OSI:

In OSI reference model terminology -the TCP/IP protocol suite covers the network and transport layers.

TCP/IP can be used on many data-link layers (can support many network hardware implementations).

Ethernet - A Real Data-Link Layer

Supported by a variety of physical layer implementations.

It will be useful to discuss a real data-link layer.

Ethernet (really IEEE 802.3) is widely used.

Multi-access (shared medium).

Every Ethernet interface has a unique 48 bit address (a.k.a. hardware address).

Example:   C0:B3:44:17:21:17

The broadcast address is all 1’s.

Addresses are assigned to vendors by a central authority

CSMA/CD
 Carrier Sense Multiple Access
with
Collision Detection

Carrier Sense: can tell when another host is transmitting

Multiple Access: many hosts on 1 wire

Collision Detection: can tell when another host transmits at the same time.

An Ethernet Frame

8 byte                          6                              6                         2                     0-1500                     4

The preamble is a sequence of alternating 1s and 0s used for synchronization.

CRC is Cyclic Redundency Check

Ethernet Addressing

Each interface looks at every frame and inspects the destination address. If the address does not match the hardware address of the interface or the broadcast address, the frame is discarded.

Some interfaces can also be programmed to recognize multicast addresses.

Internet Protocol
The IP in TCP/IP

IP is the network layer

packet delivery service (host-to-host).

translation between different data-link protocols.

IP Datagrams

IP provides connectionless, unreliable delivery of IP datagrams.

Connectionless: each datagram is independent of all others.

Unreliable: there is no guarantee that datagrams are delivered correctly or at all.

IP addresses are not the same as the underlying data-link (MAC) addresses.

IP Addresses:

IP is a network layer - it must be capable of providing communication between hosts on different kinds of networks (different data-link implementations).

The address must include information about what network the receiving host is on. This makes routing feasible.

IP addresses are logical addresses (not physical)

32 bits.

Includes a network ID and a host ID.

Every host must have a unique IP address.

IP addresses are assigned by a central authority (the NIC at SRI International).

Network and Host IDs:

A Network ID is assigned to an organization by a global authority.

Host IDs are assigned locally by a system administrator.

Both the Network ID and the Host ID are used for routing.

Host and Network Addresses:

A single network interface is assigned a single IP address called the host address.

A host may have multiple interfaces, and therefore multiple host addresses.

Hosts that share a network all have the same IP network address (the network ID).

IP Broadcast and Network Addresses:

An IP broadcast addresses has a host ID of all 1s.

IP broadcasting is not necessarily a true broadcast, it relies on the underlying hardware technology.

An IP address that has a  host ID of all 0s is called a network address and refers to an entire network.

Subnet Addresses:

An organization can subdivide it’s host address space into groups called subnets.

The subnet ID is generally used to group hosts based on the physical network topology.

Subnetting:

Subnets can simplify routing.

IP subnet broadcasts have a hostID of all 1s.

It is possible to have a single wire network with multiple subnets.

Reverse Address Resolution:

The process of finding out the IP address of a host given a hardware address is called

Reverse Address Resolution

Reverse address resolution is needed by diskless workstations when booting.

ARP:

The Address Resolution Protocol is used by a sending host when it  knows the IP address  of the destination but needs the Ethernet address.

ARP is a broadcast protocol - every host on the network receives the request.

Each host checks the request against it’s IP address - the right one responds.