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

• 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.

Back to TCP/IP

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

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

IP Address includes network address

• 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

• 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).

The four formats of IP Addresses

• Class A
  • 128 possible network IDs
  • over 4 million host IDs per network ID
• Class B
  • 16K possible network IDs
  • 64K host IDs per network ID
• Class C
  • over 2 million possible network IDs
  • 254 host IDs per network ID

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.

IP Address Representation

• IP Addresses are usually shown in dotted decimal notation:

  1.2.3.4        00000001 00000010 00000011 00000100
  

• cs.williams.edu is 137.165.8.2

  10001001 10100101 00001000 00000010
  

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 relys 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.

Mapping IP Addresses to Hardware Addresses

• IP Addresses are not recognized by hardware.
• If we know the IP address of a host, how do we find out the hardware address ?
• The process of finding the hardware address of a host given the IP address is called Address Resolution

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 and some X-Terminals 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 itŐs IP address - the right one responds.

ARP

• ARP does not need to be done every time an IP datagram is sent - hosts remember the hardware addresses of each other.
• Part of the ARP protocol specifies that the receiving host should also remember the IP and hardware addresses of the sending host.

ARP

RARP

Services provided by IP

• Connectionless Delivery (each packet is treated individually).
• Unreliable (delivery is not guaranteed).
• Fragmentation / Reassembly (based on hardware MTU).
• Routing.
• Error detection.

IP Datagram

IP Datagram Fragmentation

• Each fragment (packet) has the same structure as the IP datagram.
• IP specifies that datagram reassembly is done only at the destination (not on a hop-by-hop basis).
• If any of the fragments are lost - the entire datagram is discarded (and an ICMP message is sent to the sender).

IP Datagram Fragmentation

• If packets arrive too fast - the receiver discards excessive packets and sends an ICMP message to the sender (SOURCE QUENCH).
• If an error is found (header checksum problem) the packet is discarded and an ICMP message is sent to the sender.

ICMP - Internet Control Message Protocol

• ICMP is a protocol used for exchanging control messages.
• ICMP uses IP to deliver messages.
• ICMP messages are usually generated and processed by the IP software, not the user process.

ICMP Message Types

• Echo Request
• Echo Response
• Destination Unreachable
• Redirect
• Time Exceeded
• Redirect (route change)
• there are more ...

Transport Layer & TCP/IP

A: No.

TCP is only part of the TCP/IP transport layer - the other part is UDP (User Datagram Protocol).

UDP

• UDP is a transport protocol
• communication between processes
• UDP uses IP to deliver datagrams to the right host.
• UDP uses ports to provide communication services to individual processes.

Ports

• TCP/IP uses an abstract destination point called a protocol port.
• Ports are identified by a positive integer.
• Operating systems provide some mechanism that processes use to specify a port or access it.

Ports

UDP

• Datagram Delivery
• Connectionless
• Unreliable
• Minimal

TCP - Transmission Control Protocol

• TCP is an alternative transport layer protocol supported by IP.

  Connection-Oriented

  • Connection oriented means that a virtual connection is established before any user data is transferred.
  • If the connection cannot be established - the user program is notified.
  • If the connection is ever interrupted - the user program(s) is notified.

  Reliable

  • Reliable means that every transmission of data is acknowledged by the receiver.
  • If the sender does not receive acknowledgement within a specified amount of time, the sender retransmits the data.

  Byte Stream

  • Stream means that the connection is treated as a stream of bytes, just like the named pipes we all know far too much about.
  • The user application does not need to package data in individual datagrams (as with UDP).

  Buffering

  • TCP is responsible for buffering data and determining when it is time to send a datagram.
  • It is possible for an application to tell TCP to send the data it has buffered without waiting for a buffer to fill up.

  Full Duplex

  • TCP provides transfer in both directions.
  • To the application program these appear as 2 unrelated data streams, although TCP can piggyback control and data communication by providing control information (such as an ACK) along with user data.

TCP Ports

• Interprocess communication via TCP is achieved with the use of ports (just like UDP).
• UDP ports have no relation to TCP ports (different name spaces).

TCP Segments

• The chunk of data that TCP asks IP to deliver is called a TCP segment.
• Each segment contains:
  • data bytes from the byte stream
  • control information that identifies the data bytes

TCP Segment Format

Addressing in TCP/IP

• Each TCP/IP address includes:
• Internet Address
• Protocol (UDP or TCP)
• Port Number

TCP vs. UDP

A: It depends on the application.

UDP offers minimal datagram delivery service (as little overhead as possible).

TCP/IP Summary

• IP: network layer protocol
  unreliable datagram delivery between hosts.
• UDP: transport layer protocol
  unreliable datagram delivery between processes.
• TCP: transport layer protocol
  reliable, byte-stream delivery between processes.

TCP or UDP ?

• Internet commerce ?
• Video server?
• File transfer?
• Email ?
• Chat groups?
• Robotic surgery controlled remotely over a network?