Lecture 3 Overview

Protocol An agreed upon convention for communication both endpoints need to understand the protocol. Protocols must be formally defined and unambiguous! Protocols define format, order of msgs sent and received among network entities, actions taken on msg transmission, receipt We will study lots of existing protocols and perhaps develop a few of our own. 2 Lecture 3: Protocol Layers

Client - Server A server is a process - not a machine ! A server waits for a request from a client A client is a process that sends a request to an existing server and (usually) waits for a reply Servers are generally more complex Basic types of servers:  Iterative - server handles one client at a time  Concurrent - server handles many clients at a time 3 Lecture 3: Protocol Layers

OSI 7 Layer Model: 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data-Link 1 Physical Lecture 3: Protocol Layers 4 High level protocols Low level protocols TCP/IP Model

The Physical Layer Responsibility: – transmission of raw bits over a communication channel Issues: – mechanical and electrical interfaces – time per bit – distances 5 Lecture 3: Protocol Layers

The Data Link Layer Responsibility: – provide an error-free communication link – Sublayers Data Link Control Medium Access Control Issues: – framing (dividing data into chunks) header & trailer bits – addressing Lecture 3: Protocol Layers

The Network Layer Responsibilities: – path selection between end-systems (routing). – flow control. – fragmentation & reassembly – translation between different network types. Issues: – packet headers – virtual circuits 7 Lecture 3: Protocol Layers

The Transport Layer Responsibilities: – provides virtual end-to-end links between peer processes. – end-to-end flow control Issues: – headers – error detection – reliable communication 8 Lecture 3: Protocol Layers

The Application Layer Responsibilities: – anything not provided by any of the other layers – TCP/IP model Session and Presentation Layer functions Issues: – application level protocols – appropriate selection of “type of service” 9 Lecture 3: Protocol Layers

Layering & Headers Each layer needs to add some control information to the data in order to do it’s job. This information is typically prepended to the data before being given to the lower layer. Once the lower layers deliver the data and control information - the peer layer uses the control information. 10 Lecture 3: Protocol Layers

What are the headers? Physical: – no header - just a bunch of bits Data Link: – address of the receiving endpoints – address of the sending endpoint – length of the data – checksum 11 Lecture 3: Protocol Layers

What are the headers? Network: – Protocol – Protocol version – type of service – packet identifier – time to live – source network address – destination network address – length of the data – fragment number – header checksum 12 Lecture 3: Protocol Layers

Lecture 1 Internet CPE 401 / 601 Computer Network Systems slides are modified from Dave Hollinger Lecture 4 TCP / IP model

Ethernet Data Link Layer protocol Ethernet (IEEE 802.3) is widely used Supported by a variety of physical layer implementations Multi-access (shared medium) TCP/IP model 14

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. TCP/IP model 15

An Ethernet Frame The preamble is a sequence of alternating 1s and 0s used for synchronization. CRC is Cyclic Redundency Check TCP/IP model 16 8 bytes Preamble Destination Address Source Address LenCRC DATA

Ethernet Addressing 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. – Assigned to vendors by a central authority 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. TCP/IP model 17

Internet Protocol IP is the network layer – packet delivery service (host-to-host). – translation between different data-link protocols 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 even delivered at all. TCP/IP model 18

Why ? Why ? IP addresses are not the same as the underlying data-link (MAC) 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 is what makes routing feasible. IP Addresses TCP/IP model 19

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 – American Registry for Internet Numbers for North America TCP/IP model 20 IPv4 (version 4)

The four formats of IP Addresses TCP/IP model 21 0 NetID NetID 1110 Multicast Address HostID NetIDHostID Class A B C D 8 bits 128 possible network IDs, over 4 million host IDs per network ID 16K possible network IDs, 64K host IDs per network ID Over 2 million possible network IDs, 256 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. TCP/IP model 22

IP Addresses IP Addresses are usually shown in dotted decimal notation: cse.unr.edu is TCP/IP model 23 CSE has a class B network

Host and Network Addresses A single network interface is assigned a single IP address called the host address A host may have multiple interfaces – therefore multiple host addresses Hosts that share a network all have the same IP network address (the network ID) An IP address that has a host ID of all 0s is called a network address and refers to an entire network TCP/IP model 24

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 TCP/IP model 25 10NetIDSubnetIDHostID

Subnetting TCP/IP model 26 router Subnet x Subnet x Subnet x

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 TCP/IP model 27

Mapping IP 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 TCP/IP model 28

ARP Address Resolution Protocol is used by a sending host when it knows the IP address of 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 Hosts remember the hardware addresses of each other TCP/IP model 29

ARP conversation TCP/IP model 30 HEY - Everyone please listen! Will please send me its Ethernet address? not me Hi Green! I’m , and my Ethernet address is 87:A2:15:35:02:C3

IP Datagram TCP/IP model 31 VERSHL Fragment Offset Fragment LengthService Datagram IDFLAG TTLProtocolHeader Checksum Source Address Destination Address Options (if any) Data 1 byte

IP Datagram Fragmentation Packets are fragmented due to link’s Maximum Transmission Unit (MTU) 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 TCP/IP model 32