DCN286 Introduction to Data Communication Technology Session 11

Review 1) What function allows routers to evaluate available routes to a destination and to establish the preferred direction in which to forward a packet? a) Data Linkage b) Path Determination c) SDLC interface protocold) Frame relay 2) The protocol that matches an IP address to a MAC address is? a) DHCPb) BOOTP c) ARPd) Routed Protocol 3) Which of the following is not an unreliable protocol? a) IPb) UDP c) Ethernet d) Frame Relay 4) Which of the IP header fields is changed every time a packet passes through a router? a) Header checksum, TTL fieldb) TTL field, Source address c) DS field, TTL fieldd) None of the above 5) Which of the following is not a routing protocol? a) OSPFb) EGP c) BGPd) RIP

Objectives Subnets TCP/IP Transport TCP/IP Application

Three scenarios to be investigated How to find out the subnet numbers, and usable IP address in each subnet, when using a particular IP network and subnet mask Given an existing IP address and subnet mask, determine the subnet in which the address resides How to determine the number of subnets required for a given internetwork topology, and how to determine a subnet mask that reserves at least that many subnets 4 Default Subnets

Classes and default subnet masks Class A – Range, subnet? Class B – Range, subnet? Class C – Range, subnet? What do the '1' bits in the subnet mask represent? What do the '0' bits in the subnet mask represent? Given the Class C network Which octet is used for the hosts? What do I set the octet to in order to name the network (Network ID)? What do I set the octet to in order to broadcast to the network? 5 Default Subnets

Subnets To break a Class A, B or C network into more networks (subnetworks), the bits reserved for the hosts must be turned over to the network. Class C Network nnnnnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh Give 2 bits from the hosts to the network: nnnnnnnn.nnnnnnnn.nnnnnnnn.sshhhhhh The subnet mask is now: The two bits given up by the hosts are the subnet bits. 6 Subnets

How to find out the subnet numbers, and usable IP address in each subnet, when using a particular IP network and subnet mask IP Network = Mask = Broadcast = = Usable IP addresses = through Reserved “IP addresses”: Network ID (host bits are all '0') Broadcast ID (host bits are all '1') Usable IP addresses start at Network ID + 1, and end at Broadcast ID Subnets

Given an existing IP address and subnet mask, determine the subnet in which the address resides What do the '1' bits in the subnet mask represent? What do the '0' bits in the subnet mask represent? IP = Mask = NetID = = From the previous slide, we also determine: Broadcast = = IP addresses = through Subnets

How to determine the number of subnets required for a given internetwork topology, and how to determine a subnet mask that reserves at least that many subnets You must have one unique subnet for each broadcast domain. What delineates a broadcast domain? 9 Subnets

What are the broadcast domains? How many subnets are required? 10 Subnets

SLSM vs VLSM SLSM – Single Length Subnet Mask VLSM – Variable Length Subnet Mask SLSM uses a single subnet mask for the entire internetwork. For example, all networks will have a subnet mask of or The text book assumes this approach. VLSM uses various subnets based on the number of hosts that will require addresses. For example, why assign a subnet mask of (126 hosts) to a network that has 4 department routers being connected to the Internet through a VPN? Why not use ? (6 hosts) 11 Subnets

Subnets and Hosts vs Networks Assume a Class C network: being subnetted- the last subnet octet can be: Subnet#Networks#Hosts/Network A Class A network has 23 bits to play with. 12 Subnets

Logarithms High School Math Revisited What is a logarithm? if exponents take us one way: 2 3 = 2 x 2 x 2 = 8 logarithms reverse the process: log 2 8 = 3 (the base to what exponent gives us the value) what is log 2 16? 13 Subnets

SLSM Since the text is assuming SLSM, the question you must ask yourself is: How many subnets do I need? Then go about figuring out the subnet mask that will provide at least that many subnets. If a Class C network needs 8 subnets, then we know from the table that the mask will be or log 2 8 = 3, therefore 3 subnet bits are required: Subnets

SLSM Assume a Class A network, I need a minimum of 2000 networks: log = 10.9 (take the next highest integer) = 11 bits, therefore 11 subnet bits are required: = This provides 2048 subnets with 8190 hosts For those that use calculators that only perform log 10, and cannot perform logarithms in their heads: log x y = (log z y / log z x) log = (log / log 10 2) 15 Subnets

SLSM What are all the networks within the subnet? Assume Class C: , subnet NetworkSubnetHostNetworkID Subnets

Counting Hosts Instead of subnetting based on number of subnets desired, you want to subnet based on the number of hosts per subnet. Same, process, except you must add 2 for the network ID and broadcast address: #host bits = log 2 (h + 2) 30 hosts desired #host bits = log 2 (30 + 2) = log 2 32 = 5 17 Subnets

Transport Layer What are the two major protocols at the TCP/IP transport layer? Which is connection-oriented? Which provides flow control? Which provides error recovery? Which provides segmentation and reassembly of data? Which provides in-order delivery of data? Which identifies applications using port numbers? 18 TCP/IP Transport

Transport Layer The typical sequence of events in the establishment and tear down of a TCP session: Sender Receiver SYN SYN/ACK ACK DATA ACK DATA ACK FIN/ACK ACK 19 TCP/IP Transport

Flow Control and Windowing The receiver of the data controls the speed at which the sender transmits (throughput, not bandwidth) through flow control. If a receiver is getting data faster than it can process, it can run out of memory as things backup. Flow Control Through Sliding Windows The receiver sets a window size (in the TCP header) – the sender can send no more than this amount of data before it must stop and wait for an acknowledgment. The receiver can dynamically change the size of the window at any time in a TCP header that is part of an acknowledgment. 20 TCP/IP Transport

Flow Control Through Withhold Acknowledgments The receiver can simply not acknowledge receipt of a packet, until it knows it can accept more data. Differs from Dynamic Sliding Windows, where the ACK is sent immediately. 21 TCP/IP Transport

Establishing TCP Connection - a closer look 3 way connection Sender Receiver SYN SYN/ACK ACK Packet#1: SYN flag is set, SEQ field is set to an initial value Packet#2: SYN and ACK flags set. SEQ field is set to an initial value, ACK value = Packet#1 SEQ + 1 Packet#3: ACK flag set. SEQ = Pack#1 SEQ + 1, ACK = Packet#2 SEQ TCP/IP Transport SEQ=1234 SEQ=9876, ACK=1235 SEQ=1235, ACK=9877

TCP Error Recovery (Forward Ack. or Exceptional Ack.) Uses the ACK and SEQ fields just discussed. SEQ always contains the number of bytes sent (modulo 2 32 ) ACK always contains the next expected byte number (modulo 2 32 ) SEQ=1Bytes SEQ=1001Bytes SEQ=2001Bytes ACK=1001 SEQ=1001Bytes By ACKing 1001, the next SEQ to be sent is TCP/IP Transport

Segmentation, Reassembly, In-Order Delivery Uses SEQ to manage the In-order delivery TCP segments the data into MSS (Maximum Segment Size) chunks – typically 1460 bytes (when TCP and IP headers added, the packet is 1500 bytes- the MTU (Maximum Transmission Unit) of Ethernet) Segments are tracked using the SEQ field. Regardless of the order in which they arrive at the destination, they are reassembled into the correct order in the receive buffer 24 TCP/IP Transport SEQ=1SEQ=4 SEQ=3SEQ=5SEQ=2 RECEIVE BUFFER

TCP / UDP Headers TCP UDP Source Port – Identifies the application sending the data Destination Port – Identifies application process on receiving host for which the data is intended Sequence Number – As discussed 25 TCP/IP Transport Source Port Destination Port SEQ number ACK number Offset (Hlen) ReservedFlagsWindow Size Checksum Urgent Options PAD Source Port Destination Port LengthChecksum

TCP / UDP Headers TCP UDP Acknowledgment number – As discussed Offset or Header length – Number of 4 byte blocks in header Reserved – reserved for future use Flags – 1 bit for each flag (SYN, ACK, FIN...) 26 TCP/IP Transport Source Port Destination Port SEQ number ACK number Offset (Hlen) ReservedFlagsWindow Size Checksum Urgent Options PAD Source Port Destination Port LengthChecksum

TCP / UDP Headers TCP UDP Window size – Maximum amount of unacknowledged data allowed before mandatory ACK Checksum – similar to FCS (16 bits) Urgent – Used to point to the SEQ number of sent data for which sender requests immediate ACK from receiver 27 TCP/IP Transport Source Port Destination Port SEQ number ACK number Offset (Hlen) ReservedFlagsWindow Size Checksum Urgent Options PAD Source Port Destination Port LengthChecksum

TCP / UDP Headers TCP UDP Options – Additional headers used to expand the protocol – seldom used today Pad – To round header into multiple of 4 bytes 28 TCP/IP Transport Source Port Destination Port SEQ number ACK number Offset (Hlen) ReservedFlagsWindow Size Checksum Urgent Options PAD Source Port Destination Port LengthChecksum

Application Processes Using Port Numbers TCP/IP applications have been assigned a number: FTP 20 (data) and 21(control) Telnet 23 DNS 53 HTTP The number is referred to as a “port number”. When sending to data to an application, you address it with a Destination Port. 29 TCP/IP Transport

Application Processes Using Port Numbers Sending data to an application on a server: Client: ● Determine the application's port number (/etc/services) ● Select a port number to use for the client (must be currently unused number) ● Send the data, with TCP/UDP source and destination ports set Receiving data from server: Server: ● Uses client's source port as the destination for the data packet to be sent ● Uses application port number as the source port for the data packet to be sent ● Send the data, with TCP/UDP source and destination ports set 30 TCP/IP Transport

Application Processes Using Port Numbers Port Numbers Range from 0 – (8 bits) 0 – 1023: Well-Known ports. These are reserved port numbers that are published : Dynamic Ports. The range of ports available that the client can select as the Source Port 1024 – 49151: Registered Ports. The range of ports that can be allocated as part of the configuration of software. An IT department may make these decisions. 31 TCP/IP Transport

End Users Domain Name System (DNS) - UDP Hypertext Transfer Protocol (HTTP) - TCP Simple Mail Transfer Protocol (SMTP) - TCP Post Office Protocol Version 3 (POP3) - TCP File Transfer Protocol (FTP) - TCP 32 TCP/IP Application Layer

Network Management Trivial File Transfer Protocol (TFTP) - UDP Simple Network Management Protocol (SNMP) - UDP Telnet - TCP 33 TCP/IP Application Layer