Jan 29, 2008CS573: Network Protocols and Standards1 NAT, DHCP Autonomous System Network Protocols and Standards Winter

Jan 29, 2008CS573: Network Protocols and Standards2 IPv4 IP Datagram Format IPv4 Addressing ARP and RARP IP Routing Basics Subnetting and Supernetting ICMP Network Address Translation (NAT) Dynamic Addressing

Jan 29, 2008CS573: Network Protocols and Standards3 Private Networks Private networks have no “direct” connection to the Internet Blocks of addresses have been reserved for the private networks (RFC 1918) Blocks in different classes – (1 class A) – (16 class B) – (256 class C)

Jan 29, 2008CS573: Network Protocols and Standards4 Purpose Machines in the protected network can access the Internet normally Packets coming from the protected network all appear to be coming from IP1 Addresses in the protected network are in the private range Host 1 Host 2 Host N Protected Network Firewall Internet IP1IP2

Jan 29, 2008CS573: Network Protocols and Standards5 Implementation Hosts inside the private network are configured to use the firewall (IP2) as their gateway The firewall rewrites the IP datagram header for the outbound packets, replacing the source IP with IP1 All packets “seem” to be coming from IP1 The destination IP in the packets received from the Internet is IP1; it is rewritten replacing IP1 with the IP address of the internal destination Problem: How to figure out what is the right destination in the private network?

Jan 29, 2008CS573: Network Protocols and Standards6 Demultiplexing Incoming Packets There is not enough information in the IP header to demultiplex incoming packets It is necessary to use information from the higher layers (transport layer) Common transport layers: TCP and UDP Transport layer has the concept of port which identifies which process in the host should finally get the packet

Jan 29, 2008CS573: Network Protocols and Standards7 Ports 16-bit numbers identifying which process should get the packet UDP and TCP ports exist in different spaces Each packet carries two port numbers The source port of the process which generated it in the source host The destination port of the process which should get it at the destination IP TCPUDP TelnetFTP

Jan 29, 2008CS573: Network Protocols and Standards8 Implementation (revisited) Upon receiving an outbound packet from a host in the private network, the firewall: Rewrites the source IP with its own IP (IP1) Generates a local source port and rewrites the source port in the packet as this port and makes a record of it Upon receiving an inbound packet from the Internet, the firewall checks whether the destination port in the packet is in the list of local ports: If not, the packet is dropped Can not initiate connections from outside! If yes, the firewall knows where to send this packet

Jan 29, 2008CS573: Network Protocols and Standards9 IPv4 IP Datagram Format IPv4 Addressing ARP and RARP IP Routing Basics Subnetting and Supernetting ICMP Network Address Translation (NAT) Dynamic Addressing

Jan 29, 2008CS573: Network Protocols and Standards10 BOOTP Alternative to RARP RARP operates at a low level, requesting direct access to the network hardware Difficult for an application programmer to build a server RARP gives “only” the IP address

Jan 29, 2008CS573: Network Protocols and Standards11 BOOTP Devised to allow a machine to obtain: Its IP address Address of a router Subnet mask to use Address of a name server Can be implemented with an application program Uses UDP/IP for communication

Jan 29, 2008CS573: Network Protocols and Standards12 BOOTP Reliability in communication is based on UDP checksum Timeout and retransmissions To minimize collisions among many clients, use random timeouts Increase timeouts with each retransmission Starting with the interval 0-4 seconds Doubling interval each retransmission up to 60s

Jan 29, 2008CS573: Network Protocols and Standards13 BOOTP Message Format OPHTYPEHLENHOPS SecondsUnused Transaction ID Client IP Address Your IP Address Server IP Address Client Hardware Address (16 octets) Router IP Address Boot File Name (128 octets) Server Hostname (64 octets) Vendor-specific area (64 octets) bits

Jan 29, 2008CS573: Network Protocols and Standards14 BOOTP Message Field OP Specifies whether a request(1) or reply(2) HTYPE and HLEN Hardware type and address length (For Ethernet, HTYPE is 1 and HLEN is 6) HOPS Client passes 0 in this field; BOOTP server increments it if the request is passed to another server across a router Transaction ID Contains an integer that machines use to match requests with responses Seconds Number of seconds since the client started to boot

Jan 29, 2008CS573: Network Protocols and Standards15 BOOTP Message Remaining fields in the message To allow the greatest flexibility Clients fill in as much information as they know; unknown fields are set to zero Example If server IP or server hostname are non-zero, only the server with matching address/name will answer the request If they are zero, any server that receives the request will reply

Jan 29, 2008CS573: Network Protocols and Standards16 BOOTP Message Format BOOTP can be used by a client that already knows its IP address (e.g., to obtain boot file information) A client that knows its IP address places it in the client IP address field; other clients set this field to zero If the client’s IP address in the request message is zero, a server returns the client IP address in the “your IP address” field

Jan 29, 2008CS573: Network Protocols and Standards17 DHCP Dynamic Host Configuration Protocol RARP and BOOTP designed for relatively static environment Each host a permanent network connection Manager creates a BOOTP configuration file specifying BOOTP parameters for each host Manager configures server with mapping of host identifier to IP address New Requirements Portable computers Number of computers exceeds available IP host addresses (although not all will be up and running at the same time)

Jan 29, 2008CS573: Network Protocols and Standards18 DHCP DHCP allows: Manual configuration Automatic configuration Managers let DHCP server assign a permanent address when a computer first attaches to the network Dynamic configuration Loaning IP addresses for a limited time

Jan 29, 2008CS573: Network Protocols and Standards19 IP Routing Protocols

Jan 29, 2008CS573: Network Protocols and Standards20 IP Routing Autonomous System Domain Intra-domain Routing Interior Gateway Protocols Inter-domain Routing Exterior Gateway Protocols IP Multicast Routing MPLS

Jan 29, 2008CS573: Network Protocols and Standards21 Routing in the Internet Routing Algorithms Bellman-Ford Dijkstra Routing Protocols Distance Vector Link State Routing Hierarchy Interior Gateway Protocols (RIP, OSPF, IGRP) Exterior Gateway Protocols (EGP, BGP, CIDR, Policy Routing) Multicasting (IGMP)

Jan 29, 2008CS573: Network Protocols and Standards22 Internet from the start First, there was ARPANET Routers had complete information about all the possible destinations – core routers GGP (gateway-to-gateway) protocol was used for routing – a distance vector protocol RR R R H H H

Jan 29, 2008CS573: Network Protocols and Standards23 Internet from the start Then, LANs were connected to ARPANET R RR ARPANET LAN Core Routers

Jan 29, 2008CS573: Network Protocols and Standards24 Internet from the start Problems with above configuration: Routing overhead increased with the number of connected routers Number of routes increased with the number of connected segments Frequency of routing exchanges increased Higher likelihood that something went wrong somewhere requiring updates Number of different types of routers increased Slow deployment of new versions of routing algorithms

Jan 29, 2008CS573: Network Protocols and Standards25 Internet from the start Backbone Network R1R1 Local Network Core Router R2R2 R3R3 Local Network R4R4

Jan 29, 2008CS573: Network Protocols and Standards26 Autonomous System R RR Backbone Network AS Core Routers AS: Autonomous System

Jan 29, 2008CS573: Network Protocols and Standards27 Autonomous System What is an autonomous system? A set of routers and networks under the same administration. Examples: A single router directly connecting one local network to the Internet A corporate network linking several local networks through a corporate backbone A set of client networks served by a single ISP NOTE: From a routing point of view, all parts of an AS must remain connected

Jan 29, 2008CS573: Network Protocols and Standards28 Autonomous System Internal connectivity within the AS means: All routers must be connected Parts of network connected through core AS (yes, core is an AS!) cannot form an AS All routers must exchange routing information in order to maintain the connectivity (normally achieved by using a single routing protocol) Routers inside an AS are called “interior gateway” and the protocol they use is called Interior Gateway Protocol (IGP)

Jan 29, 2008CS573: Network Protocols and Standards29 Autonomous System In 1982, the IGP of choice was GGP IGPs in use today are: RIP OSPF IGRP Each AS is identified by a 16-bit number Number is assigned by the numbering authorities

Jan 29, 2008CS573: Network Protocols and Standards30 Autonomous System: Benefits Routing overhead is lower Network management becomes easy Easier computation of new routes Distribution of new software versions is easier Failing elements can be isolated easily AS use an Exterior Gateway Protocol to exchange information about reachability