Too Much of a Good Thing? Hosts have a 15-441/641: Computer - PowerPoint PPT Presentation

9/14/2019 Too Much of a Good Thing? • Hosts have a 15-441/641: Computer Networks • host name Domain Name System • IP address Application DNS • MAC address Presentation 15-441 Spring 2019 Session Profs Peter Steenkiste & Justine Sherry Transport ARP • There is a reason .. Network • Remember? Data link Fall 2019 • But how do we translate? Physical https://computer-networks.github.io/sp19/ 2 IP to MAC Address Translation Caching ARP Entries • Efficiency Concern • How does one find the Ethernet address of a IP host? • Would be very inefficient to use ARP request/reply every time • Address Resolution Protocol - ARP need to send IP message to machine • Broadcast search for IP address • Each Host Maintains Cache of ARP Entries • E.g., “who-has 128.2.184.45 tell 128.2.206.138” sent to Ethernet broadcast (all FF address) • Add entry to cache whenever you get ARP response • Destination responds (only to requester using unicast) with • “Soft state”: set timeout of ~20 minutes appropriate 48-bit Ethernet address • E.g, “reply 128.2.184.45 is-at 0:d0:bc:f2:18:58” sent to 0:c0:4f:d:ed:c6 3 4 1

9/14/2019 ARP Cache Example Challenge: Broadcast! • Overhead scales (roughly) as N 2 for an N host network • Show using command “arp -a” Interface: 128.2.222.198 on Interface 0x1000003 • N host does an ARP broadcast for each (new) destination Internet Address Physical Address Type 128.2.20.218 00-b0-8e-83-df-50 dynamic • Each broadcast is delivered to N hosts Network Network 128.2.102.129 00-b0-8e-83-df-50 dynamic 128.2.194.66 00-02-b3-8a-35-bf dynamic • Remember the solution? Internet 128.2.198.34 00-06-5b-f3-5f-42 dynamic Router 128.2.203.3 00-90-27-3c-41-11 dynamic • Subnetting! 128.2.203.61 08-00-20-a6-ba-2b dynamic 128.2.205.192 00-60-08-1e-9b-fd dynamic • Break up network into networks 128.2.206.125 00-d0-b7-c5-b3-f3 dynamic connected by router 128.2.206.139 00-a0-c9-98-2c-46 dynamic 128.2.222.180 08-00-20-a6-ba-c3 dynamic BIG Network 128.2.242.182 08-00-20-a7-19-73 dynamic • Not always a good idea 128.2.254.36 00-b0-8e-83-df-50 dynamic • Extra complexity, management overhead, cost, … 5 Subnetting is an Option Proxy ARP • Limit the scope of ARP requests/responses inside an L2 • Subnetting! • Proxy ARP makes it look like ne network: • Break up network into networks Network Network connected by router Network Network • Host1 in N1 sends ARP for host 2 in N2 Internet Internet • Limits the scope of ARP • Proxy ARP looks up MAC address Router Router requests/responses inside smaller • May require discovery using ARP Proxy ARP Proxy ARP L2 networks • Responds to host 1’s request • But not always a good always a • Acts as proxy for host 2 N1 N2 N3 N4 N5 N1 N2 N3 N4 N5 good idea • Also forwards packets from host 1 • Extra complexity, management to host 2 at layer 2 overhead, cost, … • Acts as a switch • Example: WiFi network 2

9/14/2019 Host Names & Addresses Why bother? • Host addresses: e.g., 169.229.131.109 • Convenience • a number used by protocols • Easier to remember www.google.com than 74.125.239.49 • conforms to network structure (the “where”) • Host names: e.g., linux.andrew.cmu.edu • Provides a level of indirection! • mnemonic name usable by humans • Decoupled names from addresses • conforms to organizational structure (the “who”) • Many uses beyond just naming a specific host • The Domain Name System (DNS) is how we map from one to the other • a directory service for hosts on the Internet DNS: Early days DNS provides Indirection • Mappings stored in a hosts.txt file (in /etc/hosts) • Addresses can change underneath maintained by the Stanford Research Institute (SRI) • • Move www.cnn.com to a new IP address new versions periodically copied from SRI (via FTP) • • People and applications are unaffected • As the Internet grew this system broke down • Name can map to multiple IP addresses SRI couldn’t handle the load • • Enables l oad-balancing conflicts in selecting names • • Multiple names for the same address hosts had inaccurate copies of hosts.txt • • E.g., many services (mail, www, ftp) collocated on the same machine • The Domain Name System (DNS) was invented to fix this • Allowing “host” names to evolve into “service” names 3

9/14/2019 Obvious Solutions (1) Goals? • Scalable Why not centralize DNS? • many names • Distant centralized database • many updates • Traffic volume • many users creating names • Single point of failure • many users looking up names • Single point of update • Highly available • Single point of control • Correct • no naming conflicts (uniqueness) • consistency • Doesn’t scale! • Lookups are fast 13 How? Key idea: hierarchical distribution • Partition the namespace – Hierarchy! Three intertwined hierarchies • Hierarchical namespace • Distribute the administration of each name space partition • As opposed to original flat namespace • Autonomy to update a network’s own (machines’) names • Translation of cmu.edu names is done by CMU • Hierarchically administered • Don’t have to track everybody’s updates • As opposed to centralized administrator • Distribute name resolution for each partition • Hierarchy of servers • As opposed to centralized storage • How should we partition things? 4

9/14/2019 DNS Design: Zone Definitions DNS Design: Hierarchy Definitions • Each node in hierarchy stores a list of • Zone = contiguous section of name space names that end with same suffix • E.g., Complete tree, single node or subtree • Suffix = path up tree • A zone has an associated set of name root root • E.g., given this tree, where would org org servers ca following be stored: net edu com uk net edu com uk • Must store list of names and tree links • Fred.com gwu ucb cmu bu mit • Fred.edu gwu ucb cmu bu mit • Fred.cmu.edu cs cs ece ece Subtree • Fred.cmcl.cs.cmu.edu cmcl cmcl Single node • Fred.cs.mit.edu Complete Tree 17 18 Server Hierarchy Server Hierarchy • Top of hierarchy: Root servers • Every server knows the address of the root name server • Location hardwired into other DNS servers • Root servers know the address of all TLD servers • … • Next Level: Top-level domain (TLD) servers New TLDs started in 2012 • An authoritative DNS server stores name-to-address mappings (“resource • .com, .edu, .uk, etc. … expect to see more records”) for all DNS names in the domain that it has authority for in the future. • Managed professionally  Each server stores a subset of the total DNS database • Bottom Level: Authoritative DNS servers  Each server can discover the server(s) responsible for • Actually store the name-to-address of devices mapping any portion of the hierarchy • Maintained by the corresponding administrative authority 5

9/14/2019 DNS Root DNS Root Servers • 13 root servers (labeled A-M; see http://www.root-servers.org/ ) • Located in Virginia, USA A Verisign, Dulles, VA Verisign, Dulles, VA C Cogent, Herndon, VA D U Maryland College Park, MD G US DoD Vienna, VA K RIPE London H ARL Aberdeen, MD J Verisign I Autonomica, Stockholm E NASA Mt View, CA F Internet Software Consortium M WIDE Tokyo Palo Alto, CA B USC-ISI Marina del Rey, CA L ICANN Los Angeles, CA DNS Root Servers Anycast in a nutshell  13 root servers (labeled A-M; see http://www.root-servers.org/ ) • Routing finds shortest paths to destination  Each server is replicated via any-casting A Verisign, Dulles, VA C Cogent, Herndon, VA (also Los Angeles, NY, Chicago) • What happens if multiple machines advertise the same address? D U Maryland College Park, MD G US DoD Vienna, VA K RIPE London (plus 16 other locations) H ARL Aberdeen, MD J Verisign (21 locations) I Autonomica, Stockholm (plus 29 • The network will deliver the packet to the closest machine with that other locations) E NASA Mt View, CA address F Internet Software Consortium, M WIDE Tokyo Palo Alto, CA plus Seoul, Paris, (and 37 other locations) San Francisco • This is called “anycast” • Very robust B USC-ISI Marina del Rey, CA L ICANN Los Angeles, CA • Requires no modification to routing algorithms 6