DNS: Domain Name System People: many identifiers: – SSN, name, Passport # Internet hosts, routers: – IP address (32 bit) - used for addressing datagrams – “name”, e.g., hermite.cs.smith.edu - used by humans Q: map between IP addresses and name ? Domain Name System: distributed database implemented in hierarchy of many name servers application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) – note: core Internet function implemented as application- layer protocol – complexity at network’s “edge”

DNS DNS services Hostname to IP address translation Host aliasing – Canonical and alias names Mail server aliasing Load distribution – Replicated Web servers: set of IP addresses for one canonical name Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn’t scale!

Structure of DNS Names Each name consists of a sequence of alphanumeric components separated by periods Examples: – – ssuet.edu.pk – khi.comsats.net.pk – aurangzeb.ssuet.edu.pk Names are hierarchical, with most-significant component on the right Left-most component is, generally, a computer name

Structure of DNS Names Top Level Domains (right-most components; also known as TLDs) are defined by global authority Organizations apply for names in a top-level domain: – fsu.edu – macdonalds.com Organizations determine own internal structure – eng.fsu.edu – cs.purdue.edu

Distributed, Hierarchical Database Client wants IP for 1 st approx: Client queries a root server to find com DNS server Client queries com DNS server to get amazon.com DNS server Client queries amazon.com DNS server to get IP address for Root DNS Servers com DNS servers org DNS serversedu DNS servers poly.edu DNS servers umass.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers

DNS: Root name servers contacted by local name server that can not resolve name root name server: – contacts authoritative name server if name mapping not known – gets mapping – returns mapping to local name server 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) i Autonomica, Stockholm (plus 3 other locations) k RIPE London (also Amsterdam, Frankfurt) m WIDE Tokyo a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations)

TLD and Authoritative Servers Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. –Network solutions maintains servers for com TLD Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail). –Can be maintained by organization or service provider Local DNS servers: organization’s DNS servers located on various subnets to provide DNS lookups for hosts on the subnet. May not be accessible from outside the subnet. Their IP addresses are part of the host's network configuration (manual or DHCP).

Local Name Server Does not strictly belong to hierarchy Each ISP (residential ISP, company, university) has one. – Also called “default name server” When a host makes a DNS query, query is sent to its local DNS server – Acts as a proxy, forwards query into hierarchy.

Inverse domain Country domains

DNS Name Resolution Example

DNS Operation

DNS: caching and updating records once (any) name server learns a mapping, it caches the mapping (Domain’s DNS = IP) – cache entries timeout (disappear) after some time (usually 20 minutes) – TLD servers typically cached longer in local name servers Thus root name servers not often visited update/notify mechanisms under design by IETF – RFC 2136 –

DNS records DNS: distributed db storing resource records (RR) Type=NS – name is domain (e.g. foo.com) – value is IP address of authoritative name server for this domain RR format: (name, value, type,ttl) r Type=A  name is hostname  value is IP address r Type=CNAME  name is an alias name for some “canonical” (the real) name  value is canonical name r Type=MX  value is hostname of mailserver associated with name

DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header r identification: 16 bit # for query, reply to query uses same # r flags: m query or reply m recursion desired m recursion available m reply is authoritative

DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative servers additional “helpful” info that may be used

wireshark Display of DNS Response ID is random nonce used to authenticate Response to Query 18

Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses

File Distribution: Server-Client vs P2P

BitTorrent Overview Website allowing peers to share music, video and other media files Central server helps users find initial set of peers that have pieces of the file Tracker server keeps track of peers possessing content of individual files Users download the file by participating in exchange: – They exchange pieces that they have – for pieces that they don’t have Therefore, for the system to work, users must have incentive to give Users who just get, but do not give are called free riders Protocol must discourage free riding

BitTorrent

P2P: Searching for Information

P2P: centralized directory

P2P: problems with centralized directory Single point of failure Performance bottleneck Copyright infringement: “target” of lawsuit is obvious file transfer is decentralized, but locating content is highly decentralized

P2P: decentralized directory Each peer is either a group leader or assigned to a group leader. Group leader tracks the content in all its children. Peer queries group leader; group leader may query other group leaders.

Decentralized Indexing: Query Flooding

Query Flooding

Skype IP telephony system Allows users to make phone calls – to Skype users – to regular phone users Calls are routed via Skype nodes Central login server Super-nodes – Nodes that know about location of other nodes

Skype P2P Network Super-nodes are nodes with powerful CPU and network bandwidth One has to qualify to become a super-node Super-nodes know about locations of other nodes Ordinary nodes contact super-nodes to place calls

Skype Components Skype client – the client program used to make phone calls Host cache – list of other known Skype users, maintained at each Skype client Skype login server – a centralized component. Processes account information, authentication

Skype User Search Procedure A Skype client making a phone call needs to find other users It contacts super-nodes from its host cache, asking them to help find the user Super-nodes return a list of nodes to contact The client contacts those nodes If unsuccessful, the client asks for more nodes Guarantees to find any user that has logged in within the last 72 hours Not much specific information on Skype protocol is available…