2: Application Layer 1 Chapter 2: Application layer r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail  SMTP, POP3, IMAP r 2.5 DNS r 2.6 P2P applications r 2.7 Socket programming with TCP r 2.8 Socket programming with UDP r 2.9 Building a Web server

2: Application Layer 2 DNS: Domain Name System People: many identifiers:  SSN, name, passport # Internet hosts, routers:  IP address (32 bit) - used for addressing datagrams  “name”, e.g., ww.yahoo.com - used by humans Q: map between IP addresses and name ? Domain Name System: r distributed database implemented in hierarchy of many name servers r 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”

2: Application Layer 3 DNS Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale! DNS services r hostname to IP address translation r host aliasing  Canonical, alias names r mail server aliasing r load distribution  replicated Web servers: set of IP addresses for one canonical name

2: Application Layer 4 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 Distributed, Hierarchical Database Client wants IP for 1 st approx: r client queries a root server to find com DNS server r client queries com DNS server to get amazon.com DNS server r client queries amazon.com DNS server to get IP address for

2: Application Layer 5 DNS: Root name servers r contacted by local name server that can not resolve name r 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 36 other locations) i Autonomica, Stockholm (plus 28 other locations) k RIPE London (also 16 other locations) m WIDE Tokyo (also Seoul, Paris, SF) a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations)

2: Application Layer 6 TLD and Authoritative Servers r 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  Educause for edu TLD r Authoritative DNS servers:  organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).  can be maintained by organization or service provider

2: Application Layer 7 Local Name Server r does not strictly belong to hierarchy r each ISP (residential ISP, company, university) has one.  also called “default name server” r when host makes DNS query, query is sent to its local DNS server  acts as proxy, forwards query into hierarchy

2: Application Layer 8 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server DNS name resolution example r Host at cis.poly.edu wants IP address for gaia.cs.umass.edu iterated query: r contacted server replies with name of server to contact r “I don’t know this name, but ask this server”

2: Application Layer 9 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server 3 recursive query: r puts burden of name resolution on contacted name server r heavy load? DNS name resolution example

2: Application Layer 10 DNS: caching and updating records r once (any) name server learns mapping, it caches mapping  cache entries timeout (disappear) after some time  TLD servers typically cached in local name servers Thus root name servers not often visited r update/notify mechanisms under design by IETF  RFC 2136 

2: Application Layer 11 DNS records DNS: distributed db storing resource records (RR) r Type=NS  name is domain (e.g. foo.com)  value is hostname 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 alias name for some “canonical” (the real) name is really servereast.backup2.ibm.com  value is canonical name r Type=MX  value is name of mailserver associated with name

2: Application Layer 12 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:  query or reply  recursion desired  recursion available  reply is authoritative

2: Application Layer 13 DNS protocol, messages Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used

Multimedia Networking 7-14 SIP: Session Initiation Protocol [RFC 3261] SIP long-term vision: r all telephone calls, video conference calls take place over Internet r people are identified by names or addresses, rather than by phone numbers r you can reach callee, no matter where callee roams, no matter what IP device callee is currently using

Multimedia Networking 7-15 SIP Services r Setting up a call, SIP provides mechanisms..  for caller to let callee know she wants to establish a call  so caller, callee can agree on media type, encoding  to end call r determine current IP address of callee:  maps mnemonic identifier to current IP address r call management:  add new media streams during call  change encoding during call  invite others  transfer, hold calls

Multimedia Networking 7-16 Setting up a call to known IP address  Alice’s SIP invite message indicates her port number, IP address, encoding she prefers to receive (PCM ulaw)  Bob’s 200 OK message indicates his port number, IP address, preferred encoding (GSM)  SIP messages can be sent over TCP or UDP; here sent over RTP/UDP.  default SIP port number is 5060.

Multimedia Networking 7-17 Setting up a call (more) r codec negotiation:  suppose Bob doesn’t have PCM ulaw encoder.  Bob will instead reply with 606 Not Acceptable Reply, listing his encoders Alice can then send new INVITE message, advertising different encoder r rejecting a call  Bob can reject with replies “busy,” “gone,” “payment required,” “forbidden” r media can be sent over RTP or some other protocol

Multimedia Networking 7-18 Example of SIP message INVITE SIP/2.0 Via: SIP/2.0/UDP From: To: Call-ID: Content-Type: application/sdp Content-Length: 885 c=IN IP m=audio RTP/AVP 0 Notes: r HTTP message syntax r sdp = session description protocol r Call-ID is unique for every call.  Here we don’t know Bob’s IP address.  intermediate SIP servers needed.  Alice sends, receives SIP messages using SIP default port 506  Alice specifies in header that SIP client sends, receives SIP messages over UDP

Multimedia Networking 7-19 Name translation and user location r caller wants to call callee, but only has callee’s name or address. r need to get IP address of callee’s current host:  user moves around  DHCP protocol  user has different IP devices (PC, PDA, car device) r result can be based on:  time of day (work, home)  caller (don’t want boss to call you at home)  status of callee (calls sent to voic when callee is already talking to someone) Service provided by SIP servers: r SIP registrar server r SIP proxy server

Multimedia Networking 7-20 SIP Registrar REGISTER sip:domain.com SIP/2.0 Via: SIP/2.0/UDP From: To: Expires: 3600  when Bob starts SIP client, client sends SIP REGISTER message to Bob’s registrar server (similar function needed by Instant Messaging) Register Message:

Multimedia Networking 7-21 SIP Proxy r Alice sends invite message to her proxy server  contains address r proxy responsible for routing SIP messages to callee  possibly through multiple proxies. r callee sends response back through the same set of proxies. r proxy returns SIP response message to Alice  contains Bob’s IP address r proxy analogous to local DNS server

Multimedia Networking 7-22 Example Caller with places a call to (1) Jim sends INVITE message to umass SIP proxy. (2) Proxy forwards request to upenn registrar server. (3) upenn server returns redirect response, indicating that it should try (4) umass proxy sends INVITE to eurecom registrar. (5) eurecom registrar forwards INVITE to , which is running keith’s SIP client. (6-8) SIP response sent back (9) media sent directly between clients. Note: also a SIP ack message, which is not shown.