chapter 2: application layer
DESCRIPTION
2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS. 2.6 P2P applications 2.7 Socket programming with TCP 2.8 Socket programming with UDP 2.9 Building a Web server. Chapter 2: Application layer. People: many identifiers: - PowerPoint PPT PresentationTRANSCRIPT
2: Application Layer 1
Chapter 2: Application layer
2.1 Principles of network applications
2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
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: 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”
2: Application Layer 3
DNS
Why not centralize DNS? single point of failure traffic volume distant centralized
database maintenance
doesn’t scale!
DNS services hostname to IP
address translation host aliasing
Canonical, alias names
mail server aliasing 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 servers edu DNS servers
poly.eduDNS servers
umass.eduDNS servers
yahoo.comDNS servers
amazon.comDNS servers
pbs.orgDNS servers
Distributed, Hierarchical Database
Client wants IP for www.amazon.com; 1st 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 www.amazon.com
2: Application Layer 5
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 worldwideb USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
e NASA Mt View, CAf 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, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)
2: Application Layer 6
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
Educause for edu TLD 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
does not strictly belong to hierarchy each ISP (residential ISP, company,
university) has one. also called “default name server”
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 hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
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authoritative DNS serverdns.cs.umass.edu
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TLD DNS server
DNS name resolution example
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
iterated query: contacted server
replies with name of server to contact
“I don’t know this name, but ask this server”
2: Application Layer 9
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
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authoritative DNS serverdns.cs.umass.edu
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TLD DNS server
3recursive query: puts burden of
name resolution on contacted name server
heavy load?
DNS name resolution example
2: Application Layer 10
DNS: caching and updating records 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
update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html
2: Application Layer 11
DNS records
DNS: distributed db storing resource records (RR)
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)
Type=A name is hostname value is IP address
Type=CNAME name is alias name for some
“canonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com value is canonical name
Type=MX value is name of
mailserver associated with name
2: Application Layer 12
DNS protocol, messagesDNS protocol : query and reply messages, both with same message format
msg header identification: 16 bit #
for query, reply to query uses same #
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 responseto query
records forauthoritative servers
additional “helpful”info that may be used
Multimedia Networking 7-14
SIP: Session Initiation Protocol [RFC 3261]
SIP long-term vision:
all telephone calls, video conference calls take place over Internet
people are identified by names or e-mail addresses, rather than by phone numbers
you can reach callee, no matter where callee roams, no matter what IP device callee is currently using
Multimedia Networking 7-15
SIP Services 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
determine current IP address of callee: maps mnemonic
identifier to current IP address
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.time time
Bob'stermina l rings
A lice
167.180.112.24
Bob
193.64.210.89
port 38060
Law audio
G SMport 48753
Multimedia Networking 7-17
Setting up a call (more) 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
rejecting a call Bob can reject
with replies “busy,” “gone,” “payment required,” “forbidden”
media can be sent over RTP or some other protocol
Multimedia Networking 7-18
Example of SIP message
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 167.180.112.24
From: sip:[email protected]
To: sip:[email protected]
Call-ID: [email protected]
Content-Type: application/sdp
Content-Length: 885
c=IN IP4 167.180.112.24
m=audio 38060 RTP/AVP 0
Notes: HTTP message syntax sdp = session description protocol Call-ID is unique for every call.
Here we don’t know Bob’s IP address.
intermediate SIPservers 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 caller wants to call
callee, but only has callee’s name or e-mail address.
need to get IP address of callee’s current host: user moves around DHCP protocol user has different IP
devices (PC, PDA, car device)
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 voicemail when callee is already talking to someone)
Service provided by SIP servers:
SIP registrar server SIP proxy server
Multimedia Networking 7-20
SIP Registrar
REGISTER sip:domain.com SIP/2.0
Via: SIP/2.0/UDP 193.64.210.89
From: sip:[email protected]
To: sip:[email protected]
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
Alice sends invite message to her proxy server contains address sip:[email protected]
proxy responsible for routing SIP messages to callee possibly through multiple proxies.
callee sends response back through the same set of proxies.
proxy returns SIP response message to Alice contains Bob’s IP address
proxy analogous to local DNS server
Multimedia Networking 7-22
ExampleCaller [email protected] with places a call to [email protected]
(1) Jim sends INVITEmessage to umass SIPproxy. (2) Proxy forwardsrequest to upenn registrar server. (3) upenn server returnsredirect response,indicating that it should try [email protected](4) umass proxy sends INVITE to eurecom registrar. (5) eurecom registrar forwards INVITE to 197.87.54.21, 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.
SIP client217.123.56.89
SIP client197.87.54.21
SIP proxyum ass.edu
SIP registrarupenn.edu
SIPregistrareurecom .fr
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