Transcript ppt - Courses

Application Layer
Socket API, HTTP, VoIP, DNS
IS250
Spring 2010
John Chuang
Application Layer & Socket API
end-to-end
Appl
Appl
Software
Socket API
end-to-end
Trans
port
Net
work
Net
work
Link
Link
Host A
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Router 1
point-to-point
point-to-point
Trans
port
Net
work
Net
work
Link
Link
Router 2
Operating
System
Hardware
Host B
2
Socket
 Socket: an API (application programming
interface) that allows the abstraction of the
underlying network (i.e., layers 4 and lower)
 Sockets support send and receive methods
 Sockets identified by <IP addr, TCP/UDP port>
 Two common types of sockets:
- Stream socket (supported by TCP)
- Datagram socket (supported by UDP)
 Read: Comer 3.13-3.23
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Network Layers
 Application (layer 7): specific to
application need
 Transport (layer 4): end-to-end
delivery, congestion and flow
control
 Network (layer 3): addressing,
routing
 Data Link (layer 2): framing, error
detection
 Physical (layer 1): bits (0/1),
voltages, frequencies, wires, pins,
…
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HTTP, FTP, NNTP, SMTP,
telnet, ...
TCP, UDP
IP
Wi-Fi
Ethernet
FDDI, SONET
coax, twisted pair, fiber,
wireless, ...
4
Representation v. Transfer
 Application layer protocols specify two aspects
of interaction:
- Data representation: Syntax of data items that are
exchanged, e.g., HTML
- Data transfer: Interaction between client and server,
e.g., HTTP
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L7 Alphabet Soup
 Applications:
-
HTTP (Comer 4.4-4.10)
FTP (Comer 4.11)
Email, SMTP (Comer 4.12-4.16)
telnet, SSH
RTP, SIP (Comer 29)
…
 Network support:
- DNS (Comer 4.17-4.25)
- BGP, RIP, OSPF (Comer 27)
- …
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Hypertext Transfer Protocol
 HTTP specification (RFC 2616):
- http://www.ietf.org/rfc/rfc2616.txt
 HTTP uses TCP
 Two types of HTTP messages
- HTTP Request
- HTTP Response
HTTP Request
Client
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HTTP Response
Server
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HTTP Request Format
Request line
General header
Request header
<method><resource identifier><HTTP version><CRLF>
[<Header>:<value><CRLF>]
…
Entity header
[<Header>:<value><CRLF>]
Blank line
<CRLF>
Entity body
[Message body]
Note:
• <CRLF> = carriage-return-line-feed
• optional fields in []
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HTTP Header Fields

HTTP Methods
-

GET
POST
HEAD
OPTIONS
PUT
DELETE
TRACE
CONNECT
General Header Fields
-
Cache-Control
Connection
Date
Pragma
Trailer
Transfer-Encoding
Upgrade
Via
Warning
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
Request Headers
-
Accept
Accept-Charset
Accept-Encoding
Accept-Language
Authorization
Cookie
Expect
From
Host *
If-Match If-Modified-Since
If-None-Match
If-Range
If-Unmodified-Since
Max-Forwards
Proxy-Authorization
Range
Referrer
TE
User-Agent
* Mandatory in HTTP/1.1

Entity Headers
-
Allow
Content-Encoding
Content-Language
Content-Length
Content-Location
Content-MD5
Content-Range
Content-Type
Expires
Last-Modified
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HTTP Request: Example
Request line
GET /index.html HTTP/1.1<CRLF>
Request header
Host: www.sims.berkeley.edu <CRLF>
Blank line
<CRLF>
HTTP message
TCP header
HTTP message
TCP segment
IP header TCP
Ethernet header IP
port
IP datagram/packet
Ethernet frame
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Adapted from Coulouris, Dollimore and Kindberg
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Note:
• <CRLF> = carriage-return-line-feed
• optional fields in []
HTTP Response

Status line
<HTTP version><status code><Reason phrase><CRLF>
General header
Response header
Entity header
[<Header>:<value><CRLF>]
…
Blank line
<CRLF>
Entity body
[Message body]
Response Header Fields
- Accept-Ranges
- Age
- Etag
- Location
- Proxy-Authenticate
- Retry-After
- Server
- Set-Cookie
- Vary
- WWW-Authenticate
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
Status Codes
-

1xx: Informational
2xx: Success
3xx: Redirection
4xx: Client error
5xx: Server error
Some common codes:
-
200 OK
401 Unauthorized
404 Not Found
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HTTP Response: Example
Status line
General header
Response header
Entity header
Blank line
Entity body
John Chuang
HTTP/1.1 200 OK
Date: Mon, 12 Nov 2007 20:14:04 GMT
Server: Apache/2.0.52 (Red Hat)
X-Powered-By: PHP/4.3.9
Set-Cookie: PHPSESSID=f49e0b5ec; expires=Wed, 05-Dec-2007 23:47:24 GMT; path=/
Last-Modified: Mon, 12 Nov 2007 20:14:04 GMT
Cache-Control: no-store, no-cache, must-revalidate
Cache-Control: post-check=0, pre-check=0
Pragma: no-cache
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en”>
…
</html>
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HTTP/1.1 and Persistent
TCP Connections
 An HTML object often has other
embedded objects (e.g., images)
 HTTP/1.0 requires a new TCP session for
each object
- Overhead of session establishment
 HTTP/1.1 permits reuse of one TCP
session for multiple requests
- Use “Connection: Keep-Alive” header
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L7 Alphabet Soup
 Applications:
-
HTTP (Comer 4.4-4.10)
FTP (Comer 4.11)
Email, SMTP (Comer 4.12-4.16)
telnet, SSH
RTP, SIP (Comer 29)
…
 Network support:
- DNS (Comer 4.17-4.25)
- BGP, RIP, OSPF (Comer 27)
- …
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Real-Time Multimedia
 How to support real-time multimedia
applications (e.g., video, audio) over a besteffort IP network?
- Packets may be delivered out of order or lost
- Retransmission is too slow to be helpful
 Technique: delayed playback using timestamps
and jitter buffer
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Source: Douglas Comer
Real-Time Transport
Protocol (RTP)
 Protocol for transport of real-time multimedia data
- Sequence number and timestamp in RTP
header
Source: Douglas Comer
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Real-Time Transport
Protocol (RTP)
 RTP messages encapsulated in UDP
datagrams
- Why not TCP?
Source: Douglas Comer
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RTP Companion Protocols
 RTCP: Real-time Transport Control
Protocol
- Control packets sent periodically to report
quality such as loss rate and jitter
 RTSP: Real-time Transport Streaming
Protocol
- Used by streaming media players to provide
remote control like capabilities (e.g., play,
stop, fast forward)
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IP Telephony (VoIP)
 Companies around the world are replacing traditional
telephone switches with IP routers
- Routers cost much less than traditional telephone switches
- Sending both data and voice as IP packets lowers cost because
underlying infrastructure is shared
 The basic idea behind IP telephony:
- continuously sample audio, convert to digital (PCM)
- send digitized stream across IP network (RTP)
- convert stream back to analog for playback
 Complications arise in signaling and backward
compatibility with PSTN
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IP Telephony (VoIP)
 IP telephone
 Media gateway
controller
 Media gateway
 Signaling gateway
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Source: Douglas Comer
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VoIP Protocols
 RTP for user multimedia data
 Multiple protocols for signaling:
- SIP: Session Initiation Protocol [RFC 2543] (IETF)
- H.323 (ITU)
- ENUM: converts telephone number to URI; facilitates DNS lookup
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Source: Douglas Comer
Example
SIP
Session
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Source: Douglas Comer
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L7 Alphabet Soup
 Applications:
-
HTTP (Comer 4.4-4.10)
FTP (Comer 4.11)
Email, SMTP (Comer 4.12-4.16)
telnet, SSH
RTP, SIP (Comer 29)
…
 Network support:
- DNS (Comer 4.17-4.25)
- BGP, RIP, OSPF (Comer 27)
- …
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Domain Name Service (DNS)
 Routers and end-hosts use IP addresses
- e.g., 128.32.226.87
 Human beings remember hostnames
- e.g., ischool.berkeley.edu
 Need translation service!
 Client sends query message (hostname) to
domain name server using UDP
 Name server sends response message (with
resolved IP address) back to client
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DNS Query
Local Name Server
(ns1.berkeley.edu)
(1)
(2)
(3)
Client
(client.ischool.berkeley.edu)
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Server
(www.publisher.com)
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DNS Query
Root Name Server
(f.root-servers.net)
Local Name Server
(ns1.berkeley.edu)
(2)
(3)
Remote Name Server
(ns.publisher.com)
(4)
(5)
(1)
(6)
(7)
Client
(client.ischool.berkeley.edu)
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Server
(www.publisher.com)
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Hostnames: [a-m].root-servers.net
Source: http://www.wia.org/pub/rootserv.html
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Next Time: Overlays
 Build your own networks at the
application layer
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