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Internetworking
Application Layer Protocols
Literature:Forouzan, TCP/IP Protocol Suite: Ch 18-22, 25
The application layer
● Networked applications give content to the Internet
● 70s-80s
– Text email, remote login, file transfers, newsgroups
● Killer app 90s:
– World-wide-web
● 2000s:
– Instant messaging, peer-to-peer sharing of MP3s
– Streaming video, Internet telephony, Distributed gaming
IP
Ethernet PPP
CSMA async SDH
Copper Fiber Radio
email www phone
SMTP HTTP RTP
TCP UDP
Clients, servers, peers
Computers connected to the Internet are end-systems or hosts (they "host" application programs running on them). Hosts are traditionally divided into clients and servers - the difference nowadays unclear.
But from a program point of view, it is easier:
– Client program - requests a service.
– Server program - provides a service.
– Peer - bot a client and a server program.
Application process, sockets and underlying transport
● Typical networked application: processes communicating over a network
● The processes send messages via a socket interface to ”the network”– API – Application Programming Interface)
● Underlying transport layer takes care of end-to-end communication between two hosts.
host/server
Process
TransportOperatingsystem
Userspace
host/server
Process
Transport
Process
Socket
Internet
SocketMessages Messages
Client-server model
● Client (local application) requests service from server (remote application)
● Many clients use one server● Client runs only when needed● Server is always running● NATs are traversed by initiating all traffic from the client● Peer-to-peer: Every client is also a server
clients server
Concurrent TCP server● Used by most application-servers based on TCP
● Child servers use well-known port and client’s port & IP address. Parent server uses wild-card for client’s port & IP address.
well-known port
well-known port
Application layer protocolsApplication protocols are a vital subset of a networked application
The TCP/IP application layer contains protocols that enable applications to communicate.
The TCP/IP application layer roughly maps to three OSI layers:
– Session: session establishment, dialog control, synchronization
– Presentation: syntax and semantics of data: higher level data structures
– Application: application-specific information and protocols
Some application-protocols exists as RFCs, most nowadays in other forums.
Many are not open or documented
Datatype classification
● Binary format
– Mainly used for ”low level” protocols
– Efficient, but need byte-swapping
– Eg: Ethernet, IP, DNS
● 7-bit ASCII (NVT)
– Primitive control encoding
– Eg: TELNET, FTP, SMTP, ...
● BNF “RFC 822”
– Structured ASCII data
– Eg: HTTP, SIP
● Tree-based
– Hierarchical/recursive structure
– TLV – Type Length Value● Eg: IP options, DHCP
– ASN.1● Eg. SNMP
– XML● Eg: HTML
Binary fixed field: example
The DNS header, taken from RFC 1035.
1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ID | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |QR| Opcode |AA|TC|RD|RA| Z | RCODE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QDCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ANCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | NSCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ARCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Fixed binary format
Most common in the underlying layers of the TCP/IP stack.Predefines exactly what information is to be where in the message.The semantics is hard-coded into the application.
Requires common alignment (ie on 16, 32 or 64 -bit boundaries)Requires byte-swapping: How the CPU loads its registers from memory.
Little endian (eg Intel): LSB (Least Significant Byte) first
Big endian (eg Motorola): MSB (Most Significant Byte) first
Network byte order is big endian --> You need to byte-swap on i386 PCs.
Pros and cons
When you feel the urge to design a [...] complex binary application protocol, it is generally wise to lie down until the feeling passes
Eric Raymond: ``The Art of UNIX Programming''
Pros: ● Compact: Efficient computer processing● Fixed syntax and simple semantics
Cons:
● Not extendable● Not human readable.● Byte order, alignment problems
Augmented BNF: example
● In RFC 2048, the HTTP URL is defined as:– URL = “http:” “//” host [ “:” port ] [abs\_path ]
– host = A legal Internet host domain name or IP address (in dotted-decimal form) as defined by Section 2.1 of RFC 1123
– port = *DIGIT
– abs_path = “/” rel_path
– rel_path = [ path ] [ “;” params ] [ “?”query ]
● Classical Internet format defined by Backus-Naur Form (BNF) – derived from context-free grammars
● RFC822 is “syntax-heavy”: keywords are introduced for parsing, requires specific parsers.
Type, Length, Value: TLV
● Binary format usually used as an extensible part of a protocol.
– Type: contains a predefined code, indicating what kind of data the value field contains.
– Length: Contains the size (in bytes) of the value field.
– Value: Contains the payload.
● Examples: IS-IS and OSPFv3, DHCP, and IP options.
● TLVs can be recursive (value field contains new TLVs).
Example: DHCP router option (RFC 2132)
Code Len Address 1 Address 2 +-----+-----+-----+-----+-----+-----+-----+-----+-- | 3 | n | a1 | a2 | a3 | a4 | a1 | a2 | ... +-----+-----+-----+-----+-----+-----+-----+-----+--
Abstract Syntax Notation #1
● A general way to define data types - ASN.1 is as powerful as a typed programming language.
● In ASN.1 the type information is inherent in the data - no external specification necessary.
● Used frequently in ISO protocols, but also to a certain extent in TCP/IP protocols.
– Examples: SNMP, UMTS, LDAP, NFSv4 and many security protocols.
PDU ::= SEQUENCE { request-id Integer32, error-status INTEGER (
noError(0),tooBig(1),noSuchName(2),badValue(3) ),
error-index INTEGER (0..max-bindings), variable-bindings VarBindList}
XML
● Plain-text markup language: simple syntax, easy to parse.
● Definition declared externally by XML Schema or DTD.
● Well suited for complex data formats with recursive and nested structures.
● Cons mainly its textual nature: parsing can be inefficient.
<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE note SYSTEM "InternalNote.dtd"><note> <to>Eva</to> <from>Phil</from> <heading>Reminder</heading> <body>Remember to go to the store!</body></note>
Specific applications in Forouzan
Protocol Chapter Transp Datatype
Telnet 18 TCP NVT
FTP File Transfer Protocol 19 TCP NVT
TFTP Trivial File Transfer Protocol 19 UDP Binary
SMTP Simple Mail Transfer Protocol 20 TCP BNF
SNMP Simple Network Mngmnt Prot 21 TCP ASN.1
HTTP Hypertext Transfer Protocol 22 TCP BNF
RTP Real Time Protocol 25 UDP Binary
SIP Session Initiation Protocol 25 TCP BNF
TELNET – TErminal NETwork● Virtual Terminal – local terminal appears to be a terminal on a
remote system– Nice tool to test other text-based protocols (FTP, HTTP, SMTP, etc)
● Client/Server using TCP, port 23● Good example of interactive TCP application
– Silly window syndrome, (Nagle, Delayed ack,...)
● TELNET uses out-of-band signaling
– eg TCP URG for ^C
● TELNET is text-based and sends data according to NVT
– Shares data and control in same character stream
– Network Virtual Terminal – simple encoding
● ”Security challenged”: use TELNET with Kerberos or use SSH
TELNET session
NVT – Network Virtual Terminal
0/1
● For simple information encoding ● First bit: 0 – data, 1 – control● Bits 1-7: 7-bit ASCII
Some NVT control characters
Char Decimal Description
EOF 236 End of file
IP 244 Interrupt process
AO 245 Abort output
EC 247 Erase character
GA 249 Go ahead
WILL 251 Agreement to enable option
DO 253 Approval to option request
IAC 255 Interpret next character as control
TELNET Examplehunerik> telnettelnet> toggle optionsWill show option processing.telnet> open 127.0.0.1Trying 127.0.0.1...Connected to 127.0.0.1.Escape character is '^]'.SENT DO ENCRYPTSENT WILL ENCRYPTSENT DO SUPPRESS GO AHEADSENT WILL TERMINAL TYPE...RCVD DO ECHOSENT WONT ECHORCVD WILL ECHOSENT DO ECHO...RCVD WILL ENCRYPTSENT IAC SB ENCRYPT REQUEST-STARTSENT IAC SB ENCRYPT SUPPORT DES_CFB64 DES_OFB64 RCVD DO ENCRYPTRCVD IAC SB ENCRYPT SUPPORT DES_CFB64 DES_OFB64 RCVD WILL SUPPRESS GO AHEAD
OpenBSD/i386 (hunerik) (ttyp4)
User not authenticated. Using plaintext username and passwordlogin: olofPassword:
FTP – File Transfer Protocol
● Standard Internet file transfer protocol● FTP uses two TCP connections
– One for control (port 21)– One for data (port 20)
● Datformat: Textual NVT over the control channel● Client defines which file data type to transfer
– ASCII/binary, stream/block mode, file/record structure,..● Control messages
– Clients generate commands: USER/LIST/RETR/...– Response: 3-digits (+ text): 200 (OK)
● FTP runs in two modes:
– Active: Server starts data TCP connection
– Passive: Client creates TCP data connection. Works if client is behind NAT.
Some FTP Commands
Control commands sent on control channel
CWD <arg> Change working directoryRMD <arg> Remove directoryPWD Print working directoryTYPE [I|A|E|L <arg>] Set the data transfer typeRETR <arg> Download a file.STOR <arg> Upload a file.LIST Download the current working directory's content list.
FTP Status codesStatus codes are sent as replies to commands: a number and an explaining text (Also in HTTP and SIP)
1xx Positive Preliminary reply The requested action is being initiated; expect another reply before proceeding with a new command.
2xx Positive Completion replyThe requested action has been successfully completed. A new request may be initiated. (Example: 200 Command OK)
3xx Positive Intermediate replyThe command has been accepted but the requested action is waiting for further information before being completed.
4xx Transient Negative Completion reply The command was not accepted and the requested action did not take place, but the error condition is temporary and the action may be requested again. (Example: 450 File not available)
5xx Permanent Negative Completion replyThe command was not accepted and the requested action did not take place. (Example: 500 Syntax error; unrecognized command)
Example: Login and file transfer
TFTP – Trivial File Transfer Protocol
● Much simpler than FTP● UDP port 69● Datatypes: Fixed Binary ● Stop and Go protocol
– Send data, wait for ack.
● Small implementations
– typically: implemented in boot prom for boot loading for diskless clients
● Five message types
– RRQ – Read ReQuest
– WRQ – Write ReQuest
– DATA
– ACK
– ERROR
SMTP – Simple Mail Transfer Protocol
● The Internet Email protocol● Client/server - TCP port 25● BNF data format● Protocol used to transfer email from hosts to mail servers and
between mail servers.
– Several steps: relaying– Spooling on sender-site– Mailbox and mail-access protocol on receiver
● User Agent (UA) – end hosts● Mail Transfer Agent (MTA) – mail servers● Addressing:
– <mailbox>@<domain name>
– Results ina DNS MX request giving name of MTA.
SMTP – Simple Mail Transfer Protocol (2)
● Mail Access Protocols– From MTA to UA– POP or IMAP
● Messages: – HELO/ELHO – sender host– MAIL FROM – sender user– RCPT TO – intended receiver– DATA – mail body– Status codes similar to FTP
Email system
SM TP
SM TP
M AILACCESS
PROTOCOL
SMTP Example
MIME Multipurpose Internet Mail Exchange
Classical email messages must be written in 7-bit US-ASCII.
MIME extends this with:
– Textual message bodies in other character sets
– Extensible set of different formats for non-textual message bodies,
– Multi-part message bodies,
– Textual header information in other character sets
Related header fields:
– Content-Type - what kind of data the content carries. Examples: text/plain, text/html, audio, video, application/pdf, extension-token, multipart.
– Content-Transfer-Encoding - how data is encoded. Examples: 7bit, 8bit, binary, quoted-printable, base64,...
Internet Network Management
● It is complex to build internetworks and we need to manage them.– Monitoring– Debugging– Control routers and other network devices
● SNMP - Internet management– No special control messages – use TCP/IP itself – Management is on TCP/IP application level
+ Same protocol is used for all managed devices- If IP does not work correctly,...
- SNMP uses TCP/IP- Datatypes: Abstract Syntax Notation ASN.1- Powerful datatype definition language
SNMP Architecture
● Client/server architecture● Client software on
manager’s workstation● Server software on system
being managed
– Agent
● Clients query agents● Agents respond to clients● Agents send traps to
clients
Router/Device being m anaged
Agent
Agent
Agent
Client
Manager’sHost
query
resp
onse
t rap
HTTP - Hypertext Transfer Protocol
● TCP port 80, RFC 2616, BNF dataformat
● HTTP is the main protocol used to download resources from the world wide web.
● Simplest form: a requestor establishes a TCP connection to the web server on port 80 and sends a string describing what resource it wants, and receives the resource in reply.
● The most modern version today is HTTP/1.1.
Some HTTP commands
● GET http_url: Download an http resource.
● POST http_url: Upload data to an http resource.
● PUT http_url: Write an http resource.
● DELETE http_url: Delete an http resource.
HTTP – example
GET /stuff/blah.html HTTP/1.1Host: zipf.pilsnet.sunet.seUser-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.5) Gecko/20031214 Firebird/0.7Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,[...]Accept-Language: en-us,en;q=0.5Accept-Encoding: gzip,deflateAccept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7Keep-Alive: 300Connection: keep-alive
HTTP/1.1 200 OKDate: Tue, 27 Jan 2004 20:18:28 GMTServer: Apache/1.3.27 (Unix) (Gentoo/Linux) PHP/4.3.4Last-Modified: Tue, 27 Jan 2004 19:53:47 GMTETag: "bb4047-2c-4016c1cb"Accept-Ranges: bytesContent-Length: 44Keep-Alive: timeout=15, max=100Connection: Keep-AliveContent-Type: text/html
<html><b> Hello there </b></html>
HTTP 1/1 persistent connections
● In HTTP 1/0, all HTTP requests generated a new TCP connection.
● But most html documents contain sub-parts --> one TCP connection for each sub-request.
● But TCP congestion control is made for longer connections --> they can adapt to congestion in the network.
● When http traffic grew when the web exploded, these small flows were said to kill the Internet!
● HTTP 1/1 supports persistent connections: keep the TCP connection during the complete session: send all requests on the same TCP connection.
● Now, these longer TCP connection can perform congestion control algorithm in a proper way.
Real-time multimedia
● Time-sensitive, interactive applications (eg telephony, conferencing): use real-time protocol.
– You cannot make retransmissions
– RTP Real-Time Protocol over UDP.● Limited time-sensitivity (eg Video-on-demand,
Radio): use streaming protocols.
– You can buffer at receiver
– RSTP Real-Time Streaming Protocol over TCP● Alternative:
– IPTV can use MPEG over UDP/IP multicast.
Delay Jitter
What happens if the packets arrive with different delays?● There is a gap between first and second packet● This phenomenon is called jitter
RTP: Real-time Transport Protocol
● Designed to carry out variety of real-time data: e.g., audio and video.
● Sequence number for receiver to detect out-of-order delivery
● Timestamp allowing receiver to control playback● Typically run on top of UDP, ● No mechanisms to ensure timely delivery
– Just provides the mechanisms to build a real-time service
SIP Session Initiation Protocol
TCP or UDP port 5060, ABNF
Terminology is similar to SMTP, but is a synchronous protocol (no delays).
SIP uses URI's (Uniform Resource Identifiers) as addresses:
<sip:[email protected]>
<sip:[email protected]>
SIP uses transactions, usually three-way (as TCP connections).
Example:
INVITE --> 200 OK --> ACK
SIP Example
From RFC 3261: softphone proxy proxy SIP Phone
| | | | | INVITE F1 | | | |--------------->| INVITE F2 | | | 100 Trying F3 |--------------->| INVITE F4 | |<---------------| 100 Trying F5 |--------------->| | |<-------------- | 180 Ringing F6 | | | 180 Ringing F7 |<---------------| | 180 Ringing F8 |<---------------| 200 OK F9 | |<---------------| 200 OK F10 |<---------------| | 200 OK F11 |<---------------| | |<---------------| | | | ACK F12 | |------------------------------------------------->| | Media Session | |<================================================>| | BYE F13 | |<-------------------------------------------------| | 200 OK F14 | |------------------------------------------------->|
SIP message example
INVITE sip:[email protected] SIP/2.0Via: SIP/2.0/UDP 192.36.125.167:5060;branch=z9hG4bK0e4415eaFrom: "6534" <sip:[email protected]>;tag=000e38a3b7e8001d597d1d53-1bfa7620To: <sip:[email protected]>Call-ID: [email protected]: Mon, 03 Jan 2005 14:16:06 GMTCSeq: 101 INVITEUser-Agent: CSCO/6Contact: <sip:[email protected]:5060>Expires: 180Content-Type: application/sdpContent-Length: 251Accept: application/sdp
Implementing it: The socket interface
The socket interface is used for programming applications with a network component.
Sometimes called BSD sockets - it was first implemented in C in BSD.
Variants exist for most programming languages.
Winsock is almost the same but not quite!
Other programming interfaces include:– Corba, Streams, Remote Procedure Calls (RPC), etc.
The sockets API is a de facto standard for network programming.
Socket function calls
● socket() – create a socket.● bind() – bind the socket to a local address.● connect() – connect the socket to a remote address.● listen() – ready to accept incoming connections.● accept() – accept an incoming connectiong.● sendto()/write() – send a message on a socket.● recvfrom()/read() – receive a message on a socket.● select() – input multiplexing: wait for input on several
sockets/files or timeout.● close() – close a socket.
UNIX Networking code is organized into 3 layers; socket, protocol, and interface layer
● Socket layer – protocol independent interface to the protocol dependent layer
● Protocol layer – TCP/IP implementation
● Interface layer - device driver
Queues● Socket queues – 1 per socket● Interface queues – 1 per IF● Protocol queue – 1 single IP
queue
Typical IP stack implementation
process
socket layer
protocol layer(TCP, UDP, IP, ICMP, IGMP)
interface layer
protocol queue(IP input queue)
socket queues
system calls
interfacequeues
funct ioncall
startoutput
Summary
● Applications vs Application-layer protocols
● Clients, Servers, Peers
● Processes, Sockets, Messages
● Datatypes: Binary, NVT, BNF, ASN.1, XML, TLV
● The specific applications,...
