Protocol stacks

OSI-viitekehys, standardointi, MSC-kaaviot, lukuohje tenttiin

Kirja sivut 6-39

Layered Protocols

• Protocols connect entities on same level, within the layer

• Higher layers use services provided by lower layers

• Layers are independent and defined by interface provided to the higher layer and required from the lower layer

• Entities communicate by exchanging Protocol Data Units (PDUs) with entities on the same level

Entity

Entity Peer

Entity Peer Entity Protocol

Entity provides an interface

Entity uses an interface

Why layers?

• Layers may be exchanged as long as interfaces stay unchanged

– WWW was built on top of existing TCP/IP implementations – WWW does not have to care about the media (modem,

Ethernet, radio link etc.)

– ATM can be used to transport IP packets without any changes to applications

– IPv6 will replace the entire IP part of the TCP/IP layer

• Layering makes development easier and adds flexibility

Protocol stacks

• Term protocol stack refers to all layers of a protocol family – E.g. having TCP/IP stack implemented in an operating system

means that part of the OS software uses device drivers (Ethernet, PPP+serial driver etc.) to receive IP packets and provides socket services to applications programs

– Protocol stack has nothing to do with push/pop stacks (data structures)

• TCP/IP by IETF is the most popular protocol stack in data communications

• Signaling System 7 is the most popular telecoms stack

• OSI protocol stack is hardly used in entirety except as a theoretical model (parts of it are in use)

• Implementations of protocol stacks often co-exist (OSI X.500 directory system over TCP/IP, TCP/IP

communications over telephone network and SS7)

OSI Stack

Physical Fyysinen

Physical 1

Link Siirto

Data link 2

Network (IP) Verkko

Network 3

Transport (TCP & UDP) Kuljetus

Transport 4

Yhteys Session

5

Esitys Presentation

6 Application

Sovellus Application

7

IETF OSI

OSI Model

• Two (N)-level entities in different systems communicate using (N)-level protocol

• The services give by layer (N) to layer (N+1) are realized in (N)-level protocol by encapsulation and decapsulation

• Encapsulation means embedding each layer's Service Data Units (SDU) into the Protocol Data Units (PDU) of the layer immediately below it, decapsulation is the reverse process

• Entities at the same level in separate systems are called peer entities

• (N+1)-level entities are using (N)-level services through (N)- level Service Access Points ((N)-SAPs)

• One (N+1)-entity can be simultaneously connected to one or more (N)-SAPs

• One (N)-SAP is connected to one (N)-entity

A data packet

Eth IP TCP SMTP CRC IP TCP SMTP

SMTP

Eth IP TCP SMTP CRC

IP TCP SMTP

PPP IP TCP SMTP FCS PPP IP TCP SMTP FCS

IP TCP SMTP ...

Host

Router

Next router

• Encapsulation and decapsulation over different types of links

• Application contents do not change

• IP protocol crosses over different link layer protocols

Service Interface

• Protocols are usually internationally standardized

• Service interface implementations are usually operating system specific

– Different implementations can communicate, with common protocol

– Application programs need to be modified to use different interfaces

• Unix socket interface example:

– 1. open socket – 2. parse address – 3. connect to server – 4. write request

– 5. read response – 6. close socket

What Protocols do?

• A protocol shall be:

– Completely and unambiguously defined – Free of dead-locks and live-locks

– Able to recover from all error conditions

• Some possible functions of protocols – Addressing

– Connections and confirmations – Error detection and correction – Flow control

– Prioritization

– Multiplexing, segmentation and blocking

Protocol Addressing

• On the Internet:

– People usually use domain names (max 255 chars, e.g.

www.nixu.fi)

– IP-protocol uses numeric addresses (four octets, e.g.

194.197.118.20)

– TCP and UDP use 16 bit port addresses (e.g. 80), separate address spaces

– LANs and data link level protocols have their own addressing (e.g. Ethernet, six octets)

– Application level programs must be able to handle both IP addresses and port addresses (Unix socket service), but not data link addresses

– Application level protocols sometimes have their own addresses (e-mail, URL)

– E.g. Telnet protocol does not do any addressing

• Other

– Telephone numbers – Postal addresses

Connections

• Some protocols, like TCP provide a connection from end to end, some, like UDP are connectionless

• TCP must store the state and sub-states of the connection – No connection

– Link set-up

– Data transmission – Link disconnect

• Three packets to open a TCP connection and four to close

• Stateless and connectionless UDP is easier to implement and lighter on the network

– Applications must worry about data loss, e.g. domain name re- requests

Confirmation

• Protocol can provide a receipt

• Example:

– TCP header contains the sequence numbers of traffic – UDP itself provides only a single datagram transmission – SMTP server acknowledges when an e-mail message is

received

– SNMP traps are not acknowledged and may be lost

Error control

• Lowest level transmission channels are analog and somewhat unreliable

• Data may be changed, entire frames (packets) may be lost

• For reliable communications we need:

– Error detection – Error correction

– Data retransmission

• Having no error control is also option – E.g. old modems and terminals

Checksums

• Trivial checksum example

– 1+2+3+4=10, we transmit 1,2,3,4,10

– We receive: 1,3,3,4,10 and check 1+3+3+4=11 -> Error detected

– Our algorithm is not very good, how about: 1,3,2,4,10?

• Actual algorithms are more complex

– CRC-CCITT catches certainly all burst errors of 16 bits or less – And is most likely to catch all other errors, too

• Parity bits

– Odd parity, make data element always to have odd number of ones, e.g.: 11100010 -> 111000101

• Cryptographic checksums (often called also hashes) make it very hard or impossible to replace or change the data

• Checksum design depends on the type of error expected – Physical media errors often come in bursts

Error Detection

• Checksums can be done at different protocol levels and for different sized chucks of data

– Parity bit for old terminals – IP has header checksum

– TCP and UDP checksums check both header and payload – TCP/IP application level protocols do not usually do error

detection, they trust TCP and UDP

– Encryption protocols (e.g. SSH) usuallyt have internal checksums to protect against tampering

Error Detected, then What?

• Ignore the faulty data

– Used with redundant real time data, like voice

• Freeze totally (e.g. old IBM PC and memory parity check)

• Report error and let higher level protocol decide

• Correct error

– The usual response

• And how about losing an entire frame (packet, cell) in transmission

– Nothing received, nothing detected

– Sender must wait and after a timeout retransmit – Or receiver polls for more data

– Or it does not matter (real time voice and picture transmission)

Error correction

• Forward Error Control

– Add enough redundancy to data to correct errors without delay (this costs)

– E.g. Hamming code

– Usually used for time critical or one way protocols – Several low level protocols also do this

• Backward Error Control – Catch error

– Discard data

– Ask for a retransmission – TCP does this

Flow Control

• What to do when the receiver’s memory is full?

• Special characters in data stream – E.g. XON/XOFF, ctrl-S/ctrl-Q

– Used by terminals

• Limited receive window (e.g. TCP, Kermit) – Sliding window increases efficiency

– The sender can not send more than a certain amount of data above what is acknowledged as received

• Receiver polls for individual data units – Not quite a flow control, really

– UDP-based SNMP does this

• No flow control – E.g. IP

Prioritization and Quality of Service

• Currently not implemented in TCP/IP

• Will likely change in future

– Operators offer QoS inside their own networks

• Several technologies

– RSVP, reservation of bandwidth – Diffserv, higher service classes first – MPLS, Multi Protocol Label Switching

• QoS services compete with trivial solutions

– Dedicated circuits to define the bandwidth to one customer – Massive bandwidth (e.g. Gigabit Ethernet)

• 3G (UMTS) telephone networks will use Voice over IP (VoIP) and require QoS

Segmentation and Concentration

• Underlying layers might have special limitations

– Typically (Ethernet) packet maximum size is less than IP packet maximum size

• A protocol can perform segmentation and concentration operations to the higher level SDUs

– IP packet fragmentation and reassembly is a sample of this – A large SDU is split to numbered smaller chunks

– The chunks are sent separately

– The receiver constructs the original SDU

Multiplexing and Blocking

• Multiplexing

– The joining of several data streams in one connection – Usually done at low level of the network (at telephone

system level)

– Packet data networks do statistical multiplexing inherently

– Everybody does not usually send at same time

• Blocking

– Packing several SDUs into one PDU

Transparency

• How to transmit any data?

• How to do in-band signaling?

– Out of band signaling = we have a separate channel for signaling

– E.g. classical telephony

– In-band signaling = control and data share same channel – E.g. Internet

• Encapsulate data in frames

– Frames can have a start and stop marker – Markers in data have to be passed

– Frame header can hold the count of objects in the frame – This is what TCP/IP and Ethernet do

– Frames can be of fixed size (or time) – ATM cells

Transparency, Stuffing and Counting

• Data Stuffing

– The frame is limited by markers

– The marker starts a control sequence

– One control sequence just passes data looking like the marker

– E.g. send the marker twice to pass the marker – E.g. & > < in HTML

– Telnet has control codes in the data stream

• Data counting

– The header of a frame holds count of the amount of data – Used by HTTP

Routing and switching

• How to get a packet to a socket over the net?

• Routing and forwarding:

– Grab each packet, read its’ address and push it to right direction

• Switching (two operations):

– 1. Reserve a path between the endpoints – 2. Transmit data along the path

Routing

• Router is a computer with two or more network interfaces

• Router receives a packet at its’ network interface

• Router reads the recipient address on the packet

• Routing table tells the router which interface to forward this packet to

– Tables can be dynamic or static

• Each packet is an individual case

• IP routers can peek into TCP data, too, for security reasons

• A routing network can usually recover from link loss, sometimes without data loss

• Terms:

– Routing: the decision about where to forward the packet – Forwarding: the actual task of moving the data

Circuit Switching

• A channel is allocated over the network for each host to host connection before any data is transferred

– Channels are reserved, even if no data is transmitted – Used in telephone systems

• Other kinds of switching

– IP packet switching is used on LANs for performance and security

– Data paths are formed on the fly

– ATM cell switching uses small, 53-byte cells

– Easier to implement in hardware than variable length IP packets

– ATM supports bandwidth reservation

• A switching network can usually recover from link loss, but connections over the missing link will be broken

Discovering the Features of the Transmission Path

• A protocol can optimize its behaviour

– TCP discovers the Maximum Transmittable Unit for efficiency

• Is likely to become more important in future with

the mobile devices, which move from low speed

high latency 2G mobile networks to high speed

low latency wireless LAN networks

Discovering the Features of the Protocol Entities

• Enables optimal use of features

– E.g. a handheld device might not support colour

– E.g. two implementations of an encryption protocol must agree on which algorithm to use

– E.g. compression algorithm to be used

• “None” is often considered a feature and provides a lowest common level between implementations

– Several encryption protocols have “none” as one of the standard encryption algorithms which must be

implemented, while this is useful for debugging it is also potentially dangerous

Standardointi

Perustuu Bengt

Sahlinin materiaaliin

Standardit

• Standardi on yleisesti hyväksytty mahdollisimman yksiselitteinen määrittely

• Standardointi edistää eri valmistajien tuotteiden – Ennustettavuutta

– Yhteensopivuutta – Vaihtokelpoisuutta

• Standardointi voi vakiinnuttaa olemassa olevia käytäntöjä tai luoda pohjan uusien tuotteiden suunnittelulle

• Liki kaikkea voi standardoida – Ohjelmistorajapintoja

– Tietoliikenneprotokollia

– Fyysisiä liittimiä, jännitetasoja, virtarajoja – Tuotekehitysprosesseja

– Hyväksi havaittuja käytäntöjä (best practice -standardit)

De facto -standardit

• Tyypillisesti valmistaja, jolla on merkittävä asema markkinoilla tai syntyy luonnostaan

– Tuote on ensimmäinen laatuaan – Avaintuote

– Markkinajohtajuus

– Varsinaisten standardien puute

• Esimerkiksi:

– Windows, PC-arkkitehtuuri kokonaisuutena

• Edut:

– Tässä ja nyt käytössä oleva menettely

• Riskit:

– Usein puutteellisesti määritelty

– Standardin haltija voi hyötyä tilanteesta

Internet Engineering Task Force (IETF)

• Organisaatio, joka standardoi Internetiin liittyviä asioita – Etenkin protokollia

• Koostuu työryhmistä – Määritelty tavoite

– Ryhmä lakkauttaa itsensä kun tavoite on saavutettu

• Avoin prosessi

– Kuka tahansa voi osallistua IETF:n toimintaan ja työryhmiin – Suurin osa työstä sähköpostilistoilla

– Tapaamisia kolmesti vuodessa

• Alkuperäinen ohjausperiaate: “running code and rough consensus”

– Nykyään IETF kärsii kasvusta

• http://www.ietf.org/

Internet Engineering Task Force (IETF)

• Kaikki julkaisut ovat saatavilla maksuttomasti

– Request for Comments -sarja (RFCt) – ftp://ftp.funet.fi/pub/doc/rfc/

– Internet Draftit

– Työdokumentteja, yleensä ideoita tai ehdotuksia protokolliksi tai arkkitehtuureiksi

– Kuka tahansa voi julkaista Draftin – Työryhmä voi jatkaa kehitystä

RFC-lajit

• Standards track

– Standardeiksi pyrkivät RFC:t

• Informational

– Tiedoksi, eivät edusta IETF:n konsensusta

• Experimental

– Tutkimustuloksia, Internetiin liittyvää tietoa

• Historical

– Vanhentuneet RFC eivät poistu vaan jäävät olemaan

• Best Current Practice (BCP)

– Dokumentoivat hyviksi havaittuja käytäntöjä

• STDs

– Muutamia erityisen tärkeitä Internet-standardeja

– Vain perusprotokollilla kuten TCP, UDP ja IP on STD- status

3rd Generation Partnership Program (3GPP)

• Määrittelee kolmannen sukupolven mobiileja solukkoverkkoja (WCDMA)

• Julkaisee kokonaisia standardiperheitä jaksoissa (parhaillaan määritellään release 7:ää)

• http://www.3gpp.org/

• Jäsenet ovat organisaatioita

– Yrityksiä, julkishallintoa

• Määrittelyt luodaan vaiheittain

– Vaihe 1 (stage 1): Vaatimukset – Vaihe 2 (stage 2): Arkkitehtuuri

– Vaihe 3 (stage 3): Yksityiskohtainen määrittely

3rd Generation Partnership Program (3GPP)

• Toiminta on jaettu osa-alueisiin

– RAN: 3G-radioverkko (Radio Access Network) – GERAN:GSM ja EDGE radioverkko

– CT: Runkoverkko ja päätelaitteet

– SA: Palvelut ja järjestelmäarkkitehtuuri

• Alueilla on tarkemmin fokusoituja työryhmiä – Esim. SA WG1 keskittyy palveluihin

– RAN WG1 keskittyy radioverkon tasoon 1

• Kaikki dokumentit ovat virallisia määrittelyjä olennaisille toiminnoille

– UMTS – IMS – MBMS

– WLAN interworking

3GPP2

• Määrittelee myös kolmannen sukupolven radioverkkoa, mutta eri teknologialla (CDMA)

• Toimii kuten 3GPP

• Standardit kohtuullisen harmonisoituja 3GPP:n kanssa

• http://www.3gpp2.org/

Institute of Electrical and Electronics Engineers (IEEE)

• Tietoliikennepuolella keskittyy OSI-mallin kahteen alimpaan kerrokseen

– Ethernet, WLAN...

• http://www.ieee.org/

Open Mobile Alliance (OMA)

• Standardoi mobiilipalveluita

– Esim. Push to Talk och DRM

• Jäsenet organisaatioita

• http://www.openmobilealliance.org/

European Telecommunications Standards Institute (ETSI)

• On standardoinut mm. mobiiliverkkoja

– Olennainen GSM:n määrittelyssä

• http://www.etsi.org

• Edelleenkin aktiivinen

– ETSI TISPAN

– Määrittelee seuraavan sukupolven nopeita

lankaverkkoja (Next Generation Networks, NGN) – Tavoitteena nopea IP-pohjainen (puhelin)verkko – 3GPP IMS (IP Multimedia Subsystem) perustana

International Telecommunication Union (ITU)

• YK:n alainen standardointiorganisaatio

• Jäseninä valtioita ja suuria organisaatioita

• On standardoinut mm. ISDN:n, B-ISDN:n och GSM:n

• Rakentuu alueista ja työryhmistä

• Suljettu prosessi

• http://www.itu.int/

Lisää tietoliikennestandardoijia

• World Wide Web Consortium (W3C) – WWW:hen liittyvät standardit

– http://www.w3c.org/

• Liberty Alliance

– Sähköisen indentiteetin menetelmiä – http://www.projectliberty.org/

• CERT Coordination Center (CERT/CC) – Ei standardoi

– Edistää Internetin tietoturvaa

– http://www.cert.org/, http://www.cert.fi/

• ATM Forum

– Edistää ATM-verkkojen kehitystä julkaisemalla standardeja – http://www.atmforum.com/

• WAP Forum

Muita standardointiorganisaatioita

• International Organisation for Standardization (ISO) – Standardoi kaikkia aloja

– Kehittänyt Open Systems Interconnection (OSI) -mallin – http://www.iso.org/

• Allliance for Telecommunications Industry Solutions (ATIS) – USA:-lainen

– Suunnittelee mm. seuraavan sukupolven lankaverkkoja

• American National Standards Institute (ANSI) – Julkaisee USA:n kansallisia standardeja

– http://www.ansi.org/

• National Institute of Standards and Technology – Kehittää USA:n kansallisia standardeja eri alueille – http://www.nist.gov/

• Ja paljon lisää kansallisia organisaatioita

Standardoinnin merkitys

• Tekninen yhteensopivuus ja laatu

• Kommunikointi ostajan ja myyjän välillä

– Standardeja noudattavilla tuotteilla yleensä kilpailuetu

• Insinööriosaamisen dokumentointi

Message Sequence Charts

• An useful graphical tool for identifying the

sequence and actors of an signaling operation

• Notation

– Entities: vertical lines with names – Signaling messages: named arrows

– Can lock participants – Condition requirements

– Internal actions (links to other MSCs) – Time constraints

– Order of messages – Time flows down

– No ordering can be indicated

A MSC Sample

MS BTS-old BTS-new BSC

Channel_activate

Channel_activate_ack Handover_command

Handover_command

Handover_access

Handover_detect Physical_information

Handover_complete

Handover_complete Move_to_new_channel

Another MSC Sample (Incomplete)

Browser DNS Proxy WWW-server

URL_from_user

Cache_lookup(URL)

Cache_store(URL, www_page) Get_IP(proxy)

Get_www(URL)

Get_IP(URL_host)

Get_www(URL_path)

Return(www_page)

Not_Found

Return(www_page)

Alt

MSC use

• Typically MSC charts are used to describe most important usage cases

– The charts are not always complete – There are variations in the notation

– MSC supports also complete descriptions

• Protocol Data Units (PDUs) can be derived from the MSC charts

– Relevant information must be described

• PDU design

– Either bit/byte maps

– Or abstract notation of information contents and binary format derived from it

Lukuohje tentteihin ja välikokeisiin

Tenttialue

• Keskeiset asiat

– Luennoista

– Luentokalvoista

– Kirjan vastaavista alueista – Harjoitustehtävistä

Kysymystyypit

• Konseptit ja akronyymit

– K: DNS Primary server

– V: Nimipalvelun palvelin, jossa ylläpidetään alueen dataa – K: TCP

– V: Transmission Control Protocol, tarjoaa tavuvirran sovelluksille – Lyhyt selitys riittää

• Perustele väittämä oikeaksi tai vääräksi

– K: TCP tarjoaa aina samanlaisen rajapinnan ohjelmoijalle

– V: Väärin, TCP tarjoaa saman palvelun, mutta rajapinnat saattavat vaihdella eri käyttöjärjestelmissä

– K: Internetissä on yhteyksiä

– V: Oikein, vaikka verkko ei yleensä ole tietoinen yhteyksistä, esim.

TCP-yhteyden osapuolet näkevät olevansa yhteydessä

– V: Väärin, Internet-verkko välittää vain IP-paketteja, yhteydet ovat näennäisiä

• Toisinaan sekä "oikein" että "väärin" saattaa olla hyväksyttävä vastaus, pisteet annetaan perustelun mukaan

Kysymystyyppejä ...

• Lyhyet kysymykset, esim. "Miten TCP-yhteys avataan?"

– Kirjoitettu vastaus, kuva jos sopii kysymykseen, saa käyttää ranskalaisia viivoja

– Tentit pyritään laatimaan mittaamaan soveltavaa osaamista, jolloin kysymys saattaisi olla "Miten Internet toimisi, jos IP paketin otsaketiedoista poistettaisi Time To Live-kenttä?"

– Vastauksen informaatiosisällön laajuudesta voi tehdä oletuksia pistemäärän perusteella

• Esseet

– Kirjoitettava esseemuotoon, kaaviot ja luettelot eivät saa olla pääosa vastauksessta. Esseen tulisi näyttää joltain, jonka voisi julkaista vaikkapa ammattijulkaisussa.

Kurssin toinen puolisko

• Luennot jatkuvat 28.3.

• Käsitellään televerkkojen ja tietoturvan perusteet

Conclusion

If the label on the cable on the table at your house,

Says the network is connected to the button on your mouse, But your packets want to tunnel on another protocol,

That's repeatedly rejected by the printer down the hall,

And your screen is all distorted by the side effects of gauss, So your icons in the window are as wavy as a souse,

Then you may as well reboot and go out with a bang, 'Cause as sure as I'm a poet, the sucker's gonna hang!

http://www.netfunny.com/rhf/jokes/96q1/seuss.html