Message Transport With The User Datagram Protocol

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1 Message Transport With The User Datagram Protocol

2 User Datagram Protocol (UDP) Use During startup For VoIP an some vieo applications Accounts for less than 10% of Internet traffic Blocke by some ISPs Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

3 UDP Characteristics En-to-en Connectionless communication Message-oriente interface Best-effort semantics Arbitrary interaction Operating system inepenence No congestion or flow control Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

4 En-To-En Communication UDP provies communication among applications Sening UDP Accepts outgoing message from application Places message in a User Datagram Encapsulates User Datagram in an IP atagram an sens Receiving UDP Accepts incoming User Datagram from IP Extracts message an elivers to receiving application Note: message is unchange by the network Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

5 Connectionless Communication An application using UDP can Sen a message to any receiver (universal) Sen at any time (asynchronous) Stop sening at any time (unterminate) That is, a sener oes not Inform the network before sening (ie, oes not establish a communication channel) Inform the other enpoint before sening Inform the network or other enpoint that no more messages will be sent Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

6 Message-Oriente Interface UDP Accepts an elivers messages (blocks of ata) Does not require all messages to be the same size, but oes efine a maximum message size Places each outgoing User Datagram in a single IP atagram for transmission Always elivers a complete message to receiving application Sening application must ivie outgoing ata into messages; UDP sens what it is given (or reports an error if the message is too large) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

7 UDP Message Size UDP allows up to 64K octet messages As a practical limit, the size of a User Datagram is limite by payloa area in IP atagram Maximum IP payloa is 64K octets minus size of IP heaer Therefore, the maximum UDP payloa is 64K octets minus size of IP an UDP heaers (usually 64K octets minus 28) Application can choose any message size up to the maximum UDP payloa Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

9 Large An Small Messages What happens if an application sens a 10K octet message? The message fits into an IP atagram, but Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

10 Large An Small Messages What happens if an application sens a 10K octet message? The message fits into an IP atagram, but network frames have a smaller MTU (typically 1500 octets) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

11 Large An Small Messages What happens if an application sens a 10K octet message? The message fits into an IP atagram, but network frames have a smaller MTU (typically 1500 octets) So, the result of sening a large message is Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

12 Large An Small Messages What happens if an application sens a 10K octet message? The message fits into an IP atagram, but network frames have a smaller MTU (typically 1500 octets) So, the result of sening a large message is IP Fragmentation! Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

13 Large An Small Messages What happens if an application sens a 10K octet message? The message fits into an IP atagram, but network frames have a smaller MTU (typically 1500 octets) So, the result of sening a large message is IP Fragmentation! What happens if an application chooses a small message size, such as 20 octets? Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

14 Large An Small Messages What happens if an application sens a 10K octet message? The message fits into an IP atagram, but network frames have a smaller MTU (typically 1500 octets) So, the result of sening a large message is IP Fragmentation! What happens if an application chooses a small message size, such as 20 octets? Inefficiency! Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

17 Choosing An Optimal Message Size What size messages shoul an application sen? Optimal UDP message size is S = M H M is the path MTU (ie, minimum MTU on the path) H is the size of IP an UDP heaers Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

18 Choosing An Optimal Message Size What size messages shoul an application sen? Optimal UDP message size is S = M H M is the path MTU (ie, minimum MTU on the path) H is the size of IP an UDP heaers Fining M requires an application to Violate layering an obtain forwaring information from IP Note: for IPv4, only the local MTU is known Bottom line: it may be ifficult/ impossible for an application to compute S Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

21 UDP Semantics UDP uses IP for elivery an offers the same semantics! UDP packet can be Lost Duplicate Delaye Delivere out of orer Delivere with ata bits altere Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

22 UDP Semantics UDP uses IP for elivery an offers the same semantics! UDP packet can be Lost Duplicate Delaye Delivere out of orer Delivere with ata bits altere Note 1: UDP oes not introuce such errors; the errors arise from the unerlying networks Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

23 UDP Semantics UDP uses IP for elivery an offers the same semantics! UDP packet can be Lost Duplicate Delaye Delivere out of orer Delivere with ata bits altere Note 1: UDP oes not introuce such errors; the errors arise from the unerlying networks Note 2: UDP oes inclue an optional checksum to protect the ata (but the checksum may be isable) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

24 Using Best-Effort Semantics Questions Do best-effort semantics make any sense for applications? Why woul a programmer choose UDP? Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

25 Using Best-Effort Semantics Questions Do best-effort semantics make any sense for applications? Why woul a programmer choose UDP? Answers Retransmitting a lost message oes not make sense for real-time auio an vieo applications because a retransmitte packet arrives too late to be use Aitional real-time protocols can be ae to UDP to hanle out-of-orer elivery (we will cover later in the course) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

26 Arbitrary Interaction UDP permits arbitrary interaction among applications 1-to-1 1-to-many Many-to-1 Many-to-many Application programmer chooses interaction type Ability to sen a single message to multiple recipients can be valuable Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

27 Efficient Implementation Of Interaction Key point: UDP can use IP broacast or multicast to eliver messages Provies efficient elivery to a set of hosts Example: UDP packet sent to IPv4 estination aress is elivere to all hosts on the local network (IPv6 has an all noes multicast aress) No nee for sener to transmit iniviual copies Allows application to fin a server without knowing the computer on which the server runs Broacast is a significant avantage of UDP over TCP for some applications Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

28 Operating System Inepenence Goal is to allow applications on heterogeneous computers to interact Must avoi OS-specific ientifiers, such as Process IDs Task names Instea, create application ientifiers that are not erive from any OS Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

29 UDP Application Ientifiers 16-bit integer known as UDP protocol port number Each application using UDP must obtain a port number Sening UDP Places a port number in UDP heaer to ientify estination application on receiving host Also inclues port number of sening application Receiving UDP Uses value in heaer to select appropriate application UDP protocol port numbers are universal across all computers, an o not epen on the operating system Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

30 Ientifying An Application Both sening an receiving applications nee a port number Assignment of port numbers epens on the type of application Application that offers a stanarize service (server) Uses a well-known port number for the service Value is less than 1024 Example: TFTP service uses UDP port 69 Other applications (client) Request a port number from the local operating system Value is greater than Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

31 Steps Taken To Contact A Service Request an unuse local port number from the local operating system Obtain the IP aress of the local computer from the operating system Look up the port number of the service to be contacte Obtain the omain name of a computer that runs the service an map to an IP aress Form a UDP atagram with a source port fiel set to the local port number an the estination port fiel set to the port number of the service Request that the UDP atagram be encapsulate in an IP atagram an sent using the source an estination IP aresses obtaine above Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

32 Examples Of Well-Known UDP Ports Port Number Description 0 Reserve (never assigne) 7 Echo 9 Discar 11 Active Users 13 Daytime 15 Network Status Program 17 Quote of the Day 19 Character Generator 37 Time 42 Host Name Server 43 Who Is 53 Domain Name Server 67 BOOTP or DHCP Server 68 BOOTP or DHCP Client 69 Trivial File Transfer 88 Kerberos Security Service 111 Sun Remote Proceure Call 123 Network Time Protocol 161 Simple Network Management Protocol 162 SNMP Traps 514 System Log Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

33 UDP Datagram Format Extremely thin layer User Datagram is ivie into heaer an payloa Heaer contains only 8 octets: UDP SOURCE PORT UDP DESTINATION PORT UDP MESSAGE LENGTH UDP CHECKSUM PAYLOAD (MESSAGE DATA) Question: why is length neee? Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

36 UDP Checksum 16-bit 1s-complement checksum Covers entire UDP packet, incluing ata (recall: IP oes not checksum the payloa) Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

37 UDP Checksum 16-bit 1s-complement checksum Covers entire UDP packet, incluing ata (recall: IP oes not checksum the payloa) Is optional: value of zero means sener i not compute a checksum Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

38 UDP Checksum 16-bit 1s-complement checksum Covers entire UDP packet, incluing ata (recall: IP oes not checksum the payloa) Is optional: value of zero means sener i not compute a checksum Inclues extra pseuo heaer that contains IP aresses Example of IPv4 pseuo heaer: IP SOURCE ADDRESS IP DESTINATION ADDRESS ZERO PROTO UDP LENGTH Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

39 Purpose Of A Pseuo Heaer Receiver can verify that message arrive at correct computer as well as correct application on that computer Consequence for NAT: if it changes the IP source or estination aress, NAT must recompute UDP checksum Note: pseuo heaers provie another example of layering violations Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

40 UDP Encapsulation User Datagram travels in IP atagram Two levels of encapsulation occur UDP Hr UDP Payloa IP Heaer IP Payloa Frame Heaer Frame Payloa Note: the message the application places in the UDP Payloa fiel may also have heaer an payloa fiels Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

41 Transmission Control Protocol (Stream Transport)

42 Transmission Control Protocol (TCP) The primary transport-layer protocol use in the Internet Accounts for about 90% of all Internet traffic (some estimates are higher) Provies reliability Appeals to programmers Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

43 TCP Characteristics En-to-en communication Connection-oriente paraigm Point-to-point connections Complete reliability Full-uplex communication Stream interface Reliable connection startup Graceful connection shutown Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

44 En-To-En Communication TCP provies communication among pairs of applications Allows an application on one host to communicate with an application on another host Permits multiple applications on a given computer to communicate simultaneously without interference Uses protocol port numbers to istinguish among applications Note: TCP ports are completely inepenent of UDP ports Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

46 En-To-En Principle An Transport Protocols Transport protocols operate in en systems, an view the unerlying Internet as a virtual network Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

47 En-To-En Principle An Transport Protocols Transport protocols operate in en systems, an view the unerlying Internet as a virtual network Host A appl TCP IP net iface communication system as viewe by TCP router IP The Internet net iface net 1 net 2 Host B appl TCP IP net iface Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

48 En-To-En Principle An Transport Protocols Transport protocols operate in en systems, an view the unerlying Internet as a virtual network Host A appl TCP IP net iface communication system as viewe by TCP router IP net iface net 1 net 2 Host B appl TCP IP net iface IP oes not rea or interpret TCP packets Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

49 En-To-En Principle An Transport Protocols Transport protocols operate in en systems, an view the unerlying Internet as a virtual network Host A appl TCP IP net iface communication system as viewe by TCP router IP net iface net 1 net 2 Host B appl TCP IP net iface IP oes not rea or interpret TCP packets When forwaring atagrams, router only processes layers 1 through 3 Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

50 TCP Protocol Port Numbers 16-bit integers use to ientify applications Each application nees a port number TCP well-known port assignments are inepenent of UDP assignments However, to help humans, the same value chosen if service available via either transport Examples Both UDP an TCP assign port 53 to the Domain Name System (DNS) Both UDP an TCP assign port 7 to the echo service Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

51 Protocol Ports, The Four-Tuple, An Flows Key concept: because a TCP connection correspons to a pair of enpoints, the connection is ientifie by four items IP source aress TCP source port IP estination aress TCP estination port Commonly calle the four-tuple Explains how an application such as a web server can communicate with multiple clients at the same time Interestingly, more than four values must be extracte from a frame to ientify a TCP flow Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

52 TCP s Connection-Oriente Paraigm Analogous to a telephone call Pair of applications must Establish a TCP connection before communicating Terminate the connection when finishe Important insights A TCP connection is virtual because only the two enpoints know a connection is in place TCP oes not have keep-alive messages: no packets are exchange unless applications are sening ata Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

53 Limite Interaction A TCP connection only provies communication between a pair of applications Known as a point-to-point communication TCP connection oes not support Reception from an arbitrary set seners Multi-point connections with more than two enpoints Broacast or multicast elivery Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

54 The TCP Reliability Guarantee TCP provies full reliability Compensates for Loss Duplication Delivery out of orer Does so without overloaing the unerlying networks an routers TCP makes the following guarantee Data will be elivere or sener will (eventually) be notifie Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

55 TCP Reliability Uses timeout-an-retransmission Receiver returns an acknowlegement (ACK) to sener when ata arrives Sener waits for acknowlegement an retransmits ata if no acknowlegement arrives Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

56 Illustration Of TCP Retransmission Events at Host 1 Events at Host 2 sen message 1 receive ack 1 sen message 2 receive ack 2 sen message 3 receive message 1 sen ack 1 receive message 2 sen ack 2 retransmission timer expires retransmit message 3 packet lost receive message 3 sen ack 3 Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

57 Why TCP Retransmission Is Har TCP esigne for Internet Roun-trip elays iffer among connections Roun-trip elays vary over time Waiting too long introuces unnecessary elay Not waiting long enough sens unnecessary copies Key to TCP s success: aaptive retransmission Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

60 How Ba Is The Internet? In the ol ays: elays in secons, high variability Now: elays in secons, high variability Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

61 How Ba Is The Internet? In the ol ays: elays in secons, high variability Now: elays in secons, high variability Example roun-trip measurements from Irelan to California, 2009 Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

62 Aaptive Retransmission Continually estimate roun-trip time of each connection Set retransmission timer from roun-trip estimate Illustration of timeout on two connections: est 1 est 2 timeout est 1 est 2 timeout packet lost packet lost Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

63 Review Of Sliing Winow Transport protocols use sliing winow mechanism Iea is to sen multiple packets before waiting for an acknowlegment Winow size is relatively small (tens of packets, not millions) Motivation is to increase throughput Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

64 Illustration Of TCP s Sliing Winow winow initial position still unsent alreay acknowlege winow winow intermeiate position winow moves as acknowlegements arrive final position Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

65 How Sliing Winow Improves Data Rate sen packet host 1 host 2 sen packet sen packet sen packet one sen ack sen ack sen ack sen ack stop-an-go Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

66 How Sliing Winow Improves Data Rate host 1 host 2 host 1 host 2 sen packet sen packet sen packet sen ack sen ack sen four packets one sen four acks sen packet sen ack one sen ack stop-an-go sliing winow Winow size of K improves ata rate by a factor of K Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

67 TCP Flow Control An TCP Winow Flow control mechanism coorinates ata being sent with receiver s spee Buffer size use instea of ata rate Receiver tells sener size of initial buffer Each acknowlegement specifies space remaining in buffer Known as winow avertisement Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

68 Illustration Of TCP Flow Control Sener Events Receiver Events avertise winow=2500 sen ata octets sen ata octets sen ata octets receive ack for 1000 receive ack for 2000 receive ack for 2500 ack up to 1000, winow=1500 ack up to 2000, winow=500 ack up to 2500, winow=0 application reas 2000 octets ack up to 2500, winow=2000 sen ata octets sen ata octets receive ack for 3500 receive ack for 4500 ack up to 3500, winow=1000 ack up to 4500, winow=0 application reas 1000 octets ack up to 4500, winow=1000 receive ack for 4500 Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

69 TCP Congestion Control An Slow Start TCP uses loss or changes in elay to infer congestion in the network When congestion is etecte, sening TCP temporarily reuces the size of the winow When a packet is lost, TCP temporarily reuces the effective winow to one half its current value Later, TCP slowly increases the winow again Congestion avoiance also use when a connection starts Temporarily use a winow size of one segment Double the winow size when ACK arrives Known as slow start Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

70 Full-Duplex Communication TCP connection between A an B provies two inepenent ata streams, one from A to B an the other from B to A Each sie Has a receive buffer Avertises a winow size for incoming ata Uses sequence numbers to number outgoing ata bytes Implements timeout-an-retransmission for ata it sens Application can choose to shut own communication in one irection Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

71 Full-Duplex Communication (continue) Each TCP packet contains fiels for both forwar an reverse ata streams Sequence number for ata being sent in the forwar irection Acknowlegement number for ata that has been receive Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

72 Stream Interface After connection is establishe, TCP accepts a stream of ata bytes from the sening application an transfers them Sening application can choose amount of ata to pass on each request Surprise: TCP ecies how to group bytes into packets Known as stream interface Consequence Data may be passe to a receiving application in chunks that iffer from the chunks that the sening application generate Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

73 Connection Startup An Shutown Difficult problem Packets can be Lost Duplicate Delaye Delivere out-of-orer Either en can crash an reboot Nee to know that both sies have agree to start/ terminate the connection Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

74 Reliable Connection Startup TCP guarantees reliable connection startup that avois replay problems Performe with 3-way hanshake Events at Host 1 Events at Host 2 sen SYN receive SYN + ACK sen ACK receive SYN sen SYN + ACK receive ACK Each sie chooses starting sequence number at ranom Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

75 Graceful Connection Shutown Analogous to 3-way hanshake for startup Guarantees no ambiguity about connection termination Events at Host 1 Events at Host 2 sen FIN + ACK receive FIN + ACK sen ACK receive FIN + ACK sen FIN + ACK receive ACK Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

76 TCP Segment Format TCP packet is calle a segment Segment is encapsulate in IP for transmission Single format use for SYNs, FINs, ACKs, an ata SOURCE PORT DESTINATION PORT SEQUENCE NUMBER ACKNOWLEDGEMENT NUMBER HLEN NOT USED CODE BITS WINDOW CHECKSUM URGENT POINTER OPTIONS (if any) BEGINNING OF DATA Computer Networks an Internets -- Moule Spring, 2014 Copyright 2014 All rights reserve

MODULE V. Internetworking: Concepts, Addressing, Architecture, Protocols, Datagram Processing, Transport-Layer Protocols, And End-To-End Services

MODULE V. Internetworking: Concepts, Addressing, Architecture, Protocols, Datagram Processing, Transport-Layer Protocols, And End-To-End Services MODULE V Internetworking: Concepts, Aressing, Architecture, Protocols, Datagram Processing, Transport-Layer Protocols, An En-To-En Services Computer Networks an Internets -- Moule 5 1 Spring, 2014 Copyright