Transcript TCP/IP How it Works Les Cottrell
TCP/IP How it Works
Les Cottrell – SLAC
Lecture # 1 presented at the Workshop on Scientific Information in the Digital Age: Access and Dissemination ICTP, Trieste, Italy October , 2009
www.slac.stanford.edu/grp/scs/net/talk09/ictp-tcpip.ppt
• • • • • •
Overview
• This is not a lecture on how to program TCP/IP, rather an introduction to how major portions works, it also does not cover IPv6.
IP Addressing: IP addresses, ARP, routing ICMP UDP TCP: flow control, error recovery, establishment, diconnect References:
– “Internetworking with TCP/IP, volume I, principles, protocols & Architecture”, by Douglas Comer – “TCP/IP Illustrated: the protocols”, by W. Richard Stevens – Most information also available free via Web searches 2
Internet Protocol (IP RFC-791)
TCP/IP Internet provides 3 layers of service Application services Transport Services Connectionless packet delivery service •Layering allows one to replace one service without affecting others •IP layer (basic unit of transfer in TCP/IP) provides: •
Best-effort
(does not discard capriciously),
unreliable
(no guarantees) •Packet may be lost, duplicated, out-of-order with no notification •
Connectionless
(each packet treated independently) •IP software provides routing 3
Internet datagram (“packet”)
• Basic transfer unit Datagram header Datagram data area • Format of Internet datagram 0 Vers 4 Hlen 8 Type of serv.
16 19 24 Total length Identification TTL Protocol Flags Fragment offset Header Checksum Source IP address Destination IP address IP Options (if any) Data … Padding 31 4
•
IP Datagram format (cont.)
Source & destination IP address
(32 bits each): contain IP address of sender and intended recipient •
Options
(variable length): Mainly used to record a route, or timestamps, or specify routing 5
IP Fragmentation
• How do we send a datagram of say 1400 bytes through a link that has a
Maximum Transfer Unit (MTU)
of say 620 bytes?
• Answer the datagram is broken into fragments Net 1 MTU=1500 Net 2 MTU=620 Net 3 MTU=1500 – Router fragments 1400 byte datagrams • Into 600 bytes, 600 bytes, 200bytes (note 20 bytes for IP header) • Routers do NOT reassemble, up to end host 6
Fragmentation Control
•
Identification
: copied into fragment, allows destination to know which fragments belong to which datagram •
Fragment Offset
(12 bits): specifies the offset in the original datagram of the data being carried in the fragment – Measured in units of 8 bytes starting at 0 •
Flags
(3 bits): control fragmentation – Reserved (0-th bit) – Don’t Fragment – DF (1 st bit): • useful for simple (computer bootstrap) application that can’t handle • also used for MTU discovery (see later) • if need to fragment and can’t router discards & sends error to source – More Fragments (least sig bit): tells receiver it has got last fragment • TCP traffic is hardly ever fragmented (due to use of MTU discovery). About 0.5% - 0.1% of TCP packets are fragmented .
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Fragment series composition
Offset=0 More frags Offset=1480 More frags Offset=2960 More frags Offset=3440 Last frag NB. If data segment contains its own header that is not replicated 8
Internet Addressing
• IP address is a 32 bit integer – Refers to interface rather than host – Consists of network and host portions • Enables routers to keep 1 entry/network instead of 1/host – Class A, B, C for unicast – Class D for multicast – Class E reserved – Classless addresses • Written as 4 octets/bytes in decimal format – E.g. 134.79.16.1, 127.0.0.1
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Internet Class-based addresses
• Class A: large number of hosts, few networks – 0nnnnnnn hhhhhhhh hhhhhhhh hhhhhhhh • 7 network bits (0 and 127 reserved, so 126 networks), 24 host bits (> 16M hosts/net) • Initial byte 1-127 (decimal) • Class B: medium number of hosts and networks – 10nnnnnn nnnnnnnn hhhhhhhh hhhhhhhh • 16,384 class B networks, 65,534 hosts/network • Initial byte 128-191 (decimal) • Class C: large number of small networks – 110nnnnn nnnnnnnn nnnnnnnn hhhhhhhh • 2,097,152 networks, 254 hosts/network • Initial byte 192-223 (decimal) • Class D: 224-239 (decimal) Multicast [RFC1112] • Class E: 240-255 (decimal) Reserved 10
Subnets
• A subnet mask is applied to the host bits to determine how the network is subnetted, e.g. if the host is: 137.138.28.228, and the subnet mask is 255.255.255.0 then the right hand 8 bits are for the host (255 is decimal for all bits set in an octet) • Host addresses of all bits set or no bits set, indicate a broadcast, i.e. the packet is sent to all hosts.
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Prefix Length
Subnet Mask Conversions
Prefix Subnet Mask Subnet Mask Length /1 /2 /3 /4 /5 /6 /7 /8 /9 /10 /11 /12 /13 /14 /15 /16 128.0.0.0
192.0.0.0
224.0.0.0
240.0.0.0
248.0.0.0
252.0.0.0
254.0.0.0
255.0.0.0
255.128.0.0
255.192.0.0
255.224.0.0
255.240.0.0
255.248.0.0
255.252.0.0
255.254.0.0
255.255.0.0
/17 /18 /19 /20 /21 /22 /23 /24 /25 /26 /27 /28 /29 /30 /31 /32 255.255.128.0
255.255.192.0
255.255.224.0
255.255.240.0 255.255.248.0
255.255.252.0
255.255.254.0
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252
255.255.255.254
255.255.255.255
Decimal Octet Binary Number 128 192 224 240 248 252 254 255 1000 0000 1100 0000 1110 0000 1111 0000 1111 1000 1111 1100 1111 1110 1111 1111 12
Address depletion
• In 1991 IAB identified 3 dangers – Running out of class B addresses – Increase in nets has resulted in routing table explosion – Increase in net/hosts exhausting 32 bit address space • Four strategies to address – Creative address space allocation {RFC 2050} – Private addresses {RFC 1918}, Network Address Translation (NAT) {RFC 1631} – Classless InterDomain Routing (CIDR) {RFC 1519} – IP version 6 (IPv6) {RFC 1883} 13
Creative IP address allocation
• Class A addresses 64 – 127 reserved – Handle on individual basis, got some back (eg Stanford) • Class B only assigned given a demonstrated need • Class C – divided up into 8 blocks allocated to regional authorities – 208-223 remains unassigned and unallocated • Four main registries handle assignments – APNIC – Asia & Pacific www.apnic.net
– ARIN – N. & S. America, Caribbean & sub-Saharan Africa www.arin.net
– RIPE – Europe and surrounding areas www.ripe.net
– AFRINIC 14
Private IP Addresses
• IP addresses that are not globally unique, but used exclusively in an organization • Three ranges: – 10.0.0.0 - 10.255.255.255 a single class A net – 172.16.0.0 - 172.31.255.255 16 contiguous class Bs – 192.168.0.0 – 192.168.255.255 256 contiguous class Cs • Connectivity provided by Network Address Translator (NAT) – translates outgoing private IP address to Internet IP address, and a return Internet IP address to a private address – Only for TCP/UDP packets 15
Class InterDomain Routing (CIDR)
• Many organization have > 256 computers but few have more than several thousand • Instead of giving class B (16384 nets) give sufficient contiguous class C addresses to satisfy needs – < 256 addresses assign 1 class C – … – < 8192 addresses assign 32 contiguous Class C nets 16
CIDR & Supernetting
• Since assigned contiguously, class C CIDR has same most significant bits & so only needs one routing table entry • CIDR block represented by a prefix and prefix length –
Prefix
= single address representing block of nets, e.g
• 192.32.136.0 = 11000000 00100000 10001000 00000000 while • 192.32.143.0 = 11000000 00100000 10001111 00000000 – 21 bit prefix (2048 host addresses)
Prefix length
indicates number of routing bits, e.g.
192.32.136.0/21 means 21 bits used for routing Mask = 255.255.248.0
• CIDR collects all nets in range 192.32.136.0 through 143.0 into a single router entry –
reduces router table entries
• Removes address classes A, B & C boundaries • For more details see RFC 1519 17
Address Recognition Protocol (ARP)
• IP address is at network layer, need to map it to the MAC (Ethernet address) link layer address • Use ARP to map 48 bit Ethernet address to 32 bit IP – IP requests MAC address for IP address from local ARP table – If not there, then an ARP request packet for IP address is sent using physical broadcast address (all FFFs) – Host with requested IP address responds with its MAC address as a unicast packet – On return, host updates ARP table and returns MAC address – ARP cache times out – ARP packets are on top of Ethernet 18
ARP cont.
• ARP requests are local only, do not cross routers Subnet 1 Subnet 2 134.79.10.17
134.79.10.1
134.79.15.1
134.79.15.3
User A User B • Compare local IP and subnet mask => local subnet • Compare local subnet to destination IP – if local, ARP for MAC address – else remote so • if ROUTE entry, ARP for router to subnet • if default route, ARP for default gateway • otherwise, drop packet & return error 19
Routing
• Routers must select next hop for packet • Get route information from other routers via a routing protocol (RIP, OSPF, EIGRP, BGP etc.) • Note the following are non-routable: – private networks: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 – Loopback 127.0.0.0/24 20
ICMP Purpose (RFC 792)
• Communicates control & error information – Between routers and hosts – Only reports to original source, suggests corrections – Error messages about error messages are not generated – Never generated due to multicasts • Packet format 0 Type 8 Code 16 24 Checksum ICMP data (depends on type/code) 31 21
Main ICMP request types
4 5 8 11 12 Type 0 3 ICMP
Echo reply, ping Destination unreachable (code 1 host, code 3 port) DF and must fragment (code 4)
Source quench Redirect (change a route)
Echo request Time exceeded (code 0 ttl=0
, code 1 reassembly) Parameter problems 22
ICMP Echo/Ping
• Very commonly used diagnostic tool • Implementations vary between OS’ • Build echo request 0 Type=8 8 Code=0 Identifier 16 Checksum Sequence number Optional data 24 31 – Identifier used to match request to replies (e.g. pid) – Sequence number, starts at 0 increments by 1 for each ping packet • Used to detect loss, reorder, duplicates – Optional data, sent by requester, returned by replier • Usually contains a timestamp when the request was sent plus pad data 23
What do we learn from Ping
• Host reachable – Host may respond to ping but not be running services • Round trip timing • Lost packets • Packet reordering duplicate packets • Example: 13cottrell@noric05:~>ping -c 4 lhr.comsats.net.pk
PING lhr.comsats.net.pk (210.56.16.10) from 134.79.125.205 : 56(84) bytes of data.
64 bytes from lhr.comsats.net.pk (210.56.16.10): icmp_seq=0 ttl=242 time=716.962 msec 64 bytes from lhr.comsats.net.pk (210.56.16.10): icmp_seq=1 ttl=242 time=720.375 msec 64 bytes from lhr.comsats.net.pk (210.56.16.10): icmp_seq=2 ttl=242 time=725.907 msec 64 bytes from lhr.comsats.net.pk (210.56.16.10): icmp_seq=3 ttl=242 time=710.734 msec --- lhr.comsats.net.pk ping statistics -- 4 packets transmitted, 4 packets received, 0% packet loss round-trip min/avg/max/mdev = 710.734/718.494/725.907/5.566 ms 24
0 8 Type 11 Code
Time Exceeded
16 24 31 Checksum Unused Internet header & 8 bytes of data • Time-to-live has expired at a router (code=0) – ttl sets bound on number routers datagram can transit • Prevents infinite routine loops • Initialized by sender, decremented by 1 each time passes router • When ttl = 0 datagram thrown away & sender notified by ICMP message • Fragment reassembly timer (code=1) 25
MTU Discovery
• Path MTUs vary • Fragmentation is bad • Small transmission units are bad • SO need to discover optimum MTU (largest without fragmentation) • Host sends a packet with the Don’t Fragment bit set – Length is lesser of local MTU and MSS announced by remote system – If MTU between hosts requires fragmentation (e.g. at an intermediate router), then • if an ICMP DF bit set & must fragment then an ICMP message is sent back to source, saying “I can’t fragment” • try again with smaller size.
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User Datagram Protocol - UDP
• RFC 768, Protocol 17 Demux on App.
Port 1 Port 2 Port 1 Port 2 Port number Transport TCP UDP Demux on Network IP IP protocol • Provides unreliable, connectionless on top of IP • Minimal overhead, high performance – No setup/teardown, 1 datagram at a time • Application responsible for reliability – Includes datagram loss, duplication, delay, out-of sequence, multiplexing, loss of connectivity 27
0
UDP Datagram format
8 16 24 31 Source port Destination port UDP message len Checksum (opt.) Data … • Source/destination port: port numbers identify sending & receiving processes – Port number & IP address allow any application in any computer on Internet to be uniquely identified – Used to demultiplex datagrams to processes – Ports can be static or dynamic • Static (< 1024) assigned centrally, known as well known ports • Dynamic • Message length in bytes includes the UDP header and data 28
UDP applications
• Message oriented, e.g. SNMP, DNS, time, some Real Time data (e.g. VoIP data, but not setup) • Some File systems, e.g. NFS, AFS • Lightweight file transfer, e.g. tftp, bootp 29
Transmission Control Protocol -TCP
• RFC 768 & host requirements RFC 1122 – Reliable stream transport • Connection oriented (full duplex virtual circuit) – Conceptually place call, two ends communicate to agree on details – After agreeing application notified of connection – During transfer, ends communicate continuously to verify data received correctly – When done, ends tear down the connection – If UDP is like regular mail, TCP is like phone call • Provides buffering and flow control • Takes care of lost packets, out of order, duplicates, long delays • Isolates application program from network details • Jargon – Segment = TCP packet – Socket= source (address + port) + destination (address + port) 30
TCP layering
App.
Port 1 Port 2 Port 1 Port 2 Demux on Transport TCP UDP Port number Demux on IP port 6 Network IP IP protocol • To ID connection need: – Source: (address, port) AND Destination: (address, port) – Only need one port on host to allow multiple connections, since each connection will have different (host, port) at other end • E.g. single host can serve multiple telnet connections • Passive open: application contacts OS & indicates will accept incoming connection, OS assigns port and listens • Active open: application requests OS to connect to an (host, port) 31
TCP – providing reliability
• Positive acknowledgement (ACK) with retransmission – Sender keeps record of each packet sent – Sender awaits an ACK – Sender starts timer when sends packet Sender site Send pkt 1 Receiver site Rcv pkt 1 Send ACK 1 Rcv ACK 1 Send pkt 2 Rcv pkt 2 Send ACK 2 Rcv ACK 2 Network messages 32
TCP – simple lost packet recovery
Sender site Send pkt 1 Start timer ACK normally arrives Timer expires Retransmit pkt 1 start timer Rcv ACK 1 Receiver site Loss Pkt should arrive ACK should be sent Rcv pkt 1 Send ACK 1 Network messages 33
TCP – improving performance
• BUT simple ACK protocol wastes bandwidth since it must delay sending next packet until it gets ACK • Use sliding window Window slides Initial window of 4 packets 1 2 3 4 5 6 7 8 … 1 2 3 4 5 6 7 8 … Packets successfully sent Packets to be sent Packets sent, awaiting ACK • Sender can send 4 packets of data without ACK – When sender gets ACK then can send another packet – Window = unacknowledged packets/bytes – Keeps timer for each packet 34
Tuning to fill pipe
• Optimal window size depends on: – Bandwidth end to end, i.e. min(BW links ) AKA bottleneck bandwidth – Round Trip Time (RTT) – For TCP keep pipe full • Window (sometime called pipe) ~ RTT*BW –
Can increase bandwidth by
Src
orders of magnitude
Rcv • Windows also used for flow control t = bits in packet/link speed RTT 35
Implementation
• Sliding window operates at byte level, NOT packet Current window 1 2 3 4 5 6 7 8 … Highest byte that can be sent Highest byte sent Bytes sent and acknowledged 3 pointers • Receiver keeps similar window to put stream back together • Since full duplex, altogether 4 windows & pointer sets 36
TCP flow control
• Windows vary over time – Receiver advertises (in ACKs) how many it can receive • Based on buffers etc. available – Sender adjusts its window to match advertisement – If receiver buffers fill, it sends smaller adverts • Used to match buffer requirements of receiver • Also used to address congestion control (e.g. in intermediate routers) 37
0 4 Source port 8
TCP Segment format
10 16 24 31 Destination port Hlen Sequence number Acknowledgement number Resv Code Checksum Options (if any) Window Urgent ptr Padding Data if any … • •
Source/Dest port
: TCP port numbers to ID applications at both ends of connection
Sequence number
: ID position in sender’s byte stream 38
•
TCP segment format – cont.
Acknowledgement
: identifies the number of the byte the sender of this segment expects to receive next • • •
Hlen
: specifies the length of the segment header in 32 bit multiples. If there are no options, the Hlen = 5 (20 bytes)
Reserved Code
for future use, set to 0 : used to determine segment purpose, e.g. SYN, ACK, FIN, URG 39
•
TCP Segment format- cont
Window
: Advertises how much data this station is willing to accept. Can depend on buffer space remaining.
• • •
Checksum
: Verifies the integrity of the TCP header and data. It is mandatory.
Urgent pointer
: used with the URG flag to indicate where the urgent data starts in the data stream. Typically used with a file transfer abort during FTP or when pressing an interrupt key in telnet.
Options
: used for window scaling, SACK, timestamps, maximum segment size etc.
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TCP timeout
• Need a timeout estimate that will work for LANs (RTT < msec.) to satellite WANs (hundreds of msec. to secs). RTT can vary a lot with time of day, day of week, or one second to next.
– TCP records time segment sent – and time ACK received – Then calculates RTT sample – Smooth & use to estimate timeout, e.g.
• Timeout=beta * RTT s • Timeout= RTT s + eta{=4}*f(dev(RTT s )) – Needs to take account of losses, e.g.
• New_timeout=gamma{2} * timeout May 12th Time of day 41
TCP connection establishment
• 3 way handshake Site 1 Send SYN seq x Site 2 Rcv SYN segment Send SYN seq=y, ACK x+1 Rcv SYN/ACK Send ACK y+1 Rcv ACK segment • Initial sequence numbers (x, y) are chosen randomly • Guarantees both sides ready & know it, and sets initial sequence numbers, also sets window & mss • Once connection established, data can flow in both directions, equally well, there is no master or slave 42
TCP close connection
• Modified 3 way handshake (or 4 way termination) Site 1 (App closes) Send FIN seq=x, ACK=y Site 2 Rcv FIN segment FIN Wait1 Rcv ACK segment Close Wait Send seq=y, ACK x+1 (inform app) Rcv FIN + ACK seg Send ACK y+1 FIN Wait2 Last ACK (app closes connection) Send FIN seq=y, ACK x+1 Time Wait Closed Receive ACK segment • App tells TCP to close, TCP sends remaining data & waits for ACK, then sends FIN • Site 2 TCP ACKs FIN, tells its application “end of data” • Site 2 sends FIN when its app closes connection (may be long delay (e.g. require human interaction).
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–
More Information
• Lectures, tutorials etc: www.nv.cc.va.us/home/joney/tcp_ip.htm
– www.cs.pdx.edu/~jrb/tcpip.lectures.html
– www.raleigh.ibm.com/cgi-bin/bookmgr/BOOKS/EZ306200/CCONTENTS – – – www.cisco.com/univercd/cc/td/doc/product/iaabu/centri4/user/scf4ap1.htm
www.cis.ohio-state.edu/htbin/rfc/rfc1180.html
www.jbmelectronics.com/tcp.htm
• Encylopaedia – – – – – http://www.freesoft.org/CIE/index.htm
• TCP/IP Resources www.private.org.il/tcpip_rl.html
• Understanding IP addresses http://www.3com.com/solutions/en_US/ncs/501302.html
• Configuring TCP (RFC 1122) ftp://nic.merit.edu/internet/documents/rfc/rfc1122.txt
• Assigned protocols, ports etc (RFC 1010) http://www.es.net/pub/rfcs/rfc1010.txt
& /etc/protocols 44
Example: 3 way handshake
• atlas> telnet sunstats.cern.ch
– atlas is a WNT PC, sunstats is a Sun Solaris 5.6 host – MSS is set in TCP option in a SYN segment, communicates the MSS the sender wants to receive – len=ip_hlen/tcp_hlen:ip_total_len – Initial Sequence Numbers are randomly selected – Telnet = port 23 – W=Receive window size advertises how much data this host will accept 45
Example: 3 way handshake - cont.
• TCP from atlas:1174 to sunstats:23 seq=180839, A=0, W=8192, SYN [len=5/6:44, opt=020405B4
Session start
SLAC>CERN: 256kbyte window,1 stream, full speed > 30msec, 13MBytes in 20s, 5.1MBytes/s Congestion window Rcvr Advertised window Segments sent Acks returned by Rcvr 47
Unreachable
76cottrell@flora06:~>ping islamabad-server2.comsats.net.pk
ICMP 13 Unreachable from gateway 207.45.205.18
for icmp from FLORA06.SLAC.Stanford.EDU (134.79.16.101) to islamabad-server2.comsats.net.pk (210.56.8.8) What does this mean, see exercise?
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