Download How the TCP/IP Protocol Works

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
1
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
2
– Most information also available free via Web searches
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)
3
•IP software provides routing
Internet datagram (“packet”)
• Basic transfer unit
Datagram header
Datagram data area
• Format of Internet datagram
0
4
8
16
19
24
31
Vers Hlen Type of serv.
Total length
Identification
Flags Fragment offset
TTL
Protocol
Header Checksum
Source IP address
Destination IP address
IP Options (if any)
Padding
Data
…
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 (1st 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 .
7
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
9
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.
11
Subnet Mask Conversions
Prefix
Length
Subnet Mask
/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
Prefix
Length
/17
/18
/19
/20
/21
/22
/23
/24
/25
/26
/27
/28
/29
/30
/31
/32
Subnet Mask
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
134.79.10.17
134.79.10.1
Subnet 2
134.79.15.1
User A
134.79.15.3
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
8
Type
Code
16
24
31
Checksum
ICMP data (depends on type/code)
21
Main ICMP request types
Type
0
3
4
5
8
11
12
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
8
16
Type=8
Code=0
Identifier
24
31
Checksum
Sequence number
Optional data
– 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
Time Exceeded
0
8
16
Type 11 Code
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.
26
User Datagram Protocol - UDP
• RFC 768, Protocol 17
App.
Transport
Network
Port 1
Port 2
Port 1
Port 2
Demux on
Port number
UDP
TCP
IP
Demux on
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-ofsequence, multiplexing, loss of connectivity
27
UDP Datagram format
0
8
Source port
16
24
31
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
Transport
Port 2
Port 1
TCP
Port 2
UDP
IP port 6
IP
Network
• To ID connection need:
Demux on
Port number
Demux on
IP protocol
– 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
Rcv ACK 2
Rcv pkt 1
Send ACK 1
Time
Rcv ACK 1
Send pkt 2
Receiver site
Rcv pkt 2
Send 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 …
Packets successfully sent
1
2 3 4 5 6 7 8 …
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(BWlinks) AKA bottleneck
bandwidth
– Round Trip Time (RTT)
– For TCP keep pipe full
• Window (sometime called pipe) ~ RTT*BW
– Can increase bandwidth by
orders of magnitude
Src
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
TCP Segment format
0
4
8 10
Source port
16
24
31
Destination port
Sequence number
Acknowledgement number
Hlen Resv Code
Window
Checksum
Urgent ptr
Options (if any)
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 for future use, set to 0
• Code: 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.
40
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.
May 12th
–
–
–
–
TCP records time segment sent
and time ACK received
Then calculates RTT sample
Smooth & use to estimate timeout, e.g.
Time of day
• Timeout=beta * RTTs
• Timeout= RTTs + eta{=4}*f(dev(RTTs))
– Needs to take account of losses, e.g.
• New_timeout=gamma{2} * timeout
41
TCP connection establishment
• 3 way handshake
Site 1
Send SYN seq x
Rcv SYN/ACK
Send ACK y+1
Site 2
Rcv SYN segment
Send SYN seq=y, ACK x+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
Rcv ACK segment
Site 2
FIN
Wait1
Close
Wait
FIN
Wait2
Rcv FIN + ACK seg
Send ACK y+1
Time
Wait
Last
ACK
Closed
Rcv FIN segment
Send seq=y, ACK x+1
(inform app)
(app closes connection)
Send FIN seq=y, ACK x+1
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 43
(e.g. require human interaction).
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
<opt=2, len=4, mss=0x5B4=1460>]
• TCP from sunstats:23 to atlas:1174
seq=1383568304, A=180840, W=64240, SYN/ACK
[len=5/6:44, opt=020405B4]
• TCP from atlas:1174 to sunstats:23 seq =180840,
A=1383568305, W=8760 [len=5/5:40, opt=nul]
– Notice window size can vary from segment to segment depending
on buffer space available
– Notice smaller PC window advertisement
– Notice ephemeral port selected by telnet client
– Notice acknowledge next expected byte (=seq+1)
– 0x020405B4: 02 = option type, 04=len, 0x5B4=1460
46
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?
48