Experiential Learning Workshop on Basics of Transport/Network Layer

Transcription

1 Experiential Learning Workshop on Basics of Transport/Network Layer July 05, 2018 Dr. Ram P Rustagi Professor, CSE Dept KSIT, Bangalore rprustagi@ksit.edu.in

2 Resources & Acknowledgements Resources Learning.html Articles in ACCS Journal Slides Example web pages, and programs Acknowledgements: Computer Networking: Kurose, Ross!2

3 Day 1: Basics of Networking Overview Introduction to basic networking Tools Handson 1: using networking tools IP and TCP Headers Analysis of layers in IP, TCP/UDP Handson-2: Analyze IP and TCP headers Fragementation and PMTU Discovery ICMP Errors, NAT Handson-3: ICMP errors, NAT, PMTU ARP, DHCP, Proxy, Gratuituous ARP Handson-4: ARP protocol Summary!3

4 Day 2: Basics of HTTP Overview: HTTP and Versions Request and Response Format, Basic headers Handson-1: Analyze HTTP headers, status codes HTTP persistent and non-persistent connections Apache config support for persistent connections Handson-2: Configuring persistent connections Web caching, HTTP headers for cache control Handson-3: Cachecing, E-tags HTTP cookies mechanisms Secure cookies, sub-domains, HTTP only cookies Handson-4: using cookies, secure cookies Summary!4

5 Day 3: Basics of Transport Layer Overview: Transport layer, requirements Connection less and connection oriented transport Handson-1: Analyze TCP 4-tuple and UDP 2-tuple Pseudo headers in TCP/UDP Concurrent communications : UDP and TCP Handson-2: Using data with same checksum TCP and UDP Error control, TCP flags Handson-3: Connection Mgmt, Queues, and states TCP Streaming, Reliability misnomer, UDP message boundaries Handson-4: TCP Streams and UDP messages Summary!5

6 Day 4: Basics of Web Security Overview: HTTPS protocol Server certificate and server authentication Mixed content and browser warnings Locks icons and HTTP Status Handson-1: HTTPS website with mixed content MITM attack and ARP spoofing MITM with browser and information stealing Understanding HSTS, CSP Handson-2: Implementing ARP Spoofing Summary!6

7 Day 3: Basics of Transport Layer Overview: Transport layer, requirements Connection less and connection oriented transport Handson-1: Analyze TCP 4-tuple and UDP 2-tuple Pseudo headers in TCP/UDP Concurrent communications : UDP and TCP Handson-2: Using data with same checksum TCP and UDP Error control, TCP flags Handson-3: Connection Mgmt, Queues, and states TCP Streaming, Reliability misnomer, UDP message boundaries Handson-4: TCP Streams and UDP messages Summary!7

8 Internet transport-layer protocols reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP no-frills extension of besteffort IP services not available: delay guarantees bandwidth guarantees application transport network data link physical network data link physical network data link physical logical end-end transport network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical Source: Kurose, Ross: Computer Networking, A Top Down Approach!8

9 Port Numbers IANA ranges for port numbers Well known ports: 1 to 1023 Assigned and controlled by IANA Reserved ports: 1024 to Only registered with IANA Not assigned or controlled by IANA Dynamic ports: to Can be used by any process Also used as ephemeral ports!9

10 Transport layer protocol vhow would you design it vwhat would you like to achieve At simplest level Multiplex/de-multiplex At advanced level Reliable delivery i.e. Data integrity Include error detection and retransmissions Sequential delivery Would need buffer Message boundaries Security!10

11 Multiplexing/demultiplexing multiplexing at sender: handle data from multiple sockets, add transport header (later used for demultiplexing) demultiplexing at receiver: use header info to deliver received segments to correct socket application application P1 P2 application socket P3 transport P4 process transport network transport network link link physical network link physical physical Source: Kurose, Ross: Computer Networking, A Top Down Approach!11

12 Connectionless demux: example clientsocket = (9157); application P3 transport network link physical application P1 transport network link physical clientsocket = (5775); application P4 transport network link physical source port: 6428 dest port: 9157 source port:? dest port:? source port: 9157 dest port: 6428 source port:? dest port:? Source: Kurose, Ross: Computer Networking, A Top Down Approach!12

13 Connection-oriented demux Transport layer socket identified by 4(or 5)- tuple: source IP address source port number dest IP address dest port number (Protocol (TCP)) demux: receiver uses all four (or five) values to direct segment to appropriate socket v server host may support many simultaneous TCP sockets: each socket identified by its own 4-tuple v web servers have different sockets for each connecting client non-persistent HTTP will have different socket for each request!13

14 Connection-oriented demux: example Source: Kurose, Ross: Computer Networking, A Top Down Approach application application P3 transport network link physical P4 P5 transport network link physical P6 server: IP address B application P2 P3 transport network link physical host: IP address A source IP,port: B,80 dest IP,port: A,9157 source IP,port: C,5775 dest IP,port: B,80 host: IP address C source IP,port: A,9157 dest IP, port: B,80 three segments, all destined to IP address: B, dest port: 80 are demultiplexed to different sockets source IP,port: C,9157 dest IP,port: B,80!14

15 Connection-oriented demux: example threaded server application application P3 transport network link physical P4 transport network link physical server: IP address B application P2 P3 transport network link physical host: IP address A source IP,port: B,80 dest IP,port: A,9157 source IP,port: C,5775 dest IP,port: B,80 host: IP address C source IP,port: A,9157 dest IP, port: B,80 source IP,port: C,9157 dest IP,port: B,80 Source: Kurose, Ross: Computer Networking, A Top Down Approach Transport Layer3-14!15

16 Transport Layer Protocol Characteristics Connection less May arrive out of order In order delivery requires pkts to be numbered. No acknowledgement, Packets may be lost No prior handshake Connection oriented Setup, data transfer and teardown phase Provides reliability, ordered delivery Handles error control in a better way *!16

17 Transport Layer Reliability Support Reliability Needs error and flow control, loss detection Compels slower service Unreliable protocol No extra overheads Reliability at data link layer Provides error and flow control Why do we need it at Transport layer when Link layer provides the same *!17

18 UDP Design Requirement How to design a simple transport layer? Just provide transport on top of IP Multiplexing and demultiplexing Little bit of error checking No handshake Rest all has to be managed by application Application practically talks to IP DNS uses UDP What happens when query/response is lost? *!18

19 UDP: segment header 32 bits source port # dest port # length checksum length, in bytes of UDP segment, including header includes pseudo header application data (payload) UDP segment format Source: Kurose, Ross: Computer Networking, A Top Down Approach!19

20 Pseudo header for checksum source address destination address zero protocol UDP length Source Destination Port Port Length Checksum data octets Src: RFC 768 Pseudo Headers Actual Headers!20

21 Internet checksum: example RFC 1071 Consider 3 words x x x8F0C x14AC1 Wrapping around the overflow bit makes it x4AC2 1 s complement will be xB53D!21

22 Data with Same Checksum Adding 0xFFFF to checksum results in same value In UDP checksum computation length is used twice Once in UDP header, once in psuedo headers Consider the data addition that adds up to 0xFFFF First thought: use 0x5555 (i.e. UU ) three times This increases length by 6, (which is counted twice) Thus addition of 6 bytes should add to 0xFFF3 Add 0xUQUQUQ (0x ) Yields same checksum Any other combination?!22

23 Data with Same Checksum Given the data text ABCDEF, How it can be changed to use same checksum Swapping two bytes? e.g. CDABEF, or EFCDAB, or ABEFCD etc. Modifying the data without swapping BCBCEF? AAAAIL?!23

24 TCP Characteristics * point-to-point: One sender, one receiver reliable, in-order byte steam: No message boundaries pipelined: TCP congestion and flow control set window size full duplex data: Bi-directional data flow in same connection MSS: maximum segment size, determined from link/ frame size connection-oriented: Handshaking (exchange of control msgs) inits sender, receiver state before data exchange flow controlled: Sender will not overwhelm receiver!24

25 Basis of TCP Handshake 2-way handshake: Let s talk OK ESTAB ESTAB Q: will 2-way handshake always work in network? v variable delays v retransmitted messages (e.g. req_conn(x)) due to message loss v message reordering v can t see other side choose x ESTAB req_conn(x) acc_conn(x) ESTAB Source: Kurose, Ross: Computer Networking, A Top Down Approach!25

26 Basis of TCP Handshake 2-way handshake failure scenarios: Source: Kurose, Ross: Computer Networking, A Top Down Approach choose x req_conn(x) ESTAB choose x req_conn(x) ESTAB retransmit req_conn(x) acc_conn(x) retransmit req_conn(x) acc_conn(x) ESTAB req_conn(x) ESTAB retransmit data(x+1) data(x+1) accept data(x+1) client terminates connection x completes server forgets x client terminates connection x completes req_conn(x) server forgets x ESTAB data(x+1) half open connection! (no client!) ESTAB accept data(x+1)!26

27 TCP 3-Way Handshake client state server state LISTEN SYNSENT choose init seq num, x send TCP SYN msg SYNbit=1, Seq=x choose init seq num, y send TCP SYNACK msg, acking SYN LISTEN SYN RCVD SYNbit=1, Seq=y ACKbit=1; ACKnum=x+1 ESTAB received SYNACK(x) indicates server is live; send ACK for SYNACK; this segment may contain client-to-server data ACKbit=1, ACKnum=y+1 received ACK(y) indicates client is live ESTAB Source: Kurose, Ross: Computer Networking, A Top Down Approach!27

28 TCP Timeouts On timer expiration Retransmits the segment that is not yet acked Sets the TimeoutInterval double of previous value Example: First time out 0.75s 2nd/3rd/ timeout will be 1.5s,3.0s, On receipt of Ack, it is computed again Provides a limited form of congestion control *!28

29 TCP Flow Control to application process RcvBuffer rwnd buffered data free buffer space TCP segment payloads * receiver-side buffering src: Computer Networking : Kurose, Ross!29

30 Typical Understanding of TCP src: ftp:// class/330/sockets/sockets.html!30

31 TCP Client/Server Communication TCP Client socket() connect() write() 3-way handsake Conn Establishment Data Request TCP Server { socket() bind() listen() accept() read() close() Data Response Connecion Close read() write() close()!31

32 \ active OPEN CLOSED \ < \ \ create TCB ^ \ \ snd SYN passive OPEN CLOSE \ \ \ \ create TCB delete TCB \ \ V \ \ CLOSE \ LISTEN delete TCB rcv SYN SEND V snd SYN,ACK / \ snd SYN < > SYN rcv SYN SYN RCVD < SENT snd ACK rcv ACK of SYN \ / rcv SYN,ACK x snd ACK V V CLOSE ESTAB snd FIN TCP State Transition Diagram src: RFC 793!32

33 CLOSE ESTAB snd FIN CLOSE rcv FIN V snd FIN / \ snd ACK FIN < > CLOSE WAIT WAIT rcv FIN \ rcv ACK of FIN CLOSE snd ACK V x V snd FIN V FINWAIT-2 CLOSING LAST- ACK rcv ACK of FIN rcv ACK of FIN rcv FIN Timeout=2MSL x V x V \ snd ACK delete TCB > TIME WAIT > CLOSED TCP State Transition Diagram src: RFC 793!33

34 TCP Connection State Use netstat -nat to know the current state LISTEN SYN-RECD SYN-SENT ESTABLISHED CLOSED -> LAST_ACK FIN_WAIT1 FIN_WAIT2 TIME_WAIT Note: you may have to install net-tools sudo apt install net-tools!34

35 Setup Requirement: TCP/UDP S1 S2!35

36 Hands-On 1a: UDP Checksum Between two machines communicate UDP packets using nc (-u option). Send data content as below and compute the checksum using the IP Address and port number used and verify it with checksum value in wireshark capture. ABCDE and ABCDEUQUQUQ Are the two checksum same? Why or why not? What other characters can you attach to ABCDE to have the same checksum value DrAIT/Basics of Socket Programming RPR!36

37 Hands-On 1b: UDP Msg Boundary Data Transfer Clients sends data (100 bytes) every 2s (10 times) AA.. (1 st pkt), BB..(2 nd pkt),, JJ..(10 th pkt) Server reads 40 bytes at a time What would server receive and display? S1 S2 DrAIT/Basics of Socket Programming RPR!37

38 Hands-On 1c: UDP Packet Loss Data Transfer Clients sends data (100 bytes) every 2s (10 times) AA.. (1 st pkt), BB..(2 nd pkt),, JJ..(10 th pkt) Break the link between switch at 7th sec and restore after 12 sec Server reads full 100 bytes. What would server receive and display? S1 S2 DrAIT/Basics of Socket Programming RPR!38

39 Hands-On 1d: TCP Streaming Data Transfer Clients sends data (100 bytes) every 2s (10 times) AA.. (1 st pkt), BB..(2 nd pkt),, JJ..(10 th pkt) Server reads 30 bytes at a time What would server display S1 S2 DrAIT/Basics of Socket Programming RPR!39

40 Hands-On 1e:TCP Recovery Data Transfer Clients sends data (100 bytes) every 2s (10 times) AA.. (1 st pkt), BB..(2 nd pkt),, JJ..(10 th pkt) Break the link between switch at 7 th sec and restore after 12 th sec Server reads 30 bytes at a time What would server receive: S1 S2 DrAIT/Basics of Socket Programming RPR!40

41 Hands-On 1f:TCP Timeouts TCP timeout study T0: break the link between S1 and S2 Connect client to server How many SYN packets are transmitted What is the time difference between SYN pkts When does client give up Repeat the experiment, but with restoring the link after 1 minute. S1 S2 DrAIT/Basics of Socket Programming RPR!41

42 Hands-On 1g:TCP Recovery & Streaming Data Transfer Clients sends data (100 bytes) every 2s (10 times) AA.. (1 st pkt), BB..(2 nd pkt),, JJ..(10 th pkt) Break the link between switch at 7 th sec and restore after 12 th sec Server reads 30 bytes at a time Study the timeouts of retransmission Analyze the segments on retransmission Do multiple segments combine in a single transmit What happens if combination is more than MTU? DrAIT/Basics of Socket Programming RPR!42

43 Handson 1h: Study of TCP Queuing Run TCP server (nc -l <port>) Connect multiple clients to it Look the TCP connections at both clients and server All should show ESTABLISHED Exchange data from all clients When server responds, which client gets it What happens to data of other clients What happens to other clients when communicating client closes the connection!43

44 Session 2 : setup A!44

45 Session 2: Setup B N1 1 2 Router N2 Switch Ha: /24 Hc: /24 Hb: /24 2!45

46 ARP - Address Resolution Protocol Packet delivery to a host requires two addresses Logical address - IP Address Physical address - MAC address Need to find mapping from logical to physical ARP is used - RFC 826 Src: Forouzan - Computer Networking RPR/Experiential Learning - Network Basics!46

47 ARP - 4 cases MAC MAC RPR/Experiential Learning - Network Basics Src: Forouzan - Computer Networking!47

48 ARP ARP Request and Reply ARP Request is broadcast ARP Reply is Unicast Other forms of ARP Proxy ARP (RFC 1027) Reverse ARP (RFC 903) Gratuitous ARP RPR/Experiential Learning - Network Basics!48

49 Proxy ARP Router (Proxy ARP Server) replied to all requests Used when Splitting a network w/o changing hosts netmask Mobile IP RPR/Experiential Learning - Network Basics Src: Forouzan - Computer Networking!49

50 Reverse ARP Reverse ARP (RARP) : RFC 903 Used for diskless stations Organization does not have enough IP Address Target as MAC Bcast does not cross the router Needs one RARP server for each subnet BOOTP Improvement over RARP Has a relay agent to forward across network Static mapping of MAC to IP Manageability issues DHCP - replaces BOOTP RPR/Experiential Learning - Network Basics!50

51 Gratuitous ARP Ref: Gratuitous ARP Request Both src and dstn IP is set to that of m/c Dstn MAC is broadcast i.e. ff:ff:ff:ff:ff:ff Ordinarily, no reply will occur normally if a m/c exists, it may respond Gratuitous ARP Reply (for HA, LB, Spoofing) A reply to which no request has occurred RPR/Experiential Learning - Network Basics!51

52 Gratuitous ARP Why Gratuitous ARP Help detect IP conflicts if a m/c receives G-ARP req which is its own, implies IP conflict Helps in updating other m/cs ARP tables Used in clustering solutions, when IP is moved Helps inform the switch to update its port table Each time an i/f comes up (after down), sends G-ARP RPR/Experiential Learning - Network Basics!52

53 Life of Packet in Internet DNS Record: -> Q: User at host A types ping -c1 What is the packet flow? RPR/Experiential Learning - Network Basics!53

54 DHCP Goal: allow host to dynamically obtain its IP address Renew its lease on address in use, on lease expiry Preferably gets the same address, not guaranteed Support for mobile users who join network Guarantee: only one address to only one client Retain DHCP client address across reboots not guaranteed Retain DHCP client configs across server reboot Must coexist with statically assigned addresses Should work with DHCP server failovers Interoperate with BOOTP relay agents RPR/Experiential Learning - Network Basics!54

55 DHCP: more than IP addresses DHCP can return more than just allocated IP address on subnet: Address of first-hop router for client Name and IP address of DNS sever Network mask (indicating network versus host portion of address) RPR/Experiential Learning - Network Basics!55

56 DHCP: Dynamic Host Configuration Protocol DHCP overview: An extension of BOOTP mechanism Host broadcasts DHCP Discover msg [optional] DHCP server responds with DHCP Offer msg more than one server can make the offer client can choose which server to send request to Host requests IP address: DHCP Request msg DHCP server sends address: DHCP Ack msg Renewal happens with DHCP Request/ack On completion, client sends DHCP Release msg Practically not seen RPR/Experiential Learning - Network Basics!56

57 Handson-2a: ARP Understand ARP working. Know current ARP working arp -an ping H x (where H x is not in ARP table) but live See the ARP table to have Hx entry ping H y (where H y is not live) What does ARP table shows!57

58 Handson-2b: ARP Create state ARP entry for H z not in ARP table sudo arp -s <IP Addr> <MAC Addr> ping H z No ARP Request should be transmitted. ping -b -c5 <Broadcast address>!58

59 Handson-2c: Gratuituos ARP Use arping to issue gratuituous ARP On Ha, assign IP of Hc sudo ip addr del /24 dev eth0 sudo ip addr add /24 dev eth0 sudo arping -A -I eth Analyze ARP table of Hb!59

60 Handson-2d: ICMP Redirect Ha: 10.x.1.1/ Switch Hc: 10.x.1.201/24 lo:10.x.3.201/24 Hb:10.x.1.101/24 lo:10.x.2.101/24 On Ha sudo ip route add 10.x.2.0/24 via 10.x sudo ip route add 10.x.3.0/24 via 10.x On Hb sudo ip route add 10.x.3.0/24 via 10.x On Hc sudo ip route add 10.x.3.0/24 via 10.x RPR/Experiential Learning - Network Basics!60

61 Handson-3a: Basic Routing Ha: /24 Hc: / Router Hb-e1: /24 Hb-e2: /24 Enable routing on Hb sudo sysctl -w net.ipv4.ip_forward=1 Define routing of /24 on Ha Define routing of /24 on Hc Ping Hc from Ha Note down the ARP table of Ha and Hc. It should show MAC addresses of Hb!61

62 Handson-3b: Proxy ARP Ha: /22 Hc: / Router Hb-e1: /24 Hb-e2: /24 Enable proxy ARP, and routing on Hb sudo sysctl -w net.ipv4.conf.all.proxy_arp=1 sudo sysctl -w net.ipv4.ip_forward=1 Ping Hc from Ha Note down the ARP table of Ha and Hc. It should show MAC addresses of Hb!62

63 Handson-3c: PMTU Discovery Ha: 10.x.1.1/24 Hc: 10.x.3.201/ Router Router Hb-e1:10.x.1.101/24 Hb-e2:10.x.3.1/24 On Hb sudo ip link set dev eth2 mtu 1000 sudo sysctl -w net.ipv4.ip_forward=1 On Ha ping -c 2 s p RPR/Experiential Learning - Network Basics!63

64 Handson-3d: IP Fragmentation Ha: 10.x.1.1/24 Hc: 10.x.3.201/ Router Router Hb-e1:10.x.1.101/24 Hb-e2:10.x.3.1/24 On Hb sudo ip link set dev eth2 mtu 1000 sudo sysctl -w net.ipv4.ip_forward=1 On Ha./udp_client -b s p c 2 RPR/Experiential Learning - Network Basics!64

65 Handson-4a: Longest Prefix Match LAN: eno1 eno1 Hb eno1 Router Router USB Routing table of Hb USB 10.x.1.65/24 10.x.1.34/27 10.x.1.129/26!65

66 Summary Transport Layer Multiplexing UDP Message boundary TCP Streaming TCP Reliability: timeouts and retransmits ICMP errors: Redirect, TTL expiry ARP, Gratuituous ARP IP routing, Longest prefix match!66

67 Thank You!67