Experiential Learning Workshop on Transport & IP Routing
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1 Experiential Learning Workshop on Transport & IP Routing July 28, 2018 Dr. Ram P Rustagi Professor, CSE Dept KSIT, Bangalore rprustagi@ksit.edu.in
2 Resources Learning.html Articles in ACCS Journal Source code and examples for articles Slides for this talks Example web pages Example programs Computer Networks: A Top Down Approach - Kurose, Ross!2
3 Day 2: Transport Layer & IP Routing Overview: Transport Layer TCP Streaming, Reliability and UDP Msg Boundary Handson-1: Transport layer Basics of IP routing Overlapping subnets, Longest prefix match Handson-2: Understanding IP Routing ICMP errors ICMP Redirect, PMTU discovery Handson-3: Experiment ICMP error Understanding ARP, Proxy ARP, Gratuitous ARP Handson-4: working with ARP Summary!3
4 Day 3: Basics of Web Security Overview: HTTPS protocol Server certificate and server authentication Handson-1: Deploying a SSL certificate Mixed content and browser warnings Locks icons and HTTP Status Handson-2: Creating website with mixed content MITM attack and ARP spoofing MITM with browser and information stealing Handson-3: Implementing MITM with arpspoofing Understanding HSTS, CSP Handson-4: Implementing CSP and HSTS Summary!4
5 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!5
6 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!6
7 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!7
8 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 *!8
9 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 *!9
10 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? *!10
11 UDP: segment header source port # dest port # length 32 bits application data (payload) checksum UDP segment format length, in bytes of UDP segment, including header includes pseudo header no connection establishment (which can add delay) simple: no connection state at sender, receiver small header size no congestion control: UDP can blast away as fast as desired Source: Kurose, Ross: Computer Networking, A Top Down Approach!11
12 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!12
13 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!13
14 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?!14
15 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?!15
16 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!16
17 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!17
18 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!18
19 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 *!19
20 TCP segment structure URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) head len 32 bits source port # dest port # sequence number acknowledgement number not used checksum U A P R S F application data (variable length) receive window Urg data pointer options (variable length) counting by bytes of data (not segments!) # bytes rcvr willing to accept Source: Kurose, Ross: Computer Networking, A Top Down Approach!20
21 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!21
22 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!22
23 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!23
24 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!24
25 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!25
26 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!26
27 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?!27
28 Day 2: Transport Layer & IP Routing Overview: Transport Layer TCP Streaming, Reliability and UDP Msg Boundary Handson-1: Transport layer Basics of IP routing Overlapping subnets, Longest prefix match Handson-2: Understanding IP Routing ICMP errors ICMP Redirect, PMTU discovery Handson-3: Experiment ICMP error Understanding ARP, Proxy ARP, Gratuitous ARP Handson-4: working with ARP Summary!28
29 Life of Packet in Internet DNS Record: -> Q: User at host A types ping -c1 What is the packet flow?!29
30 IP datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) ver head. len 16-bit identifier time to live type of service upper layer 32 bits flgs length 32 bit source IP address fragment offset header checksum total datagram length (bytes) for fragmentation/ reassembly upper layer protocol to deliver payload to how much overhead? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead 32 bit destination IP address options (if any) data (variable length, typically a TCP or UDP segment) e.g. timestamp, record route taken, specify list of routers to visit. Source: Kurose, Ross: Computer Networking, A Top Down Approach!30
31 Simple Routing!31
32 Multi Network Routing!32
33 TTL Expiry Send a ping (ICMP) pkt with TTL of 1 on Ha ping -c1 -t1 10.x.3.1 Send a ping (ICMP) Pkt with TTL of 2 on Ha ping -c1 -t2 10.x.3.1 Send a ping (ICMP) Pkt with TTL of 3 on Ha ping -c1 -t3 10.x.3.1 This should succeed.!33
34 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!34
35 Handson-3b: Multi N/W Routing Enable routing on R1 and R2 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!35
36 Day 2: Transport Layer & IP Routing Overview: Transport Layer TCP Streaming, Reliability and UDP Msg Boundary Handson-1: Transport layer Basics of IP routing Overlapping subnets, Longest prefix match Handson-2: Understanding IP Routing ICMP errors ICMP Redirect, PMTU discovery Handson-3: Experiment ICMP error Understanding ARP, Proxy ARP, Gratuitous ARP Handson-4: working with ARP Summary!36
37 IP fragmentation, reassembly network links have MTU (max.transfer size) - largest possible link-level frame different link types, different MTUs large IP datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams Source: Kurose, Ross: Computer Networking, A Top Down Approach!37
38 IP fragmentation, reassembly example: 4000 byte datagram MTU = 1500 bytes length =4000 ID =x fragflag =0 one large datagram becomes several smaller datagrams offset = bytes in data field length =1500 ID =x fragflag =1 offset =0 offset = 1480/8 length =1500 ID =x fragflag =1 offset =185 length =1040 ID =x fragflag =0 offset =370 Source: Kurose, Ross: Computer Networking, A Top Down Approach!38
39 Example : Re-fragmentation Re-fragmentation? Src: Forouzan - Data Communication and Networking!39
40 Example : Re-fragmentation Re-fragmentation Src: Forouzan - Data Communication and Networking!40
41 IP fragmentation, reassembly Questions A pkt has arrived with an M bit of 0. Is this the first, last of middle segment? Can we say if pkt was fragmented? A pkt has arrived with an M bit of 1. Is this the first, last of middle segment? Can we say if pkt was fragmented? A pkt has arrived with an M bit of 1 and offset value 0. Is this the first, last of middle segment? Can we say if pkt was fragmented? A pkt has arrived with offset value 100, and HLEN value of 5, value of total length field is 100. What are the numbers of the first byte and last byte?!41
42 ICMP Redirect Src: Forouzan - Computer Networking!42
43 Path MTU Discovery It is a mechanism It is not a protocol Mechanism Intermediate router informs the sender error : destination not reachable code : Fragmentation required!43
44 PMTU Discovery Change MTU of link from R1 to R2 Change MTU of link from R2 to Hb. Ping Hb from Ha.!44
45 ICMP: internet control message protocol used by hosts & routers to communicate networklevel information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer above IP: ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error Source: Kurose, Ross: Computer Networking, A Top Down Approach Query Messages: Type Code description 0/8 0 Echo reply/request (ping) 13/14 0 Timestamp request/reply 10/9 0 Router solicitation/advt Error Reporting Messages 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 5 0 Redirect 11 0 TTL expired 12 0 bad IP header!45
46 ICMP Messages Few points to note No ICMP error msg will be generated for Response to datagram carrying an ICMP error message Fragmented datagram that is not the first fragment Datagram having multicast address Datagram having special address e.g , or !46
47 Handson-3: 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.1.101!47
48 Handson-3: PMTU Discovery Ha: 10.x.1.1/24 Hc: 10.x.3.201/ 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 !48
49 Handson-3b: 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.1.101!49
50 Handson-3c: 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!50
51 Handson-3d: Longest Prefix 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!51
52 Day 2: Transport Layer & IP Routing Overview: Transport Layer TCP Streaming, Reliability and UDP Msg Boundary Handson-1: Transport layer Basics of IP routing Overlapping subnets, Longest prefix match Handson-2: Understanding IP Routing ICMP errors ICMP Redirect, PMTU discovery Handson-3: Experiment ICMP error Understanding ARP, Proxy ARP, Gratuitous ARP Handson-4: working with ARP Summary!52
53 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!53
54 ARP - 4 cases MAC MAC Src: Forouzan - Computer Networking!54
55 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!55
56 Proxy ARP Router (Proxy ARP Server) replied to all requests Used when Splitting a network w/o changing hosts netmask Mobile IP Src: Forouzan - Computer Networking!56
57 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!57
58 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!58
59 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!59
60 Handson-4a: 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!60
61 Handson-4b: 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>!61
62 Handson-4c: 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!62
63 Handson-4d: 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!63
64 Day 2: Transport Layer & IP Routing Overview: Transport Layer TCP Streaming, Reliability and UDP Msg Boundary Handson-1: Transport layer Basics of IP routing Overlapping subnets, Longest prefix match Handson-2: Understanding IP Routing ICMP errors ICMP Redirect, PMTU discovery Handson-3: Experiment ICMP error Understanding ARP, Proxy ARP, Gratuitous ARP Handson-4: working with ARP Summary!64
65 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!65
66 Thank You!66
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