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Understanding Local and Remote Network Communication

This article explores the process of network communication, including sending and receiving packets, ARP, routing, and more.

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Understanding Local and Remote Network Communication

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  1. IP: putting it all togetherPart 1 G53ACC Chris Greenhalgh

  2. Contents • Scenario • Local network communication • Sending a packet • ARP • Receiving a packet • Remote network communication • Routing

  3. Book coverage • Assumed from CCN: • Comer ch. 13 (routing), 17 (internet), 22 (IPv6), 25 (TCP) • Reviewed: • Comer ch. 18 (addresses), 20 (datagram), 21 (frag.) • Additional: • Comer ch. 19 (ARP), 23 (ICMP), 24 (UDP), 26 (NAT), 27 (Internet routing, part)

  4. Scenario • Sending a UDP packet • E.g. the ReverseClientUnicast, or DNS client • Pre-configured machine • On an Ethernet • Connected to the Internet • Running the IP protocol suite • How does communication "really" work?

  5. Scenario (& see text dump) 128.243.22.61 (monet) Start Here  128.243.22.1 Ethernet switch(es) 128.243.21.1 128.243.21/24 Ethernet switch(es) Router 128.243.22/24 Internet 128.243.21.16 (DNS server) 128.243.22.35 (mcclean) 128.243.21.19 155.198.5.83 (www.ic.ac.uk)

  6. e.g. ReverseClientUnicast.java • …int port = Integer.parseInt(args[1]);InetAddress server = InetAddress.getByName(args[0]);DatagramSocket socket = new DatagramSocket();…byte [] data = requestByteStream.toByteArray();DatagramPacket request = new DatagramPacket(data, data.length, server, port);socket.send(request);…

  7. TCP/IP reference model You are here ReverseClient, DNS, … TCP, UDP IP IEEE802 Ethernet, WiFi, … Comer Fig. 17.4

  8. What have you got?(what does the machine know?) • An array of bytes • Application layer data • A destination IP address (not name) • E.g. 128.243.22.35 (case 1); 128.243.21.19 (case 2) • A destination UDP port • A sending UDP socket (=> source UDP port) • Local IP configuration (in OS) • See later

  9. Goal: send that data to the destination machine • But physical network transports Ethernet frames (only!)… You are here: Application Data

  10. Source port =sending socket Dest. port =from request Data = app. Data Length = data length Checksum = error check (CRC) Transport layer, UDP:add UDP header

  11. TCP/IP reference model You are here ReverseClient, DNS, … TCP, UDP IP IEEE802 Ethernet, WiFi, … Comer Fig. 17.4

  12. What have you got now?(what does the machine know?) • A UDP header and datagram payload • Includes source & dest. UDP ports • Application layer data • A destination IP address (not name) • E.g. 128.243.22.35 (case 1); 128.243.21.19 (case 2) • Local IP configuration (in OS) • See later

  13. Network Layer, IP:add IP header

  14. Version = 4 Type = UDP Source IP address =a local IP, probably not filled in yet Destination IP =from request TTL = “Time To Live” (network hops), initially high Header checksum = error check for header Fragment offset – see fragmentation Selected IP header fields

  15. You have: IP packet With IP destination You need: Ethernet (or other network) Interface to send it Ethernet frame With Ethernet destination So…

  16. Local IP configuration • For now assume set by hand, e.g. on monet: • Own IP address, e.g. 128.243.22.61 • Netmask (range of IP addresses on physical segment), e.g. 255.255.255.0 • Default router, e.g. 128.243.22.1 • DNS server address • (See later notes on auto-configuration) • Built in: • Own Ethernet address (in Network Interface Card (NIC) PROM)

  17. IP layer: what shall I do with this packet? • IP datagram forwarding rule: • Send it to another physically reachable machine which is believed to be closer to the IP destination • But: • Which machine is closer? • Which physical interface can be used to reach that machine? •  Consult OS IP routing table…

  18. Routing table • Lists a set of rules: •  What to do with a packet addressed to any destination IP address • Which physical interface to use • Whether the destination is directly reachable • If not, which directly reachable machine to pass the packet on to

  19. Routing table example (from scenario, monet) G = gateway (not direct) # netstat -r Destination Gateway Genmask Fl M R U Iface 128.243.22.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo 0.0.0.0 128.243.22.1 0.0.0.0 UG 0 0 0 eth0 # ifconfig eth0 [windows: ipconfig /all]eth0 Link encap:Ethernet HWaddr 00:01:02:AD:0F:08 inet addr:128.243.22.61 Bcast:128.243.22.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

  20. Routing table example: windows (128.243.22.74 - not from the scenario) • >netstat –rNetwork Destination Netmask Gateway Interface Metric • 0.0.0.0 0.0.0.0 128.243.22.1 128.243.22.74 10 • 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1 • 128.243.22.0 255.255.255.0 128.243.22.74 128.243.22.74 10 • 128.243.22.74 255.255.255.255 127.0.0.1 127.0.0.1 10 • … • >ipconfig /all • … • Ethernet adapter Local Area Connection: • Connection-specific DNS Suffix . : • Description . . . . . . . . . . . : 3Com Gigabit NIC (3C2000) • Physical Address. . . . . . . . . : 00-0A-5E-54-2B-65 • IP Address. . . . . . . . . . . . : 128.243.22.74 • Subnet Mask . . . . . . . . . . . : 255.255.255.0 • … This machine(direct)

  21. Case 1: send to 128.243.22.35 128.243.22.61 (monet) Start Here  128.243.22.1 Ethernet switch(es) 128.243.21.1 128.243.21/24 Ethernet switch(es) Router 128.243.22/24 Internet 128.243.21.16 (DNS server) 128.243.22.35 (mcclean) 128.243.21.19 155.198.5.83 (www.ic.ac.uk)

  22. Case 1: Send to 128.243.22.35 • Find routing table entry matching destination IP address (128.243.22.35) •  gateway (if any) and interface • No gateway (directly reachable destination) send directly to 128.243.22.35 • Interface “eth0” • Need Ethernet address for direct destination to send on Ethernet…

  23. Address Resolution Protocol (ARP) • Internet standard, RFC 826 • Protocol for dynamic mapping of (local) IP addresses to (local) Ethernet address

  24. ARP request/response packet • Construct ARP request “who has IP 128.243.22.34”: • “H” = “hardware” (Ether.); “P” = “protocol” (IP)

  25. ARP/Ethernet encapsulation • Place ARP request in Ethernet frame • Type 806

  26. ARP query • Broadcast on identified outgoing interface: Broadcast request Unicast response

  27. ARP query handling • all hosts on that Ethernet receive the broadcast request and packet to OS • Each host inspects Ethernet frame type and passes to relevant handler (in OS) • ARP handler inspects request: is this my IP address? • Host 128.243.22.35 sees match and builds and sends back ARP response“128.243.22.35 = Ethernet address 00:a0:c9:ca:1d:d7” • Sending host caches this information for (near) future re-use in an ARP table…

  28. ARP cache • Try #arp –a • table of IP address  Ethernet (MAC) address

  29. TCP/IP reference model You are here ReverseClient, DNS, … TCP, UDP IP IEEE802 Ethernet, WiFi, … Comer Fig. 17.4

  30. (Finally) Build Ethernet frame • Source IP = sending interface IP • Source MAC address = sending interface MAC address • Destination IP address = original destination • Destination MAC address = next hop MAC address • Ethernet frame type = 0800

  31. Send on identified network interface • Ethernet LAN is (logical) broadcast • Packet typically seen by the Ethernet card of every machine on that LAN • (give or take Ethernet switches which learn and route by MAC address)

  32. Receiving a packet…

  33. Incoming Ethernet frame: NIC • Check dest. Ethernet (MAC) address • Accept if broadcast or = NIC’s MAC address • Interrupt OS…

  34. Incoming Ethernet frame: OS • OS is interrupted by NIC and retrieves received Ethernet frame • Inspects frame type field and handles contents (payload) accordingly: • 0806  ARP (already considered) • 0800  IP v.4…

  35. Incoming IP packet (OS) • Inspects IP header • Check header checksum, discard if corrupted • Check destination IP address • If (one of our) local address(es), continue local processing… • Otherwise, consider for forwarding • Forwarding enabled (e.g. router)  see later • Forwarding disabled (e.g. most hosts)  discard

  36. Incoming IP packet with local destination (OS) • (Fragments reassembled first – see later) • Check IP packet type: • 1 ICMP Internet Control Message • 2 IGMP Internet Group Management • 4 IP in IP (encapsulation) • 6 TCP Transmission Control • 17 UDP User Datagram • Pass to relevant handler…

  37. Incoming (e.g.) UDP datagram with local IP address (OS) • Inspects UDP header • Check UDP checksum, discard if corrupt • Check destination UDP port • If not bound to an application Send an error response (ICMP Destination Unreachable – see later) • If currently bound to an application socket pass payload plus source IP and source UDP port to application socket…

  38. Receiving application (e.g. ReverseServerUnicast.java) • int port = Integer.parseInt(args[0]);DatagramSocket socket = new DatagramSocket(port);…byte [] requestBytes = new byte[65536];DatagramPacket request = new DatagramPacket(requestBytes, requestBytes.length);socket.receive(request);…

  39. Case 2: send to 128.243.21.19 (different network) 128.243.22.61 (monet) Start Here  128.243.22.1 Ethernet switch(es) 128.243.21.1 128.243.21/24 Ethernet switch(es) Router 128.243.22/24 Internet 128.243.21.16 (DNS server) 128.243.22.35 (mcclean) 128.243.21.19 155.198.5.83 (www.ic.ac.uk)

  40. Case 2: Send to 128.243.21.19 • Begins as before: • Construct UDP header • Construct IP header •  Complete IP datagram

  41. Case 2: Routing at sender G = gateway (not direct) • Find routing table entry matching destination IP address (128.243.22.35) (may appear as “default”): # netstat -r Destination Gateway Genmask Fl M R U Iface 128.243.22.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo 0.0.0.0 128.243.22.1 0.0.0.0 UG 0 0 0 eth0 • Doesn't match local network, So use default route • Gateway (next directly reachable hop) = router 128.243.22.1 • Outgoing network interface = eth0 (IP 128.243.22.61)

  42. Case 2: Sending non-local • Destination IP address is 128.243.21.19 • Destination Ethernet address is Ethernet address of gateway/next hop machine •  do ARP to find Ethernet address corresponding to IP 128.243.22.1 • Router replies with its MAC address on that LAN • Send IP packet in Ethernet frame on LAN to router MAC address…

  43. Routing handling of packet • Initially as for normal host receive • NIC accepts Ethernet frame addressed to it • Passes to OS via interrupt • OS determines frame type and passes for handling as IP • Checks packet is not corrupt • Checks destination IP address • If local, continue processing for local delivery • If not local…

  44. Router packet forwarding • OS checks packet Time To Live (TTL) • may discard packet, else decrement TTL • Now send as a normal packet • router OS checks own routing tables • finds next hop IP destination for network portion of IP address • resolves low-level address of next hop (e.g. Ethernet, using ARP) • sends packet on next hop interface

  45. Routing in routers • Routers are pre-configured with details of directly connected networks • Routers exchange routing packets with all directly connected routers • e.g. RIP, BGP, OSPF • Routers progressively discover all networks and which interface is "closest" to them, i.e. what the next hop interface should be.

  46. Wide-area routing • For scalability (localisation of information) routing is normally divided: • Within a site or organisation = “autonomous unit” • Between autonomous units Note: different protocols, different levels of granularity

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