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Subnetting, ICMP, NAT, BOOTP

Subnetting, ICMP, NAT, BOOTP. Network Protocols and Standards Autumn 2004-2005. Subnet Routing. Conventional routing table entry (network address, next hop address) Network address format is predetermined for a given class (e.g., first 16 bits for class B addresses!)

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Subnetting, ICMP, NAT, BOOTP

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  1. Subnetting, ICMP, NAT, BOOTP Network Protocols and Standards Autumn 2004-2005 CS573: Network Protocols and Standards

  2. Subnet Routing • Conventional routing table entry • (network address, next hop address) • Network address format is predetermined for a given class (e.g., first 16 bits for class B addresses!) • With subnetting, routing table entry becomes • (subnet mask, network address, next hop address) • Then compare with network address field of entries to find next hop address • Subnet mask indicates the network address! CS573: Network Protocols and Standards

  3. Subnet Routing • The use of mask generalizes the subnet routing algorithm to handle all the special cases of the standard algorithm • Routes to individual hosts • Default route • Routes to directly connected networks • Routes to conventional networks (that do not use subnet addressing) • Merely combine the 32-bit mask field with the 32-bit IP address • Example: To install a route for: • Individual host (Mask of all 1’s, Host IP address) • Default Route (Mask of all 0’s, network address all 0’s) • Class B network address (Mask of two octets of 1’s and two of 0’s) CS573: Network Protocols and Standards

  4. Subnet Routing • Algorithm • Extract destination IP (D) from datagram • Compute IP address of destination network N • If N matches any directly connected network address • Send datagram over that network (obviously encapsulated in a frame) • Else • For each entry in the routing table, do • N* = bitwise-AND of D and subnet mask • If N* equals the network address field of the entry, then route the datagram to the specified next hop CS573: Network Protocols and Standards

  5. Subnetting: Example • Consider a corporate network assigned a class C address P.Q.R.00000000 • The company needs 5 subnets: • 2 subnets of 16 hosts each • 3 subnets with 32, 64, and 128 hosts • External routers reach the corporate via single routing table entry • P.Q.R.0 network and 255.255.255.0 mask (if any) • What about internal routers? CS573: Network Protocols and Standards

  6. Subnetting: Example IP addresses Subnet Mask Network/Subnet address Subnet Name { P.Q.R.0000 hhhh 255.255.255.1111 0000 P.Q.R.0000 0000 S1 { P.Q.R.0001 hhhh 255.255.255.1111 0000 P.Q.R.0001 0000 S2 P.Q.R.001 0 hhhh 255.255.255.111 00000 P.Q.R.001 00000 S3 P.Q.R.001 1 hhhh 255.255.255.111 00000 P.Q.R.01 00 hhhh 255.255.255.11 000000 P.Q.R.01 000000 S4 P.Q.R.01 01 hhhh 255.255.255.11 000000 P.Q.R.01 10 hhhh 255.255.255.11 000000 P.Q.R.01 11 hhhh 255.255.255.11 000000 P.Q.R.1 000 hhhh 255.255.255.1 0000000 P.Q.R.1 0000000 S5 P.Q.R.1 001 hhhh 255.255.255.1 0000000 P.Q.R.1 010 hhhh 255.255.255.1 0000000 P.Q.R.1 011 hhhh 255.255.255.1 0000000 P.Q.R.1 100 hhhh 255.255.255.1 0000000 P.Q.R.1 101 hhhh 255.255.255.1 0000000 P.Q.R.1 110 hhhh 255.255.255.1 0000000 P.Q.R.1 111 hhhh 255.255.255.1 0000000 CS573: Network Protocols and Standards

  7. Subnetting: Example IP addresses Subnet Mask Network/Subnet address Subnet Name P.Q.R.0000 hhhh 255.255.255.1111 0000 P.Q.R.0000 0000 S1 P.Q.R.0001 hhhh 255.255.255.1111 0000 P.Q.R.0001 0000 S2 P.Q.R.001 hhhhh 255.255.255.1110 0000 P.Q.R.001 00000 S3 P.Q.R.01 hhhhhh 255.255.255.11 000000 P.Q.R.01 000000 S4 P.Q.R.1 hhhhhhh 255.255.255.1 0000000 P.Q.R.1 0000000 S5 CS573: Network Protocols and Standards

  8. Subnetting: Routing Table Subnet Mask Network/Subnet address Next Hop/Port 255.255.255.1111 0000 P.Q.R.0000 0000 P1 255.255.255.1111 0000 P.Q.R.0001 0000 P2 255.255.255.1110 0000 P.Q.R.0010 0000 P3 255.255.255.11 000000 P.Q.R.0100 0000 P4 255.255.255.1 0000000 P.Q.R.1000 0000 P5 CS573: Network Protocols and Standards

  9. Subnetting: Routing Table Network/Subnet address Next Hop/Port Number after / indicates number of bits to look at! CS573: Network Protocols and Standards

  10. Subnetting: Routing Table Subnet S4 has 64 hosts. Can we make two subnets? 16+48? P.Q.R.01 hhhhhh 255.255.255.11 000000 P.Q.R.01 000000 S4 Old mask Old subnet New mask { P.Q.R.01 00 hhhh 255.255.255.11 000000 P.Q.R.0100 0000 P.Q.R.01 00 hhhh 255.255.255.1111 0000 P.Q.R.01 01 hhhh 255.255.255.11 000000 P.Q.R.01 hhhhhh 255.255.255.11 000000 P.Q.R.01 10 hhhh 255.255.255.11 000000 P.Q.R.01 hhhhhh 255.255.255.11 000000 P.Q.R.01 11 hhhh 255.255.255.11 000000 P.Q.R.01 hhhhhh 255.255.255.11 000000 P.Q.R.0100 hhhh 255.255.255.1111 0000 P.Q.R.0100 0000 S41 P.Q.R.01 hhhhhh 255.255.255.11 000000 P.Q.R.01 000000 S42 CS573: Network Protocols and Standards

  11. P.Q.R.0100 hhhh 255.255.255.1111 0000 P.Q.R.0100 0000 S41 P.Q.R.01 hhhhhh 255.255.255.11 000000 P.Q.R.01 000000 S42 Subnetting: Routing Table What if an IP in S42 is received? It will match on the second entry! What if an IP in S41 is received? It will match both entries! Which entry should be used? USE LONGEST PREFIX MATCH CS573: Network Protocols and Standards

  12. Subnetting: Routing Table Where else longest prefix match can be used? Subnet Mask Network/Subnet address Next Hop/Port 255.255.255.1111 0000 P.Q.R.0000 0000 P1 255.255.255.1111 0000 P.Q.R.0001 0000 P2 255.255.255.1110 0000 P.Q.R.0010 0000 P345 Router Aggregate 255.255.255.11 000000 P.Q.R.0100 0000 P345 255.255.255.1 0000000 P.Q.R.1000 0000 P345 Subnet Mask Network/Subnet address Next Hop/Port S1 Router S2 255.255.255.1111 0000 P.Q.R.0000 0000 P1 255.255.255.1111 0000 P.Q.R.0001 0000 P2 255.255.255.0000 0000 P.Q.R.0000 0000 P345 S3 S5 S4 CS573: Network Protocols and Standards

  13. Supernet Addressing • Use of many IP network addresses for a single organization • Example: • To conserve class B addresses, issue multiple class C address to the same organization • Issue: increase in the number of entries in the routing tables for routers outside the network • Solutions: • Collapse a block of contiguous class C address into the pair: (network address, count) where network address is the smallest number in the block CS573: Network Protocols and Standards

  14. Supernet Addressing • It requires each block to be a power of 2 and uses bit mask to identify the size of the block • Example Dotted decimal32-bit binary equivalent • Lowest: 234.170.168.0 11101010 10101010 10101000 00000000 • Highest: 234.170.175.255 11101010 10101010 10101111 11111111 • A block of 2048 addresses • 32-bit mask is 11111111 11111111 11111000 00000000 • Do we really need address classes when we have masks? • Answer: NO  CIDR (Classless Inter Domain Routing) CS573: Network Protocols and Standards

  15. Supernet Addressing • In the router, the entry consists of: • The lowest address and the 32-bit mask • A block of addresses can be subdivided, and separate route can be entered for each subdivision • When looking up a route, the routing software uses a longest-match paradigm to select a route CS573: Network Protocols and Standards

  16. ICMP: Internet Control Message Protocol Network Protocols and Standards Autumn 2004-2005 CS573: Network Protocols and Standards

  17. ICMP Motivation • Questions in Routing: • What if a router cannot route or deliver a datagram? • What if a router experiences congestion? • What if the TTL expires? • Router needs to inform the source to take action to avoid or correct the problem • ICMP – error reporting mechanism • Can only report condition back to the original source • Routers and hosts send error or control messages to others • Specified in RFC 792 CS573: Network Protocols and Standards

  18. ICMP • ICMP messages are encapsulated in IP datagrams, with protocol type 1 • In the data portion of the datagram, first byte indicates the ICMP message type and the format for the rest of the message • Some ICMP packets have a code that further qualifies the type • Most ICMP messages include the full IP header plus the first 8 bytes of the data portion of the datagram they refer to • Helps sender identify the packet • To avoid explosion of ICMP messages • No ICMP packets are generated to report errors on ICMP packets • If an ICMP message is generated about a fragmented datagram, it is generated only for the first fragment (fragment 0) CS573: Network Protocols and Standards

  19. Some ICMP Message Types Reference: RFC 1700 CS573: Network Protocols and Standards

  20. Echo Request/Reply • Testing destination reachability and status • Echo Request Message • Echo Reply Message • Command used to send ICMP echo request is, in most systems, called “ping” • Echo request may contain some data, which is returned unchanged in the reply • The ICMP Echo Request/Reply header also contains a sequence number and identifier, to aid the host in matching the request with the reply CS573: Network Protocols and Standards

  21. Echo Request/Reply ICMP Echo Request or Reply Message Format TYPE(0/8) CODE(0) CHECKSUM IDENTIFIER SEQUENCE NUMBER OPTIONAL DATA … … … CS573: Network Protocols and Standards

  22. Destination Unreachable • Reports of unreachable destinations • When a router can not forward or deliver an IP datagram, it sends a “destination unreachable” message back to the original source • Code determines specific condition (see table) CS573: Network Protocols and Standards

  23. Destination Unreachable ICMP Destination Unreachable Message Format TYPE(3) CODE(0-12) CHECKSUM UNUSED (MUST BE ZERO) INTERNET HEADER+FIRST 8 BYTES OF DATA … … … CS573: Network Protocols and Standards

  24. Destination Unreachable Codes CS573: Network Protocols and Standards

  25. ICMP Source Quench • Congestion and datagram flow control • Report congestion to the original source • Request to source to reduce current rate • Usually sent for each datagram discarded • Can be sent by a host or a router • Some routers may be more sophisticated • Monitor incoming traffic • Quench sources that have the highest rates • Avoid congestion by quenching before datagrams are lost CS573: Network Protocols and Standards

  26. Source Quench ICMP Source Quench Message Format TYPE(4) CODE(0) CHECKSUM UNUSED (MUST BE ZERO) INTERNET HEADER+FIRST 8 BYTES OF DATA … … … CS573: Network Protocols and Standards

  27. ICMP Redirect Message • Host sends a datagram to router R1 to be forwarded to a certain destination • Router R1 looks at its routing table, and finds the next router in the path as R2 • If R2 is directly accessible to the sending host, R1 generates an ICMP Redirect Message back to the sender. R1 also forwards the datagram to R2 normally • The purpose is to inform the host that there is a better route to that destination CS573: Network Protocols and Standards

  28. ICMP Redirect Message TYPE(5) CODE(0-3) CHECKSUM SUGGESTED ROUTER INTERNET ADDRESS INTERNET HEADER+FIRST 8 BYTES OF DATA … … … CS573: Network Protocols and Standards

  29. ICMP Time Exceeded TYPE(11) CODE(0/1) CHECKSUM UNUSED (MUST BE ZERO) INTERNET HEADER+FIRST 8 BYTES OF DATA … … … A router sends this message whenever a datagram is discarded because the TTL field in the datagram has reached zero or because its reassembly timer Expired while waiting for fragments CS573: Network Protocols and Standards

  30. Address Mask Request/Reply • Obtaining a subnet mask • ICMP address mask request message • ICMP address mask reply message • Request • Sent directly to the router (if known) • Broadcast (if router unknown) • Response is unicast if the request contains a valid IP address; otherwise, it is a broadcast • Any host can respond (see RFC 950) CS573: Network Protocols and Standards

  31. Address Mask Request/Reply TYPE(17/18) CODE(0) CHECKSUM IDENTIFIER SEQUENCE NUMBER ADDRESS MASK ICMP address mask request or reply message format. Usually, hosts broadcast a request without knowing which specific router will respond. CS573: Network Protocols and Standards

  32. Router Advertisement/Solicitation • Options for the host to learn the router address(es) • Manually enter entries • Not up to date and cumbersome • Host listens to routing protocol messages • Protocols and their messages differ • Complexity is introduced at the host • Use of ICMP messages as defined in RFC 1256 • Routers periodically send an ICMP “router Advertisement” – either broadcast or multicast • Hosts may solicit such advertisements with a Router Solicitation message CS573: Network Protocols and Standards

  33. Router Advertisement TYPE(9) CODE(0) CHECKSUM NUM ADDRS ADDR ENTRY SZIE = 2 LIFETIME (SEC) ROUTER ADDRESS [1] PREFERENCE LEVEL [1] ROUTER ADDRESS [2] PREFERENCE LEVEL [2] … … … CS573: Network Protocols and Standards

  34. Router Solicitation TYPE(10) CODE(0) CHECKSUM RESERVED – Default advertisement rate is once every 7-10 minutes – The router solicitation message causes the routers to send their advertisements earlier – Lifetime of advertisements is typically 30 minutes CS573: Network Protocols and Standards

  35. Application: Traceroute • Goal: Find the path a packet takes between two hosts • Originator host sends a series of packets, starting with TTL=1 and increasing the TTL for each packet • The first router in the path will drop the TTL=1 packet and send back an ICMP Time Exceeded • Host learns who is the first hop • Second router in the path will drop the packet that originated with TTL=2 and send back an ICMP Time Exceeded • Third router will do the same upon receiving packet that originated with TTL=3 • By collecting the ICMP responses, the host can figure out the path taken by the packet. Will this work? CS573: Network Protocols and Standards

  36. Application: Traceroute • Current method described above requires 2N messages for a N-hop path • Will also give wrong results if path changes • ICMP Traceroute (RFC 1393) can do it in N+1 messages • Idea: Define a traceroute IP option • Send an IP packet with this option set • Every intermediate system handling this packet will send back an ICMP traceroute to the source CS573: Network Protocols and Standards

  37. Application: Traceroute Tracing route to nova.stanford.edu [171.64.90.123] over a maximum of 30 hops: 1 <10 ms <10 ms <10 ms shahalami.lums.edu.pk [203.128.0.1] 2 1938 ms 1890 ms 1860 ms 202.125.139.29 3 1515 ms 1875 ms 1938 ms 202.125.139.249 4 1812 ms 1672 ms 1578 ms 202.125.159.53 5 1969 ms 1672 ms 1953 ms 203.208.147.85 6 1437 ms 1641 ms 1594 ms p5-2.nycmny1-cr11.bbnplanet.net [4.25.14.41] 7 1593 ms 1688 ms 1719 ms p3-0.nycmny1-nbr1.bbnplanet.net [4.24.10.78] 8 1859 ms * 1687 ms so-6-0-0.chcgil2-br2.bbnplanet.net [4.24.4.17] 9 1610 ms 1718 ms 1625 ms so-1-0-0.dnvtco1-br2.bbnplanet.net [4.24.9.62] 10 1516 ms 1718 ms 2000 ms p15-0.snjpca1-br2.bbnplanet.net [4.0.6.225] 11 1922 ms 1844 ms 1562 ms p2-0.paix-bi3.bbnplanet.net [4.24.7.38] 12 1562 ms 1813 ms 1812 ms p2-0.paix-bi2.bbnplanet.net [4.0.3.174] 13 1828 ms 1625 ms 1688 ms p6-0.paloalto-nbr1.bbnplanet.net [4.0.6.97] 14 1844 ms 1734 ms 2016 ms p1-0.paloalto-cr1.bbnplanet.net [4.0.6.74] 15 2031 ms 1813 ms 1687 ms p1-0-0.paloalto-cr13.bbnplanet.net [4.0.2.222] 16 2109 ms 1985 ms 1937 ms sunet-gateway.stanford.edu [198.31.10.1] 17 * * * Request timed out. 18 * * * Request timed out. 19 2078 ms 2203 ms 2078 ms nova.Stanford.EDU [171.64.90.123] CS573: Network Protocols and Standards

  38. Application: Traceroute traceroute to suraj.lums.edu.pk (203.128.0.6): 1-30 hops, 38 byte packets 1 quad-rtr.Stanford.EDU (171.64.90.1) 1.49 ms (ttl=64!) 1.25 ms (ttl=64!) 1.32 ms (ttl=64!) 2 default-gateway-2.Stanford.EDU (198.31.86.129) 2.27 ms 1.98 ms 2.82 ms 3 sunet-gateway.Stanford.EDU (198.31.86.1) 2.18 ms 1.18 ms 1.25 ms 4 g1.ba21.b003123-1.sfo01.atlas.cogentco.com (66.250.7.137) 3.27 ms 3.79 ms 3.04 ms 5 g1-1.core01.sfo01.atlas.cogentco.com (66.28.6.9) 4.23 ms 3.40 ms 3.18 ms 6 p5-0.core03.sfo01.atlas.cogentco.com (66.28.4.146) 3.44 ms 3.51 ms 5.33 ms 7 ds3.st-paix.ix.singtel.com (198.32.176.50) 10.9 ms (ttl=248!) 10.2 ms (ttl=248!) 12.1 ms (ttl=248!) 8 p6-1.plapx-cr1.ix.singtel.com (203.208.172.45) 12.9 ms 13.5 ms 13.6 ms 9 POS2-0.above-core1.ix.singtel.com (202.160.250.45) 14.6 ms 14.0 ms 13.3 ms 10 203.208.154.94 (203.208.154.94) 63.7 ms 58.7 ms 57.6 ms 11 203.208.154.97 (203.208.154.97) 78.1 ms (ttl=244!) 80.7 ms (ttl=244!) 82.0 ms (ttl=244!) 12 203.208.154.102 (203.208.154.102) 79.9 ms 80.5 ms 78.3 ms 13 203.208.147.86 (203.208.147.86) 373 ms (ttl=241!) 323 ms (ttl=241!) 310 ms (ttl=241!) 14 202.125.159.46 (202.125.159.46) 326 ms (ttl=240!) 329 ms (ttl=240!) 328 ms (ttl=240!) 15 202.125.139.250 (202.125.139.250) 328 ms (ttl=239!) 326 ms (ttl=239!) 326 ms (ttl=239!) 16 202.125.139.30 (202.125.139.30) 2075 ms (ttl=238!) 2146 ms (ttl=238!) 2216 ms (ttl=238!) 17 suraj.lums.edu.pk (203.128.0.6) 2395 ms (ttl=237!) 2294 ms (ttl=237!) 2209 ms (ttl=237!) CS573: Network Protocols and Standards

  39. Network Address Translation Network Protocols and Standards Autumn 2004-2005 CS573: Network Protocols and Standards

  40. Private Networks • Private networks have no “direct” connection to the Internet • Blocks of addresses have been reserved for the private networks (RFC 1918) • Blocks in different classes • 10.0.0.0 – 10.255.255.255 (1 class A) • 172.16.0.0 – 172.31.255.255 (16 class B) • 192.168.0.0 – 192.168.255.255 (256 class C) CS573: Network Protocols and Standards

  41. Purpose • Machines in the protected network can access the Internet normally • Packets coming from the protected network all appear to be coming from IP1 • Addresses in the protected network are in the private range Internet Host 1 Host 2 Firewall IP1 IP2 Host N Protected Network CS573: Network Protocols and Standards

  42. Implementation • Hosts inside the private network are configured to use the firewall (IP2) as their gateway • The firewall rewrites the IP datagram header for the outbound packets, replacing the source IP with IP1 • All packets “seem” to be coming from IP1 • The destination IP in the packets received from the Internet is IP1; it is rewritten replacing IP1 with the IP address of the internal destination • Problem: How to figure out what is the right destination in the private network? CS573: Network Protocols and Standards

  43. Demultiplexing Incoming Packets • There is not enough information in the IP header to demultiplex incoming packets • It is necessary to use information from the higher layers (transport layer) • Common transport layers: TCP and UDP • Transport layer has the concept of port which identifies which process in the host should finally get the packet CS573: Network Protocols and Standards

  44. Ports • 16-bit numbers identifying which process should get the packet • UDP and TCP ports exist in different spaces • Each packet carries two port numbers • The source port of the process which generated it in the source host • The destination port of the process which should get it at the destination Telnet FTP TCP UDP IP CS573: Network Protocols and Standards

  45. Implementation (revisited) • Upon receiving an outbound packet from a host in the private network, the firewall: • Rewrites the source IP with its own IP (IP1) • Generates a local source port and rewrites the source port in the packet as this port and makes a record of it • Upon receiving an inbound packet from the Internet, the firewall checks whether the destination port in the packet is in the list of local ports: • If not, the packet is dropped • Can not initiate connections from outside! • If yes, the firewall knows where to send this packet CS573: Network Protocols and Standards

  46. Dynamic Addressing Network Protocols and Standards Autumn 2004-2005 CS573: Network Protocols and Standards

  47. BOOTP • Alternative to RARP • RARP operates at a low level, requesting the direct access to the network hardware • Difficult for an application programmer to build a server • RARP gives “only” the IP address CS573: Network Protocols and Standards

  48. BOOTP • Devised to allow a machine to obtain: • Its IP address • Address of a router • Subnet mask to use • Address of a name server • Can be implemented with an application program • Uses UDP/IP for communication CS573: Network Protocols and Standards

  49. BOOTP • Using IP to determine an IP address • Request from a client is broadcast on the local network using IP address all 1’s • Since the client does not know its IP address (yet!), the reply from the server must also be broadcast; otherwise • Using clients IP address would require use of ARP to map IP address to a hardware address, which in turn requires client to already know its IP address • Using client’s request to manually add an entry to its ARP cache – Not desirable CS573: Network Protocols and Standards

  50. BOOTP • Reliability in communication is based on • UDP checksum • Timeout and retransmissions • To minimize collisions among many clients, use random timeouts • Increase timeouts with each retransmission • Starting with the interval 0-4 seconds • Doubling interval each retransmission up to 60s CS573: Network Protocols and Standards

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