Subnetting, ICMP, NAT, BOOTP

Subnetting, ICMP, NAT, BOOTP
Autumn Nov 02, 2004 Subnetting, ICMP, NAT, BOOTP Network Protocols and Standards Autumn Nov 02, 2004 CS573: Network Protocols and Standards CS573: Network Protocols and Standards

CS573: Network Protocols and Standards
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! Nov 02, 2004 CS573: Network Protocols and Standards

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) Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

Subnetting: Example Consider a corporate network assigned a class C address P.Q.R 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 mask (if any) What about internal routers? Nov 02, 2004 CS573: Network Protocols and Standards

Subnetting: Example IP addresses Subnet Mask Network/Subnet address Subnet Name { P.Q.R.0000 hhhh P.Q.R S1 { P.Q.R.0001 hhhh P.Q.R S2 P.Q.R hhhh P.Q.R S3 P.Q.R hhhh P.Q.R hhhh P.Q.R S4 P.Q.R hhhh P.Q.R hhhh P.Q.R hhhh P.Q.R hhhh P.Q.R S5 P.Q.R hhhh P.Q.R hhhh P.Q.R hhhh P.Q.R hhhh P.Q.R hhhh P.Q.R hhhh P.Q.R hhhh Nov 02, 2004 CS573: Network Protocols and Standards

Subnetting: Example IP addresses Subnet Mask Network/Subnet address Subnet Name P.Q.R.0000 hhhh P.Q.R S1 P.Q.R.0001 hhhh P.Q.R S2 P.Q.R.001 hhhhh P.Q.R S3 P.Q.R.01 hhhhhh P.Q.R S4 P.Q.R.1 hhhhhhh P.Q.R S5 Nov 02, 2004 CS573: Network Protocols and Standards

Subnetting: Routing Table
Subnet Mask Network/Subnet address Next Hop/Port P.Q.R P1 P.Q.R P2 P.Q.R P3 P.Q.R P4 P.Q.R P5 Nov 02, 2004 CS573: Network Protocols and Standards

Network/Subnet address Next Hop/Port P.Q.R / 28 P1 P.Q.R / 28 P2 P.Q.R / 27 P3 P.Q.R / 26 P4 P.Q.R / 25 P5 Number after / indicates number of bits to look at! Nov 02, 2004 CS573: Network Protocols and Standards

Subnet S4 has 64 hosts. Can we make two subnets? 16+48? P.Q.R.01 hhhhhh P.Q.R S4 Old mask Old subnet New mask { P.Q.R hhhh P.Q.R P.Q.R hhhh P.Q.R hhhh P.Q.R.01 hhhhhh P.Q.R hhhh P.Q.R.01 hhhhhh P.Q.R hhhh P.Q.R.01 hhhhhh P.Q.R.0100 hhhh P.Q.R S41 P.Q.R.01 hhhhhh P.Q.R S42 Nov 02, 2004 CS573: Network Protocols and Standards

P.Q.R.0100 hhhh P.Q.R S42 P.Q.R.01 hhhhhh P.Q.R 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 Nov 02, 2004 CS573: Network Protocols and Standards

Where else longest prefix match can be used? Subnet Mask Network/Subnet address Next Hop/Port P.Q.R P1 P.Q.R P2 P.Q.R P345 P.Q.R P345 Aggregate Router P.Q.R P345 Subnet Mask Network/Subnet address Next Hop/Port S1 Router S2 P.Q.R P1 P.Q.R P2 P.Q.R P345 S3 S4 S5 Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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 decimal 32-bit binary equivalent Lowest: Highest: A block of 2048 addresses 32-bit mask is Do we really need address classes when we have masks? Answer: NO  CIDR (Classless Inter Domain Routing) Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

ICMP: Internet Control Message Protocol
Autumn Nov 02, 2004 ICMP: Internet Control Message Protocol Network Protocols and Standards Autumn Nov 02, 2004 CS573: Network Protocols and Standards CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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) Nov 02, 2004 CS573: Network Protocols and Standards

Some ICMP Message Types
Type Field ICMP Message Type Echo Reply 3 Destination Unreachable 4 Source Quench 5 Redirect (change a route) 8 Echo Request 9 Router Advertisement 10 Router Solicitation 11 Time Exceeded for a Datagram 12 Parameter Problem on a Datagram 13 Timestamp Request 14 Timestamp Reply 17 Address Mask Request 18 Address Mask Reply Reference: RFC 1700 Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

Echo Request/Reply ICMP Echo Request or Reply Message Format TYPE(0/8) CODE(0) CHECKSUM IDENTIFIER SEQUENCE NUMBER OPTIONAL DATA … … … Nov 02, 2004 CS573: Network Protocols and Standards

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) Nov 02, 2004 CS573: Network Protocols and Standards

Destination Unreachable
ICMP Destination Unreachable Message Format TYPE(3) CODE(0-12) CHECKSUM UNUSED (MUST BE ZERO) INTERNET HEADER+FIRST 8 BYTES OF DATA … … … Nov 02, 2004 CS573: Network Protocols and Standards

Destination Unreachable Codes
Code Value Meaning Network Unreachable 1 Host Unreachable 2 Protocol Unreachable 3 Port Unreachable 4 Fragmentation Needed and DF Set 5 Source Route Failed 6 Destination Network Unknown 7 Destination Host Unknown 8 Source Host Isolated 9 Communication with Destination Network Administratively Prohibited 10 Communication with Destination Host Administratively Prohibited 11 Network Unreachable for Type of Service 12 Host Unreachable for Type of Service Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

Source Quench ICMP Source Quench Message Format TYPE(4) CODE(0)
CHECKSUM UNUSED (MUST BE ZERO) INTERNET HEADER+FIRST 8 BYTES OF DATA … … … Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

ICMP Redirect Message TYPE(5) CODE(0-3) CHECKSUM
SUGGESTED ROUTER INTERNET ADDRESS INTERNET HEADER+FIRST 8 BYTES OF DATA … … … Code Value Meaning Redirect datagrams for the Net (now obsolete) 1 Redirect datagrams for the Host 2 Redirect datagrams for the Type of Service and Net 3 Redirect datagrams for the Type of Service and Host Nov 02, 2004 CS573: Network Protocols and Standards

ICMP Time Exceeded TYPE(11) CODE(0/1) CHECKSUM UNUSED (MUST BE ZERO)
INTERNET HEADER+FIRST 8 BYTES OF DATA … … … Code Value Meaning Time-to-live count Exceeded 1 Fragment reassembly time exceeded 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 Nov 02, 2004 CS573: Network Protocols and Standards

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) Nov 02, 2004 CS573: Network Protocols and Standards

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. Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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] … … … Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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? Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

Tracing route to nova.stanford.edu [ ] over a maximum of 30 hops: 1 <10 ms <10 ms <10 ms shahalami.lums.edu.pk [ ] ms ms ms ms ms ms ms ms ms ms ms ms ms ms ms p5-2.nycmny1-cr11.bbnplanet.net [ ] ms ms ms p3-0.nycmny1-nbr1.bbnplanet.net [ ] ms * ms so chcgil2-br2.bbnplanet.net [ ] ms ms ms so dnvtco1-br2.bbnplanet.net [ ] ms ms ms p15-0.snjpca1-br2.bbnplanet.net [ ] ms ms ms p2-0.paix-bi3.bbnplanet.net [ ] ms ms ms p2-0.paix-bi2.bbnplanet.net [ ] ms ms ms p6-0.paloalto-nbr1.bbnplanet.net [ ] ms ms ms p1-0.paloalto-cr1.bbnplanet.net [ ] ms ms ms p1-0-0.paloalto-cr13.bbnplanet.net [ ] ms ms ms sunet-gateway.stanford.edu [ ] * * * Request timed out. * * * Request timed out. ms ms ms nova.Stanford.EDU [ ] Nov 02, 2004 CS573: Network Protocols and Standards

traceroute to suraj.lums.edu.pk ( ): 1-30 hops, 38 byte packets 1 quad-rtr.Stanford.EDU ( ) ms (ttl=64!) ms (ttl=64!) ms (ttl=64!) 2 default-gateway-2.Stanford.EDU ( ) ms ms ms 3 sunet-gateway.Stanford.EDU ( ) ms ms ms 4 g1.ba21.b sfo01.atlas.cogentco.com ( ) ms ms ms 5 g1-1.core01.sfo01.atlas.cogentco.com ( ) ms ms ms 6 p5-0.core03.sfo01.atlas.cogentco.com ( ) ms ms ms 7 ds3.st-paix.ix.singtel.com ( ) ms (ttl=248!) ms (ttl=248!) ms (ttl=248!) 8 p6-1.plapx-cr1.ix.singtel.com ( ) ms ms ms 9 POS2-0.above-core1.ix.singtel.com ( ) ms ms ms ( ) ms ms ms ( ) ms (ttl=244!) ms (ttl=244!) ms (ttl=244!) ( ) ms ms ms ( ) 373 ms (ttl=241!) 323 ms (ttl=241!) 310 ms (ttl=241!) ( ) 326 ms (ttl=240!) 329 ms (ttl=240!) 328 ms (ttl=240!) ( ) 328 ms (ttl=239!) 326 ms (ttl=239!) 326 ms (ttl=239!) ( ) ms (ttl=238!) ms (ttl=238!) ms (ttl=238!) 17 suraj.lums.edu.pk ( ) ms (ttl=237!) ms (ttl=237!) ms (ttl=237!) Nov 02, 2004 CS573: Network Protocols and Standards

Network Address Translation
Autumn Nov 02, 2004 Network Address Translation Network Protocols and Standards Autumn Nov 02, 2004 CS573: Network Protocols and Standards CS573: Network Protocols and Standards

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 – (1 class A) – (16 class B) – (256 class C) Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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? Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

Autumn 2004-2005 Network Protocols and Standards Autumn 2004-2005
Nov 02, 2004 Dynamic Addressing Network Protocols and Standards Autumn Nov 02, 2004 CS573: Network Protocols and Standards CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

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 Nov 02, 2004 CS573: Network Protocols and Standards

BOOTP Message Format 0 8 16 24 31 bits OP HTYPE HLEN HOPS
Transaction ID Seconds Unused Client IP Address Your IP Address Server IP Address Router IP Address Client Hardware Address (16 octets) Server Hostname (64 octets) Boot File Name (128 octets) Vendor-specific area (64 octets) Nov 02, 2004 CS573: Network Protocols and Standards

BOOTP Message Field OP Specifies whether a request(1) or reply(2) HTYPE and HLEN Hardware type and address length (For Ethernet, HTYPE is 1 and HLEN is 6) HOPS Client passes 0 in this field; BOOTP server increments it if the request is passed to another server across a router Transaction ID Contains an integer that machines use to match requests with responses Seconds Number of seconds since the client started to boot Nov 02, 2004 CS573: Network Protocols and Standards

BOOTP Message Remaining fields in the message To allow the greatest flexibility Clients fill in as much information as they know; unknown fields are set to zero Example If server IP or server hostname are non-zero, only the server with matching address/name will answer the request If they are zero, any server that receives the request will reply Nov 02, 2004 CS573: Network Protocols and Standards

BOOTP Message Format BOOTP can be used by a client that already knows its IP address (e.g., to obtain boot file information) A client that knows its IP address places it in the client IP address field; other clients set this field to zero If the client’s IP address in the request message is zero, a server returns the client IP address in the “your IP address” field Nov 02, 2004 CS573: Network Protocols and Standards

DHCP Dynamic Host Configuration Protocol RARP and BOOTP designed for relatively static environment Each host a permanent network connection Manager creates a BOOTP configuration file specifying BOOTP parameters for each host Manager configures server with mapping of host identifier to IP address New Requirements Portable computers Number of computers exceeds available IP host addresses (although not all will be up and running at the same time) Nov 02, 2004 CS573: Network Protocols and Standards

DHCP DHCP allows: Manual configuration Automatic configuration Manager let DHCP server assign a permanent address when a computer first attaches to the network Dynamic configuration Loaning IP addresses for a limited time Nov 02, 2004 CS573: Network Protocols and Standards

Subnetting, ICMP, NAT, BOOTP

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Subnetting, ICMP, NAT, BOOTP

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