IP - The Internet Protocol

Transcription

1 IP - The Internet Protocol 1

2 Orientation IP s current version is Version 4 (IPv4). It is specified in RFC 891. TCP UDP Transport Layer ICMP IP IGMP Network Layer ARP Network Access Link Layer Media 2

3 IP: The waist of the hourglass IP is the waist of the hourglass of the Internet protocol architecture Multiple higher-layer protocols Multiple lower-layer protocols Only one protocol at the network layer. Applications HTTP FTP SMTP TCP UDP IP Data link layer protocols Physical layer protocols 3

4 IP Service Delivery service of IP is minimal IP provide provides an unreliable connectionless best effort service (also called: datagram service ). Unreliable: IP does not make an attempt to recover lost packets Connectionless: Each packet ( datagram ) is handled independently. IP is not aware that packets between hosts may be sent in a logical sequence Best effort: IP does not make guarantees on the service (no throughput guarantee, no delay guarantee, ) Consequences: Higher layer protocols have to deal with losses or with duplicate packets Packets may be delivered out-of-sequence 4

5 IP supports the following services: one-to-one (unicast) one-to-all one-to-several IP Service (broadcast) (multicast) unicast broadcast multicast IP multicast also supports a many-to-many service. IP multicast requires support of other protocols (IGMP, multicast routing) 5

6 IP Datagram Format bit # version header length Identification DS ECN total length (in bytes) Fragment offset time-to-live (TTL) protocol header checksum 0 D F M F source IP address destination IP address options (0 to 40 bytes) payload 4 bytes 20 bytes Header Size < 2 4 x 4 bytes = 60 bytes 20 bytes Total Length < 2 16 bytes = bytes 6

7 Fields of the IP Header Version (4 bits): current version is 4, next version will be 6. Header length (4 bits): length of IP header, in multiples of 4 bytes DS/ECN field (1 byte) This field was previously called as Type-of-Service (TOS) field. The role of this field has been re-defined, but is backwards compatible to TOS interpretation Differentiated Service (DS) (6 bits): Used to specify service level (currently not supported in the Internet) Explicit Congestion Notification (ECN) (2 bits): New feedback mechanism used by TCP 7

8 Fields of the IP Header Identification (16 bits): Unique identification of a datagram from a host. Incremented whenever a datagram is transmitted Flags (3 bits): First bit always set to 0 DF bit (Do not fragment) MF bit (More fragments) Will be explained later Fragmentation 8

9 Fields of the IP Header Time To Live (TTL) (1 byte): Specifies longest paths before datagram is dropped Role of TTL field: Ensure that packet is eventually dropped when a routing loop occurs Used as follows: Sender sets the value (e.g., 64) Each router decrements the value by 1 When the value reaches 0, the datagram is dropped 9

10 Fields of the IP Header Protocol (1 byte): Specifies the higher-layer protocol. Used for demultiplexing to higher layers. 4 = IP-in-IP encapsulation 6 = TCP 17 = UDP 1 = ICMP 2 = IGMP IP Header checksum (2 bytes): A simple 16-bit long checksum which is computed for the header of the datagram. 10

11 What is an IP Address? An IP address is a unique global address for a network interface Exceptions: Dynamically assigned IP addresses ( DHCP) IP addresses in private networks ( NAT) An IP address: - is a 32 bit long identifier - encodes a network number (network prefix) and a host number

12 Network prefix and host number The network prefix identifies a network and the host number identifies a specific host (actually, interface on the network). network prefix host number How do we know how long the network prefix is? Before 1993: The network prefix is implicitly defined (see class-based addressing) or After 1993: The network prefix is indicated by a netmask.

13 Dotted Decimal Notation IP addresses are written in a so-called dotted decimal notation Each byte is identified by a decimal number in the range [0..255]: Example: st Byte 2 nd Byte 3 rd Byte 4 th Byte = 128 = 143 = 137 =

14 Example Example: ellington.cs.virginia.edu Network address is: (or ) Host number is: Netmask is: (or ffff0000) Prefix or CIDR notation: /16» Network prefix is 16 bits long

15 Special IP Addresses Reserved or (by convention) special addresses: Loopback interfaces all addresses are reserved for loopback interfaces Most systems use as loopback address loopback interface is associated with name localhost IP address of a network Host number is set to all zeros, e.g., Broadcast address Host number is all ones, e.g., Broadcast goes to all hosts on the network Often ignored due to security concerns Test / Experimental addresses Certain address ranges are reserved for experimental use. Packets should get dropped if they contain this destination address (see RFC 1918): Convention (but not a reserved address) Default gateway has host number set to 1, e.g., e.g.,

16 Subnetting Problem: Organizations have multiple networks which are independently managed Solution 1: Allocate a separate network address for each network Difficult to manage From the outside of the organization, each network must be addressable. Solution 2: Add another University Network Engineering School Library Medical School Subnetting

17 Address assignment with subnetting Each part of the organization is allocated a range of IP addresses (subnets or subnetworks) Addresses in each subnet can be administered locally /16 University Network / /24 Engineering School Medical School /24 Library /24

18 Basic Idea of Subnetting Split the host number portion of an IP address into a subnet number and a (smaller) host number. Result is a 3-layer hierarchy network prefix host number network prefix subnet number host number extended network prefix Then: Subnets can be freely assigned within the organization Internally, subnets are treated as separate networks Subnet structure is not visible outside the organization

19 Subnetmask Routers and hosts use an extended network prefix (subnetmask) to identify the start of the host numbers network prefix host number network prefix extended network prefix subnet number host number subnetmask

20 Advantages of Subnetting With subnetting, IP addresses use a 3-layer hierarchy:» Network» Subnet» Host Reduces router complexity. Since external routers do not know about subnetting, the complexity of routing tables at external routers is reduced. Note: Length of the subnet mask need not be identical at all subnetworks.

21 Classful IP Adresses (Until 1993) When Internet addresses were standardized (early 1980s), the Internet address space was divided up into classes: Class A: Network prefix is 8 bits long Class B: Network prefix is 16 bits long Class C: Network prefix is 24 bits long Each IP address contained a key which identifies the class: Class A: IP address starts with 0 Class B: IP address starts with 10 Class C: IP address starts with 110

22 The old way: Internet Address Classes Class A 0 bit # Network Prefix 8 bits Host Number 24 bits bit # Class B 1 network id host 31 Network Prefix 16 bits Host Number 16 bits bit # Class C network id host Network Prefix 24 bits Host Number 8 bits

23 The old way: Internet Address Classes Class D bit # multicast group id Class E bit # (reserved for future use)

24 Problems with Classful IP Addresses By the early 1990s, the original classful address scheme had a number of problems Flat address space. Routing tables on the backbone Internet need to have an entry for each network address. When Class C networks were widely used, this created a problem. By the 1993, the size of the routing tables started to outgrow the capacity of routers. Other problems: Too few network addresses for large networks Class A and Class B addresses were gone Limited flexibility for network addresses: Class A and B addresses are overkill (>64,000 addresses) Class C address is insufficient (requires 40 Class C addresses)

25 CIDR - Classless Interdomain Routing IP backbone routers have one routing table entry for each network address: With subnetting, a backbone router only needs to know one entry for each Class A, B, or C networks This is acceptable for Class A and Class B networks 2 7 = 128 Class A networks 2 14 = 16,384 Class B networks But this is not acceptable for Class C networks 2 21 = 2,097,152 Class C networks In 1993, the size of the routing tables started to outgrow the capacity of routers Consequence: The Class-based assignment of IP addresses had to be abandoned

26 CIDR address blocks CIDR notation can nicely express blocks of addresses Blocks are used when allocating IP addresses for a company and for routing tables (route aggregation) CIDR Block Prefix /27 32 /26 64 / / / /22 1,024 /21 2,048 /20 4,096 /19 8,192 /18 16,384 /17 32,768 /16 65,536 /15 131,072 /14 262,144 /13 524,288 # of Host Addresses

27 Private Network Private IP network is an IP network that is not directly connected to the Internet IP addresses in a private network can be assigned arbitrarily. Not registered and not guaranteed to be globally unique Generally, private networks use addresses from the following experimental address ranges (non-routable addresses):

28 Private Addresses H1 H2 H3 H Private network Private network 1 R1 Internet R H5 28

29 Network Address Translation (NAT) NAT is a router function where IP addresses (and possibly port numbers) of IP datagrams are replaced at the boundary of a private network NAT is a method that enables hosts on private networks to communicate with hosts on the Internet NAT is run on routers that connect private networks to the public Internet, to replace the IP address-port pair of an IP packet with another IP address-port pair. 29

30 Basic operation of NAT Private network Internet Source = Destination = Source = Destination = private address: public address: NAT device public address: H1 Source = Destination = Source = Destination = H5 Private Address Public Address NAT device has address translation table 30

31 Concerns about NAT End-to-end connectivity: NAT destroys universal end-to-end reachability of hosts on the Internet. A host in the public Internet often cannot initiate communication to a host in a private network. The problem is worse, when two hosts that are in a private network need to communicate with each other. 31

32 Concerns about NAT IP address in application data: Applications that carry IP addresses in the payload of the application data generally do not work across a private-public network boundary. Some NAT devices inspect the payload of widely used application layer protocols and, if an IP address is detected in the application-layer header or the application payload, translate the address according to the address translation table. 32

33 IPv6 - IP Version 6 IP Version 6 Is the successor to the currently used IPv4 Specification completed in 1994 Makes improvements to IPv4 (no revolutionary changes) One (not the only!) feature of IPv6 is a significant increase in of the IP address to 128 bits (16 bytes) IPv6 will solve for the foreseeable future the problems with IP addressing addresses per square inch on the surface of the Earth.

34 IPv6 Header 32 bits version (4 bits) Traffic Class (8 bits) Payload Length (16 bits) Flow Label (24 bits) Next Header (8 bits) Hop Limits (8 bits) Source IP address (128 bits) Destination IP address (128 bits)

35 IPv6 vs. IPv4: Address Comparison IPv4 has a maximum of billion addresses IPv6 has a maximum of = (2 32 ) 4 4 billion x 4 billion x 4 billion x 4 billion addresses

36 DHCP Dynamic Host Configuration Protocol (DHCP) o o o Supports temporary allocation ( leases ) of IP addresses DHCP client can acquire all IP configuration parameters needed to operate DHCP is the preferred mechanism for dynamic assignment of IP addresses 36

37 DHCP Message Type Message type is sent as an option. Value Message Type DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPDECLINE DHCPACK DHCPNAK DHCPRELEASE DHCPINFORM 37

38 DHCP Operation DHCP Client 00:a0:24:71:e4:44 DHCP Server DCHP DISCOVER DHCPDISCOVER Sent to DHCP Server DHCP Client 00:a0:24:71:e4:44 DHCPOFFER DHCP Server DCHP OFFER DHCPOFFER DHCP Server 38

39 DHCP Operation DCHP REQUEST DHCP Client 00:a0:24:71:e4:44 DHCPREQUEST DHCP Server After this, the client can start to use the IP address DHCPACK Renewing a Lease (sent when 50% of lease has expired) DHCP Server If DHCP server sends DHCPNACK, then address is released. DHCP Client 00:a0:24:71:e4:44 DHCP Server DCHP RELEASE DHCPRELEASE At this time, the client has released the IP address DHCP Server 39