1. Fall 2010CEG 41882-1 CEG-4188 Lecture 2: Internetworking and the Internet Protocol (IP) Prof. Gregor v. Bochmann SITE - University of Ottawa These course notes are based on slides prepared by Drs. Makrakis and Shirmohammadi

2. Fall 2010CEG 41882-2 Network Layer  Provides the upper layers with independence from the data transmission and physical networking technologies.  Responsible for sending data from source to destination.  This includes the nodes in- between (and therefore it is not end-to-end)  Responsible for requesting network facilities, such as priority, bit-rate, etc…  Responsible for routing. Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer

3. Fall 2010CEG 41882-3 Basic network functions Data transfer (in packet switching mode) Two modes of transfer: –Connection-oriented (end-to-end connection must be established before data transfer can occur) –Connection-less Addressing –Address identifies destination –Multicasting (broadcasting only over small networks) Additional features: –Ordered delivery –Flow control –Error control

4. Fall 2010CEG 41882-4 Connection-oriented vs. Connection-less Debate in the 1970ies The connection-less IP protocol was adopted –Simpler to realize, especially for inter-networking –Cannot provide ordered delivery, flow control and error control (if this is required by application, TCP must be used) Circuit-switched networks favor connection- oriented service. Also in optical networks, technology favors connection-orientation. However, IP is expected to remain the main internetworking protocol.

5. Fall 2010CEG 41882-5 Internetworking  Internetworking is a scheme for interconnecting multiple networks of not identical technologies  Uses both hardware and software Extra hardware positioned between networks Software on each attached computer  System of interconnected networks is called an internetwork or an internet

6. Fall 2010CEG 41882-6 TCP/IP Protocol Suite vs. OSI

7. Fall 2010CEG 41882-7 Internetworking architecture

8. Fall 2010CEG 41882-8 Autonomous System (AS) AS is a set of routers and networks managed by a single organization. AS consists of a group of routers exchanging information via a common routing protocol. Claiming that an AS is “connected”, means that (excluding times of failures) there is always a “path” between any pair of nodes.

9. Fall 2010CEG 41882-9 Example: A 2-AS formed Internet

10. Fall 2010CEG 41882-10 A “possible” Internet Architecture(1)

11. Fall 2010CEG 41882-11 A “possible” Internet Architecture(3) AS-5 AS-1 AS-4 AS-3AS-2

12. Fall 2010CEG 41882-12 Routing Autonomous System (AS): set of networks and routers operated by a single organization. Interior Router Protocol (IRP): passing routing information within and AS. Exterior Router Protocol (ERP): passing routing information between different ASs.

13. Fall 2010CEG 41882-13 Routing Approaches Distance Vector Routing: each node exchanges information with its neighboring nodes; e.g. Routing Information Protocol (RIP). Link-state Routing: sends link costs of each of its network interfaces to all routers (not just neighboring). Typically used with a Dijkstera- based algorithm; e.g., Open Shortest Path First (OSPF). Path-vector Routing: router provides information about which networks can be reached by a given router and the ASs that must be crossed; e.g. Border Gateway Protocol (BGP).

14. Fall 2010CEG 41882-14 Internet Protocol (IP) v4 defined in RFC 791 part of TCP/IP suite will (eventually) be replaced by IPv6

15. Fall 2010CEG 41882-15 IP Header

16. Fall 2010CEG 41882-16 IP Header Fields Version (4 bits): currently 4 Internet Header Length (4 bits) –Minimum is 5, for 20 octets –Header may include options and padding Type of Service (8 bits) – rarely used, for differentiated services and congestion notification Total Length (16 bits) of datagram, in octets (header & data) Identification (16 bits) –Sequence number –Together with addresses and user protocol, this field identifies the datagram uniquely (used for fragmentation) Flags (3 bits) –Only 2 bits used for fragmentation: More bit, and Don’t Fragment bit

17. Fall 2010CEG 41882-17 IP Header Fields (…) Fragmentation offset ( 13 bits) Time to Live (8 bits) Protocol (8 bits) –Next higher layer to receive data field at destination Header Checksum (16 bits) –Re-verified and recomputed at each router –16 bit ones-complement sum of all 16 bit words in the header Source Address (32 bits) Destination Address (32 bits) Options (Security, timestamp, …) Padding, to fill to multiple of 32 bits long Following the header: Data field (contains user data): maximum lengths 65 535 octets

18. Fall 2010CEG 41882-18 Internetworking Requirements Are they satisfied by IP ? Design requirements: accommodate differences in constituting sub-networks: 1.Different maximum packet size 2.Different addressing schemes 3.Different network access mechanism 4.Different maximum packet lifetime 5.Different transmission modes (connection-oriented, connectionless) 6.Error control 7.Flow control

19. Fall 2010CEG 41882-19 (1) Fragmentation and Reassembly: Why? Main reason: Different maximal packet sizes –Lower-level protocols may need to break data up into smaller blocks, an action called fragmentation –Each network only accepts blocks of a certain size, or it has a minimum and maximum limit for the allowed size of data blocks (e.g. ATM: 53 bytes cell size (48 payload + 5 control) Ethernet frames: minimum size = 72 bytes; maximum size = 1526 bytes Other reasons –more efficient error control & smaller retransmission units –fairer access to shared facilities –Less waiting times of packets of higher priority in queues –smaller buffers Disadvantages –more bandwidth wasted in overhead related data –more interrupts & processing time

20. Fall 2010CEG 41882-20 PDUs and Fragmentation

21. Fall 2010CEG 41882-21 Fragmentation Example

22. Fall 2010CEG 41882-22 (2) Different Addressing Schemes Introduce IP address as a global address All hosts on the Internet must have a unique IP address –Exception: techniques such as NAT (network address translation) allow private IP addresses that might be duplicated somewhere else. –NAT is very common (because IP v4 does not have enough address space)

23. Fall 2010CEG 41882-23 IP Address  Each IP address is divided into a prefix and a suffix prefix identifies the network to which the computer is attached suffix identifies the computer within that network we allocate some bits for prefix, some for suffix (total of 32 bits) »large prefix, small suffix - many networks, few hosts per network »small prefix, large suffix - few networks, many hosts per network  Network numbers are unique assignment of network numbers must be coordinated globally; assignment of host addresses can be managed locally

24. Fall 2010CEG 41882-24 IP Address format 0 netidhostid 8 Class A 10 netidhostid 16 Class B 110 netidhostid 24 Class C 32 1110 Class D 32 multicast

25. Fall 2010CEG 41882-25 IP Address format 11110 Class EReserved for future use Address ClassNo. of NetworksNo. of HostsComments A12616777214 Very Large Networks B1638465534 Medium Size Network C2097151254 Large number of small networks Host id 0 is never assigned to an individual host. It refers to the network itself.

26. Fall 2010CEG 41882-26 26 Example Class A Class C Class B

27. Fall 2010CEG 41882-27 Subnets and Subnet Masks Allow arbitrary complexity of internetworked LANs within organization. Insulate overall internet from growth of network numbers and routing complexity. To rest of internet, site looks like single network.

28. Fall 2010CEG 41882-28 Routing Using Subnets

29. Fall 2010CEG 41882-29 Address Mapping (ARP) Sometimes, in order to reach a destination, there is no need to go through an IP router. –E.g.? In that case, the physical address can be used directly.

30. Fall 2010CEG 41882-30 ARP (Address Resolution Protocol)

31. Fall 2010CEG 41882-31 (4) Different maximum packet lifetime Why limiting the maximum packet lifetime ? Datagrams could loop indefinitely –Consumes resources –Transport protocol may need upper bound on datagram lifetime Solution proposed for IP: –Time To Live (TTL) field in IP header the value represents the maximum hop count It is decremeted each time the packet passes through a router –When the value of TTL becomes zero, the datagram is discarded (not forwarded) –Note: This does not solve the problem of the Transport protocol, since the time waiting in a router is not bounded, so there is no enforced maximum lifetime, only a maximum hop count.

32. Fall 2010CEG 41882-32 Internetworking Requirements Design requirements: accommodate differences in constituting networks: 1.Different maximum packet size 2.Different addressing schemes 3.Different network access mechanism (implemented in each router, as required) 4.Different maximum packet lifetime 5.Different transmission modes (connection-oriented, connectionless) 6.Error control (not provided by IP) 7.Flow control (not provided by IP)

33. Fall 2010CEG 41882-33 IP v6 Why Change IP ? Address space exhaustion (this is the main reason) –growth of networks and the Internet Requirements for new types of services –new addressing features –flow identification –features for resource allocation New header structure for more efficient processing

34. Fall 2010CEG 41882-34 IPv6 Enhancements (1) Expanded IP address: 128 bit address space –increase of address space by a factor of 2 96 –allows (on the order of) 6  10 23 unique addresses per square meter of the surface of the earth, which seems inexhaustible. Improved (flexible) option mechanism –options are placed in separate optional headers ( between IPv6 header & transport- layer header). –most optional headers are not examined/processed by any internet router on the packet's path. –simplifies and speeds up IPv6 (vs. IPv4) packet routing processing. –Easier to add additional options.

35. Fall 2010CEG 41882-35 IPv6 Enhancements (2) dynamic address assignment (using address auto-configuration) Increased addressing flexibility –includes anycast & multicast –anycast: packet is delivered to just one of a set of nodes. –scalability of multicast routing is improved by adding scope field to multicast addresses. Support for resource allocation –labelled packet flows –distinguishes different flows coming from the same (IP address) source (e.g. can identify a Video over IP or Voice over IP session (having real- time constraints) from a file transfer or web browsing session (which are fine with best effort treatment).