Lecture #16: Network Layer and Internetworking

1. Lecture #16: Network Layer and Internetworking C o n t e n t s • Network Layer: functions and services • Network Layer: technologies • Internetworking • Concatenated Virtual Circuits • Connectionless internetworking • Fragmentation • Firewall technology 2 6 7 10 12 15 19

2. OSI Network Layer User application 1 ... Application layer Encryption/ decryption compression/ expansion Choice of syntax Presentation layer Session control Session synch. Session to transport mapping Session management Session layer Layer and flow control Error recovery Multiplexing Transport layer Connection control Routing Addressing Network layer Data link establishment Error control Flow control Framing Synch Link layer Access to transm. media Physical and electrical interface Activation/ deactivation of con. Physical layer 1/18 Connection control: establishment, maintaining and terminating network connections between source and destination open systems Routing: considerations associated with hop-by-hop services transparent to the underlying resources such as data link connections . Addressing: globally unique identification of a service access point of an end system (transparent to subnet technology (routers/LANs…) and topology (# of hops) including naming

3. NL Services to the Transport Layer • The basic service of the network layer is to provide the transparent transfer of data between transport entities. This service allows the structure and detailed content of submitted data to be determined exclusively by layers above the network layer. • The network layer contains functions necessary to provide the transport layer with a firm network/transport layer boundary which is independent of the underlying communications media in all things other than quality of service. • Thus the network layer contains functions necessary to mask the differences in the characteristics of different transmission and subnetwork technologies into a consistent network service.

4. Services provided to the transport layer • Transparent transfer of data between transport entities. This service allows the structure and detailed content of submitted data to be determined exclusively by layers above the network layer. • Firm network/transport layer boundary which is independent of the underlying communications media in all things other than quality of service. • Mask the differences in the characteristics of different transmission and subnetwork technologies into a consistent network service.

5. Network Layer Service Types • Connection oriented - virtual circuit (VC) - supported by the lower network layers (DLL): • setup and release of the connection • connection parameters negotiation • sequenced delivery of packets • receiver’s overflow prevented by flow control • options: • priority of delivery • confirmation of delivery • reliable • unreliable (rare usage) • Examples: most popular X.25 • Connectionless oriented - datagrams exchange - reliability issues (if present) supported by the transport layer • send/receive directives (confirmed/nonconfirmed services) • independent packets’ (“datagrams”) delivery with full destination address • Examples: most popular IP (required when using TCP/IP) 16/1 16/2

6. Network Layer Technologies • Datagram Exchange • Addressing: full source and destination address in each datagram • State information: not needed nor hold • Routing: independent routing of the subsequent packets • Node Failure effects: packets loss • Congestion control: not typical, rarely applied • Complexity: in transport layer (above the subnet!) • Application: connectionless services but also connection oriented • Virtual Circuit • Addressing: short VC number in each packet • State information: kept in the subnet table for each VC • Routing: only during the VC setup • Node Failure effects: VCs termination • Congestion control: consists of and depends on buffering • Complexity: in the network layer (in the subnet!) • Application: connection oriented services 5/2

7. Internetworking - Terms • Internetworking - multinet structure including different types of networks and protocols • Internetworking glossary: • Communication network: a facility providing data transfer service among stations attached to the network • Internet: a collection of communication networks connected by bridges and/or routers • Subnetwork: a constituent network of an internet • Intermediate system (IS): a connection device between any two subnetworks • Repeater: IS that connect two identical subnetworks on the physical level, repeats the bit sequence without storing of any data. • Bridge: IS that connects two LANs with identical protocols. Bridges are address filters that use store-and-forward mechanism without modifying the packets’ contents. It operates on DLL level • Router: IS that connects two networks with potentially different protocols (“multiprotocol router”); store-and-forward address filter operating on the Network Layer • Gateway: internetworking protocol converters acting on the Transport and Application layers. Modifications: full and half gateways 5/33 5/34

8. Networks Characteristics 5/35 • Protocol stack: OSI/IP/Novel/DECnet/AppleTalk/... • Addressing scheme: flat files (802.X) vs. hierarchical (IP), implementation of directory services • Service types incl. QoS: connectivity, confirmed/ /nonconfirmed services, special features support (e.g.real time) • Parameters:system of timeouts, buffer sizes etc. • Flow/error control: level of ordering and error protection • Security: levels of privacy, encryption, identification etc. • Routing and congestion control: different mechanisms • Broadcasting and multicasting: yes/no • Packet size: maximum size varies substantially • Accounting rules: yes/no; by traffic/time 9

9. Addressing Uniqueness: Addressing allows the DTE to be uniquely identified so that data may be routed globally to the correct destination. Levels of addressing Network Level (and above) SAP: Uniquely identifies the DTE within the internet DTE may have more than one SAP, each of them is unique to that particular DTE Global Internet Address (GNA) = (network, host or station) parameters Form: (network identifier, end system identifier) Subnet Level A unique address for each DTE attached to the subnet Referred to as the Subnetwork Attachment Point Address (SAPA) Host parameter of GNA and SAPA may be the same but are often not Different networks use different addressing formats and lengths (ARP, RARP) Some host have more than one attachment point to the subnet Host parameter (GNA) has global significance, SAPA has local significance 16/3

10. Concatenated Virtual Circuits CVCis End-to-End connection that consists of several consecutive Point-to-Point links between: source host and subnet subnet and multiprotocol router (“full gateway”) [subnet and subnet, connected by shared “half-gateways”]) subnet and destination host Features: the data routes are identified by VC numbers during the session data packets traverse the same sequence of GWs and arrive in order the routes are supported by VC tables containing the ID number of the actual VCs the next destination for each VC the number of the next concatenated VC Application: internetworking in set of subnets of similar type of services (e.g. either reliable or unreliable). Usually implemented on Transport layer (e.g. TCP - End-to-End transport protocol) 5/36

11. Concatenated Virtual Circuits Pro’s • reservation of buffers and communication capacity in advance • guaranteed sequencing, delivery and stable delays • possible implementation of any type services • short addressing (small communication overload due to the headers) • small communication overload due to packets retransmission and losses Contra’s • waste of buffer space (table space) for each open connection • static routing during the session i.e. bad congestion control • vulnerability to router failures • complicated implementation in unreliable datagram subnetworks

12. Connectionless Internetworking Applies Datagram model Features: • independent routing for each packet thus optimizing the the congestion • not-in-order delivery • datagram packets can be routed around network failure points in d.g. subnetworks • requires universal addressing system - Internet, IPX, OSI, SNA, AppleTalk address standards 5/37

13. Connectionless Internetworking Contra’s • communication overhead due to longer address fields, repeated in each datagram • communication overhead due to unreliable unordered services • dispersed delay duration • requires universal addressing system Pro’s • adaptive dynamic routing and adaptive congestion control • low buffer space needed at routers • robustness to router failures • applicable for any type of subnets incl. unreliable ones

14. Tunneling • Tunnelingis a technique for connection of two similar networks through the arbitrary type[s] of intermediate network[s] • Data entities (datagrams, packets) of two ends are packed together with their control information (addressing, ordering, error control fields, etc.) into the payload field of the intermediate network’ NL packets • The original control information is not being interpret anywhere in the intermediate network but in both ends • Therefore, tunneling needs multiprotocol routers only on the both ends of the “tunnel” where the original data entities are constructed/restored 5/38

15. Fragmentation • Fragmentationis the process of splitting of the data structures into the entities that are suitable to transmit over the various networks and the reverse process of restoring the original structures out of the fragments. • Fragmentation factors: • Transmission method (bit error rate, multiplexing method, etc.) • Operating system (read/write blocks of 0.5 kB) • Protocols (packet length field limitation) • Standardization • Service discipline and resource sharing in the end stations and intermediate systems (IS): routers, gateways (e.g. SJF “shortest job first”, RR “Round Robin” etc.) • Examples of payload size: • ATM cell carries 48B • IP packet carries 64kB • Data packets are broken into fragments and each fragment is sent in separate internet packet.

16. Fragmentation Methods • Each network in the internet is bounded by gateways which are the entry point and the output point of the packets traversing that network • 1st approach: transparent fragmentation. Large packets are fragmented (if needed!) into fragments at the small-packet-network entry point (gateways G1, G3) and resembled back at the network output point (G2, G4). Note that all the fragments should reach the same network output point! • Example: ATM networks hardware fragmentation/defragmentation of the packets into ATM cells at each entry/output point • Requirements/features: • additional counting of the number of fragments in connectionless networks or End-of-the-packet flag in the last fragment in the connection-oriented networks • congestion control and performance are affected by the requirement for similar routing of all the fragments • multiple fragmentation/defragmentation cycles may occur during an internet route of a long packet 5/41a

17. Fragmentation Methods (2) • 2nd approach: nontransparent fragmentation. Large packets are fragmented (if needed!) at the small-packet-network entry point (gateway G1), then traverse the internet as independent packets and are resembled back only at the destination host. • Requirements/features: • defragmentation capabilities of each host • communication overhead for each fragment during the whole route • better possibility for congestion control and dynamic routing (in the datagram model) • only one fragmentation/defragmentation cycle (if any!) may occur during an internet route of a long packet • possibility for hierarchical fragmentation: fragmentation of already fragmented packets in case the route passes network of even smaller packets: tree-numbering of the fragments that can be extended hierarchically (e.g. [0.]  [0.0, 0.1, 0.2 ...]  [0.0.0, 0.0.1, 0.0.2 … 0.1.0, 0.1.1 ...] ... 5/41b 18

18. Fragmentation Methods (3) Requirements/features (cont.): • fragmentation to some elementary frame size. Fragments are short enough to be carried by any intermediate network. An internet packet contains one or more elementary frames. Additional flagging: • packet ID number • ordering number of the first elementary fragment in the packet • end-of-the-packet flag (1 bit: end/no_end) 5/42