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Congestion Control Algorithm • Preallocation of Buffers e.g., – Allocate Buffer to Each Virtual Circuit in Each IMP. Note: This May be Expensive, and Only Used Where Low Delay & High Bandwidth are Essential (e.g., Digitized Voice) 1 Packet Discarding • Datagram Service: Packet Discard at Will • Virtual Circuit Service: A Copy of Packet Must be Kept Note: If Congestion is to be Avoided by Discarding Packets, A Rule is Needed Input Lines Output Lines Free Buffer Congestion can be reduced by putting an upper bound on the number of buffers queued on an output file 2 Packet Discarding (cont.) (Irland) Discovered Simple Rule of Thumb (Not Optimal) for Determining Max Q Length, m, for an IMP With k Buffers m = k / s, Where s # of Output Lines 3 Internetworking • Many Different Networks Exist • Different Networks Have Radically Different Technology • Still Desirable to Connect These Networks Examples of This Follow: 1. LAN-LAN: A Computer Scientist Downloading a File to Engineering 2. LAN-WAN: A Computer Scientist Sending Mail to a Distant Physicist 3. WAN-WAN: Two poets Exchanging Sonnets 4. LAN-WAN-LAN: Engineers at Different Universities Communicating 4 Four Common Types of Relays • Layer 1: Repeaters Copy Individual Bits Between Cable Segments • Layer 2: Bridges Store and Forward Frames Between LANs • Layer 3: Gateways Store and Forward Packets Between Dissimilar Networks • Layer 4: Protocol Converters Provide Interfacing in Higher Layers 5 Network interconnection The boxes marked B are bridges. Those marked G are gateways. 6 Gateways Operate at The Network Level Two Styles: • Connection-Oriented e.g., Virtual Circuit • Connectionless-Oriented e.g., Datagram 7 Internetworking (A Full Gateway) Network 1 Net 1 to Internet 1 Internet to Net 1 B U F F E R Net 2 to Internet Network 2 2 Internet to Net 2 Machine owned jointly by both network 8 Internetworking (Two half-Gateways) Communication Line Network 1 Net 1 to Internet Net 2 to Internet 1 Network 2 2 Internet to Net 1 Internet to Net 2 Machine owned by Network 1 Machine owned by Network 2 9 The Network Layer Source Host Destination Host Networks Gateway (a) Internetworking using concatenated virtual circuits 10 The Network Layer (cont) Source Host Destination Host (b) Internetworking using datagrams 11 A datagram moving from network to network 12 A datagram moving from network to network (cont.) Frame 1 DH1 IP TH DT1 Frame 2 DH2 IP TH DT2 Frame 3 DH3 IP TH DT3 DHX: Data link Header for network X DTX: Data link Trailer for network X IP : Internet Protocol header TH : Transport Header Internet packet 13 Transparent Fragmentation Network 2 Network 1 Packet G1 fragments a large packet G2 reassembles the fragment G3 G4 fragments reassembles again again 14 Non-transparent Fragmentation Packet G1 fragments a large packet The fragments are not reassembled until the final destination (a host) is reached 15 Firewall •A dedicated gateway machine with special security precautions on it. •Out-going/in-coming packets may be blocked. •IP address and a port number may be used to block the packets. The IP Protocol 32 Bits Version IHL Type of Service DM F F Identification Time to Live Total Length Protocol Fragment Offset Header Checksum Source Address Destination Address Options (0 or more words) The IP (Internet Protocol) header 17 The IP Addresses 32 Bits Range of host address Class A 0 Network B 10 C 110 D 1110 E 11110 1.0.0.0 to 127.255.255.255 Host Network 128.0.0.0 to 191.255.255.255 Host Network Host Multicast Address Reserved for Future use 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 240.0.0.0 to 247.255.255.255 IP address formats 18 The IP Addresses 00000000000000000000000000000000 This host 00 A host on this network … 00 Host 11111111111111111111111111111111 Network 127 1111 … 1111 (Anything) Special IP addresses Broadcast on the local network Broadcast on the distant network Loopback The IP Addresses 32 Bits Subnet mask 10 Network Subnet Host 11111111 11111111 11111100 00000000 One of the ways to subnet a class B network ARP Provides a mapping between the two forms of addresses: 32-bit IP addresses and data link addresses (e.g., 48-bit Ethernet addr.) 32-bit Internet Addr ARP RARP 48-bit Ethernet Addr ARP Request/Reply V W X Y Z V W X Y Z V W X Y Z ARP (cont’d) Fund. Concept: The network interface has a hardware address, and frames exchanged at the hardware level must be addressed to the correct interface. TCP/IP works with its own addresses (i.e., 32-bit IP addresses). Knowing a host’s IP addresses does not let the kernel (i.e., Ethernet driver) must know the hardware address to send the data. Summary Protocol addresses cannot be used when transmitting frames across physical network hardware, because the hardware does not understand IP addressing. So, a frame sent across a given physical network must use hard ware’s frame format, and all addresses in the frame must be hardware address 192.31.65.7 192.31.65.5 1 2 E1 E2 CS Ethernet 192.31.65.0 Router has 2IP addresses Router has 2IP addresses 192.31.60.4 192.31.65.1 192.31.60.7 192.31.63.3 192.31.63.8 F2 F1 3 F3 E3 E4 Campus FDDI ring 192.31.60.0 E5 4 E6 EE Ethernet 192.31.63.0 Three interconnected class C networks: two Ethernets and an FDDI ring Ethernet addresses Internet Control Protocols • • • • ICMP (Internet Control Message Protocol) ARP (Address Resolution Protocol) RARP (Reverse Address Resolution Protocol) BOOTP (Bootstrap Protocol) 26 BOOTP (cont’d) BOOTP uses UDP and is intended as an alternative to RARP For bootstrapping a diskless system to find its IP address. Bootp can also return additional information such as the IP address of a router, the client’s subnet mask, and the IP address of a name server. BOOTP (cont’d) BOOTP DHCP (Dynamic Host Configuration Protocol) Unlike BOOTP, DHCP does not require an administrator to add an entry for each computer to the database that a server uses. Instead, DHCP provides a mechanism that allows a computer to join a new network and obtain an IP address without manual intervention. Note: An administrator can configure a DHCP server to have 2 types of addresses: Permanent addresses and a pool of addresses to be allocated on demand. Operation of ARP (2) FTP Host Name (3) TCP establish connection with IP address IP (1) addr IP send IP datagram to IP address Resolver (5)ARP (6) (8) (4) (9) Ethernet Driver ARP request (Ethernet broadcast) Ethernet Driver Ethernet Driver (7) ARP ARP IP TCP Operation of ARP when user types “ftp hostname” 29 Goals: IPv6 1. Support billions of hosts, even with inefficient address space allocation 2. Reduce the size of the routing tables 3. Simplify the protocol, to allow routers to process packets faster 4. Provide better security (authentication and privacy) than current IP 5. Pay more attention to type of service, particularly for realtime data IPv6 (cont’d) 6. Aid multicasting by allowing scopes to be specified 7. Make it possible for a host to roam without changing its address 8. Allow the protocol to evolve in the future 9. Permit the old and new protocols to coexist for years 32 Bits Version Priority Payload length Flow label Next header Source address (16 bytes) Destination address (16 bytes) The IPv6 fixed header (required) Hop limit Examples of The Network Layer 7 6 Application protocol (not defined by X.25) 7 Presentation protocol (not defined by X.25) 6 5 Session protocol (not defined by X.25) 5 4 Transport protocol (not defined by X.25) 4 3 2 1 DTE X.25 layer 3 X.25 layer 2 X.25 layer 1 3 2 1 3 2 1 3 2 1 X.25 layer 3 X.25 layer 2 X.25 layer 1 Internal protocols are not defined by X.25 3 2 1 DTE The place of X.25 in the protocol hierarchy 33 Two Forms of Connections • Virtual Calls --- A Connection is Established, Data Are Transferred, & Then The Connection is Released • Permanent Virtual Calls --- Like A Leased Line, DTE at Either End LAN Just Send Data Whenever It Wants, Without Any Setup • Note: The Choice of Circuit # on Outgoing Calls is Determined by The DTE, and on Incoming Calls by The DCE, May Lead to A Call Collision. 34 The Three Phases of an X.25 connection 35