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Transcript
Guide to TCP/IP, Third Edition Chapter 3: Data Link and Network Layer TCP/IP Protocols Objectives • Understand the role that data link protocols, such as SLIP and PPP, play for TCP/IP • Distinguish among various Ethernet and token ring frame types • Understand how hardware addresses work in a TCP/IP environment, and the services that ARP and RARP provide for such networks Guide to TCP/IP, Third Edtion 2 Objectives (continued) • Appreciate the overwhelming importance of the Internet Protocol (IP), and how IP packets behave on TCP/IP networks • Understand the lifetime of an IP datagram, and the process of fragmentation and reassembly • Appreciate service delivery options • Understand IP header fields and functions Guide to TCP/IP, Third Edtion 3 Data Link Protocols • Key jobs of Data Link layer – Media Access Control (MAC) – Logical Link Control (LLC) • Point-to-point data transfer – Shipping data from one MAC layer address to another Guide to TCP/IP, Third Edtion 4 Data Link Protocols (continued) • WAN encapsulation of frames at Data Link layer involves one or more of the following services – – – – Addressing Bit-level integrity check Delimitation Protocol identification (PID) Guide to TCP/IP, Third Edtion 5 Serial Line Internet Protocol (SLIP) • Original point-to-point protocol • Sometimes used to – Manage communications or networking equipment through a dial-up serial port connection • Simple packet-framing protocol described in RFC 1055 • Uses a special END character (0xC0) – Placed at the beginning and end of each IP datagram to delimit, or separate, each payload Guide to TCP/IP, Third Edtion 6 Point-to-Point Protocol • Provides – Frame delimitation – Protocol identification and bit-level integrity check services • RFC 1661 includes – Encapsulation methods – A special Link Control Protocol (LCP) – A collection of negotiation protocols Guide to TCP/IP, Third Edtion 7 Point-to-Point Protocol (continued) • Fields in the PPP header and trailer include – Flag – Protocol identifier – Frame Check Sequence (FCS) • Supports a default MTU of 1,500 bytes – Which makes it ideal for interconnecting Ethernetbased networks (or peers) Guide to TCP/IP, Third Edtion 8 Special Handling for PPP Links • For switched technologies – Bidirectional connections must be negotiated between peers that wish to exchange data • X.25: RFC 1356. X.25 – Standard set of protocols defined in the 1970s by the International Telecommunications Union (ITU) • Frame relay: RFC 2427 – Assumes that digital-quality transmission lines are available for creating WAN links Guide to TCP/IP, Third Edtion 9 Special Handling for PPP Links (continued) • ATM: RFCs 1577 and 1626 – High-speed, long-haul, broadband, cell-switched networking technology – Offers astonishing and ever-increasing bandwidth • PPPoE: RFC 2516 – Protocol used by Internet service providers to authenticate and manage broadband subscribers Guide to TCP/IP, Third Edtion 10 Frame Types • At Data Link layer – Protocol data units are called frames • Frame – Represents same data that appears in digital form at the Network layer in an IP datagram Guide to TCP/IP, Third Edtion 11 Ethernet Frame Types • Ethernet II frame type – De facto standard frame type used for IP datagram transmissions over Ethernet networks – Has protocol identification field • Ethernet frame types that TCP/IP can use – Ethernet II – Ethernet 802.2 Logical Link Control – Ethernet 802.2 Sub-Network Access Protocol (SNAP) Guide to TCP/IP, Third Edtion 12 Ethernet II Frame Structure • Ethernet II frame type fields and structure – – – – – – Preamble Destination Address Field Source Address Field Type Field Data Field Frame Check Sequence Field Guide to TCP/IP, Third Edtion 13 Ethernet II Frame Structure (continued) Guide to TCP/IP, Third Edtion 14 Ethernet II Frame Structure (continued) Guide to TCP/IP, Third Edtion 15 Ethernet 802.2 LLC Frame Structure • Unique fields – – – – – – – – – – Preamble Start Frame Delimiter Field: Length Field Destination Service Access Point (DSAP) Field: Source Service Access Point (SSAP) Field: Control Field Destination Address Source Address Data Frame Check Sequence Guide to TCP/IP, Third Edtion 16 Ethernet 802.2 LLC Frame Structure (continued) Guide to TCP/IP, Third Edtion 17 Ethernet 802.2 LLC Frame Structure (continued) Guide to TCP/IP, Third Edtion 18 Ethernet SNAP Frame Structure • Fields – – – – – – – – – Organization Code Field Ether Type Field Preamble Start Frame Delimiter Destination Address Source Address Length Destination Service Access Point Source Service Access Point Guide to TCP/IP, Third Edtion 19 Ethernet SNAP Frame Structure (continued) Guide to TCP/IP, Third Edtion 20 Token Ring Frame Types • IEEE 802.5 standard – Defines token ring networking • Token ring networks – Rely on a physical star design, although they use a logical ring transmission path • On a token ring network – Each token ring workstation acts as a repeater • Variations of token ring frames – Token Ring 802.2 LLC frames – Token Ring SNAP frames Guide to TCP/IP, Third Edtion 21 Token Ring Frame Types (continued) Guide to TCP/IP, Third Edtion 22 Hardware Addresses in the IP Environment • IP addresses – Identify individual IP hosts on a TCP/IP internetwork • TCP/IP networking uses ARP to – Determine the hardware address of the local target for the packet • ARP cache – Table of hardware addresses learned through the ARP process Guide to TCP/IP, Third Edtion 23 Hardware Addresses in the IP Environment (continued) Guide to TCP/IP, Third Edtion 24 Hardware Addresses in the IP Environment (continued) Guide to TCP/IP, Third Edtion 25 Hardware Addresses in the IP Environment (continued) Guide to TCP/IP, Third Edtion 26 ARP Packet Fields and Functions • Basic ARP packets – Broadcast ARP request packet – Directed, or unicast, ARP reply packet • Most confusing part of ARP – Interpretation of the sender and target address information Guide to TCP/IP, Third Edtion 27 ARP Packet Fields and Functions (continued) Guide to TCP/IP, Third Edtion 28 ARP Packet Fields and Functions (continued) Guide to TCP/IP, Third Edtion 29 ARP Cache • ARP information – Kept in an ARP cache in memory on most operating systems • Windows-based systems – Command arp -a is used to view the table contents – Have utility to view IP and hardware addresses Guide to TCP/IP, Third Edtion 30 ARP Cache (continued) Guide to TCP/IP, Third Edtion 31 Proxy ARP • Method that allows IP host to use a simplified subnetting design • Enables a router to “ARP” in response to an IP host’s ARP broadcasts • Most network configurations – May never need to use proxy ARP Guide to TCP/IP, Third Edtion 32 Proxy ARP (continued) Guide to TCP/IP, Third Edtion 33 Reverse ARP • Used to obtain an IP address for an associated data link address • Initially defined to – Enable diskless workstations to find their own IP addresses upon booting or startup • BOOTP, and eventually DHCP, replaced RARP Guide to TCP/IP, Third Edtion 34 About Internet Protocol • Network layer communications – End-to-end communications • Internet Protocol – Network layer protocol used in the TCP/IP suite • IP version 4 (IPv4) – Widely implemented • Internet Protocol version 6 (IPv6) – Most used in pilot or experimental implementations Guide to TCP/IP, Third Edtion 35 Sending IP Datagrams • Requirements for building an IP datagram packet to transmit on the wire – IP addresses of the source and destination – Hardware address of the source and next-hop router • IP host – Can use a manually entered destination IP address or the DNS to obtain a destination’s IP address Guide to TCP/IP, Third Edtion 36 Route Resolution Process • Enables IP host to determine if desired destination is local or remote • Local or Remote Destination? – Upon determination of IP address • IP host compares network portion of destination address to its own local network address Guide to TCP/IP, Third Edtion 37 Route Resolution Process (continued) Guide to TCP/IP, Third Edtion 38 If Remote, Which Router? • Types of route table entries – Host route entry – Network route entry • Receiving gateway typically does one of the following – Forwards packet – Sends an ICMP reply – Sends an ICMP reply indicating that it is unclear where to send the packet Guide to TCP/IP, Third Edtion 39 Lifetime of an IP Datagram • IP packets – Have a pre-defined lifetime indicated in each packet’s Time to Live (TTL) field • 64 – Recommended starting TTL value • 128 – Default TTL in Windows 2000, Windows 2003, and Windows XP Guide to TCP/IP, Third Edtion 40 Lifetime of an IP Datagram (continued) Guide to TCP/IP, Third Edtion 41 Fragmentation and Reassembly • IP fragmentation – Enables a larger packet to be automatically fragmented by a router • Once fragmented – No reassembly occurs until fragments arrive at destination – All fragments are given the same TTL value Guide to TCP/IP, Third Edtion 42 Service Delivery Options • Precedence – Used by routers to determine what packet to send • Type of Service – Used to select routing path when multiple paths exist – Routing protocols • OSPF and Border Gateway Protocol (BGP) Guide to TCP/IP, Third Edtion 43 Differentiated Services and Explicit Congestion Notification • RFC 2474, RFC 2475, and RFC 3168 – Offer a new use of the TOS field bits – Suggest that TOS and Precedence field bytes be replaced by a Differentiated Services Code Point (DSCP) field • Diffserv – Uses DSCP value to enable routers to offer varying levels of service to traffic based on marker placed in the DSCP field Guide to TCP/IP, Third Edtion 44 IP Header Fields And Functions • Version Field – First field in IP header • Header Length Field – Denotes the length of the IP header only • Type of Service Field – Has two components: precedence and Type of Service • Total Length Field – Defines length of the IP header and any valid data Guide to TCP/IP, Third Edtion 45 IP Header Fields And Functions (continued) Guide to TCP/IP, Third Edtion 46 IP Header Fields And Functions (continued) • Identification Field – Each packet is given a unique ID value when sent • Flags Field – Three bits long – Typically, fragmentation is allowed • Fragment Offset Field – Shows where to place packet’s data when fragments are reassembled Guide to TCP/IP, Third Edtion 47 IP Header Fields And Functions (continued) • Time to Live (TTL) Field – Denotes the remaining lifetime of the packet • Protocol Field – Indicates what is coming up next • Header Checksum Field – Provides error detection on the contents of the IP header only • Source Address Field – The IP address of the IP host that sent the packet Guide to TCP/IP, Third Edtion 48 IP Header Fields And Functions (continued) • Destination Address Field – Can include a unicast, multicast, or broadcast address – Final destination of the packet • Options Fields – Exist primarily to provide additional IP routing controls – Can be useful when testing or debugging code or specific connections Guide to TCP/IP, Third Edtion 49 Summary • Data link protocols – Manage transfer of datagrams across the network • At Data Link layer – Protocols must deliver services, such as delimitation, bit-level integrity checks, addressing, and protocol identification • Ethernet II frames – Most common frame type on LANs Guide to TCP/IP, Third Edtion 50 Summary (continued) • Understanding frame layouts – Crucial for proper handling of contents • At the lowest level of detail – Important to understand the differences in field layouts and meanings • Imperative to understand how TCP/IP manages the translation between MAC layer addresses and numeric IP addresses Guide to TCP/IP, Third Edtion 51 Summary (continued) • Proxy ARP – Permits router to interconnect multiple network segments • Network layer protocols – Make their way into the Data Link layer through a process known as data encapsulation • Important characteristics of IP datagrams – Time to Live (TTL) values – Fragmentation of incoming frames – Service delivery options Guide to TCP/IP, Third Edtion 52