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 3
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
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 5
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)
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 7
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
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 Ethernet- based networks (or peers)
Guide to TCP/IP, Third Edtion 9
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
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 11
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
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 13
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
Ethernet II Frame Structure
(continued)
Guide to TCP/IP, Third Edtion 15
Ethernet II Frame Structure
(continued)
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 17
Ethernet 802.2 LLC Frame Structure
(continued)
Ethernet 802.2 LLC Frame Structure
(continued)
Guide to TCP/IP, Third Edtion 19
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
Ethernet SNAP Frame Structure
(continued)
Guide to TCP/IP, Third Edtion 21
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 framesToken Ring Frame Types (continued)
Guide to TCP/IP, Third Edtion 23
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
Hardware Addresses in the IP
Environment (continued)
Guide to TCP/IP, Third Edtion 25
Hardware Addresses in the IP
Environment (continued)
Hardware Addresses in the IP
Environment (continued)
Guide to TCP/IP, Third Edtion 27
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
ARP Packet Fields and Functions
(continued)
Guide to TCP/IP, Third Edtion 29
ARP Packet Fields and Functions
(continued)
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 31
ARP Cache (continued)
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 33
Proxy ARP (continued)
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 35
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
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 37
Route Resolution Process
• E
nables 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
Route Resolution Process (continued)
Guide to TCP/IP, Third Edtion 39
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
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 41
Lifetime of an IP Datagram (continued)
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 43
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)
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 45
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
IP Header Fields And Functions
(continued)
Guide to TCP/IP, Third Edtion 47
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
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 49
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
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 51
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
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