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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
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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
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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
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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)
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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
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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
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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)
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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
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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
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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
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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)
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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
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Ethernet II Frame Structure
(continued)
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Ethernet II Frame Structure
(continued)
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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
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Ethernet 802.2 LLC Frame Structure
(continued)
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Ethernet 802.2 LLC Frame Structure
(continued)
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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
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Ethernet SNAP Frame Structure
(continued)
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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
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Token Ring Frame Types (continued)
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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
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Hardware Addresses in the IP
Environment (continued)
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Hardware Addresses in the IP
Environment (continued)
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Hardware Addresses in the IP
Environment (continued)
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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
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ARP Packet Fields and Functions
(continued)
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ARP Packet Fields and Functions
(continued)
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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
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ARP Cache (continued)
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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
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Proxy ARP (continued)
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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
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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
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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
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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
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Route Resolution Process (continued)
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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
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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
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Lifetime of an IP Datagram (continued)
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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
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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)
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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
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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
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IP Header Fields And Functions
(continued)
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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
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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
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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
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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
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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
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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
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