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Guide To TCP/IP, Second Edition
Chapter 3
Data Link And Network Layer TCP/IP
Protocols
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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
• Appreciate the overwhelming importance of the
Internet Protocol (IP) and how IP packets behave
on TCP/IP networks
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Objectives (cont.)
• 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
• Data Link layer performs several key jobs:
– Media Access Control (MAC)
– Logical Link Control (LLC)
• Point-to-point data transfer
• Wide area network (WAN) links and WAN
protocols
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Data Link Protocols (cont.)
• Data encapsulation techniques
• Special handling for X.25, frame relay, and
Asynchronous Transfer Mode (ATM) WAN links
• WAN encapsulation of frames at the Data Link
layer involves
–
–
–
–
Addressing
Bit-level integrity check
Delimitation
Protocol identification (PID)
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Serial Line Internet Protocol (SLIP)
•
•
•
•
•
Original point-to-point protocol
Management through a dial-up serial port
Supports only TCP/IP
0xC0, 0xDB, 0xDC
compressed SLIP (C-SLIP)
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Point-to-Point Protocol (PPP)
• WAN data link encapsulation
• PPP encapsulation and framing techniques
• Fields in the PPP header and trailer include the
following values:
– Flag
– Protocol Identifier
– Frame Check Sequence (FCS)
• Synchronous technologies use bit substitution
• Support for a multi-link PPP implementation
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Special Handling for PPP Links
• Additional control and addressing in PPP headers
to manage X.25, frame relay, or ATM
• X.25: RFC 1356
– Public packet-switched data network using noisy,
narrow-bandwidth, copper telephone lines
• Frame Relay: RFC 2427
– Logical point-to-point and multi-point connections
through a single physical interface
• ATM: RFC 1577 and 1626
– High-speed cell-switched networking technology
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Frame Types
• Ethernet frames types
– Ethernet II
– Ethernet 802.2 Logical Link Control (LLC)
– Ethernet 802.2 Sub-Network Access Protocol (SNAP)
• The de facto standard is Ethernet II frame type
• Ethernet II frame fields and structure
–
–
–
–
Preamble
Source/Destination Address
Type/Data
Frame Check Sequence
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Frame Types (cont.)
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Frame Types (cont.)
• Ethernet 802.2 LLC frame structure
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–
Preamble
Start Frame Delimiter (SFD)
Destination Address/Source Address
Length
Destination Service Access Point (DSAP)
Source Service Access Point (SSAP)
Control
Data
Frame Check Sequence (FCS)
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Frame Types (cont.)
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Frame Types (cont.)
• Ethernet SNAP frame structure
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Preamble/Start Frame Delimiter (SFD)
Destination Address/Source Address
Length
Destination Service Access Point (DSAP)
Source Service Access Point (SSAP)
Control
Organization Code
Ether Type
Data
Frame Check Sequence (FCS)
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Frame Types (cont.)
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Frame Types (cont.)
• Token Ring frame
–
–
–
–
–
IEEE 802.5
Physical star design
Logical ring transmission path
Token ring workstation acts as a repeater
Two variations of token ring frames
• Token Ring 802.2 LLC frames
• Token Ring SNAP frames
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Frame Types (cont.)
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Frame Types (cont.)
• Token Ring 802.2 LLC frame format
–
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Start Delimiter
Access Control/Frame Control
Destination Address/Source Address
Destination Service Access Point (DSAP) (LLC 802.2)
Source Service Access Point (SSAP) (LLC 802.2)
Control (LLC 802.2)
Data
Frame Check Sequence
End Delimiter/Frame Status
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Frame Types (cont.)
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Frame Types (cont.)
• Token Ring SNAP frame format
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–
Start Delimiter
Access Control/Frame Control
Destination Address/Source Address
Destination Service Access Point (DSAP) (LLC 802.2)
Source Service Access Point (SSAP) (LLC 802.2)
Control (LLC 802.2)/Organization Code
Ether Type/Data
Frame Check Sequence
End Delimiter/Frame Status
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Frame Types (cont.)
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Hardware Addresses In The IP Environment
•
•
•
•
•
ARP
ARP Cache
Test for a duplicate IP address
Routing tables
Route resolution process
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Hardware Addresses In The IP
Environment (cont.)
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Hardware Addresses In The IP Environment (cont.)
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ARP Packet Fields and Functions
• Field types
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Hardware Type Field
Protocol Type Field
Length of Hardware Address Field
Length of Protocol Address Field
Opcode Field
Sender’s Hardware Address Field
Sender’s Protocol Address Field
Target Hardware Address Field
Target Protocol Address Field
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ARP Packet Fields and Functions (cont.)
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ARP Packet Fields and Functions (cont.)
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ARP Cache
• Kept in memory
– Windows 2000 and Windows XP systems, 120 seconds
– Other kinds of networking equipment, 300 seconds
• ARP cache entries
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–
–
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Automatic
Manual adding or deletion
WINIPCFG
IPCONFIG
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ARP Cache (cont.)
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Proxy ARP and Reverse ARP
• Proxy ARP
– Enables a router to “ARP” in response to an IP
host’s ARP broadcasts
• Reverse ARP (RARP)
– Obtain an IP address for an associated data link
address
– Diskless Workstations
– RARP Server
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About Internet Protocol
•
•
•
•
A Network Layer protocol
Datagrams or Packets
End-to-end communications
IPv4/IPv6
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Sending IP Datagrams
• Connectionless service
• Certain requirements to send a datagram
– IP addresses of the source and destination
– Hardware address of the source and next-hop
router
• Manually entered destination IP address
• DNS to obtain a destination’s IP address
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Sending IP Datagrams (cont.)
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Route Resolution Process
• Local or remote destination?
• If Remote, which router?
– Two types of route table entries
• Host route entry
• Network route entry
– Default Gateway
• Gateway does one of the following:
– Forwards the packet
– Sends an ICMP reply - an ICMP redirect
– Sends an ICMP reply - destination is unreachable
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Lifetime of an IP Datagram
• Time to Live (TTL)
– Cannot indefinitely circle a looped internetwork
– Routing protocols prevent loops
• TTL Value
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Defined as number of seconds or hop counts
Recommended TTL of 64
Windows 2000/XP is 128
Switches and hubs do not decrement the TTL value
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Fragment and Reassembly
• Large packet fragmented by a router into
smaller packets
• Reassembled at the Transport layer at the
destination
• Same TTL value
• Fragment retransmission process causes
more traffic
• Takes processing time
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Service Delivery Options
• Packet priority and route priority
• Precedence
– Eight levels from 0-7
• Type of Service (TOS)
– Six possible types of service
• Differentiated Services (Diffserv)
• Early Congestion Notification (ECN)
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IP Header Fields And Functions
• IP Header fields
– Version Field
– Type of Service Field
• New TOS Field Function: Differentiated Services and
Congestion Control
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Total Length Field/Flags Field
Fragment Offset Field/Time to Live (TTL) Field
Protocol Field/Header Checksum Field
Source/Destination Address field
Options Field
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IP Header Fields And Functions (cont.)
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Chapter Summary
• Because they manage access to the networking
medium, data link protocols also manage the
transfer of datagrams across the network
Normally, this means negotiating a connection
between two communications partners and
transferring data between them
• Such transfers are called point-to-point because
they move from one interface to another on the
same network segment or connection
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Chapter Summary (cont.)
• When WAN protocols, such as SLIP or PPP, come into
play, it’s possible to use analog phone lines; digital
technologies that include ISDN, DSL, or T-carrier
connections; or switched technologies, such as X.25, frame
relay, or ATM, to establish links that can carry IP and other
datagrams from a sender to a receiver
• At the Data Link layer, this means that protocols must
deliver services, such as delimitation, bit-level integrity
checks, addressing (for packet-switched connections), and
protocol identification (for links that carry multiple types
of protocols over a single connection)
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Chapter Summary (cont.)
• Ethernet II frames are the most common frame
type on LANs, but a variety of other frame types
exist that carry TCP/IP over Ethernet or token ring
networks
• Other Ethernet frame types that can carry TCP/IP
include Ethernet 802.2 LLC frames and Ethernet
802.2 SNAP frames; token ring frame types
include Token Ring 802.2 LLC frames and Token
Ring SNAP frames
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Chapter Summary (cont.)
• Understanding frame layouts is crucial for proper handling
of their contents, regardless of the type of frame in use
• Such frame types typically include start markers or
delimiters (sometimes called preambles), destination and
source MAC layer addresses, a Type field that identifies
the protocol in the frame’s payload, and the payload itself,
which contains the actual data inside the frame
• Most TCP/IP frames end with a trailer that stores a Frame
Check Sequence field used to provide a bit-level integrity
check for the frame’s contents
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Chapter Summary (cont.)
• By recalculating a special value called a Cyclical
Redundancy Check (CRC), and comparing it to
the value stored in the FCS field, the NIC can
accept the frame for further processing, or silently
discard it when a discrepancy occurs
• At the lowest level of detail, it’s important to
understand the differences in field layouts and
meanings when comparing various frame types for
any particular network medium
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Chapter Summary (cont.)
• You should understand the differences between Ethernet II
frames, Ethernet 802.2 LLC frames, and Ethernet SNAP
frames, and the differences between Token Ring 802.2
LLC frames and Token Ring SNAP frames
• Because hardware/MAC layer addresses are so important
when identifying individual hosts on any TCP/IP network
segment, it’s imperative to understand how TCP/IP
manages the translation between MAC layer addresses and
numeric IP addresses
• For TCP/IP, the Address Resolution Protocol (ARP)
provides this all-important role and helps create and
manage the ARP cache
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Chapter Summary (cont.)
• Because ARP can check the validity of the address
assigned to any machine by performing an ARP
request for a machine’s own address, ARP can also
detect IP address duplication when it occurs on a
single network segment
• Understanding ARP packet fields greatly helps to
illuminate the address resolution process,
particularly the use of the “all-zeroes” address in
the Target Hardware Address field to indicate that
a value is needed
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Chapter Summary (cont.)
• ARP also includes information about hardware
type, protocol type, length of hardware address
(varies with the type of hardware), length of
protocol address, and an Opcode field that
identifies what kind of ARP or RARP packet is
under scrutiny
• A more advanced mechanism called proxy ARP
permits a router to interconnect multiple network
segments and make them behave like a single
network segment
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Chapter Summary (cont.)
• Because this means that hardware addresses are required
from all segments that act like a single network segment,
proxy ARP’s job is to forward ARP requests from one
actual network segment to another, when required; enable
hardware address resolution; and then to deliver
corresponding replies to their original senders
• Also, when a router configured for proxy ARP receives an
ARP broadcast, it responds with its own address
• When it receives the subsequent data packet, it forwards
this along, according to its routing tables
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Chapter Summary (cont.)
• Network layer protocols make their way into the Data Link
layer through a process known as data encapsulation
• Building IP datagrams, therefore, depends on
understanding how to map the contents of an IP packet into
a datagram that carries an IP packet as its payload
• This process requires obtaining a numeric IP address for
the destination (and may involve initial access to name
resolution services such as DNS), and then using ARP (or
the ARP cache) to map the destination address to a
hardware address
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Chapter Summary (cont.)
• It is possible to use the hardware address of a known router
or a default gateway instead, which can then begin the
routing process from the sending network to the receiving
network
• When a frame must travel from one network segment to
another, a process to resolve its route must occur
• Local destinations can be reached with a single transfer at
the Data Link layer, but remote destinations require
forwarding and multiple hops to get from sender to
receiver
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Chapter Summary (cont.)
• Thus, it’s important to understand the role of local
routing tables that describe all known local routes
on a network, and the role of the default gateway
that handles outbound traffic when exact routes
are not known
• Here, ICMP comes into play to help manage best
routing behaviors and report when destinations
may be unreachable
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Chapter Summary (cont.)
• Other important characteristics of IP datagrams include:
Time to Live (TTL) values, which prevent stale frames
from persisting indefinitely on a network; fragmentation of
incoming frames when the next link on a route uses a
smaller MTU than the incoming link (reassembly of
fragments always occurs when frames ultimately arrive at
the destination host); and service delivery options to
control packet and route priorities (seldom used, but worth
understanding)
• IP traffic can be prioritized using Differentiated Services or
Type of Service designations
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Chapter Summary (cont.)
• Although Type of Service was defined in the original
specification, current network prioritization
implementations are based on Differentiated Services
functions that place a DSCP value in the IP header
• This DSCP value is examined by routers along a path, and
the traffic is forwarded according to the router
configuration for that DSCP traffic type
• In addition, Explicit Congestion Notification enables
routers to notify each other of congested links before they
must drop packets
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Chapter Summary (cont.)
• These services streamline IP traffic to ensure minimal
delay for high-priority traffic and a minimum of packet
loss
• The chapter concludes with an overview of all fields in an
entire IP header
• It brings together all the topics discussed in earlier
sections, and permits inspection of entire IP datagram
headers to map out their contents
• Ultimately, this provides the map by which it is possible to
examine and decode the addressing and handling
instructions associated with any IP datagram
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