Guide to TCP/IP, Third Edition : Guide to TCP/IP, Third Edition Chapter 3:
Data Link and Network Layer TCP/IP Protocols
Objectives : Guide to TCP/IP, Third Edtion 2 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
Objectives (continued) : 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 : Guide to TCP/IP, Third Edtion 4 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
Data Link Protocols (continued) : 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) : Guide to TCP/IP, Third Edtion 6 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
Point-to-Point Protocol : 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) : Guide to TCP/IP, Third Edtion 8 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)
Special Handling for PPP Links : 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) : Guide to TCP/IP, Third Edtion 10 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
Frame Types : 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 : Guide to TCP/IP, Third Edtion 12 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)
Ethernet II Frame Structure : 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
Slide 14 : Guide to TCP/IP, Third Edtion 14 Ethernet II Frame Structure (continued)
Slide 15 : Guide to TCP/IP, Third Edtion 15 Ethernet II Frame Structure (continued)
Ethernet 802.2 LLC Frame Structure : Guide to TCP/IP, Third Edtion 16 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
Slide 17 : Guide to TCP/IP, Third Edtion 17 Ethernet 802.2 LLC Frame Structure (continued)
Slide 18 : Guide to TCP/IP, Third Edtion 18 Ethernet 802.2 LLC Frame Structure (continued)
Ethernet SNAP Frame Structure : 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
Slide 20 : Guide to TCP/IP, Third Edtion 20 Ethernet SNAP Frame Structure (continued)
Token Ring Frame Types : 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 frames
Slide 22 : Guide to TCP/IP, Third Edtion 22 Token Ring Frame Types (continued)
Hardware Addresses in the IP Environment : 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
Slide 24 : Guide to TCP/IP, Third Edtion 24 Hardware Addresses in the IP Environment (continued)
Slide 25 : Guide to TCP/IP, Third Edtion 25 Hardware Addresses in the IP Environment (continued)
Slide 26 : Guide to TCP/IP, Third Edtion 26 Hardware Addresses in the IP Environment (continued)
ARP Packet Fields and Functions : 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
Slide 28 : Guide to TCP/IP, Third Edtion 28 ARP Packet Fields and Functions (continued)
Slide 29 : Guide to TCP/IP, Third Edtion 29 ARP Packet Fields and Functions (continued)
ARP Cache : Guide to TCP/IP, Third Edtion 30 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 : Guide to TCP/IP, Third Edtion 32 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
Slide 33 : Guide to TCP/IP, Third Edtion 33 Proxy ARP (continued)
Reverse ARP : Guide to TCP/IP, Third Edtion 34 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
About Internet Protocol : 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 : Guide to TCP/IP, Third Edtion 36 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
Route Resolution Process : Guide to TCP/IP, Third Edtion 37 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
Slide 38 : Guide to TCP/IP, Third Edtion 38 Route Resolution Process (continued)
If Remote, Which Router? : 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 : Guide to TCP/IP, Third Edtion 40 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
Slide 41 : Guide to TCP/IP, Third Edtion 41 Lifetime of an IP Datagram (continued)
Fragmentation and Reassembly : Guide to TCP/IP, Third Edtion 42 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
Service Delivery Options : 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 : Guide to TCP/IP, Third Edtion 44 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
IP Header Fields And Functions : 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
Slide 46 : Guide to TCP/IP, Third Edtion 46 IP Header Fields And Functions (continued)
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) : Guide to TCP/IP, Third Edtion 48 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
IP Header Fields And Functions (continued) : 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 : Guide to TCP/IP, Third Edtion 50 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
Summary (continued) : 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) : Guide to TCP/IP, Third Edtion 52 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