1. Guide to TCP/IP, Third Edition
Chapter 3:
Data Link and Network Layer TCP/IP Protocols

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
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
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
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)
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
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
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)
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 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

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
Guide to TCP/IP, Third Edtion

11. Frame Types At Data Link layer Frame
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

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 Logical Link Control
Ethernet 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
Guide to TCP/IP, Third Edtion

14. Ethernet II Frame Structure (continued)
Guide to TCP/IP, Third Edtion

15. Ethernet II Frame Structure (continued)
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
Guide to TCP/IP, Third Edtion

17. Ethernet 802.2 LLC Frame Structure (continued)
Guide to TCP/IP, Third Edtion

18. 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
Guide to TCP/IP, Third Edtion

20. Ethernet SNAP Frame Structure (continued)
Guide to TCP/IP, Third Edtion

21. Token Ring Frame Types IEEE 802.5 standard Token ring networks
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 LLC frames
Token Ring SNAP frames
Guide to TCP/IP, Third Edtion

22. Token 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
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. 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
Guide to TCP/IP, Third Edtion

28. ARP Packet Fields and Functions (continued)
Guide to TCP/IP, Third Edtion

29. ARP Packet Fields and Functions (continued)
Guide to TCP/IP, Third Edtion

30. ARP Cache ARP information Windows-based systems
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)
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
Guide to TCP/IP, Third Edtion

33. Proxy ARP (continued)
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
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
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
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
Guide to TCP/IP, Third Edtion

38. 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
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
Guide to TCP/IP, Third Edtion

41. Lifetime of an IP Datagram (continued)
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
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)
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
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
Guide to TCP/IP, Third Edtion

46. 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
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
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
Guide to TCP/IP, Third Edtion

50. Summary Data link protocols At Data Link layer Ethernet II frames
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
Guide to TCP/IP, Third Edtion

52. Summary (continued) Proxy ARP Network layer protocols
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