Guide to TCP/IP, Second Edition1 Guide To TCP/IP, Second Edition Chapter 3 Data Link And Network...

Guide to TCP/IP, Second Edition 1

Guide To TCP/IP, Second 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

• Appreciate the overwhelming importance of the Internet Protocol (IP) and how IP packets behave on TCP/IP networks


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


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


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)


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)


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


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


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


Frame Types (cont.)


Frame Types (cont.)

• Ethernet 802.2 LLC frame structure– 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)


Frame Types (cont.)


Frame Types (cont.)

• Ethernet SNAP frame structure– 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)


Frame Types (cont.)


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


Frame Types (cont.)


Frame Types (cont.)

• Token Ring 802.2 LLC frame format– 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


Frame Types (cont.)


Frame Types (cont.)

• Token Ring SNAP frame format– 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


Frame Types (cont.)


Hardware Addresses In The IP Environment

• ARP

• ARP Cache

• Test for a duplicate IP address

• Routing tables

• Route resolution process


Hardware Addresses In The IP Environment (cont.)


Hardware Addresses In The IP Environment (cont.)


ARP Packet Fields and Functions

• Field types– 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


ARP Packet Fields and Functions (cont.)


ARP Packet Fields and Functions (cont.)


ARP Cache

• Kept in memory– Windows 2000 and Windows XP systems, 120 seconds

– Other kinds of networking equipment, 300 seconds

• ARP cache entries– Automatic

– Manual adding or deletion

– WINIPCFG

– IPCONFIG


ARP Cache (cont.)


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


About Internet Protocol

• A Network Layer protocol

• Datagrams or Packets

• End-to-end communications

• IPv4/IPv6


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


Sending IP Datagrams (cont.)


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


Lifetime of an IP Datagram

• Time to Live (TTL)– Cannot indefinitely circle a looped internetwork

– Routing protocols prevent loops

• TTL Value– 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


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


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)


IP Header Fields And Functions

• IP Header fields– Version Field– Type of Service Field

• New TOS Field Function: Differentiated Services and Congestion Control

– Total Length Field/Flags Field– Fragment Offset Field/Time to Live (TTL) Field– Protocol Field/Header Checksum Field– Source/Destination Address field– Options Field


IP Header Fields And Functions (cont.)


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


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)



• 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



• 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



• 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



• 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



• 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



• 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



• 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



• 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



• 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



• 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



• 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



• 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



• 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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