lecture 2 tcp/ip link layer

Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking

Lecture 2

TCP/IP Link Layer


Outline

• understand principles behind data link layer services:– error detection, correction– sharing a broadcast

channel: multiple access

• instantiation and implementation of various link layer technologies:– Ethernet– SLIP– PPP

• link layer addressing– ARP– RARP


Link Layer: IntroductionSome terminology:• hosts and routers are nodes• communication channels that

connect adjacent nodes along communication path are links– wired links– wireless links– LANs

• layer-2 packet is a frame, encapsulates datagram

• data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

• Involves device driver in OS and network interface card

• There are various standards (protocols) to realize link layer services, e.g. Ethernet, token rings, etc.

“link”


Link layer: context

• Datagram transferred by different link protocols over different links:– e.g., Ethernet on first

link, frame relay on intermediate links, 802.11 on last link

• Each link protocol provides different services

transportation analogy• trip from your home to Cox’s Bazar

– car: home to train station– train: Dhaka to Chittagong– plane: Chittagong to Cox’s Bazar

• tourist = datagram• transport segment = communication

link, e.g. Dhaka to Chittagong• transportation mode = link layer

protocol, e.g. train, plane• travel agent = routing algorithm


Link Layer Services• Framing, link access:

– encapsulate datagram into frame, adding header, trailer– implement channel access if shared medium,– ‘physical addresses’ used in frame headers to identify source,

destination (different from IP address!)

• Flow Control:– pacing between sender and receivers

• Error Detection:– errors caused by signal attenuation, noise.– receiver detects presence of errors:

• signals sender for retransmission or drops frame

• Error Correction:– receiver identifies and corrects bit error(s) without resorting to

retransmission


Outline






Error Detection• EDC= Error Detection and Correction bits (redundancy)• D = Data protected by error checking, may include header fields

• Error detection not 100% reliable!– protocol may miss some errors, but rarely– larger EDC field yields better detection and correction


Error Detection: Parity Checking

Single Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

0 0


Error Detection: Cyclic Redundancy Check• view data bits, D, as a binary number• choose r+1 bit pattern (generator), G • goal: choose r CRC bits, R, such that

– <D,R> exactly divisible by G (modulo 2) – receiver knows G, divides <D,R> by G. If non-zero remainder: error

detected!– can detect all burst errors less than r+1 bits

• widely used in practice (ATM, HDLC)


Outline






Multiple Access Links and Protocols

Two types of “links”:

• point-to-point– PPP for dial-up access– point-to-point link between Ethernet switch and host

• broadcast (shared wire or medium)– traditional Ethernet– upstream HFC– 802.11 wireless LAN


Multiple Access protocols

• single shared communication channel• two or more simultaneous transmissions by

nodes: interference– only one node can send successfully at a time

• multiple access protocol:– distributed algorithm that determines how stations

share channel, i.e., determine when station can transmit

– what to look for in multiple access protocols:• synchronous or asynchronous• information needed about other stations• robustness (e.g., to channel errors)• performance


Ideal Multiple Access ProtocolBroadcast channel of rate R bps

1. When one node wants to transmit, it can send at rate R.

2. When M nodes want to transmit, each can send at average rate R/M

3. Fully decentralized:– no special node to coordinate transmissions– no synchronization of clocks, slots

4. Simple


MAC Protocols: a taxonomyThree broad classes:• Channel Partitioning

– divide channel into smaller “pieces” (time slots, frequency, code)

– allocate piece to node for exclusive use– example TDMA, FDMA

• Random Access– channel not divided, allow collisions– “recover” from collisions– example CSMA, CSMA/CD

• “Taking turns”– Nodes take turns, but nodes with more to send can

take longer turns– example polling, token ring


Outline






Ethernet• Dominant widely used wired LAN technology

• First published in 1982 by Digital Equipment Corp., Intel Corp., and Xerox Corp.

• Simpler and cheaper.

• Ethernet uses CSMA/CD:– No slots– adapter doesn’t transmit if it senses that some other adapter is

transmitting, that is, carrier sense– transmitting adapter aborts when it senses that another adapter

is transmitting, that is, collision detection– Before attempting a retransmission, adapter waits a random

time, that is, random access


Ethernet Frame Structure• Sending adapter encapsulates IP datagram (or other

network layer protocol packet) in Ethernet frame


Ethernet Frame Structure• Preamble:

– Most often preamble is used before a frame to synchronize receiver, sender clock rates

– 7 bytes with pattern 10101010 followed by one byte with pattern 10101011

• Addresses: 6 bytes– if adapter receives frame with matching destination address, or

with broadcast address (e.g. ARP packet), it passes data in frame to net-layer protocol

– otherwise, adapter discards frame• Type:

– indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk). Also type field is used to distinguish ARP/RARP request (reply) frames from datagram frames.

• CRC: – checked at receiver, if error is detected, the frame is simply

dropped


Outline






SLIP• Serial Line IP (SLIP) is a simple form of encapsulation for IP

datagram on serial lines, and it has become popular through RS-232 serial port and high-speed modems

• The rules specify the framing used by SLIP:

• Special character called END (0xc0) is used at the beginning and end of the datagram.

• If the END character appears inside the datagram, the 2 byte sequence 0xdb 0xdc is transmitted. 0xdb is called the SLIP ESC character.

• If SLIP ESC character appears inside datagram, the 2 byte sequence 0xdb 0xdd is transmitted instead.


SLIP

• Deficiencies of SLIP– Each end must know the other’s IP address

– There is no type field, it can’t be used for some other protocol at same time

– There is no checksum added by SLIP. This makes it essential that the upper layers provide some form of CRC


Outline






PPP• Point to point protocol (PPP) is a way to encapsulate IP datagram on a

serial link and it addresses all the deficiencies of SLIP. PPP supports either an asynchronous link with 8 bits of data and no parity or bit-oriented synchronous links.

• A link control protocol (LCP) to establish, configure, and test the data-link connection

• A family of network control protocols (NCPs) specific to different network layer protocols


PPP• Flag: delimiter (framing), 0x7E (i.e. 01111110)

• Address: does nothing, constant value 0xFF, reserved for future use.

• Control: does nothing; constant value 0x03. In the future possible multiple control fields

• Protocol: upper layer protocol to which frame delivered, e.g.– PPP-LCP value C021, – IP value 0021– IPCP value 8021

• info: upper layer data being carried

• check: cyclic redundancy check for error detection


PPP

• How to escape flag byte?– On a synchronous link: done by the hardware using a

technique called bit stuffing– On asynchronous link: use 0x7d as an escape

character, the next character of 0x7d had its sixth bit complemented as follows:

• 0x7e is transmitted => 0x7d 0x5e• 0x7d is transmitted => 0x7d 0x5d• ASCII control character (less than 0x20), for example, the

byte 0x01 is transmitted => 0x7d 0x21

• PPP using the link control protocol– omit the constant flag and address fields and to

reduce the size of the protocol field from 2 bytes to 1 byte


Loopback Interface• A loopback interface allow a client and server on the same host to

communicate with each other using TCP/IP• most system assign the IP address of 127.0.0.1, named localhost• in brief, it is just doing a test job


Outline






Link Layer Addressing (LAN Addressing)

• 32-bit IP address: – network-layer address– used to get datagram to destination IP subnet

• LAN or physical or Ethernet address: – used to get datagram from one interface to

another physically-connected interface (same network)

– 48 bit MAC address (for most LANs) burned in the adapter ROM


LAN Addresses

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)


LAN Address• MAC address allocation administered by IEEE

• manufacturer buys portion of MAC address space (to assure uniqueness)

• Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address

• MAC flat address ➜ portability – can move LAN card from one LAN to another

• IP hierarchical address NOT portable– depends on IP subnet to which node is attached


ARP: Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table (ARP Cache)

• ARP Table: IP/MAC address mappings for some LAN nodes

• <IP address; MAC address; TTL>– TTL (Time To Live): time

after which address mapping will be forgotten (typically 20 min) (also known as ARP Cache Timeout)

Question: how to determineMAC address of Bknowing B’s IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237.196.7.23

237.196.7.78

237.196.7.14

237.196.7.88

A B


ARP protocol: Same LAN (network)

• A wants to send datagram to B, and B’s MAC address not in A’s ARP table.

• A broadcasts ARP query packet, containing B's IP address – Dest MAC address = FF-

FF-FF-FF-FF-FF– all machines on LAN

receive ARP query • B receives ARP packet,

replies to A with its (B's) MAC address– frame sent to A’s MAC

address (unicast)

• A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) – soft state: information

that times out (goes away) unless refreshed

• ARP is “plug-and-play”:– nodes create their ARP

tables without intervention from net administrator


Routing to another LAN• walkthrough: send datagram from A to B via R, assume A knows B’s IP

address

• Two ARP tables in router R, one for each IP network (LAN)• In routing table at source Host, find router 111.111.111.110• In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc

A

RB


Routing to another LAN• A creates datagram with source A, destination B • A uses ARP to get R’s MAC address for 111.111.111.110• A creates link-layer frame with R's MAC address as dest, frame

contains A-to-B IP datagram• A’s adapter sends frame • R’s adapter receives frame • R removes IP datagram from Ethernet frame, sees its destined to B• R uses ARP to get B’s MAC address • R creates frame containing A-to-B IP datagram sends to B

A

RB


ARP Packet Format

• The first two fields in the Ethernet header are the source and destination Ethernet addresses.

• frame type specifies the type of data that follows. For an ARP request or an ARP reply, this field is 0x0806, for IP datagram it is 0x0800.

• hard type field specifies the type of hardware address (i.e. hardware used in the LAN). E.g. for Ethernet it is 1, for Frame relay it is 15, for fiber channel it is 18

• Prot type specifies the type of network protocol address being mapped. Its value is 0×0800 for IP addresses


ARP Packet Format

• hard size and prot size, specify the sizes in bytes of the hardware addresses and the protocol addresses. For an ARP request or reply for an IP address on an Ethernet they are 6 and 4, respectively.

• The op field specifies whether the operation is an ARP request (a value of 1), ARP reply (2), RARP request (3), or RARP reply (4).

• The next four fields that follow are the sender's hardware, the sender's protocol address, the target hardware address, and the target protocol address


More on ARP

• Proxy ARP:Proxy ARP lets a router answer ARP requests on one of its networks for a host on another of its networks. The router that is configured for Proxy ARP can respond to the ARP and map the router's MAC address with the destination IP address and fool the sending station that it has found its destination. The router acts as a proxy as the name suggests, and forwards the packets to the correct destination since it has the relevant information.

• Gratuitous ARP is an ARP request sent by a host for its own IP

address. This is usually done when the interface is configured at bootstrap time, to initialize its IP stack. It helps to check for a duplicate IP address.

• Arp command – try out yourself in windows/unix.


RARP• Reverse Address Resolution Protocol (RARP) is a network layer

protocol used to obtain an IP address for a given hardware address.

• When a system with a local disk is bootstrapped it normally obtains its IP address from a configuration file that's read from a disk file. But a system without a disk, such as an X terminal or a diskless workstation, RARP is used to obtain its IP address.

• The format of an RARP packet is almost identical to an ARP packet (Figure 4.3). The only differences are that the frame type is 0×8035 for an RARP request or reply, and the op field has a value of 3 for an RARP request and 4 for an RARP reply.

• As with ARP, the RARP request is broadcast and the RARP reply is normally unicast.

• It has been rendered obsolete by BOOTP, DHCP (will be discussed later)


Reading Material• Chapter 2, 4 – text1 (Stevens)

• Chapter 5 – text3 (Kurose)

lecture 2 tcp/ip link layer

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