CSEC 701 Computer Networks 2

data link layer

transforms the physical layer to a reliable link, includes framing, physical addressing, flow control, error control, access control, hop-by-hop delivery

data link control LLC

communication between two adjacent nodes, framing, flow and error control

media access control MAC

how to share the link among multiple devices, subject to lots of collision

multiple access protocol 

determines how nodes share a channel, 

MAC address

48 bit, allocated by IEEE or burned in adapter ROM

IP address

network layer address, 32-bit, associated with each interface, not with the host, composed of network ID and host ID

CSMA/CD

senses if line is free before transmitting, used for ethernet 

ethernet preamble

8 bytes, used to synchronise receiver, serves as start of frame

ethernet address

6 bytes (48 bit) if adapter recieves frame with matching destination address or with broadcast address, it passes data to network layer or it is discarded

promiscuous mode

allows listening to all frames 

ethernet type

2 bytes, indicates the higher layer protocol

ethernet data payload

46-1500 bytes

ethernet FCS

4 bytes, checked at receiver, if error is detected then the frame is dropped

minimum frame length

chosen as 64 bytes, padding is added if less than 46

maximum frame length

1518 for the whole frame, 1500 bytes data; limits buffer memory requirements and prevents monopolizing

maximum transmission unit

size in bytes of the largest packet that a given layer of a communications protocol can pass onwards, larger packets means fragmentation 

IEEE 802.3 size

1492

ethernet II

1500

if length is grater than 1536

it is ethernet II frame 

repeater

can extend the phsycial length of a LAN, connects two segments of the same LAN

hub

repeats the signal to all connected segments, creating a big collision domain

bridge (DL)

separates collision domain, bandwth is shared only in one collision domain and bridge passes the frames destined to other domains 

switch

N-port bridge, between two hosts and direct connection=no collisions and full duplex, divides the collision domain

switch table

includes MAC address, interface, and time stamp, stale entries in table dropped

protocol stack 

includes application, transport, internet, link & hardware 

network layer

end to end delivery

IP address delivery

packet is delivered to the organization first (with network ID), then the organization finds the host (with host ID)

classful addressing

used the concept of classes (A, B, C, D, E)

how does computer get an IP address

hard coded by system admin or DHCP

DHCP

dynamically get address from a server

how does an org get a block of IP address

get allocated portion of its ISP’s address space

how does ISP get block of addresses

RIR assigns it

how does RIR get the IP address block

it is assigned by IANA under ICANN

IP address shortage

due to mobile devices, more internet connectivity, insufficient address usage

permanent IP solution

IPv6 as 128 bit address

temporary IP address

get rid of classes (CIDR) and dont assign static (public) IP address to everyone (use private address)

classless addressing

any number of addresses can be granted to an org

private address

given to devices within the internal network; not globally unique and not routed; they are invisible to outside world 

ARP table

IP address to MAC address

MAC address vs IP address

MAC address works only in LAN, to send it across multiple LAN need to use IP address

static address mapping

creating a table manually

dynamic mapping

logical to physical: ARP

physical ot logical: RARP, BOOTP, DHCP

ARP

IP/MAC address mappings for some LAN modes, kept for limited time to allow dynamic change

RARP

a client sends RARP request, a RARP server replies with the answer, the data link layer broadcast worls only in one LAN

BOOTP

application layer protocol, uses UDP (port 67), communcation between client and remote server

relay agent

retransmits the request packets and who knows the BOOTP server address

BOOTP problem

phsycial to logical relation is predetermined and permanent

DHCP

backward compatible with BOOTP, dynamic address allocation

ICMP

error reporting mechanism, router discarding a packet, destination is unreachable

ICMP route

1. source sends series of UDP segemnts to dest

2. when the n-th datagram arrives to the n-th router: router discards datagram

3. when ICMP message arrives, source calculates RTT

routing table

indicates to which direction to send a packet

autonomous system AS

a collection of IP networks and routers under the control of one entity, typically an internet service provider or a large organization

within an AS

interior gateway protocol: ASPF, RIP

between ASes

exterior gateway protocol: BGP, routers in different AS can run different IGP

fragmentation

when the packet is alrger than MTU, it gets broken up and only reassembled at the dest

fields in header

identification (16 bit), flags, offset (13 bit)

offset

beginning byte location / 8

MF field

0 for last packet