CS代写 SCI 3001 - PowCoder代写

NETWORKS AND
APPLICATIONS
COMP SCI 3001
Faculty of Engineering, Computer and Mathematical Sciences

LAN Addressing

Data Link Layer introduction
LAN technologies
Data Link Layer so far
• Services, error detection/correction, multiple access
Next: LAN technologies • Addressing
• Ethernet
• Switches

Data Link Layer introduction
LAN addresses and ARP
We know 32 bit IP addresses
• Network Layer address
• Usedtogetdatagramtodestinationnetwork(recallIPnetwork definition)
LAN (or MAC (media access control) or physical) address
• used ‘locally” to get frame from one interface to another physically- connected interface (same network, in IP-addressing sense)
• At LAN level:
• 48 bit MAC address (for most LANs) burned in the adapter ROM – e.g.: 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation (each “number” represents 4 bits)

Data Link Layer introduction
MAC address allocation
• MAC address allocation administered by IEEE
• Manufacturer buys portion of MAC address space (to assure uniqueness)
(a) MAC address – like Tax File 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 network to which one attaches

Data Link Layer introduction
LAN addresses and ARP
each adapter on LAN has unique LAN address 1A-2F-BB-76-09-AD
LAN (wired or wireless)
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98

Data Link Layer introduction
Earlier routing discussion?
Starting at A, given IP datagram addressed to B:
• look up net. address of B, find B on same net. as A
• link layer send datagram to B inside link layer frame
frame source, dest address
223.1.1.3 223.1.3.27 223.1.3.1
223.1.2.1 223.1.2.9
223.1.2.2 E 223.1.3.2
datagram source, dest address
B’s MAC addr
A’s MAC addr
A’s IP B’s IP addr addr
IP payload
How do we work out the MAC address of the destination host?

Data Link Layer introduction
Address Resolution Protocol (ARP)
• Each IP node (host, router) on LAN has ARP module and table
• ARP Table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL>
Time To Live (TTL):
time after which address mapping will be forgotten (typically 20 min)
137.196.7.23
LAN 71-65-F7-2B-08-53
137.196.7.88
137.196.7.78 1A-2F-BB-76-09-AD
137.196.7.14
58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98

Data Link Layer introduction
ARP protocol: same LAN
1. A wants to send datagram to B
– B’s MAC address not in A’s ARP table.
2. A broadcasts ARP query
packet, containing B’s IP
– dest MAC address = FF-FF-FF-FF- FF-FF
– all nodes on LAN receive ARP query
3. 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”:
– nodescreatetheirARP tables without intervention from net administrator

Data Link Layer introduction
Earlier routing discussion?
Starting at A, given IP datagram addressed to E:
223.1.1.3 223.1.3.27 223.1.3.1
223.1.2.1 223.1.2.9
223.1.2.2 E 223.1.3.2
frame source, dest address
datagram source, dest address
A’s MAC addr
A’s IP B’s IP addr addr
IP payload
How do we work out the MAC address of the destination host?

Data Link Layer introduction
Addressing: routing to another LAN
Example: send datagram from A to B via R
– focus on addressing – at IP (datagram) and MAC layer (frame)
– assume A knows B’s IP address
– assume A knows IP address of first hop router, R (how?) – assume A knows R’s MAC address (how?)
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F

Data Link Layer introduction
Addressing: routing to another LAN
 A creates IP datagram with IP source A, destination B
 A creates link-layer frame with R’s MAC address as dest, frame
contains A-to-B IP datagram
MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B
IP src: 111.111.111.111
IP dest: 222.222.222.222
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer

Data Link Layer introduction
Addressing: routing to another LAN
 framesentfromAtoR
 frame received at R, datagram removed, passed up to IP
MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-EI9P-0s0rc-:1171-B1.B1-141B.111.111
IP dest: 222.222.222.222 IP src: 111.111.111.111
IP dest: 222.222.222.222
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer

Data Link Layer introduction
Addressing: routing to another LAN
 R forwards datagram with IP source A, destination B
 R creates link-layer frame with B’s MAC address as dest, frame
contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP Eth Phy
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer

Data Link Layer introduction
Addressing: routing to another LAN
 R forwards datagram with IP source A, destination B
 R creates link-layer frame with B’s MAC address as dest, frame
contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A
IP src: 111.111.111.111
IP dest: 222.222.222.222
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer

Data Link Layer introduction
NETWORKS AND
APPLICATIONS
COMP SCI 3001
Faculty of Engineering, Computer and Mathematical Sciences

Data Link Layer introduction
“dominant” wired LAN technology:
• cheap $20 for NIC
• first widely used LAN technology
• simpler, cheaper than token LANs and ATM • kept up with speed race: 10 Mbps – 10 ’s Ethernet sketch

Data Link Layer introduction
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
 used to synchronize receiver, sender clock rates
dest. address
source address
data (payload)

Data Link Layer introduction
Ethernet: physical topology
• bus: popular through mid 90s
– all nodes in same collision domain (can collide with each other)
• star: prevails today
– active switch in center
– each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other)
bus: coaxial cable

Data Link Layer introduction
– Every one was able to share the wire because it used a media
access control protocol called what? CSMA/CD
• star: prevails today (also called switched networks)
– active switch in center
– each “spoke” runs a (separate) Ethernet protocol (nodes do not
collide with each other)
– Full duplex communication, separate wires for sending and receiving
– So collisions domains are isolated and the CD part of CSMA/CD is not needed any more.
– The switch is transparent => as if there is a direct wire from A to B
• NOTE original Ethernet specification are half-duplex

NETWORKS AND
APPLICATIONS
COMP SCI 3001
Faculty of Engineering, Computer and Mathematical Sciences

Data Link Layer introduction
Ethernet switch
• link-layer device: takes an active role
– Filtering, Storing, Forwarding Ethernet
– examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment
• transparent
– hosts are unaware of presence of switches
• plug-and-play, self-learning
– switches do not need to be configured

Data Link Layer introduction
How do switches know where to send information? Switch filtering
• Switches learn which hosts can be reached through which interfaces: maintains switch table
– when frame received, switch learns location of sender: incoming LAN segment
– records sender location in switch table
• Switch table entry
– [node LAN address, switch interface (incoming), time stamp, TTL]
– stale entries in filtering table dropped (TTL can be 60 minutes)

Data Link Layer introduction
Switch: self-learning
• switch learns which hosts can be reached through which interfaces
– when frame received, switch “learns” location of sender: incoming LAN segment
– records sender/location pair in switch table
Source: A Dest: A’
Switch table (initially empty)

Data Link Layer introduction
Filtering and Forwarding
if destination is on LAN on which frame was received then drop the frame
lookup switch table
if entry found for destination
then forward the frame on interface indicated;
else flood; /* forward on all but the interface on which the frame arrived */

Data Link Layer introduction
Self-learning, forwarding: example
Source: A Dest: A’
frame destination, A’, locaton
unknown: flood C’
destination A location known: selectively
send on just one link
switch table
(initially empty)
Link Layer

Data Link Layer introduction
Switches – spanning trees
• Increased reliability
– desirable to have redundant, alternate paths from source to destination
• With multiple simultaneous paths, cycles result
– switches may multiply and forward frame forever
• Solution: organize switches in a spanning tree by disabling subset of interfaces
Switch Switch Switch

Data Link Layer introduction
Switches vs. routers
application
application

Data Link Layer introduction
Switches vs. routers
both are store-and-forward:
 routers: network-layer devices (examine network- layer headers)
 switches: link-layer devices (examine link-layer headers)
both have forwarding tables:
 routers: compute tables using routing algorithms, IP addresses
 switches: learn forwarding table using flooding, learning, MAC addresses
application
application

Data Link Layer introduction
Routers versus switches
Switches + and –
+ Switch operation is simpler requiring less processing bandwidth (only
+ Plug and Play devices
– Topologies are restricted with switches (a spanning tree must be built to avoid cycles)
– Switches do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a switch)
– Large networks will lead to nodes with large ARP tables and a lot of ARP traffic

Data Link Layer introduction
Routers versus switches (cont.)
Routers + and –
+ Arbitrary topologies can be supported, cycling is limited by TTL
counters (and good routing protocols)
+ Provide firewall protection against broadcast storms
– Require IP address configuration (not plug and play)
– Require higher processing bandwidth (e.g. routing algorithms)
Switches do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

Data Link Layer introduction
NETWORKS AND
APPLICATIONS
COMP SCI 3001
Faculty of Engineering, Computer and Mathematical Sciences

Data Link Layer introduction
MPLS – where did it come from?
• IP over ATM was a mechanism to get a good layer 3 protocol working over fast and efficient lower level hardware
– buttherewerescalabilityproblems
• Toshiba produced a Cell Switching Router (CSR) that had an ATM switching fabric controlled by IP protocols
• A little later Ipsilon produced their IP Switching solution. Their rationale:
– wantadeviceasfastasanATMswitchthatroutes
– the Internet needs fast routers, not switches, because IP is dominant
– ATMsignallingandmappingtoIPiscomplex;ditchATMcontrolprotocols!
• Cisco followed this up with Tag Switching
– didn’tneeddatatraffictoflowtopopulateitstables
– worked for link layer technologies other than ATM
– after revision and consultation this became MPLS

Data Link Layer introduction
Multiprotocol label switching (MPLS)
Initial goal: high-speed IP forwarding using fixed length label (instead of IP address)
 fast lookup using fixed length identifier (rather than longest prefix matching)
 borrowing ideas from Virtual Circuit (VC) approach  but IP datagram still keeps IP address!
PPP or Ethernet header
MPLS header
remainder of link-layer frame
Need MPLS capable routers: Label-Switched Router (LSR)

Data Link Layer introduction
MPLS capable routers
• a.k.a. label-switched router
• forward packets to outgoing interface based only on label value
(don’t inspect IP address)
– MPLS forwarding table distinct from IP forwarding tables

Data Link Layer introduction
MPLS versus IP paths
 IP routing: path to destination determined by destination address alone

Data Link Layer introduction
MPLS versus IP paths (main interest now: traffic engineering) entry router (R4) can use different MPLS
routes to A based, e.g., on source address
 IP routing: path to destination determined by destination address alone
 MPLS routing: path to destination can be based on source and dest. address
 fast reroute: precompute backup routes in case of link failure
IP-only router
MPLS and IP router

Data Link Layer introduction
MPLS Example
out interface
out interface
Label Edge Routers (LERs)
label dest
out interface
out interface

Data Link Layer introduction
MPLS – how it works
• Routers work out routing tables and forward packets through their interfaces
– for unicast packets we use a longest prefix match on the destination IP address
• This partitions all possible packets that a router can forward into a finite number of disjoint subsets. Why disjoint?
– from a forwarding point of view anything sent to the same next hop is the same
– we refer to these subsets as Forwarding Equivalence Classes (FECs)

Data Link Layer introduction
MPLS – how it works (cont.)
• If we know that a set of IP prefixes are all going out the same interface we can throw all of them into the same Forwarding Equivalence Class
• Our forwarding table is now a set of FECs and a next hop for each FEC
• We can set the granularity of these as we wish – a FEC can model many entries or just one – this is where the scalability comes from
• We need to map into the FEC consistently or we can get some odd results

Data Link Layer introduction
MPLS – label swapping
• When a packet arrives, the label is extracted and compared to a forwarding table
• The table will have subentries matching each incoming label
– these contain the outgoing label, the outbound interface and the designated next hop
• You can have multiple subentries for each incoming label to implement (among other things) traffic management capabilities
• The forwarding table may also specify resource usage, such as which outgoing queue to use
– this allows the implementation of Quality of Service requirements
• The incoming label is replaced by the outgoing label in the subentry and sent out over the designated interface to the specified next hop
– this is called label swapping

Data Link Layer introduction
MPLS Features
• Single forwarding algorithm
– multicast is the same as unicast, just with more subentries – Just make sure swapping works correctly
• Multiprotocol support: up and down
– the forwarding component is not network layer specific
– we can stick a label onto the front of any packet
– similarly, we can also use an underlying link layer protocol through the use of existing link layer fields or ‘shim’ headers

Data Link Layer introduction
MPLS Features
• flexibility: MPLS forwarding decisions can differ from those of IP
– use destination and source addresses to route flows to same destination differently (traffic engineering)
– re-route flows quickly if link fails: pre-computed backup paths (useful for VoIP)
• MPLS can support multiple levels of connection tunnelling through label stacking
– VPN support

Data Link Layer introduction
MPLS signaling (distributing labels)
• modify link-state flooding protocols to carry info used by MPLS routing,
– e.g., link bandwidth, amount of “reserved” link bandwidth
 IETF effort: entry MPLS router uses RSVP-TE signaling
protocol to set up MPLS forwarding at downstream routers
modified link state flooding

程序代写 CS代考加微信: powcoder QQ: 1823890830 Email: powcoder@163.com

Related Posts