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Network Layer 4-1
Network Layer
Goals: understand principles behind network
layer services: routing (path selection) dealing with scale how a router works advanced topics: IPv6, mobility
instantiation and implementation in the Internet
Network Layer 4-2
Topics: Datagram vs Virtual Circuit Router IP: Internet Protocol
Datagram format, IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP, OSPF, BGP
Network Layer 4-3
Network layer transport segment from sending to receiving
host on sending side encapsulates segments into
datagrams on rcving side, delivers segments to transport
layer network layer protocols in every host, router Router examines header fields in all IP
datagrams passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Network Layer 4-4
Key Network-Layer Functions
forwarding: move packets from router’s input to appropriate router output
routing: determine route taken by packets from source to dest.
Routing algorithms
analogy:
routing: process of planning trip from source to dest
forwarding: process of getting through single interchange
Network Layer 4-5
1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Network Layer 4-6
Connection setup 3rd important function in some network arch.: Virtual circuits network provides network-layer conn
service used in ATM, frame-relay, X.25 Signaling protocols used to setup, maintain teardown VC
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1. Initiate call 2. incoming call
3. Accept call4. Call connected5. Data flow begins 6. Receive data
Network Layer 4-7
VC implementation
A VC consists of:1. Path from source to destination2. VC numbers, one number for each link along
path3. Entries in forwarding tables in routers along
path Packet belonging to VC carries a VC
number. VC number must be changed on each
link. New VC number comes from forwarding table
Network Layer 4-8
Forwarding table in VC
12 22 32
1 23
VC number
interfacenumber
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …
Forwarding table innorthwest router:
Routers maintain connection state information!Forwarding table is modified whenever there’s conn setup or teardown(happen at a microsecond timescale in a tier-1 router)
Network Layer 4-9
Network service model
Q: What service model for “channel” transporting datagrams from sender to rcvr?
a service model defines the characteristics of end-to-end transport of packets between
Example services for individual datagrams:
guaranteed delivery Guaranteed delivery with
less than certain delay (e.g. 40 msec)?
Example services for a flow of datagrams:
In-order datagram delivery
Guaranteed minimum bandwidth to flow
Restrictions on changes in inter-packet spacing
Network Layer 4-10
Case study: ATM ABR congestion control
two-byte ER (Explicit Rate) field in RM cell congested switch may lower ER value in cell sender’ send rate thus minimum supportable rate on path
across all switches EFCI (Explicit Forward Congestion Indication) bit in data cells: set to
1 in congested switch to indicate congestion to destination host. when RM arrives at destination, if most recently received data
cell has EFCI=1, sender sets CI bit in returned RM cell
Network Layer 4-11
Network layer service models:
NetworkArchitecture
Internet
ATM
ATM
ATM
ATM
ServiceModel
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constantrateguaranteedrateguaranteed minimumnone
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestionfeedback
no (inferredvia loss)nocongestionnocongestionyes
no
Guarantees ?
CBR: constant bit rateVBR: variable bit rateABR: available bit rateUBR: unspecified bit rate
Network Layer 4-12
Datagram or VC network: why?
Internet data exchange among
computers “elastic” service, no strict
timing req. “smart” end systems
can adapt, perform control, error recovery
simple inside network, complexity at “edge”
Additional func built in higher levels
many link types different characteristics uniform service difficult
VC network (e.g. ATM) evolved from telephony human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems telephones complexity inside
network (e.g. network-assisted congestion control)
Network Layer 4-13
Topics:
Router IP: Internet Protocol
Datagram format, IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP, OSPF, BGP
Network Layer 4-14
Router Architecture Overview
Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link E.g. Cisco 12K, Juniper M16, Foundry SuperX
Network Layer 4-15
Input Port Functions
Decentralized switching: given datagram dest., lookup output
port using forwarding table in input port memory
goal: complete input port processing at ‘line speed’
queuing: if datagrams arrive faster than forwarding rate into switch fabric
Physical layer:bit-level reception
Data link layer:e.g., Ethernetsee chapter 5
Network Layer 4-17
Switching Via MemoryFirst generation routers: traditional computers with switching under direct control of CPUpacket copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram)
InputPort
OutputPort
Memory
System Bus
Recent development: Processors in input line cards perform lookup and storing packets into memory: shared mem multiprocessorsE.g. Cisco’s Catalyst 8500
Network Layer 4-18
Switching Via a Bus
datagram from input port memory to output port memory via a shared
bus bus contention: switching speed
limited by bus bandwidth 1 Gbps bus, Cisco 1900: sufficient
speed for access and enterprise routers (not regional or backbone)
E.g. 1Gbps bw supports up to 10 T3 (45- Mbps) links
Network Layer 4-19
Switching Via An Interconnection Network overcome bus bandwidth limitations A crossbar switch is an interconnection network
consisting of 2n buses that connect n input to n output ports.
Advanced design: fragmenting datagram into fixed length cells at the input port, switch cells through the fabric and assemble at output ports.
Cisco 12000: switches 60 Gbps through the interconnection network
Omega
Network Layer 4-20
Output Ports
Buffering required when datagrams arrive from fabric faster than the transmission rate Queueing and Buffer management
Scheduling discipline chooses among queued datagrams for transmission
Network Layer 4-21
Output port queueing
buffering when arrival rate via switch exceeds output line speed (switching fabric speed: rate of moving pkt from in-ports to out-ports)
queueing (delay) and loss due to output port buffer overflow! Buffer size = RTT times Link Capacity
A packet scheduler at output port must choose among queued to transmit using FIFO or more sophisticated such as weighted fair queuing (WFQ) that shares the outgoing link fairly among different end-to-end connections.
Network Layer 4-22
Input Port Queuing If fabric slower than input ports combined then queueing
may occur at input queues. It can be eliminated if the switching fabric speed is at least n times as fast as the input line speed, where n is the number of input ports
Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward. Only occurs at input ports. As soon as the packet arrival rate on the input lines reaches 58% of their capacity, the input queue will grow to unbounded length, due to HOL blocking
queueing delay and loss due to input buffer overflow!
Network Layer 4-23
Active Queue Management Drop-Tail policy
Drop arrival packets due to overflow Random Early Detection (RED)
Maintain a weighted average for the length of the output queue
If queue length < Threshold_min, admit it If queue length > Threshold_max, drop it Otherwise, drop it with a probability (a function
of the average queue length) RED drops packets before the buffer is full
in order to provide congestion signals to senders
Router Processor Execute routing protocols Maintain the routing information and
forwarding tables Perform network management functions
Network Layer 4-24
CISCO 12000 Gigabit Router Processor (GRP)
Network Layer 4-25
Forwarding table
Destination Address Range Link Interface 11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111
otherwise 3
packets forwarded using destination host addressThe tables are modified by routing alg anytime (every 1~5 minutes)
packets between same source-dest pair may take diff paths
Network Layer 4-26
Longest prefix matching
DA: 11001000 00010111 00011000 10101010
Examples
DA: 11001000 00010111 00010110 10100001 Which interface?
Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3
Forwarding table with 4 entries and using longest prefix match:
Network Layer 4-27
Lookup in an IP Router
Unicast destination address based lookup
Dstn Addr
Next Hop
--------
---- ----
--------
Dstn-prefix Next HopForwarding Table
Next Hop Computation
Forwarding Engine
Incoming
Packet
HEADER
Need to be as fast as line speed!! e.g OC48 link runs at 2.5Gbps, packet=256bytes 1 million lookups/sLow storage : ~100K entriesFast updates: few thousands per second, but ideally at lookup speed
Network Layer 4-28
Route Lookup Using CAM
PriorityEncoder
Location 0
1
2
3
4
5
6
103.23.122.7 P1
P1 103.23.122/23 171.3.2.22
P2
P3
P4
P5
103.23/16
101.1/16
101.20/13
100/9
171.3.2.4
120.33.32.98
320.3.3.1
10.0.0.111
Prefix Next-hop1
1
0
0
0
0
0
To find the longest prefix cheaply, need to keep entries sorted in order of decreasing prefix lengthsK. pagiamtzis, Intro to CAM pagiamtzis.com/cam/camintro.html
Content-Address Memory: Fully associative mem: Cisco 8500
Exact match (fixed-length) search op in a single clock cycle
Network Layer 4-29
Topics:
Router IP: Internet Protocol
Datagram format, IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP, OSPF, BGP
Network Layer 4-30
The Internet Network layer
forwardingtable
Host, router network layer functions:
Routing protocols•path selection•RIP, OSPF, BGP
IP protocol•addressing conventions•datagram format•packet handling conventions
ICMP protocol•error reporting•router “signaling”
Transport layer: TCP, UDP
Link layer
physical layer
Networklayer
Network Layer 4-31
IP datagram format
ver Datagram length
32 bits
data (variable length,typically a TCP
or UDP segment)
16-bit identifier
Header checksum
time tolive
32 bit source IP address
IP protocol versionnumber
header length (bytes)
max numberremaining hops
(decremented at each router)
forfragmentation/reassembly
total datagramlength, bytes)
upper layer protocolto deliver payload to6 for tcp, 17 for udp
head.len
type ofservice
“type” of data flgsfragment
offsetupper layer
32 bit destination IP address
Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.
how much overhead with TCP?
20 bytes of TCP 20 bytes of IP = 40 bytes +
app layer overhead
Network Layer 4-32
IP Fragmentation & Reassembly network links have MTU
(max.transfer size) - largest possible link-level frame. different link types,
different MTUs large IP datagram divided
(“fragmented”) within net one datagram becomes
several datagrams “reassembled” only at
final destination IP header bits used to
identify, order related fragments
fragmentation: in: one large datagramout: 3 smaller datagrams
reassembly
Network Layer 4-33
IP Fragmentation and Reassembly
ID=x
offset=0
fragflag=0
length=4000
ID=x
offset=0
fragflag=1
length=1500
ID=x
offset=185
fragflag=1
length=1500
ID=x
offset=370
fragflag=0
length=1040
One large datagram becomesseveral smaller datagrams
Example 4000 byte IP
datagram MTU = 1500 bytes (4000-20 bytes
header)=3980 bytes of data to be fragmented
3 fragments (1480+1480+1020=3980)
amount of data in all but last fragment must be multiples of 8 1480 bytes in
data fieldoffset =1480/8
Network Layer 4-34
IP Addressing: introduction IP address: 32-bit
identifier for host, router interface, in dotted-decimal notation
interface: connection between host/router and physical link router’s typically have
multiple interfaces host typically has one
interface IP addresses associated
with each interface
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
Network Layer 4-35
Subnets (aka IP networks) IP address:
subnet part (high order bits)
host part (low order bits)
What’s a subnet ? device interfaces with
same subnet part of IP address
can physically reach each other without intervening router
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 subnets
subnet
To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.
Network Layer 4-36
SubnetsHow many? 223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Network Layer 4-37
IP addressing: CIDRCIDR: Classless InterDomain Routing
subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet
portion of address. Notation /x is subnet mask. The high order x bits are
the network prefix.
Before CIDR, classful addressing: A (/8), B(/16), C(/24). Replaced by CIDRized address.
11001000 00010111 00010000 00000000
subnetpart
hostpart
200.23.16.0/23
Network Layer 4-38
IP addresses: how to get one?
Q: How does host get IP address?
hard-coded by system admin in a file Wintel: control-panel->network->configuration->tcp/ip-
>properties UNIX: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play”
5: DataLink Layer 5-39
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server.
Network Layer 4-40
IP addresses: how to get one?
Q: How does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….
Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
Network Layer 4-41
Hierarchical addressing: route aggregation
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”
200.23.20.0/23Organization 2
...
...
Hierarchical addressing allows efficient advertisement of routing information.
Two example businesses
Network Layer 4-42
Hierarchical addressing: more specific routesAssume ISPs-R-Us has been acquired by FBN-ISP and Org1 be transferredto ISPs-R-Us:
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”
200.23.20.0/23Organization 2
...
...
Network Layer 4-43
IP addressing: the last word...
Q: How does an ISP get a block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers: www.icann.org allocates address space Top-level domain name system management manages DNS root servers Protocol identifier assignment assigns domain names, resolves disputes
Network Layer 4-44
NAT: Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4
138.76.29.7
local network(e.g., home network)
10.0.0/24
rest ofInternet
Datagrams with source or destination in this networkhave 10.0.0/24 address for
source, destination (as usual)
All datagrams leaving localnetwork have same single source
NAT IP address: 138.76.29.7,different source port numbers
Network Layer 4-45
NAT: Network Address Translation
Motivation: local network uses just one IP address as far as outside world is concerned: no need to be allocated range of addresses from
ISP: - just one IP address is used for all devices can change addresses of devices in local network
without notifying outside world can change ISP without changing addresses of
devices in local network devices inside local net not explicitly
addressable, visible by outside world (a security plus).
Network Layer 4-46
NAT: Network Address Translation
Implementation: NAT router must:
outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using (NAT IP
address, new port #) as destination addr.
remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
Network Layer 4-47
NAT: Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
S: 10.0.0.1, 3345D: 128.119.40.186, 80
1
10.0.0.4
138.76.29.7
1: host 10.0.0.1 sends datagram to 128.119.40.186, 80
NAT translation tableWAN side addr LAN side addr
138.76.29.7, 5001 10.0.0.1, 3345…… ……
S: 128.119.40.186, 80 D: 10.0.0.1, 3345
4
S: 138.76.29.7, 5001D: 128.119.40.186, 80
2
2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table
S: 128.119.40.186, 80 D: 138.76.29.7, 5001
3
3: Reply arrives dest. address: 138.76.29.7, 5001
4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345
Network Layer 4-48
NAT: Network Address Translation
16-bit port-number field: 60,000 simultaneous connections with a
single LAN-side address! NAT is controversial:
routers should only process up to layer 3 but NAT router need to change the transport port.
violates end-to-end argument• NAT possibility must be taken into account by app
designers, eg, P2P applications address shortage should instead be solved by
IPv6
Skype through NAT
NAT prevents a connection from being initiated from outside.
How can Alice call Bob, both residing behind NAT (NAT traversal) ?? Alice sign-in with its super-peer (Sa) Bob sign-in with its super-peer (Sb) Alice calls Bob: Alice SaSbBob If Bob takes the call, Sa and Sb select a non-
NAT super-peer for voice relay See chapter 2 (4th ed) for details
Network Layer 4-49
Network Layer 4-50
Recap: Internet Network layer
forwardingtable
Host, router network layer functions:
Routing protocols•path selection•RIP, OSPF, BGP
IP protocol•addressing conventions•datagram format•packet handling conventions
ICMP protocol•error reporting•router “signaling”
Transport layer: TCP, UDP
Link layer
physical layer
Networklayer
Network Layer 4-51
ICMP: Internet Control Message Protocol used by hosts & routers to
communicate network-level information error reporting:
unreachable host, network, port, protocol
echo request/reply (used by ping)
network-layer “above” IP: ICMP msgs carried in IP
datagrams ICMP message: type, code,
different fields depending on the type/code. If it’s a reply type then it would have IP Header and first 8 bytes of IP datagram data.
Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header
Taxonomy 1-52
Recap: “Real” Internet delays and routes What do “real” Internet delay & loss look like? Traceroute program (in Unix) or Tracert (MS-DOS):
provides delay measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path
towards destination router i will return packets to sender sender times interval between transmission and reply.
3 probes
3 probes
3 probes
Network Layer 4-53
Traceroute and ICMP
Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number
(depends on implementation) When nth datagram arrives
to nth router: Router discards datagram And sends to source an ICMP
message (type 11, code 0 which means TTL expired)
Message includes name of router& IP address
When ICMP message arrives, source calculates RTT
Traceroute does this 3 timesStopping criterion UDP segment eventually
arrives at destination host Destination returns ICMP
“host unreachable” packet (type 3, code 3) if port is sent. When source gets this ICMP, stops.
Other Tracert implementation stops when ping reply is received from destination.
Network Layer 4-54
IPv6 Initial motivation: 32-bit address space
soon to be completely allocated. Expanded addressing capabilities: 128 bit
Additional motivation: header format helps speed
processing/forwarding• fixed-length 40 byte header• no fragmentation allowed
header changes to facilitate QoS• Flow label and priority
These are also three most important changes
Network Layer 4-55
IPv6 Header (Cont)Priority (8-bits): identify priority among datagrams in flowFlow Label (20-bits): identify datagrams in same “flow.” (concept of“flow” not well defined).Next header (8-bits): identify upper layer protocol for data (similar to Upper-layer protocol in IPv4)
Traffic Class is similar to Type Of Service in IPv4
Similar to TTL in IPv4 (8-bits)
Network Layer 4-56
Other Changes from IPv4
Checksum: removed entirely to reduce processing time at each hop
Options: allowed, but outside of header, indicated by “Next Header” field
ICMPv6: new version of ICMP additional message types, e.g. “Packet Too
Big” multicast group management functions
Network Layer 4-57
Transition From IPv4 To IPv6
Not all routers can be upgraded simultaneous no “flag days” How will the network operate with mixed IPv4 and
IPv6 routers? Two proposed solutions:
Dual-stack approach: IPv6 to IPv4 and vice versa translation of datagrams at routers that can understand IPv6 and IPv4. Some fields data will be lost.
Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers
Network Layer 4-58
TunnelingA B E F
IPv6 IPv6 IPv6 IPv6
tunnelLogical view:
Physical view:A B E F
IPv6 IPv6 IPv6 IPv6
C D
IPv4 IPv4
Flow: XSrc: ADest: F
data
Flow: XSrc: ADest: F
data
Flow: XSrc: ADest: F
data
Src:BDest: E
Flow: XSrc: ADest: F
data
Src:BDest: E
A-to-B:IPv6
E-to-F:IPv6
B-to-C:IPv6 inside
IPv4
D-to-E:IPv6 inside
IPv4
Network Layer 4-59
Summary
Network Layer Services: forwarding, routing and connection setup in some networks
Best effort network Service Models Router Architecture Overview
Input/Output ports and queuing Switching via Memory/Bus/Interconnected network
Network Layer Functions: forwarding via routing protocols, routing via IP error reporting via ICMP
IP Datagram Format IP Fragmentation and Reassembly IP Addressing: subnets, CIDR, assignments, Hierarchical addressing Network Address Translation (NAT) Internet Control Message Protocol (ICMP) usage IPv6 motivation, datagram format and transition to IPv4 through
Tunneling
Recommended