NAT: network address translation - WordPress.com
Transcript of NAT: network address translation - WordPress.com
▪ 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• address shortage should be solved by IPv6• violates end-to-end argument
• NAT possibility must be taken into account by app designers, e.g., P2P applications
• NAT traversal: what if client wants to connect to server behind NAT? (More detail later)
NAT: network address translation
1Network Layer: Data Plane
ver length
32 bits
data (variable length, typically a TCP
or UDP segment)
16-bit identifierheader
checksumtime to
live
32 bit source IP address
head. len
type of service
flgs fragment offset
upper layer
32 bit destination IP address
options (if any)
IP datagram formatIP protocol version
numberheader length
(bytes)
upper layer protocol to deliver payload to
total datagram length (bytes)
“type” of data for fragmentation/ reassemblymax number
remaining hops (decremented at
each router)
e.g. timestamp, record route taken, specify list of routers to visit.
2Network Layer: Data Plane
Differentiated Services and is called the Diff Serv Code Point (DSCP).
0x0 is default (best effort)
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Yeah No checksum. We already have error detection a data link layer
Internet Checksum: example
Transport Layer: 3-18
example: add two 16-bit integers
sum
checksum
Note: when adding numbers, a carryout from the most significant bit needs to be added to the result
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/
1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1wraparound
1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 00 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
4
How do you check using the checksum.
sum
checksum
1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1wraparound
1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 00 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
Add up all of the numbers that you receive. Then add the check some. If you don’t get all ones packet is corrupted.
5
WIRE SHARK CAPTURE INSPECTION EXAMPLES
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 datagram out: 3 smaller datagrams
reassembly
…
…
6Network Layer: Data Plane
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 becomes several smaller datagrams
example:❖ 4000 byte datagram❖ MTU = 1500 bytes
1480 bytes in data field
offset = 1480/8
IP fragmentation, reassembly
7Network Layer: Data Plane
Why is 1480 and not 1500Length includes the bytes associated with the header field
Original checksum is 20 bytes
4.1 Overview of Network layer• data plane• control plane
4.2 What’s inside a router4.3 IP: Internet Protocol
• datagram format• fragmentation• IPv4 addressing• network address
translation• IPv6
Chapter 4: outline
8Network Layer: Data Plane
9
DCHP explore
10
Bring the Ideas of Together. ( Modem/Router/Switch)
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Dial-up Modems:
Traditional telephone lines can carry frequencies between 300 and 3300 Hz,
Trelliscoded modulation. Trellis is essentially QAM plus a redundant bit.
https://www.brainkart.com/article/Dial-up-Modems_13438/
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32-QAM with a baud rate of 2400
Trellis-coded modulation. Trellis is essentially QAM plus a redundant bit.
Baud rate is symbols per second. 2400 Symbols per second
Number of bits 32 QAMlog2(32) = 5
One Redundant bit5-4
How many bits per second4*2400 = 9600 bps
How many bits per second Could you get if 128-QAM
Why the redundant bit? -> Helps with error detection
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Similar Ideas
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IPv6: motivation▪ initial motivation: 32-bit address space soon to
be completely allocated. ▪ additional motivation:
• header format helps speed processing/forwarding• header changes to facilitate QoS
IPv6 datagram format: • fixed-length 40 byte header• no fragmentation allowed•
16Network Layer: Data Plane
Why no fragmentation. (Fragmentation open you up a fragmentation attacks)
IPv6 datagram formatpriority: identify priority among datagrams in flowflow Label: identify datagrams in same “flow.” (concept of“flow” not well defined).next header: identify upper layer protocol for data
data
destination address (128 bits)
source address (128 bits)
payload len next hdr hop limitflow labelpriver
32 bits17Network Layer: Data Plane
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19
20
21
22
Traffic Class
https://tools.ietf.org/html/rfc2460
Request for Comments (RFC) 2460
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24
25
26
27
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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
29Network Layer: Data Plane
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ver length
32 bits
data (variable length, typically a TCP
or UDP segment)
16-bit identifierheader
checksumtime to
live
32 bit source IP address
head. len
type of service
flgs fragment offset
upper layer
32 bit destination IP address
options (if any)
Something Has been removed from IPV6. Is it important?
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Transition from IPv4 to IPv6▪ not all routers can be upgraded simultaneously
• no “flag days”• how will network operate with mixed IPv4 and
IPv6 routers? ▪ tunneling: IPv6 datagram carried as payload in
IPv4 datagram among IPv4 routers
IPv4 source, dest addr IPv4 header fields
IPv4 datagramIPv6 datagram
IPv4 payload
UDP/TCP payloadIPv6 source dest addr
IPv6 header fields
32Network Layer: Data Plane
Transition from IPv4 to IPv6
▪ not all routers can be upgraded simultaneously• no “flag days”• how will network operate with mixed IPv4 and
IPv6 routers?
33Network Layer: Data Plane
So how could we address this issue.
Tunneling
physical view:IPv4 IPv4
A B
IPv6 IPv6
E
IPv6 IPv6
FC D
logical view:
IPv4 tunnel connecting IPv6 routers
E
IPv6 IPv6
FA B
IPv6 IPv6
34Network Layer: Data Plane
flow: X src: A dest: F
data
A-to-B: IPv6
Flow: X Src: A Dest: F
data
src:B dest: E
B-to-C: IPv6 inside
IPv4
E-to-F: IPv6
flow: X src: A dest: F
data
B-to-C: IPv6 inside
IPv4
Flow: X Src: A Dest: F
data
src:B dest: E
physical view:A B
IPv6 IPv6
E
IPv6 IPv6
FC D
logical view:
IPv4 tunnel connecting IPv6 routers
E
IPv6 IPv6
FA B
IPv6 IPv6
Tunneling
IPv4 IPv4
35Network Layer: Data Plane
IPv6: adoption
▪ Google: 30% of clients access services via IPv6▪ NIST: 1/3 of all US government domains are IPv6
capable
▪ Long (long!) time for deployment, use•20 years and counting!•think of application-level changes in last 20 years: WWW, Facebook, streaming media, Skype, …•Why?
36Network Layer: Data Plane
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https://www.google.com/intl/en/ipv6/statistics.html
Computer Networking: A Top Down Approach
A note on the use of these Powerpoint slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:
▪ If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!)
▪ If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2016 J.F Kurose and K.W. Ross, All Rights Reserved
7th edition Jim Kurose, Keith RossPearson/Addison WesleyApril 2016
Chapter 4 Network Layer:The Data Plane
38Network Layer: Data Plane
4.1 Overview of Network layer• data plane• control plane
4.2 What’s inside a router4.3 IP: Internet Protocol
• datagram format• fragmentation• IPv4 addressing• network address
translation• IPv6
Chapter 4: outline
39Network Layer: Data Plane
Chapter 4: network layer
chapter goals: ▪ understand principles behind network layer
services, focusing on data plane:• network layer service models• forwarding versus routing• how a router works• generalized forwarding
▪ instantiation, implementation in the Internet
40Network Layer: Data Plane
Network layer▪ transport segment from
sending to receiving host ▪ on sending side
encapsulates segments into datagrams
▪ on receiving side, delivers segments to transport layer
▪ network layer protocols in every host, router
▪ router examines header fields in all IP datagrams passing through it
application transport network data link physical
application transport network data link physical
network data link physical network
data link physical
network data link physical
network data link physical
network data link physical
network data link physical
network data link physical
network data link physical
network data link physical
network data link physicalnetwork
data link physical
41Network Layer: Data Plane
Two key network-layer functions
network-layer functions:▪forwarding: move packets from router’s input to appropriate router output▪routing: determine route taken by packets from source to destination
• routing algorithms
analogy: taking a trip▪ forwarding: process of
getting through single interchange
▪ routing: process of planning trip from source to destination
42Network Layer: Data Plane
Network layer: data plane, control plane
Data plane▪local, per-router function▪determines how datagram arriving on router input port is forwarded to router output port▪forwarding function
Control plane▪network-wide logic▪determines how datagram is routed among routers along end-end path from source host to destination host▪two control-plane approaches:
• traditional routing algorithms: implemented in routers
• software-defined networking (SDN): implemented in (remote) servers
1
23
0111
values in arriving packet header
43Network Layer: Data Plane
Per-router control plane
Routing Algorithm
Individual routing algorithm components in each and every router interact in the control plane
data plane
control plane
44Network Layer: Control Plane
1
2
0111
values in arriving packet header
3
data plane
control plane
Logically centralized control planeA distinct (typically remote) controller interacts with local control agents (CAs)
Remote Controller
CA
CA CA CA CA
45Network Layer: Control Plane
1
2
0111
3
values in arriving packet header
4.1 Overview of Network layer• data plane• control plane
4.2 What’s inside a router4.3 IP: Internet Protocol
• datagram format• fragmentation• IPv4 addressing• network address
translation• IPv6
4.4 Generalized Forward and SDN• match• action• OpenFlow examples
of match-plus-action in action
Chapter 4: outline
46Network Layer: Data Plane
Router architecture overview
high-seed switching
fabric
routing processor
router input ports router output ports
forwarding data plane (hardware) operttes in
nanosecond timeframe
routing, management control plane (software) operates in millisecond
time frame
▪ high-level view of generic router architecture:
47Network Layer: Data Plane
line termination
link layer
protocol (receive)
lookup, forwarding
queueing
Input port functions
decentralized switching: ▪ using header field values, lookup output
port using forwarding table in input port memory (“match plus action”)
▪ goal: complete input port processing at ‘line speed’ (ex line speed 10Mbs)
▪ queuing: if datagrams arrive faster than forwarding rate into switch fabric
physical layer: bit-level reception
data link layer: e.g., Ethernet see chapter 5
switch fabric
48Network Layer: Data Plane
line termination
link layer
protocol (receive)
lookup, forwarding
queueing
Input port functions
decentralized switching: ▪ using header field values, lookup output
port using forwarding table in input port memory (“match plus action”)
▪ destination-based forwarding: forward based only on destination IP address (traditional)
physical layer: bit-level reception
data link layer: e.g., Ethernet see chapter 5
switch fabric
49Network Layer: Data Plane
Destination Address Range
11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111
11001000 00010111 00011000 00000000 through 11001000 00010111 00011111 11111111
otherwise
Link Interface
0
1
2
3
Destination-based forwardingforwarding table
50Network Layer: Data Plane
Longest prefix matching
Destination Address Range 11001000 00010111 00010*** *********
11001000 00010111 00011000 *********
11001000 00010111 00011*** ********* otherwise
DA: 11001000 00010111 00011000 10101010
examples:DA: 11001000 00010111 00010110 10100001 which interface?
which interface?
when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.
longest prefix matching
Link interface
0
1
2
3
51Network Layer: Data Plane
Longest prefix matching▪ longest prefix matching: often performed using ternary
content addressable memories (TCAMs)• content addressable: present address to TCAM:
retrieve address in one clock cycle, regardless of table size
• Cisco Catalyst: can hold up ~1M routing table entries in TCAM
52Network Layer: Data Planehttps://www.pagiamtzis.com/cam/camintro/
X don’t affect the line (don’t care)
Switching fabrics▪ transfer packet from input buffer to appropriate
output buffer▪ switching rate: rate at which packets can be
transfer from inputs to outputs• often measured as multiple of input/output line rate• N inputs: switching rate N times line rate desirable
▪ three types of switching fabrics
memory
memory
bus crossbar
53Network Layer: Data Plane
Memory Based Routersfirst generation routers:▪ traditional computers with switching under direct control of CPU▪ packet copied to system’s memory▪ speed limited by memory bandwidth (2 bus crossings per datagram)
input port (e.g.,
Ethernet)
memoryoutput port (e.g.,
Ethernet)
system bus
54Network Layer: Data Plane
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
▪ 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers
bus
55Network Layer: Data Plane
Switching via interconnection network
▪ overcome bus bandwidth limitations
▪ banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor
▪ advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.
▪ Cisco 12000: switches 60 Gbps through the interconnection network
crossbar
56Network Layer: Data Plane
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Hi-Z high impedance. Very little current flow
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What control signal would result in result shown below.
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1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1
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Network Layer 61
A Big Banyan Tree at Bangalore
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Given some banyan switch draw the configuration that gets the output
Input port queuing
▪ fabric slower than input ports combined -> queueing may occur at input queues • queueing delay and loss due to input buffer overflow!
▪ Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward
output port contention:only one red datagram can be
transferred.lower red packet is blocked
switch fabric
one packet time later: green packet experiences HOL
blocking
switch fabric
63Network Layer: Data Plane
Output ports
▪ buffering required when datagrams arrive from fabric faster than the transmission rate
▪ scheduling discipline chooses among queued datagrams for transmission
line termination
link layer
protocol (send)
switch fabric
datagram buffer
queueing
This slide is HUGELY important!
Datagram (packets) can be lost due to congestion, lack of buffers
Priority scheduling – who gets best performance, network neutrality
64Network Layer: Data Plane
Output port queueing
▪ buffering when arrival rate at switch exceeds output line speed
▪ queueing (delay) and loss due to output port buffer overflow!
at t, packets more from input to output
one packet time later
switch fabric
switch fabric
65Network Layer: Data Plane
How much buffering?▪ RFC 3439 rule of thumb: average buffering
equal to “typical” RTT (say 250 msec) times link capacity C• e.g., C = 10 Gpbs link: 2.5 Gbit buffer
▪ recent recommendation: with N flows, buffering equal to
RTT C.N
66Network Layer: Data Plane
Scheduling mechanisms
▪ scheduling: choose next packet to send on link▪ FIFO (first in first out) scheduling: send in order of
arrival to queue• discard policy: if packet arrives to full queue: who to
discard?• tail drop: drop arriving packet• priority: drop/remove on priority basis• random: drop/remove randomly
queue (waiting area)
packet arrivals
packet departureslink
(server)
67Network Layer: Data Plane
Scheduling policies: prioritypriority scheduling:
send highest priority queued packet
▪ multiple classes, with different priorities• class may depend on
marking or other header info, e.g. IP source/dest, port numbers, etc.
• Q: real world example?
high priority queue (waiting area)
low priority queue (waiting area)
arrivals
classify
departures
link (server)
1 3 2 4 5
5
5
2
2
1
1
3
3 4
4arrivals
departures
packet in
service
68Network Layer: Data Plane
Scheduling policies: still moreRound Robin (RR) scheduling:▪ multiple classes▪ cyclically scan class queues, sending one complete packet from each class (if available)
1 23 4 5
5
5
2
3
1
1
3
2 4
4arrivals
departures
packet in
service
69Network Layer: Data Plane
Weighted Fair Queuing (WFQ): ▪ generalized Round Robin▪ each class gets weighted amount of service in
each cycle
Scheduling policies: still more
70Network Layer: Data Plane
4.1 Overview of Network layer• data plane• control plane
4.2 What’s inside a router4.3 IP: Internet Protocol
• datagram format• fragmentation• IPv4 addressing• network address
translation• IPv6
4.4 Generalized Forward and SDN• match• action• OpenFlow examples
of match-plus-action in action
Chapter 4: outline
71Network Layer: Data Plane
The Internet network layer
forwarding table
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
network layer
72Network Layer: Data Plane