tcom 509: tcp/ip - internet protocols
DESCRIPTION
TCOM 509: TCP/IP - Internet Protocols. Instructor: Scott T. Tran. * Obtained permission to use Raj Jain’s technical material. Understand concept of datagram processing and delivery (layering and encapsulation) Understand the client-server model as applied to networking - PowerPoint PPT PresentationTRANSCRIPT
TCOM 509:TCP/IP - Internet Protocols
Instructor: Scott T. Tran
* Obtained permission to use Raj Jain’s technical material
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Course Objectives
Understand concept of datagram processing and delivery (layering and encapsulation)
Understand the client-server model as applied to networking
Understand IP Addressing and Subnet Masking Schemes (CIDR/VLSM)
Understand IP routing (RIP, OSPF, IS-IS) Understand service (e.g., application)
addressing and access to services across an IP network
Understand TCP performance parameters and metrics
Advanced topics (IP Multicast, IP Tunneling, NAT, DHCP, IP Security, etc…)
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On-Line Course Info
Look at the website at least once per week http://osf1.gmu.edu/~stran4
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IP Header
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UDP Header
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TCP Header
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Communications Between LAN Hosts (TCP/IP) Via Wide Area Networks (IP)
Chapter 1:Introduction
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Concept of Peer Entities – Logical Relationships
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Protocols
A protocol is a set of rules and formats that govern the communication between communicating peers set of valid messages meaning of each message
A protocol is necessary for any function that requires cooperation between peers
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What does a protocol tell us?
Syntax of a message what fields does it contain? in what format?
Semantics of a message what does a message mean? for example, not-OK message means receiver got
a corrupted file Actions to take on receipt of a message
for example, on receiving not-OK message, retransmit the entire file
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The Internet
Standards-based – the TCP/IP protocol suite An Open System – not linked to a single
vendor US Gov’t research resulted in quite and
extensible set of protocols Best spent tax money I know of
Evolution from gov’t-orientation to research-orientation to business-orientation
Why is it so good? Why did it beat out “OSI Networking”? Let’s start in on the details…
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Getting Data from Here to There IP is a form of packet switching
Data is broken up into discrete chucks and then sent toward destination
Each packet has to find its own route to the destination. There is no predetermined path; the decision as to which node to hop to in the next step is taken only when a node is reached.
Each packet finds its way using the information it carries, such as the source and destination IP addresses.
Network resources (routers, links) are shared between different data streams - multiplexing
The phone network: circuit switching Sender calls receiver and establishes a logical connection The connection is maintained for the duration of the data flow
Two distinct paradigms Both have value
The TCP part of TCP/IP provides a logical connection, when necessary
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Motivation behind OSI Model
Is a conceptual, reference model.
Is the primary architectural model for inter-computer communications.
Is the only common language spoken by different manufacturers.
Mastery of the OSI model is mandatory
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Motivation behind OSI Model The goal of an OSI layer is to communicate with its peer
layer on another host.
The information exchanged is called a Protocol Data Unit (PDU).
7 Application6 Presentation5 Session4 Transport3 Network2 Data Link1 Physical
7 Application6 Presentation5 Session4 Transport3 Network2 Data Link1 Physical
7 Application6 Presentation5 Session4 Transport3 Network2 Data Link1 Physical
MessagesMessagesMessagesSegmentsPackets or DatagramsFramesBits
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Communication between OSI Layers
7 Application6 Presentation5 Session4 Transport3 Network2 Data Link1 Physical
7 Application6 Presentation5 Session4 Transport3 Network2 Data Link1 Physical
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Motivation behind OSI Model – Protocol Functionality Fragmentation and Reassembly
Breaking up data into pre-defined sized blocks Encapsulation
Adding control info to data (address, error detection code, etc…) Connection Control
Providing connection establishment, data transfer, connection termination Flow Control
Throttling of data rate exchanged between source and destination Error Control
Error detection Synchronization
Timeouts, Send state, Receive state. Etc… Sequencing
Numbering of data blocks (applicable only for connection-oriented mode) Addressing
Has local and global significance, Used for routing purposes in IP
Multiplexing Allowing multiple logical connections to use one physical connections Mapping of connections from one protocol layer to another
Transmission Services Security, Priority, Grade of Service
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The OSI Reference Model
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Why seven layers?
Need a top and a bottom -- 2 Need to hide physical link, so need datalink --
3 Need both end-to-end and hop-by-hop
actions; so need at least the transport (TCP) and network (IP) layers -- 5
Session and presentation layers are not so important, and are often ignored
So, we need at least 5, and 7 seems to be excessive
Note that we can place functions in different layers
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Physical layer
Moves bits between physically connected end-systems
Standard prescribes coding scheme to represent a bit shapes and sizes of connectors bit-level synchronization Supported transmission: electric voltages, radio frequencies,
pulses of infrared or ordinary light Postal network
technology for moving letters from one point to another (trains, planes, vans, bicycles, ships…)
Internet technology to move bits on a wire, wireless link, satellite
channel etc.
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Datalink layer Introduces the notion of a frame
set of bits that belong together Idle markers tell us that a link is not carrying a frame Begin and end markers delimit a frame On a broadcast link (such as Ethernet)
end-system must receive only bits meant for it need datalink-layer address also need to decide who gets to speak next these functions are provided by Medium Access sublayer (MAC)
Some data links also retransmit corrupted packets and pace the rate at which frames are placed on a link part of logical link control sublayer layered over MAC sublayer
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Datalink layer (contd.)
Datalink layer protocols are the first layer of software
Very dependent on underlying physical link properties
Usually bundle both physical and datalink layer on host adaptor card example: Ethernet
Postal service mail bag ‘frames’ letters
Internet a variety of datalink layer protocols most common is Ethernet others are FDDI, SONET, HDLC
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Network layer
Logically concatenates a set of links to form the abstraction of an end-to-end link
Allows an end-system to communicate with any other end-system by computing a route between them
Hides idiosyncrasies of datalink layer Provides unique network-wide addresses Found both in end-systems and in intermediate
systems At end-systems primarily hide details of
datalink layer segmentation and reassembly error detection
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Network layer (contd.)
At intermediate systems participates in routing protocol to create
routing tables responsible for forwarding packets scheduling the transmission order of
packets choosing which packets to drop
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Two types of network layers In datagram (connection-less) networks
provides both routing and data forwarding Ex: Internet using IP protocol
In connection-oriented networks we distinguish between data plane and control plane data plane only forwards and schedules data
(touches every byte) control plane responsible for routing, call-
establishment, call-teardown (doesn’t touch data bytes)
Ex: TCP protocol running over IP
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Network layer
Postal network set up internal routing tables forward letters from source to destination static routing multiple qualities of service
Internet network layer is provided by Internet Protocol found in all end-systems and intermediate systems segmentation and reassembly packet-forwarding, routing, scheduling unique IP addresses can be layered over anything, but only best-effort service
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Transport layer - TCP
Network provides a ‘raw’ end-to-end service Transport layer creates the abstraction of an
error-controlled, flow-controlled and multiplexed end-to-end link
Error control message will reach destination despite packet loss,
corruption and duplication retransmit lost packets; detect, discard, and retransmit
corrupted packets; detect and discard duplicated packets Flow control
match transmission rat to rate currently sustainable on the path to destination, and at the destination itself
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Transport Layer - TCP
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Transport Layer (TCP) – Relationships with Other Layers Below
Process-to-process delivery
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Transport Layer Addressing
Addresses•Data link layer MAC address•Network layer IP address•Transport layer Port number (choose among multiple processes running on destination host)
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Transport layer (contd.)
Multiplexes multiple applications to the same end-to-end connection adds an application-specific identifier (port
number) so that receiving end-system can hand in incoming packet to the correct application
Some transport layers provide fewer services e.g. simple error detection, no flow control,
and no retransmission lightweight transport layer
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Transport layer (contd.)
Postal system doesn’t have a transport layer implemented, if at all, by customers detect lost letters (how?) and
retransmit them Internet
two popular protocols are TCP and UDP TCP provides error control, flow
control, multiplexing UDP provides only multiplexing
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Session layer
Not common Provides full-duplex service, expedited data
delivery, and session synchronization Duplex
if transport layer is simplex, concatenates two transport endpoints together
Expedited data delivery allows some messages to skip ahead in end-system
queues, by using a separate low-delay transport layer endpoint
Synchronization allows users to place marks in data stream and to
roll back to a pre-specified mark
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Example
Postal network suppose a company has separate shipping and
receiving clerks chief clerk can manage both to provide
abstraction of a duplex service chief clerk may also send some messages
using a courier (expedited service) chief clerk can arrange to have a set of
messages either delivered all at once, or not at all
Internet doesn’t have a standard session layer
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Presentation layer
Unlike other layers which deal with headers, presentation layer touches the application data
Hides data representation differences between applications e.g. endian-ness
Can also encrypt data Usually ad hoc Postal network
translator translates contents before giving it to chief clerk
Internet no standard presentation layer only defines network byte order for 2- and 4-byte
integers
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Application layer
The set of applications that use the network Doesn’t provide services to any other layer Postal network
the person who uses the postal system suppose manager wants to send a set of recall letters translator translates letters going abroad chief clerk sends some priority mail, and some by
regular mail mail clerk sends a message, retransmits if not acked postal system computes a route and forwards the
letters datalink layer: letters carried by planes, trains,
automobiles physical layer: the letter itself
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OSI Reference Model vs. TCP/IP Protocol Stack
OSI TCP / IP
Application (Layer7)ApplicationPresentation (Layer6)
Session (Layer 5)
Transport (Layer 4) Transport
Network (Layer 3) Internet
Data Link (Layer 2)Subnet
Physical (Layer 1)
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Layering and Encapsulation
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Why Layering Required For Networking? A system that is too complex to comprehend
in its entirety.
A system that is difficult to maintain.
A system whose least stable elements are not isolated.
A system whose most reusable elements are difficult to identify.
A system that is to be built by different teams, possibly with different skills.
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Why Need Encapsulation?
Preserve content of layers which is private but allow interfacing between them
Allows management of complexity and change within layers
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What is a Router?
A specialized computer Interconnects multiple physical
networks Allows construction of a LOGICAL
network topology that is independent from the PHYSICAL networks
Notation Show-and-tell
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How does a router's forwarding logic differ from a bridge's forwarding logic? 1. Packet Filtering: A router examines only those data packets specifically addressed to it, as
opposed to a bridge, which reads the destination address of every data packet on the LAN to which it is attached.
2. Route Determination: A bridge checks the frame's data-link protocol for source as well as destination address. It then checks its table of known local nodes. The destination address is compared with the contents of the known local nodes in order to determine whether the frame should be allowed to cross the bridge or not whether the destination is local or not). The bridge does not determine the path; it merely allows or disallows the packet to cross. Destination routes must be obtained through other network devices, such as the originating workstation for source routing bridges. Bridges are thus sometimes known as "forward if not local" devices. Routers actually maintain dynamic tables of "best routes", which depend on network conditions. Based the latest traffic conditions, the router chooses the best path for the data packet to reach its destination, and sends the data packet on its way. After reading the network layer destination address and the protocol of the network layer data, the router consults its routing tables in order to determine the best path on which to forward this data packet. Having found the best path, the router has the ability to repackage the data packet as required for the chosen delivery route. For example, if the packet were to be sent out over an X.25 packet-switched network, the router would encapsulate the packet in an X.25-compliant envelope.
3. Routing Logic: A bridge reads the destination address of each data frame on a LAN, decides whether the address is local or remote (on the other side of the bridge), and only allows those data frames with non-local destination addresses to cross the bridge. A router is more discriminating. The router first confirms the existence of the destination address as well as the latest information on available network paths to reach that destination. Unlike a bridge, which merely allows access to the internetwork (forward-if-not-local logic), a router specifically addresses the data packet to a distant router. However, before a router actually releases a data packet on to the internetwork, is confirms the existence of the destination address to which the data packet is bound. Only once the router is satisfies with the viability of the destination address as well as with the quality of the intended path, will it release the packaged packet. The router's meticulous processing is known as "forward if proven remote" logic.
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TCP/IP Protocol Suite Layers
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TCP/IP Layering
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Multiplexing and Demultiplexing
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The Client-Server Model
Two hosts interact in a predefined manner One side is the client – it wants
information One side is the server – it provides the
information EX: WWW – web browser is the client,
website is the server A host is not “locked down” to be
only a client or only a server Multiple client and server programs can
be running on a single host at the same time
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Client/Server on the same LAN
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Client/Server on two different LANs
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Service Addresses: Port Numbering Port numbers are “Layer 4” addresses
(TCP or UDP in the TCP/IP suite) They allow multiple services on a
single host to have unique addresses E.g., one host can be running servers for
FTP, HTTP, and telnet Each service listens on it’s own port
The combination of IP address plus TCP/UDP can uniquely identify a connection (a “socket”)
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How Do You Get a Port Number? Standards and Standards Processes The Internet Engineering Task Force
(IETF) most directly controls the development of standards for the TCP/IP protocol suite
Those standards are called Requests for Comment (RFCs) Relatively collegial process Different from IEEE, ITU, ANSI, etc.
http://www.ietf.org http://www.rfc-editor.org See section 1.11, pp. 14-15
Some of these RFCs have been updated
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IP Protocol and Its Associated Upper Layer: IP Protocol Numbershttp://www.iana.org/assignments/protocol-numbers
unix-host% more /etc/protocols## Internet protocols## $FreeBSD: src/etc/protocols,v 1.13.2.1 2000/09/24 11:26:39 asmodai Exp $# from: @(#)protocols 5.1 (Berkeley) 4/17/89## See also http://www.isi.edu/in-notes/iana/assignments/protocol-numbers#ip 0 IP # internet protocol, pseudo protocol number#hopopt 0 HOPOPT # hop-by-hop options for ipv6icmp 1 ICMP # internet control message protocoligmp 2 IGMP # internet group management protocolggp 3 GGP # gateway-gateway protocolipencap 4 IP-ENCAP # IP encapsulated in IP (officially ``IP'')st2 5 ST2 # ST2 datagram mode (RFC 1819)tcp 6 TCP # transmission control protocolcbt 7 CBT # CBT, Tony Ballardie <[email protected]>egp 8 EGP # exterior gateway protocoligp 9 IGP # any private interior gateway (Cisco: for IGRP)<snip>udp 17 UDP # user datagram protocolipv6 41 IPV6 # ipv6sdrp 42 SDRP # Source Demand Routing Protocolipv6-route 43 IPV6-ROUTE # routing header for ipv6ipv6-frag 44 IPV6-FRAG # fragment header for ipv6idrp 45 IDRP # Inter-Domain Routing Protocolrsvp 46 RSVP # Resource ReSerVation Protocolgre 47 GRE # Generic Routing Encapsulation<etc.>
Chapter 2:Link Layer
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Ethernet Encapsulations – 2 Types
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Loopback Interfaces Special logical IP address (127.0.0.1) Any IP traffic sent to loopback interface must not appear on any
network Used to allow a client and a server on the same host to
communicate with each other using TCP/IP
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Maximum Transmission Unit (MTU)
Chapter 3:IP: Internet Protocol
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IP Header
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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
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IP Fragmentation and Reassembly
ID=x
offset=0
fragflag=0
length=4000
ID=x
offset=0
fragflag=1
length=1500
ID=x
offset=1480
fragflag=1
length=1500
ID=x
offset=2960
fragflag=0
length=1040
One large datagram becomesseveral smaller datagrams
IP header has identification (x), flag, and fragmentation fields
Example: 4000byte d’gram (20byte header + 3980 IP payload).
MTU = 1500bytes Frag 1: 1480bytes +
20byte header Frag 2: 1480bytes +
20byte header Frag 3: 3980-2*1480
bytes + 20byte header
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IP Fragmentation processing at a Router Fragmentation is performed when packet size is larger than MTU size of the
outgoing interface
To fragment/segment a long internet packet, an Intermediate System using the Internet Protocol (for example, a router), creates two new IP packets and copies the contents of the IP header fields from the long packet into BOTH new IP headers.
The data of the long packet is divided into two portions on a 8 byte (64 bit) boundary. All packets which have a more fragments (MF) flag set, must have an integral multiple of 8 bytes, but those that do not have this flag set need not do.
If we call the number of 8 byte blocks in the first portion NFB (for Number of Fragment Blocks). The first portion of the data is placed in the first new IP packet, and the total length field is set to the length of the FIRST IP packet. The more-fragments flag (MF) is set to one.
The second portion of the data is placed in the second new IP packet, and the total length field is set to the length of the SECOND packet. The more-fragments flag (MF) carries the same value as the long packet. The fragment offset field of the second new IP is set to the value of that field in the long IP packet plus the NFB.
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IP Addresses in TCP/IP Model
Addresses provide UNIQUENESS Different from other types of address
Layer 1 – physical address: hardware manufacturer assigns, hardly ever changes; MAC addresses
Layer 2 – logical address: you assign, and reassign as changes are made in the network; IP addresses
Layer 3 – “service” address: standards bodies assign, software manufacturers must abide by them for interoperability; TCP/UDP addresses
Why 32 bits for IP? 2^32 = 4+ billion – enough addresses (???)
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Addressing, Numbering, and Notation
Computers care about BINARY On/off, hi/low, signal/no signal, etc.
Humans care about DECIMAL IP addresses are truly 32-bit unsigned integers,
represented in dotted-decimal (a.k.a. dotted-quad) for our convenience
Electrical Engineers care about HEXADECIMAL Neither computer nor human? Compact representation of binary info Often used for Layer 2 (hardware) addresses
YOU need to care about ALL 3 Discuss some examples here…
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Bit Positions and Their Values
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IP Address Classes
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IP Address Class Ranges
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Address Class Characteristics
Class
Network Bits Host Bits Total Networks Total Addresses
A 8 24 127 16,777,216
B 16 16 16,384 65,536
C 24 8 2,097,152
256
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IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers (guidelines in RFC 2050) allocates addresses manages DNS assigns domain names, resolves disputes
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Subnetting - To divide the standard classful host-number field into two parts - the
subnet-number and the host-number on that subnet.
Motivation: Efficient use of available network addresses Flexibility in planning network growth and design Capability to contain broadcast traffic (ARP, RARP, etc…) Subnets under local administrative control
Mechanism: Define/assign a subnet mask for addresses in a network that has been sub-netted Subnet mask tells router which octets of an IP address to pay attention to when
comparing the destination address of a packet to its routing table entries A subnet mask identifies the subnet field of network addresses Correct routing requires that all subnets of a network be physically contiguous. In
other words, the network must be set up such that it does not require traffic between any two subnets to cross another network
Most implementations require that all subnets of a network have the same number of subnet bits.
Example
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Arrangement of Subnets
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How To Calculate Subnet Address with a Given Subnet Mask
Resulting subnet address is 171.16.1.0
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Example Subnet Masks
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Using Subnet Masks
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Supernetting – CIDR and VLSM
Motivation: Address issues with current IP Address Depletion
Subnetting allows you to take a (classful) block of addresses and break it up into usable portions Subnetting >>> Segregation
Supernetting allows you to implement classless addressing scheme and combine address blocks for the purposes of efficiency in routing updates Supernetting >>> Aggregation Rationale: More flexible use of IP addresses and
reduces entries in the routing table
Two ways to implement Supernetting Organizations need Variable Length Subnet Mask
(VLSM) to provide flexibility and address efficiency The Internet needs Classless Interdomain Routing
(CIDR) for scalability
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Requirements for Deploying CIDR or VLSM
The successful deployment of VLSM has three prerequisites: The routing protocols must carry extended network
prefix information with each route advertisement.
All routers must implement a consistent forwarding algorithm based on the “longest match.”
For route aggregation to occur, addresses must be assigned so that they have topological significance
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Comparing CIDR to VLSM
CIDR and VLSM both allow a portion of the IP address space to be recursively divided into subsequently smaller pieces. The difference is that with VLSM, the recursion is performed on the address space previously assigned to an organization and is invisible to the global Internet.
CIDR, on the other hand, permits the recursive allocation of an address block by an Internet Registry to a high-level ISP, a mid-level ISP, a low level ISP, and a private organization’s network.
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Classless Interdomain Routing (CIDR) With subnet addressing, we can have higher
flexibility within a domain/AS
However, the rigidity of classful IP addresses is still very inflexible (e.g. HostIDs in a domain is limited to 256, 66048, 16908288)
CIDR – use arbitrary prefix length of Network ID E.g. 205.100.0.0/22 means that network ID length is 22
bits, i.e. netmask is 255.255.252.0
Also allows RECURSION allocation of an address block provided by the Internet Registry to a high-level ISP, to a mid-level ISP, to a low-level ISP, and finally to a private organization’s network
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Classless Interdomain Routing (CIDR) Changes to routing table
Each entry must specify a 32-bit mask together with the 32-bit IP address
Use longest prefix match to find a suitable entryE.g. a packet with destination IP addr: 205.100.1.2, and the
routing has two entries as 205.100.0.0/22 and 205.100.0.0/20. Both entries match the destination IP addr, which one should be chosen? Ans: Choose the one with longest matched bits
205.100.0.0/22 = 11001101.01100100.00000000.00000000
205.100.0.0/20 = 11001101.01100100.00000000.00000000
205.100.1.2 = 11001101.01100100.00000001.00000010
Longest match
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Classless Interdomain Routing (CIDR) Advantages:
An organization can “buy” the number of IPs according to its needs (not confined to 256, 66048, 16908288)
Reduce routing table size significantly as multiple “continuous” networks following the same route can be combined to form a single routing entry
E.g. original 4 entries for destinations as137.188.0.0, 137.189.0.0, 137.190.0.0,137.191.0.0 Now, we can combine them into one
entry of137.188.0.0/14
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An Example of How CIDR Is Used
CIDR Reduces the Size of Internet Routing Tables
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CIDR: Partitioning of IP addresses
Q: How does network get network 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 (allocated to ISP). It is divided into 8 equal sized blocks.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
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CIDR 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: “Fly-by-night-ISP requests that all datagrams whose first20 address bits match 200.23.16.0/20. The world doesn’t know thatwithin this there are 8 other orgs. each with their own networks.
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Hierarchical addressing: more specific routes
Suppose Org. 1 dislikes Fly-by-night-ISP’s service and wants to move to ISPs-R-Us? Org.1 keeps its addresses in 200.23.18.0/23 but now ISPs-R-Us advertises 200.23.18.0/23.
Organization 0
“Send me anythingwith addresses beginning 200.23.16.0/20” Internet
“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”
Fly-By-Night-ISP
ISPs-R-Us
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Organization 7
Organization 1
200.23.20.0/23Organization 2
...
...
When other routers see 200.23.16.0/20 & 200.23.18.0/23 and want to route to 200.23.18.0/23 They will use the longest prefix matching rule and send to ISPs-R-Us
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Supernetting – VLSM
In 1987, RFC 1009 specified how a subnetted network
could use more than one subnet mask. When an IP
network is assigned more than one subnet mask, it is
considered a network with (VLSM) since the extended
network prefixes have different lengths.
Allows RECURSIVE division of a network prefix (subnets
of subnets)
Allows detailed structure of routing info for one subnet
group to be hidden from routers in another subnet group
VLSM is different than CIDR because the recursion is
performed on the address space previously assigned to
an org. and is INVISIBLE to the global Internet
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VLSM Design Considerations
When developing a VLSM design, the network designer must recursively ask the same set of questions as for a traditional subnet design. The same set of design decisions must be made at each level of the hierarchy:
1 How many total subnets does this level need today?2 How many total subnets will this level need in the future?3 How many hosts are on this level’s largest subnet today?4 How many hosts will be on this level’s largest subnet be in the future?
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An Example of How VLSM Is Used
VLSM Permits Route Aggregation - Reducing Routing Table Size
Recursive Division
Detailed Structure Of Recursion is hidden
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IP Design Considerations
Addressing (Impact of Subnetting) Routing (Topology dependent) Fragmentation and reassembly
(MTU size for different layers) Datagram lifetime (impact of TTL
setting) Error control (Related to MTU size) Flow control (limited via ICMP)
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IP Futures
In addition to Stevens’ observations: IP version 6 IPSec MPLS IP Multicast These aren’t really futures anymore;
they’re here today Think about what you’d like to
cover in last week Time permitting I’m leaning toward IPSec
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IPv4 addressing - Summary
There are three types of IPv4 addressing environments. Original Classful. Classful and subnet mask (RFC 950). Classless.
An internetwork can be a mix of several environments.
Chapter 4:ARP: Address Resolution
Protocol
To ARP or Not to ARP?That is the question.
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TCP/IP Layering
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Multiplexing and Demultiplexing
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TCP/IP suite: ARP
Maps IPv4 addresses to MAC addresses. An ARP request is a local broadcast. ARP broadcasts are not propagated through
routers. Entries in the ARP table are deleted when a
timeout expires. Several scenarios exist:
You know the IP address to send to and you need the corresponding MAC address (ARP)
You know your MAC address and you need to know your IP address (RARP)
You’re hiding physical networks (Proxy ARP) You need to verify that your IP address isn’t being used
by another host (Gratuitous ARP)
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Packet Delivery on a single LAN
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Use of ARP - Packet Delivery across multiple LANs
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The Purpose of ARP
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An Example:ARP with TFTP
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ARP Frame Format
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ARP Notes ARP generally only occurs on a
single physical network ARP request is a layer 2 broadcast, and
routers block these broadcasts by default
ARP is designed to work for protocols other than IP A generic solution Some other protocols (e.g., IPX) were
designed such that they don’t need ARP
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More ARP Notes Dynamic nature of ARP is very flexible
MAC addresses tend to stay the same, but IP addresses can change (e.g., DHCP, change in logical structure of IP network)
Sometimes MAC addresses can change (e.g., change a broken NIC, administratively change MAC address)
ARP allows for dynamic (re-)mapping What happens if you ARP every time?
Lots of overhead Use a cache mechanism with timeouts
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Notes on Proxy ARP Also known as “promiscuous ARP” Accommodates older TCP/IP stacks. A technique by which a router replies to
an Address Resolution Protocol (ARP) request from a host on behalf of the ARP target host.
Proxy ARP (Address Resolution Protocol) is a technique by which a network host answers to the ARP queries for the network address that it does not have configured on the receiving interface. Proxying ARP requests on behalf of another host effectively directs all LAN traffic destined for that host to the proxying host/router. The "captured" traffic is then typically routed to the destination host via another interface or via a tunnel.
When you see same MAC address in ARP cache for 2 different IP addresses, that’s a hint that Proxy ARP is being used
Proxy ARP can create DoS attacks on networks if misconfigured. For example a misconfigured router with proxy ARP has the ability to receive packets destined for other hosts (as it gives its own MAC address in response to ARP requests for other hosts/routers), but may not have the ability to correctly forward these packets on to their final destination, thus blackholing the traffic.
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Uses of Gratuitous ARP
When a computer starts, a packet is broadcast on the network containing the computer's TCP/IP address to prevent the use of duplicate addresses on the same network
When a computer starts and its Ethernet hardware address has changed due to interface card replacement, a packet is broadcast to other host to signal an update to the IP-to-MAC address mapping
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Issues With ARP
Weak Security a bogus host can issue a gratuitous ARP
and change cache entries on other router’s cache table
a bogus host can send replies giving its own hardware address (instead of the target) – re-directing traffic
Broadcasting can be expensive excessive use of bandwidth CPU costs
Chapter 5:RARP: Reverse Address
Resolution Protocol
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What is RARP For? A workstation without a hard drive
(e.g., diskless workstation, X-terminal, “thin client”) may have no means to “remember” an IP address
However, it will have a NIC that has a MAC address burned-in on an EEPROM
RARP allows this host to broadcast and request it’s IP address A RARP Server must be configured on the
local subnet to assign this particular MAC address with a unique IP address
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Issues with RARP
More difficult to implement than ARP ARP is needed for basic IP
communications and requires no configuration (mostly)
RARP config normally resides in a static text file
Coordination between multiple RARP servers requires that those text files are always in sync
Improvements over RARP BOOTP DHCP – most commonly used today
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TCP/IP suite: DHCP vs. RARP
RARP is based on a table that needs to be configured in the RARP server.
Static, one-to-one address mapping: The same MAC address will always acquire the same IP address.
RARP does provide IP addresses to devices, but there is no much gain in administrative overhead.
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TCP/IP suite: DHCP vs. RARP
RARP is obsolete and almost never seen.
DHCP is implemented in many devices such as Windows NT servers, Novell servers, Cisco routers, NAT boxes…
Although there are plans for DHCPv6, IPv6 has some auto-configuration mechanisms that will probably make DHCP obsolete in the long run.
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TCP/IP suite: DHCP
This reservation mechanism provides a functionality similar to RARP: the IP address is obtained from the server and is always the same.
A typical IP set is a combination of static, reservations, and dynamic DHCP addresses.
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TCP/IP suite: DHCP
DHCP is an evolution of BOOTP Provides the same basic functionality
as RARP, but the underlying mechanism is not the same.
Can provide additional functionality such as the address of the WINS server or the node-type.
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TCP/IP suite: DHCP vs. RARP
Unless a reservation is made, there is no guarantee that a device will obtain the same IP address each time.
Servers are typically configured with static IP addresses.
DHCP does take some of the administrative burden out (for workstations).
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DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address from network server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address
while connected an “on”Support for mobile users who want to join
network
DHCP overview: host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg
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DHCP client-server scenario
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DHCP server
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TCP/IP suite: DHCP vs. RARP
There are some circumstances where the use of DHCP would be nice, but the devices needs a fixed IP address. Example: Print Servers.
The DHCP solution is called reservation. The MAC address of the device is configured in the DHCP server.
Chapter 6:ICMP: Internet Control
Message Protocol
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Where ICMP in The TCP/IP Layering
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What is ICMP?
Every protocol suite needs mechanisms for control and error messaging Phone network to end user: dial tone,
ringing tone, etc. (note: in-band) Phone network between switches: SS7
network for call management (note: out-of-band)
ICMP is the set of messages that handle basic control and error messaging for the TCP/IP protocol suite
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ICMP Message Types
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Key ICMP Uses
Time Stamp Ping Traceroute Source Quench
Indication that flow control needs to be activated at the source
MTU size determination ICMP Destination Unreachability
Can be used to detect malicious port scanning activity
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ICMP Message Transport in IP
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ICMP Message Format
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ICMP Timestamp
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Round Trip Time (RTT) Concept
Chapter 7:ping
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What is ping?
Uses ICMP Echo Request and Reply
Tests reachability – make sure that the network connection is in tact
Don’t use it for fine-grained measurements of network performance
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ICMP packets used for ping
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Ping output
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Ping with IP Options: Record Route
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ping with Record Route
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ICMP Source Quench
When to send an ICMP Source Quench Standards says: when a packet is dropped inside a
router due to depleted buffer space Real life: when ½ of the buffer space is used up
What to do when an ICMP Source Quench is received Implementation dependent Ex: Reset the window size to 0 for n number of ACKS
have been received Security Concern: Source Quench messages
are used by attackers in ICMP flood attacks
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MTU Size Determination
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ICMP Destination Unreachability
16 different categories of Destination Unreachable ICMP messages
Ex: ‘port unreachable’ (type 3, code 3) where a local host requests information from a remote host using TCP or UDP, and the remote host doesn’t have an application listening on the required port. The remote host replies with the type 3, code 3, ICMP messages declaring the problem
Security Concern: These messages outbound will enable an attacker the ability to easily map network topology
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ICMP Port Unreachable – Example msg
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ICMP Port Unreachable - Format
Chapter 8:traceroute
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IP Routing Processes The IP routing processes on all nodes involved in the delivery of an IP packet includes: the sending host, the intermediate
routers, and the destination host.
IP on the Sending Host When a packet is sent by a sending host, the packet is handed from an upper layer protocol (TCP, UDP, or ICMP) to IP. IP on the sending
host does the following: Sets the Time-to-Live (TTL) value to either a default or application-specified value. IP checks its routing table for the best route to the destination IP address.
If no route is found, IP indicates a routing error to the upper layer protocol (TCP, UDP, or ICMP). Based on the most specific route, IP determines the forwarding IP address and the interface to be used for forwarding the packet. IP hands the packet, the forwarding IP address, and the interface to Address Resolution Protocol (ARP), and then ARP resolves the
forwarding IP address to its media access control (MAC) address and forwards the packet.
IP on the Router - When a packet is received at a router, the packet is passed to IP. IP on the router does the following: IP verifies the IP header checksum.
If the IP header checksum fails, the IP packet is discarded without notification to the user. This is known as a silent discard . IP verifies whether the destination IP address in the IP datagram corresponds to an IP address assigned to a router interface.
If so, the router processes the IP datagram as the destination host (see step 3 in the following "IP on the Destination Host" section). If the destination IP address is not the router, IP decreases the time-to-live (TTL) by 1.
If the TTL is 0, the router discards the packet and sends an ICMP Time Expired-TTL Expired message to the sender. If the TTL is 1 or greater, IP updates the TTL field and calculates a new IP header checksum. IP checks its routing table for the best route to the destination IP address in the IP datagram.
If no route is found, the router discards the packet and sends an ICMP Destination Unreachable-Network Unreachable message to the sender.
Based on the best route found, IP determines the forwarding IP address and the interface to be used for forwarding the packet. IP hands the packet, the forwarding IP address, and the interface to ARP, and then ARP forwards the packet to the appropriate MAC address. This entire process is repeated at each router in the path between the source and destination host.
IP on the Destination Host - When a packet is received at the destination host, it is passed up to IP. IP on the destination host does the following:
IP verifies the IP header checksum. If the IP header checksum fails, the IP packet is silently discarded.
IP verifies that the destination IP address in the IP datagram corresponds to an IP address assigned to the host. If the destination IP address is not assigned to the host, the IP packet is silently discarded.
Based on the IP protocol field, IP passes the IP datagram without the IP header to the appropriate upper-level protocol. If the protocol does not exist, ICMP sends a Destination Unreachable-Protocol Unreachable message back to the sender.
For TCP and UDP packets, the destination port is checked and the TCP segment or UDP header is processed. If no application exists for the UDP port number, ICMP sends a Destination Unreachable-Port Unreachable message back to the sender. If no application exists for the TCP port number, TCP sends a Connection Reset segment back to the sender.
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What is traceroute?
A program ported to just about anything with a TCP/IP stack
Shows the path packets take across the network Takes advantage of the ICMP “time
exceeded” message “tracert” in Windows products
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How Traceroute Works
Traceroute creates an UDP packet with the time to live (TTL) in the IP Header set to 1 and addresses the packets set to the destination computer's IP address
Traceroute process waits for a response. This response will be: An ICMP Time Exceeded message - this means the host responding is not the destination. An ICMP Port Unreachable - this means the UDP layer at the destination host responding
doesn't not recognize the UDP port info in the received UDP packet.
The computer on which the messages die because the time to live expired (somewhere between the Source and Destination hosts ) sends back ICMP Time Exceeded (ICMP Type '11') responses. These messages indicate to the soure that the traceroute messages have not yet reached the destination host
The source increments the TTL in the IP Header by one, then repeats steps the previous six steps (creates 3 packets, sets the Time to Live to the next highest number, starts a timer, transmits the packets, waits for a response). This process is repeated until the packets reach the destination computer which the source host is tracing the route to.
When the ICMP message reaches the destination computer, the UDP layer will get to process it and will find out that the UDP port specified is invalid which will trigger an ICMP Port Unreachable message back to the source host.
The Port Unreachable error message indicates to traceroute that the destination has been reached.
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ICMP time exceeded message
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How to read traceroute output
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