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 <A.Ballardie@cs.ucl.ac.uk>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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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

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DHCP server

arriving DHCP client needsaddress in thisnetwork

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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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