Lecture 3: IPv4 cont’d, ICMP, and UDP - KTH · lecture_3 Lecture 3: IPv4 cont’d, ICMP, and UDP ... – Data arriving before a process is ready to accept is placed in a ... lecture_3.

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Lecture 3: IPv4 cont’d, ICMP, and UDP

Literature:Forouzan, TCP/IP Protocol Suite: Ch 8-9, 11

Internetworking

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IPv4 Options• IPv4 options are intended for network testing or debugging• Options are variable size and comes after the fixed header. • Contiguous – no separators• Not required fields, but all IP implementations must include

processing of options– In practice many implementations do not!

• Max 40 bytes - very limited use– Max header length is 60 bytes (fixed part is 20 bytes)

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IP Options Encoding• Two styles

– Single byte (only code)– Multiple byte

• Option Code: 1 byte– Copy (to fragments) (1 bit)– Class (2 bits)

• 0 (00): Datagram or network control

• 2 (10): Debugging and measurement

– Number (5 bits)

• Option Length (len): 1 byte, defines total length of option (including code and len fields)

• Data: option specific

code len data

copy class option number

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Categories of IP Options

©The McGraw-Hill Companies, Inc., 2000

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IP options types

Strict source route9Record route7

Timestamp4

Loose source route3

No option1

End of option0

OptionNumber

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IP Options: Record Route• Each router records its address • The destination processes the trace

– E.g. sends the result back to the sender

• Pointer is ”next available slot”• Source creates an empty list• Every router adds its address.

– Increments pointer

• Limited to nine hops – IP header size limit

len pointercode

First IPv4 addr

Second IPv4 addr

...

First IPv4 addr

Second IPv4 addr

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IP Options: Record Route Example

Note that pointer is an index, starting with code at index 1


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IP Options: Source Route• The sender dictates a path through the network• Strict Source Routing

– The path is exactly as specified.

• Loose Source Routing– Allows multiple hops between successive

addresses.

• The routers records their addresses– Just like record route

len pointercode

First IPv4 addr

Second IPv4 addr

...

First IPv4 addr

Second IPv4 addr

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IP Options: Source Route Example


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IP Options: Timestamp• Similar to record route, but also adds a timestamp• Source creates an empty list• Every router adds its address and a timestamp.• An overflow field (O-flow) specifies how many routers could

not specify a timestamp.• A flags field specifies the visited router responsibilities

– E.g., add only timestamp or add timestamp + outgoing IP address

Code: 68 Length (total) Pointer O-flow Flags

Data

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IP Options: Timestamp cont’d


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ICMP

Internet Control Message Protocol - RFC 792

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ICMP• ICMP is a signalling protocol for IPv4.

– Report IP problems back to sender

– Control and Management

– Considered a part of IP, but uses IP for transfers.

• Query ICMPs– Control purposes

– Examples: Echo, Router advertisement, Timestamp, etc.

• Error ICMPs– Sent when an error in IP detected

– Includes the first 8 bytes of the data field of the original datagram which caused the error.

– Not sent for: icmp errors, broadcasts, fragments, etc . – Examples: Dest unreachable, Redirect, etc.

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ICMP Messages

Redirection5Parameter problem12Time exceeded11Source quench4Destination unreachable3

MessageType

Router solicitation/advertisement10/9Address mask request/reply17/18Timestamp request/reply13/14Echo request/reply8/0

MessageType

ICMP messagesICMP messages

Error-reportingError-reporting QueryQuery

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General Format of ICMP Messages

• Type: specifies type of message• Code: specifies reason for the particular message type


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ICMP Header

• ICMP Header varies depending on type– Example: ICMP Destination Unreachable (type 3)

type (3) code (0-15) checksum

IP header (including options) + first 8 bytes of original IP data

Unused (all 0s)

• ICMP error messages returns original IP datagram• Original IP header (+ options) and 8 bytes of payload

• Example: ICMP Destination Unreachable (UDP packet)

Ethernetheader

IPheader

ICMPheader

IP header of datagram that generated the error

UDPheader

ICMP Message

0 7 8 15 3116

Common for all types

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ICMP Error Reporting• One of the main responsibilities of ICMP

– Recall that IP is an unreliable protocol, and errors may occur

• ICMP does not correct errors– Left to higher level protocols

• Error messages are always sent back to the original source– Because the only information available in the datagram about the

route is the source and destination IP addresses

• ICMP uses the source address of the IP packet to send the error message back to the source (originator)

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ICMP Error RestrictionsAn ICMP Error is not returned in response to:

– A datagram carrying another ICMP Error

– A datagram destined to IP broadcast or multicast address

– A datagram sent as link-layer broadcast (e.g., Ethernet)

– An IP fragment other than the first

– A datagram whose source address does not define a single host (e.g., 0.0.0.0)

• Reason is the risk of creating:– Loops

– Packet explosions (broadcast storms)

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ICMP Error Reporting Messages


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ICMP Destination UnreachableDifferent types (Code 0-15):

• Code 0 – network unreachable– Returned by routers

• Code 1 – host unreachable– Returned by routers

• Code 3 – port unreachable– Returned by hosts when UDP/TCP port does not

exist

• ... (Code 0 – 15 are defined)

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ICMP Source Quench Error• Attempt to add a kind of flow control to IP!• ICMP source quench may be generated if the system

receives data faster than it can process it• New Router Requirements RFC: routers should not

generate source quench errors– Consumes network bandwidth

– An ineffective and unfair fix for congestion

• Thus, for reliability you have to do end-to-end (transport level) flow control, error checking, and use acknowledgements– TCP

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ICMP Time ExceededThis type (11) of error message is sent in 2 cases• Code 0: when TTL is zero after decrementation, the router

discards the datagram and sends an ICMP Time Exceed back to the source

• Code 1: when all fragments of a datagram do not arrive at the destination host within a certain time limit– Timer is started at reception of first fragment

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Tool Using ICMP: Traceroute• Traceroute traces a path to a destination by exploring every

IP hop on the way– Note: only receiving interfaces are traced, not sendig.

• Traceroute algorithm uses two steps:1. Set small TTL fields and receive ICMP time exceeded incrementally

2. When final host reached, use unlikely UDP port and get ICMP portunreachable back

• Alternative: use an IP datagram with record route option– But this is not always implemented

– Limited number of hops can be traced due to maximum size of IP options

– (Record route records IP addresses if outgoing interfaces)

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ICMP Parameter Problem• Code 0: Main IP header field problem

– Pointer points to byte with problem

• Code 1: Problem in IP option field– Pointer not used

• IP routers and hosts do sanity checks on IP header


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ICMP Redirect – concept • ICMP Redirect is sent by a router (R1) to the sender of an

IP datagram (host) when the datagram should have been sent to a different router (R2)

(1) IP datagram

(2) IP datagram

(4) Subsequent IP datagrams

(3) ICMP Redirect

R2R1

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ICMP Redirect – message format• Code 0: Redirection for network specific route• Code 1: Redirection for host specific route• ...


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ICMP Query Messages


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Echo Request and Reply

• Can you think of a widely used program that uses ICMP Echo request/reply?


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Ping uses ICMP Echo Request/Reply• Ping tests host reachability. • Uses ICMP echo request/response,

– Almost all IP implementations support Ping server.

• Sends an ICMP echo request to a node• Server replies with ICMP echo response• With IP record route (RR) option, the route of the ping

datagram can be traced

...ICMP Echo Respond

ICMP Echo Request

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Timestamp Request and Reply

• Can be used to calculate round-trip time– Even if clocks are not synchronized

• Can be used to synchronize clocks if one-way duration is known


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Address-Mask Request and Reply

• Can be used by diskless clients to find out the address mask– Recall that RARP gives only the IP address

• Not used very much– Other protocols (DHCP, BOOTP) are used for autoconfiguration

(later lecture)


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Router Solicitation/Advertisement

• On booting, hosts send ~3 ICMP router solicitation messages (~3 seconds apart) to find a default router. – Dynamic discovery of the default router.

• Routers periodically broadcast or multicast advertisements of their existence and desire to provide routing service

• Advertisements typically every 450 - 600 seconds

• Advertisements have a stated lifetime (typically 30 minutes)• Seldom implemented

– Mobile IP discovery mechanisms

– IPv6

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ICMP Summary• Destination Unreachable

– Network/Host/Protocol/Port/...

• Time Exceeded – TTL expired– Used in the traceroute tool

• Parameter problem– IP header error

• Source Quench – Requests source to decrease its

data rate.

• Redirect – Tell source to send its

messages to a “better address”

• Echo Request/Echo reply– For testing (e.g., “ping” program

sends an Echo request)

• Timestamp Request/Reply– Clock synchronization– RTT

• Address Mask Request/Reply– Diskless systems

• Router Solicitation and Advertisment– Hosts query routers– Routers advertise presence and

routes

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IP and ICMP Summary• IPv4 is engineered to solve problems encountered at the

network level• Each field in the IPv4 header addresses a networking issue

– Logical addressing– Different L2 characteristics (MTUs)– QoS– Bit errors– Multiplexing

• The control and error mechanism of IP is provided by ICMP• IPv4 is a very successful protocol, but there are many flaws

and unused features– IPv6 has cleaned up the IP layer considerably

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Transport Layer

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Transport LayerResponsible for end-to-end delivery of entire messages – Service-point addressing (Protocol Port or Port Number)

• Address the specific running process on a computer

– Segmentation and Reassembly

• Divide message into transmittable segments and reassemble message at receiver

– Connection Control

• For connection-oriented transport protocols

– End-to-end Flow Control (in contrast to link level flow control)

– End-to-end Error Control (in contrast to link level error control)

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TCP/IP Transport Layer Protocols2 transport layer protocols in the TCP/IP stack• UDP – User Datagram Protocol

– Connectionless unreliable service

• TCP – Transmission Control Protocol– Connection-oriented reliable stream service

Telnet FTP DNS. . .TCP UDP

IPICMP ARP

Underlying link technology

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Protocol PortsUltimate source/destination of/for a transport level message is a protocol

port

• A process sends/listens to a protocol port (identified with an integer)

• Most operating systems provide synchronous access to ports– A process gets blocked if it attempts to extract data from a port prior to

arrival of data

• In general, ports are buffered

– Data arriving before a process is ready to accept is placed in a (finite) queue

• To communicate with a port, sender needs to know both the IP address of the machine and the protocol port number within the machine

• The combination of an IP address and a port number is called a socket

• Each message must carry destination port and source port

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Port Numbers in Three Groups

• Servers are normally known by their well-known port number (e.g., 80 for HTTP). Assigned and controlled by IANA

• Dynamic ports are ephemeral and can be used by any process (normally used by client processes)

PurposeRange

Dynamic ports49152 .. 65535

Registered ports1024 .. 49151

Well-known ports0 .. 1023

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UDP

User Datagram Protocol - RFC 768

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UDPUDP – User Datagram Protocol

• Datagram-oriented transport layer protocol• Provides connectionless unreliable service

• Provides optional end-to-end checksum covering header and data

• Provides no feedback to control data rate• An UDP datagram is silently discarded if checksum errors

• UDP messages can be lost, duplicated, or arrive out of order

• Application programs using UDP must deal with reliability problems– DNS, DHCP, SNMP, NFS, VoIP, etc. use UDP

– An advantage of UDP is that it is a base to build your own protocols on

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UDP Message Format

16 bit source port number 16 bit destination port number

16 bit UDP length 16 bit UDP checksum

data (if any)

0 15 16 318 byte header + possible data

UDP dataUDP headerIP header

20 bytes 8 bytes 0..(216 – 1) – (20 + 8 ) = 65507 bytes

UDP datagramIP datagram

• UDP length field is redundant, since the IP software can pass this info to UDP

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UDP Checksum and Pseudo-header• UDP checksum covers

– application data, UDP header,a pseudoheader, and pad byte (if needed)

• Purpose with pseudo-header:– double-check that packet arrived to correct destination

– check that IP delivered the packet to the correct protocol (UDP/TCP)

• Pseudoheader and pad byte not transmitted, only used for computation

32-bit src IP addr

32-bit dst IP addr

All 0s 8-bit protocol 16-bit UDP total length

16-bit src port number 16-bit dst port number

16-bit UDP total length 16-bit checksum

Data, padded to multiple of 2 bytes (16 bits)

Pseudoheader

UDP header

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Maximum UDP Datagram Size• Theoretical limit: 65,535 bytes - due to (IP’s) 16-bit total length field

– with 20 bytes of IP header + 8 bytes of UDP header ⇒ 65,507 bytes of user data

• Two limitations:

– sockets API limits size of send and receive buffer; generally 8 kbytes, but you can call a routine to change this

– TCP/IP implementation - Stevens found various limits to the sizes - even with loopback interface

• Hosts are required to handle at least 576 byte IP datagrams lots of protocols limit themselves to 512 bytes or less of data to avoidfragmentation

– DNS, TFTP, BOOTP, and SNMP

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UDP Summary

• Transport Layer Basics

– end-to-end delivery of messages

• UDP

– a fairly simple connectionless protocol

Lecture 3: IPv4 cont’d, ICMP, and UDP - KTH · lecture_3 Lecture 3: IPv4 cont’d, ICMP, and UDP ... – Data arriving before a process is ready to accept is placed in a ... lecture_3.

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