cmpt 371

© Janice Regan, CMPT 128, 2007-20121

CMPT 371Data Communications and Networking

Network LayerICMP and fragmentation

© Janice Regan, 2007-2012 2

IPv4 Protocol Header

Protocoltransport layer

Headerchecksum

TTL

Source IP address

Destination IP address

Options(if any)

Flags.

Identifier.

Datagram length (bytes).

13 bit fragmentation offset

Header Length

Version#

Type of service

Padding.

0 4 8 16 32 bits

© Janice Regan, 2007-20123

Header Fields (1) Version(4 bits): Now 4 (IP v6 being

phased in) Internet header length or IHL( 4 bits):

length of IP header in 32 bit words. Minimum is 20 octets, so header length would be at least 5. Used to locate the start of the payload

Type of service (8 bits): Contains bits to set priority (0 lowest to 7 highest) and to select routing based on optimization of reliability, precedence, delay or throughput parameters (TOS replaced by Differential Services)


Header Fields (2) Total length (16 bits): This includes the header and the

data payload. Packet length is measured in octets. Maximum length of a packet is 216 -1 = 65,535 octets

Identification (16 bits): Identifies a particular datagram or packet. The same Identification is used for each fragment of a fragmented datagram. The final receiver will use the Identification for reassembly

Flags(3 bits): More bit, Don’t fragment bit, third bit is undefined


Header Fields (3) Fragmentation offset (13 bits): Position of the

fragment in the present packet within the unfragmented payload. (Must be a multiple of 64 bits from start of the unfragmented payload)

Time to live (8 bits): Measured in seconds, but must decrement by at least 1 at each intermediate station. Since transmission time in modern system are very rarely in excess of one second this is essentially a hop counter (Default 64)

Protocol (8 bits): protocol of next higher layer (transport layer) to receive data field at destination


Header Fields (4) Header checksum (16 bits): ones complement of ones

complement sum of all 16 bit words in header (assuming header checksum is zero). Reverified and recomputed at each intermediate station (IS) because TTL, some options and some other fields may change at each intermediate station. IP packet is discarded if checksum does not match. Note this is a checksum of the header not the entire datagram.

Source address (32 bits): IP address of originating station

Destination address (32 bits): IP address of final destination


Header Fields (5) Options (variable):

Security, Strict source routing (specify all ISs), Loose source routing (Specify some ISs), record route (records address at each hop), timestamp (records address and timestamp at each hop)

Padding (variable) To fill to multiple of 32 bits long after options added

Data field Carries user data from next layer up Integer multiple of 8 bits long (octet) Max length of datagram (header plus data) 65,535 octets


IP Fragmentation (1) Uses fields in header

Data Unit Identifier (ID): uniquely identifies end system originated datagram and contains Source and destination address Protocol layer generating data (e.g. TCP) Identification supplied by that layer

Data length: Length of user data in octets (datagram length – header length)

Fragmentation Offset: Position of fragment of user data in original datagram, (offset from start of original datagram) in multiples of 64 bits (8 octets)

More flag: Indicates that this is not the last fragment


IP Fragmentation (2) Copy the header frame of the incoming

datagram into each fragment Divide the incoming user data field into

equal parts along 64bit boundaries (last fragment may be shorter). Length of each equal part is MTU

For of each datagram except the last, set Data Length to the length of each data fragment and set more flag to 1. Add the length of the previous data segment in octets to the Offset.

For the last datagram set Data Length to the length of the remaining data, Add the length of the previous data segment in octets to the Offset.

Fragmentation: example 1

offset=0 more=1

offset=208/8=26 more=1

offset=52 more=0

© Janice Regan, 2007-2012 10

IP header

IP header

IP header

UDP header

IP data 612 octets

UDP header

208 octets

IP header

208 octets

196 octets

Fragmentation: example 1

offset=0 more=1

offset=600/8=75 more=1

offset=150 more=0

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IP header

IP header

IP header

TCP header

IP data 1400 octets

TCP header

600 octets

IP header

600 octets

200 octets

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Dealing with Failure Re-assembly may fail if some fragments

get lost Need to detect failure Re-assembly time out

Assigned to first fragment to arrive If timeout expires before all fragments arrive,

discard partial data Use packet lifetime (time to live in IP)

If time to live runs out, kill partial data Fragmentation is needed but Don’t

fragment bit is set: packet is dropped


Dealing with IP errors IP datagram delivery (network level) has a header

checksum to detect transmission errors in the IP header TCP has a checksum which covers the TCP header,

pseudo header and data Higher level protocols (for example TCP) also handle

more types of errors Higher level protocols need to be informed that the

errors have occurred in order to handle them more efficiently that with only timeouts and retransmissions

Within IP need an error reporting mechanism to report such errors to TCP, one such mechanism is the ICMP protocol.

Why fragment The maximum size of a TCP (or UDP) segment can be configured

for each network Therefore different networks can have different MSS (maximum

segment size) When a segment from a network with a larger MSS must pass

through a network with a smaller MSS the segment must be broken into parts small enough to pass through the network with the smaller MSS

The parts will be reassembled at the destination any intermediate network may have multiple routers, not all segments will leave the intermediate network through the

same router, so they cannot be reassembled as they leave the intermediate network

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Errors in Packet Switching Networks Possible causes of errors include

Hardware failure Network congestion Inability to fragment (DF set) Routing loops Unavailable host (disconnected or failed) Queue overrun on routers

IP offers best effort delivery, it needs a mechanism to inform the source of packets dropped because of errors (except transmission errors). In the remainder of this lecture errors will mean errors not cause by transmission impairments


ICMP encapsulation

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ICMP message format Internet Control Message Protocol

(RFC792) There are several types of ICMP

messages designed to report different types of errors

Each ICMP message has its own format, but all start with the same three fields A type field (1 octet) indicating which type of

ICMP message follows A 1 octet code following the type that further

defines the message (see text for list) For example type specifies destination

unreachable, code specifies router or host The 3rd common field is a 2 octet checksum.

© Janice Regan, 2007-2012 20

What ICMP messages do Time exceeded

Lifetime of datagram expired Parameter problem

IP header parameter cannot be interpreted Timestamp/timestamp reply

Measures transmission time (not time in queue) for strict source routing Redirect message

Sends router a ‘better’ route to the destination that it currently has Router discovery / solicitation

Can be used by a booting host on the network to find the routers wait for discovery message which lists routers and is periodically

transmitted ask for immediate discovery message by sending a solicitation message

21© Janice Regan, 2007-2012

ICMP Echo Request/Reply Echo request is sent by the ping command to test for

reachability Echo reply is sent in response to a received echo reply

to confirm reachability Type: request 8, reply 0, Code 0 : no additional

qualifying codes Identifier and sequence number are optional, they can

be used to match replies with requests The optional data in a echo request must be returned in

the resulting echo reply Linux ping has a record route and a timestamp option

traceroute The unix traceroute command is an example of the use

of the time exceeded message A UDP packet with a hop count of 1 is sent The first router reached sends back a time exceeded

message, identifying itself A packet with a hop count of 2 is sent The second router in the path sends back a time

exceeded message identifying itself This is repeated, incrementing the hop count by 1 until

the packet reaches its destination Ubuntu sends 3 copies of each packet sent in the description above

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ICMP destination unreachable Sent when a router or host cannot deliver a

datagram due to an identified failure (not all failures are identified)

Can be disabled, not all hosts or routers will send ICMP messages

The codes indicate what destination could not be reached and why (see table in text)

The header and datagram information is provided to identify the packet needing retransmission (port numbers and sequence number for TCP UDP)

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ICMP Source Quench Message Used to help control congestion When a packet must be dropped due to

congestion a source quench packet may be sent to the source

When the source receives a source quench message it may reduce the rate at which it transmits to the network 1 quench message per round trip travel time

should cause change

cmpt 371

Documents

length of ip header

bits janice regan

header fields 1version4

header checksum

reassemblyflags3 bits

packet length

length of user data

fragmentation janice