UDP : User Datagram Protocol

2005. 4. 25백 일 우

steigensonne@hufs.ac.kr

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Concept Introduction UDP Header Format Pseudo-header IP Fragmentation MTU discovery using UDP Interaction Between UDP and ARP Maximum UDP Size ICMP Source Quench Error

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Introduction Connectionless Oriented Service

Out of Order Just Information Application process location Info of Two hosts No service for Reliability

When UDP? For Not reliability, but SPEED : Realtime Multimedia Transmission

Not Needed to wait for ACK Can communicate to MANY : Multicast, Broadcast For periodic Transmission

Routing Protocol should communicate adjacent routers to spread info periodically (every 30 secs)

Very inefficient for TCP to do as above Simple And Light

So make CPU & memory light Good to do service for the unspecific

DNS, WINS service could be good examples to this

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UDP Header Format

IP Header UDP Header UDP Data

UDP datagram

IP datagram

20 Bytes 8 Bytes

16-bit source port number 16-bit destination port number

16-bit UDP length 16-bit UDP checksum

data( if any )

0 15 16 31

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UDP Header Format – Cont’ UDP length = UDP Header + UDP data UDP checksum

Range : UDP header + UDP data IP checksum : IP header

Unit : 16 bit words The shortage is filled with Padding 0

When Checksum, Must Include pseudo-header Purpose : UDP double-check

If Calculated Checksum 0, Set 1 in Checksum field If Checksum field is 0 at Receiver, means that Sender did not

calculate checksum Checksum Error detected at receiver, Discarded immediately

Not generated error message

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Pseudo-header

16-bit source port number 16-bit destination port number

16-bit UDP length 16-bit UDP checksum

data( if any )

0 15 16 31

32-bit source IP address

32-bit destination IP address

zero 8-bit protocol(17) 16-bit UDP length

Pad byte(0)

UDP pseudo header

UDP header

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IP Fragmentation When fragment,

Flags bits field uses one bit as the “more fragments” bit Fragment offset field contains the offset of this fragment from the

beginning of the original datagram Total length field of each fragment is changed to be the size of that

fragment size Surely, “don’t fragment” bit set

If this filed tuned off and transmitted exceeded size than MTU, ICMP error would be generated.

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Example - Fragmentation Test

Send data size : 1471, 1472, 1473, 1474

offsetsizeID

UDP header+ UDP data

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Example - Fragmentation

Test From 220.67.124.138 to www.naver.com

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MTU discovery using UDP

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MTU discovery using UDP - Cont’FAIL

FAIL-again

Retry-frag’OK

FAIL ?

> IP turns on the DF bit to see if the path MTU has increased, every 30secs

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RFC 1191 Says,.. 30 secs is too small, RFC 1191 recommends a value of

10mins Can be changed by modifying the parameter ip_ire_pathmtu_interval

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Interaction Between UDP and ARP

If we send UDP packet (size : 8192),.. Would generate six fragments on an ethernet

1480*5(7400) < 8192 < 1480 * 6(=8880) Make ARP cache be empty before run

IP datagram

IP Header UDP Header UDP Data(1473 bytes)

20 Bytes 8 Bytes

IP Header UDP Header

1472 bytes20 bytes 8 bytes

IP Header

20 bytes 1 bytes

packet packet

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Interaction Between UDP and ARP

6 ARP request (before the first reply returned) -> 6 fragment’s request When first ARP reply received (line 7) only the last fragment sent (line

9) It appears that first five fragments have been discarded Most implementation keep only the last packet sent to a given destination

while for an ARP reply

Figure 11.17 Packet exchange when an 8192-byte UDP datagram is sent on an Ethernet

verify ARP cache is empty

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Interaction Between UDP and ARP (con’t)

ICMP “time exceeded during reassembly” error IP layer must start a timer when the first fragment appears (first

fragment ≠ fragment offset of 0) Normal timeout value is 30 or 60 seconds If all the fragments for this datagram have not arrived when the

timer expires, all these fragments are discarded Why don’t see the ICMP message here?

Most Berkeley-derived implementations never generate this error The first fragment-the one with an offset of 0 containing the UDP

header-was never received Couldn’t tell which user process sent the datagram that was discarded

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Interaction Between UDP and ARP The Host Requirements RFC says,..

Require an implementation to prevent this type of ARP flooding That is, Repeatedly sending an ARP request for the same IP address

at high rate Recommended rate is one per second

Here we see six ARP previous in 4.3ms(approximately 0.7ms) Too fast than recommended

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Interaction Between UDP and ARP

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Maximum UDP Datagram Size Theoretically, 65535 bytes, imposed by the 16-bit total length

field in the IP header, but Most provide less than that First, Application may be limited by its programing interface

The socket API provide a function that application can call to set the size of the send and receive buffer

Most system provide a default of just over 8192 bytes (Default NFS read and write)

Second, the kernel’s implementation of TCP/IP Whether it is feature or bug,..

Limit the size of an IP datagram to less than 65535 bytes

A host is required to receive at least a 576-byte IP datagram Many UDP applications are designed to restrict their application data

to 512 bytes or less, to stay below this

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ICMP Source Quench Error Receives datagrams at a rate that is too fast to be

processed is not always not required to send a source quench , even if it runs out of buffers

and throws datagrams away

Figure 11.18 ICMP source quench error

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ICMP Source Quench Error (con’t) BSD implementations normally ignore received source

quenches if the protocol is UDP What Problem?

process already terminated when the source quench is received UDP is an unreliable protocol

Possible discarded by the router Need for acknowledgment into the application

Unless you do not as above, the sender has no idea whether the receiver really got the data

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ICMP Source Quench Error (con’t) RFC 1009[Braden and postel 1987] requires router to

generate source quenches when it runs out of buffers

But new Router Requirements RFC[Almquist 1993] changes this and says that a router must not originate source quench error since it consumes network BW and is an inefficient and unfair fix

for congestion