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ECE453 – Introduction to
Computer Networks
Lecture 12 – Network Layer (IV)
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IP Datagram Format
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An Example (tcpdump)
4500 0054 0000 4000 3401 eb82 982d 0469 a024 1e6c 0800 57a3 ce1b 0000
||| | | || | | | | | | | ||| | | || | | | 32-bit source IP: 152.45.4.
||| | | || | | Header checksum (16 bits)
||| | | || | Upper layer protocol: 01
||| | | || TTL
||| | | |13-bit fragmentation offset
||| | | 3-bit flag
||| | identifier
||| datagram length:
||type of service (1 byte) ||header:4*5=20bytes (4 bits)
V4 (4 bits)
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Analyze Network: tcpdump
You need root privilege
-i : listen to a specific interface, e.g., eth
-w: write the raw packet to a file rather than print
them out, -r can be used to read packet from a file
-s num: get num bytes of data from each packet rather
than the default value of 68
src host xyz: true if IP destination field of the packet is
xyz
dst host xyz: true if IP source field of the packet is xyz
ip proto xyz: true if the packet is an IP packet and
protocol type is xyz
Example: /usr/sbin/tcpdump src host 152.45.4.11 and
icmp
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IP Fragmentation & Reassembly
Link-layer protocols can only carry packets of a limited size Different link-layer protocols may carry packets of different size Ethernet: 1,500 bytes Others: 576 bytes MTU: maximum transfer unit 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 datagram out: 3 smaller datagram
reassembly
IPv6 doesn’t allow fragmentation at routers
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IP Fragmentation and
Reassembly
ID
=x
offset
fragflag
length
ID
=x
offset
fragflag
length
ID =x
offset
fragflag
length
ID =x
offset
fragflag
length
One large datagram becomes several smaller datagrams
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ICMP Message Format
Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header
Type 8,0Code 0 checksum
identifier Sequence #
Optional data (header plus 64 bits)
…
Echo request / reply
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Test Reachability (Ping)
A host or router sends an ICMP echo request
message to a specified destination
Any machine that receives echo request must
formulate an echo reply message and send to
sender
Successful receipt of a reply verifies that
major pieces of transport system work
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Traceroute
traceroute: Warning: cn.yahoo.com has multiple addresses; using 61.135.128. traceroute to cn.yahoo.com (61.135.128.50), 30 hops max, 38 byte packets 1 r6hm01v163.ns.utk.edu (160.36.30.1) 1.373 ms 0.332 ms 0.322 ms 2 bsm01v200.ns. utk .edu (160.36.1.104) 0.417 ms 0.515 ms 0.393 ms 3 atl-edge-19.inet. qwest .net (216.207.16.33) 5.452 ms 5.547 ms 5.484 ms 4 atl-core-03.inet.qwest.net (205.171.21.125) 5.486 ms 5.688 ms 5.520 ms 5 atl-core-01.inet.qwest.net (205.171.21.153) 5.836 ms 5.905 ms 5.830 ms 6 iah-core-03.inet.qwest.net (205.171.8.145) 25.322 ms 25.348 ms 25.325 ms 7 iah-core-02.inet.qwest.net (205.171.31.41) 25.321 ms 25.419 ms 25.299 ms 8 bur-core-01.inet.qwest.net (205.171.205.25) 56.697 ms 56.746 ms 56.713 ms 9 lax-core-01.inet.qwest.net (205.171.8.41) 57.019 ms 57.058 ms 57.022 ms 10 lax-brdr-01.inet.qwest.net (205.171.19.38) 57.064 ms 57.099 ms 57.020 ms 11 202.97.48.65 (202.97.48.65) 264.265 ms 259.337 ms 257.330 ms 12 202.97.51.193 (202.97.51.193) 492.494 ms 470.912 ms 464.106 ms 13 p-13-0-r1-c-bjbj-1.cn.net (202.97.33.9) 958.715 ms 1012.859 ms 1016.328 ms 18 202.108.61.2 (202.108.61.2) 298.953 ms 293.484 ms 300.453 ms 19 cn.yahoo.com (61.135.128.50) 1908.846 ms 1892.476 ms 1953.833 ms
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ARP: Address Resolution
Protocol
Each node on LAN has ARP module,
maintaining ARP table
ARP Table: IP/MAC address mappings for
some LAN nodes
< IP address; MAC address; TTL>
TTL (Time To Live): time after which address
mapping will be forgotten (typically 20 min)
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ARP protocol
A knows B's IP address, wants to learn physical
address of B
A broadcasts ARP query pkt, containing B's IP
address
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's)
physical layer address
A caches (saves) IP-to-physical address pairs until
information becomes old (times out)
soft state: information that times out (goes away) unless refreshed
/sbin/arp
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Example
/sbin/arp
[hqi@panther hqi]$ /sbin/arp Address HWtype HWaddress Flags Mask Iface panda.ece.utk.edu ether 00:C0:4F:2D:81:29 C eth lion.mail.utk.edu ether 00:D0:04:77:4F:FC C eth miranda.org ether 00:D0:04:77:4F:FC C eth ns0.utk.edu ether 00:D0:04:77:4F:FC C eth
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IPv
Initial motivation: 32-bit address space
completely allocated by 2008.
Additional motivation:
header format helps speed
processing/forwarding
header changes to facilitate QoS
The concept of flow
new “anycast” address: route to “best” of
several replicated servers
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IPv6 Header
ver (^) length
data (variable length, typically a TCP or UDP segment)
16-bit identifier Internet checksum
time to live 32 bit source IP address
head. len
type of service flgs
fragment offset upper layer
32 bit destination IP address
Options (if any)
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Other Changes from IPv
Length field: fixed-length 40 byte header
No fragmentation allowed
Checksum: removed entirely to reduce
processing time at each hop
Options: allowed, but outside of header,
indicated by “Next Header” field
ICMPv6: new version of ICMP
additional message types, e.g. “Packet Too Big”
Subsumes multicast group management functions
(IGMP – Internet Group management Protocol)
“Unrecognized IPv6 option”
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Transition From IPv4 To IPv
Flag day?
Dual stack
some routers with dual stack (v6, v4) can
“translate” between formats (IPv6/IPv
nodes)
Tunneling:
IPv6 carried as payload of IPv4 datagram
among IPv4 routers
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Dual Stack Approach
IPv6/IPv4 nodes must have both IPv6 and IPv
addresses
Be able to determine whether another node is IPv6-
capable or IPv4-only
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Tunneling
IPv6 inside IPv4 where needed
