datagram fragmentation, icmp & ipv6cis.k.hosei.ac.jp/~jianhua/course/com/lecture10.pdf ·...
TRANSCRIPT
Datagram Fragmentation, ICMP & IPv6
• IP Datagram Encapsulation
• Network Maximum Transmission Unit (MTU) • IP Datagram Fragmentation • ICMP (Internet Control Message Protocol) - Error Report Mechanism - Information Query Mechanism - ICMP Message format and Transmission - ping and traceroute Utilities
• IPv6 - IPv6 Features - IPv6 Header and Format - IPv6 Address
Lecture 10
Lecture 10
Internet Transmission Paradigm
• Source host - Forms datagram with destination address - Sends to nearest router • Intermediate routers - Forward datagram to next router • Final router - Delivers to destination host
Note: Datagram must be passed to network interface & sent across physical network. Network hardware does not recognize IP datagram format and IP address !! How is datagram transmitted across physical network ?? Address Resolution (ARP) and Encapsulation !!
network network network network
router router router Source host
Destination host
IP D IP D IP D IP D IP D
Routing Table --------- ## *** …………
Routing Table --------- ## *** …………
Routing Table --------- ## *** …………
Routing Table --------- ## *** …………
Routing Table --------- ## *** …………
IP Datagram Encapsulation
Lecture 10
Frame Header Frame Data
IP Header IP Data Area
• Entire datagram treated like data encapsulated in a frame for transmission
• Frame type (0800 for Ethernet) identifies contents as IP datagram
• Frame destination address gives next hop
• Next hop Frame/Hardware Address is obtained by address resolution protocol (ARP)
• IP address will not be changed while frame address is different in different network
IP Datagram/Packet
Hardware Network Frame/Packet
Ethernet Frame
Encapsulated into a frame/packet in lower layer
Lecture 10
Encapsulation Across Multiple Hops
• Each router extracts datagram, discard frame, determines next hop via ARP, encapsulates datagram in outgoing frame
• Frame headers may differ depended upon network types • Datagram survives in entire trip, but frame only survives one hop
Animation
Lecture 10
Maximum Transmission Unit (MTU)
• Every hardware technology specification includes the definition of the maximum size of the frame data area - called maximum transmission unit (MTU) • IP datagrams can be larger than most hardware MTUs - IP: (216 – 1) bytes = 64K bytes - Ethernet: 1500 bytes - Token ring: 4464 bytes - FDDI: 4352 bytes - X.25: 576 bytes - PPP: 296 bytes (Point-to-Point Protocol) • Any datagram encapsulated in a hardware frame must be smaller than the MTU for that hardware • An internet may have networks with different MTUs
Ethernet Frame
Lecture 10
Datagram Fragmentation
• Fragmentation: a technique to limit datagram size to smallest MTU of any network • IP uses fragmentation – split datagrams into pieces to fit in network with small MTU • Router detects datagram larger than network MTU - Splits into pieces called fragments - Each piece smaller than output network MTU • Each fragment has datagram header and is sent separately • Ultimate destination reassembles fragments
Fragmentation Fragmentation No-fragmentation Assemble fragments
No-assemble No-assemble
> MTU
Each <= MTU Fragment 1 Fragment 2 Fragment 3
Network links have MTU
- Different link types
with Different MTUs
* 1500 bytes for Ethernet
* 296 bytes for PPP
large IP datagram divided (“fragmented”) within net
one datagram becomes several datagrams
“reassembled” only at the final destination
IP header bits used to identify, order related fragments
Fragmentation: in: one large datagram out: 3 smaller datagrams
Reassembly
Lecture 10
Datagram Fragmentation & Reassembly
Lecture 10
Fragment Related Fields in IP Header
Identification - Datagram ID - 16 bits counter Flag - Signal fragment. - 3 bits, ABC A: reserved B: 1 – no fragment 0 - fragmented C: 1 - not last fragment 0 - last fragment Fragment offset - Payload data location - Numbers of 8 bytes - 13 bits
ID =x
offset =0
fragflag =0
length =4020
ID =x
offset =0
fragflag =1
length =1500
ID =x
offset =185
fragflag =1
length =1500
ID =x
offset =370
fragflag =0
length =1040
One large datagram becomes several smaller datagrams
Example
MTU = 1500 bytes
4020 byte IP datagram
20 byte IP header
4000 byte payload
3 fragments: F1, F2, F3
4000=1480+1480+1040
1480 bytes in data field
offset = multiple of 8 bytes so 1480/8 = 185 185+185 = 370
ID: set by sending host IP layer; typically increments ID num for each datagram it sends. Last fragment sent has flag field set to 0 to indicate it’s the last fragment; all other fragments have flag set to 1
If one fragment is lost, IP discards all fragments
Lecture 10
An Example of Datagram Fragmentation
F1
F2
F3
Lecture 10
Sub-fragmentation and Fragment Loss
• Fragment may encounter a subsequent network with even smaller MTU • Router fragments the fragment to fit • Resulting (sub)fragments look just like original fragments (except for size) • No need to reassemble hierarchically; (sub)fragments include position in datagram
• IP may drop fragment • What happens when a fragment is lost? Destination drops entire original datagram
• How does destination identify lost fragment? - Sets timer with each fragment - If timer expires before all fragments arrive, fragment assumed lost - Datagram dropped • Source (transport/application layer protocol) assumed to retransmit
IP Hdr21 data21 IP Hdr22 data22
(sub)fragments
Lecture 10
IP Datagram Errors and ICMP
IP provides best-effort delivery Datagrams will be dropped if the following errors are detected - corrupted bits detected by header checksum - illegal address detected by routers (routing table) and ARP reply - routing loop detected by Time-To-Live (TTL) field - fragment loss detected by timeout
IP ignores errors, but reports some errors !!
Internet Control Message Protocol (ICMP) is a protocol to report errors and provide some information. - Error reporting function Report problems that a router or a destination host encounters when it processes an packet via sending an ICMP message TO a source host - Information query function Help a source host or a network manager get specific information from a router or another host
Lecture 10
Error Report and Information Query Mechanism
Error report mechanism
Source
Host Destination
Host
Router
Router
Router x
with error
with error
IP datagram
ICMP datagram
for error report
ICMP datagram
for error report
Information query mechanism
Source
Host Destination
Host
Router
Router
Router q
ICMP datagram
for information query
r
ICMP datagram
for reply
ICMP datagram
for reply
q q q
r r r
X
x
Dropped
Dropped
Lecture 10
ICMP Message Format and Transmission
- ICMP includes both error messages and information messages - ICMP message consists of ICMP header and ICMP data - ICMP encapsulates message in IP data area for transmission - ICMP datagram is processed and forwarded like conventional IP datagram
ICMP Header ICMP Data Area
ICMP Message
IP Header IP Data Area
ICMP Datagram
Type Checksum Code
Identifier Sequence Num.
ICMP Header
0 8 16 24 31
IP Header: type=1 for ICMP message
Type: error/information type
Code: detailed error type
Encapsulated
Encapsulated
ICMP Message Types
• Error messages:
- Source quench (type=4)
too many datagrams to buffer in a router
- Time exceeded (type=11)
TTL becomes zero in a router (code=0)
fragment reassembly timer expires
in a host (code=1)
- Destination unreachable (type=3, code=1~15)
network disconnection or
destination host is powered down or
TCP/application not run, firewall, etc
• Information query messages: (a pair)
- Request/reply
(type=8: request, type=0: reply)
- Timestamp request/reply
(type=13: request, type=14: reply)
- Address mask request/reply
(type=17: request, type=18: reply)
Lecture 10
ICMP, Host Reachability and Internet Route
An internet host A is reachable from another host B if datagrams can be delivered from A to B
ping utility tests reachability
- Sends datagram from B to A that A echoes back to B - Uses ICMP echo request and echo reply messages Command format: ping IP-address/Host-name
List of all routers on path from A to B is called the route from A to B
traceroute uses UDP to non-existent port and TTL field to find route
- Sends ICMP echo messages with increasing TTL - Router that decrements TTL to 0 sends ICMP time exceeded message, with router's address as source address - First, with TTL=1, gets to first router, which discards and sends time exceeded message - Next, with TTL=2, gets through first router to second router - Continue, with TTL=3, 4, …, until message from destination received Command format for Unix/Linux: traceroute IP-address/Host-name
Command format for Windows: tracert IP-address/Host-name
ICMP & TraceRT Anim1
ICMP & TraceRT Anim2
ping & other network utilities
Lecture 10
Motivation for Change from IPv4 to IPv6
- Current version of IPv4 - is more than 30 years old - IPv4 has shown remarkable success !!! - Then why change? Address space - 32 bit address space allows for over a million networks - But...most are Class C and too small for many organizations - 214 = 16384 Class B network addresses already almost exhausted Type of service - Different applications have different requirements for delivery reliability & speed - Current IPv4 has type of service that's not often implemented - Effective multimedia communication - Data encryption and authentication Multicast One next version is called IPv6 !
Lecture 10
New Features in IPv6
- Large address size – 128 bits = 16 bytes - Better header format - entirely different - Base header – 40 bytes - Extension headers - Additional information stored in optional extension headers - Support for resource allocation (QoS) - flow labels and quality of service allow audio and video applications to establish appropriate connections - Support for more security - Extensible - new features can be added more easily - No checksum field - to reduce processing time in a router - No fragmentation - to reduce load of routers - Potential for the Internet of Things (IoT)
40 bytes
Lecture 10
IPv6 Base Header Format
It contains less information than IPv4 header - VERS = 6 for IPv6 - PRIORITY (8 bits) for traffic classes, such as delay, jitter, reliability requirements - PAYLOAD LENGTH (16 bits): Length excluding the base header - NEXT HEADER points to first extension header - HOP LIMIT (8 bits) same as TTL in IPv4 - FLOW LABEL (20 bits)
- used to associate datagrams belonging to a flow or communication between two applications - Specific path - Routers use FLOW LABEL to forward datagrams along prearranged path
Lecture 10
IPv6 Next Header
Purpose of multiple headers: economy and extensibility
Next header codes 0 - Hop-by-hop option 2 - ICMP 6 - TCP 17 - UDP 43 - Source routing 44 - Fragmentation 50 - Encrypted security payload 51 - Authentication 59 - Null (no next header) 60 - Destination option
Lecture 10
IPv6 Addressing - 128-bit addresses: Type + Rest of address - Groups of 16-bit numbers in hex separated by colons - colon hexadecimal (or colon hex) 69DC:8864:FFFF:FFFF:0:1280:8C0A:FFFF
- Special types of addresses: unicast, multicast, anycast - collection of computers with same prefix - Type: 0000 0000 - Reserved 0000 000 - ISO network addresses 0000 010 - IPX (Novell) 010 - Provided-based unicast addresses 100 - Geographic unicast addresses 1111 1111 - Multicast address - Provider-based unicast addresses for normal host -------------------------------------------------------------------------------------------------------------- | 010 | RegID(5) | ProviderID(16) | SubscriberID(24) | SubnetID(32) | HostID(48) | ------------------------------------------------------------------------------------------------- ------------- - Register ID: 11000 - INTERNIC for North America 01000 - RIPNIC for European countries 10100 - APNIC for Asian and Pacific countries - Address hierarchy - Reserved addresses - Loopback address: 000...0001 - IPv4 address: 000...000+IPv4 address = Ipv6 address
IPv6 Introduction Video
A B E F
IPv6 IPv6 IPv6 IPv6
Tunnel Logical view:
Physical view: A B E F
IPv6 IPv6 IPv6 IPv6
C D
IPv4 IPv4
Flow: X Src: A Dest: F data
Flow: X Src: A Dest: F data
Flow: X Src: A Dest: F data
Src:B Dest: E
Flow: X Src: A Dest: F data
Src:B Dest: E
A-to-B: IPv6
E-to-F: IPv6
B-to-C: IPv6 inside
IPv4
B-to-C: IPv6 inside
IPv4
Lecture 10
Tunneling – Transition from IPv4 toIPv6
Not all routers can be upgraded simultaneous
How will the network operate with mixed IPv4 and IPv6 routers?
Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers
http://en.wikipedia.org/wiki/IPv6
http://ja.wikipedia.org/wiki/IPv6
Exercise 10
1. 100 byte data is sent using IP across an Ethernet. Before sent, the data will be first formed an IP datagram and then the datagram will be encapsulated into an Ethernet Frame. Calculate the percentage of headers in sending the 100 byte data. Assume no optional field in IP header.
2. Suppose a file of 20 Kbytes to be sent from host H1 to host H2 across three networks as shown in the following figure. How many IP datagrams will be sent from H1? And how many IP datagrams will be received by H2? Assume no datagram loss, duplication and disorder during the transmissions. 3. Host A sends a message to host B and never receive reply from B. However, host A receives an ICMP message with a header in hexadecimal format as the follows 03 01 1A C8 31 00 B7 Give possible reasons that A does not receive reply from B.
4. Explain how traceroute utility works. Use the utility in a Windows OS environment to probe the Internet organization web server. The command is tracert www.ietf.org . How many routes
have been passed when your packet travel to the web server? Which one is the slowest?
5. Summarize main features of IPv6 as compared with IPv4.
Toking Ring MTU=4464
Ethernet MTU=1500
FDDI MTU=4352 R1 R2 H1 H2