Lecture 10
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 Internet Transmission Paradigm
IP D IP D IP D IP D IP D
Source router router router Destination host Routing Routing Routing host Routing network Table network Table network Table network Routing Table ------Table ------## *** ## *** ## *** ------## *** ………… ………… ………… ## *** ………… …………
• 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 !! Lecture 10 IP Datagram Encapsulation
IP Datagram/Packet IP Header IP Data Area Encapsulated into a frame/packet in lower layer Frame Header Frame Data Hardware Network Frame/Packet
• Entire datagram treated like data encapsulated in a frame for transmission
• Frame type (0800 for Ethernet) identifies contents as IP datagram
Ethernet Frame • 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 Lecture 10 Encapsulation Across Multiple Hops
Animation
• 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 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 Ethernet Frame - 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 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
> MTU
Each <= MTU Fragment 1 Fragment 2 Fragment 3
Fragmentation Fragmentation No-fragmentation Assemble fragments
No-assemble No-assemble Lecture 10 Datagram Fragmentation & Reassembly
Network links have MTU - Different link types with Different MTUs Fragmentation: * 1500 bytes for Ethernet in: one large datagram * 296 bytes for PPP out: 3 smaller datagrams
large IP datagram divided (“fragmented”) within net Reassembly one datagram becomes several datagrams “reassembled” only at the final destination IP header bits used to identify, order related fragments 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
Lecture 10 An Example of Datagram Fragmentation
Example length ID fragflag offset MTU = 1500 bytes =4020 =x =0 =0 4020 byte IP datagram If one fragment is 20 byte IP header One large datagram becomes lost, IP discards all 4000 byte payload several smaller datagrams fragments 3 fragments: F1, F2, F3 length ID 4000=1480+1480+1040 fragflag offset F1 =1500 =x =1 =0
1480 bytes in data field length ID fragflag offset F2 =1500 =x =1 =185 offset = multiple of 8 bytes so 1480/8 = 185 length ID fragflag offset F3 185+185 = 370 =1040 =x =0 =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 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? (sub)fragments - Sets timer with each fragment IP Hdr21 data21 IP Hdr22 data22 - If timer expires before all fragments arrive, fragment assumed lost - Datagram dropped • Source (transport/application layer protocol) assumed to retransmit
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
IP datagram with error Router X Dropped with error x Router x Dropped Router Source Destination ICMP datagram Host ICMP datagram Host for error report for error report
Information query mechanism ICMP datagram for information query Router q q q Router q r r Router r Source ICMP datagram r Destination Host for reply ICMP datagram Host for reply 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 Message ICMP Header ICMP Data Area ICMP Datagram Encapsulated IP Header IP Data Area
IP Header: type=1 for ICMP message ICMP Header 0 8 16 24 31 Type Code Checksum Identifier Sequence Num.
Encapsulated Type: error/information type Code: detailed error type 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 ping & other network utilities
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 ICMP & TraceRT Anim1 - First, with TTL=1, gets to first router, which discards and sends time exceeded message ICMP & TraceRT Anim2 - 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 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
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 IPv6 Introduction Video - Loopback address: 000...0001 - IPv4 address: 000...000+IPv4 address = Ipv6 address Lecture 10 Tunneling – Transition from IPv4 toIPv6
A B E F Logical view: Tunnel IPv6 IPv6 IPv6 IPv6
A B C D E F Physical view: IPv6 IPv6 IPv4 IPv4 IPv6 IPv6
Not all routers can be Flow: X Src:B Src:B Flow: X upgraded simultaneous Src: A Dest: E Dest: E Src: A Dest: F Dest: F How will the network Flow: X Flow: X operate with mixed Src: A Src: A IPv4 and IPv6 routers? data Dest: F Dest: F data
Tunneling: IPv6 carried as payload in IPv4 data data datagram among IPv4 routers A-to-B: E-to-F: B-to-C: B-to-C: IPv6 IPv6 http://en.wikipedia.org/wiki/IPv6 IPv6 inside IPv6 inside IPv4 IPv4 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.
H1 Toking Ring Ethernet FDDI MTU=4464 R1 MTU=1500 R2 MTU=4352 H2
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.