1 K. Salah Module 5.2: Internet Protocol CO vs. CL protocols IP Features –Fragmentation –Routing IP Datagram Format IPv6

2 K. Salah CO vs. CL CO – Connection Oriented –Modeled after the telephone system –When PDU are sequenced, I.e. logical connection CL – Connectionless –Modeled after the postal system –When PDUs are not sequenced. Each PDU is treated independently from each other. IP is a CL protocol! Advantages –Flexibility –Robust –Smaller Buffers Needed –No unnecessary overhead Unreliable –Not guaranteed delivery  packets can be lost, duplicated, damaged. –Not guaranteed order of delivery  Packets can take different routes –Reliability is responsibility of next layer up (e.g. TCP)

3 K. Salah IP Features IP has two primary responsibilities: –Routing:  Providing CL, best-effort delivery of datagrams through an internetwork; and –Fragmentation:  Providing fragmentation and reassembly of datagrams to support data links with different maximum transmission unit (MTU) sizes.

4 K. Salah Routing IP relies on two tools to help it route datagrams: –Subnet mask –IP routing table If source and destination network and subnet parts are the same, then the destination host is in the same network and the routing is direct. The datagram is wrapped in a frame and transmitted directly to its destination on the local LAN. The destination address that is placed in the frame header must be the physical address of the destination. ARP (Address Resolution Protocol) will be used to find the physical address of the destination. If destination is not on the local subnet, IP must consult its local routing table. In such a case, the datagram is sent to the router specified in the routing table. If no router (or default gateway) is found in the routing table, report error.

5 K. Salah Fragmentation Each LAN and WAN technology imposes a different size limit on its frames. –For example, the maximum frame size of the ethernet (MTU) is 1500 bytes, which is far below the maximum size of an IP datagram. –Maximum IP packet size is (65537) or 2 16 bytes. IP solves the size problem by chopping the datagram into several smaller datagrams called fragments. Fragmentation is performed by routers and hosts. It is up to IP in the destination host to gather up the incoming fragments and rebuild the original datagram, before passing it to the upper layer. Fragmentation most often is performed in a router. Fragmentation is a performance killer.

6 K. Salah Fragmentation (Cont.) When to re-assemble –At destination  Results in packets getting smaller as data traverses internet –Intermediate re-assembly  Need large buffers at routers  Buffers may fill with fragments  All fragments must go through same router  Inhibits dynamic routing IP re-assembles at destination only

7 K. Salah Fragmentation (Cont.) Uses fields in header –Data Unit Identifier (ID)  Identifies end system originated datagram  Source and destination address  Protocol layer generating data (e.g. TCP)  Identification supplied by IP layer –Data length  Length of user data in octets –Offset  Position of fragment of user data in original datagram  In multiples of 64 bits (8 octets) –More flag  Indicates that this is not the last fragment

8 K. Salah Fragmentation Example

9 K. Salah 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 (remaining time to live in IP) –If time to live runs out, kill partial data

10 K. Salah IP: Datagram format

11 K. Salah Header Fields (1) Version –Currently 4 –IP v6 - see later Internet header length (HLEN) –In 32 bit words –Including options Type of service Total length –Of datagram (header+data) in octets Identification –unique integer –Used with addresses and user protocol to identify datagram uniquely –This parameter is needed for reassembly and error reporting.

12 K. Salah Header Fields (2) Flags (only 2 bits used) –More bit –Don’t fragment  If a node does not know how to reassemble  Useful in bootstrapping. The node initially has a lightweight IP stack Fragmentation offset Time to live Protocol –Next higher layer to receive data field at destination

13 K. Salah Header Fields (3) Header checksum –Reverified and recomputed at each router –16 bit ones complement sum of all 16 bit words in header –Set to zero during calculation Source address Destination address Options (variable) Padding (variable) –Used to ensure that the IP header is a multiple of 32 bits in length. Data (variable) –Must be an integer multiple of 8 bits in legth –The maximum length of datagram (data+header) is 65,535 bytes

14 K. Salah Type of Service Precedence –Measurement of packet’s relative importance. –8 levels Reliability –Try not to drop the packet. Delay –Try to minimize the delay for this packet. Throughput –Choose a network with high bandwidth. Cost –Choose a network with least cost

15 K. Salah Options Security –Attach classified information level to packet. For DOD military application. RFC Source routing –List of all routers. Route recording –List of routers visited. Stream identification –For special handling of voice and data Timestamping –Add a timestamp at each router

16 K. Salah IPv6 IP v 1-3 defined and replaced IP v4 - current version IP v5 - streams protocol IP v6 - replacement for IP v4 –During development it was called IPng –Next Generation Why Change IP? –Address space exhaustion  2 32 different addresses gives over 4 billion addresses is not enough!  Due to growth of wireless, PDA, and Internet. –Other enhancements

17 K. Salah IPv6 vs. IPv4 The changes from IPv4 to IPv6 are primarily in: –expanded addressing capabilities; –header format simplification; –flow labeling capability; –Support for resource allocation –improved support for extensions, options, and QoS; –Support for more authentication and security.

18 K. Salah IPv6 Format

19 K. Salah Comparison The header length field is eliminated. The service type field is eliminated in IPv6. The total length field is eliminated. The identification, flag, and offset fields are eliminated. The TTL field is called hop limit. The protocol field is replaced by the next header field. The header checksum is eliminated. The option fields in IPv4 changed to extension headers.

20 K. Salah Extension Headers

21 K. Salah Extension Headers

22 K. Salah Status of IPv6 Smooth transition is key factor in success of IPv6: –Dual stack –IPv6 Tunneling for IPv4 packets. –Header translation In reality, we have a slow adoption of IPv6. This is due to the invention of NAT. NAT may work only with certain styles of applications, but not adequate for say IP telephony. Also, it does not scale very well. The urge is not there yet, but surely growing!