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Internet Networking Spring 2005

Internet Networking Spring 2005. Tutorial 2 IP Checksum, Fragmentation. IP Header Diagram. Ver. IHL. TOS. Total Length. Identification. Flags. Fragment Offset. TTL. Protocol. Checksum. Source Address. Destination Address. Option. Padding. IP Checksum.

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Internet Networking Spring 2005

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  1. Internet Networking Spring 2005 Tutorial 2 IP Checksum, Fragmentation

  2. IP Header Diagram Ver. IHL TOS Total Length Identification Flags Fragment Offset TTL Protocol Checksum Source Address Destination Address Option Padding

  3. IP Checksum • Ensuring integrity of IP header • Reducing processing time at routers. • Does not check data integrity. • Allowing higher level protocols to choose their own checksum scheme for the data.

  4. Checksum (RFC 1071) • Adjacent octets to be checksummed are paired to form 16-bit words. The checksum field is cleared. • The 16-bit 1's complement sum is computed over the 16-bit words • Any overflows are added to the sum. • The 1's complement of this sum is placed in the checksum field.

  5. Checksum • To verify a checksum, the 1's complement sum is computed over the same set of octets, including the checksum field. • If the result is all 1 bits, the check succeeds.

  6. Checksum • Insensitive to: • Byte order. • Zero padding. • Word width (2 bytes, 4 bytes, etc.). • Hardware oriented (simple addition). • Not robust (unlike CRC).

  7. Example

  8. Incremental Checksum Update (RFC 1141) • Updating part of the IP header data doesn’t require recomputing the entire Checksum field. (For example: when a router change the TTL field.) • The technique for speed up checksum recomputing in this case is called - incremental checksum update.

  9. Incremental Checksum Update • Notation: • C - Old 1’s complement sum. • m - Old data. • C’ - New 1’s complement sum. • m’ - New data. C’ = C + (-m) + m’ = C + ( m’ – m) • The checksum is the 1’ complement of C’: ~C' = ~(C + (-m) + m') = ~C + (m - m') = ~C + m + ~m‘ • Note, the second equality does not work if the sum in the brackets is 0xFFFF (see RFC 1624).

  10. Incremental Internet Checksum • Subtracting 1 from TTL field (common case) ~C’ = ~C + (m - m’) = ~C + 0100H (TTL byte is higher byte of integer) If ~C=220dH then ~C’ = 220dH + 0100H = 230dH

  11. IP Fragmentation • The internet is used to connect network from different technologies. • In particular every technology has its own maximum packet size. • This maximum packet size is called MTU (Maximum Transfer Unit). • In Ethernet the MTU is 1514 bytes. • Large IP packets may traverse network in which the MTU is smaller than the packet sized. • These packets must be fragmented.

  12. IP Fragmentation • IP fragmentation is done in the entry point of the (physical) network that requires the fragmentation. • IP layer automatically performs fragmentation of a datagram when its too large to be sent on physical network. • A datagram could be fragmented more than one time. • Fragments are counted in units of 8 octets. • Reassemble is done in the IP layer at the destination. • Transparent to upper protocol.

  13. IP Fragmentation Ver. IHL TOS Total Length Identification Flags Fragment Offset TTL Protocol Checksum Source Address Destination Address Option Padding

  14. Identification field • Unique integer that identifies the datagram • The originator of the IP packet (i.e. the source) sets the identification field to a value that must be unique for that source-destination pair and protocol for the time the datagram will be active in the internet system. • Implemented by counter, which increments by one. • Ensures that fragments of different datagrams are not mixed.

  15. Fragment Offset • Tells the receiver the position of a fragment in the original datagram. • Identifies the fragment location, relative to the beginning of the original unfragmented datagram. • The fragment offset and length determine the portion of the original datagram covered by this fragment. • The fragment offset is measured in units of 8 octets (64 bits). • Enables Additional fragmentation. • It does not include the length of the IP header. • Reason: 13 bits are used to map 16 bits (IP length). • The first fragment has offset zero.

  16. Flag • Bit 0: reserved, must be zero. • Bit 1: (DF) 0 = May Fragment, 1=Don't Fragment. • If is set, then internet fragmentation of this datagram is NOT permitted • If fragmentation required, but this bit is set than the packet is discarded and ICMP is returned. • Bit 2: (MF) 0 = Last Fragment, 1=More Fragments. • Set if the datagram is not the last fragment.

  17. Datagram header Data1 600 octets Data2 600 octets Data3 200 octets a Datagram header Data1 600 octets Fragment 1 offset = 0 Datagram header Data2 600 octets Fragment 2 offset = 75 b Datagram header Data3 200 octets Fragment 3 offset = 150 Fragmentation - Example

  18. Fragmentation • Fragmentation must be supported by every IP entities (routers, host, etc.). • Fragmentation should be avoided. • Loss of one fragment requires retransmission of the entire packet. • Advanced IP forwarding entities (e.g HW based forwarding entities) does not support IP fragmentation (i.e. IP fragmentation is an exception). • MTU discovery protocol (RFC 1191) that uses the DF bit, is used to avoid the necessity of IP fragmentation.

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