July 3-5 overheads

Anatomy of an Internet Datagram

Precedence	Normal prio to high (control)
Delay
Throughput	Indications to routing algorithm
Reliability

Identification number
• Each fragment the same ID as the original datagram (uniqueness)
• Retransmitted datagram often different ID

Do not fragment	Error if not possible
More fragments	Allows reassembly
Fragment offset

Internet datagram options

Copy	into each fragment of the datagram
Class	normal; network control	0
Number

Option list end
no-op (octet boundary alignment)
Security restrictions


Loose source routing	Some hops
Strict source routing	Every intermediate
Record route	Included in routing
Time stamp	Class 2

Routing IP datagrams

• Direct delivery on the same network does not involve routing decisions, only

  physical address

  encapsulation into physical frames(s)

  
  Obvious efficient check of destination network identity
• Indirect delivery forwarding a datagram to a neighboring network towards destination.
  routedatagram(datagram,routingtable)
extract destination IP address from datagram D
compute the network prefix N
if N matches adjacent network
then directly deliver
elseif RT contains host specific route for D
then send datagram to the next hop address
elseif RT contains a route for network N
then send datagram to the next hop address
elseif RT contains a default route
then send datagram to default router
else declare a routing error


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  Reverse address resolution

  • A host's IP address resides often on a secondary memory (disk, server) and has to be determined at start-up.
  • PARP provides a solution to this problem in a manner similar to ARP.
    • Sender broadcasts a RARP request identifying itself by its physical address. (Target's physical address may be different.)
    • The appropriate RARP servers (that have knowledge of the requested IP address) send a RARP reply directly to the sender.

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  • hlen
    The length in bytes of the datalink addresses. For Ethernet, this is set to 6.
  • plen
    The length in bytes of the network addresses. For IP, the length is always 4.
  • opcode
    0x0001 indicates an ARP request; 0x0002 indicates an ARP reply.
  • sender datalink and sender network
    For both a request and a reply, set by the sender of the ARP packet, which fills in its own datalink and network addresses.
  • target network
    On a request, ,filled in with the IP address of the target; on a reply, filled in with the original sender's IP address.
  • target datalink
    On a request, filled in with 0; on a reply, filled in with the original sender's datalink address.

  Anatomy of a RARP frame

  • The same frame fields as ARP
  • Different ethernet frame type (0x8025)
  • Extended protocol allows querying for an arbitrary target's IP address
  • RARP issues
    • Time out for resending a request (when reply not forthcoming)
      • Transmission failure
      • Network failure
      • Server failure
    • Multiple servers
      • Only primary server replies to the first request
      • Secondary servers kick in when observing repeated request

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  Some of the diagrams copied from S. Carl-Mitchell & J.S. Quarterman, Practical Internetworking with TCP/IP and UNIX , Addison-Wesley 1993.

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  ser@cs.uoregon.edu