Review/Perspective: IP operations

1.1.d Explain IP operations

  • 1.1.d [i] ICMP unreachable, redirect
  • 1.1.d [ii] IPv4 options, IPv6 extension headers

ICMP Unreachable – This ICMP message type is sent when a router in the transport path to a destination doesn’t know how to get to the destination.  It’s received a packet destined for a network it lacks a route for.  The router process this as an unroutable network and send back an ICMP unreachable message to the originating device indicating a problem in connectivity to that network.

ICMP Redirect – these ICMP messages are used by routers to notify hosts that a better router is available to a particular destination.

Cisco routers send ICMP redirects when all of these conditions are met:

  • The interface on which the packet comes into the router is the same interface on which the packet gets routed out.
  • The subnet or network of the source IP address is on the same subnet or network of the next-hop IP address of the routed packet.
  • The datagram is not source-routed.
  • The kernel is configured to send redirects.

ICMP redirects are enabled by default…however they are disabled by default if HSRP is configured on the interface and you can manually turn ICMP redirects back on if HSRP is enabled.

IPv4 Options – RFC 791 goes over the specifics of the various IP options you can set in the header of a packet.  Options typically provide control functions needed or useful in some situations but unecessary for most common communications.  Options include provisions for timestamps, security, and special routing. 

IPv6 Extension Headers (EH)– Like IPv4 Options, the operations they performed are crucial and had to be preserved when translating to IPv6.  The functionality was added to IPv6 in the form of Extension Headers (EH).  The standard IPv6 header remains the same fixed size (40 bytes) while customized EHs are added as needed.

extension headers.PNG

RFC2460 defines extension headers and recommends a specific order in which extension headers are chained on top of the IPv6 headers in a packet. Where the only requirement is that the Hop-by-Hop Options Header MUSt be the first one following the main header if present. The EHs are considered a powerful tool in extending IPv6 to adapt to future protocol requirements and service needs. It is expected the other uses will be identified for the existent EHs and that new EHs will be defined.

Extension headers are an intrinsic building block of IPv6. It is critically important to understand their role and mode of use as well as the processing requirements they have on various network devices. Deployed IPv6 networks must be capable to handle IPv6 traffic containing extension headers. They must forward such IPv6 traffic at optimal, production level performance in the presence of adv anced features such as Access Lists



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