Category Archives: 1.1.d Explain IP operations

1.1.d Explain IP operations

1.1.d [v] IP MTU

IPv4 allows fragmentation: dividing the datagram into pieces, each small enough to pass over the single link that is being fragmented for, using the MTU parameter configured for that interface. This fragmentation process takes place at the IP layer (OSI layer 3) and marks packets it fragments as such, so that the IP layer of the destination host knows it should reassemble the packets into the original datagram. This method implies a number of possible drawbacks:

● All fragments of a packet must arrive for the packet to be considered received. If the network drops any fragment, the entire packet is lost.

● When the size of most or all packets exceed the MTU of a particular link that has to carry those packets, almost everything has to be fragmented. In certain cases the overhead this causes can be considered unreasonable or unnecessary.

For example, various tunneling situations cross the MTU by very little as they add just a header’s worth of data. The addition is small, but each packet now has to be sent in two fragments, the second of which carries very little payload. The same amount of payload is being moved, but every intermediate router has to do double the work in terms of header parsing and routing decisions.

● As it is normal to maximize the payload in every fragment, any further fragmentation that turns out to be necessary will increase the overhead even more.

● There is no simple method to discover the MTU of links beyond a node’s direct peers.

● The Internet Protocol requires that hosts must be able to process IP datagrams of at least 576 bytes (for IPv4) or 1,280 bytes (for IPv6). However , this does not preclude Data Link Layers with an MTU smaller than IP’s minimum MTU from conveying IP data. For example, according to IPv6′ s specification, if a particular Data Link Layer physically cannot deliver an IP datagram of 1,280 bytes in a single frame, then the link layer must provide its own fragmentation and reassembly mechanism, separate from IP’s own fragmentation mechanism, to ensure that a 1280-byte IP datagram can be delivered , intact, to the IP layer.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1086-1098).  . Kindle Edition.


1.1.d Explain IP operations

1.1.d [iv] TTL

Time to live (TTL) or hop limit is a mechanism that limits the lifespan or lifetime of data in a network. TTL prevents a data packet from circulating indefinitely. If the TTL field reaches zero before the datagram arrives at its destination, then the datagram is discarded and an ICMP error datagram (11 – Time Exceeded) is sent back to the sender.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1074-1075).  . Kindle Edition.


1.1.d Explain IP operations

1.1.d [iii] IPv4 and IPv6 fragmentation

IP implements datagram fragmentation, breaking it into smaller pieces, so that packets can pass through a link with a smaller maximum transmission unit (MTU) than the original datagram size. The Identification field, and Fragment offset field along with Don’t Fragment (DF) and More Fragment (MF) flags in the IP protocol header are used for fragmentation and reassembly of IP datagrams.

The details of the fragmentation mechanism, as well as the overall architectural approach to packet fragmentation, are different between IPv4 and IPv6. In IPv4, routers perform fragmentation, whereas in IPv6, routers do not fragment, but drop the packets that are larger than the MTU. While the header formats are different for IPv4 and IPv6, analogous fields are used for fragmentation , so the algorithm can be reused for fragmentation and reassembly.

In IPv4, hosts must make a best-effort attempt to reassemble fragmented IP datagrams with a total reassembled size of up to 576 bytes – equal to the minimum MTU for IPv4. They may also attempt to reassemble fragmented IP datagrams larger than 576 bytes. In IPv6, this minimum MTU is increased to 1,280 bytes larger than the minimum MTU for IPv4.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1064-1071).  . Kindle Edition.

1.1.d Explain IP operations

1.1.d [ii] IPv4 options, IPv6 extension headers

Main/ Regular IPv6 Header IPv6 Extension Headers

The main IPv6 header is equivalent to the basic IPv4 one despite some field differences that are the result of lessons learned from operating IPv4.

Extension headers are an intrinsic part of the IPv6 protocol and they support some basic functions and certain services. The following is a list of situations where EHs are commonly used:

● Hop-by-Hop EH is used for the support of Jumbo-grams or, with the Router Alert option, it is an integral part in the operation of Multicast Listener Discovery (MLD). Router Alert is an integral part in the operations of IPv6 Multicast through MLD) and RSVP for IPv6.

● Destination EH is used in IPv6 Mobility as well as support of certain applications.

● Routing EH is used in IPv6 Mobility and in Source Routing. It may be necessary to disable ipv6 source routing using ipv6 source-route command on routers to protect against DDoS.

● Fragmentation EH is critical in support of communication using fragmented packets (in IPv6, the traffic source must do fragmentation-routers do not perform fragmentation of the packets they forward)

● Mobility EH is used in support of Mobile IPv6 service

● Authentication EH is similar in format and use to the IPv4 authentication header

● Encapsulating Security Payload EH is similar in format and use to the IPv4 ESP header. All information following the Encapsulating Security Header (ESH) is encrypted and obfuscated and for that reason, it is invisible to intermediary network devices.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1049-1060).  . Kindle Edition.



1.1.d Explain IP operations

1.1.d [i] ICMP unreachable, redirect

If a router or a layer 3 switch receives a non-broadcast packet destined for itself that uses an unknown protocol, it sends an ICMP protocol unreachable message back to the source. Similarly, if the software receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it will send an ICMP host unreachable message to the source. This feature is enabled by default.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 996-999).  . Kindle Edition.