Fragmentation statistics in DS-Lite can be observed by issuing the following command:
show isa nat-group <grp-id> mda <slot-id/mda-id> statistics
The command output displays only relevant DS-Lite fragmentation counters are shown. The remaining counters are removed from the output for easier reading.
show isa nat-group 1 mda 1/2 statistics
===============================================================================
ISA NAT Group 1 MDA 1/2
===============================================================================
--snip--
too many fragments for IP packet : 0
too many fragmented packets : 0
too many fragment holes : 0
too many frags buffered : 0
fragment list expired : 0
Reassembly Failures : 0
Fragments RX DSL : 0
Fragments RX DORMANT : 0
Fragments TX DSL : 0
Fragments TX DORMANT : 0
Fragments RX OUT : 0
Fragments TX OUT : 0
too many frag. lists for flow : 0
frag. list cleanup in progress : 0
--snip--
===============================================================================
To interpret these counters, familiarity with the following terms in the context of fragmentation is required:
packet
An IP packet that is split into fragments (multiple frames) because of its original size being larger than the MTU configured on any node servicing this packet.
fragment
A fragment comprises the frames that make up a packet. Multiple fragments (frames on the wire) are eventually reassembled into the original packet.
fragment list
MS-ISA maintains a list of fragments belonging to a single packet. Each list represents a single fragmented packet and the list contains multiple fragments.
fragment hole
A hole refers to a fragment or a group of consecutive fragments in a fragment list. Fragments of a packet are sequentially numbered from first to last and they must be reassembled in the same order in which they are fragmented. For example, if a packet contains 9 fragments but only fragments [1,3,5,9] are received by MS-ISA, then there are 3 holes in this list [2],[3,4],[6,7,8].
flow
This is identified by 5 tuple <src IP, dst IP, src Port, dst Port, protocol>. Flows can have many packets and each packet of a flow can be fragmented.
Table: Counter names and descriptions describes counter names.
| Counter name | Description |
|---|---|
too many fragments for IP packet |
This counter increments if there are more than 20 fragments of a received single packet (the maximum number of fragments per packet is 20). In this case, all fragments of the packet are dropped. |
too many fragmented packets |
This counter increases if the maximum number of fragmented packets per MS-ISA is reached. See the MS-ISA Scaling Guides for the maximum number of fragmented packets per MS-ISA (specifically, the max num of frag lists parameter). |
too many fragment holes |
This counter increments if there are more than 11 holes tracked for a single packet. In this case, all fragments are be dropped. |
too many frags buffered |
This counter increases if the maximum number of fragments that can be stored on MS-ISA is reached. |
fragment list expired |
This counter increases if all fragments of single packets are not received within two seconds. In this case, all fragments of this packet are dropped. |
lists for flow |
This counter increases when more than five fragmented packets per flow are being maintained simultaneously in MS-ISA. In this case, the fragments of the sixth packets are dropped. |
Reassembly Failures |
This counter increases when the reassembly of a packets (when all fragments are received) failed. This can attributed to the size of the first DS-Lite IPv6 fragment is smaller than 1280B, or the total reassembled packet is too big (greater than 9212B). |
Fragments RX DSL |
This counter increments only when a DS-Lite packet/fragment is received in the upstream direction that contains an IPv4 fragment. In other words, this counter is relevant only to IPv4 fragments inside of the DS-Lite packet/fragment that is received from the subscriber, and is not affected by DS-Lite fragments. |
Fragments TX DSL |
This counter increments only in case that DS-Lite packet/fragment sent in the downstream direction contains an encapsulated IPv4 fragment (which is received from the public side). In other words, this counter is relevant only to IPv4 fragments inside of the DS-Lite packet/fragment that is sent toward the subscriber, and is not affected by DS-Lite fragments. |
Fragments RX OUT |
This counter increments when an IPv4 fragment is received in the downstream direction, toward the subscriber. |
Fragments TX OUT |
This counter increments when an IPv4 fragment is transmitted in the upstream direction (public side). |