MESSAGE
| DATE | 2017-08-23 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [Hangout - NYLXS] One of the best reviews of Packet switching I've
|
https://blog.cloudflare.com/ip-fragmentation-is-broken/?utm_content=bufferd16b5&utm_medium=social&utm_source=linkedin.com&utm_campaign=buffer
Broken packets: IP fragmentation is flawed
18 Aug 2017 by Marek Majkowski.
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As opposed to the public telephone network, the internet has a Packet
Switched design. But just how big can these packets be?
CC BY 2.0 image by ajmexico, inspired by
This is an old question and the IPv4 RFCs answer it pretty clearly. The
idea was to split the problem into two separate concerns:
What is the maximum packet size that can be handled by operating
systems on both ends?
What is the maximum permitted datagram size that can be safely
pushed through the physical connections between the hosts?
When a packet is too big for a physical link, an intermediate router
might chop it into multiple smaller datagrams in order to make it fit.
This process is called "forward" IP fragmentation and the smaller
datagrams are called IP fragments1.
Image by Geoff Huston, reproduced with permission
The IPv4 specification defines the minimal requirements. From the RFC791:
Every internet destination must be able to receive a datagram
of 576 octets either in one piece or in fragments to
be reassembled. [...]
Every internet module must be able to forward a datagram of 68
octets without further fragmentation. [...]
The first value - permitted reassembled packet size - is typically not
problematic. IPv4 defines the minimum as 576 bytes, but popular
operating systems can cope with very big packets, typically up to 65KiB.
The second one is more troublesome. All physical connections have
inherent datagram size limits, depending on the specific medium they
use. For example Frame Relay can send datagrams between 46 and 4,470
bytes. ATM uses fixed 53 bytes, classical Ethernet can do between 64 and
1500 bytes.
The spec defines the minimal requirement - each physical link must be
able to transmit datagrams of at least 68 bytes. For IPv6 that minimal
value has been bumped up to 1,280 bytes (see RFC2460).
On the other hand, the maximum datagram size that can be transmitted
without fragmentation is not defined by any specification and varies by
link type. This value is called the MTU (Maximum Transmission Unit)2.
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