C H A P T E R 4 F E A T U R E S
V100 Versatile Multiplexer Technical Manual Version 2.2
Page 174 of 231
Each virtual port requires a DBA (Dynamic Bandwidth Allocation) and Destination configured. The DBA is
the maximum bandwidth (in bits per second) of IP traffic that can be sent to the peer V100 – the actual
amount is determined by the data routers in the V100 network according to other offered loads). The
destination is the virtual port identifier on the remote device that we are peering with.
The IP field indicates that we are running unnumbered IP across this link – the IP address and mask
should match the ENET address and mask to allow for host operations from this virtual port. The MTU
(maximum transmission unit) is discussed below.
One quirk of IP unnumbered operation is that IP routes that are configured for IP networks across the
V100 network must be configured with a next-hop that identifies the unnumbered link to transfer that
data across, instead of an IP address of the next-hop gateway. This is discussed further in the section on
Static Routes.
4.13.6
MTUs
The Maximum Transmission Unit or MTU for an IP network specifies the largest datagram that may be
transmitted onto that network. Routed packets that exceed the MTU for the onward network are
fragmented before transmission over the V100 network and are reassembled by the peer V100 unit. The
default MTU for the Ethernet is 1514 – this allows transmission of the maximum sized Ethernet frame (the
4-byte CRC is not included in the MTU).
Most servers are on segments with large MTUs, but it is increasingly common for internet users to be
connected via links with reduced MTUs, so it is becoming common for some packets to be too big. How
the problem of oversize packets has been handled has evolved considerably over time. The original
approach was to send only small packets corresponding to the TCP/IP default MTU (576 bytes). (To this
day, a sending system needs permission from the receiving system to send larger packets, but that
permission is given as a matter of routine.) For some packets, especially those sent by older equipment,
an oversize packet can be sent by breaking it into fragments and sending the fragments as smaller
packets. The fragments can be reassembled downstream to reconstruct the original large packet, but this
packet fragmentation has several problems involving both efficiency and security.
Newer servers try to optimise their transmissions by discovering the path MTU and sending packets of the
maximum size when there's enough data to fill them. The procedure for doing this was standardized and
published as
RFC 1191
in 1990, but it did not become widely deployed until years later. By mid 2002,
80% to 90% of computers on the internet used Path MTU Discovery.
The basic procedure is simple - send the largest packet you can, and if it won't fit through some link get
back a notification saying what size will fit. The notifications arrive as ICMP (Internet Control Message
Protocol) packets known as "fragmentation needed" ICMPs (ICMP type 3, subtype 4). The notifications are
requested by setting the "do not fragment" (DF) bit in packets that are sent out.
Some network and system administrators view all ICMPs as risky and block them all, disabling path MTU
discovery, usually without even realizing it. Of the several dozen ICMP types and subtypes, some do pose
