time according to the remote host. The remote host
uses an ICMP timestamp reply message to respond to the request.
The type field on an ICMP timestamp message can be either 13
(timestamp request) or 14 (timestamp reply). The code field
value is always set to 0 because there are no additional
parameters available. The ICMP timestamp request contains an
originate timestamp, which is the time on the requesting host
just before the timestamp request is sent. The receive
timestamp is the time that the destination host receives the
ICMP timestamp request. The transmit timestamp is filled in
just before the ICMP timestamp reply is returned. Originate,
receive and transmit timestamps are computed in numbers
milliseconds elapsed since midnight Universal Time (UT). All
ICMP timestamp reply messages contain the originate, receive
and transmit timestamps. Using these three timestamps, the host
can estimate transit time across the network by subtracting the
originate time from the transit time. It is only an estimate
however, as true transit time can vary widely based on traffic
and congestion on the network. The host that originated the
timestamp request can also estimate the local time on the
remote computer. While ICMP timestamp messages provide a simple
way to estimate time on a remote host and total network transit
time, this is not the best way to obtain this information.
Instead, more robust protocols such as Network Time Protocol
(NTP) at the upper layers of the TCP/IP protocol stack perform
clock synchronization in a more reliable manner. Web
Links Clock Synchronization Algorithms for Network
Measurements http://www.ieee-infocom.org/2002/ papers/329.pdf
Content 8.2 TCP/IP Suite Control Messages
8.2.4 Information requests and reply message
formats The ICMP information requests and reply messages
were originally intended to allow a host to determine its
network number. Figure shows the format for an ICMP information
request and reply message. Two type codes are available in this
message. Type 15 signifies an information request message, and
type 16 identifies an information reply message. This
particular ICMP message type is considered obsolete. Other
protocols such as BOOTP and Dynamic Host Configuration Protocol
(DHCP) are now used to allow hosts to obtain their network
numbers.
Content 8.2 TCP/IP Suite Control
Messages 8.2.5 Address mask requirements
When a network administrator uses the process of subnetting to
divide a major IP address into multiple subnets, a new subnet
mask is created. This new subnet mask is crucial in identifying
network, subnet, and host bits in an IP address. If a host does
not know the subnet mask, it may send an address mask request
to the local router. If the address of the router is known,
this request may be sent directly to the router. Otherwise, the
request will be broadcast. When the router receives the
request, it will respond with an address mask reply. This
address mask reply will identify the correct subnet mask. For
example, assume that a host is located within a Class B network
and has an IP address of 172.16.5.2. This host does not know
the subnet mask, thus it broadcasts an address mask
request:
Source address: 172.16.5.2 Destination address:
255.255.255.255 Protocol: ICMP = 1 Type: Address Mask Request =
AM1 Code: 0 Mask: 255.255.255.0 This broadcast is received by
172.16.5.1, the local router. The router responds with the
address mask reply: Source address: 172.16.5.1 Destination
address: 172.16.5.2 Protocol: ICMP = 1 Type: Address Mask Reply
= AM2 Code: 0 Mask: 255.255.255.0 The frame format for the
address mask request and reply is shown in Figure . Figure
shows the descriptions for each field in the address mask
request message. Note that the same frame format is used for
both the address mask request and the reply. However, an ICMP
type number of 17 is assigned to the request and 18 is assigned
to the reply. Web Links IP Routing Tech Notes
http://www.cisco.com/en/US/tech/ tk826/tk365/ tech_tech_notes_
list.html
Content 8.2 TCP/IP Suite Control
Messages 8.2.6 Router discovery message
When a host on the network boots, and the host has not been
manually configured with a default gateway, it can learn of
available routers through the process of router discovery. This
process begins with the host sending a router solicitation
message to all routers, using the multicast address 224.0.0.2
as the destination address. Figure shows the ICMP router
discovery message. The router discovery message may also be
broadcast to include routers that may not be configured for
multicasting. If a router discovery message is sent to a router
that does not support the discovery process, the solicitation
will go unanswered. When a router that supports the discovery
process receives the router discovery message , a router
advertisement is sent in return. The router advertisement frame
format is shown in Figure and an explanation of each field is
shown in Figure . Web Links ICMP Router Discovery
Messages http://www.cis.ohio-state.edu/cgi-bin/
rfc/rfc1256.html
Content 8.2 TCP/IP Suite
Control Messages 8.2.7 Router solicitation
message A host generates an ICMP router solicitation
message in response to a missing default gateway. This message
is sent via multicast and it is the first step in the router
discovery process. A local router will respond with a router
advertisement identifying the default gateway for the local
host. Figure identifies the frame format and Figures gives an
explanation of each field. Web Links ICMP
Router-Discovery Protocol (IDRP)
http://www.cisco.com/en/US/products/sw/ iosswrel/ps1831/
products_configuration_guide_ chapter09186a00800d97f7.html#xtocid15
Content 8.2 TCP/IP Suite Control Messages
8.2.8 Congestion and flow control messages If
multiple computers try to access the same destination at the
same time, the destination computer can be overwhelmed with
traffic. Congestion can also occur when traffic from a high
speed LAN reaches a slower WAN connection. Dropped packets
occur when there is too much congestion on a network. ICMP
source-quench messages are used to reduce the amount of data
lost. The source-quench message asks senders to reduce the rate
at which they are transmitting packets. In most cases,
congestion will subside after a short period of time, and the
source will slowly increase the transmission rate as long as no
other source-quench messages are received. Most Cisco routers
do not send source-quench messages by default, because the
source-quench message may itself add to the network congestion.
A small office home office (SOHO) is a scenario where ICMP
source-quench messages might be used effectively. One such SOHO
could consist of four computers networked together using CAT-5
cable and Internet connection sharing (ICS) over a 56K modem.
It is easy to see that the 10Mbps bandwidth of the SOHO LAN
could quickly overwhelm the available 56K bandwidth of the WAN
link, resulting in data loss and retransmissions. With ICMP
messaging, the host acting as the gateway in the ICS can
request that the other hosts reduce their transmission rates to
a manageable level, thus preventing continued data loss. A
network where congestion on the WAN link could cause
communication problems is shown in Figure . Web Links
BECN for Congestion Control in TCP/IP Networks: Study and
Comparative Evaluation http://www.sce.carleton.ca/faculty/lambadaris/ recent-