Additional API functions and data-structures

1
Additional API functionsand data-structures

• A look at how some information from lower-layers
  of the TCP/IP protocol stack could be accessed

2
Two more socket functions

• Besides the sendto() and recvfrom()
  socket-functions (or send() and recv()),
  there are two further functions which will be
  essential to make use of in particular
  programming situations

ssize_t sendmsg( int sock, struct msghdr
msghdr, int flags ) ssize_t recvmsg( int
sock, struct msghdr msghdr, int flags )
3
More data-structures

• The sendmsg() and recvmsg() functions make
  use of a special data-structure that collects a
  group of parameters into a neat package, called a
  message header, and supports use of
  scatter-gather operations by using an array of
  I/O-vector objects

struct iovec void iov_base size_t iov_len
Each I/O-vector describes a memory-buffer, giving
its address and its length
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Message-header
struct msghdr void msg_name // optional
address socklen_t msg_namelen // size of
address struct iovec msg_iov // scatter/gather
array int msg_iovlen // no. of
members void msg_control // ancillary data
buffer int msg_controllen // ancillary buffer
length int flags // flags on received
message This structure is used as a
value-result parameter in the recvmsg()
function, and is used as a value-only parameter
in the sendmsg() function.
5
Idea for a scatter example

• If most of your applications messages are short
  ones, but sometimes there could be a larger one,
  then you could allocate one oversized buffer to
  handle the occasional overflow from your
  small-size buffers

char smallbuf 128 , largebuf 1400 struct
iovec myiovec 2 smallbuf, 128 ,
largebuf, 1400
Data scattering The arriving data will be
accumulated in the small buffer until it is
filled, then any overflow will be accumulated in
the large buffer.
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Ancillary data

• Our main interest in using the recvmsg()
  socket-function will be to obtain ancillary
  information specifically, we want to see
  details about message-delivery failures which
  normally are not available outside the Linux
  kernels networking subsystem
• Some socket options will make the extra
  information available to an application

7
ICMP

• The Internet Control Message Protocol is used for
  relaying information between the code-modules at
  the network level of the TCP/IP protocol stack
  (e.g., for reporting errors and for various
  system queries)

Application Layer
Application Layer
Transport Layer
Transport Layer
Network Layer
Network Layer
Link Layer
Link Layer
Physical Layer
Physical Layer
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ICMP packet-format
Ethernet Header
Internet Protocol Header
ICMP Header
Data
Checksum
Code
Type
Rest of the ICMP Header (layout varies with the
Type)
32 bits
9
ICMP messages
Type
Message
Echo request or reply
8 or 0
Timestamp request and reply
13 or 14
ICMP Query messages
Address Mask request and reply
17 or 18
Router solicitation and advertisement
10 or 9
Destination unreachable
3
Source quench
4
ICMP Error reporting messages
Time exceeded
11
Parameter problem
12
Redirection
5
10
Type 3 Destination unreachable
Code
Reason
0
The network is unreachable
1
The host is unreachable
2
The protocol is unreachable
3
The port is unreachable
4
Fragmentation needed (but DF is set)
5
Source routing cannot be accomplished
6
Destination network is unknown
7
Destination host is unknown
Several additional code-values can occur for
Type 3
11
Our msgserver and msgclient

• We posted a new client-and-server demo in which
  exchanges of network messages is accomplished by
  using the sendmsg() and the recvmsg() socket
  functions and accompanying message-header
  objects
• With nicwatch we can use these demos to
  illustrate the ICMP protocol in action

12
ICMP protocol
Type 8 Echo Request
ICMP protocol
Type 0 Echo Reply
13
UDP protocol
ICMP protocol
Type 11 Time exceeded Code 0 (always zero for
Type 11)
14
UDP protocol
ICMP protocol
Type 3 Destination unreachable Code 3 The port
is unreachable