Download IPv4: Understanding the Internet Protocol Version 4 and more Study notes Computer Science in PDF only on Docsity!
copyright 2005 Douglas S. Reeves 1
The Internet Protocol,
Version 4 (IPv4)
Internet Protocols CSC / ECE 573 Fall, 2005 N.C. State University copyright 2005 Douglas S. Reeves 2
Today’s Lecture
I. IPv4 Overview
II. IP Fragmentation and Reassembly
III. IP and Routing
IV. IPv4 Options
copyright 2005 Douglas S. Reeves 3
IPv4 Overview
copyright 2005 Douglas S. Reeves 4
Internet Protocol v4 (RFC791) Functions
• A universal intermediate layer
• Routing
• Fragmentation and reassembly
copyright 2005 Douglas S. Reeves 5
“IP over Everything, Everything Over IP”
- Everything over IP
- TCP, UDP
- Appletalk
- Netbios
- SCSI
- ATM
- X.
- SNA
- Sonet
- Fibre Channel
- Frame Relay…
- Remote Direct Memory Access
- Ethernet
- IP over everything
- Dialup
- ISDN
- X.
- Ethernet
- Wi-Fi
- FDDI
- ATM
- Sonet
- …
• Even IP over IP!
copyright 2005 Douglas S. Reeves 6
IP = Basic Delivery Service
• Connectionless delivery simplifies router design
and operation
• Unreliable, best-effort delivery. Packets may be…
- lost (discarded)
- duplicated
- reordered
- and/or corrupted
copyright 2005 Douglas S. Reeves 7
IPv4 Datagram Format
version (^) length (x4)header prec^ type of service | D T R C 0 total length (in bytes) 0 4 8 16 31 identification (^) 0 DF MFflags fragment offset (x8) time-to-live (hop count) (next) protocol identifier header checksum source IP address destination IP address IP options (if any) payload 20 bytes ≤40 bytes ≤65515 bytes copyright 2005 Douglas S. Reeves 8
IPv4 Header Contents
- Functions 1. universal intermediate layer
- routing
- fragmentation and reassembly
- Options
- Version (4 bits)
- Header Length x4 (4)
- Type of Service (8)
- Total Length (16)
- Identification (16)
- Flags (3)
- Fragment Offset ×8 (13)
- Time-to-Live (8)
- Protocol Identifier (8)
- Header Checksum (16)
- Source IP Address (32)
- Destination IP Address (32)
- IP Options (≤ 320) copyright 2005 Douglas S. Reeves 9
IPv4 “Universal Common Layer” Fields
- Version: 4 (i.e., IPv4)
- Header Length ×4 (i.e., header length is always a
multiple of 4 bytes)
- normally = 5 (×4 = 20)
- at most = 15 (×4 = 60)
- Total Length (incl. IP header) < 2^16 -1 (65535 10 )
- Protocol Identifier: how to interpret the payload
- e.g., TCP = 6, UDP = 17, … copyright 2005 Douglas S. Reeves 10
Header Checksum
- Only for detecting errors in the IP header
- Algorithm
- add (ones-complement addition) consecutive 16-bit words to generate a 16 bit sum
- then one’s-complement this sum
- (for purposes of computation, assume an “initial” checksum value of all zeros) copyright 2005 Douglas S. Reeves 11
Header Checksum (cont’d)
- Receiver generates checksum on received header
and compares. If differs from received
checksum…
- IP packet is discarded
- no error messages are sent (why not?)
- What type of errors is this guaranteed to detect? copyright 2005 Douglas S. Reeves 12
Checksum Code
- Given… a) IP header b) length of the header (in units of 16 bit words) **u_short checksum(u_short header, int length) { register u_long sum = 0; while (length--) { sum += header++; _/ This is twos-complement addition /_ if (sum & 0xFFFF0000) { _/ carry occurred, wrap around /_ sum &= 0x0000FFFF; sum++; } } return ~(sum & 0x0000FFFF); _/ 1 ’s complement the sum /_ }
copyright 2005 Douglas S. Reeves 19
Fragmentation Example
- Example below: path MTU = 620 host A host B router R router R Network 1, MTU= Network 2, MTU= Network 3, MTU= 20 1480 (^20 ) 20 600 20 280 20 600 20 600 20 280 copyright 2005 Douglas S. Reeves 20
- Fragment payload size must be a multiple of 8
bytes, except for the last one
1 packet 3 fragments (Stored as 75 (= 600/8) in the header) (Stored as 150 (= 1200/8) in the header) offset = 0^ offset = 600^ offset = 1200 copyright 2005 Douglas S. Reeves 21
Fragmentation Fields
- Identification field uniquely identifies each
datagram
- allows fragments of a datagram to be matched together
- Each fragment has the same IP header as the
original IP datagram, except for the following:
- Fragment Offset
- More Fragments flag
- Options
- IP Header Length
- Total Length
- Header Checksum copyright 2005 Douglas S. Reeves 22
Fragmentation Fields (cont’d)
- IP Checksum will of course be different in
fragment than for original datagram
- More Fragments flag = 0 if this is the last (or only)
fragment of the datagram, 1 otherwise
- The Fragment Offset field gives offset of the data
(payload) portion of the fragment relative to the
start of data in the original IP datagram
- in units of 8 bytes
- 13 bits are enough to represent a maximum datagram length of 2^13 * 8 = 2^16 copyright 2005 Douglas S. Reeves 23
Fragmentation Fields (cont’d)
- IP Options may or may not be included in fragment
IP headers (option-dependent)
- IP Header Length may therefore be different than in original datagram
- Total Length is length of the fragment, not length
of the original datagram
copyright 2005 Douglas S. Reeves 24
Reassembly
- Fragments reassembled at final destination in a
reassembly buffer
- good? bad?
- What if some fragments never arrive? Problems?
- ???
- What if two fragments overlap?
- ???
copyright 2005 Douglas S. Reeves 25
Avoiding Fragmentation
- Is fragmentation even a good idea?
- Do Not Fragment flag “forbids” fragmentation by
the network. If datagram exceeds MTU of the
outgoing router interface, the router…
- discards the datagram, and
- sends ICMP error message back to the source
- Better approach: Path MTU Discovery (we’ll
discuss later)
copyright 2005 Douglas S. Reeves 26 IP Routing Fields copyright 2005 Douglas S. Reeves 27
Basic IP Routing Fields
- Source IP Address, Destination IP Address
- Time-to-Live (TTL) (max allowable “hop” count)
- max of 255, usually initialized to 128 or greater
- decremented by each router the datagram passes through
- When TTL=0…
- datagram will be discarded
- error message sent back to source by ICMP
- purpose?
- What’s the longest valid IP path length??? copyright 2005 Douglas S. Reeves 28 IP Options copyright 2005 Douglas S. Reeves 29
IP Options
- Basic protocol property: extensibility
- IP options mainly used for testing / debugging
- infrequently used; 40 bytes doesn’t give you much to work with
- Every IP option must start with:
- Code (i.e., option type)
- Option Length (maximum of 40 bytes) copyright 2005 Douglas S. Reeves 30
What Options Are Used?
- Record Route [RFC791]
- Loose Source Route [RFC791]
- Strict Source Route [RFC791]
- Time Stamp [RFC791]
- MTU Probe and Reply [RFC1191, we’ll discuss in
ICMP lecture]
copyright 2005 Douglas S. Reeves 37
IP Strict Source Route Option Example
copyright 2005 Douglas S. Reeves 38
Option #3: Timestamping
- Allows intermediate routers to insert 32-bit (ms
since midnight UT) timestamps in the option
- right now: 6,480,000,
- If IP addresses filled by source, only specified
routers will insert timestamp
Code Length Pointer First IP address (may be filled by source) First timestamp Last IP address (may be filled by source) Last timestamp
32 bits Overflow Flags copyright 2005 Douglas S. Reeves 39
Option #3: Timestamping (cont’d)
- How many entries possible?
- Problem of unsynchronized clocks?
- Not copied on fragmentation; in first fragment only copyright 2005 Douglas S. Reeves 40
Option #5: Router Alert
- Alerts routers to more closely examine the
contents of a “special” IP packet
- example protocol benefiting from this: RSVP
- Value has only one interesting interpretation: “pay
attention to this packet”
- All fragments carry the option Code Length Value 32 bits copyright 2005 Douglas S. Reeves 41
Summary
1. IP provides a universal intermediate layer,
routing, and fragmentation and reassembly
- IP is unreliable, best-effort delivery
- Fragmentation is infrequent, undesirable; is it
necessary?
- IP is extensible through the 40-byte Options field copyright 2005 Douglas S. Reeves 42
Next Lecture
- The Address Resolution Protocol (ARP)
