IP datagram reassembly algorithms
RFC 815

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RFC:  815

                   IP DATAGRAM REASSEMBLY ALGORITHMS

                             David D. Clark
                  MIT Laboratory for Computer Science
               Computer Systems and Communications Group
                               July, 1982

     1.  Introduction

     One of the mechanisms of IP is fragmentation and reassembly.  Under

certain  circumstances,  a  datagram  originally transmitted as a single

unit will arrive at its final destination broken into several fragments.

The IP layer at the receiving host must accumulate these fragments until

enough have arrived to completely reconstitute  the  original  datagram.

The  specification  document  for IP gives a complete description of the

reassembly mechanism, and contains several examples.  It  also  provides

one  possible  algorithm  for  reassembly,  based  on  keeping  track of

arriving fragments in a vector of bits.    This  document  describes  an

alternate approach which should prove more suitable in some machines.

     A  superficial  examination  of  the reassembly process may suggest

that it is rather complicated.  First, it is necessary to keep track  of

all the fragments, which suggests a small bookkeeping job.  Second, when

a  new fragment arrives, it may combine with the existing fragments in a

number of different ways.  It may precisely fill the space  between  two

fragments,  or  it  may  overlap  with existing fragments, or completely


                                   2

duplicate  existing  fragments,  or  partially  fill a space between two

fragments without abutting either of them.  Thus, it might seem that the

reassembly  process  might  involve  designing  a   fairly   complicated

algorithm that tests for a number of different options.

     In  fact,  the  process  of  reassembly  is  extremely simple. This

document describes a way of dealing with reassembly  which  reduces  the

bookkeeping  problem  to  a minimum, which requires for storage only one

buffer equal in size to the final datagram being reassembled, which  can

reassemble a datagram from any number of fragments arriving in any order

with  any  possible  pattern  of  overlap  and duplication, and which is

appropriate for almost any sort of operating system.

     The reader should consult the IP specification document to be  sure

that  he  is completely familiar with the general concept of reassembly,

and the particular header fields and vocabulary  used  to  describe  the

process.

     2.  The Algorithm

     In  order  to  define this reassembly algorithm, it is necessary to

define some terms.  A partially reassembled datagram consists of certain

sequences of octets that have already arrived, and certain  areas  still

to  come.    We will refer to these missing areas as "holes".  Each hole

can be characterized by two numbers, hole.first, the number of the first

octet in the hole, and hole.last, the number of the last  octet  in  the

hole.    This pair of numbers we will call the "hole descriptor", and we

will assume that all of the hole descriptors for a  particular  datagram

are gathered together in the "hole descriptor list".


                                   3

     The  general  form  of  the  algorithm  is  as follows.  When a new

fragment of the datagram arrives, it will possibly fill in one  or  more

of  the existing holes.  We will examine each of the entries in the hole

descriptor list to see whether the hole in  question  is  eliminated  by

this incoming fragment.  If so, we will delete that entry from the list.

Eventually, a fragment will arrive which eliminates every entry from the

list.    At this point, the datagram has been completely reassembled and

can be passed to higher protocol levels for further processing.

     The algorithm will be described in two phases. In the  first  part,

we  will  show  the  sequence  of  steps  which  are executed when a new

fragment arrives, in order to  determine  whether  or  not  any  of  the

existing  holes  are  filled by the new fragment.  In the second part of

this description, we will  show  a  ridiculously  simple  algorithm  for

management of the hole descriptor list.

     3.  Fragment Processing Algorithm

     An arriving fragment can fill any of the existing holes in a number

of ways.  Most simply, it can completely fill a hole.  Alternatively, it

may  leave some remaining space at either the beginning or the end of an

existing hole.  Or finally, it can lie in  the  middle  of  an  existing

hole,  breaking the hole in half and leaving a smaller hole at each end.

Because of these possibilities, it might seem that  a  number  of  tests

must  be  made  when  a  new  fragment  arrives,  leading  to  a  rather

complicated algorithm.  In fact, if properly  expressed,  the  algorithm

can compare each hole to the arriving fragment in only four tests.


                                   4

     We  start  the algorithm when the earliest fragment of the datagram

arrives.  We begin by creating an empty data buffer area and putting one

entry in its  hole  descriptor  list,  the  entry  which  describes  the

datagram  as  being completely missing.  In this case, hole.first equals

zero, and hole.last equals infinity. (Infinity is presumably implemented

by a very large integer, greater than 576, of the implementor's choice.)

The following eight steps are then used to insert each of  the  arriving

fragments  into  the  buffer  area  where the complete datagram is being

built up.  The arriving fragment is  described  by  fragment.first,  the

first  octet  of  the fragment, and fragment.last, the last octet of the

fragment.

   1. Select the next hole  descriptor  from  the  hole  descriptor
      list.  If there are no more entries, go to step eight.

   2. If fragment.first is greater than hole.last, go to step one.

   3. If fragment.last is less than hole.first, go to step one.

         - (If  either  step  two  or  step three is true, then the
           newly arrived fragment does not overlap with the hole in
           any way, so we need pay no  further  attention  to  this
           hole.  We return to the beginning of the algorithm where
           we select the next hole for examination.)

   4. Delete the current entry from the hole descriptor list.

         - (Since  neither  step  two  nor step three was true, the
           newly arrived fragment does interact with this  hole  in
           some  way.    Therefore,  the current descriptor will no
           longer be valid.  We will destroy it, and  in  the  next
           two  steps  we  will  determine  whether  or  not  it is
           necessary to create any new hole descriptors.)

   5. If fragment.first is greater than hole.first, then  create  a
      new  hole  descriptor "new_hole" with new_hole.first equal to
      hole.first, and new_hole.last equal to  fragment.first  minus
      one.


                                   5

         - (If  the  test in step five is true, then the first part
           of the original hole is not filled by this fragment.  We
           create a new descriptor for this smaller hole.)

   6. If fragment.last is less  than  hole.last  and  fragment.more
      fragments   is  true,  then  create  a  new  hole  descriptor
      "new_hole", with new_hole.first equal to  fragment.last  plus
      one and new_hole.last equal to hole.last.

         - (This   test  is  the  mirror  of  step  five  with  one
           additional feature.  Initially, we did not know how long
           the reassembled datagram  would  be,  and  therefore  we
           created   a   hole   reaching  from  zero  to  infinity.
           Eventually, we will receive the  last  fragment  of  the
           datagram.    At  this  point, that hole descriptor which
           reaches from the last octet of the  buffer  to  infinity
           can  be discarded.  The fragment which contains the last
           fragment indicates this fact by a flag in  the  internet
           header called "more fragments".  The test of this bit in
           this  statement  prevents  us from creating a descriptor
           for the unneeded hole which describes the space from the
           end of the datagram to infinity.)

   7. Go to step one.

   8. If the hole descriptor list is now empty, the datagram is now
      complete.  Pass it on to the higher level protocol  processor
      for further handling.  Otherwise, return.

     4.  Part Two:  Managing the Hole Descriptor List

     The  main  complexity  in  the  eight  step  algorithm above is not

performing the arithmetical tests, but in adding  and  deleting  entries

from  the  hole descriptor list.  One could imagine an implementation in

which the storage management package was  many  times  more  complicated

than  the rest of the algorithm, since there is no specified upper limit

on the number of hole descriptors which will exist for a datagram during

reassembly.   There  is  a  very  simple  way  to  deal  with  the  hole

descriptors, however.  Just put each hole descriptor in the first octets


                                   6

of  the  hole  itself.    Note  that by the definition of the reassembly

algorithm, the minimum size of  a  hole  is  eight  octets.    To  store

hole.first  and  hole.last  will presumably require two octets each.  An

additional two octets will be required to thread together the entries on

the hole descriptor list.  This leaves at least two more octets to  deal

with implementation idiosyncrasies.

     There  is  only  one obvious pitfall to this storage strategy.  One

must execute the eight step algorithm above before copying the data from

the fragment into the reassembly buffer.  If one were to copy  the  data

first,  it might smash one or more hole descriptors.  Once the algorithm

above has been run, any hole descriptors which are about to  be  smashed

have already been rendered obsolete.

     5.  Loose Ends

     Scattering  the  hole  descriptors throughout the reassembly buffer

itself requires that they be threaded onto some sort  of  list  so  that

they can be found.  This in turn implies that there must be a pointer to

the head of the list.  In many cases, this pointer can be stored in some

sort  of  descriptor block which the implementation associates with each

reassembly buffer.  If  no  such  storage  is  available,  a  dirty  but

effective  trick  is  to  store  the  head  of the list in a part of the

internet header in the reassembly buffer which is no longer needed.   An

obvious location is the checksum field.

     When  the final fragment of the datagram arrives, the packet length

field in the internet header should be filled in.


                                   7

     6.  Options

     The preceding description made one unacceptable simplification.  It

assumed that there were no internet options associated with the datagram

being  reassembled.    The  difficulty  with  options  is that until one

receives the first fragment of the datagram, one cannot tell how big the

internet header will be.  This is because,  while  certain  options  are

copied  identically  into  every  fragment of a datagram, other options,

such as "record route", are put in the first fragment only.  (The "first

fragment"  is  the  fragment  containing  octet  zero  of  the  original

datagram.)

     Until  one  knows how big the internet header is, one does not know

where to copy the data from each fragment into  the  reassembly  buffer.

If  the  earliest  fragment  to arrive happens to be the first fragment,

then this is no problem.  Otherwise, there are two  solutions.    First,

one  can  leave  space in the reassembly buffer for the maximum possible

internet header.  In fact, the  maximum  size  is  not  very  large,  64

octets.    Alternatively,  one can simply gamble that the first fragment

will contain no options.  If, when the first fragment  finally  arrives,

there  are  options,  one  can  then  shift  the  data  in  the buffer a

sufficient distance for allow for them.  The only peril in  copying  the

data  is  that  one  will  trash  the  pointers  that  thread  the  hole

descriptors together.  It is easy to see how to untrash the pointers.

     The source and record route options have  the  interesting  feature

that,  since  different  fragments  can follow different paths, they may

arrive with different return routes  recorded  in  different  fragments.


                                   8

Normally,  this  is  more information than the receiving Internet module

needs.  The specified procedure is to take the return route recorded  in

the first fragment and ignore the other versions.

     7.  The Complete Algorithm

     In addition to the algorithm described above there are two parts to

the reassembly process.  First, when a fragment arrives, it is necessary

to  find  the  reassembly  buffer  associated  with that fragment.  This

requires some  mechanism  for  searching  all  the  existing  reassembly

buffers.   The correct reassembly buffer is identified by an equality of

the following fields:  the  foreign  and  local  internet  address,  the

protocol ID, and the identification field.

     The  final  part  of  the  algorithm  is  some  sort of timer based

mechanism which decrements the time to  live  field  of  each  partially

reassembled  datagram,  so that incomplete datagrams which have outlived

their usefulness can be detected and deleted.  One can either  create  a

demon  which  comes alive once a second and decrements the field in each

datagram by one, or one can read the  clock  when  each  first  fragment

arrives,  and  queue  some  sort  of  timer  call, using whatever system

mechanism is appropriate, to reap the datagram when its time has come.

     An implementation of the complete algorithm  comprising  all  these

parts  was  constructed  in BCPL as a test.  The complete algorithm took

less than one and one-half pages of listing, and generated approximately

400 nova machine instructions.  That portion of the  algorithm  actually

involved with management of hole descriptors is about 20 lines of code.


                                   9

     The   version  of  the  algorithm  described  here  is  actually  a

simplification of the author's original version, thanks to an insightful

observation by Elizabeth Martin at MIT.